Siegmund Pastoor and Matthias Wöpking

Ongoing developments in information technology have led to an increasing demand for 3-D displays. Indeed, a broad range of fairly mature 3-D equipment is already on the market. The available systems, however, suffer from the drawback that users have to wear special devices to separate the left eye's and right eye's images. Such "aided viewing" systems have been firmly established in many professional applications. Yet further expansion to other fields will require "free viewing" systems with improved viewing comfort and closer adaptation to the mechanisms of binocular vision. The respective technologies are still under development. This article reviews the state-of-the-art of both aided viewing and free viewing systems.

Keywords: 3-D displays, stereoscopy, spatial vision

The proliferation of 3-D technology is currently gaining impetus from a number of activities. The driving impact originates not only from the traditional entertainment sector (television and cinema), but is also fed by many new sources: content providers in the multimedia field have spotted a market for true 3-D games and adventure rides in virtual reality, software-developers have recognized the opportunity to launch a new generation of three-dimensional graphic user interfaces, the Internet community is specifying data structures for the transmission of 3-D objects (the Virtual Reality Modeling Language), and EC-sponsored R&D programs (RACE and ACTS) are aimed at preparing the infrastructure for 3-D videoconferencing. Moreover, the base technologies for a future 3-D market have made considerable progress. As will be shown by various examples, 3-D display development has profited enormously from recent advances in LC-technology. Other factors which will play a leading role are the new programmable multimedia signal processors, the rapid growth of highspeed digital networks and the supply of ever more computing power at low costs.

Therefore, the chances for success are high, if and when 3-D displays become available which enable immediate space perception without any impairing side effects. This is the crucial technological challenge. Appropriate solutions require a thorough understanding of the mechanisms of binocular vision. The next section will outline the relevant perceptual processes. This is followed by a taxonomy of 3-D display techniques. The main part of the paper reviews the presently known forms of implementation.

SPATIAL VISION

Spatial vision is based on monocular and binocular depth cues contained in the scene observed. Monocular cues are visible with one eye and are used to create an illusion of volume and depth on flat image surfaces. Common examples are linear and aerial perspective, texture gradients, overlap of objects, and the distribution of light and shadows.

Binocular depth perception is based on local displacements between the projections of a scene onto the left and right retina (disparity). As long as these displacements do not exceed a certain magnitude, the two projections are merged into a single percept with structured depth (fusion). This condition is safeguarded by the operation of several interacting mechanisms:

Depth-of-focus is an integral constituent of spatial vision. Automatic focusing of the eye's lens (accommodation) makes the currently fixated object stand out against its surroundings; details in the near foreground and in the far background "disappear" in increasing optical blur. This way, disparity is kept within physiological limits. In order to maintain a moving object within the zone of sharpest vision, the eyeballs perform rotational movements (convergence). Thus, accommodation and convergence are used to carrying out coordinated changes. As a consequence, all objects which are in focus are also within the limits of fusion - independent of their absolute distance from the observer.

Most of the currently feasible 3-D display techniques neither support the gliding depth-of-focus nor the linkage between accommodation and convergence (Fig. 1). This failure is a potential cause of visual strain, and generally limits the comfortable viewable depth volume [1].

 

Fig. 1: 3-D displays usually force the observer to  "freeze" accommodation in the screen plane, irrespective of the fixated object's  distance. Consequently, all details of the scene appear within unvarying depth-of-focus.  Normally, accommodation distance corresponds to convergence distance and the  depth-of-focus range is centered at the fixated object.

Taxonomy of 3-D displays

3-D displays can be categorized by the technique used to channel the left and right images to the appropriate eyes (Table 1): some require optical devices close to the observer's eyes, while others have the eye-addressing techniques completely integrated into the display itself. Displays of the latter category are called autostereoscopic and are technically much more demanding than the type with viewing aids (stereoscopic displays).

Table 1 Classification of 3-D display techniques.

Principle of eye addressing		Effective  origin of waves	Number  of different views	Eye-point  dependent perspective
aided viewing (stereoscopic)	color multiplex, polarization multiplex, time multiplex, location multiplex	fixed image plane, gaze-controlled image plane	two	optional (for a single observer)
free viewing	direction multiplex (e.g., by diffraction, refraction, reflection, occlusion)	fixed image plane, gaze-controlled image plane	³ two	optional (for a small number of observers)
(autostereoscopic)	volumetric display, electro-holography	distinct depth planes  (slices), entire space	unlimited	inherent (for a small number of observers)

With stereoscopic displays two different perspective views are generated (quasi-)simultaneously. The waves of light entering the eyes may have their origin on a common, fixed or movable (gaze-controlled) image plane. Several multiplexing methods have been proposed to carry the optical signals to the appropriate eyes. It is possible to adapt the image content to the current head position (eye-point dependent perspective; e.g. via corresponding camera movements or signal processing).

With autostereoscopic systems, the only exploitable constraint for addressing the left and right eye is the fact that they occupy different points in space. The waves of light forming the 3-D image may originate from fixed or gaze-controlled image planes. In both cases, direction-multiplex is the only way to channel the information of the left and right views into the appropriate eyes. Compared to stereoscopic techniques, it is possible to multiplex more than two views at a time. Thus, individual (eye-point depending) perspective views can be delivered to different observers. The volumetric and the electro-holographic approaches produce 3-D images where the effective origin of the waves entering the observer's eyes match with the apparent spatial position of the corresponding image points. Thus, the fundamental mechanisms of spatial vision are perfectly supported (in principal there is no difference to natural viewing conditions).

Stereoscopic techniques

Current approaches to 3-D displays are described in detail in the following sections, starting with stereoscopic techniques. Except for a recently proposed location-multiplex approach, all stereoscopic techniques force the observer to focus on a fixed image plane (Fig. 1).

Color-multiplexed (anaglyph) displays

In anaglyph displays the left and right-eye images are filtered with near-complementary colors (red and green, red and cyan or green and magenta, and the observer wears respective color-filter glasses for separation. Combining the red component of one eye's view with the green and blue components of the other view allows some limited color rendition (binocular color mixture). Color rivalry and unpleasant aftereffects (transitory shifts in chromatic adaptation) restrict the use of the anaglyph method.

Polarization-multiplexed displays

The basic setup consists of two monitors arranged at a right angle and the screens are covered by orthogonally oriented filtersheets (linear or circular polarization). The two views are combined by a beam combiner (half-silvered mirror) and the observer wears polarized glasses. This frequently used technique provides full color rendition at full resolution and very little cross-talk in the stereo pair (less than 0.1% with linear filters). However, over 60% of the light emmited is lost through the filters, and the remaining light flux is halved by the mirror.

Polarization techniques are very well suited for videoprojection. When using CRT-projectors with separate optical systems for the primary colors, the left and right-view color beams should be arranged in identical order to avoid rivalry. The light flux in LC-projectors is polarized by the light-valves. Commercial LC-projectors can be fitted for stereo display by twisting the original polarization direction via half-wave retardation sheets to achieve, e.g., the prevalent V-formation.

Stereo projection screens must preserve polarization. Optimum results have been reported for aluminized surfaces and for translucent opalised acrylic screens. Typical TV rear-projection screens (sandwiched Fresnel lens and lenticular raster sheet) depolarize the passing light. LCD-based, direct-view displays and overhead panels have recently been marketed [2]. Their front sheet consists of pixel-sized micro-polarizers, tuned in precise register with the raster of the LCD. The left and right-eye views are electronically interlaced line-by-line and separated through a line-by-line change of polarization.

Time-multiplexed displays

The human visual system is capable of merging the constituents of a stereo pair across a time-lag of up to 50 ms. This "memory effect" is exploited by time-multiplexed displays. The left and right-eye views are shown in rapid alternation and synchronized with an LC-shutter, which opens in turns for one eye, while occluding the other eye. The shutter-system is usually integrated in a pair of spectacles and controlled via an infrared link. When the observer turns away from the screen, both shutters are switched to be transparent. Time-multiplexed displays are fully compatible for 2-D presentations. Both constituent images are reproduced at full spatial resolution by a single monitor or projector, thus avoiding geometrical and color differences. The monitor-type systems have matured into a standard technique for 3-D workstations.

Impairing cross-talk may result from the persistence of CRT phosphors, particularly of the green phosphor. Ideally, image extinction should be completed within the blanking interval. Using "fast" phosphors like P46 leads to noticeable loss of brightness (50%) and life-span. P43 yields a somewhat optimum performance. With the standard phosphor P22, cross-talk can amount to over 20% (the perception threshold is at 0.3% [3]). Since the two views are displayed with a time-delay, they should be generated with exactly this delay. Otherwise, moving objects will appear at incorrect positions in depth.

Time-sequentially controlled polarization

Tektronix has developed a display which combines the time and polarization-multiplex techniques. The monitor's faceplate is covered with a modulator, consisting of a linear polarizer, a liquid-crystal p -cell and a quarter-wave retardation sheet (to turn linear polarization into circular polarization). The p -cell switches polarization in synchronism with the change of the left and right-eye views. Circular polarizing glasses serve for de-multiplexing.

This approach offers three advantages over systems with active shutter-glasses: First, polarizing glasses are inexpensive and light-weight. Second, the p -cell can be constructed from several segments which operate independently on the active portions of the screen, hence ensuring that each eye is only reached by the intended image contents. This way, cross-talk can be greatly reduced (down to 3.6% in a system with five segments and the P22 standard phosphor [4]). Third, multiple display arrays can be operated without any extra synchronizing circuitry.

Location-multiplexed displays

Location multiplex means that the two views are created at separate places and relayed to the appropriate eye through separate channels (e.g. by means of lenses, mirrors and optical fibres). In the following examples, the image plane appears either in a fixed accommodation distance (HMD and BOOM displays) or in a variable, gaze-controlled position (3DDAC).

Helmet Mounted Display (HMD): With HMDs, the perceived images subtend a large viewing angle, typically up to 120° (horizontal) by 80° (vertical). As the natural surroundings are occluded from sight (sometimes optional, by a visor), HMDs are apt to convey a feeling of total immersion in the scene displayed. Attached headtracking devices are used to create eye-point dependent changes in perspective when the user moves. Accommodation distance is usually below 2 m, or can be adjusted in see-through displays to the distance of the fixated real world objects [5]. Meanwhile, numerous manufacturers offer a variety of systems with different performance [6].

HMDs allow free head movement without losing screen contact, thus avoiding musculo-skeletal problems. There is no need to dim ambient light (visor closed), and peripheral vision must not necessarily be obstructed (visor open). On the other hand, latencies and tracking-errors tend to provoke odd, uneasy sensations through conflicting visual stimulation and postural feedback, and adaptation can lead to reciprocal aftereffects. A disadvantage of see-through HMDs is that the observer must direct his regard towards a dim background in order to perceive the HMD image.

BOOM-Display: The Binocular Omni-Orientation Monitor (BOOM [7]) was designed to release the user from the encumbrance of wearing an HMD. Two miniature CRTs are accommodated in a case, which is mechanically supported by a counterweighted, six degrees of freedom arm. The observer regards the stereo images as if looking through binoculars. Tracking is implemented by optical shaft encoders at the joints of the boom. The monitor case is moved either with hand-held handles or with the head (like an HMD).

3DDAC-Display: ATR Labs (Kyoto) have developed an HMD concept with movable relay lenses interposed between the screen and the eyepiece lenses [8]. The convergence distance is sensed by a gaze-tracker. The position of the relay lenses is constantly adjusted, so that the screen surface (image plane) appears in the same distance as the convergence point of the gaze-lines (Fig.2). Thus, the "3-D display with accommodative compensation" (3DDAC) supports the natural link between accommodation and convergence.

 

 

Fig. 2: Location-multiplexed display with gaze-controlled image plane

Autostereoscopic techniques

The present situation regarding autostereoscopic displays is marked by a variety of competitive approaches. Therefore, this survey mainly focuses on fundamental trends. The range of currently recognized techniques is presented in the following order: electro-holography, volumetric displays and direction-multiplexed displays.

Electro-holography

Holographic techniques can record and reproduce the properties of light waves - amplitude (luminance), wavelength (chroma) and phase differences - almost to perfection, making them a very close approximation of an ideal free viewing 3-D technique. Recording requires coherent light to illuminate both the scene and the camera target (used without front lenses). For replay, the recorded interference pattern is again illuminated with coherent light. Diffraction (amplitude hologram) or phase modulation (phase hologram) will create an exact reproduction of the original wavefront.

Video-based holographic techniques are still in their infancy, although they have received much attention over the past five years. Organised by TAO (Telecommunications Advancement Organization), several Japanese research institutes are working towards adapting the principle of holography to an LCD-based video electronics environment [9]. However, the spatial resolution of today's LC-panels is a serious bottleneck (minimum requirements are 1000 lp/mm). A possible solution is to partition the hologram among several LC-panels and to reassemble the image with optical beam combiners. It is yet an open question how to store and transmit the enormous amount of data contained in a hologram. The source rate is estimated to exceed 10¹² bit/sec [10]. Specific data compression methods are required [11]. Currently, the scope of this approach is limited to very small and coarse monochromatic holograms (width of field 1 cm [12]).

Holograms cannot be recorded with natural (incoherent) lighting - a decisive shortcoming. Therefore, they will remain confined to applications where the scene is available in the form of computer generated models. An approach based on computer generated holograms has been pursued since the late 1980s at MIT's Media Lab [13].

The MIT approach makes intense use of data reduction techniques (elimination of vertical parallaxes, subsampling of horizontal parallaxes). Yet, the pixel rate for a monochrome display with a diameter of 15 cm, a depth of 20 cm, and a viewing zone of 30 degrees amounts to 2 Gigapixel/sec (at 30 Hz frame rate). The hologram is displayed with acousto-optical modulators (tellurium dioxide crystals transversed by laser light, Fig. 3). At any one instant, the information of nearly 5000 pixels travels through a crystal. For optical stabilization, synchronized oscillating mirrors are interposed between the modulator and the observer's eyes. Vertical sweep is accomplished with a nodding mirror. Current work aims at increasing the image diameter to 25 cm by assembling multiple basic elements in parallel.

 

 

 

Fig. 3: MIT holographic display. Light modulation is effected by  frequency-modulated soundwaves which propagate through the acousto-optical modulator and  cause local changes of the refractive index due to pressure variations. Phase shifts in  the lightwaves cause local retardations used to create a phase hologram.

Volumetric displays

Volumetric displays project image points to definite loci in a physical volume of space where they appear either on a real surface, or in translucent (aerial) images forming a stack of distinct depth planes. With the first type of system, a self-luminous or light reflecting medium is used which either occupies the volume permanently or sweeps it out periodically. Technical solutions range from the utilization of fluorescent gas (with external excitation through intersecting rays of infrared light) over rotating or linearly moved LED-panels to specially shaped rotating projection screens. Rotating screens have been implemented in the form of a disc, an Archimedian spiral or a helix, winding around the vertical axis [14].

Among the displays with a real collecting surface, the helical design has reached maturity. The most elaborate equipment uses a double-helix filling an 91-cm-diameter by 46-cm-high volume at 10 revolutions per second with a maximum of 40,000 color pixels per frame (or 120,000 pixels in the color primaries) [15]. Observers can walk around the display and see the imaged objects from different angles.

The second type creates aerial images in free space which the observer perceives as cross sections of a scene lined-up one behind the other (multiplanar display). The images belonging to different depth layers are written time-sequentially to a stationary CRT. The observer looks at the screen via a spherical mirror with a varying focal length (varifocal mirror [16]). Rendition of the depth layers is synchronized with changes of the mirror's shape, so that slices of the 3-D volume are successively created in planes of varying distance. The oscillating process is repeated at 30 Hz (which is insufficient to avoid flickering). Due to phosphor persistence, only a limited number of planes can be displayed without visible image smear.

Usually, the mirror is a circular flexing membrane with a metalized surface which is forced to oscillate by means of an acoustical woofer. The deformation of the mirror (maximum 4 mm) yields an optical leverage of approximately 70:1. Variations of optical magnification are compensated by reciprocal magnification of the CRT-images. A volumetric display with additional display of high-definition 2-D background images has been recently announced [17]. In volumetric displays the portrayed objects appear transparent, since the light energy addressed to points in space cannot be absorbed by foreground pixels. Practical applications seem to be limited to fields where the objects of interest are easily iconized or represented by wireframe models.

Direction-multiplexed displays

Direction-multiplexed displays apply optical effects like diffraction, refraction, reflection and occlusion in order to direct the light emitted by pixels of different perspective views exclusively to the appropriate eye. Recent technical approaches outlined in the rest of the paper make use of this concept.

Diffraction-based approaches

DOE approach: With the diffractive-optical-elements (DOE) approach, corresponding pixels of adjacent perspective views are grouped in arrays of "partial pixels" (ICVision Display [18] and 3D Grating Image Display [19]). Small diffraction gratings placed in front of each partial pixel direct the incident light to the respective image's viewing area (first order diffraction; A1 to A4 in Fig. 4). Current prototypes yield images of less than 1.5 inches in diameter. Advanced concepts provide for the integration of image modulation and diffraction of light within a single, high-resolution spatial light modulator [20].

 

 

 

Fig. 4: Principle of a DOE 3-D display (after [19]).

HOE approach: Holographic optical elements (HOE) model the properties of conventional optical elements (e.g. lenses) by holographic methods. Thus, an HOE contains no image information, but serves to diffract rays of light modulated elsewhere. In a recent prototype display, the HOE is an integral part of the light box of a modified LCD [21]. It consists of the hologram of an even diffuse plane which is rastered so as to direct the light of alternating lines to specified viewing zones. Outside the zone for stereoscopic viewing, both eyes receive a 2-D view. The stereo zone can be made to follow the observer's head movement by moving the light source.

Making the above HOE takes two exposures of a hologram with the same illumination setup. Between the exposures, the object, which in this case is a diffuse plane, is shifted horizontally by its own width. For each exposure, parts of the hologram corresponding to the odd or even image lines are occluded.

Refraction-based approaches

Numerous display concepts have been proposed based on conventional, refractive optical elements (such as picture-sized large lenses or small lenslets) to address the observer's eyes. These concepts are discussed in the following.

Integral imaging: With integral imaging, the spatial image is composed of multiple tiny 2-D images of the same scene, captured with a very large number of small convex lenslets (fly's-eye lens sheet). Each lenslet captures the scene from a slightly different perspective. A lens sheet of the same kind is used for display (between capture and replay, the image has to be inverted for orthoscopic depth rendition [22]). As the image plane is positioned into the focal plane of the lenslets, the light from each image point is emitted into the viewing zone as a beam of parallel rays at a specific direction. Therefore, the observer perceives different compositions of image points at different points of view.

The individual lenslets must be very small, since each pixel is spread to the lens diameter at replay, and the image formed behind each lenslet should be as complete and detailed as possible. As a consequence, the display must provide an extremely high spatial resolution. Basic research on electronic video equipment for integral imaging is being carried out at De Montfort University (Leicester) [23].

Lenticular imaging: Lenticular techniques use arrays of vertically oriented cylindrical lenslets, and can be considered a one-dimensional version of integral techniques. The light from each image point is emitted at a specific direction in the horizontal plane, but non-selectively in the vertical plane. Therefore, changes of perspective in accordance with vertical head movements cannot be achieved by optical means (but by headtracking and computational image processing). Based on lenticular techniques, direct-view and projection-type, 3-D displays have been implemented [24].

The working principle of a direct-view display is shown in Fig. 5. Two stereo half images are presented simultaneously, with two columns of pixels (one for the left and one for the right-eye image) behind a lenslet. The observer's head position is constantly sensed, and the lenticular sheet is shifted mechanically to track any lateral and frontal movements. Because of the cylindrical lenses' horizontal selectivity, the imaging panel's color-filter stripes must be aligned one above the other. Otherwise, the observer would see separated color components from different view points.

Fig. 5: Direct-view lenticular raster display with headtracking. L  and R denote corresponding columns of the left and right-eye images.

Headtracking can also be achieved by moving the complete display to and fro and rotating it around its vertical axis [25]. Alternatively, a large number of pixel-columns of the different perspective views has to be displayed simultaneously behind each lenslet [26]. The principle of a purely electronical headtracking display is shown in Fig. 6 [27]. An LC-shutter with movable vertical slits is placed in the focal plane of the lens sheet, and the left and right-eye views are time-sequentially displayed on a CRT screen. The slit position is switched in sychronism with the change of views and shifted laterally according to the observers head position.

Fig. 6: Direct-view display with purely electronic headtracking.

Fig. 10: Components of a depth-of-interest display.

Reflection-based approach

This approach uses a retro-reflective screen for direction multiplexing. Retro-reflective means that the incident rays of light are reflected only into their original direction [22]. In a recent prototype [36], dual video projectors are mounted on a laterally movable stage (Fig. 11). The screen reflects the two images through a large half mirror to the observer's eyes. The system locates the current head position and adjusts the position of the projectors and the angle of the half-mirror accordingly.

Fig. 11: Schematic view of the Xenotech display [36].

Occlusion-based approaches

These approaches have one thing in common: due to parallax-effects, parts of the image are hidden from the one eye but visible for the other eye. The technical solutions differ in the number of viewing slits (ranging from a dense grid to a single vertical slit), in presentation mode (time-sequential vs stationary) and in whether the opaque barriers are placed in front of or behind the image screen (parallax barrier vs parallax illumination techniques).

Basically, any of the lenticular display designs can also be implemented with parallax effects, replacing the raster of cylindrical lenslets by a raster of vertical slit openings (cf. Fig. 5 and 12). Parallax barriers are much easier to manufacture than lenticular screens (e.g. by printing or electro-optical methods). They can be moved electronically for headtracking and time sequential presentation (moving slit technique). Moreover, the dark barriers increase the maximal contrast under ambient illumination (black-matrix effect). The following paragraphs list some recent developments of parallax displays.

Barrier-grid displays: Sanyo has recently optimized the barrier-grid design for LCD-based direct-view displays [37]. One barrier is in front of the LCD and an additional one is placed between the LCD panel and the backlight case. The additional barrier has a reflective coating facing the backlight. It serves to exploit the light which would otherwise be blocked by the black matrix of the LCD panel. This way, lighting efficiency was improved by a factor of 1.4.

Fig 12: Principle of a parallax-barrier direct-view 3-D display and grid vs pixel  arrangement for standard direct-view panels with vertical color-filter stripes.

As the rays of light pass through several adjacing slits, they produce a number of additional viewing windows for the left and right views. This can lead to the disturbing effect that the left eye perceives the right view and vice versa (i.e. the stereo-depth is inverted). Ideally, a headtracker should be used to switch the pixel-positions of the multiplexed left and right-eye images as soon as the observer enters a "wrong" viewing window [38]. In a prototype of NHK Labs [39], the barrier grid is generated by an LCD panel. This way, it can be limited to software-selected areas of the panel to create scalable "3-D windows". The grid is switched off in the 2-D area, making the full resolution of the imaging panel available.

Parallax-illumination displays: Dimension Technologies create the parallax-effect by a lattice of very thin vertical lines of light, placed at a distance behind a standard LC panel [40]. As indicated in Fig. 13, parallax causes each eye to perceive light passing through alternate image columns only.

Fig. 13: Basic concept of a parallax illumination 3-D display

Fig. 16: Moving-slit approach of the Holotron.

Conclusions

Recent developments in information technology have stimulated a growing demand for mature 3-D displays. As a consequence, 3-D display technology has rapidly gained momentum in terms of research and commercial activities. On the one hand, these projects concentrate on the utilization of novel base technologies in conjunction with rather conventional 3-D concepts for the custom design of marketable equipment for special applications (aided viewing systems). On the other hand, there is a rising interest in new concepts for universal purpose systems that do not fall behind familiar 2-D displays in terms of image quality and viewing comfort (autostereoscopic displays). Overall, the research scene, especially in the field of autostereoscopic displays, is characterized by a large variety of competing concepts, each having particular advantages and flaws. Thus, there are still enormous challenges waiting for future R&D work: the ultimate 3-D display has yet to be invented.

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36 The Xenotech 3-D auto stereoscopic display system. Technical Information.  Xenotech Australia, Suite 1,41 Walters Drive, Osborne Park, Western Australia 6017

37 Hamagishi, G, Sakata, M, Yamashita, A, Mashitani, K, Nakayama, E, Kishimoto, S,  Kanatani, K 'New stereoscopic LC displays without special glasses.' Asia Display '95 1995, 791-794

38 Shiwa, S, Tetsutani, N, Akiyama, K, Ichinose, S, Komatsu, T 'Development of  direct-view 3D display for videophones using 15 inch LCD and lenticular sheet.' IEICE  Trans. Information and Systems 1994, Vol. E77-D(9), 940-948

39 Isono, H, Yasuda, M, Sasazawa, H 'Autostereoscopic 3-D LCD display using  LCD-generated parallax barrier.' Japan Display '92 1992, 303-306

40 Eichenlaub, J 'An autostereoscopic display with high brightness and power  efficiency.' Proc. SPIE 2177, 1994, 4-15

41 Eichenlaub, J, Hollands, D, Hutchins, J 'A prototype flat panel hologram-like  display that produces multiple perspective views at full resolution.' Proc. SPIE 2409,  1995, 102-112

42 Travis, A R L, Lang, S R, Moore, J R, Dodgson, N A 'Time-multiplexed  three-dimensional video display.' SID 95 Digest 1995, 851-854

43 Hattori, T 'Electro-optical autostereoscopic displays using large cylindrical  lenses.' Proc. SPIE 1457, 1991, 283-289

44 Sombrowsky, R Verfahren und Vorrichtung zur Erzeugung stereoskopischer  Darstellungen. German patent DE 42 28 111 C1 (25.8.1992)

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