A method is given for the measurement of the optical thickness of nonabsorbing dielectric films when it is inconvenient to apply an overlay film to measure the thickness by Fizeau fringe shift. The method is based on fringe-step displacements which have been observed in CdS films evaporated over reflecting metal films. The fringe-step structure is accounted for by a Fresnel reflection coefficient analysis of double-layer multiple reflection within the dielectric film and an air film, formed between the dielectric and a half-silvered mirror.
J. G. GOTTLING and W. S. NICOL, "Double-Layer Interference in Air-CdS Films," J. Opt. Soc. Am. 56, 1227-1230 (1966)