incandescent lamps are too slow for clear stroboscopic effects. Yet when operated from an AC source they are mostly fast enough to cause audible hum (at double mains frequency) on optical audio playback such as on film projection. The frequency of the flash is adjusted so that it is an equal to, or a unit fraction of the object's cyclic speed, at which point the object is seen to be either stationary or moving slowly backward or forward, depending on the flash frequency. Neon lamps or light-emitting diodes are commonly used for low-intensity strobe applications, Neon lamps were more common before the development of solid-state electronics, but are being replaced by LEDs in most low-intensity strobe applications. Xenon flash lamps are used for medium- and high-intensity strobe applications. Sufficiently rapid or bright flashing may require active cooling such as forced-air or water cooling to prevent the xenon flash lamp from melting. History Joseph Plateau of Belgium is generally credited with the invention of the stroboscope in 1832, when he used a disc with radial slits which he turned while viewing images on a separate rotating wheel. Plateau's device became known as the "Phenakistoscope". There was an almost simultaneous and independent invention of the device by the Austrian Simon Ritter von Stampfer, which he named the "Stroboscope", and it is his term which is used today. The etymology is from the Greek words στρόβος - strobos, meaning "whirlpool" and σκοπεῖν - skopein, meaning "to look at". As well as having important applications for scientific research, the earliest inventions received immediate popular success as methods for producing moving pictures, and the principle was used for numerous toys. Other early pioneers employed rotating mirrors, or vibrating mirrors known as mirror galvanometers. In 1917, French engineer Etienne Oehmichen patented the first electric stroboscope, building at the same time a camera capable of shooting 1,000 frames per second. Harold Eugene Edgerton ("Doc" Edgerton) employed a flashing lamp to study machine parts in motion. General Radio Corporation then went on to produce this device in the form of their "Strobotac", an early example of a commercially successful stroboscope. Edgerton later used very short flashes of light as a means of producing still photographs of fast-moving objects, such as bullets in flight. Applications Stroboscopes play an important role in the study of stresses on machinery in motion, and in many other forms of research. Bright stroboscopes are