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Sodium Lamps

A Sodium Lamp is a gas discharge lamp which uses sodium in an excited state to produce light. These are two varieties of those lamps:

1. Low pressure consist of an outer vacuum envelope of glass coated with an infrared reflecting layer of indium tin oxide, a semiconductor material that allows the visible light wavelengths out and keeps the infrared back. It has two inner borosilicate glass U-pipes that hold solid sodium and a small amount of neon and argon gas Penning mixture to start the gas discharge. When the lamp is turned on, it emits a dim red/pink light to warm the sodium metal and it turns into the common bright orange/yellow colour as the sodium vaporises.

They are the most efficient electrically powered light source when measured for photographic lighting conditions. They are widely used for outdoor lighting such as street lights and security lighting.

These lamps are closely related to fluorescent lamps since they have a low-pressure, low-intensity discharge source and a linear lamp shape. They do not exhibit a bright arc but rather emit a softer luminous glow.

2. High pressure are smaller and contain some other elements (mercury, for example). They produce a dark pink glow when first turned on and a pinkish orange light when warmed up. The sodium D-line is the main source of light from the HPS lamp and it is pressure broadened by the high sodium pressures in the lamp.

These lamps are quite efficient. They can be used for outdoor lighting such as streetlights and security lighting.