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Cheryl Guttman
Speaking at the 26th annual meeting of the Macula Society, Martin A Mainster MD said HID bulbs may make good headlights but the glare they cause could visually disable oncoming drivers, particularly the elderly. “Xenon headlight systems project more light than conventional halogen headlights. Therefore, drivers of cars equipped with HID headlights benefit from increased road visibility and greater driving safety because they can see hazards earlier and have more time to respond. “But for the elderly in particular, who are already handicapped by night-time driving difficulties, glare from HID headlight encounters on two-lane roads can be dangerous,” Dr Mainster said. His interest in the area was stimulated about three years ago when a 65-year-old diabetic patient of his, with 20/20 vision and minimal retinopathy, told him he had been involved in a road accident after having being temporarily dazzled by the lights of an oncoming car. The patient remarked the lights were the brightest he’d ever seen, with a bluish hue. Dr Mainster began to investigate headlight design issues to find out if there was anything unique about the lights that might cause more problems for oncoming drivers. His research led him to a manufacturer’s website which acknowledged that HID headlights are brighter than halogen headlights but added that they were an irritation only because people tended to stare into them due to their novel bluish tint. “The truth, however, has nothing to do with a ‘photophilia’ effect, nor do the blue-white HID headlights have greater potential to produce disability glare simply because of their colour. Rather xenon headlights are potentially more visually disabling than halogens for drivers who confront them because they are three times brighter,” Dr Mainster said. He explained that the brighter light of the xenon bulbs creates more disability glare for onlookers by several mechanisms. Increased brightness is associated with more light being scattered in the eye, which leads to reduced retinal image contrast and therefore greater disability glare from the optical phenomena of dazzle and veiling. Brighter light sources also increase scotomatic glare, known more commonly as photostress. Photostress occurs because it takes time for visual sensitivity to recover after eyes are exposed to brilliant light flashes. “Veiling glare makes it difficult for the eye to see targets between bright light sources, while dazzle glare from oncoming headlights makes it difficult to identify the edge of a curving two-lane highway when driving at night. Photostress can startle and disorient drivers, and also cause after-images that interfere with vision,” Dr Mainster added. Older drivers, in particular, are adversely affected by visual problems from increased headlight brightness because of age and pre-existing disease-related ocular changes which make them more susceptible to veiling, dazzle and scotomatic glare. “People over 50 years are more prone to disability glare because they have increased intraocular light scattering. Their photostress recovery time is also increased. “And disability glare is increased by findings which are more common in older people, including cataract, IOLs and retinal disease,” Dr Mainster said. The HID headlight systems available in Europe differ somewhat from those used in the United States. The problem of glare for oncoming drivers is lessened with European systems which feature a sharper horizontal cut-off in luminous intensity above the height of the headlights. Nonetheless, with the European design on bumpy roads, the sharp cut-off bounces up and down, flashing into and out of an oncoming driver’s field of view. For drivers on divided highways, glare screens on lane separators can provide protection from oncoming HID headlights. This solution is not possible on two-lane highways. While other counter-measures are available in the form of technologies to produce less bothersome headlight systems, consumer disinterest, manufacturer resistance and lack of legislative resolve have obstructed their implementation, Dr Mainster said. “Acceptance or rejection of the current generation of HID xenon headlights will depend on their record in traffic and litigation,” he commented. Alternative headlight systems for lessening glare include adaptive headlights, which adjust optically and mechanically for changes in road and weather conditions, and ultraviolet headlights, which offer the added benefit of improving visibility of fluorescent highway markers and signage without serving as a glare source for oncoming drivers. However, polarising headlight systems represent an even better solution for reducing the problem of highway glare. That technology places polarising filters in front of the automobile headlights and in front of the eyes of drivers. The latter filter, known as an analyser, decreases light from oncoming traffic while transmitting light scattered from its own headlights by roadside objects. “Implementation of these systems will be slowed by the fact that their cost would be borne by the owners of cars with HID headlights and, until they become widely used, only oncoming drivers would derive their benefits,” Dr Mainster said. HID
headlight systems have been available for several years in the US
and Europe. They are now common on several luxury car models and
can be retrofitted on some cars. Martin
A Mainster PhD, MD, FRCOphth |
