Epithelial damage thresholds for multiple-pulse exposures to 80 ns pulses of CO₂ laser radiation

Corneal epithelial damage thresholds for exposures to 80 ns pulses of 10.6 μm infrared radiation produced by a CO₂-TEA laser were investigated. Thresholds were determined for sequences of 1, 2, 8, 32, 128, and 1,024 pulses at pulse repetition frequencies of 10 and 16 Hz. Threshold damage is correlated by an empirical power law of the form ED_th = CN^-α, in which ED_th is the threshold radiant exposure per pulse, and N is the number of pulses. The constants C and α are similar for the two pulse repetition frequencies. For the combined data set, C = 2,955 J m^-2 pulse^-1 (295.5 mJ cm^-2 pulse^-1) and α = 0.178. This value of the constant C is within 5% of the measured damage threshold for a single 80 ns pulse exposure. Temperature calculations reveal that the maximum temperature increase on the beam axis, 10 μm beneath the anterior tear surface, resulting from the different threshold exposuresis constant to within ±10% of the mean values. This result is consistent with a critical temperature damage model. Damage threshold measurements on cooled corneas indicate that the damage mechanism indeed has a substantial thermal component.