UV Transmission

• Ultraviolet rays are particularly dangerous to eyes and skin. Though you can't see them, they can burn your eyes and cause problems such as cataracts and eye fatigue. To protect from all types of ultraviolet rays, look for sunglasses that are labeled for 99 percent to 100 percent UV protection. According to the Glaucoma Research Foundation, lens tint and price play no role in the sunglasses' ability to block UV rays, so look at the label. UV absorption of "up to 400nm" equates to 100 percent protection, according to the foundation.
  
  

Visible Light Transmission

• Lenses can block 100 percent of ultraviolet light but vary in how much visible light they let through. Different lens tints have different ratings of visible light transmission, or VLT, usually shown as a percentage. The higher the percentage, the more light that the lenses allow to pass through. High-VLT lenses are better for low-light conditions, where you need the extra light to see. Low-VLT lenses are better for bright conditions in which you want to block as much light as possible. Lens tints usually correlate intuitively to VLT, with dark lenses (black, brown) having the lowest VLTs, and light lenses (clear, yellow, light orange) having the highest. Photochromic lenses adjust the tint and VLT based upon light conditions.
  
  

Construction

• Sunglasses can protect only against light that passes through their lenses. If your sunglasses don't fit correctly or if they have large gaps, both ultraviolet and visible light can reach your eyes. Choose sunglasses that are larger and offer fuller protection. Wraparound designs protect your eyes from the sides, and goggles offer some of the best protection if light is coming in from multiple angles, such as skiing, in which light reflects off the snow. Wear a hat to protect from light coming from above.
  
  

Features

• Many features on sunglasses can be confusing. Polarization blocks glare from reflective non-metallic sources such as water, snow and ice. This helps for water sports, skiing or other activities in which glare can be a problem. Mirrored lenses reduce glare and visible light. Sport-specific lenses are designed to optimize contrast and certain colors to improve performance. For instance, certain rose and yellow lenses are used in ski goggles and sunglasses to increase contrast with snow, maximizing visibility.