How do HID lamps produce light?

High intensity discharge lamps produce light when the gases inside the fused alumina tube are heated to the point of evaporation. Tungsten electrodes are kept in the alumina tube, and when the metal elements begin to heat up, they strike the tungsten electrodes, creating an arc of light.

Similar to fluorescent lights, HID lamps need a ballast (a device used to start gas discharge in a lamp and limit the flow of electrical current) to even start producing heat. If they do not have the ballast, the reaction will not begin, and there won be any light.