The bidirectional reflectance distribution function (BRDF;  ) is a function of four real variables that defines how light is reflected at an opaque surface. It is employed both in the optics of real-world light, in computer graphics algorithms, and in computer vision algorithms. The function takes an incoming light direction, , and outgoing direction, (taken in a coordinate system where the surface normal  lies along the z-axis), and returns the ratio of reflected radiance exiting along to the irradiance incident on the surface from direction . Each direction is itself parameterized by azimuth angle  and zenith angle , therefore the BRDF as a whole is a function of 4 variables. The BRDF has units sr^-1, with steradians (sr) being a unit of solid angle.