Pulfrich

The Pulfrich phenomenon, also known as the Pulfrich effect, is a depth illusion in binocular vision caused by interocular differences in neural processing speed. It occurs most commonly when one eye receives less light than the other, for example by placing a neutral density filter over one eye. In moving objects, the delayed processing from the dimmer eye creates a binocular disparity that the brain interprets as depth.

Origin and explanation: In 1922, Carl Pulfrich demonstrated the effect while observing a pendulum and a rotating disk. He showed that with a filter over one eye, a pendulum swinging in the frontal plane appears to move in depth, either toward or away from the observer, depending on the direction of motion. The effect arises because the brain receives the two images at slightly different times and computes depth from their apparent parallax.

Applications and notes: The Pulfrich effect is used as a demonstration of stereo vision, as a simple diagnostic tool for interocular processing delays, and in some novelty 3D viewing setups known as Pulfrich glasses. It is not a true depth cue based on real-world geometry, and it depends on motion; stationary objects do not create the illusion. The effect can cause dizziness or confusion in some viewers, particularly if the motion is rapid or the viewing conditions are inconsistent.

Relation and significance: The phenomenon illustrates how luminance and neural latency can influence perceived depth and is distinct from the geometric cues exploited in standard stereopsis or in conventional 3D cinema. It remains a topic of study in vision science and optometry, highlighting the interaction between temporal processing and depth perception.