Stroboscopic Artefacts 

What to observe

For slow speeds, below about 3 rps (rps = rotations/s) there is fairly smooth movement. For higher speeds we see jumbled segments, backward motion etc. At some high speeds the motion disappears entirely, leaving only a bit of flicker.

Comment

Motion demonstrations, especially when fast motion is involved, suffer from stroboscopic artefacts. A grander designation would be "temporal aliasing”. So I decided to make a teaching point out of a nuisance – my original goal was to demonstrate additive colour mixture…

The disk does not rotate smoothly (even if it might look so) but rather is presented in rapidly succeeding still frames, each with a different rotation angle. This produces the percept of smooth motion (within certain limits), the "Phi Phenomenon” of Wertheimer. The current demo is set up with a frame rate of 120 Hz and, initially, an angle increment ∆ of 5°. Were the angle increment 365°, it would not appear to rotate very fast, but at the same slow speed with ∆ = 365°–360° = 5° (try it). Physically these two alternatives are identical, and our perceptual system chooses the ‘closer’ alternative. An angle increment of 355° leads to the appearance of a slow backwards rotation. This is well known as the "wagon wheel effect”.

However, here in your browser another aspect comes into play: the frame rate of your display. Many LCDs have a refresh rate of 60 Hz, monitors come in rates from 60 to 200 Hz. So what exactly you see, depends on the interaction of the two frame rates and the angle increment, and the intermediate effects may not be pretty ;-).

Sources

Bach M, Meigen T, Strasburger H (1997) Raster-scan cathode ray tubes for vision research – limits of resolution in space, time and intensity, and some solutions. Spatial Vision 10:403–414