Retinal injury threshold for high repetition rate multiple-pulse laser exposures

Pulsed lasers operating in the nanosecond regime are used in a variety of applications, but safety data for the wide variety of laser parameters such as repetition rate, pulse length, and wavelength are limited. Knowledge of laser-induced thresholds at these varying parameters informs the establishment of maximum permissible exposure limits in the American National Standards Institute Z136.1 standard. In this study, the ED50 damage values for multiple-pulse retinal exposures to a Q-switched laser with 63 ns pulses at a wavelength of 1064 nm was measured in a Yucatan minipig (Sus scrofa domestica) model for a collimated beam incident on the cornea (estimated 1/e2 46 m spot size on the retina). Pulse trains from N = 100 to N = 20 000 pulses were investigated at a pulse-repetition frequency of 2 kHz (total exposure durations from 0.05 to 10 s). This is one of the few experiments to date examining the cumulative damage effects of multiple-pulse retinal exposures for a large number of pulses. This study demonstrated the ED50 decreased by approximately N−1/2 for exposures from N = 100 to N = 1000 pulses (0.05–0.5 s). Between N = 1000 and N = 20 000 pulses (0.5–10 s), the ED50 decreased as N−0.15. This latter trend, when expressed as average power vs. total exposure duration, replicates trends observed in thermal models of retinal damage for small retinal beam spot sizes. Although the safety standards indicate the N−1/4 rule (Rule 3) should provide more restrictive exposure limits, thermal effects are important at this higher pulse-repetition rate for exposures longer than a few hundred milliseconds (ms).