Swimming - Applies to lens and frame. Swim goggles with water-tight seals, designed specifically for swim use. Water Sports - Applies to lens. Polycarbonate - Polycarbonate is a light weight lens material that has the highest impact resistance of any lens material available in a sunglass. In fact polycarbonate lenses are 10 times more impact resistant than conventional plastic or glass lenses.
Polycarbonate lenses are also much thinner and lighter than glass or plastic lenses.
Gargoyles, Smith, Adidas and Nikon lines are made exclusively with polycarbonate lenses. CR-39 lenses are much lighter and less breakable than glass lenses, but not as impact resistant as polycarbonate. Be sure to check out the Costa Del Mar line for more information. Glass - Glass is the heaviest lens material available, but it also has the best scratch resistance available. Glass must be hardened by chemicals or heat tempering to meet U.S. Food and Drug Administration standards for impact resistance.