Photochromic lenses have an adjustable tint that changes with the amount of available sunlight. In bright, outdoor sunlight, they darken; indoors, they become nearly clear. They are available in the thinnest, lightest high-index material, plastic, and glass.  Glare-free lenses allow for greater light transmission, allowing for the best light and dark change.  Common names for these lenses are: Transitions, SunSensors, and InstaShades.