VALIDATION OF ICNIRP ESTIMATES OF TOXICITY THRESHOLDS FOR NIR (785 NM) LIGHT IN THE RETINAS OF PIGMENTED RABBITS

Abstract

The International Commission on Non-Ionizing Radiation Protection (ICNIRP) established eye-safe exposure limits for laser light based on estimates of the 50% exposure dose (ED50) for retinal damage in animals. The ED50 has not been determined for continuous-wave near-infrared (NIR) light, as may be used in instruments for spectroscopic analysis of the eye. The purpose of this study is to determine the ED50 of 785-nm light delivered by a Raman spectroscopy instrument on the retinas of pigmented rabbits, and to compare that ED50 to the ICNIRP estimates. Light of varying power was focused into the anterior chambers of rabbits for 4, 20, or 300 s. Estimates of spot size on the retina were determined by modifying the schematic rabbit eye with actual axial length and refractive error measurements. Masked examiners determined retinal burns from fundus photographs and fluorescein angiograms made immediately after exposure and again 48-72 h later. Thirty-five of 86 exposed eyes had retinal burns. In 12 of these the burn was not visible until 48-72 h. In 6 the burn was visible on color photographs, but not on fluorescein angiography. The ED50 retinal power density from probit analysis was 14,513 mW cm(-2), 10,369 mW cm(-2), and 5,237 mW cm(-2) for 4-s, 20-s, and 300-s exposures, respectively. Thus, the ED50 for retinal burns from 785-nm light is approximately 7 and 7.5 times the ICNIRP permitted limits for 4- and 20-s exposures, and 5.6 times the limit for 300-s exposures. This is consistent with the ICNIRP intention to set power limits several times less than ED50.