by Chris Woodford. Last updated: November 28, 2016.
What makes night-time cities fizz and crackle
with life? Brightly colored neon lamps play a huge part. If you've ever seen the lights
dancing in Tokyo, New York City, or London, you'll know
exactly what I mean. Whole streets seem to leap alive the minute
the neon switches on. Strictly speaking, lamps filled with neon gas
can make only red light and you need other gases to make other
colors. In fact, by mixing different gases, it's possible to make
over 150 different colors of "neon" light—and paint the night
sky with almost any color you like! Let's take a closer look at how
these things work.
Photo: Neon lights in the Shinjuku district of Tokyo, Japan.
Photo by Lynn Jenkins courtesy of Defense Imagery.
How atoms make light come to life
If you've read our article on light, you'll know that atoms
produce light when they absorb energy and become "excited". In
their excited state, they're also unstable—so they rapidly give out
the energy they absorbed to get themselves back to normal again. They
do this by giving out tiny packets of light energy
You can use this idea to make
an electric light. Suppose you fill a tube full of atoms and
seal it up at both ends. Now put some sort of electrical
device inside the tube that can keep feeding energy to the atoms.
When you switch on the power, the atoms will constantly get
excited and give out light. That's pretty much how a fluorescent lamp
works—and it's how a neon lamp works as well.
(Let's also note in passing that it's how a laser works too, although in a laser
the outgoing light is made into a super-concentrated beam.)
Photo: Neon lights in the Shinjuku district of Tokyo, Japan.
Look closely and you'll see that each "stripe" in the display is made from a separate
glass tube, a bit like the fluorescent strip lights you might have in your kitchen or classroom.
The "stars" contain single tubes that have been heated, bent round into angles while hot,
then allowed to cool again. You can make all kinds of letters, characters, and
other shapes by bending tubes in this way. Photo by Lynn Jenkins courtesy of Defense Imagery.
Why is neon light red?
Photo: When electrons in neon atoms return from their "excited" state to their "ground" (unexcited) state,
they give out packets of energy called quanta that our eyes see as red light.
In argon atoms, the quanta are bigger and our eyes see them as higher-frequency blue light.
The energy levels inside atoms are a bit like rungs on a ladder or
steps on a staircase. Electrons can only be on the rungs or steps,
not on the gaps in between. That means atoms can absorb or release
energy only in fixed-sized packets (called quanta, which is
the plural of quantum) and atoms of different chemical elements will
give out quanta that are bigger or smaller, depending on their
precise inner structure. Atoms that give out bigger quanta of energy
make higher-frequency (bluer) light than atoms that give out smaller quanta.
In neon, the quanta of energy that are given out correspond exactly
with light that we see as red. Other noble gases make light of
different colors. Argon, for example, makes blue light—so when you
see "neon" lamps shining blue, you're actually looking at tubes
filled with argon, not neon. There are two ways to make other colors.
You can put more than one gas in the same "neon" tube. To make green tubes,
you need neon and argon together. For purple, you'd use argon and
xenon. You can also change the color that a tube makes by painting its walls
with phosphors of different colors. So you can use a blue phosphor painted on a red
neon tube to make pink light, or a green phosphor with red neon to make orange light.
Why "cold cathode"?
You'll sometimes see neon signs referred to as an example of cold-cathode lighting. That doesn't make
sense unless you understand that various other electrical devices use a hot cathode. But what is a cathode anyway...?
In a lamp with two electrical terms, the positive terminal is called the anode (the green terminal in the artwork up above) and the negative one is called the cathode (the blue terminal on the left). In a hot-cathode device, the cathode has to be heated with a filament (a small heating element) so electrons "boil off" its surface—and then do something useful. Hot cathodes were used in vacuum tubes, which were used as computer switches before transistors were invented. They're also used in cathode-ray tubes, such as the ones that make the picture in an old-fashioned TV (one of those really old ones that sticks out at the back) and the
graph-traces on oscilloscopes. The light you see on this kind of TV screen comes from energy released when the cathode is heated up so it gives off electrons. The electrons (which were historically known as "cathode rays") make your TV picture when they fly down the tube and smash into the phosphor-coated screen at the front. In a neon lamp, light is made by exciting gas atoms in the space between two electrodes and there's no need for a hot cathode. But just because the cathode isn't heated, it doesn't follow that a neon lamp is cold; indeed, you'll find neon lamps surprisingly hot if you stand anywhere near them!
Who invented neon lamps?
Artwork: From US Patent 1,125,476: System of illuminating by luminescent tubes by Georges Claude. Jan 19, 1915, courtesy of US Patent and Trademark Office.
French chemical engineer Georges Claude (1870–1960) invented the neon lamp, filing his original French patent on March 7, 1910. The same year, he made his first, spectacular public demonstration of neon lighting at the Paris Motor Show, using two gigantic 12m (39ft) lamps. By 1913, neon lighting was being used in French advertising signs, though it didn't find its way to the United States until the 1920s.
Here's a fascinating illustration from Claude's US patent, filed in 1915. You can see the main neon tube at the top (which I've
colored red), with its two electrodes (yellow) at either end. Claude had discovered that impurities in neon gas could seriously affect how well his lamps worked, so the bits of apparatus in the middle were designed to purify the neon before the tube was electrically lit, then removed. The blue line is a link to a vacuum pump; the green flask contains wood or charcoal to absorb impurities; and the larger orange vessel underneath contains liquid air. Together, these things carried out what Claude described as "an exceedingly efficacious purification process on the spot in order to obtain a high degree of purity of the neon."