Gas discharge

A gas discharge is 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.

A glow discharge and arc discharge in low-pressure gas produces visible light with emission spectrum depending on the gas used.

Gas	Color	Notes
Helium	Whitish orange; under some conditions may be grayish, bluish, or green-bluish	Used by artists for special purpose lighting.
Neon	Red-orange	Intensive light. Used frequently in neon signs and neon bulbs.
Argon	Violetish pale lavender blue	Often used together with mercury vapor.
Krypton	Grayish dim off-white. May be greenish. At high peak currents bright blue-white.	Used by artists for special purpose lighting.
Xenon	Grayish or bluish-gray dim white, at high peak currents very bright green-bluish	Used in xenon flash lamps, xenon HID headlamps, and xenon arc lamps, and by artists for special purpose lighting.
Nitrogen	Similar to argon, duller, more pinkish; at high peak currents bright bluish-white, whiter than argon
Oxygen	Violet-lavender, dimmer than argon
Hydrogen	Lavender at low currents, pinkish magenta over 10 mA
Water vapor	Similar to hydrogen, dimmer
Carbon dioxide	Slight bluish-white, in lower currents brighter than xenon
Mercury vapor	Light blue, intense ultraviolet	In combination with phosphors used to generate many colors of light. Widely used in mercury-vapor lamps and Hydrargyrum Medium-Arc Iodide lamps. Often used together with argon.
Sodium vapor	Bright yellow	Widely used in sodium vapor lamps.