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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.
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, [1] optic neuritis, [2] [3] or multiple sclerosis. [4] 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. [5] [6] Carl Pulfrich was the brother-in-law of Heinrich Hertz. [7] The effect has been exploited as the basis for some television, film, and game 3D presentations.
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.
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. [8] [9] [10] [11] These delays increase monotonically with decreased luminance over a wide (> 6 log-units) range of luminance. [8] [9] The effect is also seen with bright targets on a black background and exhibits the same luminance-to-latency relationship.
The Pulfrich effect has been utilized to enable a type of stereoscopy, or 3-D 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.
The effect achieved a small degree of popularity in television in the late 1980s and 1990s. On Sunday, January 22, 1989 the Super Bowl XXIII halftime show and a specially produced commercial for Diet Coke were telecast using this effect. In the commercial, objects moving in one direction appeared to be nearer to the viewer (actually in front of the television screen) and when moving in the other direction, appeared to be farther from the viewer (behind the television screen). 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). [12] The right eye's filter was grayed purple (resembling red wine color), while the left was very light amber (resembling white wine color). These colors complemented each other to produce the Pulfrich effect while avoiding distortion in the broadcast's natural colors. 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 commercial was said to be created using Nuoptix 3D technology to create the Pulfrich effect.
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 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 petrol stations. These short films were mainly travelogues of Dutch localities. A Power Rangers Lightspeed Rescue movie called Power Rangers in 3D: Triple Force (later broadcast as two-part Trakeena's Revenge) [13] sold on VHS through McDonald's purportedly used "Circlescan 4D" technology, which is based on the Pulfrich effect, but there was very little 3D present. In the United States and Canada, six million 3D Pulfrich glasses were distributed to viewers for an episode of Discovery Channel's Shark Week in 2000. [14] Animated programs that employed the Pulfrich effect in specific segments of its programs include Yo Yogi! , The Bots Master , and Space Strikers ; they typically achieved the effect through the use of constantly moving background and foreground layers.
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". [15]
Some episodes of the Italian/German TV game show "Tutti Frutti" utilised the effect. 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. It is not known how viewing glasses were distributed. Episodes are widely available on the internet, but only a few use the Pulfrich effect.
The video game Orb-3D for the Nintendo Entertainment System used the effect (by having the player's ship always moving) and came packed with a pair of glasses. So did Jim Power: The Lost Dimension in 3-D for the Super NES, using constantly scrolling backgrounds to cause the effect. Also on the first day of Super Mario 64's release, Toys R US offered glasses to view Super Mario 64 through. There were only 2 2D compatible technologies which use clear lenses, ChromaDepth and Pulfrich. Since ChromaDepth would make Mario (a red-and-blue colored character) look odd, and most walking scenes were filmed with Mario in the foreground slightly left of center, the left having the perpendicular to the long horizon, and the right line of sight extending into the horizon, Pulfrich glasses were assumed to be the type included in the Toys R Us pack.
Stereoscopy is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision. The word stereoscopy derives from Greek στερεός (stereos) 'firm, solid', and σκοπέω (skopeō) 'to look, to see'. Any stereoscopic image is called a stereogram. Originally, stereogram referred to a pair of stereo images which could be viewed using a stereoscope.
Depth perception is the ability to perceive distance to objects in the world using the visual system and visual perception. It is a major factor in perceiving the world in three dimensions. Depth perception happens primarily due to stereopsis and accommodation of the eye.
A stereoscope is a device for viewing a stereoscopic pair of separate images, depicting left-eye and right-eye views of the same scene, as a single three-dimensional image.
An autostereogram is a two-dimensional (2D) image that can create the optical illusion of a three-dimensional (3D) scene. Autostereograms use only one image to accomplish the effect while normal stereograms require two. The 3D scene in an autostereogram is often unrecognizable until it is viewed properly, unlike typical stereograms. Viewing any kind of stereogram properly may cause the viewer to experience vergence-accommodation conflict.
A 3D display is a display device capable of conveying depth to the viewer. Many 3D displays are stereoscopic displays, which produce a basic 3D effect by means of stereopsis, but can cause eye strain and visual fatigue. Newer 3D displays such as holographic and light field displays produce a more realistic 3D effect by combining stereopsis and accurate focal length for the displayed content. Newer 3D displays in this manner cause less visual fatigue than classical stereoscopic displays.
An active shutter 3D system is a technique of displaying stereoscopic 3D images. It works by only presenting the image intended for the left eye while blocking the right eye's view, then presenting the right-eye image while blocking the left eye, and repeating this so rapidly that the interruptions do not interfere with the perceived fusion of the two images into a single 3D image.
A polarized 3D system uses polarization glasses to create the illusion of three-dimensional images by restricting the light that reaches each eye.
Anaglyph 3D is the stereoscopic 3D effect achieved by means of encoding each eye's image using filters of different colors, typically red and cyan. Anaglyph 3D images contain two differently filtered colored images, one for each eye. When viewed through the "color-coded" "anaglyph glasses", each of the two images reaches the eye it's intended for, revealing an integrated stereoscopic image. The visual cortex of the brain fuses this into the perception of a three-dimensional scene or composition.
Stereopsis is the component of depth perception retrieved through binocular vision. Stereopsis is not the only contributor to depth perception, but it is a major one. Binocular vision happens because each eye receives a different image because they are in slightly different positions on one's head. These positional differences are referred to as "horizontal disparities" or, more generally, "binocular disparities". Disparities are processed in the visual cortex of the brain to yield depth perception. While binocular disparities are naturally present when viewing a real three-dimensional scene with two eyes, they can also be simulated by artificially presenting two different images separately to each eye using a method called stereoscopy. The perception of depth in such cases is also referred to as "stereoscopic depth".
Carl P. Pulfrich was a German physicist, noted for advancements in optics made as a researcher for the Carl Zeiss company in Jena around 1880, and for documenting the Pulfrich effect, a psycho-optical phenomenon that can be used to create a type of 3-D visual effect. Carl Pulfrich was the brother-in-law of Heinrich Hertz.
Autostereoscopy is any method of displaying stereoscopic images without the use of special headgear, glasses, something that affects vision, or anything for eyes on the part of the viewer. Because headgear is not required, it is also called "glasses-free 3D" or "glassesless 3D". There are two broad approaches currently used to accommodate motion parallax and wider viewing angles: eye-tracking, and multiple views so that the display does not need to sense where the viewer's eyes are located. Examples of autostereoscopic displays technology include lenticular lens, parallax barrier, and may include Integral imaging, but notably do not include volumetric display or holographic displays.
Phantograms, also known as Phantaglyphs, Op-Ups, free-standing anaglyphs, levitated images, and book anaglyphs, are a form of optical illusion. Phantograms use perspectival anamorphosis to produce a 2D image that is distorted in a particular way so as to appear, to a viewer at a particular vantage point, three-dimensional, standing above or recessed into a flat surface. The illusion of depth and perspective is heightened by stereoscopy techniques; a combination of two images, most typically but not necessarily an anaglyph. With common (red–cyan) 3D glasses, the viewer's vision is segregated so that each eye sees a different image.
Parallax scanning depth enhancing imaging methods rely on discrete parallax differences between depth planes in a scene. The differences are caused by a parallax scan. When properly balanced (tuned) and displayed, the discrete parallax differences are perceived by the brain as depth.
Chromostereopsis is a visual illusion whereby the impression of depth is conveyed in two-dimensional color images, usually of red–blue or red–green colors, but can also be perceived with red–grey or blue–grey images. Such illusions have been reported for over a century and have generally been attributed to some form of chromatic aberration.
3D television (3DTV) is television that conveys depth perception to the viewer by employing techniques such as stereoscopic display, multi-view display, 2D-plus-depth, or any other form of 3D display. Most modern 3D television sets use an active shutter 3D system or a polarized 3D system, and some are autostereoscopic without the need of glasses. As of 2017, most 3D TV sets and services are no longer available from manufacturers.
A 3D stereo view is the viewing of objects through any stereo pattern.
Dolby 3D is a marketing name for a system from Dolby Laboratories, Inc. to show three-dimensional motion pictures in a digital cinema.
Wiggle stereoscopy is an example of stereoscopy in which left and right images of a stereogram are animated. This technique is also called wiggle 3-D, wobble 3-D, or sometimes Piku-Piku.
Stereo photography techniques are methods to produce stereoscopic images, videos and films. This is done with a variety of equipment including special built stereo cameras, single cameras with or without special attachments, and paired cameras. This involves traditional film cameras as well as, tape and modern digital cameras. A number of specialized techniques are employed to produce different kinds of stereo images.
The Van Hare Effect is a 3D stereoscopic viewing technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision using psychophysical percepts. The Van Hare Effect creates the illusion of dimensionality, rather than actual dimensionality in the subject being viewed. The Van Hare Effect is achieved by employing the stereoscopic cross-eyed viewing technique on a pair of identical images placed side-by-side. In doing so, it artificially tricks the human brain and optical center into seeing depth in what is actually a two-dimensional, non-stereoscopic image. The illusion of depth is interesting in that even if the image pair is not itself originally stereoscopic, the brain perceives it as if it is.