Lamps that produce light as a result of an electrical glow discharge, generated between two electrodes, in a low-pressure neon, argon, krypton, helium, xenon, or carbon dioxide vapor, which is contained in a transparent bulb or tube.
Note that also glow discharge lamps using no neon but one of the other vapors mentioned in the definition are referred to as neon lamps.
Neon lamps are usually not used for general lighting purposes, and therefore only the fundamentals of their working principle and of their properties are described in this encyclopedia.
Working Principle of Glow Discharge Lamps
In a glow discharge, the electrons taking part in the initial discharge are made free by external sources like thermal collisions between atoms, ultraviolet, or cosmic radiation. In a discharge bulb or tube with a gas at very low pressure and a voltage of several hundred volts applied across cold electrodes, the electrodes made free by the external sources move at high speed toward the anode and ionize some gas atoms creating positive charged ions and more free electrons. Some of the positive ions hit the cathode at high speed, thanks to the low pressure of the gas of less than 0.05 atmosphere. They release by their impact free electrons from the cathode surface. Electron production is thus not by external means like heated cathodes but by secondary emission as a result of positive ions hitting the cathode surface. Atoms can also be excited to a higher-energy state by the moving electrons and ions, and in releasing the extra energy so obtained, photons are created that are responsible for the typical soft and transparent luminous glow. The color characteristics of the glow light are dependent on the type of gas used.
The glow discharge process is only stable at small current densities in the order of milliamps. By increasing the voltage, the current is increased, and the process will change into that of an arc discharge that is typical for most discharge lamps used for lighting: low- and high-pressure sodium, mercury, and metal halide lamps. Some arc discharge lamps make use of the glow light process to facilitate ignition of the lamp (the red neon glow light of a starting low-pressure sodium lamp). Glow light starters (used in some fluorescent lamp systems) also make use of this process.
Tubular Glow Discharge Lamps
Tubular neon lamps can be shaped into any form and produced in many different color versions. They are therefore particularly well suited for advertising signs. LED strings now offer an alternative. Tubular neon signs are also popular in decorative art.
Lead glass is usually used for the tube because it can easily be bent and fused. Sometimes red- or yellow-tinted glass is used to adapt the original color of the glow light. So will yellow-tinted glass change the color of blue glow light into green.
The length of the tube usually is less than 2 m, and its diameter varies between some 7 and 35 mm. Larger signs are constructed from various pieces of individual tubes.
The electrodes are large rings of pure iron with a diameter of 15–20 mm and a length of about 80 mm.
Pure neon as fill gas results in red light; most other colors can be produced with argon-mercury mixtures. Helium results in warm white light.
Fluorescent powders on the inner wall are often used to extend the range of colors or to improve light output.
System Luminous Efficacy
Luminous efficacy of clear glass tubes is low with some 5 lm/W. Fluorescent-covered tubes have efficacies depending on the color between some 10 lm/W for red, 30 lm/W for blue, more than 80 lm/W for green, and 75 lm/W for white. The values vary quite a bit with the type of fluorescent powder used.
Lumen Package Range
The luminous flux of neon tubes is expressed in lumen per meter of tube length. Values vary according to type of gas, type of fluorescent powder (and thus color), lamp current, and tube diameter between 100 lm/m (clear glass neon filled) and 1600 lm/m (fluorescent-coated tube producing green light).
Applying different fill gasses, different tinted glass tubes, and different fluorescent powders enables the production of all the colors of the spectrum.
Neon tubes have electrically a very long life. There practical life is often indicated as the time after which the light output has dropped to 50 %: 10,000–20,000 h.
Run-Up and Reignition
Ignition of the cold lamp is instantaneous and hot reignition is immediate as well. This makes it possible to produce flashing neon sign installations.
Compact Glow Discharge Lamps
Neon compact glow lamps were used as indicator lamps on all kinds of electrical and electronic equipment. LED signal lamps have taken over this application. They were also, and still are, used for low-level lighting in night lamps in children’s and hospital bedrooms and corridors. Also, here LEDs are an alternative. The lamp’s electrodes can be shaped in many different forms so that the glow light can be used to display letters, numbers, and symbols. By making the lamp flashing a flickering neon glow, for example, in the form of a candle, flame can be produced.
Compact glow discharge lamps consist of a compact glass bulb with two electrodes closely positioned together. The nickel or iron electrodes can take any form like strip, disk, ring, cross, etc. The filling gas usually is neon giving an orange-red glow. Fluorescent coating is used for green-emitting lamps. Miniature indicator lamps in sizes from some 5 to 15 mm operate on a current of 0.5–1 mA. The operating current of the larger types in the size of an incandescent bulb goes up to 50 mA.
System Luminous Efficacy
Luminous efficacy is 0.3–1 lm/W which for indicator lamps is reasonable enough.
The lumen output, less to much less than one lumen, is low but for indicator lamp applications sufficient.
Clear glass bulbs produce orange-red light. With fluorescent coating of the bulb, many more colors can be produced, especially also green which for indicator lamps is indispensable.
Lamp life is 10,000–100,000 h.
Run-Up and Reignition
Cold start and reignition are both instantaneous which of course for indicator lamps is a first requirement.