a photochromic materials in modern glasses

Photochromic glass lenses exposed to ultraviolet light to dark. A photochromic glass several common brands of light, you can online source, such as the conversion (which is the market leader) or reaction, Reactolite or graduated prescription glasses.
They were first introduced made of glass, but today most of the transition from the plastic glasses. From the change of optical DARL lens manner, in the essence is the same - through the lens is embedded or coating and microcrystalline or UV response of low molecular organic elements.
The modern conversion lens is very effective, and for many years in the chemical improvements have significantly improved from light to dark, then return to achieve a very significant change in a very short period of time.
Glasses lens zoom lens surface to buy the "change for varifical lens, electrochromic coating the thickness of the lens light, this can be a problem.
It is also much less of a problem in modern plastic lens, because these are often color photographic materials coated with uniform thickness.
Usually containing a photochromic materials in modern glasses, glasses are not exposed to UV light environment (such as indoor) a clear (or nearly clear) state transition for a short time - about 60 seconds to reach the dark state. Turn the lens more dark, because they are still exposed to the sun, most likely process takes 15 minutes to complete.
One of the effects is more positive, transition lenses are often used for skiing, but not on the beach, their transition and shading degree.
There are even some manufacturers who photochromic process is embedded into the sunglasses, or prescription sunglasses, let them go skiing or used in similar environments, more dark.