Effects of changing spectral radiation distribution on the performance of photodiode pyranometers

The effect of changes in the spectral distribution of the incident solar radiation on the direct normal responsivity of the LI-COR photodiode pyranometer is examined using DNI spectral data from a PMOD spectroradiometer and the generic spectral response of a LI-COR pyranometer. The direct normal responsivity is found to vary over the day under all weather conditions at Payerne, Switzerland in a systematic manner that can be modeled as a function of air mass or solar zenith angle. This suggests that scattering of other atmospheric constituents plays as a subsidiary role to Rayleigh scattering that preferentially scatters more blue light as the path through the atmosphere increases. The SMARTS2 model is used to examine the effect on the full spectral response range of the photodiode based pyranometer and to refine the estimated response changes. The use of this information is discussed relative to improving adjustments made to Rotating Shadowband Irradiometers measurements. Similar methodology can be used to estimate the spectral effect on the performance of solar modules.