Review of thresholds and recommendations for revised exposure limits for laser and optical radiation for thermally induced retinal injury

Abstract

Exposure limits (ELs) for laser and optical broadband radiation that are derived to protect the retina from adverse thermally-induced effects vary as a function of wavelength, exposure duration, and retinal irradiance diameter (spot size) expressed as the angular subtense α. A review of ex vivo injury threshold data shows that, in the ns regime, the microcavitation-induced damage mechanism results in retinal injury thresholds below thermal denaturation-induced thresholds. This appears to be the reason that the injury thresholds for retinal spot sizes of about 80 μm (α = 6 mrad) and pulse durations of about 5 ns in the green wavelength range are very close to current ELs, calling for a reduction of the EL in the ns regime. The ELs, expressed in terms of retinal radiant exposure or radiance dose, currently exhibit a 1/α dependence up to a retinal spot size of 100 mrad, referred to as αmax. For α ≥ αmax, the EL is a constant retinal radiant exposure (no α dependence) for any given exposure duration. Recent ex vivo, computer model, and non-human primate in vivo threshold data provide a more complete assessment of the retinal irradiance diameter dependence for a wide range of exposure durations. The transition of the 1/α dependence to a constant retinal radiant exposure (or constant radiance dose) is not a constant αmax but varies as a function of the exposure duration. The value of αmax of 100 mrad reflects the spot size dependence of the injury thresholds only for longer duration exposures. The injury threshold data suggest that αmax could increase as a function of the exposure duration, starting in the range of 5 mrad in the μs regime, which would increase the EL for pulsed exposure and extended sources by up to a factor of 20, while still assuring an appropriate reduction factor between the injury threshold and the exposure limit.