Laser-induced retinal injury thresholds: Variation with retinal irradiated area.

The retinal injury threshold for exposure to a laser source varies as a function of the irradiated area on the retina. Currently accepted guidelines for the safe use of lasers provide that the MPE will increase as the diameter of the irradiated area for retinal diameters between 25 ¥ìm and 1700 ¥ìm, based on the ED50 data available in the late 1970s. Recent studies by Zuclich and Lund produced data showing that the ED50 for ns-duration exposures at 532 nm and ¥ìs duration exposures at 590 nm varied as the square of the diameter of the irradiated area on the retina. This paper will discuss efforts to resolve the disagreement between the new data and the earlier data though an analysis of all accessible data relating the retinal injury threshold to the diameter of the incident beam on the retina and through simulations using computer models of laser-induced injury. The results show that the retinal radiant exposure required to produce retinal injury is a function of both exposure duration and retinal irradiance diameter and that the current guidelines for irradiance diameter dependence do not accurately reflect the variation of the threshold data.