An incandescent lamp works in a very simple principle. When a current flows through a resistance, the ohmic loss occurs. This ohmic loss contributes heat to the resistance. Actually the heat is nothing but the infrared electromagnetic wave. If the current continues to flow, the temperature of the resistance increases due to the generated heat. After a certain temperature, the wavelength of the radiated electromagnetic waves become so shorter that the waves become visible. That means the resistance starts glowing by emitting light energy.
Working Principle of Incandescent Lamp
Depending upon the above-mentioned principle, an incandescent lamp works. Actually the filament of the lamp offers resistance. Therefore it becomes red hot and then emits visible light when current flows through the filament. For obtaining brighter light energy we use tungsten filament in the lamp.
What Does Incandescent Mean?
When a metallic resistance becomes hotter above a certain limit, it starts radiating visible light. We know this phenomenon as the incandescent. As the tungsten filament emits bright light after becoming sufficiently hot, we call the lamp as an incandescent lamp.
Types of Incandescent Lamp
These lamps are of two types. One is vacuum type and another is gas filled type.
Construction of Incandescent Lamp
One glass bulb holds the filament inside it. The lamp has a metal cap. This metal cap provides the electrical connectivity to the filament from an external electrical circuit. This cap also helps the holder to hold the lamp. In the vacuum type variant, the air is sucked out from the bulb. But in the gas-filled variant we fill the bulb with argon or nitrogen like inert gases.
Working of Vacuum Type Incandescent Lamp
The vacuum inside the bulb reduces the probability of the oxidation on the filament. Because no air means no oxygen, hence no oxidation.
Again due to the absence of any air or gaseous medium inside the bulb the possibility of the convectional flow of heat is minimal. So the heat loss is minimum.
Disadvantage of Vacuum Incandescent Lamp
Due to the vacuum, the filament gets vaporized at a lower temperature than the boiling point of tungsten. As a result during the lifespan of the lamp, the vapor of the filament is slowly deposited on the inner wall of the bulb as a thin layer. Therefore luminous efficiency of the lamp decreases. At the same time, the filament becomes thinner and weaker day by day and ultimately the filament breaks or fuses.
This is the reason for what we cannot make a vacuum type incandescent lamp for more than 25 watt rating.
Gas Filled Incandescent Lamp
When we fill the bulb of a lamp with inert gas at high pressure, the high-temperature losses are reduced. The temperature of the filament of a gas-filled lamp can be as higher as 2500° C. This is because the melting and boiling points of the filament become higher than its normal value. In the gas-filled incandescent lamp, the filaments are normally in the form of coiled-coil. Here also tungsten is the main material of the filament. The wattage of this type of lamps maybe 5, 15, 25, 40, 60, 100, 200, 500, 1000 watt.
Generally, voltage ratings of the lamp are of 220 V A.C. But 24V and 110V lamps are also available in the market.
Effects of High Voltage in Incandescent Lamp
The brightness of the lamp is very voltage sensitive. It is seen that far increase of 5% supply voltage, the brightness of the lamp increases by around 20%. But this reduces the lifespan of the filament by around 50%. On the other hand, if we operate this type of lamps with a lower voltage than its actual rating, the lifespan of the lamp increases significantly. Generally, a 5% decrease in supply voltage doubles the lifespan of the lamp.
Performance of Incandescent Lamp
The efficiency or performance of the lamp is somewhat from 10 to 18 lumen per watt.
Metal Cap of Incandescent Lamp
The metal cap is mainly designed for holding the lamp in the holder. These are two types of the metal cap. One is a bayonet type and another is screw type. In bayonet-type, there are two pins projected on two opposite sides of the cap. These pins facilitate the holding of the lamp in the holder. But in the screw-type metal cap the outer surface of the cap consists of screw grooves for screwing it in the holder. There are two screw cap types in incandescent lamps.
- Edison Screw Type Cap
- Goliath Screw Type Cap
Edison screw-type cap is used up to 200 Watt lamps. Beyond that, we use Goliath screw-type cap.