Normalization of Sun/View Angle Effects Using Spectral Albedo-Based Vegetation Indices

C u r r e n t vegetation indices are normally computed with directional spectral reflectances and are subjected to many external perturbations such as soil background variations, atmospheric conditions, geometric registration, and especially sensor viewing geometry. Subsequent use of these indices to estimate vegetation amounts would result in substantial uncertainties. To reduce the uncertainties due to sun~view angle variations, spectral albedos, which are integrated reflectance values over a hemisphere of the surface within the specific spectral waveband, were derived from multidirectional measurements and bidirectional reflectance distribution function (BRDF) models and were subsequently used in vegetation index computations. The albedo-based vegetation indices were then compared with those computed with spectral reflectances using ground-, aircraft-, and satellite-based remote sensing measurements over harvested alfalfa, full-cover cotton canopy, pecan orchards, and bare soil surfaces. The results showed that spectral albedo-based vegetation indices were independent of view angles while the spectral reflectance vegetation indices varied substantially with sensor viewing geometry. Therefore, the view angle effects on spectral vegetation indices can be normalized, and the sun angle effects can be further reduced with a limited number of multidirectional measurements and BRDF models.