Bidirectional transmittance distribution function measurements on ZnSe

The bidirectional transmittance distribution function (BTDF) is used to evaluate the scattering properties of chemically vapor-deposited (CVD) zinc selenide (ZnSe). Conceptually, the BTDF is a logical extension of the bidirectional reflectance distribution function (BRDF) relation. A working equation for BTDF based on the practical limitations of the instrumentation has been developed. The practical limitations inherent in making BTDF measurements are the finite detector size, the two scattering surfaces of the sample, and the linear radiometric responses of the detector. The instrumentation used to measure BTDF and the data collection procedure are described. Plots of BTDF vs angle from the sample normal are presented for CVD ZnSe at room temperature. The plots are for three thicknesses of ZnSe (6, 19, and 25 mm) at three laser wavelengths (0.6328, 3.39 and 10.6 microm).