The bidirectional reflectance distribution function (BRDF), symbol f r ( ω ω i , ω ω r ) , is a function of four real variables that defines how light from a source is reflected off an opaque surface. It is employed in the optics of real-world light, in computer graphics algorithms, and in computer vision algorithms. The function takes an incoming light direction, ω ω i , and outgoing direction, ω ω r (taken in a coordinate system where the surface normal n lies along the z -axis), and returns the ratio of reflected radiance exiting along ω ω r to the irradiance incident on the surface from direction ω ω i . Each direction ω ω is itself parameterized by azimuth angle ϕ ϕ and zenith angle θ θ , therefore the BRDF as a whole is a function of 4 variables. The BRDF has units sr, with steradians (sr) being a unit of solid angle.