New entrance optics for solar spectral UV measurements

The investigation of the impact of solar UV radiation on the biosphere requires spectral measurements of solar UV radiation of high accuracy. However, the accuracy of current measurements is limited, and this can partly be attributed to the entrance optics of the instruments used for these examinations. The angular response of spectroradiometers measuring spectral global UV irradiance should be given by the cosine of the incidence angle. Intercomparison campaigns have shown that deviations from this ideal cosine response lead to uncertainties in solar measurements of more than 10 per cent. Here we present recently developed entrance optics that reduce these uncertainties to +/- 4 per cent in the UV. The new entrance optics have been characterized with respect to their angular response, transmission, weather durability, fluorescence and dependence of the angular response on wavelength and polarization. Solar spectroradiometric measurements carried out with the new optics were compared w ith simultaneously performed measurements of a second spectroradiometer that was equipped with a conventional diffuser. The deviations of up to 12 per cent between both systems are quantitatively explained to within 3 per cent.