Photochromic lenses change from light to dark depending on the amount of ultraviolet light they are exposed to. Early photochromics were strictly glass lenses, but today, you can choose from regular plastic, polycarbonate and high-index glass and plastic. Thirty-five years after their invention, traditional PhotoGray and PhotoBrown lenses from Corning Medical Optics are still on the market.
As their names suggest, the lenses are available in either gray or brown colors that are light enough to wear indoors and darken to a sunglass shade when exposed to ultraviolet light.The active ingredient that causes the lenses to transform is called silver halide and is mixed evenly throughout the lens. This means the whole lens will change when exposed to light. It also means that if a particularly strong prescription is made, the strongest, thickest part of the lens will be darker than the thinner parts.
Also, if there is a large difference in prescription between the two eyes, the lens with the stronger prescription will be darker than the weaker one. Corning has updated its offerings to include Thin and Dark photochromic glass lenses, which are up to 30 percent lighter in weight than traditional glass lenses and also thinner. These lenses are said to change from clear eyeglass to dark sunglass lenses in 60 seconds. Thin and Dark lenses are also available in gray and brown. Corning’s newest photochromics are SunSensors mid-index gray and brown plastic lenses.
A bit lighter in weight and thinner than regular plastic, these lenses have photochromic molecules mixed throughout the material, rather than just in the top layer. However, unlike their glass counterparts, SunSensors change within 60 seconds to a uniform color density throughout the lenses, regardless of the prescription or lens thickness, according to the company.