Reduced uncertainties of field pyrheliometers through improved sensor calibration

A decisive influence on the overall uncertainty of common field pyrheliometers is caused by their laboratory calibration using artificial light sources and instruments of the same kind as references. Alternative outdoor calibrations allow the use of an absolute cavity radiometer as a more accurate reference. As the sun is used as light source for these calibrations actual sky conditions can influence the outdoor calibration and have to be monitored. If test and reference instruments have different angular acceptance functions circumsolar radiation can have a crucial effect on the calibration accuracy. With outdoor calibrations carried out by DLR the calibration uncertainty of K&Z pyrheliometers could be reduced from 0.9% to approximately 0.4%.