About a decade ago, those in
the metrology world were anticipating a seemingly inevitable
battle for the crown of the dimensional measurement division.
The champion --the optical comparator --was well-liked and
capable, but since its birth in 1920, improvements had reached
something of a plateau. The door seemed wide open to the
new kid on the block: vision (more accurately but less often
described, video) technology, whose ostensibly comparable
capabilities drew attention to its apparently boundless
In an effort to maintain --if not increase --market
share, manufacturers in all sectors routinely revise
and append their products, often adding functionality
previously only available as part of their competitors'
products. In the highly competitive dimensional measurement
industry, this is being accomplished via multisensor
technology (i.e., adding camera-based functionality
to optical products, touch probes to vision systems
and even noncontact options to traditional CMMs. The
result: the introduction of machines that have blurred
--if not completely disregarded --the lines representing
traditional product classification that held largely
true just a few years ago.
"Additional sensors extend measurement capabilities
so more measurements can be done in a single setup
on one machine," explains Fred Mason of Optical
Gaging Products. "For video systems, lasers provide
surface contours, and touch probes measure surface
boundaries not easily measured by video. Optical comparators
have added automation and automatic edge detection,
but they're still based on a single optical technology.
Collectively these additional features provide a more
complete characterization of a part."
One such product, the OV2 from the L.S. Starrett
Co., aims to bring the advantages of video inspection
to those who have already invested in optical measurement
without forcing them to purchase an entire additional
machine. "OV2 will allow existing Starrett comparator
users a simple, elegant and economical upgrade path
from the comparator that they already have into newer
video measuring technology," explains Craig Smith
of the company's Metrology Systems division.
The OV2 incorporates a 6.5:1 zoom lens with 32 mm
of working distance, allowing for maximum usage of
the stage travel of the comparator. It dovetails into
the projector body and is prealigned for linear accuracy.
In the right configuration, it even allows simultaneous
use of video and optical technologies.
Nikon Instruments pairs its MM40 measuring microscope,
an optical solution, with its E-Max software to provide
a manual version of the same software that drives
the company's Nexiv automated measuring vision systems.
"This software is a 'bridge' between the old
manual method of measuring and the automated measuring
systems," explains the company's Merrill Brenner.
"This setup replaces the data processor with
a computer and software."
VideoGage Inc. has recently finished the development
of an attachment for its vision product --whose horizontal
structure provides the simplicity of optical systems
--that will convert it to the more conventional vertical
orientation. The company is also currently developing
a touch probe accessory.
- Ease of use
- Low maintenance
- Image processing
- 3-Axis measurement
- Surface- and blind-feature measurement
- High-magnification performance
The media provided exhausting coverage of the prefight
hype. Metrology experts weren't shy about their predictions
of the likely victor, and a profound buzz of the looming
battle invaded the industry. "In the early 1990s, many
video system manufacturers scoffed at the proposition that
projectors would make it through the turn of the century,"
recalls industry expert Craig Smith of the L.S. Starrett
Co. The stage seemed set for a showdown in the grandest
venue of all, the marketplace. Nevertheless, some 10 years
later, the long-awaited face-off has yet to take place.
It was thought that optical comparators would be displaced
by camera-based alternatives much the same as word processing
has supplanted typewriters. Yet it seems that some time
during the prefight banter and preparation, that phenom
of a contender went on to bigger and better challenges,
leaving the simple optical system stuck in its niche, where
it remains comfortably today.
Head to head, the comparator holds its own against today's
video systems, so long as it stays within its comfort zone.
Not unlike the shadowgraph projectors used even before World
War II, today's comparators excel in measuring via a part's
profile. "If it can be seen in the silhouette, a comparator
is probably the best choice to measure it," Smith says
"Optical systems will continue to exist," predicts
Fred Mason, marketing communications manager of QVI, the
parent company of Optical Gaging Products. "They're
relatively inexpensive, they're adequate for a wide variety
of parts still being manufactured, and they have a track
However, if a company needs to inspect large production
runs, a video system becomes an alternative. But, as already
stated, video systems have become much more than viable
alternatives. In many situations, they're the hands-down
"One reason you'd go to video would be for measuring
a lot of parts or features very quickly," explains
Smith. "When it comes to automation, particularly as
related to throughput, the vision system will quickly outperform
Mason agrees. "As production volumes increase, the
benefits of automatic video measurement become advantageous,"
Automation has been added to modern optical solutions
in the form of fiber-optic edge detection. "A CNC projector
will use this technology, gathering a single point at a
time by observing a light-to-dark transition as the work
piece is moved past the sensor on the machine," Smith
describes. But there are drawbacks. "To measure a radius
based on eight points, the projector would need to make
eight separate machine moves (regardless of the measured
part's size). In contrast, an automated video system is
capable of measuring hundreds of data points and/or features
displayed on the monitor without the need for any stage
movement." This translates to the ability to gather
an almost countless number of data points while also increasing
throughput by shortening cycle time.
It was once thought that automation would be the next
evolution for comparators, but camera-based video systems
quickly exerted their dominance in this role. Notwithstanding
their limited success in that progression, optical systems
have evolved substantially since their invention during
the Roaring '20s. Although outshined by today's video options,
surface detail can be inspected with contemporary comparators.
Yesterday's optical solutions used overlays to give a
go/no-go determination, but industry today demands much
more objective and accurate data. "An operator's 'good/bad'
opinion of whether or not a part is within specs isn't enough
anymore," agrees Smith. "Actual quantitative data
is required. To provide the less subjective hard numbers
now often required, today's comparators utilize digital
Another, and perhaps the video system's best, advantage
over its optical counterpart is the ability to measure surface
detail. Although this can be done --on some level --with
a comparator using surface illumination, this technique
has its drawbacks. First, speed is severely affected because
comparators only measure one discreet point at a time, and
when measuring via surface illumination, they do so without
the benefit of edge detection. Second, the resultant representation
is significantly degraded at high magnifications. Smith
says this typically becomes an issue at about 50X magnification,
whereas video systems can deliver sharp images in excess
Although it hasn't accomplished the complete replacement
of optical systems as predicted, video technology hasn't
remained stagnant. "It's come a long way," says
Bipin Mukherji, president of VideoGage Inc. "It has
become more affordable, and there's greater consumer awareness
about its reliability, speed and convenience."
"Video technologies offer a number of advantages
over optical comparators," concurs Mason. "Beyond
the fully automatic measurement possible on CNC-based video
machines, they can use zoom optics with large magnification
ranges, as opposed to fixed magnifications of comparators.
And images of nearly any size can be displayed on video
monitors for viewing from a wide angle, again, as opposed
to the limited viewing angle of optical comparators."
Nevertheless, optical comparators are arguably as common
in machine shops today as they were decades ago. Clearly,
there have to be some saving graces.
For Smith, the issue comes down to that old craftsman
tenet of using the right tool for the job. "Measuring
profile features with an optical comparator is certainly
a case of using a wheelbarrow for a wheelbarrow's job,"
he declares. "You could do the job with a truck, but
you probably wouldn't choose to push the family pickup back
and forth across the backyard." Indeed, an optical
comparator used exclusively for edge detection and related
measurements might cost as little as half of the price of
a manual video system capable of the same inspection function.
And like an old, worn wheelbarrow, optical systems represent
a trusted and robust technology, one that's been reliable
in tough shop-floor environments for the better part of
a century. Smith explains that this long history gives operators
a sense of comfort that might not accompany the use of a
video system. "Let's face it, a comparator's design
is fundamentally quite simple," he explains. "We're
basically talking about a couple of high-quality mirrors,
a lens and a screen, whereas a video system is perceived
by the typical user as less like a mechanical tool and more
like an electronic gizmo."
Additionally, the simplicity of design inherent to optical
comparators translates into ease of use and less maintenance.
"When a comparator's light source burns out --a typical
maintenance issue --the average user will feel plenty comfortable
replacing it," describes Smith. "But if a video
system's camera goes out, now you're talking about a service
"Likewise, an entire group of inexperienced operators
can be trained very quickly to use a comparator. Conversely,
video systems training is usually more involved and typically
only completed by a couple of people in a given machine
shop, for instance."
"The main reason optical systems are still being
used is familiarity," suggests Mason. "Because
optical comparators have been used for so many years, many
companies have documentation and procedures based on those
machines. As long as dimensional tolerances have not changed,
there's no compelling reason to change the metrology."
Perhaps no longer the preeminent dimensional measurement
solution, optical comparators still have a place --one that
no longer appears threatened by video technology --on the
shop floor. In fact, as touch probes are added to video
systems, and CMM manufacturers continue the trend of offering
noncontact options, Smith suggests video technology may
be more of a threat to traditional CMMs in the long term.
Is another battle looming? We'll pass on the opportunity
for conjecture this round; only time will tell.
Robert Green is Quality Digest's managing editor.
Letters to the editor about this article can be e-mailed
For more information about optical and video measuring
systems, read Kennedy Smith's article, "Shedding
Light on Optical Comparators," which is available