Methods are described for measuring accurately the half-widths of frustrated total reflection niters (see reference 1) of very narrow pass bands, and for detecting extremely small non-uniformities in the spacer layers of these filters. The latter method, permitting observation and measurement of thin film non-uniformities appreciably less than one unit cell provides a very sensitive means of studying thin film structures and the mechanism of formation.
ALAN E. GEE and HARRY D. POLSTER, "A Method for Measuring Extremely Small Non-Uniformities in the Optical Thickness of Evaporated Films," J. Opt. Soc. Am. 39, 1044-1045 (1949)