Photochromic lenses are clear lenses that react with UV and produce a tint effect based on the amount of UV present.
When wearing Photochromic lenses even a dull day some reaction will occur producing a very pale tint and then on a very bright day where more UV is present they will turn quite dark. When wearing them indoors they will become clear.
The idea behind photochromic lenses is that they will provide the correct amount of protection for the varying light conditions you experience. However, when driving, your car windscreen will block a certain amount of UV which is required to make the lens react so they will not work as well as they would outside of the car.
Transitions are available in Standard Plastic, Poly Carbonate, 1.6 and 1.67 Lenses. They are the fastest reacting lens on the market and will react into sunglasses within 30 seconds. However, they will take longer than that to go clear again when you go inside.
Reactolite is a brand name given to a Glass photochromic lens. They do not react as well as the transitions and are much heavier to wear. However, one good thing about glass reactolite is that they work much better behind a car windscreen than the transitions do. (If only we could have the best of both worlds.)
In most cases the Transitions lens is the preferred option due to the over all comfort and performance.
Photocromic lenses are ideal in the sense that they can save you having to purchase two pairs of glasses ie, Clear glasses and a pair of Sunglasses. However, many people who wear transitions also have a cheaper tinted pair just to use in the car.