Geometrical modelling of soil bidirectional reflectance incorporating specular effects

A geometrical model, taking into account the diffuse, as well as the specular component of energy leaving soil surfaces in the visible and near-infrared, is discussed here. The model computes the bidirectional reflectance of soils illuminated by a single source. A rough soil surface is simulated by equal-sized spheroids regularly spaced on a horizontal surface. The model was tested using soil bidirectional reflectance data obtained in laboratory conditions by Jacquemoud et al in 1992. Two parameters describing soil surface geometry were used for modelling the soil relative reflectance in laboratory conditions: the relative distance (d/a) between spheroids (relative to their horizontal radii (a)), and the shape of spheroids (b/a) (as proportion of their vertical (b) to horizontal radii (a)). The simulation of reflectance for soil surfaces of pebbles and sand, containing simple dense particles with rounded edges, can be carried out using the d/a and b/a ratios which nearly described their actual geometry. The reflectance of more geometrically complicated soil surfaces, such as clay and peat with irregular secondary porous aggregates, can be simulated by surfaces of effective geometry of vertically elongated spheroids.