# Study on the Performance of Optical Lenses under High Fluxes of Solar Radiation
## 3 Results and Discussion  
### Results from Tests with Xenon Arc Lamp  
The results of irradiation tests performed with the xenon arc lamp are summarized in Table 4.  
It should be emphasized that the lenses were tested using different types of support in order to detect possible problems caused by the behavior of the support along the irradiation test. The type of support is mentioned in the 2 nd column of Table 4: the letter \"M\" means that the lens was carried on a metallic holder (made of brass); the letter \"A\" means that the lens was simply surrounded by an alumina foam. As previously mentioned, details of the experimental setup of the irradiation tests performed with a xenon arc lamp are presented in Figure 5. To confine the radiation beam, making it uniquely incident in a  
Figure 8: Details of experimental setup of the irradiation tests using the Very High Concentration Solar Tower (VHCST). (**a**) Heliostat field; (**b**) central tower; (**c**) image provided by infrared camera software, showing the temperature distribution on the target (made of alumina foam) at the top of the tower; (**d**) photo showing the test setup used for a lens (with 76.2 mm diameter) simply surrounded by alumina foam and aspect of the Gardon-type radiometer; (**e**) photo taken during an irradiation test on a lens (with 50.8 mm diameter) simply surrounded by alumina foam and aspect of the Gardon-type radiometer below the lens. central zone of the lens (thus restricting as much as possible the unnecessary heating of the supports), we have used a mask (or screen) made of alumina felt with a circular aperture of 1 inch (25.4 mm) diameter (as depicted in Figure 5c). The converging lenses (plano-convex) that were tested in this work are larger (diameter = 76.2 mm) than the diverging (biconcave) lenses (diameter = 50.8 mm) and, as the radiation beam generated by the xenon arc lamp can be confined to the central zone of a lens with 76.2 mm in diameter, all tests conducted on converging lenses were undertaken without a mask. As revealed in Table 4, no changes were observed in the lenses made of silica glass, even in those situations where hypothetically the risk was higher, such as the case of a diverging (biconcave) lens inserted on a holder made of brass, heated up to \(\approx\)340 \({}^{\circ}\)C, when the lens was irradiated during 60 min with an estimated power of 852 W.  
The only problems that were detected during the testing campaign using a xenon arc lamp were assigned to the lenses made of borosilicate glass, because they fractured. In our opinion, the borosilicate (BK-7) lenses collapsed due to an internal vibrational phenomenon, occurring in the glass material, and not due to mechanical efforts induced by the support or by thermal shock. In all our experiments with the lenses, there was no thermal shock because the radiation was not applied suddenly. The thermal lensing effect [20] induced by temperature gradients may also occur in our experiments, but the fracture of the lenses must be attributed to internal mechanical stresses. As can be seen in Table 4, even the CB100 lens that was simply surrounded by an alumina foam was fractured after being exposed to the radiation generated by the xenon arc lamp. It should be noted that it is common at the laboratories that make use of high-flux solar radiation to employ reaction chambers (spherical or tubular) made of borosilicate glass, so it was surprising to observe the premature fracture of the lenses made with this type of glass. The origin of the fracture can only be explained by differences between the solar radiation and the radiation emitted by a xenon arc lamp. Figure 9a depicts the solar radiation spectrum before entering the Earth's atmosphere (in yellow color) and the solar radiation spectrum at sea level (in red color) [21]. After absorption by the atmosphere, the solar radiation shows wavelengths starting only at 290 nm, at the ultraviolet region.