## 4 Conclusions  
This study investigated whether the average photon energy (APE) uniquely represents the direct normal irradiance (DNI) spectrum over the 350-1050 nm band at Jaen, a Spanish sunny site with Mediterranean-Continental climate. No previous study focusing on the application of APE to the direct spectrum is known except at a Japanese location in a different climate. The methodology used here is based on a procedure that was previously applied to assess the possible bijectivity between APE and the global tilted irradiance (GTI) spectrum at both Jaen and Madrid. This procedure centers on the analysis of the coefficient of variation (CV) of the relative importance of contributions of 50-nm bands compared to the integrated DNI (from 350 to 1050 nm) of the spectral distributions binned in specific APE intervals. DNI spectra from 350 to 1050 nm were collected over the course of a 12-month experimental campaign. Over the range 450-900 nm, spectral distributions obtained for an APE of \(\approx\)1.85 eV and air mass (AM) of \(\approx\)1.5 match quite well that of the standard AM1.5D spectrum used as reference in concentrating photovoltaic applications. Nevertheless, some scatter is noticed outside of this central band. Likewise, spectral distributions corresponding to measurements binned in the same APE interval are well aligned within 450-900 nm, where CV remains below \(\approx\)3.5%. In contrast, this concordance does not hold outside of that spectral range, considering that CV reaches values of up to \(\approx\)10% and 13% within 350-450 nm and 900-1050 nm, respectively. This significant scatter at shorter wavelengths is only partially ascribed to the measurement uncertainty of the spectroradiometer reported by the manufacturer, which is relatively large below 450 nm. Theoretical considerations reveal that another explanation for the high CV there is related to the temporal variance in aerosol optical depth, which is significant at Jaen, and results in a more or less pronounced red shift in the spectrum. In contrast, the measuring uncertainty is far smaller beyond 900 nm, but steep temperature excursions during the day may influence measurements. Additionally, weak signal levels around the water vapor absorption band (940 nm) are especially vulnerable to noise, thus conducing an increased uncertainty related to the measurements in such band. Temporal variance in atmospheric precipitable water also induces a more or less significant blue shift of the spectrum. From a theoretical standpoint, the results presented here confirm previous radiative transfer simulations that rejected the APE bijectivity by showing how a single APE value could be obtained by combining different values of air mass, aerosol optical depth, and precipitable water, thus shaping significantly different DNI spectral distributions. To summarize, the results presented in this study disprove the currently assumed one-to-one relationship between APE and the spectral DNI within the _whole_ 350-1050 nm waveband. Rather, APE may only be assumed to uniquely characterize the DNI spectrum distribution _only_ from \(\approx\)450 to 900 nm for Jaen in practical terms. Hence, this limited band is recommended when using APE to model the spectral impact on concentrating PV devices. The present findings confirm those from a previous work that focused rather on the GTI spectrum, which is the reference for flat-plate PV applications. The present conclusion that bijectivity is valid over only a part of the DNI spectrum presumably holds for sites with a similar climate as Jaen, although further verification is needed. Hence, further collection and analysis of spectrally resolved DNI data are desirable at more sites worldwide to better evaluate the impact of local climate, since this type of experimental methodology has been done so far at only two sites in relatively temperate climates, which do not allow drawing general conclusions on a worldwide basis. Conceptualization, methodology, and writing--review and editing, G.N.