Sunglass Guide and terminology

　　Guide to Sunglass Use and Selection
　　Your Eyes Work Hard
　　Seeing is a highly active function. Your eyes continually move and adjust, receiving a constant flow of visual impressions. Normally, all this activity happens routinely and without noticeable strain.
　　The Burden of Glare
　　Glare is nearly always present. This brilliant sometimes blinding light is intensified when it reflects off water, snow, concrete highways, glass buildings or automobile brightworks. Glare is bothersome and discomforting, causing your eyes to work even harder Your pupils contract, your eyelids narrow and the muscles around your eyes contract. Your eyes must strain to see well, causing eye fatigue and even headaches.
　　The Benefits of Top Quality Sunglasses
　　This is why top quality sunglasses are such an important part of our outdoor life -- to help us see comfortably and clearly in sunlight -- to counteract the discomfort caused by glare. And to protect our eyes from the threat of permanent damage that could lead to cataracts and other eye diseases. Lesser quality sunglasses are often more of a hindrance than a help. Lenses may not provide proper protection from glare and are often marked with wavy, irregular surfaces that distort and disturb vision, causing added eye strain and fatigue. Good, top quality sunglasses provide cool comfort and complete protection even during lengthy exposure to harsh glare.
　　Year-Round Protection
　　Your eyes need glare protection all year round. The winter sun is often brighter and snow-reflected glare can be more intense than the brightest summer day. Damaging UV rays are present year-round and throughout all daylight hours.
　　Night Driving
　　Wearing sunglasses during the day can help your night driving. Exposure to strong sunlight without adequate glare protection can sharply reduce your night vision. Even a few hours of exposure can slow your eyes' adaptation process as darkness falls.
　　Repeated exposure can delay this adaptation even longer and your night vision could be cut by 50% or more. Never wear sunglasses when driving at dusk or after sundown. Sunglasses lenses will reduce light to dangerously low levels.
　　Different Types of Light
　　Visible Light
　　Visible light is that portion of the light spectrum which the eye perceives as color. Excessive amounts of visible light are irritating to unprotected eyes and can reduce your visual perception by as much as 50%. It can also hinder your eyes' ability to adjust to darkness (night blindness) and cause difficulty in depth perception.
　　Blue Light

　　Blue light is dispersed through the air by dust particles, which is why the sky appears blue. It also causes glare and is the most demanding type of light for the retina. However, if blue light is eliminated, we lose the ability to recognize colors; traffic lights for example. So, one of the primary tasks of quality sunglasses is to block the correct amount of blue light and to sharpen contrast without distorting color perception. Neutral gray lenses provide the least color distortion of all lens colors.
　　Ultraviolet Light
　　UV light is the most dangerous kind of light. It can cause "sunburn" on eyelids, damaging the cornea and membranes in the eyelids. Most ultraviolet rays are absorbed by the atmosphere before they reach the earth. Some do get through however. And at higher altitudes even greater amounts reach ground level.
　　There are three kinds of ultraviolet light.
　　UVC is below the visible spectrum and is effectively absorbed by the ozone layer.
　　UVA at the low end of the visible spectrum, passes through the outer structure of the eye and is absorbed by our eye's lens. Extended exposure to UVA can lead to the formation of catatracts.
　　UVB causes sunburned skin. These rays are absorbed in the outer surface of the eye. Extended exposure can cause "snow blindness," a burning of the eye's outer tissues and can also lead to cataracts.
　　Infrared Rays
　　Infrared Rays are heat rays. If you are exposed to intense sunlight for a lengthy period of time (a day at the beach, for example) without infrared protection, you may experience a burning or stinging sensation in your eyes and a sense of fatigue.
　　Infrared rays can be especially discomforting if you wear contact lenses. If your sunglasses fail to stop infrared light, it can be absorbed by your contacts causing them to "warm up."
　　Selecting the Right Lenses
　　The Right Lenses
　　To provide full, day-long, comfort, sunglasses should eliminate all problems of glare and reduce the amount of visible light to a comfortable, visibly efficient level. The lenses should also reduce infrared rays and nearly eliminate UV light.
　　Light Transmission
　　Light Transmission is the percentage of light that passes through the lens and reaches the eye. Most optical experts agree that the best light transmission range for sunglasses is from 10% to 30%. For example, a lens with a visible light transmission of 20% allows 20% of the light to pass through the lens, blocking the other 80%.
　　When selecting sunglasses, consider how they will be used. Most people do not need lenses rated less than 10%. While driving, for example, lenses that are too dark can cause visibility problems when driving from very bright conditions into shady-dark areas; driving into a tunnel could cause complete loss of vision. Some people can become "addicted" to dark lenses and find they cannot tolerate even moderate glare without dark glasses.
　　UV Transmission
　　For outside activities in direct sunlight, sunglasses should block at least 95% of UV light. For driving, higher UV transmission levels are acceptable, since the windshields on most cars will block some of this harmful light.
　　Optical Quality
　　Simply stated, Optical Quality means the lens is free from distortion. Lines reflected in the lens will follow in straight lines the even contours of the lens, versus wavy, eye-straining distortions found in non-optical lenses.
　　Lens Types
　　Optical Glass
　　Optical Glass Lenses are ground and polished to the exact thickness of the lens to assure a distortion-free lens. Glass is the most scratch-resistant lens material.
　　Polycarbonate
　　Polycarbonate plastic lenses are made of the toughest, most shatter-resistant material commonly used for sunglass lenses. This is the same material used for aircraft windshields. It is scratch resistant (though not scratch proof), lighter than glass and 50 times more impact resistant.
　　Polarized Lenses
　　Light striking flat surfaces, such as water, snow, glass or pavement, is reflected perpendicular to that surface. This reflected glare or polarized light is much more intense than normal sunlight, irritating your eyes and inhibiting vision. Polarized lenses, through the horizontal alignment of polarizing micro crystals, block all vertical light. Polarized lenses are particularly suitable for water sports, cycling and driving.
　　Photochromic or Photochromatic Lenses
　　Photochrornic or Photochromatic Lenses are light sensitive lenses that grow lighter or darker as lighting conditions change. Some photochromic lenses react to temperature as well as lighting changes.
　　Lens Colors
　　Lens colors have a lot to do with the performance of your sunglasses. Colors are chosen based on the way your eye reacts to them. Different colors cause different visual sensations. Following is a description of the basic characteristics of the more popular lens colors.
　　Grey
　　The most popular color for general purpose use. Colors remain true, with no distortion. Reds remain red, blues blue and greens green. Light is flattened to maintain normal depth perception. Grey lenses absorb UV and IR well.
　　Green
　　Like grey lenses, colors remain true and UV and IR is absorbed well.
　　Brown
　　Especially effective for filtering scattered blue light commonly found in hazy, foggy or low light conditions. Improves depth perception and contrast in variable light conditions. Popular for skiing, boating, driving and general use.
　　Yellow
　　Enhances depth perception, although colors are somewhat distorted. Provides 100% UV protection. However, yellow lenses require special coatings to absorb IR. Yellow lenses are a popular fashion color.
　　Rose
　　Like brown, effectively filters scattered blue light and improves contrast (much like a "haze" filter on a camera). Many people consider rose most soothing and comfortable over a longer period of time.