Solar UV measurements with Robertson-Berger type instruments: influence of the detector's internal humidity status

The solar light biometer model 501 is one of the most common detectors for routine measurements of biologically effective solar UV radiation. In addition to an already known sensitivity of the instrument to ambient temperature, the laboratory investigations presented in this manuscript revealed a significant influence of the detector's internal humidity status on its spectral sensitivity. An extreme variation of internal relative humidity from 9 to 70% H-r resulted in a 5% reduction of the instrument's sensitivity at its maximum in the UVB range, whereas its sensitivity in the UVA range was reduced by more than a factor of 2. Even for a moderate variation of internal relative humidity from 9 to 35% H-r, which might well occur under outdoor conditions, the sensitivity reduction in the UVA range amounted to 40%. Consequences of these results for routine field measurements were estimated by weighting of global solar irradiance spectra with the respective sensitivity functions. At an internal relative humidity of 35% H-r, the detector's sensitivity to solar UV irradiance was found to be reduced by 13% at 20degrees solar elevation and by 7% at 60degrees solar elevation, relative to an internally dry state (9% H-r). During an outdoor observation period of 50 days, the detector's internal relative humidity gradually increased from 20 to 30% H-r, corresponding to a reduction of sensitivity by about 5% for solar elevations above 20degrees. Although other biometers showed quantitatively different behaviour, these results lead to the conclusion that long-term consistency of solar light biometer data can only be achieved by careful control of the instrument's humidity status during field measurements. (C) 2003 Elsevier B.V. All rights reserved.