COMMENTS ON RECIPROCITY IN THE DIRECTIONAL REFLECTANCE MODELING

Reciprocity, not in its original meaning of Helmholtz's principle, is widely generalized in some textbooks 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 univer sally applicable principle. Whether it can be applied to bidirectional reflectance distribution function (BRDF) of structured earth surface is still a subject to be investigated. Siegel and Howell's thermodynam ic proof of BRDF reciprocity contains a circular argument and is thus incorrect. A corrected version of such a proof, however, indicates tha t hemispherical directional reflectance is reciprocal to directional h emispherical reflectance. The systematic and accurate experimental res ults of Clarke and Parry agree with this conclusion. Their laboratory measurements, as well as presently available field BRDF measurements, however, do not support BRDF reciprocity in general. More theoretic an d experimental investigations should be conducted to conclude whether reciprocity can be used as a principle in practice of BRDF modeling.