RECIPROCITY OF THE BIDIRECTIONAL REFLECTANCE DISTRIBUTION FUNCTION (BRDF) IN MEASUREMENTS AND MODELS OF STRUCTURED SURFACES

The bidirectional reflectance distribution function (BRDF) is one of t he most important surface properties for terrestrial remote sensing, b ut its definition for structured surfaces is not fully understood, The BRDF of flat surfaces has a straightforward definition and is usually considered to be reciprocal, which means the value is the same when t he source and detector angles are switched. Structured surfaces, such as forest canopies and grasslands, require an extension of the definit ion of BRDF and some additional measurement conditions, In this paper, a definition for the BRDF of structured surfaces is proposed, and it is shown that with this definition, the BRDF is reciprocal, In additio n, some of the related geometrical measurement requirements are discus sed. It is concluded that reciprocity should apply for both measuremen ts and models of structured surfaces and that field measurements viola te reciprocity not because the BRDF itself is nonreciprocal, but becau se of uncorrected geometric and radiometric factors.