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The stroboscopic effect is when the brain interprets movement based on rapid but episodic retinal information. Strobes can control what the eye sees of a moving object, creating an optical illusion. The effect is most clearly demonstrated by repetitive objects, including those that move in cycles. The “wagon wheel effect” occurs when a video camera shoots a wheel spinning at a slightly slower rate than the camera’s frame rate. The stroboscopic effect can be seen in nightclubs, analyzing engines, and visualizing water fountains.
Stroboscopic effect is a phenomenon of human visual perception in which motion is shown to be interpreted by a brain that receives successive discrete images and stitches them together with automatic aliases for temporal continuity. In short, movement is an artifact. Whether with a flashing light source or by opening and closing an aperture, a strobe can control what the eye sees of a moving object. Although it actually moves, if each retinal image is that of an object in exactly the same location, it will be perceived as stationary. Stroboscopic control of repetitive or predictive motion, such as the rotation of a wheel, can create an optical illusion that is completely contrary to actual motion.
The first stroboscope was an innovative toy in which a lampshade with successive images of something in motion, such as a horse’s gait, was spun while another outer lampshade with a series of radial viewing slots was rotated in the opposite direction, creating the illusion of a moving still image. Motion picture film uses the same principle with a projector light and lens housing a high-speed shutter that alternately illuminates and occludes a long, rotating reel of successive still images. Rotating or swinging mirrors can also create the stroboscopic effect. Electronic strobe lights, first invented in 1931, are gas-filled bulbs that discharge at a rate governed by the frequency, or cycles, of the electric current that alternates their polarity. In fact, fluorescent lighting is a strobe that turns on and off at a rate too fast for humans to detect.
Researchers had long ago discovered that humans perceive indiscernibly real motion at 24 frames per second: faster speeds offer no improvement in likelihood, and slower speeds produce a recognizable illusion of motion. Numerous theories have developed from this observation. One is the discrete frame theory which assumes that this speed is related to the physical speed of neural impulses and that each signal constitutes an instantaneous, instantaneous retinal image. The human brain then subjectively produces movement by processing successive images through temporal aliasing, filling the empty moments with ghost images according to both hard-wired laws and learned rules of space and time.
This theoretical framework is the most accepted explanation of the stroboscopic effect. Humans do not see physical movement; rather, the brain interprets movement based on rapid but episodic retinal information. The effect is most clearly demonstrated by repetitive objects, including those that move in cycles. An apt analogy is that if a picture of a working clock is taken every 60 seconds, a person can rightly, albeit wrongly, conclude that the second hand is broken and hasn’t moved. Any such object whose movement is perfectly stroboscopically synchronized will appear motionless.
Extrapolating from this visual phenomenon, if a video camera, operating at 24 frames per second, shoots an automatic wheel spinning 23 times per second, or its fractional equivalent, each subsequent video frame will capture the wheel at a position just a little behind to a whole revolution of its previous image. The frame-by-frame evidence clearly indicates that the wheel has moved backwards, and indeed, human vision will perceive that it has rotated backwards at one revolution per second. The optical illusion, made familiar from movies depicting horse-drawn carriages, is called the “wagon wheel effect,” and occurs to varying degrees with any video recording of a rotating object.
The stroboscopic effect can be seen elsewhere. Popularized by nightclubs, a relatively slow flashing light will animate a person’s dance movements in seemingly slow motion. A race car engine spinning at 9,000 rpm can be synchronized with a strobe light to freeze and analyze the static state of the engine at that speed. A water fountain with a known flow rate can be visualized to seemingly defy gravity by illuminating it with a time-shifted stroboscope. Principles derived from the stroboscopic effect, such as sample rate and aliasing algorithms from one sample to another, have been applied to optical devices such as pulsed lasers that read a rotating digital data disc.
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