Photochromic lenses first appeared on the market in the late 1960s, but only became popular in the 1990s. When exposed to UV radiation, photochromic lenses undergo a chemical reaction that changes the colour of the glass. These lenses contain a million tiny molecules that change shape based on the intensity of the UV radiation, which is why the lenses are darker in direct sunlight than in cloudy weather.
In the past, the degree of darkness of lenses made from mineral glass tended to be inconsistent and noticeable, but today this problem no longer exists – the colour coating is even and the transitions smooth. It is important to know that photochromic lenses are darkened by UV radiation, not visible light, and because of this will not darken sufficiently in a car that has a windscreen with a UV filter. There are photochromic lenses which darken in a car, but their initial degree of darkness is around 15%, while ordinary photochromic glasses are completely transparent at first.
Photochromic lenses are very comfortable and suitable for use by anyone. They are available in a variety of materials (such as plastic, mineral glass and polycarbonate) and can be supplemented with several types of coating (such as anti-reflective coating, mirror coating and hydrophobic surface). The photochromic feature can be added to all lenses and can be fitted to any frame. Their maximum degree of darkness is 88%, which is achieved at 23°C, and they are available in brown or grey.
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