The word photochromic comes from two Greek words "photos" meaning light and "Chroma" meaning colour, so photochromatic simply means something that changes colour in response to light. In relation to sunglasses, photochromic lenses darken or lighten dependent on their exposure to ultraviolet (UV) radiation. For example, once the UV is removed by walking indoors or reduced by cycling through a forest, the lenses will gradually return to their clear state or a lighter tint respectively.
What are photochromic sunglasses and how do they work?
Photochromic lenses darken when exposed to UV light and gradually return to the original state once the light is reduced. They protect the eyes in a wide variety of lighting conditions. In addition, photochromic lenses provide 100% protection from harmful solar radiation. The correct term for these lenses is photochromic or photochromatic, which refers to a specific chemical reaction the lenses have to ultraviolet (UV) radiation.
Photochromic lenses have millions of molecules of substances such as silver chloride or silver halide embedded in them. The molecules are transparent to visible light in the absence of UV light, which is normal for artificial lighting. But when exposed to UV rays, as in direct sunlight, the molecules undergo a chemical process that causes them to change shape. The new molecular structure absorbs portions of the visible light, causing the lenses to darken. The number of molecules that change shape varies with the intensity of the UV rays.
When you go indoors and out of the UV light, a different chemical reaction takes place. The absence of the UV radiation causes the molecules to "snap back" to their original shape, resulting in the loss of their light absorbing properties. In both directions, the entire process happens very rapidly. Because they react to UV light and not to visible light, there are circumstances under which the darkening will not occur. A perfect example of this is in your car. Because the windshield blocks out most UV light, photochromic lenses will not darken. For this reason, most sunglasses with photochromic lenses also have a certain amount of tint already applied to them.
Because photochromatic compounds fade back to their clear state by a thermal process, the higher the temperature, the less dark photochromatic lenses will be. This thermal effect is called "temperature dependency" and prevents these devices from achieving true sunglass darkness in very hot weather. Conversely, photochromatic lenses will get very dark in cold weather conditions, which makes them more suitable for snow skiers than beachgoers while outside. Once inside, away from the triggering UV light, the cold lenses take longer to regain their transparency than warm lenses.
Photochromatic (light sensitive) lenses were originally developed in the 1960’s and made from glass. They worked a bit like pieces of old-fashioned, photographic film. Modern photochromatic lenses are now made out of polycarbonate or plastic and instead of silver chemicals they contain organic (carbon-based) molecules that react to light in a slightly different way.
Today's photochromic lenses can offer different categories of sun protection to meet specific needs. For example, you can buy lenses that will start as a tint and become progressively darker which are ideal for sunglasses.
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