Phase angle dependence of sand density observable in hyperspectral reflectance

This paper describes measurements of the phase angle dependence of sand density observable in hyperspectral reflectance. Bi-directional reflectance distribution measurements in the principal scattering plane were recorded for sand samples prepared near minimum and maximum relative densities. Radiative transfer theory for granular media of a single constituent would predict increased reflectance with increased density. However, sands are usually composite materials, and we observed that reflectance can actually decrease with increasing density when the dominant constituents of the sand are semi-translucent, and a darker fraction with typically smaller grains is also present. We postulate that under these circumstances, as density increases, multiple scattering modes are being suppressed as the darker absorbing fraction occupies more optimally the available pore space.