Application of the Clear-Sky Spectral Error for Radiometer Classification in ISO 9060: Preprint

A recent update of the ISO 9060 standard for the classification of solar radiometers introduces the use of the clear sky spectral error. The spectral error is the change in responsivity that may occur when the spectral distribution of the incident solar radiation differs from the spectral distribution present at the time of calibration. Spectral errors may occur if the radiometer does not have a completely uniform spectral responsivity. This use of the clear sky spectral error is a significant change compared to the previous version of the 9060 standard as now not only thermopile radiometers, but also photodiode radiometers are covered by the standard. We explain the method used to derive the spectral error and present spectral errors for several radiometers of different technologies. The results demonstrate that the new method is helpful to classify radiometers, distinguishing between different radiometer types and excluding inappropriate instruments from the classification as pyranometers and pyrheliometers. We recommend that the method is also used for WMO's radiometer classification which up to our knowledge is currently not fulfilled by any field pyrheliometer and many pyranometers on the market due to its demanding requirements for the spectral responsivity.