A Low Pressure Sodium lamp contains a U-shaped arc tube constructed of sodium-resistant lime borate glass inside an evacuated clear
outer envelope. This glass envelope has an internal coating of heat-retaining
indium oxide, which contributes to the extremely high efficacy of the LPS lamp.
Inside the arc tube there is a mixture of neon and argon gases, which are used
to start the lamp, together with pure sodium metal.
The arc tube surface features a series of dimples that serve as reservoirs to assure even distribution of the sodium. This promotes maximum lumen maintenance throughout the life of the lamp. The arc discharge takes place between triple-coil tungsten electrodes mounted at each end of the arc tube near the base of the lamp. The electrodes are coated with a special emission material.
