Vol. 4, No. 2, 191-202, 2005

Abstract
Analysing databases, field and airborne spectrometer data, modelling studies and publications, a lack of consistency in the use of definitions and terminology of reflectance quantities can be observed. One example is the term ‘BRDF’ (bidirectional reflectance distribution function) assigned to significantly differing quantities, ranging from the bidirectional reflectance distribution function to hemispherical-conical reflectance factors. Our contribution summarizes basic reflectance nomenclature articles. Secondly differences of reflectance products are quantified, with special emphasis on wavelength specific effects, 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. Differences of these quantities are exemplified using modelling results of a black spruce forest canopy, snow cover, as well as an artificial target. 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. As these effects are highly wavelength-dependent, the imaging spectroscopy community has to become more specific on the application and definition of reflectance quantities. As of today most delivered reflectance products from imaging spectrometers include the hemispherical illumination component. Thus, product algorithms based on surface reflectance data have to include the actual atmospheric conditions even for nadir view angles, e.g., in the form of a wavelength-specific indication of the ratio of diffuse to direct illumination. The results urge the community to treat reflectance quantities with outmost care and consistency to reduce uncertainties of derived products.

ISSN 1729-3782