Polarization, Directional Distribution, and Off-Specular Peak Phenomena in Light Reflected from Roughened Surfaces

The directional distribution of light reflected from surfaces of varying roughness is explored experimentally. Measurements were made of the plane-polarized components of the reflected light as well as of the mixed radiation (containing all components of polarization). The test materials included a magnesium oxide ceramic and aluminum coated ground glass, thereby permitting a study of reflection in the presence and in the absence of subsurface scattering. The angle of incidence was varied from 10° to 87°, while the angle of reflection extended from 0° to 89°. The roughness of the test surfaces ranged from optically smooth to 5.8 μ, while the measurements were performed monochromatically at a wavelength of 0.5 μ. The measured directional distributions affirm that the diffuse limit (Lambert’s cosine law) does not hold when roughened surfaces are illuminated at moderate to large angles of incidence. Rather, it is found that a maximum in the distribution of the reflected radiance occurs at reflection angles larger than the specular-ray direction. The contributions of the s and p components of polarization to this off-specular peak are made clear. The degree of polarization imparted by reflection at surfaces of varying roughness is also investigated. It is shown that the absence or presence of subsurface scattering is an important factor. The various findings of the experiments are subjected to interpretation by a model of the reflection process which pictures the surface as being composed of elementary mirror-like facets.