Stroboscopic Effect

an illusion of apparent motion or absence of motion that arises when an object or picture is viewed not continuously but during separate time intervals that succeed one another in a periodic manner. An example is the projection of a picture on a screen through a shutter consisting of a rotating disk with slits that alternately passes and shuts off the projecting light. Another example is the illumination of a dark room by periodic flashes of light.

Stroboscopic effects are a result of persistence of vision—that is, the retention in the viewer’s consciousness of a perceived visual image for a short time after the picture or object producing the image disappears. If the time between successive intervals when the picture or object is viewed is shorter than the visual-persistence time, then the images resulting from the discrete acts of viewing are fused into a single image, and the viewer thinks he continuously sees the picture or object.

Two types of stroboscopic effects may be distinguished. In one type, an illusion of apparent motion results when separate pictures are viewed intermittently and the positions of the objects in each picture are slightly shifted relative to the positions in the preceding picture. This type of stroboscopic effect is responsible for the perception of motion in motion pictures and television.

In the second type of stroboscopic effect, an illusion of apparent lack of motion or of slowed motion occurs when a moving object periodically, with the frequency f₁, takes up a previous position. For the object to appear to be totally still, the frequency f of the moments when it is viewed must be equal to f₁ If f and f₁ are not equal but differ by a small amount, then the frequency of the apparent motion of the object is f – f_t. In this case, the apparent motion may be much slower than the actual motion and may differ from it in direction. The instruments used to achieve this type of stroboscopic effect are known as stroboscopes.