The Bidirectional Reflectance Distribution Function (BRDF) is a fundamental concept in optics and remote sensing that describes how light is reflected at an opaque surface. It characterizes the way in which light is scattered from a surface when illuminated by a light source. Mathematically, the BRDF is defined as the ratio of reflected radiance in a specific direction to the incident irradiance from a specific direction.
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A function relating reflected radiance in one direction to incident irradiance from another direction at a surface location. A local nonemissive, nonfluorescent optical reflection model integrates over incident solid angle to obtain outgoing radiance. Transmission, wavelength conversion and nonlocal transport need additional models.