###### Abstract  
A simple index is desirable to assess the effects on both flat-plate and concentrating photovoltaics of natural changes in the solar spectrum. Some studies have suggested that the relationship between the Average Photon Energy (APE) and the shape of individual global tilted irradiance, global horizontal irradiance, or direct normal irradiance (DNI) spectra is bijective and can therefore be used as a single number to unequivocally replace a complete spectral distribution. This paper reevaluates these studies with a modified methodology to assess whether a one-to-one relationship really exists between APE and spectral DNI. A 12-month dataset collected in Jaen (Spain) using a sun-tracking spectroradiometer provides the necessary spectral DNI data between 350 and 1050 nm. After quality control and filtering, 78,772 valid spectra were analyzed. The methodology is based on a statistical analysis of the spectral distributions binned in 0.02-eV APE intervals, from 1.74 to 1.90 eV. For each interval, both the standard deviation and coefficient of variation (CV) are determined across all 50-nm bands into which the 350-1050-nm waveband is divided. CV stays below 3.5% within the 450-900-nm interval but rises up to 13% outside of it. It is concluded that APE may be approximately assumed to uniquely characterize the DNI spectrum distribution for Jaen (and presumably for locations with similar climates) only over the limited 450-900-nm waveband. s 2021 11 897 20  
Photovoltaics (CPV). Such a higher sensitivity of the latter technology to spectral effects is especially noticeable when concentrated sunlight of 1000 suns or more is used in High Concentrating Photovoltaics (HCPV) [6], where multi-junction solar cells based on series-connected subcells with different bands of absorption are used in conjunction with optical elements [7; 8]. The present contribution is mostly related to the latter type of application. Spectral effects may be quantified using two methods aimed at estimating a spectral mismatch (MM) factor [9]. The first is based on measurements of the outdoor short-circuit current of the PV device under investigation. The second is based on combining its spectral response with spectra recorded with spectroradiometers--although synthetic or modeled spectra may also be used. One hindrance in the first method is that spectral effects might be difficult to isolate the impacts of device temperature, soiling, or degradation and, in the case of flat-plate PV, angular losses. Hence, the second method is often preferred but requires knowledge of (i) the actual spectrum under the atmospheric conditions at any specific moment (in the case of outdoor exposure) and (ii) the actual spectral response of the PV device under test. The procedure also implies numerical integrations in a 2D domain (wavelength and time). This can become cumbersome if this needs to be repeated at high frequency and can be inexact if the spectral response of the PV device is not known precisely, which is often the case. To overcome the above difficulties, a simplifying approach--recourse to 1D (time only) spectral indexes that would be independent of the cell's spectral response--has been proposed in the literature. Indeed, such kinds of indexes are often preferred to the integrated useful fraction [10], the spectrally weighted ratio [11], the spectrally corrected global irradiance [12], or other metrics used in non-concentrating PV that depend on the specific device under scrutiny. The same preference holds for some other PV device-dependent indexes used in CPV characterization, such as the Z-parameter [13], the spectrally corrected direct normal irradiance [14], etc. A comprehensive review of spectral indexes has been recently presented [8]. In view of the above, finding a PV device-independent spectral index that could uniquely characterize individual spectra would be most useful for both PV science and engineering.