High index lenses absorb all harmful UV light and can be tinted to any shade or colour. These are lenses that darken as you go from indoor to outdoors. They are available in glass, hard resin and polycarbonate. Originally made to darken to a moderate shade, new versions are available that darken to a true sunglass. Ask for a demonstration of these lenses.
Polycarbonate lenses are high index and usually provide the lightest, most comfortable lenses.
They absorb all harmful UV light and can be made ultra thin because of their unique strength. For years glass was the only lens material available and glass still offers superior optics. The most scratch-resistant material, the primary disadvantage of glass are its weight, generally twice that of hard resin, and it's potential danger from breakage. Glass lenses can be heat or chemically tempered to increase impact resistance. Persons who normally wear bifocals or trifocals can use single vision for distance or near glasses.