Low pressure sodium lamps provide the highest luminous efficacy of all lamps in general use. The construction of the lamp is shown in Figure 1.
The inner U shaped glass tube is filled with a neon and argon mixture at low pressure. A small quantity of metallic sodium is also added to the mixture. The outer glass envelope creates a vacuum to keep the inner discharge tube at the optimum operating temperature so that sodium particles can vaporize.
A high voltage is supplied by the control gear to start the lamp. The initial discharge is in the neon and argon gas (glows red). Heat from the discharge gradually vaporizes the metallic sodium, causing the discharge to change from red to yellow. A significant reduction in lamp voltage occurs when complete vaporization of sodium takes place.
Lamp operation is achieved by using a high magnetic leakage autotransformer or a ballast and ignitor. Power factor correction capacitors are used in both circuits (Figure 2 and 3).
Low pressure sodium lamp circuit data
. lamp efficacy 70-180 lumens per watt
. poor colour rendering
. low power consumption and running costs
. lamp takes time to reach its full brilliance
. low wattage lamps tend to re-strike quickly after an interruption in supply but hiaghr voltage lamps may take approximately 10 minutes
. average lamp life 16000 hours
. applications – used where colour rendering is not of major importance, for example, roadways motorways (good performance in foggy conditions), car parks, floodlighting of buildings, general outdoor commercial and industrial uses
. lamp designation - SOX