What’s an optical comparator?

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An optical comparator is a device that magnifies and projects an image of an object placed on its stage onto a screen so that the part can be compared and measured against a set of known data. These machines are used to compare parts at various stages of production to ensure compliance with standards and can also be used to establish degrees of wear on working parts. This measurement is obtained by placing the part in question on a glass stage table under powerful lighting. A complex arrangement of lenses then magnifies and transfers the image to a viewing screen. This image can then be compared to a reference grid or model of the part attached to the screen.

The optical comparator was developed in the early 1920s as an aid for measuring and comparing the complex profiles of screw threads. These machines use a brightly lit horizontal glass set-up table and vertical display screen. The object to be checked or compared is held securely in a stage set up and then placed on the set up table, where it is illuminated by a powerful halogen or cold fiber optic light source. The optical elements of the machine, which can be adjusted for different magnifications and for fine focusing, then project the image onto the viewing screen. These images can then be used to verify the conformity of production parts or the wear or degradation of working items.

The viewing screen may have a protractor grid or part sample outline attached as a transparent overlay that allows the operator to compare the projected image to a series of standard measurements. The image projected onto the viewing screen can be a simple outline of the part or include surface details depending on the chosen lighting location and process requirements. This measurement method allows you to compare small and complex profiles with great precision. Some optical comparator systems allow the part to be rotated to allow for multi-axis comparisons and can capture and store projected images for later reference or analysis.

One of the disadvantages of the optical comparator is the subjectivity in interpreting the projected image. This has led to industry advances such as automatic edge detection which allows a digitized computer model to be used to compare expected results. Another problem with older systems was the inability to compare a part in more than two dimensions. Modern comparison systems have addressed this problem with the inclusion of non-contact laser readheads and probes that allow for the addition of Z-axis measurement.

The latest advances in the optical comparison area are video measurement systems that allow a computer measurement program to compare images down to the pixel level for ultra-accurate measurements. These video systems are fast and allow extremely precise surface and dimensional mapping of the samples on 3 axes. While still a fledgling technology, advances in the video comparator arena are likely to render the conventional optical comparator obsolete in the near future.

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