PD ISO/TR 9901:2021 Solar energy — Pyranometers — Recommended practice for use.
Pyranorneters classified in ISO9fl6Q as spectrally flat” have a spectral selectivity of less than 3% (guard hands 2 %) in the 0,35 urn to 1,5 pm spectral range. This is the same requirement as in the previous ISO 9060:1990 for secondary standard pyranometers. Spectrally flat pyranorneters are typically more accurate over a wide range of conditions, and applicable not only for horizontal measurement of global horizontal irradiance, GHI, but also for measurements of plane of array irradiance, POA, and reflected irradiance, RI, as well as for artificial solar sources such as lamps. IEC 61724-1 requires use of instruments of a specified accuracy class for its class A and B monitoring systems. There is consensus that the spectral selectivity specifications of ISO 9060 “spectrally flat” pyranometers have a negligible (zero) spectral error and that they can be used for all the common outdoor measurements in solar energy studies with the same calibration (typically performed with the clear sky solar spectrum as the source) without significant loss of accuracy. The clear sky solar spectrum is one of the reference operating conditions for pyranometers if it is the source under which an instrument is calibrated or the source under which a calibration reference standard has been calibrated.
Pyranorneters employing photodiodes (otherwise known as silicon-pyranometers), are not classified as “spectrally flat” in ISO 9060. The spectral error of pyranorneters is defined for a set of clear sky solar spectra only. This implies that their spectral error for other than clear sky spectra cannot be based on the classification alone. The spectral error of pyranometers, in particular if they are not spectrally flat, may be larger for measurements of LJHI, POA or RI than for clear sky GHI. The user may perform an individual uncertainty evaluation depending on the manufacturer specification of the instrument and the spectra of the measured radiation. The factory calibration of non spectrally flat instruments is typically valid for a set of clear sky solar spectra. Their sensitivity and uncertainty of their sensitivity may both change for different conditions.
Non spectrally flat pyranometers also may offer specific advantages; they generally are inexpensive, small and have a fast response time. They may be used for example for temporally highly resolved measurements, when overall accuracy requirements are not too high, or where constant spectrum conditions exist (for example, working with artificial sources, or only working under clear sky conditions). They also may be used for high-accuracy applications when calibrated under the working conditions.
In summary, spectrally flat pyranometers can be used for the most common solar testing applications, including GI-Il, POA, RI and albedo measurements using traceability to the same clear sky spectrum calibration. When using non-spectrally flat pyranometers for other than clear-sky GHI measurements, the spectral error may be larger than the spectral error specified in ISO9Q0.
If a higher measurement accuracy is required than may be attained with a class A pyranometer. there also are class A pyranometers with improved directional error- and zero-offset specifications.PD ISO/TR 9901 pdf download.