Optical comparators are instruments that project a magnified image or profile of a part onto a screen for comparison to a standard overlay profile or scale. They are noncontact devices that function by producing magnified images of parts or components, and displaying these on a glass screen using illumination sources, lenses and mirrors for the primary purpose of making 2-D measurements. Optical comparators are used to measure, gage, test, inspect or examine parts for compliance with specifications.
Optical comparators are available in two configurations, inverted and erect, defined by the type of image that they project. Inverted image optical comparators are the general standard, and less advanced type. They have a relatively simple optical system, which produces an image that is inverted vertically (upside-down) and horizontally (left-to-right). Adjustment and inspection requires a trained or experienced user (about two hours of practice time and manipulation). Erect models have a more advanced optical system that renders the image in its natural or "correct" orientation. The image appears in the same orientation as the part being measured or evaluated.
Optical comparators are similar to micrometers, except that they are not limited to simple dimensional readings. Optical comparators can be used to detect burrs, indentations, scratches and incomplete processing, in addition to length and width measurements. In addition, a comparator's screen can be simultaneously viewed by more than one person and provide a medium for discussion, whereas micrometers provide no external viewpoints. The screens of optical comparators typically range from 10"-12" diameters for small units to 36"-40" for larger units. Even larger screen sizes are available on specialized units. Handheld devices are also available, which have smaller screens as would be expected.