The initially high reflection from zinc sulphide is still further enhanced by constructive interference in thin films. By distilling sphalerite in vacuum, thin films showing first order interference and reflecting white light brilliantly are produced. The application of such films to the metallographic microscope, Michelson interferometer and other instrument is discussed.
A. H. PFUND, "Highly Reflecting Films of Zinc Sulphide," J. Opt. Soc. Am. 24, 99-102 (1934)