# Strobe effect

Apparently slowly turning propeller in a video recording

The stroboscopic effect (in the cinematic context also known as the cartwheel effect ) is the apparently slowed down or reverse sequence of periodic processes that are only observed at certain, regularly successive time intervals, for example by means of light flashes ( stroboscopes ) or a rotating disk with windows that only temporarily release the view.

## Explanation

If, in a sequence of individual images, these follow one another with sufficient frequency, movements due to the beta movement are perceived as realistic and continuous. In the cinema, for example, 24 frames per second are used. The individual images are repeated two to three times in order to appear flicker-free, but this is irrelevant for the stroboscopic effect. If a process that has a constant period is shown on the images shown, the stroboscopic effect can occur, depending on the relationship between the frequencies . For example, if a wagon wheel with 24 radial spokes rotates exactly once per second in a movie with 24 frames per second, the wheel appears to be standing still because each spoke in the following image is exactly in the position of the next spoke. If the wheel spins a little slower, it appears to be spinning backwards. If it rotates a little faster, it appears to be moving in the right direction, but much more slowly than in reality.

## history

The discovery of the stroboscopic effect goes back to an observation by the English doctor Peter Marc Roget (1779–1869). Through the cracks of a dark fence, he saw a carriage driving past on the sunlit street and noticed that the wheel spokes took on strangely curved and immobile shapes. He then tried to explain this optical illusion and built a test model in which he replaced the fence with a band with gaps and the wheel with a sector -like disk. Looking through the cracks in the tape at the rotating disc, he saw his observation confirmed. He explained this phenomenon graphically and mathematically and published his findings in 1825.

The Belgian physicist Joseph Plateau was the first to use this knowledge to imitate motion sequences and constructed the phenakistiscope , which contained 16 drawings on the rotating circular disk that imitated a motion sequence. The Austrian geodesist Simon Ritter von Stampfer, on the other hand, brought the similarly functioning magic disks , also called “optical magic disks” or “stroboscopic magic disks”, onto the market. As his creation became more widespread, his name for the “stroboscopic disc”, which was later simplified to “stroboscope”, also caught on.

## Examples

### machinery

The stroboscopic effect is used in many areas of technology to slow down rapidly running processes or to make them visible as a still image. For example, vibrations of components can be observed, or rotating parts (shafts, gears) can be observed during operation.

The effect when working on machines with moving parts is dangerous when the lighting flickers. This is usually the case with fluorescent lamps or other gas discharge lamps with conventional ballasts . With these, the luminous flux fluctuates in the double rhythm of the mains AC voltage (i.e. 100 or 120 Hz corresponding to a mains frequency of 50 or 60 Hz). This can lead to the rotation or the direction of rotation of rotating parts being misjudged. Due to the associated risk of accidents, incandescent lamps or other light sources with little or no flickering (for example fluorescent lamps or energy-saving lamps with electronic ballasts) must be used. Another possibility to prevent this effect is to connect the lighting system to the three different outer conductors, since here the oscillations of the mains frequency have a phase shift of 120 °, or the so-called dual circuit , which enables a phase shift of 90 °.

### Communications engineering

In communications engineering, the stroboscopic effect is an undesirable effect that often occurs when the Nyquist-Shannon sampling theorem is not complied with when a signal is sampled (undersampling).

If an attempt is now made to reconstruct the original signal from the discrete-time sampling values, it can happen that this is caused by aliasing effects, e.g. B. is reconstructed with twice the period.

### Movie and TV

Due to the superimposition of the speed of rotating parts shown in films or television (for example a rotating spoked wheel) with the frame rate, it may appear that the movement appears slower or in the opposite direction of rotation (cartwheel effect).

Flicker-free is achieved in film and television, despite the image content only changing at 16, 24, 25 or 30 Hz, by showing the same image two or three times in a row.

### vinyl record

Line pattern on the platter circumference
Strobe lighting (bottom right)

In order to play records at the correct speed, the speed of the turntable must correspond to the intended speed of the record. In order to be able to control the speed as the user, many turntables have a stroboscopic device: A glow lamp, usually operated on the AC mains voltage, illuminates line patterns made up of circular sector sections along the circumference, the number of lines being selected so that the pattern appears to stand still at the exact speed. If the speed is too low, the pattern seems to move slowly backwards, if it is too high it moves forward.