by Robert Green

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 potential.

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 concisely.

"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 record."

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 winner.

"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 a comparator."

Mason agrees. "As production volumes increase, the benefits of automatic video measurement become advantageous," he explains.

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 geometric readouts."

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 of 200X.

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 call.

"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.

About the author

Robert Green is Quality Digest's managing editor. Letters to the editor about this article can be e-mailed to letters@qualitydigest.com. For more information about optical and video measuring systems, read Kennedy Smith's article, "Shedding Light on Optical Comparators," which is available at www.qualitydigest.com/may02/html/optcomp.html.