Sodium vapor lamp

Sodium Vapor Lamp is a gas discharge 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 Sodium Lamps' also known as SOX Lamps consist of an outer vacuum envelope of glass coated with an infrared reflecing layer of Indium (allows the light wavelengths out and keeps the infrared (heat)in). It has an inner borosilicate 2 ply glass U shaped tube containing sodium metal and a small amount of neon and argon gas to start 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 minutes it turns into the common bright yellow color as the sodium metal vaporizes. These lamps produce a virtually monochromatic light (589 nm wavelength).

"Low Pressure Sodium Lamps" are the most efficient light source — up to 210 lm/W. Therefore they are widely used for outdoor lighting such as streetlights and security lights where colour rendition is less important.

Wattages are available from 18 [Watts] up to 180 [watts].

High pressure sodium lamps are smaller and contain some other elements (e.g. mercury) and produce a pinkish orange light. These lamps produce continuous spectrum light (not monochromatic).

High pressure Sodium lamps are quite efficient — about 100 lm/W. Therefore they are widely used for outdoor lighting such as streetlights and security lighting where colour rendition is more important.