Persistence of vision

Sparkler's trail effect

The fact that a glowing coal appears as a line of light when it is moved around quickly has been used to illustrate the idea of persistence of vision. It is known as the "sparkler's trail effect", named after the trail that appears when a sparkler is moved around quickly.

The effect has been applied in the arts by writing or drawing with a light source recorded by a camera with a long exposure time.

Rubber pencil trick

A pencil or another rigid straight line can appear as bending and becoming rubbery when it is wiggled fast enough between fingers, or otherwise undergoing rigid motion.

Persistence of vision has been discarded as sole cause of the illusion. It is thought that the eye movements of the observer fail to track the motions of features of the object.

The effect is known as a magic trick for children.

Thaumatrope

In April 1825 the first Thaumatrope was published by W. Phillips (in anonymous association with John Ayrton Paris). The fact that the image of one side of the disc blends with the image of the other side when it is looked at while it is twirled very fast, is often used as an illustration of persistence of vision.

History

Although the theory of persistence of vision as the reason we see film as motion has been disproved since 1912, film historians have persisted in citing the theory as well as historical references to afterimages and similar illusions. In 1824 the theory was used to explain the illusion of still spokes seen in a rotating wheel. Further research into this phenomenon resulted in the invention of animated moving images in 1832. The optical illusion of motion appearing as a still image led to an optical illusion of still images appearing as motion.

Historical references to afterimages

Aristotle (384–322 BCE) noted that the image of the sun remained in his vision after he stopped looking at it.

The discovery of persistence of vision is sometimes attributed to the Roman poet Lucretius (c. 15 October 99 BCE – c. 55 BCE), although he only mentions it in connection with images seen in a dream.

Leonardo da Vinci and Isaac Newton were also curious about the phenomenon.

1820-1866: Revolving wheels

In 1820 a letter to Quarterly Journal of Science noted that the spokes of a rotating wheel seen through fence slats appeared with peculiar curvatures and asked readers for an explanation. Four years later Peter Mark Roget addressed this observation in a paper: “However rapidly the wheel revolves, each individual spoke, during the moment it is viewed, appears to be at rest." Roget claimed that the illusion is due to the fact “that an impression made by a pencil of rays on the retina, if sufficiently vivid, will remain for a certain time after the cause has ceased.”

As a university student Joseph Plateau noticed in some early experiments that when looking from a small distance at two concentric cogwheels which turned fast in opposite directions, it produced the optical illusion of a motionless wheel. He later read Peter Mark Roget's 1824 article and decided to investigate the phenomenon further. He published his findings in Correspondance Mathématique et Physique in 1828 and 1830.

On December 10, 1830 Michael Faraday presented a paper at the Royal Institution of Great Britain called On a Peculiar Class of Optical Deceptions about the optical illusions that could be found in rotating wheels. He referred to Roget's paper and described his associated new findings. Much was similar to what Plateau had published and Faraday later acknowledged this, but some experiments were new to Plateau. Faraday's experiments of turning wheels in front of the mirror inspired Plateau with the idea for new illusions. In July 1832 Plateau sent a letter to Faraday and added an experimental circle that produced a “completely immobile image of a little, perfectly regular horse” when rotated in front of a mirror. After several attempts and many difficulties Plateau constructed a working model of the phénakisticope in November or December 1832. Plateau published his invention in a January 21, 1833 letter to Correspondance Mathématique et Physique.

Simon Stampfer simultaneously and independently invented his very similar Stroboscopischen Scheiben oder optischen Zauberscheiben (stroboscopic discs or optical magic discs) soon after he read about Faraday's findings in December 1832.

Stampfer also mentioned several possible variations of his stroboscopic invention, including a cylinder (basically the later zoetrope) or a long, looped strip of paper or canvas stretched around two parallel rollers (similar to film). In January, 1834 William George Horner also suggested a cylindrical variation of Plateau's phénakisticope, but he did not manage to publish a working version. William Ensign Lincoln invented the definitive Zoetrope in 1865 and had it published by Milton Bradley and Co. in December 1866.

Other theories for motion perception in film

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 but persists in citations in many classic and modern film-theory texts. 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. 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.

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).

Film systems

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.

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, respectively.

Printed media

Flip books similarly create an illusion of smooth motion when the book is flipped at a fast enough speed.

Persistence of vision displays

A class of display device described as "POV" is one that composes an image by displaying one spatial portion at a time in rapid succession (for example, one column of pixels every few milliseconds). A two-dimensional POV display is often accomplished by means of rapidly moving a single row of LEDs along a linear or circular path. The effect is that the image is perceived as a whole by the viewer as long as the entire path is completed during the visual persistence time of the human eye. A further effect is often to give the illusion of the image floating in mid-air. A three-dimensional POV display is often constructed using a 2D grid of LEDs which is swept or rotated through a volume. POV display devices can be used in combination with long camera exposures to produce light writing.

A common example of this can be seen in the use of bicycle wheel lights that produce patterns.