
Photochromic lenses
Last updated: February 22, 2008.
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 to take this photo. The lenses darken much more given time.
How normal sunglasses work

Photo: Ordinary sunglasses are simple light filters.
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.
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,
the lens 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!