Photochromic lenses are lenses that automatically adjust to changing light conditions. These lenses get darker in bright conditions and lighter when conditions get darker.
Photochromic lenses reduce eyestrain and glare and protect against UV rays. As a combination of clear-lens prescription glasses and sunglasses, they make carrying two pairs of glasses at all times unnecessary.
Photochromic lenses work by changing from dark in bright conditions to clear inside or in dark conditions. This tint change is caused by a chemical reaction, as the slightly tinted dye molecules in the lenses are activated and become darker when exposed to UV wavelengths. Once the UV radiation source (light) is removed, the dye molecules revert to their indoor tint.
Photochromic lenses take 30 seconds to a minute to darken when exposed to sunlight. They will return to their near-clear state within 2 minutes in the absence of UV rays.
Photochromic lenses are sensitive to temperature. In extreme heat, a photochromic lens is unlikely to become completely dark no matter the lighting condition. Cold has the opposite effect, as lenses will become very dark in cold weather.
Most photochromic lenses do not darken when inside a car due to the UV-blocking properties of windshield glass.
Photochromic lenses are good for people who wear prescription lenses with lifestyles that take them to outdoor locations frequently. They may also be useful for athletes and adventurers in variable lighting conditions as the lenses will adjust their tint levels accordingly.