Indoor Direct Comparison between High-Accuracy Pyrheliometer and Room Temperature Absolute Radiometer

Abstract This article proposes a practicable indoor method by directly comparing the pyrheliometer with the room temperature absolute radiometer (RTAR) to overcome the inconvenience of outdoor pyrheliometer calibration caused by the limitation of weather conditions. The RTAR serves as the primary standard for radiant power, irradiance, illuminance, and candela at the Center for Measurement Standards/ITRI. Given that the RTAR and pyrheliometer exhibit different sensitivity curves, the key to a successful comparison lies in designing an incident beam that enables both the RTAR and pyrheliometer to operate within their respective uniform sensitivity regions. While the indoor incident beam’s irradiance is considerably lower than solar radiation, the comparison results demonstrate a quite well measurement consistence between the RTAR and the pyrheliometer. As the indoor method is insusceptible to weather, the complex data acquisition procedures for the outdoor method can be simplified due to the stable incident beam indoors. Despite current documentary standards recommending outdoor calibration and comparison for pyrheliometer, the indoor method presented in this article can serve as a supplementary approach for assessing the accuracy of pyrheliometers during the intervals between two calibration or comparison events.