Persistence of vision or visual persistence is the optical illusion that occurs when the visual perception of an object continues for some time after the rays of light proceeding from it have ceased to enter the eye. The illusion has also been described as "retinal persistence" when attributed strictly to the retina, and "persistence of impressions" as a more general term that can also apply to the other senses. An example of the phenomenon is the fiery trail from a glowing coal or burning stick that is whirled around in the dark. Some explanations of the illusion describe either positive afterimages or motion smear (comparable to motion blur in photography, film and video). Recent theories about visual sensory memory differentiate between higher-level psychological 'informational persistence' and lower-level phenomenal 'visible persistence'. The human visual system depends on temporal integration in general; it continuously processes visual inputs over time. "Persistence of vision" can be understood to be the same as "flicker fusion", the effect that images persist when the light that enters the eyes is interrupted at short, regular intervals. When the frequency is too high for the visual system to discern differences between moments, light and dark impressions fuse together into a continuous image of the scene with intermediate brightness (defined by the Talbot-Plateau law). Since its introduction, the term "persistence of vision" has often been claimed to be the explanation for motion perception in optical toys like the phenakistiscope and the zoetrope, praxinoscope, mutoscope, theatre optique (optical theater) and later in cinema. This theory has been disputed since long before cinematography's breakthrough in 1895. The illusion of motion as a result of fast intermittent presentations of sequential images is a stroboscopic effect, as explained in 1833 by Simon Stampfer, one of the inventors of the stroboscopic disc (phenakistiscope). If the retention of images lasts longer than the interruptions, it does explain the apparent continuity of the image, which looks weaker, depending on the ratio between exposures and interruptions. Early descriptions of the illusion attributed the effect purely to the physiology of the eye, particularly the retina. The nervous system later became accepted as an important factor.
Neurology
Although the persistence of a visual impression can be pathological (palinopsia), it can commonly be attributed to incidental yet normal physiological afterimages, or to the aspect of visual sensory memory that is a standard element of human vision. Vision has a delayed response to stimulus onset, with varying durations of the store, integration and decay after offset. In recent theories about visual sensory memory, a distinction is made between visible persistence and informational persistence. Visible persistence has an inverse relation to the duration and intensity of the stimulus, and presumably depends on neural persistence in the visual pathway. Informational persistence is a subsequent element of higher-level (cortical) processing.
Natural occurrences and applications Impressions of several natural phenomena and the principles of some optical toys have been attributed to persistence of vision. In 1768, Patrick D'Arcy recognised the effect in "the luminous ring that we see by turning a torch quickly, the fire wheels in the fireworks, the flattened spindle shape we see in a vibrating cord, the continuous circle we see in a cogwheel that turns with speed". Basically everything that resembles motion blur seen in fast moving objects could be regarded as "persistence of vision".
Sparkler's trail effect The apparent line of light behind a fast moving luminous object such as a sparkler is known as the "sparkler's trail effect".
Light painting
The effect has occasionally been applied in the arts by writing or drawing with a light source recorded by a camera with a long exposure time. This technique has been further developed into media with computer-controlled moving light sources (typically LED light), known as S.W.I.M. (Sequential Wave Imprinting Machine). However, like video and television, the technology actually gets rid of the visual trail of fast-moving lights by presenting a stroboscopic sequence of very short visual cues (resulting in a sharp image, still or animated).
Color-top / Newton disc
Colors on spinning tops or rotating wheels mix together if the motion is too fast to register the details. A colored dot then appears as a circle and one line can make the whole surface appear in one uniform hue. The Newton disc optically mixes wedges of Isaac Newton's primary colors into one (off-)white surface when it spins fast.
Thaumatrope In April 1825, the first set of the Thaumatrope was published by W. Phillips (in anonymous association with John Ayrton Paris). The fact that the pictures on either side of the twirling disc are seen as a combined or superposed single image has traditionally been presented as an illustration of retinal persistence. Given the common association with motion streaks of whirled around burning coals or sticks, the principle would sooner be expected to render the fast-moving pictures as unrecognizably smeared across the retina, but this is prevented by the brief visibility of each side. Just like the last frame of one shot in a movie and the next frame of an entirely different shot are clearly perceived as separate images, the alternating pictures of the thaumatrope don't just appear superposed if the depicted figures aren't mentally compatible. Informational persistence would be the more likely cause, and the effect depends on principles of Gestalt psychology, inspired by study of the differences between impressions of quick alternations of two figures –depending on tachistoscope frequencies, distance between the figures, and/or variance in shapes– as studied by Max Wertheimer in 1912.
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