# Study on the Performance of Optical Lenses under High Fluxes of Solar Radiation
## 4 Conclusions  
In this work we have examined the performance of optical lenses made of silica glass or of borosilicate glass (BK-7) when submitted to high-flux radiation emitted by a xenon arc lamp or provided by a high-concentration solar tower. All lenses made of silica glass perfectly withstood the tests, even when the power applied on the surface of a 50.8 mm diameter lens was of the order of 852 W and a temperature of 342 \({}^{\circ}\)C (measured by thermocouples located in the back of the lens, at shadow, but very close to the lens border) was registered. Concerning the lenses made of borosilicate glass, all of them fractured, sooner or later, when tested with the xenon arc lamp projector; but they resisted unaffected when tested with concentrated natural sunlight. We concluded that the borosilicate (BK-7) lenses collapsed due to an internal phenomenon (occurring at the glass material) and not due to mechanical efforts induced by the support. There is a discrepancy in the wavelength region between 250 and 290 nm when we compared the spectrum of natural solar radiation at sea level with the unfiltered spectrum of the xenon arc lamp. The solar radiation at sea level shows wavelengths starting at only around 290 nm at the ultraviolet region; but the unfiltered spectrum of the xenon arc lamp shows wavelengths already starting from circa 250 nm. The collapse of the borosilicate lenses is attributed to the radiation within this interval between 250 and 290 nm, in the UV region. Absorption spectroscopy analyses in the UV-visible light wavelength range and also Raman spectroscopy analyses were conducted on lenses that were previously irradiated and corroborate the conclusion  
Figure 11: Raman spectra: (**a**) standard spectra for silica glass, measured with different excitation wavelength; (**b**) standard spectra for a borosilicate glass; (**c**) obtained Raman spectra in silica glass lenses of different focal length; (**d**) obtained Raman spectra in borosilicate glass lenses of different focal length.  
that both types of glass, either silica glass or borosilicate glass, seem not to be affected by high-flux natural solar radiation at sea level (i.e., after passing through the Earth's atmosphere).  
L.G.R.: methodology, data curation, validation, project administration and funding acquisition, writing--review and editing. J.C.G.P.: methodology, data curation, validation, original draft preparation. K.R.: data curation, original draft preparation. G.D.A.: selection of optical lenses, formal analysis, optical and mechanical diagrams. L.F.S.: Vibrational spectral analyses of lenses. All authors have read and agreed to the published version of the manuscript.  
The authors were beneficiaries of the Solar Facilities for the European Research Area (SFERA) Programme (SFERA-III Project Grant Agreement no. 823802) which allowed them to carry out a test campaign at the IMDEA Energy Institute. This research was partially funded by the Portuguese Foundation for Science and Technology (FCT), through IDMEC, under LAETA, project UIDB/50022/2020.  
We thank the IMDEA Energy Institute (Mostoles, Spain) for providing access to its installations, the support of its scientific and technical staff, namely, [PERSON], [PERSON], and [PERSON], as well as the financial support of the SFERA-III project (Grant Agreement No 823802). Special thanks are also due to [PERSON] (Marinha Grande, Portugal) for his help in the acquisition process of the tested lenses, and construction of metallic holders.  
The authors declare no conflict of interest.