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, Pi, is computed. It was particularly shown that Pi 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 Pi 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.