A sodium vapor lamp is a gas release lamp which uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure.

Low pressure / LPS / SOX

A LPS / SOX streetlight at full power LPS Lamps (Low Pressure Sodium), also known as SOX Lamps (Sodium Oxide), consist of an outer vacuum cover of glass coated with an infrared reflecting layer of indium-tin oxide, a semi-conductor material that allows the visible light wavelengths out and keep the infrared (heat) back. It has an inner borosilicate 2 ply glass U shaped tube containing sodium metal and a small amount of neon and argon gas to begin the gas discharge, so when the lamp is turned on it emits a dim red/pink light to warm the sodium metal and within a few action it turns into the common bright orange/yellow color as the sodium metal vaporizes. These lamps create a virtually monochromatic light in the 589 nm wavelength. As a result, objects have no color rendition below a LPS light, only the reflection of the 589 nm light.

LPS lamps are the most capable electrically-powered light source — up to 200 lm/. As a result they are widely used for outdoor lighting such as streetlights and security lighting where color rendition is less significant. LPS lamps are available with power ratings from 18 W up to 180 W.

High pressure / HPS

An HPS bulb at full power High pressure sodium (HPS) lamps are lesser and contain some other elements (for example, mercury), produce a dark pink glow when first struck, and create a pinkish orange light when warmed up. These lamps create continuous spectrum light (not monochromatic); hence colors of objects under them can be notable. This leads them to be used in areas where good color interpretation is important, or desired (such as to identify the color of a fleeing suspect's car).

They are widely used for outdoor lighting such as streetlights and security lighting where color piece is more important because of the very high chemical activity of the high pressure sodium arc; the arc tube is typically made of translucent aluminum oxide (alumina). This construction led common Electric to use the trade name "Lucalox" for their line of high-pressure sodium lamps.

Light pollution considerations

For placements where light pollution is of prime meaning (for example an observatory parking lot), low pressure sodium is preferred. As it emits light on only one wavelength, it is the easiest to filter out.

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