3-D Vector Radiative Transfer for Vegetation Cover Polarized BRDF Modeling

In this study, we try to propose a 3-D Vector radiative transfer (VRT) model based on Monte Carlo (MC) forward ray tracing simulation to analyze vegetation canopy reflectance. Two kinds of scattering are taken into account: lambertian reflectance and transmittance as well as specular reflection. A new method to estimate the condition on leaf orientation to produce reflection is proposed, and its probability to occur, P-l, is computed. It was particularly shown that Pl is low, but when reflection happen, the corresponding radiance vector, I-o, is very high. Such a phenomenon increases dramatically the MC variance and produces an irregular reflectance distribution function with a number of peaks corresponding to the specular effect. To be reduced, we propose a new MC approach that simulates reflection for each sunny leaf assuming that its orientation is random. It is shown in this case that the average canopy reflectance is proportional to P-l x I-o. Our experimental results confirm that in forward direction, canopy polarizes horizontally light. In addition, they show that in inclined forward direction, diagonal polarization can be observed.