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Mirror-scatter degradation by particulate contamination

Measurements of the BRDF values of naturally collected mirror surfaces contaminated with particles are compared with predictions from Mie scattering theory. The principles of the scattering theory are set forth, and the test apparatus is described which includes an IR scatterometer and a visible scatterometer. The three metal mirrors are contaminated by normal laboratory air for a period of 95 hours, and the particles are characterized by count and size distribution for the Mie scattering calculation. The results of the Mie data reduction are compared to the experimental measurements, and the measured BRDF is found to be in fairly good agreement with the calculations. The scattering from the particles is primarily forward scattering which degrades the BRDF significantly, but because the particle optical properties do not greatly affect the forward scattering calculations, the measurements are close to predicted values.