Spectral dependence in the cosine response of broadband UV instruments

Abstract

The erythemal UV biometer, a product of the original Robertson‐Berger meter, is now widely used across the globe for routine UV measurements. They are employed to measure the erythemal response to radiation, however, the filter present in such instruments rarely mimics the erythemal curve. As such, the relative spectral response of each instrument is generally different from the erythemal response curve. Thus calibration of the detector is dependent upon wavelength as well as solar zenith angle. In this study, measurements were compared for the purpose of characterizing a new cosine response function (CRF) for the broadband detector. Concurrent measurements of spectral and broadband erythemal UV data have been undertaken at the University of Tasmania in Hobart, Australia. Measurements spanned solar zenith angles (SZA) of 21°‐74°, total ozone values of 220–320 Dobson units and relatively small aerosol optical depths. The CRF is both wavelength and SZA dependent and applicable to all broadband UV‐B instruments based on the original Robertson‐Berger design. A methodology is presented for incorporation of the CRF into the calibration scheme. The concept of spectral multipliers is introduced, which are unique to each broadband instrument and correct for the wavelength dependence of the CRF. A method is then described for ongoing calibration of the detector based on model data alone. The scheme is then tested with real data to validate its reliability. Overall, by nature of the simultaneous characterisation of both spectral and cosine response, the CRF allows for a more robust calibration standard by which measurements and intercomparisons can be made.