FLASH LAMP

any of several devices that produce brief, intense emissions of light useful in photography and in the observation of objects in rapid motion. The first flash lamp used in photography was invented in Germany in 1887; it consisted of a trough filled with Blitzlichtpulver (flashlight powder), a mixture of magnesium, potassium chlorate, and antimony sulfide. Upon ignition the powder burned quickly, providing a brilliant white light, but it also released a dense cloud of white smoke and was hazardous. The flashbulb, developed in the 1920s, is a transparent envelope filled with oxygen and a tangle of fine aluminum, magnesium, or zirconium wire ignitable by an electrically heated filament or, rarely, a chemical deflagrator. Luminous combustion of the metal is complete within a few hundredths of a second. Most flashbulbs are coated with tinted lacquer or plastic to prevent shattering and to adjust the colour of the light. The electronic flash lamp, commonly called a flashtube, or speedlight, consists of a transparent glass or quartz tube filled with xenon (or, occasionally, other noble gases) and fitted with electrodes. High voltage from a capacitor charges the electrodes and causes the gas to ionize; when an ionization path is complete, a pulse of current passes between the electrodes, causing the gas to flash and discharging the capacitor. The duration of the flash can be as short as one microsecond, and circuitry can be arranged to cause the lamp to operate several thousand times per second. The flashtube was invented in 1931 by Harold Edgerton of the Massachusetts Institute of Technology.