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The phi phenomenon is the optical illusion of perceiving continuous motion between separate objects viewed rapidly in succession. The phenomenon was defined by Max Wertheimer in the Gestalt psychology in 1912 and along with persistence of vision formed a part of the base of the theory of cinema, applied by Hugo Münsterberg in 1916. It is part of a larger process called Motion perception.
Beta movement is often confused with the phi phenomenon but they are different. The phi phenomenon is the apparent motion caused by a changing static image, as in a motion picture. The beta phenomenon is the apparent motion between different light sources that are periodically switched on, as in chase lighting. In phi, there are different images or lights in a single place, whereas in beta the images or lights are in different locations. In both cases the images or lights are turned on and off rapidly to produce the effect. In the animation on the right, you see pink dots going on and off. But look at the cross in the middle and you will see a green dot moving around.
Persistence of vision
Persistence of vision, that is popularly taught as the reason for motion illusion, is in reality merely the reason that the black spaces that come between each "real" movie frame are not perceived, which makes the phi phenomenon the true reason for motion illusion in Cinema and animation, including the phenakistoscope, zoetrope, and others.
Experiment of the phi phenomenon
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. In these cases, the line that seems to move is actually a figure that first appears in the right of the screen and then in the left.
- Wertheimer, M. (1912). Experimentelle Studien uber das Sehen von Bewegung (Experimental studies on the seeing of motion)
- H. Münsterberg: The Photoplay, A Psychological Study
- The Myth of Persistence of Vision Revisited – 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.
- Phi phenomenon activity – Application that lets us change some parameters to experiment with the phi phenomenon.
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