December 20, 2006
Glare and Vision
When a patient sits down in my examination room, I see only part of the story of his or her vision. Vision problems in the real world can be quite different. One of the most difficult problems for me to quantify and evaluate is effect of glare on a patient. In the broadest of definitions, glare is a difficulty seeing in the presence of bright light. Glare can be from direct sunlight or another bright light source. It can also be a reflection off a surface. Let's examine these two very different sources and how the eye reacts to them.
When bright light is directed into the eye, the retina (the back surface of the eye where images are processed) is temporarily affected. Light is absorbed by receptor cells called rods and cones. Each rod or cone releases a chemical called rhodopsin when stimulated by light. Rhodopsin acts upon deeper layers of the retina to relay visual messages to the brain. It takes a brief period of time for these cells to replenish their rhodopsin supply. Until then, that cell cannot relay any image that it sees.
The brighter the light source, the more rods and cones are rendered temporarily non-functional. A perfect example of this is a camera flash. The spot where the flash hits the retina will appear as a dark spot in your vision until the rods and cones replenish with rhodopsin. Or, when I evaluate your eye health with that annoyingly bright light, everything in that eye will appear darker than the other (sorry!). As we get older, glare recovery takes longer. Patients with virtually any degree of macular degeneration will have significant glare recovery issues.
Glare is exacerbated when the material it's passing through is not perfectly transparent. Let's use a windshield as our illustration here. If our windshield wiper is in bad shape and we get clear and smudgy areas, we're constantly moving around to find a clear spot. If sunlight hits a dirty area, the sun literally explodes through the glass, dangerously reducing our view. It's time for new wipers, and don't wait for the first snowstorm!! What's happening? As light passes through the glass, non-transparent particles scatter the light in different directions. Thus light is diffused through what should be a transparent medium.
Let's look at the eye now: If cataracts are developing, you will get a similar dispersion of light. There are many problems that can occur in the cornea (the clear tissue on the front surface of the eye) that can also increase glare. These include infections, corneal dystrophy and scarring. As a matter of fact, any opacities or opaque particles that are in the path of light between the cornea and the retina can be a problem. Inflammation of any of the tissues within the eye will do this. If you see a sudden change in glare sensitivity, see your eye doctor ASAP.
Sunglasses and indoor tints are invaluable to patients with the above problems. If there is a distance prescription, prescription sunglasses will be better. Ultraviolet (UV) coating will slow cataract development and is very important. Cataract surgery should not be avoided. When reduced vision and/or glare is hindering your daily activities, have it done. Any inflammatory illnesses causing glare should always be evaluated: never ignore sudden changes.
Reflected glare is a more universal problem. Indoor lights on computer screens, and sunlight off cars, water, ice and even leaves, share a common characteristic: the reflected light is mostly polarized. Without getting into the physics of how and why, polarization occurs when the angle of light waves are all the same. This is why polarized sunglasses are so good. Not only are they darkened for direct glare reduction, but reflected — polarized — light is also virtually eliminated. For people who live and work on the water or drive for a living, it's more than a good; it's almost a necessity.
The last type of reflected glare is the type you get off your eyeglasses. Some people see reflected images in their lenses, or more commonly, light from above or behind. Anti-reflective coatings on your eyeglass lenses will greatly reduce or eliminate this annoying and distracting problem.
Dr. David Eilbert, Optometrist
North Fork Optical Center
P.O. Box 1419
Mattituck Center, Main Rd