Risk assessment of optically aided viewing

Abstract

The definition and measurement of Class 1 and Class 3A accessible emission limits relates very much to one simple question: “How much energy will be collected by magnifying optics?” To simulate the increased risk of optically aided viewing by telescopes and binoculars, a 50 mm aperture (at one time 80 mm) has been used in ANSI, CDRH, and IEC standards. The current measurement criterion found in IEC 60825-1 was a compromise, and makes use of a 50 mm aperture at a distance of 100 mm to define a geometrical collection efficiency to simulate not only that of telescopes and binoculars, but also that of highly diverging laser and light emitting diode emission patterns as well. It was later recognized that this was a rather arbitrary mathematical construct designed to mimic both a high-power eye loupe at a very close distance (14 mm) and a 7×50 binocular at a much greater viewing distance. This approach is not followed in the U.S. standards, and the two types of sources are treated differently. While the IEC approach appears at a first glance to have the advantage of simplicity and requires little laboratory space for measurements, it fails to accurately describe the real hazard for both conditions of optically aided viewing. Hand-held magnifiers and eye loupes increase the retinal or corneal hazard by effectively collimating the divergent beam and directing more energy into the true limiting aperture: the eye’s pupil (simulated by the 7 mm aperture for dark-adapted viewing). By contrast, telescopes and binoculars which can produce a sharp image only at distances beyond at least 2 m, collect more radiant energy. A better simulation of risk is afforded by a 50 mm aperture at 2 m and a 7 mm aperture at about 10 cm.