A method to measure the broadband longwave irradiance in the terrestrial direct solar beam

• Measuring the broadband longwave irradiance in the solar beam, WLW.
• Discrepancy in calibrating shortwave radiometers using Absolute Cavity Radiometers.
• Lack of a daytime reference for longwave radiometers, i.e. pyrgeometers.
• Addresses calibration discrepancy of shortwave and longwave radiometers.
• Use to develop method quantifying effect of dome heating without dome thermistors.

Shortwave radiometers such as pyranometers, pyrheliometers, photovoltaic cells, and longwave radiometers such as pyrgeometers are calibrated with traceability to consensus References, which are maintained by Absolute Cavity Radiometers (ACRs) and the World InfraRed Standard Group (WISG), respectively. Since the ACR is an open cavity with no window, and was developed to measure the extended broadband spectrum of the terrestrial direct solar beam irradiance, then there would be discrepancy in calibrating the shortwave radiometers because of their limited spectral band. On the other hand, pyrgeometers are calibrated during the nighttime only, because no consensus reference has yet been established for the daytime longwave irradiance. This article describes a method to measure the broadband longwave irradiance in the terrestrial direct solar beam from 3 µm to 50 µm. The method might be used in developing calibration methods to address the mismatch between the broadband ACR and shortwave radiometers, and the lack of a daytime reference for pyrgeometer calibration. We used the described method to measure the irradiance from sunrise to sunset; the irradiance varied from approximately 1 W m−2 to 16 W m−2 with an estimated uncertainty of 1.46 W m−2, for a solar zenith angle range from 80° to 16°, respectively.