Comments on reciprocity in the directional reflectance modeling

Reciprocity, not in its original meaning of Helmholtz’s principle, is,widely generalized in some text-books as "when the wave source and the detector exchange positions, the response of the detector remains the same"However, reciprocity (misunderstood as above) is not a universally applicable principle. Whether it can be applied tobidirectional reflectance distribution function (BRDF) of structured earth surface is still a subject to be investigatedSiegel and Howell’s thermodynamic proof of BRDF reciprocity contains a circular argument and is thus incorrect Acorrected version of such a proof, however, indicates that hemispherical directional reflectance is reciprocal to direc-tional hemispherical reflectance. The systematic and accurate experimental results of Clarke and Parry agree with thisconclusion. Their laboratory measurements, as well as presently available field BRDF measurements, however, do notsupport BRDF reciprocity in general. More theoretic and experimental investigations should be conduct