Light transmission through a sand layer: modeling changes due to illumination direction

Direct observations of transmission through a thin layer of quartz sand indicate that the transmitted radiation – from the visible through the shortwave infrared – is essentially diffuse after little more than one attenuation length. Except for an anomalously high transmission in a dry, 3-mm deep quartz sample when the detector was directly aligned with the light source, no complex forward scattering features were apparent. A simple model designed to describe the observations is explored for insight into the angular dependence and the spectral distribution of the transmitted radiation. The model suggests that the observed variation of the transmittance with illumination angle can be attributed to surface effects (including absorption), that much of the transmitted light has passed through the sand particles, and that a wavelengthdependence of the attenuation in the visible is consistent with scattering within the sand particles.