Motion - Computer Graphics: Principles and Practice - page 977

25 Hz, and is completely insensitive to flickering at frequencies higher than 80 Hz.
This is partly why the 50 Hz (Europe) or 60 Hz (U.S.) flickering of a fluorescent
light is just barely perceptible, and is why most computer displays refresh at 60 to
85 Hz.
The Beta phenomenon [Wer61], sometimes casually referred to by the over-
loaded term “persistence of vision,” is the phenomenon that allows the brain to
perceive object movement. At around 10 Hz, the threshold of motion perception
is crossed and overlapping objects in sequential frames may appear to be a single
object that is in motion. This is only the minimum rate for motion perception. If
the 2D shapes are not suitably overlapped they will still appear as separate objects.
This means that fast-moving objects in screen space require higher frame rates for
adequate presentation.
The combination of fast-moving objects and the limitations of afterimages to
conceal flickering cause a phenomenon called strobing. Here, motion perception
breaks down even at frame rates higher than 30 Hz. A classic example of strobing
is a filmed or rendered roller-coaster ride from a first-person perspective. Because
points on the coaster track can traverse a significant portion of the screen, even
at high frame rates the individual frames may appear as actual separate flashing
images and not blend into perceived motion of an object. This can be a disturbing
artifact that causes nausea or headache if prolonged. Above 80 Hz, afterimages
appear to completely conceal the strobing effect and the motion becomes apparent,
although the actual images may be blurred by the visual system.
The human perception of motion and flickering creates a natural range for
viable animation rates, from about 10 Hz to about 80 Hz. Table 35.2 shows that
various solutions are currently in use throughout that viable range.

Table 35.2: Common image display rates.

Frequency Phenomenon or Technology

10 Hz Approximate threshold of human motion perception

24 Hz U.S. film

25 Hz Approximate point where afterimages begin to conceal

flickering

25 Hz PAL film (Europe)

25 Hz PAL television and video (progressive rate)

29.92 Hz NTSC television and video (progressive rate)

50 Hz PAL television and video (interleaved rate)

59.94 Hz NTSC television and video (interleaved rate)

60 Hz U.S. power duty cycle; fluorescent light flicker rate

65 Hz Typical LCD monitor refresh rate

72 Hz U.S. film projector refresh (24 Hz with each frame shown

three times)

80 Hz Approximate limit of human flickering perception; strobing

ceases

85 Hz Typical CRT monitor refresh rate

120 Hz Stereo-vision LCD monitor refresh (both views)