Stroboscopic effect

As a stroboscopic effect ( in the context of film as Wagenradeffekt ) refers to the seemingly slow or reverse sequence of periodic processes that are observed only at certain, regular successive intervals of time, for example by light flashes ( strobe ) or by a rotating disk with windows the views only temporarily release.

Basics

When the frequency at which successive time intervals is at least about 16 Hz, then the observed phase of the process for merging the image persistence or Phi - effect to a seemingly continuous sequence.

If the distance of the time intervals equal to the period of the process ( or a multiple of it ), then the process is always observed in the same phase of the periodic process, and he seems to stand still. The spacing of the time intervals is a little larger, then the process will always be observed in a slightly later stage than in the previous time interval, and the process appears to slowly expire. However, if the distance of the time intervals a little smaller than the period, then the phase is always a little earlier and the process seems to be running slowly backwards.

History

The discovery of the stroboscopic effect is an observation of the English physician Peter Marc Roget back ( 1779-1869 ). This saw through the cracks of a dark fence a passing carriage on the sunlit street and noticed that the wheel spokes assumptions strangely curved and immobile forms. He then tried to explain this optical illusion and built a test model in which he replaced the fence having through a column provided with tape and the wheel by a sector-like openings disc. Looking through the columns of the tape on the rotating disc he saw confirmed his observation. He explained this phenomenon graphically and mathematically and published his findings in 1825.

The Belgian Joseph Plateau made ​​these findings was the first to imitate movements advantage and constructed the Phenakistiscope that on the rotating disc 16 drawings included, which imitated a movement sequence. The Austrian Simon Ritter von Stampfer in turn brought the work-alike magic discs, also called " optical discs magic " or " magic stroboscopic disks " called on the market. Since its creation more widespread, sat down, ultimately, its designation of " stroboscopic disc ," which was later simplified to " Strobe " by.

Examples

Machinery

The stroboscopic effect is applied in many areas of engineering to make slow fast processes or visible as a static image. For example, vibrations of components be observed or observed rotating parts (shafts, gears ) in operation.

Harmful is the effect when working with moving machinery when the lighting flickers. This is usually the case with fluorescent or other discharge lamps with conventional ballast. In these, the light flux in the double rhythm of the AC voltage fluctuates (that is 100 or 120 Hz corresponding to a network frequency of 50 or 60 Hz). This can cause the rotation or the direction of rotation of rotating parts is misjudged. Due to the risk of accidents here filament lamps or other low or non- flickering light sources must be used (for example, fluorescent lamps and energy saving lamps with electronic ballasts ). Another way to prevent this effect, provides the connection of the lighting system to the three phase conductors, since the vibration of the line frequency having a phase shift of 120 °.

Communications Engineering

In the news technology of the stroboscopic effect is an unwanted effect that often occurs when when sampling a signal (sampling ) the Nyquist -Shannon sampling theorem has not been met ( undersampling ).

Is then attempts to reconstruct the original signal from the time-discrete sampling values, it is possible that this is reconstructed by aliasing effects such as at twice the period of time.

Film and Television

By superposition of the speed of shown in film or television rotating parts (for example, a rotating spoked wheel ) with the frame rate, it may appear that the movement seems to be slower or in the opposite direction of rotation ( Wagenradeffekt ).

Flicker is achieved in film and television, despite the varying only with 16, 24, 25 or 30 Hz image content in that two or three times the same image is shown.

Record

To play records at the right speed, the speed of the turntable of the intended speed must match the plate. In order to control the speed as a user, many turntables have a stroboscopic device: A mostly operated on the AC line voltage neon lamp lit line pattern of circular sector - sections along the circumference, the line number is chosen so that the pattern seems to stand still with exact speed. When speed is too slow, the pattern appears to move slowly backwards, at too high a contrast forward.