Stroboscopic effect

The stroboscopic effect is a visual phenomenon caused by aliasing that occurs when continuous motion is represented by a series of short or instantaneous samples. It occurs when the view of a moving object is represented by a series of short samples as distinct from a continuous view, and the moving object is in rotational or other cyclic motion at a rate close to the sampling rate. It also accounts for the "wagon-wheel effect", so-called because in video or film, spoked wheels on horse-drawn wagons sometimes appear to be turning backwards.

A strobe fountain, a stream of water droplets falling at regular intervals lit with a strobe light, is an example of the stroboscopic effect being applied to a cyclic motion that is not rotational. When viewed under normal light, this is a normal water fountain. When viewed under a strobe light with its frequency tuned to the rate at which the droplets fall, the droplets appear to be suspended in mid-air. Adjusting the strobe frequency can make the droplets seemingly move slowly up or down.

Explanation
Consider the stroboscope as used in mechanical analysis. This may be a "strobe light" that is fired at an adjustable rate. For example, an object is rotating at 60 revolutions per second: if it is viewed with a series of short flashes at 60 times per second, each flash illuminates the object at the same position in its rotational cycle, so it appears that the object is stationary. Furthermore, at a frequency of 60 flashes per second, persistence of vision smooths out the sequence of flashes so that the perceived image is continuous.

If the same rotating object is viewed at 61 flashes per second, each flash will illuminate it at a slightly earlier part of its rotational cycle. Sixty-one flashes will occur before the object is seen in the same position again, and the series of images will be perceived as if it is rotating backwards once per minute.

The same effect occurs if the object is viewed at 59 flashes per second, except that each flash illuminates it a little later in its rotational cycle and so, the object will seem to be slowly rotating forwards.

In the case of motion pictures, action is captured as a rapid series of still images and the same stroboscopic effect can occur.

Audio conversion from light patterns
The stroboscopic effect also plays a role in audio playback. Compact discs rely on strobing reflections of the laser from the surface of the disc in order to be processed (it is also used for computer data). DVDs and Blu-ray Discs have similar functions.

The stroboscopic effect also plays a role for laser microphones.

Wagon-wheel effect
Motion-picture cameras conventionally film at 24 frames per second. Although the wheels of a vehicle are not likely to be turning at 24 revolutions per second (as that would be extremely fast), suppose each wheel has twelve spokes and rotates at only two revolutions per second. Filmed at 24 frames per second, the spokes in each frame will appear in exactly the same position. Hence, the wheel will be perceived to be stationary. In fact, each photographically captured spoke in any one position will be a different actual spoke in each successive frame, but since the spokes are close to identical in shape and color, no difference will be perceived.

If the wheel rotates a little more slowly than two revolutions per second, the position of the spokes is seen to fall a little further behind in each successive frame and therefore the wheel will seem to be turning backwards.

Dangers
Because of the illusion that the stroboscopic effect can give to moving machinery, it is advised that single-phase lighting is avoided. For example, a factory that is lit from a single-phase supply with basic lighting will have a flicker of 100 or 120Hz (depending on country, double the nominal frequency), thus any machinery rotating at multiples of 50 or 60rpm may appear to not be turning, increasing the risk of injury to an operator. Solutions include deploying the lighting over a full 3-phase supply, or by using high-frequency controllers that drive the lights at safer frequencies.