Ceramic discharge metal-halide lamp
The ceramic discharge metal-halide (CDM) lamp, often referred to as Ceramic Metal Halide lamp (CMH) is a source of light that is a type of metal-halide lamp which is 10-20% more efficient than the traditional quartz metal halide and produces a superior color rendition (80-96 CRI).
Applications for these lamps include television and film making, shop lighting, digital photography, street lighting, architectural lighting and agricultural lighting including grow lights. A CMH light was first exhibited by the Thorn Lighting Group in 1981 at the Hannover World Light Fair, and the first commercial CMH lamps were distributed by Philips in 1994.
The term "Light Emitting Ceramic" or "LEC" is sometimes improperly used to describe ceramic discharge metal-halide grow lights in general, though that term is actually the registered trademark of a specific brand of ceramic discharge metal halide light.
The ceramic metal halide is a variation of the metal-halide lamp which is itself a variation of the old (high-pressure) mercury-vapor lamp. A CMH uses ceramic instead of quartz like a traditional metal halide lamp. Ceramic arc tubes allow higher arc tube temperatures, which some manufacturers claim results in better efficacy, color rendering, and color stability.
The discharge is contained in a ceramic tube, usually made of sintered alumina, similar to what has been used in the high pressure sodium lamp. During operation, the temperature of this ceramic tube can exceed 1200 kelvins. The ceramic tube is filled with mercury, argon and metal-halide salts (for example, sodium iodide). Because of the high wall temperature, the metal halide salts are partly vaporized. Inside the hot plasma, these salts are dissociated into metallic atoms and iodine.
The metallic atoms are the main source of light in these lamps, creating a bluish light that is close to daylight with a CRI (color rendering index) of up to 96. The exact correlated color temperature and CRI depend on the specific mixture of metal halide salts. There are also warm-white CDM lamps, with somewhat lower CRI (78-82) which still give a more clear and natural-looking light than the old mercury-vapour and sodium-vapour lamps when used as street lights, besides being more economical to use.
The ceramic tube is an advantage in comparison to earlier fused quartz. During operation, at high temperature and radiant flux, metal ions tend to penetrate the silica, depleting the inside of the tube. Alumina is not prone to this effect.
CDM lamps use one fifth of the power of comparable tungsten incandescent light bulbs for the same light output (80–117 lm/W) and retain color stability better than most other gas discharge lamps. Like other high-intensity discharge lamps, they require a correctly rated electrical ballast in order to operate.
- Turner, Wayne C.; Doty, Steve (2007-01-01). Energy Management Handbook. The Fairmont Press, Inc. p. 376. ISBN 9780881735437.
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