is a function of 4 variables. The BRDF has units sr^-1, with steradians (sr) being a unit of solid angle.

Definition

The BRDF was first defined by Fred Nicodemus around 1965. The definition is:

where is radiance, or power per unit solid-angle-in-the-direction-of-a-ray per unit projected-area-perpendicular-to-the-ray,