by Chris Woodford. Last updated: August 1, 2015.
Now here's a bright idea—a lamp that
saves you money and helps the environment! It lasts 10 times longer than a standard electric
lamp and uses 80 percent less energy. If you
care about tackling global warming, lamps like this
are a great place to start. During
its lifetime, a typical energy-saving lamp will stop about one ton of
carbon dioxide from entering the atmosphere and pay for itself many
times over. So it's good for your pocket and kind to the Earth as
well. But how exactly does it work?
Photo: A typical energy-saving compact fluorescent lamp (CFL). Light comes from the twin fluorescent tubes on the left. The cube-shaped base of the lamp contains a transformer and other electronics, as explained below.
Why ordinary (incandescent) lamps waste energy
To understand what's so good about energy-saving lamps, we first
need to understand what's so bad about ordinary ones.
Most lamps are incandescent. This means they give off light
because they are hot. A typical electric light bulb is a glass globe
with a very thin piece of wire inside it. The thin wire, called a
filament, gets extremely hot when electricity flows through it. Now,
hot things often give off light. Fires, for example, look red,
orange, yellow, or white because they are hot. Put an iron bar in a
fire and it will glow red when the temperature reaches about 950°C
(1750°F); this is what we mean by "red hot." If the temperature
rises to about 1100°C (2000°F), the bar glows yellow. If it
gets hotter still, say about 2500°C (4500°F) it will glow with a
bright, white light. The filament in a lightbulb looks white because
it is glowing white hot.
Photo: An incandescent lamp makes light when the filament gets white hot. It's hard to see this normally since you can't look at a lamp for very long without it hurting (or even damaging) your eyes. But this short exposure photo makes it clear exactly what's happening.
Hot iron looks red, yellow, or white because it is giving off
light—but why should it give off light at all? When you heat iron,
the atoms inside it absorb the heat energy you supply. The electrons inside the atoms push out farther from the nucleus to soak up this
extra energy. But this makes them unstable, so they quickly return to
their original or "ground" state. When they do so, they have to
get rid of some energy and do so by giving off a tiny packet
of light called a photon. Depending on how much energy they get
rid of, the photon appears as light of a particular color.
See our article on light for a fuller explanation of how atoms make light.
You might think heating up a bit of wire is a pretty inefficient
way to make light—and you'd be right. A fire, a hot iron bar, and
the wire filament in a lamp all give off light, but they also give
off heat. If making light is our only objective, any heat we make is
wasted energy. If you've ever put your hand near a typical
incandescent lamp, you'll know it gets incredibly hot—far too hot
to touch, so don't try it! In fact, an incandescent bulb wastes about
90 percent of the electricity it uses by getting hot.
Find out more in our article on incandescent lamps.
How compact fluorescent lamps (CFLs) work
Energy-saving lights save energy by making light without the
heat using a completely different process called
fluorescence. This is a trick similar to the one used by
creatures like fireflies and glow-worms, whose bodies contain chemicals that make
"cool light" without any heat. The general name for light made this way
From the outside, a fluorescent lamp looks simple enough, with two main
parts: a squarish base out of which two or more white glass tubes
emerge. Plug in the base and the tubes light up. What could be simpler? Inside, things are a bit more complex!
Here's how it all works:
- You plug the base into the power outlet.
- Just inside the base, where the case widens out, there's a small electronic
circuit, containing a transformer, that boosts the
voltage of the incoming electricity. (You can see a photo of the circuit below.) This means the lamp can produce more light than it would otherwise do and also helps to reduce flicker.
- The circuit is connected to a couple of electrical contacts called electrodes.
- When electricity flows into the electrodes, electrons (shown here as red
dots) boil" from their surface and shoot off down the thin white tubes, which contain
mercury gas, shown here as bigger blue dots.
- As the electrons hurtle down the tubes, they collide with atoms of the mercury.
The collisions give the mercury atoms energy so their electrons jump to higher energy levels. But this
makes the mercury atoms unstable, so the electrons quickly return to
their ground states. When they do so, they give off photons of
invisible ultraviolet light (slightly higher frequency than the blue
light we can see), shown here as a purple wiggly line.
- If fluorescent lights make invisible light, how come they glow
white? Here's the clever part. The thin glass tubes of a fluorescent
light are covered in white-colored chemicals called phosphors. When
the ultraviolet light strikes atom of the phosphors (shown here as gray dots), it excites
their electrons in just the same way that the mercury atoms were
excited. This makes the phosphor atoms unstable, so they give off
their excess energy as photons—which, this time, happen to be visible,
white light (indicated here with yellow wiggly lines).
So, in short, fluorescent lights make their energy in a three-step process:
- Electrodes take electrical energy from the power supply and generate moving electrons.
- The moving electrons collide with mercury atoms in the tubes to make ultraviolet light.
- The white phosphor coating of the tubes converts the ultraviolet light into visible light (that we can see).
Photo: The electronic circuit inside an energy-saving lamp.
The transformer is the big orange/gold thing in the center. The black cylinder
on the left is a capacitor. The four silver colored contacts
on the extreme right are where the electrodes attach.
What's inside a compact fluorescent lamp?
In case you're wondering, here's what a compact fluorescent light actually looks like inside.
(Don't break one apart yourself; there is some health risk from the mercury inside if you do so.)
Sorry the photo is a bit blurred. Next time one of my lamps breaks, I'll take a better photo!
Photo: Inside a compact fluorescent lamp. The numbers on this photo correspond to the numbers in the artwork up above: 1) Connection to power socket; 2) Transformer circuit; 3) Electrodes; 4-6) Glass tubes with white phosphor coating inside.
How much do they save?
You often hear people say that lamps like this pay for themselves quite quickly, but is that really true?
Let's crunch the numbers and find out!
Suppose you buy an incandescent lamp rated at 100 watts and it lasts for 1000 hours. In its lifetime, it will use 0.1 kilowatts (100 watts) × 1000 hours = 100 kilowatt hours, which is equivalent to 100 units of electricity or 360 megajoules. If you're in the USA, electricity might cost you 15–20 cents per kilowatt hour, making the total lifetime electricity cost around $15–20. (If you're in the UK, electricity would cost you probably 15–20p per unit, making the total lifetime cost of electricity £15–20.) A quick look around online
reveals that you could buy a packet of 10 energy-saving lamps for that price. So it would take only 100 hours' worth
of electricity to buy one equivalent, energy-saving lamp. For the sake of easy math, let's say an energy-saving lamp equivalent to an old-style 100 watt lamp uses 20 watts and lasts for 10,000 hours (so it will use five times less electricity and cost only a fifth as much to run). For every 1000 hours that it runs, it will save you 80 units of electricity. Ten lamps running for 1000 hours will save you 800 units of electricity or roughly $120–160.
In other words, switching to low-energy lamps is definitely a no-brainer: they easily pay for themselves.
CFL or LED?
That's not to say that energy-saving fluorescent lamps aren't without drawbacks. As you can see from the photo up above, there's lots of electronic circuitry inside them and that's hard to recycle at the end of their life, so most lamps like this end up in landfills. The mercury inside them is toxic, so if you break a bulb inside your home, there's a small risk to your health (and the mercury pollutes the landfill as well). Since compact fluorescent lamps were invented, better alternatives have started to appear. LED lamps, for example, are even more energy efficient, last quite a bit longer, contain far fewer electronic components, and don't contain toxic mercury, though, for the moment at least, are quite a bit more expensive to buy.
Find out more
On this website
On other sites
- All About Fluorescent Lighting by Evan Powell, Popular Science, August 1975. An old but nonetheless interesting article about the general principles of fluorescent lamps.
- How it Works: Fluorescent Lamp by Merle Henkenius, Popular Science, October 1991. Explains the workings of a fluorescent strip lamp.
- Light bulbs: Not such a bright idea by Matt Prescott, BBC News, 3 February 2006. Matt explains his Ban the Bulb campaign to scrap incandescent bulbs and help the world save energy.
Fluorescent lamps go green by BBC News, January 1999. Explains efforts to develop safer, mercury-free fluorescent lamps.
- Light by David MacKay. This short chapter from David's excellent book Sustainable Energy Without the Hot Air explains the science behind home lighting, concluding that you should switch over immediately because "continuing to use an old lightbulb is throwing good money after bad."
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.