While excimer lasers have been commercially available since the late 1970s, their main use until recently has been as powerful UV photon sources in research and development laboratories. In the R&D environment, large amounts of experimental data can be obtained with few laser pulses (thousands, tens of thousands, and, in rare cases, several millions). The durability of optical components used in these experiments has therefore not been a source of concern. UV-grade lenses, mirrors, and vacuum windows would be incorporated in the experimental setup with the implicit assumption that their optical properties would be invariable over time, as are the properties of visible-grade elements used with, for instance, frequency-doubled pulsed Nd:YAG lasers. Furthermore, not much thought was given to the details of the UV transmission of optical materials. As long as the vendor specified a nominal transmission at the appropriate wavelength (say, 98% excluding surface reflections), the material was frequently deemed adequate for its intended purpose. And indeed it often was.