Competing Photochemical Retinal Damage Mechanisms From Visible Light: Implications for Human Retinal Exposure Limits

Abstract

Purpose: : To evaluate if recently published retinal injury thresholds (Morgan et al., 2008) make it necessary to revise current light safety guidelines for human retinal exposure to protect against photochemical injury.

Methods: : We reviewed known retinal injury thresholds from studies of mammalian retinae that provided reliable dosimetric information. Of particular value were those that attempted to determine action spectra or explored the potential photochemical damage mechanism. Threshold data from thermal injury were excluded.

Results: : Traditionally, the guidelines for limiting human exposure to bright light have been the retinal-thermal limits (400-1400 nm) and the blue light hazard (BLH) limits (380-550 nm). The BLH has a well-defined action spectrum peaking near 445 nm for the normal phakic eye. Two types of photochemically induced retinal injury have been well reported: Type 1 and Type 2. Type 1 is typically seen in animals exposed for many hours to fluorescent lamps (L_V ~ 1 cd•cm^-2, retinal irradiance approx 0.1 mW•cm^-2). Since the action spectrum indicates that injury results from saturation of the visual pigment regeneration cycle (i.e., from a full bleach), exposure limits were not needed. Type 2 results from focal exposures to short-wavelengths for fractions of an hour, but the chromophore is still debated. The luminous efficacy of radiation at 568 nm is 654 lm/W, near the max of 683 lm/W, which suggests the damage mechanism could be due to saturation of cone-opsin regeneration in the RPE. Retinal changes reported by Morgan et al (2008) at 568 nm were similar to those found in full-bleach, flash-blindness studies (retinal illuminance > 7 logTd-sec).

Conclusions: : Although previously developed retinal exposure limits are still relevant for the awake, task-oriented eye, this is not the case for ophthalmic instrument exposure--particularly to the anesthetized eye during surgery. Until the action spectrum for this kind of damage is determined, we propose that the retinal radiant exposure shall not exceed 5/V() J/cm², where V() is the CIE photopic sensitivity function.

Keywords: radiation damage: light/UV • laser • retina