Spectral transmittance of a translucent sand sample with directional illumination

The nadir transmittance of sand samples was monitored as the illumination direction moved from 0° to 70°. Measurements were made for high- and low-density samples, at two depths and under both air-dry and saturated conditions. Transmittance decreased monotonically, but slowly, with increasing illumination angle at all wavelengths. A peak in transmittance appeared only at the 0° (zenith) illumination angle, but only for the low-bulk-density, dry sample at the shallower, 3-mm depth. This indicates that directional radiation transmitted through a sand layer becomes diffuse with few millimeters-thin sand layer. For the saturated samples, the influence of water on light transmittance was opposite in the visible and near-infrared (VNIR) (350-1300 nm) and shortwave infrared (1330-2500 nm) wavelength regions. In the VNIR region, transmittance increased in the saturated sample relative to dry sample, while transmittance decreased sharply after 1330 nm, with obvious spectral absorption features characteristic of water absorption. In the VNIR region, water absorption is low, and the low relative index of refraction enhanced transmittance through the sand sample. In contrast, water absorption became dominant at longer wavelengths, leading to the strongly reduced transmittance. Analysis of transmittance by quartz sand samples with the same bulk density but a different depth suggests that most of the directional variation due to the change in illumination angle can be attributed to surface reflective loss. The implication is that the directional reflection may be treated as a surface phenomenon, with the volume reflectance contributing a diffuse component.