Bi-directional Reflectance Distribution Functions (brdf) of Benthic Surfaces in the Littoral Zone

The bi-directional reflectance distribution function (BRDF) describes the angular distribution of reflectance for any angle of light incident on a surface. It is a crucial parameter for modeling the light field near the benthic surface, and is important for understanding the remote sensing signal from shallow waters. We have been making measurements of this parameter for varied benthic surfaces in the Bahamas during the ONR sponsored Coastal Benthic Optical Properties experiment (CoBOP) for the last several years. While this parameter is generally assumed to be Lambertian, meaning that the radiance reflected from the surface does not depend on viewing angle, we have found that in general this is not the case. Even simple surfaces, such as sand, exhibit nonLambertian behavior in two directions. The first is in the specular direction, where there can be a small enhancement in the reflectance for many natural surfaces. The largest non-Lambertian feature seems to be in the "hotspot" or backward direction. In almost all the samples we have seen, for an incident illumination polar angle(θi )>25°, there is an enhancement of the reflectance in the hotspot.