## 3 Results and Discussion  
The recorded spectral instances that are outside the eight APE intervals stated in step 4 and removed according to step 5 of Section 2.2 account for less than 17% of the whole 12-month experimental database. After filtration, 65,881 spectral instances remained. It is worth investigating to what extent the local spectral distribution for values of APE close to its standard value (1.846 eV) align with that of the standard spectrum when AM values are close to 1.5. Figure 2 shows the mean percentage contributions of every 50-nm band considered in this work relative to the integrated DNI for measured spectra in which APE and AM are within 1.84-1.86 eV and 1.45-1.55, respectively. The values of \(R_{c}\) for each of the fourteen bands are shown for the reference spectrum as well. The latter seems to coincide reasonably well with local AM1.5D distributions. The fact that spectral DNI measurements with APE values in the vicinity of 1.85 eV closely resemble the distribution of the reference spectrum is according to expectations and confirms what was already noted by [PERSON] et al. [34] based on experimental measurements at Kusatsu, Japan. Nevertheless, a careful analysis shows that, despite being apparently small, the SD values obtained at both Jaen and Kusatsu are not negligible when compared to the \(<\)\(R_{c}\)\(>\) values across the 50-nm band intervals used here--or 10-nm bands for the Japanese site--that lie outside the central wavelength range (450-900 nm). This dispersion of percentage contributions within the 350-450 nm and 900-1050 nm wavebands was also noted in local GTI spectral distributions for APE and AM around 1.88 eV and 1.5, respectively(corresponding to the AM1.5G spectrum) for Jaen[32]. These two results are mutually consistent because the AM1.5G spectrum is essentially composed of direct irradiance (see, e.g., Figure 10 of [45]). The AM values corresponding to the spectral measurements recorded over the course of the 12-month experimental campaign spread over a large interval, 1.03-16.00; the calculated average AM is 2.19 and, hence, significantly above the standard 1.5 value. In parallel, the broadband DNI values obtained from the pyrheliometer range from 50 to 1047 W\(\cdot\)m\({}^{-2}\) and average 715 W\(\cdot\)m\({}^{-2}\), far lower than the 900 W\(\cdot\)m\({}^{-2}\) standard value. This could be expected because DNI decreases rapidly when AM increases or when thin clouds partly obscure the sun's disc. Figure 3 shows the values of \(<\)\(R_{c}\)\(>\) for every 50-nm band according to each APE bin into which the spectral measurements have been grouped. Higher values of APE imply shorter values of \(\lambda_{\mathrm{eff}}\) and, thus, higher relative contributions of shorter wavelengths relative to the integrated DNI within 50-nm bands. Conversely, values of \(<\)\(R_{c}\)\(>\) corresponding to 50-nm bands with longer wavelengths are enhanced as APE decreases (and \(\lambda_{\mathrm{eff}}\) increases). Interestingly, note that \(<\)\(R_{c}\)\(>\) tends to remain constant in the 600-650-nm spectral band, possibly because it corresponds to the \"balance\" wavelength for the 350-1050-nm spectrum, close to the effective wavelength of the AM1.5D standard spectrum. It is also worth noting that the results shown in Figure 3 are quite well aligned with those obtained by [PERSON] et al. [34] in their investigation of the purported one-to-one relationship between APE and the DNI spectrum in Kusatsu (see their Figure 5a). Figure 2: Mean percentage contributions to direct normal irradiance (DNI)—integrated from 350 to 1050 nm—across all 50-nm bands for spectral measurements with Average Photon Energy (APE) and air mass (AM) values of 1.85 \(\pm\) 0.01 eV and 1.5 \(\pm\) 0.05, respectively, recorded at Jaén (blue line): error bars indicate the standard deviation (SD) values related to each of these bands. The “reference” \(R_{c}\) values calculated for each of these bands from the standard AM1.5D spectrum are also shown (green line).