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Photochromic lenses are lenses that darken on exposure to specific types of light, most commonly ultraviolet (UV) radiation. Once the light source is removed (for example by walking indoors), the lenses will gradually return to their clear state. Photochromic lenses may be made of glass, polycarbonate, or another plastic.
Photochromic lenses were developed by Roger Araujo at the Corning Glass Works Inc. in the 1960s, and the process was used in the first mass-produced variable tint lenses.
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The glass version of these lenses achieve their photochromic properties through the embedding of microcrystalline silver halides (usually silver chloride), or molecules in a glass substrate. Plastic photochromic lenses rely on organic photochromic molecules (for example oxazines and naphthopyrans) to achieve the reversible darkening effect. The reason these lenses darken in sunlight but not indoors under artificial light, is that room light does not contain the UV (short wavelength light) found in sunlight. Automobile windows also block UV so these lenses would darken less in a car. Lenses that darken in response to visible (rather than UV) light would avoid these issues, but they are not feasible for most applications. In order to respond to light, it is necessary to absorb it, thus the glass could not be made to be clear in its low-light state. This correctly implies photochromic lenses are not entirely transparent: they filter out UV light. This does not represent a problem, because the human eye does not see in the UV spectrum.
With the photochromic material dispersed in the glass substrate, the degree of darkening depends on the thickness of glass, which poses problems with variable-thickness lenses in prescription glasses. With plastic lenses, the material is typically embedded into the surface layer of the plastic in a uniform thickness of up to 150 µm.
Typically, photochromic lenses darken substantially in response to UV light in less than one minute, and then continue to darken very slightly over the next fifteen minutes. The lenses fade back to clear along a similar pattern. The lenses will begin to clear as soon as they are away from UV light, and will be noticeably lighter within two minutes and mostly clear within five minutes. However, it normally takes more than fifteen minutes for the lenses to completely fade to their non-exposed state. A study by the Institute of Ophthalmology at the University College London has suggested that even in dark conditions photochromic lenses can absorb up to 20% of ambient light.
Because photochromic compounds fade back to their clear state by a thermal process, the higher the temperature, the less dark photochromic lenses will be. This thermal effect is called "temperature dependency" and prevents these devices from achieving true sunglass darkness in very hot weather. Conversely, photochromic 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.
A number of sunglass manufacturers/retailers (Intercast, Oakley, Serengeti Eyewear, Persol to name a few) offer products that use photochromism to make lenses that go from a dark to a darker state. Because these products are tinted in the bleached state, they are typically used only outdoors and are not considered general-purpose lenses.