Stroboscope

Stroboscope, device for viewing a revolving or oscillating object by making the object appear to be at rest. In its simplest form, a stroboscope is a revolving disk with one or more slits or holes in its periphery through which the observer can view the object. By changing the rate of revolution of the disk, the disk can be synchronized with the moving object, so that each time the slit passes before the eye of the observer, the moving object is in the same position. The simplest way of obtaining synchronization is to revolve a disk with a single slit or hole at precisely the same speed as the revolution or oscillation of the object; the same effect would be obtained using a disk with three evenly spaced holes running at exactly one-third the speed of the object. The observer, looking through the stroboscope disk, sees the object in exactly the same position each time the slit passes the observer's eye. Because of the persistence of vision, the individual glimpses blend into a single image that appears to be stationary, just as the individual frames of a motion picture are blended when the film is projected. If the stroboscope disk is rotating slightly slower than the moving object, the moving object will appear to move slowly in the direction of its actual motion; if the disk is turning faster than the moving object, the object will appear to move slowly in the direction opposite to its actual motion.

Modern stroboscopes have departed from the rotating wheel with slits; they generally utilize a light that gives short flashes at the same rate that the moving object is revolving or oscillating. The neon or other vapor lamp (see Electric Lighting; Neon Lamp) used in such stroboscopes is turned on and off by either mechanical means or an alternating current of variable frequency.

The stroboscope is useful in engineering. Such devices can be used to study vibration, wear, and distortion of moving parts while a machine is actually running. Stroboscopes also can be used to adjust the running speed of a machine to the exact rate desired. For example, a stroboscopic disk is used to check the speed of phonograph turntables. A paper disk is printed with a pattern of lines and placed on the turntable. The lines are so spaced that, if the turntable is revolving at standard speed, they appear stationary when viewed by the light of a lamp flashing at the regular 60-cycle frequency of the AC power line.

High-speed gas discharge stroboscopic lamps, developed from around 1926 to 1931 by Harold Eugene Edgerton and his associates at the Massachusetts Institute of Technology, have made possible both still and motion picture photography of swiftly moving objects (see Photography: Lenses: Flash Photography). These high-speed stroboscopic lamps are capable of producing flashes whose duration is as short as .000001 second.