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 a ceramic arc tube instead of the fused quartz arc tube of 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 that 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 white light 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 ceramic metal halide 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. CMH lights have a long life of up to 24,000 hours. == Efficacy ==