Corneal epithelial injury thresholds for multiple-pulse exposures to Tm:YAG laser radiation at 2.02 microm

Corneal epithelial damage thresholds for exposures to sequences of pulses of 2.02 microm infrared radiation produced by a Tm:YAG laser were investigated. Thresholds were determined for sequences of pulses at frequencies of 1, 10, 20, and 100 Hz. The duration of the individual pulses was 0.300 s at 1 Hz, 0.025 s at 10 and 20 Hz, and 0.005 s at 100 Hz. Threshold damage is correlated by an empirical power law of the form H(th) = CN(-alpha), in which H(th) is the threshold radiant exposure per pulse, and N is the number of pulses. The constant C differs depending on the pulse repetition frequency and individual pulse duration. The exponent alpha has values between 0.22 and 0.29. For some Tm:YAG exposures the empirical power law underestimates the damage threshold for small numbers of pulses. An empirical critical temperature model in which the critical damage temperature has a weak dependence on the duration of the single pulses or the entire train of pulses also correlates injury thresholds for both single and multiple pulses.