Stereoscopic display

Abstract: A stereoscopic display (100;200) capable of displaying monochromatic or coloured views of moving, three dimensional scenes comprises means (102;202) for sequentially projecting alternate ones of successive pairs of substantially monochromatic images corresponding to right-eye and left-eye perspectives of the scene at a display rate sufficient to avoid a perception of flicker in the images. A variable polariser (107;207) is used to circularly polarize alternate images in respectively opposite senses synchronously at their projection rate. The images are analysed by highly transmissive eyewear (118;218) comprising at least one pair of oppositely sensed cholesteric liquid crystal circular polarisers (120;220) tuned to the particular colour wavelength of the images and disposed, one over each eye, to transmit only appropriately-polarized images to the corresponding eye.

Description: This invention pertains to stereoscopic viewing systems in general, and in particular to a high efficiency stereoscopic display capable of producing colour images. An observer with normal binocular vision sees a slightly different perspective of a real-world, three-dimensional scene with each eye. This relative perspective shift between eyes is a function of the scene-object distance and results in a perception of depth in the scene by the observer. Over the years, various techniques have been developed to present a three-dimensional scene to an observer utilising a two-dimensional format. These typically involve the display, either simultaneously or time-sequentially, of a pair of two-dimensional images corresponding to right-eye and left-eye-perspective views of a three-dimensional subject, which images are each "encoded" with right-eye or left-eye information that permits them to be seen only by an eye equipped with, or shuttered by an appropriate "decoder". A well known example is the "stereoscope" viewer, known since Victorian times, which first records a pair of static, two-dimensional images, usually photographs, corresponding to right-eye and left-eye perspectives of a three-dimensional scene in fixed, side-by-side relationship, and then presents them for viewing through apparatus which confines the vision of each eye of the viewer to the appropriate, corresponding image using optics. An improved form of this device is available today in most toy shops. Another technique which has been used to present moving scenes, as in motion pictures, involves recording the right-eye and left-eye perspectives in different colours on the same frame of film, then viewing the projected frame through eyewear comprising a pair of colour filters, each of which is disposed within the eyewear to pass the appropriate image to a corresponding right or left eye and to block reception by that eye of the other, simultaneously-presented, but differently-coloured image. A variation on this technique is to record the right-eye and left-eye perspectives on superposed sheets of orthogonally polarised dichroic polarising materials, and then to view the perspectives projected simultaneously onto a non-depolarising screen through eyewear having a pair of orthogonally polarised lenses. A description of the two foregoing techniques, along with a modification of the them said to be more suitable for presenting stereoscopic images for viewing on television, is presented in US-A-4431265. US-A-4 559 556, describes polarising eyewear usable with a method of split-screen image presentation in which the right-eye and left-eye perspectives are presented simultaneously in an "over-and-under", orthogonally polarised fashion on a television screen for viewing. US-A- 4 583 117 describes a similar "over-and-under" type of image presentation in which a pair of conventional videocameras are modified to utilise only half of the horizontal sweep lines to form the perspectives, which are then presented time-sequentially. Electronically shuttered and synchronised eyewear is employed to view the scenes without flicker. "Shuttering" can be accomplished either mechanically or electro-optically. The latter technique is typically based on liquid crystal devices. A liquid crystal shutter consists of a sandwich of a first plane polariser, an active liquid crystal device, and an "analyser", i.e. a second plane polariser. In one voltage state, the liquid crystal device is optically neutral, and in a second state serves to rotate the plane of polarisation of light transmitted through it by 90°. By appropriate initial alignment of the axes of polarisation of the three elements, the shutter can be thereafter electrically switched between "open" and "closed" conditions. A pair of goggles employing two such shuttering devices over each eye and synchronised by means of a cable interconnecting them to a display of sequentially-presented right-eye and left-eye images is described in US-A-4 021 846. It is possible to split the shutter and to position the first polariser and liquid crystal device in front of the image generating device. In such a configuration, the two elements, when switched synchronously with the sequential image display rate, operate as a ""switchable polariser" to polarise right-eye and left-eye images alternatively in orthogonal directions, to be analysed appropriately by the second polariser, worn by the observer as a pair of polarising spectacles having orthogonal axes of polarisation. Stereoscopic displays employing such a technique are discussed in US-A-3 858 001 and US-A-4 281 381. US-A-4 670 744 describes an entirely different means for displaying a three-dimensional image using a cathode ray tube ("CRT") to sequentially project a series of images corresponding to different depth planes of a three-dimensional subject. These images are then selectively transmitted or reflected from appropriate positions along an optical path by means of a plurality of "light direction modulators", which are notable for their use of "chiral cells", i.e. cholesteric liquid crystal cells, and circularly-polarized light, to achieve the desired reflections and transmissions. It is to be noted that the foregoing art has timely application in the field of avionics, wherein visual information of a type useful to a pilot or electronics warfare operator, such as navigation, fire-control, or threat-environment information, can be displayed in three-dimensions and within the context of the pilot's or operator's three-dimensional view of the real world. However, the prior art techniques discussed above present certain problems in terms of their practical utilisation in an avionics system. For example, those which employ shutter elements in the operator's eyewear are bulky and clumsy to wear, have a low see-through transmission (less than about 20%), and must be tethered to the display by means of a cable or radio signal to carry the synchronising signals. Those which use coloured eyewear also transmit relatively low levels of light and present a view of the real world that is both dim and strangely coloured. Even those which employ eyewear consisting of a simple pair of "clear", linear polarisers with crossed polarising axes are not without such problems because, 1) the latter are still capable of transmitting only about 40% of the incident real world light to the operator's eyes, a level that is considered unacceptable in many avionics applications where high "real world" situational awareness must be maintained, and 2) it is difficult to keep the first polariser element and the "analyser" elements properly orientated with respect to each other, particularly when the observers head is subject to being tilted sideways, as in a manoeuvring aircraft. Such disorientation can cause loss of shutter efficiency and create "ghosting" on the display in which the ghost image is seen by the "wrong" eye. Displays which synthesise three-dimensional images from a plurality of depths planes lack sufficient definition unless a large number of such planes are generated, leading to very complicated and expensive displays. The present invention teaches a way to overcome these problems in a three-dimensional display that is well suited to avionics applications and may be provided in either monochromatic or full-colour embodiments. According to the present invention a stereoscopic display is characterised by those features in the characterising clause of Claim 1. A better understanding of the stereoscopic display of the present invention will be obtained from a consideration of the following detailed description of some exemplary preferred embodiments thereof, particularly when read in conjunction with the figures of the accompanying drawings, a brief description of which now follows Figure 1 is an isometric, schematic view of a stereoscopic display of the prior art; Fig 2 is an expanded, isometric, schematic view of another kind of prior art stereoscopic display; Fig 3 is an expanded, isometric, schematic view of a preferred embodiment of a stereoscopic display in accordance with the present invention; Fig 4 is an expanded, isometric, schematic view of an alternative preferred embodiment of a stereoscopic display in accordance with the present invention; and Fig 5 is an enlarged cross-sectional view into one of the preferred eyewear elements of the present invention, as revealed by the section 5-5 taken in Fig 4. A prior art display 10 of a type discussed above is illustrated diagrammatically in Fig 1, and comprises means 12, such as the CRT illustrated, for projecting, either simultaneously or sequentially, successive pairs of apparently superimposed images 14R and 14L corresponding respectively to right-eye-and left-eye-perspective views of a three-dimensional subject. In one such known system, the images are projected simultaneously and are "coded" to a corresponding eye of the observer by colouring them each a different colour eg red for the left eye image 14L and blue for the right-eye image 14R. The observer is provided with eyewear 16 containing a pair of "decoding" elements comprising a red filter 18R positioned over the right eye and a blue filter 18L positioned over the left eye. The colour filters block correspondingly-coloured images so that the eyes see only their appropriate perspective, which are then integrated in the observer's brain to produce the perception of depth. Such a display is described in US-A- 4 431 265. A modified form of the display 10 can be obtained by projecting the image pair 14R, 14L in monochrome or colour and encoding them for their corresponding eyes by linearly polarizing them in othogonal directions. this may be accomplished, for example, by using the split screen technique described in US-A-4 559 556, in combination with a pair of polarizing layers fitted appropriately to the screen of the CRT 12. The images are then alalysed by eyewear 16 containing a pair of plane polarisers 18R and 18L having their axes or polarization in alignment with the respective axes of their corresponding polarized images. Each polariser acts to pass a corresponding-polarized images. Each polariser acts to pass a correspondingly-polarized image and to block an orthogonally-polarized image. It is to be noted that, when the split screen technique is employed additional optics are necessary within the eyewear 16 to converge the split images into a single, whole image. In Fig 2, another form of a known stereoscopic display system 50 is illustrated of a type described in US-A-3 858 001 and in US-A-4 281 341, and to which the display of the present invention is more closely related. The display system 50 comprised an image projection means 52 eg a CRT capable of sequentially projecting non-polarized right-eye and left-eye images 54 at a rate sufficient to avoid a perception of flicker, ie, an image rate of 120 Hz or greater, or a frame rate (consisting of a right-eye filed plus a left-eye field of at least 60 Hz. A "switchable polariser" 56 is disposed in front of the CRT to receive the images, where they first pass through a plane-polarizing layer 58 of a type known per se, and are thereby linearly polarized in a plane indicated by the arrow 59. Disposed adjacent to the front of polariser 58 is a twisted nematic liquid crystal cell 60 of a type known per se. The liquid crystal cell has two states "ON and "OFF", obtained by imposing a distributed high or low voltage field, respectively, across the liquid crystal material. By convention, the ON state can be described as optically neutral, while the OFF state causes polarized light transmitted by the cell to be rotated by 90°. Thus, unpolarized images passing through both the polariser 58 and the liquid crystal cell 60 while the cell is ON emerge therefrom linearly polarized in the plane indicated by the solid arrow 61, while images passing through them while the cell is OFF emerge polarized in a plane normal to the first plane, as indicated by the dotted arrow 62. Electronic means, labelled "SYNC" in Fig 2, synchronize the switching of the cell 60 with the image projection rate of the CRT 52 so that the right-eye and left-eye images are appropriately coded with orthogonal polarizations for analysis by the eyewear 64, which contains a pair of linear polarisers 66R, 66L each with an axis of polarization in alignment with one of the orthogonal axes of polarization 61 or 62 of the two images. If the switchable polariser 56 adjacent to the CRT 52 were replaced with a pair of smaller such devices located within the eyewear 64, one in front of each of the polarisers 66R and 66L, a form of shuttered goggles of the type described in US-A-444 021 846 would result. While each of the prior art displays discussed above in connection with FIGS 1 and 2 has some features that are applicable to an avionics stereoscopic system, they also possess some drawbacks. Analysing eyewear that utilizes differently coloured filters tints the pilot's view of the actual environment to a degree that in unacceptable, and additionally has relatively poor light transmission. Displays incorporating linear polarisers, while capable of transmitting colour fairly accurately, provide an unacceptably-low transmission of light. For example, the shuttered goggles described above are not only bulky to wear and require a "tether" in the form of a synchronising cable, but are also not capable of transmitting more than about 20% of the total light incident upon them. While eyewear consisting of only a pair of plane polarisers, such as those illustrated in Fig 2, are less restrictive to wear and will transmit up to 40% of incident light, this figure is still considered too low for most avionics applications. Finally, they have low tolerance for misalignment between their axes and that of a remotely-located first polariser, such as would be caused by a cocking or tilting of the pilot's head. An exemplary first preferred embodiment of a stereoscopic display 100 capable of overcoming at least some of these disadvantages is illustrated in Fig 3. the display 100 comprises a CRT 102 having a screen 104 coated with a rare earth (narrow emission band) phosphor layer which emits visible, substantially monochromatic light having a relatively sharp emission peak at a wavelength of about L o when struck by an electron beam from the CRT'S gun. Means not shown are provided to "paint" the screen 104 with an electron beam from the gun such that alternate ones of successive pairs of substantially monochromatic, unpolarized images 106, corresponding to right-eye-and left-eye-perspective views of a three dimensional subject or scene, are projected from the screen at a rate of at least about 120 images per second to avoid flicker. The display 100 also includes a variable polariser 107 including a circular polariser 108, which comprises a linear polariser 108a having an optic axis, and a quarter wave plate 108b which has a "slow" axis set at 45° with respect to the polariser's axis, so that the pair together functions as a circular polariser. The circular polariser 108 may be constructed as either a right-hand-circular polariser (RHCP) or a left-hand-circular-polariser (LHCP), illustrated, and acts on the image 106 from the CRT 102 to polarize it circularly in the same sense, or "chirality", as that of the polariser. The variable polariser 107 also comprises a birefringent nematic liquid crystal cell 110 which is disposed in front of the circular polariser 108. The nematic liquid crystal cell 110 also has a slow axis and is used as a variable "retarder" plate, ie one whose differential retardation between fast and slow axes is variable bewtween zero and a half wave, depending on the voltage applied across it. Thus, by convention, when the retarder plate is ON (high voltage applied), the plate is optically neutral, and a right-handed-circularly-polarized image 112 (solid arrow) emerging from the polariser 108 will be transmitted through the optically-clear retarder plate 110, as the RHCP image 114 with its polarization unaffected (solid arrow). In the OFF state (low voltage applied), the retarder plate 110 is adusted to be a half-wave retarder, and the image 112 is transformed by it to a LHCP image 116 (dotted arrrow). Thus, when the variable retarder 110 is switched between its OFF and ON states synchronously with the image projection rate of the CRT 102 by means of block labelled "SYNC", the right-eye and left-eye perspectives emerge from the variable polariser 107 encoded with circular polarizations of opposite senses. In the exemplary first preferred embodiment illustrated, the eyewear 118 utilized to decode or analyse the right-hand and left-hand perspectives for their corresponding eyes comprise a pair of cholesteric liquid crystal elements 120R and 120L "tuned" to the particular colour or wavelength of the images. By "tuned" it is meant that the layer of cholesteric liquid crystal material in the elements is adjusted to have a refractive index of n and a pitch of p, where np is approximately equal to L o the wavelength of the images. A cholesteric liquid crystal functions as an efficient circular polariser of light of wavelengths at or near L o . Thus in the spectral region near L o only one sense of circularly polarized light at or near L o is transmitted, while circularly polarized light of the opposite sense at or near L o is reflected. Light of other wavelengths is unaffected by the cholesteric layer. Since the direction of twist of the cholesteric liquid crystal material can be made either right-handed or left-handed, the cholesteric can be made into either a RHCP or a LHCP. By convention, the former has a film or layer which reflects right-handed-circularly-polarized light, and transmits, with high efficiency, left-handed-circularly-polarized light, while the latter functions in just the opposite manner. Thus, the eyewear 118 of the exemplary embodiment illustrated in FIG 3 comprises a cholesteric liquid crystal RHCP for one eye and a LHCP for the other, both tuned to L o , the emission peak of the CRT'S phosphor. Where the right-eye images are circualrly polarized in the right-hand direction and the left-hand images polarized opposite thereto, the right-eye element 12OR will comprise a LHCP and the left-eye element 12OL a RHCP. When ambient surroundings are viewed with the cholesteric eyewear 118, light transmission is close to 100%, except for the set of wavelengths near L o , where transmission is near 50%, due to the nearly-perfect transmission of one sense of circular polarization and nearly-perfect reflection of the other. In this regard, the cholesteric elements 120R and 120L may be thought of as very narrow passband, or "notch", filters centered on the frequency f o = C/L o . Since the active range around L o can be kept quite narrow, high overall light transmission results, thus overcoming one of the major drawbacks of the prior art. The resulting eyewear 118 appears virtually clear, can be made very light in weight, eg, as a pull-down visor in a pilot's helmet, and requires no electronic tether to the CRT. It should be further noted that, since the polarisers used in the variable polariser 107 and the eyewear 118 are circular polarisers, cocking of the observer's head to either side will not result in any misalignment of their respective axes, and the "ghosting" problem of the prior art displays discussed above is also overcome. An alternative embodiment, comprising a high-brightness display, is achived by replacing the circular polariser 108 in the variable polariser 107 with a cholesteric liquid crystal plate, similar to the ones in the eyewear 118, which is also tuned to the emission peak L o of the CRT'S phosphor. This results in a significant increase in the amount of light reaching the observer from the CRT, making the display ore suitable for, eg, high ambient light conditions. Yet another exemplary alternative embodiment, a multicolour of full colour stereoscopic display 200, is illustrated schematically in FIG 4, with elements similar to those in FIG 3 numbered correspondingly, plus 100. The display 200 comprises a CRT 202 having a screen 204 with a matrix of rare earth phosphor elements deposited on it, each one capable of emitting light of different one of a plurality of narrowband colours when irradiated with electromagnetic energy, eg an electron beam. For full colour display, the phosphor elements can be deposited on the screen 204 in adjacent groups, eg conventional triads, of primary-coloured phosphor elements, ie, Red, Blue and Green, and conventional television techniques employed to generate full colour, right-eye and left eye image pairs or frames 206, each image of field consisting of the sum of three monochromatic, primary-coloured image components. The high brightness, full colour variable polariser 207 uses a variable retarder plate 210 identical to that discussed in connection with the first embodiment, as well as a "stack" of individual cholesteric liquid crystal circular polarisers 208A, 208B and 208C, equal in number to the number of different image primary colours, each tuned to a different one of the primary colours of the image components. In the exemplary embodiment illustrated, three LHCP's are illustrated, respectively tuned to the colours red, green and blue. thus, the encoding of right-eye and left-eye images with their respective oppositely-sensed polarizations within the variable polariser 207 can be seen to occur on a monchromatic component-by-component basis. The analyzing eyewear 218 used in the display 200 includes right-eye and left-eye analysers 222R and 222L, and likewise comprises "stacks" of pairs of oppositely-sensed cholesteric liquid crystal polarisers, each pair within the stack tuned to one of the colour components and disposed within the eyewear to transmit appropriately encoded images to their appropriate eyes, and to reflect inapproriately encoded images therefrom. An enlarged, partial cross section through the right eye analyser 222R is whon in FIG 5, and comprises three cholesteric liquid material layers or films 226R, 226B and 226G, corresponding to right-eye analysers tuned to the colours red, blue and green, respectively. Those skilled in the art will recognize that various modifications in terms of materials, construction and procedure are possible in the embodiments discussed and illustrated herein above. For example, in a head-up-display application, it may be desirable to invert the CRT and variable polariser and rotate them 90° relative to a pilot's forward line of sight. If a half silvered mirror is then disposed at 45° to the line of sight between the variable polariser and the analysing eyewear, three dimensional images can be diplayed to the pilot within the context of the pilot's forward view. Likewise, it is to be noted that the image source need not necessarily be a CRT but can comprise any type of refreshed display panel, eg, liquid crystal, light-emitting diode (LED), vacuum fluorescent, etc other modifications are, of course, also possible.

Claims: A stereoscopic display (110;200) including means (102;202) for sequentially projecting pairs of successive images (106;206) corresponding to left-eye and right-eye perspective views of a scene; variable polariser means (107;207) arranged to be switched sequentially with the projecting means (102;202) to circularly-polarise the respective images of each pair in opposite senses; and viewing means (118;218) comprising a pair of circular polariser means (120; 220), one for each eye of a user, characterised in that each said image is substantially monochromatic and each circular polariser means is arranged to transmit only one of each pair of circularly polarised images, light of frequencies other than those of the said substantially monochromatic images being substantially unaffected by the viewing means (118;218). A display as claimed in Claim 1 in which each circular polariser (120;220) means comprises a cholesteric liquid crystal element. A display as claimed in Claim 1 or Claim 2 in which the variable polariser means (107;207) comprises a circular polariser (108;208) and a variable retarder (110;210) arranged to be switched between a first mode which passes one of the said pair of images with its polarisation substantially unaffected and a second mode which passes the other of the said pair with a retardation of substantially one half wave. A display as claimed in Claim 3 in which the variable retarder (110;210) includes a layer of nematic liquid crystal material. A display as claimed in Claim 2 or Claim 3 in which the circular polariser (108;208) comprises a linear polariser (108a) and a quarter-wave plate (108b) having a slow axis at substantially 45° to the linear polariser's axis. A display as claimed in Claim 2 or Claim 3 in which the circular polariser (108;208) comprises a cholesteric liquid crystal element. A display as claimed in any one of the preceding claims, for multicolour imaging, in which the projector means (202) is arranged to project a plurality of differently coloured generally monochromatic image components, together forming successively left-eye and right-eye multicoloured images, the viewing means (218) comprising a plurality of pairs of circular polariser means (220) equal in number to the number of image components and each being effective for a different one of the image component colours. A display as claimed in Claim 7 when dependent upon Claim 3 in which the variable polariser means (207) comprises a plurality of said circular polarisers (208) equal in number to the number of image components and each being effective for a different one of the image component colours. A display as claimed in any one of the preceding claims in which the projector means (102;202) comprises a cathode ray tube having a narrow band, rare-earth phosphor layer. A display as claimed in any one of Claim 1 to 8 in which the projector means comprises a liquid crystal display or a light-emitting diode display. A display as claimed in any one of the preceding claims in which the viewing means (118;218) is mounted to eyewear.

Inventors: KALMANASH MICHAEL H, KALMANASH, MICHAEL H.
Applicants: KAISER AEROSPACE & ELECTRONICS, KAISER AEROSPACE & ELECTRONICS CORPORATION
Application year: 1988
Main classification: H04N13
IPC codes: H04N13, G02B27