Photochromic lenses
by Chris Woodford. Last updated: June 7, 2011.
Wearing eyeglasses can often be a pain. If it's raining, you're wiping water off the lenses; if it's humid, the lenses mist up; and if it's sunny, you don't know whether to wear your normal glasses or your shades and you may have to keeping switching between the two! Many people who wear eyeglasses have found a solution to the last of these problems by changing over to photochromic lenses. They look clear indoors or in poor light, but in sunlight they darken automatically and effectively turn your normal glasses into shades. It's pretty cool technology—but how exactly does it work?
Photo: Definitely darker. I took my photochromic eyeglasses out into the light with the left lens covered, then returned a few minutes later to take this photo. The lenses darken much more given time.
How normal sunglasses work
Photo: Ordinary sunglasses are simple light filters: by restricting the wavelengths of light that get through, they effectively reduce the quantity of light passing through as well—giving you a darkened picture of the world.
Normal sunglasses work by blocking out some of the light in one of two ways. Most of them are really just colored filters: they let through only light of a certain color (the color of the lens) and block out the rest. Since only a fraction of the light gets through, you see a darkened (and colored) picture. The other type of sunglasses use polarisation. Light travels in a wave motion—a bit like the waves on the sea. But where ocean waves vibrate only up and down, light waves wriggle in every direction. Polarising lenses are a bit like slits that let through only light waves vibrating in a single direction. So, just like colored lenses, they let through only a fraction of the light and you see a darkened view of the world (typically grayer, rather than colored).
Photochromic lenses are completely different, because they work by reacting to ultraviolet (UV) light—the light that's just too blue for our eyes to see. Indoors, there is hardly any UV light so photochromic lenses remain clear; outdoors, where there's quite a bit of UV light coming down from the sun, they darken.
How photochromic lenses work
First let's cut through the jargon. The word "photochromic" comes from two Greek words: "photos" meaning light and "chroma" meaning color. So photochromic simply means something that changes color in response to light. Photochromic lenses have been around since the 1960s. In those days, they were made of glass and worked a bit like pieces of old-fashioned, photographic film. Film darkens because it contains silver-based chemicals that clump together when light falls on them. Early photochromic lenses contained similar silver chemicals and darkened the same way. Unlike photographic film, which darkens permanently, the photochromic lenses could change back again and clear when the light level fell back to normal.
Modern photochromic lenses tend to be plastic and instead of silver chemicals they contain organic (carbon-based) molecules that react to light in a slightly different way. Instead of clumping together, they change their color and size when ultraviolet light strikes them. Although each molecule changes by only a fraction of a nanometer, when many molecules respond in the same way the effect can be more dramatic. Imagine lots of molecules suddenly darkening and fattening up inside a clear lens. It's a bit like closing the blinds in front of your window on a sunny day: as the slats turn, they progressively block out more and more light.
Photo: Molecules inside photochromic lenses cut off light like rapidly closing blinds. As the molecules change size, the space between them is reduced, and that's what shuts out the light.
You might think all of this would take quite a bit of time, but photochromic lenses respond remarkably quickly. About half the darkening happens within the first minute and they're cutting out about 80% of sunlight within 15 minutes.
Drawbacks of photochromic glasses
Unfortunately, it takes a little bit longer for photochromic lenses to clear than it does for them to darken in the first place. Generally, they let through about 60% of light again after you've been back indoors for five minutes. However, it can take up to an hour for them to clear completely. You might also be surprised to find that your photochromic lenses darken more or less every time you go outside whether it's sunny or not; that's because they're reacting to ultraviolet light—and there's always plenty of that about even on cloudy days.
A more serious drawback is that the photochromic molecules react to temperature as well as light: they darken much more in cold conditions. This means your photochromic sunglasses will give really effective performance in winter (when you probably don't need it) and work somewhat less well in summer (when effective sunglasses are more of a priority). This temperature effect can sometimes be a real problem: the lenses can darken so much that they make driving dangerous in really cold and snowy places, so you're recommended not to wear photochromic lenses for something like driving a snowmobile!
A related problem is that photochromic lenses don't always work effectively in cars, because windscreens naturally screen out most of the ultraviolet light. That means drivers really need a second pair of tinted or polarised sunglasses just for driving in.
One final difficulty is that photochromic lenses don't last forever. After three years or so of continuous darkening and lightening, they become noticeably less reactive. This is less of a problem than it sounds, since many people change their eyeglasses at least this often. (Generally, you should get your eyes tested at least every two years and more often if you're older.)
But all these things aside, photochromic lenses are a brilliant solution for people who need different glasses for different conditions and hate constantly switching between their normal glasses and their shades. Come the summer, I certainly wouldn't be without mine!
