双向反射分布函数（bidirectional reflectance distribution function、BRDF）是一个定义光线在不透明表反射的四次元函数，基本式为：，在这里是指光线的反方向，另外是指光线反射的方向，除此之外，还有一个代表法线，这个值的意义是在方向的反射光线的 辐射率 和同一点上 从方向射入的光线的辐射率的比值。每一个 　方向可以被参数化　为 方位角 和 天顶角 , 因此BRDF是一个四维函数。 BSDF的单位是 sr, 其中 (sr) 是 球面度的单位.

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, with steradians (sr) being a unit of solid angle.