Filament and Fluorescent Lamp

A filament lamp works on the principle of incandescence, a general term meaning light produced by heat. In an incandescent type of bulb, an electric current is passed through a thin metal filament, heating the filament until it glows and produces light.

Filament bulb was invented by Thomas Alva Edison. It uses thin tungsten filament which on heating at 3400^oC produces light. The filament is kept inside glass bulb filled with nitrogen or argon gas to protect from oxidation. Tunsten metal has high light emissivity and high melting point so it is best suitable for using as filament in bulb. It is compatible for both AC and Dc current,  cheap and easy to manufacture ideal for wide range of applications.

Drawbacks: It has low efficiency(around 10%), short life (around 1,000 hours) and high operating cost.

Fluorescent lamp consists of a glass tube coated with fluorescent powder inside which little mercury vapor is filled. Both end are fitted with electrodes. Choke coil controls current and starter provides temporary switching till mercury vapor is exited. When current is passed through electrodes mercury vapour get exited and emits ultraviolet radiation which is invisible to our eye. When ultraviolet radiation strikes fluorescent powder coated on wall of glass tube it produces flash of visible light and hence fluorescent light works as shown in fig below.

Compared to filament lamp fluorescent lamp is expensive but it has advantages for long term use. It has long life(about 3,000 hours), higher efficiency (about 30%) and low operating cost. As filament lamp, it is not compatible to both AC and DC it operates only on AC.