The effect of ocular aberrations on retinal laser damage thresholds in the human eye

For the dark-adapted human eye the diffraction limited retinal image is approximately 2.8 microm in diameter for green light, although the estimation of the size of the retinal image resulting from the incidence of a collimated beam on the cornea is problematical and has been estimated to be anywhere from 10 to 30 microm. The resolution of this difference is important for the accurate determination of the retinal hazards of optical sources and for setting safety limits for laser-retinal exposure. Using literature results for the aberrations measured in a population of healthy young adults, beam propagation calculations of retinal images are presented for different pupil diameters. Using the concept of a generalized Strehl ratio, retinal damage thresholds, EDx, are derived for exposures in the thermal confinement regime (exposure times approximately less than 10 micros). The most vulnerable eyes are predicted to be those with pupil sizes 2-3 mm such as would be found under daylight illumination. The results also suggest that populations with particularly small ocular aberrations and correspondingly high visual acuity may be significantly more vulnerable than a "normal" population.