## 3 Results and Discussion
Moreover, the results depicted in Figure 3 appear to be a good match for those obtained by [PERSON] et al. [19] (also for Kusatsu), [PERSON] et al. [31] (for Ispra and Golden, Colorado), [PERSON] et al. [32] (for Jaen and Madrid), and [PERSON] et al. [33] for three Japanese cities: Kusatsu, Tsukuba, and Miyazaki. It should be kept in mind, though, that the work presented here is not strictly comparable with these studies, as they were carried out to elucidate the possible bijectivity between APE and either the GTI or GHI spectral distribution. Table 1 provides details about the distribution of the spectral measurements binned by their corresponding APE interval. For each of these APE intervals, Table 1 also indicates the corresponding effective wavelength, in conjunction with the maximum, minimum, and mean SD values related to the relative contribution to the integrated 350-1050-nm DNI across the fourteen 50-nm bands under scrutiny. Note that only three APE bands (1.79 \(\pm\) 0.01 eV, 1.81 \(\pm\) 0.01 eV, and 1.83 \(\pm\) 0.01 eV) account for 80% of the spectral instances recorded. Moreover, only less than 1% of the spectral measurements are grouped in bins with APE greater than 1.85 eV, which indicates prevailing red-shifted spectra for the site under investigation. As expected from fundamentals of statistics, the scatter around the mean is larger in APE intervals with fewer samples. In spite of this, the maximum values of SD stay below 0.40%. In comparison, the SD values obtained by [PERSON] et al. [34] at Kusatsu across all the seventy-one 10-nm bands within the 350-1050 nm spectral range do not exceed 0.20%. Remarkably, SD peaks in the same wavebands (350-400 nm and 900-950 nm) at Kusatsu and Jaen, which deserves scrutiny. As discussed before [32], SD alone is not enough to suitably assess the dispersion of \(R_{c}\) around its mean for each APE interval across the 350-1050 nm range. Instead, CV is a more meaningful parameter intended for such purpose. Figure 4 shows the CV values obtained for each 50-nm band grouping defined above. Within the 450-900-nm waveband, CV stays below \(\approx\)3.5% in contrast with the 900-1050-nm waveband, where CV is found to reach much larger values, close to 10%. Higher spikes even occur in the 350-450-nm waveband, where CV reaches up to 13%. Figure 3: Mean values of relative contributions to DNI—integrated from 350 to 1050 nm—across all 50-nm bands for spectral measurements binned in 0.02-eV-width APE intervals over the range 1.75–1.89 eV (central values), using direct irradiance spectra recorded at Jaén. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Central Value of** & \begin{tabular}{c} \(A_{\text{eff}}\) **for the Central** \\ **Value of APE** \\ **Interval (nm)** \\ \end{tabular} & **SD\({}_{\text{max}}\) (\%)** & **SD\({}_{\text{min}}\) (\%)** & **SD\({}_{\text{med}}\) (\%)** & **Number of Samples** & **\
\begin{tabular}{c} **Waveband where** \\ **SD\({}_{\text{max}}\) Occurs (nm)** \\ \end{tabular} & **
\begin{tabular}{c} **Waveband where** \\ **SD\({}_{\text{min}}\) Occurs (nm)** \\ \end{tabular}** \\ \hline
1.75 & 709 & 0.39 & 0.09 & 0.21 & 4123 & 900-950 & 800-850 \\
1.77 & 701 & 0.38 & 0.09 & 0.20 & 7195 & 900-950 & 800-850 \\
1.79 & 693 & 0.36 & 0.08 & 0.19 & 13,962 & 900-950 & 800-850 \\
1.81 & 685 & 0.26 & 0.08 & 0.15 & 21,116 & 900-950 & 800-850 \\
1.83 & 678 & 0.21 & 0.06 & 0.12 & 17,727 & 900-950 & 800-850 \\
1.85 & 670 & 0.25 & 0.07 & 0.17 & 1317 & 350-400 & 800-850 \\
1.87 & 663 & 0.35 & 0.07 & 0.19 & 338 & 350-400 & 800-850 \\
1.89 & 656 & 0.31 & 0.09 & 0.19 & 103 & 350-400 & 800-850 \\ \hline \end{tabular}
\end{table} 
Table 1: Maximum, minimum, and mean values of SD of \(R_{c}\) for every 50-nm band according to their APE interval and corresponding effective wavelength, \(\lambda_{\text{eff}}\). In practical terms, such results suggest that APE can be assumed to _uniquely_ represent the shape of the direct spectrum within the central 450-900-nm wavelength range _only_.