Halogen lightbulbs contain a gas called a halogen, which makes them brighter, whiter, more efficient and longer-lasting. And, of course, much more expensive.
A halogen lamp is a variation on the standard incandescent, as opposed to fluorescent, lamp. An incandescent lamp contains a tungsten filament enclosed in a glass bulb filled with gas. An electric current heats the filament to incandescence, a white-hot glow. It may look very bright, but in reality only 10 to 12 percent of the energy it emits is visible light; about 70 percent of it is invisible infrared radiation, which heats, rather than illuminates.
In a regular bulb, the gas inside is an inert (unreactive) one such as argon or krypton with some added nitrogen. These inert gases keep the tungsten from oxidizing, or “burning up,” as it would in air. Some smaller bulbs solve the problem by being completely evacuated; there’s practically no gas inside at all.
In a halogen bulb, the gas is usually iodine or occasionally bromine, two highly reactive chemical elements in the family that chemists call halogens. They perform a two-step chemical dance that makes the filament last twice as long. But first, we have to understand how the standard bulb works.
The filament is a coil of thin tungsten wire. Tungsten is used because it has the highest melting point of all metals, 6200 degrees Fahrenheit (3400 degrees Celsius), and it stays strong even at white-hot temperatures of 4500 degrees Fahrenheit (2500 degrees Celsius) or higher. Moreover, it has the lowest vapor pressure of all metals, meaning that it evaporates less than any others. Yes, even metals evaporate a few atoms now and then, but so slowly that we never notice it except at very high temperatures. (Never fear; your gold jewelry isn’t going to dry up.)
When it is white-hot, even tungsten will evaporate enough so that the filament gets thinner and thinner as the bulb burns, until it finally breaks apart and interrupts the electric circuit. That’s when your bulb burns out. For some time before this disaster strikes, you can see the evaporated tungsten as a dark coating on the inside of the glass, where it has condensed because of the glass’s relatively low temperature. This darkening, of course, progressively cuts down on the amount of light that the bulb puts out as it ages.
Sometimes a bulb’s filament will have developed such a thin spot that it will blow out suddenly when you turn on the switch. The blue flash that you see is an electric arc, leaping across the widening gap as the thin spot evaporates completely under the heat stress of the power surge.
Tip: When a bulb burns out, try tapping or shaking it gently while the power is on. Sometimes you can get the broken ends close enough together so that an arc will flow between them and weld them back together, rewarding you with perhaps an hour or so of life-after-death experience.
What halogen-filled bulbs do is to cut down the evaporation rate of the tungsten in a very interesting way. First, the iodine vapor reacts with the evaporated tungsten atoms before they can condense out on the glass and converts them to tungsten iodide, a gaseous chemical compound.
The molecules of tungsten iodide then float around inside the bulb until they happen to encounter the white-hot filament, whereupon the high temperature breaks them back down again into iodine vapor and metallic tungsten, which deposits itself back on the filament. The released iodine is then free to apprehend and deliver more tungsten atoms, and the cycle continues, with the iodine atoms continually capturing evaporated tungsten atoms and returning them to the filament. This recycling process approximately doubles the life of the filament, and hence of the bulb.
The halogen process allows the lamp to be operated at a much higher temperature without excessive deterioration of the filament, and that makes a brighter, whiter light. In fact, the temperature of the bulb’s inside wall has to be high, above about 480 degrees Fahrenheit (250 degrees Celsius), to keep the tungsten atoms from condensing on it before the iodine vapor can grab them.
Halogen bulbs are made of quartz, which withstands much higher temperatures, and is more expensive, than ordinary glass. They are usually tube-shaped and closely surround the filaments to stay hot. In fact, tungsten lamps burn so hot that they can be a fire hazard if used too close to flammable materials such as curtains.