All-weather comparison between spectral and broadband (Robertson-Berger) UV measurements

Spectral measurements of UV irradiance at the Fraunhofer Institute at Garmisch-Partenkirchen, Germany, are intercompared with data from two different Robertson-Berger-type meters. More than 21000 spectra gathered during 20 months of continuous operation were used for this analysis, sampling a large variety of atmospheric conditions. At low solar zenith angles the agreement (+/- 10 per cent) was quite satisfactory, whereas at high solar zenith angles the broadband instruments gave systematically lower readings than the erythemally weighted spectral data. The deviation of the spectral response of the broadband instruments from the Commission Internationale de I'Eclairage (CIE) erythema function and the cosine error provide an explanation of the observed differences that are similar for both broadband meters. Model calculations agree quite well with the observations, if an accurate measurement of the broadband spectral response is available. An analytical fit curve was established for the ratio between broadband and weighted spectral data, as a function of solar zenith angle, enabling quick and easy checks of the stability of the spectroradiometric system. The relative standard deviation of the data from the fit function was only 2-3 per cent in the case of cloudless sky conditions and 5 per cent for all sky conditions, including all types of systematic and instrumental errors.