Intercomparison and harmonization of UV Index measurements from multiband filter radiometers

Multiband filter radiometers (MBFRs) are extensively used in national measurement networks for UV climate monitoring and for informing the public about potential health risks from excessive solar UV exposure. Results from the first international intercomparison of MBFRs, arranged in Oslo in 2005, are presented. Forty-three radiometers of type GUV, NILU-UV, and UVMFR-7 were assembled, representing monitoring stations on several continents. The first objective was to conduct a blind intercomparison of Global UV Index (UVI) processed by the instrument owners. Eleven independent data sets were compared, eight of which agreed with the reference to within +/- 5% and ten to within +/- 10%. The second objective was to provide a harmonized calibration scale for all instruments. When this scale was applied, the UVI agreed to within +/- 5% (2-sigma) for solar zenith angles (SZAs) up to 90 degrees. The results demonstrate that MBFRs provide accurate UVI measurements for realistic sky conditions and a wide range of SZAs, provided the calibration functions are optimized. The harmonized UVI scale is traceable to the European QASUME reference spectroradiometer.