Comparison of Vector Scattering Theory and BRDF Measurements on a Thick Silica Overcoated Aluminized Substrate

Abstract

An aluminized substrate overcoated with a 1.83 μm thick silica film was used to test the correlation properties of the two rough film surfaces using BRDF measurements in the visible spectral region. Thin overcoating films contour the low spatial frequency roughness of the substrate but can add small amounts of high spatial frequency roughness because of the lumpiness of the coating. Thick overcoating films can destroy the low spatial frequency correlation, making the two rough surfaces completely uncorrelated. The outer silica film surface and a comparable aluminized substrate (inner surface) were measured using a mechanical stylus profiler to obtain rms roughness and autocovariance functions for both surfaces. The thick film system was then modeled by assuming a Gaussian autocovariance function for the highest frequency roughness (that could not be directly measured by the mechanical profiler) and an exponential autocovariance function for the lower spatial frequency roughness for both surfaces. This film model was used in theoretical calculations of BRDF for the 1.83 μm physical thickness silica film for s and p polarization at various angles of incidence in the visible. The calculations showed measurable differences in BRDF for correlated and uncorrelated roughness on the two surfaces. BRDF measurements suggested that the interfacial roughnesses were uncorrelated for the thick silica film.