About the Importance of the Definition of Reflectance Quantities - Results of Case Studies

In the remote sensing user community there is a lack of consistency in definitions and properties of reflectance quantities. On one hand, more recent satellite programs such as NASA's MODIS and MISR sensors take into account the directional dimension of the different reflectance products. On the other hand, many published studies still remain unspecific on the reflectance quantities they are based on, or do not follow common definitions. One example is the term 'albedo' assigned to significantly differing products. This fact makes it difficult and confusing to evaluate and compare published results. Our contribution briefly summarizes basic reflectance nomenclature articles. The main aim is to quantify differences of reflectance products to stress the importance of adequate usage of reflectance definitions and quantities. Results from the comparison of directional-hemispherical reflectance versus bihemispherical reflectance and bidirectional reflectance factors versus hemispherical-directional reflectance factors are shown. We exemplify differences of these quantities using modelling results of a black spruce forest canopy and snow cover, as well as selected biome-specific MISR reflectance products of the year 2001. The presented case studies can only give an insight into the dimension of the problem. The actual differences in the reflectance products of a remotely sensed surface depend on the atmospheric conditions, the surroundings, topography, and the scattering properties of the surface itself. Never the less the presented results are urging the user community to be more specific on the application and definition of reflectance quantities.