Searchlights

A searchlight is an optical device for projecting a narrow, high-intensity light beam over distances up to many miles. The beam is projected by means of a concentrated light source of extremely high brightness placed at the focal point of a lens or mirror.

Photo by Frank Hermers
Photo by Frank Hermers

Uses

By World War I, powerful searchlights had been developed for military applications such as coast defense and aircraft detection, for long-range signaling, and for aids to navigation. Electronic techniques developed in World War II made the searchlight obsolete for aircraft detection and other detection applications. However, new applications have been found for them, and they are still widely used. Searchlights are used on aircraft in antisubmarine warfare, on tanks for tactical combat applications, and in advance combat areas for indirect illumination of battlefields by atmospheric scattering of light. They also are used as solar simulators in space research and as components of a variety of measuring and signaling devices. In everyday life, searchlights have a variety of uses, such as helping firemen fight fires or illuminating buildings to highlight them at night.

Design and Performance

Most searchlights use parabolic reflectors rather than lenses because reflectors with large light-collecting angles are free from spherical and chromatic aberrations, which severely limit the capability of lenses. A theoretical-point source of light at the focal point of a perfect parabolic reflector would produce a beam of exactly parallel rays. In fact, however, all light sources, however small, have extent and thus are not actually point sources.

Searchlight beams are fully formed at a relatively short distance from the light. From there on, they have characteristic angular distributions of light intensity, with divergences ranging from about 1° to 10°. If a fully formed beam is projected through a vacuum, the illumination at any point is inversely proportional to the square of the distance between the point and the light source. In such circumstances, the attenuation, through air, the beam is also attenuated by scattering and absorption caused by dust, moisture, and the air molecules themselves.

Attenuation due to the inverse square law and the atmosphere drastically limits the useful range of a searchlight. When the range of a searchlight under a given set of conditions is several miles, the increase in light-source intensity required to obtain a relatively small increase in range is very large. If the atmosphere is at all hazy, the required increase in intensity may be extremely high.

The most effective light sources for searchlights are those with very high brightness, notably the carbon arc. The high-intensity carbon arc, developed by the German inventor Heinrich Beck between 1906 and 1910 is the most intense light source for searchlights. Water-cooled carbon arcs have been operated with brightnesses of 2,000 candles per square millimeter (c/mnr). High-intensity carbon arcs generally operate in the range from 500 to 1,500 c/mm2. For comparison, the brightness of the sun as seen from the earth's surface, is about 1,600 c/mm2.

Mercury and mercury-xenon short-arc lamps are widely used as light sources in small searchlights. They do not attain the brightness of carbon arcs, but they do not require complex carbon-feed mechanisms and can operate for many hours without attention.

Where simplicity and reliability are of most importance and high intensity and small beam divergence of less importance, incandescent lamps may be used as light sources in searchlights, even though the maximum brightness of a tungsten-filament incandescent lamp is only about 20 c/mm2. Incandescent light sources have been used in tank-mounted searchlights and in airborne searchlights for search and rescue.

Research and Development

The development of large powerful searchlights, with reflectors 10 feet (3 meters) in diameter and carbon arcs consuming as much as 600 kilowatts of power, reached a peak in World War II. The maximum beam intensity of many of the more powerful searchlights was several billion candles. Since then, researchers have concentrated on the development of new light sources, new reflector designs, and new fabrication techniques. For instance, research in plasma physics may eventually result in the development of a practical light source with a brightness greater than that of the high-intensity carbon arc.

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