Bidirectional reflectance distribution function of gold-plated sandpaper.

Gold-plated sandpaper was investigated for use as a Lambertian standard reference reflector for the IR spectrum. Various grit sizes from 3 to 400 microm and material types (i.e., silicon carbide and aluminum oxide) were studied. The different gold-plated sandpaper grit sizes were measured in the same way using three laser wavelengths (0.6328, 3.39, and 10.6 microm) at five angles of incidence of the source (0, 10, 20, 30, and 60 degrees ). All the scattering measurements were performed in the plane of incidence. The best choices of sandpaper grit sizes were 9-microm A1(2)O(3) for 0.6328- and 3.39-microm radiation and 600 grit by Armak Co. for 10.6-microm radiation. These choices were compared with other commonly used reflectors such as magnesium oxide, halon, sintered bronze, and flowers of sulfur. An attempt was made to correlate surface roughness (size of grit) to the degree of approximation to a good Lambertian reflector, but it was found that grit size is not as important as the filling factor, or density of particles, over a given area. It was found that fairly good approximations to Lambertian behavior result when the angle of incidence is small but not when the angle of incidence is as large as 60 degrees .