If you hate carrying one pair of glasses for indoors and another for outdoors, eyeglass lenses that darken in sunlight may be the answer to your prayers. These light-changing lenses are called photochromic or photochromatic, but the Transitions brand of lenses has become so popular, the light-changing lenses are sometimes referred to as transition lenses.
History
Photochromic glass was first developed by the Corning glass and ceramics company in the early 1960s and was released commercially in 1964. The photochromic lenses were popularized in the 1990s by Transitions Optical, and that's why many people now refer to the lenses as "transition" lenses. Scientifically they're known as photochromic or photochromatic. The terms refers to the chemical reaction that takes place when the lenses are exposed to ultraviolet, or UV, light.
Effects
Transitions Optical explains that photochromic lenses can change color because the lenses are coated with photochromic dyes. The dyes cause the lenses to darken when they're exposed to UV rays---not visible light. That's why they don't change color indoors. As the amount of UV light diminishes, the lenses fade back to clear. The amount of darkening depends on the amount of UV rays, so the glasses will get darker on a very bright day.
Materials
The original photochromic glasses were made of glass, but today you can find them in a variety of materials. AllAboutVision.com notes that in addition to standard glass and plastic, you can also get polycarbonate and high-index photochromic lenses. Polycarbonate lenses are recommended for children because they're 10 times more impact-resistant than glass or standard plastic lenses. High-index lenses are thinner and lighter than regular lenses to make them more comfortable to wear.
Tints
Photochromic lenses come in a variety of tints. AllAboutVision.com reports that depending on the manufacturer, you can find photochromic lenses in brown, gray and green. Some manufacturers also make photochromic lenses that begin with a light tint indoors and get very dark outdoors. The darkness of the lens helps make vision more comfortable and helps protect the retina of the eye.
Benefits
There are benefits to wearing darker lenses outdoors. Transitions Optical notes that they can help reduce glare, diminish eye strain and fatigue, and help you to distinguish contrast more easily. AllAboutVision.com adds that your risk for cataracts and age-related eye problems is linked to the amount of exposure your eyes have to UV rays over your lifetime. It notes that makes photochromic lenses a good choice for children.
