An optical comparator, sometimes called an optical projector, is an inspection device that projects a two-dimensional, magnified silhouette of a part onto a screen using light, lenses, and mirrors. Optical comparators simplify the inspection of otherwise difficult-to-measure parts, small parts, or part components.
Optics, or the physics of visible light, has been used for visual evaluation and measurement since the 1600s, following introduction of the telescope and the microscope. The idea of using optics to inspect machined parts came about just after World War 1 when James Hartness and Russell Porter were awarded the first patent for a screw thread comparator. The screw thread comparator projected the shadow of the part being inspected onto a screen across the room from a projection device. The inspector could measure the shadow and compare the results against draft designs. Because the screw thread comparator proved to be versatile and reliable, its use quickly spread to other industries. As a result, the screw thread comparator became known as the optical comparator.