The persistence of vision

By Michael McKinney 

OUR EYES are one of the five specialized ways our mind uses to form a picture of world. The eye is a remarkable instrument that has certain characteristics that help us to process the light we see in such a way that our minds can create meaning from it. 

The motion picture, the scanning of an image for television, and the sequential reproduction of the flickering visual images they produce, work in part, because of an optical phenomena called the persistence of vision and its psychological partner, the phi phenomenon—the mental bridge that the mind forms to conceptually complete the gaps between the frames or pictures. Persistence of vision also plays a role in keeping the world from going pitch black every time we blink our eyes. 

Whenever light strikes the retina, the brain retains the impression of that light for about a tenth of a second—depending on the brightness of the image—after the source of that light is removed from the eye. This is due to a prolonged chemical reaction. As a result, the eye cannot clearly distinguish fast changes in light that occur faster than this retention period. The changes either go unnoticed or they appear to be one continuous picture to the human observer. This fundamental fact of the way we see has been used to our advantage. 

When we go to the movies, we know that a motion picture creates an illusion of a constantly lit screen by flashing separate, individual photographs in rapid succession. Even though the movie screen appears to be constantly lit, it is in fact dark about half the time. This flickering image on the screen gave rise to the old term “flicks” in the early days of movies. Today’s motion pictures flash a picture on the screen at flicker-free 24 frames per second. 

Television too, uses a complicated form of intermittent light impulses to literally build the picture we see. If a picture can be built up in less than a tenth of a second, the eye will be unaware that this process is even occurring. In fact, it will and does appear as if the picture is constantly lit all the time. 

Simply put, if you could create at least ten pictures per second, you could maintain the illusion of a continuous image or picture. You may remember the little “flipbooks” you might have made as a kid that worked on this same principle. The more images per second the smoother the picture looks. American television actually transmits and recreates 30 complete pictures per second to give the illusion of a single continuous picture.