Kodak model comparator, 1945
Edward C. Polidor (left), founder of Optical Gaging Products,
and Bill DeBoer of OGP
by Kennedy Smith
Since their invention in 1920, optical comparators have changed very little.
Improvements during the years have added higher accuracy and user-friendliness, but all in all, optical comparators look and function much the same as they always have.
"The fact that they haven't changed much over time is a major reason why they remain so popular," explains Fred Mason, marketing communications manager at
Optical Gaging Products. "Familiarity and experience breed confidence." Simply put, people are sticking to what works.
So what's in store for the future? Is it even possible to improve upon a workhorse that's hardly changed in 80 years? This article attempts to answer these questions.
Evolution of the optical comparator
The Evolution of Optical Comparators
The optical comparator is invented by James Hartness, president of J&L Machine Co., in 1922. It projects
the shadow of an object onto a screen a few feet away and can be compared with a chart showing tolerance levels for the part. By the end of the decade, the optical
comparator is an integrated machine, small enough to fit on a desktop. Comparators also begin to be used to examine wear of a part as well as for setup phases in
J&L Machine Co. weathers the Great Depression by exporting optical comparators to the Soviet Union,
which finds itself undergoing rapid industrialization under the lead of Josef Stalin. At home, comparators are being used more and more in small-part manufacturing plants,
including those that produce razor parts, toothbrushes, dental burrs, bottle molds and more. Comparator sales reach a little more than 300 per year.
Optical comparator sales skyrocket as optical comparators are adopted as a standard for U.S. artillery specifications.
They're used in the manufacture of just about every part used in World War II, including rivets and firing pins. About this time, comparators become commonplace in the
automobile and aircraft manufacturing industries. Accuracy of the optical comparator improves because of better glass grinding technology, creating better lenses for image
Automatic edge detection is introduced, making it possible for the machine, rather than the operator, to
determine the part edge. This provides more accuracy by eliminating subjectivity. Accuracy is also improved with the introduction of Vernier encoder-type scales on the
comparator's stage, which converts the stage into an additional measurement instrument with which to measure the part.
Digital readouts are
introduced, as is programmable motorized stage control. As machines become more automated, developers start to incorporate programmable functions into the optical
comparator. This paves the way for complete automation of an optical comparator machine.
Incorporated software becomes
standard optical comparator equipment. Computers can be interfaced with optical comparators to run image analysis. Points from manual or automatic edge detection are
transferred to an external program where they can be directly compared to a CAD data file.
Sources: Fred Mason, Optical Gaging Products; Peter Klepp, Dorsey Metrology; The Optical Comparator and the Fight Against Scrap
("Tools & Technology" newsletter, Winter 1998–1999) by Ruth Ann Groh.
It seems that James Hartness had it right the first
time when he introduced his staff at the Jones and Lamson Machine Co. to his invention, the shadow graph (now known as the optical comparator). Hartness developed the optical
comparator out of a need to standardize screw thread sizes. Serving on the National Screw Thread Commission, Hartness who would later become governor of Vermont set out to create a
machine that could measure the complex curves of a screw. This first optical comparator projected the shadow of the object onto a screen a few feet away. Hartness and his
employees could then measure the shadow image of the screw thread against draft designs. By the end of the 1920s, an optical comparator was a single integrated unit able to fit on a table top.
In the 1940s, it became obvious that optical comparators were indispensable tools of the design and production process. As the United States became involved in World War II, the
defense industry used optical comparators for weapons and special equipment. This, and a booming automobile industry, helped the optical comparator become a staple in part
measurement by the 1950s.
Automatic edge detection was added in the 1960s, and the 1970s brought about digital readout capabilities. About 15 years ago, the
issue of backlash was resolved when manufacturers started using linear scales. Before then, measuring accuracy relied upon threads on a rotary encoder (using a hand crank to turn and
move the stage). "When you're using threads and turning a gear, the threads can actually wiggle a little bit between the meshing of the gears," says
Mike Metzger, measuring department manager at Nikon Instruments. This problem was eliminated when operators switched from rotary encoders to linear encoders.
Despite a host of technological advances and dozens of sizes, magnifications and special features, the optical comparator still relies on a relatively complex optical assembly that
accurately magnifies and projects the image of the part with minimal distortion. "Plus, because it's such a mature technology, there are a number
of manufacturers," says Mason. "This means a wide range of prices and capabilities."
Peter Klepp of Dorsey Metrology agrees,
adding, "This basic principle has remained unchanged since its invention. Manufacturers continue to apply emerging technologies to the foundation to increase the accuracy and
versatility of their instruments."
In the last decade or so, changes to optical comparators have focused on additional
functionality, improvement of the quality of imaging, creating fully automated machines and integrating computer technology into the system.
By now, most measurement equipment
operators are proficient with computers, so using them with an optical comparator system doesn't add any difficulty. "As computers have become reasonably priced and the average operator has
become comfortable with them, they've become almost standard equipment on an optical comparator," says Klepp. "It's very seldom that a comparator without a digital electronics
package is requested by a customer." In addition, software increases the capabilities of the optical comparator and makes it easier for operators to use special features of the system,
such as transferring selected points of measurement into a program that can directly compare it to CAD file data.
Software aside, Klepp notes two major
developments in comparator technology at Dorsey. In 2000, the company released an optical comparator with reflective scale technology, which is capable of eliminating any
backlash from linear encoders. Additionally, Dorsey recently released the first comparator with three-axis touch probing. "Historically, the Achilles heel of an optical comparator has been
its limitation to measure only two axes," says Klepp. By integrating either a touch probe or a laser noncontact device, the optical comparator is now capable of measurement in the Z axis.
Nikon's Metzger says the best (recent) improvement to optical comparator technology has been to the quality of lenses.
"This is a result of computer-controlled manufacturing of the lenses. If you make a lens better, more light transmits through it and your image becomes brighter." This
allows surface illumination so the operator is capable of seeing not only a profile of the part but features on the surface of the part as well.
"The latest comparators feature motorized stages, automatic edge detection, geometric measurement computers and 0.5-micron or better stage measurement,"
adds Walt Wardzala, precision measuring instruments product manager at Mitutoyo.
The size of today's optical comparators typically range from 12-inch screens to
30-inch screens, although there have been exceptions (at one time, Optical Gaging Products produced a 60-inch comparator). These can be tabletop models
or floor models, depending on the needs of the company. Small, low-end manual machines are available for a few thousand dollars, while large, fully automated,
high-quality machines can cost upward of $100,000. Other factors that affect price are the size and weight of the part that can be measured. For example, a
14-inch machine designed to hold heavy machined parts will cost more than a comparably sized system designed for sheet metal parts.
Predictions on new developments
Although the optical comparator has changed little over time, its original design is
still subject to constant improvement. There's always something in the works for a better and more efficient product. Some experts speculate on what's in store for
the future of optical comparators.
"Removing operator subjectivity from the measurement process will continue to
be a goal," says Mason. "So, automation of edge detection, stage motion, etc., will be an emphasis for comparator developers."
Klepp predicts that optical comparators will move toward three-axis measurement, incorporating video and touch probing systems. "These will
become standard options, and three-axis software development will further improve operator capabilities."
Perhaps the most noteworthy prediction, however, is that optical comparators will be replaced by video measuring systems.
Next-generation optical comparators
"The video measuring tools that are out today are the hottest products on the
market, and they've been hot for about 10 years," says Metzger. Some experts even go so far as to say that video measurement systems will make optical
comparators obsolete in the not-too-distant future.
Bipin Mukherji, president of VideoGage Inc., says his company's product is
slated to become the next generation beyond optical comparators. "Comparators are nice basic tools, which have done their job over the years," explains Mukherji,
"But, with industry evolving the way it is, there is a need for something superior to optical comparators."
Among the advantages to utilizing a video measuring system, Mukherji notes:
Fully corrected (i.e., not inverted) image
Image processing capabilities
Variable zoom lens, offering multiple magnification levels
Variable illumination with a cool fiber-optic light source, plus three light sources
Color and profile images
Capability to capture images of work pieces or part features
Capability to archive and document measurements
With a video measuring system such as this one, the operator can click the mouse on any given feature on the part and because it utilizes a video camera and computer
technology measure every pixel in that area. "One could never do this with an optical comparator," says Mukherji. "It's so quick, you can do this with the blink of an eye."
"Video measuring is booming right now," agrees Metzger. This is partly because such systems are reasonably priced and offer more advanced capabilities than do their
optical comparator counterparts.
Another product combines optical comparator technology with video and
microscope technology. Rick Coyle of Vision Engineering Inc. describes the company's product as a more advanced alternative to the optical comparator. Like the optical comparator, Vision
Engineering Inc.'s DynaScope series of products utilizes a ground-glass screen. However, the company has patented a method of rotating the glass at 2,800
revolutions per minute, which reduces graininess and produces a smoother image for accurate edge detection. Additionally, Coyle notes the DynaScope is capable
of advanced illumination of the part, easy upgrades to include photographic capabilities for documentation, and several magnifications.
Optical comparator phaseout?
For several reasons, companies still have a need for optical comparators.
Comparing a part profile to a chart is easy to understand, easy to teach and easy to implement. Newer machines have more capabilities than before. Electronic
geometric processors allow more accurate dimensional and angular measurements. Moving the image of a feature to a particular location on the
screen, zeroing the readout and moving to the next feature provides a direct readout of distance. Some edge-detection systems remove subjectivity in the
process. They also provide a low-cost solution for operators interested in quick go/no-go measurement. "They're ideal for someone with a limited budget, an
extreme variety of parts or just looking for a quick check of a feature," adds Wardzala.
Despite the prediction that optical comparators are on their way out, experts
agree that they've proven their place in industry. There will always be people who want the nonautomatic, basic tool. "Those markets will always be there, although
they're not the growing markets," says Metzger. "They're easy to use, they're reliable and they work for the job that they're intended to work for."
About the author
Kennedy Smith is Quality Digest's assistant editor. E-mail her at email@example.com . E-mail letters to the editor regarding this article to firstname.lastname@example.org .
Optical Comparator Companies
A.A. Jansson Inc.
- Phone: (248) 674-4811
- Phone: (847) 437-0022
- Phone: (800) 451-6922
Dorsey Metrology International
- Phone: (845) 454-3111
- Phone: (707) 838-6272
Mitutoyo America Corp.
- Phone: (630) 820-9666
Nikon Instruments Inc.
- Phone: (800) 526-4566
Optical Gaging Products Inc.
- Phone: (800) 647-4243
- Phone: (800) 326-2039
The L.S. Starrett Co.
- Phone: (800) 784-2887
- Phone: (248) 391-7800
- Phone: (818) 768-2700
Vision Engineering Inc.