Vision Meets Metrology
By Harald Weiss
November 2, 2012
Vision metrology systems are driven by advances in optics, electronics and software.
Vision metrology systems with a moving stage and a video camera have been in use since the 1960s, when
cameras were mounted on a toolmaker’s microscope. These systems have now proven themselves for feature
measurements and surface inspection in a wide range of applications.
Early vision metrology systems made use of crosshair reticles or an external crosshair generator to
superimpose the image of a crosshair on the video image. The stage was moved manually using micrometers,
and stage positions were written down as image features passed across the crosshairs. The micrometers, and
then the stage itself, became equipped with encoders to produce a numeric display of position on a digital
readout (DRO) unit. In the late 1970s, the advent of the video frame grabber board and machine vision
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software allowed automatic edge detection, which led the way to the development of vision metrology
software.
The early cameras were analog and produced a black and white image, with a theoretical resolution of 480
lines across the video monitor, but actual resolution was often closer to 300 lines. While primitive by today’s
standards, these metrology systems set the standard by providing a highly magnified image of optical edges,
and they allowed edge measurements with better than one micron resolution.
Branching Out
During the 30 years since their introduction, commercial vision metrology
systems have branched into a wide range of products differing in size and
capability for different applications and industries. The smallest bench-top
systems have a compact stage commonly with 2 inch by 2 inch of manual X-Y
travel, while larger systems can offer CNC travel to accommo-date large
parts. The smaller systems tend to be manual, with X-Y stage positioning via
hand wheels. The larger and more expensive systems are normally motorized
and driven by CNC software, but they can also be operated manually for
individual measurements and short jobs using a joystick and trackball
pendant.
Vision metrology systems are now available in the form of multi-sensor CNC
metrology systems, where the video lens “probe” can be augmented by a
touch probe and/or a laser probe. In these systems, the video camera is used
for high-speed measurement of edges within the field of view, while the touch
probe handles contours, vertical surfaces and under hangs. The laser probes
can rapidly collect a large number of data points to characterize complex
shapes. Lasers may be suitable for translucent and soft surfaces, as well as
difficult geometries such as narrow slots, grooves and blind holes. Metrology
software can invoke all probe types as needed for measurements on the same
part.
TECH TIPS
The combination of
telecentric optics with
an ultra-high
resolution video
camera is
revolutionizing the
field of video
metrology.
These systems have a
wide field of view,
close to zero optical
distortion, highresolution, and realtime video imaging.
Today, customers
have seen increasing
overlap between
optical comparators
and vision metrology
systems.
Zoom optics, with a continuously adjustable magnification ratio of 6.5:1 or 12:1, have long been used as the
imaging optics for their effectiveness and convenience. These optics are first used at a low magnification and
large field of view zoom setting to locate the features of interest. Then a high magnification zoom setting is
used to measure these features with highest accuracy. The characteristic of zoom optics known as
parcentricity ensures that the image remains centered at all magnifications. However, the convenience of the
zoom optics is not without some limitations: the field of view is relatively small, even at the lowest zoom
setting, optical distortion and optical resolution is limited.
A Look at Telecentric Lenses
The combination of telecentric optics with an ultra-high resolution video camera is now revolutionizing the
field of video metrology by simultaneously offering a wide field of view, close to zero optical distortion,
high-resolution, and real-time video imaging.
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Telecentric optics are compound lenses that process the chief ray from each point in the field of view as if
that point were on the optical axis of the lens, with the result that all chief rays are collimated and parallel to
the optical axis. Parallel rays provide the benefit that magnification is the same for all distances from the
center of the lens, eliminating perspective effects and most optical distortion. The image remains sharp with
irregular, non-flat surfaces.
If the lens is to accept all parallel rays from within the field of view, the entrance aperture of the lens must be
at least as large as the field of view, making telecentric lenses larger and more expensive than conventional
lenses of the same focal length. While telecentric lenses have been available for many years, only recently
have they come down in price to be affordable for vision metrology.
Telecentric optics are now also being offered in conventional vision metrology systems with a horizontal XY stage and a Z column. Current telecentric optics are fixed focus, but can be mounted so that they can be
changed by the user in seconds.
Field-of-view (FOV) measurements, where all measurements are performed from video data within the field
of view with no need for stage motion, have been made possible by the combination of telecentric optics with
a high-resolution camera, high-speed processing, and new FOV metrology software. For example, a 5
megapixel camera divides the field of view into 2448 by 2058 picture elements and can provide 0.00075 inch
(19 µm) of optical resolution across a 2.4 inch by 2.0 inch (60 by 51 millimeters) field view. FOV
measurements on a small part or on features of a large part can be seamlessly integrated with stage motion for
larger parts and be compared to CAD files by DXF/FOV metrology software.
History & Product Evolution
New, more powerful controllers are becoming available. We are now also seeing increasing overlap between
optical comparators and vision metrology systems.
Optical comparators were patented in the United States as far back as 1929. Known as shadowgraphs and
profilometers outside of the United States, these are rugged machines that are still being sold in large
numbers for use in machine shops worldwide.
In the basic optical comparator, a part is brightly illuminated from the rear, and its profile, or silhouette, is
projected on a screen at a precisely known magnification from 10X to 100X. The projected image is then
compared to a screen overlay with pass-fail limits. Modern optical comparators also incorporate a moving
stage with digital encoders so that dimensions can be read as the projected edges move past crosshairs etched
on the screen. A common refinement is an optical edge detector, which measures brightness at a specific
screen location and removes operator subjectivity in locating edges.
A new class of horizontal digital video comparators is a hybrid between an optical comparator and a vision
metrology system. With a horizontal optical axis, an all-steel housing, and a stage with load capacity up to
110 pounds (50 kilograms), such a system is built like a traditional horizontal comparator for heavy-duty
applications; however, it uses telecentric optics, a multi-megapixel video camera, and video image
processing. It can import DXF CAD files and make direct high-speed electronic go/no-go comparisons to the
engineering design, thereby eliminating the need for screen overlays. It can also measure and record
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deviations for statistical process control (SPC).
Driven by advances in optics, cameras, electronics and software, vision metrology systems have increased in
performance, speed and versatility. Expect more from these systems for both measurement and quality
assurance.
For more on vision metrology,
visit www.qualitymag.com:
What Machine Vision Solution is Right for You?
The Future of Machine Vision
Measure with Vision
Harald Weiss, MSEE, is a consultant for Starrett Kinemetric Engineering (Laguna Hills, CA). For more
information, call (949) 348-1213, email sales@starrettkinemetric.com or visit www.starrett.com.
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