What is Persistence of vision

What is Persistence of vision

Persistence of vision refers to the optical illusion whereby multiple discrete images blend into a single image in the human mind and believed to be the explanation for motion perception in cinema and animated films. Like other illusions of visual perception, it is produced by certain characteristics of the visual system.

History

Narrowly defined, the theory of persistence of vision is the belief that human perception of motion (brain centered) is the result of persistence of vision (eye centered). That version of the theory was disproved in 1912 by Wertheimer[2] but persists in citations in many classic and modern film-theory texts.[3][4][5] A more plausible theory to explain motion perception (at least on a descriptive level) are two distinct perceptual illusions: phi phenomenon and beta movement.

A visual form of memory known as iconic memory has been described as the cause of this phenomenon.[6] Although psychologists and physiologists have rejected the relevance of this theory to film viewership, film academics and theorists generally have not. Some scientists nowadays consider the entire theory of iconic memory a myth.[7]

When contrasting the theory of persistence of vision with that of phi phenomena, an understanding emerges that the eye is not a camera and does not see in frames per second. In other words, vision is not as simple as light registering on a medium, since the brain has to make sense of the visual data the eye provides and construct a coherent picture of reality. Joseph Anderson and Barbara Fisher argue that the phi phenomena privileges a more constructionist approach to the cinema (David Bordwell, Noël Carroll, Kirstin Thompson) whereas the persistence of vision privileges a realist approach (André Bazin, Christian Metz, Jean-Louis Baudry).[7]

The discovery of persistence of vision is attributed to the Roman poet Lucretius, although he only mentions it in connection with images seen in a dream.[8] In the modern era, somestroboscopic experiments performed by Peter Mark Roget in 1824 were also cited as the basis for the theory.[9]

Film systems

Main article: Frame rate

Persistence of vision is still the accepted term for this phenomenon in the realm of cinema history and theory. Early practitioners tried different frame rates, and chose a rate of 16 frames per second (frame/s) as high enough to cause the mind to stop seeing 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 cinema 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 (single-bladed shutters make two rotations per frame – double-bladed shutters make one rotation per frame), which is less visible. (Some three-bladed 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.

Computer monitors

Aside from some configurations used until the early 1990s, computer monitors do not use interlacing. They may sometimes be seen to flicker, often in a brightly lit room, and at close viewing distances. The greater flickering in close-up viewing is due to more of the screen being in the viewer's peripheral vision, which has more sensitivity to flickering. Generally, a refresh rate of 85 Hz or above (as found in most modern CRT monitors) is sufficient to minimize flicker in close viewing, 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 lower. This is because an LCD pixel generates a continuous stream of light as long as that part of the image is supposed to be lit (see also ghosting). With each scan, the monitor determines whether a pixel should be light or dark and changes the state of the pixel accordingly. In a CRT, by comparison, each pixel generates a temporary burst of light, then darkening, in each periodic scan. The monitor activates a phosphor on the screen during each scan if the pixel is supposed to be light, but the phosphor fades before the next scan.[10]



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