Xenon lamps and arc lamps
Last updated: June 20, 2010.
You may have only a fraction of a second to catch a vital
photograph, but what if it's
too dark to see? Flash lamps, filled with a gas called xenon, are the
answer. Press the button on your camera, wait a few moments for
the flash to charge, hit the shutter button to take your picture
and—ZAP!—you suddenly have all the light you need. What exactly
are xenon lamps and how do they work? They're examples of what we call
arc lamps and they work in a very different way
to ordinary lamps. Let's take a closer look!
Photo: Lighthouse lamp: It takes an extremely bright light to throw a beam miles out to sea, even with the help of a powerful Fresnel lens (the concentric circles you can see in the background). That's why many lighthouses are powered by super-bright xenon lamps. Photo by Gary Nichols courtesy of
US Navy.
How do xenon lamps work?
Photo: An arc lamp is like a small, contained, bolt of lightning. Picture by Dave Parsons courtesy of US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).
All lamps make light, but they don't all work the same way. Incandescent lamps (our traditional household lights) make light by passing electricity through a thin metal filament (wire), so it gets really hot and burns brightly. Fluorescent lamps are very different: they zap electricity through a gas to make invisible ultraviolet
light, which gets converted into light we can see (visible light) when it passes through the white inner coating of the lamp's glass tube, making it glow brightly (or fluoresce).
Like neon lamps, xenon lamps are examples of
arc lamps. An arc lamp is a bit
like a small bolt of lightning happening under very controlled
conditions inside a glass tube
filled with a gas under either very low or very high pressure
(depending on the type of lamp). At the two ends of the tube there are
metal contacts called electrodes, connected to a high-voltage power
supply.
Where does the light come from? When the power supply is turned on, the gas
atoms suddenly find themselves under incredible, electrical force
and split into smaller parts. This is called ionization (or ionizing the gas).
The broken bits of atoms (positively charged ions and negatively charged electrons) then hurtle in
opposite directions along the tube, with electrons rushing to the positive electrode and ions going the other way,
forming an electric current. The ions and electrodes crash into one another and into the electrodes,
giving off energy as a flash of light called an arc that
effectively leaps the gap between the electrodes—just like a lightning bolt.
More light is produced by the electrodes themselves, which become incredibly hot and burn brightly in the process.
Temperatures of over 3000°C or 5400°F are typical, which is why the electrodes are generally made of tungsten, the metal with the highest
melting point (approximately 3400°C or 6200°F). The color of the light depends on the atomic structure of the gas that's used (we explain this in more detail in our article on neon lamps). In a neon lamp, the light produced is red; in a xenon lamp, it's a blueish, greyish, whiteish light not that different from natural daylight.
Photo: Electric arcs generate so much heat and such
high temperatures that they can also be used to weld metals together.
This is known as arc welding.
Picture taken at Idaho National Laboratory, Idaho Falls, courtesy of US Department of Energy (DOE Photo).
The original arc lamps
Strictly speaking, we use the term arc lamp to mean one, particular type of
arc lamp with carbon electrodes and air in between them. Before Edison, Swan, and their contemporaries perfected the
incandescent lamp, arc lamps like this were really the only type
of electric light available. They were invented in 1807 (about 70 years before Edison perfected his lamp) by British chemist Sir
Humphry Davy (1778–1829).
Davy found he could make electric light by connecting two carbon electrodes (a bit like pencils) to a high-voltage power supply.
Initially, he kept the electrodes touching one another. Gradually, as he moved them apart, he found an arch-shaped beam of light spanning the gap between them—which is how "arc" lamps got their name. Arc lamps weren't very practical: they needed
huge electric current to make them work and the high temperature of the arc quickly burned the carbon electrodes away in
the air. "Huge" electric current is no exaggeration: Davy had to use a battery with 2000 separate cells to make a 10cm (4-inch) arc.
Modern incandescent lamps developed when arc lamps were improved in two ways. The air gap was replaced by a
filament, so lower voltages and currents could be used. The whole lamp was also sealed inside a glass bulb filled with a noble
gas to prevent the filament from burning up in the air's oxygen. That made the lamp last very much longer.
Photo: The basic concept of the arc lamp. An electric discharge passes between two carbon electrodes,
giving off light.
What are the different kinds of xenon lamps?
In xenon photographic flash lamps, the light is literally a flash: it lasts anything from a
microsecond (one millionth of a second) to about a twentieth of a second. There's no real
need for it to last any longer since it only takes this long to
capture a photo. Other kinds of xenon lamps work more like neon lamps
and produce smaller amounts of light continually. Instead of passing
a huge amount of electricity through the gas very briefly to produce
a sudden "arc" of light, they use smaller, steadier voltages to
produce a constant discharge of bright light.
Movie-projector lamps, lighthouse lamps, and HID (high-intensity
discharge) car headlamps are all examples of xenon lamps that work in
this way.
Photo: Here's a very small xenon flash lamp from inside a digital camera. The black and red wires connect the two electrodes at opposite ends of the lamp to a
capacitor (the black cylinder just visible on the top left). The capacitor's job is to build up a high-voltage charge that's big enough to make a discharge in the flash tube using only the camera's puny, low-voltage
batteries. That takes time—which is why you often have to wait a few seconds to take a flash photo. The camera lens is the black circle underneath the flash. Flash lamps that work this way were
invented in 1931 by American electrical engineer and photographer Harold E. Edgerton (1903–1990).
What is xenon anyway?
You've heard of neon? Xenon is similar. Helium, neon, argon, krypton, xenon, and
radon are the chemical elements from the part of the periodic table that we call the
noble gases (once called the "inert gases" because they don't really react that well
with other elements).
What's xenon like? It has no color, taste, or smell, but it is present in the air around us in minutely small
quantities—roughly one molecule of xenon for every 20 million molecules of other gases. Xenon
atoms have an atomic number of 54 (much heavier than oxygen or nitrogen atoms), so xenon gas is
about 4½ times heavier than air: if you're hunting for xenon, look near the ground!
Xenon is a gas on Earth because it melts at roughly −111°C (−168°F) and boils at −107°C (−161°F).
Who discovered xenon?
Most of the nobel gases, including xenon, were discovered by Scottish chemist
Sir William Ramsay
(1852–1916), who won the Nobel Prize in Chemistry in 1904 for his work. According to
the Royal Swedish Academy of Sciences, which awarded the prize:
"The discovery of an entirely new group of elements, of which no single representative had been known with any certainty, is something utterly unique in the history of chemistry, being intrinsically an advance in science of peculiar significance. The more remarkable is this advance when we recollect that all these elements are components of the atmosphere of the earth, and that, though apparently so accessible for scientific research, they have for so long a time baffled the acumen of eminent scientists..."
Quoted from Presentation Speech by Professor J.E. Cederblom, President of the Royal Swedish Academy of Sciences, on December 10, 1904.
Further reading
- Sir William Ramsay: His Life and Works: An online exhibition from University College London, England where Ramsay was professor of chemistry. Includes
some fascinating photos of Ramsay's experiments and notebooks.
Photo: "Hmmm, maybe xenon is not so unreactive after all?" That's what chemists John Malm, Henry Selig and Howard Claassen
from Argonne National Laboratory concluded in October 1962 when they successfully produced these sparkling, square crystals of xenon tetrafluoride—the first simple, manmade compound of xenon ever produced. One of Malm's favorite jokes was that chemists would hang up their lab coats the day someone discovered a solid compound of a noble gas—exactly what he and his colleagues achieved. Photo by courtesy of Argonne National Laboratory
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