Stroboscopy

A stroboscope also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. It consists of either a rotating disk with slots or holes or a lamp such as a flashtube which produces brief repetitive flashes of light. Usually the rate of the stroboscope is adjustable to different frequencies. When a rotating or vibrating object is observed with the stroboscope at its vibration frequency (or a submultiple of it), it appears stationary. Thus stroboscopes are also used to measure frequency.

The principle is used for the study of rotating, reciprocating, oscillating or vibrating objects. Machine parts and vibrating strings are common examples. A stroboscope used to set the ignition timing of internal combustion engines is called a timing light.

In its simplest mechanical form, a rotating cylinder (or bowl with a raised edge) with evenly spaced holes or slots placed in the line of sight between the observer and the moving object. The observer looks through the holes/slots on the near and far side at the same time, with the slots/holes moving in opposite directions. When the holes/slots are aligned on opposite sides, the object is visible to the observer.

Alternately, a single moving hole or slot can be used with a fixed/stationary hole or slot. The stationary hole or slot limits the light to a single viewing path and reduces glare from light passing through other parts of the moving hole/slot.

Viewing through a single line of holes/slots does not work, since the holes/slots appear to just sweep across the object without a strobe effect.

The rotational speed 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. Typically a gas-discharge or solid-state lamp is used, because they are capable of emitting light nearly instantly when power is applied, and extinguishing just as fast when the power is removed.

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