stroboscope

 

(strŏb'əskōp) , optical instrument for making a moving object appear to be slowed down or stationary. This effect is created by interrupting the observer's view so that the object is seen only at regularly spaced intervals rather than continuously. In its simplest form the stroboscope is a rotating disk; along its edge are evenly spaced holes through which the moving object is observed. If the object's motion is cyclic, the speed of the disk can be synchronized with it so that the object always appears in the same position when viewed through one of the holes. During the time that a solid area is blocking the line of sight, the persistence of vision enables the eye to retain the image previously seen, while the object moves to the same or a similar position by the time the next hole is in front of the eye. The effect is thus one of a stationary object.

If the stroboscope is not quite synchronized with the object's motion, the object will appear to move slowly either backward or forward, depending upon whether the stroboscope's rotation is too fast or too slow. For more accurate observation a flashing light (stroboscopic light) is used instead of a disk. When used in conjunction with a camera a stroboscopic light can also be used to study motion that is not cyclic, e.g., a speeding bullet; the resulting photograph shows a series of still images whose separations are proportional to the object's speed.

The stroboscope was invented and improved upon by H. E. Edgerton starting in 1931. It has various uses in scientific research, teaching, and industry, where it is used to study stresses on parts of machines while in motion.

Bibliography

H. Edgerton et al., Stopping Time: The Photographs of Harold Edgerton (1987).

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.

See also

• Electrotachyscope
• Episcotister
• Flip book
• Phenakistoscope
• Praxinoscope
• Strobe light
• Tachometer
• Thaumatrope
• Zoetrope

External links

• Checkline, Hand-Held Stroboscope manufacturer

• Demonstration of Phenakistoscope and Stroboscope at North Carolina School of Science and Mathematics
• (French) an ancestor of stroboscope : the palissadoscope

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