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Photochromic glasses can often be a [url=http://www.shewyne.com/peutereyoutlet.html]peuterey outlet[/url] very convenient alternative to [url=http://www.lcdmo.com/hollister.php]hollister pas cher[/url] a second pair of tinted prescription spectacles, as they turn dark when worn outside.
There are several common brands of photochromic glasses that you can source online, for example Transitions (which is the market leader) or Reactions, Reactolite or Graduations prescription glasses.
They were originally introduced made of glass, but today most transitions glasses are made out of plastic. The process by which they turn dark is basically the same - by embedding or coating the lenses with microcrystalline or molecular organic elements that react with ultra violet light.
Modern transitions lenses are very effective and the improvement in the chemistry over the years has improved dramatically realising a very significant transition from light to dark and back again [url=http://www.rtnagel.com/louboutin.php]louboutin[/url] in a very short period of time.
When the chemistry has been embedded or coated onto the glass lens itself to make the glasses, how much the lenses actually change in degree of darkness depends a [url=http://www.fayatindia.com/giuseppe-zanotti.html]giuseppe zanotti pas cher[/url] lot on the thickness of the glass being used to construct the glasses.
This can be a problem with varifocal lenses, as the thickness of the glass can vary quite significantly across the optical field of vision, but this issue is usually addressed by the optician ordering special lenses which null out this distortion in the chemistry.
It is also much less of an issue with [url=http://www.shewyne.com/woolrichoutlet.html]woolrich outlet[/url] modern plastic lenses, as these tend to be coated with a photochromatic material at a uniform thickness..
Usually in modern glasses that contain photochromic material, the time for the glasses lenses to convert from a clear (or close to clear) state having not been exposed to ambient UV light (so indoors for example) is [url=http://www.lcdmo.com/hollister.php]hollister france[/url] very short - of the order of a a minute or less to reach 85% of the capability. They will continue to darken when remaining exposed to the UV light over ten to twenty minutes until the molecules reach saturation.
When you come indoors with your glasses, or are no longer exposed to the UV light, [url=http://www.jordanpascherofficiele.com]air jordan[/url] the reaction reverses and the lenses in the glasses will start to lighten.
This reversal process tends to take longer, and can be quite significantly dependent on the ambient temperature as well as the amount of ambient UV. The colder it is, the faster the particles (in the case of glasses lenses) or molecules (in [url=http://www.fayatindia.com/giuseppe-zanotti.html]giuseppe zanotti[/url] the case of plastic glasses lenses) will react and either go darker or lighter depending on the exposure.
One effect of this is that transitions glasses tend to be much [url=http://www.shewyne.com/hoganoutlet.html]hogan outlet[/url] more aggressive in terms of their transition and level of [url=http://www.jeremyparendt.com/Barbour-Paris.php]barbour pas cher soldes[/url] darkness when used for skiing rather than at the beach.
There are even some manufacturers who embed the photochromic process into sunglasses, or prescription sunglasses so that they go even darker when used for skiing or in a similar environment.
If you are considering [url=http://www.rtnagel.com/airjordan.php]jordan pas cher[/url] transitions glasses for your beach holiday however, you will find them less useful - they will certainly reduce glare and add some eye protection, but they will not be as dark as fully tinted prescription glasses made as sunglasses.
Transitions glasses can be used for the beach or skiing, They are available in single vision glasses, bifocal and varifocal glasses. They can be made of glass or plastic, but there [url=http://www.mxitcms.com/abercrombie/]abercrombie[/url] are some things you should consider when buying transitions glasses.
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