Gas-discharge lamp

Gas-discharge lamps are a family of artificial light sources that generate light by sending an electric discharge through an ionized gas, a plasma.

Germicidal lamps are simple low-pressure mercury vapor discharges in a fused quartz envelope.

Typically, such lamps use a noble gas (argon, neon, krypton, and xenon) or a mixture of these gases. Some include additional substances, like mercury, sodium, and metal halides, which are vaporized during startup to become part of the gas mixture.

Single ended self-starting lamps are insulated with a mica disc and contained in a borosilicate glass gas discharge tube (arc tube) and a metal cap.[1][2] They include the sodium-vapor lamp that is the gas-discharge lamp in street lighting[3][4].[1][2]

In operation, some of the electrons are forced to leave the atoms of the gas near the anode by the electric field applied between the two electrodes, leaving these atoms positively ionized. The free electrons thus released flow onto the anode, while the cations thus formed are accelerated by the electric field and flow towards the cathode.

Typically, after traveling a very short distance, the ions collide with neutral gas atoms, which transfer their electrons to the ions. The atoms which lost an electron during the collisions ionize and speed toward the cathode while the ions which gained an electron during the collisions return to a lower energy state while releasing energy in the form of photons. Light of a characteristic frequency is thus emitted. In this way, electrons are relayed through the gas from the cathode to the anode.

The color of the light produced depends on the emission spectra of the atoms making up the gas, as well as the pressure of the gas, current density, and other variables. Gas discharge lamps can produce a wide range of colors. Some lamps produce ultraviolet radiation which is converted to visible light by a fluorescent coating on the inside of the lamp's glass surface. The fluorescent lamp is perhaps the best known gas-discharge lamp.

Compared to incandescent lamps, gas-discharge lamps offer higher efficiency,[5][6] but are more complicated to manufacture and most exhibit negative resistance, causing the resistance in the plasma to decrease as the current flow increases. Therefore, they usually require auxiliary electronic equipment such as ballasts to control current flow through the gas, preventing current runaway (arc flash).

Some gas-discharge lamps also have a perceivable start-up time to achieve their full light output. Still, due to their greater efficiency, gas-discharge lamps were preferred over incandescent lights in many lighting applications, until recent improvements in LED lamp technology.

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