The spectral BRDF at wavelength is defined as the surface radiance of a sample in a particular viewing direction, due to the scattering of radiation incident from a particular direction of irradiation. Display Formula
(1)with (, ) being the spherical coordinates of the radiation incident on the surface, (, ) being the spherical coordinates of the light scattered from the surface, being the differential spectral radiance, and being the differential spectral irradiance. All spherical coordinates are referenced to the surface normal. To determine the BRDF from goniospectrophotometer measurement data, the American Society for Testing and Materials has proposed the following practical formula:16Display Formula
(2)with and being the scattered radiant flux toward the detector and the incident radiant flux on the sample, respectively, and being the solid angle of the detector aperture.