32 Metal Halide Lamps

Metal Halide lamps are the natural evolution to the mercury vapor lamp, in that they are similar in construction to the mercury vapor lamps, with a shorter arc tube being their chief identifying feature, but contain additional compounds of metals and halogen gas. These compounds improve the efficacy and color rendition of the lamp, and for this reason, metal halide lamps are the first choice among HIDs for good color discrimination.

While the efficacy of metal halide lamps is much higher than when compared to mercury vapor lamps, they have a much shorter average rated life, between 5,000 and 15,000 hours, depending upon wattage rating. Towards the end of the lamp’s useful lifespan, the lumen output can be expected to decrease up to 30%, beyond which point the lamp should be quickly replaced.

The arc tube includes a starting electrode near the first main electrode. The lamp begins the ignition process by initiating a small arc between them. This starts heating the arc tube and vaporizing the compounds inside, however the metal halide lamp usually requires a higher striking voltage than a mercury vapor lamp of the same power rating.

The strike-up time is also longer, typically between five and seven minutes, but because the arc tube operates at a higher temperature than the mercury lamp, it requires a longer cooling period before re-striking, which can be as long as 15 minutes.