How Does an ARC Lamp Create Light?
Arc lamps produce an intense light used for the projection of films or computer images, in movie and stage lighting, and searchlights, for solar simulation and ultraviolet (UV) sterilization. In addition to visible light, arc lamps emit high levels of UV that can, even with short exposure, burn your skin and eyes.
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History
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In the early 19th century, the English scientist Sir Humphry Davy created the first arc light using a large battery and two carbon electrodes. When the electrodes were moved from contact with each other, an electrical arc formed. The development of electrical generators allowed continuous arcs to be created. Continuous arcs resulted in erosion of the electrodes. To combat erosion and variations in the power supply, automatic electrical spacing of electrodes was devised.
Modern Arc Lamps
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Modern arc lamps overcome the problems of arc lights by using metallic electrodes in a glass bulb containing a nonreactive gas such as argon, xenon or krypton. To avoid the need for the electrodes to be in contact to start the lamp, an igniter is used. A power supply or ballast is used to regulate the current and voltage supplied to the lamp.
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Power Supplies
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The power supply converts an alternating current (AC) line voltage into the direct current (DC) voltage required for the operation of the igniter and the arc lamp. It is important that the voltage output have ripple of less than 0.5 percent as operation of the arc lamp is sensitive to small variations in voltage.
Igniters
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The igniter is needed to generate the 20,000-volt spike needed to ignite the arc lamp. The igniter is placed in the circuit between the power supply and the lamp and can sense that the lamp is connected. When the power supply is turned on, the igniter will “fire” as many as five times to light the lamp.
Arc-Lamp Bulbs
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The high-voltage spike from the igniter ionizes a column of gas in the bulb between the two electrodes. This ionized gas is a conductor of electricity and allows a high current to flow between the electrodes. This heats up the gas to the equivalent of about 6,000 degrees Kelvin, so that it radiates light in the visible and UV spectrum. This light is so like the sun’s light that some bulbs are used to simulate the sun for laboratory experiments. The UV light can be of such high frequency and intensity that it can create ozone in the air in its beam.
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References
- Photo Credit projecteur de tournage image by Jerome Dancette from Fotolia.com
