Combined field and laboratory goniometer system - FIGOS and LAGOS

Ground level measurements of surface directional reflectance properties can be performed either in the field or within a laboratory setup. The latter has the advantage of independence on weather conditions, constant illumination and neglectable atmospheric disturbances. On the other hand, the artificial laboratory light sources usually are less parallel and less homogeneous than the clear sky solar illumination. In order to compare these two types of measurements (or replace one by the other) a careful correction of the data (BRDF retrieval) with respect to the mentioned sources of error is necessary. For the field case, this can be done following the well known procedures proposed by Martonchick and others. The purpose of this paper is to present our laboratory goniometer system and a corresponding BRDF retrieval solution. The RSL laboratory goniometer system (LAGOS) is based on the field goniometer (FIGOS) with the addition of a 1000 W brightness-stabilized quartz tungsten halogen lamp and lens system, placed in a dark room for minimization of stray light. The inhomogeneity of the illuminated area has been directly measured and found to be within 10% mean deviation for the zenith position. A simulation of the complete geometry of LAGOS, including the angular distributions and inhomogeneity of the light source as well as the changing position of the radiometer footprint allows us to estimate the measurement error for any target with known BRDF. The same algorithm can be used as core for the BRDF retrieval.