Abstract
Two computationally fast Monte Carlo algorithms (models) are described for calculating the bidirectional reflectance factors (BRFs) from architecturally realistic canopies and from statistically characterized canopies. Both methods use a novel concept, called photon spreading. In this technique, the photon is treated as a particle when it travels through the canopy, but with each collision it is also treated like a wave, contributing to the canopy reflectance. This concept allows one to obtain canopy reflectance of the same accuracy with 30,000 photons that is obtained with 10 to 50 million photons in the traditional Monte Carlo method, leading to computational speed. The PS model takes about 10 min on a Pentium based PC and the SPS model a few minutes, for calculating BRF for 81 viewing directions. The two algorithms are compared against the SAIL model, another architecturally realistic model (for pure Lambertian scattering), Diana, and an analytical model for horizontal layers. The algorithms show good agreement. The potential of the statistical algorithm (SPS) for calculating canopy reflectance for large‐scale scenes is demonstrated by calculating the reflectance of a “forest scene” which consists of statistically defined vegetation extending over several acres.
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