A geometric-optical forest canopy model that treats conifers as cones casting shadows on a contrasting background explains the major anisotropies in bidirectional reflectance measurements of a conifer forest canopy taken from the literature. The model uses parallelray geometry to describe the illumination and viewing of conifers as three-dimensional cones. Cones are randomly placed and overlap freely. Cone size (height) is distributed lognormally, and cone form, described by the apex angle of the cone, is a negative exponential function of height. The cones are first presumed to be solid dark gray Lambertian objects, located on a lighter gray Lambertian background. To add realism, "translucence" is added and light is allowed to pass through cones with negative exponential attenuation. Both computer simulation and analytical closed-form expressions are implemented. The results show a good qualitative agreement with the directional reflectance measurements of the conifer stand, indicating that the three-dimensional nature of the canopy is a key factor in determining its directional reflectance.
Li et al. (1986) studied this question.