Persistence of vision is the optical phenomenon where the human eye continues to perceive a visual image for a short period after the physical stimulus has disappeared. This sensory lag in our visual system is what allows the world to appear continuous, preventing our perception from going dark every time we blink or when an object momentarily passes out of view. Understanding this effect was historically fundamental to early experiments in human sight, revealing how the brain processes sequential inputs to create a unified visual experience.
The Core Concept Defined
The lingering visual impression is not instantaneous, but rather persists for a measurable duration after the light source is removed. Generally, this retention period is estimated to be approximately 1/10th to 1/25th of a second, depending on factors like the brightness of the image and the field of view. When a new image is presented to the retina before the previous one has completely faded, the two impressions merge in the visual system. A common example is the “sparkler’s trail” effect, where waving a light source in the dark causes the bright point to appear as a continuous line because the light remains in perception slightly longer than it exists in a given position.
The Underlying Biological Process
The persistence of a visual impression is rooted in the physiological processes of the eye and the central nervous system. When light strikes the retina, photoreceptor cells convert the light energy into electrical signals that travel along the optic nerve to the visual cortex in the brain. This conversion and transmission process is not immediate, and the nerve signal continues to fire briefly even after the stimulating light is gone.
This sensory retention, often referred to as visible persistence, is a lower-level function linked to the delayed decay of neural activity in the visual pathway. The photoreceptors, specifically the rods and cones, undergo a prolonged chemical reaction before fully resetting, which contributes to the lingering impression. The delay in the processing of the signal by the visual cortex further ensures that the image information is held in a sensory memory store.
The Role in Motion Pictures and Animation
The technical foundation of motion pictures and animation relies heavily on exploiting the sensory lag inherent in human vision. By presenting a series of slightly different still images, or frames, in rapid succession, creators can generate the illusion of movement.
In cinema, the standard projection rate of 24 frames per second (fps) is sufficient to ensure that each image replaces the last before the visual impression fades completely. If the frame rate drops too low, the viewer would perceive a choppy sequence of images rather than fluid motion. Early devices such as the thaumatrope and the zoetrope were invented to demonstrate and utilize this visual principle.
Animators use techniques like “animating on ones,” where 24 unique drawings are used for every second of film, to achieve the smoothest possible motion. Alternatively, “animating on twos” uses one drawing for every two frames, resulting in 12 unique images per second, which is a common practice that still maintains the illusion of continuity.
Persistence of Vision Versus Related Visual Effects
While persistence of vision explains why the eye does not perceive the dark gaps between individual film frames, it is not the sole explanation for the perception of smooth motion. Modern visual science distinguishes this simple sensory retention from more complex cognitive effects.
The perception that a sequence of still images is moving smoothly is more accurately attributed to the Phi phenomenon, which describes the brain’s tendency to perceive two sequential, stationary lights flashing on and off as a single light moving between the two points.
Furthermore, the Flicker Fusion Rate is a related concept that dictates the frequency at which a flickering light source appears to be continuous. For humans, this rate is typically around 50 to 90 flashes per second.
This explains why film projectors often use a shutter to flash each 24 fps frame two or three times to achieve a flicker rate of 48 to 72 flashes per second. This prevents the distracting perception of flicker, working in tandem with the visual persistence that blends the images together.