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(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. EdgertonEdgerton, Harold,
1903–90, American inventor and educator, b. Fremont, Nebr. He was educated at the Univ. of Nebraska and the Massachusetts Institute of Technology (D.Sc.
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 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.


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



originally, a toy consisting of two disks that rotate on a common axis (Figure 1). On one disk, as on the face of a clock, there are drawn figures in different phases of some repetitive process—for example, individual positions in the motion of a walking man. In the second disk, which is coupled to the first, thin radial slits are cut; the picture located behind each slit can be seen through the slit.

When the disks are rotated, a person looking through the viewing hole and the slits of the rotating disk sees in succession, for a short time, each of the pictures. Although the motion of the object represented by the figures has been broken down, with respect to time, into discrete phases, the viewer perceives a fused image performing continuous motion. This fusion, into a single visual image of a moving object, of images that are shifted relative to each other and are successively presented at certain intervals for a short time is one type of stroboscopic effect.

The principle of operation of the toy is based on fundamental properties of the human visual-perception apparatus. For this reason, the principle has been made use of in a number of applications in science and technology. For example, the reproduction of moving images in cinematography and television is based on this principle.

Another type of stroboscopic effect involves the illusion not of the motion but of the absence of motion of an object that is, in actuality, moving. In order for a stroboscopically observed object that is executing periodic motion with frequency fobj to appear stationary, this frequency must equal or be a multiple of the frequency of stroboscopic illumination fstr.

Suppose, for example, the frequency of the flashes of light illuminating a rotating spoke (Figure 2) is equal to the number of revolutions of the spoke in one second. The spoke will then be illuminated each time in the same position 0—that is, in the same phase of its rotational motion—and will appear stationary to the eye. If the flash frequency is decreased somewhat, then the interval between flashes will be increased, and the spoke will complete slightly more than a single full revolution in this interval. With each successive flash, the spoke will appear displaced somewhat in the direction of rotation: it will be successively at positions 1, 2, 3, and so on—that is, it will appear to be rotating slowly in the direction shown in Figure 2,a.

If the flash frequency is somewhat greater than the number of revolutions of the spoke per second, each successive flash will illuminate the spoke before it has had a chance to complete a full revolution. The spoke will be seen successively at positions 0, 1, 2, 3, and so on (Figure 2,b) and will appear to be rotating slowly

Figure 1

in the direction opposite to its actual motion. Such apparent backward motion is also produced when the flash frequency is slightly more than one-half, one-third, or one-fourth of the spoke’s speed of rotation. This phenomenon is an example of the stroboscopic effect sometimes seen in motion pictures.

It should be noted that when the flash frequency is two, three, four, or more times greater than the speed of rotation of the spoke, the number of apparently stationary spokes seen will be two, three, four, or more, respectively. These spokes will be located at equal angular distances around the path of rotation.

Figure 2

A source of periodic, intermittent light with a controlled frequency is required to make use of the stroboscopic effect. The term “stroboscope” is applied to instruments for making moving objects visible intermittently, either by illuminating the object with flashes of light or by imposing an intermittent shutter between the viewer and the object. At the present time, in the last quarter of the 20th century, a great variety of optical and electronic shutters—for example, the Kerr cell—are used to provide periodic transmission of light; various sources of pulsed light with controlled parameters are also employed.

The development of stroboscopic methods led to gating, which involves the isolation of an individual phase of the motion of some object by transmitting light from the object to the eye of the viewer with a certain on-off ratio. In this way, the phase is separated from the other phases of the object’s motion and from other types of noise that hamper the viewer.

Stroboscopes have found broad application in all areas of science and technology involving the use of stroboscopic effects. For example, the effect wherein a moving body appears stationary is used in studying periodic motions of such objects as rotating disks, moving rulers, wheels, and axles. The stroboscopic tachometer is a well-known example of a stroboscope-based measuring instrument.



An instrument for making moving bodies visible intermittently, either by illuminating the object with brilliant flashes of light or by imposing an intermittent shutter between the viewer and the object; a high-speed vibration can be made visible by adjusting the strobe frequency close to the vibration frequency.


1. an instrument producing a flashing light, the frequency of which can be synchronized with some multiple of the frequency of rotation, vibration, or operation of an object, etc., making it appear stationary. It is used to determine speeds of rotation or vibration, or to adjust objects or parts
2. a similar device synchronized with the opening of the shutter of a camera so that a series of still photographs can be taken of a moving object
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