Gas-discharge lamps are a family of artificial light sources that generate light by sending an electric discharge through an ionised gas.
Typically, such lamps use a noble gas (argon, neon, krypton, and xenon) or a mixture of these gases. Some include additional substances, such as mercury, sodium, and metal halides, which are vaporized during start-up to become part of the gas mixture.
In operation the gas is ionized, and free electrons, accelerated by the electrical field in the tube, collide with gas and metal atoms. Some electrons circling around the gas and metal atoms are excited by these collisions, bringing them to a higher energy state. When the electron falls back to its original state, it emits a photon, resulting in visible light or ultraviolet radiation.
Discharge lamps consist of several types of lamp depending on the application the lamp is employed.
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Simple glass tubes 'shaped' and filled with noble gasses. Used for shop signs and advertising.
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Low pressure mercury types, used for UV sources and also, when coated with a flourescent phosphor, employed for office lighting etc.
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High pressure mercury types, used for high intensity UV sources and external lighting, with some lamps having a phosphor coating to either improve lamp efficaciy or colour rendering.
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Low pressure sodium lamps give a monochromatic light (prodominently yellow), but are very efficent.
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High pressure sodium lamps give a much better spectrum, whilst giving a reletivley efficent light source.
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