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.
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).
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.
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.
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.
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!
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.
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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?
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.
Photodegradation
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.
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."
Photochromic Materials: Preparation, Properties and
Applications by He Tian and Junji Zhang (eds). Wiley-VCH, 2016. A very comprehensive review of photochromic science and technology, including the latest cutting-edge applications (such as nanomaterials and quantum dots).
Photochromism: Molecules and Systems by Heinz Dürr and Henri Bouas-Laurent (eds). Gulf Publishing, 2003. Introduction to the various applications of photochromic technology, with an emphasis on the chemistry of photochromic molecules.
Physics and Chemistry of Photochromic Glasses by Alexander Victorovich Dotsenko et al. CRC Press, 1998. Describes how photochromic technology is used in eyeglasses and other optical instruments.
Brighter future for cricketers by Pranav Soneji. BBC News, 26 July 2007. Cricketers are now wearing photochromic sunglasses that help to show up red balls in bright sunlight.
Through A Glass, Darkly by Jim McCraw. The New York Times, February 11, 2000. Photochromic technology could soon be coming to a car near you!
Photochromic and Photosensitive Glass
by Donald M. Trotter Jr., Scientific American, Vol. 264, No. 4, April 1991, pp.124–129. An illustrated overview of photochromics and a quick review of some of their interesting applications. Also looks at photosensitive glass, which
changes permanently to produce interesting structural and decorative effects.
Photochromic Glass by Norbert J. Kreidl, Leonardo, Vol. 3, No. 4 (October 1970), pp. 429–432.
Patents
If you're looking for deeper technical details of how photochromic lenses work, these patents will be useful:
US Patent #3,208,860: Phototropic material and article made therefrom by William H. Armistead and Stanley D. Stookey, Corning Glass Works. Applied for July 31, 1962 and granted September 28, 1965. This is the original invention of photochromic glass: it describes the basic concept of reversible, photosensitive glass made using small amounts of silver crystals.
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