The stroboscopic effect is a phenomenon of human visual perception in which motion is shown to be interpreted by a brain that receives successive discreet images and stitches them together with automatic aliases for temporal continuity. In short, motion is an artifact. Whether with a flashing light source or through an aperture opening and closing, a strobe can control what the eye sees of a moving object. Despite actually moving, if each retinal image is that of an object in the same exact position, it will be perceived as being 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 the true motion.
The first stroboscope was a novelty toy in which a lampshade with successive images of something in motion, such as the gait of a horse, was spun while another outer lampshade with a series of radial viewing slits was spun in the opposite direction, creating the illusion of a moving still picture. Motion picture film employs the same principle with a projector light and a lens housing a high-speed shutter that alternately illuminates and occludes a long, spinning reel of successive still images. Rotating or oscillating mirrors can also create the stroboscopic effect. Electronic strobe lights, first invented in 1931, are bulbs containing gases that discharge at a rate adjusted by the frequency, or cycling, of electrical current alternating its polarity. Fluorescent lighting, in fact, is a strobe that flashes on and off at a speed too fast for humans to discern.
Researchers had long ago discovered that humans perceive indiscernibly real motion at 24 frames per second — a rate that is greater affords no improvement in verisimilitude, and a lesser rate produces a recognizable illusion of movement. A number of theories evolved from this observation. One is the discreet frame theory that assumes that this rate correlates with the physical speed of neural impulses and that each signal constitutes a still, snap-shot retinal image. The human brain then subjectively manufactures motion by processing the successive images through temporal aliasing, filling in the blank moments with ghost imagery according to both hardwired 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 motion; rather, the brain interprets motion based on rapid but episodic, nonetheless, retinal information. The effect is most clearly demonstrated by repetitive — including cyclically moving — objects. An apt analogy is that if a photo of a working clock is taken every 60 seconds, a person can justly, though incorrectly, conclude that the second hand is broken and has not moved. Any such object whose movement is perfectly synchronized stroboscopically will appear to be motionless.
Extrapolating from this visual phenomenon, if a video camera, operating at 24 frames per second, shoots an auto wheel revolving 23 times per second or its fractional equivalent, each successive video frame will capture the wheel at a position just a bit lagging behind a full revolution of its preceding image. The frame-by-frame evidence clearly indicates that the wheel has moved backward, and indeed, human vision will thus perceive it to have spun in reverse at one revolution per second. The optical illusion, familiarized by movies depicting horse-drawn carriages, is called the “wagon-wheel effect,” and occurs to a varying degree with any video recording of a rotating object.
The stroboscopic effect can be witnessed elsewhere. Popularized by dance clubs, a light THAT strobes relatively slowly will animate a person’s dance movements in seemingly slow motion. A race car engine turning at 9,000 revolutions per minute can be synchronized with a strobe light to freeze and analyze the engine’s static state at that speed. A water fountain with a known flow rate can be displayed to apparently defy gravity by illuminating it with a temporally offset strobe. Principles derived from the stroboscopic effect, such as the sampling rate and aliasing algorithms from one sample to the next, have been applied to optic devices such as pulsing lasers that read a spinning digital data disc.
