An optical comparator is a device which magnifies and projects an image of an object placed on its stage onto a screen so that the part may be compared and measured against a set of known data. These machines are used to compare parts at various stages of production to ensure conformity to standards and may also be used to establish degrees of wear and tear in working parts. This measurement is achieved by placing the part in question on a glass staging table under powerful lighting. A complex arrangement of lenses then magnifies and transfers the image onto a viewing screen. This image may then be compared with a reference grid or template of the part attached to the screen.
The optical comparator was developed in the early 1920s as an aid to measuring and comparing the complex profiles of screw threads. These machines make use of strongly illuminated horizontal glass staging table and a vertical viewing screen. The object to be checked or compared is held firmly in a staging fitting and then placed on the staging table where it is illuminated by a powerful halogen or cool fiber optic light source. The machine's optic elements, which can be adjusted for different magnifications and for fine focus, then project the image onto the viewing screen. These images may then used to check production parts for conformity or working items for wear or degradation.
The viewing screen may have a protractor grid or a sample profile of the part attached as a transparent overlay which allows the operator to compare the projected image against a set of standard measurements. The image projected onto the viewing screen may be a simple profile of the part or include surface detail depending on the lighting position chosen and the process requirements. This method of measurement makes it possible to benchmark small, complex profiles with great accuracy. Some optical comparator systems allow the part to be rotated to allow for multi-axis comparisons and may capture and store projected images for reference or later analysis.
One of the disadvantages of the optical comparator is subjectivity in the interpretation of the projected image. This has lead to industry advancements such as automatic edge detection which allows a digitized computer model to be used to benchmark the projected results. Another problem experienced with older systems has been the inability to compare a part in more than two dimensions. Modern comparator systems have addressed this issue with the inclusion on non-contact laser readers and touch probes which allow for the addition of Z axis measurement.
The latest advances in the area of optical comparison are video measuring systems which allow a computer measurement program to compare images down to pixel level for ultra-accurate measurement. These video systems are quick and allow for extremely precise, 3 axis surface and dimensional mapping of samples. Although still a fledgling technology, advancements made in the video comparator arena are likely to make the conventional optical comparator obsolete in the near future.
