Field and landscape BRDF optical wavelength measurements: Experience, techniques and the future

There is a greater need to measure the bidirectional reflectance distribution function (BRDF) from surfaces at various scales. Landscape BRDF measurements capture the different aspects of vegetation heterogeneity which complicate scaling from individual materials to spatially distributed products. This is forcing experimenters to review measurement protocols for BRDF from ground‐based and airborne radiometers. Such a review benefits investigators via insights on such issues as angular sampling, atmospheric corrections, and spectral, bidirectional and temporal variability. This paper briefly reviews past BRDF‐related field campaigns that contributed to our present knowledge of surface directional effects and guided the development of most analytical models. While isolated and often disconnected, in situ BRDF measurements collected from plots in different ecosystems have been very useful in contributing to our understanding of radiative mechanisms that led to the development of early reflectance models. It now appears that accuracy assessment of most past measurements is consistent with the models employed for data processing for global applications. There is an evolution in progress towards new demands for measurements that consider user requirements of the earth science communities and validation issues. Once data archiving demands are met, an emphasis on validation efforts within the context of the new generation sensor systems is needed. Some recommendations are also given for future investigations to acquire BRDF measurements that will account for the necessary compromises between angular and spectral, spatial, and temporal observation samples, thereby better answering the needs of end users.