Directional display

Abstract: A three-dimensional display comprises a spatial light modulator (13), such as a liquid crystal display, sandwiched between first and second lenticular screens (12, 16). The pitch of the lenticules of the second screen (16) is an integral multiple of that of the first screen (12). The spatial light modulator (13) comprises a plurality of cells aligned with the lenticules of the first screen (12). A linear array of sequentially illuminated light sources (1-8) is focused by an optical system (10) into a plurality of collimated light beams with different angles of incidence on the first screen (12). For each illumination of the light sources, the spatial light modulator carries a plurality of 2D interlaced views.

Description: The present invention relates to a directional display. Such a device may be used to convert spatial and temporal information into directional information, for instance so as to provide an autostereoscopic (no viewing aids) three-dimensional display. Known three-dimensional (3D) display devices which can create images of opaque moving objects rely on creating the perception of a 3D image to a human observer by displaying a number of 2-dimensional (2D) images. Each of the 2D images is a view of the object from a particular direction and is "replayed" in that direction. The accuracy and effectiveness of these 3D images and the maximum display size and freedom of viewer location increase as the number of 2D views displayed increases. Two known techniques for providing 3D displays are lenticular methods and time multiplexed methods. For effective operation, such systems must display a large number of 2D views of an object. In direct view lenticular systems, the number of views is determined by the resolution of a spatial light modulator (SLM) used in the system whereas, in multiple projector lenticular systems, the number of views is determined by the number of separate SLMs used. In time multiplexed systems, the frame rate of the SLM determines the number of views. With known arrangements, to display a large number of views, the maximum frame rates of available SLMs are insufficient for time multiplexed displays and the maximum resolution of presently available SLMs is insufficient for direct view lenticular methods. Multiple projector lenticular methods with a large number of SLMs are expensive and bulky. Thus, known 3D display systems are incapable of or inconvenient for accurately producing a moving opaque colour autostereoscopic 3D image from a range of perspectives. GB-A-2206763 discloses a 3D display apparatus comprising a 2D display device, such as a cathode ray tube, disposed in the focal plane of a lens arranged behind a SLM, such as a LCD. Different views are provided in a time-multiplexed manner on the SLM but each view is visible only from a direction dependent on the position of a spot of light emitted from the 2D display device. Thus, the apparatus disclosed in this patent specification provides a rate of 2D views which is limited by the frame rate of the SLM. An early example of a time multiplexed system is disclosed in an article entitled "The Stereoptiplexer; Competition for the Hologram" by R. B. Collender in Information Display, November/December 1967. SLMs for use in 3D displays are generally provided by liquid crystal displays (LCD). For certain applications, a large screen size, perhaps of the order of one metre, is desirable in order to provide an effective impression of a 3D image. However, manufacturing difficulties presently limit the size of LCDs to around 400 mm. Known projection displays produce large images optically from relatively small SLMs. Such known arrangements use lenticular screen techniques which require a respective projector for each 2D view. Thus, an accurate 3D image would require a large number of projectors, which would be expensive and bulky. Another known 3D technique is disclosed by J. Hamasaki in a paper entitled "Electronic Registration for Autostereoscopic Lenticular 3D TV on a CRT: An Equivalent of a Verifocal Lens for an Electronic 3D Display" at the First International Symposium on 3D images, in 1991. Known 2D displays using lens arrays and SLMs are disclosed in EP-A-0366462, EP-A-0425266, EP-A-0431886, EP-A-0440495, and EP-A-0444872. EP-A-0 431 886 discloses a 2D liquid crystal colour projection display in which separate liquid crystal devices are used for red, green and blue. A microlens array is associated with each liquid crystal device and has electrically controllable refractive power to allow variations in refractive index with colour to be compensated. Each microlens array acts as a condenser lens and each liquid crystal device is associated with a projection lens for projecting an image onto a screen. GB 1 093 232 discloses a system which attempts to give a visual impression of relief from a single image. Viewing of the side strips of the image is made directional using, for instance, a lens array associated with a parallax screen. According to first to fourth aspects of the invention, there are provided optical devices as defined in the appended Claims 1, 20, 21, and 22. Preferred embodiments of the invention are defined in the other appended claims. It is thus possible to provide optical devices which may be used in direct view and projection display systems and which combine spatial and temporal multiplexing to increase the rate of display of 2D images or views. By increasing the number of views, it is possible to improve the accuracy and appearance of a 3D image. It is also possible to produce a larger 3D display with a larger range of observer positions. No viewing aids are necessary in order to see the 3D effect i.e. the display is autostereoscopic. Furthermore, known image capture techniques may be used so that colour, movement, and opaque images may all be displayed. The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a direct view display constituting a first embodiment of the invention; Figures 2 and 3 are enlarged plan views of part of the display of Figure 1 illustrating the operation thereof; Figure 4 is a plan view of a direct view display constituting a second embodiment of the invention; Figure 5 is a plan view of a projection display constituting a third embodiment of the invention; Figure 6 is a plan view of a projection display constituting a fourth embodiment of the invention; Figure 7 is an enlarged plan view of part of the projection display of Figure 6 illustrating the operation thereof; Figure 8 is a plan view of a direct view display constituting a fifth embodiment of the invention; Figure 9 is an enlarged plan view of part of the display of Figure 8 illustrating the operation thereof; Figure 10 shows a beam splitter not constituting an embodiment of the invention; and Figures 11 and 12 are enlarged views similar to Figures 3 and 9, respectively, showing modified embodiments of the invention. The direct view display shown in Figure 1 comprises a linear array of N light sources 1 to 8, where N=8 in this embodiment. The light sources 1 to 8 are contiguous so as to form a continuous strip. The light sources are connected to a control circuit 9, which causes the light sources to be illuminated one at a time repetitively in order, Figure 1 indicating that the light source 5 is illuminated. The linear array of light sources 1 to 8 is disposed in the focal plane of an illumination or collimating system 10, which may comprise a plano-convex lens having a cylindrical convex surface as shown in Figure 1. The collimating system 10 produces collimated light from each point of each of the light sources 1 to 8. Because the light sources have finite dimensions, the light output of the collimating system has a spread of angles. Collimated light from the lens of the collimating system 10 is directed towards a hybrid sandwich 11 at an angle which is determined by which of the light sources 1 to 8 is presently illuminated. The hybrid sandwich comprises a lenticular screen (LS1) 12 formed by a plurality of contiguous cylindrical converging lens elements or lenticules having a horizontal pitch p. Alternatively or additionally, the lenticules may have a graded refractive index profile. The lenticular screen 12 is followed by a SLM 13 in the form of a 2D LCD, for instance of the ferroelectric or TFT twisted nematic liquid crystal type, which is connected to the control circuit 9. The screen 12 may be mounted directly onto the surface of the LCD of the SLM 13. The SLM 13 comprises a 2D array of picture elements whose light transmission properties (and colour transmission properties for a colour display) are controlled by the control circuit 9. For a colour display, red, green, and blue elements for each spot are arranged vertically above each other. The SLM is followed by a diffuser 14, for instance of the Lambertian type or of the high gain type, and a further lenticular screen 16. The screen 16 is similar to the screen 12 but has a horizontal pitch equal to Mp, where M is greater than 1 and, for most applications, is an integer. In alternative embodiments, the diffuser 14 may be omitted or replaced by an array of field lenses disposed in the focal planes of the lenticules of the screens 12 and 16. Figure 2 is an enlarged view of part of the display illustrated schematically in Figure 1. As shown more clearly in Figure 2, the SLM comprises a plurality of vertical strip portions disposed substantially in alignment with the lenticules of the screen 12. Each strip portion comprises a linear array of picture elements which are individually controllable by the control circuit 9 in respect of their light attenuations and, for colour displays, their colour transmission properties. The diffuser 14 is located at or near the focal plane of the lenticules of the screen 12 and at or near the focal plane of the lenticules of the screen 16. Each of the lenticules of the screen 16 corresponds to M strip portions of the SLM 13, where M=2 in the embodiment shown. Figure 2 illustrates light ray paths which are produced by the display when the light source 5 is illuminated. For purposes of illustration, the light source is shown as an ideal point source. The light rays from the source 5 are collimated into parallel light rays by the illumination lens 10 and are focused by the screen 12 through the SLM 13 onto the diffuser 14. The picture elements of the SLM 13 are controlled by the control circuit 9 so as to provide two views of the image taken from different directions during image capture. The two views are interlaced such that alternate strip portions correspond to a respective one of the views. The picture elements of the SLM control the amount (and colour for a colour display) of light passing through the SLM so that a 2D array of images of the light source 5 is formed on the diffuser 14 corresponding to the two interlaced views. Each of the lenticules of the screen 16 converts the images on the diffuser 14 into output ray bundles whose angles of emission from the hybrid sandwich 11 depend on the lateral locations of the images on the diffuser 14 with respect to the optical axes of the lenticules. For purposes of illustration, the light sources 1 to 8 are shown as point light sources in Figure 2 and the two interlaced images represented on the SLM are illustrated by different densities of shading. Thus, for point light sources, the screen 12 forms point light images on the diffuser 14 which are imaged by the screen 16 into monodirectional output ray bundles as shown by the broken lines in Figure 2. The two views represented on the SLM 13 are therefore visible from different angles corresponding to the angles of the object from which the views were taken during image capture. After the light source 5 has been actuated for a predetermined time, the control circuit 9 deactivates the light source 5 and causes the SLM 13 to display the next pair of interlaced views, illustrated in Figure 3 by light and dark regions. The next light source 4 is then activated as shown in Figure 3. The illumination lens 10 directs collimated light at a different predetermined angle from that shown in Figure 2 on to the lenticular screen 12. The lenticular screen 12 images the light through the SLM 13 on to regions of the diffuser 14 which are laterally displaced with respect to the images formed when the light source 5 is illuminated as shown in Figure 2. Thus, the lenticular screen 16 provides output ray bundles directed at different angles from those shown in Figure 2, these different angles corresponding to the directions of the views of the object during image capture. This sequence of operation continues until each of the light sources of the linear array has been illuminated in turn, with the views represented on the SLM corresponding to a single "frame" of the 3D image. The whole sequence is then repeated for new sets of views representing consecutive frames constituting consecutive 3D images, the rate of repetition or "refresh" being sufficiently large to provide a substantially flicker-free image. For the embodiment shown in Figures 1 to 3, the number of views making up each 3D image is equal to MxN, i.e. 2x8=16 views per image. Thus, by combining spatial and temporal multiplexing, a relatively large number of views per 3D image is displayed so as to improve the accuracy and appearance of the 3D image presented to an observer. The embodiment shown in Figure 4 differs from that shown in Figures 1 to 3 in that the illumination arrangement comprises a plurality of linear arrays of light sources 20, 21, each having a corresponding illumination lens 22, 23, for instance in the form of a lenticular arrangement. The light sources of the arrays 20, 21 are controlled in synchronism by the control circuit 9. Such an arrangement may, for instance, be desirable in order to reduce the thickness of the device, since the lenses 22, 23 can have a shorter focal length than the lens 10, or in order to provide greater illumination by increasing the number of individual light sources simultaneously illuminated. A single grid or barrier 24 is provided so as to prevent light from one of the arrays being received by the lens or lenticule corresponding to the other array. Figure 5 shows a projection display comprising a single projector 30 and a lenticular screen arrangement 31. The projector 30 comprises a powerful source of light, such as a tungsten halogen bulb, and an optical system containing a SLM 32 of the type shown in Figures 1 to 4 of the drawings for simultaneously encoding two interlaced views of a 3D image. The lenticular screen arrangement 31 comprises a lenticular screen 33, a diffuser 34, and a lenticular screen 36 substantially identical to the screen 12, the diffuser 14, and the screen 16, respectively, of Figures 1 to 4. However, the lenticular screen arrangement 31 may be substantially bigger than the hybrid sandwich 11. Also, the diffuser 34 may be omitted or replaced by a field lens array. The SLM 32 controls the intensity (and colour for a colour display) of light through the optical system of the projector 30 so as to project the interlaced views on to the lenticular screen 33. Alternate lenticules of the screen 33 receive alternate vertical strip portions of a respective one of the two views encoded by the SLM 32. The lenticules of the screen 33 image the picture elements on to the diffuser 34, which is located in or near the focal plane of the lenticules of the screen 36. The screen 36 produces output ray bundles which are emitted with different directions corresponding to the two views encoded by the SLM so as to provide a 3D image. The views encoded by the SLM 32 are repeatedly refreshed so as to display a sequence of 3D images. The embodiment of Figure 6 differs from that of Figure 5 in that a plurality of projectors 30, each having a respective SLM 32, is provided. The projectors 30 project pairs of views on to the lenticular screen 33 from different angles so as to increase the number of views for each 3D image. Figure 7 shows in more detail the angular directions of the output ray bundles of the display corresponding to the MxN views making up the 3D image, where M is the ratio between the pitch of the screen 36 and the pitch of the screen 33 and N is the number of projectors. In order to ensure that the two views encoded by each SLM are correctly registered on to the screen 33 for each of the projectors, high quality projection lenses may be used. Alternatively, optical errors may be compensated during the manufacture of the lenticular screen 33. In the case of mask manufacturing schemes for lenticular arrays, this may be achieved by imaging the mask through the projection lens during the writing stage. It is thus possible to provide direct view and projection displays for 3D images which are capable of displaying images of moving, coloured, and opaque objects to provide a convincing 3D effect to an observer, who does not need to wear any viewing aids in order to see the 3D effect. Such displays have a large number of possible applications, for instance as large screen 3D displays which may be used in television, computer aided design, medical imaging, video games, and virtual reality displays. In addition, the hybrid sandwich 11 may be used in optical information processing. The embodiments described hereinbefore illustrate 3D displays in which horizontal parallax is created by means of a horizontal one dimensional array of light sources and lenticular screens having cylindrical converging lenticules which extend vertically. For 3D displays in which vertical parallax is also required, a 2D array of light sources may be used, with the lenticular screens replaced by 2D arrays of converging lenses, such as spherical convex lenses. The lens arrays may be in the form of spherical microlens arrays with lenslets arranged in any convenient pattern. For instance the vertical and horizontal pitches of the lenslets may be similar or equal. Similarly, the SLM comprises a 2D array of cells, each of which corresponds to and is aligned with a lenslet of the array of smaller pitch. Also, it is possible to replace the lenticular screens or lens arrays with parallax screens, for instance as shown in Figures 8 and 9. The display shown in Figures 8 and 9 is of the same type as that shown in Figures 1 to 3, with like reference numerals corresponding to like parts. However, the display of Figures 8 and 9 differs from that of Figures 1 to 3 in that the hybrid sandwich 11 is replaced by a hybrid sandwich 51 in which the lenticular screens 12 and 16 are replaced by parallax screens 52 and 56, respectively. Each of the parallax screens comprises a plurality of elongate parallel slits spaced apart horizontally with a pitch p. Each of the slits corresponds to and is aligned with the longitudinal axis of a respective one of the elongate picture elements of the SLM 13. Similarly, the slits of the parallax screen 56 are spaced apart horizontally with a pitch P equal to Mp, where M is greater than one and, for most applications, is an integer. Each of the slits of the parallax screen 56 corresponds to a group of M picture elements or strip portions of the SLM 13. Figure 9 corresponds to Figure 2 and illustrates the light ray paths where the light source 5 is illuminated. The substantially collimated light from the lens 10 is restricted by the parallax screen 52 such that images of the slits are projected through respective strip portions of the SLM 13 on to the diffuser 14. The lateral positions of the images dependent on which of the light sources 1 to 8 is currently illuminated so that the images of the slits for different light sources are laterally displaced with respect to each other. The slits of the parallax screen 56 limit the viewing angles of the images of the slits formed on the diffuser 14 such that each image is visible only from a predetermined angle corresponding to the angle of the view during image capture. Thus, the ray paths substantially correspond to those illustrated in Figure 2. Although the hybrid sandwich 51 using parallax screens permits 3D image display to be achieved, the light output, for a given power of illumination, of the display shown in Figures 8 and 9 is substantially less than the light output of the display shown in Figures 1 to 3. In order to provide both vertical and horizontal parallax, the parallax screens 52 and 56 may be replaced by parallax screens having arrays of holes in place of the slits. Although hybrid sandwiches either using two arrays of lenses, such as lenticular screens, or two parallax screens have been described and illustrated, it is also possible to provide mixed embodiments in which one of the elements is a lens array and the other is a parallax screen. In the embodiments shown in Figures 1 to 4, 8, and 9, the SLM 13 is disposed between the lenticular screens 12 and 16 or the parallax screens 52 and 56. However, it is also possible to dispose the SLM 13 between the light sources 1 to 8 and the lenticular screen 12 as shown in Figure 11 or between the light sources 1 to 8 and the parallax screen 52 as shown in Figure 12. Otherwise, the modified embodiments shown in Figures 11 and 12 are identical to the embodiments shown in Figures 3 and 9, respectively. Figure 10 illustrates another possible application of a lenticular screen arrangement 31 of the type shown in Figure 5. Like reference numerals refer to like parts, so that the screen arrangement comprises a first lenticular screen 33, a diffuser 34, and a second lenticular screen 36. In this application, the screen arrangement is used as a beam splitter. An input beam 60 is incident on the lenticular screen 33 and is focused by the lenticules of the screen 33 into a plurality of images on the diffuser 34. Each of the lenticules of the screen 36 receives light from images formed by M lenticules of the screen 33, where M=2 in the embodiment shown. Thus, the lenticular screen 36 forms two output beams 61 which are emitted from the screen at different angles of emission. The lenticular screen 33 effectively spatially samples the input beam 60 and the lenticular screen 36 reconstructs the two substantially identical output beams from the samples. Thus, if the input beam 60 carries lateral information, the usual precautions concerning the interaction of such information and the sampling frequency should be observed.

Claims: A directional display comprising : a first array (12; 33) of lenses arranged as columns which are regularly spaced apart with a pitch p; a spatial light modulator (13; 32) comprising light modulating elements arranged in columns so that each column of the modulator (13; 32) is aligned with a respective column of the first array (12; 33); and a second array (16; 36) of lenses facing the first array (12; 33) and arranged as columns which are regularly spaced apart with a pitch P where P>p, each column of the second array (16; 36) facing a respective group of Z columns of the first array (12; 33), where Z is an integer greater than one. A display as claimed in Claim 1, wherein P=Zp. A display as claimed in Claim 1 or 2, wherein each of the first and second arrays (12, 16; 33, 36) comprises a lenticular screen, each of whose lenticules constitutes a respective one of the columns. A display as claimed in any one of the preceding claims, wherein the modulator (13) is disposed adjacent the first array (12) and the columns of light modulating elements are regularly spaced apart with a pitch p. A display as claimed in Claim 1 or 2, wherein the lenses of the first array (12; 33) are arranged as rows which are regularly spaced apart with a pitch q and the lenses of the second array (16;36) are arranged as rows which are regularly spaced apart with a pitch Q, where Q>q. A display as claimed in Claim 5, wherein Q=Yq, where Y is an integer greater than one. A display as claimed in Claim 5 or 6, wherein the modulator (13) is disposed adjacent the first array (12) and each picture element is optically aligned with a respective one of the lenses of the first array (12). A display as claimed in any one of the preceding claims, wherein the lenses of the first array (12; 33) have a common image plane which coincides with a common object plane of the lenses of the second array (16; 36). A display as claimed in Claim 8, further comprising a diffuser (14; 34) located at the common image and object planes of the lenses of the first and second arrays (12, 16; 33, 36), respectively. A display as claimed in Claim 8, further comprising a field lens array located at the common image and object planes of the lenses of the first and second arrays (12, 16; 33, 36), respectively. A display as claimed in any one of the preceding claims, further comprising illumination means (1-8; 20; 21) for illuminating the first array (12) with collimated light whose angle of incidence on the first array (12) varies with time. A display as claimed in Claim 11 when dependent on any one of Claims 1 to 4, wherein the illumination means comprises at least one set of sequentially illuminatable light sources (1-8) arranged as a one-dimensional array extending perpendicularly to the columns of the first array (12) and disposed in the focal plane of a respective collimating optical system (10; 22, 23) for the or each set. A display as claimed in Claim 11 when dependent on Claim 3 or 4, wherein the illumination means comprises at least one set of sequentially illuminatable light sources (1-8) arranged as a two-dimensional array in the focal plane of a respective collimating optical system (10; 22, 23) for the or each set. A display as claimed in Claim 12 or 13, wherein each light source (1-8) of the or each set is contiguous with the or each adjacent light source (1-8) of the respective set. A display as claimed in any one of Claims 12 to 14, further comprising control means (9) for sequentially illuminating the light sources (1-8) of the or each set, each lens of the second array (16) being associated with M picture elements of the modulator (13), the control means (9) being arranged to control the supply of image data to the modulator (13) so that the modulator (13) represents M images simultaneously, with the M picture elements associated with each lens of the second array (16) representing elements of different ones of the M images. A display as claimed in Claim 15, wherein the control means (9) is arranged to supply fresh image data to the modulator (13) substantially synchronously with changing the illumination of the light sources (1-8) of the or each set. A display as claimed in any one of Claims 1 to 3, 5 and 6, further comprising at least one projection means (30) containing the modulator (32) and arranged to project Z spatially interlaced images onto the first array (33). A display as claimed in Claim 17, wherein the or each projection means (30) comprises means for illuminating the spatial light modulator and means for projecting an image of the illuminated spatial light-modulator onto the first array (33). A display as claimed in Claim 17 or 18, wherein the projection means (30) comprises a plurality of projection means arranged to project onto the first array (33) at different angles of incidence. A directional display comprising: a first parallax screen (52) comprising a plurality of apertures arranged as aperture columns which are regularly spaced apart with a pitch p; a spatial light modulator (13) comprising light modulating elements arranged in columns so that each column of the modulator (13) is aligned with a respective column of the first screen (52); and a second parallax screen (56) facing the first screen (52) and comprising a plurality of apertures arranged as aperture columns which are regularly spaced apart with a pitch P where P>p, each aperture column of second screen (56) facing a respective group of Z aperture columns of the first screen (52), where Z is an integer greater than one. A directional display comprising: an array (12) of lenses arranged as columns which are regularly spaced apart with a pitch p; a spatial light modulator (13) comprising light modulating elements arranged in columns so that each column of the modulator (13) is aligned with a respective column of the array (12); and a parallax screen (56) facing the array (12) and comprising a plurality of apertures arranged as aperture columns which are regularly spaced apart with a pitch P where P> p, each aperture column of the screen (56) facing a respective group of Z columns of the array (12), where Z is an integer greater than one. A directional display comprising: a parallax screen (52) comprising a plurality of apertures arranged as aperture columns which are regularly spaced apart with a pitch p; a spatial light modulator (13) comprising light modulating elements arranged in columns so that each column of the modulator (13) is aligned with a respective column of the screen (52); and an array (16) of lenses facing the screen (52) and arranged as columns which are regularly spaced apart with a pitch P where P>p, each column of the array (16) facing a respective group of Z aperture columns of the screen (52), where Z is an integer greater than one.

Inventors: EZRA DAVID, WOODGATE GRAHAM JOHN, EZRA, DAVID, WOODGATE, GRAHAM JOHN
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1993
Main classification: G02B27
IPC codes: H04N13, G02B27, G02F1