Epithelial damage thresholds for sequences of 80 ns pulses of 10.6 μm laser radiation

Infrared radiation from a CO₂ laser operating at 10.6 μm is almost totally absorbed by the tear film and corneal epithelium. Epithelial damage thresholds for single and multiple pulse exposures have been well characterized for exposures having pulse durations ≥ 1 ms and a few damage thresholds have been reported for much shorter duration pulses. However until recently, no studies have been done to determine damage thresholds from sequences of very short pulses. The purpose of this study is to begin to fill this gap. New Zealand white rabbit corneas were exposed to sequences of 80 ns pulses from a CO₂-TEA laser operating in the TEM₀₀ mode (Gaussian beam profile). The damage threshold was defined as the energy density, ED_th, that produced minimal superficial epithelial lesions. Thresholds have been determined for sequences of 2, 8, 32, 128, and 1024 pulses at a rate of 10 Hz. The damage data are shown to be correlated by empirical laws relating either the threshold energy density per pulse to the number of pulses, or the total threshold energy density to the duration of the pulse train. Research is underway to obtain data for pulse sequences at greater pulse rates and to examine the relative importance of thermal and acoustic damage mechanisms.