In early 1800s, Sir Humphry Davy invented the first arc lamp. In that first lamp, two electrodes of carbon were used. The arc was produced between the electrodes in air. It was used in searchlights, movie projectors (high-intensity light).
Nowadays, gas discharge lamps are widely used. It is preferred to carbon arc lamp due to high efficiency. Here, the light is produced by the arc as in the case of carbon arc lamp but an inert gas is filled between the electrodes. These are enclosed in a glass tube under low pressure. The ionization of this inert gas is the reason for the formation of arc here. Xenon arc lamp, mercury arc lamp, neon arc lamp, krypton long arc lamp, mercury-xenon arc lamp are the examples. Xenon lamps are the widely used lamp.
Working Principle of Arc Lamp
In carbon arc lamp, the electrodes are in contact at first which is in air. This causes a low voltage to attain an arc. Then the electrodes are detached slowly. As a result of this, the electric current gets heated and the arc is maintained between the electrodes. By the process of heating, the tip of the carbon electrodes gets evaporated. The high-intensity light is produced by this carbon vapor which is highly luminous in the arc. The color of the light produced depends on the temperature, time and electrical characteristics.
In gas discharge lamps, the arc is produced in the space between the electrodes. Here, space is filled with any inert gas. The arc is produced by the ionization of this particular gas. The electrodes and the gas together are covered with a glass tube. When the electrodes are given high voltage power supply, the atoms in the gas experienced an inconceivable electric force and it results in the splitting of the atoms into free electrons and ions. Thus the ionizing of gas takes place (ionization process).
The broken atoms (free electrons and ions) move in contrasting directions. The two charges (free electrons and ions) collapse with each other and also with the electrodes. As a result, energy is emitted in the form of a flash of light. This flash of light is called an arc. This is known as the formation of the arc and is done by the discharging process. So it is also known as discharge lamps. The name of the arc lamp and color of light emitted will directly depend on the atomic structure of the inert gas that is filled in the glass tube.
The typical temperature of arc is over 3000oC or 5400oC. The color of the light emitted by the xenon arc lamp is white (similar to natural daylight) which is used widely. From the neon arc lamp, we get red color and from the mercury arc lamp, bluish color is obtained. The combination of inert gasses are also used. They will give more even light spectrum in a wider range of wavelength.
Application of Arc Lamps
Arc lamps are commonly used in:
- Outdoor lighting
- Flashlights in cameras
- Flood lights
- Microscope lighting (and other research applications)
- Projectors (including cinema projectors)