Abstract:
The projected beam radiance of several common ophthalmologic instruments was measured, and potential hazard to the patient from light exposure was analyzed with reference to safety standards for coherent light. The indirect ophthalmoscopes tested appear to be "safe" under moderate voltage settings, provided exposure is reasonably brief Slit-lamp biomicroscopy of the fundus, however, merits caution. It produces a three-times-higher retinal irradiance than the indirect ophthalmoscope. Overhead surgical lamps produce a retinal irradiance about one-third that of the indirect ophthalmoscope (for clear media and dilated pupil). This could, be dangerous, since an operation may take long enough to exceed the maximal permissible exposure by several orders of magnitude. Major design changes are indicated for surgical illuminators to extend the "safe time" to the 40 to 60 min range.
Keywords: phototoxicity, light damage, laser safety, ophthalmoscopes, slit lamps, surgical illuminators, retinal burns, retinal irradiance, rhesus monkey, radiance
From the Wilmer Ophthalmological Institute, The Johns Hopkins University Hospital, Baltimore, Md. and the Applied Physics Laboratory, The Johns Hopkins University, Laurel, Md.
Introduction
When the ophthalmoscope was first introduced over 130 years ago, some doctors thought it might be dangerous to admit "naked" light into diseased eyes.[1] Even today, this objection has not been settled. The purposes of this study are to quantitate retinal light exposure from several ophthalmologic instruments and to make estimates of their relative safety. With the advent of the laser in the early 1960s, hundreds of researchers have tried to define retinal damage thresholds to laser light, using monkeys as the principal model.
The American National Standards Institute (ANSI) guidelines for the safe use of lasers conveniently compiled much of this work. Work is just beginning on white light (non-coherent) safety standards. The laser guidelines, however, correlate closely enough with white light studies of retinal damage thresholds so that first approximation comparisons seem justified.
We describe a method of measuring the beam radiance of ophthalmologic instruments and, from this, a method of calculating the retinal irradiance. Our results for the indirect ophthalmoscope agree quite closely with estimates made by an entirely different method.[3] We then make use of the ANSI laser guidelines to specify the time required to reach the presumed safety limit (the maxi- mum permissible exposure, MPE). An example of noncoherent white light retinal changes is also shown to substantiate predicted hazards.
Conclusions
Slit-lamp biomicroscopy produces over three times greater Hret than the indirect ophthalmoscope. This is cause for concern because one does not usually perform such an examination on a macula unless the macula is diseased. Yet a diseased macula is undoubtedly more susceptible to light damage than is a healthy one.
Overhead surgical lights probably present a greater hazard than ophthalmoscopes or slit lamps. Whereas the latter instruments are used for brief intervals for diagnostic purposes, the surgical lamps may expose the retina for prolonged periods of time. Exposure for various types of surgery could stretch from 5 to 45 min or more. Yet (for a dilated pupil) only about 2 minutes would be "safe."
For a 3 mm pupil, one has about 11 min of "safe time." The large, smooth reflector lamps illuminate an area 5 disc diameters in size. Therefore some areas of retina may be continuously exposed even though the eye is constantly moving. Aphakia or other extreme refractive errors do not protect the patient (for smooth reflector lamps) because the source is so large. For ribbed reflectors, however, the concentric rings may be blurred enough to provide some protection.
One would not consider aiming a direct ophthalmoscope steadily at the macula for 15 min. Yet surgical lamps produce two thirds of the Hret of direct ophthalmoscopes (dilated pupils) and frequently for longer intervals than this.
We urge manufacturers to design safe surgical illuminators, hopefully with a "safe time" of 45 to 60 min. Subtracting the especially hazardous portions of the blue spectrum and the unnecessary infrared spectrum will be a great help. Diagnostic instruments could hopefully be filtered as well. The primary impetus should come from the ophthalmologist in restricting nonessential light exposure, especially to the macula.
Keywords: phototoxicity, light damage, laser safety, ophthalmoscopes, slit lamps, surgical illuminators, retinal burns, retinal irradiance, rhesus monkey, radiance
From the Wilmer Ophthalmological Institute, The Johns Hopkins University Hospital, Baltimore, Md. and the Applied Physics Laboratory, The Johns Hopkins University, Laurel, Md.
Supported by NIH grant EY-01312. Submitted for publication May 25, 1979. Reprint requests: Joseph L. Calkins, M.D. The Johns Hopkins Hospital 601 N. Broadway, Baltimore, Md. USA 21205
0146-0404/80/091009+07$00.70/0 © 1980 Assoc. for Res. in Vis. and Ophthal., Inc. 1009 1010 Calkins and Hochheimer, Invest. Ophthalmol. Vis. Sci. September 1980