26-August-2007, 12:02 AM
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Senior Member
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Join Date: Dec 2005
Location: Adelaide, Australia
Posts: 893
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Quote:
Originally Posted by 01101001
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Yes, excellent reply, thanks.
Quote:
Many people report seeing the wagon-wheel effect on car wheels under continuous illumination. The effect can be produced with a special pattern of lug nut orientation, but often there are other explanations. Some cars have special wheels called spinners; these can truly rotate backwards. With conventional wheels, there's always the possibility of stroboscopic illumination. At night, it can come from artificial light sources. During the day, it can come from reflections from another car's wheels that are rotating at a slightly different rate from that of the observed wheel, or even from another wheel of the observed car if its diameter is not exactly the same as that of the observed wheel. The same caution needs to be exercised for propellors if other propellors are spinning nearby.
Rushton (1967[3]) observed the wagon-wheel effect under continuous illumination while humming. The humming vibrates the eyes in their sockets, effectively creating stroboscopic conditions within the eye. By humming at a frequency of a multiple of the rotation frequency, he was able to stop the rotation. By humming at slightly higher and lower frequencies, he was able to make the rotation reverse slowly and to make the rotation go slowly in the direction of rotation. A similar stroboscopic effect is now commonly observed by people eating crunchy foods, such as carrots, while watching TV. The crunching vibrates the eyes at a multiple of the frame rate of the TV. Besides vibrations of the eyes, the effect can be produced by observing wheels via a vibrating mirror. Rear-view mirrors in vibrating cars can produce the effect.
The first to observe the wagon-wheel effect under truly continuous illumination (such as from the sun) was Schouten (1967[4]). He distinguished three forms of subjective stroboscopy which he called alpha, beta, and gamma: Alpha stroboscopy occurs at 8-12 cycles per second; the wheel appears to become stationary, although "some sectors [spokes] look as though they are performing a hurdle race over the standing ones" (p. 48). Beta stroboscopy occurs at 30-35 cycles per second: "The distinctness of the pattern has all but disappeared. At times a definite counterrotation is seen of a grayish striped pattern" (pp. 48-49). Gamma stroboscopy occurs at 40-100 cycles per second: "The disk appears almost uniform except that at all sector frequencies a standing grayish pattern is seen ... in a quivery sort of standstill" (pp. 49-50). Schouten interpreted beta stroboscopy, reversed rotation, as consistent with there being Reichardt detectors in the human visual system for encoding motion. Because the spoked wheel patterns he used (radial gratings) are regular, they can strongly stimulate detectors for the true rotation, but also weakly stimulate detectors for the reverse rotation.
Purves, Paydarfar, and Andrews (1996[5]) also reported reversed rotation of radial gratings. They concluded however, that this was evidence that human visual perception takes a series of still frames of the visual scene and that movement is perceived much like a movie. This can be called the discrete-frame theory.
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Seems it might not be as simple as a stroboscopic light source though. Vibration of the eyes in the eye sockets?
clop
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