Sodium Vapour Lamp consists of a discharge tube made from a heat resistant glass, containing a small amount of metallic sodium, neon gas and two electrodes, Neon gas is added to start the discharge and to develop enough heat to vaporize the sodium. Because of law pressure inside the tube, a sufficiently long tube required to obtain more light. To reduce the overall dimension of the lamp, this tube is generally bent into U-shape.
The light produced by this lamp is yellowish which is produced at its optimum pressure of about 0.004mm of mercury. This pressure is obtained at a temperature of about 280 C and so it becomes necessary to maintain this temperature. For this purpose the U-tube is enclosed in a double walled flask to prevent lose of heat. The double walled flask is interchangeable and can be fitted on to another U-tube. While replacing the inner U-tube one must be very careful because if it is broken and sodium comes in contact with moisture, it may result in fire.
All electric discharge lamps require a higher voltage at the time of starting and low voltage during operation. Generally, sodium vapour lamps are operated by a high leakage reactance transformer. At starting a high voltage of about 450 volts is applied across the lamp which is sufficient to start the discharge. When the lamp is fully operative after 10 - 15 minutes, the voltage across it falls to about 150 volts. Because of the high reactance of circuit, the power factor is low and hence a p.f improvement capacitor is connected.
The efficiency of a low pressure sodium vapour lamp is very high (about 40 - 50 lumens/watt) and it produces a light of particular wavelength having yellow color. Sodium lamps are mainly employed for street, high way and airfield lighting where color distinction is not so necessary.