Laboratory and Field Reflectivity Measurements of Beach and Desert Sands within the Hapke BRDF Model Using Portable Spectrometers.

Bidirectional Reflectance Distribution Function (BRDF) measurements using a field spectrometer have been made in the laboratory and analyzed within variants of Hapke's particulate BRDF model for six ocean beach sites and five desert sands sites for the Nonconventional Exploitation Factors Data System (NEFDS) program. In-plane BRDF field measurements, made at night using a portable light source, have been made for two of these beach sites and three of the desert sites. Analysis of the spectrometer field data use a variant of Hapke's Shadow Hiding Opposition Effect (SHOE) model as given by Shepard and Helfenstein. In this variant a series expansion of Legendre Polynomials is substituted for the angularly dependent backscatter function. We show that if out-of-plane scattering and the backscatter are small, this model provides an adequate description of the BRDF, except within a small region near backscatter. This allows inclusion of field BRDF measurements within the NEFDS database. Measurement and analysis techniques are discussed and sample results of both the lab and field data are shown. The determination of soil porosity from measured BRDF within Hapke's model is discussed.

The authors wish to acknowledge Tim Boyce, Carol May, and Jim Crowley for their support.