Low-pressure sodium (LPS) lamps have a borosilicate glass gas discharge tube containing solid sodium and a small amount of neon and argon gas in a Penning mixture to start the gas discharge. The discharge tube may be linear (SLI) lamp or U-shaped. When the lamp is first started, it emits a dim red/pink light to warm the sodium metal; within a few minutes as the sodium metal vaporises, the emission becomes the common bright yellow. These lamps produce a virtually monochromatic light.
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LPS lamps have an outer glass vacuum envelope around the inner discharge tube for thermal insulation, which improves their efficiency. Earlier LPS lamps had a detachable dewar jacket (SO lamps). Lamps with a permanent vacuum envelope (SOI lamps) were developed to improve thermal insulation. Further improvement was attained by coating the glass envelope with an infrared reflecting layer of indium tin oxide, resulting in SOX lamps.
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LPS lamps are among the most efficient electrical light sources when measured in photopic lighting conditions, producing above 100 and up to 206 lm/W. This high efficiency is partly due to the light emitted being at a wavelength near the peak sensitivity of the human eye. They are used mainly for outdoor lighting (such as street lights and security lighting) where faithful color rendition is not important.
Internal Layout of a Low Pressure Sodium Lamp
GEC 35w Super SOX low presure sodium lamp with B22 base. These had widespread use for street-lighting on minor roads, have since been replaced with HID lamps and later LED's.
Osram 55w SOX with B22 Base, which were used on minor roads; there was one of these outside my house, but that has been replaced with LEDs.
18w SOX lamp, which does not have any make on it, which does work and is fitted with a B22 Base.