Photochromic lenses, otherwise known as transitions, are a marvel of the modern eyewear industry. Invented in 1968 and available only in glass lenses (they’ve since become available and very popular in the plastic lens format), transitions have the seemingly miraculous ability to automatically darken and lighten depending on the lighting conditions. Indoors the lenses remain crystal clear; walk out into the bright sun and within a minute they darken to protect your eyes. Go back inside and they soon clear again.
What Makes Transition Lenses Change?
The cause of this remarkable transformation is a photosensitive reaction on a molecular level. That may sound technical and perhaps a bit complicated, but it’s really quite a simple process.
Photochromic lenses are embedded with millions of molecules that are transparent when exposed to most forms of artificial light, such as the kind put out by incandescent, halogen, or fluorescent bulbs. That’s why your transitions remain clear indoors or at night. They won’t darken with exposure to any form of non-ultraviolet light – not TV or computer screens, not oncoming headlights, not even the reflected light of the moon or the stars. Thus they always remain clear when you need them to remain clear.
Exposure to the sun is a different matter. Walking out into a bright, sunny day, you want the lenses to darken, and they’re up to the task. This happens because one of the components of sunlight, invisible to human eyesight, is ultraviolet (UV) light. When ultraviolet light rays strike the photochromic molecules, those molecules react by changing shape. As they do, they begin to absorb more and more of the light spectrum. The practical effect of this reaction is a darkening of the glass or plastic in which these molecules are present.
As UV light increases, more and more molecules change shape, so the level of shade produced by your transitions varies with the amount of UV light to which they’re exposed. The lenses reach their darkest state in full sunlight. As UV light lessens, the molecules revert back to their transparent form, and the lenses lighten up again.
This process repeats over and over again with virtually no degradation to the molecules. So not only will your transitions convert to sunglasses and back again, they’ll also work for the lifespan of your lenses without wearing out. Visit myeyewear2go to see the hundreds of frame styles available with photochromic lenses, and try a pair today.