Pulfrich effect
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The Pulfrich effect is a consequence of the fact that at low light levels the eye-brain visual response is slower. 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.
The observation made by Pulfrich was that if a pendulum is swung across the visual field (i.e., perpendicular to the line of sight) and one eye is viewing through a light-reducing filter, that the pendulum will be perceived to be swinging in an eliptical orbit, rather than the linear arc in which it actually swings.
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 again in a special TV episode of 3rd Rock from the Sun.