Process and apparatus for sensing defects on a smooth surface

Abstract

Method and apparatus for sensing the effective magnitude and/or number and/or position of individual depression and/or protrusion defects on a high-precision smooth surface of an article which may have said defects randomly distributed and relatively spatially distant each from the other, comprising continuously scanning the surface with a laterally-moving interference fringe pattern adjusted in size to have a cross-sectional area substantially smaller than the surface and no larger than an area which includes about an average predetermined spatial incidence of about one defect per pattern area; adjusting the fringe period to a size substantially larger than the effective cross-sectional dimension of a predetermined, maximum size defect; continuously sensing the AC and DC or AC signal components of the backscattered light; and substantially determining the effective magnitude and/or number and/or surface position of said defects by determining the magnitude of the AC or AC and DC signals obtained at each instantaneous relative position of the fringe pattern and the smooth surface. Said scan can thus provide information as to effective magnitude, number of defects, and topography of the defects on the surface. By the use of appropriate electronics, the information can be recorded and/or displayed.