Soil directional (biconical) reflectance in the principal plane with varied illumination angle under dry and saturated conditions

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.

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