The technology used for the most popular brands of photochromic lenses today is owned by Transitions Optical. This has led some eye care practitioners and consumers to call all photochromic lenses "transition" lenses. But though Transitions photochromic lenses outsell other brands, a number of lens manufacturers have their own photochromic lens technology. Transitions Optical partners with other lens manufacturers to make photochromic versions of their lenses. For example, progressive lenses designed by lens manufacturers Essilor, Hoya and Shamir are available with the same Transitions photochromic lens technology. Sources: http://www.allaboutvision.com/lenses/photochromic.htm
The best quality photochromic lenses can offer very quick transition times. Julbo's Zebra lens for example boast high speed activation of between 22 to 28 seconds.However, as photochromic compounds fade back to their clear state by a thermal process, the higher the temperature, the less dark photochromic lenses will be. This thermal effect is called "temperature dependency" and prevents these devices from achieving their full effect in very hot weather. Conversely, photochromic lenses will get very dark in cold weather conditions, which make them more suitable for snow skiers than beachgoers while outside. Once inside, away from the triggering UV light, the cold lenses take longer to regain their clear color than warm lenses.
The brand manufacturers have created many brand names for photochromic lenses. These include Transitions, Reactolite, Reactions and Graduations. However, they are all photochromic lenses. The most common photochromic brands are 1. Transitions Lenses. They are the most popular brand of photochromic lenses today. They are available in a variety of options including Standard Plastic, Polycarbonate, 1.6 and 1.67 Lenses. They are the fastest reacting lens on the market, reacting into sunglasses within 30 seconds of UV exposure. 2. Reactolite Lenses. 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, they work much better behind a car windscreen than the Transitions. Source: http://www.eyekit.co/information/sunglasses/about-sunglass-lenses/what-are-photochromic-lenses.html