Double Focus Interferometers and the Influence of Their Sample Setting Errors

Abstract

Double focus interferometers are attractive for reducing the influence of ambient conditions due to their common-path configuration. This paper describes the influence of sample setting errors in these interferometers. They have been divided into two types, i.e. the normal illumination type and the oblique illumination type. For each type, an equation of the measurement error due to sample setting error has been derived with Gaussian optics. The measurement error of the normal illumination type becomes a parabolic function of the position on the sample surface. The derived error equations have been verified by ray tracing and by experiment. The error of the oblique type is smaller than that of the normal type.