Photochromic lenses are lenses which darken each time they are exposed to ultraviolet light and radiation. Thus, photochromic lenses change from clear lenses to tinted whenever the user walks into a sunlit area. These lenses protect the wearer's eyes from 99.9% of the harmful UV rays as well helping the user to achieve clearer vision in areas with bright glare from the sun. Once the wearer is back inside or no longer exposed to UV rays, photochromic lenses will transition back to clear/transparent lenses.
Photochromic lenses are sometimes referred to as "Transition lenses" due to the first commercially successful photochromic lens, introduced by Transitions Optical in 1991. Bifocals, trifocals and standard lenses can all be made into "Transition" lenses with photochromic glass, polycarbonate or another plastic. Such lenses can be used in prescription or non-prescription eyewear and are ideal for individuals with a sensitivity to light.
The process by which photochromic lenses achieve their tinted state depends upon the material that they are made from. Glass photochromic lenses achieve their tinted state through the embedding of microcrystalline silver halides (usually silver chloride), into a glass substrate. When exposed to UV radiation, the silver halide molecules change shape enabling them to block out the UV rays. Plastic photochromic lenses rely on organic photochromic molecules such as oxazines and naphthopyrans to achieve their darkening effect. Regardless of material, the molecular change reverses itself when the wearer goes back indoors and into artificial light.
Unlike the photochromic lenses of the past, today's technology enables the transitional change to be a rapid one in either direction. Typically, exposure to UV light will result in the lens darkening in relative entirety in less than one minute. The lens then continues to darken slightly over the next fifteen minutes. The reverse process back toward a clear state follows a similar chronological pattern.