“…The bi-directional reflectance, or rather the bi-directional reflectance distribution function BRDF (Nicodemus, 1970), of vegetation canopies is the fundamental physical object property which eventually determines the remotely sensed radiance signal, along with other factors such as the local solar and sky irradiances, and the atmospheric conditions. Modeling the top-of-canopy BRDF has evolved in several directions (Goel, 1988;Pinty et al, 2004), making use of various numerical solution techniques like ray tracing (Chelle, 1997), Monte Carlo (North, 1996;Thompson & Goel, 1998) and radiosity (Gerstl & Borel, 1992), or analytical solutions like four-stream (Verhoef, 1985), N + 2 stream (Verhoef, 2002) and discrete ordinates (Knyazikhin et al, 1992), for 1-D as well as 3-D canopy architectures (Gastellu-Etchegory et al, 1996;Kuusk & Nilson, 2000). In hybrid models analytical and numerical techniques are combined (Gobron et al, 1997), or geometric optics (GO) approaches are combined with radiative transfer (RT) approaches (so-called GORT models, Li et al, 1995).…”