Approximate solutions to the Radiative transfer equation for granular media have been previously developed1. To apply these models to coastal sediments, modifications are needed to account for observed phenomenology. This study uses a new hyperspectral goniometer system, the Goniometer of the Rochester Institute of Technology (GRIT), designed for both field and laboratory settings, to compare observed bidirectional reflectance distribution function (BRDF) measurements with outcomes predicted by the approximate radiative transfer solutions. In previous laboratory studies,2 using a more limited hyperspectral goniometer observing in the principle plane, we had seen that the degree of optical contrast between coastal sand constituents was indicative of whether these models accurately predict the observed BRDF dependence on sediment density. Our earlier measurements using another field hyperspectral goniometer also demonstrated results consistent with the laboratory measurements as well as with CASI- 1500 airborne hyperspectral measurements3,4. In our earlier work,2 the presence of highly contrasting constituents (translucent quartz and more opaque fractions composed of minerals such as magnetite) led to greater reflectance as density decreased, exactly the opposite of what was anticipated from radiative transfer models for a more uniform sand. The present study shows that the illumination zenith angle also plays a significant role in whether or not BRDF dependency exhibits behavior predicted by current radiative transfer theory, and this distinction is directly related to the degree of multiple scattering, which depends on the illumination zenith angle. We also investigate a novel sampling paradigm that constrains the measurements to constant phase angle and reveals when the multiple scattering component of models departs from the assumptions of current theory. For the multiple scattering term, we also propose and analyze a simple modification which removes the isotropic assumption and provides a better match to BRDF observations under the constrained sampling paradigm.