The bidirectional reflectance distribution function (BRDF; ) describes the ratio of reflected
radiance exiting from a surface in a particular direction (defined by the vector ) to the
irradiance incident on the surface from direction over a particular waveband. It was first defined by Edward in mid sixties. BRDF has units sr-1, with
steradians (sr) being a unit of
solid angle. The spectral BRDF (units of sr-1nm-1) describes the BRDF as a function of
wavelength, .
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