Deriving Lambertian BRDF from first principles

[JeGX]

This is a short exercise on integration. We will derive the Lambertian BRDF from first principles to understand the origin of PI in it.

We start by stating that the Lambertian BRDF is R/PI, where R (albedo) is the measure of diffuse reflection. It is the proportion of incident light that is reflected from a surface and is used to denote how bright a surface is (not to be confused with how reflective a surface is). Albedo maps are used ubiquitously in lighting, and look like this.

Link: https://sakibsaikia.github.io/graphics/2019/09/10/Deriving-Lambertian-BRDF-From-First-Principles.html