Pulfrich effect

The Pulfrich effect is a psychophysical percept wherein lateral motion of an object in the field of view is interpreted by the visual cortex as having a depth component, due to a relative difference in signal timings between the two eyes.

In the classic Pulfrich effect experiment, a subject views a pendulum swinging in a plane perpendicular to the observer's line of sight.

The widely accepted explanation of the apparent depth is that a reduction in retinal illumination (relative to the fellow eye) yields a corresponding delay in signal transmission, imparting instantaneous spatial disparity in moving objects.

The Pulfrich effect has typically been measured under full field conditions with dark targets on a bright background, and yields about a 15 ms delay for a factor of ten difference in average retinal illuminance.

On Sunday, January 22, 1989 the Super Bowl XXIII halftime show and a specially produced commercial for Diet Coke were telecast using this effect.

Forty million pairs of paper-framed 3D viewing "glasses" were distributed by Coca-Cola USA for the event (though they were originally produced and intended for a May 1988 3D episode of Moonlighting that never finished production due to a writer's strike).

The commercial was in this case restricted to objects (such as refrigerators and skateboarders) moving down a steep hill from left to right across the screen, a directional dependency determined by which eye was covered by the darker filter.

The effect was also used well throughout the whole 1993 Doctor Who charity special Dimensions in Time and in dream sequences of the 1997 3rd Rock from the Sun two-part season 2 finale Nightmare on Dick Street.

In France, "Le Magazine de la Santé", a long-lasting popular medicine TV-show, has extensively presented the effect in October 2016, inviting its viewers "to see the program in 3D for the first time".

One of the showgirls stripped topless while others danced around her in an anticlockwise pattern, while two additional rear layers were created by graphics moving at different speeds.

Plan view of a pendulum swinging in a plane (solid black) that appears to swing in an ellipse (dashed black) when one eye views it through a neutral-density filter – A denotes the actual position of the bob, A′ its previous position (as perceived by the darkened eye) and A* its apparent position; similarly for B and C.
Imagine a camera which starts at position X and moves right to position Y. If a viewer then watches this segment with a dark lens over the left eye, then when the right eye sees the image recorded when the camera is at Y, the left eye will be a few milliseconds behind and will still be seeing the image recorded at X, thus creating the necessary parallax to generate right and left eye views and 3D perception, much the same as when still pictures are generated by shifting a single camera.