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To support refinement of the Maximum Permissible Exposure (MPE) safety limits, a series of experiments were
conducted in vivo on Dutch Belted rabbit corneas to determine corneal minimum visible lesion thresholds for 2.0 &mgr;m
continuous-wave laser irradiation. Single pulse radiant exposures were made at specified pulse durations of 0.1 sec, 0.25
sec, 0.5 sec, 1.0 sec, 2.0 sec and 4.0 seconds for spot 1/e2 diameters of 1.17 mm and 4.02 mm. Lesions were placed in rows without overlap on rabbit cornea. The effect of each irradiation was evaluated within one minute post exposure
and the final determination of lesion formation was made using a slit lamp one hour post exposure. Threshold lesions
were defined as the presence of a superficial surface whitening one hour after irradiation. Probit analysis was conducted
to estimate the dose for 50% probability (ED50) of laser-induced damage. Approximately 20 different radiant exposures
were made for each exposure duration-spot size combination. At the threshold level, the diameters of barely visible
opaque white lesions were smaller than the Gaussian 1/e2 beam diameter. In selected survival animals, most of the
threshold lesions were still visible 24 hours after exposure. The average lesion radius was approximately 0.4 ± 0.12 mm
diameter for the 1.17 mm spot size and 1.0 ± 0.20 mm diameter for the 4.02 mm spot size. The exposure duration
dependence of threshold average radiant exposure was described by an empirical power law equation: Threshold radiant
exposure[J/cm2] = a x exposure duration[s] b, experimentally derived coefficient a was 9.79 and b was 0.669 for the
1.17 mm spot diameter; values of a and b were 4.57 and 0.456 respectively for the 4.02 spot diameter. Based on the
experimental data and the empirical power law, the safety factors which were defined as threshold radiant exposure
divided by MPE values were predicted for the 2.0 &mgr;m wavelength at various exposure durations and spot diameters.
The minimum limit of the safety factor was approximately a factor of four for both 4.02 mm and 1.17 mm spot
diameters. Due to the very sharp boundary and small uncertainties of damage threshold determination, it is suggesting
that a factor of 4 "padding" is adequate and safety standard may not need to be changed.
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