BIDIRECTIONAL REFLECTANCE DISTRIBUTION FUNCTION (BRDF)

A description of the angular distribution of reflected electromagnetic radiation from a surface, with respect to the distribution and wavelength of the incident radiation. In remote sensing, knowledge of the BRDF is useful for several reasons. First, it provides a more accurate measure of albedo than estimates derived by assuming that the reflectance of the surface is Lambertian. Albedo is an important input parameter to generalcirculation models (GCMs). Second, uniform changes in brightness across remotely sensed images are often due to the directional reflectance properties of the surface and consequently the more accurately the brdf is known the better these effects can be corrected. Finally, physical models of the BRDF of vegetation are typically parameterised by the biophysical properties of the vegetation canopy, such as leaf area index (LAI), stand height and leaf-angle distribution (LAD). Hence, with sufficient angular samples of the canopy reflectance, it is possible mathematically to invert such models and retrieve estimates of these biophysical parameters. For remote sensing, this offers a quantitative method of information retrieval not available from more commonly used spectral or temporal methods.