There does not appear to be any “standard” in use in the literature, indeed the situation would seem to be quite the opposite, many authors making up their own ad hoc “reflectance” or “scattering” fun...There does not appear to be any “standard” in use in the literature, indeed the situation would seem to be quite the opposite, many authors making up their own ad hoc “reflectance” or “scattering” functions to suit the problem at hand. (This can make for very frustrating reading, especially when words such as “flux”, ”intensity” and “brightness” are used loosely, as, sadly, is often the case).