Calibration of erythemally weighted broadband instruments: A comparison between PMOD/WRC and MSL

A Yankee Environmental Systems (YES) UVB-1 ultraviolet pyranometer, designed to measure erythemally weighted total solar irradiance, was calibrated by the Measurement Standards Laboratory (MSL) in Lower Hutt, New Zealand during August 2010. The calibration was then repeated during July and August 2011 by the Physikalisch-Meteorologisches Obervatorium Davos, World Radiation Center (PMOD/WRC) located in Davos, Switzerland. Calibration results show that measurements of the relative spectral and angular response functions at the two institutes are in excellent agreement, thus providing a good degree of confidence in these measurement facilities. However, measurements to convert the relative spectral response into an absolute calibration disagree significantly depending on whether an FEL lamp or solar spectra are used to perform this scaling. This is the first serious comparison of these scaling methods to formally explore the potential systematic errors which could explain the discrepancy.