Optical inspection device for measuring depthwise variations from a

Abstract

A scanning laser measurement system for measuring depthwise variations of surfaces relative to a focal plane of an optical system. Laser light is directed to a test surface, generally aligned with the focal plane, through focusing optics having an optical axis. Some of the light from a beam spot is retro scattered from the test surface and reimaged along a path generally parallel to the optical axis. Depthwise variations in the test surface with respect to the focal plane cause spatial displacements in the retrobeam relative to the optical axis. The retrobeam is directed to a filter which is partially reflective and partially transmissive. A pair of photodetectors is used to intercept reflected and transmitted components of the retrobeam and the intensity of these components is measured and used to compute centroid values for the retrobeam on the filter for various beam spots on the test surface. The computed centroid values are directly proportional to depthwise surface deviations from the focal plane. The beam is directed to various points on the test surface by an optical scanner. If the deviations in the test surface from the focal plane are so great that a retrobeam cannot be formed, the entire optical system is translated until beam focus can be achieved and a retrobeam formed. The extent of translation is a coarse measurement of depthwise variations in the test surface relative to the focal plane, while the previously mentioned centroid values yield a fine measurement of depthwise variations relative to the focal plane.