Optical comparators have been used for more than 50 years and remain a versatile and cost-effective technology for monitoring the processes and quality of a broad range of manufactured parts. Originating from static overhead projectors that displayed magnified images of screw threads onto a wall for manual measurement, optical comparators have evolved into full-featured machines that use modern mechanical, electrical and optical technology to minimize inspection time and maximize cost savings. Believing after seeing is appropriate when referring to optical comparators. Because these measurement tools display a magnified image of a part, a tremendous amount of information about that part can be gathered in a short time simply by looking at the image. Optical comparators, for those unfamiliar with them, are inspection machines that project magnified images of parts onto a glass screen using illumination sources, lenses and mirrors for the primary purpose of making 2-D measurements. One advantage of optical comparators can provide more information than just simple dimensions. Length and width measurements of the part shown above, for example, can be quickly obtained from two separate measurements by using a micrometer.

These superficial measurements, however, might not reveal burrs, scratches, indentations or undesirable chamfers. Such imperfections are best detected on a comparator. In addition, the screen of comparator can be simultaneously viewed by more than one person and provide a medium for discussion, just as a white board might facilitate a conference. Another advantage of comparators is their ability to measure in 2-D space. Unlike micrometers and calipers which measure one dimension at a time, comparators measure length and width simultaneously. To do this, the operator lines up the lower left-hand corner of the image with the screen centerline to establish a zero point, as illustrated above, and then checks the upper right-hand corner to get a simultaneous reading of both length and width. The straight-line distance from corner to corner can be obtained with a single keystroke. Optical comparators are among the easiest measurement instruments to use. In less than two hours, users with only a minimal amount of gagging experience can make accurate measurements using these devices. Because a comparator displays a part’s 2-D image on-screen, the image can be easily associated with the part’s 2-D CAD drawing.

This simplifies the process of developing measurement procedures for parts from drawings and minimizes the subjectivity of more complex measuring methods. For example, using a sine plate and a height gage to measure an angle might require a detailed procedure, whereas the same measurement on a comparator might not require a written procedure at all. Optical comparators are non contact gages, another key advantage. Nothing but light touches a part during measurement, which means that delicate parts won’t change shape from any direct forces during the process. Non contact gages also eliminate the feel factor and resulting human error of hand gages. Contact measuring methods such as micrometers or plug gages are subject to the amount of pressure applied during measurement, particularly a problem on more delicate parts. Also, the size and location of certain dimensions might preclude the use of contact inspection tools. A tiny radius of 0.010 in., for example, would be difficult to measure with a contact gage. When magnified on a comparator with a 100X lens, however, this radius would be 1 in. on-screen–and easily measured with a radius chart. Moreover, optical comparators save time. Ease-of-use factors and ergonomic designs reduce the inspection time, retraining costs and operator fatigue, all while increasing throughput. And because comparators generally allow parts to be held in one orientation for each 2-D view, costs associated with parts handling and setup times are significantly reduced. Custom hard gages are subject to wear and need frequent recertification, which takes them out of service. Additionally, high-volume production can require multiple sets of fixed gages, increasing costs further.

The inflexibility of these fixed gages means that minor product-design changes can render hard gages obsolete or require costly modifications. Optical comparators, on the other hand, are general-purpose measurement devices, easily adaptable to design changes and appropriate for either first-article or repetitious inspections. Dimensioning techniques designed to give more leeway to parts in relation to their true functional purpose, such as profile tolerance and true-position tolerance with bonuses, reduce the reject rate of good parts that might have passed had their tolerances been assigned differently. For example, an arc by itself might be out of specification, whereas that same arc combined with other features as part of a profile tolerance might pass. The optical comparator is ideal for such dimensioning.