The phi phenomenon is an optical illusion defined by Max Wertheimer in the Gestalt psychology in 1912, in which the persistence of vision formed a part of the base of the theory of the cinema, applied by Hugo Münsterberg in 1916. This optical illusion is based in the principle that the human eye is capable of perceiving movement from pieces of information, for example, a succession of images. In other words, from a slideshow of a group of frozen images at a certain speed of images per second, we are going to observe constant movement.
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The phi phenomenon is an optical illusion of our brains and eyes that allows us to perceive constant movement instead of a sequence of images. We are supplying information that does not exist (between image and image) that creates the illusion of a smooth movement. The phi phenomenon, which might be considered the basis of the correct working of the cinema, is only a limitation of the human eye, which depends on the persistence of visual sensations.
The "persistence of vision" refers to that phenomenon in which the human perception of the decay of a visual stimulus is slower than the true decay of that stimulus. An image will stay on one's eye for a brief amount of time after its cause has, in reality, disappeared.
The phenomenon of the persistence of vision is popularly taught as the reason that humans perceive motion in such things as zoetropes and classically projected films, but it is in reality not connected with motion perception. It is merely the reason that we do not see the black frames that come between each "real" frame while watching a movie. The true reason for motion perception is the phi phenomenon.
Cinema and other popular forms of animation are, of course, the best example of this phenomenon at work. However, some of its predecessors are as follows:
The phenakistoscope was an early animation device. It preceded the zoetrope.
Meaning "wheel of life," the zoetrope is a device that produces the illusion of motion by presenting static pictures in quick succession. It accomplishes this by way of a spinning drum with slits in the top through which one watches the animation, which has usually been drawn on a strip of paper that sits at the bottom of the drum.
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 a 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.
Although both cause sensation of movement, the phi phenomenon can be considered to be an apparent movement caused by luminous impulses in sequence, whereas the beta movement is an apparent movement caused by luminous stationary impulses [1]