Luminescence

Last updated: March 22, 2009.

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 are dim? Let's take a closer look!

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).

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.

Photo: An energy-saving compact fluorescent lamp (CFL). The fluorescent chemical is a kind of chalky coating on the inside of the thin glass tubes.

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 called photons. Shine ultraviolet (UV) 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 light.

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 glows. 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 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 different kinds of energy. Scientists almost have an entire A-Z (well a B-T anyway!) of words to describe the different kinds of luminescence:

• 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 X-rays.)
• Sonoluminescence: made by passing energetic sound waves through liquids.
• Thermoluminescence: made when photons are emitted from hot materials.
• Triboluminescence: made by rubbing, scratching, or physically deforming crystals.

What can we use luminescence for?

"Luminous" (phosphorescent) paints, energy-saving fluorescent 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 of crime.

Photo (left): The greenish glow of this oscilloscope screen is caused by phosphorescent chemicals that make green light when a beam of electrons strikes them, briefly "charging" them with energy. Photo by Ed McKenna courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

Photo (right): Safety stripe: fluorescent white paint makes this black jacket show up at night in car headlights or, in this case, in the flash of my camera.

Some uses of luminescence are even more surprising. Many washing detergents contain ingredients known as optical brighteners, which are really just phosphorescent chemicals. When UV light falls on recently washed clothes, atoms of these chemicals, left behind by the detergents, become excited and give off a kind of blueish light. Apparently this makes white clothes look cleaner and brighter, which is why TV commercials used to talk about "bluey whiteness" and holding 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!