Intercomparison of field and laboratory goniometer measurements

Abstract

Field and laboratory goniometers are widely used in the remote sensing community to assess spectrodirectional reflectance properties of selected targets. Even when the same target and goniometer system are used, field and laboratory results cannot directly be compared due to inherent differences, mainly in the illumination conditions: typically goniometers measure a hemispherical-conical reflectance in the field and a biconical reflectance in the laboratory. Yet, the ability to compare and combine measurements from different instrumental designs is critical to ensure sensor cross-calibration. It is also critical for all applications that rely on measurements obtained with both types of instruments. One solution is to retrieve the BRDF of the targets of interest for each experimental setup individually and to compare those, since theoretically they are independent from the particular conditions of illumination and observation. This involves a correction for diffuse incoming radiation in the case of field measurements, and a correction for the conicality and inhomogeneity of illumination in the case of laboratory measurements. We present a BRDF retrieval scheme for typical laboratory goniometers as well as results of measurements and BRDF retrievals using the field and laboratory goniometer systems (FIGOS/LAGOS) of the University of Zurich and the same artificial target for both goniometer setups.