Ceramic discharge metal-halide lamp
The ceramic discharge metal-halide (CDM) lamp, mostly referred to as Ceramic Metal Halide lamp (CMH), is a relatively new source of light that is a variation of the metal-halide lamp, which itself is a variation of the old (high-pressure) mercury-vapor lamp. 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. 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 colour 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.
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