Computer Graphic Rendering of Material Surfaces
One of the most general means to characterize the reflection properties of a surface is by use of the bi-directional reflection distribution function (BRDF), a function which defines the spectral and spatial reflection characteristic of a surface. The BRDF of a surface is the ratio of reflected radiance to incident irradiance at a particular wavelength:
where the subscripts i and r denote incident and reflected respectively, is the direction of light propagation, is the wavelength of light, L is radiance, and E is irradiance. The geometry used by the BRDF is shown below in Figure 1.
Figure 1: Light reflection geometry