Stroboscope

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 string are common examples. A stroboscope used to set the ignition timing of internal combustion engines is called a timing light.

Mechanical

In its simplest mechanical form, a stroboscope can be a rotating cylinder or disc with evenly spaced holes or slots placed in the line of sight between the observer and the moving object. If the rotational speed is perfectly synchronised with the speed of the observed system or with an exact multiple of that speed, the system appears motionless, while it can be seen in apparently slowed-down motion if the rotational speed is adjusted to be slightly out of synch. The illusion is caused by temporal aliasing, commonly known as the stroboscopic effect.

Electronic

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.