Contrasts among Bidirectional Reflectance of Leaves, Canopies, and Soils

The apparent brightness of a natural surface depends on the characteristics and direction of incident radiation, surface radiative properties, and the direction from which the surface is viewed. The bidirectional reflectance distribution functions (BRDF) for soils, vegetation canopies, and individual leaves have common features that arise from an anisotropic diffuse scattered component as well as specular reflections from irregular surfaces. The diffuse scattered component, however, tends to dominate canopy and soil BRDF's where as specular reflection tends to dominate leaf BRDF's. In this paper, simple models are presented for predicting soil and canopy BRDF's, and predicted results are compared with some measurements. This effort represents an attempt to illustrate the important features that cause observed BRDF's. The results of the models are compared with a simple three coefficient empirical equation that may be easier to invert than the causitive models so that radiation measurements can be used to obtain surface features. Some BRDF measurements from corn and soybean leaves provide a contrast for canopy and soil distributions, and also provide needed leaf properties that are important to vegetation canopy models. The knowledge of soil, canopy, and leaf BRDF's is combined into a single model, called Cupid, to predict the resultant BRDF of complex natural surfaces.