A low-pressure sodium lamp is coded as SOX (SO stands for 'sodium vapour', X stands for 'standard single-ended lamp of integral construction'). A typical lamp shape and its circuit are shown in Figure 22.7.
The lamp has a U-shaped arc tube containing metallic sodium and an inert gas such as neon. Common power ratings of the lamp are 35W, 55W, 90W, 135W, and 180W with luminous efficiencies in the range of 120-175 lm/W and with an average operating life span of 6000 hours.
The low-pressure sodium lamp needs a high voltage (480-650V) provided by a special transformer ballast circuit. When first ignited, the SOX lamp gives a red glow as the discharge is initially through the neon gas. As the lamp warms up, the sodium begins to evaporate to take over the discharge from the neon while the lamp colour changes from red to yellow. The time taken to reach maximum intensity is approximately 6-15 minutes.
