Increasing Comfort
Sunglasses reduce glare and brightness, whether from the sun directly or from water, snow, sand or reflective surfaces. Bright flashes off water or shiny surfaces can subject the eye to 10 times more light than needed to see. Glare can be painful and dangerously distracting while driving or playing sports.

To be sure that sunglasses will block glare, select lenses that are dark enough to hide your eyes. For comfortable vision on sunny days, sunglasses should block 75 to 90 percent of visible light. If dark enough, sunglasses will eliminate eye strain and squinting when in bright light.

Improving Visibility
Sunglasses improve vision by enhancing contrast in hazy or overcast conditions.

Protecting the Eyes
Sunglasses protect the eyes by blocking harmful light. Although the eye depends on light to see, wavelengths below the color blue (ultraviolet light or UV) on the color spectrum can harm the unprotected eye. These rays are invisible to the human eye. UV light is potentially harmful to the eye and is strongly linked to cataracts and macular degeneration. It can "sunburn" the eyes, causing temporary blindness (photokeratitis). The cornea, lens and retina are all vulnerable.

There are two types of ultraviolet light. UVA are the light rays that "age" eye tissues and skin. They contribute to wrinkling and cataracts. They are closer to visible light than UVB. UVB rays are the "burning" rays that cause skin cancer, degenerate the macula, burn the cornea, and cause photokeratitis.

Glasses can be treated to filter out both types of UV rays. Sunglasses that are not treated for UV light may actually be detrimental to the eyes. Dark lenses reduce light entering the eye, causing the pupil to dilate and exposing the inside of the eye to more UV radiation than without the sunglasses.

UVA light is constant throughout the year, while UVB light increases intensity in the summer. UV light passes through glass, water, clouds and some clothing. Up to 85 percent of UV light shining on sand and snow is reflected back off these surfaces. UVB light is greater at high altitudes.

Ozone depletion in the upper atmosphere has increased the amount of UV radiation that reaches earth. For every 1 percent of ozone depletion, UVB rays reaching the earth increase by 2 percent.

The negative effects of exposure accumulate over time, throughout our entire lives. For this reason, parents should start protect their children's eyes very early in life with UV-blocking sunglasses. Anyone who spends a lot of time outdoors should be extra cautious about UV radiation.

Fit and Function
Be thorough when evaluating sunglasses. Inexpensive sunglasses from discount or department stores may not provide the protection or quality you really need. Whether purchasing sunglasses from an optical store or not, look for specific characteristics that provide proper fit and function.

High quality, well fitting sunglasses provide comfort, sharp vision and the best protection possible. Sunglasses should fit according to the same guidelines as regular eyeglasses. Wraparound or side-screen models block UV light and glare that would otherwise reach the eyes from over, under and around the sides of the frames.

But these styles may cause vision distortion. To test the quality of the lenses, put on the sunglasses and look at a vertical edge or line (a door frame, floor tiles, etc.). Move your head back and forth, sweeping your eyes across the width of the lenses. If you notice any wiggle in the line, the lenses may be defective and could distort your vision. Your eye doctor can also test the optical quality of your sunglass lenses.

Lens Treatments
For the best protection, choose sunglasses that block UVA and UVB rays between 290 and 400 nanometers. You can select from shades and tints that meet your personal needs, and special treatments like polarization and photochromic lenses. The shade and color of sunglass lenses has no effect on their ability to block UV light - UV coating is a separate treatment.

Light to medium shades are good for daily wear. For extra bright conditions and outdoor sports, darker shades will be more comfortable. It is wise to have multiple pair to choose from for different occasions.

Gradient tinting refers to lenses that are darker at the top and lighter at the bottom. It protects the eyes from glare when looking up but allows clear vision when looking down. Double-gradient tinting is also available, in which the upper and lower parts of the lens are dark and the center is light. These are comfortable in situations where light is reflected up from snow or water.

Different tints filter different wavelengths of light. Some may enhance or distort colors and affect contrast. Select your tint based on your lifestyle and personal needs.

• Gray - Allows true color perception, but does not enhance contrast. Good for golf, cycling, or running.

• Green - Allows true color perception and good contrast in bright light. Reduces eye strain in bright light.

• Brown - Good in hazy sun, enhances contrast. Good for high-glare sports such as skiing, fishing or sailing.

• Amber - Brightens cloudy, hazy, or foggy skies. Excellent for contrast. Minimizes eye strain. Good for hunters, pilots and snow skiers. Distorts color (images look yellow orange).

• Yellow - Improves contrast and depth perception in low light. Good for snow skiing and trap shooting, especially on overcast days.

• Red - Excellent depth perception in low light. Contrasts objects against blue or green backgrounds. Good for skiing and hunting.

• Mirrored - Reflect high-intensity light to reduce glare. Available in a variety of colors.

Polarization
Polarized lenses are the ultimate sun lenses, blocking glare by absorbing light from scattered angles. Polarized filters are added to the lenses during the manufacturing process. Golfers, fishermen and outdoors enthusiasts prefer these lenses. They are also recommended for driving. Polarization is a separate feature from shade or tint; various colors filter out different light. Polarized filters also do not block UV light, although the lenses may be treated for this too.

Photochromic Lenses
Photochromic lenses are sensitive to light. They automatically darken within 30 seconds of exposure to bright light. Once out of the sun, they lighten in about five minutes. They provide freedom from switching between prescription glasses and sunglasses, or from removing and replacing non-prescription sunglasses when going in and out of the sun.

Safety Standards
Among other issues, the impact of ultraviolet light on eye health is a worldwide concern. Several countries have set voluntary standards for sunglass manufacturers in order to protect their citizens'eyes. In Australia, these standards are mandatory.

Manufacturing and labeling regulations cover characteristics such as refractive properties like distortion and blur, impact resistance, UV protection, color transmission and appropriateness for driving. Check labels when purchasing sunglasses and contact regulatory organizations for more information.

The most widely recognized regulatory agencies are:

• American National Standards Institute
• Canadian Standards Association
• CEN (Europe)
• British Standards
• Australian Standards

All sunglasses must be impact resistant, but they are not shatterproof. They are not tough enough to protect the eyes for high-impact sports or industrial safety purposes. Sunglasses cannot provide protection against intense light sources such as sunlamps, lasers, welding torches or solar eclipses.