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. 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 a unit fraction of the object's cyclic speed, at which point the object is seen to be stationary.

History
Joseph Plateau of Belgium is generally credited with the invention the stroboscope in 1832, when he used a disc with radial slits which he turned while viewing images on a separate rotating wheel. Plateau called his device the 'Phenakistoscope'. There was a simultaneous and independent invention of the device by the Austrian Simon von Stampfer, which he named the 'Stroboscope', and it is his term which is used today. The etymology is from the Greek words strobo(s), meaning 'whirling' and scope meaning 'to look at'.

As well as having important applications for scientific research, the earliest inventions received immediate popular success as methods for producing moving pictures, and the principle was used for numerous toys.

Other early pioneers employed rotating or vibrating mirrors. The electronic strobe light stroboscope was invented in 1931, when Harold Eugene Edgerton ("Doc" Edgerton) employed a flashing lamp to study machine parts in motion. General Radio Corporation then went on to productize this invention in the form of their "Strobotach".

Edgerton later used very short flashes of light as a means of producing still photographs of fast-moving objects, such as bullets in flight.

Applications
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. 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 by incandescent lighting powered at mains frequency, and provided the platter is rotating at the correct speed, the marks appear to be stationary.

Flashing lamp strobes are also adapted for pop use, as a lighting effect for discotheques and night clubs where they give the impression of dancing in slow motion.

Other effects
Rapid flashing can give the illusion that white light is tinged with colour, known as Fechner colour. Within certain ranges, the apparent colour can be controlled by the frequency of the flash, but it is an illusion generated in the mind of the observer and not a real colour. The Benham's top demonstrates the effect.

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