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The concept of the “virtual age” forms a useful means of comparison of different IOLs with regard to short‐wavelength spectral properties
In this issue, van Norren and van de Kraats1 propose a novel means for comparisons of different IOLs with regard to short‐wavelength spectral properties. (see pages 1374–5) They have developed the concept of defining an equivalent age for comparing the spectral properties of any IOL with those properties of the ageing natural crystalline lens, which they term a “virtual age.” Recognising the need to balance short‐wavelength protection with visual performance in the blue‐violet, they have defined two “virtual ages,” one for “photoprotection” and one for “photoreception.” Of course, the reference age is based upon properties of northern European lenses, but the concept allows the ophthalmologist and patient an understandable means for comparison.
Arguments will remain as to the best spectral transmission curve—the best choice of spectral age—but the concept allows for a quick comparison in terms of degree of protection against some types of light‐induced retinal damage. The debate for optimum spectral filtration will continue, because there is no easy answer, and some will still question the hypothesis that chronic light exposure accelerates the ageing of the retina and plays any aetiological role in retinal disease. Others will argue for a greater virtual age for photoprotection because of the impaired cellular and molecular repair mechanisms with increasing age, but it is well to remember that the eye appears remarkably well designed by evolution to minimise the risk of excessive retinal exposure to sunlight. In open sunlight, the pupil readily constricts to 2 mm and even smaller, and the upper lid moves down as an automatic “awning” as ambient light levels increase. Both pupil constriction and upper‐lid movement can be mathematically expressed as a function of scene luminance (brightness). Since the degree of response for each individual varies, two persons looking at the same sky may have a difference in retinal exposure varying by a factor of twofold, thus making epidemiological studies of sunlight and age‐related macular diseases very difficult to design. Since more than one type of acute retinal light toxicity exists, and each has a different action spectrum, it is well to remember that the virtual age for protection is most specific for acute photic maculopathy—that seen when one stares at the sun or welding arc, but as Richard Young emphasised, the shorter the wavelength, the greater the photon energy and the greater the number of molecules potentially sensitive to damage.
To a large extent, the atmosphere already performs remarkably well in minimising the risk of an acute “blue‐light” maculopathy from staring at the sun, since ultraviolet and blue light are selectively scattered out of the direct beam of sunlight, thus producing the diffuse, blue sky. When the sun is directly overhead, it takes only 90 s fixation to produce a retinal injury, yet it is perfectly safe to stare for many minutes at the rising or setting sun. Because of the changing atmospheric pathlength as the sun arcs across the sky, the spectrum changes from orange or red when near the horizon to yellow at mid‐elevation angles, to white when overhead. The yellow macular pigment provides enhanced protection in that critical retinal region, which is always illuminated—regardless of ambient light level. The inferior retina receives little illumination during the day as the upper lid blocks light to that region. Just like a very well‐designed, modern, automated camera, the pupil and selective filtration maintain a relatively stable retinal illumination level over a great range of ambient light levels.
It was 40 years ago that Noell first reported light damage of the retina from white light and 30 years ago that Ham, Mueller and Sliney identified the short‐wavelength action spectrum of photic maculopathy. International standards for product manufacturers have been issued and revised in the last decade to minimise short‐wavelength light emissions from operating microscopes and other ophthalmic instruments, but the best way to approach filtration recommendations for ophthalmic lenses, sunglasses and IOLs has defied the standardisation process. Perhaps the suggestion of a “virtual age” will help.