Sensitivity Analysis and Quality Assessment of Laboratory BRDF Data

A detailed quality and sensitivity analysis of hyper-spectral BRDF data, acquired under controlled laboratory conditions at the European Goniometric 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 sampling, the movement of the Sun, and the Lambertian assumption of a Spectralon 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 analyzing the wavelength dependency of the effects. The results of the sensitivity analysis show that in a first step a moderate resolution of 15° and 30° in zenith and azimuth, respectively, is adequate to capture the bidirectional reflectance distribution function (BRDF) of vegetated targets. Only in the hot spot area a higher resolution is desirable. The movement of the light source during data acquisition is found to be critical, and should be kept within ±1° source zenith angle in order to obtain homologous BRDF data sets. Due to the wavelength dependence of BRDF effects, the impact of the light source may vary significantly between different wavelength ranges. A normalization of the irradiance with the help of frequent reference measurements is recommended, and should be carried out using a panel with known BRDF characteristics. The Spectralon panel examined showed deviations from a Lamber-tian panel of up to 5% and more, but obeyed Helmholtz’s reciprocity law. Corresponding calibration coefficients are given for correcting the non-Lambertian behavior in field applications. Laboratory specific constraints such as the nonparallelism and heterogeneity 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.