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Metal Halide Lamps:
The Halide Cycle in a Discharge Lamp

Diagram of the halide cycle in a discharge lamp

Step 1: Metal atoms move from the hot electric arc toward the cooler arc tube wall where the halides are.

Step 2: Near the wall, the temperature and vapor pressure allow the metals and halides to form a stable molecule which will not corrode the arc tube.

Step 3: When the metal halides approach the hot arc, the molecule breaks apart.

Step 4: The halides move away from the arc, while the metals are energized and radiate light.

Sometimes a metal atom will not combine with a halide, but instead migrates through the arc tube. Over time, when enough metal atoms are lost, the lamp will fail.


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