Changes in directional (biconical) spectral reflectance with varied illumination and observing angles were monitored for three soil samples under air dry and saturated conditions in the laboratory. The illumination angle was set at -10°, -40°, and -70° (left side of sample in the principal plane), and the observing angle ranged from -60° to +60° (both side of sample in the principal plane) in 5° increments. The samples were chosen to represent various soil properties. The nadir spectral reflectance was relatively stable for all illumination angles, however, the directional reflectance was more variable. When soil samples were dry, the directional reflectance changed obviously with phase angle with a stronger backward reflectance, while the forward reflectance was generally lower. For saturated soil samples, the directional spectral reflectance of dry soil feature was reduced, and the strong backward scattering was weakened. Indeed, the directional spectral reflectance became less sensitive to illumination angle and observation angle changes, especially for dark soils. The added water not only darkened the soil reflectance, but also reduced the directional variation difference of soil. A simple sketch was introduced to suggest an explanation for the difference between directional reflectance between air dry and saturated samples. When illumination was from one direction, the convex soil surface forms a distinct shadow on the opposite side, leading to a low forward reflectance. However, with a water layer coating on the soil surface, the chance of light propagating to the opposite side of illumination was increased, increasing reflectance in the forward direction.
Given the importance of penetration of light in the soil for seed germination, soil warming, and the photolytic
degradation of pesticides, directional transmission of thin sand samples are studied in this paper under both dry and
saturated conditions. The detector views upward through a glass-bottom sample holder, filled to 3 or 4 mm with a
coarse, translucent, quartz sand sample. Transmission through the samples was measured as the illumination zenith angle
moved from 0 to 70° in 5° intervals. In the most cases, transmission decreased monotonically, but slowly with increasing
illumination angle at all wavelengths. A peak in transmission only appeared at 0° illumination for the low bulk density,
dry sample at 3 mm depth. The 0° peak disappeared when the sample was wetted, when the bulk density increased, or
when the depth of the sample increased, which indicates that the radiation transmitting through a sand layer can be
diffused thoroughly with a millimeters-thin sand layer. For the saturated samples, water influences light transmission in
contrasting ways in shorter and longer wavelength. Transmission increased in the VNIR when saturated relative to dry,
while transmission decreased sharply after 1300 nm, with spectral absorption features characteristic of water absorption.
In VNIR region, water absorption is low and the low relative index of refraction enhanced transmission through sand
sample. In contrast, water absorption became dominant at longer wavelengths region leading to the strongly reduced
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.
The spectral reflectance of a sample of quartz sand was monitored as the sample progressed from air-dry to fully saturated, and then back to air-dry. Wetting was accomplished by spraying small amounts of water on the surface of the sample, and collecting spectra whenever change occurred. Drying was passive, driven by evaporation from the sand surface, with spectra collected every 5 minutes until the sample was air dry. Water content was determined by monitoring the weight of the sample through both wetting and drying. There was a pronounced difference in the pattern of change in reflectance during wetting and drying, with the differences being apparent both in spectral details (i.e., the depth of absorption bands) and in the magnitude of the reflectance for a particular water content. The differences are attributable to the disposition of water in the sample. During wetting, water initially occurred only on the surface, primarily as water adsorbed onto sand particles. With increased wetting the water infiltrated deeper into the sample, gradually covering all particles and filling the pore spaces. During drying, water and air were distributed throughout the sample for most of the drying period. The differences in water distribution are assumed to be the cause of the differences in reflectance and to the differences in the depths of four strong water absorption bands.