The bidirectional reflectance distribution function (BRDF) is a 4-dimensional function that defines how light is reflected at an opaque surface. The function takes an incoming light direction, and outgoing direction, both defined with respect to the surface normal, and returns the ratio of reflected radiance exiting along to the irradiance incident on the surface from direction. Physically based BRDFs have additional restrictions, including Helmholtz reciprocity, and energy conservation. The BRDF has units sr-1, with steradians (sr) being a unit of solid angle.
The BRDF is a fundamental radiometric concept, and accordingly is used in computer graphics for photorealistic rendering of synthetic scenes, as well as in computer vision for many inverse problems such as object recognition.