Stroboscopic effect

The stroboscopic effect is a visual phenomenon caused by aliasing that occurs when continuous rotational or other cyclic motion is represented by a series of short or instantaneous samples (as opposed to a continuous view) at a sampling rate close to the period of the motion. It accounts for the "wagon-wheel effect", so-called because in video, spoked wheels (such as on horse-drawn wagons) sometimes appear to be turning backwards. A strobe fountain, a stream of water droplets falling at regular intervals lit with a strobe light, is an example of the stroboscopic effect being applied to a cyclic motion that is not rotational. When viewed under normal light, this is a normal water fountain. When viewed under a strobe light with its frequency tuned to the rate at which the droplets fall, the droplets appear to be suspended in mid-air. Adjusting the strobe frequency can make the droplets seemingly move slowly up or down. Depending upon the frequency of illumination there are different names for the visual effect. Up to about 80 hertz or the flicker fusion threshold it is called visible flicker. From about 80 hertz to 2000 hertz it is called the stroboscopic effect (this article). Overlapping in frequency, but from 80 hertz up to about 6500 hertz a third effect exists called the phantom array effect or the ghosting effect, an optical phenomenon caused by rapid eye movements (saccades) of the observer. Simon Stampfer, who coined the term in his 1833 patent application for his stroboscopische Scheiben (better known as the "phenakistiscope"), explained how the illusion of motion occurs when during unnoticed regular and very short interruptions of light, one figure gets replaced by a similar figure in a slightly different position. Any series of figures can thus be manipulated to show movements in any desired direction.