A study was made of evaporated ZnS films on glass, including method of preparation, optical properties, and aging effects. The influence of rate of evaporation and pressure during evaporation was noted. Thicknesses and indexes of refraction of the films were determined from measurements of reflected intensity as a function of wavelength. ZnS films are quite soft just after preparation but become harder if aged in a dry atmosphere.
JOSEPH L. ROOD, "Evaporated Zinc Sulfide Films," J. Opt. Soc. Am. 41, 201-202 (1951)