What is a Fluorescent Lamp? – Construction, Working and Applications

A fluorescent lamp, also called fluorescent tube, is a low pressure mercury vapor gas-discharge lamp that works on the principle of fluorescence to emit visible light.

When an electric current is passed through the fluorescent tube, it excites the mercury vapor which produces UV rays that then causes a phosphor coating on the inside of the lamp to glow.

Construction of Fluorescent Lamp

The basic construction of a fluorescent tube lamp is shown in Figure-1.

The fluorescent lamp is a low pressure mercury vapor lamp. Thus, due to low pressure, the lamp is made in the form of a long tube whose inside walls are coated with some phosphor. The tube is filled with a small amount of mercury vapor and a small amount of argon gas.

At both ends of the tube, the electrodes are attached. The electrodes are of spiral form, made of tungsten and coated with an electron emitting material. A choke is also connected in series with the tube filament that provides a voltage impulse for starting the fluorescent lamp and once the lamp is started, it acts as a ballast. The lamp filament is connected to a starter switch which is a small cathode glow lamp with bimetallic strip at the electrodes.

Working of Fluorescent Lamp

The circuit diagram of a fluorescent lamp is shown Figure-2.

When the circuit is energized, nearly full supply voltage appears across the starter terminals due to low resistance of the filaments and a negligible current flows through the choke. The starter switch is filled with argon gas. This argon gas ionized and a glow appears inside the starter switch, which heats up the bimetallic strip carrying a moving contact. In a while, the bimetallic strip bends and short circuits the starter terminals. This results in a high current to flow through the filaments F1 and F2 and the choke circuit. The filaments are coated with oxides of barium and strontium, resulting in thermionic emission which can ionize the argon gas inside the tube.

In the meanwhile, the bimetallic strip of the starter switch cools down and breaks open the starter circuit. This sudden opening of the starter circuit causes an abrupt change of current or flux linking the choke coil. As a result, a high voltage surge is induced in the choke which makes one of the filament at a very high potential with respect to the other filament. This momentary high potential difference between the filaments ionizes the mercury and argon present inside the lamp tube and results in the passes of current between the two electrodes inside the tube.

The excited mercury vapor in the tube produces ultraviolet rays, which are falling on the phosphor coating results in fluorescence, i.e., visible light is emitted from the phosphor coating.

Advantages of Fluorescent Lamp

The main advantages of the fluorescent lamps are given as follows −

• Fluorescent lamps have high luminous efficiency.

• They have long life span.

• There running cost is low.

• The heat output is also low for a fluorescent lamp.

Disadvantages of Fluorescent Lamp

Following are the disadvantages of the fluorescent lamps −

• Fluorescent lamps suffer from stroboscopic effect.

• The choke used in fluorescent lamps produces magnetic hum causing disturbance.

• For fluorescent lamps, small wattage requiring a large number of fittings.

Applications of Fluorescent Lamp

Fluorescent lamps can be used in many industrial, commercial and residential applications. Some applications of the fluorescent lamps are as −

• Fluorescent lamps can provide light output in large area, thus these lamps are suitable for lighting in industrial applications.

• Fluorescent lamps are used for lighting in offices as they provide uniform light level.

• In residential applications, the fluorescent lamps provide effective lighting for kitchens, valences, and fascia, etc.

• Fluorescent lamps are also used in classroom lighting and retail lighting, etc.