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-prependicular-to-the-ray, is irradiance, or power per unit surface area, and is the angle between and the surface normal, The index indicated incident light, whereas the index indicates reflected light.
The reason the function is defined as a quotient of two differentials and not directly as a quotient between the undifferentiated quantities, is because other irradiating light than which are of no interest for might illuminate the surface which would unintentionally affect whereas is only affected by
Read more about this topic: Bidirectional Reflectance Distribution Function
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