Photochromic lenses made of glass or polycarbonate darken on exposure to ultraviolet (UV) radiation. Once the UV radiation is removed the lenses will gradually return to their clear state. UV radiation can be removed by walking out of the sun. 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 15 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 2 minutes and mostly clear within 5 minutes. However, it normally takes more than 15 minutes for the lenses to completely fade to their non-exposed state.
The glass version of this type of lens was first developed by Corning in the 1960s- glass lenses achieve their photochromic properties through the embedding of microcrystalline silver halides (usually silver chloride), or molecules in a glass substrate. 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 the first commercially successful plastic photochromic lens introduced by Transitions Optical in 1991, the plastic versions are getting increasingly commercialized.
The first generation of plastic lens used of organic dyes known as blue pyridobenzoxazines. The later generations used naphthopyrans, and presently are based on indenonaphthopyrans. When a photochromic dye is exposed to UV radiation, a chemical bond is broken. The molecule then rearranges into a species that absorbs at longer wavelengths in the visible region, making the lens to darken. Improvements in the dyes over the years have led to faster activation and clearing, as well as longer lifetimes. Fusion of a bicyclic hydrocarbon group, called an indene group, to one of the benzene rings in a naphthopyran dye to make indenonaphthopyrans does not break it for a long time. The lens material, which supports a layer of photochromic dyes, also affects how fast the lens darkens and fades. Photochromic dyes do not work well in common plastic materials. New polymers compatible with dyes are now used. PC and thiourea are suitable with the photochromic dyes. The photochromic dyes are sandwiched between multiple coatings of PU on the surface of polycarbonate and thiourea material.
Plastic photochromic lenses rely on organic photochromic molecules 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. 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 plastic lenses the material is typically embedded into the surface layer of the plastic in a uniform thickness of up to 150 µm. 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 make 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 clear color than warm lenses.
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