# Study on the Performance of Optical Lenses under High Fluxes of Solar Radiation
## 3 Results and Discussion
### Results from Vibrational Spectral Analyses of Lenses  
The UV-visible spectra measured on the lenses of silica glass and of borosilicate glass, after sustaining intense radiation are reported in Figure 10c,d, respectively. The silica glass lenses that were analyzed were DS151 (focal length f = \(-\)150 mm) and DS201 (focal length f = \(-\)200 mm). DS151 was previously irradiated during 60 min at the xenon arc lamp projector +60 min at the solar tower; and DS201 was only irradiated during 60 min at the xenon arc lamp projector. On the other hand, the borosilicate glass lenses that were analyzed were DB100 (focal length f = \(-\)100 mm) and DB250 (focal length f = \(-\)250 mm); both were previously irradiated during 60 min at the solar tower. The obtained UV-visible spectra (Figure 10c,d) were similar to the standard spectra for these types of glass (see Figure 10a [23] and Figure 10b [24]). This seems to indicate that the optical behavior of the lenses was not affected by the irradiation tests. For each pair of silica glass or borosilicate glass lenses, the obtained spectra for lenses of different focal length, with different thermal and radiation history, were almost matching and were only slightly different. The lenses that were thicker (i.e., silica glass lens with f = \(-\)150 mm, and borosilicate glass lens with f = \(-\)100 mm) presented higher absorbance compared to their thinner partner. It is known [25; 26] that the absorption in the ultraviolet region for oxide glasses is mainly determined by the O atoms, steadily increasing with the weakness of the bonds established by these atoms. Pure silica glass has only bridging O\({}^{2}\) - ions, strongly bonded, so absorption is very low above 200 nm. Modified glasses have increasing fractions of terminal O\({}^{-}\) oxygens, covalently bonded only to one Si\({}^{4+}\) cation, thus absorption increases substantially in the 200-300 nm region. Pure borate glasses, B\({}_{2}\)O\({}_{3}\), have also an excellent transmission in the ultraviolet, above 170 nm. The addition of small quantities (below 15%) of Na\({}_{2}\)O does not change absorption significantly because the O bonds are not affected (the B coordination number increases from 3 to 4, but each O remains bonded to two Si). For larger amounts of Na\({}_{2}\)O, some of the O atoms become terminal, bonded only to one Si, and the absorption increases sharply [25; 26]. However, if we compare the scale of absorbance in the graphs shown in Figure 10c,d, we can notice that, for the range of wavelengths up to \(\sim\)290 nm, the absorbance values of borosilicate glass are approximately 10 times higher than the corresponding ones measured in silica glass. In what concerns the region between 800 and 1000 nm where a series of intense peaks appear in the spectrum \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Lens Type**} & **Type of Support** & **Irradiation** & **Irradiation** & **Maximum** & **Comment** \\  & **and Mask** & **Power (W)** & **Time (min)** & **Temperature \({}^{1}\) (\({}^{o}\)C)** & **Comment** \\ \hline DB100 & A, 1 inch & 200 & 60 & \(\approx\)507 & No changes observed in the lens \\ DB250 & A, 1 inch & 253 & 60 & \(\approx\)577 & No changes observed in the lens \\ DS151 & A, 1 inch & 202 & 60 & \(\approx\)537 & No changes observed in the lens \\ CB100 & A, 2 inch & 314 & 60 & \(\approx\)241 & No changes observed in the lens \\ \hline \hline \end{tabular} \({}^{1}\) Measured by thermocouples located in the back of the lens, at shadow, but very close to the lens border.  
\end{table} 
Table 5: Summary of tests performed with concentrated natural sunlight. of the xenon arc lamp, the graphs depicted in Figure 10c,d indicate that for wavelengths above 800 nm pure silica glass as well as borosilicate glass are both quite transparent and, therefore, these glasses are not affected by those peaks.