Thickness measurements of thin transparent coatings on thick transparent bases by reflection interferometry are made possible through use of an immersion medium of the same refractive index as the base to intensify interference. A convenient interferometer comprising three essential parts, a “pocket” spectroscope, an incandescent lamp, and a cylindrical immersion tank, is described. Its use for rapid, precise measurements of thickness of single and double layer coatings and films in the range of 0.2 to 50 microns is illustrated with spectra.
M. F. BECHTOLD, "Coating Thickness Measurement by Interferometry," J. Opt. Soc. Am. 37, 873-876 (1947)