Optical detection and measurement system

An optical detection and measurement system for selectively detecting the plane of the reflective surfaces of a workpiece. A laser source provides a low power, single wavelength collimated beam of light which is directed onto a workpiece. The beam is passed through a converging lens to a reflected focus on a quad detector. The light beams reflected from the workpiece pass through a diverging lens to the quad detector. The non-diverging optical axis center of the diverging lens is aligned with the center of the quad detector so that a reflected beam passing through the non-diverging optical axis center of the diverging lens creates equal photocurrent outputs from each of the four photosensitive elements of the quad detector to produce a null condition. Where the workpiece is made up of translucent or transparent layers which produce multiple reflected beams, reflected beams from surfaces other than the surface of interest are deflected away from the active surface of the quad detector by the diverging lens. The selectivity of beam detection of the system allows detection of the location individual reflective surface planes of the workpiece by positioning the workpiece so that each of the reflective surfaces individually pass their reflected beams through the non-diverging optical axis center of the diverging lens. Storing a numerical value indicative of the location of these planes permits calculation of layer and workpiece thickness.