Abstract:
Autostereoscopic image displays provide highly realistic three-dimensional images to one or a plurality of viewers without the need for wearable appliances. In some embodiments, the images are viewed through a beamsplitter, while in other embodiments the viewer observes the images on a display screen. A viewer-tracking system monitors the viewer&#39;s movements and directs each image of a stereopair to the proper eye of the viewer. In some embodiments, the stereoimages are kept independent and separately directed through differential polarization. In other embodiments, this is accomplished through selective intensity modulation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to creation and viewing of three-dimensional moving or still images, and in particular to “autostereoscopic” systems that do not require special wearable peripheral devices. 
     BACKGROUND OF THE INVENTION 
     For well over a century, researchers have proposed and developed a variety of devices that allow viewing of images or graphic designs in three dimensions. The concept of true “stereoscopic” viewing holds tremendous potential for applications ranging from entertainment to scientific visualization and basic research. Stereoscopically portrayed stories and events carry the impact and immediacy of true realism, allowing viewers to be drawn into an experience with the sense that they are truly there. 
     Efforts to date have largely been confined to specific, controlled environments. For example, for decades movie-goers have donned special glasses to view stereoscopic films; for this application, the restricted presentation atmosphere and a captive audience willing to tolerate special viewing aids facilitated use of relatively unsophisticated optical arrangements. Similar audience receptivity underlies the current generation of commercial “virtual reality” devices, which require the user to wear full-vision headgear that imparts an “immersive” three-dimensional environment. 
     Displays involving eyeglasses or headgear control what enters the eyes rather than what exits the display. The wearable apparatus covering the user&#39;s eyes allows separate information to be provided to each eye. The left-eye and right-eye images differ in perspective but not content, so the viewer integrates the images into a single, stereoscopic picture. Early three-dimensional film systems displayed left-eye and right-eye images in separate colors, which were directed to the appropriate eye by special glasses having lenses tinted with one or the other of these colors. More recent adaptations of this approach code the left-eye and right-eye images with orthogonal polarizations, and utilize eyeglasses with orthogonally oriented polarizers. 
     The disadvantages to systems that require eyeglasses or wearable headgear are numerous and well-known. Beyond the inconvenience and unnaturalness of wearing an appliance, the user may also experience headaches or eye strain. Head-mounted displays suffer the additional disadvantage of being single-user devices, isolating viewers from one another and preventing them from sharing the three-dimensional experience with others. 
     Another popular form of stereoscopic display relies on lenticular optical technology, which utilizes a linear array of narrow cylindrical lenses to create separate spatial viewing zones for each eye. Image information for the different view zones is spatially separated in the back focal plane of the cylindrical lenslets, allowing the lenslets to direct this information only to a narrow area of the viewing plane. Recent adaptations of this approach utilize liquid crystal display (LCD) panels or LCD-projected images to provide an updatable display medium for creating the spatially separated information behind the cylindrical lenses. Lenticular displays also suffer from certain drawbacks, however, such as poor image resolution (due both to the need to divide the overall resolution of the single image-producing device over a plurality of view zones, and to diffraction). Lenticular designs are also difficult to adapt to multi-user environments. 
     Other approaches to stereoscopic image presentation include so-called “volumetric” displays (which utilize a medium to fill or scan through a three-dimensional space, small volumes of which are individually addressed and illuminated), and electronic holography displays. Both of these types of display require rapid processing of enormous quantities of data, even for lower resolution images, and both have significant obstacles to overcome when the displays are scaled up to accommodate larger image sizes. In addition, the volumetric displays produce transparent images which, while suitable for applications (such as air-traffic control or scientific visualization) where the illusion of solidity is less important than a wide viewing zone, do not typically provide a fully convincing experience of three-dimensionality. 
     Macro-optic display systems utilize large-scale optics and mirrors, as opposed to lenticular displays, to deliver each image of a stereopair to a different viewing zone. A system designed by Hattori et al. (see Hattori et al., “Stereoscopic Liquid Crystal Display,”  Proc. Telecom. Advance. Org . (TAO) 1st Int&#39;l. Symp. (1993)) utilizes two LCDs, one providing left-eye information and the other providing right-eye information. The outputs of both LCDs are combined by a beamsplitter, with the light passing through each LCD being focused to a separate viewing zone. The Hattori et al. system utilizes a monochrome cathode-ray tube (CRT) monitor behind each LCD as the illuminating light source. Each monochrome monitor is driven by a To camera that records the viewing area in front of the display, capturing a picture of the viewer. A pair of infrared (IR) illuminators, each emitting at a different wavelength, are angled toward the viewer from different sides. Each recording camera is equipped with a bandpass filter tuned to the emitting frequency of one or the other IR illuminator. Because the illuminators each face the viewer at an angle, one of the cameras records an image of the left side of the viewer&#39;s face, while the other records an image of the right side. A fresnel lens near each LCD projects the left-side or right-side image of the viewer&#39;s face, as appropriate, onto the corresponding side of the viewer&#39;s actual face. As a result, the image information from each LCD reaches only the appropriate left or right eye; for example, because light passing through the left-eye image LCD goes only to the left side of the viewer&#39;s face, the viewer&#39;s left eye sees only left-eye information. As the viewer moves within the viewing space, the image on the monitors moves with him, so the view zones for the left-eye and right-eye images remain properly positioned over the viewer&#39;s left and right eyes. 
     This type of display offers a number of advantages over prior designs. It is “autostereoscopic,” so that the user receives a three-dimensional image without special wearable peripheral devices. It is capable of delivering three-dimensional images to a moving viewer anywhere within the viewing area, and can accommodate several viewers at once. In addition, because the system uses LCDs as the primary image source for the stereopairs, it is capable of generating strongly realistic, full-color images of naturalistic scenes (as well as displaying ordinary two-dimensional television or other information). And since only two LCDs are used, the total amount of information needed to drive the display is only twice that of a standard television or monitor. 
     The Hattori et al. design poses problems in terms of scalability, however. Because the viewer-tracking system is implemented with CRTs, larger displays will require proportionally larger CRTs and more powerful lenses. As a result, the display size, cost, and complexity expand dramatically with increase in the size of the stereoscopic image. Moreover, because this design requires a final beamsplitter to be placed between the display medium and the viewer, the resulting three-dimensional images give the psychological impression of being inaccessible to the viewer; this arises from the fact that stereoscopic images display the least distortion when the three-dimensional content is localized at or near the surface of the display medium, which is positioned behind the final beamsplitter. Other limitations of the design stem from shortcomings that generally affect CRT displays, such as varying image intensities. 
     Another macro-optical design was recently proposed by Ezra et al. (see Ezra et al., “New Autostereoscopic Display System,” SPIE Proc. #2409 (1995)). Starting once again with two image LCDs combined by a beamsplitter, an additional system of fold mirrors and another beamsplitter were added to allow both LCDs to be backlit by a single light source. A lens disposed near each LCD images the single light source to the eyes of the viewer. These lenses are laterally offset by a do slight amount from the optical axis of the LCDs so that two separate images of the light source are created in the viewing area. Light passing through the left-eye LCD forms an image of the light source near the viewer&#39;s left eye, while light passing through the right-eye LCD forms an image of the light source near the viewer&#39;s right eye. By moving the light source behind the two LCDs, the left-eye and right-eye view zones can be moved to follow a viewer within the viewing area. To accommodate additional viewers, additional “dynamic light sources” can be added so as to create further view zones. More recently, this group proposed handling multiple viewers with a single, specialized illumination component rather than multiple individual light sources. This specialized component consists of a number of thin, vertical cold-cathode sources arranged in a one-dimensional array. See Woodgate et al., “Observer Tracking Autostereoscopic 3D Display Systems,” SPIE Proc. #3012A (1997). 
     This system shares many of the advantages of the Hattori et al. design described above, and overcomes the difficulties stemming from multiple illumination sources. Once again, however, the ability to scale the Ezra et al. system can be problematic. The two LCDs and the beamsplitter occupy a large space for a display of only modest three-dimensional size. As in the Hattori et al. system, the three-dimensional image is “trapped” behind the output beamsplitter, making the images seem inaccessible. Finally, the array of cold-cathode sources have a limited on/off switching speed, creating possible lags in tracking speed. 
     DESCRIPTION OF THE INVENTION 
     Brief Summary of the Invention 
     The present invention addresses the foregoing limitations using a variety of innovative approaches to viewer tracking and image presentation. Although the various embodiments of the invention are well-suited to presentation of complementary stereoimages to each of the viewer&#39;s eyes, it should be stressed that the invention is not limited to this use. More broadly, the invention is useful in any context requiring the directing of separate images to distinct spatial regions. For example, the images generated by the invention can be directed to different viewers (rather than the left and right eyes of a single viewer), so that viewers observing the same display device can view different images. Although the ensuing discussion is directed toward generation of stereoimages for a single viewer, it should be understood that this is for convenience of presentation only. 
     In a first embodiment, an improved “through-the-beamsplitter” approach (in which the viewer sees a stereoscopic image through a final beamsplitter located at the output of the display) utilizes light polarization to separate left-eye and right-eye stereoimages. Each of the complementary images is projected so as to be visible to only one of the viewer&#39;s eyes. This may be accomplished by means of a viewer-locating system that acquires the locations of the viewer&#39;s eyes, and a viewer-tracking system that directs each stereoimage to the proper location. 
     In accordance with this embodiment, a viewer-locating means acquires a facial image fragment of the viewer; as used herein, the term “facial image fragment” refers to a recognizable portion of the user&#39;s face, and may include, or be limited to, a first one of the viewer&#39;s eyes. For example, a camera at the front of the display may capture a picture of the viewer with one side of his face illuminated by an IR source. The system further comprises means for generating a tracking output image, the output image comprising a first region of light polarized in a first polarization direction and substantially conforming to the facial image fragment, and a second region of light polarized in a second polarization direction orthogonal to the first polarization direction. Two displays produce complementary stereoimages, and the tracking output image is directed through each of the displays so as to illuminate them in accordance with the polarizations of the tracking output image. The facial image fragment is focused onto a corresponding portion of the viewer&#39;s face through the first display. Each of a pair of polarizers is interposed between the tracking output image and each of the displays; one of the polarizers is oriented in the first polarization direction and the other in the second polarization direction. This arrangement presents illumination from the appropriate display to the first eye of the viewer and illumination from the other display to the viewer&#39;s other eye. Naturally, as with all embodiments of the invention, the stereoimages may represent a single, still stereopair or may instead change rapidly over time to convey movement. 
     In a second embodiment of the invention, polarization is used to segregate stereoscopic images for presentation on a display screen. In accordance with this embodiment, the stereoimages are combined through a projection lens onto a rear-projection display that directs each image component to the proper eye of the viewer. 
     In accordance with the second embodiment, a tracking system acquires a facial image fragment of the viewer, the facial image fragment including a first one of the viewer&#39;s eyes. First and second complementary stereoimages are polarized in first and second polarization directions, respectively, and then combined (e.g., by a beamsplitter) into a composite image. A projection lens system projects the composite image onto a viewable display. Before the projected composite image reaches the display, however, it passes through means for separating the first and second images from the projected composite image. The image projection and/or display are controlled such that the viewer&#39;s first eye receives light only from the first stereoimage and the viewer&#39;s other eye receives light only from the second stereoimage. 
     In a preferred implementation of this embodiment, the means for separating the first and second images generates a tracking polarization pattern. This pattern has a first region substantially correlated to the facial image fragment (i.e., having the same general contour as the facial image fragment, although at a different scale) and a second region separate from the first region (e.g., the remainder of the pattern), and the pattern operates to alter the polarization state of the composite image. In particular, the pattern rotates the first or second region to a first polarization direction but rotates the other region to a second polarization direction. The altered light then passes through an output polarizer (disposed between the tracking polarization pattern and the display) that passes only that portion of the pattern-modified composite image polarized in the first or second direction. Which direction the polarizer passes depends on the polarizations initially applied to the two stereoimages, as discussed below. 
     The image exiting the polarizer reaches the display, which may be, for example, a lens functioning as a projection screen. The lens is situated so as to direct that portion of the composite image which has passed through the first region of the pattern onto the region of the viewer&#39;s face from which the first region was derived. Suppose, for example, that the facial image fragment is drawn from the left side of the viewer&#39;s face, the first region of the pattern (which is defined by this fragment) rotates the polarization of the incoming light by 90°, and that the output polarizer is oriented vertically. Assuming the left stereoimage is initially polarized horizontally, the viewer&#39;s left eye will receive only light originating with the left stereoimage. This is because light directed toward the viewer&#39;s left eye has been rotated 90° by the first region of the pattern; the horizontally polarized light from the left stereoimage now passes through the vertical output polarizer, while the light from the right stereoimage, which was polarized vertically but has now been rotated 90°, is absorbed by the output polarizer. The opposite effect occurs with respect to the remainder of the composite image, which is directed toward the viewer&#39;s right eye. Since this light is unrotated, only the vertical component—originating with the right stereoimage—can pass through the output polarizer. As a result, the proper image is continuously directed toward the proper eye of the viewer. 
     A third embodiment of the invention provides a projection system that is polarization-independent. In accordance with this embodiment, two LCDs are used as “light valves” to restrict the output of separate projection subsystems, each of which projects one image of a stereopair onto a beamsplitter that combines the images. The combined images are viewed through a display such as a projection screen (as in the second embodiment). Each LCD passes a small region of light corresponding to a viewing zone. The size and position of each region, combined with the geometry and optics of the display, operate to ensure that each of the viewer&#39;s eyes receives light only from the proper stereoimage. It should be stressed that, as used herein, the term “light valve” connotes either variable illumination restriction or generation. For example, in the restrictive case described above, a source of illumination is positioned behind the light valve, which allows the light to pass only through a defined region. In the variable-illumination case, the light valve itself generates the desired region of illumination. 
     In a fourth embodiment, a single image source presents left-eye and right-eye images in rapid succession, and a light valve, in cooperation with the image source, synchronously presents the alternating images to the proper eye of the viewer (whose position is tracked). So long as the device performs this cycle of alternating images and light-valve regions at a high enough frequency, the viewer will see a proper three-dimensional image through the single image display. 
     One implementation of this embodiment includes an image source that successively displays left and right stereoimages, a light valve for providing a controllably sized and positioned region of light, and a focusing system for passing light from the light valve through the image source so as to present an image. Based on the tracked position of a viewer, a controller actuates the light valve so as to alternately define, in synchrony with display by the image source of left and right stereoimages, a pair of light regions comprising a left-eye region through which an image from the image source will appear in a left view zone visible to the viewer&#39;s left eye, and a right-eye region through which an image from the image source will appear in a right view zone visible to the viewer&#39;s right eye. 
     In a second implementation, the image appears on a projection screen or similar display device. In accordance with this implementation, light from the image source is directed through a projection lens onto the display screen, which may itself be a lens or lens system. The light valve is disposed between the image source and the display screen, at or near the projection means, which directs toward the left view zone light provided by the left-eye region of the light valve and directs toward the right view zone light provided by the right-eye region. 
     The invention also embodies methods relating to the above-described systems and various components thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing discussion will be understood more readily from the following detailed description of the invention, when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic plan showing an implementation of the first embodiment of the invention; 
     FIG. 2A is an exploded isometric view of a viewer-tracking polarizer in accordance with the invention; 
     FIG. 2B schematically depicts a preferred tracking system; 
     FIG.  2 C,illustrates operation of the viewer-tracking polarizer shown in FIG. 2A in accordance with the output of the tracking system shown in FIG. 2B; 
     FIG. 3 is a schematic plan showing an implementation of the second embodiment of the invention; 
     FIG. 4 is a schematic plan of an angle-widening display system; 
     FIG. 5 is a schematic plan of a compact version of the optical arrangement illustrated in FIG. 4; 
     FIG. 6 is a schematic plan showing an implementation of the third embodiment of the invention; and 
     FIGS. 7 and 8 are schematic plans showing alternative implementations of the fourth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, an implementation of the first embodiment of the invention comprises a light source  100 , a diffuser  1   02 , and a viewer-tracking polarizer  105  (the operation of which is described in greater detail below). Light exiting from viewer-tracking polarizer  105  encounters a beamsplitter  110  that directs the light toward a pair of fold mirrors  115   r ,  115   l . Light reflected by mirror  115   r  may pass through a focusing lens  120   r  (which, if employed, may be biconvex as shown), a polarizer  122   r  oriented in a first polarization direction, and a first image LCD  125   r  that is illuminated by the light passing through it. The light from image LCD  125   r  strikes and is reflected from a second beamsplitter  130  toward the viewer. 
     Analogously, light reflected by mirror  115   l  passes through a second focusing lens  120   l , a second polarizer  122   l  oriented in a second polarization direction orthogonal to the first polarization direction, and a second image LCD  125   l . The light from image LCD  125   l  passes through beamsplitter  130  toward the viewer. In operation, image LCD  125   r  displays a right-eye stereoimage, while image LCD  125   l  displays a left-eye stereoimage. 
     Viewer-tracking polarizer  105  creates a tracking output image having a region of light polarized in the first direction and a region of light polarized in the second direction; it is these regions that are differentially focused to laterally separated viewing zones. A representative viewer-tracking polarizer is shown in FIG.  2 . The operation of this device is best understood with reference to conventional LCD displays. Such a display ordinarily comprises a thin layer of liquid crystal material sandwiched between two glass plates, each of which has an electrical conducting layer deposited thereon. When the inside surfaces of the glass plates have been properly treated, the molecules of the liquid crystal material are forced to align so as to rotate helically from one glass plate to the other. Light passing through this sandwich is forced to rotate its polarization in accordance with the rotated conformation of the liquid crystal molecules, this degree of rotation often being 90°. When a voltage of sufficient magnitude is applied between the two glass plates, however, the liquid crystal molecules no longer form a rotational path from one plate to the other, and the polarization of light passing through the material is therefore unaffected. The liquid crystal material is selectably addressable (e.g., through arrangement in a pattern of cells or “pixels,” which create the resolution of the display) so that the molecules can be electrically stimulated or left unstimulated in accordance with a desired pattern; as a result, the polarization of light passing through the liquid crystal material is altered or left unaffected in accordance with that pattern. 
     In many conventional LCDs used for display purposes, the sandwiched liquid crystal material is placed between two crossed polarizers. The first polarizer preconditions the light so that only one polarization passes through the liquid crystal sandwich. When the polarized light passes through a pixel across which no voltage is applied (an “off” pixel), the polarization of the light rotates 90° and, as a result, exits through the final polarizer. Light passing through an activated (or “on”) pixel, however, is not altered in polarization and is therefore absorbed by the final polarizer. The LCD acts as a restrictive light valve with respect to a source of illumination on either side of the LCD, selectively passing or blocking the light in accordance with the pixel pattern. 
     With reference to FIG. 2A, viewer-tracking polarizer  105  may comprise a plane polarizer  160  and an addressable liquid crystal sandwich element  162 . Polarizer  160  allows only one polarization component of incoming light to pass through; in the figure, plate  160  is configured to pass vertically polarized light. A controller  150  (see FIG.  1 ), which receives position data from a viewer-tracking system as described below, controls the pixels of liquid crystal element  162 . As shown in FIG. 2A, element  162  may be operated by controller  150  such that the polarization of light passing through a small rectanglar region  165  rotates by 90° the light passing therethrough, while the polarization of light passing through the remainder of element  162  is unaffected. Light source  100  and diffuser  102  are located behind polarizer  160 , so that polarizer  160  preconditions the light before it reaches element  162 . Exiting element  162 , therefore, is a light image having separate regions of crossed polarizations. 
     Due to the optical geometry of the arrangement shown in FIG. 1, the position of rectangle  165  within the area of element  162  determines where that rectangle  165  will be focused within a view zone. Accordingly, the rectangle of light whose polarization has been rotated may be directed to any desired point within the view zone merely by choosing its position on element  162 . 
     Viewer-tracking polarizer  105  works in conjunction with polarizers  125   r ,  125   l  to direct each of the stereoimages to the proper eye of the viewer. Suppose, for example, that rectangular region  165  is horizontally polarized as shown while the remainder of the light passing through element  105  is vertically polarized; that polarizer  122   r  passes only the horizontally polarized component of incoming light; and that polarizer  122   l  passes only the vertically polarized component of incoming light. As a result of these assumptions, polarizer  122   r  is aligned with the “off” pixels of rectangle  165 , while polarizer  122   l  is aligned with the “on” pixels outside rectangle  165 . 
     Therefore, although light emanating from the entire face of viewer-tracking polarizer  105  passes through beamsplitter  110  and reaches polarizer  122   r , only the light from rectangle  165  actually passes through polarizer  122   r  to illuminate display  125   r ; and because rectangle  165  has been positioned so that its image reaches the viewer&#39;s right eye via beamsplitter  130 , the stereoimage from display  125   r  (which is illuminated by light from rectangle  165 ) also reaches the viewer&#39;s right eye. Similarly, only the light emanating from regions other than rectangle  165  can pass through polarizer  122   l . This light passes through beamsplitter  130  to reach a view zone that includes the viewer&#39;s left eye but not his right eye, so the viewer&#39;s left eye perceives the stereoimage from display  125   l.    
     This arrangement can accommodate multiple viewers. When a new viewer enters the view zone, controller  150  creates another polarization-altering rectangle within viewer-tracking polarizer  105  and positions this to follow the new viewer&#39;s right eye. Additional viewers can be accommodated so long as the rectangles do not occupy an excessive amount of the area of element  162 . 
     Refer now to FIG. 2B, which illustrates a tracking system useful in conjunction with this embodiment of the invention. A source  170  of non-visible (e.g., infrared (IR), ultraviolet (UV), etc.) radiation is aimed at the viewer so that, when the viewer is within the view zone (i.e., the physical boundaries within which the invention can operate), source  170  illuminates one side of his face. A camera  172  that is sensitive only to the non-visible radiation is positioned in front of the display so as to monitor the entire view zone (for example, if source  170  emits IR radiation, camera  172  may be fitted with an IR bandpass filter). The output of camera  172 , then, is a facial image fragment corresponding to one side of the viewer&#39;s face (and including, most critically, one of the viewer&#39;s eyes). The output of camera  172  is provided to controller  150 . The controller is programmable and is provided with the camera and viewing geometry. Based on the location of the facial image fragment within the camera field and the known camera and viewing geometry, controller  150  is able to straightforwardly create a pixel pattern  175  on element  162  (see FIG. 2C) conforming to the facial image fragment, and to position pattern  175  such that light passing through the pixels will be directed back onto the actual viewer&#39;s face to overlie the region illuminated by source  170 . 
     Like all “through-the-beamsplitter” approaches-meaning that the image must be viewed through an output beamsplitter-the foregoing embodiment produces images that appear removed from the viewer and inaccessible. The second embodiment utilizes polarization to segregate stereoscopic images for presentation on a display screen rather than through a beamsplitter. An implementation of this embodiment, illustrated in FIG. 3, comprises a pair of light sources  200   r ,  200   l ; a pair of diffusers  202   r ,  202   l ; and a pair of image LCDs  205   r ,  205   l . Image LCDs  205   r ,  205   l  each display one image of a stereopair (LCD  205   r  displaying the right-eye image and LCD  205   l  displaying the left-eye image). The So output polarizers of image LCDs  205   r ,  205   l  are oriented in orthogonal directions. 
     Light from the image LCDs  205   r ,  205   l  is combined by a beamsplitter  207  that directs the light toward a projection lens or lens assembly  210 . Lens  210  projects the composite image through a viewer-tracking polarizer  215  onto a display element  220 , which presents a viewable image to the viewer. A controller (not shown) analogous to controller  150  discussed in connection with the first embodiment controls viewer-tracking polarizer  215 ; this controller may receive tracking information from a viewer-locating system in accordance with the first embodiment. Viewer-tracking polarizer  215  is similar in construction to viewer-tracking polarizer  105 , except that the liquid crystal element  162  (rather than polarizer element  160 ) faces the light source—i.e., projection lens  210 . 
     Suppose, for purposes of example, that light exiting from image LCDs  205   r ,  205   l  is polarized vertically or horizontally, with the output polarizer of LCD  205   r  oriented so that light exiting this LCD is polarized vertically, while light exiting LCD  205   l  is polarized horizontally. Suppose, further, that the output polarizer of viewer-tracking polarizer  215  is oriented horizontally. In this configuration, light exiting LCD  205   l  can pass through viewer-tracking polarizer  215  only where are its pixels are “on”—that is, where the polarization of incoming light is unrotated, since the output polarizers of LCD  205   l  and viewer-tracking polarizer  215  are both horizontally oriented. Conversely, light exiting LCD  205   r  can pass through viewer-T) tracking polarizer  215  only where its pixels are “off” and therefore rotate light 90° in polarization. 
     Separate view zones V r , V l  can thus be formed through display element  220 , which acts as a lens. The controller uses information from the tracking system to define an “on” area of viewer-tracking polarizer  215  conforming to the left side of the viewer&#39;s face (continuing the exemplary convention established above) and whose size and position, given the optics and geometry of display element  220  and the position of the viewer, focuses light from projection lens  210  onto the left side of the viewer&#39;s face—i.e., into viewing zone V l . Since only light originating with image LCD  205   l  can pass through this “on” area of viewer-tracking polarizer  215 , only the left stereoimage reaches the viewer&#39;s left eye. Analogously, light originating with image LCD  205   r  is directed toward the viewer&#39;s right eye. And once again, the system can accommodate multiple viewers by locating and tracking them, and defining appropriate “on” areas of viewer-tracking polarizer  215 . 
     It should also be emphasized that this design does not require LCD displays. Instead, virtually any form of display (television, rear-projected images, etc.) can be used in conjunction with an appropriate polarizer disposed between the display and beamsplitter  207 . Vivid moving images can readily be transmitted to image sources  205   r ,  205   l  using current broadcast technologies: by linking together two television channels, by compressing the information for the two images into a single television channel, or by using digital HDTV channels to deliver the two images over a single channel. This embodiment is also backward-compatible with standard (two-dimensional) television, allowing viewers to watch traditional television images and programs. 
     Display element  220  may be as simple as a focusing lens or, more realistically, a large fresnel lens. In these cases, the angular size of the total viewing zone as seen from the display will be the same as the angular size of the viewer-tracking polarizer  215 . Increasing the size of the view zones requires proportionate increase in the size of projection lens  210  and viewer-tracking polarizer  215  (with consequent increases in cost, complexity and overall equipment size). 
     A display device was therefore designed to increase the viewing angle without increasing the size of either projection lens  210  or viewer-tracking polarizer  215 . This display device, illustrated in FIG. 4, serves as an angle multiplier for light projected thereon, taking rays impinging at a small angle and widening the angle for the view zones. The display device comprises a Galilean telescope arrangement  250  disposed between two lenses  252 ,  254  of positive focal length. Galilean telescope  250  includes lens  257  having a positive focal length and a lens  260  having a negative focal length, lenses  257  and  260  being spaced apart such that they share a common focal point. In this manner, telescope arrangement  250  serves as an angle multiplier for light passing between the two outside lenses  252 ,  254 . Lens  252  collimates the light exiting viewer-tracking polarizer  215 , telescope arrangement  250  magnifies the ray angles of this collimated light, and lens  254  focuses the light to a magnified image of viewer-tracking polarizer  215  at the proper viewing distance, which corresponds to the front focal point of lens  254 . Light rays entering the display at an angle a exit the display at an angle β. 
     This optical arrangement, while suited to the purpose described, would nonethless be bulky if implemented with the standard optical elements illustrated in FIG.  4 . Accordingly, a compact version of the arrangement was designed. This screen system, indicated generally at  265  in FIG. 5, utilizes a pair of cylindrical lenslet arrays  270 ,  272 . The elements of array  270  have positive focal lengths, while the elements of array  272  have negative focal lengths. These microlens arrays are sandwiched between two outer lenses, which may be fresnel lenses for compactness. Lens  275  collimates light exiting viewer-tracking polarizer  215 , which is placed at the focal point of lens  275 . The collimated light encounters lenslet arrays  270 ,  272 , which act as a multiplicity of Galilean telescopes. The light passing through a positive lenticule of array  270  forms an image of viewer-tracking polarizer  215  a short distance beyond the facing negative lenticule of array  272 . This negative lenticule recollimates the light before it becomes an actual image. Output lens  277  receives the collimated light leaving all of the negative lenticules of array  272  and forms at its focal point a single image of viewer-tracking polarizer  215 . The proper viewing distance, then, is at the plane of this image (which is one focal length away from lens  277 ). 
     The magnification achieved with system  265  can be considered in two parts, namely, the fresnel lens magnification of lenses  275 ,  277  and the lenticular magnification of lenslet arrays  270 ,  272 . The angular magnification of light passing through system  265  is determined solely by the lenticular magnification, while the size magnification of the view zone with respect to the size of viewer-tracking polarizer  215  is determined by a combination of the lenticular and fresnel lens magnifications. 
     The magnification arising from lenticular arrays  270 ,  272  is given by the ratio of the focal lengths of the positive and negative lenslets:          m   lenticular     =       f   positive       f   negative                              
     The magnification due to input fresnel lens  275  and output fresnel lens  277  is similarly given by a ratio of focal lengths:          m   fresnel     =       f     output                 fesnel         f     input                 fresnel                                
     The total size magnification of the system  265  is found by multiplying these equations: 
     
       
           m   size =( m   fresnel )( m   lenticular ) 
       
     
     while the angular magnification m angle  is the same as the lenticular magnification m lenticular . 
     A third embodiment of the invention, illustrated in FIG. 6, utilizes left and right image sources that do not require polarization. Rather than utilizing a single projection lens and viewer-tracking polarizer for both image sources, in this embodiment each image source has its own projection lens and viewer-tracking element, the latter not based on polarization. 
     In the illustrated implementation, each image source  300   r ,  300   l  comprises a light source  302   r ,  302   l ; a diffuser  304   r ,  304   l ; and an image LCD  306   r ,  306   l  (although, once again, this embodiment is least restrictive in terms of the nature of the image sources, which can be any form of display). The stereopair images from sources  300   r ,  300   l  are received by a pair of projection lens or lens arrangements  315   r ,  315   l , which are directed toward different sides of an angled beamsplitter  320 . The combined image from beamsplitter  320 —that is, the light from lens  315   l  passing through the beamsplitter and the light from lens  315   r  reflected from it—is directed onto a viewable display element  325 . The display element  325  may once again be as simple as a focusing lens, but is preferably the lenticular arrangement illustrated in FIG.  5 . 
     Intervening between projection lenses  315   r ,  315   l  and beamsplitter  320  are a pair of viewer-tracking intensity modulators  330   r ,  330   l . Rather than creating regions of different polarizations, as in the second embodiment, viewer-tracking intensity modulators  330   r ,  330   l  instead selectively pass or restrict light from image sources  300   r ,  300   l . Thus, the intensity modulators may be conventional liquid crystal displays, with “off” pixels passing light and “on” pixels blocking light. A controller (not shown) analogous to controller  150  discussed in connection with the first embodiment controls both intensity modulators  330   r ,  330   l ; this controller may receive tracking information from a viewer-locating system in accordance with the first embodiment. 
     In operation, the controller creates a light-passing window in each intensity modulator, the position of each window being such that light passing through it is directed, through display  325 , onto the appropriate viewing zone V l , V r . For example, the tracking system discussed above may be used to form an image of one side of the viewer&#39;s face (e.g., as shown in FIG. 2B, the right side), with the window in intensity modulator  330   r  shaped to conform to this image and positioned appropriately within the LCD display, and intensity modulator  330   l  assuming the inverse pattern (that is, passing light everywhere except through a region in the shape of the window in intensity modulator  330   r , so that light from projection lens  315   l  is excluded only from the region containing the viewer&#39;s right eye). 
     Alternatively, the tracking system can be configured to keep track of the locations of both of the viewer&#39;s eyes, with the controller positioning windows in intensity modulators  330   r ,  330   l  in accordance with these locations. In one approach, the tracking system discussed in connection with the first embodiment is retained, but the controller is programmed to approximate the location of the viewer&#39;s unilluminated eye based on the pattern of illumination and typical face To sizes. That is, for most viewers, a tracked location of one eye suggests the location of the other eye with an acceptable degree of precision. The observed image of one eye (or side of the viewer&#39;s face) and the computed complementary image are used to define the windows in intensity modulators  330   r ,  330   l.    
     In another approach, two separate illumination systems may be used in the manner proposed by Hattori et al. With renewed reference to FIG. 2B, a second source of illumination is directed toward the viewer from the opposite side so as to illuminate the other side of his face (i.e., the side not illuminated by source  170 ). Each of the sources emits non-visible light of a different wavelength, and a pair of cameras (of known locations and optical geometries) each equipped with a bandpass filter is focused on the viewer. One of the bandpass filters is tuned to the wavelength emitted by source  170 , so this camera provides controller  150  with an image of the right side of the viewer&#39;s face; and the other bandpass filter is tuned to the wavelength emitted by the other source, the associated camera providing controller  150  with an image of the left side of the viewer&#39;s face. These left-side and right-side images are used to control the intensity modulators  330   r ,  330   l.    
     The foregoing embodiments require separate image displays, one for the left-eye image and one for the right-eye image. The fourth embodiment of the invention utilizes a single display that presents alternating left-eye and right-eye images in rapid succession, directing each stereoimage to the proper eye and presenting the images with sufficient rapidity to confer the illusion of a unified image. 
     In a first implementation of this embodiment, illustrated in FIG. 7, images are presented by a single LCD  400 , which is illuminated by a light source  402  situated behind a diffuser  404 . Intervening between the light source and LCD  400  are a lens  410  and an LCD  415  that acts as an intensity-modulating light valve. LCD  415  is operated by a controller  420  and a viewer-tracking system (not shown); the tracking system may be one that defines separate locations for each of the viewer&#39;s eyes, as discussed above in connection with the third embodiment of the invention, or controller  420  is programmed to use the actual location of one of the viewer&#39;s eyes to approximate the location of the other eye. 
     Controller  420  also determines the image appearing on LCD  400 . Thus, controller  420  may be a programmable computer equipped for computation and control of LCD image displays. In operation, controller  420  computes the proper size and location on LCD  415  of two light-passing windows, each of which is displayed separately as determined by controller  420 . A left-eye window passes light emanating from diffuser  404  such that, given the location of the viewer and the optical geometry of lens  410 , the light is focused over the viewer&#39;s left eye (i.e., in view zone V l ). Analogously, light passing through a right-eye window appears in view zone V r . The left-eye and right-eye windows may simply be inverse patterns; that is, a first window passes light directed onto one of the viewer&#39;s eyes and blocks light directed everywhere else, while the other window passes light directed everywhere except where directed through the first window. Controller  420  alternates the stereoimage appearing on LCD  400  in synchrony with the window it defines on LCD  415 . Thus, for example, when the left image of a stereopair is displayed on LCD  400 , the left-eye window is defined on LCD  415  (with the remainder of LCD  415  blocking light). 
     The image that controller  420  places on LCD  400  may originate from any number of sources. For example, controller  420  may acquire stereoimages from a permanent storage device, such as a hard disk or CD-ROM drive; as streamed data from a computer network or the Internet; or from a television broadcaster over one or two communication channels. A single still, three-dimensional image may be maintained by persistent alternating presentation of its two stereoscopic image components; or a moving image may be presented by successively displaying each stereoscopic component of each successive image frame with sufficient rapidity to convey the illusion of motion. 
     Naturally, this embodiment of the invention requires adequate display refresh rates to avoid the appearance of flickering. Ordinarily, an image LCD displaying a single channel of information must be updated at about 60 Hz to provide an image free of flicker. Because LCD  400  must handle two channels of information—that is, successive display of both stereoscopic components of each image frame—refresh rates of at least 120 Hz are desirable. It should also be noted that, because a single display alternates between images rather than displaying one of two images simultaneously, the illumination power of LCD  400  must be greater than that required of displays in the previously described embodiments. 
     A second implementation of the fourth embodiment, illustrated in FIG. 8, utilizes projection rather than direct display of an LCD image, thereby allowing for practical display of larger images. In this case, light emanating from diffuser  404  is directed through a projection lens or lens system  425 . Light exiting lens  425  passes through intensity-modulating LCD  415  before striking display  430  (which, yet again, may be as simple as a focusing lens but is preferably the lenticular arrangement illustrated in FIG.  5 ). The windows defined by controller  420  (see FIG. 7) alternately direct light through display  430  to one or the other of the viewer&#39;s eyes. Once again, the controller alternates the stereoimage appearing on LCD  400  in synchrony with the window it defines on LCD  415 . 
     Both implementations of the fourth embodiment are capable of accommodating multiple viewers by creating additional windows within LCD  415 . 
     It will therefore be seen that the foregoing approaches to stereoscopic image display are straightforwardly implemented, providing highly realistic images without the need for the viewer to wear special equipment or to remain motionless at a designated location. The terms and expressions employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. For example, as noted earlier, it is not necessary to use light valves based on LCDs that selectively pass a pattern illumination from behind; instead, the pattern of illumination can be directly generated by an imaging element.