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 is the result of persistence of vision. That version
of the theory was disproved in 1912 by Wertheimer but persists in many
citations in many classic and modern film-theory texts. A more plausible
theory to explain motion perception 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 a myth.
In contrasting persistence of vision theory with phi phenomena, a critical
part of understanding that emerges with these visual perception phenomena is
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,
whereas the persistence of vision privileges a realist approach.
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. In the modern era, some stroboscopic experiments performed by
Peter Mark Roget in 1824 were also cited as the basis for the theory.
Film systems Persistence of vision is still the
accepted term for this phenomenon in the realm of cinema history and theory. In
the early days of film innovation, it was scientifically 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, 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, which is less visible.
In digital film systems, the scan rate may be decoupled from the image update
rate. In some systems, such as the Digital Light Processing 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 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 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
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. 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, 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 use this principle. If the
book is flipped at a fast enough speed, the illusion of smooth motion is
created. Sparkler's trail effect
The sparkler's trail effect occurs when one waves around a lit sparkler,
creating a trail of light. Although it appears that this trail is created by
the light left from the sparkler as it is waved through the air, there is, in
fact, no light along this trail. The lighted trail is a creation of the mind,
which retains a perception of the sparkler's light for a fraction of a
second in sensory memory. 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. 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.
See also Flicker fusion threshold
Motion perception Light writing, a physical animation
technique that has the appearance of persistence of vision.
Beta movement Phi phenomenon
Afterimage Notes and references
External links A Study of the Persistence of Vision –
Analysis by Arthur C. Hardy at MIT Persistence of Vision
The Myth of Persistence of Vision Revisited – commentary on whether the
concept is really a myth. Winkler, Robert. "The Need for Speed".
The New York Times. Winkler, Robert. "The Flicker Fusion
Factor: Why we can't drive safely at high speed". Archived from the original
on 2010-12-05, repost on author's personal website.
I get it, I know I'm inferior, November 9, 2006, Pharyngula – comments
Burns, Paul The History of the Discovery of Cinematography An Illustrated
Chronology Video of a 2D POV display integrated
into a bicycle wheel Build a SpokePOV: LED Bike Wheel Images
MiniPOV: build your own instructions – a project designed for beginners to learn
soldering, electronics assembly, and programming microcontrollers
Visual Perception 8 – Visual Perception Lecture 8, The Moving Image.
Newsreel film of persistence of vision – 1936 Newsreel film explaining how
persistence of vision was thought to work.
Physics Stack Exchange on Persistence of Vision – Physics discussion of
persistence of vision. TestUFO Eye Tracking Animation Animation
demonstrating persistence of vision