The phi phenomenon is a perceptualillusion described by Max Wertheimer in his 1912Experimental Studies on the Seeing of Motion, in which a disembodied perception of motion is produced by a succession of still images. In discussions of the perception of film and video it is often confused with beta movement, but it is a distinct phenomenon not directly involved in the perception of motion pictures.
The classic phi phenomenon experiment involves a viewer or audience watching a screen, upon which the experimenter projects two images in succession. The first image depicts a line on the left side of the frame. The second image depicts a line on the right side of the frame. The images may be shown quickly, in rapid succession, or each frame may be given several seconds of viewing time. Once both images have been projected, the experimenter asks the viewer or audience to describe what they saw.
At certain combinations of spacing and timing of the two images, a viewer will report a sensation of motion in the space between and around the two lines, even though the viewer also perceives two distinct lines and not the continuous motion of objects referred to as Beta movement. The phi phenomenon looks like a moving zone or cloud of background color surrounding the flashing objects. The discovery of the phi phenomenon was a significant milestone in Gestalt psychology.
Phi is not Beta (http://www.psych.purdue.edu/Magniphi/PhiIsNotBeta/phi1.html) – An explanation of the difference between the beta and phi phenomena, with online demonstrations of both, and a stronger version of phi than that found by Wertheimer.
The Myth of Persistence of Vision Revisited (http://www.uca.edu/org/ccsmi/ccsmi/classicwork/Myth%20Revisited.htm) – A detailed explanation of how the perception of motion in film and video differs from the simplest notions of "persistence of vision", with mention of the erroneous use of phi as a revised explanation.
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