Dual field-of-view goniometer system FIGOS

Abstract

Almost all natural surfaces exhibit an individual anisotropic reflectance characteristic due to contrasting optical properties of surface elements and background as well as an uneven distribution of illuminated and shadowed areas. The bidirectional reflectance distribution function BRDF is a basic quantity which describes the reflectance characteristic. Many applications, such as BRDF correction of remote sensing data and quantitative retrieval of vegetation parameters require accurate knowledge of the spectrodirectional surface reflectance properties. However, the directly observed quantity in field experiments is affected by atmospheric conditions which need to be accounted for when retrieving the target specific BRDF. The most exact BRDF retrieval from field goniometry measurements can be achieved by considering the diffuse irradiance at angular resolution. However, most goniometer systems are not able to simultaneously collect the reflected and incoming radiances at high spectral and angular resolution. The dual field-of-view (FOV) goniometer system FIGOS of the Remote Sensing Laboratories (RSL, Switzerland) has this capability and is presented here for the first time along with a full field BRDF retrieval concept proposed by Martonchik et al. (Martonchik, 1994). The dual FOV FIGOS is based on the proven field goniometer system FIGOS extended by two ASD FieldSpec-3, each pointing in opposite directions. Reflected and incoming radiance measurements are performed simultaneously and at the same angular and spectral resolution. The first spectrodirectional dual field-of-view dataset was obtained in summer 2006 in Gilching, Germany. Both a natural and an artificial target were measured. The evaluation showed that the dual field-of-view goniometer system FIGOS substantially supports not only data collection for full field BRDF retrievals but also direct comparisons of field and laboratory measurements. Due to its reliable performance and known characteristics it has the potential to be a reference instrument for extensive field and laboratory experiments.