Silvered Mirrors for Interferometric Measurement

Abstract

THE simple interferometric devices used in Great Britain¹ and on the Continent for testing the surface quality of workpieces of less than 20 micro-in. Surface roughness have recently been improved by applying a partly reflecting mirror as suggested by L. Leinert². This method has been compared with the usual practice of using non-silvered glass¹ and the results are quite interesting. Whereas with non-silvered glass the interference bands are relatively weak and the underlying surface with its imperfections is easy to recognize, the silvered glass of the same surface shows the outlines of the interference bands very clearly (see accompanying reproductions). This is due to the fact that in this case the scattered light from the workpiece cannot penetrate the silver layer. A much better topographic picture of the surface is thus obtained; but it is open to question whether for practical inspection purposes a combined picture of surface and interferences would not be more desirable.