Persistence of vision

According to the theory of persistence of vision, the perceptual processes of the brain or the retina of the human eye retains an image for a brief moment of time. A visual form of memory known as iconic memory has been described as the cause of this phenomenon. Persistence of vision is said to account for the illusion of motion which results when a series of film images are displayed in quick succession, rather than the perception of the individual frames in the series.

Although psychologists and physiologists have rejected the relevance of this theory to film viewership, film academics and theorists generally have not.

Persistence of vision should be compared with the related phenomena of beta movement and phi movement. A critical part of understanding these visual perception phenomena is that the eye is not a video camera: there is no "frame rate" or "scan rate" in the eye: instead, the eye/brain system has a combination of motion detectors, detail detectors and pattern detectors, the outputs of all of which are combined to create the visual experience.

The frequency at which flicker becomes invisible is called the flicker fusion threshold, and is dependent on the level of illumination.

Film systems
Through experience in the early days of film innovation, it was determined that a frame rate of less than 16 frames per second caused the mind to see flashing images. Audiences still interpret motion at rates as low as ten frames per second or slower (as in a flipbook), but the flicker caused by the shutter of a film projector is distracting below the 16-frame threshold.

Modern theatrical film runs at 24 frames a second. This is the case for both physical film and digital film systems.

It is important to distinguish between the frame rate and the flicker rate, which are not necessarily the same. In physical film systems, it is necessary to pull down the film frame, and this pulling-down needs to be obscured by a shutter to avoid the appearance of blurring; therefore, there needs to be at least one flicker per frame in film. To reduce the appearance of flicker, virtually all modern projector shutters are designed to add additional flicker periods, typically doubling the flicker rate to 48 Hz, which is less visible. (Some newer projector shutters even triple it to 72 Hz.)

In digital film systems, the scan rate may be decoupled from the image update rate. In some systems, such as the Digital Light Processing (DLP) system, there is no flying spot or raster scan at all, so there is no flicker other than that generated by the temporal aliasing of the film image capture.

The new film system MaxiVision 48 films at 48 frames per second, which, according to film critic Roger Ebert, offers even a strobeless tracking shot past picket fences. The lack of strobe (as opposed to flicker) is due to the higher sampling rate of the camera relative to the speed of movement of the image across the film plane. This ultra-smooth imaging is called High motion. It is critical for sports and motion simulation, but unpopular for drama.

Video systems
Video records at 50 (Eurasia) or 60 (US & Japan) images per second (ips) depending on the national system used; The flicker or refresh rate on a television screen is fixed to one or the other nationally chosen standards. A technique called interlace uses persistence of vision to combine two consecutive images (or fields) to create one frame with higher detail in non-moving areas. Because the fields are exposed and displayed separately, a single TV "frame" can potentially contain motion or even two distinct images. This is a case of confusing terminology.

With ordinary video from video cameras, the flicker rate and the image rate are the same. However, when footage shot on 24 hz film is shown on 60 hz TV, each film frame is repeated for 2 to 3 consecutive fields to produce 60 fields per second. (see 3:2 pulldown) In countries using 50 hz TV, 24 Fps film is sped up by 4% to produce 25 frames (50 fields) per second.

Some modern video systems also decouple display from image update, for example systems using LCD monitors with continuous light output, or intermediate frame buffers that increase the display rate to 100 or 120 fields per second.

Computer monitors
Computer monitors do not use interlacing. They may sometimes seem to flicker, especially in a brightly lit room. This is due to the greater likelihood that a computer monitor will occupy the viewer's peripheral vision, where sensitivity to flickering is greater. Generally, a refresh rate of 75 Hz or above (as found in most modern monitors) is sufficient to minimize flicker at close viewing distances, and all recent computer monitors are capable of at least that rate. Flat-panel Liquid Crystal Display (LCD) monitors do not suffer from flicker even if their refresh rate is 60 Hz or even lower, since the light transmission changes in the liquid crystals are slower than that.

Cartoon animation


In drawn animation, moving characters are often shot "on twos", that is to say, one drawing is shown for every two frames of film (which usually runs at 24 frames per second), meaning there are only 12 drawings per second. Even though the image update rate is low, the fluidity is satisfactory for most subjects. However, when a character is required to perform a quick movement, it is usually necessary to revert to animating "on ones", as "twos" are too slow to convey the motion adequately. A blend of the two techniques keeps the eye fooled without unnecessary production cost.

Animation for most "Saturday morning cartoons" is produced as cheaply as possible, and is most often shot on "threes", or even "fours", i.e. three or four frames per drawing. This translates to only 8 or 6 drawings per second. Persistence of vision does not work at such a low image rate, and the motion may be objectionably jerky.

Optical toys
Persistence of vision was used to create a number of optical toys that were popular before movies. Most used simple mechanical methods, such as spinning disks or turning spindles, to quickly transpose a series of pictures and so create the illusion of motion. Examples include the zoetrope and the thaumatrope.