Stroboscopes

A stroboscope, also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. The principle is used for the study of rotating, reciprocating, oscillating or vibrating objects. Machine parts and vibrating strings are common examples.

In its simplest form, a rotating disc with evenly-spaced holes is placed in the line of sight between the observer and the moving object. The rotational speed of the disc is adjusted so that it becomes synchronised with the movement of the observed system, which seems to slow and stop. The illusion is caused by temporal aliasing, commonly known as the stroboscopic effect.

In electronic versions, the perforated disc is replaced by a lamp capable of emitting brief and rapid flashes of light. The frequency of the flash is adjusted so that it is an equal to, or a unit fraction below or above the objects cyclic speed, at which point the object is seen to be either stationary or moving backward or forward, depending on the flash frequency.

Stroboscopes play an important role in the study of stresses on machinery in motion, and in many other forms of research. They are also used as measuring instruments for determining cyclic speed.

As a timing light they are used to set the ignition timing of internal combustion engines.

In medicine, stroboscopes are used to view the vocal cords for diagnosis of conditions that have produced dysphonia (hoarseness). The patient hums or speaks into a microphone which in turn activates the stroboscope at either the same or a slightly different frequency. The light source and a camera are positioned by endoscopy.

Another application of the stroboscope can be seen on many gramophone turntables. The edge of the platter has marks at specific intervals so that when viewed under fluorescent lighting powered at mains frequency, provided the platter is rotating at the correct speed, the marks appear to be stationary. This will not work under incandescent lighting, as incandescent bulbs dont strobe. For this reason, some turntables have a neon bulb next to the platter.

At certain frequencies, flashing light can trigger epileptic seizures in some people.

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