Temperature effects correction in a Brewer MKIV spectrophotometer for solar UV measurements

The increasing demand for accuracy in the measurement of the solar UV-spectral irradiance, has motivated an investigation of the dependence of the spectral responsivity of our Brewer MKIV spectrophotometer on the ambient temperature. The study has been performed using a set of 50 W tungsten-halogen lamps whose specific intensities have been calibrated by a National Institute of Standards and Technology referenced lamp. The main observed feature is a decrease of the responsivity of the instrument with increasing temperature: this effect is greater at low wavelengths and can produce mean inaccuracies over the 290-325 nm spectral interval ranging from 2% in the cold season up to 8% in summer. The proposed correction methodology makes use of a family of responsivity files (uvres fries) made at different temperatures and normalized to a calibration performed with a referenced lamp at a given temperature. Each UV spectrum taken along the day will be calculated using the uvres files corresponding to the same temperature at which the spectrum has been measured. The appropriate uvres will be obtained by linear interpolation of the already determined responsivity files.