THE EFFECT OF CHANGES OR DIFFERENCES IN ROBERTSON-BERGER RADIOMETER RESPONSIVITY ON SOLAR ULTRAVIOLET-B MEASUREMENT

Two long-term solar UV measurement campaigns, in the USA and in Austri a, using Robertson-Berger radiometers found opposite trends for measur ed levels of ultraviolet-B radiation reaching the earth's surface. Thi s could be a consequence of the method used to calibrate the radiomete rs. Changes or differences in responsivity were compensated for by adj usting the sensitivity of the field radiometers to match the output of a reference radiometer. This radiometer intercomparison procedure has been evaluated in terms of the normalization wavelength to which the Robertson-Berger effective irradiance refers. There are small differen ces in spectral responsivities apparent in the radiometers used in the USA campaign, which require the field radiometers to be normalized at different wavelengths to match the response of the reference radiomet er. This normalization wavelength is shown to depend on the time-avera ged spectrum experienced by the instruments during the intercomparison . As a result there are substantial interradiometer variations in thei r calculated response to solar radiation when the measured spectral di stribution is different from the spectrum used for the radiometer inte rcomparison procedure.