Shortwave radiometers such as pyranometers, pyrheliometers, and photovoltaic cells are calibrated with traceability to consensus reference, maintained by Absolute Cavity Radiometers (ACRs). The ACR is an open cavity with no window that measures the extended broadband spectrum of the terrestrial direct solar beam irradiance, unlike shortwave radiometers that cover a limited range of the spectrum. The difference between the two spectral ranges may lead to calibration bias that can exceed 1%. This article describes a method to reduce the calibration bias resulting from using broadband ACRs to calibrate shortwave radiometers by using an ACR with Schott glass window to measure the reference broadband shortwave irradiance in the terrestrial direct solar beam from 0.3 μm to 3 μm. Reducing the calibration bias will result in lowering the historical solar irradiance by at least 0.9%. The published results in this article might raise the awareness of the calibration discrepancy to the users of such radiometers, and open a discussion within the solar and atmospheric science community to define their expectation from such radiometers to the radiometers’ manufacturers and calibration providers.
Müllejans, H., Zaaiman, W., Merli, F., Dunlop, E.D. and Ossenbrink, H.A. (2015) Comparison of Traceable Calibration Methods for Primary Photovoltaic Reference Cells. Journal of Progress in Photovoltaics: Research and Applications, 13, 661-671. https://doi.org/10.1002/pip.625
Reda, I., Konings, J. and Xie, Y. (2015) Method to Measure the Broadband Longwave Irradiance in the Terrestrial Direct Solar Beam. Journal of Atmospheric and Solar-Terrestrial Physics, 129, 23-29. https://doi.org/10.1016/j.jastp.2015.04.003
Reda, I., Hickey, J., Long, C., Myers, D., Stoffel, T., Wilcox, S., Michalsky, J.J., Dutton, E.G. and Nelson, D. (2005) Using a Blackbody to Calculate Net Infrared Responsivity of Shortwave Solar Pyranometers to Correct for Their Thermal Offset Error during Outdoor Calibration Using the Component Sum Method. Journal of Atmospheric and Oceanic Technology, 22, 1531-1540. https://doi.org/10.1175/JTECH1782.1