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Photochromic eyeglasses with one lens darker than the other

Photochromic lenses

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—sold under popular brand names such as Transitions® and (some years ago) Reactolite Rapide. 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, but how effectively they work depends on the ambient temperature and how old they are.

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  1. How normal sunglasses work
  2. Who invented photochromic lenses?
  3. How modern photochromic lenses work
  4. Drawbacks of photochromic glasses
  5. What are the alternatives?
  6. Photodegradation
  7. Find out more

How normal sunglasses work

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 polarization. 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. Polarizing 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).

Rotating two pairs of polarizing sunglasses (or two polarizing filters), you can block out light waves.

Photo: Polarizing sunglasses block all light waves except those vibrating in one direction. So if you hold two pairs in front of one another and slowly rotate one of them, you'll see the overlapping lenses gradually turn black, then lighten again.

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 (ordinary glass windows generally filter it out) so photochromic lenses remain clear; outdoors, where there's quite a bit of UV light coming down from the sun, they darken.

Who invented photochromic lenses?

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 glass has been around since the early 1960s, when it was invented by William H. Armistead and Stanley Donald Stookey of Corning Glass Works (their US patent 3,208,860 describing the idea, titled Phototropic material and article made therefrom, was filed on July 31, 1962). In those days, photochromic glass worked a bit like pieces of old-fashioned, photographic film. Film darkens because it contains silver-based crystals that clump together when light falls on them. Early photochromic lenses contained similar silver crystals and darkened in a similar way: when light hit them, some of the silver crystals changed into microscopic bits of silver.

A piece of 35mm photographic film.

Photo: Early photochromic lenses contained silver compounds (such as silver chloride) and reacted to light much the same way as old-fashioned photographic film, like this. Unlike film, the reaction was reversible: the lenses soon lightened again.

How can you see through lenses made with opaque silver? As Armistead and Stookey explain in their original patent, only tiny quantities of silver crystals are needed (less than 0.1 percent by volume), and each crystal is less than 0.1 microns (one ten millionth of a meter—or about 100 times thinner than a human hair) in diameter. Unlike photographic film, which darkens permanently, the photochromic lenses could change back again and clear when the light level fell back to normal. In the 1970s, the British Pilkington glass company helped to popularize photochromic lenses by introducing brands called Reactolite and Reactolite Rapide (according to the US Patent and Trademark Office, these two trademarks were applied for in the United States in 1978 and both have since been cancelled).

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How modern photochromic lenses work

Modern photochromic lenses tend to be plastic and instead of silver chemicals they contain organic (carbon-based) molecules called naphthopyrans that react to light in a slightly different way: they subtly change their molecular structure when ultraviolet light strikes them. In this altered form, they soak up more ordinary light as it tries to pass by (technically, we say they have a different absorption spectrum), which is what makes the lenses darken. Imagine lots of molecules suddenly darkening 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.

Window blinds open, half-closed, and fully closed

Photo: Molecules inside photochromic lenses cut off light like rapidly closing blinds. As the molecules change their structure, they absorb more of the light passing through, and that's what darkens your lenses.

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.

An example of a reversible chemical reaction that makes napthopyran molecules photochromic.

Artwork: When napthopyran molecules are exposed to UV light, their structure changes reversibly. This diagram shows a generic example of such a reaction and is redrawn from Joseph Framingham et al's description in US Patent #3,627,690: Photochromic naphthopyran compounds. You can see that the ultraviolet light powers a restructuring of the molecule on the left, which is cleaved between the carbon and the oxygen atom.

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!

Temperature dependency of photochromics: simplified artwork showing typical darkening performance of photochromic lenses in winter and summer.

Chart: Temperature matters: You'll probably find your photochromic lenses darken much more in winter than in summer: they get darker, faster in the cold. Chart shows: percentage light transmission after the same time period in hot (top) or cold (bottom) temperatures. Drawn using data from Figure 2.10 of "Chapter 2: Spirooxazines" by Shuichi Maeda in Organic Photochromic and Thermochromic Compounds by John C. Crano and Robert J. Gugliemetti (eds), Kluwer, 2002.

A related problem is that photochromic lenses don't always work effectively in cars, because ordinary glass windscreens naturally screen out most of the ultraviolet light. That means drivers really need a second pair of tinted or polarized sunglasses just for driving in. Although you can get "hybrid" sunglasses that combine photochromism and polarizing in a single lens, the polarization only really works effectively when the photochromism has already darkened the lenses, so they don't work that well indoors (or inside cars) either.

One final difficulty is that photochromic lenses don't last forever. Although they don't actually wear out, after a few years of continuous darkening and lightening, they become noticeably less reactive, particularly indoors. [1] 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.) If, like me, you have a stable eye prescription and don't wear your eyeglasses too often, you might find it more of a nuisance: my photochromic lenses seem to have stopped reacting as effectively roughly five years after I bought them.

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.

What are the alternatives?

Experimental electrochromic Fast-Tint Protective Eyewear (FTPE) lenses from the ONR.

Photo: Electrochromics (electrically changing lenses) are a promising alternative to photochromics. These are experimental electrochromic (FTPE) sunglasses developed by the Office of Naval Research (ONR). Photo by John F. Williams courtesy of US Navy and DVIDS.

If inventors always think along the same lines, they'll never come up with anything radically new: every idea they have will be a matter of evolution, not revolution. Just as photochromic lenses were a departure from colored filters and polarizing lenses, so there are other options for keeping the sun out of your eyes.

One promising possibility is electrochromic technology, in which small currents of electricity are used to change the orientation of liquid crystals in a thin film, allowing more or less light to get through. LCD sunglasses like this can switch between light and dark much more quickly than traditional photochromics and, because they're manually controlled (at the push of a button), tend to work better indoors (in things like cars) where photochromics struggle to perform in the lack of ultraviolet light. A few years ago, The Office of Naval Research (ONR) developed lenses like this that it called Fast-Tint Protective Eyewear (FTPE), which could be used as protective eyeglasses by Navy SEALs.


Photochromic lenses are plastics that change color reversibly: they lighten up as soon as you take them away from the Sun. Lots of ordinary plastics also change color after exposure to sunlight, but not in a reversible way. Some transparent plastics gradually turn opaque, while others that start off a clear, "white" color gradually turn yellow. This slow, unhappy color-change of plastics is called photodegradation and it's caused by the infrared and ultraviolet wavelengths in sunlight, which chop large plastic molecules into smaller pieces.

Two plastic containers, one that has photodegraded to a yellow color and one that is still transparent

Photo: Photodegradation in action. Left: This piece of plastic packaging is about 20 years old and has photodegraded so much that it's almost turned brown! It's also very brittle now and pieces break off it easily. It started off completely clear, like the packaging on the right, which is less than a year old.

Photo-degradation can be a real nuisance. In 2007, thousands of brand new, bright red seats had to be replaced at London's Wembley Stadium after the Sun rapidly turned them an unattractive pink color, not long after they'd been installed. Sometimes, though, photo-degradation can be much more helpful. By breaking down the big molecules in plastics into small pieces, it can help to destroy waste that would otherwise survive in the environment for several hundred years.

Some bioplastics and biodegradable plastics are deliberately designed to degrade this way. For example, incorporating a tiny proportion of carbonyl groups into the polymers (repeating, long-chain molecules) from which plastics are made can make them start to photodegrade within days of exposure to sunlight. But that doesn't necessarily help when a lot of our trash is still buried in landfills—deep, dark valleys of garbage where sunlight never sets foot. According to the US FTC, you should only believe claims that plastics are truly biodegradable if the manufacturers provide "scientific evidence that their product will completely decompose within a reasonably short period of time under customary methods of disposal."

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If you're looking for deeper technical details of how photochromic lenses work, these patents will be useful:


  1.    See System for Ophthalmic Dispensing by Clifford W. Brooks, Elsevier Health Sciences, 2023, p.451.

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Text copyright © Chris Woodford 2008, 2020. All rights reserved. Full copyright notice and terms of use.

Transitions is a registered trademark of Transitions Optical, Inc.

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