1.4.2 Persistence of Vision

Persistence of vision, mentioned in [Creative computing I, vol. 1,
section 8.3], can mean one of two things. The strict meaning relates
to the fact that the response to a visual stimulus can persist after that
stimulus has disappeared; a simple example of this can be seen by
moving a luminous object (such as a torch or sparkler) in the dark:
the light leaves an apparent ‘trail’ everywhere that the stimulus has
been observed in the previous second or so. This occurs because, as
discussed in section 1.1.1, the photoreceptors in the eye have a high
response time; the chemical reactions involved in detection of light
are not instantaneous, but happen over a period of time.
The second meaning of “persistence of vision” is the overall
processing by the visual system – eye and brain – which allows the
perception of motion from a series of still images shown in rapid
succession. The persistence of visual response to a momentary
stimulus is only one small part of the overall effect of inducing
motion where none is present; some other perceptual effects playing
their part in the perception of motion, and in particular beta motion,
are introduced in the next section.
Persistence of vision has implications for the design of projection
apparatus, for example for use in cinemas. There are two rates of
importance in such an apparatus. The first is the frame rate: the rate
at which different images are displayed on the screen; for smooth
motion to be perceived, the frame rate should be above about 16
hertz (16 frames per second – modern film runs at 24 frames per
second); frame rates lower than this do not give a convincing
illusion of smooth motion when rapid movements are being
conveyed.
The second rate is the flicker rate: the visual system can perceive a
regular flicker (between frames) as a distraction even at relatively
high frequencies. The flicker rate is at least as high as the frame
rate, as there must be one interruption between frames; however,
modern projectors are designed to add additional interruptions, to
increase the flicker rate (and so decrease its detectability). Typically,
the projector shutter is made doublebladed,
so that one of the
blades obscures the light beam when the projected image is being
updated, and the other at the halfway point; for a frame rate of
24Hz, this would give a flicker rate of 48Hz; a triplebladed
apparatus would give a flicker rate of 72Hz.