A gas discharge lamp consists of a transparent gas discharge enclosure containing a small quantity of gas or vapour at a low pressure and two electrodes provided for connecting the lamp to the supply mains. When the electrodes establish a potential difference, the gas gets ionized. Consequently, an electrical current flows and a luminous discharge is obtained from the excited atom of gas.
Ionisation of gas.
Gas discharge lamps are preferred to incandescent lamps due to higher efficiency, longer life, the better color of lighting and uniform intensity of light.
(a) High initial cost, (b) Low power factor, (c) Complicated starting requires choke to give voltage surge and to limit the current, (d) Condenser is required to improve power factor, (e) Full brilliancy comes after a considerable time, (f) These can not be used in any position,(g) Light output fluctuates producing stroboscopic effect & (h) Multiple images are formed on the moving objects.
In discharge lamps cathodes emitting electrons by heat are termed as hot cathodes and where no heating is employed they are called as cold cathodes.