If you've ever noticed someone wearing sunglasses outdoors and as he goes indoors, the lenses lighten, the person is most likely wearing what are commonly called "transition" lenses. Photochromic, or transition, lenses use a special technology that causes lenses to lighten and darken depending on the amount of sunlight on the lens.
Significance
Eyeglass lenses that change from clear indoors and at night to darker outdoors are called photochromic or transition lenses. The name of the company that invented the lens-changing technology has led to the common reference of all photochromic lenses to be called "transition" lenses. Many companies, however, manufacture photochromic lenses.
Function
Photochromic lenses react to ultraviolet, or UV, sunlight. The lenses contain a special dye that darkens when the lenses are exposed to UV light. The color of the lens adjusts depending on the amount of UV radiation. If sunlight is obscured by clouds, the level of darkness adjusts slightly and reduces the tint.
Types
Photochromic lenses were originally made in only in glass lenses which meant the lenses were heavier and prevented people with thicker or complicated prescriptions from getting transitioning lenses. Today, the lenses are available in all lens material including glass, plastic and polycarbonate. The lenses are lighter, thinner and available for most prescriptions.
Benefits
In addition to changing color, the technology that causes photochromic lenses to transition also protects the eyes from ultraviolet radiation. Photochromic lenses absorb 100 percent of UV light so they protect the eye in the same way sunscreen protects the skin. Ultraviolet radiation has been shown to cause cataracts, so protecting the eyes with a UV filtering lens is important.
Considerations
Transition lenses also work by helping night vision. As the lenses absorb UV radiation during the day, the pigments of the retina are prevented from experiencing overbleaching. The effect for the eyeglass wearer is better adaption and better ability to see at night.
Warning
Car windshields absorb UV radiation, so many photochromic lenses will not work well for driving during the day. However, newer lenses are available for use with a windshield that combine photochromic lenses with a slight tint or a polarized lens which will cut down glare while driving.
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