Photochromatic glasses turn dark by exposing the glasses lenses to ultra violet light.
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 way that the glasses lenses change from light to darl is essentially 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 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 lot on the thickness of the glass being used to construct the glasses.
This can be an issue when buying varifocal glasses with a photochromic coating as the thickness of the lens used to make the varifical lenses changes over the surface of the glasses lens.
It is also much less of an issue with 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 very short – of around 60 seconds to reach a substantially dark state. The lenses continue to turn even darker as they remain exposed to sunlight and may take up to 15 minutes to complete the process.
When you come indoors with your glasses, or are no longer exposed to the UV light, 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 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 more aggressive in terms of their transition and level of 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.
The drawback though when using transitions glasses for the beach or in a warmer climate is that – 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 are some things you should consider when buying transitions glasses.