An electric current passing through a gas can produce visible light. This is a completely different process to the way incandescent sources produce light. The excitation of the gas by the electricity causes collisions between atoms and these collisions result in the release of energy in the form of ultraviolet or visible light. In the natural world, the most common electrical discharge we experience is lightning—lightning is an electrical discharge through air. It lasts only for a short time, but it produces a very intense fl ash of visible light.
As the gas discharge process does not involve heating materials as with incandescent light sources, it is generally a much more effi cient way of producing visible light. Gas discharge lamps can offer a much longer life than most incandescent sources. Combined with their greater effi ciency, this makes them an attractive alternative to incandescent lamps in many situations.
Creating a gas discharge that produces visible light is a much more involved process than the relatively simple heating of an incandescent fi lament. It requires a lot of energy to start the discharge in the fi rst place, and the energy fl ow then has to be reduced and controlled very precisely to maintain a steady discharge. This means complex electrical control devices are needed to operate discharge lamps. The devices are commonly called ballasts or control gear.
There are many different kinds of discharge lamp. They have a wide range of functions, from general lighting to producing colored light or being used in tanning booths. The different types contain different combinations of gas and additives such as metallic compounds. Commonly used gases include helium, neon, argon, xenon, krypton, and nitrogen. The gases are often combined with small amounts of metals such as sodium and mercury. When activated by an electrical discharge, the different gases and combinations of materials produce radiation in different parts of the spectrum. This means different colors of visible light can be produced by different gases/metal halides. By combining the gases, the different colors can be mixed to produce a whiter light source.
Discharge light sources are more effi cient than incandescent sources because they produce more visible light for the energy used. However, they are most effi cient at higher wattages, and it is diffi cult to produce very low- power discharge lamps that have high effi ciency.
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Left
When an electrical discharge is passed through different gases they produce visible radiation in different parts of the spectrum. Helium (He) produces a very pink light while neon (Ne) produces an orange-red light that is characteristic of classic American motel and bar signs. Argon (Ar) is one of the most commonly used gases in cold cathode lamps and naturally produces a purple-blue glow. Krypton (Kr) creates a bright white light. Xenon (Xe) is one of the rarest elements on earth; it produces a very intense bluish white light and is often used in automobile headlight lamps.
Low-intensity (or low-pressure) discharge lamps operate at internal pressures lower than atmospheric pressure—the lamp is a partial or total vacuum. They include fluorescent lamps (compact fluorescent lamps are essentially the same as straight linear ones, except they are twisted and coiled to fit a smaller space); cold cathode lamps (typically used for signage, these are commonly known as neon lamps, though this is erroneous since they often contain argon and not neon gas); and sodium lamps (these produce a very orange light and are most commonly used for street lighting).
Fluorescent lamps are perhaps the most common modern light source. They come in a range of physical shapes and sizes, different wattages, and different colors —though usually some form of white. At its heart, a fluorescent lamp is actually producing ultraviolet radiation, not visible light. The white coating that we see on the inside of the glass tube is a layer of phosphors and minerals that react to UV radiation. The phosphors absorb the high-energy UV radiation and reradiate some of it as lower-energy visible light. This process is called fluorescence. Fluorescent lamps can be produced in a wide range of tints of white. A different mix of phosphors is used to create each different tint.
Right
A standard CD can be used as a simple spectroscope. When seen reflected in a CD, the light from a low-voltage tungsten halogen spotlight (top) and compact fluorescent lamp (bottom) is diffracted by the fine grooves in the CD’s surface, splitting the white light into its component colors. The low-voltage tungsten halogen lamp is an incandescent light source; the white light it produces contains all the colors of the spectrum. By contrast, the typical low-wattage compact fluorescent lamp has a very broken spectrum. It produces white light, but there are large gaps in its spectral output. This means that although its light appears white, some colors will not be rendered correctly by this light source.
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Left
Cold cathode lamps are discharge light sources similar to fluorescent lamps. While fluorescent lamps are produced almost exclusively in white colors, cold cathode lamps are made in a large range of colors and tints of white. The colors can be produced by using different gases in the tube (neon gas glows red, argon glows blue); phosphor coatings can be used to modify the output of a blue or ultraviolet discharge; colored glass can further modify the light produced. By combining these techniques, the manufacturer of these cold cathode tubes has a range of around 55 different colors. In the sample color set shown here, the valentine red, shocking pink, electric blue, and sea green lamps look quite different when switched off. Although neon-filled tubes are the best-known reds, all the lamps in this set use argon. The red and green tubes at each end use phosphors and colored glass, while the pink and blue ones use only phosphors to create the colors.
Left
With discharge lamps that produce white light with the aid of phosphor coatings, such as fluorescent lamps and the white cold cathode lamps illustrated here, different combinations of phosphors can create different tints of white light. In this example the three white, cold cathode lamps contain phosphor mixes designed to match the quality of white light available from incandescent light sources operating at 4,200 K, 3,500 K, and 3,000 K. In the left- hand image the colors are boosted for printing purposes, but when the lamps are seen in real life, it is clear that the 4,200 K lamp on the left is bluer than the warm white of the 3,000 K lamp on the right. The second image has no color boost, but the exposure was reduced to demonstrate the subtlety of the color tints.
High-intensity (or high-pressure) discharge lamps operate with an internal pressure greater than atmospheric pressure. They include high-pressure sodium lamps (used for street lighting, these produce an orange light that is slightly whiter than that created by low-pressure sodium lamps); metal halide lamps (a wide-ranging category of lamps that produce anything from low-quality whitish light through to very high-quality white light); and mercury vapor lamps (a relatively old technology that produces a slightly green form of white light).
Above
These luminaires use 400 W high- intensity discharge lamps to create a daylight quality in a perimeter void for an office building. The camera is less tolerant of color tints in white light than the human visual system: where the camera records a distinct green tinge to the lamps, the eye sees something closer to cool white daylight at 6,500 K.
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