When I updated my eyeglass prescription recently, I decided to outfit my frames with photochromic lenses. What are photochromic lenses? They are lenses that change from transparent to dark as they and their wearer move from indoors to outdoors. We usually call them transitions lenses or just Transitions for the same reason we call synthetic floor coverings Linoleum and soda pop Coke; the people who make them know how to market their product. Unlike Linoleum and Coke, the brand name Transitions tells us something meaningful about the product, too.
But how do they do it?
At first I imagined millions of tiny shutters embedded in the lenses, opening and closing with the light. And, it turns out, that’s not far off. Instead of little Levolors, however, the outer layer of my plastic eyeglass lenses is embedded, to the depth of just 150 microns*, with millions of molecules of a light sensitive organic compound known as an oxazine. Organic compounds are those that include at least one carbon and an oxazine is an organic compound with one oxygen and one nitrogen arranged in a ring. When indoors or otherwise shielded from ultraviolet light, the molecules do not absorb visible light, and the lenses remain transparent. But when exposed to ultraviolet light, such as that from the sun, the molecules change shape, which causes them to absorb visible light, and the lenses gradually darken. The reverse happens when ultraviolet light is removed; the molecules return to their original shape and the lenses gradually become transparent again.
Had my eyeglasses been made of glass instead of plastic, they would have been embedded with the inorganic compound silver chloride, which has the same photochromic properties. Whatever the material, if eyes are windows to the soul, then photochromic lenses are blinds on those windows to the soul.
*How deep is that? A micron (or micrometre) is one one-millionth of a meter. A human hair is about 100 microns in diameter. So the oxazines in my glasses go down about a hair and a half.