SENSITIVITY ANALYSIS AND QUALITY ASSESSMENT OF LABORATORY BRDF DATA

A derailed quality and sensitivity analysis of hyperspectral BRDF data , acquired under controlled laboratory conditions at the European Goni ometric Facility (EGO) of the Joint Research Center, Ispra/Italy, has been performed. In regard to bidirectional reflectance measurements in the field with the FIGOS goniometer, the impact of angular data sampl ing, the movement of the Sun, and the Lambertian assumption of a Spect ralon panel have been analyzed. An erectophile grass lawn canopy and a planophile watercress surface were chosen as main targets. A GER-3700 spectroradiometer providing hyperspectral resolution allowed for anal yzing the wavelength dependency of the effects. The results of the sen sitivity analysis show; that in a first step a moderate resolution of 15 degrees and 30 degrees in zenith and azimuth, respectively, Is adeq uate to capture the bidirectional reflectance distribution function (B RDF) of vegetated targets. Only in the hot spot area a higher resoluti on is desirable. The movement of the light source during data acquisit ion is found to be critical, and should be kept within +/-1 degrees so urce zenith angle in order to obtain homologous BRDF data sets. Due to the wavelength dependence of BRDF effects, the impact of the light so urce may vary significantly between different wavelength ranges. A nor malization of the irradiance with the help of frequent reference measu rements is recommended, and should be carried out using a panel with k nown BRDF characteristics. The Spectralon panel examined showed deviat ions from a Lambertian panel of up to 5% and more, bur obeyed Helmholt z's reciprocity law. Corresponding calibration coefficients are given for correcting the non-Lambertian behavior in field applications. Labo ratory specific constraints such as the nonparallelism and heterogenei ty of the lamp are assessed and corrected where necessary and feasible . The reproducibility of the BRDF data obtained lies within 1-9% rmse, depending on target type, wavelength range, and measurement duration. (C)Elsevier Science Inc., 1998.