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The dependence of retinal damage threshold on laser spot size was examined for two pulse width regimes; nanosecond-duration Q-switched pulses from a doubled Nd:Yttrium–aluminum–garnet laser and microsecond-duration pulses from a flashlamp-pumped dye laser. Threshold determinations were conducted for nominal retinal image sizes ranging from 1.5 to 100 mrad of visual field, corresponding to image diameters of ∼22 μm to 1.4 mm on the primate retina. In addition, base line collimated-beam damage thresholds were determined for comparison to the extended source data. Together, this set of retinal damage thresholds reveals the functional dependence of threshold on spot size. The threshold dose was found to vary with the area of the image for larger image sizes. This experimentally determined trend was shown to agree with the predictions of thermal model calculations of laser-induced retinal damage for spot sizes ≳150 μm. The results are compared to previously published extended source damage thresholds and to the ANSI Z136.1 laser safety standard maximum permissible exposure levels for diffuse reflections.

We describe a new model for laser-induced retinal damage. Our treatment is prompted by the failure of the traditional approach to accurately describe the image size dependence of laser-induced retinal injuries and by a recently reported study which demonstrated that laser injuries to the retina might not appear for up to 48 h post exposure. We propose that at threshold a short-duration, laser-induced, temperature rise melts the membrane of the melanosomes found in the pigmented retinal epithelial cells. This results in the generation of free radicals which initiate a slow chain reaction. If more than a critical number of radicals are generated then cell death may occur at a time much later than the return of the retina to body temperature. We show that the equations consequent upon this mechanism result in a good fit to the recent image size data although more detailed experimental data for rate constants of elementary reactions is still required. This paper contributes to the current understanding of damage mechanisms in the retina and may facilitate the development of new treatments to mitigate laser injuries to the eye. The work will also help minimize the need for further animal experimentation to set laser eye safety standards.

In earlier studies, we examined the dependence of the laser induced retinal damage threshold on retinal image size for extended-source ocular exposures. We reported the spot-size dependence of the retinal threshold (based on ophthalmic observations at 24 h postexposure) for two pulsewidth regimes: nanosecond-duration (Q-switched) pulses from a doubled Nd:yttrium–aluminum–garnet laser and microsecond-duration pulses from a flashlamp-pumped dye laser at 590 nm. In either case, the retinal threshold was shown to vary with the area (i.e., diameter squared) for image diameters >5 mrad. More recently, we have collected additional data for the intermediate spot-size range (1.5–10 mrad) and have compared both the absolute values and the spot-size trend of retinal thresholds determined via ophthalmoscopic observation at 1 h postexposure to the analogous threshold data collected with observations at 24 h postexposure. These additional data and analyses reinforce our earlier conclusions regarding the threshold vs. spot-size trend and are compared to and reconciled with results from previously published extended-source ocular damage studies. The experimental spot-size trends are also contrasted to the existing laser safety standard treatments of maximum permissible exposure levels for extended-source ocular exposures.