Bidirectional reflectance measurement of black absorber layers for use in optical instrument design

The bidirectional reflectance distribution function (BRDF) plays a fundamental role in the optical characterization of a surface. The BRDF is a measure of the amount of light incident from one direction which is scattered by a surface in another direction. Integrating the BRDF over specified incidence and reflected solid angles defines the bihemispherical reflectance, which can be related to the absorptance and emissivity of a surface. We have designed and fabricated a high-accuracy bidirectional reflectometer and used it to measure the bidirectional reflectance of a smooth silicon substrate coated with the black absorber Aeroglaze Z302®. Two different coating thicknesses displaying different degrees of surface roughness were studied. A BRDF model consisting of diffuse, glossy, and specular components was then fitted to the experimental results. Finally, the Monte Carlo ray-trace (MCRT) method was used to split an incident beam into a group of reflected rays whose power and direction were determined by the BRDF model. The combined model is capable of simulating the performance of any optical instrument which has Z302 coated on its active surfaces. As a demonstration, the combined model is used to simulate the performance of the bidirectional reflectometer experiment used to obtain the original data.