This tube is filled with noble gas and often also contains suitable metal or metal salts. The noble gas enables the arc’high intensity discharge lamps pdf initial strike.
Once the arc is started, it heats and evaporates the metallic admixture. Its presence in the arc plasma greatly increases the intensity of visible light produced by the arc for a given power input, as the metals have many emission spectral lines in the visible part of the spectrum.
High-intensity discharge lamps are a type of arc lamp. Brand new high-intensity discharge lamps make more visible light per unit of electric power consumed than fluorescent and incandescent lamps, since a greater proportion of their radiation is visible light in contrast to infrared. Many modern vehicles use HID bulbs for the main lighting systems, some applications are now moving from HID bulbs to LED and laser technology.
However, this HID technology is not new and was first demonstrated by Francis Hauksbee in 1705. Mercury-vapor lamps were the first commercially available HID lamps.
Originally they produced a bluish-green light, but more recent versions can produce light with a less pronounced color tint. However, mercury-vapor lamps are falling out of favor and being replaced by sodium-vapor and metal-halide lamps.