Temporal Aliasing (Wagon Wheel/Stroboscopic Effect)

Temporal Aliasing is where artefacts appear in video due to the interaction between the time/speed of sampling and motion of objects being imaged.

When the frequency of the frame rate used matches any cyclic motion in the scene being captured an effect known as ‘The Wagon Wheet Effect’ occurs. The most recognisable instance of this is when the wheels on a car (or wagon!) appear to be stationary or moving backwards (or simply moving at a slower speed to reality).

It’s not just wheels that exhibit this type of temporal aliasing. Helicoptor rotors when running at just the right speed have resulted in videos that look faked.

It’s important to highlight that for the effect to work the shutter speed must be very short (i.e. the period of time light is falling on the camera’s sensor), otherwise blurring will occur making the effect less impressive or removing it altogether.

An interesting related artefact, going back to wheels, is how spokes on moving wheels blur more at the top than at the bottom due to the relative speed with which they are travelling.