Correction of bi-directional effects in video imagery

Bidirectional reflectance (BDRF) from vegetation is a function of solar zenith, view azimuth, and view zenith angles. Bidirectional effects confound surface reflectance measurements using airborne video imagery, where the sensor viewing angle changes across the image. Several semi-empirical BDRF distribution function models relating the ratio between off-nadir to nadir radiance to the geometrical arrangement between the vegetated surface, the sun, and the sensor have been developed. However, these models are based on site-specific coefficients, which limits the validity of the approach to a limited set of conditions and surface-sensor geometries. The purpose of this study was to develop a mechanistic model of the bidirectional effect for the purpose of improving the use of airborne video imagery for studying vegetated surfaces. Although modeling BDRF profiles within canopies can be complex, we used a simplified model of BDRF that neglects radiation in different azimuths for the purpose of correcting airborne imagery. The effects due to sensor view angle or view azimuth were determined from airborne data. Three correction coefficients were developed: NBDR, due to canopy BDRF; Z(v) due to changing sensor view zenith angle; and A(v) due to changing view azimuth angle with respect to the sun.