by Chris Woodford. Last updated: November 23, 2016.
When you wake up in the middle of the night, not sure where you
are, there's nothing more reassuring than the luminous dial of a
watch. You don't need to find a light: just glance at your wrist and
you know exactly what time it is. Watches like this glow all day
long—we just don't notice their ghostly shine in the daytime. What
makes them glow at night, long after all the other sources of light
Photo: This "luminous" watch dial is coated with phosphorescent paint so it glows in the dark. It's surprisingly hard to photograph (without cheating!) because it gives off very little light.
What is luminescence?
Luminous simply means giving off light; most things in our
world produce light because they have energy that originally came
from the Sun, which is the biggest, most luminous thing we can see.
Strictly speaking, although the Moon appears to give off light, it's
not actually luminous because it's simply reflecting light from the
Sun like a giant mirror made of rock. Luminous is quite a vague word
really. Arguably, even a flashlight bulb is luminous, because it turns
electricity (electrical energy) into light and shines it toward us. But bulbs like
this are incandescent and make light by making heat. Luminescent
things, by contrast, make light when their atoms become excited in a process that needs little or no heat
to make it happen.
Photo: Luminous doesn't mean "glows in the dark": it means an object is giving off light it produces itself. Strictly speaking, that means the Sun (left) is luminous but the Moon (right) is not.
Pictures courtesy of NASA Goddard Space Flight Center (left) and NASA Jet Propulsion Laboratory (right), via NASA on the Commons.
What's the difference between luminescence, fluorescence, and phosphorescence?
When we talk about "luminous" watches and paint,
what we really mean is phosphorescence, which is
very similar to fluorescence: the process by which energy-saving
lamps make light.
Fluorescent materials produce light instantly, when the atoms
inside them absorb energy and become "excited." When the atoms return
to normal, in as little as a hundred thousandth of a second, they
give out the energy that excited them as tiny particles of light
Shine ultraviolet (UV) light light on a stolen
TV or camera and you might find someone's address shining back at you, written in invisible ink. The ink is made of
fluorescent chemicals that absorb energy from the UV light, become
excited, and then give out the energy as photons of visible light.
Switch off the UV light and the ink disappears again. You can read
more about how atoms make light in the feature box in our article on
Photo: An energy-saving compact fluorescent lamp (CFL). The fluorescent chemical is a kind of chalky white coating on the inside of the thin glass tubes. You might have noticed that lamps like this continue glowing a little bit even after you switch them off? Like luminous watches, the phosphor chemicals are still excited enough to give off light for some time after they've been stimulated.
Phosphorescent materials work in much the same way as fluorescent
ones, except that there's a delay between them absorbing energy and
giving out light. Sometimes phosphorescence lasts for a few seconds
after the stimulating energy has been removed; sometimes—as in
luminous watches—it lasts for hours. You've probably noticed that it
takes a bit of time to "charge up" a luminous watch with energy
before it will glow in the dark. You might have also noticed that a luminous watch
shines most in the early part of the night. By the time dawn breaks,
it's typically run out of energy and stopped glowing.
That should come as no real surprise. A watch can't make
light out of nothing at all without violating one of
the most basic laws of physics—the
conservation of energy.
What other types of luminescence are there?
Shine light on a luminous watch and it shines straight back at you. That's
an example of what we call photoluminescence: luminescence made by
light. But you can make things give off light by exciting their atoms
with many other kinds of energy. You give the atoms one kind
of energy (light, heat, sound, or whatever) and they give the same energy back
to you as light. Scientists almost have an entire A-Z
(well a B-T anyway!) of words to describe the different kinds of
- Bioluminescence: made by living creatures such as
fireflies, glow-worms, and many marine creatures.
- Chemoluminescence: made by a chemical reaction. Glow
sticks work this way.
- Electroluminescence: made by passing electricity
through something like a gas.
- Photoluminescence: made by shining light at
"luminous" (phosphorescent) paints.
- Röntgenoluminescence: made by shining X-rays at
things. (The curious name comes from Wilhelm Röntgen (1845–1923), the discoverer of
- Sonoluminescence: made by passing energetic sound waves
- Thermoluminescence: made when photons are emitted from
- Triboluminescence: made by rubbing, scratching, or
physically deforming crystals.
Lights in the night
Fireflies and glow-worms
Fireflies and glow-worms (their larvae) are the best-known examples of bioluminescent
creatures. They use a complex reaction to make light from a
pair of chemicals called luciferin and luciferase stored in their
tails. Bioluminescence is a special kind of chemoluminescence that
happens inside living things.
Creatures of the deep
Squid, shrimp, sardines, plankton, starfish, and all kinds of
other marine creatures use bioluminescence for communication,
camouflage, or defense—flashing to attract mates or warn off
Photo: Bioluminescence in action. Left:
crinoids bioluminescing in the North Atlantic.
Photo courtesy of Bioluminescence 2009 Expedition, NOAA/OER,
published on Flickr
under a Creative Commons Licence.
Right: A bioluminescent ctenophore.
Photo courtesy of NOAA Okeanos Explorer Program, Gulf of Mexico 2012 Expedition,
published on Flickr
under a Creative Commons Licence.
What can we use luminescence for?
"Luminous" (phosphorescent) paints,
lamps, and fluorescent (high-visibility) jackets are obvious
examples. But there are many other ways we use luminescence too.
Old-style, cathode-ray television sets (and
oscilloscopes) make pictures by
firing electron guns at a screen coated with phosphors
(phosphorescent chemicals). Lasers make their powerful beams by a
process called stimulated emission, which happens when atoms are
forced to give off photons over and over again. UV lights are used to
produce phosphorescence in a variety of medical tests, in
archaeological research, and in forensic science to aid the detection
Photo (left): The ghostly glow of this oven timer is caused by phosphors that make green light when electrons strike them, briefly "charging" them with energy. It's an example of what's called
a vacuum fluorescent display..
Photo (right): Safety stripe: old-style, fluorescent silver paint makes this black jacket show up at night in car headlights or, in this case, in the flash of my camera. This is the sort of low-tech, high-visibility that has been around for decades and its big drawback is that it quickly dulls and loses its reflectiveness. Newer high-visibility jackets and vests have retroflective fabric sewn directly into them. It's made from materials such as 3M™ Scotchlite™, which uses tiny reflective beads to throw back more light. It's far brighter than old-style paint and lasts much longer.
Some uses of luminescence are even more surprising. Many
washing detergents contain ingredients known as optical
brighteners, which are actually phosphorescent chemicals. Sunlight contains a mixture of ordinary, visible light
(which our eyes can see) and ultraviolet light (which we can't see). When sunlight falls on recently washed clothes, atoms of the optical-brightener chemicals, left behind by the detergents, become excited and convert the sun's ultraviolet
light into ordinary light. As a result, when you look at freshly washed white
clothes, you're supposed to see brighter, slightly bluer reflected light produced
by the optical brighteners. The idea is that your clothes look cleaner and
brighter, which is why TV commercials used to talk about "bluey
whiteness" and featured smiling people holding their clothes up to a window (where there's more
UV-rich sunlight) to see it. It's amazing some of the places where
you find science—even lurking in your laundry!
How fluorescent light beats burglars!
The best intruder alarm in the world can't always keep thieves out
of your home and if your valuables get stolen they're often gone
for good. Even if the police catch the crooks and recover some of
their loot, how can they ever return it to its rightful owners? Who
knows which camera or TV belongs to which person? Science offers a
really easy solution! All you have to do is mark your property with
an invisible, fluorescent ink that shows up only in
ultraviolet light. When the police recover stolen property, they wave an ultraviolet lamp
over it, the markings (maybe your name or zip code) show up, and they
instantly find out to whom it belongs.
Photo: Ultraviolet lamps like this can be
used to show up the "invisible" security inks that deter thieves.
Photo by Warren Gretz courtesy of
US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
Now, if the ink is invisible and shows up only in invisible
ultraviolet light, how come you can see it when you shine one of
those special lights on it? As we've already seen, atoms make light when they absorb energy,
then emit (give out) the same energy a few moments later. What happens with invisible security ink is that the atoms absorb
ultraviolet light, but then give out a slightly
different, blueish light that our eyes can see. (This is like the
process that happens in the white outer coating of a
fluorescent lamp, which converts ultraviolet light made inside the tube into
visible light that brightens up our homes.)
Photo: How invisible security inks and paints work, compared to normal inks and paints. 1) In ordinary
white light (colored yellow here so it shows up), normal inks show up because they absorb all light rays except those of their own color, which they reflect. So red ink looks red in white light. 2) In UV light, ordinary inks tend to turn black. 3) When white light (again colored yellow in this diagram) shines on invisible UV ink, the ink reflects the light as light our eyes can't see—so it remains invisible. 4) In UV light, the invisible ink reflects visible light—so it shows up red or another color.
If you liked this article...
You might like my new book, Atoms Under the Floorboards: The Surprising Science Hidden in Your Home, published worldwide by Bloomsbury.