Sodium Vapour Lamp

出典: くみこみックス

Sodium Vapour Lamp consists of a discharge tube produced from a heat resistant glass, containing a small quantity of metallic sodium, neon gas and two electrodes, Neon gas is added to begin the discharge and to create sufficient heat to vaporize the sodium. Due to the fact of law pressure inside the tube, a sufficiently long tube needed to obtain a lot more light. To minimize the general dimension of the lamp, this tube is typically bent into U-shape.

The light produced by this lamp is yellowish which is produced at its optimum pressure of about .004mm of mercury. This pressure is obtained at a temperature of about 280 C and so it becomes required to sustain this temperature. For this purpose the U-tube is enclosed in a double walled flask to stop lose of heat. The double walled flask is interchangeable and can be fitted on to another U-tube. Whilst replacing the inner U-tube one need to be quite careful due to the fact if it is broken and sodium comes in speak to with moisture, it may result in fire.

All electric discharge lamps need a higher voltage at the time of starting and low voltage for the duration of operation. Typically, 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 adequate to start off the discharge. When the lamp is completely operative immediately after ten - 15 minutes, the voltage across it falls to chicken houses about 150 volts. Because of the high reactance of circuit, the power aspect is low and hence a p.f improvement capacitor is connected.

The efficiency of a low pressure sodium vapour lamp is quite high (about 40 - 50 lumens/watt) and it produces a light of specific wavelength having yellow color. Sodium lamps are mainly employed for street, high way and airfield lighting where color distinction is not so necessary.