Polarised bidirectional reflectance factor measurements from vegetated land surfaces

This study measured the spectral bidirectional reflectance factors of selected vegetated land surfaces in two linear polarisation directions. It was observed that the degree of linear polarisation is usually weakest near nadir, turning 1-5% negative in the backward direction, and growing larger in the forward and Brewster directions. Polarisation was found to be inversely but non-linearly proportional to reflectance. In addition, a wavelength-dependent trend was found to exist in some data, but in general, the wavelength dependence was smooth. In most of the practical applications, a few well-selected channels would give all the available information.The authors conclude that polarisation observations may be useful in land surface remote sensing, but that only far-forward angles (60-70^°) contain a strong enough signal for easy interpretation. Polarisation near nadir is low for all known samples, and can thus be safely ignored. Despite a strong theoretical interest, no clearly discriminating signal was found in the negative polarisation branch near backward direction, providing few opportunities for quantitative analysis. There must be a better understanding of the directional effects in all applications.