In HID lamps the power dissipation of an electric current passing through a gaseous medium at a pressure greater than or equal to 1 atm is converted into radiation. Much higher radiating temperatures can be achieved than in any incandescent lamp. Appropriate selection of the gaseous medium results in favourable spectral distribution of radiated power, with a much smaller fraction of IR rays. Therefore, these light sources are very bright, and are up to 10 times as efficient as incandescent lamps.
The following are in use:
· Mercury high pressure lamps
· Sodium high pressure lamps
· Metal-halide lamps
· Xenon- and Xenon-Mercury short arc lamps
Each system comprises an inner discharge tube (arc tube) containing the high pressure gas or vapour enclosed in a hermetically sealed outer envelope. The outer jacket is required for thermal insulation, protection of the arc tube seals from oxidation, and absorption of any short wavelength UV rays that may be emitted from the arc tube. Arc tubes for mercury and metal halide lamps are quartz; arc tubes for high pressure sodium lamps are fabricated from translucent polycrystalline alumina (PCA) to withstand corrosion by hot molten and gaseous sodium. The energy coupling takes place via tungsten electrodes.
HID lamps are for high output, high luminous efficiency and long operating times. Their main applications are lighting of roads, outdoor areas, and halls, shopping areas, floodlighting, plant irradiation, photography, medical technology and automobiles.
Recent advancements of Metal Halide discharge lamps with ceramic arc tubes and high pressure sodium lamps with efficacies of up to 150 lm/W and improved electronic ballast characteristics are capable of a 60% reduction in energy consumption and 30% in maintenance costs.