Effect of surface roughness on the total hemispherical and specular reflectance of metallic surfaces

Measurements have been made of the hemispherical and specular reflectance of metallic surfaces of controlled roughness. The surfaces, which were ground nickel rectangles, were irradiated at various angles of incidence by a beam of black-body radiation, the temperature of which was also varied. The instrumentation which was devised to perform the experiments is described. The measurements show that beyond a certain surface roughness, the hemispherical reflectance is virtually independent of further increases in roughness. On the other hand, the specular reflectance decreases steadily with increasing roughness. Additionally, the hemispherical reflectance is found to be quite insensitive to the angle of incidence, while the specular reflectance increases with angle of incidence for the rougher surfaces.