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
![](http://info.babylon.com/onlinebox.cgi?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=2555)
is itself
parameterized by azimuth angle ![](http://info.babylon.com/onlinebox.cgi?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=241)
and
zenith angle ![](http://info.babylon.com/onlinebox.cgi?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=2691)
, 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.