Eight definitions of reflectance are compared with experimental results presented in an earlier paper. Two types of reflection, diffuse and specular, usually take place simultaneously at all incidence and viewing angles. Only one of the eight definitions that we study, the equation of Spencer and Gray, agrees entirely with experiment. Half of the definitions apply only for purely diffuse reflection (i.e., reflection devoid of any specular component). Apparently, many illuminating engineers treat all reflecting surfaces as though they were purely diffuse. We also show that some of the definitions that do not agree with experiment are nevertheless valid for the extreme cases of purely diffuse and purely specular reflection. Definitions that apply to these special cases do not necessarily apply in general.
Domina Eberle Spencer and Eugene A. Gaston, "Current definitions of reflectance," J. Opt. Soc. Am. 65, 1129-1132 (1975)