The Bidirectional Reflectance Distribution Function (BRDF) is defined as the quotient between the spectral radiance reflected by a sample and the spectral irradiance from the source that illuminates it. It depends on both the incidence and viewing angles. From this point of view it represents an absolute definition of reflectance whose value, as is known, depends on the geometry of the illumination and observations directions.
Illustrate why we refer to the BRDF as an absolute definition of spectral reflectance
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