Corneal damage thresholds from sequences of short pulses of CO2 laser radiation

Purpose. Many laser systems emit sequences of pulses. The Army Medical Research and Material Command (USAMRMC) has a mission to assess the health effects and hazards of nonionizing electromagnetic radiation from such laser syslems for developing injury prevention criteria. There is a paucity of corneal damage threshold data for sequences of very short pulses of highly absorbed radiation. The purpose of this study is to begin to fill this gap. Methods. New Zealand white rabbit corneas were exposed to sequences of 80 ns pulses from a CC2-TEA laser operating in the TEMoo mode (Gaussian beam profile). The 1/e beam diameter was determined at each session with a linear pyroelectric array. The damage threshold was determined by exposing the cornea at energy densities, ED, above and below the amount needed to produce minimal superficial epithelial lesions. The bracket was narrowed until the £D's differed by only ~10£. The damage threshold ,EDth, is taken to be at the center of the bracket. Temperature calculations were made as described previously.1 Results. Thresholds were determined for sequences of 2 pulses at rates of 5 and 10 Hz which had ED± values of 275 mJ/cm2/pulse and 235 mJ/cm2/pulse respectively. These exposures each produced a maximum temperature increase on the beam axis 10 [im below the anterior tear surface of 26.2 °C. The single-pulse ED\ obtained previously' was 360 mJ/cm2 with a temperature increase of 35.5 C. Conclusions. The temperature increases associated with these thresholds are lower than those for pulses with durations >lms where the damage is described by a purely thermal model.' These results suggest that acoustic damage may also play a role. Research is underway to obtain data for sequences with larger numbers of pulses and to examine the importance of these damage mechanisms. 1. R.A. Farrell, C.B. Bargeron, R.L. McCally and W.R. Green, in Lauer Safetv. Eyesafe Laser Systems, and Laser Eye Protection, SPIE 1207, 59-70 (1990).