Gas-discharge lamp

Gas discharge lamps are a family of artificial light sources that generate light by sending an electrical discharge through an ionized gas, i.e. a plasma. The character of the gas discharge critically depends on the frequency or modulation of the current: see the entry on a frequency classification of plasmas. Typically, such lamps use a noble gas (argon, neon, krypton and xenon) or a mixture of these gases. Most lamps are filled with additional materials, like mercury, sodium, and/or metal halides. In operation the gas is ionized, and free electrons, accelerated by the electrical field in the tube, collide with gas and metal atoms. Some electrons circling around the gas and metal atoms are excited by these collisions, bringing them to a higher energy state. When the electron falls back to its original state, it emits a photon, resulting in visible light or ultraviolet radiation. Ultraviolet radiation is converted to visible light by a fluorescent coating on the inside of the lamp's glass surface for some lamp types. The fluorescent lamp is perhaps the best known gas discharge lamp.

Gas discharge lamps offer long life and high light efficiency, but are more complicated to manufacture, and they require electronics to provide the correct current flow through the gas.

History
Francis Hauksbee first demonstrated a gas discharge lamp in 1705. He showed that an evacuated or partially evacuated glass globe, while charged by static electricity could produce a light bright enough to read by. Sir Humphry Davy demonstrated in 1802 the first electric arc at the Royal Institution of Great Britain. Since then, discharge light sources have been researched because they create light from electricity considerably more efficiently than incandescent light bulbs.

Later it was discovered that the arc discharge could be optimized by using an inert gas instead of air as a medium. Therefore noble gases neon, argon, krypton or xenon were used, as well as carbon dioxide historically.

The introduction of the metal vapor lamp, including various metals within the discharge tube, was a later advance. The heat of the gas discharge vaporized some of the metal and the discharge is then produced almost exclusively by the metal vapor. The usual metals are sodium and mercury owing to their high vapor pressures that increase efficiency of visible spectrum emission.

One hundred years of research later led to lamps without electrodes which are instead energized by microwave or radio frequency sources. In addition, light sources of much lower output have been created, extending the applications of discharge lighting to home or indoor use.

Color
Each gas, depending on its atomic structure emits certain wavelengths which translates in different colors of the lamp. As a way of evaluating the ability of a light source to reproduce the colors of various objects being lit by the source, the International Commission on Illumination (CIE) introduced the color rendering index. Some of gas discharge lamps exhibit indexes below 100 which means that the colors appear completely different from, for instance with sun-light illumination. Some people are unconsciously aware of this phenomenon and when buying clothes, they try to illuminate them with sun-light in order to know the "real" color.

Low pressure discharge lamps

 * Fluorescent lamps, the most common lamp in office lighting and lots of other applications, produces up to 100 lumens/watt
 * Low pressure sodium lamps, the most efficient gas discharge lamp type, producing up to 200 lumens/watt, but at the expense of very poor color rendering. The almost monochromatic yellow light is only acceptable for street lighting and similar applications.

High pressure discharge lamps

 * Metal halide lamps. These lamps produce almost white light, and attain 100 lumen/watt light output. Applications include indoor lighting of high buildings, parking lots, shops, sport terrains.
 * High pressure sodium lamps, producing up to 150 lumens/watt. These lamp produce a broader light spectrum than the low pressure sodium lamps. Also used for street lighting, and for artificial photoassimilation for growing plants
 * High pressure mercury-vapor lamps. This lamp type is the oldest high pressure lamp type, being replaced in most applications by the metal halide lamp and the high pressure sodium lamp.

Other examples

 * Neon signs may use either direct illumination or (to obtain certain colors), indirect phosphor excitation.


 * Xenon flash lamp. This lamp is commonly found in film and digital cameras, even in single-use cameras. These lamps have produced interesting illumination effects in theatre and dancing. More robust versions of this lamps can produce short intense flashes repeatedly, allowing the stroboscopic examination of repetitive motion (useful in certain balancing applications). These were at one time popular, "freezing" the motion of the actors or dancers. This type of lamp was also used to demonstrate persistence of vision, where an entire room would be illuminated by multiple lamps behind diffusing wall panels. In this otherwise darkened room a periodic flash would cause every detail of the occupants to be imaged on the observer's retina, completely frozen in motion.