Infrared birefringence spectra for cadmium sulfide and cadmium selenide

Measurements of the birefringence spectra for cadmium sulfide and cadmium selenide from 2.5 to 16.5µm obtained with a rotating sample spectropolarimeter are presented. Because of the similarity in the birefringence spectra for cadmium sulfide and cadmium selenide, a highly achromatic IR retarder can be constructed from a combination of these materials. The ordinary and extraordinary refractive indices for cadmium sulfide are estimated in the region from 10.6 to 15 µm and for cadmium selenide from 10.6 to 16.5 µm by combining these birefringence data with an extrapolation of previous dispersion relations.