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.
Overview The effect is generally induced by placing a dark filter over one eye but can also occur spontaneously in several eye diseases such as cataract, optic neuritis, or multiple sclerosis. In such cases, symptoms such as difficulties judging the paths of oncoming cars have been reported. The phenomenon is named for German physicist Carl Pulfrich, who first described it in 1922. Carl Pulfrich was the brother-in-law of Heinrich Hertz. The effect has been exploited as the basis for some television, film, and game 3D presentations.
Demonstration
In the classic Pulfrich effect experiment, a subject views a pendulum swinging in a plane perpendicular to the observer's line of sight. When a neutral density filter (a darkened lens—typically gray) is placed in front of, say, the right eye, the pendulum seems to take on an elliptical orbit, appearing closer as it swings toward the right and farther as it swings toward the left, so that if it were to theoretically be viewed from above, it would appear to be revolving counterclockwise. Conversely, if the left eye is covered, the pendulum would appear to be revolving clockwise-from-top, appearing closer as it swings toward the left and farther as it swings toward the right. A similar effect can be achieved by using a stationary camera and continuously rotating an otherwise stationary object. If the movement stops, the eye looking through the dark lens (which could be either eye depending on the direction the camera is moving) will "catch up" and the effect will disappear. One advantage of this system is that people not wearing the glasses will see a perfectly normal picture.
Explanation 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. This seems to occur because visual system latencies are generally shorter (i.e., the visual system responds more quickly) for bright targets as compared to dim targets. This motion with depth is the visual system's solution to a moving target when a difference in retinal illuminance, and hence a difference in signal latencies, exists between the two eyes. 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. These delays increase monotonically with decreased luminance over a wide (> 6 log-units) range of luminance. The effect is also seen with bright targets on a black background and exhibits the same luminance-to-latency relationship.
Use in stereoscopy The Pulfrich effect has been utilized to enable a type of stereoscopy, or 3D visual effect, in visual media such as film and TV. As in other kinds of stereoscopy, glasses are used to create the illusion of a three-dimensional image. By placing a neutral filter (e.g., the darkened lens from a pair of sunglasses) over one eye, an image, as it moves right to left (or left to right, but not up and down) will appear to move in depth, either toward or away from the viewer. Because the Pulfrich effect depends on motion in a particular direction to instigate the illusion of depth, it is not useful as a general stereoscopic technique. For example, it cannot be used to show a stationary object apparently extending into or out of the screen; similarly, objects moving vertically will not be seen as moving in depth. Incidental movement of objects will create spurious artifacts, and these incidental effects will be seen as artificial depth not related to actual depth in the scene. Many of the applications of Pulfrich involve deliberately causing just this sort of effect, which has given the technique a bad reputation. When the only movement is lateral movement of the camera then the effect is as real as any other form of stereoscopy, but this seldom happens except in highly contrived situations. It can, however, be effective as a novelty effect in contrived visual scenarios. One advantage of material produced to take advantage of the Pulfrich effect is that it is fully backward-compatible with "regular" viewing; unlike stereoscopic (two-image) video, a 3D Pulfrich effect only has one image and as a result does not produce the ghosting effect for those not wearing glasses or the color distortion of technologies such as anaglyph. The Pulfrich effect can also be achieved by wearing a sunglass lens over one eye, and since sunglasses are very common, the need to distribute "special" 3D glasses is reduced.
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