searchlight

searchlight, device, usually swiveled, using a lens and reflecting surface to direct a powerful beam of light of nearly parallel rays. In 1892 such apparatus was used along the English Channel in coastal defense and later, in the South African War, as an aid to infantry movement. It was also used to illuminate vessels in order to identify them and for possible bombardment, for dazzling the enemy, for illuminating the coast in an attack, and to locate targets, at sea or ashore, for the guns. After 1900 acetylene came into use as an illuminant, and in 1916 Edison invented a portable electric apparatus powered by storage batteries. During World War I powerful searchlights mounted on trucks and railroad cars came into use. The electric arc was generally employed after the American inventor E. A. Sperry introduced (1915) his high-intensity arc lamp based on principles that still predominate in modern searchlight equipment. Searchlights of 1,500 million candle power, visible for 150 mi (241 km), have become common. Revolving searchlights as beacons spaced along air routes have yielded to radio beacons. Similarly, the use of powerful lights coordinated with antiaircraft guns developed during World War II has been outmoded by radar-directed artillery. Small searchlights, which are usually employed for signaling, use incandescent lamps. These lamps are often of the quartz-halogen type in which the filament is run at very high temperatures.