On the conditions for applying light scattering methods to rough surface evaluation

Abstract

Discusses light scattering techniques as applied to surface quality evaluation and presents results obtained both in elaborating the theory and in practising the method. The theory indicates the existence of two roughness height ranges for which the spatial distribution of the light flux scattered from a surface, can be easily related to reliable geometrical features. The first domain dealing with small heights is suitable for deriving the roughness power spectrum from the diffraction theory. The second one, where greater heights are concerned, makes it possible to determine the facet slope distribution in accordance with the microreflection theory. In between is a rather narrow range whose mean location depends on the set-up used for the exploration. Experimental conditions are examined with special attention given to some typical dimensions. The measurements carried out on glass surfaces diversely smoothed and polished yield results which allow empirical relations to be derived, and that are valid at least for this type of surface. One is appropriate for a rapid determination of the r.m.s. roughness and the second is a general formula giving the scattered flux over the three roughness height ranges.