[7.3.3.18] The ratio of the flux actually reflected by a sample surface to that which would be reflected into the same reflected-beam geometry by an ideal (glossless), perfectly diffuse (Lambertian), completely reflecting standard surface irradiated in exactly the same way as the sample.
The analogies to reflectance may be noted in the fact that: nine canonical forms are possible paralleling the hemispherical, conical and directional reflectance terms; spectral may be applied as a modifier; and it may be a luminous or a radiant reflectance factor. It should also be noted that reflectance cannot exceed unity, but reflectance factor may have any value from zero to values approaching infinity.
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