Traceability of Solar UV Filter Radiometers
by Dr. Gregor Hülsen, Dr. Julian Gröbner

Abstract

SI-traceable solar UV radiation measurements with broadband solar UV filter radiometers poses several unique challenges to their calibration: • The solar irradiance can be up to 100 times larger than standard laboratory irradiance sources. • The spectral responsivity of UV-B weighted solar radiometers changes by up to 3 orders of magnitude within a very narrow spectral range with a sensitivity optimised for solar intensities. • The current accepted calibration methodology is to use a mixture of laboratory-based instrument characterisations, combined with an outdoor calibration using the sun as source. • Neither NMIs nor instrument manufacturers currently offer SI-traceable calibrations of solar UV broadband filter radiometers. In view of improving the status of the solar UV measurements within the UV monitoring community, the World Calibration Center for UV (WCCUV) organised three solar ultraviolet broadband radiometer campaigns (2006, 2017 and 2022, Hülsen et al., 2018, Hülsen et al., 2020). We see the following components aﬀecting the uncertainty of solar UV measurements using broadband radiometers on diﬀerent time scales: First, the calibration factors and measurement equations used by the participating institutes not always follow the recommendations of the WMO (Seckmeyer et al., 2008). Secondly, we see scarce to non-existent recalibration frequency of the solar UV radiometers. Third, instrument maintenance guidelines such as cleaning and humidity management inside the device are not followed (Webb, 2007). References Hülsen G., and Gröbner J. (2018), Report of the Second International UV Filter Radiometer Calibration Campaign UVC-II, GAW Report- No. 240, WMO Hülsen G., Gröbner J., Bais A., Blumthaler M., Diemoz H., Bolsee D., Diaz A., Fountoulakis I., Naranen E., Schreder J., Stefania F., and Vilaplana Guerrero J. M. (2020), Second solar ultraviolet radiometer comparison campaign UVC-II, Metrologia 57 Seckmeyer G., Bais A., Bernhard G., Blumthaler M., Booth C.R., Lantz K., and McKenzie R.L. (2008), Instruments to Measure Solar Ultraviolet Radiation - Part 2: Broadband Instruments Measuring Erythemally Weighted Solar Irradiance (GAW Report- No. 164, WMO TD No. 1289)