Working with systems in the UV is extremely challenging and demanding. In the thermal infrared, specifically the MWIR and the LWIR spectral bands, we found that the wavelength was 8 and 20 times the visible wavelength, respectively, which in some ways made IR lens systems more forgiving. In the UV we find that we now have a wavelength about one-half that of a visible system. In addition to the Airy disk expressed in micrometers being about one-half the f/number, we also find a limited number of viable optical materials are available. These include fused silica, several of the fluorides (barium fluoride, calcium fluoride, and lithium fluoride), UBK7 glass, and sapphire. The index of refraction of these materials is generally not very high. Many of these materials (especially the fluorides) are very difficult to work with and have other problems such as being hygroscopic. This leads to extreme care in manufacturing and assembly, and you may need to nitrogen purge your system to prevent moisture damage. Even sodium chloride can be used in the UV, but we recommend that you "take it with a grain of salt."

Figure 12.33 shows two deep UV lens systems, the first of which is a relatively wide-angle lens using calcium fluoride and Ultran 30 materials (the latter is no longer available). Note the relatively steep radii which is due to the inherently lower refractive indices of the materials. The second lens shown is a wafer stepper lens from a...