Reducing Broadband Shortwave Radiometer Calibration-Bias Caused by Longwave Irradiance in the Reference Direct Beam

Ibrahim Reda,Afshin Andreas,Mike Dooraghi,Mark Kutchenreiter,Aron Habte,Manajit Sengupta

doi:10.4236/acs.2017.71004

Abstract

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.

Highlights

Shortwave radiometers such as pyranometers, pyrheliometers, and photovoltaic cells are calibrated with traceability to the World Radiometric Reference (WRR) (ISO, 1990) [1]
The Absolute Cavity Radiometers (ACRs) 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 WRR is maintained by six Absolute Cavity Radiometers (ACR) that form the World Standard Group (WSG), which are located at the Physikalisch-Meteorologisches Observatorium in Davos, Switzerland [3]

Summary

Introduction

Shortwave radiometers such as pyranometers, pyrheliometers, and photovoltaic cells are calibrated with traceability to the World Radiometric Reference (WRR) (ISO, 1990) [1]. Pyranometers and pyrheliometers measure broadband shortwave irradiance from approximately 0.3 μm to 3 μm [4] [5] and the silicon-based photovoltaic cells are limited to the spectral range of approximately 0.3 μm to 1 μm. The method described in Reda, et al, 2015 summarizes using two pyrgeometers: one shaded with a Schott glass shading mechanism to block the spectral range greater than 3 μm from the direct solar beam, and the other unshaded with spectral range from approximately 3 μm to 50 μm. The reference broadband longwave irradiance in the direct solar beam (WLW) is calculated by subtracting the irradiance measured by the shaded pyrgeometer from that measured by the unshaded pyrgeometer

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Conclusion