A modeling study of bidirectional reflectance from soil surfaces

An examination of soil particles from very fine to medium sand surfaces has indicated that they are generally on the order of 50 to 500 microns. Thus, at an incident wavelength around 0.6 micron, the incident light should “see” microscopic roughness features on the particle rather than its macroscopic features. It is anticipated that the macroscopic features of a soil particle are responsible for the shadowing and tilting of the much smaller roughness scales that ride on it. Note that these smaller scales of roughness may still be larger than the incident wavelength. In view of this physical structure, a soil particle is modeled as a layer with two arbitrarily oriented surface boundaries to simulate the overall roughness effect. A scattering phase function is then developed for this layer by considering wave scattering from and propagation through it. A probability distribution function for the orientation of the layer boundaries is assumed for the calculation of this phase function. After the phase function is developed, it is incorporated into a matrix doubling algorithm to calculate the polarized bidirectional reflectance distribution function (BRDF) for a half space of soil particles. This quantity is then converted to the polarized bidirectional reflectance factor (BRF) so that the calculated BRF can be compared with polarized BRF measurements. Preliminary results indicate that backscattering is dominated by the small scales of roughness riding on the particle, and that large scales of roughness are responsible for tilting