It is a low pressure mercury vapour lamp. It consists of a glass tube 25 mm in diameter and 0.6 m, 1.2 m and 1.5 m in length. The inner portion of the tube is coated with phosphorous. The tube contains argon gas at low pressure and a drop of mercury is added. The choke, two filaments and the starter are connected in series as shown in the figure 2.8.
a. Working principle
Fluorescent lamp function based on the principle of current passing through air medium. In the air medium the resistance falls down heavily. When the current passes to the lamp, circuit is closed through choke, filament and starter. So the 230V supply voltage is applied between the starter terminals. Due to this voltage, current starts flowing through the inert gas in the starter. Now the bimetallic strips raised touch each other as the temperature of inert gas has increased. Once the circuit is completed through the bimetallic strips and filaments in the tube light, the bulb will start glowing. The inert gas in the starter cools down and the bimetallic strip opens again.
Therefore the current through the choke decreases and hence the magnetic flux decreases. The decreasing (or alternating) flux is cut by the choke winding which causes self induced EMF in the choke coil.
The induced EMF is nearly about 1200V. This voltage is applied between the filaments of the tube light, causing electron flow between the filament and the inert gas of the tube light. The electron collision in to the inert gas produces the ultra violet rays. These rays impinge on the phosphorous coating. Light is emitted by the coating. After light started at 110V is enough for the light to retain the supply voltage 120 volt, which is dropped across the choke. The luminous efficiency of a fluorescent lamp is 60 lumens / watt.
1. Voltage fluctuation has very small effect on light output.
2. The luminous efficiency is more as length of rod is more.
3. It gives light close to natural light.
4. Heat radiations are negligible.
1. Its brightness is less.
2. Initial cost is more
3. Overall maintenance cost is high.
1. Fluorescent lamp is available in required designs and sizes. Hence it is used largely in residential areas.
2. It is used for good lighting.
3. Special fluorescent lights are used in stage lighting for films and in video camera lighting.