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The bidirectional reflectance distribution function {\displaystyle f_{\text{r}}} ] is a function of four real variables that defines how light is reflected at 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 i {\displaystyle \omega _{\text{i}}} , and outgoing direction, ω r {\displaystyle \omega _{\text{r}}} , and returns the ratio of reflected radiance exiting along ω r {\displaystyle \omega _{\text{r}}} to the irradiance incident on the surface from direction ω i {\displaystyle \omega _{\text{i}}}. Each direction ω {\displaystyle \omega } is itself parameterized by azimuth angle ϕ {\displaystyle \phi } and zenith angle θ {\displaystyle \theta } , therefore the BRDF as a whole is a function of 4 variables. The BRDF has units sr, with steradians being a unit of solid angle.