The electrical light source which works on the principle of incandescent phenomenon is called Incandescent Lamp. In other words, the lamp working due to glowing of the filament caused by electric current through it, is called incandescent lamp.
How do Incandescent Lamps Work?
When an object is made hot, the atoms inside the object become thermally excited. If the object does not melt, the outer orbit electrons of the atoms jump to higher energy level due to the supplied energy. The electrons on these higher energy levels are not stable, they again fall back to lower energy levels. While falling from higher to lower energy levels, the electrons release their extra energy in a form of photons. These photons are then emitted from the surface of the object in the form of electromagnetic radiation.
This radiation will have different wavelengths. A portion of the wavelengths is in the visible range of wavelengths, and a significant portion of wavelengths are in infrared range. The electromagnetic wave with wavelengths within the range of infrared is heatenergy and the electromagnetic wave with wavelengths within visible range is light energy.
Incandescent means producing visible light by heating an object. An incandescent lamp works in the same principle. The simplest form of the artificial source of light using electricity is an incandescent lamp. Here we use electric current to flow through a thin and fine filament to produce visible light. The current rises the temperature of the filament to such extent that it becomes luminous.
History of Incandescent Lamp
It is normally considered that Thomas Edison was the inventor of the incandescent lamp, but the actual history was not like that. There were numbers of scientists who worked and designed prototype for the incandescent lamp before Edison did. One of them was British physicist Joseph Wilson Swan. From the record, it is found that he got the first patent for the incandescent lamp. Later Edison and Swan merged to produce incandescent lamps in commercial scale.
Construction of Incandescent Lamp
The filament is attached across two lead wires. One lead wire is connected to the foot contact and other is terminated on the metallic base of the bulb. Both of the lead wires pass through glass support mounted at the lower middle of the bulb. Two support wires also attached to glass support, are used to support filament at its middle portion. The foot contact is isolated from metallic base by insulating materials. The entire system is encapsulated by a colored or phasphare coated or transparent glass bulb. The glass bulb may be filled with inert gases or it is kept vacuum depending upon rating of the incandescent lamp.
The filament of incandescent lamps is air-tightly evacuated with a glass bulb of suitable shape and size. This glass bulb is used to isolate the filament from surrounding air to prevent oxidation of filament and to minimize convention current surrounding the filament hence to keep the temperature of the filament high.
The glass bulb is either kept vacuum or filled with inert gases like argon with a small percentage of nitrogen at low pressure. Inert gases are used to minimize the evaporation of filament during service of the lamps. But due to convection flow of inert gas inside the bulb, there will be greater chances of losing the heat of filament during operation.
Again vacuum is a great insulation of heat, but it accelerates the evaporation of filament during operation. In the case of gas-filled incandescent lamps, 85% of argon mixed with 15% of nitrogen is used. Occasionally krypton can be used to reduce filament evaporation because the molecular weight of krypton gas is quite higher.
But it costs greater. At about 80% of atmospheric pressure, the gasses are filled into the bulb. Gas is filled in the bulb with the rating more than 40 W. But for less than 40 W bulb; there is no gas used.
The various parts of an incandescent lamp are shown below.
Filament of Incandescent Lamp
In present days, incandescent lamps are available in different wattage ratings such as 25, 40, 60, 75, 100 and 200 watts etc. There are different shapes of bulbs, but basically, all are rounded in shape. There are mainly three materials used for producing the filament of incandescent lamps, and these are carbon, tantalum, and tungsten. Carbon was previously used for filament material, but presently tungsten is used most for the purpose.
The melting point of carbon filament is about 3500oC, and the operating temperature of this filament is about 1800oC hence the chance of evaporation is quite less. Because of that carbon filament, incandescent lamps are free from darkening due to filament evaporation. Darkening of filament lamp occurs when molecules of filament material are deposited on the inner wall of the glass bulb due to evaporation of filament during operations.
This darkening becomes prominent after the long life span of the lamp. The efficiency of carbon filament lamp is not good it is about 4.5 lumens per watt. Tantalum was used as the filament, but its efficiency is much poor, it is about 2 lumens per watt. This is because tantalum is very rarely used as filament element.
The most widely used filament material now a day is tungsten because of its high luminous efficacy. It can give 18 lumens per watt when it operates at 2000oC. This efficacy can be up to 30 lumens per watt when it operates at 2500oC. The high melting point is a major criterion for filament material as it has to work at very high temperature without being evaporated.
Although tungsten has the little bit poorer melting point than that of carbon but still tungsten is more preferable as filament material. This is because of high operating temperatures which makes tungsten much luminous efficient. The mechanical strength of tungsten filament is quite high to withstand mechanical vibrations.
Life Span of Incandescent Lamps
Whatever may be the technology of manufacturing, each type of incandescent lamps has some approximate life span. This is because of filament evaporation phenomenon which can be minimized but can not be avoided completely.
Due to filament evaporation, the glass bulb darkens over a period. Due to filament evaporation the filament becomes thinner which makes the filament less luminous efficient and at last, the filament is broken. As the filament lamps are directly connected to the power supply line, the voltage fluctuations in the line, affect the performance of the bulb.
It is found that luminous efficacy of an incandescent lamp is directly proportional to the square of supply voltage but at the same time, the life span of the lamp is inversely proportional to 13th to 14th power of supply voltage. The main advantages of incandescent lamps are that these are cheap enough and very suitable for lighting at small areas. But these lamps are not energy efficient and about 90% of input electrical energy is lost as heat.
Availability of the Incandescent Bulbs in the Market
There are various attractive shapes and sizes of the bulbs available in the market. PS30 lamps have a pear shape, T12 bulb is tubular with diameter 1.5 inch, R40 bulb is with reflector bulb envelope with a diameter of 5 inches. Based on availability of wattage the bulbs are common in the market with 25, 40, 60, 75, 100, 150 and 200W etc. We can follow the table below to get important data about the incandescent lamp.