Sunglass lenses are made from either glass or plastic. Plastic lenses are typically made from acrylic, polycarbonate, CR-39 or Polyurethane. Glass lenses have the best optical clarity and scratch resistance, but are heavier than plastic lenses. They can also shatter or break on impact. Plastic lenses are lighter but are more prone to scratching. Plastic offers more resistance to shattering than glass. Polycarbonate plastic lenses are the lightest, and are also almost shatterproof, making them good for impact protection. CR-39 is the most common plastic lens, due to their low weight, high scratch resistance, and low transparency for ultraviolet and infrared radiation.