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An experiment has been performed to determine the directional reflection characteristics of electric nonconductors. The test material was fused polycrystalline magnesium-oxide ceramic. Measurements were made for surface roughnesses ranging from 0.16 to 5.8 microns over a wavelength range extending from 0.5 to 12 microns. The test specimens were illuminated by a narrow pencil of rays inclined at either 10 or 45 deg to the surface normal. The reflected radiation was collected at various directions in the hemispherical space over the surface. The experimental results indicate that the reflection characteristics of a given surface approach a diffuse distribution at short wavelengths and a specular distribution at long wavelengths. Increased surface roughness accelerates the former and retards the latter. Moreover, a given surface becomes effectively smoother when illuminated at larger angles of incidence. For the case of incidence at 45 deg, it was found that maxima in the biangular reflectance distribution occurred in angular directions other than the specular; within the knowledge of the authors, such off-specular peaks have not been discussed previously. The measured specular reflectance could be correlated in a restricted parameter range using a theory originally derived for electric conductors.
Torrance et al. (Sat,) studied this question.