High Intensity Discharge Lighting
High Intensity Discharge is the term commonly used to designate four distinct types of lamps that actually have very little in common. They are high pressure sodium, low pressure sodium, metal halide, and mercury vapor. Each requires a few minutes (one to seven) to come up to full output. Also, if power to the lamp is lost or turned off, the arc tube must cool to a given temperature before the arc can be re-struck and light produced. Up to seven minutes (for mercury vapor lamps) may be required.
In all High Intensity Discharge lamps, light is produced by passing a current through a metal vapor. Free electrons colliding with an atom in the vapor momentarily knock an electron into a higher orbit of the atom. When the displaced electron falls back to its former level, a quantum of radiation is emitted. The wavelength of radiation depends on the energy zone of the disturbed electron and on the type of metal vapor used in the arc tube.
Containing mercury and/or sodium in significant amounts, HID lamps require special care to protect the environment.
Technology Types (Resource)
By using special ballasts and/or controller circuits, it's possible to dim HID lamps over a wide range of light output. Lamp performance hasn't been fully established and depends on the dimming system and on the degree and duration of dimming.
High Intensity Discharge lamps can be operated at high frequencies, but with little gain in light output. Savings in ballast cost, size and weight, reduction of stroboscopic effect or other considerations may make such operation desirable. However, each lamp is likely to have several points of acoustic resonance. These may cause lamp outage or, in the most extreme cases, actual destruction of the lamp.
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