In general measurements of UV-radiation are related to horizontal surfaces, as e.g. also done for the internationally standardized and applied UV-index. In order to get more information on biologically relevant UV-exposure, there is a need for quantitative data of radiation fluxes on tilted surfaces. UV exposure of the human skin is one of the most essential issues in UV research, and therefore UV-irradiance for surface orientations typical for the human body should be known for all kinds of meteorological conditions. To measure these fluxes the new automatic system ASCARATIS (Angle SCAnning RAdiometer for determination of erythemally weighted irradiance on TIlted Surfaces) was developed and built. With three units of ASCARATIS hundreds of thousands of sets of UV-index measurements (each set consisting of measurements in 27 directions) have been made at different sites (urban, rural, mountain) during the last three years. The measurements cover the whole variability of weather conditions for all seasons. The results show large differences between UV-irradiation of horizontal surfaces (measuring standard) and inclined surfaces especially during the time of the year with lower sun elevations. The measuring data are used to fit a radiation model for inclined surfaces and to model the UV-irradiance of the human body.
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