page_title: High-intensity discharge lamp
section_title: Construction
hierarchical_section_title: High-intensity discharge lamp / Construction
caption_attribution_description: English: High Intensity Discharge ballasts for Natrium and Metal Halide lamps
context_page_description: High-intensity discharge lamps are a type of electrical gas-discharge lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube. This tube is filled with noble gas and often also contains suitable metal or metal salts. The noble gas enables the arc's 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. However, the lumen output of HID lighting can deteriorate by up to 70% over 10,000 burning hours.
  Many modern vehicles use HID bulbs for the main lighting systems, although some applications are now moving from HID bulbs to LED and laser technology.
context_section_description: Various types of chemistry are used in the arc tubes of HID lamps, depending on the desired characteristics of light intensity, correlated color temperature, color rendering index (CRI), energy efficiency, and lifespan. Varieties of HID lamp include:
  Mercury-vapor lamps
  Metal-halide (MH) lamps
  Ceramic MH lamps
  Sodium-vapor lamps
  Xenon short-arc lamps
  The light-producing element of these lamp types is a well-stabilized arc discharge contained within a refractory envelope arc tube with wall loading in excess of 3 W/cm² (19.4 W/in²).
  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.
  Metal-halide and ceramic metal-halide lamps can be made to give off neutral white light useful for applications where normal color appearance is critical, such as TV and movie production, indoor or nighttime sports games, automotive headlamps, and aquarium lighting.
  Low-pressure sodium-vapor lamps are extremely efficient. They produce a deep yellow-orange light and have an effective CRI of nearly zero; items viewed under their light appear monochromatic. This makes them particularly effective as photographic safelights. High-pressure sodium lamps tend to produce a much whiter light, but still with a characteristic orange-pink cast. New color-corrected versions producing a whiter light are now available, but some efficiency is sacrificed for the improved color.
  Like fluorescent lamps, HID lamps require a ballast to start and maintain their arcs. The method used to initially strike the arc varies: mercury-vapor lamps and some metal-halide lamps are usually started using a third electrode near one of the main electrodes, while other lamp styles are usually started using pulses of high voltage.
  Replacements for the toxic mercury in the HID lamps have been investigated and are a matter of ongoing research. Experiments show promising results and widespread future applications are expected.
caption: Ballasts for discharge lamps
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