Chapter 1
Visual Testing
Table of Contents
Fundamentals of Visual Testing
Vision
The Cornea
The Cornea
Crystalline Lens
Aqueous Humor
Vitreous Humor
Cataracts
Accommodation
Modeling Accommodation
Ciliary Muscle and Fibers
Optic Nerve
The Retina
The Fovea Centralis
Measuring the electromagnetic spectrum
Example:
Solution:
E1 = (d2 / d1)2 * E2
2 and 3mm Semi-Rigid Needle Scopes
DIAL CALIPER GAUGES
DIAL SPRING CALIPERS,Internal
RECESS GAUGE SLOT GAUGE
GEAR TOOTH VERNIER CALIPERS
Vernier Height Gage
VERNIER DEPTH GAUGE
Creep (Stress Rupture) Voids In A 304ss Stainless Steel Furnace Tube
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Standard Terminology for Visual and Optical Testing and Materials Processing
Angstrom unit (A) - Unit of length, equal to 0.1 nm.2
Base material - The material to be welded, brazed, soldered, or cut. See Base metal
Bertrand Lens - in polarizing microscopes a special lens used to view interfaced figures.
Butt weld or butt joint - Weld joining two metal pieces in the same plane.2
Crack, transverse - Cracks at right angles to the length of the test object.6
Graybody - Radiator whose spectral emissivity is uniform for all wavelengths.2
Groove face - The portion of a surface or surfaces of a member included in a groove.5
N-type semiconductor - See Semiconductor.
Photometer - The basic measuring instrument of photometry.2
P-type semiconductor - See Semiconductor.
Quality of lighting - Level of distribution of luminance in a visual task or environment
Seam - (1) On the surface of metal, an unwelded fold or lap that appears as a crack, usuall resulting
from a discontinuity obtained in casting or working. (2) Mechanical or welded joints. (3)
Longitudinal surface discontinuity on metal originating from a surface crack or blowhole near the
surface of the ingot, that is drawn out during rolling and follows the rolling direction. Also due to
overfill while rolling. After forging, seams generally follow the direction of flow lines.
Specular - Pertaining to a mirror-like reflective finish, as of a metal
Toe of weld - The junction between the face of a weld and the base metal
Glossary References
Visual Testing
Fundamentals of Visual Testing
Visual testing (VT) is perhaps the oldest and most widely used inspection technique. Often, the eyes of
the inspector are the only “equipment” used for the inspection. VT is applicable to virtually any
material, at any stage of manufacture, at any point in its service life. To perform a successful direct
visual examination, adequate lighting and good inspector eyesight is required. If access to specific
areas of the test article is limited, borescopes, fiberscopes, video probes, and closed-circuit television
(CCTV) can be used to perform remote VT For a thorough understanding of the VT method, the listed
references and resource materials are recommended.
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Vision
The inspector’s eyes are the most important element in VT The eye is a complex organ composed of
many structures - all of which must function properly for reliable examinations to be performed. The
components of the human eye are shown in Figure 1.1. Reflected light enters the eye through a tear
film in the cornea where the majority of light refraction occurs. This refracted light then passes
through the iris, the thin colored membrane that controls the amount of light entering the eye. Past the
iris is the crystalline lens, which changes shape in order to focus the light on the retina at the back of
the eyeball to maintain its shape, the eyeball is filled with a clear gel called the vitreous humor. The
retina is covered with specialized cells, called rods and cones that convert the incoming light into
nerve impulses through a photochemical process. These nerve impulses travel to the brain through the
optic nerve, where they are decoded.
- Sphincter pupillae
- Dilator pupillae
Figure 1.1: Components of the human eye
The Cornea
The cornea represents the strongest part of the refracting power of the eye, providing about
80% of the power of the system. The index of refraction of the cornea is about 1.376. Rays
pass from the cornea into the watery fluid known as the aqueous humor which has an index
of refraction of about 1.336, so most of the refraction is at the cornea-air interface.
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The Cornea
The cornea represents the strongest part of the
refracting power of the eye, providing about 80% of
the power of the system. The index of refraction of
the cornea is about 1.376. Rays pass from the cornea
into the watery fluid known as the aqueous humor
which has an index of refraction of about 1.336, so
most of the refraction is at the cornea-air interface.
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Crystalline Lens
About 9mm in diameter and 4 mm thick, the
crystalline lens provides perhaps 20% of the refracting
power of the eye. Hecht likens it to a tiny transparent
onion with some 22,000 fine layers. The index ranges
from about 1.406 at the center to about 1.386 in outer
layers, making it a gradient index lens. It is pliable,
and changes shape to accomplish accommodation for
close focusing.
The term cataract is used to describe the condition of
clouding or darkening of this lens.
Aqueous Humor
The anterior chamber of the eye is filled
with the watery “aqueous humor” which
has an index of refraction of about 1.336.
It is positioned immediately behind the
cornea. The larger chamber of the eye is
filled with the gelatinous “vitreous
humor”, which has an index of refraction
of about 1.337 .
Vitreous Humor
The large chamber of the eye is filled with the gelatinous “vitreous humor”, which has an
index of refraction of about 1.337. The front chamber of the eye, immediately behind the
cornea, is filled with the watery “aqueous humor” which has an index of refraction of
about 1.336.
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Cataracts
When the inner lens of the eye becomes darkened or
opaque, the condition is called a cataract. The lens may be
surgically replaced with a plastic lens. This can have
dramatic results in restoring vision to the eye. The
implanted lens is of fixed focal length, so it is not capable
of accommodation like the natural lens. This is usually not
a major concern, because persons who develop cataracts
after age 60 do not have much accommodation remaining
anyway because the inner lens has become less pliable with
age.
Accommodation
When the eye is relaxed and the interior lens is the least rounded, the lens has its maximum
focal length for distant viewing . As the muscle tension around the ring of muscle is
increased and the supporting fibers are thereby loosened, the interior lens rounds out to its
minimum focal length.
Modeling Accommodation
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To model the accommodation of the eye, the scale
model eye was used with the cornea through the
front surface of the lens held constant at the
model values. The matrix for that part of the eye
was calculated and then the thickness d and back
surface power P were varied.
Ciliary Muscle and Fibers
When the ciliary muscles contract, they loosen the ciliary fibers which are attached to the envelope
of the crystalline lens. Because the lens is pliable, it relaxes into a more curves shape, increasing
it’s refractive power to accommodate for closer viewing. The iris serves as the aperture stop for the
eye, closing to about 2mm in diameter in bright light and opening to a maximum of about 8mm in
dim light.
Optic Nerve
The optic nerve is the cable of nerve fibers with carries the electrical signals from the retina to the
brain for processing. The point of departure of that optic nerve through the retina does not have
any rods or cones, and thus produces a “blind spot”.
The Retina
The retina is a light-sensitive layer at the back of the eye that covers about 65 percent of its
interior surface. Photosensitive cells called rods and cones in the retina convert incident light
energy into signals that are carried to the brain by the optic nerve. In the middle of the retina is a
small dimple called the fovea or fovea centralis. It is the center of the eye’s sharpest vision and the
location of most color perception.
“A thin layer (about 0.5 to 0.1mm thick) of light receptor cells covers the inner surface of the
choroid. The focused beam of light is absorbed via electrochemical reaction in this pinkish
multilayered structure. The human eye contains two kinds of photoreceptor cells; rods and cones.
Roughly 125 million of them are intermingled nonuniformly over the retina. The ensemble of rods
(each about 0.002 mm in diameter in some respects has the characteristics of a high-speed, black
and white film(such as Tri-X). It is exceedingly sensitive, performing in light too dim for the
cones to respond to, yet it is unable to distinquish color, and the images it relays are not well
defined.”(Hecht)
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“In contrast, the ensemble of 6 or 7 million cones (each about 0.006 mm in diameter) can be
imagined as a separate, but overlapping, low-speed color film. It performs in bright light, giving
detailed colored views, but is fairly insensitive at low light levels.”(Hecht)”
The Fovea Centralis
Though the eye receives data from a field of about 200 degrees, the acuity over most of that range is poor. To form
high resolution images, the light must fall on the fovea, and that limits the acute vision angle to about 15 degrees. In
low light, this fovea constitutes a second blind spot since it is exclusively cones which have low light sensitivity. At
night, to get most acute vision one must shift the vision slightly to one side, say 4 to 12 degrees so that the light falls
on some rods.
A “dimple on the retina” provides our highest resolution vision.
“Just about at the center of the retina is a small depression from 2.5 to 3 mm in diameter known as the yellow spot, or
macula. There is a tiny rod-free region about 0.3mm in diameter at its center, the fovea centralis. (In comparison the
image of the full Moon on the retina is about 0.2 mm in diameter.) Here the cones are thinner (with diameters of
0.0030mm to 0.0015mm) and more densely packed than anywhere else in the retina. Since the fovea provides the
sharpest and most detailed information, the eyeball is continuously moving, so that light from the object of primary
interest falls on this region. ...the rods are multiply connected to nerve fibers, and a single such fiber can be activated
by any one of about a hundred rods. By contrast, cones in the fovea are individually connected to nerve fibers. The
actual perception of a scene is constructed by the eye-brain system in a continuous analysis of the time-varying retinal
image.”(Hecht)
Eyesight can be adversely affected by a malfunction of anyone of these structures. Glaucoma, an
increase in the fluid pressure within the eye, may produce slight aberrations or complete b1indness.
Myopia, hyperopia and astigmatism occur when light is focused on a plane other than the retinal plane.
As an individual ages, the crystalline lens loses its ability to flex for focusing, and cataracts may occur.
To ensure that the eyesight of a visual inspector is adequate, periodic vision examinations are
performed to verify visual acuity, depth perception, and color discrimination. An individual diagnosed
with a deficiency that can be remedied by corrective lenses will be required to wear them during any
visual examination.
Visual Perception
Visual perception is the comparison of what the eyes see with what the mind sees. Although there is a
tremendous amount of information available in any given image that is directed to the retina, only a
small percentage is used for detail recognition. This is due to the overall make-up of the human vision
system. The eye is nothing more than a receptor that gathers and focuses the incoming information
contained in the entering photons, while the brain takes the provided information and processes it into
a mental image that will be interpreted.
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The individual does not necessarily notice all of the information available in a particular field of view.
The portion of the image that appears on the fovea is the sharpest because of the large grouping of rods
and cones in this area. It is this information that the brain processes regarding the qualities of the
object of attention. The parts of the image that extend farther away from the fovea toward the retina
decrease in resolution as the distance from the fovea increases due to a decrease in the number of rods
and cones. Peripheral vision is not completely ignored, but it serves largely as a reference for
orientation and motion and does not enhance perception.
For this reason, an object being examined should be observed with a series of intermittent scans,
where the inspector looks at a small area, scans a few degrees and looks again, repeating this sequence
over the entire area of interest. Even though the field of view may completely encompass the entire
object being examined; only a small portion of that field is usable for detailed information.
Lighting
To understand the importance of lighting ill an inspection atmosphere, it is essential to know the
fundamentals of light, how it is measured, and the recommended lighting levels for inspections.
Fundamentals of Light
There are several existing theories that describe the phenomena of radiant energy. The wave theory
and the quantum theory are the two widely-accepted theories. The wave theory proposes that radiation
originates from accelerating charged particles (i.e., vibrating electrons) and travels through space and
time in wave-like movements. The quantum theory, developed through modem physics, proposes that
energy is emitted and absorbed in discreet quanta or packets of energy (photons). While both models
are based on a mass less, charge less transfer of energy at a speed of 3 X 1010 cm/s, each proposes a
different explanation of light’s interaction with matter.
Visible light, the energy that stimulates the receptors of the human eye, is generally defined as energy
in the wavelength range of 380 to 770 nm. Visible light exhibits properties of both the wave model and
the quantum model
The electromagnetic spectrum is a convenient way to graphically depict electromagnetic radiation. It is
based on the wave theory model. The electromagnetic spectrum encompasses the range of energies
from extremely short wavelength cosmic rays to long wavelength electrical waves as shown in Figure
1.2.
Visible light is produced from the electron cloud of an atom when an external force disturbs its
electrons. Energy from the external force removes an electron from its original energy level and, upon
its return to that energy level, the excess energy is emitted as quanta of light. This light travels in a
nearly straight line until it encounters a medium or force that reflects, refracts, or diffracts it.
Visible light, known as “white” light, is actually a broad spectrum of frequencies. When white light is
passed through a prism, it is separated into the constituent frequencies that produce the vision
sensation of color. An important point to remember is that all electromagnetic radiation is similar in
nature and that it is the specialized properties of the eye that allow the visible portion of the spectrum
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to stimulate sight.
Figure 1.2: The electromagnetic spectrum
Listed below are the approximate wavelength, frequency, and energy limits
of the various regions of the electromagnetic spectrum.
Wavelength (m)
Frequency (Hz)
Energy (J)
Radio
> 1 x 10-1
< 3 x 109
< 2 x 10-24
Microwave
1 x 10-3 - 1 x 10-1
3 x 109 - 3 x 1011
2 x 10-24- 2 x 10-22
Infrared
7 x 10-7 - 1 x 10-3
3 x 1011 - 4 x 1014
2 x 10-22 - 3 x 10-19
Optical
4 x 10-7 - 7 x 10-7
4 x 1014 - 7.5 x 1014
3 x 10-19 - 5 x 10-19
UV
1 x 10-8 - 4 x 10-7
7.5 x 1014 - 3 x 1016
5 x 10-19 - 2 x 10-17
X-ray
1 x 10-11 - 1 x 10-8
3 x 1016 - 3 x 1019
2 x 10-17 - 2 x 10-14
Gamma-ray
< 1 x 10-11
> 3 x 1019
> 2 x 10-14
Measuring the electromagnetic spectrum
You actually know more about it than you may think! The electromagnetic (EM) spectrum is just a
name that scientists give a bunch of types of radiation when they want to talk about them as a group.
Radiation is energy that travels and spreads out as it goes—visible light that comes from a lamp in
your house or radio waves that come from a radio station are two types of electromagnetic radiation.
Other examples of EM radiation are microwaves, infrared and ultraviolet light, X-rays and gammarays. Hotter, more energetic objects and events create higher energy radiation than cool objects. Only
extremely hot objects or particles moving at very high velocities can create high-energy radiation like
X-rays and gamma-rays.
Radio: yes, this is the same kind of energy that radio stations emit into the air for your boom box
to capture and turn into your favorite Mozart, Madonna, or Coolio tunes. But radio waves are
also emitted by other things ... such as stars and gases in space. You may not be able to dance to
what these objects emit, but you can use it to learn what they are made of.
Microwaves: they will cook your popcorn in just a few minutes! In space, microwaves are used
by astronomers to learn about the structure of nearby galaxies, including our own Milky Way!
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Infrared: we often think of this as being the same thing as ‘heat’, because it makes our skin feel
warm. In space, IR light maps the dust between stars.
Visible: yes, this is the part that our eyes see. Visible radiation is emitted by everything from
fireflies to light bulbs to stars ... also by fast-moving particles hitting other particles.
Ultraviolet: we know that the Sun is a source of ultraviolet (or UV) radiation, because it is the
UV rays that cause our skin to burn! Stars and other “hot” objects in space emit UV radiation.
X-rays: your doctor uses them to look at your bones and your dentist to look at your teeth. Hot
gases in the Universe also emit X-rays .
Gamma-rays: radioactive materials (some natural and others made by man in things like nuclear
power plants) can emit gamma-rays. Big particle accelerators that scientists use to help them
understand what matter is made of can sometimes generate gamma-rays. But the biggest gammaray generator of all is the Universe! It makes gamma radiation in all kinds of ways.
Measurement of Light
Photometry, the measurement of light, is a means of quantifying the radiant energy of visible light.
Measurements are obtained with a photometer, which converts the radiant energy of the light into a
measurable electrical signal When measuring visible light, the inverse square law and the Lambert
Cosine law are frequently used. The inverse square law (Equation 1 and Figure 1.3) states that the
illumination of a surface varies inversely as the distance between the light source and surface is
squared.
or E1 d12 = E2 d22 --- 1
where:
I
E
d
= luminous intensity
= illuminance
= distance between the point and source
For example, a light source with a luminance of 1000 lm measured at 0.3 m (1 ft) will be reduced to 250 lm at a distance of
0.6 m (2 ft). The Lambert Cosine law (Equation 2) states that the luminance of a surface varies as the cosine of the angle of
incidence. Combining the inverse square law with the Lambert Cosine law (Figure 1.4) allows illuminance at angles other
than normal to be calculated.
E = I cos θ (Eq. 2)
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where:
I
= source illuminance
E = surface illuminance
θ = angle of incidence
Both laws can be combined to yield the following relationship: E =
Cosine Cubed Law:
An extension of the cosine law is the cosine cubed law, which may be convenient alternative in certain
calculations. By substituting a/Cosθ for d, the above equation may be written as:
E=
Figure 1.3: The inverse square law
Example:
You measure 10.0 lm/m2 from a light bulb at 1.0 meter. What will the flux density be at half the
distance?
Solution:
E1 = (d2 / d1)2 * E2
E0.5 m = (1.0 / 0.5)2 * 10.0 = 40 lm/m2
Figure 1.4: The Lambert Cosine law
Figure 1.4: The Lambert Cosine law
Measurements of visible light arc made in reference to primary standards established by national
physical laboratories. In the United States, the National Institute of Standards and Technology (N1ST)
maintains most physical standards. From these primary standards, working standards are prepared for
use in calibrating photometry equipment. Using the basic laws of photometry with readings from
photometric or photoelectric instruments, measurements of unknown light sources may be made.
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Color Models and Photometry
Most models of perceived color contain three
components: hue, saturation, and lightness. In the CIE
L*a*b* model, color is modeled as a sphere, with
lightness comprising the linear transform from white to
black, and hues modeled as opposing pairs, with
saturation being the distance from the lightness axis.
Photometry is concerned with the measurement of
optical radiation as it is perceived by the human eye.
The CIE 1931 Standard Observer established a
standard based on the average human eye response
under normal illumination with a 2° field of view. The
tristimulus values graphed below represent an attempt
to describe human color recognition using three
sensitivity curves. The y(l) curve is identical to the CIE
V(l) photopic vision function. Using three tristimulus
measurements, any color can be fully described.
Tristimulus values
Measurable characteristics of light, light sources and lighting materials
Characteristic
Dimensional Unit
Equipment
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light
Wavelength1
Color’
Illuminance (flux density)2
Orientation of polarization1
Degree of polarization1
meter
none
lux
degree (angle)
percent (dimensionless ratio)
spectrometer
spectrophotometer and colorimeter
photometer
analyzing prism
polarization photometer
Energy radiated1
Color temperature2
Luminous intensity2
Luminance2
Spectral power distribution1
joule per square meter
kelvin
candela
candela per square meter
watts per nanometer
Power consumption2
watt
lumen
lumen or candela
calibrated radiometer
colorimeter or filtered photometer
photometer
photometer or luminance meter
spectroradiometer
wattmeter, or voltmeter and ammeter (for
direct current and for unity power factor
alternating current circuits)
integrating sphere photometer
distribution meter or goniometer
percent (dimensionless ratios)
percent (dimensionless ratios)
percent (at specific wavelengths)
dimensionless number
reflectometer
photometer
spectrophotometer
densitometer
light Sources
Luminous flux (light output)1
Zonal distribution1
Lighting Materials
Reflectance2
Transmittance2
Spectral reflectance and transmittance1
Optical density
1. CAN BE MEASURED IN THE LABORATORY.
2. CAN BE MEASURED IN THE FIELD OR THE LABORATORY.
Quantity
Symbols, equations, and units for photometry and radiometry
Radiometry
Symbol
Equation
Units
Radiant Energy
Qe
Radiant Flux
Φ
W
Ee
Φ = dQ/dt
E
Me = dΦ/dA
Ee = dΦ/dA
Ie
Ie = dΦ/dω
W/sr
Le
Le = d2Φ/(dω dA cos θ)
W/sr-1.m-2
E
Me
Radiant Exitance
Irradiance
Radiant Intensity
Radiance
J (erg)
W/cm2
W/m2
Photometry
Quantity
Symbol
Equation
Units
Luminous Energy
Qv
Luminous Flux
Φ
v
Φ = dQldt
v
lm
Luminous Exitance
Illuminance
Mv
Ev
Mv= dΦ/dA
Ev= dΦ/dA
lm/ft2
lx (ftc)
Luminous Intensity
Iv
Iv= dΦ/dω
cd
lm.h
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Luminance
where:,
J = joule
W = watt
m = meter
Lv= d2Φ/dω (dA cosθ)
Lv
2
cd/m
lx = lux (lumen/m2)
h = hour
ftc = footcandles
w = solid angle
lm = lumen
s = second
sr = steradian
cd = candela
ft = feet
Recommended Lighting Levels
Adequate lighting at the inspection surface is essential for the proper identification of indications.
Often, the general illumination of the work area is sufficient for visual examination; however, the
governing code or specification should be referenced for the minimum lighting level required.
Recommended maximum luminance ratios
Between tasks and adjacent darker surroundings
Between tasks and adjacent lighter surroundings
Between tasks and more remote darker surfaces
Between tasks and more remote lighter surfaces
3 to 1
1 to 3
20 to 1
1 to 20
Illuminance values for some visual inspection activities
Activity
Range of Illuminance
Lux (lx)
Footcandles (ftc)
100-200
10-20
Occasional visual tasks
Tasks with high contrast 200-500
or large size
Tasks with medium
500-1,000
contrast or small size
Tasks with low contrast 1,000-2,000
or very small size
20-50
Lighting Type
General lighting
50-100
Task lighting
100- 200
Lighting Techniques
When the illumination level at the inspection surface is determined to be inadequate, every effort
should be made to provide the necessary lighting. To provide the necessary illumination during VT,
flashlights, portable shop lights, and high-intensity lamps should be considered. Another option would
be to move the test place to a brighter inspection area if possible.
In addition to the illumination intensity at the inspection site, the color of the light is also important.
Color plays a significant roll in increasing contrast in the inspection area, For example, the inspection
of chromium plating over nickel may be enhanced by using a bluish light such as that provided by
“daylight” fluorescent lamps.
Surfaces and the detectability of indications can vary greatly due to the characteristics of the light
source; therefore, the characteristics of the light source used during an inspection should be as close as
practical to the light source used to examine reference standards.
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Whatever illumination source is chosen, consideration should be given to its location. The distance of
the light source from the test piece and its angular position determine the intensity of the light and the
amount or absence of glare. As with the lighting characteristics, the physical configuration of the
equipment should closely approximate the conditions that were used during the examination of the
reference standard.
Room Lighting
Area or room lighting can be direct, semi-direct, or general diffuse lighting. Direct lighting is where the light source directs
90-100 percent of the output downward. This type of lighting produces high illuminance in the vertical directioQ. but it can
also produce shadows, glare, and veiling reflections. Highly reflective walls are generally recommended to control these
effects.
Semi-direct lighting directs 60-90 percent of the total light output downward with the balance of the light directed upwards.
This type of lighting eliminates ceiling shadows, but the use of any bright ceilings should be avoided.
General diffuse lighting directs approximately equal amounts of light upward and dowQward. This type of lighting
generally provides good brightness relationships throughout the room and produces horizontal illuminance to soften
shadows. Glare is minimal :-vith this type of lighting.
Task Lighting
Task lighting may be classified based on its layout or orientation to the task to be performed. General task lighting provides
approximately uniform illuminance over a broad area. It is generally produced by the area or room lighting.
Localized general lighting provides uniform general lighting and is oriented to provide optimum1illumination in the
inspection area. Commonly, ceiling lights are positioned close to the inspection area.
Local lighting includes desk lamps and other portable lighting equipment. Local lighting produces ,very high illuminance
and is useful when shadow formation produces enhanced contrast. Local lighting can cause direct and reflected glare. To
prevent eye fatigue due to constant eye adaptation, the general room lighting should provide at least 20-30 percent of the
tota1 illumination.
Optical Inspection and Machine Vision Lighting
When choosing lighting for optical inspection, techniques and machine vision applications, the goal is to optimize the
contrast between a potential discontinuity and its surroundings by providing stable illumination that reduces signal
processing complexity. Lighting that takes advantage of the spectral sensitivity of the light sensor and the optical
characteristics of the test material are recommended.
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Optical Inspection and Machine Vision Lighting (cont’d)
The spectral sensitivity of sensors are quite different from that of the human eye. This difference
allows for the use of light sources that are not effective during direct visual inspection, or those that
are harmful to the eye. This includes polarized light, monochromatic light, and coherent or laser light.
Sensors that are optimized for the inspection of self radiating materials such as high-temperature
metals or glass can also be used. Most light sensors cannot adapt to differing levels of illumination as
easily as the human eye, which makes consistent illumination a critical factor in the inspection process.
When determining the lighting geometry and the type of light source for an inspection, the reflective,
absorptive, and transmitive characteristics of the test material arc the primary factors to be considered.
Reflective characteristics describe a diffuse or specular surface and selective and nonselective
reflecting materials. Spectrally selective reflectors reflect uneven amounts of the incident light’s initial
wave length and absorb or transmit the balance. The non-uniform reflective characteristics of a
spectrally selective reflector may be illuminated by the use of a monochromatic illumination source.
Absorptive material characteristics include selective and selective absorption. Nonselective materials
appear black or gray in appearance. Selective materials absorb some wavelengths more than others.
They appear to have a distinctive color.
Transmitive characteristics describe transparent and translucent materials. Transparent materials
transmit the light spectrum with no apparent scatter. Transparent materials transmit a large percentage
of the light but scatter some portion due to diffusion opaque materials transmit no light - the entire
spectrum is absorbed or reflected.
The placement of the light source or sources and the amount of direct or diffuse light produced by
each light source are critical for accurate inspections. The type of illuminator controls the amount of
direct and diffuse light produced. Types of illuminators include condenser reflectors, spot projectors,
diffuse light sources and collimators
Condenser and spot projectors have mirrors or reflective surfaces designed to produce a field of light
that is a gradually expanding cone. The condenser’s reflector (front lens) concentrates the cone of light
while a spot or flood projector allows for the gradual expansion of the light cone. Collimators use a
small hole to produce a direct cone of light that expands very little. Diffuse surface light sources use a
translucent material to scatter the incident light.
Common illumination geometries for optical inspections such as microscopic examinations and
machine vision include diffuse front illumination, dark field specular illumination, 1igl1t held specular
illumination diffuse rear field illumination, and rear offset illumination.
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Diffuse front illumination is used” to flood the area of interest with as much light as possible and to minimize shadow
formation. It typically is used on opaque materials where there are prismatic reflections or where the contrast values are
very high. By making the light source more directional, differences in the surface texture become more apparent.
Diffuse Front Illumination
Specular materials are inspected for differences in contrast due to the reflection angle using dark field or light field
specular illumination. With dark field specular illumination, the sensor is positioned at an angle other than the angle of
reflection. Under normal circumstances, the camera shows only a dark field. The sensor receives a signal only when an
irregularity in the surface changes the angle of reflection. This technique is normally used for surface flaw recognition.
Dark Field Specular Illumination
Light Field Specular Illumination
With light field specular illumination, the light sensor is positioned in line with the reflected light. An irregularity causes
the light to be deflected away from the sensor. The contrast value produced by this method are much lower than those
produced by dark field geometry. This method is most commonly used to assess the characteristic’s of the surface such as
color and gloss or when contrast values are extremely high. It generally minimizes the effects of surface irregularities.
Diffuse rear illumination is normally used with translucent materials or to detect opaque objects in a translucent or
transparent medium. With most ‘microscopes, the diffuser is placed directly below the test object. Directional rear
illumination is used for edge detection on opaque objects such as those in optical comparators.
Diffuse Rear Illumination
Rear Offset Illumination
Rear offset illumination (Figure 1.16) is used when materials are highly transparent and the difference in the light
transmission between the object of interest and the background is small. As with dark field front illumination, the lighting
and camera geometry normally require the camera to show a dark field. Scattered light, produced by the area of interest,
provides greater inspe9tion contrast than diffuse rear illumination.
Light Sources
As mentioned previously, light sources used to provide adequate illumination range from the penlight
flashlight to the brilliant high intensity sources that are used with video probes. Although a candle will
provide light (its luminance used to be the standard measure of light), candles are inadequate for the
purpose of visual examination. Electric light sources are generally used to enhance visual
examinations. There are three types of artificial light - incandescent light, fluorescent light, and
discharge (arc) light.
Incandescent Lighting
Incandescent light is produced by passing a current through a tungsten filament that is heated to
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incandescence. The common tungsten incandescent lamp is basically a thin coiled tungsten wire
surrounded by a vacuum in a sealed envelope. As a current is passed through the wire, it heats and
glows. The halogen lamp is a refinement of the tungsten lamp, which uses an inert gas with an active
halogen compound within the envelope instead of a vacuum. The halogen lamp design maximizes the
service life of the filament because the inert gas minimizes the evaporation of tungsten from the
filament and its subsequent deposition on the lamp wall, and any tungsten that does evaporate is
combined with the halogen and redeposited on the filament when the lamp is de-energized. This also
minimizes the blackening of the lamp as is commonly observed with incandescent lamps.
Watt Metal Halide
C7-C9 Lamps
1000 Watt Metal Halide
5 to 60 Watt
EXCLUSIVE Long-Life Halogen Pars
LITETRONICS
Colored Bulbs
Reflector Type Lamps
Heat Lamps
Plant Lights
Colored Flood Bulbs
Tubular Exit Bulbs
3 Way Bulbs
Metal Halide
High Pressure Sodium
Mercury Vapor
PAR Halogen
Flood & Spots
PAR36
Sealed Beam
Large PAR Lamps
Stage & Studio
MR11 & MR16
Halogens
Mini
Halogens
Quartz
Halogen
Xenon
Krypton
Decoratives
Incandescents
Fluorescent Lighting
Fluorescent light is produced by a gas within a glass envelope that fluoresces when it is excited by an
electron discharge. Electrons are discharged by filaments at one or both ends of the tube and their
interaction with the gas atoms causes the gas atoms to emit radiation in the infrared, visible, and
ultraviolet frequency range. The powder coating on the inside surface of the tube is excited by, the
ultraviolet radiation, and in turn emits visible light.
Candelabra and Standard Base
Torpedo’s
R20 - R30 - R40
Reflector Floods
Indoor or Outdoor
BR38 Weather Proof Reflectors
Household Shape
4 Watt Micro Springs
Globes
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Air Purification Lamp
Bug Lights
Bi-Pin
Discharge (Arc) Lighting
Discharge (arc) lamps use an electric arc to produce light. This lamp type is used in some video probe
imaging systems as a source of high-intensity illumination. The electrodes are housed in a vacuum or
gaseous filled envelope and a reflector focuses the light on a specific exit point. Sapphire and quartz
are commonly used at the exit point because of their light transmission and thermal properties. The
electrode gap, arc voltage, reflector shape, and material used at the light exit point determine the
intensity and efficiency of this type of lamp. When a sufficient voltage is applied, a rapid transfer of
electrons crosses the electrode gap and produces the visible light.
Discharge (Arc) Lighting
Factors that Affect Visual Testing
Factors that affect VT include material attributes, the inspection environment, and physiological
factors that affect the inspector.
Material Attributes
The physical size and condition of the object(s) to be examined play a significant role in the outcome
of an examination. Knowledge of how each variable influences the examination, will aid the inspector
in alleviating the possibility of missed indications.
Surface Conditions
Some surface conditions that affect VT include cleanliness, color, condition, shape, Size, temperature,
and texture.
Cleanliness
One basic requirement for an effective visual examination is a clean test item. Unwanted foreign
material such as dirt, oil, grease, etc. can mask the surface of actual discontinuities or present false
indications. A clean inspection surface helps to prevent the possibility of missed indications.
Color
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The color of the incident light relative to the color of the object being examined can play a significant
roll in the detection of discontinuities. The color of the light can be used to increase contrast by
intensifying or subduing certain colors. To intensify a color, the light source should be strong in that
color. Conversely, to subdue a color, the light source should have a relatively low output of the color.
For example, as previously mentioned, when examining chromium plating over nickel plating, a blue
light such as that provided by daylight fluorescent lamps can enhance any imperfections in the
chromium.
Texture
The surface of a material is important in relation to the amount and quality of light reflected from it to
the examiner’s eye. A surface that is reflective can produce unacceptable glare, which may interfere
with the examination of the test surface. In this case, light applied during the examination should be
considered carefully. Conversely, an extremely rough surface may also require special lighting to
sufficiently illuminate the area without masking. Glare can be reduced by increasing the angle between
the glare source and line of sight or by dimming the light source. Decreasing the angle between the
light source and the line of sight is also helpful when examining rough surfaces because it decreases
the shadow effect of surface irregularities.
Physical Conditions
Physical conditions such as specimen condition, shape, and size can act as limiting factors during VT
Specimen Condition:
The stages of the manufacturing process, service environment, and applied surface coatings, all
influence the condition of the item being examined. Mill scale and weld slag remaining after
manufacture can mask discontinuities, and painted or plated surfaces can mask other surface defects.
Shape
The shape of an object also affects the outcome of an examination. Complex-shaped objects, such as
keyways and splines, may hinder an examination and care should be taken in these areas.
Size
When examining objects of substantial Size certain precautions should be taken to ensure that a
complete examination has been performed.
Temperature
Elevated temperatures limit the service life of many metal components. Using metal components in
high temperatures can result in creep, thermal fatigue, and overload failure. Creep is the deformation
of a metal under stress, generally at higher temperatures than normal. Thermal fatigue cracks are the
result of repeated thermal cycles that cause expansion and contraction within the metal If thermal
cycles are caused by friction, as in the case of brake components, thermal fatigue cracks called heat
checking can occur. Engine exhaust manifolds that are restrained during repeated heating and cooling
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cycles may develop fatigue cracks due to residual tensile stress. Repeated thermal cycles of certain
materials in an oxidizing atmosphere can create scaly oxide layers over the material These flaky layers
may mask surface discontinuities and care should be taken when inspecting areas of this nature.
Environmental Factors
Environmental factors that can affect a visual inspection include atmosphere, cleanliness of the object
being inspected, and the position of object in relation to the inspector.
Atmosphere
Atmosphere in this context refers to the portion of the environment that has a physical or psychological
influence on the examiner. An atmosphere free of high noise levels, dust, smoke, and other distractions
is more conducive to the performance of the examiner.
Cleanliness
As previously mentioned, the cleanliness of the item to be examined is important when performing a
visual examination. Of equal importance is the cleanliness of the examination environment. The
primary source of contamination in the inspection environment is the movement of parts from one area
to another. While the movement of parts during an examination is necessary, dirt and other
contaminants can be transferred to the part surface if the surrounding area is not reasonably clean. An
examination environment can become contaminated in many different ways. One such way would be
smoke and vapors in the air that settle on reflective surfaces and reduce the contrast needed to locate
potential discontinuities.
Humidity and Temperature
Humidity and temperature are environmental factors that affect the proper performance of visual
examinations. While people differ in their ability to tolerate heat and humidity, it is known that
increased body core temperature decreases the mental ability of an inspector. The National Institute for
Occupational Safety and Health (NIOSH) recommends a wet globe bulb (WGB) temperature of 32° C
(90°F) as a maximum for the typical amount of time between work breaks (approximately 2 hours). If
a WGB temperature is not available at the combined temperature and humidity seem excessive, sound
judgment should be exercised regarding the length of time that an individual is exposed to the
conditions.
Safety
Of all environmental factors relating to an examination, safety is the key issue. There are very few
truly safe places in any industry; however, precautions can be taken to minimize potential hazards and
risks involved.
Information obtained before the work is to begin can be used by inspection personnel to prepare for
and lessen the potential risks. Knowledge about the actual item or component (i.e., physical size and
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shape), the location of the inspection, time of the inspection, and accessibility can help the inspection
personnel gather the appropriate equipment to perform the task or have arrangements made for the
necessary equipment to be provided. Inspection personnel should also have knowledge of or access to
the appropriate regulations concerning safety and all necessary personal protective equipment should
be issued to them or it should be accessible.
Physiological Factors
Physiological variables that may influence the outcome of an examination are the physical comfort,
health, and mental attitude of the inspector, fatigue, and the position of the test item to be examined in
relation to the inspector.
Physical Comfort
Although physical comfort is determined by the tolerance of the person involved, a comfortable
arrangement for the inspector will result in greater attention to detail and less attention to distractions
and annoyances. An inspection performed from an awkward position, or from the same position for an
extended period of time, may result in missed indications, because the inspector will tend to hurry the
examination in order to return to a more comfortable position.
Health
Many physical conditions can affect one’s eyesight. Some perceptual problems are inherited, some are
affected by one’s emotional state or circulation, and others may be the result of ocular structure
deterioration due to tumors, cataracts, glaucoma, or hemorrhaging. Age also plays a role in vision. As
one gets older, the accommodation response of the eye decreases due to a stiffening of the lens. This
condition is called presbyopia, the inability to
focus on near objects. Other factors that compromise inspection integrity include blurring, light
sensitivity, watery eyes, pain, burning, or other discomfort. If these symptoms occur, a thorough eye
examination is necessary.
Mental Attitude
The inspector’s mental attitude may affect an inspection because preconceived mental images may be
formed prior to the inspection. Preconceived mental images may alter the way in which an image
entering the eye is perceived and interpreted, therefore, affecting inspection results.
Vision is a selective sense that is strongly guided by what the observer wants and expects to see. The
intent of the inspector can affect perception if the inspector is looking only for certain aspects of a
particular scene. Even though there is a large amount of information available at the time of viewing,
many new characteristics may not be recognized until another viewing is performed. For example, a
person may walk into a crowded room and immediately recognize a familiar face across the room
while completely missing another familiar face that was not expected to be present. The same holds
true for visual inspection. If an inspector is prepared to only find porosity on the surface of a
weldment, fusion problems at the weld toes may be inadvertently ignored.
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Fatigue
The effect that the general well being of the inspector has on VT cannot be overstated. Fatigue not only
affects the overall physical feeling of the inspector, but it reduces the efficiency and accuracy of the
interpretation of the visual data. Fatigue of the eye muscles, caused by poor illumination and awkward
body positions, leads to oscillation of the eye and eyelids and causes ineffective examinations.
Test Item Position
The position of the item and its distance from the inspector has an effect on the inspection outcome.
Again, ~he recommended viewing distance and angle for visual examination is to have the eye within
610 mm (24 in.) of the object and positioned at an angle not less than 30° to the inspection surface as
shown in Figure 1.5. If the examination surface is immovable and situated so that the eye cannot be
placed within this region, suitable visual aids, such as mirrors, must be used.
Elevated objects present another obstacle to proper examination. A component elevated to a point
beyond the reach of the inspector cannot be thoroughly examined without adequate means to provide
access except in cases such as component location or object verification.
Figure 1.5: The Lambert Cosine law
Viewing Angle Range
Equipment
Visual inspection equipment includes borescopes, fiberscopes, rules, calipers, mechanical gages, weld
gages, magnifiers, mirrors, automated systems, computer-based systems, imaging systems, special
optical systems, and closed-circuit television.
Borescopes
Rigid borescopes allow visual inspection to be performed on areas inside assemblies, on remote
objects that arc out of the normal inspection environment or on remote objects or assemblies with
restricted physical access. Using a system of lenses and prisms, borescopes can magnify the image of
the area of interest while permitting examination. Light at the inspection site is provided by a bulb at
the distal end of the scope or through a fiber optic bundle or liquid light guide from a remote, highintensity light source.
Borescopes are available in many diameters and working length combinations. Borescopes can use a
convex lens relay system, a hybrid rod and convex lens relay system, or a rigid glass rod, plus
objective and ocular lenses. The convex lens relay system transmits images by relaying them from one
lens to another along the length of the borescope.
The hybrid rod lens consists of a series of rod lenses to relay the image. Rod lens trains have an
advantage over simple lens trains in that there are fewer refraction points resulting in less light
scattering, and they may be made in smaller a diameter, which in turn reduces the borescope diameter.
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2 and 3mm Semi-Rigid Needle Scopes
Images shown L to R Eyepiece side view, Eyepiece end view, Close up of prism tip, shown with video
accessories. Bottom: Lighting
The slimmest borescope available uses a single rod to transmit the image along the length of the scope
without the need for lenses. The image refracts within the rod at certain intervals similar to that with
lenses. Because the diameter of the rod is minimized, the effective aperture is small and results man
infinite depth of field – eliminating the need for focusing.
Regardless of the light transmitting system used internally, borescopes also have lenses at each end of
the scope. An objective lens at the distal end (far end) gathers the image light and relays it to the
optical train. After the light has been relayed along the length of the scope, it is picked up by the ocular
lens (eyepiece) and subsequently relayed to the eye. The ocular and objective lenses are made
moveable on focusing scopes to compensate for observer eyesight differences, to expand the depth of
field, and to present a crisp image to the Viewer.
Fiberscopes
Fiberscopes are similar to borescopes because they allow access to normally inaccessible areas;
however, they operate on a different principle. The borescope uses a system of lenses to transmit the
Image from the object to the eye, while the fiberscope uses a bundle of light transmitting fibers made
of glass or quartz. This bundle is called the image guide. The flexible nature of the image guide gives
the fiberscope the ability to negotiate bends and corners while the more rigid borescopes can only
travel in a straight path.
The fiberscope collects the image with an objective lens and relays the light through a bundle of
extremely thin fibers, some as small as 8 µm (0.0003 in.). These fibers transmit light along their entire
length by total internal reflection of the light within the fiber (total internal reflection occurs when
medium-2 have lower refractive index than medium-1). This is possible because the fibers are coated
with a thin layer of glass, which has a lower refractive index than that of the fiber. This causes the light
to reflect within the strand without escaping into the air or into other adjacent fibers. Because each
fiber transmits only a small portion of the image, the bundle must be arranged so that each fiber ends at
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the ocular lens in the same position it originates in at the objective lens. This is called coherent
alignment, which allows for a cohesive image at the eyepiece. If the fibers were randomly placed at
each end, the resultant image would be an unrecognizable jumble of dots or pixels.
Also, a fiberscope can be focused by a moveable objective lens, or fixed focus where the objective lens
cannot be moved.
Light for the inspection area is generally provided by another bundle of fibers that transmits light
produced by an external, high intensity source. These fibers have a larger diameter than the image
fibers and they do not need to be aligned coherently because they do not transmit an image.
An articulating tip is an optional piece of equipment that can be used on a fiberscope. Articulating tips
are mechanical or pneumatic devices that allow the distal end of the scope to achieve a greater field of
view.
Visual Inspection Instruments
Many different instruments are available as aids in visual inspection. These instruments are used to
determine length, diameter, height, surface texture, thread pitch, and many other characteristics that are
not quantifiable by the eye alone.
Some common instruments that are useful for examination include the steel rule, caliper, mechanical
gage, depth gage, weld fillet gage, Cambridge weld gage, and pit gage.
Rules
The simplest mechanical instrument for linear measurement is the 6 in. machinists steel rule. While
accurate to 0.016 in. if applied properly, misinterpretation is common. Accuracy is also limited by the
inherent width of the graduation etching itself.
To obtain accurate measurements with a steel rule, use the 1 in. mark for the beginning reference
instead of the zero end, and then subtract 1 in. from the obtained dimension. This method is suggested
because it is more difficult to align the zero end with the edge of the work piece than it is to align the
graduations of the rule. Also, the possibility of unseen damage to the zero end of the rule could affect
the measurement.
When performing the measurement, the rule should be aligned perpendicular to the test piece so the
graduations are as close as possible to the, measurement area as shown in Figure 1.6. This alignment
also minimizes parallax error. Because steel rules have several scales with different graduations,
attention should be paid to the selection of the proper scale to avoid approximation. If the
measurement made with the selected scale falls between graduations, the next finer scale should be
used. Steel rules come in lengths other than 6 in. and they can be made of rigid or flexible material.
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Figure 1.6: Steel Rule
Calipers
Calipers are used to obtain accurate linear measurements. Calipers come in a wide variety of sizes and
configurations for measuring length, width, height, diameter, and depth, and they can be either direct
reading or indirect reading, Indirect, or transfer type, reading calipers (Figure 1.7) are used to
“transfer” the dimension of an item from the item to a steel rule. For example, the measurement of an
outside diameter is made by adjusting the caliper so that both legs lightly touch the widest portion of
the item. This distance is then transferred to a steel rule to obtain the measurement. If performed
properly, this type of measurement is accurate to 1/64 in.
Indirect calipers
Figure 1.7: Indirect calipers
DIAL CALIPER GAUGES
DIAL SPRING CALIPERS,External
DIAL SPRING CALIPERS,Internal
RECESS GAUGE
SLOT GAUGE
Direct reading calipers are available in a variety of types. A direct reading caliper can be simple - a
rule with jaws for coarse measurements; or it could be of the vernier, dial, or electronic digital type,
which are used for very accurate measurements, as shown in Figure 1.8. All types of direct reading
calipers consist of a fixed jaw on a beam along which a moveable jaw slides. The measurements are
taken with the item between the jaws of the instrument.
Dial and electronic calipers are simple to use and read. Electronic calipers are the easiest to read
because the actual measurement is displayed on a digital readout. The dial caliper requires some
interpretation between the beam scale, which is divided in graduations of 0.1 in., and the rotating
indicator on the dial that represents 0.1 in. for every revolution and is divided into 100 segments each
representing 0.001 in.
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Vernier Calipers
Vernier Dial Calipers
Vernier Digital Caliper
GEAR TOOTH VERNIER CALIPERS
Figure 1.8: Vernier reading
Figure 1.9: Direct reading calipers
Vernier Height Gage
VERNIER DEPTH GAUGE
Vernier calipers are more difficult to use because the scale used requires more care during
interpretation. The vernier consists of the fixed main scale, which is etched into the beam, and the
sliding vernier scale that is attached to the moveable jaw. Each inch of the main scale is divided into
10 equal divisions, which are further divided into quarters so that each graduation represents 1/40 in.
The vernier scale on the sliding jaw is divided into 25 subdivisions and is arranged so that the total
span of those 25 divisions equals the span of 24 divisions on the main scale. This is so that the
difference between the divisions of each scale is 1/25 of a main scale division. Therefore, because the
main scale divisions are 1/40 in., and the vernier scale division is 1/25 of the main scale division, the
caliper is capable of reading to l/l 000 in. as 1/25th of 1/40th equals 1/1000 or 0.001 in.
To take a measurement with the vernier caliper, open the jaws larger than the maximum dimension of
the item to be measured and slowly close the jaws around the item until light contact is made. For the
greatest accuracy, the item must make even contact all along the thickness of the jaw faces. Note the
number of inches and subdivisions of an inch that the zero mark of the vernier scale has passed. This
gives a reading to 1/100 in. The additional thousandths are obtained by observing the graduation of the
vernier scale that coincides with a graduation on the main scale, and adding this to the tenths obtained.
Figure 1.9 shows a reading of 1.659 in.
Gages
Some gages that are commonly used during VT include mechanical gages, weld fillet gages, and
Cambridge gages.
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Mechanical Gages (Micrometers)
Mechanical gages perform extremely accurate measurements of linear dimensions. Mechanical gages
are available in a wide variety of configurations for inside and outside measurements of flat, curved,
threaded, and cylindrical dimensions. The mechanical gage is a form of caliper that operates by
determining how far the end of a screw travels after one complete revolution.
To better understand the operation of the mechanical gage, it is useful to know the components of the
instrument. This includes the frame, anvil, spindle, barrel, thimble, screw, ratchet, and clamp ring.
These components are illustrated in Figure 1.10.
Measurement with the mechanical gage occurs between the anvil and spindle. The screw of the spindle
has a thread pitch of 40 threads per inch, so 1 revolution of the screw equals 0.025 in. The thimble is
divided around its circumference into 25 graduations, so that 1/25 of a revolution equals (1/1,000 in.)
because 1/25 of 1/40 is 1/1,000. A 1 inch section of the micrometer barrel is divided into tenths of an
inch, and each tenth is divided into four equal divisions. As the thimble is rotated counterclockwise,
the spindle and anvil separate and the barrel graduations are revealed in succession.
Figure 1.10: Mechanical gage (micrometer)
Digital Micro Meter (External)
Micro Meter (Internal)
To make a measurement with a standard mechanical gage, rotate the thimble counter clockwise until
the spindle is a far enough away from the anvil to allow the item to be measured to fit in between
them. With the part between the anvil and spindle, slowly turn the thimble clockwise to obtain contact
between the anvil, part, and spindle. A gentle pressure is all that is necessary to make an accurate
measurement - too much pressure could distort the frame reducing accuracy, and too little pressure
could result in improper contact with the part, producing an inaccurate measurement. The part should
be able to be rotated about the spindle axis with the feeling of a slight drag.
Barrel graduations are revealed by the outward travel of the thimble. To read the dimension, note the
largest major division uncovered, the graduation closest to the thimble, and the thimble division
aligned with the barrel reference line. Referring to Figure 1.11, the “2” or 0.2 in. division is revealed,
and the closest graduation to the thimble is the second, or 0.05 in. and the “12” or 0.012 in. graduation
on the thimble is aligned with the reference line of the barrel. Adding these together, 0.2 plus 0.05 plus
0.012 gives the dimension 0.262 in. as shown in Figure 1.11.
Figure 1.11: Mechanical gage reading
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Weld Gages
A common tool used in visual examination of weldments is the weld fillet gage. This simple, easy-touse device measures leg lengths and determines if there is sufficient throat in weld fillets. This gage is
basically a
Comparator - the acceptable size is etched into the gage and arcs are cut into the gage to allow space
for the weld bead. The gage is placed square against the welded components and the actual weld is
compared to the standards of the gage as shown in Figure 1.12. This type of gage offers a, quick and
precise means of measuring concave and convex fillet welds from 3 mm (0.13 in.) to 25 mm (1 in.).
Figure 1.12: Fillet weld gage
Automatic Weld Size Weld Gauge
Check Reinforcement Check The Throat Check Leg Size
Of Fillet Weld
Of Fillet Weld
of Butt Weld
The Palmgren weld gage, shown in Figure 1.13, can be used to measure the size of fillet welds, the
actual throat size of convex and concave fillet welds, reinforcement of butt welds, and root openings of
8 mm (0.3 in.) and 3 mm (0.13 in.)
Figure 1.13: Palmgren weld gage
Figure 1.14: Cambridge gage
The following measurements are possible either in inches or millimeters
• Angle of preparation, 0º to 60º
• Excess weld metal (capping size)
• Depth of undercut
• Depth of pitting
Excess Weld Metal
Scale reads in Inches and Millimeters.
Gage is placed on lower piece and dial
rotated until it contacts higher piece
• Fillet weld throat size
• Fillet weld length
• Misalignment (high-low)
Fillet Weld Throat
Angle of Preparation
Small sliding pointer reads to 20mm and
¾”. Gage also fits flush in 90° corners.
Scale reads 0° to 60° in 5°
increments. Angle is read against the
chamfered edge of the dial.
7 Piece Fillet Weld Set
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Checking Fillet Throat Size
Checking Fillet Leg Size
Adjustable Fillet Weld Gauge
Adjustable Fillet Weld Gauge is easy to use.
To measure fillet welds place irregular curve edge flush to horizontal toe
of weld so the straight edge is in line with the horizontal member.
To measure weld throat thickness place the 45º angle end flush to the
horizontal and vertical members. Loosen the thumb screw and slide the
pointer until it touches the face of the weld.
Combination 0” - 10” Dial/Pit Depth Gauge
Gauges Shown With Welding Base And 0.062 Microprobe Attached
Measures Undercut
Socket Weld
Measures Undercut
Butt Weld
Measures Crown Height
Butt Weld
“Mini Sub” Socket Weld Gauge
Economy Single Purpose HI-LO Welding Gage
Skew-T Fillet Weld Gauge
• • CALCULATOR • •WG-9b
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A handy compilation of mathematical relationships between leg length,
throats, skew angles and inspection dimensions. A must for designers
and inspectors. The perfect companion to the Skew-T Fillet Weld
Gauge, (Gauge measures in English & Metric.)
Checking Angle of Checking Weld Checking Weld
Vertical Member Acute Side
Obtuse Side
WTPS Gauge
Measure Undercut 0.010 Deep
Undercut shall be no more than
0.010 in. (0.25 mm) deep when the weld
is transverse to the primary stress
in the part that is undercut.
The WTPS Gauge is precision made from
stainless steel all markings and dimensions
are laser engraved for ease and clarity
when reading.
Gauge set comes with a precision ground
calibration block as shown above, each
block has been surface ground to 0.0005
tolerance for exceptional accuracy.
Another more versatile device used for weld inspection is the welding gage, commonly referred to as
the Cambridge gage. The Welding Institute of Cambridge, England developed this versatile tool, hence
the name. With this device, joint preparation angles, joint misalignment, weld fillet size and depth
measurements can be easily obtained. Figure 1.14 shows some typical applications.
Magnifiers
Occasionally, the visual range of the human eye is not sufficient when observing for desired
characteristics. In these cases, visual aids such as magnifiers are employed to enhance natural vision.
Magnifiers are available in magnification powers that range from 1.5x to 2,000x with fields of view
from 89 mm (3.5 in.) to 0.15 mm (0.006 in.). Resolving power can range from 0.05 mm (0.002 in.) to
2 ).lm (0.000008 in.). Microscopes, loupes, and optical comparators are variants of magnifiers.
When selecting magnifiers, the attributes of magnification power, working distance, and field of view
should be considered. These are related in that a high power magnifier has a short working distance
and narrow field of view. Conversely, a low power magnifier has a long working distance and wide
field of view.
Magnifiers
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Microscopes
Hand Held Microscopes
Handheld Inspection Microscope w/Coax Illumination
These handheld inspection microscopes with coaxial illumination send light down the optical path (axis) so that
it comes out the tip of the objective and strikes the sample perpendicular to the fiber end face. This process
produces excellent detail of scratches and contamination on the fiber ferrule, but requires backlight to examine
core condition. Excellent for inspecting a connector ferrule for polish quality, cleave quality, and cleanliness. A
universal connector is included with each microscope that accepts any 2.5mm diameter ferrule. Specific
connector adapters sold separately. WARNING: DO NOT use the fiber microscope to view active fiber signals
under ANY circumstances.
Mirrors (reflectors)
The use of mirrors is sometimes necessary when all or portions of the inspection area cannot be placed
easily within the recommended viewing range (610 mm or 24 in. at 30°). Many types of inspection
mirrors are available with articulating joints, extendible handles, and self-contained light sources to
allow placement of the mirror and sufficient illumination at the inspection site.
Automated Systems
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Automated visual examination systems make use of the recently developed “machine vision” that
“sees” and interprets information about the test object. The apparatus examines the object with a
vidicon camera or a charged-coupled device (CCD), laser, thermometer, etc., it processes the data in a
microprocessor, it compares this data with a known acceptable example stored in memory, and it
determines the acceptability of the object. Manufacturers using automated systems can obtain costeffective, reliable inspections that are not affected by the vagaries of the human condition such as
fatigue or attitude.
For example, the steel industry uses CCD cameras to scan hot slabs of steel for discontinuities as they
pass by on roller tables. The automobile industry is using automated systems to examine the paint
finish of automobiles for characteristics such as mottling, blotch, and distribution of the metal particles
in metallic paint.
Computer-based Systems
Some visual examination systems use computers to enhance and manipulate the image of the test
specimen. These instruments digitize the image and convert each picture cell (pixel) into a number,
generally binary, which can be interpreted by computer software. Using this digitized image data,
extremely accurate measurements are made of selected areas, and the image can be electronically
enhanced to compensate for poor lighting or variations in surface texture. The digitized data may also
be saved to a storage medium (magnetic tape, disk, etc.) for future retrieval.
Imaging Systems
The equipment used to obtain an image of a test surface and relay it electronically to a display device
is called an imaging system. A video probe borescope is a good example of an imaging system. The
video probe borescope combines a borescope with video processing technology, which reduces some
of the difficulties encountered when using typical borescopes or fiberscopes. Difficulties such as
eyestrain and operator fatigue, induced by the need to assume awkward positions, are lessened when
the image is transferred from the eyepiece to a display such as a CRT video monitor. Imaging systems
that combine a video probe with an image processor are ideal for displaying, manipulating, and storing
obtained images.
Early imaging systems simply used a camera mounted on the eyepiece of a borescope or fiberscope,
with the resulting image displayed on a monitor. Video probes place the camera in the distal end of the
probe behind the objective lens and connect a monitor in place of an eye piece.
The video probe is similar in appearance to the fiberscope; however, the video probe uses fiber optic
strands only to deliver illumination to the observation site. Reflected light (the image) is collected on a
CCD and transmitted as electric signals to the video processor. The signals are transmitted along a
wire assembly (this replaces the fiberscope image bundle) to the video processor. The processor then
decodes and amplifies the data for display on a
CRT screen. Video probes are also available with pneumatic or mechanically operated articulating
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ends for a greater field of view.
The CCD is an array of semiconductor capacitors that gains a charge as photons of light impinge on
them. Color is provided by a color wheel that rotates between the light source and light guide or by
placing a primary color filter over each chip and grouping the primary colors as a pixel of information.
While both of these are effective at providing “false” color, the use of color filters requires a CCD
approximately two times larger than sequential color lighting (use of the rotating wheel).
The operating system software installed in these units allows the operator to perform several functions
- adjust the image (i.e., edge and contrast enhancement) and zoom of selected areas. Measurement
capabilities are available, along with the ability to add text and pointers, compare selected images, and
store, retrieve, and duplicate images. A character generator (keyboard) is provided with special
function keys to facilitate the data entry and cursor functions.
Special Optical Systems
Optical flats and optical comparators are two common visual examination tools that are used to
enhance certain characteristics of the item being examined.
Optical Flats
Optical flats are used to check the flatness of surfaces that require a high degree of accuracy. An
optical flat is a cylinder of glass or quartz that has been made with one or more extremely flat surfaces.
When applied to the inspection surface, any deviation of that surface will result in an air a between the
flat and the specimen, causing patterns to appear in the optical flat. These patterns are interference
patterns created when light reflected from the inspection surface interferes with light transmitted
through the flat.
HOW AN OPTICAL FLAT WORKS
An optical flat utilizes the property of interference to exhibit the flatness on a desired
surface. When an optical flat, also known as a test plate, and a work surface are placed in
contact, an air wedge is formed. Areas between the flat and the work surface that are not
in contact form this air wedge. The change in thickness of the air wedge will dictate the
shape and orientation of the interference bands. The amount of curvature that is shown by
the interference bands can be used to determine the flatness of the surface. If the air
wedge is too large, then many closely spaced lines can appear, making it difficult to
analyze the pattern formed. Simply applying pressure to the top of the optical flat
alleviates the problem. The determination of the flatness of any particular region of a
surface is done by making two parallel imaginary lines; one between the ends of any one
fringe, and the other at the top of that same fringe. The number of fringes located
between the lines can be used to determine the flatness. Monochromatic light is used to
create sharp contrast for viewing and in order to specify the flatness as a function of a
single wavelength.
Optical Comparators
Optical comparators are available in several varieties and they are used to compare a characteristic of
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the inspection surface to a known standard. Surface finish, threaded fasteners, and gear teeth are some
of the conditions that may be inspected with comparators.
14” OPTICAL COMPARATORS
• Surface illumination through duplex fiber optics.
• Digital LCD scales on x and Y axis provide
readings
to 0.0005”.
• SPC Output from scales is compatible with
most
SPC Systems.
• 3-Position rotary switch selects profile,
surface, OR combination surface and profile
illumination modes.
• High capacity 4” x 8” travel stage with 4.5” x 19”
work surface.
• Ball way construction handles loads to 100 lbs.
• Single long life tungsten-halogen light source.
• Heavy-duty monorail type, welded steel frame.
• Rotary frosted glass screen with 90° crosslines
and 360° protractor rim.
• Vernier readings to 5 minutes of arc.
• 10 power lens system.
• Overlay chart clips.
• Weighs 245 lbs.
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Some comparators are small enough to be taken to the inspection site, while others are large, stationary
pieces of equipment into which test items must be placed. This type of comparator is also called a
contour projector. The item to be inspected is placed in the unit and a light source behind the item
projects a magnified shadow of the item onto a screen over which a standard profile is laid. A
comparison is then made between the standard and the item.
Closed-circuit Television
Closed-circuit television (CCTV) uses a television camera tube to convert reflected light from the
observed object into an electrical image that is transmitted and displayed on a cathode ray tube (CRT).
Camera tubes are either the image orthicon tube that works by photoemission or the vidicon tube-that
works by photoconduction.
Photoemission occurs when electrons are emitted by a photosensitive surface that is stimulated by
light. Photoconduction is the process by which the conductivity of the photosensitive surface changes
in relation to the intensity of the light striking it.
Both processes produce an electric current that is the video signal This signal is processed, amplified,
and displayed on the CRT.
Discontinuities
VT is used to locate discontinuities in many stages during the manufacturing process. VT is commonly
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performed on castings, forgings, and welds and it is perfoITI1ed after machining processes as well.
Castings
Castings are made by pouring molten metal into a prefoITI1edmold. They are subject to various
discontinuities inherent in the process. Typical casting discontinuities that appear on the surface
include inclusions, porosity, hot tears, shrink cracks, and cold shuts.
Inclusions may result from the sand used to make the mold, slag from the poured metal, refractory
materials, or other materials that do not melt and alloy with the molten metal Inclusions appear
anywhere on the surface and give the surface a rough, porous appearance.
Porosity is the result of gasses emanating from the molten metal that have been trapped during freezing.
Hot tears and shrink cracks are caused by an uneven rate of freezing of the metal or drastic changes in
material thickness due to an irregular shape of the pattern. Hot tears and shrink cracks are usually
evident in areas of tension such as corners, areas where portions of the casting are restrained, or areas
where large thickness transitions occur.
Cold shuts are an incomplete fusion of the cast metal that results from an interrupted pour of metal, or
in areas where freezing has occurred before the pour is complete. Cold shuts can appear anywhere on a
casting but are generally found near an area of interrupted flow near the core.
Forgings
Forgings are made by placing a heated blank of metal between two dies and forming the blank into the
finished part using great pressure or repeated blows. Several discontinuity types may be evident during
visual examination, notably laps, bursts, and flash line tears.
Laps are produced when material folds over due to the forging process. The material is subsequently
pressed down onto adjacent material but does not fuse due to surface oxidation. Laps can appear
almost anywhere on a forging, but they usually appear along the flash line. A burst is a rupture of the
material that occurs due to forging at an improper temperature or a drastic change in section thickness
as the material is forced throughout the die. Flash line tears are cracks along the flash line caused by
improper trimming of flash.
Machining Processes
A machining process performed on raw material as the name implies, forms machined components.
Turning on a lathe, milling, grinding, drilling, and cutting are various machine processes, and may
result in discontinuities. Grinding cracks result from excessive heat generated in the item during the
grinding process and they appear as fine, tight cracks. Tears occur due to improper stock or tool bit
speed and they result in a tearing of material rather than the desired cut.
Welds
Welding, the process of joining members together by applying enough heat to melt and fuse the
material, results in a number of discontinuities apparent during visual examination. Porosity, cracks,
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slag, fusion, penetration, undercut, and overlap are the discontinuities that may appear on a completed
weldment.
Porosity, as in the casting process, is the result of gasses emanating from the molten weld pool that are
trapped during the freezing of the metal.
Cracks can occur due to the portion of the base material being restrained during the cooling process.
As the weld metal cools it contracts, and any restraint may cause cracking.
Slag is the remains of the flux shielding used to protect the molten weld pool from atmospheric
contaminants. If slag is not completely removed prior to depositing additional weld metal, it may get
trapped in the weld. Slag appears as an elongated indication between weld beads.
Fusion is the melting and mixing of weld metal and base metal, or previous weld layers. If complete
fusion does not occur, a linear indication may be evident between weld beads or between the weld
bead and the base metal.
Penetration is the term used to define the degree to which the weld metal has entered the weld joint.
Penetration is important for determining if the opposite side of the weldment is accessible to the
inspector. A weld root that is at least as high as the base metal that blends smoothly with the base
metal will evidence the correct amount of penetration.
Undercut is caused by a failure of the weld metal to flow into the reduced area vacated by the melting
base metal. This creates a notch type discontinuity at the transition between weld and base material or
weld toe. Overlap occurs when the weld metal flows over the underlying material but does not fuse
with it. The resulting notch type discontinuity produced results in a concentration of stresses at the
base of the notch.
Visual Testing Documents
There are may types of documents involved in the regulation of VT examinations including codes,
standards, and specifications, and others designed to record inspection results.
Welding Specifications
Because certain specifications require a more thorough examination of welds than simply observing
for surface discontinuities, this section will discuss some of the more common welding symbols in use.
Welding symbols are used to represent the desired weld on a drawing, and the American Welding
Society (AWS) has developed ANSI/AWS A2.4 to standardize welding symbols throughout industry.
It should be noted that a “welding” symbol differs from weld symbol in that a weld symbol indicates
the type of weld while a weld symbol is incorporated into the welding symbol on the drawing. The
welding symbol has many elements that describe the characteristics of the desired weld.
Characteristics such as weld type, size, length, preparation, penetration, finish, and location are all
delineated on the welding symbol.
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Wire Rope Inspection
All wire ropes will wear out eventually and gradually lose work capability throughout their service
life. That’s why periodic inspections are critical. Applicable industry standards such as ASME B30.2
for overhead and gantry cranes or federal regulations such as OSHA refer to specific inspection criteria
for varied applications.
Three purposes for inspection
Regular inspection of wire rope and equipment should be performed for three good reasons:
v It reveals the rope’s condition and indicates the need for replacement
v It can indicate if you’re using the most suitable type of rope
v It makes possible the discovery and correction of faults in equipment or operation that can
cause costly accelerated rope wear
How often
All wire ropes should be thoroughly inspected at regular intervals. The longer it has been in service or
the more severe the service, the more thoroughly and frequently it should be inspected. Be sure to
maintain records of each inspection.
Appoint a qualified person to inspect
Inspections should be carried out by a person who has learned through special training or practical
experience what to look for and who knows how to judge the importance of any abnormal conditions
they may discover. It is the inspector’s responsibility to obtain and follow the proper inspection criteria
for each application inspected.
Wire Defects
Here’s what happens when a wire breaks under tensile load exceeding its
strength. It’s typically recognized by the “cup and cone” appearance at the point
of failure. The necking down of the wire at the point of failure to form the cup and
cone indicates failure has occurred while the wire retained its ductility.
This is a wire with a distinct fatigue break. It’s recognized by the square end
perpendicular to the wire. This break was produced by a torsion machine that’s
used to measure the ductility. This break is similar to wire failures in the field
caused by fatigue.
A wire rope that has been subjected to repeated bending over sheaves under
normal loads. This results in fatigue breaks in individual wires – these breaks are
square and usually in the crown of the strands.
An example of fatigue failure of a wire rope subjected to heavy loads over small
sheaves. The breaks in the valleys of the strands are caused by “strand nicking.”
There may be crown breaks, too.
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Here you see a single strand removed from a wire rope subjected to “strand
nicking.” This condition is a result of adjacent strands rubbing against one
another. While this is normal in a rope’s operation, the nicking can be
accentuated by high loads, small sheaves or loss of core support. The ultimate
result will be individual wire breaks in the valleys of the strands.
Here you see a single strand removed from a wire rope subjected to “strand
nicking.” This condition is a result of adjacent strands rubbing against one
another. While this is normal in a rope’s operation, the nicking can be
accentuated by high loads, small sheaves or loss of core support. The ultimate
result will be individual wire breaks in the valleys of the strands.
Typical evidence of wear and abuse
A “birdcage” is caused by sudden release of tension and the resulting rebound
of rope. These strands and wires will not be returned to their original positions.
The rope should be replaced immediately.
A typical failure of a rotary drill line with a poor cutoff practice. These wires have
been subjected to continued peening, causing fatigue type failures. A
predetermined, regularly scheduled cutoff practice can help eliminate this type of
problem.
This is localized wear over an equalized sheave. The danger here is that it’s
invisible during the rope’s operation, and that’s why you need to inspect this
portion of an operating rope regularly.The rope should be pulled off the sheave
during inspection and bent to check for broken wires.
This is a wire rope with a high strand—a condition in which one or more strands
are worn before adjoining strands. This is caused by improper socketing or
seizing, kinks or doglegs.
At top, you see a close-up of the concentration of wear. At bottom, you see how
it recurs every sixth strand in a 6 strand rope.
A kinked wire rope is shown here. It’s caused by pulling down a loop in a slack
line during handling, installation or operation. Note the distortion of the strands
and individual wires. This rope must be replaced.
Here’s a wire rope that has jumped a sheave. The rope “curled” as it went over
the edge of the sheave. When you study the wires, you’ll see two types of
breaks here: tensile “cup and cone” breaks and shear breaks that appear to
have been cut on an angle.
Drum crushing is caused by small drums, high loads and multiple winding
conditions.
Removal criteria
A major portion of any wire rope inspection is the detection of broken wires. The number and type of
broken wires are an indication of the rope’s general condition and a benchmark for its replacement.
Frequent inspections and written records help determine the rate at which wires are breaking. Replace
the rope when the values given in the table below are reached.
Valley wire breaks—where the wire fractures between strands or a broken wire protrudes between
strands – are treated differently than those that occur on the outer surface of the rope. When there is
more than one valley break, replace the rope.
Broken wire removal criteria cited in many standards and specifications, like those listed below, apply
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to wire ropes operating on steel sheaves and drums. For wire ropes operating on sheaves and drums
made with material other than steel, please contact the sheave, drum or equipment manufacturer or a
qualified person for proper broken wire removal criteria.
CREEP
Creep failure is essentially plastic deformation, so its appearance is often similar to that of Ductile
Fracture. Creep would be suspected if a component shows extensive plastic deformation before failure
and if the material is operating at a temperature within its creep range, which is usually about 0.3 of its
melting point (in degrees K). As with fatigue, the final failure may be ductile (by growth of voids) or
brittle (by crack propagation).
Microscopic Appearance: Creep failures are usually of the intergranular ductile type, due to the
formation of voids on the grain boundaries: see above under Brittle Fracture for an example of this
kind of fracture surface. Failure is usually preceded by a lot of microscopic damage, which can be seen
on polished sections as shown in the two photos below. This damage takes the form of voids (shown
on the left) and/or small cracks (on the right), mostly located on the grain boundaries:
CREEP
Creep (Stress Rupture) Voids In A 304ss Stainless Steel Furnace Tube
Cast 304 stainless steel shock bank tube in an ethylene cracking furnace showing stress rupture (creep)
voids and cracking. 100X
ELASTIC FAILURE
Two types of failure occur due to unsuitable elastic behavior in a material:
If the elastic modulus E is too low, the component may deflect too much in service, making it unusable. Examples are
moving parts such as engines and structures such as bridges. Alternatively some components may not deflect as much as
they should do, or may require too much force to deflect by the required amount. Examples are switches and springs. These
problems will become apparent instantly as soon as the load is applied, except in viscoelastic materials such as polymers,
where elastic strain can build up gradually over time. Elastic deflection can be distinguished from plastic strain and creep
strain because it is totally recoverable on removal of the load.
Buckling: Though buckling leads to a failure by another mechanism (usually ductile failure), the process of buckling is
essentially an elastic instability, which is controlled by the material’s elastic modulus. A classic example is the failure of
slender columns loaded in compression. This type of failure generally occurs instantaneously; it can be difficult to predict
because it depends on the shape of the part and how it is loaded and restrained.
DUCTILE FAILURE
A failure is termed “ductile” if it occurs by plastic deformation, i.e. if the yield strength of the material
is exceeded. This causes a permanent change in shape of the part, which becomes apparent if one
attempts to reassemble broken components. Tensile stress causes necking, reducing the cross section
of the part, as shown dramatically in this failed bolt:
Shear stress causes deformation of the type shown below in this sketch of a failed rivet:
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Some plastic deformation also occurs in other types of failure: for example fatigue failure always
involves some plasticity, and brittle fracture may be preceded by plastic strain. However, in these cases
the amount of plasticity is small: ductile fracture is characterized by gross plasticity which leads to an
obvious change in shape of the component.
Microscopic Appearance: Below are two photos taken using a scanning electron microscope (SEM)
showing the appearance of ductile fracture surfaces at high magnification. They have a distinctive
‘dimpled’ appearance, the surface being covered with small dimples or craters. These dimples result
from the fracture of spherical pores which open up due to plastic deformation (see more about this
under Brittle Fracture below). The dimples may contain inclusions and other small particles from
which the pores nucleated.
Above: SEM photograph of part of a fracture surface, at high magnification. The total width of this
picture is about 50µm (0.05mm)
SEM photograph of another ductile fracture surface. The dotted line (bottom left) shows the scale (in
this case 10µm).
BRITTLE FAILURE
Brittle fracture is the term given to static failures caused by crack propagation. Crack propagation also
figures in fatigue and creep failure, but brittle fracture occurs when a pre-existing crack propagates
very rapidly through the material. Such fractures release a lot of energy and can be very loud,
explosive and dangerous, as fragments of the material may be thrown long distances. In many cases,
especially if the micromechanism of cracking is cleavage (see below), a macroscopic examination of
the fracture surface may show lines (called “chevron marks”) which lead back to the failure origin.
Here’s an example (the arrow shows the origin):
A big difference between brittle fracture and ductile fracture is that in brittle fracture there is no
macroscopic deformation. So you can take the broken parts and put them back together again, like
making a jigsaw. You can’t do this after a ductile fracture because the shape of the pieces has changed.
For example, compare the fracture of this bolt with the one shown above in the section on Ductile
Fracture:
Microscopic Appearance: The appearance of the fracture surface at high magnification depends on the
mechanism of crack propagation. This can be either ductile or brittle. Ductile crack extension occurs if
the crack grows by causing local plasticity ahead of the crack tip, which pulls the material apart. Pores
form and grow in the material just ahead of the crack. The crack grow occurs by linking of these pores
as shown in the sketch below.
This gives give a fracture surface identical to that described in the section above on Ductile Fracture
(see the SEM photos in that section).
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Brittle crack extension, also called “cleavage”, occurs by the splitting apart of planes in the atomic
lattice. Each grain cleaves like a gemstone, giving a perfectly flat surface, as shown in the sketch
below:
Because the cleavage plane for each grain will be at a different angle, the final fracture surface will
consist of a series of small flat planes, each corresponding to one grain. The following two pictures
show what this looks like at high magnification, in the SEM:
This photo shows cleavage fracture in a surface coating of Cadmium (applied to a steel component to
prevent corrosion and wear); the total thickness of the coating is about 95µm.
The two mechanisms of crack extension described above are both “transgranular”, i.e. the crack grows
through each grain. If, alternatively, the crack propagates by traveling around the grains (i.e. along the
grain boundaries), then the mechanism is termed “intergranular”. Intergranular fractures can be either
ductile or brittle; a brittle intergranular failure looks like this:
Here you see cleavage planes again as in the pictures above, but now you are looking at the outsides of
the grains. The existence of transgranular failure is often an indication that something is wrong with
the material (e.g. an incorrect heat treatment causing a brittle phase to form on the grain boundaries).
Most materials, in good condition, fail in the transgranular mode.
In a ductile intergranular failure you still see the outsides of the grains, but they have dimples on them
as shown here:
Ductile intergranular failures are quite rare - the only situation in which they commonly occur is
Creep. Also certain incorrect heat treatments applied to certain metal alloys can cause them to fail in
this way even at room temperature, but the phenomenon is rare.
The existence of the cleavage mode is an indication that the crack grew very easily in the material,
needing very little energy to propagate. This is often a clue to something wrong with the material, such
as an incorrect heat treatment or the use of a sub-standard material. Some metals actually change from
ductile fracture to brittle fracture if you lower the temperature. However some materials are just
inherently brittle and will always fail by this mode.
The terminology we use here is rather confusing, because we use the words ‘ductile’ and ‘brittle’ to
mean two different things. In classifying the different types of failure we talk about Brittle Fracture
(meaning fracture due to crack growth) and Ductile Fracture (sometimes called ‘ductile failure’)
meaning fracture due to the yield strength being exceeded. But within the category of Brittle Fracture
we also talk about cracks propagating by either ductile or brittle mechanisms, as you see above. Sorry
for the confusion - that’s just the way it is!
FATIGUE
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Fatigue failures occur due to cyclic loading; the mechanism of failure is the initiation and propagation
of cracks. When a crack has reached a critical length, final fast fracture will occur by either Brittle
Fracture or Ductile Fracture. Fatigue fracture surfaces can usually be recognized because they have
one or more of the following features (see also the sketch below):
• A relatively flat, smooth region where fatigue crack growth took place, and a rougher, fast
fracture region.
• lines on the fracture surface, known as “beach marks” or “clam shell marks”, which lie
parallel to the crack front, perpendicular to the direction of crack growth.
• Corrosion (e.g. rusting) on the fatigue region of the fracture surface if the crack has been
exposed to a source of corrosion during growth. This can be increased by fretting action if
compressive stresses are involved.
• Wear on the fatigue region surface if compressive stresses are involved, causing the two
surfaces of the crack to rub together.
The following two photographs show examples of fatigue failures, illustrating these features:
Microscopic Appearance: The SEM photograph below is typical of a fatigue fracture surface. The
failure is usually ductile in appearance (i.e. it does not show cleavage facets) but the scale of the
ductility is small - not sufficient to create dimples as in Ductile Failure.
This type of fracture surface is quite difficult to recognize; the main thing to remember is that it does
not show the features which are typical of fast fracture, i.e. Ductile Fracture or Brittle Fracture. In
some cases - usually very ductile metals such as aluminum, a pattern of parallel lines known as
“striations” occurs, marking the position of the crack front on successive load cycles, as shown here:
The total width of this SEM picture is only 12µm (0.012mm). This specimen was cycled with
alternating blocks of 10 cycles at high stress followed by 10 cycles at low stress, creating groups of
striations with two different spacing.
If you see striations like this then you know it is definitely a fatigue failure, but in many materials
these striations don’t occur.
STRESS-CORROSION CRACKING
Stress-corrosion cracking (SCC), also called Environmentally Assisted Cracking, is a process of crack
initiation and growth which occurs in the presence of a corrosive environment; it can occur even if the
stress is constant. If the stress is cyclic you can get a related phenomenon called ‘Corrosion Fatigue’.
Many different mechanisms are involved and this type of failure may be difficult to distinguish from
other types, but some clues to the existence of a SCC failure are:
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- Multiple crack initiation and crack branching, as shown here:
- Corrosion which is present on the fracture surface even when the rest of the part is
uncorroded.
- Intergranular fracture.
WEAR
Wear causes loss of material from the surface as a result of rubbing contact with another surface. This
may cause failure directly, as when an engine part can no longer function, or it may contribute to other
types of failure such as fatigue. We will not be considering wear failures specifically in this course
PUMP
The condition of cavitation is essentially an indication of an abnormality in the pump suction system,
whereas the condition of low flow indicates an abnormality in the entire pumping system or process.
The two conditions are also interlinked such that a low flow situation can also induce cavitation.
Mechanism of cavitation:
General symptoms of cavitation and its effects on pump performance and pump parts.
Types of cavitation:
Vaporous cavitation
i. Classic cavitation
ii. Internal re-circulation cavitation
Gaseous cavitation
i.
Air ingestion induced cavitation
Concept of Cavitation
Cavitation is a common occurrence but is the least understood of all pumping problems. Cavitation
means different things to different people. Some say when a pump makes a rattling or knocking sound
along with vibrations, it is cavitating. Some call it slippage as the pump discharge pressure slips and
flow becomes erratic. When cavitating, the pump not only fails to serve its basic purpose of pumping
the liquid but also may experience internal damage, leakage from the seal and casing, bearing failure,
etc.
In summary, cavitation is an abnormal condition that can result in loss of production, equipment
damage and worst of all, personnel injury.
In the context of centrifugal pumps, the term cavitation implies a dynamic process of formation of
bubbles inside the liquid, their growth and subsequent collapse as the liquid flows through the pump.
Generally, the bubbles that form inside the liquid are of two types: Vapor bubbles or Gas bubbles.
1. Vapor bubbles are formed due to the vaporisation of a process liquid that is being pumped. The
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cavitation condition induced by formation and collapse of vapor bubbles is commonly referred to as
Vaporous Cavitation.
2. Gas bubbles are formed due to the presence of dissolved gases in the liquid that is being pumped
(generally air but may be any gas in the system). The cavitation condition induced by the formation
and collapse of gas bubbles is commonly referred to as Gaseous Cavitation.
Both types of bubbles are formed at a point inside the pump where the local static pressure is less than
the vapor pressure of the liquid (vaporous cavitation) or saturation pressure of the gas (gaseous
cavitation).
Vaporous cavitation is the most common form of cavitation found in process plants. Generally it
occurs due to insufficiency of the available NPSH or internal recirculation phenomenon. It generally
manifests itself in the form of reduced pump performance, excessive noise and vibrations and wear of
pump parts. The extent of the cavitation damage can range from a relatively minor amount of pitting
after years of service to catastrophic failure in a relatively short period of time.
Gaseous cavitation occurs when any gas (most commonly air) enters a centrifugal pump along with
liquid. A centrifugal pump can handle air in the range of ½ % by volume. If the amount of air is
increased to 6%, the pump starts cavitating. The cavitation condition is also referred to as Air binding.
It seldom causes damage to the impeller or casing. The main effect of gaseous cavitation is loss of
capacity.
The different types of cavitation, their specific symptoms and specific corrective actions shall be
explored in the next part of the article. However, in order to clearly identify the type of cavitation, let
us first understand the mechanism of cavitation, i.e. how cavitation occurs. Unless otherwise
specified, the term cavitation shall refer to vaporous cavitation.
CAVITATION PITTING
CAVITATION DAMAGES
Figure 10: Impeller Cavitation Regions
Vaporization of the liquid (bubble formation) occurs due to the reduction of the static pressure to a
value below that of the liquid vapor pressure. The reduction of static pressure in the external suction
system occurs mainly due to friction in suction piping. The reduction of static pressure in the internal
suction system occurs mainly due to the rise in the velocity at the impeller eye.
Figure 11: Collapse of a Vapor Bubble
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After the bubble collapses, a shock wave emanates outward from the point of collapse. This shock
wave is what we actually hear and what we call “cavitation”. The implosion of bubbles and emanation
of shock waves (red color) is shown in a small video clip available
DEFINITIONS:
Static Pressure, ps: The static pressure in a fluid stream is the normal force per unit area on a solid
boundary moving with the fluid. It describes the difference between the pressure inside and outside a
system, disregarding any motion in the system. For instance, when referring to an air duct, static
pressure is the difference between the pressure inside the duct and outside the duct, disregarding any
airflow inside the duct. In energy terms, the static pressure is a measure of the potential energy of the
fluid.
Dynamic pressure, pd: A moving fluid stream exerts a pressure higher than the static pressure due to
the kinetic energy (½ mv2) of the fluid. This additional pressure is defined as the dynamic pressure.
The dynamic pressure can be measured by converting the kinetic energy of the fluid stream into the
potential energy. In other words, it is pressure that would exist in a fluid stream that has been
decelerated from its velocity ‘v’ to ‘zero’ velocity.
Total pressure, pt : The sum of static pressure and dynamic pressure is defined as the total pressure. It
is a measure of total energy of the moving fluid stream. i.e. both potential and kinetic energy.
Relation between ps, pd & pt : In an incompressible flow, the relation between static, dynamic and
total pressures can be found out using a simple device called Pilot tube (named after Henri Pilot in
1732) shown in Figure 1.
Figure 1: A Simple Sketch of a Pilot Tube
Vapor pressure, pv: Vapor pressure is the pressure required to keep a liquid in a liquid state. If the
pressure applied to the surface of the liquid is not enough to keep the molecules pretty close together,
the molecules will be free to separate and roam around as a gas or vapor. The vapor pressure is
dependent upon the temperature of the liquid. Higher the temperature, higher will be the vapor
pressure.
How does pressure reduction occur in a pump system?
The reduction in local static pressure at any point inside the pump can occur under two conditions:
1. The actual pressure drop in the external suction system is greater than that considered during
design. As a result, the pressure available at pump suction is not sufficiently high enough to
overcome the design pressure drop inside the pump.
2. The actual pressure drop inside the pump is greater than that considered during the pump design.
Mechanism of Cavitation
The phenomenon of cavitation is a stepwise process as shown in Figure 2.
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Figure 2: Phenomenon of Cavitation
COMPOSITES:
EXAMPLES
The success of a nondestructive inspection depends primarily of the correct choice of a testing method.
Regarding complex components like a composite structure the results from any NDT method must
often be completed by the results of another measurement technique. The goal of optical techniques is
not to simply substitute conventional methods but to offer, in a complementary way, a potential to
gather data rapidly and cost-effectively in a non-destructive inspection program or for experimental
verification of calculated models. It is impossible to define which optical method is the best. There is
no winning technique, as it all depends of the application. Several criterias must be considered which
all depend from the individual needs.
D-Sight: Inspection of a wing part, composite material (CRP) shows the layer structure and a dent in
the upper section (Courtesy of Diffracto Ltd.)
Laser-US: Edge of a horizontal stabilizer of a ready-to-flyCRP CF-18 plane, wing with metallic edge,
in one scan
(Courtesy of UltraOptec Inc. & Royal Canadian Air Force)
Acceptance/Rejection Criteria
Due to the wide variety of accept/reject criteria among the different codes and specifications, it is
impractical to list the differences and similarities this document. The inspector should be aware of the
applicable governing document prior to performing the examination so that the appropriate criteria
may be employed in determining acceptability.
Examination Results
VT examination results are recorded on forms designed for the purpose of permanently recording
pertinent data regarding the examination procedure and results. Supplemental forms are available
when the primary record is insufficient and sketch forms are available to record illustrations of specific
items of interest for future reference.
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Inspection of Galvanized Products
The last step in the hot-dip galvanizing process is inspection of the galvanized piece. Inspection is
critical to measure the thickness of a galvanized coating and to ensure it meets specification
requirements. The coating thickness determines the expected service life of the item. Service life for
galvanized products is defined as the time to 5% rusting of the steel surface.
According to ASTM standard A 123, the thickness of the zinc should be between 1.4 and 3.9 mils,
depending on the coating grade, which is generally a function of steel thickness. This coating thickness
should result in a service life between 35 and 75 years, depending upon the type of environment in
which the finished piece is placed. The galvanized piece is usually inspected at the galvanizer’s plant.
Inspection covers a statistical sampling lot as prescribed in ASTM B 602.
There are several different ways to check the actual thickness of the zinc coating on a galvanized
piece. Magnetic thickness measurements can be taken with several different measuring devices. ASTM
A 123 describes other options for measuring coating thickness including stripping and weighing a
piece, weighing before and after galvanizing and microscopy testing. Magnetic balance gauges are
based upon the variation in the magnetic force of attraction between two ferromagnetic bodies as a
function of the distance between them. Pull-off magnetic gauges measure the magnetic attraction of
the underlying steel.
Electronic thickness gauges use a temperature compensated magnetic transducer to measure the
magnetic flux changes that occur when the probe (a magnet) is separated from the coated ferrous
substrate. Coating thickness is just one facet of the inspection process. Coating uniformity, adherence
and appearance also are evaluated, as are possible defects arising from fabrication and design.
Unique to galvanizing is the variety of coating appearances that may result. It is important to note that
the steel composition and surface condition greatly influence the coating appearance. However,
whether a bright, spangled, matte gray, or mottled finish, the exposed coating will turn to a matte gray
stable patina and the long-term corrosion protection is the same.
Difference Between Peeling and Flaking
Peeling is the detachment of the outer zinc layer from the underlying layers of iron-zinc alloy. These
iron-zinc alloy layers stay firmly attached to the steel. One cause of peeling is a metallurgical change
that occurs between the outer zinc layer and the intermetallics layers. While the part is still near the
galvanizing temperature a considerable quantity of zinc can diffuse into the alloy layers creating a
separation as voids on the boundary of the two layers. This is called the Kirkendall Effect. The result is
that the outer zinc layer will separate from the alloy layer and peeling will occur. Peeling can occur
when the galvanized part has been cooled too slowly. The following diagram shows how the diffusion
of zinc from the outer layer can create a series of voids and completely separate the outer layer from
the intermetallics.
Peeling
Flaking
Peeling can also happen when galvanized parts are stacked on top of each other soon after removal
from the galvanizing kettle. The stacked parts can have their free zinc layers bond together and, when
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separated, the free zinc sticks to one part and pulls off of the other part. When galvanized parts are
exposed for long periods of time to temperatures of 400° F or more, again, peeling can occur. To
prevent peeling, very slow cooling of the galvanized parts should be avoided. If possible, quench the
galvanized parts immediately after galvanizing. When stacking parts, be sure the parts are cooled
below 300° F before they are stacked or bundled.
Flaking occurs when nearly the entire zinc coating, including the iron-zinc alloy layers, has separated
from the base metal. Flaking is usually caused by galvanized coatings that are thicker than normal
coatings. Besides high-silicon steels, killed or semi-killed steels can develop a thicker than normal
coating. Also, low-silicon steels can develop coatings with thicker than normal delta alloy layers when
galvanized for long times. Thick galvanized coatings are often brittle and less adherent than normal
coatings. Flaking occurs often when galvanized parts receive blows during handling.
The above picture shows an example of flaking of the galvanized coating. Shortening the amount of
time the part is in the galvanizing kettle can minimize flaking. Flaking usually occurs on coatings that
are over 10 mils in coating thickness. To tell the difference between flaking and peeling of the
galvanized coating, a thickness measurement of the remaining coating on the steel should be made. If
the thickness gauge records a near zero reading, there is only the gamma layer of intermetallic left on
the steel. This is the surface coating problem of flaking. If the thickness gauge records a reading of two
to six mils, there is still some intermetallic material covering the base steel. This is referred to as
peeling of the galvanized coating. Peeling can be handled by touching up the surface, while flaking is a
total coating rejection.
Appendix 2 – Glossary
Standard Terminology for Visual and Optical Testing and Materials Processing
Accommodation- The process by which the eye changes focus from far to near objects.
Abbe Condenser - see condenser
Abbe Error -Additional linear off-axis error introduced through amplification of tilt and wobble with
a long moment arm. This type of error occurs when the point under measurement is a relatively long
distance from the axis of motion. For example, XYZ stages incorporating an angle bracket between the
moving elements will exhibit measurable Abbe error since the Z stage is significantly displaced above
the X and Y axes. It appears as runout, but unlike true runout, Abbe error can be minimized by
reducing the lever arm.
Abbe-Number (Abbe Constant)- Quantifies the amount of dispersion in an optical medium. A material with a high Abbe
number means that different wavelengths will have nearly the same index of refraction in that medium, yielding less
separation between wavelengths of light. See also Index of Refraction.
Aberration
In optics, aberrations are imperfections in a transmitted image compared with that predicted by paraxial limits.
Absolute Efficiency
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Regarding diffraction gratings, the actual percentage of incident monochromatic radiation diffracted into a specific order.
Grating efficiency is generally expressed as either Absolute Efficiency or Relative Efficiency. All curves listed in the
product pages are first order absolute efficiency curves.
Accuracy
For positioners, the difference between the target move position and the actual realized position measured at the defined
reference. Should not be confused with resolution.
Achromat Lens (Achromatic Doublet)
Two lenses, usually one of crown glass and one flint glass, cemented together. Corrects for chromatic and spherical
aberrations. Can also be air spaced.
Achromatic -term referring to the lens. A lens which brings in light from two parts of the spectrum
(red and blue wavelengths) to the same focus, reducing Chromatic Aberration. This is the most
common lens on a microscope. Color fringes may appear when viewing under bright white light
because not all wave lengths are brought within an acceptable focus range. If you use filtered light
(monochromatic) as in phase contrast the image will be sharper.
Achromatic Objective
The most common type of microscope objective. An achromatic objective has a flat field in approximately 65% of the
center of the image, compared to the 80% of a Semi-Plan objective and the 95% of a Plan objective. See also Semi-Plan
Objective, Plan Objective.
Acuity - See Vision acuity.
Adaptation - The process by which the retina becomes accustomed to more or less light than it was
exposed to during an immediately preceding period. It results in a change of the sensitivity of the
eye to light
Adhesive bonding - A materials joining process in which an adhesive, placed between the faying
surfaces (adherents), solidifies to produce an adhesive bond.5
AGC -See Automated Gain Control.
Aliasing - Visible as jagged effects on the vertical edges of the image and occurs in a static image
when the, scan or sample rate is too low for the frequency being digitized.
Ambient light - Light in the environment, as opposed to illumination, provided by a visual or optical
testing system.2
Angstrom unit (A) - Unit of length, equal to 0.1 nm.2
Arc strikes - Localized burn damage to an object from the arc caused by breaking an energized
electric circuit. Also called arc burns.4,6
Arc welding - See Electric arc welding.
Aspect ratio - The ratio of the width of the raster to its height.
As-welded - The condition of the welded metal, welded joints, and weldments after welding but
prior to any subsequent thermal, mechanical, or chemical treatments.5
Austenite - A solid solution with iron as the solvent in a face centered cubic structure formed by
slow cooling of delta ferrite. Characteristic lattice structure is stable between 906°C (1663°F) and
1,390 °C (2535°F).Also called gamma iron.2
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Automatic welding (AU) - Welding with equipment that performs the welding operation without
adjustment of the controls by a welding operator. The equipment mayor may not perform the
loading and unloading of the work. See Machine welding (MA).5
Back gouging - The removal of weld metal and base metal from the other side of a partially welded
joint to ensure complete penetration upon subsequent welding from that sides Background - Short
for electrical background noise. Signals from a test object that create noise, or false indications. The
higher the level of background, the more difficult it is to distinguish an indication.
Background: Short for electrical background noise. Signals from a test object that create noise or
false indications. The higher the level of background, the more difficult it is to distinguish an
indication.
Backing - In welding, a material placed under or behind a joint to enhance the quality of the weld at
the root. It may be a metal backing ring or strip; a pass of weld metal; or a nonmetal such as carbon,
granular flux, or a protective gas.5
Bainite - A metastable aggregate of ferrite and cementite resulting from the transformation of
austenite at temperatures below the pearlite range but above Ms, the martensite start temperature.
Upper bainite is an aggregate that contains parallel lath-shape units of ferrite, produces the so-called
“feathery” appearance in optical microscopy, and is forn1ed above approximately 350°C (660°F).
Lower bainite, which has an acicular appearance similar to tempered martensite, is formed below
approximately 350°C (660 °F).5
Barrel Focus - the body tube of the microscope moves to focus the objective lenses and the stage is
fixed.
Base material - The material to be welded, brazed, soldered, or cut. See Base metal
Base metal - (1) The metal present in the largest proportion in an alloy; brass, for example, is a copperbase alloy. (2) The metal to be brazed, cut, soldered, or welded. (3) After welding, the part of the metal
which was not melted. (4) A metal that readily oxidizes, or that dissolves to form ions.5
Bertrand Lens - in polarizing microscopes a special lens used to view interfaced figures.
Blackbody: A theoretical object that radiates more total power and more power at any wavelength than
any other source operating at the same temperature.
Bevel angle - The angle formed between the prepared edge of a member and a plane perpendicular to
the surface of the member.5
Body Tube Length - the distance between the objective and the top of the body tube, usually 160mm.
Objectives are designed for a specific length, if miss-matched, spherical aberration will occur.
Borescope - A periscope or telescope using mirrors, prisms, lenses, optic fibers, or television wiring to
transmit images from inaccessible areas for visual testing.2
Brazing - Joining of metals and alloys by fusion of nonferrous alloys that have melting points above
430°C (806°F), but below melting points of materials being joined.6
Bright field Illumination - bright white light which illuminates a transparent or translucent specimen
which appears dark against a bright or white background.
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Burn-through - A term erroneously used to denote excessive melt-through or a hole. See Meltthrough.5
Burst - (1) A signal whose oscillations have a rapid increase in amplitude from an initial reference
level (generally that of the background noise), followed by a decrease (generally more gradual) to a
value close to the initial value.5 (2) In metal, external or internal rupture caused by improper forming.2
Butt weld or butt joint - Weld joining two metal pieces in the same plane.2
CIE - An abbreviation for the French title of the International Commission on Illumination
(Commission lntemationale de I’Eclairage).
Candela (cd) - Base unit of measure in the SI system for luminous intensity. The luminous intensity in
a given direction of a source that emits monochromatic radiation of frequency 540x1012 Hz and that
has a radiant intensity in that direction of 1.4641 milliwatt per steradian.2
Chroma - The difference from gray (the achromatic equivalent) of a color.
chromatic - Lacking any amount of a chromatic primary.
Chromatic Aberration - failure of lens to bring light of different wavelengths to a common focus.
This can be compensated by using an achromatic lens.
Condenser (substage) - provides an even cone of light that illuminates the specimen. Light from the
condenser converges on the specimen, passes through it, and diverges to from an inverted illumination
cone that is captured by the objective lens. The Abbe condenser is the most common. The condenser
numerical aperture (NA) should be equal to or greater than the highest objective NA., usually 1.25 to
1.32 for a 100x oil objective. The resolving power of the optical system composed of condenser,
objective ocular lens is limited to the lowest NA of its individual components.
Coaxial - focusing system where the coarse and fine focus are mounted together on a common axis.
Cold shut - (1) Casting discontinuity caused by two streams of semi molten metal coming together
inside a mold but failing to fuse. Cold shuts are sometimes called misruns but the latter term correctly
describes incomplete filling of the mold. (2) A discontinuity that appears on the surface of test metal as
a result of two streams of liquid meeting and failing to unite. A crack like discontinuity caused by
forging, where two surfaces of metal fold against each other to produce a discontinuity at the point of
folding. This is usually at some angle to the surface. It may also be a separate piece of metal forged
into the main component. See Lap. (3) A portion of the surface of a forging that is separated in part
from the main body of metal by oxide.6
Color - Sensation by means of which humans distinguish light of different intensities (brightness) and
wavelengths (hue)2
Complete fusion - Fusion that has occurred over the entire base material surfaces intended for welding
and between all layers and weld beads.5
Complete joint fusion - Joint penetration in which the weld metal completely fills the groove and is
fused to the base metal throughout its total thickness.5
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Concavity - The maximum distance from the face of a concave fillet weld perpendicular to a line
joining the toes.5
Convexity - The maximum distance from the face of a convex fillet weld perpendicular to a line
joining the toes.5
Carburization: The absorption of carbon into a metal surface; may or may not be desirable.
Crack - (1) A break, fissure or rupture, usually V-shaped and relatively narrow and deep. A
discontinuity that has a relatively large cross section in one direction and a small or negligible cross
section when viewed in a direction perpendicular to the first. (2) Propagating discontinuities caused by
stresses such as heat treating or grinding. Difficult to detect unaided because of fineness of line and
pattern (may have a radial or latticed appearance).6
Crack, transverse - Cracks at right angles to the length of the test object.6
Crater - (l) In machining, a depression in the cutting tool face eroded by chip contact. (2) In arc or gas
fusion welding, a cavity in the weld bead surface, typically occurring when the heat source is removed
and insufficient filler metal is available to fill the cavity.6
Crevice Corrosion: Localized corrosion of a metal surface at, or immediately adjacent to, an area that
is shielded from full exposure to the environment because of close proximity between the metal and
the surface of another material.
Dark current - The unwanted charge that accumulates in each pixel due to the natural thermal
processes that occur above absolute zero.
Dark field Illumination - An optical technique where the specimen is seen as a bright object against a
dark background.
Datum - A theoretically perfect point, axis, or plane that is derived from actual features.
Defect - A condition or discontinuity having a size, shape, orientation, nature, frequency, or location
that impairs the useful service of the part or that is rejectable according to a specification or standard.3
Depth of Field - the distance along the optical axis throughout which the object can be located and yet
be imaged with satisfactory clarity. This is used in stereo microscopy.
Differential Interference Contrast (DIC) -A mode of contrast generation in microscopy that yields
an image with a shadow relief. The relief reflects the gradient of optical path difference. DIC, which is
a form of interference microscopy that uses polarizing beam splitters.
Diopter Adjustment - adjustment ring of the eyepiece that fine focuses the eye lens element.
Discontinuity - An intentional or unintentional interruption in the physical structure or configuration
of a part.4
Disc Diaphragm - a circular disc, located under the stage platform containing five or six apertures of
various sizes which controls the size of illumination cone converging on the specimen.
DIN - a term referring to optics which is the abbreviation for Deutsche Industrie Normen, an industry
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standard for optics.
Effective throat - In welding, the weld throat including the amount of weld penetration but ignoring
excess metal between the theoretical face and the actual face.2
Electric arc welding - Joining of metals by heating with electric arc. Also called arc welding.2
Electromagnetic spectrum - A continuum or electric and magnetic radiation encompassing all
wavelengths.1
Electroslag welding (ESW) - A fusion welding process in which the welding heat is provided by
passing an electric current through a layer of molten conductive slag (flux) contained in a pocket
formed by water-cooled dams that bridge the gap between the members being welded. The resistance
heated slag not only melts filler-metal electrodes as they are fed into the slag layer, but also provides
shielding for massive weld puddle characteristic of the process.5
Endoscope - Device for viewing the interior of objects. The term endoscope is usually used for
medical instruments that are equivalent to borescopes.2
Evaluation - A review, following interpretation of the indications noted, to determine whether they
meet specified acceptance criteria.2
False indication - An indication that is not produced by a discontinuity. Compare Dcfect.2
Far vision - Vision of objects at a distance, generally beyond arm’s length.2
Faying surface - The surfaces of materials in contact with each other and joined or about to be joined.5
Field-Of-View - the visible area through the eyepiece when the microscope is in focus.
Field of vision - The range or area where objects can be perceived organoleptically (using the eyes
only), assuming that the eye’s position is fixed.
Filler metal - Metal added in making a brazed, soldered, or welded joint.5
Fillet weld - Weld at the corner of two metal pieces.2
Flat Field Objectives or Optics - lenses which are corrected to eliminate curvature. Flat field
optics are a higher grade then achromatic.
Flatness of Travel -Deviation from ideal straight line travel in a vertical plane, also referred to as
vertical runout.
Flicker - When the frame repetition rate is not high enough to provide an image that is perceived as
continuous by a human observer.
Fluorescence - Luminescence produced by a material that is excited by light, electricity, or
radiation. The luminescence ceases as soon as the source of excitation is removed.
Flux cored arc welding (FCAW) - An arc welding process that joins metal by heating them with
an arc between continuous tubular filler-metal electrode and the work. Shielding is provided by a
flux contained within the consumable tubular electrode. Additional shielding mayor may not be
obtained from an externally supplied gas or gas mixture.5
Focal distance (lens) - See Focal length.
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Focal length (lens) - The distance from the principal plane to the focal plane.
Focal plane - Where parallel incident light rays converge after being diffracted by the lens.
Foot candle (ftc) - Former unit of measure for illumination, equivalent to 1 lumen/ft2 or 1 candle of
luminance measured at 1 ft.
Fusion (welding) - the melting together of filler metal and base metal (substrate), or of base metal
only, which results in coalescence.5
Gas metal arc welding (GMAW) - An arc welding process that produces coalescence of metals by
heating them with an arc between a continuous filer metal (consumable) electrode and the work.
Shielding is obtained entirely from an externally supplied gas or gas mixture. Variations of the
process include short-circuit arc GMAW, in which the consumable electrode is deposited during
repeated short circuits, and pulsed arc GMAW, in which the current is pulsed.5
Gas tungsten arc welding (GTAW) - An arc welding process that produces coalescence of metals
by heating them with an arc between a tungsten (non consumable) electrode and the work.
Shielding is obtained from a gas or gas mixture)
Glare - Excessive brightness (or brightness varying by more than 10: 1 within the field of view)
that interferes with clear vision, critical observation, and judgment2
Graybody - Radiator whose spectral emissivity is uniform for all wavelengths.2
Grayscale - A means of defining the image brightness as a function of the brightness of the object
being imaged. It is specified by the highlight brightness, contrast ratio, and gamma
Groove (welding) - An opening or channel in the surface of a part or between two components that
provides space to contain a weld.5
Groove angle - The total included angle of the groove between parts to be joined. This the sum of two
bevel angles, either or both of which may be zero degrees.5
Groove face - The portion of a surface or surfaces of a member included in a groove.5
Heat affected zone (HAZ) - Base metal not melted during brazing, cutting, or welding, but whose
microstructure and physical properties were altered by the heat. 6
Highlight brightness - The brightness of the brightest area of the image.
Hot tear - A fracture formed in a metal during solidification because of hindered contraction. Surface
cracks on castings produced by contraction of the metal during cooling. Hot tears often occur where
areas of different thickness adjoin.2
Hue - Attribute of color perception by which a color is perceived to be red, yellow, blue, or an
intermediate color between these. White, black, and grays possess no hue.
Huygenian - a type of eyepiece that has two planoconvex lens elements with the eyepiece diaphragm
between them. Usually with a narrow field of view
Hydrogen Embrittlement - A condition of low ductility in metals resulting from the absorption of
hydrogen.6 A process resulting in a decrease of the toughness or ductility of a metal due to the
presence of atomic hydrogen. Hydrogen embrittlement has been recognized classically as being of two
types. The first known as internal hydrogen embrittlement, occurs when the hydrogen enters molten
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metal which becomes supersaturated with hydrogen immediately after solidification. The second type,
environmental hydrogen embrittlement, results from hydrogen being absorbed by solid metals.
Illuminance (Ev = dΦ/dA) - The density of luminous flux on a surface. It is the equivalent to the
measure of irradiance in radiometry.2
Image - The visual impression of an object produced by a lens, mirror, or optical system. A real image
is formed by convergence of light and can be produced on a screen. A virtual image is one from which
the rays of light appear to diverge. The virtual image cannot be produced on a screen.
Incandescence - The emission of visible radiation due to thermal excitation.2
Indication - An NDT response that requires interpretation to determine its relevance.2
Indication, false - An NDT indication that is interpreted to be caused by a condition other than a
discontinuity or imperfection.4
Indication, non relevant - An NDT indication that is caused by a condition or type of discontinuity
that requires evaluation.4
Indication, relevant - An NDT indication that is caused by a condition or type of discontinuity that
requires evaluation.4
Interpretation - The determination of whether indications are relevant, nonrelevant, or false.4
Immersion Objective - most common is the 100x oil objective. Oil is placed on the cover glass of the
slide which (and sometimes on the top element of the condenser) to produce a high magnification and
high resolving power of the objective when immersed in the oil. This produces the full NA of objective
lens.
Iris Diaphragm - usually mounted under the condenser this controls the amount of light converging
on the specimen by opening or closing the leaf diaphragm.
Irradiance (Ee = dΦ/dA) - Power of electromagnetic radiant energy incident on a surface of given unit
area.2
IshiharaTM plates - Trade name for a kind of pseudoisochromatic plates.2
Eyepiece - also known as ocular, it produces the second stage of magnification enlarging the image
magnified by the objective lens. Eyepieces vary in magnification from 5x to 30x. The standard is 10x.
Eyepiece Diaphragm - the piece inside the eyepiece which holds gracticules and reticules. It also
defines the round field of view that is seen through the microscope.
Koehler Illumination - a technique for uniformly illuminating a field from non-uniform light such as
a coiled filament lamp.
Lag - measure of the rate of change of the video signal at a fixed point on the raster.
Lamellar tearing - Occurs in the base metal adjacent to weldments due to high through- thickness
strains introduced by weld metal shrinkage in highly restrained joints. Tearing occurs by decohesion
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and linking along the working direction of the base metal; cracks; usually run roughly parallel to the
fusion line and are steplike in appearance. Lamellar tearing can be minimized by designing joints to
minimize weld shrinkage stresses and joint restraints
Lamination - Discontinuity in plate, sheet, or strip caused by pipe, inclusions, or blowholes in the
original-ingot. After .rolling, laminations are usually flat and parallel to the outside surface.
Laminations may also result from pipe, blisters, seams, inclusions, or segregation elongated and are
made directional by working. Lamination discontinuities may also occur in metal powder compacts.
May appear in the form of rectangles or plates as inclusion stringers between rolled surfaces. Short,
intermittent laminations may be detrimental if the object is subjected to high bending stresses in
service.6
Lap - Surface discontinuity, usually parallel to the surface, appearing as a fold or tangential seam
in a wrought product and caused by the folding over of a hot metal fin or sharp comer in a thin
plate, then rolling or forging it into the surface but not welding it. See also Cold shut.6
Leg of fillet weld - (1) Actual - the distance from the root of the joint to the toe of the fillet weld. (2)
Nominal - the length of a side of the largest right triangle that can be inscribed in the cross section of
the weld.5
Lens - Translucent object that refracts light passing through it in order to focus the light on a target.2
Light - Radiant energy between approximately 380 nm and 770 nm. This portion of the
electromagnetic spectrum produces a response in the human retina.
Lumen (1m) - Unit of measure in the SI system for luminous flux, equivalent to candela times
steradian (cd.sr).2
Luminance [L = d2Φ/(dω dA cos θ)] - The ratio of a surface’s luminous intensity in a given direction
to a unit of projected area. Measured in candelalm2.
Luminous flux - Radiant energy’s time rate of flow. Measured in lumens.
Luminous intensity (Iv = dΦ/dw) - Luminous flux on a surface normal to the direction from its light
source, divided by the solid angle the surface subtends at the source. Measured in candela2
Lux (lx) - Unit of measure for illuminance in the SI system. Equivalent to lm/m2.
Machine vision - Automated system function of acquiring, processing, and analyzing images to
evaluate a test object or to provide information or interpretation for human evaluation.2
Machine -welding (ME) - Welding with equipment that performs under the continual observation and
control of a welding operator. The equipment mayor may not load the work. Compare with automatic
welding (AW).5
Macular lutae - Irregular, diffuse ring of yellow pigment surrounding the fovea that absorbs blue light
and changes the spectral energy distribution of the light reaching the receptors under it.
Magnification - the enlargement of an object through the lens system. This is determined by
multiplying the magnifying power of the objective by the eyepiece.
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Magnifying Power - magnification, which is the ability of the lens to make an object appear larger. It
is the number of times the image is seen through the microscope is larger than the item appears to the
unaided eye.
Manual welding (MA) - Welding wherein the entire welding operation is performed and controlled by
hand.5
Mechanical Stage - a device on the platform for holding and moving the slide (or specimen) on an X
or Y axis.
Melt-through - Complete joint penetration for a joint welded from one side. To prevent melt-through,
the welding current and the width of the root opening should be reduced, and travel speed increased.
Metallograph - A metallographic microscope designed for metallography.
Metallography - The science and practice of microscopic testing, inspection, and analysis of a metal’s
structure, typicality at magnifications in the range of 50X to 2500X.2
Micrometer Disc - a glass disc with a scale or grid mounted to the eyepiece diaphragm used for
measurement.
Microscope - An instrument that provides enlarged images of very small objects.2
Monochromatic - Color composed of a single hue, having a very narrow band of wavelength.
Monochromator - Device that uses prisms or gratings to separate or disperse the wavelengths of the
spectrum into non continuous lines or bands.2
Near vision - Vision of objects near to the eye when accommodative ability is required. Generally
within arm’s length.
Nearsightedness - Vision acuity that is adequate for near vision.
N-type semiconductor - See Semiconductor.
Numerical Aperture - (N.A.) a term representative of the angle included by a cone of light accepted
by the objective of a microscope. The higher the N.A., the greater the resolving power.
Objective lens - forms the primary image of the microscope which is seen through the eyepiece. The
markings on the objective lens are the magnifying power (such as 10x), followed by the NA (0.25) and
the tube length. Other numbers which appear on the objective lens may refer to the manufactures
catalog number of the particular item.
Oculars -see “eyepiece”
Orthogonality
- The degree of perpendicularity, or squareness, between the two axes in an X-Y
or X-Z table This parameter is usually measured in arc-seconds or microradians.
Overlap - (1) Pultrusion of weld metal beyond the toe, face, or root of a weld. (2) In resistance
seam welding, the area in a given weld remelted by the succeeding weld. See also Face of weld,
Root of weld, and Toe of weld.
Oxyacetylene welding (OAW) - An oxyfuel gas welding process in which the fuel gas is acetylene.
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Oxidation: A reaction in which there is an increase in valence resulting from a loss of electrons.
Contrast with reduction. (2) A corrosion reaction in which the corroded metal forms an oxide.
Usually applied to reaction with a gas containing elemental oxygen, such as air.
Parcentered - When all the elements of the optical system are aligned on a single axis thus
reducing aberration.
Parfocal (parfocality) - a term used describing the property of a microscope where the subject
stays in focus when the objective lenses are changed. Less then ½ of a revolution from the fine
adjustment is usually acceptable.
Pearlite - Platelet mixture of cementite and ferrite in steels or in alpha and beta phases in
nonferrous alloys.
Phase Contrast - the optical technique used to view the structure of transparent objects whose
varying but invisible differences in thickness result in differences in the phase of transmitted light.
This is done when the transmitted light changes its optical path by about 1/3 wavelength.
Photometer - The basic measuring instrument of photometry.2
Photometry - The science and practice of the measurement of light or photon emitting
electromagnetic radialion.2
Photons - Discrete particles of light or electromagnetic radiation hypothesized to explain the
corpuscular theory of radiant energy.
Photopic vision - Vision adapted to daylight and mediated mainly by the cones. Vision is wholly
photopic when the luminance of the test surface is above 0.034 cd.m-2 (0.0032 cd.ft-2)
Pipe - (1) The central cavity formed during solidification of metal, especially ingots, by thermal
contraction. (2) The discontinuity in wrought or cast products resulting from such a cavity (3) An
extrusion discontinuity due to the oxidized surface of the billet flowing toward the center of the rod at
the back end. (4) A cast, wrought, or welded metal tube.6
Pitch -An angular deviation possible in positioning systems, in which the tables’ leading edge rises or
falls as the table translates along its direction of travel. This represents rotation around a horizontal
axis, perpendicular to the direction of travel.
Pixel - Short for picture element, a pixel is an individual light sensor.
Planachromat - an achromat lens which has been corrected for a flat field.
Planoconcave Mirror - a two sided mirror 50mm in dia., with one side flat (plano) and the other
curved, (concave). The concave side is used for low NA when no condenser is used, the plano surface
is used with a substage condenser.
Plasma arc welding (PAW) - An arc welding process that produces coalescence of metals by heating
them with a constricted arc between an electrode and the work piece (transferred arc) or the electrode
and the constricting nozzle (non transferred arc). Shielding is obtained from hot, ionized gas issuing
from an orifice surrounding the electrode and may be supplemented by an auxiliary source of shielding
gas, which may be an inert gas or a mixture of gasses. Pressure mayor may not be used, and filler
metal mayor may not be supplied.
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Polarizer - transparent material which can absorb all vibrations of light passing through it except those
in a single plane.
Porosity - A discontinuity in metal resulting from the creation or coalescence of gas. Very small pores
are called pinholes.
Principal focus - See Focal plane.
Process - Repeatable sequence of actions to bring about a desired result.
Pseudoisochromatic plates - Color plates used for color vision examinations.2
P-type semiconductor - See Semiconductor.
Quality of lighting - Level of distribution of luminance in a visual task or environment
Radiance [Le = d2Φ/(dω.dA cos θ)] - Radiant flux per unit solid angle and per unit projected area of
the source. Measured in watts per square meter steradian.
Rack and Pinion - term used to describe the gear system for lowering and raising the stage or barrel
when focusing. The coarse adjustment control (knob), usually moves the barrel or stage.
Radiant energy (Qe) - Energy transmitted through a medium by electromagnetic waves. Also known
as radiation.
Radiant flux (Φ = dQ/dt) - Radiant energy’s rate of flow, measured in watts.
Radiant intensity (Ie = dΦ/dω) - Electromagnetic energy emitted per unit time per unit solid angle.
Measured in watts per steradian.2
Raster - The repetitive pattern whereby the scanning electron beam follows a path of adjacent parallel
lines.
Remote visual inspection - Viewing of an object not in the viewer’s field of vision.
Refractive Index (R I ) - the ratio of the speed of light in a vacuum to its speed in some other
medium. This will determine how much light rays are bent. When using immersion objectives it is
important to keep the values as close together as possible.
Resolution - An aspect of image quality pertaining to a system’s ability to reproduce objects, often
measured by resolving a pair of adjacent objects or parallel lines. A measure of the ability of a lens to
image closely spaced objects so they are recognized as separate objects.
Resolving power - The ability of vision or other detection systems to separate two points, Resolving
power depends on the angle of vision and the distance of the sensor from the test surface. Resolving
power is often measured using parallel lines. The capacity of any optical system to distinguish and
separate details in a specimen.
Retractable Objective (XR) - usually on the 40x and 100x objectives, a spring is loaded inside the
objective so that minimal damage is done to the slide and the top element of the lens should it be
racked down beyond its normal stop.
Rhodopsin - Retinal receptor that responds at low light levels.
Root face - The portion of a weld groove face adjacent to the root of the joint.
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Root mean square (MIS) - The square root of the average of the squares of a number of values.
Root of joint - The portion of a weld joint where the members are closest to each other before
welding. In cross section” this may be a point, a line, or an area.
Root opening - In a weldment, the separation between the members at the root of joint prior to
welding.
Root of weld - The points at which the weld bead intersects the base-metal surfaces either nearest to or
coincident with the root of joint.
Runout -The linear (versus angular) portion of off-axis error. It is the deviation between ideal straight
line motion and actual measured motion in a translation stage. Runout has two orthogonal components,
straightness, a measure of in-plane deviation, and flatness, the out-of-plane deviation.
SI - The International System of units of measurement. Includes most of the base units formerly called
metric.
Saturation - Relative or comparative color characteristic resulting from a hue’s dilution with white
light.
Scab - A flat volume of metal joined to a casting through a small area. Usually set in a depression, a
flat side being separated from the metal of the casting proper by a thin layer of sand.
Scan lines - See Trace.
Scotopic vision - Dark adapted vision, using only the rods in the retina, where differences in
brightness can be detected but the differences in hue cannot. Vision is wholly scotopic when the
luminance of the test surface is below 3 x 10-5 cd.m-2 (2.7 x 10-6 cd.ft-2). Also known as parafoveal
vision.
Seam - (1) On the surface of metal, an unwelded fold or lap that appears as a crack, usuall resulting from a discontinuity
obtained in casting or working. (2) Mechanical or welded joints. (3) Longitudinal surface discontinuity on metal originating
from a surface crack or blowhole near the surface of the ingot, that is drawn out during rolling and follows the rolling
direction. Also due to overfill while rolling. After forging, seams generally follow the direction of flow lines.
Semiautomatic arc welding (SW) - Arc welding with equipment that controls only the filler metal
feed. The advance of the welding is manually controlled.
Semiconductor - A solid with an electrical conductivity that is intennediate between those of
insulators and metals. Semiconductors are usually metalloid elements such as Germanium or Silicon.
n-type semiconductors - a semiconducting material where most of the circuit is carried by electrons
when deliberately placed donor impurities in the material form positive ions and release electrons.
p-type semiconductor - semiconducting material where acceptor impurities require extra electrons for
stable bonding. The ionization of the impurity creates a positive hole creating the current flow.
Shielded metal arc welding (SMAW) – A manual arc welding process in which the heat for -welding
is generated by the arc established between a flux-covered consumable electrode and a work piece.
The electrode tip, molten weld pool, arc, and adjacent areas of the work piece are protected form
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atmospheric contamination by a gaseous shield obtained from the combustion and decomposition of
the electrode covering. Additional shielding is provided from the molten metal in the weld pool by a
covering of molten flux or slag, Filler metal is supplied by the core of the consumable electrode and
from the metal powder mixed with the electrode covering of certain electrodes. SMAW is often
referred to as arc welding with stick electrodes, manual metal arc welding, and stick welding.
Size of weld - (1) The joint penetration in a groove weld. (2) The lengths of the nominal legs of a fillet
weld. (3) The weld metal thickness measured at the root of a flange weld.
Smear - When the phase response of the imaging system is not capable of reproducing a rapid change
in shade or contrast in a static object.
Specification - A set of instructions or standards invoked by a specific customer to govern the results
or performance of a specific set of tasks or products.
Spectral power distribution - The radiant power per unit wavelength as a function of wavelength.
Also known as spectral energy distribution, spectral density, and spectral distribution.
Spectrophotometer - Instrument used for spectrophotometry.
Spectrophotometry - Measurement of electromagnetic radiant energy as a function of wavelength,
particularly in the ultraviolet, visible, and infrared wavelengths.
Spectroradiometer - Instrument used for spectroradiometry.
Spectroradiometry - Measurement of electromagnetic radiant power and spectral emittance, used
particularly to examine colors and to measure the spectral emittance of light sources.
Spectrum - Representation of radiant energy in adjacent bands of hues in sequence according to the
energy’s wavelengths or frequencies. A rainbow is a well known example of a visible spectrum.
Specular - Pertaining to a mirror-like reflective finish, as of a metal
Spherical Aberration - the failure of a lens system to image the central and peripheral rays at the
same focal point.
Stage - the platform on the microscope where the slide or specimen is placed. The stage may be
square, circular, fixed, rotating or even interchanged.
Stage Focus - a specific type of focus where the stage moves and the body tube is stationary when
focusing.
Standard - Document to control and govern practices in an industry or application, applied on a
national or international basis and usually produced by consensus.
Straightness of Travel -Deviation from straight line motion in a horizontal plane. Also referred to as
horizontal runout. This error is usually traceable to an underlying angular error of the ways.
Stress corrosion cracking - Failure by cracking under combined action of corrosion and stress, either
applied or residual. Cracking may be either intergranular or transgranular, depending on the meta] and
corrosive medium.
Stress Corrosion Cracking: A cracking process that requires the simultaneous action of a corrodent
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and sustained tensile stress. This excludes corrosion-reduced sections that fail by fast fracture. It also
excludes intercrystalline or transcrystalline corrosion, which can disintegrate an alloy without applied
or residual stress. Stress-corrosion cracking may occur in combination with hydrogen embrittlement.
Stud welding (SW) - An arc welding process in which the contact surfaces of a stud, or similar
fastener, and a work piece are heated and melted by an arc drawn between them. The stud is then
plunged rapidly onto the work piece to form a weld. Partial shielding may be obtained by the use of a
ceramic ferrule surrounding the stud, Shielding gas or flux may or may not be used. The two basic
methods of stud welding arc known as stud arc welding, which produces a large amount of weld metal
around the stud base and a relatively deep penetration into the base metal, and capacitor discharge stud
welding which produces a very small amount of weld metal around the stud base and shallow
penetration into the base metal
Submerged arc welding (SAW) - Arc welding in which the arc, between a bare metal electrode and
the work, is shielded by a blanket of granular, fusible material overlying the joint. Pressure is not
applied to the joint, and filler metal is obtained from the consumable electrode (and sometimes from a
supplementary welding rod).
Throat, actual - Shortest distance from the root of a fillet weld to its face, as opposed to theoretical
throat or weld size.
Throat, theoretical - The distance from the beginning of the root of the weld perpendicular to the
hypotenuse of the largest right triangle that can be inscribed within the cross section of the fillet weld.
Compare Size of weld.
Throat, weld - Distance from the root of a fillet weld to its face. Compare Size of weld and Throat,
actual.
Tilt and Wobble -The angular portion of off-axis error. It is the deviation between ideal straight line
motion and actual measured motion in a translation stage. Tilt and wobble have three orthogonal
components commonly referred to as roll, pitch, and yaw. These terms usually dominate the overall
error due to the geometry of the motion system.
Toe of weld - The junction between the face of a weld and the base metal
Trace - Line formed by electron beam scanning from left or right on a video screen to generate a
picture.
Tungsten inert gas (TIG) welding - See Gas tungsten arc welding (GTAW).
Ultraviolet radiation - Electromagnetic radiation with wavelengths from 4-400 nm, between visible
light and X-rays.
Undercut - Undesirable depression or groove left unfilled by weld metal, created by melting during
welding and located in base material at the toe of a weld.
Underfill - In weldments, a depression on the face of the weld of root surface extending below the
surface of the adjacent base metal.
Video - Pertaining to the transmission and display of images in an electronic format that can be
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displayed on a cathode ray screen.
Visibility - The quality or state of being perceivable by the eye. In many outdoor applications,
visibility is defined in terms of the distance at which an object can be just perceived by the eye. In
indoor applications, it usually is defined in terms of the contrast or size of a standard test object,
observed under standardized view conditions, having the same threshold as the given object.
Vision - Perception by eyesight.
Vision acuity - The ability to distinguish fine details visually. Quantitatively, it is the reciprocal of
the minimum angular separation in minutes of two lines of width subtending one minute of arc
when the lines are just resolvable as separate.
Visual acuity - See Vision acuity.
Visual angle - The angle subtended by an object or detail at the point of observation. It usually is
measured in minutes of arc.
Visual perception - The interpretation of impressions transmitted from the retina to the brain in
terms of information about a physical world displayed before the eye. Visual perception involves
anyone or more of the following - recognition of the presence of something (object, aperture, or
medium); identifying it; locating it in space; noting its relation to other objects; or identifying its
movement, color, brightness, or form.
Visual performance - The quantitative assessment of the performance of a visual task, taking into
consideration speed and accuracy.
Visual purple - See Rhodopsin.
Visual task - The appearance and immediate background of those details and objects that must be
seen for the performance of a given activity.
Visual testing - Method of NDT using electromagnetic radiation at visible frequencies.
Wide field Oculars (eyepieces) - a term referring to the size of the field of view on the ocular. A
wide field ocular may be 19-21mm compared to a lower measure.
Working Distance - the distance between the cover glass or object and the tip of the objective. This
governs the allowable movement of the objective in obtaining critical focus of the specimen.
Weld - A localized coalescence of metals or nonmetals produced either by heating the materials to
suitable temperatures, with or without the application of pressure, or by the application of pressure
alone and with or without the use of filler material.
Yaw -An angular deviation from ideal straight line motion, in which the positioning tables rotates
around the Z (vertical) axis as it translates along its travel axis.
Zoom - a lens system that provides for variable magnification capability while keeping the specimen
in focus.
Glossary References
1. IES Lighting Handbook - Reference & Application, 8th Edition. New York, NY - The Illuminating Engineering
Society of North America. 1993.
2.
Allgaier, Michael W., Stanley Ness, Paul McIntire, and Patrick O. Moore, eds. Nondestructive Testing Handbook,
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second edition - Volume 8, Visual and Optical Testing. Columbus, OH - The American Society for Nondestructive Testing.
1993.
3.
EPRI Learning Modules. Charlotte, NC - The Electric Power Research Institute
4. ASTM E J316, 1997 Annual Book of ASTM Standards, Section 3, Metals Test Methods and Analytical Procedures,
Volume 03.03, Nondestructive Testing. West Conshohocken, PA : The American Society for Testing and Materials. 1997.
5.
Davis, J.R., ed. ASM Materials Engineering Dictionary. ASM International 1992.
6. Ness, Stanley, Charles N. Sherlock, Paul McIntire, and Patrick O. Moore, eds. Nondestrtlctive Testing Handbook,
second edition - Volume 10, Nondestructive Testing Overview. Columbus, OH - The American Society for Nondestructive
Testing. 1996.
Alignment (Centration)
Beam alignment for a laser is a measure of how well the optical axis coincides with the mechanical axis of the laser
(usually expressed as an angle). It is often critical to know the alignment of the beam output to the mechanical housing of a
laser for alignment or sighting applications. A simple test to check the alignment of a laser is to rotate the laser about its
mechanical axis and observe the circle traced out on a target some known distance away. The center of the circle defines
the mechanical axis and the diameter of the circle in combination with the distance from the laser can be used to determine
an angular alignment error.
Alnico Magnets
Short for Aluminum Nickel Cobalt magnets.
Amici (Right Angle Roof) Prism
Inverts the image and changes the line of sight by 90 degrees, still having left to right correct. Used in telescopes to erect an
inverted image making them useful for terrestrial viewing.
Analog vs. Digital Signal
An analog signal continuously changes over time with respect to a reference level or standard; while a digital signal
changes in regular steps, the signal level at each step represented by a number. Analog-to-digital signal conversion (as in
PC image capture boards) involves sampling an analog signal at high frequency and representing each sample level by a
number, stored as binary data. CCD cameras output analog video signals unless explicitly specified as digital cameras, in
which case analog-to-digital conversion takes place in the camera rather than in a computer.
Anamorphic Lenses
Since the symmetries of traditional lenses are not always desirable, anamorphic lenses are designed to have an axially
dependent imaging characteristic. Such lenses can be used to transform the elliptical beam profile formed by a diode
junction into a circular beam profile by treating the major and minor axes differently.
Angular Field of View
The field of view (FOV) and working distance (WD) are related by the tangent of the viewing angle.
Anodize
Chemical oxidation of the surface of our aluminum parts to prevent corrosion and increase abrasion resistance. Black
anodizing is not only cosmetically appealing, but also does not reflect light in optical set-ups.
Aperture (Soft/Sharp)
Important in some machine vision applications, soft aperture filters the normal (sharp) video signal to diffuse sharp edges
or features that may cause unwanted harmonics during image processing. See also Iris Diaphragm.
AR (Anti-Reflection) Coating
A coating applied to optics designed to minimize back reflection and maximize throughput. There are various AR coatings
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designed for different wavelengths and wavelength ranges.
Argon-Ion Laser
Gas laser that uses argon ions as the amplification (lasing) medium. Although argon ion lasers are generally larger and less
efficient than HeNe lasers, these lasers can be used to produce a wider variety of wavelengths.
Aspect Ratio
The ratio of width to height in a video device. NTSC and EIA video signals use 4:3 aspect ratio; cinematic and HDTV
systems use 16:9.
Aspheric
In terms of lenses, a surface that is not spherical.
a. Lens aberration that results in the tangential and sagittal image surfaces being of different curvatures and in
different planes.
b. Although light propagation for many laser systems can be described as radially symmetric about the beam axis,
laser diodes often have beams that must be described in terms of two orthogonal planes. For example, a HeNe laser
beam can be accurately described in terms of a single divergence specification about the propagation axis, while many
diodes require two divergence specifications for orthogonal axes within the beam (leading to an elliptical beam spot).
Astigmatic lenses can be used to correct the beam characteristics by introducing an equal and opposite amount of
astigmatic effect. See Anamorphic Lens.
Attenuation
Loss of average optical power, usually given as dB per unit distance.
Autocollimator
A single instrument combining the functions of a telescope and a collimator to detect small angular displacement of an
optical flat by means of its own generated collimated light. See also Collimation, Collimated Light.
Automatic Gain Control (AGC)
Used in cameras to maintain a constant video signal output when light levels become very low, especially below minimum
sensitivity. Applying gain to a signal lowers the signal-to-noise ratio by intensifying noise as well as the signal.
Average Extinction
When two polarizers are placed in front of a light source with transmission axes at 90 degrees to each other, the theoretical
transmission should reach zero percent of the input. Because real polarizers will not reach this limit, average extinction is
cited as a measure of how closely polarizers approach the theoretical limit.
Axial Illumination
Light that strikes an object along the optical axis, generally accomplished by using a beamsplitter.
Axial Runout
For rotary stages, this is the amount of vertical motion of the stage as it rotates, measured with respect to the center of
rotation.
Back Focal Length (BFL)
The distance between the last surface of a lens to its image plane. See also Focal Length.
Backlash
The amount of play (lost motion) between a set of moveable parts when changing the direction of travel.
Back Light Compensation
In electronic cameras, an automatic function that gives higher photometric weight to objects in the center of the field of
view than those in the periphery. Increases local contrast to help avoid silhouetting.
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Ball Bearing Slides
Ball slides are mechanically simple linear motion devices comprised of a stationary base member with a mobile carriage
riding on top. Two rows of hardened steel balls on both sides of the base provide the smooth, accurate, low friction sliding
motion between the stationary base and the top slide. Each row of bearings is contained between four hardened precision
steel rods. These bearing assemblies are factory pre-loaded to eliminate wobble and play in the system. Ball slides are
functionally more reliable than simple dovetail slides, since there is no direct sliding contact between the top and bottom
members. Ball slides eliminate rapid wear problems, regular lubrication requirements, and “stiction” (skipping and jumping
caused by the increased force needed to initiate movement) characteristic of dovetail slides.
Ball Bearing Stages
Linear positioning stages that provide controlled, precise point-to-point positioning along a linear axis. Stages are
comprised of two basic components: a precision linear ball slide which serves as a linear bearing and guide, and a drive
mechanism which accurately moves and positions the slide top along the linear axis. These linear positioners operate in a
simple manner: a bracket which supports the drive screw is attached to the slide base. The end of the drive screw rests
against the end of the moveable top. There are two extended springs “pulling” the slide top toward the screw so that the top
will always be held firmly against the screw end. When the screw is turned clockwise, it advances and pushes the slide top
along the linear axis; when turned counter clockwise, the screw retracts and the slide top follows due to the spring pressure.
The result is a very smooth linear motion, accurately controlled by rotation of the drive mechanism.
BBAR Coating
Short for Broadband Anti-Reflection Coating. See AR Coating.
Beam Divergence
See Divergence.
Beam Expander
A component commonly used with lasers to expand the beam diameter. One advantage is that expanding a beam will
decrease divergence by the expansion factor. The decreased divergence is a major advantage for long distance applications
despite the initial increase in beam diameter. For example, a 1mm laser beam expanded 10X will be 10mm in diameter and
will have one tenth the divergence of the original beam.
Beamsplitter
An optical device designed to split collimated light into a specific ratio of transmissions. Examples include cube
beamsplitters, plate beamsplitters, and pellicle beamsplitters.
Bend Radius
In an optical fiber, the minimum radius you can bend the fiber before introducing damages to either the core or the cladding.
Bevel
On a lens, a bevel is a slanted edge (chamfer) ground into the side and is used to prevent chipping or sharp edges.
Bezel
Many laser modules have an optional mounting bezel to allow the laser to accept 1”-32 TPI components such as line
generators or beam expanders. Mounting Bezels can also adapt the laser for use with C-Mount Integrated Mounting
Components.
BFL
See Back Focal Length.
Blaze Angle
In diffraction gratings, the angle between the longer leg of the sawtooth profile and the plane of the grating surface (as if it
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were polished flat). The efficiency of a grating into a desired order can be maximized by altering the blaze angle. The
wavelength of peak efficiency is generally referred to as the blaze wavelength.
Blaze Wavelength
The wavelength of peak grating efficiency based upon the blaze angle. The maximum efficiency occurs when the incident
angle from the blaze normal and diffracted rays from the blaze normal are equal (Littrow configuration). Using the grating
equation, one can easily determine the appropriate blaze angle for a specific peak efficiency (blaze wavelength) or
determine the blaze wavelength from the blaze angle.
Blocking
This is amount of light that is rejected-not passed-by a filter outside the passband. Expressed as the ratio of total energy
transmitted outside the passband to the total energy transmitted within the passband. Blocking values are given over a
specified wavelength range.
BNC (Bayonet Niell-Concelman) Connector
See Connectors.
Board-Level Cameras
CCD cameras that are not yet housed or connected to particular terminations. These devices are completely functioning
units.
Bolt Circle (BC)
Term used for metal parts used to define the location of holes. The circular line containing the center of holes about a
common center.
Borofloat™
Borosilicate glass manufactured by Schott Glass Technologies using the float method. BorofloatTM is a highly chemical
resistant glass with low thermal expansion. It replaces the drawn borosilicate glass, Tempax.
C vs. CS-Mount
Threads are identical on these mounts: 1” diameter x 32 threads per inch. C-Mount uses 17.52 mm flange back distance;
CS-Mount uses 12.5 mm. CS-Mount cameras can be converted to C mount by using a 5mm spacer element; however, CSMount lenses cannot be used with C-Mount cameras.
Candela
Unit of luminous intensity. 1 candela is equal to 1 lumen per solid angle.
Cathode Ray Tube
Video display based on vacuum tube with electron gun and annular anodes at one end and a cathode and phosphor screen at
the other. Charged plates or electromagnets deflect the electron beam such that the image is scanned line by line across the
face of the tube. Signal response is nonlinear with input, gamma usually between 2 and 3.
Cavity
a. With interference filters, a cavity is a spacer layer between two stacks of dielectric layers. The number of
cavities determines the overall shape of the transmittance curve. Our interference filters use three cavities, which
results in steep slopes, improved blocking near the passband, and relatively flat tops.
b. With lasers, the structure in which the act of lasing begins.
CCD : See Charge-Coupled Device.
CCIR (Consultive Committee for International Radio standard)
See PAL and SECAM.
CDAR™/TDAR™ (Clear Display Anti-Reflection/Teal-Gray Display Anti-Reflection)
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CDAR™ and TDAR™ is a specific highly transmissive anti-reflection coating used on our High Efficiency Anti-Reflection
Windows and our Conductive Windows respectively. CDAR™ has a transmission of 96 to 99% between 400 to 700nm,
and is clear in nature. TDAR™ has a transmission of approximately 55% between 400 and 700nm and is teal-gray in
appearance so as to improve contrast and minimize glare.
Center Air Spacing
The spacing between the crown and the flint of an achromat when the two are not cemented together. See Achromat Lens.
Central Wavelength (CWL)
Typically (but incorrectly) used to denote the peak transmitting wavelength of a filter. CWL is actually the midpoint
determined by the half bandwidth. For example, interference filters typically have peaks that do not occur at the CWL.
Centration (Relative to Housing)
See Alignment.
Charge-Coupled Device (CCD)
Two-dimensional self-scanning electronic analog imaging device. The rectangular imaging area consists of rows and
columns of rectangular photosensitive pixels that accumulate and store electric charge. Each column is separated by a shift
register, or masked charge storage area, to which pixel charge is transferred. Charge is read out of the device sequentially
from the shift register (masked) as the photosensitive pixels (unmasked) collect charge for the next field. Pixel charge is
then transferred to the shift register, and the cycle continues. (Compare with frame transfer cameras in which charge is
transferred to an entirely separate storage array at one time, eliminating the dead space between active pixels.)
CE Marking
The CE Mark is a European proof of conformity that allows manufacturers and exporters to circulate products freely within
the European Union. The letters “CE”, French for “Conformite Europeenne,” indicate that the manufacturer has satisfied all
assessment procedures specified by law for its product. Products requiring the CE Mark in Europe include, but are not
limited to: high voltage devices, toys, construction products, equipment that generates electromagnetic interference,
personal protection Equipment, and Medical Devices.
Circular Polarizer
A polarizing component that uses a linear polarizer in combination with a wave retarder. The retarder is used to introduce a
wave shift between the orthogonal components of the polarized light. In this way, two equal components of light oscillating
perpendicular to each other with a relative phase difference add as vectors to yield a rotating linear polarization state. If the
amplitude components of the orthogonal states are not equal, the vector addition of the two components will produce an
elliptical polarization. For this reason, the linearly polarized light is input into the retarder at 45 degrees to the fast axis to
ensure that both components are of equal amplitude.
Cladding
A low refractive index material that surrounds the core of an optical fiber. The difference in index between the core and the
cladding contains light travelling through the core, while also serving to protect the fiber against surface contaminant
scattering.
Class (Laser Class)
Classifications issued by the United States Center for Devices and Radiological Health (CDRH) categorize lasers based on
laser output power, wavelength, and other safety considerations. For more information, see the Laser Institute of America
website http://www.laserinstitute.org/
Clear Aperture
The useable portion of a lens (or other component), also called free aperture. Specified by diameter, clear aperture limits the
height of the ray bundles that can enter the component.
Clearance (CLR)
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Abbreviation for a clearance hole. This is a term used in component mounting to indicate that a hole is not threaded and is
oversized to receive another part (typically a screw).
Coherent (or Aligned)
a. With respect to fiber optic bundles, coherence means that fiber coordinates at the entrance and exit faces are
identical; a necessary condition for imaging as opposed to simply directing illumination.
b. (Laser reference) Since the fundamental laser process of stimulated emission produces additional photons with
the same phase as the originator, the amplifying laser medium is a source of coherent light. Noise within the laser
output is the result of spontaneous emissions that do not have the same characteristics as the stimulated emission.
During oscillation, the spontaneous emission component will also be amplified and appear in the output as random
noise.
Cold Mirror
Has a dielectric coating that is designed to reflect the visible region of the spectrum and transmit the infrared. Uses include
specimen illumination, microscopy and any application where heat needs to be removed from the reflected beam.
Collimate/Collimation
a. Process in which the optical axis of the system is aligned to the mechanical axes or surfaces
of an instrument.
b. Converting a diverging or converging beam of light into a parallel beam of light.
Collimated Light
Light in which every ray is parallel to every other ray of light. Note that light is never totally collimated due to aberrations
present in collimators and due to the finite size of light sources.
Color CCD
See Monochrome vs. Color.
Color Temperature
Colorimetric parameter associated with light sources (compared with blackbodies). Related to apparent visual color; given
in degrees Kelvin.
Coma
An off-axis lens aberration that yields a variation in magnification (and therefore image size) with aperture. In other words,
when a lens containing coma images an off-axis light bundle, each zone of the lens is focused on the image plane at a
different height and with a different spot size.
Commercial Grade
As specified by Edmund Industrial Optics, commercial grade components are a standard design with tolerances of ±2%.
Less technical information is available about them. Although generally available in large volumes for production, they are
not necessarily available on an ongoing basis.
Composite vs. Component Signal
Color video signals are composed of luminance and chroma information. Composite signals carry both luminence and
chroma on one line, whereas Y-C and RGB signals carry luminance information on one line and chroma information
separately on one or more lines. Breaking up the signal components generally improves signal fidelity, especially when
recording or balancing color. Each type of composite/component signal is only compatible with like devices.
Concentricity
For rotary stages, the maximum variance between a perfect circle and the path that the stage follows.
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Conjugate Distance
A conjugate distance is defined as the distance along the optical axis of a lens from the principle plane of that lens to either
the image plane (for an image conjugate) or the object plane (for an object conjugate). Lenses designed to focus at infinity
have an infinite conjugate distance for the object and a finite conjugate distance for the image.
Condenser
A positive lens or set of lenses designed to collect light and project it evenly across an area. Condenser lenses are
commonly found in projector systems.
Connectors
Not to be confused with the video signal format itself, connectors are fairly standardized: RCA (a bayonet), BNC (Bayonet
Niell-Concelman, named for the inventor), and 4-pin mini-DIN (Deutsche Industrie Norm). RCA and BNC types are
simple coaxial (line and ground) terminations; 4 pin offers two line connections and their respective grounds.
Consumer CCD Camera
See Industrial vs. Consumer CCD Camera.
Contrast
Comparison of shades of gray that define an object and its background. See also MTF.
Corner Cube Retroreflector
See Trihedral Prism.
Counterbore (C’bore)
A cylindrically enlarged end of a hole. Typically used to accept the head of a socket head cap screw, making the screw head
accessible but not obstructive.
Crown Glass
One of two major types of optical glass; crown glass is harder than flint glass and has a lower index of refraction and lower
dispersion.
CS-Mount
See C vs. CS-Mount.
CT
Abbreviation for the center thickness of a lens.
Cube Beam Splitter
Constructed of two cemented right angle prisms, one with a broadband multi-layer dielectric coating on the hypotenuse.
50% of incident light is transmitted, and 50% is reflected 90°. Outside surfaces have an anti-reflection coating to reduce
back reflections. No beam displacement occurs between the original and separated beams. The reflected and transmitted
beams travel through the same amount of glass, so although the optical path length of each arm is increased, both paths are
increased by the same amount. Their cubic shape makes these beamsplitters easy to mount, thus suffering less from
deformation due to mechanical stress. They are however polarization sensitive with s-pol and p-pol components differing
by approximately 70%. The effects of polarization should be carefully considered prior to their selection. We do not
currently carry non-polarizing cubes, but we do carry a line of strictly polarizing cubes (92% s-pol reflection at 632.8 nm).
Cut Edges
Edges that have been scribed and broken (or cut by other means). Usually have sharp edges.
CW
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In lasers, an abbreviation for Continuous Wave.
CWL
See Central Wavelength.
Cylinder Lens
Providing magnification in only one plane, a cylinder lens transforms a point of light into a line image. Useful for focusing
light onto a slit, and in scanning applications.
Dark Current
Thermal current produced in an operating photodetector device when no optical radiation impinges on the detector.
Dark Field Illumination
A cone of light that comes to focus in front of an optical system and then diverges from the optical axis. Unless scattered or
reflected by an object, the field would remain dark; however, a transparent object will exhibit defects, stress, and
contaminants against a dark field.
DCV
See Double-Concave Lens.
DCX
See Double-Convex Lens.
Decibel (dB)
Logarithmic measure of relative power levels.
Delrin™
Plastic material used as a soft-contact surface for optical components and is typically found in optical mounts.
Depth of Field (DOF)
The difference between the closest and farthest distances an object may be shifted before an unacceptable blur is observed
at a particular resolution.
Dichroic Coating
A filter or mirror coating that transmits or reflects light depending on wavelength rather than upon polarization. The colors
of these materials vary with angle of incidence and thickness of material. Metallic coatings tend to be more spectrally flat.
For example, compare our Mirror-Type Beamsplitter coatings for 30R/70T (metallic coating) and 70R/30T (dichroic
coating).
Dielectric Coating
Coatings consisting of materials that are electrical insulators. The reflective coatings consist of alternating layers of higher
and lower index materials (compared to the substrate) in order to achieve a certain reflectivity over a certain wavelength
region. For example, our Enhanced Aluminum mirror coatings are described as multi-layer dielectrics.
Diffraction
When light is obstructed by an object and the wavefront is changed, interference occurs between components of the altered
wavefront. The pattern formed by interference is called the diffraction pattern. Many components are designed to yield very
specific diffraction effects (diffractive optics, gratings). Other components attempt to counteract this process in order to
determine more information about the obstructing medium (electronic imaging).
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Diffraction Limited
A limit in an optical system where the effects of diffraction (see above) are the only factors in the quality of the image it
produces.
Diffractive Pattern Generator
An optical element that utilizes the principles of diffraction in a manner very similar to holography to create patterns and
manipulate beam characteristics. Since they are designed around diffraction theory, diffractive pattern generators are meant
to be used at specific wavelengths and input profiles (and thus are considered laser components).
Diffusion
Scattering light over a large solid angle. Light is diffused by reflecting from or transmitting through an irregular (rough)
surface. For example, a ground or frosted surface acts as a good diffuser, as well as a filter or screen. There are different
types of diffusers, differing mainly by how they scatter light and how evenly (or by what angle) they diffuse light.
Digital Micrometer Drive
A ball stage drive mechanism option. The 1.0 inch travel Starrett micrometer provides an LCD readout to 0.00005 inch
resolution and features incremental and/or absolute positioning modes and automatic shutdown to conserve integral battery.
The battery will power the unit for 500 hours of use.
Digital Signal
See Analog vs. Digital Signal.
Digital Signal Processing (DSP)
Found often in high-end industrial CCD cameras, digital signal processing involves analog-to-digital conversion of all or
part of a standard video signal to enhance/change the resulting signal recovered upon digital-to-analog conversion.
Enhancements usually pertain to RGB balancing for more accurate color reproduction.
DIN (Deutsche Industrie Norm)
a. for Video, see Connectors.
b. for Microscopes, an industry standard for the design of microscope objectives and eyepieces.
DIN standard microscopes have a 160mm tube length.
Diopter
Unit of optical measurement of a lens that equals the inverse of the focal length of the lens in meters.
Dispersion
The separation of radiation into its spectral components due to different wavelengths of light traveling at different speeds
through a transmissive medium (different indices of refraction). See Equilateral Prisms.
Distortion
a. Nonlinear geometrical aberration in which magnification changes with field height (i.e. no distortion at center), defined
at maximum field. If imaging a square grid, positive distortion gives a pincushion effect while negative distortion yields a
barrel effect.
b. Term used to denote that an image is not a true reproduction of the object.
Divergence
A term that describes the degree to which a light source expands as the distance from the laser increases. Divergence is
generally specified as a full angle and can be used to predict spot sizes at a given distance through tangent calculation.
Beam divergence can be reduced for long distance uses by expanding the diameter of the beam.
DOF
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See Depth of Field.
Double-Concave Lens (DCV)
Both surfaces of the lens are curved inward.
Double-Convex Lens (DCX)
Both surfaces of the lens are curved outward.
Double Gauss Lens
These low F-number lenses provide superior overall correction and produce less distortion than a standard fixed focal
length lens. However, they can be subject to residual oblique spherical aberration. Double Gauss lenses are commonly used
in CCD camera applications.
Doubler Tube
Available for some video lenses, a “doubler” is a negative focal length achromatic lens that doubles the magnification
specified for that working distance by giving half the field of view.
Dove Prism
Inverts an image; if the prism is rotated about the beam axis, an image will rotate twice as much. Aberrations are present
when not used with collimated light.
Drive Ratio
Ratio of motor revolutions per lead screw revolution.
DWDM
Dense Wavelength Division Multiplexing is a fiber-optic transmission technique that employs different light wavelengths
to transmit data on the same optical signal. Because each type of data is transmitted on a different wavelength, the receiver
can filter out wavelengths in order to isolate the desired signal.
EFL
Abbreviation for Effective Focal Length. See Focal Length.
EIA (RS-170) vs NTSC (RS-170A)
EIA and NTSC are, respectively, the standard monochrome and color signal formats for the US, Canada, Mexico, most
South American and Caribbean countries, Japan and many Southeast Asian countries. In 1941 the FCC approved the
monochrome signal format (document number RS-170) for the US at the recommendation of the Electronic Industries
Association (EIA); in 1953 the National Television Standards Committee (NTSC) of the EIA color signal format
(document number RS-170 revision A) was approved. The designations RS-170 (EIA) and RS-170A (NTSC) arise from
these documents, and are simply nomenclature.
Electroluminescence (EL) Panels
The non-thermal conversion of electrical energy into light. Electroluminescent (EL) Panels are commonly used for visual
readout displays.
Electronic Shutter
Sometimes referred to as Electronic Light Control, electronic shuttering controls the amount of time a CCD camera collects
light or charge during each field. Standard signal formats employ approximately 60 fields per second, the camera collecting
charge for 1/60 second for each field. As light levels increase the sensor overfills with charge, resulting in blooming or
streaking. Electronic shuttering circuits prevent these effects by allowing the sensor to collect charge for a shorter period
during each field. Steps in shuttering generally halve the previous collection time; for example, from 1/60 second to 1/125,
then 1/250, 1/500,1/1000, etc. (These steps correspond to unit f-stop increments, f/1, f/2, f/3...)
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Encoder
Electromechanical device that creates discrete electrical pulses directly related to the angular position of the input device
(shaft). This allows for high resolution feedback data on position.
Equilateral (Dispersing) Prism
Used to separate white light into its spectral components.
Erect Image
An image that is oriented top-to-bottom, the same as the object. It is not necessarily left to right correct (i.e. it may be
reverted).
Erecting Prism (or System)
Prism (or system) that rotates an image 180° about the axis of the system.
ESD
Both people and devices frequently develop electrical charge through the action of static electricity. When contact with
another item is made, the built up charge is often “discharged” into this item. This discharge is known as electrostatic
discharge (ESD). While ESD voltages frequently exceed 3,000 volts, the current associated with ESD is low making it safe,
though at times unpleasant, for humans.
ET
Abbreviation for the Edge Thickness of a lens.
Extinction Ratio
A common measure of the polarizing characteristics of a polarizer.
Extremely Low Frequency (ELF)
For those concerned with reducing their exposure to the electromagnetic fields associated with electronic displays,
conductive coatings provide shielding from ELF/VLF (electric field) radiation to help comply with TCO and MPRIII
standards. We currently carry a matrix of Anti-Reflection/Conductive Windows.
f-number (f/#)
Also referred to as f-stop, f/# is the ratio of the focal length of an optical system to its clear aperture. For example, a 50mm
focal length lens system with a clear aperture of 25mm is f/2. Lower f/# optics generally have high throughput compared
with those of high f/# optics. With respect to optical fibers, f-number is related to the difference in index of refraction
between the core and cladding materials as governed by numerical aperture. See also Numerical Aperture.
Fan Angle
Fan angle is a term used to describe the divergence angle of a laser line generator by the dimension of the line length. This
angle defines how long the line will be at any given distance. Large fan angles lead to longer lines over shorter distances.
Since the visibility of the laser line is directly linked to the density of light reaching the target, low power lasers should not
be used to generate long lines.
Field vs. Frame
CCD cameras and other interlaced electronic imaging devices produce “live” video by scanning odd-numbered lines in the
first pass, then even-numbered lines in the second pass. Each video frame is, therefore, made up of two fields.
Field Curvature
A lens aberration that results in a flat object being imaged onto a curved surface.
Field of View (FOV)
The basic parameter in a video system, field of view defines the rectangular area of the viewed object that is displayed on
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the monitor. FOV depends on the angular magnification of the optics in front of the camera, and on the linear
magnification, or ratio of monitor size to CCD format size. Magnification by itself is not a basic system parameter.
Fine Screw Drive
A ball stage drive mechanism option. The 64 pitch screw is used for fine resolution positioning, but the head is not labeled
for position readout.
First Surface Mirror
A mirror in which the front surface is coated. Light does not pass through any glass before it is reflected, but the coating is
not protected and is more prone to damage.
Flange Back Distance
Distance from the front of the camera mount thread, or flange, to the CCD image plane.
Flint Glass
Optical glass that is softer than crown glass (the other major type of optical glass). The flint has a higher dispersion and
typically a higher index of refraction.
Float Glass
Glass manufactured by the float process, which involves floating glass on liquid tin as the glass cools.
Focal Length (FL)
Regarding optical elements and systems: effective focal length (EFL) - Distance from the principle plane to the focal point;
front focal length (FFL) - Distance from the vertex of the first lens to the front (left) focal point; back focal length (BFL) Distance from the vertex of the last lens to the back (right) focal point.
Focusing Range
(Laser reference) Due to the optical configuration within some diode modules, the modules have a minimum focus distance
from the front of the laser. Although a laser can be used at distances shorter than the minimum focal distance, the spot size
will be larger than the focused spot at the minimum focus distance. The maximum focusing distance is commonly defined
as infinity or beyond collimation. It is important to note that the focused spot size will increase with distance from the
module. At long distances, the optimum focus condition is a collimated beam with minimum divergence (spreading of the
light).
Footcandle
Unit of illuminance. See Lux vs. Footcandle.
FOV : See Field of View.
Frame : See Field vs. Frame.
Frame Grabber : See Image Capture Board.
Free Aperture : See Clear Aperture.
Fresnel lens
Manufactured using a molding process that results in annular rings (or grooves) in thin plastic/acrylic. Each groove refracts
the light as if it were a part of a whole conventional lens. The result is a lighter lens that can have a lower f/# than a
conventional glass lens of the same focal length. The original design for this type of lens was made by A.J. Fresnel for use
in lighthouses.
Front Surface Mirror
See First Surface Mirror.
Full Width at Half Maximum (FWHM)
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See Half Bandwidth.
Gamma
The exponent of the function relating output signal to input signal in video cameras and monitors. Industrial CCD cameras
naturally have a gamma of 1, that is, they are linear in response. CRT displays, however, have a nonlinear response
corresponding to gamma values between 2 and 3. When used together the camera may incorporate gamma correction to
linearize the resulting system.
Gear Ratio
Ratio of the number of teeth on mating gears (larger gear : smaller gear). For example, a ratio of 16:1 means that the
smaller gear (or pinion) makes sixteen revolutions for every one revolution of the larger mating gear.
Gimbal
For mounting components (i.e., gimbal mount), refers to a type of mount defined by its range of motion. The mount allows
for adjustable movement about two perpendicular and intersecting axes (i.e., pitch and roll).
Glass Code
Reference code for a specific glass type that is represented by three digits, then a slash, then another three digits. The first
three digits refer to the three numbers after the decimal in the glass’ index of refraction. The second three digits refer to the
first three significant digits in the glass’ Abbe number. For instance, BK7, which has an index of refraction of 1.517 and an
Abbe number of 64.2, has a glass code of 517/642.
Grating Equation
The relationship between diffracted order, wavelength, groove spacing, and angles of incidence and diffraction. Used to
determine the diffracted angle for a given incident beam.
Groove Efficiency
See Relative Efficiency.
Grooves/mm
A term defining the groove density for a diffraction grating. Since this groove density can be used to determine the groove
spacing, the grating equation can be used to obtain information about diffracted orders and incident/diffracted angles.
Ground Edges
The perimeter of a component is ground to remove all sharp edges and corners. Ground edges are used when tighter
tolerances are required.
Ground Glass
Clear glass that is finely ground on one surface. Ground and opal glass scatter light in a large angle about the direction of
the incoming light. Ground glass has an on-axis transmission of approximately 20 - 25%, but decreases rapidly as the light
is angularly diffused. It is typically used in condenser set-ups to produce uniform illumination and is ideal for use as an
imaging screen (i.e., rear projection screen). See also Opal Glass.
Half Bandwidth
A specific wavelength region of the passband of a filter. Half bandwidth is defined by the two points of the passband where
transmittance is 50% of the peak. Often referred to as the full width half maximum (FWHM).
Half Wave Optical Thickness (HWOT)
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Refers to a layer of optical coating that is equal in optical thickness to one half the design wavelength. No change in
reflectance/transmittance results from layers of this thickness.
HDTV
See High Definition Television.
Heat Absorbing Glass
A type of glass filter that absorbs, rather than reflects, energy associated with heat. Our heat absorbing filters absorb
approximately 60% of the light from 1700 to 2500 nm.
HBW
See Half Bandwidth.
Helical Gear
A gear with angled teeth. Far superior to straight rack and pinion. Big advantage is smoothness of operation, since the mesh
between gear and rack is much tighter. Our helical gears have a one piece (shaft and gear) steel construction.
Helium Neon (HeNe) Laser
One of the most common types of lasers, the HeNe laser uses a combination of helium and neon gas as its laser medium.
Although the most common output wavelength is 632.8nm, HeNe lasers are available in several different wavelengths.
High Definition Television (HDTV)
An updated video format that provides approximately twice the horizontal and vertical resolution of NTSC, PAL or
SECAM; and uses a 16:9 aspect ratio.
Holographic Grating
A sinusoidal “blaze” pattern is generated on the surface of the diffraction grating by exposing the grating (covered with
photoresist) to an interference pattern. The sinusoidal variations in the interference pattern lead to the groove profile for the
holographic element.
Hot Mirror
Essentially a dielectric coating that reflects infrared radiation and transmits the visible region. Uses include projection
systems, illumination systems and any application where heat and near-infrared light needs to be removed from the
transmitted beam.
Huygenian Eyepiece
An eyepiece having two plano-convex lenses of the same glass separated by half the sum of their focal lengths. This
eyepiece is free of lateral chromatic aberration, but is not suitable for use with reticles.
HWOT
See Half Wave Optical Thickness.
Image Capture Board (Frame Grabber)
Computer card that samples and digitizes analog video signals so that the information may be processed, stored, or operated
on by the computer.
Index of Refraction (n)
The ratio of the speed of light in a vacuum © to the speed of light in a refractive material (v). The index of refraction of a
given medium varies with wavelength because the speed of light varies with wavelength. Common indices of refraction
include nd (the yellow helium line at 587.6nm), nf (the blue hydrogen line at 486.1nm) and nc (the red hydrogen line at
656.3nm). See also Abbe Number.
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Industrial vs. Consumer CCD Camera
Although based on the same technology, industrial CCD cameras generally offer higher resolution, higher signal-to-noise
ratio, greater sensitivity, and a standard lens mount (C or CS-Mount) for changing optics where appropriate.
Infrared (IR) Spectrum
The wavelength interval of the electromagnetic spectrum that corresponds to infrared light (light that is beyond the visible
red range). The range commonly used is 750 nm to 1,000 micron and can be divided into Near-Infrared (750 to 3000 nm),
Mid-Infrared (3 to 30 microns), and Far-Infrared (30 to 1000 microns).
Integration
Increase in charge accumulation time in imaging applications that involve light levels lower than CCD camera minimum
sensitivity (e.g. fluorescence applications). Limitations include vibration and dark current level.
Interference Filter
Filters made up of several layers of materials consisting of metals and dielectrics. The correct combination yields high
spectral transmittance over very narrow wavelength ranges due to interference effects between layers. Our filters are all 3cavity designs.
Interlaced vs. Progressive Scan
Conventional CCDs use interlaced scanning across the chip. The chip is divided into two fields: the odd field (rows 1, 3,
5..., etc.) and the even field (rows 2, 4, 6..., etc.). These fields are then integrated to produce a full frame. For example, with
a frame rate of 30 frames per second, each field takes 1/60 of a second to read. For most applications, interlaced scanning
will not cause a problem. Some trouble can develop in high-speed applications because by the time the second field is
scanned, the object has already moved. This causes ghosting or blurring effects in the resulting image. The progressive scan
CCD solves this problem by scanning the lines sequentially (rows 1, 2, 3, 4..., etc.). The progressive scan output has not
been standardized so care should be taken when choosing hardware. Some progressive scan cameras offer an analog output
signal, but few monitors are able to display the image. For this reason, capture boards are recommended to digitize the
image for display.
Interline Transfer
Also referred to as 2:1 interlaced video, interline transfer devices combine two fields to make up each video. If each field is
scanned at 60Hz, the frame rate is necessarily 30Hz. (There are other interlacing schemes, but they are not used in standard
industrial cameras.)
Inverted Image
An image that is rotated 180° about the system axis (upside down, but left-right correct).
Inverted Load
Same as normal load, except in an upward (tension) direction. Values provided by ball slides.
Iris Diaphragm
A mechanical aperture (opening) designed to smoothly change the aperture size by use of a lever arm. For lenses, the iris
changes the effective diameter and thus controls the amount of light through the lens.
Jitter
Interline transfer CCD cameras employ columns of pixels separated by columns of storage elements. Signals impinging on
the CCD sensor may fall on a pixel or on the space between pixels. Statistically speaking, the latter case may cause a
response in the pixel to the immediate left or the immediate right of the space. Due to the standard CCD scan frequency of
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60Hz, such a static image will exhibit jitter between the two pixels in question over time. Jitter, therefore limits the
accuracy of video measurement to at least two pixel separations in monochrome cameras.
Kellner Eyepiece
An eyepiece consisting of a plano-convex field lens and a cemented doublet as the eyelens.
Kinematic Mount
Optical mount capable of six degrees of freedom that are singularly constrained during alignment. Designed to minimize
stress to the optics or optical assemblies they hold.
Laser Type
Three major types of lasers are gas, semiconductor, and diode pumped. HeNe and Argon Ion lasers are examples of lasers
that apply electric excitation energy to gaseous lasing media. Semiconductor lasers take advantage of the energy gap at the
junction between two types of materials that corresponds to a specific wavelength of output. Diode pumped lasers use
semiconductor lasers to pump a glass/crystal lasing medium. The optical signal pump takes the place of the applied electric
field used in the other laser types.
LDM : Abbreviation for Laser Diode Module.
Leadscrew : Device that converts rotary motion to linear motion.
Light
Radiation generally considered “visible,” with wavelengths between 400 and 700nm. Optical components and applications
often encompass radiation just below 400nm (ultraviolet) and just above 700nm (infrared).
Linear Polarizer
A type of polarizer used to limit the oscillations of electromagnetic waves to a specific plane by absorbing components
oscillating in planes other than the specified polarization transmission axis.
Line Width
For focusable lasers, the laser can be focused prior to addition of the line generator. The width of the line created by a line
generator is equivalent to the beam spot diameter at the target without the line generating optic. The length of the line is
defined by the fan angle.
Littrow Dispersion Prism
The uncoated littrow prism will disperse white light into its spectrum; the coated littrow prism will divert the beam at a 60
angle without inverting or reverting the image. The prism is designed so that the hypotenuse (in the coated version) is at
Brewster’s angle for 514.5 nm light, resulting in retroreflection at that wavelength. Rotating the prism changes the
retroreflected wavelength and makes this prism suitable for line selection in a laser.
Load Capacity :
See Normal Load Capacity, Inverted Load, Thrust Capacity.
Longitudal Mode Spacing
The spacing between multiple frequencies of oscillation within a laser that results from a combination of the gain curve for
a laser and the resonance characteristics of its cavity. Since the mode spacing is defined as the speed of light divided by
twice the cavity length, short cavities can be used in applications that require a minimal frequency spread. A short cavity
can extend the mode spacing such that only one frequency experiences gain during oscillation.
Longpass :
A filter in which the passband is a high wavelength region (higher than the region blocked); for example, a filter that blocks
the visible spectrum (400-700 nm) and passes the Near-IR (700-1200 nm). Filters are commonly defined by the 50%
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transmittance point.
Louver Angle :
For 3M Light Control Film, the angle orientation of the closely spaced micro-louvers. The louvers simulate a tiny venetian
blind that blocks out unwanted ambient light and controls the direction of display light. Unlike blinds, the film is a solid
material.
Lux vs. Footcandle
Units of measure of luminous flux incident per unit area. One lux (lx) equals one lumen per square meter; one footcandle
(fc) equals one lumen per square foot. (1 fc = 10.764 lx).
Machine Vision
Specifically refers to computer interpretation of an image obtained by a video camera and associated optics for the purpose
of automatic machine control. Machine vision is often loosely interpreted as any video-based inspection or documentation
application.
Magnification
Optical magnification is a ratio of image angular subtense to object angular subtense. Electronic, or linear, magnification is
the ratio of monitor size to CCD sensor size. (For example, 13” monitors are between 38 and 39 times as large as ½” size
format CCD cameras when comparing horizontal, vertical, or diagonal measures.)
Maximum Drift
A specification usually relating to output power/wavelength or beam alignment of a laser. Since many characteristics of a
laser will change with time, the drift specifications will generally define allowable limits for these changes during a specific
time period.
Maximum Output Power
Laser diode modules are specified in terms of a maximum output power. Note this distinction when comparing these lasers
to gas modules that are specified in terms of a minimum output.
Maximum Working Distance
A term referring to the maximum distance at which a laser line or cross line can be viewed with comfortable visibility. The
major factors in determining the maximum working distance are the output power, ambient background illumination, target
characteristics, and fan angle of the module. It should be noted that the visibility of the lines created by cross line or line
generating components drops off more quickly for wider fan angles.
Meniscus Lens
Lens with one surface convex and the other concave.
Micrometer Drive
A ball stage drive mechanism option. The micrometer is used whenever a position readout is required. Micrometer
graduations are 0.001” for English travel (0.01mm metric). Course scale increments (one rotation) are 0.025” English
(0.5mm metric).
Micron (µ)
Unit of measurement equal to 10-6 meters. This term has conventionally changed to micrometer (µm).
Mil
Unit of measurement equal to 10-3 inches (0.001”) or 0.0254 mm.
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Minimum Focus Distance
Minimum working distance required by an optical system to produce a focused image.
Minimum Output Power
Gas lasers, especially, will output with at least this minimum value. It is important to notice the distinction of how the
output is specified when comparing lasers since some lasers are specified in terms of a maximum output (e.g. diode lasers).
Minimum Sensitivity
Minimum level of illumination required to produce a full video signal, expressed as luminous flux per unit area (lux or
footcandles) through a specified lens, without AGC. Camera manufacturers use no universal method of testing for this
specification.
Mirror-Type (or Plate) Beamsplitters
Glass plates with metallic or dichroic coating on the front side. These beamsplitters have the advantage of being available
in many sizes and R/T (Reflectance/Transmittance) ratios and are optimized at 45 degrees and 550nm. Some also have a
rear surface anti-reflection coating optimized at 45 degrees. The thin glass substrate means small beam deviation and a very
low absorption of approximately 0.5% at 550 nm for a 50/50 coating. Stress deformation is a mounting concern, as well as
45 degree angular alignment. We currently make available the average reflection/transmission curves, which do not take
into account polarization effects.
Modulation Bandwidth
The characteristic specification of a modulatable laser describing its pulse bandwidth. There is generally a minimum and
maximum bandwidth that can be achieved during operation.
Modulation Input
The input to a modulatable device that leads to the variations in the output. The relationship between modulation input and
output is often linear to preserve relationships between signal fluctuations.
Moment Load
This refers to forces that are offset (cantilevered) from the bearing centers producing uneven loading on the bearings.
Uneven loading means that some bearings are supporting more of the load than others. It is important to determine the
moment loading for a given positioner, within acceptable limits. Moment forces are categorized by the direction in which
they act: pitch, roll or yaw. When loading results in moments acting in only one of the moment directions, it is called a
single direction moment. If it is in more than one direction, it is referred to as compound moment loading.
Monochrome vs. Color CCD
Color CCD cameras start with the same sensor chip as monochrome cameras, but they must be filtered with a stripe or
mosaic color filter to reproduce color information. In the filtering process some spatial resolution is lost because the sensor
essentially is not using every pixel for every color. Color cameras necessarily filter out infrared illumination.
Mounting Bezel
See Bezel.
Modulation Transfer Function (MTF)
A measure of contrast, or visibility, with respect to spatial frequency. As a function of maximum and minimum intensity
visible in the image, MTF continuously declines with increasing spatial frequency to zero contrast at a theoretical
maximum frequency defined by the diffraction limit of the optical system.
Monochromatic
Composed of one color.
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Mounting
Term used for mounting components to indicate that the hole is used to mount that part to another part.
MTF
See Modulation Transfer Function.
MVO™
Machine Vision Optics are an Edmund trademarked family of lens systems designed specifically for use in machine vision
applications. The optical performance of MVO™ lens systems is designed around the use of CCD cameras in order to
improve contrast and resolution. MVO™ optics are also contained in housings which integrate well into most machine
vision applications.
NA
See Numerical Aperture.
Nanometer (nm)
Unit of length equal to 10-9 meters. Used to express wavelength of light.
Near-IR
See Infrared Spectrum.
Near-UV
See Ultraviolet Spectrum.
Neutral Density
Term used to describe a filter with a metallic coating that yields a constant attenuation, or optical density, value over a
range of wavelengths (i.e. spectrally flat). Transmission and absorption percentages are provided in our catalog (%
absorption + % reflection + % transmission = 100).
Noise
Although the dominant factor in laser output is light from stimulated emission, spontaneous emission is a source of noise.
Since the population inversion necessary to achieve laser function can not exist free of spontaneous emission, noise is
unavoidable.
Normal Load
This is the maximum downward (compression) load or force that can be applied to a positioner perpendicular to the
mounting surface. The center of force, or the center of gravity, of the load must be located in the center of the mounting
surface. For loads that are offset from this position refer to Moment Loads.
NM
See Nanometer.
NTSC
See EIA vs. NTSC.
Nth Order
From the grating equation one can see that there are multiple combinations of wavelength and integer order values that will
satisfy any given groove spacing, angle of incidence, and diffracted angle. Gratings are generally specified in terms of first
order diffraction since efficiency decreases with higher orders.
Numerical Aperture (NA)
The sine of the vertex angle of the largest cone of meridional rays that can enter or leave an optical system (such as a lens
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or an optical fiber).
Nyquist Limit
The limiting resolution of the CCD camera generally defined as being one half the sampling frequency. Typically measured
in number of pixels per mm.
OD
See Optical Density.
OEM (Original Equipment Manufacturer)
A manufacturer of product(s), who purchases components from other companies to make their products. Many of our
customers are OEMs.
Opal Glass
Diffuser comprised of glass plate with white coating approximately 1 mm thick. With low on-axis transmission, scattered
percentage is high off-axis and evenly diffused light is produced. It is also often used as a target (i.e. for lasers).
Transmission data as a function of angular diffusion is currently not available for opal and diffused glasses. See also
Ground Glass.
Optical Density
Term used to indicate a specific blocking value of a Neutral Density filter. The optical density (OD) is related to the amount
of energy transmitted through a filter.
Optical Fiber
Waveguide filament of drawn plastic or glass for the efficient propagation of light. Fibers are comprised of a core
surrounded by a relatively thin cladding layer of lower index material. This geometry causes internal reflection of light
inside the fiber, and defines the maximum angles accepted by the fiber. Optical fibers emit into a cone of equal angle, not
as a beam of light. Coherent fiber bundles can relay images from one end to the other.
Optical Flat
Precision flat against which the flatness of another surface may be compared.
Output Power
The power measured at the exit of a laser cavity, commonly expressed as minimum output or maximum output. The class
designation of a laser is based in part upon this characteristic.
Overscan vs. Underscan
Video monitors characteristically scan input signals about 5% larger than the area of the visible CRT screen, yielding an
image 5% smaller than wha the CCD sensor produces. Some monitors have functions for underscanning about 5% so that
the entire image is visible at one time, especially useful for calibrations and system resolution measurements.
P-Polarization
The polarization state that is parallel to the plane of incidence. The plane of incidence is the plane containing the incident,
reflected, and transmitted ray (the plane of the page for most diagrams).
Packing Fraction
Considering a cross-section of an optical fiber bundle, packing fraction is the ratio of the area of the fibers to the space
between the fibers. Bundles comprised of smaller individual fibers have larger packing fractions.
PAL and SECAM
In 1966 the XIth Plenary Assembly of the International Radio Consultative Committee (CCIR) issued recommendations for
color television signal formats, as had been done by the US National Television Systems Committee in 1953. CCIR
recommendations led to adoption of more than one signal format, depending on region. Short for Phase Alternation by
Line, PAL is the color television signal format adopted by the United Kingdom, Ireland, most of Western Europe, Brazil,
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Argentina, China, and several Middle Eastern and African countries. Short for Sequentiel Couleur avec Memoire
(sequential color with memory), SECAM is the color television signal format adopted by France, Russia, and many African
and Eastern European countries.
Paraxial
The small bundle of rays that travel through an optical system on- or nearly on-axis. Paraxial rays are useful for first-order
approximations and for defining basic system parameters.
Passband
A wavelength range used to denote a specific part of the spectrum that passes through a filter. Passbands can be very
narrow (10 nm for our Interference filters) or very large (100+ nm for color dichroic filters).
PCV
See Plano-Concave Lens.
PCX
See Plano-Convex Lens.
Peak Transmittance
The highest transmittance value for light passed by a filter.
Peak Wavelength
The wavelength of light that has the highest transmittance value.
Pellicle Beamsplitter
Constructed of a 2 micron thick nitrocellulose membrane bonded to the lapped edge of an aluminum ring, these
beamsplitters neither add distance to the optical path length, nor create ghost images due to back reflections. Although
pellicle beam splitters are more sensitive to vibrations than glass ones are, they do not add chromatic aberration or induce a
focal shift.
Penta Prism
Useful for shortening the length of an instrument, this five sided prism changes the direction of the beam 90°. Because the
beam is actually reflected twice to achieve the right angle turn, the image from a penta prism is of the same orientation as
the object (i.e. image is not inverted or reversed). Slight movement of the prism does not affect the right angle at which the
rays are reflected.
Periplan Eyepiece
An eyepiece consisting of one achromatic doublet plus one plano-convex lens (plano side facing the doublet) as the eye
lens and a second plano-convex lens (plano side also facing the doublet) as the field lens. Designed for use with Plan and
Semi-Plan objectives. See also Plan Objectives, Semi-Plan Objectives.
Pinion
The smaller of two mating gears. See also Rack.
Pitch
a. Angular rotation about the lateral axis, typically the Y-axis for X-Y-Z.
b. The spacing between consecutive threads on a threaded part. The inverse of TPI.
Pixel
Short for picture element, a pixel is generally a rectangular unit of a scene. With respect to CCD cameras, pixel count is not
a measure of resolution.
Plan Objective
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A planar microscope objective that best corrects for color and spherical aberration. See also Semi-Plan Objective.
Plano-Concave Lens (PCV)
Has one planar (flat) side and one inwardly curved (concave) side.
Plano-Convex lens (PCX)
Has one planar (flat) side and one outwardly curved (convex) side. When using a PCX to collimate or focus, face curved
side towards longer conjugate; i.e., if focusing light the convex side should face the source.
Polarization
A property arising from the wave nature of light. Unpolarized or randomly polarized light does not have a preferred plane
of oscillation. Polarized light consists of waves that are oscillating in a defined and predictable manner. For example, the
oscillations of linearly polarized light are confined to a specific axis perpendicular to the propagation. Through the use of
birefringent materials the linear polarization can be made to rotate along the propagation axis and give circularly or
elliptically polarized light. Although common light sources may not be specified as polarized, many of these sources are
actually slightly polarized due to reflections or other factors. Polarized lasers are equipped with special windows within the
cavity to introduce linear polarization. When choosing a source, it is important to consider the effect that polarization may
have on reflections from coatings and other polarization sensitive processes.
Polarizing Efficiency
Common measure of the polarizing characteristics of a polarizer (100% is ideal).
Porro prism
A 45-90-45 prism which inverts an image.
Positional Repeatability
The precision with which a positioner can reproduce a given movement or move sequence over a number of attempts. For a
repeatability curve, it is the width of the dispersion about the mean value for a large number of positioning trials.
Progressive Scan
See Interlaced vs. Progressive Scan.
Quarter Wave Optical Thickness (HWOT)
Refers to a layer of optical coating that is equal in optical thickness to one quarter the design wavelength. Coating layers at
this thickness result in the maximum destructive interference of reflected light, and consequently maximum transmittance
through the optic.
Quartz Waveplates
See Retarders.
QWOT : See Quarter Wave Optical Thickness.
Rack
A flat, straight bar (track) with teeth to engage (mate) with the gear teeth of a pinion gear, in order to move in a straight-line
orientation. Typically the gear teeth are parallel, however for a helical rack the teeth are angled. Our helical racks are
constructed of brass. See also Pinion.
RCA Connector
See Connectors.
Refraction
The term given for the bending of light rays that occurs at the interface between two materials with different indices of
refraction. A higher index of refraction results in a lower refracted angle.
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Relative Efficiency
Sometimes referred to as the groove efficiency of diffraction gratings, relative efficiency expresses the energy diffracted
into the required order as compared to the efficiency of the coating itself at the same wavelength. For example, a grating
that yields 40% diffraction into a specific order at 500nm compared to a mirror with the same coating that reflects 80% of
light at 500nm would have a relative efficiency of 50%. Theoretically, the relative efficiency for a grating can appear very
high, yet, the actual piece could yield very low diffraction into the desired order if the coating reflectivity is low at the
wavelength region of interest. Efficiency curves on this site are all first order Absolute Efficiency curves.
Relay Lens
A lens or combination of lenses used to transfer an image from one point to another at a specified magnification ratio.
Resolution
a. For positioners, the smallest measurable increment of motion.
b. Considering an image of a ruled test pattern made up of equal width black lines and intervening spaces (or white lines),
resolution is a measure of how closely spaced lines and spaces (also called lines or line pairs) may be before contrast
between white and black appears uniformly gray. Industrial video devices generally have higher horizontal than vertical
resolution. CCD cameras are specified by TV Line resolution, which is related to spatial resolution (lines/mm).
Retarders (Quartz, Film)
A class of materials that exhibit the characteristic of birefringence. Orthogonal components of light that enter a retarder will
experience a relative phase shift upon output. This phase shift is dependent upon the thickness of the retarding medium and
the degree of birefringence. The axes of the material exhibit different index characteristics and are generally referred to as
the fast (low index) and slow (high index) axes.
Reticle
A planar element located at the image plane of an optical system designed to measure the object imaged. The element is
typically made of glass, and can have a wide variety of lines, circles, or patterns either etched or chrome deposited on it.
Edmund Industrial Optics carries a wide variety of reticles for Magnifiers and Microscopes.
Retroreflector
An optical device designed to reflect light back in the direction from which it originated.
Reverted Image
A mirror image - the left side of an image is mapped to the right side and vice versa.
RGB (Red, Green, Blue)
Component level video signal in which red, green, and blue signals are separated from each other and (usually) from
synchronization information.
Right Angle Prism
Produces an image that is erect, but reverted left to right (mirror image) due to the 90° internal reflection from the
hypotenuse.
Ring Light Guide
Bundle of optical fiber that terminates in a circular pattern, canted slightly inward towards the axis of the circle. Provides
intense, near on-axis illumination for shadow-free viewing through the ring.
Rise Time
The time it takes for a signal to rise from a defined minimum to a fraction of the maximum signal. The low and high points
are commonly defined as 10% to 90% of the maximum signal output.
Roll
Angular rotation about the longitudinal axis (plane of translation), typically the X-axis for X-Y-Z.
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RS-170
See EIA vs. NTSC.
Ruled Grating
A diffraction grating with a sawtooth profile that is generated by precision mechanical ruling engines. Replicas are then
generated by depositing a coating, an epoxy layer, and polished substrate onto the master. Except in some special cases,
ruled gratings generally have higher efficiency than their holographic counterparts. Although the gratings are generally cut
from larger pieces and have grooves across the entire face, incident light should be confined to 90% of the full face during
use.
Runout
See Axial Runout.
S-Polarization
The polarization state that is perpendicular to the plane of incidence. The plane of incidence is the plane containing the
incident, reflected, and transmitted ray (the plane of the page for most diagrams).
S-VHS/S-Video
See Y-C.
Sag
The height of a curve measured from the chord.
Seamed edges
Edges that are cut and conditioned to remove sharp corners.
SECAM (Sequential Color and Memory)
See PAL and SECAM.
SHCS
Abbreviation for Socket Head Cap Screw.
Second Surface Mirror
Mirror in which the back, or second, surface is coated. Beam of light makes two passes through the glass - before and after
it is reflected. Coating is protected and therefore not easily damaged. Second surface mirrors tend to create ghost images
due to reflection off the first surface.
Semi-Plan Objective
Microscope objective with a semi-planar design where roughly 80% of the objective’s field of view appears flat. See also
Plan Objective.
Sensor Size
CCD imagers were originally designed to replace vidicon-type technology, and were categorized by the tube size they
replaced. Field of view in a video system can be changed by using a camera with a different sensor size.
Shortpass
Term used to describe a filter with a specific passband. In this case, the passband is a low wavelength range (lower than the
region blocked). For example, a IR cut-off filter passes the visible (400-700 nm) and blocks the Near-IR (700-1200 nm).
Filters are commonly defined by the 50% transmittance point.
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Signal Format
Signal formats standardize transmission and reception of television signals for a given geographic area. Standard industrial
CCD cameras and consumer video recorders, therefore, are designed to reproduce what is considered “live” video using a
particular television signal format. Signal parameters include synchronization, number of scan lines, bandwidth, black and
blanking levels, frame rate, aspect ratio, etc. For instance, US signals (EIA/NTSC) use 525 scan lines with a field rate of 60
Hz; Western European (PAL/CCIR) signals use 625 lines at 50 Hz. Video components of one signal format are not
compatible with those of another.
Signal-to-Noise Ratio (SNR)
Comparison of signal power to noise power, including dark current and other unwanted / unpredictable fluctuations. SNR is
expressed in decibel values for analog systems, and “bit” values for digital systems. Industrial CCD cameras have no less
than 46dB SNR, yielding 256 steps of contrast in the image. This corresponds to an 8 bit digital signal (28 = 256). Every
6dB of an analog signal converts to 1 bit when digitized.
SNR : See Signal-to-Noise Ratio.
Spatial Filter
An assembly that is used in conjunction with laser applications to eliminate spatial noise. A pinhole is used to select only
the central peak of the intensity pattern from a focused spot. The size of the pinhole is selected based upon the wavelength,
objective focal length, and input beam diameter.
Spatial Resolution : See Resolution.
Spherical Aberration
The only lens aberration that appears on axis. Results in light rays from the outer portion of the lens focusing either in front
of (undercorrected) or behind (overcorrected) the focus point of the rays from the center portion of the lens.
Sphericity
Tolerance relating to how spherical a lens is.
Stage Configurations
Stages are available in single or multi-axis configurations (XY, XZ, and XYZ).
Stopband
A wavelength interval used to denote a spectral region of energy that is not transmitted by a filter.
Straight Line Accuracy
For ball stages, denotes error in the horizontal plane (left and right movement in direction of travel); flatness denotes error
in the vertical plane (up and down movement in direction of travel). Both straight line and flatness accuracies are measured
at the moving carriage surface center.
Strehl Ratio
The ratio of the illuminance at the peak of an aberrated diffraction pattern to that at the peak of an aberration-free system.
Surface Accuracy
Given in terms of waves (a multiple or part of a reference wavelength), surface accuracy is an indication of the peak-tovalley flatness of a surface. For example, a specification of ¼ wave means that the flatness of the surface deviates by one
quarter of the wavelength used to test the surface. If the wavelength was 550nm, the flatness would be held to within
0.14•m in order to produce a surface accuracy of ¼ wave.
Surface Quality
Surface quality is a specification of allowable flaws in the surface on an optic. Given as a hyphenated number (ex. 60-40),
the first number is referred to as the scratch number and quantifies defects of a long nature, such as scratches, while the
second number indicates round defects such as digs and is referred to the dig number. Scratch-Dig numbers for surface
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quality conform to the US government standards MIL-O-13830.
T-Mount
Metric photo mount; 42mm diameter x 0.75mm pitch. Used in our T-Mount Integrated Mounting Components.
Tangent Arm Drive
For some rotary stages, this is the drive mechanism. Angular rotation is controlled by three control knobs. The release knob
disengages the shaft from the drive, freeing the table to be rotated by hand to a desired location. The release knob is then
tightened to re-engage the drive mechanism and transfer control to the adjustment knob which, when rotated, produces
precise angular positioning of the shaft and table top. The locking knob can then be used to positively lock the table at the
desired setting.
TDAR™
See CDAR™/TDAR™.
Tech Spec™
Edmund Industrial Optics’ line of lenses, prisms, mirrors and other optical components readily available in an expanding,
systematic, stepped matrix. We guarantee long term availability of our Tech Spec™ components and full technical data is
available.
Telecentric Lens
Lens in which the exit pupil is at infinity. Applied, this means that within a certain range of working distances there is no
viewing angle error or magnification error associated with features in the image that are at different heights or angles.
Telecentricity
See Telecentric Lens.
Tempered
Glass heat treated after fabrication in order to make it more durable and resistant to heat and thermal shock. Since heat
absorbing filters are placed in front of intense light sources, they are typically tempered to increase their resistance to
breaking. According to Schott Glass Technologies Inc., if the glass is subjected to temperatures in excess of Tg = 300K, or
Tg = 250K for short periods then the tempering will be reduced.
Threads
Threads are an integral part of mounting components. English notation is thread diameter (inches) - threads per inch (e.g. ¼20). Metric notation is thread diameter (mm) x thread pitch (mm) (e.g. 42 x 0.75mm).
Thru-Hole
Common term indicating that a hole (often threaded) goes through a part and does not bottom out. If it does “bottom out,”
the hole depth is given.
Thrust Capacity
For ball stages, this is the maximum force or load that can be applied in the direction of travel without damage to
positioning stage components. For ball stages, the mounting surface (stage carriage) is pressed against the drive mechanism
by means of a spring. Because of this the maximum thrust which the stage assembly can maintain is different when
pressing towards the spring versus away from it. When pressing towards the spring, the force is taken up by the drive
mechanism (i.e. micrometer). While pulling away, the force is being held in place by the spring. Stages with this type of
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mechanism (micrometer or fine screw) have two thrust capacities (Ta and Tb). Ta refers to the load capacity against the
micrometer and Tb is the spring load capacity.
TPI
Abbreviation for Threads Per Inch. Equivalent to the reciprocal of the thread pitch. See also Threads.
Transformation Temperature (Tg)
Although specified for our longpass filters, all optical glasses have this value. As defined by Schott Glass Technologies
Inc., the transformation range of a glass is the boundary region between brittle behavior and viscous-liquid behavior,
characterized by the transformation temperature, Tg (°C). The operating temperature of a filter may not exceed the
transformation temperature without permanent irreversible changes in glass or filtering characteristics.
Transistor-Transistor Logic (TTL)
Refers to a common digital logic family of devices used for control in most modern electronics. TTL signals have two
distinct states; “low” and “high.” A logic “low” corresponds to a maximum voltage threshold of 0.8 volts; a “high”
corresponds to a voltage greater than 2.5 volts (typically used voltage levels are 5-12V DC).
Travel
For positioners, the total movement of the stage from hard stop to hard stop.
Trihedral Prism (Corner Cube Retroreflector)
Three total internal reflections reflect incident light back onto itself (to the accuracy specified), regardless of the direction
the beam is coming from or the tilt of the prism.
Trinocular
Visual device, such as a stereo scope or microscope, that incorporates two eyepiece ports and a vertical third port for
alternative image pickup. Alternative imagers include video and film cameras.
TTL : See Transistor-Transistor Logic.
TV line (TVL) resolution
Relative resolution figure for comparing CCD cameras and compatible devices.
Underscan : See Overscan vs. Underscan.
Ultraviolet (UV) Spectrum
The wavelength interval of the electromagnetic spectrum that corresponds to ultraviolet light (light that is beyond the
visible violet range). The range commonly used is 1 to 400nm. Can be divided into Near-Ultraviolet/Ultraviolet A (320 to
400nm) and Far-Ultraviolet/Ultraviolet B (280 to 320nm).
Vernier
For rotary stages, a scale measuring the amount of angular movement by the drive. Vernier values of resolution are the
smallest angular movement that can be measured by the scale.
Vidicon
Video device based on photoconduction in a vacuum tube; used as a general term for vacuum tube imaging technology.
Viewing Angle
For 3M Light Control Film, the full angle through which the film will transmit light. This value is centered about the louver
angle of the film.
Visible Spectrum
The wavelength interval of the electromagnetic spectrum which corresponds to visible light (light that is sensitive to the
human eye). The range commonly used is 400 to 750 nm, though sensitive eyes can often see out to 780nm (in the Near-IR
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by definition).
Very Low Frequency : See ELF.
Warm-Up Time
The time required by a laser to reach a designated percentage of the final output power (typically 95%).
Wavelength
The characteristic peak-peak measure of one cycle of an electromagnetic wave.
Wavelength Shifting Fibers
Optical fibers doped with materials that absorb high-energy, short wavelength light and emit lower-energy, longer
wavelength light.
Waveplates
See Retarders.
WD
See Working Distance.
WDM
Wavelength Division Multiplexing is a fiber-optic transmission technique that employs different light wavelengths to
transmit various types of data on the same optical signal. Because each type of data is transmitted on a different
wavelength, the receiver can filter out wavelengths in order to isolate the desired signal. DWDM is an advanced format of
WDM, allowing a greater number of data streams to be transmitted by decreasing the space between transmitting
wavelengths.
Wide-Field Eyepiece
An eyepiece for a magnifier or microscope with a field-of-view greater than 50°
Working distance (WD)
Distance from the front end of a lens system to the object under inspection.
Worm Gear Drive
Drive mechanism for some rotary stages. A worm wheel (gear), which is attached to the table shaft, meshes with the worm
drive, whose shaft extends out of the housing. Controlled rotation of the worm shaft creates precise angular rotation of the
worm wheel and table shaft. The worm gear and shaft are matched sets and are pre-loaded to remove backlash. This type of
drive provides high resolution (180:1) and continuous angular positioning over a full 360 range.
WRATTEN Filters
WRATTEN filters are filters manufactured by Kodak’s Scientific Imaging products division. They are created by
dissolving organic dyes in a gelatin material to achieve the desired spectral performance. The gelatin liquid and dye
combination is then coated onto a supportive substrate until it has dried. After removal from the substrate, the film is coated
with laquer for protection. Although the filters are coated, they should be handled only by the edges or in the corners to
avoid damage.
Y-C
Component level signal in which luminance and chroma are separate, sometimes referred to as S-VHS or S-Video.
Requires two separate coaxial connections, or a 4-contact connector such as 4-pin mini-DIN. See also Connectors.
Yaw
Angular rotation about the vertical axis, typically the Z-axis for X-Y-Z.
Zerodur®
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An inorganic, non-porous material manufactured by Schott Glass Technologies, Inc., that exhibits a thermal expansion
coefficient approaching zero.
Zoom
In the context of optics, zoom refers to a variable focal length system: the field of view can be changed without changes in
working distance. A zoom is essentially a compromise in aberration and image quality over a range of focal length
solutions; therefore, zoom systems do not theoretically yield the same level of performance at every focal length setting. In
the context of computer-based image capture, zoom is a magnification (or demagnification) of the individual pixels in an
image file that yields no additional resolution.
GLOSSARY – VISUAL INSPECTION HANDBOOK
Most of the definitions in this glossary have been modified to satisfy peer review and editorial style.
For this reason, ref-erences given in this glossary are not attributions hut rather acknowledgments and
suggestions for further reading. Most definitions are adapted from the preceding text. There, published
sources are acknowledged individually on section title pages. This glossary is provided for
instructional purposes. No other use is intended.
AOQ: Average outgoing quality.
AOQL: Average outgoing quality limit.
AQL: See acceptable quality level.
ASNT: American Society for Nondestructive Testing.
ASNT Recommended Practice No. SNT-TC-IA: A set of guidelines for employers to establish and
conduct a nondestructive testing personnel qualification and cer-tification program. SNT- TC-1A was
first issued in 1968 by the Society for Nondestructive Testing (SNT, now ASNT) and has been revised
every few years since.
acceptable quality level (AQL): The maximum percent defective (or the maximum number of units
with dis-continuities per hundred units) that, for the purposes of sampling tests, can be considered
satisfactory as a pro-cess average.
accommodation: Of the eye, adjustment of focus made by changing the thickness and curvature (the
focusing power) of the lens and done by the action of tiny mus-cles attached to the lens.
acuity: See neural acuity, vision acuity.
adaptive thresholding: Threshold value varying with inconstant background gray level.
adhesive wear: See wear, adhesive.
alpha ferrite: The form of pure iron that has a body centered cubic structure stable below 910°C
(1,670°F). Also called alpha iron.
alpha iron: See alpha ferrite.
ambient light: Light in the environment as opposed to illumination provided by a visual testing
system.
angle: See field angle.
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angstrom unit (A): Unit of length, equal to 0.1 nanometer.
annealing: Process of heating and cooling a material, usually to reduce residual stresses or to make it
softer.
anode: Negatively charged terminal, which may corrode electrochemically during production of an
electric cur-rent. Compare cathode.
arc welding: See electric arc welding.
austenite: A solid solution with iron as the solvent in a face centered cubic structure formed by
slow cooling of delta ferrite. Austenite’s characteristic lattice struc-ture is stable between 906 °C
(1,663 °F) and 1,390 °C (2,535 °F). Also called gamma iron.
automated system: Acting mechanism that performs required tasks at a determined time and in a
fixed sequence in response to certain conditions.
background cylinder and difference cylinder: Two devices used to calculate illuminance by
using the equivalent sphere illumination technique.
binary system: In metallurgy, a two element alloy system. See also isomorphous binary system.
birefringence: The splitting of a light beam into two parts through a translucent material.
black body: See blackbody.
black light: Term sometimes used for ultraviolet radiation, particularly in the near ultraviolet range
of about 320 to 400 nm.
blackbody: A theoretical object that radiates more total power and more power at any wavelength
than any other source operating at the same temperature.
Black light: See black light. blind spot: Portion of the retina where the optic nerve enters, without rods and cones and hence
insensitive to light.
blotch: (1) An irregularly spaced area of color change on a surface. (2) The nonuniform condition
of a surface characterized by such blotches.
blue hazard: Exposure to high frequency visible light at intensities and durations that may damage
the retina, particularly in conjunction with overheating.
borescope: An industrial endoscope; a periscope or tele-scope using mirrors, prisms, lenses, optic
fibers or tele-vision wiring to transmit images from inaccessible interiors for visual testing.
Borescopes are so called because they were originally used in machined aper-tures and holes such
as gun bores. There are both flexible and rigid, fiber optic and geometric light borescopes.
borescope, angulated: Borescope bent for viewing at for- ward oblique, right angle or
retrospective angles for visual testing of surfaces not accessible with conven-tional borescopes.
borescope, calibrated: Borescope with gage on external tube to indicate the depth of insertion
during a test. Borescopes with calibrated reticles are used to deter-mine angles or sizes of objects in
the field when held at a predetermined working distance.
borescope, cave: Multiangulated, periscope borescope used for remote observation of otherwise
inaccessible areas.
borescope, fiber optic: Borescope that uses fiber optic materials (such as glass or quartz) in the
optical path and for transmission of light to and from the test surface.
borescope, indexing: Borescope that can be bent 90 degrees by rotation of a knob after the
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instrument has been inserted through an aperture. A knob at the eyepiece can rotate the objective
head through 360 degrees for scanning a circumferential weld seam.
borescope, micro-: Borescope with an outside diameter generally from 1 to 5 mm (0.04 to 0.2 in.),
typically using quartz filaments. Compare miniature borescope.
borescope, miniature: Borescope with an outside diameter generally less than 13 mm (0.5 in.).
Sometimes called miniborescope. See also microborescope.
borescope, panoramic: Borescope with a scanning mirror mounted in front of the objective lens
system. Rotation of the mirror is adjusted at the ocular end of the instru-ment to scan in forward
oblique, right angle and retro-spective directions.
bores cope, retrospective: Borescope that looks backwards, 180 degrees or nearly so from the distal
line of interro-gation normal to the plane of a conventional objective lens.
borescope, rigid: Borescope that does not bend, typically in order to keep the geometrical optics in
alignment through a light train system.
borescope, ultraviolet: Borescope equipped with ultraviolet lamps, filters and special transformers to
transmit radia-tion of ultraviolet wavelengths.
borescope, water proof / vapor proof: Borescope completely sealed and impervious to water or other
types of fluid, used for internal tests of liquid, gas or vapor envi-ronments.
borescope, wide field: Borescope with rotating objective prism to provide fields of view up to 120
degrees.
brinelling: Stripe indentations made by a spherical object. False brinelling refers to a type of surface
wear.
burr: A raised or turned over edge occurring on a machined part and resulting from cutting, punching
or grinding. I
burst: In metal, external or internal rupture caused by improper forming.
butt weld or butt joint: Weld joining two metal pieces in the same plane.
CCD: See charge coupled device.
candela: Base unit of measure in the 51 system for luminous intensity. The luminous intensity in a
given direction of a source that emits monochromatic radiation of fre-quency 540 X 1012 Hz and
that has a radiant intensity in that direction of 1.4641 milliwatt per steradian. Sym-bolized cd.
Formerly known as candle.
candle: Former name for candela.
case crushing: A mechanism producing fracture of the case, like subcase fatigue but attributable to
static overload-ing rather than to fatigue alone. In many instances the movement of the subcase
causes the case to crack or spall.
casing: The many strings of pipe that are used to line the hole during and after drilling of a gas or
oil well. This pipe protects the hole from formation collapse, keeps the formation fluids out of the
hole and keeps the oil well fluids out of the water tables.
casing string: Tubular structure on the outer perimeter of a gas or oil well hole. The casing string is
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a permanent part of the well and many are cemented into the formation.
cathode: Positively charged terminal in an arrangement that produces current by chemical
reactions. Compare anode.
cavitation fatigue: A form of pitting caused by erosion from vibration and movement in liquid
environments.
cementite: Iron carbide (Fe3C), present in steels.
certification: The process of providing written testimony that an individual is qualified. See also
certified.
certified: Having .written testimony of qualification. See also certification.
chafing: See wear, fretting.
channels: In biology, mechanisms functioning as band pass filters in the visual cortex of mammals,
causing sensitiv-ity to visual stimuli in particular frequencies and ranges.
charge coupled device (CCD): Solid state image sensor. CCDs are widely used in inspection
systems because of their accuracy, high speed scanning and long service life.
closing: In image processing, dilation followed by erosion. A single pixel closing connects a broken
feature separated by one pixel. See also opening.
closure: Process by which a person cognitively completes patterns or shapes that are incompletely
perceived.
cocoa: Debris (usually oxides of the contacting metals) of fretting wear, retained at or near the site
of its forma-tion-a condition especially helpful during visual tests. With ferrous metals, the debris is
brown, red or black, depending on the type of iron oxide formed. For this reason, ferrous debris is
called cocoa or, when mixed with oil or grease, red mud.
code: A standard enacted or enforced as a law.
coefficients of the filter: Values in a mask that serves as a fil-ter in image processing.
cold light: Obsolete word for fluorescence.
color: Sensation by means of which humans distinguish light of different intensities (brightness)
and wavelengths (hue).
color blindness: Deficiency in the ability to perceive or dis-tinguish hues.
color discrimination: The perception of differences be-tween two or more hues.
color temperature: Rating of a light source for color vision.
complete testing: Testing of an entire production lot in a prescribed manner. Sometimes, complete
testing entails the inspection of only the critical regions of a part. One hundred percent testing
requires the inspec-tion of the entire part by prescribed methods. Compare sampling, partial.
compound microscope: See microscope, compound.
cone: In biology, a retinal receptor that dominates the retinal response when the luminance level is
high and provides the basis for the perception of color. Compare rod.
constitution diagram: See phase diagram.
contrast: The difference between the amount of light reflected or transmitted by an object and by
the visual task or the observer’s field of view.
control: See in control, process control and quality control.
corrosion: Loss or degradation of metal because of chemical reaction.
corrosion-erosion: Simultaneous occurrence of erosion and corrosion.
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corrosion, crevice: A kind of galvanic corrosion caused by differences in metal ion concentrations
in neighboring portions of the corrodent.
corrosion, fretting: Corrosion facilitated by fretting, par-ticularly where a protective surface has
been chafed in a corrosive environment.
corrosion, poultice: Corrosion occurring under a layer of foreign material (e.g., under mud in
automobile rocker panels).
creep: Gradual and permanent change of shape in a metal under constant load, usually at elevated
temperature. Occurs in three stages: primary creep, secondary creep and tertiary creep. See also
deformation.
crevice corrosion: See corrosion, crevice.
dark adaptation: The process by which the retina becomes adapted to a luminance that is less than
approximately 0.034 candela per square meter.
dark adapted vision: See scotopic vision.
defect: A condition or discontinuity having a size, shape, ori-entation, nature, frequency or location
that impairs the useful service of the part or that is rejectable according to a specification or
standard. Compare discontinuity, indication.
deformation: Change of shape under load. See also creep and elastic deformation.
delta ferrite: Solid solution with body centered cubic struc-ture and iron as solvent. Also called
delta iron.
delta iron: See delta ferrite.
depth of field: In photography, the range of distance over which an imaging system gives
satisfactory definition when its lens is in the best focus for a specific distance.
dewetting: In soldering, the flow and retraction of solder, caused by contaminated surfaces,
dissolved surface coatings or overheating before soldering.
difference cylinder: See background cylinder.
dilation: In image processing, the condition of a binary image where the pixel in the output image
is a 1 if any of its eight closest neighbors is a 1 in the input image. See also closing, erosion and
opening.
direct photometry: Simultaneous comparison of a standard lamp and an unknown light source.
direct substitution alloy: Alloy in which the atoms of the alloying element can occupy the crystal
lattice spaces normally occupied by the atoms of the parent metal.
direct viewing: Viewing of a test object in the viewer’s immediate presence. The term direct
viewing is used in the fields of robotics and surveillance to distinguish con-ventional from remote
viewing,
direct vision instrument: Device offering a view directly for-ward. A typical scene is about 20
mm (0.75 in.) wide at 25 mm (1 in.) from the objective lens.
directional lighting: Lighting provided on the work plane or object predominantly from a preferred
direction.
discontinuity: An intentional or unintentional interruption in the physical structure or configuration
of a part. Compare defect, dislocation, indication.
discontinuity, inherent: Material anomaly originating from solidification of cast metal. Pipe and
nonmetallic inclu-sions are the most common and can lead to other types of discontinuities in
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fabrication.
discontinuity, primary processing: Material anomaly pro-duced from the hot or cold working of
an ingot into forgings, rod and bar.
discontinuity, secondary, processing: Material anomaly pro-duced during machining, grinding,
heat treating, plat-ing or other finishing operations.
discontinuity, service induced: Material anomaly caused by the intended use of the part.
dislocation: Void or discontinuity in the lattice of a metal crystalline structure.
distal: In a manipulative or interrogating system, of or per-taining to the end opposite from the
eyepiece and far-thest from the person using the system. Objective; tip.
effective throat: In welding, the weld throat including the amount of weld penetration but ignoring
excess metal between the theoretical face and the actual face.
elastic deformation: Temporary change in shape under a load. The material returns to its original
size and shape after the load is removed. Elastic deformation is the. state in which most metal
components are used in service.
elasticity: The ability of a material to resume its former shape after deformation.
electric arc welding: Joining of metals by heating with elec-tric arc. Also called arc welding.
endoscope: Device for viewing the interior of objects. From the Greek words for inside view, the
term endoscope is used mainly for medical instruments. Nearly every medical endoscope has an
integral light source; many incorporate surgical tweezers or other devices. Com-pare borescope.
equilibrium diagram: A phase diagram showing the phases present at equilibrium in a material
system.
equivalent sphere illumination: Level of perfectly diffuse (spherical) illuminance that makes the
visual task as photometrically visible within a comparison test sphere as it is in the real lighting
environment.
equivalent 20/20 near vision acuity: Vision acuity with remote viewing or other nondirect viewing
means that approximates 20/20 direct viewing closely enough to be considered the same for visual
testing purposes.
erosion-corrosion: Simultaneous occurrence of erosion and corrosion.
erosion: (1) Loss of material or degradation of surface qual-ity through friction or abrasion from
moving fluids, made worse by solid particles in those fluids or by cavi-tation in the moving fluid.
See Wear. (2) In image pro-cessing, condition of a binary image where the pixel in the output image
is a 1 if each of its eight neighbors is a 1 in the input image. See also closing, dilation and opening.
etch crack: Shallow crack in hardened steel containing high residual surface stresses, produced in
an embrittling acid environment.
eutectic liquid: A liquid having a proportion of metals such that two or more solid phases form at
the same tempera-ture during cooling.
eutectic point: Temperature and proportion of metals at which two or more phases of a eutectic
liquid form. Compare eutectoid.
eutectoid: Similar to eutectic but in a solid system during cooling.
evaluation: A review, following interpretation of the indications noted, to determine whether they
meet specified acceptance criteria.
eye sensitivity curve: Graphic expression of vision sensitivity characteristics of the human eye. In
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the case of a physi-cal photometer, the curve should be equivalent to the standard observer. The
required match is typically achieved by adding filters between the sensitive ele-ments of the meter
and the light source.
false brinelling: Fretting wear indentations. Compare brinelling.
false indication: An indication that is not produced by a dis-continuity. Compare defect.
far vision: Vision of objects at a distance, generally beyond arm’s length. Compare near vision.
farsightedness: Vision acuity functionally adequate for view-ing objects at a distance, generally
beyond arm’s length. Compare nearsightedness.
feature extraction: From an enhanced image, derivation of some feature values, usually
parameters for distinguish-ing objects in the image.
ferrite: Solid solution of one or more other elements, in alpha iron.
fiber optic borescope: See borescope, fiber optic.
fiber optics: The technology of light transmission through crystalline fibers such as glass or quartz.
fiberscope: Jargon for fiber optic borescope.
field: In video technology, one of two video picture components that together make a frame. Each
picture is divided into two parts called fields because a frame at the rate of thirty frames per second
in a standard video output would otherwise produce a flicker discernible to the eye. Each field
contains one half of the total picture elements. Two fields, then, are required to produce one
complete picture or frame so the field frequency is sixty fields per second and the frame frequency
is thirty frames per second.
field angle: The included angle between those points on opposite sides of the beam axis at which
the luminous intensity from a theatrical luminaire is 10 percent of the maximum value. This angle
may be determined from an illuminance curve or may be approximated by use of an incident light
meter.
field of view: The range or area where things can be seen through an imaging system, lens or
aperture. Compare depth of field.
field of vision: The range or area where things can be per-ceived organoleptically at a point in time,
assuming the eye to be immobile.
fillet weld: Weld on the inside comer of two metal pieces at a right angle.
filter: A processing component or function that excludes a selected kind of signal or part of a signal.
filtering: See low pass filtering.
fit up: To secure one or more joint members with special external fixturing in order to prevent
movement during welding.
flakes: Short discontinuous internal fissures in ferrous metals attributed to stresses produced by
localized transforma-tion and/or decreased solubility of hydrogen during cooling usually after hot
working. On a fractured sur-face, flakes appear as bright silvery areas; on an etched surface they
appear as short, discontinuous cracks.
fluorescence: The emission of visible light from a material in response to ultraviolet or X-radiation.
Formerly called cold light.
flux method: See lumen method.
focus: Position of a viewed object and a lens system relative to one another to offer a distinct image
of the object as seen through the lens system. See accommodation and depth of field.
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focus, principal plane of: The single plane actually in focus in a photographic scene.
focusing, automatic: (1) Feature of camera, usually incorpo-rating a range finder, whereby the lens
system adjusts to focus on an object in part of the field of view. (2) Meta-phorical attribute of a
borescopic instrument’s depth of field (the range of distance in focus). The depth of field is so great
in the case of video borescopes that focusing is unnecessary for most applications. Despite the
name, no mechanism is actively adjusted. The expanded depth of field is due both to the small
diameter of the lens aperture and to the proximity of the lens to the charge coupled device.
focusing, primary: Focusing by the lens of the image onto a fiber optic bundle at the tip of a probe.
focusing, secondary: Focusing at the eyepiece of a borescope or other optical instrument,
specifically the manual refocusing needed when the viewing distance changes.
footcandle: Former unit of measure for illumination, equiv-alent to one lumen evenly distributed
over a square foot, or to a surface illumination at a distance of one foot from a point of one candela.
Abbreviated ftc or fc. See also lux.
footlambert: Former unit of luminance. Measured in the SI system by candela per square meter.
forging crack: Discontinuity formed during mechanical shaping of metal.
fovea centralis: Region of sharpest vision in the retina, where the layer of blood vessels, nerve
fibers and cells above the rods and cones is far thinner than in periph-eral regions.
foveal vision: See photopic vision.
frame: A complete raster scan projected on a video screen. There are thirty frames per second in a
standard video output. A frame may be comprised of two fields, each displaying part of the total
frame.
fretting corrosion: See corrosion, fretting
fretting wear: See wear, fretting
friction oxidation: See wear, fretting.
galling: A type of adhesive wear more gross than fretting.
galvanic series: List of metals, alloys and graphite (a non- metal) in sequence with the most anodic
(easily cor-roded) in liquids at one end and the most cathodic (least easily corroded) at the other end.
For practical reasons, this sequence is compiled using seawater as the electrolyte -3 to 5 percent
sodium chloride and other salts dissolved in water.
gamma iron: Iron with face centered cubic structure formed by slow cooling of delta ferrite. This
characteristic lat-tice structure is stable between 906°C (1,663°F) and 1,390 °C (2,535°F). Also called
austenite.
gas tungsten arc welding (GTAW): Inert gas shielded arc welding using a tungsten electrode. Also
called tungsten inert gas (TIC) welding.
gasket seal: Resilient ring, usually virgin poly tetra fIuoro -ethylene (PTFE), in a piping or tubing
connection. Compare interference sealing thread and metal to metal seal.
general examination: A test or examination of a person’s knowledge, typically (in the case of
nondestructive test-ing personnel qualification) a written test on the basic principles of a
nondestructive testing method and gen-eral knowledge of basic equipment used in the method.
(According to ASNT’s guidelines, the general examina-tion should not address knowledge of specific
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equip-ment, codes, standards and procedures pertaining to a particular application.) Compare practical
examination and specific examination.
geometrical optics: The mathematical study of how light rays are reflected and refracted and practical
tech-niques based on such understanding, including the transmission of images by lenses and mirrors.
Also called lens optics.
glare: Excessive brightness (or brightness varying by more than 10:1 within the field of view) which
interferes with clear vision, critical observation and judgment.
glare, blinding: Glare so intense that for an appreciable length of time after it has been removed, no
object can be seen.
glare, direct: Glare resulting from high luminances or insuf-ficiently shielded light sources in the field
of view. Direct glare is usually associated with bright areas, such as luminaires, ceilings and windows
which are outside the visual task or region being viewed.
glare, reflected: Glare resulting from specular reflections of high luminances in polished or glossy
surfaces in the field of view. It usually is associated with reflections from within a visual task or
nearby areas.
gloss meter: Reflectometer used to measure specular reflectance.
gouge: Surface indentation caused by forceful abrasion or impact or flame cutting. Also called nick.
Compare tool mark.
grain: Solid particle or crystal of metal. As molten metal solidifies grains grow and lattices intersect,
forming irregular grain boundaries.
grain boundary: Interface that forms between grains of solidifying metal as the random oriented
crystal lattices meet. See grain.
gray level: Integer number representing the brightness or darkness of a pixel or, as a composite value,
of an image
comprised of pixels.
graybody: Radiator whose spectral emissivity is uniform for all wavelengths.
green rot: Form of attack due to simultaneous carburization and oxidation of stainless heating
elements common to nickel chromium and nickel chromium iron alloys, especially in furnace
environments.
grinding crack: Shallow crack formed on the surface of rela-tively hard materials due to excessive
grinding heat, to allotropic transformation of the surface material or to the high sensitivity of the
material surface containing high tensile residual stress. Grinding cracks typically are 90 degree to the
direction of grinding.
Hadfield’s steel: An austenitic manganese specialty steel that is easily work hardened.
halitation: Rings of light visible around a spot on a video screen where an electron scanning beam is
held.
heading: Upsetting wire, rod or bar stock in dies to form parts having some of the cross-sectional area
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larger than the original. Examples are bolts, rivets and screws.
heat checking: Surface cracking caused when metal rapidly heated (or cooled and heated repeatedly)
is prevented from expanding freely by colder metal below the sur-face. Friction may produce the heat.
Heat checking is sometimes called thermal fatigue.
heat wave: Thermally produced variation in flue gas density that distorts images of objects in a
firebox.
hot tear: Fracture formed in a cast metal dulling solidifica-tion and due to extensive tensile stress
associated with volumetric shrinkage. Hot tears often occur where areas of different thickness adjoin.
hue: Characteristic of light at a particular bandwidth that gives a color its name.
hundred percent testing: See one hundred percent testing.
hyperthermia: Heating so excessive that it can damage or kill plant or animal cells.
illuminance: The density of luminous flux on a surface. Mea-sured in the SI system by lux.
illuminate: Shed light on.
illumination: The act of illuminating or state of being illumi-nated. See also illuminate. Compare
illuminance.
image: Visual representation of a test object or scene.
image enhancement: Any of a variety of image processing steps, used singly or in combination to
improve the detectability of objects in an image.
image guide: Fiber bundle that carries the picture formed by the objective lens at the distal end of a
fiber optic borescope back to the eyepiece.
image orthicon: Television tube that uses the photoemission method. Compare vidicon.
image processing: Actions applied singly or in combination to an image, in particular the
measurement and alter-ation of image features by computer. Also called pic-ture processing.
image segmentation: Process in which the image is parti-tioned into regions, each homogeneous.
in control: Within prescribed limits of process control.
incandescence: The emission of visible radiation due to thermal excitation.
incandescent: Emitting visible radiation as a result of heating.
indication: A nondestructive testing response that requires interpretation to determine its relevance.
Compare defect, discontinuity, false indication.
indication, nonrelevant: An indication that has no relation to a discontinuity that might constitute
a defect.
indication, relevant: An indication from a discontinuity (as opposed to a false indication) requiring
evaluation by a qualified inspector, typically with reference to an accep-tance standard, by virtue of
the discontinuity’s size or location.
inert gas shielded arc welding: Joining of metals by heating them with an electric arc between
electrode (s) and the work piece, using an inert gas to shield the electrode (s). See also gas tungsten
arc welding.
infrared radiation: Electromagnetic radiant energy of wave- lengths longer than 770 nm.
interference fitted thread: Interference sealing thread.
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interference objective: Small, metallized glass mounted in contact with the test object and
adjustable for tilt to control fringe spacing.
interference sealing thread: Piping seal using a tapered connection made up under great pressure,
forcing the mating surfaces together more tightly than is possible by hand alone. Compare gasket
seal and metal to metal seal.
interlaced scanning: A process whereby the picture appearing on a video screen is divided into
two parts. Interlaced scanning reduces flicker by increasing the electron beam’s downward rate of
travel so that every other line is sent. When the bottom is reached, the beam is returned to the top
and the alternate lines are sent. The odd and even line scans are each transmitted at 1/60 s, totaling
1/30 s per frame and retaining the standard rate of 30 frames per second. The eye’s persis-tence of
vision allows the odd and even lines to appear as a single image without flicker.
interpretation: The determination of whether indications are relevant or non relevant or false.
interstitial alloy: Alloy in which the atoms of the alloying element fit into the spaces between the
atoms of the par-ent metal. iris: Ring of variable area around the pupil and in front of the lens of the eye. The surface area of
the iris adjusts spon-taneously to change the amount of light entering the eye.
irradiance: Power of electromagnetic radiant energy inci-dent on the surface of a given unit area.
Compare radiance.
Ishihara TM plates: Trade name for a kind of pseudoisochro-matic plates.
isomorphous binary system: A two element alloy system in which both elements are completely
soluble in each other in the liquid and the solid states, in all proportions at all temperatures.
Jaeger eye chart: An eye chart used for near vision acuity examinations.
kinetic vision acuity: Vision acuity with a moving target. Studies indicate that 10 to 20 percent of
visual efficiency can be lost by target movement.
LTPD: Lot tolerance percent defective.
laboratory microscope: Conventional compound micro-scope. See microscope.
lambertian: Having a surface that diffuses light uniformly rather than reflecting it. Matte. Most
objects have a lambertian surface. Compare specular.
lap: A forging discontinuity caused by a folding over of metal. Laps are found in rolled bar stock
and at or near diameter changes.
laser: An acronym (light amplification by stimulated emis-sion of radiation). The laser produces a
highly mono-chromatic and coherent (spatial and temporal) beam of radiation. A steady oscillation
of nearly a single electro-magnetic mode is maintained in a volume of an active material bounded
by highly reflecting surfaces, called a resonator. The frequency of oscillation varies according to
the material used and by the methods of initially exciting or pumping the material.
lens: Translucent object that refracts light passing through it in order to focus the light on a target.
lens optics: See geometrical optics.
light: Radiant energy that can excite the retina and produce a visual sensation. The visible portion
of the electromag-netic spectrum extends from about 380 to 770nm.
light adapted vision: See photopic vision.
light guide bundle: Bundle of filaments, usually glass, that carries noncoherent light from a high
intensity source through a fiber optic borescope to illuminate the object.
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lighting, back: Placement of light source and image sensor on opposite sides of the test object,
used when the sil-houette of a feature is important.
lighting, flash: See lighting, strobe.
lighting, front: Placement of light source and image sensor on the same side of the test object.
lighting, strobe: Lighting that flashes intermittently at a rate that may be adjusted and is often
perceived as a flicker, used to image moving objects or still objects with poten-tial movement.
lighting, structured: Combining a light source with optical elements to form a line or sheet of light.
limited certification: Individuals who are certified only for specific operations are usually called
limited Level (I, II or Ill) or are designated as having limited certification because they are not
qualified to perform the full range of activities expected of personnel at that level of qualification.
line pair: Pair of adjacent, parallel lines used to evaluate the resolution of a specific imaging
system. See also mini-mum line pair.
lot tolerance percent defective: In quality control, the per-cent defective at which there is a 10
percent probability of acceptance in a production run.
low pass filtering: A linear combination of pixel values to smoothen abrupt transitions in a digital
image. Also called smoothing.
lumen: Unit of measure in the 51 system for luminous flux, equivalent to candela times steradian
(cd •sr). Abbrevi-ated lm.
lumen method: A lighting design procedure used for prede-termining the relation between the
number and types of lamps or luminaires, the room characteristics and the average illuminance on
the work plane. It takes into account both direct and reflected flux. Also called flux method.
luminance: The ratio of a surface’s luminous intensity in a given direction to a unit of projected
area. Measured in candela per square meter.
luminosity: The luminous efficiency of radiant energy.
luminous efficacy: The ratio of the total luminous flux of a light source to the total radiant flux or
to the power input. Sometimes called luminous efficiency
luminous efficiency: See luminous efficacy.
luminous flux: Radiant energy’s time rate of flow. Measured in lumens.
luminous intensity: Luminous flux on a surface normal to the direction from its light source,
divided by the solid angle the surface subtends at the source. Measured in candela. Also known as
candlepower.
lux: Unit of measure for illuminance in the SI system. Equiv-alent to lumens per square meter and
symbolized lx. Formerly known as meter-candle.
machine vision: Automated system function of acquiring, processing and analyzing images to
evaluate a test object or to provide information or interpretation for human interpretation. A typical
machine vision system consists of a light source, a video camera, a video digi-tizer, a computer and an
image display.
macular lutae: Irregular, diffuse ring of yellow pigment which partly overlaps the fovea and
surrounds it out to around 10 degrees and which absorbs blue light, thus changing the color of the light
reaching receptors beneath.
martensite: (1) Acicular (needlelike) microstructure pro-duced by fast cooling or quenching of metals
and alloys such as steel. (2) The hard steel with such microstruc-ture produced by fast cooling of
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austenite.
martensite finish temperature: Temperature at which mar-tensite formation is completed as steel
cools.
martensite start temperature: Temperature at which mar-tensite starts to form as steel cools.
mask: (1) A spatial filter in the sensing unit of a surface inspection system. (2) An n X n square matrix
with dif-ferent values that selves as a filter in image processing.
match bend effect: Optical illusion whereby an area of uni-form brightness appears to be nonuniform
because of contrast with the brightness of an adjacent area.
mathematical morphology: Image processing technique of expanding and shrinking. The basic
operators in mathe-matical morphology are dilation (expanding), erosion (shrinking), opening and
closing.
matte: Tending to diffuse light rather than reflect it; not shiny. Also called lambertian. The term matte
is gener-ally applied to smooth surfaces or coatings. Compare specular.
mesopic vision: Vision adapted to a level of light between photopic at 3.4 X 10-2cd•m-2 (3.2 X 10-3
cd•ft-2) and scotopic at 3 X 1-5 cd•m-2(2.7 X 10-6cd•ft-2).
metal to metal seal: Piping seal in which the mating surfaces on the external connection (the pin) and
internal con-nection (the box) are machined to provide a pressured interference fit 360 degrees around
the connection. Compare gasket seal and interference sealing thread.
metallograph: Short term for metallographic microscope.
metallographic microscope: See microscope, metall-graphic.
metallography: The science and practice of microscopic testing, inspection and analysis of a metal’s
structure, typically at magnifications in the range of 50 X to 2,500 X
metallurgical microscope: See microscope, metallurgical. microborescope: See borescope, micro.
microscope: An instrument that provides enlarged images of very small objects.
microscope, compound: Conventional microscope, using geometrical optics for magnification. Also
called labora-tory microscope.
microscope, interference: Magnifier using the wavelength of light as a unit of measure for surface
contour and other characteristics.
microscope, metallographic: Metallurgical microscope incorporating a camera. Also called a
metallograph. Most metallographic microscopes share these features: (a) stand with concealed shock
absorbers, (b) intense light source, (c) inverted stand so that the test object is face down, (d) viewing
screens for prolonged tasks such as dirt count or grain size measurements, (e) blight, dark and
polarized illumination options.
microscope, metallurgical: Microscope designed with fea-tures suited for metallography.
microscope, phase contrast: Laboratory microscope with two additional optical elements to transmit
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both dif-fracted and undiffracted light, revealing refractive index discontinuities in a completely
transparent test object.
microscope, polarizing: Microscope with polarizing ele-ments to restrict light vibration to a single
plane for studying material with directional optical properties. As fibers, Crystals, sheet plastic and
materials under strain are rotated between crossed polarizers on the micro-scope stage, they change
color and intensity in a way that is related to their directional properties.
miniature borescope: See borescope, miniature.
miniborescope: Jargon for miniature borescope.
minimum line pair: The closest distance that a specific imaging system can resolve between a pair of adjacent, parallel
lines (line pair) that are used to evaluate sys-tem resolution.
modulus of elasticity: The ratio between stress and strain in a material deformed within its elastic
range.
monochromatic: Light from a very small portion of the spec-trum is called monochromatic.
monochromator: Device that uses prisms or gratings to sep-arate or disperse the wavelengths of the
spectrum into noncontinuous lines or bands.
morphology: See mathematical morphology.
mottle: An apparently random positioning of metallic flakes that creates an accidental pattern.
multipass weld: A weld made by more than one pass.
multiphase alloy: Alloy in which several phases are present.
NDE: (1) Nondestructive evaluation. (2) Nondestructive examination.
NDI: Nondestructive inspection.
NDT: Nondestructive testing.
near ultraviolet radiation: Ultraviolet radiation with wave-lengths ranging from about 320 to about
400 nm. Formerly called black light.
near vision: Vision of objects nearby, generally within arm’s length. Compare far vision.
nearsightedness: Vision acuity functionally adequate for viewing objects nearby, generally within
arm’s length. Compare farsightedness.
necking down: Localized reduction in area of a specimen or structural member during welding under
overload.
negative sliding: The rolling and sliding of meshing gears or rollers when the rolling and sliding are in
opposite directions.
neural acuity: The ability of the eye and brain together to discriminate patterns from background.
Discrimination is influenced by knowledge of the target pattern, by the scanning technique and by the
figure/ground relation-ship of a discontinuity. The figure/ground relationship can be referred to as
having a level of visual back-ground noise.
nick: Surface indentation caused by forceful abrasion or impact. Also called gouge. Compare tool
mark.
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nit: A former unit for measuring luminance, equivalent to one candela per square meter. Abbreviated
nt.
noble metals: Cathodic metals (such as gold, platinum and silver), which strongly resist corrosion.
nondestructive evaluation (NDE): Another term for non-destructive testing. In research and
academic commu-nities, the word evaluation is often preferred because it emphasizes interpretation by
knowledgeable personnel.
nondestructive examination (NDE): Another term for non-destructive testing. In the utilities and
nuclear industry, examination is sometimes preferred because testing can imply performance trials of
pressure containment or power generation systems.
nondestructive inspection (NDI): Another term for nonde-structive testing. In some industries
(utilities, aviation), the word inspection often implies maintenance for a component that has been in
service.
nondestructive testing (NDT): The determination of the physical condition of an object without
affecting that object’s ability to fulfill its intended function. See also nondestructive evaluation,
nondestructive examination and nondestructive inspection.
nonrelevant indication: See indication, nonrelevant.
OCTG: Oil country tubular goods.
objective: In discussion of a lens system (camera, borescope, microscope, telescope), of or
pertaining to the end or lens closest to the object of examination at the end opposite from the
eyepiece. Distal; tip.
oil country tubular goods (OCTG): Hollow cylindrical com-ponents used to convey petroleum
and related products.
one hundred percent testing: Testing of all parts of an entire production lot in a prescribed
manner. Some-times, complete testing entails the testing of only the critical portions of the part.
Compare sampling, partial.
opening: Image processing operation of erosion followed by dilation. A single opening eliminates
isolated single pix-els. See also closing.
opsin: See visual purple.
optic disk: Area in the retina through which the fibers from the various receptors cross the inner
(vitreous humor) side of the retina and pass through it together in the optic nerve bundle. This
transitional area is completely blind.
organoleptic: Relying on or using sense organs, such as the human eye.
orthicon: See image orthicon.
parafoveal vision: See scotopic vision.
parallax: The apparent difference in position of an imaged point according to two differently
positioned sensors.
pass: In welding, a single bead along the entire weld length or the process of laying down that head.
pearlite: Platelet mixture of cementite and ferrite in steels or in alpha and beta phases in nonferrous
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alloys.
peripheral vision: The seeing of objects displaced from the primary line of sight and outside the
central visual field
phase: In metallurgy, a physically homogeneous portion of a material system, specifically the
portion of an alloy char-acterized by its microstructure at a particular temperature during melting or
solidification.
phase contrast microscope: See microscope, phase contrast.
phase diagram: Graph showing the temperature, pressure and composition limits of phase fields in
a material system. Also called a constitution diagram. Compare equi-librium diagram.
photoconduction: Method by which a vidicon television camera tube produces an electrical image,
in which the conductivity of the photosensitive surface changes in relation to the intensity of the
light reflected from the scene focused onto the surface. Compare photo-emission.
photoelasticity: The effect of a material’s elastic properties on the way that it refracts or reflects
light.
photoelectric effect: Emission of electrons from a surface bombarded by sufficiently energetic
photons. Such emissions may be used in an illuminance meter and can be calibrated in lux.
photoemission: Method by which an image Orthicon televi-sion camera tube produces an electrical
image, in which a photosensitive surface emits electrons when light reflected from a viewed object
is focused on the surface. Compare photoconduction.
photometer: The basic measuring instrument of photome-try. Early photometers requiring visual
appraisal by the operator have been replaced by more accurate meters that measure radiant energy
incident on a receiver, producing measurable electrical quantities.
photometric brightness: The luminance of a light source.
photometry: The science and practice of the measurement of light or photon emitting
electromagnetic radiation. See also relative photometry.
photons: Particle of light, hypothesized to explain those behaviors of light in which its behavior is
corpuscular rather than wavelike.
photopic vision: Vision adapted to daylight and mediated mainly by the cones. Vision is wholly
photopic when the luminance of the test surface is above 0.034 cd•m-2 (0.0032 cd•ft-2). Also
known as foveal vision and light adapted vision. Compare mesopic vision and scotopic Vision.
Photoreceptor: Light sensor.
picture element: See pixel.
picture processing: See image processing.
pipe: A longitudinal centerline discontinuity inherent in ingots, imparted to some rolled metal and
consisting of a concavity or voids.
pitting: Discontinuity consisting of surface cavities. See also cavitation fatigue and pitting fatigue.
pitting fatigue: Discontinuity consisting of surface cavities typically due to fatigue and abrasion of
contacting surfaces undergoing compressive loading. See also cavita-tion fatigue and pitting.
pixel: A lighted point on the screen of a digital image. The image from a conventional computer is an
array of over 256,000 pixels, each of which has a numerical value. The higher the number for a pixel,
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the blighter it is. Formerly called picture element.
plane of focus: See focus, principal plane of
platelet: Flat crystallites in certain phases of steel.
polarizing microscope: See microscope, polarizing.
porosity: A discontinuity in metal resulting from the creation or coalescence of gas. Very small
pores are called pinholes.
positive sliding: The rolling and sliding of meshing gears or rollers when the directions of rolling
and sliding are the same.
poultice corrosion: See corrosion, poultice.
practical examination: In certification of nondestructive testing personnel, a hands-on examination
using test equipment and sample test objects. Compare general examination and specific
examination.
primary creep: First stage of creep, marked by elastic strain plus plastic strain.
principal plane of focus: See focus, principal plane of
process: Repeatable sequence of actions to bring about a desired result.
process control: Application of quality control principles to the management of a repeated process.
production string: See tubing string.
pseudocolor: Image enhancement technique wherein colors are assigned to an image at several
gray scale intervals.
pseudoisochromatic plates: Color plates used for color vision examinations. Each plate bears an
image which may be difficult for the examinee to see if his or her color vision is impaired.
psychophysics: Interaction between vision performance and physical or psychological factors. One
example is the so- called vigilance decrement, the degradation of reliabil-ity based on performing
visual activities over a period of time.
pupil: Black aperture in the center of the eye’s lens, through which light enters the lens to impinge
on the retina.
purple: See visual purple.
qualification: Process of demonstrating that an individual has the required amount and the required
type of train-ing, experience, knowledge and capabilities. See also qualified.
qualified: Having demonstrated the required amount and the required type of training, experience,
knowledge and abilities. See also qualification.
quality: The ability of a process or product to meet specifi-cations or to meet the expectations of its
users in terms of efficiency, appearance, longevity and ergonomics.
quality assurance: Administrative actions that specify, enforce and verify a quality program.
quality control: Physical and administrative actions required to ensure compliance with the quality
assurance pro-gram. Quality control may include nondestructive test-ing in the manufacturing cycle.
quality of lighting: Level of distribution of luminance in a visual task or environment.
radiance: Radiant flux per unit solid angle and per unit pro-jected area of the source. Measured in
watts per square meter steradian. Compare irradiance.
radiant energy: Energy transmitted through a medium by electromagnetic waves. Also known as
radiation.
radiant flux: Radiant energy’s rate of flow, measured in watts.
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radiant intensity: Electromagnetic energy emitted per unit time per unit solid angle. Measured in
watts per steradian.
radiant power: Total radiant energy emitted per unit time.
radiometer: Instrument for measuring radiant power of specified frequencies. Different
radiometers exist for different frequencies.
radiometric photometer: Radiometer for measuring radiant power over a variety of wavelengths.
raster: A repetitive pattern whereby a directed element (a robotic arm or a flying dot on a video
screen) follows the path of a series of adjacent parallel lines, taking them successively in turn,
always in the same direction (from top to bottom or from left to right), stopping at the end of one
line and beginning again at the start of the next line. Following a raster pattern makes it possi-ble
for electron beams to form video pictures or frames and for a sensor bearing armature to cover a
predeter-mined part of the surface of a test object.
rat’s tooth principle: (1) The tendency for hard material on a tooth’s front surface to wear more
slowly than soft material on the back surface, keeping the edge sharp. (2) Mechanism of wear
whereby adjacent hard and soft surfaces wear at different rates, producing a self sharpening edge.
recommended practice: A set of guidelines or recom-mendations.
Recommended Practice SNT-TC-IA: See ASNT Recommended Pradice No. SNT-TC-IA.
recovery: Reduced stress level and increased ductility of metal after work hardening. See creep.
recrystallization: Changes in microstructure and properties upon heating of cold worked metal.
red mud: Debris (usually oxides of the contacting metals) of fretting wear, mixed with oil or grease
and retained at or near the site of its formation. See also cocoa.
reference standard: Work piece or energy source prepared according to precise instructions by an approved agency for
tests and calibrations requiring precise and consis-tent measurements. (In the case of photometry, the standard is a light
source).
reflectance: The ratio of reflected wave energy to incident wave energy. Also known as reflectivity.
reflection: A general term for the process by which the inci-dent flux leaves a surface or medium from
the incident side, without change in frequency. Reflection is usually a combination of regular and
diffuse reflection.
reflectometer: Photometer used to measure diffuse, specu-lar and total reflectance.
reflector: A device used to redirect the luminous flux from a source by the process of reflection.
refraction: The bending of radiation’s path by the medium through which it passes.
relative photometry: (1) Evaluation of a desired photomet-ric characteristic based on an assumed
lumen output of a test lamp. (2) Measurement of an uncalibrated light source relative to another
uncalibrated light source.
relevant indication: See indication, relevant.
remote viewing: Viewing of a test object not in the viewer’s immediate presence. The word remote
previously implied either closed circuit television or fiber optic sys-tems remote enough so that, for
example, the eyepiece and the objective lens could be in different rooms.
High resolution video and digital signals can now be transmit-ted around the world with little loss of
image quality. Compare direct viewing.
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replica: Piece of malleable material, such as polyvinyl or polystyrene plastic film, molded to a test
surface for the recording or analysis of the surface microstructure.
replication: A method for copying the topography of a sur-face by making its impression in a plastic
or malleable material.
reserve vision acuity: The ability of an individual to maintain vision acuity under poor viewing
conditions. A visual system with 20/20 near vision acuity under degraded viewing conditions has
considerable reserve vision acuity compared to that of an individual with 20/70 near vision acuity,
resolution: An aspect of image quality pertaining to a sys-tem’s ability to reproduce objects, often
measured by resolving a pair of adjacent objects or parallel lines. See also minimum Line pair,
resolving power.
resolution test: Procedure wherein a line is detected to verify a system’s sensitivity.
resolution threshold: Minimum distance between a pair of points or parallel lines when they can be
distinguished as two, not one, expressed in minutes of arc. Vision acu-ity, in such a case, is the
reciprocal of one-half of the period expressed in minutes.
resolving power: The ability of vision or other detection sys-tem to separate two points. Resolving
power depends on the angle of vision and the distance of the sensor from the test surface. Resolving
power is often mea-sured using parallel lines. Compare resolution.
retina: In the eye, the tissue that senses light.
retinene: See visual purple.
rhodopsin: See visual purple.
robotic system: Automated system programmed to perform purposeful movements in variable
sequences.
rod: Retinal receptor that responds at low levels of lumi-nance even down below the threshold for
cones. At these levels there is no basis for perceiving differences in hue and saturation. No rods are
found in the fovea centralis.
SI: The International System of units of measurement. Includes most of the base units formerly called
metric.
SNT-TC-1A: See ASNT Recom1nended Practice No. SNT- TC-1A.
sampling, partial: Testing of less than one hundred percent of a production lot. See one hundred
percent testing.
sampling, random partial: Partial sampling that is fully random.
sampling, specified partial: Partial sampling in which a par-ticular frequency or sequence of sample
selection is prescribed. An example of specified partial sampling is the testing of every fifth unit.
saturation: Relative or comparative color characteristic resulting from a hue’s dilution with white
light.
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scaling: (1) Forming a layer of oxidation product on metals, usually at high temperatures. (2)
Deposition of insolu-ble constituents on a metal surface, as in cooling tubes and water boilers.
scoring: (1) Marring or scratching of any formed part by metal pickup on a punch, die or guide. (2)
Reducing the thickness of a part along a line to weaken it purposely at a specific location.
scotopic vision: Dark adapted vision, using only the rods in the retina, where differences in brightness
can be detected hut differences in hue cannot. Vision is wholly scotopic when the luminance of the test
surface is below 3 x 10-5 cd•m-2 (2.7 x 10-6 cd•ft-2). Also known as parafoveal vision. Compare
mesopic vision and photopic vision,
scuffing: A type of adhesive wear.
seam: Linear discontinuity formed by a lack of metal from folds produced by an under-filled pass
during metal roll-ing, Squeezed tight on subsequent passes, the underfill runs parallel to the
longitudinal axis of the bar.
second stage replica: A positive replica made from the first cast to produce a duplicate of the
original surface is called a second stage replica.
secondary creep: Second stage of creep, where deformation proceeds at a constant rate and less
rapidly than as in primary creep. Essentially an equilibrium condition between the mechanisms of
work hardening and recovery.
sensitization: Precipitation of chromium carbides in the grain boundaries of a corrosion resistant
alloy, resulting in intergranular corrosion that would otherwise be resisted.
shadow casting: Nondestructive technique of vapor depos-iting a thin metal film onto a replica at an
oblique angle in order to obtain a micrograph of a test surface of an opaque specimen.
shear break: Open break in metal at the periphery of a bolt, nut, rod or member at approximately a 45
degree angle to the applied stress. Shear breaks occur most often with flanged products. Also called
shear crack.
shear crack: See shear break.
shoulder: Cylindrical metal component surface, machined to receive threading indentations but in
fact not threaded, where the thread stops on the outside surface of the pipe.
signal electrode: Transparent conducting film on the inner surface of a vidicon’s faceplate and a
thin photoconduc-tive layer deposited on the film.
signal processing: Acquisition, storage, analysis, alteration and output of digital data through a
computer.
simple magnifier: A microscope having a single converging lens.
smoothing: In image processing, use of positive coefficients in a linear combination of pixel values
to smoothen abrupt transitions in a digital image. Also called low pass filtering.
spalling fatigue: See subcase fatigue.
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specific examination: In certification of nondestructive testi-ng personnel, a written examination
that addresses the specifications and products pertinent to the application. Compare general
examination and practical exami-nation.
specification: A set of instructions or standards invoked by a specific customer to govern the
results or performance of a specific set of tasks or products.
spectral power distribution: The radiant power per unit wavelength as a function of wavelength.
Also known as spectral energy distribution, spectral density and spec-tral distribution.
spectral reflectance: The radiant flux reflected from a mate-rial divided by the incident radiant
flux.
spectral transmittance: The radiant flux passing through a medium divided by the incident radiant
flux.
spectrophotometer: Instrument used for spectropho-tometry.
spectrophotometry: Measurement of electromagnetic radi-ant energy as a function of wavelength,
particularly in the ultraviolet, visible and infrared wavelengths.
spectroradiometer: Instrument used for spectroradiometry.
spectroradiometry: Measurement of electromagnetic radi-ant power and spectral emittance, used
particularly to examine colors and to measure the spectral emittance of light sources.
spectroscope: Instrument used for spectroscopy.
spectroscopy: Spectrophotometry or spectroradiometry in which the spectrum, rather than being
analyzed only by a processing unit, is presented in a visible form to the operator for organoleptic
examination.
spectrum: Representation of radiant energy in adjacent bands of hues in sequence according to the
energy’s wavelengths or frequencies. A rainbow is a well known example of a visible spectrum.
specular reflection: When reflected waves and incident waves form equal angles at the reflecting
surface.
specular: Pertaining to a mirror-like reflective finish, as of a metal. Compare lambertian.
speed of light: The speed of all radiant energy, including light, is 2.997925 x 108 meters per second
in vacuum (approximately 186,000 miles per second). In all materi-als the speed is less and varies
with the material’s index of refraction, which itself varies with wavelength.
speed of vision: The reciprocal of the duration of the expo-sure time required for something to be
seen.
standard observer response curve: See eye sensitivity curve.
standard: Document to control and govern practices in an industry or application, applied on a
national or interna-tional basis and usually produced by consensus. See also working standnrd and
reference standard.
steel: An iron alloy, usually with less than two percent carbon.
stereo photography: Close range photogrammetric tech-nique involving the capture and viewing
of two images of the same object in order to reconstruct a three dimensional image of the object.
strain: The alteration of the shape of a material by external forces.
stress: (1) In physics, the force in a material that resists exter-nal forces such as tension and
compression. (2) Load per unit area.
stress raiser: Contour or property change that causes local concentration of stress.
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stress riser: See stress raiser.
subcase fatigue: Fatigue Originating below the case depth. Compare case crushing. Also see
spalling fatigue.
subcase Origin fatigue: See sub case fatigue.
subsurface fatigue: Fatigue cracking that Originates below the surface. Usually associated with
hard surfaced or shot peened parts but may occur anytime subsurface stresses exceed surface
stresses.
TIC welding: Tungsten inert gas welding.
TIT: Time temperature transformation.
Tarasov etching technique: Way of visually inspecting for the presence of deleterious effects in
hardened steels by using specific etching solutions and methods of inspection.
temperature diagram: See time temperature transformation(TTT) diagram.
tempering: Process of heating a material, particularly hard-ened steel to below the austenite
transformation tem-perature, to improve ductility.
tertiary creep: Third stage of creep, marked by steady increase in strain to the point of fracture under
con-stant load.
threshold: See adaptive threshold, resolution threshold, threshold.
thresholding: Digital data processing technique that reduces a gray level image into a binary image.
throat, actual: Shortest distance from the root of a fillet weld to its face, as opposed to theoretical
throat or weld size.
throat, theoretical: The distance from the beginning of the root of the weld perpendicular to the
hypotenuse of the largest right triangle that can be inscribed within the cross section of the fillet weld.
Compare weld size.
throat, weld: Distance from the root of a fillet weld to its face. Compare weld size and throat, actual.
time temperature transformation (TIT) diagram: A graph showing time required at any
temperature to transform austenite to pearlite, bainite or martensite.
tip: In casual usage, the distal or objective end of a borescope.
tool mark: Shallow indentation or groove made by the movement of manufacturing tools over a
surface. Compare gouge or nick.
trace: Line formed by electron beam scanning from left to right on a video screen to generate a picture.
tracer: In leak testing, a gas that is sensed as it escapes from confinement.
transformation diagram: See time temperature transformation (TTT) diagram.
troland: A unit of retinal illuminance equal to that produced by a surface whose luminance is 1 nit
when the pupil measures 1 square millimeter. (1 nit = 1 candela per square meter.)
tubing string: Pipe with which oil or gas has contact as it is brought to the earth’s surface. Also called
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production string.
tungsten inert gas (TIC) welding: See gas tungsten arc welding.
ultraviolet borescope: See borescope, ultraviolet.
ultraviolet radiation: Electromagnetic radiation with wave- lengths ranging from about 4 to about
400 nm, between visible light and X-rays. Compare near ultraviolet radiation.
ultraviolet radiometer: A meter, usually calibrated at 365 nm, used in fluorescent liquid penetrant
and mag-netic particle testing to detect output of ultraviolet lamp.
undercut: Undesirable groove left unfilled by weld metal, created during welding and located in
base plate at the toe of a weld.
VT: Visual testing.
Vapor proof borescope: See waterproof borescope.
video: Pertaining to the transmission and display of images in an electronic format that can be
displayed on a cath-ode ray screen.
videoscope: Jargon for video borescope. See borescope, video.
vidicon tubes: Television tube that uses the photoconduc-tion method. Compare image orthicon.
vigilance decrement: Degradation of reliability during per-formance of visual activities over a period
of time. See also psychophysics.
visibility: The quality or state of being perceivable by the eye. In many outdoor applications, visibility
is defined in terms of the distance at which an object can be just perceived by the eye. In indoor
applications it usually is defined in terms of the contrast or size of a standard test object, observed
under standardized view conditions, having the same threshold as the given object.
vision: Perception by eyesight. See far vision, machine vision, mesopic vision, near vision, peripheral
vision, photopic vision, scotopic vision, speed of vision.
vision acuity: The ability to distinguish fine details visually. Quantitatively, it is the reciprocal of the
minimum angular separation in minutes of two lines of width sub-tending one minute of arc when the
lines are just resolvable as separate.
visual acuity: See vision acuity.
visual angle: The angle subtended by an object or detail at the point of observation. It usually is
measured in minutes of arc.
visual background noise: Formations on or signals from a test object that constitutes the
background to a disconti-nuity. The higher the level of visual background noise, the more difficult it
is to distinguish a discontinuity.
visual efficiency: Reliability of a visual system. The term visual efficiency uses 20/20 near vision
acuity as a base-line for 100 percent visual efficiency.
visual field: The locus of objects or points in space that can .be perceived when the head and eyes
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are kept fixed. The field may be monocular or binocular.
visual perception: The interpretation of impressions trans-mitted from the retina to the brain in
terms of informa-tion about a physical world displayed before the eye. Visual perception involves
anyone or more of the fol-lowing: recognition of the presence of something (object, aperture or
medium); identifying it; locating it in space; noting its relation to other things; identifying its
movement, color, brightness or form.
visual performance: The quantitative assessment of the per-formance of a visual task, taking into
consideration speed and accuracy.
visual purple: Chromoprotein called rhodopsin, the photo-sensitive pigment of rod vision. The
mechanism of con-verting light energy into nerve impulses is a photochemical process in the retina.
Chromoprotein is transformed by the action of radiant energy into a suc-cession of products, finally
yielding the protein called opsin plus the carotenoid known as retinene.
visual task: The appearance and immediate background of those details and objects that must be
seen for the per-formance of a given activity. The term visual task is a misnomer because it refers to
the visual display itself and not the task of extracting information from it.
visual testing: Method of nondestructive testing using elec-tromagnetic radiation at visible
frequencies.
voids: Hollow spots, depressions or cavities. See also discontinuity and dislocation.
wear: See erosion; rat’s tooth principle; wear, adhesive; and wear, fretting.
wear, adhesive: Degradation of a surface because of micro-welding and consequent fracture due to
the sliding of one surface against another. Types include fretting, galling and scuffing.
wear, fretting: Surface degradation caused by microwelding and microfractures on surfaces
rubbing each other. Also called chafing, friction oxidation and wear oxidation. See also cocoa and
false brinelling.
wear oxidation: See fretting wear.
weld size: Thickness of weld metal in a fillet weld the dis-tance from the root to the toe of the
largest isosceles light triangle that can be inscribed in a cross section of the weld.
weld throat: See throat.
welder’s flash: Clinical condition, specifically keratocon-junctivitis, commonly caused by
overexposure to ultra-violet radiation of welding arc.
white light: Light combining all frequencies in the visible spectrum.
work hardening: Increase in hardness accompanying plastic deformation of a metal. Usually
caused in a metal by repeated bending or flexing. Compare creep and recovery.
working standard: Work piece or energy source calibrated and used in place of expensive
reference standards. In the calibrating of photometers, the standard would be a light source.
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ASME SECTION –IX
QW-490 DEFINITIONS
Definitions of the more common terms relating to welding/brazing are defined in QW/QB-492. These
are identical to, or substantially in agreement with the definitions of the American Welding Society
document, AWS A3.0, Standard Welding Terms and Definitions. There are terms listed that are
specific to ASME Section IX and are not presently defined in AWS A3.0. Several definitions have
been modified slightly from A3.0 so as to better define the context/intent as used in ASME Section IX.
QW/QB-492 Definitions
arc seam weld — a seam weld made by an arc welding process
arc spot weld — a spot weld made by an arc welding process
arc strike — any inadvertent discontinuity resulting from an arc, consisting of any localized remelted
metal, heat-affected metal, or change in the surface profile of any metal object. The arc may be caused
by arc welding electrodes, magnetic inspection prods, or frayed electrical cable.
arc welding — a group of welding processes wherein coalescence is produced by heating with an arc
or arcs, with or without the application of pressure, and with or without the use of filler metal
as-brazed — adj. pertaining to the condition of brazements after brazing, prior to any subsequent
thermal, mechanical, or chemical treatments
as-welded — adj. pertaining to the condition of weld metal, welded joints, and weldments after
welding but prior to any subsequent thermal, mechanical, or chemical treatments
backgouging — the removal of weld metal and base metal from the weld root side of a welded joint to
facilitate complete fusion and complete joint penetration upon subsequent welding from that side
backhand welding — a welding technique in which the welding torch or gun is directed opposite to the
progress of welding
backing — a material placed at the root of a weld joint for the purpose of supporting molten weld
metal so as to facilitate complete joint penetration. The material may or may not fuse into the joint. See
retainer.
backing gas — a gas, such as argon, helium, nitrogen, or reactive gas, which is employed to exclude
oxygen from the root side (opposite from the welding side) of weld joints
base metal — the metal or alloy that is welded, brazed, or cut
bond line (brazing and thermal spraying) — the cross section of the interface between a braze or
thermal spray deposit and the substrate
braze — a joint produced by heating an assembly to suitable temperatures and by using a filler metal
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having a liquidus above 840°F and below the solidus of the base materials. The filler metal is
distributed between the closely fitted surfaces of the joint by capillary action.
brazer — one who performs a manual or semiautomatic brazing operation
brazing — a group of metal joining processes which produces coalescence of materials by heating
them to a suitable temperature, and by using a filler metal having a liquidus above 840°F and below
the solidus of the base materials. The filler metal is distributed between the closely fitted surfaces of
the joint by capillary action.
brazing, automatic — brazing with equipment which performs the brazing operation without constant
observation and adjustment by a brazing operator. The equipment may or may not perform the loading
and unloading of the work.
brazing, block (BB) — a brazing process that uses heat from heated blocks applied to the joint. This is
an obsolete or seldom used process.
brazing, dip (DB) — a brazing process in which the heat required is furnished by a molten chemical or
metal bath. When a molten chemical bath is used, the bath may act as a flux; when a molten metal bath
is used, the bath provides the filler metal.
brazing, furnace (FB) — a brazing process in which the workpieces are placed in a furnace and heated
to the brazing temperature
brazing, induction (IB) — a brazing process that uses heat from the resistance of the work pieces to
induced electric current
brazing, machine — brazing with equipment which performs the brazing operation under the constant
observation and control of a brazing operator. The equipment may or may not perform the loading and
unloading of the work.
brazing, manual — a brazing operation performed and controlled completely by hand. See automatic
brazing and machine brazing.
brazing, resistance (RB) — a brazing process that uses heat from the resistance to electric current flow
in a circuit of which the workpieces are a part
brazing, semiautomatic — brazing with equipment which controls only the brazing filler metal feed.
The advance of the brazing is manually controlled.
brazing, torch (TB) — a brazing process that uses heat from a fuel gas flame
brazing operator — one who operates machine or automatic brazing equipment
brazing temperature — the temperature to which the base metal(s) is heated to enable the filler metal
to wet the base metal(s) and form a brazed joint
brazing temperature range — the temperature range within which brazing can be conducted
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build-up of base metal/restoration of base metal thickness — this is the application of a weld material
to a base metal so as to restore the design thickness and/or structural integrity. This build-up may be
with a chemistry different from the base metal chemistry which has been qualified via a standard butt
welded test coupon. Also, may be called base metal repair or buildup.
butt joint — a joint between two members aligned approximately in the same plane
buttering — the addition of material, by welding, on one or both faces of a joint, prior to the
preparation of the joint for final welding, for the purpose of providing a suitable transition weld
deposit for the subsequent completion of the joint
clad brazing sheet — a metal sheet on which one or both sides are clad with brazing filler metal
coalescence — the growing together or growth into one body of the materials being joined
complete fusion — fusion which has occurred over the entire base material surfaces intended for
welding, and between all layers and beads
composite — a material consisting of two or more discrete materials with each material retaining its
physical identity
consumable insert — filler metal that is placed at the joint root before welding, and is intended to be
completely fused into the root to become part of the weld
contact tube — a device which transfers current to a continuous electrode
corner joint — a joint between two members located approximately at right angles to each other in the form of an L
coupon — see test coupon
crack — a fracture-type discontinuity characterized by a sharp tip and high ratio of length and width to opening
displacement
defect — a discontinuity or discontinuities that by nature or accumulated effect (for example, total
crack length) render a part or product unable to meet minimum applicable acceptance standards or
specifications. This term designates rejectability. See also discontinuity and flaw.
direct current electrode negative (DCEN) — the arrangement of direct current arc welding leads in
which the electrode is the negative pole and the workpiece is the positive pole of the welding arc
direct current electrode positive (DCEP) — the arrangement of direct current arc welding leads in
which the electrode is the positive pole and the workpiece is the negative pole of the welding arc
discontinuity — an interruption of the typical structure of a material, such as a lack of homogeneity
in its mechanical, metallurgical, or physical characteristics. A discontinuity is not necessarily a
defect. See also defect and flaw.
double-welded joint — a joint that is welded from both sides
double-welded lap joint — a lap joint in which the overlapped edges of the members to be joined
are welded along the edges of both members
dwell — the time during which the energy source pauses at any point in each oscillation
electrode, arc welding — a component of the welding circuit through which current is conducted
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electrode, bare — a filler metal electrode that has been produced as a wire, strip, or bar with no
coating or covering other than that incidental to its manufacture or preservation
electrode, carbon — a nonfiller material electrode used in arc welding and cutting, consisting of a
carbon or graphite rod, which may be coated with copper or other materials
electrode, composite — a generic term of multicomponent filler metal electrodes in various
physical forms, such as stranded wires, tubes, and covered electrodes
electrode, covered — a composite filler metal electrode consisting of a core of a bare electrode or
metal-cored
electrode to which a covering sufficient to provide a slag layer on the weld metal has been applied.
The covering may contain materials providing such functions as shielding from the atmosphere,
deoxidation, and arc stabilization, and can serve as a source of metallic additions to the weld.
electrode, electroslag welding — a filler metal component of the welding circuit through which
current is conducted between the electrode guiding member and the molten slag
NOTE: Bare electrodes and composite electrodes as defined under arc welding electrode are used
for electroslag welding. A consumable guide may also be used as part of the electroslag welding
electrode system.
electrode, emissive — a filler metal electrode consisting of a core of a bare electrode or a composite
electrode to which a very light coating has been applied to produce a stable arc
electrode, flux-cored — a composite filler metal electrode consisting of a metal tube or other
hollow configuration containing ingredients to provide such functions as shielding atmosphere,
deoxidation, arc stabilization, and slag formation. Alloying materials may be included in the core.
External shielding may or may not be used.
electrode, lightly coated — a filler metal electrode consisting of a metal wire with a light coating
applied subsequent to the drawing operation, primarily for stabilizing the arc
electrode, metal — a filler or nonfiller metal electrode used in arc welding and cutting that consists
of a metal wire or rod that has been manufactured by any method and that is either bare or covered
electrode, metal-cored — a composite filler metal electrode consisting of a metal tube or other
hollow configuration containing alloying ingredients. Minor amounts of ingredients providing such
functions as arc stabilization and fluxing of oxides may be included. External shielding gas may or
may not be used.
electrode, resistance welding — the part of a resistance welding machine through which the
welding current and, in most cases, force are applied directly to the workpiece. The electrode may
be in the form of a rotating wheel, rotating roll, bar, cylinder, plate, clamp, chuck, or modification
thereof.
electrode, stranded — a composite filler metal electrode consisting of stranded wires which may
mechanically enclose materials to improve properties, stabilize the arc, or provide shielding
electrode, tungsten — a nonfiller metal electrode used in arc welding, arc cutting, and plasma
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spraying, made principally of tungsten
face feed — the application of filler metal to the face side of a joint
ferrite number — an arbitrary, standardized value designating the ferrite content of an austenitic
stainless steel weld metal. It should be used in place of percent ferrite or volume percent ferrite on a
direct one-to-one replacement basis. See the latest edition of AWS A4.2, Standard Procedures for
Calibrating Magnetic Instruments to Measure the Delta Ferrite Content of Austenitic Stainless Steel
Weld Metal.
filler metal — the metal or alloy to be added in making a welded, brazed, or soldered joint
filler metal, brazing — the metal or alloy used as a filler metal in brazing, which has a liquidus
above 450°C (840°F) and below the solidus of the base metal
filler metal, powder — filler metal in particle form
filler metal, supplemental — in electroslag welding or in a welding process in which there is an arc
between one or more consumable electrodes and the workpiece, a powder, solid, or composite
material that is introduced into the weld other than the consumable electrode(s)
fillet weld — a weld of approximately triangular cross section joining two surfaces approximately at
right angles to each other in a lap joint, tee joint, or corner joint
flaw — an undesirable discontinuity. See also defect.
flux (welding/brazing) — a material used to dissolve, prevent, or facilitate the removal of oxides or
other undesirable surface substances. It may act to stabilize the arc, shield the molten pool, and may
or may not evolve shielding gas by decomposition.
flux, active (SAW) — a flux from which the amount of elements deposited in the weld metal is
dependent upon the welding conditions, primarily arc voltage
flux, alloy (SAW) — a flux which provides alloying elements in the weld metal deposit
flux, neutral (SAW) — a flux which will not cause a significant change in the weld metal
composition when there is a large change in the arc voltage
flux cover — metal bath dip brazing and dip soldering. A layer of molten flux over the molten filler
metal bath.
forehand welding — a welding technique in which the welding torch or gun is directed toward the
progress of welding
frequency — the completed number of cycles which the oscillating head makes in 1 min or other
specified time increment
fuel gas — a gas such as acetylene, natural gas, hydrogen, propane, stabilized methylacetylene
propadiene, and other fuels normally used with oxygen in one of the oxyfuel processes and for
heating
fused spray deposit (thermal spraying) — a self-fluxing thermal spray deposit which is
subsequently heated to coalescence within itself and with the substrate
fusion (fusion welding) — the melting together of filler metal and base metal, or of base metal only, to produce a weld
fusion face — a surface of the base metal that will be melted during welding
fusion line — a non-standard term for weld interface
gas backing — see backing gas
globular transfer (arc welding) — a type of metal transfer in which molten filler metal is
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transferred across the arc in large droplets
groove weld — a weld made in a groove formed within a single member or in the groove between
two members to be joined. The standard types of groove weld are as follows:
square groove weld
single-Vee groove weld
single-bevel groove weld
single-U groove weld
single-J groove weld
single-flare-bevel groove weld
single-flare-Vee groove weld
double-Vee groove weld
double-bevel groove weld
double-U groove weld
double-J groove weld
double-flare-bevel groove weld
double-flare-Vee groove weld
heat-affected zone — that portion of the base metal which has not been melted, but whose
mechanical properties or microstructures have been altered by the heat of welding or cutting
inter-pass temperature — the highest temperature in the weld joint immediately prior to welding, or
in the case of multiple pass welds, the highest temperature in the section of the previously deposited
weld metal, immediately before the next pass is started
joint — the junction of members or the edges of members which are to be joined or have been
joined
joint penetration — the distance the weld metal extends from the weld face into a joint, exclusive of
weld reinforcement
keyhole welding — a technique in which a concentrated heat source penetrates partially or
completely through a workpiece, forming a hole (keyhole) at the leading edge of the weld pool. As
the heat source progresses, the molten metal fills in behind the hole to form the weld bead.
lap or overlap — the distance measured between the edges of two plates when overlapping to form
the joint lap joint — a joint between two overlapping members in parallel planes
lower transformation temperature — the temperature at which austenite begins to form during
heating
melt-in — a technique of welding in which the intensity of a concentrated heat source is so adjusted
that a weld pass can be produced from filler metal added to the leading edge of the molten weld
metal
oscillation — for a machine or automatic process, an alternating motion relative to the direction of
travel of welding, brazing, or thermal spray device. See also weave bead.
overlay — a non-standard term, used in Section IX, for surfacing. See hard-facing and corrosionresistant overlay.
overlay, corrosion-resistant weld metal — deposition of one or more layers of weld metal to the
surface of a base material in an effort to improve the corrosion resistance properties of the surface.
This would be applied at a level above the minimum design thickness as a nonstructural component
of the overall wall thickness.
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overlay, hard-facing weld metal — deposition of one or more layers of weld metal to the surface of
a material in an effort to improve the wear resistance properties of the surface. This would be
applied at a level above the minimum design thickness as a nonstructural component of the overall
wall thickness.
pass — a single progression of a welding or surfacing operation along a joint, weld deposit, or
substrate. The result of a pass is a weld bead or layer.
pass, cover — a final or cap pass(es) on the face of a weld
pass, wash — pass to correct minor surface aberrations and/or prepare the surface for
nondestructive testing
peel test —a destructive method of testing that mechanically separates a lap joint by peeling
peening — the mechanical working of metals using impact blows
performance qualification — the demonstration of a welder’s or welding operator’s ability to
produce welds meeting prescribed standards
plug weld — a weld made in a circular, or other geometrically shaped hole (like a slot weld) in one
member of a lap or tee joint, joining that member to the other. The walls of the hole may or may not
be parallel, and the hole may be partially or completely filled with weld metal. (A fillet-welded hole
or spot weld should not be construed as conforming to this definition.)
polarity, reverse — the arrangement of direct current arc welding leads with the work as the
negative pole and the electrode as the positive pole of the welding arc; a synonym for direct current
electrode positive
polarity, straight — the arrangement of direct current arc welding leads in which the work is the
positive pole and the electrode is the negative pole of the welding arc; a synonym for direct current
electrode negative
postbraze heat treatment — any heat treatment subsequent to brazing
postheating — the application of heat to an assembly after welding, brazing, soldering, thermal spraying, or thermal cutting
postweld heat treatment — any heat treatment subsequent to welding
powder — see filler metal, powder
preheat maintenance — practice of maintaining the minimum specified preheat temperature, or
some specified higher temperature for some required time interval after welding or thermal spraying
is finished or until post weld heat treatment is initiated
preheat temperature — the minimum temperature in the weld joint preparation immediately prior to
the welding; or in the case of multiple pass welds, the minimum temperature in the section of the
previously deposited weld metal, immediately prior to welding
preheating — the application of heat to the base metal immediately before a welding or cutting
operation to achieve a specified minimum preheat temperature
pulsed power welding — any arc welding method in which the power is cyclically programmed to
pulse so that effective but short duration values of a parameter can be utilized. Such short duration
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values are significantly different from the average value of the parameter. Equivalent terms are
pulsed voltage or pulsed current welding. See also pulsed spray welding.
pulsed spray welding—an arc welding process variation in which the current is pulsed to utilize the
advantages of the spray mode of metal transfer at average currents equal to or less than the globular
to spray transition current
rabbet joint — typical design is indicated in QB-462.1©, QB-462.4, QB-463.1©, and QB-463.2(a)
retainer — nonconsumable material, metallic or nonmetallic, which is used to contain or shape
molten weld metal. See backing.
seal weld — any weld designed primarily to provide a specific degree of tightness against leakage
seam weld — a continuous weld made between or upon overlapping members in which coalescence
may start and occur on the faying surfaces, or may have proceeded from the surface of one member.
The continuous weld may consist of a single weld bead or a series of overlapping spot welds. See
resistance welding.
short-circuiting transfer (gas metal-arc welding) —metal transfer in which molten metal from a
consumable electrode is deposited during repeated short circuits. See also globular transfer and
spray transfer.
single-welded joint — a joint welded from one side only
single-welded lap joint — a lap joint in which the overlapped edges of the members to be joined are
welded along the edge of one member only
slag inclusion — nonmetallic solid material entrapped in weld metal or between weld metal and
base metal
specimen — refer to test specimen
spot weld — a weld made between or upon overlapping members in which coalescence may start
and occur on the faying surfaces or may proceed from the outer surface of one member. The weld
cross section (plan view) is approximately circular.
spray-fuse — a thermal spraying technique in which the deposit is reheated to fuse the particles and
form a metallurgical bond with the substrate
spray transfer (arc welding) — metal transfer in which molten metal from a consumable electrode
is propelled axially across the arc in small droplets
stringer bead—a weld bead formed without appreciable weaving
surfacing — the application by welding, brazing, or thermal spraying of a layer(s) of material to a
surface to obtain desired properties or dimensions, as opposed to making a joint
tee joint (T) — a joint between two members located approximately at right angles to each other in
the form of a T
test coupon — a weld or braze assembly for procedure or performance qualification testing. The
coupon may be any product from plate, pipe, tube, etc., and may be a fillet weld, overlay, deposited
weld metal, etc.
test specimen — a sample of a test coupon for specific test. The specimen may be a bend test,
tension test, impact test, chemical analysis, macrotest, etc. A specimen may be a complete test
coupon, for example, in radiographic testing or small diameter pipe tension testing.
thermal cutting (TC) — a group of cutting processes that severs or removes metal by localized
melting, burning, or vaporizing of the workpieces
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throat, actual (of fillet) — the shortest distance from the root of a fillet weld to its face
throat, effective (of fillet) — the minimum distance from the fillet face, minus any convexity, to the
weld root. In the case of fillet welds combined with a groove weld, the weld root of the groove weld
shall be used.
throat, theoretical (of fillet) — the distance from the beginning of the joint root perpendicular to the
hypotenuse of the largest right triangle that can be inscribed within the cross-section of a fillet weld.
This dimension is based on the assumption that the root opening is equal to zero.
undercut— a groove melted into the base metal adjacent to the weld toe or weld root and left
unfilled by weld metal
upper transformation temperature — the temperature at which transformation of the ferrite to
austenite is completed during heating
usability — a measure of the relative ease of application of a filler metal to make a sound weld or
braze joint
weave bead — for a manual or semiautomatic process, a weld bead formed using weaving. See also
oscillation.
weaving — a welding technique in which the energy source is oscillated transversely as it
progresses along the weld path
weld — a localized coalescence of metals or nonmetals produced either by heating the materials to
the welding temperature, with or without the application of pressure, or by the application of
pressure alone and with or without the use of filler material
weld, autogenous — a fusion weld made without filler metal
weld bead — a weld deposit resulting from a pass. See stringer bead and weave bead.
weld face — the exposed surface of a weld on the side from which welding was done
weld interface — the interface between the weld metal and base metal in a fusion weld
weld metal — metal in a fusion weld consisting of that portion of the base metal and filler metal
melted during welding
weld reinforcement — weld metal on the face or root of a groove weld in excess of the metal
necessary for the specified weld size
weld size: groove welds — the depth of chamfering plus any penetration beyond the chamfering,
resulting in the strength carrying dimension of the weld
weld size: for equal leg fillet welds — the leg lengths of the largest isosceles right triangle which
can be inscribed within the fillet weld cross section
weld size: for unequal leg fillet welds — the leg lengths of the largest right triangle which can be
inscribed within the fillet weld cross section
welder — one who performs manual or semiautomatic welding
welding, arc stud (SW) — an arc welding process that uses an arc between a metal stud, or similar
part, and the other workpiece. The process is used without filler metal, with or without shielding gas
or flux, with or without partial shielding from a ceramic or graphite ferrule surrounding the stud,
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and with the application of pressure after the faying surfaces are sufficiently heated.
welding, automatic — welding with equipment which performs the welding operation without
adjustment of the controls by a welding operator. The equipment may or may not perform the
loading and unloading of the work. See machine welding.
welding, consumable guide electroslag — an electroslag welding process variation in which filler
metal is supplied by an electrode and its guiding member
welding, electrogas (EGW) — an arc welding process that uses an arc between a continuous filler metal
electrode and the weld pool, employing approximately vertical welding progression with retainers to confine
the weld metal. The process is used with or without an externally supplied shielding gas and without the
application of pressure. Shielding for use with solid or metal-cored electrodes is obtained from a gas or gas
mixture. Shielding for use with flux-cored electrodes may or may not be obtained from an externally
supplied gas or gas mixture.
welding, electron beam (EBW) — a welding process that produces coalescence with a concentrated
beam composed primarily of high velocity electrons, impinging on the joint. The process is used
without shielding gas and without the application of pressure.
welding, electroslag (ESW) — a welding process producing coalescence of metals with molten slag
which melts the filler metal and the surfaces of the work to be welded. The molten weld pool is
shielded by this slag which moves along the full cross section of the joint as welding progresses.
The process is initiated by an arc which heats the slag. The arc is then extinguished and the
conductive slag is maintained in a molten condition by its resistance to electric current passing
between the electrode and the work. See electroslag welding electrode and consumable guide
electroslag welding.
welding, flux-cored arc (FCAW) — a gas metal-arc welding process that uses an arc between a
continuous filler metal electrode and the weld pool. The process is used with shielding gas from a
flux contained within the tubular electrode, with or without additional shielding from an externally
supplied gas, and without the application of pressure.
welding, friction (FRW) — a solid state welding process that produces a weld under compressive
force contact of workpieces rotating or moving relative to one another to produce heat and
plastically displace material from the faying surfaces
welding, friction, inertia and continuous drive — processes and types of friction welding (solid
state welding process) wherein coalescence is produced after heating is obtained from mechanically
induced sliding motion between rubbing surfaces held together under pressure. Inertia welding
utilizes all of the kinetic energy stored in a revolving flywheel spindle system. Continuous drive
friction welding utilizes the energy provided by a continuous drive source such as an electric or
hydraulic motor.
welding, gas metal-arc (GMAW) — an arc welding process that uses an arc between a continuous
filler metal electrode and the weld pool. The process is used with shielding from an externally
supplied gas and without the application of pressure.
welding, gas metal-arc, pulsed arc (GMAW-P) — a variation of the gas metal-arc welding process
in which the current is pulsed. See also pulsed power welding.
welding, gas metal-arc, short-circuiting arc (GMAWS) — a variation of the gas metal-arc welding
process in which the consumable electrode is deposited during repeated short circuits. See also
short-circuiting transfer.
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welding, gas tungsten-arc (GTAW) — an arc welding process which produces coalescence of
metals by heating them with an arc between a tungsten (nonconsumable) electrode and the work.
Shielding is obtained from a gas or gas mixture. Pressure may or may not be used and filler metal
may or may not be used. (This process has sometimes been called TIG welding, a nonpreferred
term.)
welding, gas tungsten-arc, pulsed arc (GTAW-P) — a variation of the gas tungsten-arc welding
process in which the current is pulsed. See also pulsed power welding.
welding, induction (IW) — a welding process that produces coalescence of metals by the heat obtained
from resistance of the workpieces to the flow of induced high frequency welding current with or without the
application of pressure. The effect of the high-frequency welding current is to concentrate the welding heat at
the desired location
welding, laser beam (LBW) — a welding process which produces coalescence of materials with the
heat obtained from the application of a concentrated coherent light beam impinging upon the
members to be joined
welding, machine — welding with equipment which performs the welding operation under the
constant observation and control of a welding operator. The equipment may or may not perform the
loading and unloading of the work. See automatic welding.
welding, manual — welding wherein the entire welding operation is performed and controlled by
hand
welding, operator — one who operates machine or automatic welding equipment
welding, oxyfuel gas (OFW) — a group of welding processes which produces coalescence by
heating materials with an oxyfuel gas flame or flames, with or without the application of pressure,
and with or without the use of filler metal
welding, plasma-arc (PAW) — an arc welding process which produces coalescence of metals by
heating them with a constricted arc between an electrode and the workpiece (transferred arc), or the
electrode and the constricting nozzle (nontransferred arc). Shielding is obtained from the hot,
ionized gas issuing from the torch orifice which may be supplemented by an auxiliary source of
shielding gas. Shielding gas may be an inert gas or a mixture of gases. Pressure may or may not be
used, and filler metal may or may not be supplied.
welding, projection (PW) — a resistance welding process that produces coalescence by the heat
obtained from the resistance of the flow of welding current. The resulting welds are localized at
predetermined points by projections, embossments, or intersections. The metals to be joined lap
over each other.
welding, resistance (RW) — a group of welding processes that produces coalescence of the faying
surfaces with the heat obtained from resistance of the workpieces to the flow of the welding current
in a circuit of which the workpieces are a part, and by the application of pressure
welding, resistance seam (RSEW) — a resistance welding process that produces a weld at the faying
surfaces of overlapped parts progressively along a length of a joint. The weld may be made with
overlapping weld nuggets, a continuous weld nugget, or by forging the joint as it is heated to the
welding temperature by resistance to the flow of the welding current.
welding, resistance spot (RSW) — a resistance welding process that produces a weld at the faying
surfaces of a joint by the heat obtained from resistance to the flow of welding current through the
workpieces from electrodes that serve to concentrate the welding current and pressure at the weld
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area
welding, resistance stud — a resistance welding process wherein coalescence is produced by the
heat obtained from resistance to electric current at the interface between the stud and the workpiece,
until the surfaces to be joined are properly heated, when they are brought together under pressure
welding, semiautomatic arc — arc welding with equipment which controls only the filler metal
feed. The advance of the welding is manually controlled.
welding, shielded metal-arc (SMAW) — an arc welding process with an arc between a covered
electrode and the weld pool. The process is used with shielding from the decomposition of the
electrode covering, without the application of pressure, and with filler metal from the electrode
welding, stud — a general term for the joining of a metal stud or similar part to a workpiece.
Welding may be accomplished by arc, resistance, friction, or other suitable process with or without
external gas shielding.
welding, submerged-arc (SAW) — an arc welding process that uses an arc or arcs between a bare
metal electrode or electrodes and the weld pool. The arc and molten metal are shielded by a blanket
of granular flux on the workpieces. The process is used without pressure and with filler metal from
the electrode and sometimes from a supplemental source (welding rod, flux, or metal granules).
weldment — an assembly whose constituent parts are joined by welding, or parts which contain
weld metal overlay
Glossary -Galvanizing
AASHTO - American Association of State Highway and Transportation Officials
Abrasion-resistance - the ability of the galvanized coating to resist damage caused by contact with hard, rough, or coarse
media or objects
Abrasive blasting - the process of using a forceful stream of particles, available in varying hardness, to remove residue and
contaminants from steel surfaces to prepare for galvanizing and/or to profile steel surfaces for paint application
Adherence - the act, action, or quality of zinc bonding to steel, measured in pounds per square inch (psi) or mega pascals
(MPa)
Aggressive environment - an environment that is particularly corrosive
Alloy layers - the interior layers of the galvanized coating comprised of iron/zinc intermetallics formed when molten zinc
reacts with iron in the steel
Aluminum - element found in the galvanizing bath (added to molten zinc through a product commonly called “brightener
bar”) that gives the hot-dip galvanized coating a shiny appearance
Aluminum-killed steel - steel treated with aluminum as an oxidizing agent in order to reduce the oxygen content to such a
level that no reaction occurs between carbon and oxygen during solidification
Anode - the electrode of an electrolytic cell at which corrosion (oxidation) occurs, positive current flows from the anode
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through the electrolyte to the cathode; with respect to hot-dip galvanizing, anode refers to zinc, which corrodes sacrificially
to protect steel
Anodic - exhibiting the properties of an anode; zinc is anodic to steel
Application - the act of putting to use; specifically, the use to which galvanized steel will be put
Ash - solid byproduct formed in the galvanizing process, consisting primarily of zinc oxides, that remains on the surface of
the kettle; commonly referred to as “skimmings”
Ash inclusions - ash or skimmings carried out of the kettle on parts; ash inclusions remain solidified in the coating
Assemblies - the fitting together of manufactured parts into a complete structure, machine, or unit of a machine; assemblies
sometimes require specific design features in order to be optimally galvanized
ASTM - American Society of Testing and Materials
Atmosphere - a surrounding influence or environment that affects the rate of corrosion; frequency and amount of moisture,
humidity, chlorides, sulfides, and wind are some of the atmospheric components affecting corrosion rates
AZA - American Zinc Association
Barrier protection - the protection provided by inhibiting oxidation (rust) by an insoluble top- coating such as zinc, which
isolates steel from any electrolytes that would assist the corrosion process
Bond strength - the strength with which two or more items are joined; the resistance that must be overcome in order to
separate the joined materials, e.g. steel and zinc-iron alloy layers of the galvanized coating, or galvanized reinforcing steel
and concrete
Brown staining - reaction between exposed intermetallic layers (specifically the iron portion of the layers) and oxygen,
resulting in surface color changes from gray to brown
Bracing - metal that is attached to a fabrication prior to galvanizing in order to provide support so that the steel does not
change shape during heating and cooling; can be temporary or permanent
Carbon - naturally occurring element commonly found in steel
Cast iron - a generic term for a large family of cast ferrous alloys
Cast steel - steel in the form of an object at or near finished shape, produced by pouring molten steel into a mold
Casting - an object at or near finished shape obtained by solidification of a metal or alloy in a mold
Cathode - the electrode of an electrolytic cell at which reduction occurs; positive current flows from the anode (zinc)
through the electrolyte to the cathode (steel)
Cathodic - exhibiting properties of a cathode; steel is cathodic in relation to zinc
Cathodic protection - reduction or prevention of corrosion of a metal surface by making it a cathode in an electrolytic cell,
using either a galvanic or impressed current; zinc cathodically protects steel, i.e. sacrificially giving up electrons to protect
the steel from corrosive attack
Caustic cleaning - the cleaning of steel in a solution with high alkalinity; in the hot-dip galvanizing process, organic
residues are removed by immersing steel in a tank of caustic solution
Centrifuging - the process of removing excess zinc from small hot-dip galvanized parts by placing them in a perforated,
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rapidly spinning cylindrical container
Chemical cleaning - the process of immersing steel in chemical solvents to remove (dissolve) residues that would otherwise
prevent the galvanized coating from forming
Chemical composition - the makeup of steel, broken down into naturally occurring elements; usually carbon, manganese,
phosphorous, silicon as primary elements
Chromate quenching -treating metal in a tank of containing a solution of chronium compounds to produce a conversion
coating consisting of trivalent and hexavalent chromium compounds; chromate passivations sometimes are used on
galvanized reinforcing bar to control reactions between zinc and concrete while the concrete cures, particularly the
hydrogen evolution that adversely affects bonding; chromate quenching other galvanized articles prevents the formation of
wet storage stain
Chromating - chromate quenching a galvanized article
Cleaning - the process of chemically or mechanically removing unwanted residue or contaminants (mill scale, rust, dirt, oil)
from the surface of a steel article prior to galvanizing
Cleaning solutions - liquids used to remove unwanted residue or contaminants (mill scale, rust, dirt, oil) from the surface of
steel prior to galvanizing, typically alkalai caustic solution, hydrochloric or sulfuric acid, and zinc ammonium chloride flux
solution
Coating thickness - the thickness of the zinc coating, measured in mils (0.001 inches) or micrometers (10-6 meters)
Cold-galvanizing - See zinc-rich paint
Cold-rolled steel - steel that has been produced from a hot-rolled pickled coil and given substantial cold reduction at room
temperature; cold-rolled steel is characterized by improved surface smoothness, greater uniformity in thickness and
improved mechanical properties when compared to hot-rolled steel
Cold-working -bending or forming ambient-temperature steel; this action induces stresses that may be released during the
galvanizing process
Containment - the act, process, or means to keep within prescribed limits
Contraction - the shrinkage of steel due to cooling of the part after removal from the galvanizing kettle
Corrosion - the chemical or electrochemical reaction between a material and its environment that produces a deterioration
of the material and its properties; zinc chemically reacts with elements in the atmosphere, thereby sacrificially corroding to
prevent underlying steel corrosion
Cracking (concrete) - the breaking of concrete due to the expansive forces caused by the formation of iron-oxide corrosion
products on unprotected reinforcement bars
CSA - Canadian Standards Association
Delta layer - the second layer of zinc-iron alloy growth from the base steel formed during the galvanizing process; the Delta
layer’s chemical composition is approximately 90% zinc and 10% iron; the Delta layer is 60% harder than the base steel it
protects from abrasion and corrosion
Design - to create, fashion, execute, or construct steel according to plan so that it will yield a quality hot-dip galvanized
coating
Diamond Pyramid Number -(DPN) system of assigning values to metals quantifying their hardness
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Dissimilar metals - two or more different metals in contact; due to varying surface conductivity, one or more metals may
experience accelerated corrosion; because zinc is high in the galvanic series (see Galvanic Series of Metals), it
preferentially corrodes to protect most dissimilar metals
Distortion - any deviation from an original size, shape or contour that occurs when the application of heat during the
galvanizing process releases stress from the steel induced in the fabrication process or during the steel-making process;
distortion is of concern when galvanizing asymmetric structural shapes and/or fabrications
Double-dipping (see progressive-dipping) - the act of dipping steel, too large in one dimension to completely fit into the
galvanizing kettle, more than once in cleaning solutions and molten zinc metal in order to produce a coating that covers the
entire surface of the steel
Drainage - the act, process, or mode of becoming emptied or freed of cleaning solutions and/or zinc
Dross - byproduct of the galvanizing process that forms by reactions between zinc and loose particles of iron; dross may
exist at all depths of the kettle, but usually sinks to the bottom
Dross inclusions - dross that is carried out on the work upon removal from the galvanizing kettle
Drossing - removing dross buildup from the bottom of the kettle
Dry galvanizing - dipping steel in an aqueous zinc ammonium chloride solution and then thoroughly drying before
immersing in the molten zinc bath
Ductile iron - molten iron treated with an element such as magnesium or cerium to induce a measurable degree of ductility
to the metal; these additives do not affect galvanizeability
Ductility - the ability of a material to be formed without fracturing; galvanized steel is ductile within certain recommended
bending radii
Duplex systems - galvanized steel that has been coated with an additional corrosion-inhibiting product, typically liquid or
powder paint; the two separate coating systems work synergistically to provide enhanced corrosion protection
Electrical isolation - separating two conductive materials from electrical contact; galvanized steel is sometimes electrically
isolated in order to prevent rapid consumption of the zinc coating
Electrode - see anode or cathode, whichever is appropriate
Electrolyte - an ionized chemical substance or mixture, usually liquid, that will conduct electric currents; water, soil, or
many chemical solutions
Embrittlement - reduction in the normal ductility of a metal due to a physical or chemical change that may occur when coldworked steel is immersed in molten zinc in the galvanizing kettle
Environment - the complex of physical, chemical, and biotic factors (climate, soil, living things) that act upon metal and
ultimately affect the corrosion rate
Eta layer - the fourth, outer layer of the galvanized coating solely comprised of zinc
Excess zinc - extra amounts of zinc that may accumulate on the steel because of chemical composition of the steel or the
profile/design of the steel and/or fabrication
External venting - holes that prevent high-pressure gas buildup in enclosed fabrications dipped in the molten zinc of the
galvanizing bath
Fabrication - steel configurations constructed from diverse and usually standardized steel members
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Fasteners - manufactured steel products (bolts, nails, etc.) used to connect two or more steel members
Fatigue strength - the maximum stress that can be sustained for a specified number of cycles without failure, the stress
being completely reversed within each cycle unless otherwise stated
Faying surfaces - the surface of a piece of metal (or a member) in contact with another to which it is or is to be joined
Ferrous metals - metals containing iron
Finishing - final stages of inspection and preparing galvanized steel so that it complies with specification(s)
Flux - chemicals used to protect steel from oxidation prior to entering the molten zinc-containing kettle
Flux inclusions - Flux carried out onto the work from the top flux blanket incorporated in the ‘wet’ process; occurs only in
the ‘wet’ galvanizing process
Fluxing - the process by which steel is dipped in aqueous zinc ammonium chloride to remove undesirable substances and to
protect it from further oxide formation prior to entering the galvanizing bath
Forged steels - steels formed by plastically deforming metal, usually hot, into desired shapes with compressive force, with
or without dies
Fresh water - sodium-chloride-free water, especially when considered as a natural resource
Galling - a condition whereby excessive friction between high spots on two different steel parts results in localized welding
Galvanic corrosion - corrosion associated with the current of a galvanic cell consisting of two dissimilar conductors in an
electrolyte solution, or two similar conductors in dissimilar electrolytes
Galvanic Series of Metals - a listing of metals and alloys arranged according to their relative electrode potentials in a
specified environment; indicates what metal(s) will corrode first when two or more metals are in contact
Galvanizing - the act of coating steel with zinc in order to provide barrier and cathodic protection from corrosion
Galvanizing temperature - the temperature at which the molten zinc bath is kept in order to react with the steel; typically,
this temperature is between 830 F (443 C) and 850 F (454 C)
Gamma layer - the first layer of zinc-iron alloy growth from the base steel formed during the galvanizing process; the
chemical composition of this layer is approximately 75% zinc and 25% iron; the Gamma layer has a DPN of 250, compared
to the base steel’s DPN of 159
Grinding - mechanically removing material from a work-piece with a grinding wheel or abrasive belt
Grit-blasting - abrasive blasting with small irregular pieces of steel, malleable cast iron or hard nonmetallic materials
Handling - the process by which steel articles are carried throughout the galvanizing facility, by chain, wire, hook, or
racked in a fixture
Hardness - resistance of metal to plastic deformation, usually by indentation; the term may also refer to stiffness or temper,
or to resistance to scratching, abrasion or cutting
Heat-treating - heating and cooling a solid metal or alloy in such a way as to obtain desired conditions or properties
High-strength low-alloy (HSLA) steel - steels with a high yield point and low percentages (<1.25%) of other alloying
components
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High-strength steel - steels that have a tensile strength over 150 ksi (1100 MPa)
Hinges - a jointed or flexible device upon which a door, lid, or other swinging part turns
Holding devices - fixtures used to connect fabrications/parts to be galvanized to handling equipment in the galvanizing
facility
Hot-rolled steel - steel deformed plastically at such a temperature and strain-rate that re- crystallization takes place
simultaneously with the deformation, thus avoiding strain-hardening; this is the most common type of steel galvanized
Hydrochloric acid - solution used in the cleaning stages of the galvanizing process and consisting of one hydrogen ion and
one chloride ion (chemical formula: HCl) in mixture with water
Hydrogen embrittlement - a condition of low ductility in metals resulting from the absorption of hydrogen
Identification - marking/labeling steel so that different customer products can be distinguished from one another after
galvanizing
Impact resistance - the ability to avoid damage due to contact with a forceful motion or object; galvanized coatings’
uppermost, pure zinc Eta layer is relatively soft and absorbs impact shock, protecting the underlying alloy layers
Impurities - elements or compounds in which their presence is undesired
Inspection - coating thickness and surface condition verifications
Intermetallic - interior layers of the galvanized coating that have distinct proportions of the alloying metals iron and zinc; e.
g. Delta, Gamma & Zeta layers
Internal stress - also known as residual stress, stress present in a steel member or fabrication that is free of external forces or
thermal gradients
Internal venting - holes on the inside of enclosed fabrications that allow cleaning solutions, zinc, and any gases to freely
flow throughout the structure
ISO: International Organization for Standardization. A network of national standards institutes from 140 countries working
in partnership with international organizations, governments, industry, business and consumer representatives. The source
of ISO 9000 and more than 13,000 international standards for business, government and society. Click here to view the
Foreign Specification & ASTM Cross-Reference Chart.
IZA - International Zinc Association
Kettle - molten zinc-filled tank where the metallurgical bonding of zinc and steel takes place
Lifting points - connectors (sometimes temporary) directly on the steel article that aid the galvanizer in handling the article
throughout the galvanizing process
Machined surfaces - surfaces cleaned or otherwise altered by a power-driven machine
Malleable iron - iron that permits plastic deformation in compression without rupture
Manganese - a grayish white, usually hard and brittle metallic element found in steel that resembles iron but is not magnetic
Masking - using a material to produce intentionally ungalvanized areas, typically used in areas that are to be welded, on
faying surfaces, or areas where the galvanized coating is not necessary for uniform corrosion protection
Matte - dull, lacking or deprived of shine; matte-gray galvanized appearance may result from steel chemistry or may be
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intentionally induced when the galvanized steel’s use defines reflectivity limits
Mechanical cleaning - removing residues or impurities from steel using mechanical force such as grinding or sand blasting
Mechanical properties - the properties of a material that reveal its elastic and inelastic behavior when force is applied,
thereby indicating its suitability for mechanical applications; for example, elasticity, tensile strength, elongation, hardness,
and fatigue limit
Metalizing - forming a metallic coating by atomized spraying with molten zinc or by vacuum deposition; also called spray
metalizing; applying an electrically conductive metallic layer to the surface of another material
Metallurgical bond - the bonding of iron/zinc intermetallic layers to the base steel
Mill lacquer - organic protective coating applied to steel parts, usually pipes or tubes, to protect the parts during shipping;
this material cannot be removed by the usual galvanized cleaning methods
Mill scale - a heavy, imbedded iron oxide layer formed during hot fabrication or heat- treatment of steels
Nickel - common element found in the galvanizing kettle to suppress the reactivity of silicon and phosphorus in the steel
Notching - cutting out various shapes from the edge of a strip, blank or part
Organic contaminants - surface impurities (dirt, grease, oil, paint markings) that will hinder the formation of the galvanized
coating, usually removed in the caustic cleaning stages of the galvanizing process
Overlapping surfaces - created when one area of a metal structure is covered by the surface of another metal structure
Overtapping - cutting female fastener threads of nuts or threaded holes larger than standard to account for the increased
diameter of the galvanized (male) mating part
Passivation -changing chemically active metal surfaces to a much less reactive state (see phosphating and chromating)
Patina - relatively insoluble zinc carbonate layer that forms as the galvanized coating weathers, providing added corrosion
protection and abrasion resistance
Phosphating - forming an adherent phosphate coating on a metal by immersion in a suitable aqueous phosphate solution,
commonly used to promote better adhesion of paint to galvanized steel
Phosphorus - naturally occurring element commonly found in steel, particularly reactive in molten zinc metal
Pickling - removing surface oxides from metals by immersion in ambient temperature, dilute hydrochloric acid or hot (180
F/82 C) sulfuric acid
Pinhole - small hole left in a weld area that allows low viscosity liquids to enter and become pressurized under the high
temperature conditions of the molten zinc bath
Pitch diameter - the diameter of a fastener cylinder prior to threading
Pitted surfaces - areas of metal where small, sharp cavities exist, usually formed by corrosion
Post-treatment - subjecting the steel to specific processes after it has been galvanized (see quenching, phosphating, and
chromating)
Pre-flux - the process of fluxing steel before it enters the galvanizing kettle as opposed to using a top-flux layer, which
would be located on top of the molten zinc in the kettle
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Pre-treatment - subjecting steel to specific processes before galvanizing
Progressive dipping - the act of dipping steel more than once in cleaning solutions and molten zinc metal in order to
produce a coating that covers the entire surface of the steel; commonly done when the steel article/fabrication is too large to
fit entirely into the kettle in one dip
Quenching - rapid cooling by dipping galvanized steel in a tank filled with a liquid solution; usually water or a dilute
chromate or phosphate solution
Racking - the process of arranging articles on a rack in order to transport them more efficiently through the galvanizing
process
Reinforcing steel - steel embedded in concrete to increase the concrete’s load strength
Repair - performing finishing work after galvanizing in order to meet standards or specifications, or coating areas of steel
that have been exposed due to post-galvanizing fabrication, installation, or extremely rough handling
Residue - contaminants (oil, grease, dirt, rust, mill scale) that unless removed, will prevent complete galvanizing of the
steel surface
Return current path - the path through which the current in an electric cell returns to the source
Rinsing - removing any active solution from the surface of steel by immersion in water Rust - corrosion product consisting
of hydrated iron oxides; this term is applicable only to ferrous (iron-containing) alloys
Rust - corrosion product consisting of hydrated iron oxides; this term is applicable only to ferrous (iron-containing) alloys
Rust staining - reaction between exposed intermetallic layers (specifically the iron portion of the layers) with oxygen, that
cause mild red or brown staining.
Salt water -water with high concentrations of sodium chloride or other salts
Scale - a thick layer of imbedded oxidation (rust) products on metals
Seal-welding - a weld used primarily to obtain tightness and prevent the flow of cleaning solutions and zinc into otherwise
enclosed areas, to prevent flash steaming that causes localized ungalvanized areas
Service-life - anticipated length of time zinc will protect steel; the amount of time until enough of the galvanized coating is
consumed and 5% of the substrate steel surface area shows signs of rust
Shearing - the type of force that causes or tends to cause two parts of the same body that are touching along a boundary to
slide relative to each other in a direction parallel to their plane of contact
Shot-blasting - abrasive blasting steel with metal shot, usually to remove deposits or mill scale more rapidly or more
effectively than can be done by sand-blasting or chemical cleaning
Silicon - naturally occurring element commonly found in steel; silicon is particularly reactive in molten zinc metal
Silicon-killed steel - steel treated with silicon as an oxidizing agent in order to reduce the oxygen content to such a level
that no reaction occurs between carbon and oxygen during solidification
Skimmings (ash) - galvanizing byproduct comprised mainly of zinc oxides; skimmings are recyclable
Skip-welding - alternating the weld so that it is not continuous or complete
Spalling (concrete) - the complete failure of concrete due to the expansive forces caused by the formation of corrosion
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products on unprotected reinforcement bars
Spangle - the characteristic crystalline form exhibited by the solidified, hot-dipped zinc coating
Stainless steel - type of steel alloy that contains significant amounts of chromium and/or nickel and is typically very
corrosion resistant due to its ability to passivate upon atmospheric exposure
Stamping - a general term covering almost all press operations, including blanking, shearing, hot- or cold-forming,
drawing, bending, coining
Stenciling - the process by which lettering or a design through which a substance (ink, paint, or metallic powder) is forced
onto a surface to be printed; commonly used to mark steel fabrications but generally does not remain after the galvanizing
process
Storage - the area where galvanized articles are staged for pick-up or delivery
Strain-age embrittlement - the loss in ductility accompanied by an increase in hardness and strength that occurs when lowcarbon steel (especially rimmed or capped steel) is aged following plastic deformation; the degree of embrittlement is a
function of aging time and temperature, occurring in a matter of minutes at the galvanizing temperature but requiring a few
hours to years at room temperature
Stress-relieving - heating to a suitable temperature, holding long enough to reduce residual stresses and then cooling slowly
enough to minimize the development of new residual stresses
Structural - steel member of specific cross-sectional dimensions used in fabrication and/or construction, (e.g. H-beam,
angle, I-beam, W-flange)
Structural steel shape - piece of steel of any specific design accepted as standard by the structural branch of the steel
industry
Sulfuric acid - solution used in the cleaning stages of the galvanizing process that consists of two hydrogen ions and one
sulfate ion (chemical formula: H2SO4) in a water mixture
Surface condition - state of the surface of the steel
Surface preparation - stages of cleaning that prepare the steel for finishing (galvanizing)
Tank - container for chemicals used in the galvanizing process; steel is dipped sequentially in solution-containing tanks
Temporary bracing - metal that is attached to a fabrication prior to galvanizing in order to provide added support so that the
steel does not change shape during heating and cooling; temporary bracing is removed after galvanizing.
Thermal expansion - the process by which steel becomes larger upon heating
Threaded parts - parts such as bolts and rods that allow nuts to be screwed on to one or both ends
Touch-up - performing finishing work after galvanizing in order to meet standards or specifications, or coating areas of
steel that have been exposed due to post-galvanizing fabrication, installation, or extremely rough handling
Venting - providing holes in fabrications to be galvanized to allow entrapped, heated liquids and gases to escape as pressure
increases
Vibrating - the process of removing excess zinc by rapidly shaking galvanized articles
Viscosity - the property of resistance to flow in a fluid or semi-fluid state
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Warping - twisting or curving of steel originally flat or straight
Weathering steel - corrosion-resistant steel that initially corrodes; the presence of corrosion products then limits the further
oxidation of the metal
Weepage - the leaching out of trapped liquid solutions in galvanized structures, primarily through pinholes or gaps in welds
that were not sealed over by zinc
Weld beads - deposits of filler metal from a welding pass
Weld flux - material used to prevent the formation of, or to dissolve and facilitate removal of, oxides and other undesirable
substances
Weld residue - impurities left from the welding process; weld residue will inhibit localized formation of the galvanized
coating.
Weld slag - material resulting from the combination of weld material and weld flux; weld slag will inhibit localized
formation of the galvanized coating
Welding - joining two or more pieces of material by applying heat or pressure, or both, with or without filler material, to
produce a localized union through fusion or recrystallization across the interface
Wet galvanizing -using a liquid flux layer floated on top of the molten zinc; in the galvanizing process, final cleaning
occurs as the material passes through the flux blanket before entering the molten zinc bath
Wet storage stain - white surface oxide and hydroxide that forms on newly galvanized steel when excessive moisture is
present in poorly ventilated storage
Wheel-abrading - mechanical cleaning method used to remove small amounts of residues found on steels by means of a
rough, rotating wheel
White rust -white, sticky substance comprised of basic zinc carbonate; forms when galvanized surfaces are constantly
covered by water or water containing sulfides or chlorides
Zinc - major element found in the galvanizing kettle that provides both barrier and cathodic protection for steel
Zinc ammonium chloride - typical component of the flux solution used in the cleaning phase of the galvanizing process
Zinc carbonate patina - relatively insoluble zinc carbonate layer that forms as the galvanized coating weathers, providing
added corrosion protection and abrasion resistance
Zinc hydroxide - corrosion product formed in response to the presence of moisture on galvanized articles
Zinc oxide - basic corrosion product formed almost instantaneously on freshly galvanized articles after withdrawal from the
molten zinc metal
Zinc patina - relatively insoluble zinc carbonate layer that forms as the galvanized coating weathers, providing added
corrosion protection and abrasion resistance
Zinc solder - material used to touch-up and/or repair hot-dip galvanized surfaces
Zinc-iron alloy layers - inner layers of the galvanized coating formed from interdiffusion reactions between iron in the base
steel and molten zinc metal, (e.g. Delta, Gamma, Zeta)
Zinc-rich paint - (also called “cold galvanizing”) material used to touch-up and or repair hot- dipped galvanized surfaces,
providing barrier protection and some cathodic protection (if the concentration of zinc is above 94% in dry film thickness)
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