The title HID covers the following light sources:
- Mercury Vapor (the first of this type used for commercial purposes)
- Metal Halide
- High Pressure Sodium
- Ceramic Metal Halide
- Xenon Short Arc Lamps
All of the listed lamps emit light in basically the same manner. We start with an outer glass envelope. Within this glass envelope is an arc tube that contains a gas, usually argon, krypton, or a mixture of the two. On each end of the arc tube are tungsten electrodes. There are also metal salts within the tube. When the internal temperature and internal pressure build to a predetermined level, an arc will strike across the electrodes. As the arc builds up the heat and pressure will melt the metal salts to form plasma which increases the intensity of the light and reduces the power consumption.