Empirical methods to compensate for a view-angle-dependent brightness gradient in AVIRIS imagery

A view-angle-dependent brightness gradient was observed in an AVIRIS image of a forested region in Oregonʹs Cascade Mountains. A method of removing the view-angle effect was sought that would not alter the radiometric integrity of the image, and which would require minimal ancillary information. Four methods were tested and evaluated in terms of remaining brightness gradient and in terms of retention of spectral characteristics. All methods used a quadratic fitting equation to model the changes in brightness across view angles. Other descriptive coefficients were calculated to aid in interpretation. The observed view-angle effect varied with wavelength in a manner consistent with predictions of bidirectional reflectance distribution function characteristics for vegetation. View-angle effects were determined to contain both additive and multiplicative components, with multiplicative components being strong in the chlorophyll absorption region. The view-angle effect in a given pixel was a function of both an underlying viewangle response determined by surface structure and the inherent brightness of that pixel. The most successful compensation method was the one that best accounted for broad differences between pixels in these two components. Despite the simplifying assumptions necessary for empirical view-atigle correction techniques, they can still be useful for hyperspectral remote-sensing data in situations where the view-angle brightness variations would mask variance useful for extracting scene information.