Photochromic film lenses are available in nearly all lens materials and designs, including high indexes , bifocal and progressive. An added benefit of photochromic lenses is that they shield your eyes from 100 percent of the sun's harmful UVA and UVB rays.
Because a person's lifetime exposure to sunlight and UV radiation has been associated with cataracts later in life, it's a good idea to consider photochromic lenses for children's eyewear as well as for eyeglasses for adults.
The molecules in photochromic technology work by reacting to UV light. However, temperature can have an effect on the reaction time of the molecules. When the lenses become cold the molecules begin to move slowly. This means that it will take somewhat longer for the lenses to adapt from dark to clear. When the lenses become warm the molecules speed up and become more reactive. This means that they will fade back faster. It can also mean that if you’re outside on a hot sunny day, but sitting in the shade, your lenses will be quicker to detect the diminished UV rays and lighten in color. Whereas, if you are outside on a sunny day in a cold climate, and then move into the shade, your lenses will adjust more slowly than they would in a warm climate.
Photochromic film lenses, when exposed to sunlight/UV rays, will on average take approx 30 seconds to a minute to darken and will continue to darken for a further 10 minutes to full sunglasses tint/shade, returning to an almost clear state within 2 minutes and fully cleared within 5 minutes although excess temperature will affect this.
Wearing eyeglasses can often be a pain. If it's raining, you're wiping water off the lenses, if it's humid, the lenses mist up; and if it's sunny, you don't know whether to wear your normal glasses or your shades and you may have to keeping switching between the two! Many people who wear eyeglasses have found a solution to the last of these problems by changing over to photochromic lenses