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What does an optical comparator do?

When industrial manufacturing and applied science come together, the result is the optical comparator. This simple machine was first created more than 90 years ago, and still functions today using the science behind optics. Optics involves the study of light and the general properties that guide transmission of light. With many progressions in style, usability, and precision, optical comparators today use the same design as long ago. But what do they do? Optical comparators are used in the measurement and inspection of manufactured parts.

Using light, an optical comparator functions similarly to an overhead projector that you might see in a classroom. A part is placed on the optical comparator such that a silhouetted image of the part is project on a nearby surface or screen. Importantly, the device itself creates a stable, immovable, and reliable stage. This creates a known and fixed distance between the part and the projected image. Additionally, through a series of mirrors and lenses, the image silhouette is a magnified version of the original. This makes the final image more manageable to work with and examine.

The image of the part created by an optical projector is then utilized to take the needed measurements. There are three measurement processes available when using an optical comparator. The first, from which the device gets its name, is to compare the image created to rulers, protractors, or other measurement units placed across the projective screen. Because the magnification and location of the optical comparator are known entities, then the measurement taken on the screen can be converted to an accurate measurement on the actual part. This method of measurement eventually started using upgraded chart gages, or precisely designed glass screens that overlay the part image.

The second method of measurement possible with an optical comparator uses screen rotation. Screen rotation is typically used when the user needs to measure angles of a part. The basic process starts with lining up the screen ring such that its center lines are matched up to the first edge of the part being measured. Then, after setting the angle readout to zero, the screen is rotated in order to align the second edge. The resulting displayed angle will correspond to the angle on the part. Finally, the third measurement method used with optical comparators is measurement by motion. Similar to using screen rotation, measurement by motion uses a moving worktable or a sliding fixture paired with a chart gage, mentioned above. This type of measurement is ideal for longer parts that do not fit on the screen. Modern day optical comparators are often equipped with integrated class scales and geometric processors that make these measurements even more efficient and accurate.

Optical comparators are a classic method of measurement that provide a number of unique advantages. These wonderful devices are able to show both simple dimensions as well as more detailed imperfections like scratches, indentations, or burrs. Being able to detect both broad and specific features of an industrial part is a very useful capability. Optical comparators also offer the advantage of shared observation between two metrologists simultaneously, allowing for discussion of any concerns or disagreements. Furthermore, optical comparators can take measurements in a two dimensional space, measuring both length and width at the same time. This puts them a step above micrometers and calipers which are limited to one dimensional measurements. In general, optical comparators are amazing devices that utilize optical physics to take measurements of industrial parts. This sophisticated collaboration of sciences creates a beautifully simple tool for metrologists everywhere.


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