Calibration of the broadband radiometers of the Finnish solar ultraviolet monitoring network

The present solar ultraviolet (UV) monitoring network of the Finnish Meteorological Institute consists of seven sites equipped with erythemally-weighted broadband radiometers. Two of the sites are also equipped with a Brewer spectroradiometer. The Finnish Centre for Radiation and Nuclear Safety is responsible for testing and calibration of the broadband radiometers. An Optronic 742 spectroradiometer is used as a reference instrument, and the broadband radiometers are calibrated against the Optronic 742 in solar radiation. The Optronic 742 is calibrated against 1000 W FEL lamps traceable to the NIST. The errors caused by nonideal radiometric characteristics of the Optronic 742 are numerically corrected. The uncertainty (2 sigma) of solar UV measurements with the Optronic 742 is estimated to be +/- 8 % at 310 nm and the uncertainty of the spectroradiometric solar calibration of the Solar Light Model 501 radiometers is estimated to be +/- 11%.