Sunglasses or prescription eyeglasses that darken when exposed to the sun were first developed by Corning in the late 1960s. The original PhotoBrown and PhotoGrey products are still available today. They are made of glass, with the color-changing material distributed evenly throughout the lens thickness. A potential problem with this method is that the thicker part of a prescription lens can appear darker than the thinner part. Strong nearsighted lens corrections can have a lighter central "bull's-eye", which is surrounded by a darker lens periphery. Today, Corning also offers the Thin & Dark photochromic glass lenses, which are thinner and up to 30 percent lighter in weight than traditional glass lenses.
How do Photochromics work?
In the simplest terms, a photochromic lens changes color because of a chemical reaction. When someone wears their lenses outside, exposure to UV rays affect a silver chloride compound that was added to the lens material. When triggered by UV, the silver chloride molecules undergo a chemical reaction that turns them a blue-grey color (other colors are available). When the lenses are removed from bright sunlight (on cloudy days, indoors, etc.) very little UV light is present to maintain the chemical reaction and the blue-grey lenses rapidly return to their original, clearer state.
Photochromic lens varieties
"Transition" Lenses – With the increasing popularity of plastic lenses, a new tinting method was developed in the 1990s by Transitions Optical, Inc. By immersing plastic lenses in a chemical bath, photochromatic molecules are absorbed into the body of the lenses to a uniform depth of about 150 microns. Since this absorbed layer of molecules is the same thickness across the entire lens surface, the color-change effect is also uniform across the lenses, regardless of prescription. Transitions lenses are as clear as regular eyeglasses indoors. Outdoors, they quickly darken getting as dark as medium sunglasses. They are available in regular, lightweight plastic materials as well as in high-index plastic and polycarbonate. Currently, approximately 2/3 of all photochromic prescribed produced are of the plastic variety.
Sunglass-only photochromics – These lenses are not meant to be worn indoors. They change from a medium sunglass color to a darker sunglass color when they are exposed to sunlight. They're a good choice for wearers who are especially light sensitive and want a tint darker than the "clear-to-medium" tint that standard photochromics provide. An additional advantage of "sunglass-only" photochromics is, when they are in their unchanged state, they are still dark enough to wear when driving. Regular photochromic lenses don't darken behind car windshields because the glass blocks UV rays needed to trigger the lens color change.
Unique tints – Some plastic photochromic "fun" lenses are available in unusual colors. These lenses are one color when UV light is not present but change to a totally different color when activated by UV. There are teal-blue lenses that change to green, yellow lenses that change to orange, and red lenses that change to purple.
Medical Applications – some photochromic lenses even have medical applications. Corning's CPF series consists of glass photosensitive lenses with colors in the red family. They are used to enhance the vision of patients who have various ocular pathologies. According to Corning, people with severe light sensitivity are likely to benefit from these lenses, especially when the sensitivity is due to: cataracts, macular degeneration, glaucoma, diabetic retinopathy or retinitis pigmentosa.
Photochromics "at a glance"
- Photochromics darken in sunlight and become clear when indoors and at night.
- The shade change from darkened to clear usually occurs within 1-2 minutes.
- Lenses will darken more as the temperature drops (skiing, etc.)
- Photochromics are available in plastic, glass & polycarbonate materials.
- Lenses are available in single vision, bifocals and progressive designs.
- Gray, brown and fashion tints (teal, yellow, violet, orange, blue, rose) are available.
- Lenses don't darken behind car windshields because the glass blocks out UV rays that trigger the color change but sunglass-only designs work well for driving.
- There are specialized designs for almost any activity (polarized photochromics for water sports and fishing, etc.) so be sure to mention your hobbies and interests whenever your eyes are examined or you are replacing your eyeglasses.