Multiple-shot damage thresholds of dielectric reflectors have been measured at 248 and 308 nm. Standard irradiation conditions were a 10-ns pulsewidth, 0.6-mm spot diameter and 35-Hz pulse repetition frequency. The reflectors, from various sources, were composed of oxide and fluoride films. Although damage was generally initiated at visible film defects, there was no correlation between damage susceptibility and the appearance of these defects. At levels near threshold, damage was most often observed as an increase in white-light scatter of a site with no growth upon continued irradiation; at higher levels, the damage site grew with successive shots. Test sites were subjected to at least 10/sup 3/ shots and some sites received as many as 2.5 x 10/sup 4/ shots; however, with only one exception damage was found to occur within the first few shots or not at all. Reflectors at 248 nm typically had damage thresholds in the 1.0 to 1.8 J/cm/sup 2/ range with two samples exhibiting unexpectedly high thresholds of 2.8 and 3.0 J/cm/sup 2/. In some cases, a subthreshold pre-irradiation treatment resulted in a 20 to 25% enhancement in damage resistance
