A new method has been devised for measuring refractive indexes of crystals in the visible and infrared region. In this method thermocouples with CaF2 or KBr windows have been used to detect the chopped (10 c/s) refracted beam. Single synthetic crystals of CdS and ZnS grown by the vapor phase method have been examined from their absorption cutoffs to 1.4µ. In comparing the results obtained, in the visible region, to the visual method of determining refractive indexes, the thermocouple detector method has duplicated these to within an index of refraction of ±0.001. For data extended to the infrared region it is found that the calculated results compare very closely with the experimental data. Index of refraction measurements may now be extended considerably into the infrared region being limited only by the sensitivity of the detector and the intensity of the transmitted monochromatic beam.
S. J. CZYZAK, W. M. BAKER, R. C. CRANE, and J. B. HOWE, "Refractive Indexes of Single Synthetic Zinc Sulfide and Cadmium Sulfide Crystals," J. Opt. Soc. Am. 47, 240-242 (1957)