Photochromic lenses are eyeglass lenses that darken automatically when exposed to ultraviolet (UV) light from the sun, but remain clear indoors. They are also known as transition lenses, variable tint lenses, light-adaptive lenses, and light intelligent lenses. The lenses are made of either polycarbonate or another plastic, or glass. Glass lenses use visible light to darken, while plastic lenses use organic photochromic molecules to achieve the reversible darkening effect. The first photochromic lenses were patented in the 1960s and were made of glass, while plastic versions followed in the 1980s and 1990s and are now the more popular choice among glasses wearers.
Photochromic lenses are ideal for people who need to wear glasses all the time, as they provide the benefits of glasses and sunglasses in a single frame. They protect the eyes from UV light and eliminate the need for a separate pair of prescription sunglasses. The lenses darken significantly within about a minute of exposure to bright light and take somewhat longer to clear. They are available in a range of clear and dark transmittances.
The technology behind photochromic lenses involves chemical reactions that are triggered by UV exposure. The photochromic molecules on the lens react to light and change their structure, causing the lens to darken. The molecules that adjust to light are built into the lens, so they wont rub off over time. Photochromic lenses are customizable and work with any frame, and are available in gray, dark gray, brown, and green, among other colors, depending on the brand and type.
