Bidirektionale Reflektanz von städtischen Oberflächen (Bidirectional Reflectance of Urban Surfaces) Dissertation zur Erlangung des Doktorgrades des Fachbereichs Physik der Universität Hamburg

The reflectance of a surface usually depends on the angles, at which the surface is illuminated and viewed. The dependence can be described by the Bidirectional Reflectance Distribution Function BRDF. In this study, the BRDFs of several samples typically found in urban areas (roof tiles, asphalt, etc.) were measured at the European Goniometric Facility EGO in Ispra, Italy with hyperspectral sensors. The basic characteristics of the measured BRDFs are a specular peak, an increase in backscattering direction for the roughest surfaces and a constant diffuse component for the surfaces of moderate roughness. Surfaces of moderate roughness can be well described by the BRDF model for specular reflection of rough surfaces by (Torrance & Sparrow 1967), whereas for very rough surfaces the model by (Oren & Nayar 1995) is more appropriate. Surface topography measurements suggest that the size of the scattering structure is in the 20 m range. In reflectance terminology, the principal plane is the plane formed by the surface normal and the illuminating light ray. We found that the angular width of the specular peak of rough surfaces perpendicular to the principal plane decreases proportionally to the cosine of the illumination zenith angle. This result is important in computer vision applications, but it is also of theoretical interest. The BRDF measurements were also used to develop a BRDF model for urban areas remotely sensed from spaceborne sensors with pixel-sizes above 500 m. The model is built on the street structure of urban areas. It shows a strong increase of reflectance in backscattering direction. Model predictions and simulated data derived from multispectral imagery of an airborne scanner agree well. The major areas of application are albedo calculation and landcover identification in global image data.