Gennesaret Resources Nigeria Limited
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VISUAL TESTING II BOOK
1
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CONTENTS
Chapter
1.
Physiology of VisiDn ..........................................................................
Page
ND.
1TO 32
2. Fundamental of light 6 lighting........................................................ 33 TO 57
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3. Fundamental of imaging .............................................................,........ 58 TO 82,
4. Borescopes .......................................................................................... 83 TO 104
5. Visual and Optical testing applications .......................................... 105 TO 12S'
S. Visual inspection welding .............................................ow.................. 127 TO 139
7. Weld joint and welding symbols ...................................................... 140 T[] IG9
8. Weld and Base Metal .......................................................................... 170 TO 200
9. Chapter QuestiDn Bank Answers .................................................. 201 TO 204
CHAPTER 1
PHYSIOLOGY OF VISION
The Human Eye
To ensure that the visual test is properly designed and that the eyesight of the
inspector is adequate to detect all relevant indications, a fundamental understanding of
the operation of the human eye is necessary.
The most common optical instrument used in visual and optical testing systems is
the human eye. The eye is an optical instrument with many automatic adjustments that
allow it to adapt and provide sharp vision at varying distances throughout a range of light
intensities. The components of the eye and their function are described in subsequent
paragraphs and illustrated in Figure 1.1.
Those components of the eye with the greatest mechanical relationship to vision
can be compared to the parts of a camera. The primary control of the eye’s ability to
adapt to different light conditions is due to the operation of the pupil and the iris.
The pupil is the central aperture of the eye. Although it appears black in color, it
is transparent to light. The pupil is contracted and dilated by the iris to modulate the
amount of light reaching the retina. The pupil also corrects some of the chromatic and
spherical aberrations of the retina and increases the depth of field.
The iris, named for its various flower-like colors, consists of a thin circular
curtain and the dilator and sphincter papillae muscles. These muscles expand and
contract the aperture of the pupil. Its function is comparable to the diaphragm of a
camera.
Much of the function of the eye is similar to that of a refracting instrument, as
shown is Figure 1.2. The comparison of the eye to a camera illustrates these functions as
well. The majority of the focusing ability of the eye is provided by the cornea and the
crystalline lens with minor assistance from the pupil and the refractive index of the
vitreous humor and aqueous humor. The cornea covers the iris and pupil and provides
about 70 percent of the eye’s refractive ability.
1
Figure 1.1: Components of the Human Eye
Vitreous humor
The crystalline lens provides most of the remaining 30 percent of the eye’s
refracting power. It is responsible for the adjustment of focus to maintain a sharp retinal
image as the distance between the eye and the object being viewed changes. By the
process of accommodation, the crystalline lens is allowed to relax to a thinner shape for
far vision or it is thickened in order to provide near vision. This comparable to the
movement of the focusing lenses in a camera.
2
Figure 1.2:
The eye as a refracting Instrument
Fair Object
Cornea
Relaxed Crystalline Lens
Retinal Focus Plane
Near Object
Accommodated Crystalline Lens
With age, the eye loses its accommodative ability because the crystalline lens
becomes less flexible and loses its ability to thicken, causing a loss of close-focusing
ability. This is corrected by the use of glasses or contact lenses, which provide a slight
amount of magnification.
Image formation in the eye is accomplished by the retina and its components; the
fovea centralis, the macula, the optic nerve, and the rods and cones.
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The retina is a delicate nervous membrane at the back of the eye that converts
light into an electrical signal and transmits theses signals, via the optic nerve, to the brain.
The retina faces the vitreous humor and is backed by the choroids.
Rods and cones are the visual receptors of the retina. Rods are much more
numerous in the retina than cones. When given adequate time, the rods will respond to
much lower intensities of light due to the production of visual purple, or rhodopsin. The
rhodopsin is bleached out at very low light intensities. This bleaching action probably
creates visual sensation due to an electrochemical process. Rods, which are concentrated
in the outer portion of the retina, are the components that create vision at low light levels.
Cones are concentrated at the fovea centralis, where there is almost a complete
absence of rods. There are three types of cones in the retina characterized by the
sensitivity of the pigments to a specific wavelength of light. The peaks of these
wavelengths are at 445 nm, 535 nm, and 570 nm corresponding to blue, green, and
yellow respectively.
The fovea centralis is located in the center of the retina and it has approximately
18 times as many cones as rods. This region of the eye is of primary importance for
determining vision acuity and color vision. The macula, which surrounds the fovea
centralis, is a yellow pigmented disk. The yellow pigment absorbs blue light, which
changes the perception of color in the cones of this central region.
The optic nerve is connected to the retina approximately 3 mm (0.1 in.) away
from the body centerline from the macula. This is the only portion of the retina that has
no visual imaging ability.
The structural components of the eye are the choroids, the sclera, the vitreous
humor, and the aqueous humor. The vitreous humor and the aqueous humor are the
liquids that fill the interior chambers of the eyeball. The aqueous humor is located
between the cornea and the iris. The vitreous humor fills the cavity between the retina
and the lens. Both liquids provide support to the eye’s structure and provide refracting
ability. The choroids and sclera form the outer sphere of the eye. The sclera form the
outer sphere of the eye. The sclera is visible as the white portion of the eyeball.
The six muscles of the eye’s extraoculer system provide the necessary motion for
focusing and tracking ability. These large scale eye movements are divided into version
and vergence. Version is where both eyes move simultaneously in the same direction and
vergence is where the two eyes move in equal and opposite directions.
In addition to the large scale movements, the eye makes constant small
movements. These saccadic movements are high frequency tremors of 10-60 minutes of
arc that occur even when the observer considers the vision to be fixed on an object.
Saccadic motion is necessary for the visual process to continue. If the eye remains
stationary, the cones become saturated and lose their ability to provide an image.
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Vision Acuity
Vision acuity, the ability to discern detail, is a product of the concentration of
cones in the fovea centralis. This high cone concentration extends outward from the
center of the eye approximately 2-3 degrees. Vision acuity decreases rapidly as the
retinal image of the object is moved away from the fovea. Visual acuity includes the
measurement of resolution acuity, recognition acuity and temporal resolution.
Resolution acuity is the resolving power of the eye and is a measure of the eye’s
ability to distinguish between fine lines or points. The is usually expressed as an angle.
At 254 mm (10 in.), a normal eye is capable of resolving two dots approximately 0.10
mm (0.004 in.) apart when this target is within 1 degree of are from the center of the
field of vision. Common eye tests often check recognition acuity by using letters to
measure the smallest target in angular measurement where the difference between objects
is perceived. The entire “E” on the 20/20 line of an eye examination given at 6 m (20 ft)
subtends 5 minutes of arc. Each part of the “E” subtends 1 minute of arc, the minimum
are that is visually separable. Figure 1.3 demonstrates how the resolving power of the
eye is expressed in terms of angle or angular size.
Recognition acuity is the ability to differentiate between objects such as the letter
“O” and the letter “Q”. Recognition acuity is determined by combination of
physiological, perceptive, and environmental factors. These environmental factors
include contrast, size luminance, and blur. This type of acuity is difficult to quantify but
it is the most important in terms of qualifying the visual or optical test method.
Figure 1.3:
d
Visual Acuity
D
θ
r
R
d
D
θ
r
=
=
=
=
=
the resolving power angular measurement
17 mm (0.7 in.)
distance from the front of the eye to the object
visual angle
retinal image size [17 mm (0.7 in.) tan θ]
5
R
Temporal response is the measure of the response of the eye to changes in
contrast over time. Due to the saccadic motion of the eye, spatial contrast also involves
temporal contrast. Temporal contrast is measured as the length of time and cycle time
that a flash of light must last in order to be distinguished. Above a certain duration and
length, the flicker will appear to be constant. This is an important factor in the selection
of video monitors.
Stereoscopic vision provides the ability to distinguish the depth between objects.
This is due to the lateral distance between the two eyes. This causes the eyes to see the
objects at a slightly different angle and causes vergence of the eyes. Human depth
perception is measured on a percentage basis known as percent stereopsis. Visual and
optical devices, such as binoculars, increase stereopsis. Monocular devices, such as
microscopes, borescopes, and metallographs, remove all sensation of depth perception.
The vision acuity of the inspector may be assessed at both near and far distances.
The 20/20 Snellen vision test is performed at 6 m (20 ft) and is the most common far
vision acuity test. Vision professionals usually check near vision acuity using the Jaeger
J1 or J2 vision tests at 330 mm (13 in.). For example, the Recommended Practice No.
SNT-TC-1A (1996) requirement for Jaeger chart distance is greater than or equal to 305
mm (12 in.). The reduced 20/20Snellen test at 406 mm (16 in) and the Ortho-Rater test
are also used to assess near vision acuity. The distance used during the eye examination
may be adjusted by using a scale that provides for an equivalent visual angel.
When not clearly defined by code or specification, it is necessary to clearly state
the distance to be used for vision acuity examinations.
ASME codes require 20/20 or 20/30 vision. It is not specified if this refers to near
or far vision. Aerospace standards require near vision acuity assessment using a Jaeger
J1, J2, or equivalent.
Visual Angle and Distance
The requirements for specifying and controlling the visual angle and distance
should be developed based on the minimum dimensions and contrast values of the
indications that must be recognized. Resolution acuity is determined by the distance and
the angle from the eye.
The minimum distance from the eye is governed by the ability of the eye to focus
at close distances. An 18 year-old adult is capable of focusing on objects as close as 102
mm (4 in). By age 50, this minimum distance has increased to about 229 mm (9 in.). It
is a requirement of some inspection specifications that inspection not be performed with
the unaided eye at closer than 152 mm (6 in). To prevent fatigue, visual examinations are
typically designed to be performed at a distance of about 483 mm (19 in.).
There is evidence to suggest that people who are forced to focus at close distances
for extended periods of time may be prone to myopia, a condition of nearsightedness
6
caused by a permanent thickening of the crystalline lens. This research was conducted on
submarine personnel where most visual tasks are performed at distance 0.6 m (2 ft) or
less and where it is rare to view an object more than 3 m (10 ft) in distance. Similar
results have been obtained when studying occupations in which people frequently
perform close visual tasks.
The further distance at which visual inspection should be performed is limited by
the size of potential indications and the amount of contrast they have in relation to the
surrounding area. The size of the retinal image determines the limit of detectability. This
is directly related to the angular size of the object. As the object is moved away from the
eye, the angle size decreases. This is illustrated in Figure 1.3 by noting the decreased
visual angle (θ) as the length (D) is increased for the same size test object. Some
specifications limit the maximum inspection distance to 508 mm (20 in.).
When the angle of the eye varies from normal to the surface being inspected, the
resolving power is decreased. This is due to a reduction in the size of the retinal image
and due to reduced contrast because of increased reflection and glare. Angles between 0
and 40 degrees are generally considered to be the limits for the performance of visual
tasks. Some specifications impose angle limits of 30 or 45 degrees.
Color Vision
Color is detected in the eye by the cones concentrated at the fovea centralis.
Color vision is a product of photopic vision and is dependent on the quality and quantity
of the ambient light and on the eye being properly adapted.
In 1801, Thomas Young proposed that the eye had only three types of color
receptors with which all the colors of the visible spectrum are detected. Hermann von
Helmholtz expanded this theory fifty years later. The Young-Helmholtz theory proposes
that light is detected in an additive nature and that the three primary colors are
responsible for the perception of all colors. The cones are primarily responsive to one of
the three primary colors and less responsive, or unresponsive, to the other two. This
theory has strong physical backing due to medical research on the physiology of vision.
The Young-Helmholtz theory does not fully explain many issues concerning the
perception of color. Ewald Hering performed research in 1874 based on human
perception that led him to propose the existence of four primary colors. These proposed
primary colors are red, green, yellow, and blue. While this theory is not substantiated by
physical evidence, it still is used to describe the perceptive or psychological process
because it allows the pairing of the primary colors as opposing or complimentary. No
color is described as red-green or yellow-blue but red does appear to mix with yellow to
make orange or with blue to make purple. In addition, Hering proposed that black and
white were opposing achromatic primaries.
Current theories on the perception of color involve a synthesis of these two ideas.
The tri-chromatic Youn-Helmholtz model explains the physical nature of the eye and
Hering’s theory is used to describe the manipulations of the information received by the
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brain. This model may explain the differences between the perception of color and
measured analysis supplied by colorimeters or other optical devices.
The various components of the eye are subject to chromatic aberration as the focal
plane of different colors of light occurs at different points within the eye. Blue, with the
shortest wavelength, has the shortest focal distance. Yellow is intermediary and red has
the longest focal distance. Optical correction of this aberration is not typically required
and may reduce the eye’s ability to focus.
The color temperature of the ambient light strongly affects the effective visual
assessment of color. Color inspection and color vision testing should be performed under
even lighting with a full color spectrum the approximates graybody illumination as
closely as possible. A color temperature of approximately 6,700 degrees (the color
temperature of the northern sky) is optimal.
Color vision deficiencies affect approximately 10 percent of the male population.
Of those identified as having a color deficiency, only 0.5 percent is women. The most
common cause of color deficiency is a genetic inability to discern between red and green.
Color vision is typically assessed by the use of pseudoisochromatic plates,
IshiharaTM plates, or by the use of colored caps.
Visual Perception
Visual perception in visual and optical inspection is the study of how the human
mind interprets the visually supplied information and forms an impression based on this
information. The perceptive process includes many environmental, physiological, and
psychological factors. This becomes important during inspection when the physical
reality is different from the perception. Inspection methods must be designed to
minimize the effects of factors that lead to incorrect decisions.
Other than the errors due to fatigue, disease, optical disorders, or a lack of
training, most errors in perception appear to be related to the misinterpretation of the
visual clues that the mind uses to make the correct decision most of the time. Throughout
life, the mind is conditioned to make evaluations of color, depth perception, relative size,
spacing, and motion.
Many decisions are made based on the visual clues provided by light and shadow.
In painting, photography, and other two dimensional image of a three-dimensional object,
a sizable amount of the information on depth, relative placement, curvature, and texture
comes form the perception of light and shadow. For example, there often appears to be a
linear line or crack-like indication present at the border of areas with very different colors
or shades. In Figure 1.4, gray dots seem to appear at the intersections of the white lines
between the black boxes.
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Figure 1.4: Illusion due to contrast
The appearance of texture often indicates corners, edges or gaps in an object.
Parallel lines or lines that converge or diverge may be used to represent the perception of
texture and may lead to conclusions that differ from the physical reality.
Diverging lines lead the eye away from the two parallel lines and create the
illusion that the lines bow inward. Converging lines are frequently used in paintings in
this manner. The problem in inspection comes when the converging or diverging lines
are not indicative of distance.
The orientation of the observe to the environment is based on a perception of up
and down, left, and right. When optical devices provide an image that is reversed, it may
Cause Disorientation. While a skilled observer will adapt in a few seconds, less skilled
observers may take several minutes to become oriented.
Some other visual illusions are more difficult to explain but are of interest during
visual and optical inspection. Bright objects appear to be larger than dark objects of the
same size. Circles are estimated to be smaller than other Straight lines appear to be
longer than they actually measure. Acute angles are estimated as large than the actual,
and obtuse angles are estimated as being smaller.
Pattern recognition may be used to gain insight into the performance of the visual
or optical inspection system. Pattern recognition is based on what Gestalt psychology
refers to as perceptual organization or grouping principles. These principles create the
separation of an object from its background and from other object. The more distinct the
boundary of an object is, the more readily the object is separated from its background.
The following principles of organization are not applicable in isolation, and they may
enhance or oppose each other.
Grouping by proximity (Figure 1.5) – A pattern is perceived due to the proximity
of objects to each other. For example, a series of uniform dots is described as a set of
lines or columns. The horizontal spacing is twice that of the vertical spacing.
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Figure 1.5: Grouping by Proximity
Grouping by proximity
Grouping by similarity (Figure 1.6) – A pattern is perceived due to the similarity
of object’ size, shape, color, or shade. A series of black and white dots of the same size
is described as lines or columns of black and white. Note that the horizontal spacing is
the same as the vertical to prevent proximity grouping.
Figure 1.6:
Grouping by Similarity
Grouping by similarity
Grouping by symmetry (Figure 1.7) – Patterns are perceived and distinguished
from each other due to the perceptive desire to create simple patterns such as squares,
circles and triangles. Due to symmetry and similarity, Figure 1.7 is usually described as a
set of two triangles. Symmetry also allows the pattern to be described as a circle.
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Figure 1.7:
Grouping by Symmetry
Grouping by similarity
Grouping due to continuation (Figure 1.8) – The eye tends to follow or continue
along a described path. Figure 1.8 is described as a cross or as two intersecting lines. It
is seldom described as a set of v-shaped objects that join at the points.
Figure 1.8:
Grouping due to Continuation
Groupimg due to continuation
Grouping due to closure (Figure 1.9) – Patterns with missing pieces tend to be
filled in as the mind tries to create symmetry. The dots missing from the circles in Figure
1.9 are not immediately apparent.
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Figure 1.9:
Grouping due to Closure
Grouping due to closer
VISION ILLUSION
Perception
The most remarkable thing in a remarkable universe is so commonplace that it is
accepted without wonder or understanding. This is the appearance of reality and solidity
that surrounds us when our eyes are open, which we call simply vision. Intellectual effort
is required to realize that this seeming reality is within us, distinct for each individual, but
so concordant with reality and with each other, and so stable, that it is accepted without
question. The explanation for this wonderful aspect of consciousness is completely
unknown.
The information that allows the mind to create and maintain this appearance is collected
entirely by the visual system. When the eyes are closed, the appearance vanishes. At
least, it does for me. Some people may be able to create realistic pictures with their
mind's eye, but I cannot. I can still imagine my environment more or less accurately, but
the vivid picture is gone.
The ability to make an accurate visual model of the external world is learned, not innate.
The necessary materials are there at birth, of course, but they must be trained, or
programmed, before the skill is perfected. This seems to be done in the first instance by
comparing the chaotic impressions of light with the solid evidence of touch, which gives
the perception depth and form. The visual sense ever after shows subtle indications of its
origin in touch, though it becomes completely independent of touch after perfection.
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The ability to acquire this skill vanishes early in mental development. A person
totally blind from birth whose vision may become normal at a later age can never make
sense of the visual information and arrange it in a consistent manner.
The Chain of Perception
There are three links in the chain of perception. The first is external and physical: the
propagation of electromagnetic waves from the object to the eye. The second is the
physical visual apparatus, from eye to brain, consisting of nervous tissue, although some
important preliminary processing takes place. The third, and most complex, is the
interpretation of the visual stimulus and the creation of the internal model of the world
that is used by the consciousness.
In the third step, the visual stimulus received from outside is combined with information
from the memory to create the picture. This is the most important part of vision, and how
it is done is unknown. All the really interesting parts of vision occur here. The physical
visual system from eye to brain has been studied in exquisite detail, its parts examined
and described, and even the nerve impulses observed and measured, but all this gives no
satisfying explanation of vision. It has been established, however, that important
preliminary processing takes place here, including the differencing and the coding of
stimuli. Coding is necessary to reduce the flood of information to a manageable amount.
The visual system has a bandwidth problem, indeed.
The world picture must be constructed from incomplete information, in fact inferred from
clues. The three-dimensional world is sensed by the two- dimensional retina,
emphasizing the central role of depth clues. The picture depends on the unconscious
recognition of objects, so that the remembered properties of objects can be transferred to
those they seem to be on the basis of visual hints. Recognition is what gives vision its
reality, showing the central role of mind.
Illusion and Hallucination
A picture so assembled on the basis of partial information must be expected to
occasionally be in error. The mind will always try to match stimulus and memory to
create a picture. It will make what seems to be the most likely choice, and present that to
the consciousness. An illusion occurs when the choice is incorrect. If a picture is created
solely from memory, without visual stimulus (or with only a minimal visual stimulus) the
result is hallucination, with which we shall not be concerned here, since it is a disorder of
perception, not a normal or intended part of it. Things that are not there can also appear in
illusion, it must be emphasized, but here it is normal.
An illusion can arise in any of the three links of visual perception. The mirage is an
example of an external illusion, created in the first, physical link of light rays. It is
visually interpreted as an actual scene, though we consciously recognize it as an illusion,
and understand its cause. When we stare at a brightly illuminated red disk for a time, then
transfer our attention to a white paper, we see a green disk as a result of what is called
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rather inaccurately fatigue. The green disk is an illusion created in the second partly
physical, partly mental link. When the full moon is seen at the horizon, it seems much
larger than when riding high in the sky, though physically it subtends exactly the same
angle at the eye. This familiar illusion occurs in the third, mental link of vision, and a
satisfying explanation of it is unknown.
Optical Illusions
Illusions occurring in the third link are those most generally recognized as optical
illusions. Their scientific study began with J. Oppel in Jahresberichte des physikalisches
Vereins zu Frankfurt, p. 138 (1854). Much work was done later in the century, but
tapered of after 1900, although the subject is still actively researched by psychologists.
Recent work deals largely with color and motion illusions, not on the static, black- andwhite illusions that dominated earlier work. Popular interest in optical illusions has been
sustained. The books by M. Luckeish (Visual Illusions, 1920), S. Tolansky (Optical
Illusions, 1964), and M. Fineman (The Nature of Visual Illusion, 1981) are evidence of
the continuing fascination. Each of these books gives references to further information.
All theories of optical illusion in the third link are mere jejune speculation. Feel free to
create your own theories; they will be as valid as those created by many a psychologist!
Sometimes a phenomenon is called an illusion when it really is not, but is simply a true
picture of an unexpected observation. An example is the searchlight illusion described by
Luckeish. The beam of a bright searchlight is visible because of scattering by dust and
fog in its path, so that it seems practically a physical object. When the beam is projected
up into the sky, it seems to vanish abruptly while still in full glory. When you look at this
apparent end of the beam, you are looking in the direction in which the beam is pointed.
If the beam were parallel (as your mind expects) it would, by perspective, narrow to a
point. However, a searchlight beam is actually more or less divergent, fooling this
expectation. It is only one's mental interpretation that is an illusion in this case, not the
observation. Stars can be pointed out to others by means of a strong laser using this
effect. If you view the searchlight beam from a distance, you see it diverge and become
attenuated, and perhaps penetrating the layer of dusty air.
Camouflage
Tricking the eye into recognizing one thing while observing another is often very useful
to living things. There are three different ways to do this. First, one might mimic
something dangerous or nasty-tasting, as does the fly who resembles a wasp, a brightlycolored butterfly, or an armed, uniformed policeman. Another way is to merge with the
background, as do moths, stick insects, tabby cats, or wealthy people wearing old clothes
in the street. An interesting way to do this is to break up a familiar outline by a
contrasting pattern. Warships were painted in bold, zig-zag patterns in the First World
War for this purpose. The patterns did indeed break up the outline when you were close
enough to see that they were ships, but at large distances aerial perspective (blue haze)
smoothed the pattern, revealing again that they were ships.
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The third way is to look like something else. Cylindrical snakes and lizards are dark on
top and light on the bottom, contrary to the normal modelling of a cylinder, so they
resemble flat objects containing no meat.
Deliberate Illusions
A picture drawn on a flat background is an attempt to trick the eye into perceiving a
three-dimensional scene. This is very effective, since the eye must do something similar
in its normal functioning, because the retina is two- dimensional. The skill of perceiving
depth and perspective in a painting is learned, not innate. In moving pictures, the mind
interprets the succession of static frames as continuous motion, again something it must
do in its normal functioning. There must be a temporal element in sensing a changing
world, which is revealed by the flicker frequency, the rate above which continuous
motion is perceived instead of jumps, of about 20 to 30 Hz. We are very thankful for
these illusions (if we realize what they are) and are glad to have them.
Conjurors, three-card monte men, swindlers, mediums, priests, and others interested in
influencing people sometimes make effective use of visual (and other) illusion. Stage
magicians who are only concerned with entertainment call themselves illusionists to
make it clear what they do, and to distinguish
themselves from those who ascribe their
wonders to spirits or chemicals. Illusionists,
and the the other sorts of entrepreneurs, mainly
use other kinds of illusions, but optical
illusions are not ruled out. These procedures
have been perfected through centuries and even
millenia of profitable use, and remain
evergreen owing to the continuous copious
production of fools.
Classic Static Illusions
Let's look at some classic static illusions
created by black-and-white figures. All are
third-link illusions resulting from a failure of
estimation, or from the faulty comparison of
distances or objects. In the bisection illusion,
the vertical line is the same length as the
horizontal line it bisects, though it seems about
Fig1.10
25% longer. The illusion persists if the figure is rotated 90°, so it is not due to asymmetry
of the retina, as one witless psychologist asserted. In the Müller-Lyer illusion, the line is
bisected by the center arrowhead. The segment with the diverging wings appears longer,
but it is not. In the annulus illusion, the area of the central disk is equal to the area of the
15
annulus surrounding it, though it appears greater. Distance b-c in the lozenge illusion is
equal to distance a-b, though appearing significantly longer. In the curvature illusion, all
three arcs have exactly the same radius of curvature. Poggendorff's illusion is very
famous. Line 2 is actually the continuation of the line on the left, although line 1 appears
to be. This illusion is counteracted in the British Union Flag by displacing the arms of St.
Patrick's cross on either side of St. George's cross so they appear to be in the same line.
Greek temples were designed with deliberate distortions to make the building appear
correctly. Columns were given entasis, a slight swelling in the middle, so they would
look straight, and architraves were cambered up slightly in the center so they would
appear straight. Modern buildings are not so sensitively designed.
There is no satisfying explanation for any of these illusions, or even of the reasons why
they should exist. Depth clues are not involved in any of them, at least obviously, or
ambiguous or incomplete information. They can, however, be recognized and classified,
and have some practical application.
Fig1.11
Ambiguous Figures
Sometimes a view may not contain
enough information for the mind to make
a conclusive interpretation. Where there
are only two reasonable interpretations,
the mind may alternate them, as if unable
to make up its mind. The rate of
alternation gives some idea of the length
of time between reconsiderations of input data by the visual system, or of the operation of
the short-term memory that is so necessary to avoid overload in the face of the flood of
information bombarding the mind.
In the ambiguous figures shown, the one on the left can be interpreted either as an open
book, or as a folded card with the fold towards you. The cube can be interpreted either
with the diagonal line in the lower left-hand corner out of the page, or behind it. Vision is
not really fooled here; there is simply insufficient depth information for a conclusive
choice. Modifying the figures to give better depth clues, as shown, makes the
interpretation unique. In one case, the figure was made to resemble a definite object, an
open book, and in the other hidden lines were removed to make the cube appear solid.
Other Illusions
Illusions can also arise from contrast of brightness, as the perception strives to maintain
line and shade. The well-known illusion shown at the right is an example. There are gray
patches at every crossing, except for the one you are looking at directly. This effect is
something to avoid when designing linoleum.
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Illusions of motion and color are difficult to illustrate in text, and are so extensive as to
require individual study. Color can be perceived in a rotating black-and-white disc of
suitable pattern, which is probably due to different fatigue characteristics of the colorsensitive proteins in the cone cells of the retina. Many color illusions are due to physical
causes, because of the poor spectral resolution of the eye, and differences in illuminants
and pigments. Adaptation, where the ambient illumination comes to appear as white as
possible, and color constancy, where colors are interpreted similiarly under different
conditions of illumination, are fundamental and useful properties of the color sense, not
illusions.
Stereoscopy
Stereoscopic binocular vision is a remarkable facility that provides many interesting
illusions, mostly useful and entertaining ones. When both eyes are fixated on the same
nearby object, the images on the two retinas are not identical and conflict, since each eye
sees the object from a different position. Detecting the conflict, the mind checks to see if
the conflict can be explained by the different positions of the eyes, and if so, immediately
interprets the object as located in the proper position in space. The two images are then
said to have fused. Of all depth clues, the mind regards stereoscopy as superior,
overriding all other clues. Stereoscopic vision is most effective in the same regions as
human hands work, and must be considered predominantly as an aid to such activities.
Accommodation and convergence of the axes of vision may play a part in stereoscopy,
but image conflict is the primary cause, as was realized by Charles Wheatstone, the first
to study stereoscopy around 1838.
When the conflicting images cannot be explained in this way, rivalry occurs instead of
fusion, resulting in rejection of one image, alternation of images, or double vision. When
sufficiently addled by alcohol, the mind may not feel like exerting the effort required for
fusion. When the image in one eye is markedly poorer than the image in the other, it is
usually suppressed, and the image from the good eye governs. If red is presented to one
eye, and green to the other, rivalry results, in my perception, in an intermediate state than
cannot be described as either color or any mixture of them. This allows stereoscopic
views to be presented to the eyes separated by colored lenses without color conflict.
The stereoscopic illusion is the fusion of two scenes presented separately to the two eyes
into one three-dimensional scene. The feeling of three dimensions is very strong, so that
viewing such stereopairs is quite pleasant. When the two views are printed on a page
about 65 mm apart, a normal interocular distance, a trained observer can fuse them by
simply diverging her optic axes as if viewing a distant object. When this is done, the
fused image appears between the two original images, so three images are actually seen.
This is more easily done by the older observer, since accommodation of the eye does not
hinder fusion when the axes are diverged. To allow larger stereopairs, and to help
untrained observers, stereoscopes using prisms and lenses were invented. The common
stereoscope was invented by David Brewster, and has now been reduced to spectacles
with prism lenses. If the two scenes are presented in different colors, they can be printed
17
on the same surface and separated by glasses with colored lenses, as mentioned above.
Such stereopairs are called anaglyphs.
The strength and independence of the stereoscopic facility is shown by the latelydiscovered fact that it is fully effective even when an object cannot be recognized,
provided only that corresponding points can be identified in the two views. A stereopair
can be made with random dots, identical except for small displacements that would occur
if they were located on a three- dimensional surface. Such pairs can be fused, and the
shape of the imaginary surface made visible in three dimensions. This clearly shows that
memory plays no essential role in stereoscopy, in contrast to the major role it plays in all
other visual interpretation.
Autostereograms
A related type of stereogram consists of cunningly arranged areas in a single picture,
often only small dots, such that each area does double duty, as a point of an object for
each eye at the same time, but on different objects. These stereograms fuse when the
optic axes are made parallel, so the areas do their intended double duty. The picture
appears only upon fusion, and cannot be perceived in advance. The same thing happens if
the optic axes are over- converged, but the stereoscopic effect is inverted (depth is
reversed). These single-image random-dot stereograms are generally called
autostereograms. These stereograms attracted great public interest in the early 1990's,
when they were published in newspapers and books, and even appeared in outdoor
advertising. Viewing autostereograms is excellent practice for learning how to fuse
stereograms without aid, called free fusion.
Final Remarks
A curious illusion shows how the mind does its best to interpret its data. Fixate on a
finger resting on a book at normal reading distance from your eyes. Now move the finger
toward your eyes, keeping your fixatin on the page of the book. As soon as your finger is
far enough from the page that what is obscured from the right eye is seen by the left, and
vice-versa, it will become transparent, and you will see the book unobstructed by the
finger. The finger is surely there, right in the way, but it is suppressed, probably because
your fixation shows you are looking at the book, not the finger.
Illusions show that visual perception is much more complicated than was ever imagined
in primitive views of it. One early view interpreted sight as touching by visual rays from
the eye in the presence of activating rays from the source of light. More recently, the eye
was perceived as a camera making a picture that was viewed somehow by the brain. The
most interesting aspects of vision are, however, yet unexplained.
To understand the importance of lighting in an inspection atmosphere, it is essential to
know the fundamentals of light, how it is measured, and the recommended lighting levels
for inspections.
18
Can you figure out what this is a picture of?
It is a cow's head looking at you.
Which black rectangle is bigger?
Would you believe that they are both the same size? Can you find the lowest step in this
image?
How is this possible? It just keeps on going…
19
Are the circles on the inside or the outside of this
image?
Which man is the tallest in the picture below?
Look closer. All three are exactly the same size.
20
CHAPTER 1
PHYSIOLOGY OF VISION
1. The most common optical instrument is
a. Glass lens
b. Borescope
c. Eye
d. Microscope
2. Pupil appear black in color .So it is
a. Transparent to light
b. Opaque to light
c. Reflect all light
d. Absorb all light
3. Iris controls the aperture of the pupil
a. 4 muscles
b. 6 muscles
c. 8 muscles
d. No muscles at all
4. Major refracting ability of the eye is provided by
a. Lens
b. Retina
c. Cornea and pupil
d. Optic nerves
5. In Fovia centralis
a. The number of light amplifiers are more
b. The number of cones are more
c. Both are equal
d. Neither rod nor cones are present. Only optic nerve is present
6. Visual purple is
a. Is bleached at lower light levels
b. It is nothing but rodospin
c. It is an electro chemical process
d. All of the above
e. Only (a) and (c)
7. In retina
a. The number of light amplifiers are more
b. The number of cones are more
c. Both are equal
d. Neither rod nor cones are present. Only optic nerve is present
21
8. Fovia centralis is
a. Insensitive to shorter wave length
b. More sensitive to shorter wave length
c. Same sensitivity to all the wave length
d. Sensitivity depends only on the intensity of the light and not the wave length
9. Optic nerve portion
a. Is very sensitive to light
b. It is blind to light
c. It is sensitive to blue light only
d. It is sensitive yellow green light only
10. Vergence : Two eyes moving in the different direction
11. Version : Two eyes moving in the same direction
12. If there is no saccadic motion
a. Cones become saturated and lose their ability to produce an image
b. Since there is no motion vision will be sharp
c. Saccadic motion concerns only with the frequency and not with vision
capacity
d. None of the above
13. If the threshold is 5 minutes , find the vision acuity as
a. 0.4
b. 0.6
c. 0.8
d. Always greater than 1
14. In the eye --------------------carries out the function of aperture’ in camera
a. Cornea
b. Iris
c. Lens
d. Retina
15. Human eye has cells that respond to __________ separate colors:
a. Seven
b. Five
c. Three
d. None of the above
16. The primary colors are said to be
a. Red, green ,blue
b. White, black, yellow
c. Blue ,green ,yellow
d. Red ,yellow, violet
22
17. A simple spherical lens is likely to be truncated to minimize
a. Chromatic aberration
b. Coma
c. Spherical aberration
d. Di-spherical aberration
e. None of the above
18. The view of any object as seen from above is called as the_______
a. Isometric view
b. Side view
c. Plan view
d. Down view
19. In dim light our eye is said to be in:
a. Mesopic mode
b. Scotopic mode
c. Dark adrenaline mode
d. Acuiy mode
20. Human eye has certain sensors that are capable of detecting
a. Color
b. Lines
c. Direction of movement
d. All of the above
21. The ability to see and identify what is seen is called as:
a. Vision
b. Visual interpretation
c. Vision acuity
d. Inspection
e. None of the above
22. The lens of the eye focuses
a. All wavelengths in a single plane
b. Each wavelength at a different depth in the retina
c. Light waves on the optical nerve
d. All light on fovea
23. Examinations for near and far vision are usually carried out at a distance of
a. 10 inches and 20 inches
b. 12 inches and 30 feet
c. 15 inches and 6 feet
d. 15 inches and 6 meters
23
24. Vision acuity tests are recommended to be carried out under
a. Tube light
b. Mercury vapor lamps
c. Incandescent light
d. Sunlight
25. A person is said to be truly color blind when he can not
a. See any color
b. Distinguish black and white
c. Distinguish blue and red
d. Distinguish red and green
26. The purity of a color defined by its
a. Tone
b. Hue
c. Saturation
d. None of the above
27. Electronic aids to vision are based primarily on :
a. Photosynthetic devices
b. Photothermal devices.
c. Photoelectric devices.
d. Photostat devices.
28. The three physical characteristics of color include :
a. Saturation, brightness and glare.
b. Hue, saturation and brightness.
c. Reflective index, spectral range and hue.
d. Tone, purity & Brightness.
29. Visual inspection is easy to apply, quick and relatively inexpensive, and requires
no special equipment other than :
a. A clean work area.
b. 25x magnification.
c. Good eyesight.
d. 2152 lx (200 ftc) of illumination.
30. Direct visual examination is possible when the eye can be placed within :
a. 305 mm (12”) of the inspection surface.
b. 381 mm (15”) of the inspection surface.
c. 610 mm (24”) of the inspection surface.
d. 762 mm (30”) of the inspection surface.
24
31. The human eye cannot always distinguish clearly the fine differences between
contact angles and states of wetting when inspecting soldered joints. To improve
the inspector’s ability to distinguish these differences, it is recommended that
the inspector use magnification in the range of :
a. 200x-300x.
b. 300x-400x.
c. No more than 10x.
d. 100x-200x.
32. The term used for dark adaptation vision using only the rods in the retina when
differences in brightness can be detected but differences in the hue cannot is
called :
a. Photopic vision.
b. Mesopic vision.
c. Scotopic vision.
d. Fovea vision.
33. The restriction on the angle between the eye and the test surface for general
visual testing should be :
a. Less than 60 degrees.
b. More than 60 degrees.
c. More than 30 degrees.
d. Less than 30 degrees.
34. The function of daylight vision for color and detail is performed by the :
a. Rods.
b. Cones.
c. Fovea.
d. Retina.
35. The element of light related to the characteristics of tone, purity and brightness
are called :
a. Vision.
b. Daylight vision.
c. Color.
d. Illumination.
36. Variable(s) other than lighting and target size that affect vision acuity include :
a. Inspector attitude.
b. Target movement and target angle.
c. Target angle.
d. Target movement and brightness.
25
37. The ability of the eye and brain to work together to discriminate patterns from
the background is called :
a. Near vision acuity.
b. Nueral acuity.
c. Vision acuity.
d. Pattern recognition.
38. When the point of focus is beyond the plane of the retina, this condition is
called
a. Astigmatism.
b. Nearsightedness.
c. Scotopic vision.
d. Farsightedness.
39. To form reliable images, the lens of the eye focuses light rays onto the :
a. Retina.
b. Optic nerve.
c. Sclera.
d. Cornea.
40. The condition when the point of focus is short of the retina is called :
a. Farsightedness.
b. Scotopic vision.
c. Nearsightedness.
d. Astigmatism.
41. When a primary color is mistaken for another primary color, this is an error in:
a. Discrimination.
b. Perception.
c. Color vision.
d. Sensation.
42. Exposure to high frequency visible light at intensities and durations that may
damage the retina, and does not elevate retinal temperatures enough to cause
thermal hazard is called :
a. Hyperthermia
b. Thermal shock
c. Blue hazard.
d. Birefringence.
43. The angle of vision and the distance of the eye from the test surface determine
the minimum angular separation of two points resolvable by the eye. This is
known as the eye’s :
a. Sensitivity.
b. Resolving power.
c. Vision power.
d. Discrimination
26
44. The magnifying power of alens with f = 50 mm would be
a. 2
b. 5
c. 10
d. 20
45. Which type of light can be more dangerous to human eye
a. White light
b. Red light
c. Polarized light
d. Laser light
46. In color vision testing following parameters would be important
a. Light level
b. Reading temperature of source
c. Color temperature of source
d. All of the above
e. A and b only
47. Prolonged exposure to infrared radiation can cause
a. Hypothermia
b. Hyper thermia
c. Iso thermia
d. Pseudo thermia
48. Porlonged exposure to ultra violet radiation can cause
a. Kertoconjunctivitis
b. Contract formation
c. Skin aging
d. All of the above
e. A and b only
49. The illumination at a point on a surface in relation to the luminous intensity of
the source and the point varies directly with the intensity and
a. The distance
b. Inversely with the distance
c. Inversely with the square of the distance
d. The square of the distance
50. The difference in hue and saturation between an object and its background is
usually called as
a. Chromatic contrast
b. Somatic contrast
c. Photopic contrast
d. Luminance contrast
e. None of the above
27
51. The recommended maximum luminance ratio between object and nearby darker
surrounding is
a. 1 to 6
b. 6 to 1
c. 3 to 1
d. 3 to 20
52. Which of the following would reduce glare?
a. Reduce light source intensity
b. Reduce light source size
c. Reduce angle between light source and eye
d. All of the above
e. A and b only
53. Tasks with low contrast or very small size require the following range of
illuminance
a. 100-200 lux
b. 200-500 lux
c. 1000-2000lux
d. 2000-5000lux
54. The cornea provides about…………..% of the eyes refractive ability
a. 10%
b. 30%
c. 50%
d. 70%
55. The movements of eye are classified as
a. Version
b. Vergence
c. Saccadic
d. All of the above
e. A and b only
56. The ability of eye to distinguish between fine lines or points is called
a. Resolution acuity
b. Recognition acuity
c. Reorganization acuity
d. Any of the above
e. A and c only
57. The near vision acuity can be assessed by
a. Jaguar chart
b. Reduced 20/20 snellen test
c. Ortho-rater test
d. All of the above
e. A and c only
28
58. The color vision is typically assed by the use of
a. Pseudoisochromatic plates
b. Ishihara plates
c. Colored caps
d. All of the above
e. A and b only
59. The image formation by a thin lens write the formula
Where f = focal length, D = image distance, U = object distance
a. 1/D + 1/U = 1/ F
b. 1/U + 1/F = D/1
c. 1/F + 1/U = 1/D
d. 1/F = 1/U – 1/D
60. The dispersive property of glass causes ……..when light passes through a glass
lens
a. Psuedochromatic aberration
b. Apparent aberration
c. Chronic aberration
d. Chromatic aberration
61. Spherical aberrations are caused by a lens that is
a. Spherical in shapes
b. Cylindrical in shapes
c. Large in in aperture
d. Small in aperture
e. None of the above
62. The ability of an optical device or system to gather light is measured by its
a. Aperture
b. F-number
c. Numerical apertune
d. All of the above
e. B and c only
63. If an object has chromatic contrast in addition to a luminance contrast the
perception of eye may
a. Increase
b. Decrease
c. Remain same
d. Any of the above can happen
e. A or c only
64. Human depth perception is measured on a percentage basis known as
a. Percent acuity
b. Percent depth vision
c. Percent stereopsis
d. Percent monoscopy
29
65. The 20/20 snellen vision test is usually performed at a distance of
a. 10”(250 mm)
b. 13”(330 mm)
c. 10feet(3m)
d. 20feet(6m)
66. The cornea
a. Changes the lens shape
b. Covers the eye
c. Protects the lens
d. ‘b’ and ‘c’ are correct
e. None of the above
67. Far sightedness occurs when the focal spot is
a. Focal spot is deeper into the retina
b. Focal spot is near the inner surface of the retina
c. Eyeball is elliptical and long diameter is vertical
d. ‘a’ and ‘c ’are correct
68. For the far sight vision exam the chart is placed at
a. 6m
b. 6ft
c. 20ft
d. 20m
e. ‘a’ and ‘c’ are correct answers
69. Photovoltaic cells convert radiant energy into
a. Electrical energy
b. Thermal neutrons
c. Light
d. None of the above
70. At 6’’ distance the intensity of light is 1000 at 12’’ distance it will be
approximately
a. 500 lux
b. 250 lux
c. 25ft candle
d. ‘b’ and ’c’ are correct
e. None of the above
71. For adequate inspection of surface discontinuities the angle of eyes with respect
to test surface plan should not be
a. More than 30
b. Less than 30
c. More than 45
d. Less than 45
30
72. Sluggishness of iris can be caused by
a. Age
b. Fatique
c. Drugs
d. Disease
e. All of the above
73. Presbyopia is the condition which
a. Lens stiifens with age and losses ability to focus
b. Night blindness occurs
c. Colour blindness occurs
d. Is an early stage of cataract
74. VT operator perception can be impaired by
a. Pre-disposition of hereditary
b. Enlarged cataract
c. Both ‘a’ and ‘b’
d. None of the above
75. The common raster scans have an aspect ratio of:
a. 2/3
b. 3/2
c. 4/3
d. 3/4
76. High performance computer based digital systems often have scan rates of
a. 25 frames per second
b. 30 frames per second
c. 80 frames per second
d. 200 frames per second
e. Either c or d
77. When a radiating body has e(λ) = 1 for all wavelengths such as a body is called
a. Black body
b. Gray body
c. Selective radiator
d. Black hole
78. The subtractive primary colors are
a. Cyan, yellow and magenta
b. Cyan, green and blue
c. Violet, magenta and yellow
d. Yellow, green and cyan
e. None of the above
31
79. In the formula m=10/f (magnification m of a single lens) the value of f is in
a. Cms
b. Inches
c. Mms
d. Microns
32
CHAPTER 2
FUNDAMENTAL OF LIGHT & LIGHTING
Lighting Fundamentals
Types of Reflection
Objects reflect light in two ways. In specular reflection, light from each incoming ray
reflects in a single direction (figure 12). A tinned circuit board trace or a mirror exhibits
specular reflection. In diffuse reflection, light from each incoming ray is scattered over a
range of outgoing angles. A piece of copier paper is a diffuse reflector.
In reality, objects exhibit the whole range of behaviors between the specular and diffuse
extremes. A machined metal surface scatters light over a small range of angles, and
scatters differently in directions parallel and perpendicular to the turning marks. Paper
exhibits some specular properties, as anyone who has tried to read with a high intensity
lamp can attest. Many objects have components that reflect differently. An electrical
connector includes both shiny (specular) metal pins and dull (diffuse) plastic housing
parts.
Figure 12. Types of reflection
Specular Reflections
Specular reflections are bright, but unreliable. They are bright because the intensity of the
reflection is comparable to the intensity of the light source. In many cases, a specular
reflection saturates the camera. Specular reflections are unreliable because a small change
in the angle between the illuminator, the object, and the lens may cause the specular
reflection to disappear completely. Unless these angles are well controlled, it is best to
avoid depending on specular reflections. The best method for lighting specular parts is
with diffuse lighting (figure 13). The large illumination solid angle means that the image
remains almost constant as the reflection angle changes.
33
Diffuse Reflections
Diffuse reflections are dim but stable. The intensity of the reflection is reduced from that
of the source by a factor of 10 to 1000. The reflected intensity changes slowly with the
angle (figure 14). Diffuse surfaces can be lit successfully with either diffuse or point-like
illuminators. Other considerations, such as specular elements on the object or the
influence of shadows, determine the best approach.
Figure 13. Specular objects viewed with diffuse lighting
Figure 14. Diffuse objects illumincated with point-like source
34
Lighting Techniques
The basic approach to lighting for a particular application is easily determined. It is a
function of the type of object and the features to be measured. The more detailed lighting
design builds on this basic technique (table 3).
Lighting Solid Angle: Point or Diffuse
Lighting solid angle is the area of a unit sphere, centered on the object, that the
illumination occupies (figure 15). Just as angles are measured in radians, with 2 radians
in a full circle, solid angles are measured in steradians, with 4 steradians in a full sphere.
Illumination from a small solid angle is called point-like; illumination from a large solid
angle is called diffuse.
Point-Like Lighting
Diffuse Lighting
Lighting Direction
Lighting Color
Polarization
Light Sources
Solid
Angle
Direction
Advantages
Disadvantages
Directional Front Illum
ination Incandescent
lamp or fiber bundle
illuminates object from
the top
Point
Front
Easy to implement;
good for casting
shadows; fiber-optic
delivery available in
many configurations
May create unwanted
shadows; illumination is
uneven
Coaxial Lighting
Illumination from the
precise direction of the
imaging lens, either
through the lens or
with a beamsplitter in
front of the lens
Point
Front
Eliminates shadows; Complicated to implement;
uniform across field
intense reflection from
of view
specular surfaces
Illustration Type
Diffuse Front
Diffuse
Illumination
Fluorescent lamp, fiber
illuminator with
diffuser, or
incandescent lamp with
diffuser, illuminates
object from the front
Front
35
Soft, relatively
nondirectional;
reduces glare on
specular surfaces;
relatively easy to
implement
Illuminator relatively large;
edges of parts may be
fuzzy; low contrast on
monocolor parts
Illustration Type
Solid
Angle
Direction
Advantages
Disadvantages
Light Tent
Diffuse illuminator
surrounds object
Diffuse
Front
Eliminates glare;
eliminates shadows
Must surround object
illuminator is large; can be
costly
Dark-Field
Illumination
Point-like source at
near right angle to
object surface
Point
Side
Illuminates defects;
provides a highcontrast image in
some applications
Does not illuminate flat,
smooth surfaces
Diffuse Backlighting
Source with diffuser
behind object
Diffuse
Back
Easy to implement;
creates silhouette of
part; very-highcontrast image; low
cost
Edges of parts may be
fuzzy; must have space
available behind object for
illuminator
Collimated
Backlighting
Point source with
collimating lens
behind object
Point
Back
Polarized Front
Point or
Illumination
Diffuse
Point-like or diffuse
front illumination;
polarizer on
illuminator; analyzer
in front of imaging
lens
Front
Polarized
Backlighting
Diffuse backlight;
polarizer on
illuminator; analyzer
in front of imaging
lens
Back
Diffuse
Produces sharp edges Must have space available
for gauging
behind object for
illuminator
Reduces glare
Reduces light to lens
Highlights
Only useful for birefringent
birefringent defects; defects; edges of parts may
relatively easy to
be fuzzy; must have space
implement
available behind object for
illuminator
36
Figure 15. Solid angle
Point-Like Lighting
Point-like lighting is generally easy to implement because the illuminators are small and
can be located at a distance from the object. Incandescent lamps, optical fiber bundles,
ring lights, and LEDs are examples of point-like illuminators. Some, like fiber optic
bundles, are directional, so light can be directed onto the object from a distance.
Point-like illumination provides high intensity and light efficiency. It is good for creating
sharp image edges, casting shadows, and accenting surface features. Their small size
makes the illuminators easier to mount and integrate than diffuse sources.
The same shadows and surface features that are useful in some applications can be
distractions in others. With specular objects, point-like illumination creates very bright
reflections that may saturate video cameras. Away from these reflections, specular
objects appear dark.
Diffuse Lighting
By definition, diffuse lighting must cover a large solid angle around the object.
Fluorescent lamps (both straight tubes and ring lights) are inherently diffuse. Diffusers in
front of point-like sources make them more diffuse.
Diffuse illumination of specular surfaces allows imaging without bright reflections.
Surface texture is minimized, and there is less sensitivity to surface angles on parts.
Diffuse illumination can be difficult to implement because the illuminator must surround
much of the object. For example, when reading characters stamped on textured foil,
sources with solid angles approaching 2 steradians are required. These “light tents” are
difficult to construct effectively because the lens, camera, and handling equipment must
be mounted around the illuminator. Diffuse illumination can also cause blurred edges in
images. In general, a diffuse illuminator is more complex than a point-like illuminator.
37
Lighting Direction
Bright field
In bright-field illumination, the light comes in approximately perpendicular to the object
surface (figure 16). The whole object appears bright, with features displayed as a
continuum of gray levels. Normal room lighting is bright-field illumination. This sort of
illumination is used for most general-vision applications.
An important special case of bright-field illumination is coaxial illumination. Here, the
object is illuminated from precisely the direction of the imaging lens. This requires a
beamsplitter, either within or in front of the imaging lens. Coaxial illumination is used to
inspect features on flat, specular surfaces, to image within deep features, and to eliminate
shadows.
Dark field
If the object is illuminated from a point parallel to its surface, texture and other high
angle features appear bright while most of the object appears dark. This low angle
illumination is called dark-field illumination. Dark-field illumination is useful for
imaging surface contamination, scratches, and other small raised features.
Figure 16. Lighting angles
Backlight
Backlight illumination means the illuminator is behind the object. It can be either pointlike or diffuse. Point-like lighting, projected through a collimator whose axis is parallel to
the lens axis, is similar to coaxial lighting.
There are two distinct uses of backlighting: viewing translucent objects in transmission
and silhouetting opaque objects.
38
Sheet glass is an example of a translucent product inspected using backlight. Point-like
lighting that is not coaxial with the lens highlights surface defects (scratches, gouges) as
well as internal defects (bubbles, inclusions).
Backlighting is more commonly used to silhouette opaque parts. Silhouettes are easy
images to process because they are inherently two dimensional and binary. Flexible parts
feeders frequently use back lighted images to determine the orientation of mechanical
parts to be picked up by a robot for assembly.
Lighting Color
Most machine vision applications use unfiltered light. However, in some cases,
monochromatic illumination provides better feature contrast. A narrow spectrum also
reduces the effect of any chromatic aberration in the imaging lens and therefore provides
improved resolution. Filtering does, however, reduce the amount of illumination and may
be unsuitable for applications where there is a shortage of light.
Polarization
Polarized illumination is used to reduce glare from specular surfaces. A polarizer is
placed in front of the illuminator, and another polarizer (called the analyzer), whose
polarization axis is perpendicular to that of the first, is placed in front of the imaging lens.
Light that is specularly reflected from the object retains its polarization direction, and is
therefore blocked by the analyzer. Light scattered from the object is randomly polarized
and is passed by the analyzer.
Light Sources
Several types of light sources and illuminators are available for machine vision
applications. Table 4, below, summarizes the properties:
Light Source Type
Advantages
LED Array of light-emitting Can form many configurations
diodes
within the arrays; single color
source can be useful in some
applications. Can strobe LEDs
at high power and speed
Fiber-Optic Illuminators
Incandescent lamp in
housing; light carried by
optical fiber bundle to
application
Disadvantages
Some features hard to
see with single color
source; large array
required to light large
area
Fiber bundles available in
Incandescent lamp has
many configurations; heat and
low efficiency,
electrical power remote from especially for blue light
application; easy access for
lamp replacement
39
Light Source Type
Advantages
Disadvantages
Fluorescent Highfrequency tube or ring
lamp
Diffuse source; wide or
narrow spectral range
available; lamps are
efficient and long lived
Limited range of configurations;
intensity control not available on
some lamps
Strobe Xenon arc
strobe lamp, with
either direct or fiber
bundle light delivery
Freezes rapidly moving
parts; high peak
illumination intensity
Requires precise timing of light
source and image capture
electronics. May require eye
protection for persons working near
the application
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 most 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 modern physics, proposes that energy is emitted and
absorbed in discreet quanta or packets of energy (photons). While both models are based
on a massless, chargeless 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 1.1
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.
40
Figure 2.1:
The Electromagnetic spectrum
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 to stimulate sight.
Types of Light
Light sources for visual and optical inspection may be divided into four
categories: incandescent, luminescent, polarized, and coherent light.
Incandescent Light
Incandescence is the emission of light due to the thermal excitation of the atoms
or molecules. Sources of incandescent light include filament lamps, pyroluminescence,
gas mantles, and carbon arc lamps
Luminescent Light
Luminescence results from the excitation of a single valence electron.
Luminescent light is more monochromatic in nature than incandescent light sources.
Sources of luminescent light include gaseous discharge lamps, lasers, light emitting
diodes (LED), and fluorescent lamps.
41
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 2.2) states that the
illumination of a surface varies inversely as the distance between the light source and
surface is squared.
E
=
I/d2
Where
I = luminous intensity
E = illuminance
D = 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 1m 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.
E = I cos θ
where
I = source illuminance
E = surface illuminance
θ = angle of incidence
Figure 2.2:
The Inverse Square Law
42
Combining the inverse square law with the Lambert Cosine law (Figure 2.3)
allows illuminance at angles other than normal to be calculate.
Measurements of visible light are made in reference to primary standards
established by national physical laboratories. In the United States, the National Institute
of Standards and Technology (NIST) maintains most physical standards. From these
primary standards, working standards are prepaid 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.
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.
Figure 2.3: The Lambert Cosine law
In addition to the illumination intensity at the inspection site, the color of the light
is also important. Color plays a significant role 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.
43
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.
Light Source
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
videoprobes. 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.
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.
Discharge (Arc) Lighting
Discharge (arc) lamps use an electric are to produce light. This lamp type is used
in some videoprobes imaging systems as 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.
44
CHAPTER 2
FUNDAMENTAL OF LIGHT & LIGHTING
1)
Mono chromatic light emit light of ---------------A. Single wavelength
B. Coming from same type of the source
C. Having same intensity
D. None
2) A device used to view moving object is
A. Stroboscope.
B. Dynamic scope
C. Kinetic scope
D. None of the above
3) Fluorescent material emits light of
A. Shorter wavelength
B. Longer wave length
C. Same wave length
D. Wave length does not matter at all
4) Out of these which are not luminous bodies
A. Sun
B. Moon
C. Stars
D. Candle light
5) Laser light is
A. Luminescent light
B. Polarized light
C. Coherent light
D. Incandescent light
6) Laser light has
A. Spatial coherence
B. Phase coherence
C. Both
D. Should be monochromatic only
7) The relation ship between velocity, wavelength, frequency and refractive index is
given as
V = velocity of light in the medium
v = frequency
μ = refractive index
λ = wavelength
45
A.
B.
C.
D.
V = λν/μ
V =λμ/ν
V= μ/λν
μ= λV/ν
8) Law of reflection holds good
A. For rough surface reflection
B. For specular reflection
C. For diffused reflection
D. For the medium of low refractive index
9) Photometers are of there type
A. Direct , indirect substitute photometers
B. Visual comparison, Photoelectric, photo emissive devices
C. Direct , relative, substitute photometer
D. All of the above
10) Neutral filters are
A. Wire mesh and perforates metals
B. Glass filter
C. Plastic filter
D. None of the above
11) Chromatic contrast
A. Is usually more than luminous contrast
B. Is less than luminous contrast
C. Same as luminous contrast
D. Contrast is in no way affected by the above parameters
12) Which is the best viewing light with a minimum glare
A. Direct light
B. Semi direct light
C. General diffused light
D. Monochromatic light
13) For photography under bright light the film chosen would be:
A. 100 asa
B. 200 asa
C. 400 asa
D. 800 asa
14) A source of light called ‘cold light’ is likely to be used in:
A. A camera flash bulb
B. A microscope
C. A fibroscope
D. Black light bulb
46
15) ‘Kerititis’is likely to be caused by exposure to
B. White light
C. Infrared light
D. Polarized light
E. Ultraviolet light
F. Laser light
16) Luminous intensity of a radiating source in any direction is measure in
A. Watts
B. Watt/hour
C. Lambert
D. Candela
17) The luminous flux of a source in a steradian is measured in
A. Lambert
B. Candela
C. Lumen
D. Foot candela
18) One lumen light flux falling on 1m2 area on the surface of a sphere around the
source results in an illumination level of
A. 100 lux
B. 1 lux
C. 10 lambert
D. 1 candela
19) Light can be detected using
A. Photoelectric cell
B. Bolometer
C. a and b only
D. None
20) Which distance require on a surface is 500lux .a bulb has light output of 400 lux
at a distance of 1 meter. What is the farthest distance the bulb can be placed?
A. 4 feet
B. 4 meter
C. 1.4 meters
D. 9.4 meters
E. none of the above
21) Glare light can be reduced on an inspection surface by using :
A. Visible light.
B. Spectral light.
C. Screens.
D. Polarized light
47
22) Radiant energy that excites the retina and produces a visual sensation is called :
A. Vision.
B. Light.
C. Spectrum.
D. Color.
23) In order to obtain light of a specific wavelength, use :
A. Filters.
B. Shades.
C. Reflectors.
D. Diffractive screens.
24) A device that uses synchronised pulses of high-intensity light to permit viewing
of objects moving with a rapid periodic motion is called :
A. Stereophotometer.
B. Stereoscope.
C. Stroboscope.
D. Spectrophotometer.
25) The intensity of florescence in relation to the intensity of the ultra violet
radiation that excites it is :
A. Inversely proportional to the intensity of the ultra violet radiation.
B. Directly proportional to the intensity of the ultra violet radiation.
C. Directly proportional to the square of the intensity of the ultra violet radiation.
D. Not dependent upon the ultra violet radiation
26) The principal biological effect of infra red radiation is :
A. Thermal fatigue.
B. Hyperthermia.
C. Blue hazard.
D. Ultra violet hazard.
27) The light from common sources, particularly light from incandescent lamps, is
often compared with light from a theoretical source. This theorotical source is
called a :
A. Graybody.
B. Photometer.
C. Blackbody.
D. Light comparator.
28) The simultaneous comparison of a standard lamp and an unknown light source
is called :
A. Absolute photometry.
B. Relative photometry.
C. Direct photometry.
D. Substitution photometry
48
29) The measurement of radiant energy in the visible spectrum, based on a standard
observer response, is called :
A. Photometry.
B. Spectrometry.
C. Goniometry.
D. Spectrdiometry.
30) The illumination at a point on a surface in relation to the luminous intensity of
the source and the distance between the source and the point varies directly with
the intensity and :
A. The distance.
B. Inversely with the distance.
C. Inversely with the square of the distance.
D. Square of the distance.
31) According to the illuminating engineering society, the minimum light required
for critical work should be :
A. 500 lx (46 ftc).
B. 1100 lx (102 ftc).
C. 2152 lx (200 ftc).
D. 5382 lx (500 ftc).
32) The brightness of a diffusely reflecting colored surface depends on the quantity
of incident light and :
A. The reflecting factor.
B. The quality of incident light.
C. Light intensity.
D. The amount of reflected glare.
33) High speed film requires :
A. Less light but can produce less graininess.
B. More light but can produce more graininess.
C. Less light but can produce more graininess.
D. More light but can produce less graininess
34) Compact arc sources, metal vapor, and florescent lamps are sources of :
A. Ultraviolet hazards.
B. Infrared hazards.
C. Electromagnetic hazards.
D. Visible hazards.
35) Light is defined as that
wavelength s between :
A. 280 nm and 560 nm.
B. 320 nm and 650 nm.
C. 325 nm and 780 nm.
D. 380 nm and 770 nm.
portion of the electromagnetic spectrum with
49
36) Most color deficiencies are hereditary and occur in :
A. The brown-green range.
B. The red-green range.
C. The blue-yellow range.
D. The blue-green range.
37) Color deficiencies can be hereditary and/or acquired.
deficiencies can include :
A. Trichromatism (three colors).
B. Protanopia (red lacking).
C. Tritanopia (blue lacking).
D. Protan-dueton (red-yellow).
acquired color
38) The principle of solid state image devices is based on :
A. Photoelectric effect and the free electrons that are created in a region of
silicon illuminated by photons.
B. Generation of a train of electrical pulses that represent light intensities present
in an optical image.
C. The amount of charge in each packet that stays substantially the same.
D. An electron beam that is used to scan a photoconductive target.
39) When visual inspection of finished weldments is required, the inspector should :
A. Examine the weld with a low powered magnifier.
B. Examine the weld with liquid penetrant.
C. Verify the qualification of the welder.
D. Visually examine the weld with sufficient illumination
40) A material that emits light when excited by illuminated areas of a test object is
said to be :
A. Photovoltaic.
B. Luminescent.
C. Photoresistant.
D. Radiescent.
41) A device which converts light into useful energy is called
A. Lux meter
B. Photo diode
C. Photo voltaic cell
D. None of the above
42) Measurement of property of light is called
A. Photo geometry
B. Photometry
C. Optometry
D. B or c only
50
43) Amount of light falling on a film inside the camera is controlled by
A. Aperture
B. Shutter speed
C. Speed of film
D. All of the above
E. A and b only
44) Exposure of eye to UV light can be reduced by using
A. Lead glasses
B. Polarized glasses
C. Sodium glasses
D. None of the above
E. A or c only
45) The ratio of light source output of a object to the output of the theoretical black
body radiator( at a specific wavelength) is called
A. Spectral sensitivity
B. Spectrometry
C. Spectral emissivity
D. None of the above
46) In the visible region. A gray body is approximated by
A. Tungsten incandescent bulb
B. Carbon arc lamp
C. Tube light
D. Xenon light
47) The color of light is determined by its
A. Velocity
B. Frequency
C. Wavelength
D. B or C only
48) The visible colors can be created by mixing the proper amounts of
A. Primary color
B. Secondary color
C. Shades of black and white
D. Any of the above
E. None of the above
49) As a object heats up the visible color changes from
A. Dull blue to white
B. Dull red to deep blue
C. Dull red to blue white
D. Dull red to orange
51
50) The light sources for visual and optical inspection are typically divided into
following categories
A. Luminescent
B. Incandescent
C. Polarized
D. All of the above
E. A and b only
51) Emission of light due to thermal excitation of atoms or molecules of a solid is
called
A. Luminescent light
B. Incandescent light
C. Polarized light
D. None of the above
52) Polarization could be
A. Linear
B. Circular
C. Elliptical
D. All of the above
E. A and b only
53) Polarized filters can be used control of light
A. Intensity
B. Color
C. Glare
D. All of the above
E. A and c only
54) Polarized light is used in which of the following technique
A. Bifrigence technique
B. Moiré fringe technique
C. Magnetosirctive technique
D. All of the above
E. A and b only
55) Coherent light is usually produced by
A. Ruby laser
B. Fluorescent light
C. Sodium lamp
D. All of the above
52
56) Monochromatic light is produced by
A. Ruby laser
B. Fluorescent light
C. Sodium vapor lamp
D. All of the above
E. A and c only
57) Velocity of light in vacuum is
A. Constant
B. Same for all wavelengths
C. Faster than in any other medium
D. All of the above
58) The refractive index depend upon
A. Medium of incident wave
B. Medium of refracted wave
C. Frequency of the light wave
D. All of the above
E. B and c only
59) Units of luminance are
A. Candela
B. Candela/ft2
C. Candela/m 2
D. LUX
60) The inverse square law is accurate within 0.5% when
A. The source is a point source
B. The measurement is done at a point 5 times greater than source size
C. When the source light output is small
D. All of the above
E. A and b only
61) Lamert’s cosine law is given by
A. E =I/cosø
B. E =I.secø
C. E=I.cosø
D. E=cosø/I
where I = source illuminance
E= surface illuminance
Ø= angle of incidence
62) When the incident beam of light is reflected equally in all directions, the
reflecting surface is called
A. Specular reflector
B. Diffuse reflector
C. Scatter reflector
D. Luminous reflector
53
63) Which of the following is usually used in photometry of extremely low
illuminance levels
A. Photovoltaic cell made from selenium
B. Solar cell made for silicon
C. Photo conductive cell made from cadmium sulfide
D. Any of the above
64) Minimum glare would be caused by
A. Direct lighting
B. Semi-direct lighting
C. Indirect lighting
D. General diffuse lighting
65) Any surface that reflects uneven amounts of the incident light initial wavelengths
and absorbs the balance is a called ……………..surface
A. Specular
B. Spectral
C. Spectrally selective
D. Transmitive
66) The technique used for surface flaw recognition is generally
A. Diffuse front illumination
B. Dark field specular illumination
C. Light field specular illumination
D. Diffuse rear illumination
67) The technique generally used for assessing color or gloss is
A. Diffuse front illumination
B. Dark field specular illumination
C. Light field specula illumination
D. Diffuse rear illumination
68) Typical devices used for manipulation of light to produce images for human
viewing are
A. Mirrors
B. Lenses
C. Prisms
D. All of the above
69) Scanning is typically used in
A. Electronic cameras
B. Flat bed scanners
C. Monitors
D. All of the above
E. B and c only
54
70) The aspect ratio of a raster scan display is defined as the ratio of its
A. Diagonal width to height
B. Width to height
C. Diagonal width to horizontal width
D. None of the above
71) When a radiating body had E(λ) nearly same for all wavelengths it is called
A. Black body
B. Grey body
C. Red body
D. White body
72) The color of objects created by reflected light is created using
A. Additive primary colors
B. Subtractive primary colors
C. Both a and b
D. Neither a or b
73) To get accurate color response of the imager the color ____________ of light
source must be known
A. Color temperature
B. Temperature
C. Power output
D. All of the above
74) Examples of luminous bodies would include
A. Sun, moon, and star
B. Sun, incandescent lamp and firefly
C. Aurora borealis tube light, moon
D. All of the above
75) Sources of incandescent light are
A. Tungsten filament lamp
B. Gas mantles
C. Fluorescent lamps
D. All of the above
76) Sources of luminescent light are
A. Pyroluminescence
B. Lasers
C. Light emitting diodes(LED)
D. B and c only
E. All of the above
55
77) When the vector describing direction of light wave from resembles a helix. It is
called
A. Elliptical polarization
B. Cylindrical polarization
C. Circular polarization
D. Helical polarization
78) The sensor in which electrical resistance changes as light falls on it is called
A. Photovoltaic cell
B. Photoconductive cell
C. Photoemissive cell
D. None of the above
79) Materials that appear black or grey are
A. Selective reflectors
B. Non selective reflectors
C. Diffuse reflector
D. Chromatic reflector
80) The recommended maximum luminance ratio between tasks and more remote
lighter Surfaces should be
A. 1 to 3
B. 3 to 1
C. 1 to 20
D. 1 to 100
81) Spherical aberration in a mirror is minimum for
A. Concave mirrors
B. Convex mirrors
C. Parabolic mirrors
D. It is some for all mirrors
82) The image undergoes inversion and 180 reflection when using
A. A concave lens
B. A right angle prism
C. A porro prism
D. None if the above
E. A and c only
83) The ability of an optical device or system to gather light is controlled by
A. Lens diameter
B. Aperture
C. Focal length
D. B and c
56
Unacceptabl
e fillet weld
profiles:
(a)
Insufficient
throat (b)
84) The conversion factor from lux to foot candle is
A. Multiply by 10.76
B. Divided by 10.76
C. Divided by 1076
D. Multiply by 12
85) A pocket weld magnifier has the magnification range of
A. 1.5x to 10x
B. 0-100x
C. 0-1000x
D. 0-10000x
86) The unit used for illumunance is
A. Nanometer
B. Armstrong
C. Lux
D. Footcandle
E. ‘c’ and ‘d’ are correct
87) The inverse square law and lambert cosine law can be combined as
A. E=(l/d2 ) cos ø
B. E=l cos ø
C. E=(l/d2 )
D. E=ld2
88) Visual inspection with very low contrast and very small size objects would
generally need the light of
A. 100 to 200 ftc
B. 10 to 20 ftc
C. 1000 to 2000 lux
D. ‘a’ and ‘c’ are correct answer
E. Neither of above
89) Glare can be reduced on an inspection surface by using
A. Visible light
B. Spectral light
C. Screens
D. Polarized light
57
CHAPTER 3
FUNDAMENTALS OF IMAGING
Light Manipulation
The science of optics provides an explanation for the operation of many visual and
optical tools from simple magnifying glasses to metallographs. Classical optics explains the
manipulation of light by mechanical devices to produce an image for human viewing. These
mechanical devices are categorized as lenses, mirrors, and prisms.
Lens Fundamentals
Lens
A lens is a device for either concentrating or diverging light, usually formed from a piece of
shaped glass. Analogous devices used with other types of electromagnetic radiation are also
called lenses: for instance, a microwave lens can be made from paraffin wax.
In its usual form, a lens consists of a slab of glass or other optically transparent material (such as
perspex) with two shaped surfaces of a particular curvature. It is the refractive index of the lens
material and the curvature of the two surfaces that give a particular lens its particular properties.
A lens works by refracting (bending) the light that passes through it, in a similar manner to a
prism.
Lens construction
Figure 3.1
58
The most common type of lenses are spherical lenses, which are formed from surfaces that have
spherical curvature, that is, the front and back surfaces of the lens can be imagined to be part of
the surface of two spheres of given radii, R1 and R2, which are called the radius of curvature of
each surface. The sign of R1 gives the shape of the front surface of the lens: if R1 is positive, the
surface is convex (bulging outwards from the lens). If R1 is negative, the front surface is concave
(bulging into the lens). If R1 is infinite, the surface is flat, or has zero curvature, and is said to be
plane. The same is true for the back surface of the lens, except that the sign conversion is
reversed: if R2 is positive, it is concave, and if R2 is negative, the back surface is convex. The line
joining the centers of the spheres making up the lens surfaces is called the axis of the lens; in
almost all cases the lens axis passes through the physical centre of the lens.
Figure 3.2
Lens are classified by the curvature of these two surfaces. A lens is biconvex if both surfaces are
convex, likewise, a lens with two concave surfaces is biconcave. If one of the surfaces is flat, the
lens is termed plano-convex or plano-concave depending on the curvature of the other surface. A
lens with one convex and one concave side is termed convex-concave, and in this case if both
curvatures are equal it is a meniscus lens.
If the lens is biconvex or plano-convex, a collimated or parallel beam of light passing along the
lens axis and through the lens will be converged (or focused) to a spot on the axis, at a certain
distance behind the lens (known as the focal length). In this case, the lens is called a positive or
converging lens.
59
Figure 3.3
If the lens is biconcave or plano-concave, a collimated beam of light passing through the lens is
diverged (spread); the lens is thus called a negative or diverging lens. The beam after passing
through the lens appears to be emanating from a particular point on the axis in front of the lens;
the distance from this point to the lens is also known as the focal length, although it is negative
with respect to the focal length of a converging lens.
If the lens is convex-concave, whether it is converging or diverging depends on the relative
curvatures of the two surfaces. If the curvatures are equal (a meniscus lens), then the beam is
neither converged or diverged.
The value of the focal length f for a particular lens can be calculated from the lensmaker's
equation:
,
where n is the refractive index of the lens material and d is the distance along the lens axis
between the two surfaces (known as the thickness of the lens). If d is small compared to R1 and
R2, then the thin lens assumption can be made, and f can be estimated as:
.
The focal length f is positive for converging lenses, negative for diverging lenses, and infinite for
meniscus lenses. The value 1/f is known as the power of the lens, and so meniscus lenses are said
to have zero power. Lens power is measured in dioptres, which have units of inverse meters (m1
).
60
Lenses are also reciprocal; i.e. they have the same focal length when light travels from the front
to the back as when light goes from the back to the front (although other properties of the lens,
such as the aberration [see below] are not necessarily the same in both directions).
Imaging properties
As mentioned above, a positive or converging lens will focus a collimated beam traveling along
the lens axis to a spot (known as the focal point) at a distance f from the lens. Conversely, a point
source of light placed at the focal point will be converted into a collimated beam by the lens.
These two cases are examples of image formation in lenses. In the former case, an object at an
infinite distance (as represented by a collimated beam of light) is focused to an image at the focal
point of the lens. In the later, an object at the focal length distance from the lens is imaged at
infinity.
Figure 3.4
If the distances from the object to the lens and from the lens to the image are S1 and S2
respectively, for a lens of negligible thickness they are found by the thin lens formula:
What this means is that, if an object is placed at a distance S1 along the axis in front of a positive
lens of focal length f, a screen placed at a distance S2 behind the lens will have an image of the
object projected onto it, as long as S1 > f. This is the principle behind photography. The image in
this case is known as a real image.
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Figure 3.5
Note that if S1 < f, S2 becomes negative, and the image is apparently positioned in front of the
lens. Although this kind of image, known as a virtual image, cannot be projected on a screen, an
observer looking through the lens will see the image in its apparent calculated position.
The magnification of the lens is given by:
,
where M is the magnification factor; if |M|>1, the image is larger than the object. Notice the sign
convention here shows that, if M is negative, as it is for real images, the image is upside-down
with respect to the object. For virtual images, M is positive and the image is upright.
In the special case that S1=∞, we have S2=f and M=-f/∞=0. This corresponds to a collimated
beam being focused to a single spot at the focal point. The size of the image in this case is not
actually zero, since diffraction effects place a lower limit on the size of the image (see Rayleigh
criterion).
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Figure 3.6
The formulas above may also be used for negative (diverging) lens by using a negative focal
length (f), but for these lenses only virtual images can be formed.
Aberrations
Lenses do not form perfect images, and there is always some degree of distortion or aberration
introduced by the lens which causes the image to be an imperfect replica of the object. Careful
design of the lens system for a particular application ensures that the aberration is minimized.
Figure 3.7
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There are several different types of aberration. Spherical aberration is caused because spherical
surfaces are not the ideal shape with which to make a lens, but they are by far the simplest shape
to which glass can be ground and polished and so are often used. Spherical aberration causes
beams parallel to but away from the lens axis to be focused in a slightly different place than
beams close to the axis. This manifests itself as a blurring of the image. Lenses in which closerto-ideal, non-spherical surfaces are used are called aspheric lenses, which are complex to make
and often extremely expensive. Spherical aberration can be minimized by careful choice of the
curvature of the surfaces for a particular application: for instance, a plano-convex lens which is
used to focus a collimated beam produces a sharper focal spot when used with the convex side
towards the beam.
Figure 3.8
Chromatic aberration is caused by the dispersion of the lens material, the variation of its
refractive index n with the wavelength of light. Since from the formulae above f is dependent on
n, if follows that different wavelengths of light will be focused to different positions. Chromatic
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aberration of a lens is seen as fringes of color around the image. It can be minimized by using an
achromatic doublet (or achromat) in which two materials with differing dispersion are bonded
together to form a single lens. This reduces the amount of chromatic aberration over a certain
range of wavelengths, though it does not produce perfect correction. The use of achromats was
an important step in the development of the optical microscope.
Thin-Lens Model
To understand machine vision lenses, we start with the thin-lens model. It is not an exact
description of any real lens, but illustrates lens principles. It also provides terms with which to
discuss lens performance. A ray, called the chief ray, follows a straight line from a point on the
object, through the center of the lens, to the corresponding point on the image (figure 3.10). The
lens causes all other rays that come from this same object point and that reach the lens to meet at
the same image point as the chief ray. Those rays which pass through the edge of the lens are
called marginal rays.
Figure 3.9
To understand machine vision lenses, we start with the thin-lens model. It is not an exact
description of any real lens, but illustrates lens principles. It also provides terms with which to
discuss lens performance. A ray, called the chief ray, follows a straight line from a point on the
object, through the center of the lens, to the corresponding point on the image (figure 3.10). The
lens causes all other rays that come from this same object point and that reach the lens to meet at
the same image point as the chief ray. Those rays which pass through the edge of the lens are
called marginal rays.
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Figure 3.10. Thin-lens model
The distance from the object plane to the lens is called the object conjugate. Likewise, the
distance from the lens to the sensor plane is called the image conjugate. These conjugates are
related by the lens maker’s formula:
Focal Length
If we let the object conjugate, get very large, we see
In other words, the focal length is the distance between the lens and the sensor plane when the
object is at infinity. For photographic lenses, the objects are usually far away, so all images are
formed in nearly the same plane, one focal length behind the lens.
From figure 3.10 and geometry, we can see that
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The magnification is the ratio of the image to the object conjugates. If the focal length of a lens
increases for a specified magnification, both object and image conjugates increase by the same
ratio.
Thin-Lens Example
We need a magnification of 0.5x, with a working distance of 50 mm. We want to find the correct
lens focal length and total system length (TSL). From the equations (after some algebra), we get:
so
Therefore, we need a lens with focal length of approximately 17 mm. The total system length is
approximately 75 mm.
f-Number (f/#)
The f-number describes the cone angle of the rays that form an image (figure 3.11). The fnumber of a lens determines three important parameters:

The brightness of the image

The depth of field

The resolution of the lens
For photographic lenses, where the object is far away, the f-number is the ratio of the focal
length of the lens to the diameter of the aperture. The larger the aperture, the larger the cone
angle and the smaller the f-number. A lens with a small f-number (large aperture) is said to be
“fast” because it gathers more light, and photographic exposure times are shorter. A well-
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corrected fast lens forms a high-resolution image, but with a small depth of field. A lens with a
large f-number is said to be “slow”. It requires more light, but has a larger depth of field. If the
lens is very slow, its resolution may be limited by diffraction effects. In this case, the image is
blurred even at best focus.
The f-number printed on a photographic lens is the infinite conjugate f-number. It is defined as:
where f is the focal length of the lens and A is the diameter of the lens aperture. When the lens is
forming an image of a distant object, the cone half-angle of the rays forming the image is:
Figure 3.11. f-number
This infinite conjugate f-number is only applicable when the lens is imaging an object far away.
For machine vision applications, the object is usually close and the cone angle is calculated from
the working f-number.
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F-Number (Working)
In machine vision, the working f-number describes lens performance:
where s2 and s1 are the image and object conjugates, respectively. f/#image is called the working f/number in image space, or the simply image side f-number. Similarly, f/#object is the object side fnumber.
For close objects, f/#image is larger than f/#infinity, so the lens is “slower” than) the number given
on the barrel. For example, a lens shown as f/4 on its barrel (i.e, an f-number of 4) will act like
an f/8 lens when used at a magnification of 1.
The object-side f-number determines depth of field. It is given by:
Numerical Aperture (NA)
For lenses designed to work at magnifications greater than 1 (for example, microscope
objectives), the cone angle on the object side is used as the performance measure. By convention,
this angle is given as a numerical aperture (NA). The NA (figure 3.12) is given by:
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Figure 3.12. Numerical aperture (NA)
NA is related to f-number by these exact relationships:
For N/A < 0.25 (f-number >2), these simplify to:
Real-World Lenses
Thick-Lens Model
The thin-lens model treats a lens as a plane with zero thickness. To model a real-world lens, we
divide this thin-lens plane into two planes (figure 3.13). These planes contain the entrance and
the exit pupils of the lens. Everything in front of the entrance pupil is said to be in object space.
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Everything behind the exit pupil is said to be in image space. How light gets from the entrance
pupil to the exit pupil is not considered in this model.
Aberrations
Standard Lenses
In object space, we think of the real-world lens as a thin lens located at the entrance pupil. The
entrance pupil is generally located within the physical lens, but not always. Wherever it is
located, light rays in object space proceed in straight lines until they reach the entrance pupil.
The effects of any elements in front of this position are taken into account when the entrance
pupil position is calculated. In the same way, we think of the real-world lens as a thin lens
located at the exit pupil in image space.
For many lenses, the entrance and exit pupils are located near each other and within the physical
lens. The exit pupil may be in front of or behind the entrance pupil. For certain special lens
types, the pupils are deliberately placed far from their “natural” positions. For example, a
telephoto lens has its exit pupil far in front of its entrance pupil (figure 3.14). In this way, a longfocal-length lens fits into a short package. A telecentric lens has its entrance pupil at infinity,
well behind its exit pupil (figure 3.15).
Figure 3.13. Thick-lens model
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Figure 3.14. Telephoto lens
Figure 3.15. Telecentric lens
Aberrations
If real lenses followed first order theory, lens design would be easy. Unfortunately, it is difficult
to make a real lens approximate this behavior. Diffraction sets a lower limit on image spot size.
The differences between ideal “diffraction limited” behavior and real-lens behavior are called
aberrations.
The job of the lens designer is to choose glasses, curvatures, and thicknesses for the lens’
elements that keep its overall aberrations within acceptable limits. Such a lens is said to be well
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corrected. It is impossible to design a lens that is well corrected for all conjugates, FOVs, and
wavelengths. The lens designer works to correct the lens over the small range of operating
conditions at which the lens must function. The smaller the range, the simpler the design can be.
A lens that is corrected for one set of conditions may show significant aberrations when used
under a different set of conditions. For example, a surveillance lens with a magnification of 1/10
is corrected for distant objects. By using extension tubes, the image conjugate of the lens can be
extended so that the lens forms an image at a magnification of 1. But this image may show
significant aberrations, because the lens was not corrected to work at these conjugates.
Standard Lenses
Commercial lenses, produced in high volume, are by far the best value in terms of performance
for the price. Finding a suitable stock lens is the most cost-effective solution to a machine vision
problem. Table 1 lists various lens types and their range of operating conditions. Commercial
lenses incorporate design and manufacturing techniques that are not available in custom designs.
For example, a lens for a 35-mm, single-lens reflex (SLR) camera that costs one hundred dollars
at the local camera store would cost ten thousand dollars to design and many thousands of dollars
to manufacture in small quantities. It is always best to consider commercial lens options before
starting a custom lens design.
Lens type
Magnification Image format Object FOV Focal length
Working f-number
(object side)
Surveillance
<0.1
1¼" CCD
format
large
2–50 mm
>20 (adjustable)
Standard
Machine Vision
.05–5
2/3" CCD
2–200 mm
25–75 mm
>4 (adjustable)
Telecentric
Machine Vision
.07–5
2/3" CCD
2–170 mm
N/A
>6 (adjustable)
F-mount
Lenses
<1
45 mm
large
35–100 mm
>4 (adjustable)
Large/Medium
Format
Photographic
<1
80 mm
large
50–250 mm
>4 (adjustable)
Photographic
Enlarger
2–20
500 mm
50 mm
40–150 mm
>4 (adjustable)
Microscope
5–100
requires
additional
lens
<2 mm
5–40 mm
0.1–0.95 NA (fixed)
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Real Lens Parameters
Resolution
Resolution is the ability of an optical system to distinguish between two features that are close
together. For example, if a lens images a row of pins on an electrical connector, it must have
sufficient resolution to see each pin as separate from its neighbors. A lens imaging a lot code on
a pharmaceutical bottle must have sufficient resolution to distinguish one character from
another. Resolution is also required to make sharp images of an edge. A lens with high
resolution will show an edge transition in fewer pixels than a lens with low resolution.
There are many different definitions of lens resolution. They differ by what type of test object is
measured (points, bars, sine patterns, or other objects), and by the criteria for determining when
two objects are “resolved”. A practical measurement for machine vision uses three-bar targets of
various spatial frequencies. A chrome-on-glass USAF-1951 target is a good test object. If the
contrast between bar and space is greater than 20 percent, the bars are considered to be resolved.
Resolution
Diffraction
Contrast
Depth of Field
Telecentricity
Distortion
Spectral Range
Resolution
Resolution does not determine the dimensional accuracy to which objects can be measured. The
position of a large object can be determined to within a fraction of a resolution spot under
suitable conditions. Many vision systems determine positions to one-quarter pixel. On the other
hand, if the lens has distortion, or if its magnification is not known accurately, then the measured
position of a feature may be in error by many resolution spot widths.
Diffraction
Diffraction limits the resolution possible with any lens. In most machine vision calculations, we
consider light as traveling in straight lines (rays) from object points to image points. In reality,
diffraction spreads each image point to a spot whose size depends on the f-number of the lens
and the wavelength of the light. This spot pattern is called an Airy disk. Its diameter is given by:
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where DAiry is the diameter of the inner bright spot,  is the wavelength of light, and the fnumber is the image side f-number. Since the wavelength of visible light is ~ 0.5 , this means
the diameter of the diffraction-limited spot (in m) is approximately equal to the working fnumber.
For example, a typical CCD camera has pixels that are 10 m square. To form a diffractionlimited spot of this diameter, the working f-number on the image side should be ~ 10. An f/22
lens forms an image spot larger than a pixel. Its image therefore appears less sharp than that of
the f/10 image. An f/2 lens image will not appear sharper than an f/10 image, since the camera
pixel size limits the resolution. In this case, the system is said to be detector limited.
Contrast
Contrast is the amount of difference between light and dark features in an image. Contrast (also
called modulation) is defined by:
Here, “light” is the gray level of the brightest pixel of a feature, and “dark” is the gray level of
the darkest pixel. A contrast of 1 means modulation from full light to full dark; a contrast of 0
means the image is gray with no features. Finer (higher spatial frequency) features are imaged
with less contrast than larger features. A high-resolution lens not only resolves finer features, but
generally images medium-scale features at higher contrast. A high-contrast image appears
“sharper” than a lower contrast image, even at the same resolution.
Factors other than lens resolution can affect contrast. Stray light from the environment, and glare
from uncoated or poorly polished optics reduce contrast. The angles of the lens and of the
illumination have a great effect on contrast. The contrast of some objects is dependent on the
color of the illumination.
Depth of Field
Depth of field (DOF) is the range of lens-to-object distances over which the image will be in
sharp focus. The definition of “sharp” focus depends on the size of the smallest features of
interest. Because this size varies between applications, DOF is necessarily subjective. If very fine
features are important, the DOF will be small. If only larger features are important, so that more
blur is tolerable, the DOF can be larger. The system engineer must choose the allowable blur for
each application.
In general, the geometrical DOF (figure 3.16) is given by:
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Figure 3.16. Depth of field
To find the DOF for detector limited resolution, we choose the diffraction spot size created by
the lens to be one pixel width in diameter, and the geometric blur due to defocus also to be one
pixel width in diameter. With these assumptions:
Here, we set the image side f-number of the lens equal to the pixel width in m. Wpixel is the pixel
width in m; m is the lens magnification. Thus, for a camera with 10-m pixels, operating at 0.5x
magnification, with an image side f-number of f/10, the DOF is 800 m, or 0.8 mm.
These assumptions are very conservative. Using a higher f-number reduces the resolution of the
lens slightly, but greatly increases the DOF. For example, with the lens operating at f/22 and
allowing a geometric blur of two pixel widths, the DOF is 3.2 mm, which is four times larger.
This is a better estimate if the important image features are larger than two pixels 40 m. The
choice of f-number and allowable blur depends on the requirements of the particular application.
Telecentricity
Telecentricity determines the amount that magnification changes with object distance. Standard
lenses produce images with higher magnification when the object is closer to the lens. We
experience this with our eyes. A hand held up near your face looks larger than when it is moved
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farther away. For the same field size, a longer focal length shows less magnification change than
a short focal length lens.
A telecentric lens acts as if it has an infinite focal length. Magnification is independent of object
distance. An object moved from far away to near the lens goes into and out of sharp focus, but its
image size is constant. This property is very important for gauging three-dimensional objects, or
objects whose distance from the lens is not known precisely.
A telecentric lens views the whole field from the same perspective angle. Thus, deep round holes
look round over the entire field, rather than appearing elliptical near the edge of the field. Objects
at the bottom of deep holes are visible throughout the field.
The degree of telecentricity is measured by the chief ray angle in the corner of the field (figure
3.17). In machine vision, a standard commercial lens may have chief ray angles of 10 degrees or
more. Telecentric lenses have chief ray angles less than 0.5 degree. Some telecentric lenses have
chief ray angles of less than 0.1 degree.
Telecentricity is a measure of the angle of the chief ray in object space and does not affect the
depth of field.
Figure 3.17. Telecentricity - (a) conventional camera; (b) telecentric lens
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Depth of field is determined by the angles of the marginal rays. Chief ray and marginal ray
angles are independent of each other.
The objective element of a telecentric lens must be larger than the field of view. The lens must
“look straight down” on all portions of the field. Telecentric lenses for very large fields are thus
large and expensive. Most telecentric lenses cover fields less than 6 inches in diameter.
Gauging Depth of Field
The gauging depth of field (GDOF) is the range of distances over which the object can be
gauged to a given accuracy (figure 3.18). A change in object distance changes the image
magnification and therefore the measured lateral position of the object. The gauging depth of
field describes how precisely the object distance must be controlled to maintain a given
measurement accuracy. Telecentric lenses provide larger gauging depths of field than do
conventional lenses.
Figure 3.18. Gauging depth of field
Distortion
In optics, distortion is a particular lens aberration that causes objects to be imaged farther or
closer to the optical axis than for a perfect image. It is a property of the lens design and not the
result of manufacturing errors. Most machine vision lenses have a small amount of pincushion
distortion (figure 3.19). Relative distortion increases as the square of the field, so it is important
to specify the field over which field distortion is measured.
Distortion is generally specified in relative terms. A lens which exhibits 2 percent distortion over
a given field will image a point in the corner of its field 2 percent too far from the optical axis. If
this distance should be 400 pixels, it will be measured as 408 pixels.
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Lens distortion errors are often small enough to ignore. Because distortion is fixed, these errors
can also be removed by software calibration. Lenses designed to have low distortion are
available.
Figure 3.19. Gauging depth of field
Spectral Range
Most machine vision lenses are color corrected throughout the visible range. Filters that narrow
the spectral range to a single color sometimes improve lens resolution. CCD cameras are
inherently sensitive to near-infrared (NIR) light. In most cases, there should be an NIR filter
included in the system to reduce this sensitivity. Many cameras have NIR filters built in.
Prisms
On most surfaces, incident light is partially reflected and partially refracted. The greater
the angle of incidence and the difference in the refractive indices of the material, the more light
will be reflected instead of refracted. The angle above which all light is reflected is known as the
critical angle. Prisms use the critical angle to change the direction or the orientation of the image
produced by light rays. Two common types of prisms are the right angle prism and the porro
prism, as illustrated in Figure 3.5. The right angle prism produces a 180-degree reflection. Both
prisms are common in optical instruments.
Prisms are also used to separate the frequencies of a chromatic light source by diffraction.
Because the two refracting surfaces of the prism are not parallel, the distance the light paths
travel vary from the top to the bottom of the prism. Because the index of refraction changes with
the frequency of the light, the higher frequency portions of the spectrum emerge from the base of
the prism..
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Color and Gloss
The assessment and measurement of color appearance is usually related to the assessment
of the perception of light reflected from the test object by a human observer. This assessment is
highly dependent on the illumination source and on the perceptive abilities of the observer. The
communication of color requirements and the assessment of color against requirements can be
very imprecise. Color requirements can be effectively communicated by visual comparison to a
color order system or a color collection. Color may also be assessed quantitatively.
Color order systems place colors into an orderly three-dimensional arrangement with a
standard nomenclature to describe each color in the system. The two most common are the
Natural Color System and the Munsell Color Order System. Color order systems describe color
using the terms hue, value, and saturation.
Geometry
The physical features of an object (form, profile, orientation, location, and size) must be
controlled. The blueprint or drawing for the object must specify the attributes of each
characteristic including tolerances. The most effective method of specifying the requirements
and of assessing the variations is by using geometric tolerancing techniques. These requirements
are uniformly communicated when ANSI Y 14.5M, Dimensions and Tolerancing, is used to
specify dimension and tolerance.
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CHAPTER 3
FUNDAMENTALS OF IMAGING
1. The lenses used in visual inspection equipment are described as:
a. Concave and convex
b. Converging and diverging
c. Simple, combination, or compound.
d. All of the above
2. The diffraction in a simple, ,thin convex lens conuses:
a. Light to converge, allowing the retinal plane to be placed closer to the test piece.
b. Light to converge and be magnified when the test piece is placed inside the focal
length
c. Light to diverge and be magnified
d. None of the above
3. Magnification of a divergent lens can be determined by a manipulation of the lens
law. Which of the following is the lens law?
a. f = d + v
b. m = S0/SI = D0/DI
c. m = SI/S0 = DI/D0
d. EFL = F1 x F2/F1 + F2 – S
4. The focal length of a lens:
a. Is measured from the principle plane of the lens to the focal plane
b. Determines the closest distance an object can be from the lens and project a sharp
image
c. Determines the spacing used in a compound lens system
d. None of the above
5. The probability of detecting an object against its background is directly related to:
a. The size, shape, and glare factor
b. The angle of the luminous source used during inspection
c. The amount of available localized illuminance
d. The size of the object and its luminance and chromatic contrast
6. Luminance contrast can best be defined by which of the following equations where:
Lg = greater luminance and Ll = lesser luminance
a. C = (Lg – Ll)/Lg
b. C = Ll – Lg/Ll
c. C = LgLl/Ll
d. C = Lg/Ll
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7. The variation of a surface is typically controlled by three independent
characteristics of:
a. Shape, irregularity, and slope
b. Form, waviness, and roughness
c. Shape, profile, and roughness
d. Form, level, and waviness
8. Roughness average (Ra)measures the:
a. Area under the curve between the surface profile and the surface mean after
applying a mathematical filter to eliminate the effects of waviness
b. Root mean square variation of the surface
c. Total variation of the surface profile from the surface mean
d. Average peak height of the surface profile from the mean height after applying a
mathematical filter to eliminate the effects of waviness
9. Variations in form typically controlled by:
a. Applying a filter to eliminate the effects of roughness and analyzing the waviness
b. Applying a filter to eliminate the effects of waviness and analyzing the roughness
c. Dimensional or geometric tolerance specification
d. Applying a leveling filter to the overall profile
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CHAPTER 4
BORESCOPES
Fiber Borescopes
The industrial fiber optic borescopes is a flexible, layered sheath protecting two fiber
optic bundles, each comprising thousands of glass fibers. One bundle serves as the image guide
and the other bundle helps illuminate the object. Light travels only in straight lines but optical
glass fibers bend light by internal reflection and so can carry light around corners .Such are 9 to
30um 9(0.4 TO 1.2 ml) in diameter or roughly one tenth the thickness of a human hair .
A single fiber transmits very little light , but thousand of fibbers may be bundled for
Transmission of light and images .To prevent the light from diffusing , each fiber consists of a
central core of high quality glass coated with a thin layer of another glass with a different
refractive index fig 4.2 This cladding acts as a mirror- all light.
Entering the end of the fiber is reflected internally as it travels fig 4.1 and cannot escape
by passing through the side to an adjacent fiber in the bundle.
Although the light is effectively trapped within each fiber, not all of it emerges from the
opposite end. Some of the light is absorbed by the fiber itself and the amount of absorption
depends on the length of the fiber and its optical quality. For example, plastic fiber can transmit
light and is less expensive to produce than optical glass but plastic is less efficient in it’s to
produce than optical glass but plastic is less efficient in its transmission and unsuitable for use in
fiber optic borescopes.
Fiber image guides
The fiber bundle used as an image guide carries the image guide fig 5.3 carries the
image formed by the objective lens at the distal end or tip of the borescope back to the eyepiece.
The image guide must be coherent bundle; the individual fiber must be precisely aligned so they
are in identical relative positions at their terminations.
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Figure 4.1. Internal reflection of light in an optic fiber can be used to move
the light path in a curve
Figure 4.2: Light paths in fiber bundles: (a) uncoated fibers allow light to
travel laterally through the bundle and (b) coated fibers restrict the light’s
path to its original fiber
Image guide fiber range from 9 to 17um (0.35 to 0.67 mil ) in diameter. There is one of
the factor affecting resolution. Although the preciseness of alignment is far more important.
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Figure 4.3:
Optical fiber bundle used as an image guide
Note that a real image is formed on both highly polished faces of the image guide.
Therefore, to focus a fiber optic borescope for different distance, the objective lens at the tip
must be moved in or out, usually by remote control at the eyepiece section. A separate diopter
adjustment at the eyepieces is necessary to compensate for differences in eyesight.
Fiber light guides
Another fiber bundle carries light from an external high intensity source to Illuminate the
object. This is called the light guide bundle and is noncoherent (se fig 4.4 These fibers are about
30um (1.2mil) in diameter and the size of the bundle is determined by the diameter of the scope.
Fiber optic borescopes usually have a controllable bending section near the tip so that inspector
can direct the borescope during testing and can scan an area inside the test object. Fiber optic
borescope are made in a variety of diameters, some as 3.7 mm (0.15 in) in lengths up to 10m
(30 ft), and with a choice of viewing directions at the tip.
Rigid borescopes
The rigid borescope FIG 4.5 was invented to inspect the bore of rifles and cannons. It was
a thin telescope with a small lamp at the top for illumination. Most rigid borescopes now use a
fiber optic light guide system as an illumination source.
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The image is brought to the eyepiece by an optical train consisting of an objective lens,
sometimes a prism, relay lenses and an eyepiece lens. The image is not a real image but an aerial
image; it is formed in the air between the lenses. This means that it is possible to both provide
Figure 4.4.
Diagram of a typical fiber optic borescope
Figure 4.5.
Typical lens system in a rigid borescope
Diopter correction for the observer and to control the objective focus with a single adjustment to
the focusing ring at the eyepiece.
Focusing a rigid borescope
The focus control in rigid borescope greatly expands the depth of field over nonfocusing
or fixed focus designs. At the same time, focusing can help compensate for the wide variations in
eyesight among inspectors.
Fig 4.6 and 4.7 emphasize the importance of focus adjustment for expanding the depth of field.
Fig 4.6 was taken at a variety of distance with fixed focus. Fig 4.7 was taken at the same distance
as in fig 4.6 but with a variable focus, producing much sharper images.
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Figure 4.6: Borescope images for a variety of distances with fixed focus (see
fig. 4.7): (a) at 75 mm (3 in.), (b) at 200 mm (8 in.) and (c) at 300 mm (12 in.)
Figure 4.7. Borescope Images with variable focus (see Fig. 4.6): (a) 75 mm
(3 in.), (b) 200 mm (8 in.) and (c) 300 mm (12 in.)
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Figure 4.8: Borescope direction of view: (a) direct, (b) side, (c) forward
oblique and (d) retrospective
Need for specifications
Because rigid borescope lack flexibility and the ability to scan areas, specifications
regarding length, direction of view and field of view become more critical for achieving a valid
visual test. For example, the direction of view should always be specified in degrees rather than
in letters or words such as north up forward or left. Tolerances should also be specified.
Some manufacturers consider the eyepiece to be zero degrees abed therefore a direct view
borescope fig 4.9 a is 180degrees. other the manufacturer start with the borescope tip as zero
degrees and then count back toward the eyepiece, making a direct – view 0 degrees.
Setup of rigid borescope
To find the direction and field of view during visual testing with a rigid borescope, place
a protractor scale on a board or worktable. Position the borescope carefully so it is parallel to the
zero line, with the lens directly over the center mark on the protractor. Remember that the optical
center of a borescope is usually 25 to 50mm
(1 to 2 in) behind the lens window.
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By sighting through the borescope, stick pins into the board at the edge of the protractor
to mark the center and both the left and right edges of the view field. This simple procedure
gives both the direction of view and the field of view see fig 4.9, 4.10.
Miniborescope
One variation of the rigid borescope is called the miniborescope fif 23 . In this design, the
really lens train is replaced with a single, solid fiber. The fiber diffuses ions in a parabola from
the center to the periphery of the housing, giving a graded index of refraction. Light passes
through the fiber and at specific intervals an image is formed.
The solid fiber is about 1 mm (0.4 in) in diameter, making it possible to produce high
quality and thin rigid borescopes from 1.7 to 2.7 mm (0.07 to 0.11 in) in diameter. The lens
aperture is so small that the lens has an infinite depth of field
(Like a pinhole camera) and no focusing mechanism is needed.
\Figure 4.9:
Field of view for a rigid borescope
89
Figure 4.10:
Field of view width for varying distances
Figure 4.11:
detail
Miniborescope wide angle lensa) general shape and (b) lens
90
Accessories
Many accessories are available for rigid borescopes. Instant cameras, 35mm cameras,
video cameras can be added to provide a permanent record of a visual test. Closed circuit
television display, with or without video tape, are common as well. Also available is attachment
at the eyepiece permitting dual viewing or right angle viewing for increased accessibility.
Special purpose borescopes
Angulated borescope are available with forward oblique, right angle or retrospective
visual system. These instruments usually consist of an objective section with provision for
attaching an eyepiece at right angles to the objective sections axis. Is permits inspection of
shoulders or recesses in areas not accessible with standard borescope.
Calibrated borescope are designed to meet specific test requirements. The external tubes of these
instrument can be calibrated to indicate the depth of insertion during a test.
Borescope with calibrated reticules are used to determine angles or sizes of objects
In the field when held at a predetermined working distance.
Panoramic borescopes are built with special optical systems to permit rapid panoramic scanning
of internal cylindrical surface of tubes or pipes.
Wide field borescope have rotating prisms to provide fields of view up to 120 degrees. One
application of wide field borescope is the observation of models in wind tunnels under difficult
operating conditions.
Ultraviolet borescopes are used during fluorescent magnetic particle and fluorescent penetrant
test. These borescope are equipped with ultraviolet lamps, filters and special transformers to
provide the necessary wavelengths.
Waterproof and vapor proof borescopes are used for internal test of liquid, gas or vapor
environments. They are completely sealed and impervious to water or other types of liquid.
Water cooled or gas cooled borescope are used for test of furnace cavities, jet engine test cells
and for other high temperature applications.
Typical industrial borescope application
Aviation industry
The use of borescope for test of airplane engines and other components without
disassembly has resulted in substantial saving in costs and time. A borescope of 11mm (0.44 in)
diameter by 380 mm (15 in) working length can be used by maintenance and service departments
for visual testing of engines through spark plug openings, without dismantling the engines. An
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excellent view of the cylinder wall, piston head, valves and valve seats is possible and several
hundred hours of labor are saved for each engine test. Spare engines in storage can also be
inspected for corrosion of cylinder wall surfaces.
Aircraft propeller blades are visually tested during manufacture. The entire welded seam
of a blade can be inspected internally for crack and other discontinuities. Propeller hubs, reverse
pitch gearing mechanism, hydraulic cylinders, landing gear mechanisms and electrical
components also can be inspected with borescope. Aircraft wing spars and struts are inspected
for evidence of fatigue cracks and rivets and wing section cam is tested visually for corrosion.
Borescope used for tests of internal wing tank surface and wing corrugations subject to corrosion
have saved airlines large sums of money by reducing the aircraft are out of service.
Automotive industry
Borescope are widely used in the manufacturing bad maintenance division of the
automotive industry .engine cylinder can be examined through spark plug holes without
removing the cylinder head. The cylinder wall, valves and piston head can be visually tested
for excess wear carbon deposits and surface discontinuities. Crankcases and crankshaft are
examined through wall plug opening without removing the crankcase. Transmission and
differentials are similarly inspected.
Borescope are also useful for locating discontinuities such as crack or blowholes in
casting and forgings. Machined components such as cross holes can be examined for internal
discontinuities. Borescope are used to inspect cylinders for internal surface finish after honing
.Tapped holes, shoulders or recesses also can be observed. Inaccessible areas of hydraulic
system, small pumps, motor and mechanical or electrical assemblies can be visually tested
without dismantling the engine.
Machine shop
Borescopes find applications in production machine shop tool and die departments and in
ferrous, nonferrous and alloy foundries. In production machine operations, borescopes of various
size and angles of view are used to examine internal holes, cross bored holes, threads, internal
surface finishes and various inaccessible areas encountered in machine and mechanical assembly
operations. Specific examples are visual tests of machine gun barrels, rifle bores, cannon bores,
machine equipment and hydraulic cylinders.
In tool and die shop borescopes are used to examine internal finishes, threads, shoulders
recesses, dies, jigs fixtures .fitting and the internal mating of mechanical parts. I n foundries,
borescope are widely used for internal inspection to locate discontinuities, crack, porosity and
blow holes. Borescope are also used for test of many types of defense materials, including the
internal surface finish of rocket head, rocket head seats and guided missile components.
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Power plants
In steam power plants, borescopes are used for visual tests of boiler tubes for pitting,
corrosion, scaling or other discontinuities. Borescope used for this type of work are usually made
in 2 or 3m (6 to 9 ft) section. Each section is designed so that it can be attached to the preceding
section, providing an instrument of any required length.
Other borescope are used to examine turbine blades generators, motors, pumps, and
condenser. control panels and other electrical or mechanical components without dismantling .In
nuclear plants, borescopes offer the advantage that the inspector can be in low radiation field
while the distal, or sensor, end is in a high radiation field .
Chemical industry
Visual test of high pressure distillation units are used to determine the internal condition
of tubes or headers. Evaporation tubes, fractionation units, reaction chamber, cylinders, retorts,
furnaces, combustion chambers, heat exchangers, pressure vessels and many other types of
chemical process equipment are with inspected with borescope or extension borescopes
Tank cars inspected for internal for rust, corrosion and the condition of outletvalves.
Cylinders and drums can be examined for internal conditions such as corrosion, rust or other
discontinuities.
Petroleum industry
Borescopes are used for visual test of high pressure catalytic cracking units distillation
equipment, fractionation units, hydrogenation equipment, pressure vessels, retorts, pumps and
similar process equipment. Use of the borescope in the examination of such structures is doubly
significant. Not only does it allow the examination of inaccessible areas without the expense
incurred in dismantling, it avoids breakdown and the ensuing costly repair.
Borescope Optical Systems
Borescopes are precise optical devices containing a complex system of prisms,
achromatic lenses and plain lenses that pass light to the observer with high efficiency. An
integral light at the objective end of the borescope to provide illumination for the test object.
Angle of Vision
To meet a wide range of visual testing applications, borescopes are available in various
diameters and working lengths to provide various angles of vision for special requirements. The
most common types of vision are: 1. right angle,
2. forward oblique, 3. Direct and 4. retrospective (see Fig.4.8).
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These types of vision are characterized by different angles of obliquity for the central ray
of the visual field, with respect to the forward direction of the borescope axis (see Table 3).
General Characteristics
Desirable properties of borescopic systems are large field of vision, no image distortion,
accurate transmission of color values and adequate illumination.
The brightest images are obtained with borescopes of large diameter and short length. As
the length of the borescope is increased, the image becomes less brilliant because of light losses
from additional lenses required to transmit the image. To minimize such losses, lenses are
typically coated with antireflecting layers to provide maximum light transmission.
Optical Components
The optical system of a borescope consists of an objective, a middle lens system,
correcting prisms and an ocular section (see Fig.4.12). The objective is an arrangement of
prisms and lenses mounted closely together. Its design of light gathered by the system.
The middle lenses conserve the light entering the systems conduct it through the
borescope tube to the eye with a minimum loss in transmission. Design of the middle lenses has
an important effect on the character of the image. For this reason, the middle lenses are
achromatic, each lens being composed of two elements with specific curvatures and indexes of
refraction. This design preserves sharpness of the image and true color values.
Depending on the length of the borescope, the image may need reversal or inversion or
both at the ocular. This is accomplished by a correcting prism within the ocular for borescopes
of small diameter and by erecting lenses for larger designs.
Depth of Focus, Field of View and Magnification
The depth of focus for a borescopic system is inversely related to the numerical aperture
N.
N = n sin a
(Eq.1)
Where:
n = the refractive index of the object space; and
a = the angle subtended by the half diameter of the entrance pupil of the optical system.
94
Figure 4.12:
Sectional view of a typical borescope, showing relationship of
parts in its optical system
Comparison of vision types and angles of obliquity
Type of vision
Direct
Forward oblique
Forward vision
Right angle
Retrospective
Circumferential
Angle of
Obliquity
(degrees)
0
25
45
90
135
0
90
Angular
Field
(degrees)
45
50
45
50
45
45
15
The entrance pupil is that image of any of the lens apertures, imaged in the object space,
which subtends the smallest angle at the object plane. Because the numerical aperture of
borescope systems is usually very small compared with that of a microscope, the corresponding
depth of focus is exceedingly large. This permits the use of fixed focus eyepieces in many small
and moderately sized instruments.
Field of view, on the other hand, is relatively large, generally on the order of 50 degrees
of angular field. This corresponds to a visual working field of about 25 mm (1 in.) diameter at
25 mm (1 in.) from the objective lens. At different working distances, the diameter of the field
of view varies almost directly with the working distance (see Fig.4.10).
Magnification of a bore scope’s optical system is given by the relation.
M = m1 x m2 x m3
(Eq.2)
Where m1, m2 and m3 are the magnifications of the objective, middle lenses and ocular. The
total magnification of borescopes varies with diameter and length but generally ranges from
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about 2x to 8x in use. Note that the linear magnification of a given borescope changes with
working distance and is about inversely proportional to the object distance. A borescope with 2x
magnification at 25 mm (1 in.) working distance therefore will magnify 4x at 13 mm (0.5)
distance.
Borescope Construction
A borescopic system usually consists of one or more borescopes having integral or
attached illumination, additional sections or extensions, a battery handle, battery box or
transformer power supply and extra lamps, all designed to fit in a portable case (see Fig.4.13).
The parts of a fixed length borescope for right angle vision are shown in Fig.4.14. Also shown
is a lamp at the objective end of the device. In this configuration, insulated wires are located
between the inner and outer tubes of the borescope and serve as electrical connections between
the lamp and the contacts at the ocular end. A contact ring permits rotation o the borescope
through 360 degrees for scanning the object space without entangling the electrical cord. In
other models, a fixed contact post is provided for attachment to a battery or a transformer, or the
illumination is provided by fiber optic light guides (see Fig.4.4).
Borescopes with diameters under 37 mm (1.5 in.) are usually made in sections, with
focusing eyepieces, interchangeable objectives and high power integral lamps. This kind of
borescope typically consists of an eyepiece or ocular section, a1 or 2 m (3or 6 ft) objective
section, with 1, 2 or 3 m (3, 6 or 9 ft) extension sections. The extensions are threaded for fitting
and ring contacts are incorporated in the junctions for electrical connections. Special optics can
be added to increase magnification when the object is viewed at a distance.
Figure 4.13:
Components of typical borescope system (case not shown)
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Figure 4.14.
A Typical right angle borescope
Eyepiece extensions at right angles to the axis of the borescope can be supplied, with
provision to rotate the borescope with respect to the eyepiece extension, for scanning the object
field.
Right Angle Borescopes
The right angle borescope is usually furnished with the light source positioned ahead of
the objective lens (see Fig.4.14). The optical system provides vision at right angles to the axis of
the borescope and covers a working field of about 25 mm (1 in.) diameter at 25 mm (1 in.) from
the objective lens.
Applications of the right angle borescope are widespread. The instrument permits testing
of inaccessible corners and internal surface. It is available in a wide range of lengths, in large
diameters or for insertion into apertures as small as of rifle and pistol barrels, walls of cylindrical
or recessed holes and similar components.
Another application of the right angle borescope is inspection of the internal entrance of
cross holes, where it may be critical to detect and remove burrs and similar irregularities that
interfere with correct service, immediately following the drilling operation, for blowholes or
other discontinuities that cause rejection of the component. Right angle borescopes can be
equipped with fixtures to provide fast routine test of parts in production. The device’s portability
allows occasional tests to be made at any point in a machining cycle.
Forward Oblique Borescopes
The forward oblique system is a design that permits the mounting of a light source at the
end of the borescope yet also allows forward and oblique vision extending to an angle of about
55 degrees from the axis of the borescope.
A unique feature of this optical system is that by rotating the borescope, the working area
of the visual field is greatly enlarged.
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Retrospective Borescope
The retrospective borescope has an integral light source mounted slightly to the rear of
the objective lens. For a bore with an internal shoulder whose surfaces must be accurately
tooled, the retrospective borescope provides a unique method of accurate visual inspection.
Direct Vision Borescope
The direct vision instrument provides a view directly forward with a typical visual area of
about 19 mm (0.75 in.) at 25 mm (1 in.) distance from the objective lens. The light carrier is
removable so that the two parts can be passed successively through a small opening.
Sectioned Borescopes
Borescopes under 38 mm (1.5 in.) diameter are often made in pieces, with the objective
section 1 or 2 m (3 or 6 ft) in length. The additional sections are 1, 2 or 3 m (3, 6 or 9 ft) long
with threaded connections. These sections may be added to form borescopes with lengths up to
15 m (45 ft) for diameters under 37 mm (1.5 in.).
Tables 4 through 7 list the diameters and working lengths of typical borescopes. For
special applications, custom made sizes ad designs are available.
Special Purpose Borescope
Borescope can be built to meet many special visual testing requirements. The factors
affecting the need for custom designs include: 1. the length and position of test area, 2. its
distance form the entry port, 3. the diameter and location of the entry port and 4. Inspector
distance from the entry port.
Environmental conditions such as temperature, pressure, water immersion, chemical
vapors or ionizing radiation are important design factors. The range of special applications is
partly illustrated by the examples given below.
Miniature Borescopes
Miniature borescopes are made in diameters as small a 1.75 mm (0.07 in.), including the
light source. They are useful because they can go into small holes. Inspection of microwave
guide tubing is a typical application.
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Periscopes
A large periscopic instrument with a right angle eyepiece and a scanning prism at the
objective end is shown in Fig.4.15. This instrument is 125 mm (5 in.) in diameter and 9 m (27 ft)
long. It is sectioned and provides for visual or photographic study of models in wind tunnels. A
field of view 70 degrees in azimuth by 115 degrees in elevation is covered by this design.
The cave borescope is a multiangulated, periscopic instrument used for remote
observation of otherwise inaccessible areas.
Indexing Borescope
Butt welds in pipes or tubing 200 mm (8 in.) in diameter or larger can be visually tested
with a special 90 degree in extended form through a small hole drilled next to the weld seam and
is then indexed to the 90 degree position by rotation of a knob at the eyepiece.
The objective head is then centered within the tube for viewing the weld. A second knob
at the eyepiece rotates the objective head through 360 degrees for scanning the weld seam.
Another application of this instrument is for inspecting the inside surface of cathode ray tubes.
Panoramic Borescopes
The panoramic borescopes has a scanning mirror mounted in front of the objective lens
system. Rotation of the mirror is accomplished by means of an adjusting knob at the ocular end
of the instrument. This permits scanning in one plan to cover the ranges of forward oblique,
right angle and retrospective vision (see Fig.4.16).
Another form of panoramic borescope permits rapid scanning of the internal cylindrical
surfaces of tubes or pipes. This instruments has a unique objective system that simultaneously
covers a cylindrical strip 30 degrees wide around the entire 360 degrees with respect to the axis
of the borescope. The diameter of this instrument is 25 mm (1 in.) and the working length is 1 m
(3 ft) or larger.
99
Figure 4.15. Eyepiece end of large wind tunnel periscope
Reading Borescopes
Low power reading borescopes are used in plant or laboratory setups for viewing the
scales of instruments such as cathetometers at moderately remote locations. The magnification is
about 3 X at 1 m (3 ft) distance.
Figure 4.16:
Panoramic borescope: (a) comparative ranges of vision and (b)
panoramic system components
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Photographic Adaptations
Many borescopes also include the ability to record with still photography, motion picture
or video tape. For example, still pictures on 35 mm film can be taken with a borescope fitted
with an adapter designed for the purpose. A telescopic system with a movable prim built into the
adapter operates on the reflex principle, permitting observation of the visual field of the
borescope up to the instant of photographic exposure. High intensity light sources incorporated
into the borescope provides illumination for 16 mm circular pictures on 35 mm film. Motion
pictures are possible with a fiber optic light source or a rod illuminator that eliminates electrical
connections and the heat of a lamp from the objective end of the borescope. This is especially
valuable where explosive vapors are present.
Photography of the interiors of large power plant furnaces during operation has been
done since the 1940s using a unit power periscope and camera.' The periscope extends through
the furnace wall and relays the optical image to the camera. A water cooled jacket protects the
optical system and the camera from the furnace’s high temperatures. With this equipment, still
and motion picture studies have been made of the movement of the fuel bed and the action of the
powdered fuel burner in furnaces operating at full load.
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CHAPTER 4
BORESCOPES
1. A light source that can be used to study fast moving test objects by making them
appear to move slowly is called
a. Laser source
b. Boroscope
c. Microscope
d. Stroboscope
2. The maximum diameter of a borescope that can be used for the test is determined
by :
a. Object depth.
b. Entry port size.
c. Objective distance.
d. Direction of view
3. An instrument that can be equipped with forward oblique, right angle, or
retrospective visual systems is called :
a. An angulated borescope.
b. A microscope.
c. A panoramic borescope.
d. A stereoscope.
4. The angle(s) for the fore-oblique direction of view borescope is/are :
a. 0 degrees.
b. 1-89 degrees.
c. 90 degrees.
d. 91-110 degrees.
5. A wide angle of view for a borescope provides:
a. Illumination.
b. High-magnification.
c. Shorter depth of field.
d. Greater depth of field.
6. A narrow angle of view in a boroscope is required for :
a. High magnification.
b. Low magnification.
c. A greater depth of field.
d. Greater reflectivity
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7. In visual testing, using borescopes with a narrow field of view produces :
a. High magnification and greater depth of field.
b. Low magnification and greater depth of field.
c. High magnification and shallow depth of field.
d. Low magnification and shallow depth of field.
8. In visual testing, using borescopes with a wide field of view :
a. Reduces magnification (smaller depth of field)
b. Reduces magnification (greater depth of field).
c. Increases magnification (greater depth of field)
d. Increases magnification (smaller depth of field)
9. Magnification of a borescope optical system depends on the
a. Middle lens and ocular.
b. Object lens.
c. Object lens and ocular.
d. Object lens, middle lens and ocular
10. During the performance of a visual examination, the borescope is used to :
a. Determine inside pipe diameter dimensions.
b. Examine external parts of welds.
c. Determine outside diameter dimensions.
d. Examine internal parts of pipes and components.
11. During the typical operation of a fiberoptic borescope, adequate lighting :
a. Is about the same as for reading.
b. Is often enhanced by mirrors.
c. Must be provided by artificial means.
d. Is not a problem since most work spaces generally have adequate illumination.
12. Visual examination tools that use flexible glass strands to transfer the image are
called :
a. Telescopes.
b. Fiberoptic borescopes.
c. Borescopes.
d. Binoculars.
13. Flexible fibrescopes work on the principle of:
a. Refraction of light
b. Dispersion of light
c. Total internal reflection of light
d. Resonance of light
e. Destructive interference of light
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14. Magnification of a borescope’s optical system is given by
a. M= m1*m3/m2
where m1 =magnification of objective
b. M = m1*m2/m3
m2= magnification of middle lenses
c. M =m1*m2*m3
m3 = magnification of ocular
d. M= m12 *(m3/m2)
15. In a borescope, the image is brought to the eyepiece by
a. An objective lens
b. Relay lens
c. An eyepiece lens
d. All of the above
16. An optical aid used in visual examination that brings the image to the eyepiece by a
lens trains is called
a. A fiberoptic borescope
b. A borescope
c. A mirror
d. An image guide
17. In flexible boroscope the image remains round and sharp until the tube is pent upto
an angle of
a. 45
b. 34
c. 60
d. 56
18. What is the correct statement regarding property of boroscope?
a. Image guide must be non coherent
b. Fiber guide light must be coherent
c. Image guide must be coherent
d. None of the above
19. In boroscope, the contact ring
a. Enable to connect the eyeplace
b. Permits rotation of the boroscope through 360
c. Connects two pieces of boroscope to increase the length
d. None of the above
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CHAPTER 5
VISUAL AND OPTICAL TESTING APPLICATIONS
Nondestructive examination (N D E) is comprised of a number of different
examination methods , including radiography, ultrasound , magnetic particle, liquid
penetrant, eddy current acoustic emission and visual testing , all designed to evaluate
without destroying the usefulness of a weld. Each method has its advantages and
disadvantages. However, visual inspection is the one method that can and should be
employed prior to using any other method; it is also accurate as the sole method of
inspection. Visual inspection can be performed using any one of several types of
equipment, including magnifiers, color enhancement, projectors, Rulers. Micrometers.
etc. Visual inspection is advantageous because it is economical Short of good eyesight.
However, visual inspection id limited to external or surface conduction and to the visual
acuity of the inspector.
Establishing a Routine
Before a project commence it is first important the ‘role and responsibilities of the visual
inspector be determined and clearly understood for every project’. This understanding
maybe a verbal agreement or it may be written in the form of contract document.
Examples of items and issues an inspector should be familiar with are;
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The area in which the weld is being examined should be well-lit using a
lamp or a flashlight.
Before Welding
Review applicable documentation
Check welding procedures
Check individual welder qualifications
Establish hold points
Develop inspection plan
Develop plan for recording inspection results and maintaining those records
Develop system for identification of rejects
Check condition of welding equipment
Check quality and condition of base and filler materials to be used
Check weld preparations
Check joint fit up
Check adequacy of alignment devices
Check weld joint cleanliness
Check preheat, when required
During Welding
Check welding variables for compliance with welding procedure
Check quality of individual weld passes
Check interpass cleaning
Check interpass temperature
Check placement and sequencing of individual weld passes
Check back gouged surfaces
Monitor in-process NDT, if required
After Welding
Check finished weld appearance
Check weld size
Check weld length
Check dimensional accuracy of weldment
Monitor additional NDT, if required
Monitor postweld heat treatment, if required
Prepare inspection reports
As with any task, routine procedures are the best way to ensure a thorough and
accurate inspection. Inspectors are encouraged to develop method of routine to proved
adequate coverage of the piece being examined.
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Inspecting the weld
The area in which the weld is being examined should be well- lit using a lamp or
flashlight. Weld that are inaccessible can be viewed using a bore scope or can be
examined during the progress of the work, if necessary. .
Inspection should begin prior to the onset of welding with an initial inspection of the
material. This step in an inspector’s routine can help eliminate conduction that could
lead to weld defects. Once the part has been assembled in position for welding, the
inspector should make it a point to take one final look for environmental factors that
could affect the quality of the weld. Items to check include.




Joint preparation, dimensions and finish.
Clearance dimensions, backing strips, ring or filler metal.
Alignments and Fitz up of the pieces being welded.
Verification of cleanliness.
Welding and post welding
During the welding procedure, visual inspection continues to provide yet another step
in the prevention of a low- quality weld. The inspector can best prepare by becoming
familiar with all items involved in the qualified welding procedure specification. This can
be achieved through communication with the welder or welders. The relationship
between the inspector and welder is one of major importance throughout the welding and
inspecting processes. Good communication is essential when explaining and commenting
on acceptable and unacceptable welds, Understanding each other s role and function will
also enable both parties to work together more efficiently. Once welding is complete, the
visual inspector faces the most important step in his inspection task. Using visual
inspection, the inspector will check for the following;






Verification of the dimensional accuracy of the weldment, including distortion
Conformity to drawing requirements.
Acceptability of welds with regards to appearance.
The presence of unfilled craters, pock marks, undercuts, overlap and crack.
Evidence of mishandling from center punch.
Postweld heat treatment time and temperature
Examples of common defect include cracks, undercut, overlap, excessive weld
irregularity and dimensional inaccuracies. In order for the inspector to accurately
inspect and diagnose any defect, the weld surface must be thoroughly cleaned. This is
another item to include in the inspection routine.
Marking repair welds
Finally, if an area of the weld is in need of repair, it is important the inspector be
positive and clear with the marking in accordance with a method established and
107
understood by all inspector and personnel involved in the actual repair. The markings
need to be a distinctive color to avoid confusion and permanent enough to be evident
until after the repair has been made. The inspector needs to be sure the markings will
not permanently damage the material .Once the repair is complete, the inspector once
again inspect the piece, using the initial
Markings as a guide.
Conclusion
Visual examination is the most used widely used method of nondestructive due to
its ability to work jointly with other method and its ability to provide a stand alone
method of weld inspection. Key to success as a visual inspector lies in the
establishment of routines and procedures to ensure quality and accuracy in all
inspection.
Inspection Planning and Visual Inspection Tools
108
109
110
111
112
Vernier Caliper
CHAPTER 5
113
CHAPTER - 5
VISUAL AND OPTICAL TESTING APPLICATIONS
1) When scale is partially rolled into the surface of a steel plate it is called :
A. Scabs.
B. Rolled-in scale.
C. Pits.
D. Tears.
2) A valve that provides linear motion during operation is called :
A. A gate valve.
B. A ball valve.
C. A butterfly valve.
D. All of the above.
3) An attachment to a component that is welded, cast or forged is called :
A. Non-integral attachment.
B. An integral attachment.
C. A restraint.
D. A clamp.
4) Devices that restrict the movement of hanger springs and prevent damage
during installation are called :
A. Clips.
B. Travel stops.
C. Shims.
D. Grout.
5) The combined static and friction head (vertical difference in elevation) is called:
A. Pump head.
B. Total head.
C. Brake horse power.
D. Maximum head.
6) Wear due to erosion/corrosion on a valve is typically found in :
A. The valve body.
B. The valve seating area.
C. The valve disk.
D. All of the above.
7) Which of the following is a type of component support?
A. Plate and shell.
B. Linear.
C. Component support standards.
D. All of the above.
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8) A valve is a mechanical device that :
A. Moves fluids or gases.
B. Controls flow.
C. Is rarely used in a nuclear power plant.
D. Is always welded into a system.
9) Devices that limit or allow no motion in one or more directions are called :
A. Hangers.
B. Supports.
C. Restraints.
D. Clamps.
10) Bolting failures typically occur at :
A. The thread root area.
B. The head to shank area.
C. Nicks or gouges.
D. All of the above.
11) The two major categories of pumps are :
A. Static and friction head.
B. Dynamic and displacement.
C. Single stage and multi-stage.
D. Turbine and condensate.
12) A mechanical device that raises, Transfers or pressurizes fluids is :
A. A valve.
B. A snubbed.
C. An electric motor.
D. A pump.
13) A valve that uses linear motion, which is used to regulate Flow, is called a :
A. A butterfly valve.
B. Globe valve.
C. Swing check valve.
D. Ball valve.
14) A device that restricts movement during an abnormal or seismic event is called a
A. Restraint.
B. Snubber.
C. Hanger.
D. Support.
15) A centrifugal pump is classified as :
A. A dynamic pump.
B. A reciprocating pump.
C. A displacement pump.
D. All of the above.
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16) The device in a pump that moves or compresses fluid is called:
A. A pump casting.
B. A packing gland.
C. A disk.
D. An impeller
17) The closure element of a diaphragm valve is called :
A. The closure cap
B. A flexible elastomer.
C. The bonnet.
D. The disk.
18) Component supports are divided into groups. Which of the following is not
considered to be a group of component supports?
A. Hangers.
B. Supports.
C. Integral attachments.
D. Snubbers.
19) During a visual examination, a welding discontinuity that could not be detected
would be :
A. Undercut.
B. Under fill.
C. Cracks.
D. Side wall lack of fusion.
20) For component supports, the principal movement axis is :
A. The horizontal axis.
B. The vertical axis.
C. The longitudinal axis.
D. All of the above.
21) A device that is typically installed vertically with the support member in tension
is called a :
A. Support.
B. Restraint.
C. Snubber.
D. Hanger.
22) A valve subassembly that is considered to be part of the pressure vessel assembly
is called the :
A. Drive.
B. Stem.
C. Body.
D. Disk.
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23) A device that raises, transfers or pressurizes fluids by pressing, forcing or
throwing the fluid through apertures or pipes is called a :
A. Valve.
B. Pump.
C. Snubber.
D. Bolt.
24) Valve that use rotational motion to make a seal are called :
A. Ball valves.
B. Gate valves.
C. Globe valves.
D. All of the above.
25) Service-induced discontinuities can be the result of :
A. Vibration.
B. Stress risers.
C. Corrosion.
D. All of the above.
26) A device that is typically installed vertically with the support member in
compression is called a :
A. Support.
B. Hanger.
C. Snubber.
D. Spring can.
27) Typical reportable discontinuities for component supports include :
A. Drawing anomalies.
B. Workmanship.
C. Operational.
D. All of the above.
28) A physical attribute that cannot be visually inspected during welding is:
A. The welding process.
B. The acceptability of the weld, with regard to its appearance.
C. Alignment and fit-up.
D. Joint preparation
29) An inherent discontinuity in forgings that cannot be detected using visual testing
is :
A. Bursts.
B. Cracks.
C. Seams.
D. Laps.
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30) Every inspector is affected differently by perception, fatigue and attitude. In
visual inspection, these factors are classified as :
A. Physiological factors.
B. Uncontrolled factors.
C. Production factors.
D. Classic distress factors.
31) In a casting, a visual examiner could expect to find :
A. Laminations.
B. Stringers.
C. Bursts.
D. Hot tears
32) Porosity is :
A. Material used during the welding process.
B. Gas entrapped below the surface of a material.
C. Gas entrapped below or at the surface of a material.
D. Foreign crystallized material entrapped below the surface of a material.
33) A visual examiner could expect to find a crater crack :
A. At the beginning of the weld.
B. Somewhere between the beginning and the end of the weld.
C. At either the beginning or the end of the weld.
D. At the end of the weld.
34) During the visual examination of a full penetration double bevel weld joint,
visual examination cannot locate :
A. Undercut.
B. Under fill.
C. Crater cracks.
D. Insufficient penetration.
35) When choosing a magnifier, major consideration should be given to :
A. Power or magnification.
B. Working distance.
C. Field of view.
D. All of the above.
36) Visual inspection is the most extensively used inspection method on weldments
because:
A. It is simple and relatively inexpensive.
B. It does not normally require special equipment.
C. It gives important information about conformity to specifications.
D. All of the above.
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37) In order to render valid results, visual examination must include a :
A. Trained operator.
B. Procedure for conducting the tests.
C. Standard for interpreting and reporting the results.
D. All of the above.
38) In accordance with SNT-TC-1A, certification of visual NDT personal is the
responsibility of :
A. ASNT.
B. The employer.
C. The NDT level iii.
D. An outside agency
39) When visually examining an arc strike, the inspector should inspect for :
A. Lack of fusion.
B. Craters.
C. Whiskers.
D. Cracks.
40) Joint profiles of finished welds are controlled by :
A. Acceptance standards.
B. Workmanship standards.
C. Design requirements.
D. All of the above.
41) A welding process in which shielding is provided by the electrode covering is
called :
A. Smaw.
B. Gtaw.
C. Gmaw.
D. Brazing.
42) A slag type discontinuity is produced by :
A. Smaw.
B. Gtaw.
C. Gmaw.
D. Brazing.
43) Recording information from a visual examination is accomplished using :
A. A video tape recording of the examination area.
B. Photographs.
C. A subjective report.
D. All of the above.
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44) A visual examination of the rubber elastomer seating material is performed on :
A. A gate valve.
B. A check valve
C. A diaphragm valve.
D. All of the above.
45) A mechanical device that controls flow is called :
A. A pump.
B. A valve.
C. A snubber.
D. All of the above.
46) A visual examination of a swing check valve would include
A. An examination of the hinge pin.
B. An examination for wear on the disc.
C. An examination for wear on the seat.
D. All of the above.
47) An employer shall establish a qualification and certification program based on :
A. Education and experience.
B. Training and testing.
C. Evaluation.
D. All of the above.
48) Operationally, valves are categorized as linear and :
A. Rotational.
B. Stop/start.
C. Regulatory.
D. Uni directional.
49) A restraint :
A. Allows only expensive movements.
B. Allows for only thermal movements.
C. Allows limited or no motion in one or more directions.
D. Does all of the above.
50) A typical inadequate constructions practice involving components supports
involves :
A. The use of different or wrongly sized parts.
B. Elongated bolt holes.
C. Corrosion
D. Stress corrosion cracking.
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51) Luminous energy are primarily for testing exposed or accessible surfaces of
opaque test objects and for :
A. Testing interior of transparent test objects.
B. Testing interior of test objects.
C. Verifying the adequacy of available light source.
D. Verifying the capability of a system to detect small discontinuities
52) When performing the etching process, surface finish requirements are
determined by the :
A. Etchant and its strength.
B. Material to be tested and etchant strength.
C. Discontinuities to be found and etched materials.
D. Etchant, its strength., material and discontinuities
53) Pipe crawlers are not considered to be robotic systems because they :
A. Are transported to given, ocation without operator intervention.
B. Operate on open loop control logic and respond to input from an outside
source.
C. Have closed loop control logic and respond to the environment in which
they operate.
D. Are pushed and pulled manually by an operator
54) Documents having significant influence on public health and safety are
sometimes accepted by legislative bodies or federal regulation agencies. In those
jurisdiction, such documents become law and are referred to as :
A. Standards.
B. Practices.
C. Codes.
D. Specifications.
55) The surface roughness of cold rolled steel determines :
A. Glossiness, weldability and coating properties.
B. Plating, weldability and workability.
C. Coating, weldability and plating.
D. Glossiness, coating and plating properties, and workability.
56) To determine the maximum percent defect that for the purpose of sampling test
can be considered satisfactory as a process average, use the :
A. Acceptable quality level.
B. Acceptable outgoing quality level.
C. Control chart curve.
D. Operating process curve.
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57) To determine the maximum percent defective for the tolerance of an outgoing lot
of parts, use :
A. Process control variables.
B. Acceptable outgoing level.
C. Average outgoing quality level.
D. Acceptable quality level.
58) A tool that uses the wavelength of light as a unit to measure the surface contour
is called :
A. Surface comparator.
B. Metallurgical microscope.
C. Interference microscope.
D. Polarized microscope.
59) In the steel industry, the term “surface measurement” covers :
A. Gloss and reflectance.
B. Dimensional measurement.
C. Surface roughness and properties related to roughness.
D. Both a and c above.
60) Machine vision technology is used in the automobile industry to :
A. Verify colors.
B. Calibrate speedometers.
C. Design lighting systems.
D. All of the above.
61) One of the main principles of visual and optical testing is described by :
A. Access, contact or preparation
B. Indication or recording method.
C. Process control applications.
D. Dimension and metrology.
62) When documenting the results of a visual examination, reducing the aperture
opening on a photographic lens results in :
A. An increase in depth of field.
B. A decrease in depth of field.
C. No change in depth of field.
D. A decrease in field resolution.
63) In service ’inspection of hot forging dies are likely to reveal
A. Herringbone cracks
B. Forging burst
C. Thermal checks
D. Laps
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64) Visual testing could involve
A. Measuring quantity
B. Determining shape
C. Comparing surface finish
D. All of the above
E. A or c only
65) When visually accepting a finished weld, the following factor(s) should be
considered :
A. Weld appearance.
B. Welder’s stencil mark.
C. Dimensional conformance to specification.
D. Both a and b above.
E. Both a and c above.
66) An indication of a crater crack at the start-stop of a weld was observed. This
condition:
A. Would be cause for rejection of the weld.
B. Is acceptable for all weldments when the length is less than 4 mm (0.15 in.).
C. May be acceptable if allowed by specification.
D. May be acceptable if reviewed by an owner’s representative.
67) The visual inspector evaluating the welding process should consider the
following factor(s) :
A. Preheat temperatures.
B. Filler metal control and handling.
C. Joint fit-up and bevel angle.
D. All of the above.
68) Visual surface condition for the final acceptance of weldments :
A. Is the only item to be considered.
B. May not indicate the actual condition of the weld.
C. Is based on mechanical testing of the weld.
D. None of the above.
69) Colors of weld surface can give useful information in weld inspections of
A. Authentic stainless steel
B. Titanium
C. High color steels
D. All of the above
E. A and b only
70) Joint profiles of finished welds are controlled
A. Acceptance standards
B. Workmanship standards
C. Design requirements
D. All of the above
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71) To be acceptable a concave fillet weld must have an actual leg dimension that is
A. Longer than the size
B. Shorter than the size
C. Equal to the size
D. Equal to the throat
72) When visually examining an arc strike, the inspector should inspect for
A. Lack of fusion
B. Craters
C. Whiskers
D. Cracks
73) Service – induced discontinuities can be the result of
A. Vibration
B. Stress risers
C. Corrosion
D. All of the above
74) The testing of certain numbers less than the total in a production run is called
A. Random sampling
B. Partial sampling
C. Specified partial sampling
D. Random specified sampling
75) All of the following are weld joints except
A. A butt joint
B. An edge joint
C. A groove joint
D. A lap joint
76) Embrittlement, caused by a physical or chemical change in the metal is a
reduction in
A. Ductility
B. Hardness
C. Hydrogen
D. All of the above
77) The undesirable removal of material from contacting surfaces by mechanical
action is referred to us
A. Corrosion
B. Erosion
C. Wear
D. Grinding
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78) Replication is used for
A. The analysis of fracture surfaces and microstructure
B. The evaluation of yield and tensile strengths of metals
C. The evaluation of corrosion damage and wear
D. Both a and c above
79) Guided bend test was carried out for a test coupon. What will be inspected in the
bend sample?
A. Weld discontinuities on the outside(convex)portion
B. Porosity on concave portion
C. Undercut on concave surface
D. None of the above
80) The fillet gauge will measure
A. Theoretical throat
B. Effective throat
C. Actual throat
D. None of the above
81) A machined heat treated bolt is showing an axial linear discontinuity. the
probable cause is likely to be
A. Seam
B. Lamination
C. Lamellar tearing
D. None of the above
82) Temperature indicators chalks are available over a range of
A. 38 c to 100 c
B. 38 c to 1370 c
C. Upto 250 c
D. None of the above
83) Threads are checked by
A. Profile gauge
B. Thread rollers
C. Matching parts
D. All of the above
84) Metallography is science of testing inspection and analysis of the metals
structure typically in the magnification range of
A. 50 to 2500 x
B. 50 to 100 x
C. 5 to 10 x
D. none of the above
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85) How is the creep damage in high pressure boiler tubes assessed?
A. UT
B. RT
C. Replication
D. ET
86) The electron microscope can be used to study structures upto
A. 1 micron level
B. Below the wavelength of visible light
C. Between the wavelength spectrum of visible light
D. None of the above
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CHAPTER 6
VISUAL INSPECTION WELDING
Welding Processes
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







Shielded Metal Arc Welding
Gas Metal Arc Welding
Flux Cored Arc Welding
Gas Tungsten Arc Welding
Submerged Arc Welding
Plasma Arc Welding
Electroslag Welding
Oxyacelylene Welding
Stud Welding
Laser Beam Welding
Electron Beam Welding
Resistance Welding
Brazing Processes
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
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


Torch Brazing
Fumace Brazing
Induction Brazing
Resistance Brazing
Dip Brazing
Infrared Brazing
Cutting Processes




Oxyfuel Cutting
Air Carbon Arc Cutting
Plasma Arc Cutting
Mechanical Cutting
127
128
129
130
Hardness Testing
While hardness testing is not considered to be a visual inspection, visual
inspectors frequently perform these tests when they have the necessary training and skill
required to conduct hardness tests.
Hardness is defined as a material’s resistance to penetration. It is not a
fundamental property of a material but it is related to its elastic and plastic properties.
Hardness testing of steel alloys is common; it is closely correlated to the tensile and yield
strength. The most common standards for hardness testing of metals are ASTM E-10,
Brinell Hardness of Metallic Material; ASTM E-18, Rockwell Hardness and Rockwell
Superficial Hardness of Metallic Materials; and ASTM E-92, Vickers Hardness of
Metallic Materials.
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Brinell Hardness Testing
Brinell hardness testing is the oldest and most commonly used hardness test. It
was developed by John Brinell in 1900. The indentor is a hardened steel ball. Both the
diameter of the ball and the amount and duration of the load are specified. The hardness
is determined by measuring the diameter of the indentation, using a measuring
microscope, and converting the diameter to a hardness value, using a reference chart. The
most common combination is a 3000 kg (6,614 lbs) load with a 10 mm (0.4. in.) diameter
steel indentor.
Rockwell Hardness Testing
Rockwell Testing is used almost as much as Brinell testing. The Rockwell test is
performed by forcing the indentor into the test piece using a minor load and then
measuring the increase in the depth when the major load is applied. The indentors may
be balls or diamond shaped. The Rockwell test is more accurate than Brinell tests over a
greater range of hardnesses, and it leaves a smaller impression on the test piece.
Microhardness Testing
Microhardness testing is performed on very thin parts where other techniques
would not be accurate and on precision parts where the larger
Etching
Etching is the removal of surface material by chemical means. This process is
commonly used with metallic substances. Etching removes layers of surface
contamination to clean or prepare parts for visual or penetrant examination or to prepare a
test specimen for metallography.
Profilometers
Profilometers are used to measure surface roughness or texture. The most
common profilometers use a stylus and operate on the same principle as an old
phonograph. Stylus instruments include a stylus, a transducer, and a skid that together
form the pick-up. Additionally, there is a traverse unit, a data processing chip, and a
display or recording apparatus. This type of profilometer is compact; it is a handheld
device and has a digital display as well as an output port for computer analysis.
Mohs Hardness Scale
The hardness of metals and nonmetals may be informally quantified using the
Mohs hardness scale. In this method, the material is rated for hardness based on the
softest material that will leave a scratch. The softest material, tale, is rated at 1; the
hardest material, a diamond, is rated at 10.
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There are many other methods of assessing hardness not discussed here including
the assessment of surface hardness and microhardness using eddy current and ultrasonic
methods.
Tensile Testing
Tensile testing is performed to obtain information about the ductility, tensile
strength, proportional limit, modulus of elasticity, elastic limit, resilience, yield strength,
and breaking strength of materials. Tensile tests are performed to standardized
requirements.
Tensile testing is performed using material testing machine. This machine
includes provisions for applying the strain in a controlled manner, a device to measure
and record the applied load, and an extensiometer (a gage that measures the strain and
deformation of the test piece).
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CHAPTER 6
VISUAL INSPECTION WELDING
1. To be acceptable, concave fillet weld must have an actual leg dimension that is:
a. Longer than the size.
b. Shorter than the size.
c. Equal to the size.
d. Equal to the throat.
2. The melting and fusing of the filler metal and base metal into a straight
continuous weld pass is called a :
a. Multipass weld.
b. Depressed bead.
c. Stringer bead.
d. Weave pattern.
3. A depression on the face of a fillet weld that reduces the cross section of the weld
when measure at the depression is called :
a. Depression bead.
b. Excessive convexity.
c. Insufficient throat.
d. Insufficient leg.
4. Horizontal indications on the edge of a 76mm (3 inch) plate are on several levels
and do not extend a long the whole edge. the most likely cause of these visual
indications is :
a. Pipe.
b. Poor burning practice.
c. Laminations.
d. Porosity.
5. A process that uses a filler metal with a liquids state that does not exceed 449oc
(840 of) and that does not melt the base material is :
a. Smaw.
b. Brazing.
c. Soldering.
d. Resistance welding.
6. Which of the following is a primary processing method?
a. Forging.
b. Machining.
c. Heat treating.
d. All of the above.
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7. Which of the following is a basic joint configuration?
a. A tee joint.
b. A single v-joint.
c. A single j-joint.
d. All of the above.
8. Weld metal that completely fills the groove and is fused to the base metal
throughout its total thickness is called :
a. Partial joint penetration.
b. Plate thickness.
c. Theoretical throat.
d. Complete joint penetration.
9. A disadvantage of the gmaw process is :
a. That slag removal is required.
b. That there is an excessive amount of post weld cleaning
c. That shielding gas must be protected from drafts.
d. All of the above.
10. A condition of excessive offset of the inside diameter surface is called :
a. Underfill.
b. Misalignment.
c. Overlap.
d. Excessive reinforcement
11. On a welding symbol, the flag symbol indicates :
a. A shop weld.
b. A repair weld.
c. A field weld.
d. Weld all around.
12. On a welding symbol, the horizontal line connecting the arrow and the tail is
called the :
a. Main line.
b. Reference line.
c. Symbol line.
d. Aws line.
13. For a given size weld, the theoretical throat for a concave fillet weld is :
a. The same for convex fillet weld.
b. Larger for a convex fillet weld
c. Smaller for a convex fillet weld
d. Equal to the effective throat.
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14. The theoretical throat dimension for a 10mm (0.4”) leg fillet weld is :
a. 5 mm (0.2”).
b. 7 mm (0.3”).
c. 10 mm (0.4”).
d. 13 mm (0.5”).
15. Overlap is a weld profile condition where the angle formed at the junction
between the weld and base material is :
a. Less the 90 degrees from the plate surface
b. Equal to 90 degrees from the plate surface
c. Greater than 90 degrees from the plate surface
d. An internal flaw only detectable with ultrasonic testing.
16. The most critical part of any weld is :
a. The weld reinforcement.
b. Correct heat input.
c. Polarity.
d. The root pass.
17. For concave fillet welds, the size of the weld as compared to the weld is :
a. Equal to the leg.
b. Larger than the leg.
c. Smaller than the leg.
d. Not related to the leg.
18. In convex fillet welds, the shortest distance from the root of the weld to the face
weld is called :
a. Actual throat.
b. Theoretical throat.
c. Effective throat.
d. Throat.
19. A fillet weld has size requirement of 15mm. Which of the following welds would
be acceptable if throat as measured was 11 mm?
a. Concave fillet
b. Convex fillet
c. Mitre fillet
d. A & c only
e. B & c only
20. The actual size of a groove weld is :
a. One-half of the cap width dimension.
b. 0.7 of the short leg dimension.
c. The average width of the weld.
d. The groove prep plus penetration.
136
21. Temperature monitoring during welding is commonly carried out using
a. Thermometers
b. Tempil sticks
c. Pyrometers
d. All of the above
22. Scabs are likely to occur in which operation
a. Casting
b. Rolling
c. Forging
d. Welding
23. Scarfing preparation would be done before
a. Mould preparation
b. Brazing operation
c. Soldering
d. Extrusion
24. Hydrogen inside steel plates could result in
a. Buckles
b. Blisters
c. Blow holes
d. Blow cracks
e. B and c only
25. Hydrogen induced flakes are likely in
a. Casting
b. Forging
c. Copper brazing
d. Soldering
26. When a sheet of steel is formed cylindrically, it can lead to formation of
a. Seams
b. Laps
c. Flutes
d. Chutes
e. All of the above
27. The brazing process is commonly defined as a liquid –solid phase joining method
accomplished at a temperature above:
a. 232º (450º f)
b. 343º(650º f)
c. 449º(840º f)
d. 504º(940º f)
137
28. The welding process that is sometimes referred to us “”stick welding” is
a. Saw
b. Smaw
c. Gmaw
d. Gtaw
29. The welding processes in which there is a higher degree of probabaility of
entrapping slag is
a. Gmaw
b. Gtaw
c. Smaw
d. All of the above
30. A process in which materials are joined by heating them to a suitable
temperature and by using a filler metal, which liquefies above 449ºc(840º f) and
below the solid’s of the base metal, is called
a. Welding
b. Soldering
c. Brazing
d. Solid state welding
31. The fillet weldsize is based on the
a. Effective fillet weld throat
b. Length of fillet weld
c. Theoretical throat
d. Length of fillet weld leg
32. On welding symbol, the flag symbol indicates
a. A shop weld
b. A repair weld
c. A field weld
d. Weld – all- around
33. The heat-affected zone is the portion of the
a. Metal that is added to produce the weld joint
b. Base metal that has been melted and solidified
c. Base metal that has not been melted but where properties have been alerted
by the welding heat
d. Original metal that is welded
34. Arc strike are typically caused by
a. Molten particles splashed that are splashed out of the molten puddle
b. Excessive heat during the welding process
c. The use of improper or wet electrodes
d. Welding operator error
138
35. A condition that is caused by unintentional rapid heating of the base metal or
weld metal and subsequent rapid cooling of the molten material , which results
in extremely high heat input and causes localized hardness and cracking is called
a. Undercut
b. Ark strike
c. Weld spatter
d. Overlap
36. Unequalleg fillet size is specified by
a. Short leg(a)
b. Long leg(b)
c. Short leg(a) / long leg(b)
d. Throat of the triangle
37. If the root gap is very less and the included angle is 14’’’the probable
discontinuity at the rot of the weld is –
a. lof
b. Crack
c. Globules
d. Overlap
38. Brazing is a
a. Metallurgical joint
b. Mechanical join
c. Metallic joint
d. None of the above
39. For Carbon and Alloy Steels The Etchant Generally Used Is
a. Hcl Diluted With Water
b. Hf(48%)
c. Hbr
d. None Of The Above
40. A Gambridge Weld Gauge Can Measure
a. Bevel Angle
b. Reinforcement
c. Depth Of Undercut
d. All Of The Above
e. Only ‘B’ And ‘C’ Are Correct
41. Considering The Stress Concentration At The Toe
a. Concave Fillet Is Better
b. Convex Filet Is Better
c. Miter Fillet Is Better
d. All Are Having Same Merit
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CHAPTER 7
WELD JOINT AND WELDING SYMBOLS
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157
158
159
160
161
162
163
164
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CHAPTER 7
WELD JOINT AND WELDING SYMBOLS
1) A welding symbol over the reference line refers to :
A. The area on the arrow side.
B. The area near the end of the arrow.
C. The area opposite of the arrow.
D. A field welds.
2) When the weld is to be placed on the arrow side of the joint, the weld symbol in the
drawing will be :
A. Below the line.
B. Above the line.
C. In the tail.
D. At the end of the arrow.
3) On a welding symbol the horizontal line connecting the arrow and the tail is called the
A. Main line
B. Reference line
C. Symbol line
D. Aws line
4) Brinell hardness tester will generally have----------magnification
A. 5 to 10 x
B. 20 to 40 x
C. 1000 x
D. None of the above
5)
A bar was tested for tensile test initial gauge length of 50mm.what shall be the
minimum final gauge length in mm .if it has to pass minimum 35% elongation as per
specification?
A. 67.5
B. 85
C. 32.5
D. None of the above
6) Which of the following instruments can be used to measure coating or painting
thickness?
A. Spectrometer
B. Refractometer
C. Elcometer
D. Paint”ö”meter
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7) Visual testing could involve
A. Measuring quantity
B. Determining shape
C. Comparing surface finish
D. All of the above
E. A or c only
8) The threads of a drill casing are usually inspected by
A. Microscope
B. Profile gauge
C. Replica process
D. None of the above
E. A and b only
9) Detailed drawing of small length of pipe line is referred to
A. Pipe drawing
B. Line drawing
C. Spool drawing
D. None of the above
10) A common inspection instrument that is used to visually inspect internal bore surface is
as:
A. Magnifying glass
B. Bore scope.
C. Phototube.
D. Microscope
11) To examine areas around bends inside a pipe section, the visual examiner uses a
A. Telescope.
B. Fiber optic bore scope.
C. Bore scope.
D. Microscope.
12) On a inch thickness gage, twenty thousands of an inch is represented by :
A. 0.2.
B. 0.02.
C. 0.002.
D. 0.0002.
13) When measuring plate thickness, the most accurate reading is given by :
A. Steel ruler.
B. Steel tape.
C. Mechanical gage (micrometer).
D. Feeler gage.
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14) The gage that is typically used to measure the face reinforcement of a butt joint is :
A. A Cambridge gage.
B. A tempil gage.
C. A fillet weld gage.
D. All of the above.
15) The gage used in the visual testing of threads in oil country tubular goods is called :
A. An lc gage.
B. A thread gage.
C. A profile gage.
D. A pin gage
16) The gage that provides measurements of internal misalignment on 76mm(3in) diameter
pipe is
A. A Cambridge gage
B. A fillet weld gage
C. A hi-lowelding gage
D. Both a and b above
17) Visual examination tools that use flexible glass stands to transfer the image are called
A. Telescopes
B. Fiberoptic borescopes
C. Borescopes
D. Binoculars
18) The mirror strip is provided on the dial of the pressure gauge to enable
A. To reduce the least count of the instrument
B. To enable operator to see the dirt on the dial face
C. To eliminate error due to parallax
D. To enable the operator to see the reading from any direction
19) Which of the following value indicates smoothest finish of the surface?
A. 640rms
B. 320 rms
C. 160 rms
D. 80 rms
20) Glossmeter is
A. Used for checking glare
B. Used for checking light intensity
C. Refletometer used to measure specular reflectance
D. None of the above
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21) If You Have To Check Internal Mismatch Of A Pipe To Pipe Joint Of A Pipe With 4’’
Od And ¼’’ Thickness The Gauge To Be Used Is
A. Hi-Low Gauge
B. Vernier
C. Cambridge Gauge
D. Fillet Gauge
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CHAPTER 8
WELD AND BASE METAL DISCONTINUITIES
Discussion of Discontinuities
Having provided this background on discontinuities in a general sense, let’s now
discuss some of the more common weld and base metal discontinuities found during
normal inspection activities. Those with which we will concern ourselves are listed, and
the definitions for each can be found in the AWS standard A3.0, Standard Welding Terms
and Definitions, or in the Key Terms and Definitions section at the end of this module.




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

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







Crack
Incomplete fusion
Incomplete joint penetration
Inclusion
Slag inclusion
Tungsten inclusion
Porosity
Undercut
Underfill
Overlap
Convexity
Weld reinforcement
Arc strike
Spatter
Lamination
Lamellar tear
Seam/lap
Dimensional
Transverse Cracks
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KEY TERMS AND DEFINITIONS
Arc strike – a discontinuity resulting from an arc, consisting of any localized remelted
metal, heat-affected metal, or change in the surface profile of any metal object.
Atomic hydrogen – the ionic form of hydrogen, denoted as H+ as opposed to molecular
hydrogen which contains two atoms of hydrogen and is denoted as H2. A synonym for
atomic hydrogen is nascent hydrogen.
Collet – in welding terms, a part of a welding torch forming a shroud.
Convexity – the maximum distance from the face of a convex fillet weld perpendicular to
a line joining the weld toes.
Crack – a fracture type discontinuity characterized by a sharp tip and high ratio of length
and width to opening displacement.
Crater crack – a crack forming at the termination of a weld.
Defect – a discontinuity which exceeds the permissible limit of a code; a rejectable
discontinuity requiring repair of replacement.
Delamination – the separation of a lamination under stress.
Density – the ratio of the mass of an object to its volume, usually in terms of grams per
cubic centimeter or pounds per cubic foot; also refers to the darkness of radiographs film:
the darker areas are noted as having a higher density.
Discontinuity – any irregularity in the normal pattern of a material: any interruption of
the uniform nature of an item.
Inclusion – entrapped foreign solid material, such as slag, flux, tungsten, or oxide.
Incomplete fusion – a weld discontinuity in which fusion did not occur between weld
metal and fusion faces or adjoining welds beads.
Incomplete joint penetration – a joint root condition in a groove weld in which weld
metal does not extend through the joint thickness.
Intergranular – referring to conditions which occur at or follow the grain boundaries of a
metal. An intergranular crack would initiate and propagate along a metal’s grain
boundaries.
Lamellar tear – a subsurface terrace and step-like crack in the base metal with a basic
orientation parallel to the wrought surface caused by tensile stresses in the through174
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thickness direction of the base metals weakened by the presence of small dispersed,
planar shaped, nonmetallic inclusions parallel to the metal surfaces.
Lamination – a type of discontinuity with separation or weakness generally aligned
parallel to the worked surface of a metal.
Nascent hydrogen – see Atomic Hydrogen.
Overlap – in fusion welding, the protrusion of weld metal beyond the weld toe or weld
root.
Pipe – in metal ingot casting, the severe shrinkage occurring at the top center portion of
the ingot, usually containing oxides.
Planar – of or pertaining to a plane; lying in a plane
Porosity – cavity-type discontinuities formed by gas entrapment during solidification or
in a thermal spray deposit.
Propagate – growth, or continuation of growth; to get larger.
Protrusion – a projection outward; a jutting out.
Radiograph – a film made by passing X- or gamma radiation through an object to
determine the quality of its internal structure.
Safe-ending – the practice of drilling a small hole at each end of a crack to increase the
crack end radius and stop further crack propagation.
Seam/lap – longitudinal base metal surface discontinuities on wrought product.
Shielding gas – protective gas used to prevent or reduce atmospheric contamination, as
of a molten weld metal.
Slag inclusion – an inclusion of slag.
Spatter – the metal particles expelled during fusion welding that do not form a part of the
weld.
Stress risers – conditions such as notches, cracks, or geometry which increase the applied
stress by factors of 2 to 10.
Stringer – in metallurgy, an elongated oxide or nonmetallic inclusion within the metal.
Titania – a titanium oxide; a coating type for covered electrodes in welding.
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Transgranular – referring to conditions which cross or pass through the metal’s grains.
A transgranular crack has a path across the grains as opposed to intergranular cracking
which follows a path along the grain boundaries.
Transverse – lying, situated placed across; having a path from side to side.
Tungsten inclusion – an inclusion of tungsten.
Undercut – a groove melted into the base metal adjacent to the weld toe or weld root and
left unfilled by weld metal.
Weld reinforcement – weld metal in excess of the quantity required to fill a joint; at the
face or root.
Wrought – the term applied to the working or forming of metal while it is solid to form
shapes, as opposed to a cast product which forms directly from the molten state.
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PRIMARY PROCESSING DISCONTINUITIES
1. CASTING
Discontinuity
Caused by
Cold Shut
Lack of fusion
Surface
between two
intercepting surfaces
of metal as it flows
into the cast.
Hot Tear
Difference in
cooling rates
between thin
sections and thick
sections
Lack of enough
molten metal to fill
the space created by
shrinkage
Improperly designed
mold causing
premature blockage
at mold gate.
Inability of external
gases to escape from
the mold.
Entrapped internal
gases
Shrinkage Cavity
Microshrinkage
Blow holes and
porosity
Location
Surface and
subsurface
Suitable NDT
technique
PT for another than
ferromagnetic
materials, and MT
for ferromagnetic
materials.
Some procedure
recommends both
PT and MT for
ferromagnetic
materials,
considering the
defect location.
MT/PT for surface
and RT for internal
Subsurface
RT
Subsurface
RT
Surface or
subsurface
PT/MT
RT
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2. FORGING
Discontinuity
LAP
BURST
Caused by
Folding of metal in
a thin plate on the
surface of the
forging.
Forging at improper
temperature.
Location
Surface
Suitable NDT
MT/PT, OR BOTH
Surface or
Subsurface
MT/PT
RT & UT
3. ROLLING
Discontinuity
LAMINATIONS
STRINGERS (Bar
stocks)
SEAMS (Bar
Stocks)
Caused by
Flattening and
lengthening of
discontinuities in
parent metal.
Flattening and
lengthening of
discontinuities
found in parent
metal.
Lengthening of
surface cracks found
in parent metal.
Location
Subsurface
Suitable NDT
UT
Subsurface
UT
Surface
MT/PT
4. SEAMLESS TUBES AND PIPES
Discontinuity
SEAMS
SLUGS
GOUGES
Caused by
Present in the parent
metal.
Metal buildup of
piercing Mandrel.
Sizing mandrel
dragging
Location
Outer Surface
Suitable NDT
MT/PT
Inner Surface
Bore scope-optical
fibre
Bore scope
Inner Surface
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5. EXTRUSIONS
Discontinuity
SEAMS
POROSITY
GALLINGE
(Cracks)
Caused by
Present in the parent
metal.
Present in the parent
metal.
Improper metal flow
through the die.
Location
Surface
Suitable NDT
MT/PT
Surface or
subsurface
Surface
MT/PT
RT/UT
MT/PT
6. GRINDING
CRACKS
Excess localized
heat created
between the
grinding wheel and
material
Surface
MT/PT
7. HEAT TREATING
STRESS CRACKS
Stressing built-up
by improper
processing-unequal
heating or cooling
Surface
PT/MT
8. EXPLOSIVE FORMING
CRACKS AND
TREARS
Extreme
deformation
overstresses the
material
Surface
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9. WELDING
Discontinuity
Cracks
Caused by
Location
Improper use of heat Surface or
source
subsurface
Stress cracks
Stress built up by
weld contraction (if
material is
restrained)
Entrapped gases
Porosity
Slag inclusions
Tungston inclusions
Lack of penetration
Lack of fusion
Undercut
Overlapping
Incomplete cleaning
of slag form the
weld between
passes
Excessive current
used during
tungsten are
welding
Improper welding
technique
Improper welding
technique
Improper welding
technique
Weld overlaps
parent metal-Not
fused
Surface
Suitable NDT
MT/PT
RT, PREFERRED
METHOD UT
MT/PT
Surface or
subsurface
Surface or
subsurface
PT
RT
PT/MT
UT/RT
Subsurface
RT
Surface or
subsurface
Subsurface
Preferred method
UT,RT possible if
the beam is directed
along the fusion
surface between
weld and parent
metal
MT/PT
Surface
Visual inspection
Surface
10. BENDING
CRACKS
OVERSTRESS OF
MATERIAL
SURFACE
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MT/PT
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11. MACHINING
TREARS
Working with dull
tools or cutting too
deep
Surface
MT/PT
12. PICKLING, ETCHING, AND ELECTROPLATING
CRACKS
Relief of internal
stresses
Surface
MT/PT
CONDITIONS LEADING TO FAILURE:
1. Imposed loads may be static or dynamic
2. Environment may contribute corrosion, vibration, or temperature & pressures
higher or lower than the normal.
3. Structures may also be subjected to abuse
4. Mechanical failure is always a result of stresses, above some critical value for
each material that causes deformation or fracture.
5. Such excessive stresses are setup by some combination of material defects excess
loads, improper type of operation or design error.
6. All material’s defects (surface, near surface, or (internal) can be detected, by
NDT.
7. Defects size, shape, location, or orientation (detrimental for failure) can be
8. accurately characterized by NDT.
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TABLE 6-1 PRIMARY PROCESSING DISCONTINUITIES
Process
Casting
Discontinuity
Cold Shut
Hot Tear
Shrinkage Cavity
Microshrinkage
Blow Hotels
Porosity
Forging
Rolling
Welded pipe
Seamless Pipes and
Tubes
Caused by
Lack of fusion between two
intercepting surfaces of
metal as it flows into the cast
Difference Cooling Rates
between thin sections and
thick sections
Lack of enough molten metal
to fill the space created by
shrinkage
Improperly designed mold
causing premature blockage
at mold gate
Inability of external gasses to
escape from the mold
Entrapped internal gasses
Lap
Folding of metal in a tain
plate on the surface of the
forging
Burst
Forging at improper
temperature
Laminations (Flat Flattering and Lengthening
Plate)
of discontinuities found in
parent metal
Stringers (Flat
Flattening and lengthening of
Plate)
discontinuities in parent
metal
Seams (Bar Stock) Lengthening of surface
cracks found in parent metal
Location
Surface
Surface
Subsurface
Subsurface
Surface
Surface or
Subsurface
Surface
Surface or
Subsurface
Subsurface
Subsurface
Surface
Lack of Fusion
Incomplete weld
Laminations
Present in the parent metal
(sheet or plate material)
Seams
Present in the parent metal
(Round bar stock)
Outer
Surface
Slugs
Metal buildup on piercing
mandrel
Sizing mandrel dragging
Inner Surface
Gouges
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Surface
(Inner or
Outer)
Subsurface
Inner Surface
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Grinding
Cracks
Heat Treating
Stress Cracks
Explosive Forming
Cracks and Tears
Welding
Crater Cracks
(Star, Transverse,
Longitudinal)
Stress Cracks
Porosity
Slag Inclusions
Tungsten
Inclusions
Lack of
Penetration
Lack of Fusion
Undercut
Overlapping
Excess localized heat created
between grinding wheel and
material
Stress buildup by improper
processing – unequal heating
or cooling
Extreme deformation
overstresses the material
Improper use of heat source
Stress built up by weld
contraction (If material is
restrained)
Entrapped Gasses
Incomplete cleaning of slag
from the weld between
passes
Excessive current used
during tungsten – arc
welding
Improper welding technique
Surface
Surface
Surface
Surface or
Subsurface
Surface
Surface or
Subsurface
Surface or
Subsurface
Subsurface
Bending
Machining
Cracks
Tears
Pickling and
Erching
Electric plating
Cracks
Surface or
Subsurface
Improper welding technique Subsurface
Improper welding technique Surface
Weld overlaps parent metal – Surface
not fused
Overstress of Material
Surface
Working with dull tools or
Surface
cutting too deep
Relief of internal stresses
Surface
Cracks
Relief of internal stresses
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Surface
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ARTICLE – 9
VISUAL EXAMINATION
T-910 SCOPE
a. This Article contains methods and requirements for visual examination
applicable when specified by a referencing Code Section. Specific visual
examination procedures required for every type of examination are not
included in this Article, because there are many applications where visual
examinations are required. Some examples of these applications include
nondestructive examinations, leak testing, in-service examinations and
fabrication procedures.
b. The requirements of Article 1, General Requirements, apply when visual
examination, in accordance with Article 9, is required by a referencing Code
Section.
c. Definitions of terms for visual examination appear in Article 1, Appendix I –
Glossary of Terms in Nondestructive Examination, and Article 9, Appendix I.
T-920 GENERAL
T-921 Performance
Visual examination to this Article, when required by the referencing Code Sections, shall
be performed to a written procedure prepared by the user.
T-922 Personnel Requirements
The user of this Article shall be responsible for assigning qualified personnel to perform
visual examinations to the requirement of this Article. At the option of the manufacture,
he may maintain one certification for each product, or several separate signed records
based on the area or type of work, or both combined. Where impractical to use
specialized visual examination personnel, knowledgeable and trained personnel, having
limited qualifications, may be used to perform specific examinations, and to sign the
report forms. Personnel performing examinations shall be qualified in accordance with
requirements of the referencing Code Section.
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TABLE T-923
REQUIREMENTS OF A VISUAL EXAMINATION PROCEDURE
Requirement (As Applicable
Technique used
Surface Condition
Surface preparation/cleaning
Method or Tool(s) required for surface preparation
Direct or Indirect Viewing method
Special Illumination
Equipment to be used
Sequence of performing examination
Data to be documented
Report Forms to be Completed
Personnel Qualifications
Procedure Qualification Reference
Essential
Variable
X
X
Non Essential
Variable
X
X
X
X
X
X
X
X
X
X
T-923 Procedure
T-923.1 Requirements. Visual examinations shall be performed in accordance with
a written procedure, which shall, as a minimum, contain the requirements listed in Table
T-923. The written procedure shall establish a single value of range of values, for each
requirement.
T-923.2 Procedure Qualification. When procedure qualification is specified, a
change of a requirement in Table T-923 identified as an essential variable form the
specified value, or range of values, shall require requalification of the written procedure.
Where a range is specified for an essential variable, the bounding values of the range
shall be qualified by demonstration. A change of a requirement identified as a
nonessential variable from the specified value, or range of values does not require
requalification of the written procedure. All changes of essential or nonessential
variables from the value, or range of values specified by the written procedure. All
changes of essential or nonessential variables from the value, or range of values specified
by the written procedure shall require revision of, or an addendum to, the written
procedure.
T-930 EQUIPMENT
Equipment used for visual examination techniques, for example, direct, remote, or
transluscent, shall have the capabilities as specified in the procedure. Capabilities
include, but are not limited to viewing, magnifying, identifying, measuring, and/or
recording observations in accordance with requirements of the referencing Code Section.
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T-940 MISCELLANEOUS REQUIREMENTS
T-941 Procedure Requirements
The procedure shall contain or reference a report of what was used to demonstrate the
examination procedure was adequate. In general, a fine line 1/32 in. (0.8 mm) or less in
width, an artificial imperfection or a simulated condition, located on the surface or a
similar surface to that to be examined, may be considered as a method for procedure
demonstration. The condition or artificial imperfection should be in the least discernable
location on the area surface to be examined to validate the procedure.
Note: T-941.3 is a non-essential variable (See Table T-923)
T-941.1 Visual examination shall be performed in accordance with a written procedure.
T-941.2 A written procedure, when required in accordance with T-150, shall include at
least the following:
a.
b.
c.
d.
e.
f.
g.
h.
i.
how visual examination is to be performed
type of surface condition and criteria for surface cleaning;
cleaning instructions or reference to cleaning procedures:
method or tool for surface preparation, if any;
whether direct or remote viewing is used;
special illumination, instruments, or equipment to be used, if any;
sequence of performing examination, when applicable;
data to be tabulated, if any;
report forms or general statement to be completed.
T-941.3 In some instances it is preferable to relate the procedure to a specific component
or surface such as the internal examination of a weld many feet from the open end of a
tube or tubes of several sizes, but procedures may be in a general form applicable without
adaptation to a variety of unlisted products or situations, thereby reducing the number of
written procedures required.
T-941.4 The procedure shall contain or reference a report of what was used to
demonstrate that the examination procedure was adequate. In general, a fine line 1/32 in.
(0.8 mm) or less in width, or some other artificial flaw located on the surface or a similar
surface to that to be examined, may be considered a test method for this demonstration.
The line or artificial flaw should be in the least discernible location on the area examined,
to prove the procedure.
T-941.5 Substituting one equipment manufacturer’s equipment for another, or changes
in the details of test arrangement, will not require requalification.
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T-942 Physical Requirements
Personnel shall have an annual vision test to assure natural or corrected near distance
acuity such that they are capable of reading standard J-1 letters on standard Jaeger test
type charts for near vision. Equivalent near vision tests are acceptable.
T-950 TECHNIQUE
T-951 Applications
Visual examination is generally used to determine such things as the surface condition of
the part, alignment of mating surfaces, shape, or evidence of leaking. In addition, visual
examination is used to determine a composite material’s (translucent laminate)
subsurface conditions.
T-952 Direct Visual Examination
Direct visual examination may usually be made when access is sufficient to place the eye
within 24 in. (610 mm) of the surface to be examined and at an angle not less than 30
deg. To the surface to be examine. Mirrors may be used to improve the angle of vision,
and ids such as a magnifying lens may be used to assist examinations. Illumination
(natural or supplemental white light) for the specific part, component, vessel or section
therof being examined is required. The minimum light intensity at the examination
surface/site shall be 100 footcandles (1000 lux). The light source, technique used, and
light level verification is required to be demonstrated one time, documented, and
maintained on file. Personnel shall have an annual vision test to assure natural or
corrected near distance acuity such that they are capable of reading standard J-1 letters on
standard Jaeger test type charts for near vision. Equivalent near vision tests are
acceptable.
T-953 Remote Visual Examination
In some cases, remote visual examination may have to be substituted for direct
examination. Remote visual examination may use visual aids such as mirrors, telescopes,
borescopes, fiber optics, cameras, or other suitable instruments. Such systems shall have
a resolution cpabilt8ya t least equivalent to that obtained by direct visual observation.
T954 Translucent Visual examination
Translucent visual examination is a supplement of direct visual examination. The
method of translucent visual examination uses the aid of artificial lighting, which can be
contained in an illuminator that produces directional lighting. The illuminator that
produces directional lighting. The illuminator shall provide light of an intensity that will
illuminate and diffuse the light evenly through the area or region under examination. The
ambient lighting; must be so arranged that there are no surface glares or reflections from
the surface under examination and shall be less than the light applied through the area or
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region under examination. The artificial light source shall have sufficient intensity to
permit “candling” any translucent laminate thickness variations.
T-980 EVALUATION
T-980.1 All examination shall be evaluated in terms of the acceptance standards of the
referencing Code Section.
T-980.2 An examination checklist shall be used to plan visual examination and to verify
that the required visual observations were performed. This checklist establishes
minimum examination requirements and dose not indicate the maximum examination
which the Manufacturer may perform in process.
T-990 DOCUMENTATION
T-991 Report of Examination
T-991.1 A written report of the examination shall contain the following information:
a.
b.
c.
d.
e.
the date of the examination;
procedure identification and revision used;
technique used;
results of the examination;
examination personnel identity, and, when required by the referencing Code
Section, qualification level;
f. identification of the part or component examined.
T-991.2 Even though dimensions, etc., were recorded in the process of visual
examination to aid in the evaluation, there need not be documentation of each viewing or
each dimensional check. Documentation shall include all the observation and
dimensional checks specified by the referencing Code Section.
T-992 Performance Documentation
Documentation of performance demonstration shall be completed when required by the
referencing Code Section.
T-993 Record Maintenance
Records shall be maintained as required by the referencing Code Section.
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ARTICLE 9
MANDATORY APPENDIX
APPENDIX I – GLOSSARY OF TERMS FOR VISUAL
EXAMIANTION
I-910 SCOPE
This Mandatory Appendix is used for the purpose of establishing standard terms and
definitions of terms related to Visual Examination which appear in Article 9.
I-920 GENERAL
a. Article 30, SE-1316, Section 9, provides the definition of footcandle (fc).
b. Definitions of terms for visual examination and other methods appear in
Article 1, Mandatory Appendix I, Glossary to Terms for Nondestructive
Examination.
c. The following Code terms are used in conjunction with Article 9:
Artificial flaw – an intentional imperfection placed on the surface of a material
to depict a representative flaw condition.
Auxiliary lighting – an artificial light source used as a visual aid to improve
viewing conditions and visual perception.
Candling – see translucent visual examination
Direct visual examination – a visual examination technique performed by eye
and without any visual aids (excluding light source, mirrors, and/or corrective
lenses).
Enhanced visual examination – a visual examination technique using visual
aids to improve the viewing capability, e.g., magnifying aids, borescopes,
video probes, fiber optics, etc
Lux (Lx) – a unit of illumination equal to the direct illumination on a surface
that is everywhere one meter from a uniform point source of one candle
intensity or equal to one lumen per square meter.
Remote visual examination – a visual examination technique used with visual
aids for conditions where the area to be examined is inaccessible for direct
visual examination.
Surface glare – reflections of artificial light that interfere with visual
examination.
Translucent laminate – a series of glass reinforced layers, bonded together,
and having capabilities of transmitting light.
Translucent visual examination- a technique using artificial lighting intensity
to permit viewing of translucent thickness variations (also called candling)
Visual examination- a nondestructive examination method used to evaluate an
item by observation, such as: the correct assembly, surface conditions, or
cleanliness of materials, parts and components used in the fabrication and
construction of ASME Code vessels and hardware
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TABLE K341.3.2A
ACCEPTANCE CRITERIA FOR WELDS
Type of Imperfection
Crack
Lack of fusion
Incomplete penetration
Internal porosity
Slag inclusion or
elongated indication
Undercutting
Surface porosity or
exposed slag inclusion
Concave root surface
(suck-up)
Surface Finish
Reinforcement or
internal proturusion
Criteria (A-E) for Types of Welds, and for Required
Examinations Methods [Note (1)]
Methods
Type of Weld
Visual
100%
Girth
Longitudin
Fillet
Branch
Radiograp Groove
al
[Note
Connectio
hy
Groove
(3)]
n
[Note (2)]
[Note (4)]
X
X
A
A
A
A
X
X
A
A
A
A
X
X
A
A
A
A
X
B
B
NA
B
X
C
C
NA
C
X
X
X
A
A
A
A
A
A
A
A
X
X
D
D
NA
D
E
F
E
F
E
F
E
F
X
X
GENERAL NOTE: X = required examination; NA = not applicable; … = not required
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Criterion Value Notes for Table K3413.3.2A
Criterion
Symbol
Measure
A
Extent of imperfection
B
Size and distribution of internal porosity
C
D
E
F
Slag inclusion or elongated indication
Individual length
Individual width
Cumulative length
Depth of surface concavity
Surface roughness
Height of reinforcement or internal protrusion
[Note (6)] in any plane through the weld shall
be within the limits of the applicable height
value in the tabulation at the right. Weld metal
shall be fused with and merge smoothly into the
component surfaces.
Acceptable Value Limits [Note(5)]
Zero (no evident imperfection)
See BPV Code, Section VII,
Division 1, Appendix 4
≤ Tw/4 and < 5/32 in. (4.0 mm)
≤ Tw/4 and < 5/32 in. (4.0 mm)
≤ Tw/4 any 12 Tw weld length
Total joint thickness including weld
reinforcement, > Tw
< 500 in. Ra per ASME B46.1
Wall Thickness External Weld
Reinforecement
Tw in. (mm)
or Internal Weld
Protrusion
< 2/2 (12.7)
2/26 (1.6)
> 2/2 < 2 (50.8) 2/8 (3.2)
>2
5/32 (4.0)
NOTES:
1) Criteria given are for required examination. More stringent criteria may be specified
in the engineering design.
2) Longitudinal welds include only those permitted in paras. K302.3.4 and K305. The
radiographic criteria shall be met by all welds, including those made in accordance
with a standard listed in Table K326.1 or in Appendix K.
3) Fillet welds include only those permitted in para 311.2.5(b)
4) Branch connection welds include only those permitted in para. K328.5.4.
5) Where two limiting values are given, the lesser measured value governs acceptance.
6) For groove welds, height is the lesser of the measurements made form the surfaces of
the adjacent components. For fillet welds, height is measurement from the theoretical
throat; internal protrusion does not apply. Required thickness tm shall not include
reinforcement or internal protrusion.
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CHAPTER 8
WELD AND BASE METAL DISCONTINUITIES
1. In ‘service’ inspection of hot forging dies are likely to reveal
a. Herringbone cracks
b. Forging burst
c. Thermal checks
d. Laps
2. Cracks occurring at a change of cross section, due to restrained shrinkage
during solidification of a casting are called
a. Hot cracks
b. Restraint cracks
c. Hot tear
d. Hot checks
3. A discontinuity associated with metal overflow during forging is called a :
a. Seam.
b. Flake.
c. Lap.
d. Lamination.
4. A jagged, nonintegrally bonded piece of metal that leaves a depression in
another metal after it is removed, is called a :
a. Seam.
b. Blister.
c. Scab.
d. Gouge.
5. A discontinuity that is not associated with welds is :
a. Undercut.
b. Overlap.
c. Laminations.
d. Under fill.
6. A discontinuity with a small star-shaped pattern where a weld a starts or stops is
usually an indication of :
a. Surface porosity.
b. Undercut.
c. A crater crack.
d. Slag.
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7. The initiation of a fatigue crack could occur at :
a. Weld toes.
b. Notches.
c. Section changes.
d. Thread roots.
e. All of the above.
8. A welding discontinuity typically referred to as distortion is caused by :
a. The use of a tungsten electrode in the gtaw welding process.
b. The uncontrolled heating and cooling of the weld metal.
c. Exposure to radiation and other nde techniques.
d. The excess amount of porosity in the weld metal.
9. Cracks, suckback, undercut and overlap are discontinuities found in :
a. Castings.
b. Forgings.
c. Extrusions.
d. Weldments.
10. In welding , weld metal protrusion beyond the fusion line at the weld toe is
called:
a. Overlap.
b. Undercut.
c. Reinforcement.
d. Incomplete fusion.
11. Cracking under the combined action of corrosion and tensile stress is referred to
as:
a. Fatigue cracking.
b. Creep cracking.
c. Stress corrosion cracking.
d. Tensile stress cracking.
12. A groove formed at the toe or root of a weld when the base metal is melted away
and left unfilled by weld metal is referred to as :
a. Underfill.
b. Cold lap.
c. Crack.
d. Undercut.
13. Hot tears, inclusions, porosity and cold shuts are :
a. Forging discontinuities.
b. Casting discontinuities.
c. Welding discontinuities.
d. Processing discontinuities
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14. A discontinuities that appears as a series of remelted dots beside a weld,
resembling a trial left in striking a match is called :
a. Linear porosity.
b. Are strikes.
c. Undercut.
d. Slag.
15. A protrusion or rollover of weld metal beyond the toe or root is called :
a. Overlap.
b. Undercut.
c. Reinforcement.
d. Overfilled.
16. Cheverons may occur in :
a. Plates.
b. Weldments.
c. Bar stock.
d. Valve castings
17. A discontinuities that is found in bars and forgings, which is caused by the
rupture of metal forged at either too low or too high temperatures, is called
a. Pipe.
b. Seam.
c. Cupping.
d. Internal burst.
18. A cause for undercut that occurs during the welding process is called :
a. Excessive voltage or current.
b. Slow travel speed.
c. Excessive travel speed.
d. Both a and c above.
19. A rounded discontinuity that occurs in the weld and is then distributed in line,
parallel with the weld is called :
a. Melt-through.
b. Linear porosity.
c. Cluster porosity.
d. A crack.
20. An inherent discontinuity associated with the original solidification of metal in
the ingot is called :
a. A seam.
b. Thermal fatigue.
c. A hot tear.
d. Porosity
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21. Discontinuities associated with the casting process are :
a. Inclusions.
b. Hot tears.
c. Porosity.
d. All of the above
22. A reduction in ductility due to in-service or pre-service environments is called :
a. Embrittlement.
b. Hydrogen fatigue cracking.
c. Thermal fatigue.
d. Intergranular stress corrosion cracking.
23. Metals that become weaker due to continuing deformation under steady stress at
elevated temperatures demonstrate :
a. Thermal fatigue.
b. Stress corrosion cracking.
c. Corrosion reduction.
d. Creep.
24. A nonfusion discontinuity that is located at the root area of a welded joint is
called :
a. Porosity.
b. A hot tear.
c. Incomplete joint penetration.
d. All of the above.
25. In casting process, a chaplet is :
a. A device that supports the core material.
b. A device that is used as a heat shink.
c. A ragged, irregularly shaped discontinuity.
d. All of the above.
26. The three stages of fatigue are :
a. Initiation, propagation, and failure.
b. Initiation, branching, and expansion.
c. Stress, temperature and propagation.
d. None of the above.
27. Discontinuities that originate during the melting and original solidification of
the metal in the ingot are categorized as :
a. Forming discontinuities
b. Inherent discontinuities.
c. Process discontinuities.
d. Service-induced discontinuities.
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28. All casting discontinuities are considered to be :
a. Inherent.
b. Primary processing.
c. Secondary processing.
d. Service induced.
29. Failure to adequately penetrate the weld root of a groove weld is called :
a. Lack of fusion.
b. Excessive penetration.
c. Incomplete joint penetration.
d. Undercut.
30. Repeated fluctuating stress having a maximum value less than the tensile
strength of the material is called :
a. A crack.
b. Mechanical fatigue.
c. Thermal fatigue.
d. Stress corrosion cracking.
31. Cracks can occur in :
a. Forgings.
b. Castings.
c. Welds.
d. All of the above.
32. The structure and shape of mechanical fatigue type cracking is best described
as:
a. Multiple indications or brazing.
b. Relatively straight and non branched.
c. Multiple inter granular indications.
d. All of the above.
33. A crater crack is formed :
a. At the junction between weld beads.
b. At the start and stop of a weld bead.
c. In the base material during the rolling process.
d. In the base material during the forging process.
34. A common processing discontinuity for a bolt is :
a. A burst.
b. Porosity.
c. Necking down.
d. All of the above.
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35. Primarily, piping leaks occur at components such as :
a. Integral attachments.
b. Bolted connections.
c. Valves.
d. All of the above.
e. Both b and c above.
36. A hanger assembly that is attached to a pipe with a pipe clamp is considered to
be:
a. A class 1 component support.
b. An integral attachments.
c. A restraint assembly.
d. A non integral attachment.
37. A condition that is caused by unintentional rapid heating of the base metal or
weld metal and subsequent rapid cooling of the molten material, which results in
extremely high heat input and causes localized hardness and cracking, is called :
a. Undercut.
b. Arc strike.
c. Weld spatter.
d. Overlap.
38. During the visual examination of a forging, a folded thin flap of metal was
observed. This is typically called :
a. Forging porosity.
b. A cold shut.
c. A crack.
d. A surface lap.
39. Arc strikes are typically caused by :
a. Molten particles splashed that are splashed out of the molten puddle.
b. Excessive heat during the welding process.
c. The use of improper or wet electrodes.
d. Welding operator error.
40. The heat-affected zone is the portion of the :
a. Metal that is added to produce the weld joint.
b. Base metal that has been melted and solidified.
c. Base metal that has not been melted but where properties have been
altered by the welding heat.
d. Original metal that is welded.
41. During the visual inspection of castings, chills and chaplets appear as :
a. Rounded indication s.
b. U – shaped indications.
c. Chills appear as rounded indications but chaplets will appear as u- shaped.
d. No definite description of these discontinuities is possible.
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42. Visual testing of low stress applications, such as the sheets of composite
material bonded to a honeycomb core, may reveal :
a. Cracks and voids.
b. Cracks and delamination.
c. Large voids and delamination.
d. Cracks, large voids, and delamination.
43. Cracks occurring at a change of cross section, due to restrained shrinkage
during solidification of a casting are called
a. Hot cracks
b. Restraint cracks
c. Hot tear
d. Hot checks
44. Cracks parallel to plate surfaces developed in 2 plates subjected to transverse
load are called
a. Transverse tears
b. Plate tears
c. lamellar tears
d. Tensile tears
45. Slivers are likely to be seen in a:
a. Casting(sand)
b. Rolled bar
c. Casting (investment)
d. Forged bolt
46. Misrun is a discontinuity likely to be seen in
a. An ingot
b. A casting
c. A welding
d. In service part
47. Typical composite discontinuities would be
a. Hot tear, lamination , bonds
b. Matrix cracks, lamination, voids
c. Delamination , matrix cracks, voids.
d. All of the above
48. Typical cracks in case hardened rolling surfaces would be
a. Surface micro cracks
b. Radial flakes
c. Subsurface cracks at case boundary parallel to rolling surface
d. Subsurface cracks at case boundary perpendicular to rolling surface
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49. Cavitation fatigue damage is most likely to occur in
a. Aircraft wing
b. Marine propeller
c. Pump impeller
d. All of the above
e. B or c only
50. Unfused chaplets are likely to be visually in
a. Composite materials
b. Powder metallurgy parts
c. Castings(sand)
d. Extruded pipes
51. Complete penetration is not likely in
a. Butt joint
b. Lap joint
c. Edge joint
d. All of the above
52. The difference between discontinuities and defects is that
a. A defect is a rejectable discontinuity
b. Discontinuities affect the base metal, whereas defects affect the weld metal
c. There should be no distinction between discontinuities and defects
d. A discontinuity is a rejectable
53. Tungsten inclusions are found during
a. Gmaw
b. Gtaw
c. Smaw
d. None of the above
54. Discontinuities that are produced during the hot or cold working of the ingot
into rod or bar to make studs are called
a. Inherent discontinuities
b. Primary processing discontinuities
c. Secondary processing discontinuities
d. Service – induced discontinuities
55. Discontinuities associated with welds may be classified as
a. Dimensional, process, and mechanical
b. Process,mechanical and basemetal
c. Mechanical, dimensional and process
d. Dimensional, processes, mechanical and base metal
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56. What type of discontinuity is not expected in gtaw?
a. Tungsten inclusion
b. Slag inclusion
c. Lack of penetration
d. Lack of fusion
57. Lamellar tear is the discontinuity occurring in
a. Along the fusion zone
b. Center of the weld
c. Parent metal
d. None of the above
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CHAPTER ANSWERS
CHAPTER – 1
Q.NO.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
ANS
C
A
B
C
B
D
A
B
B
DIFFERENT
SAME
A
A
B
C
A
C
C
B
A
C
B
D
C
C
C
C
B
C
D
C
A
D
B
C
B
B
D
A
C
Q.NO.
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
ANS
A
C
B
B
D
C
B
D
C
A
B
D
D
D
D
A
D
D
E
D
A
E
D
C
D
D
A
E
A
D
B
E
A
C
C
E
A
A
B
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CHAPTER – 2
Q.NO
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
ANS
A
A
B
B
C
C
A
B
C
A
B
C
A
C
D
D
C
B
C
E
CHAPTER - 3
Q.NO ANS
1
B
2
A
3
C
4
A
5
C
6
A
7
B
8
A
9
C
Q.NO
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
ANS
D
B
A
C
B
B
C
C
A
C
C
A
C
A
D
B
D
A
D
B
Q.NO
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Q.NO
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ANS
C
B
D
C
E
B
D
A
C
D
B
D
C
E
A
D
D
E
C
D
CHAPTER - 4
ANS
Q.NO
D
15
B
16
A
17
B
18
D
19
A
C
B
D
D
C
B
C
C
202
Q.NO
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
ANS
D
D
A
C
A
ANS
C
B
C
D
C
B
C
D
E
B
B
B
A
B
A
D
C
B
B
C
Q.NO
81
82
83
84
85
86
87
88
89
ANS
C
C
D
B
A
E
A
D
D
203 | P a g e
CHAPTER – 5
Q.NO ANS
1
B
2
A
3
A
4
B
5
B
6
D
7
D
8
B
9
C
10
D
11
B
12
D
13
B
14
B
15
C
16
D
17
B
18
C
19
D
20
D
Q.NO
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
Q.NO
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
CHAPTER – 6
Q.NO ANS
Q.NO ANS
21
D
41
A
22
A
23
B
24
E
25
B
26
C
27
C
28
B
29
C
30
C
31
B
32
C
33
C
34
D
35
B
36
C
37
A
38
A
39
A
40
D
ANS
A
C
C
C
C
A
A
D
C
B
C
B
A
B
C
D
C
B
D
D
ANS
D
C
B
A
D
A
D
B
A
A
D
C
C
D
D
D
D
B
D
D
Q.NO
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ANS
A
A
B
C
B
D
D
A
C
A
A
D
B
C
D
A
B
C
D
D
203
Q.NO
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
ANS
B
B
C
D
E
C
D
B
E
D
A
D
D
B
C
A
C
D
A
C
Q.NO
81
82
83
84
85
86
CHAPTER – 7
Q.NO
ANS
1
C
2
A
3
B
4
B
5
A
6
C
7
D
8
B
9
C
10
B
11
B
12
D
13
C
14
A
15
C
16
C
17
B
18
C
19
D
20
C
21
A
ANS
A
D
A
B
C
B
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CHAPTER 8
Q.NO
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
ANS
C
C
C
C
C
C
E
B
D
A
C
D
B
B
A
C
D
D
B
C
Q.NO
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
ANS
D
A
A
C
A
A
B
A
C
B
D
B
B
A
E
D
B
D
D
C
Q.NO
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
ANS
D
C
C
C
B
B
C
A
D
C
B
A
B
B
D
B
C
204