Process and apparatus for determining thicknesses of layers

Abstract

A process for determining the thickness and/or change in thickness of a thin layer applied to a carrier, the process comprising radiating the carrier coated with the thin layer with a plane polarized light beam at a single incident angle and detecting the reflectance on the carrier and layer. The light reflected by the carrier and the thin layer is split into a p-polarized part and an s-polarized part, after which the respective intensities (I.sub.s).sub.r and (I.sub.p).sub.r l of the reflected s-polarized light and p-polarized light are measured simultaneously for the two parts, and calculating a quantity M according to the equation: ##EQU1## in which formula R.sub.s is the ratio of the intensities (I.sub.s).sub.r and (I.sub.s).sub.i of the reflected and incident s-polarized light, respectively;