Transition lenses are photochromatic lenses for eyeglasses that become darker with exposure to ultraviolet rays from the sun. The lenses fade back to clear when the wearer moves inside. In some cases, the lenses eliminate the need for a separate pair of sunglasses.
The chemical responsible for darkening these lenses reacts to ultraviolet rays from the sun. UV rays are able to penetrate clouds but not glass, so the wearer may notice his glasses get dark when it is cloudy outside but are less reactive to sunlight when driving a car. Regardless of the darkness of the lens, it provides 100 percent protection against both UVA and UVB rays from the sun. This protection helps to reduce the risk of forming cataracts later in life.
Several companies manufacture photochromatic lenses. The Transitions brand is the most common, and eyeglass wearers and some providers use the name as a generic for the product. Most manufacturers use polycarbonate in the manufacturing of these lenses, although Corning continues to produce glass photochromatic lenses.
Photochromatic lenses are generally more expensive than regular lenses. Other options include tinting prescription eyewear. A solid tint on the prescription lens allows the wearer to use the glasses like normal sunglasses. A gradient tint provides some sun protection but fades to clear at the bottom of the lens.