Photochromic lenses contain chemicals that react to the presence of light in the ultraviolet light spectrum. These chemicals undergo a molecular structure transformation that causes the lens to change color. This process is reversible.
In eyeglasses and sunglasses, the lenses will turn brown or grey (a darkening effect) when they are exposed to ultraviolet light. They will activate to a much lesser extent indoors or other areas such as inside an automobile where ultraviolet light does not penetrate.
The speed at which a photochromic lens changes is dependent on several variables. These include the material of the lens, the ambient temperature, the specific type of photochromic lens used, the degree of ultraviolet light entering the lens as well as the age of the lens. In general, most modern photochromics in eyeglasses and sunglasses will achieve substantial darkening within one minute of exposure to ultraviolet light.
The time it takes to clear up a darkened lens once the ultraviolet light stimulus disappears is also dependent on the variables specified for the darkening speed. The clearing rate is generally a few minutes longer than the time it took the same lens to darken.