Bidirectional reflectance distribution function

The Bidirectional Reflectance Distribution Function (BRDF) is a fundamental concept in optics and remote sensing that describes how light is reflected at an opaque surface. It characterizes the way in which light is scattered from a surface when illuminated by a light source. Mathematically, the BRDF is defined as the ratio of reflected radiance in a specific direction to the incident irradiance from a specific direction.