Pulfrich effect

Visual system latencies are generally shorter for (the visual system responds more quickly to) bright targets compared to dim targets [ref]. Motion with depth (the Pulfrich effect -first described by the German physicist Carl Pulfrich) is the visual system’s solution to a moving target when a difference in retinal illuminance exists between the two eyes.

In the classic Pulfrich Effect experiment a subject views, binocularly, a pendulum swinging in a fronto-parallel plane (perpendicular to the observer’s line of sight). When a neutral density filter (a darkened lens – usually grey) 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. 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 [fig].

The Pulfrich effect has typically been measured under full field conditions with dark targets on a bright background, and yields about a 15ms. 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.

The effect can 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 Pulfrich effect has been utilized to enable a type of stereoscopy, or 3-D visual effect, in visual media. As in other kinds of stereoscopy, glasses are used to create the illusion of a three-dimensional image. By placing a neutral (transparent gray) filter over one eye, a moving image perceived by that eye will lag behind the image perceived by the unimpeded eye. This lag will induce a difference in the images perceived by each eye, inducing a binocular vision illusion of depth.

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. However, it can 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 compatible with "regular" viewing without glasses.

This effect was exploited in a "3D" motion television commercial in the 1990s, where objects moving in one direction appeared to be nearer to the viewer (actually in front of the television screen) due to the binocular vision of the user. To allow viewers to see the effect, the advertiser provided a large number of viewers with a pair of filters in a paper frame. One eye's filter was a rather dark neutral gray while the other was transparent. 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 used in the 1993 Doctor Who charity special Dimensions in Time and a 1997 special TV episode of 3rd Rock from the Sun. In many countries in Europe, a series of short 3D films, produced in the Netherlands, were shown on television. Glasses were sold at a chain of gas stations. These short films were mainly travelogues of Dutch localities. A Power Rangers episode sold through McDonalds used "Circlescan 4D" technology which is based on the Pulfrich effect.