Photochromic lens technology has been around for over 40 years. Photochromic lenses change from clear to dark based on the intensity of UV radiation. Remove the source of UV radiation from the lenses, and they return to their clear state.
The amount of photochromic reaction (how much a lens darkens) depends upon the intensity of the UV radiation present, combined to a lesser extent with the current temperature of the air. That means photochromics adjust automatically to indoor and outdoor light conditions. Photochromic lenses automatically adjust to outdoor lighting conditions by providing the right level of tint, and return automatically to their clear state; both indoors and at night. Polycarbonate and Trivex lenses are special high index lenses that offer superior impact resistance.
Watch a short video on photochromic lenses: