Technically they’re called photochromic lenses, although they’re also know as transition lenses – that moniker being based on both what they do and on the name of the company, Transitions, that developed and introduced them in 1968.
Photochromic lenses contain a coating that works on the molecular level. The molecules embedded in this coating are transparent when exposed to most forms of light, but alter their form when exposed to ultraviolet (UV) light, a component of direct sunlight. In the presence of UV light, these molecules change shape and begin to absorb more of the light spectrum. This results in a gradual darkening effect.
Since the molecules are layered, they’re not exposed to UV light all at once; in low levels of UV light, some of the molecules retain their original shape. In full sunlight, however, all the molecules convert, and the lenses reach their darkest shade. This is how the level of tint varies with the amount of light that is present. As the source of UV light lessens, the molecules revert back to their “transparent” shape and the lenses clear again.
Additional facts about photochromic lenses:
If you’re in the market for a new pair of prescription glasses, you might just want to give photochromics a try. The additional cost of the lens treatment is often offset by the fact that you’ll only have to purchase one pair of prescription eyeglasses to get both standard eyewear and sunglasses. Also factor in the convenience of a single pair of glasses – no searching around for lost sunglasses, no eyewear left at home or in the car when you need it. Your sunglasses will always be with you, ready to spring into action when the sun appears and disappear again when their work is done.
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