A sodium-vapor lamp is a gas-discharge lamp that 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 are highly efficient electrical light sources, but their yellow light restricts applications to outdoor lighting such as street lamps.[1] High-pressure sodium lamps have a broader spectrum of light than the low-pressure lamps, but still poorer color rendering than other types of lamps.[2] Low-pressure sodium lamps only give monochromatic yellow light and so inhibit color vision at night.
Sodium vapor lamps are mainly used for street lighting. They have low luminosity hence require glass tubes of large lengths, which makes them quiet bulky
Construction:
The lamp consists of a U shaped inner glass tube filled with neon gas at a pressure of 10mm. It also contains a small quantity of sodium and argon gas. The initial ionization voltage is reduced, as the ionization potential of argon is low. Two oxide coated tungsten electrodes are sealed into the tube at the ends. This tube is enclosed in an outer double walled vacuum enclosure to maintain the required temperature.
Working:
A voltage of the order of 380- 450 volts (depending on the wattage) is necessary to start the discharge, which is obtained from a high reactance transformer or an autotransformer. Initially the sodium vapor lamp operates as a low-pressure neon lamp emitting pink color. As the lamp gets heated and reaches a temperature of 200° C the sodium deposited on the sides of the tube walls vaporizes and radiates yellow light. It has a maximum efficiency at 220° C. Proper mounting of the lamp is to be ensured to prevent the sodium blackening the inner walls of the tube .A capacitor C is used to improve the power factor.