Photochromic lens
Photochromic lenses are lenses that darken on exposure to ultraviolet (UV) radiation. Once the UV 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.
History
The glass version of this type of lenses was first developed by Corning in the 1960s. More recently, plastic versions of these lenses have been commercialized. The first of these was the Photolite lens sold in the early 1980s by American Optical Corporation, but the first commercially successful plastic photochromic lens was introduced by Transitions Optical in 1991.[1]
Technical details
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, specifically 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.[2] 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.[citation needed]
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 clear color 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.
Advantages and disadvantages
There are both advantages and disadvantages to photochromic lenses. The main advantage is that they will darken into a sunglass when exposed to U.V. light, thus obviating the need to carry a separate pair of sunglasses for protection against harmful U.V. rays from the sun.
The main disadvantage of photochromic lenses is they do not adjust immediately. It could take up to two minutes for the lenses to completely change from light to dark or vice versa. Another possible disadvantage is that they will not darken when worn inside vehicles with glass that blocks U.V. light.
References
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- Photochromic lenses How stuff works.
- Optometry Glossary
fr:Verre photochromique it:Lente fotocromatica
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