Safety Evaluation of Light Levels in Ophthalmic Instruments and Devices
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PURPOSE: There is an absence of information regarding safe levels of light exposure in present ophthalmic devices, and data on the highest continuous usage of these devices is unavailable. Studies that explored similar safety parameters were last conducted over 35 years ago using the American National Standard Institute (ANSI) Z136.1-1976 Standard. The maximum permissible exposure (MPE) identified in the Standard has been periodically updated based on new findings in studies examining light safety and damage thresholds in the eye. Furthermore, evolving imaging techniques have given rise to a number of new ophthalmic devices that did not exist previously, such as optical coherence tomography (OCT). The purposes of this research were to assess the light exposure levels of current ophthalmic devices and to provide a quantitative safety time limit for each instrument using the most recent ANSI Z136.1-2014 American National Standard for Safe Use of Lasers. METHODS: The spectral distribution, radiant power, pulse characteristic, and visual angle of the light emitted from 15 different ophthalmic devices at the University Eye Institute were measured using a spectroradiometer and a calibrated power meter. The examined instruments included optical biometers, corneal topographers, autorefractors, wavefront aberrometers, OCT instruments, fundus cameras, slit lamps, and binocular indirect ophthalmoscopes. A combination of a custom MATLAB program and an Excel Spreadsheet with Excel Visual Basic for Applications (VBA) was developed based on the guidelines written in the 2014 ANSI standard and used to analyze all measurements. Spectral weighting functions were applied following an adjustment guideline for using the ANSI Standards to estimate the weighted MPEs of ophthalmic devices with broadband sources. RESULTS: The ophthalmic instruments and devices evaluated were measured to be safe for typical, clinically relevant usage times. Instruments that were calculated to have the shortest exposure times before exceeding the MPEs were slit lamps when used for direct illumination (~100-400 seconds for continuous usage). Exposure times were much less hazardous when using a condensing lens with a slit lamp for indirect illumination of a larger retinal area. CONCLUSIONS: All examined instruments were calculated to be light safe for normal eyes using our methods. However, caution should be exercised when using slit lamps for direct illumination as exposure levels for these devices can most quickly reach the MPEs within achievable durations. In particular, caution should be exercised when practicing with and training in the use of these instruments, as longer exposure times are expected compared with a typical clinical examination. The reason that slit lamps with direct illumination more quickly reached the MPEs is the visual angle subtended by the source is much smaller. The reason that slit lamps and BIOs with LED sources more quickly reach the MPE limits could be due to the presence of large amounts of short wavelength light in their illumination spectrum. Based on these findings, as well as the uncertainty with which these standards apply to diseased eyes with retinas that are already compromised, it is suggested that manufacturers of ophthalmic instruments reduce the recommended exposure times for instruments that emit light with wavelengths between 400 and 600 nm, as well as increase the transparency of device specifications with regards to exposure parameters and safety.