Measuring and quantifying scatter from non-isotropic sources

The usual definition for BRDF assumes that the illuminated surface is isotropic. This is why when the primary source of scatter is a surface pit or particle the differential scattering cross-section is used to quantify scatter. In these cases the DSC is independent of changes in illumination spot size and thus is a more meaningful characterization than the measured BRDF. The same thing is true for other situations. These include scatter from isolated scratches, non-isotropic roughness (such as a rolled surface) and scatter from the edge or corner of a surface. In these situations the measurements may be done differently and the quantified scatter often has different units – such as area/sr or 1/deg instead of the common 1/sr associated with BRDF. If the data is being taken for use in one of the stray radiation codes this can cause problems because those codes require BRDF as an input. This paper reviews these situations for both measurement and analysis issues.