Spectrodirectional field and laboratory measurements of an artifical target

Spectrodirectional experiments with goniometer systems are only able to observe approximations of truly directional surface reflectance properties (BRDF). The directly observed quantity in field experiments is called hemispherical directional reflectance factor (HDRF), corresponding to hemispherical illumination, which depends on the atmospheric conditions, and directional observation. Laboratory experiments suffer from imperfect illumination resulting in a biconical rather than bidirectional reflectance factor. Quantitative comparison of field and laboratory measurements is not only important to ensure effective comparability, but also to permit cross-calibration of the experimental devices and to document the degree of compatibility. It is further a prerequisite for determining for which targets a replacement of field by laboratory experiments is feasible. Preliminary studies (i) revealed that the diffuse illumination present in the field is one of the major differences between field and laboratory measurements. A goal of this study is to characterize and correct the diffuse influence in spectrodirectional field measurements more accurately and validate previously achieved results. Spectrodirectional field measurements were accomplished using a GER3700 spectroradiometer mounted on the field goniometer system (FIGOS) of the Remote Sensing Laboratories (RSL, Switzerland). Additionally, an MFR and a REAGAN sun photometer were permanently monitoring the atmospheric conditions. The laboratory goniometer system (LAGOS) uses a 1000W brightness-stabilized quartz tungsten halogen lamp as illumination source. For both field and laboratory measurements, we used an inert and highly anisotropic target. Field data were corrected for diffuse illumination following a procedure proposed by Martonchik (ii).The diffuse influence is then computed as a correction term depending on the angular characteristic of the target BRDF and of the amount of diffuse irradiance. Additionally, the diffuse sky radiance distribution is simulated through multiple runs of MODTRAN (iii) and will be implemented within the correction algorithm in further studies