Operating Principles

High Intensity Discharge" is often shortened to "HID" or "Discharge". HID lamps give out light from an intense electrical arc or “ discharge� between two electrodes. See Figure 9.1.

The process is similar to the fluorescent lamp, except that the light comes from the arc itself, and not from the phosphor coating on the tube wall. Some discharge lamps do have phosphor coated bulbs but this is only to provide supplementary colour and / or increase the apparent size of the source to reduce glare.

Figure 9.1. HID lamp operation.

Figure 9.2 shows the basic parts of Kolorlux and Metal Halide lamps:

1. Outer bulb of heat resistant glass; small amount of nitrogen fill gas at a very low pressure.
2. Quartz arc tube; fill gas of argon; small amount of mercury and other metals, depending upon the lamp type.
3. Starting electrode (not for some metal halide lamps).
4. Two main electrodes and supporting structure.
5. Cap to connect lamp to the power circuit.

Figure 9.2. Mercury and Metal Halide lamp construction.