Assessing Effects of Azimuthally Oriented Roughness on Directional Reflectance of Sand

We measured and compared hyperspectral bidirectional reflectance factors of a sand sample of varying roughness levels using the Goniometer of the Rochester Institute of Technology-Two. We developed a geotechnical routine to generate sand samples of approximately constant density and grain size distribution, but varying roughness levels. In addition, we developed sand samples of two different classes of surface roughness: wave-like and normally distributed. The samples exhibiting normally distributed roughness met several criterion outlined by Hapke in the roughness correction to his photometric model for a smooth sediment surface. We developed a method to empirically forward model the photometric model for a rough surface using roughness metrics derived from digital elevation models of the surface. Our results from empirically forward modeling the correction factor indicate that Hapke's shadowing function does not adequately describe the effects of macroscopic roughness at a subcentimeter scale. In addition, we also performed experiments in which we oriented the surface waves of the wave-like roughness profiles in different orientations to the principal plane of illumination. These results indicate that future photometric models of wave-like roughness should include a description of how multiple scattering increases reflectance within cavities, reducing the effects of shadowing within the cavities. Our results also suggest that since Hapke's model correction for macroscopic roughness assumes that the underlying distribution of surface slopes does not depend on azimuth, it cannot adequately characterize surface roughness when it is both structured and has a preferred orientation.