Display

Abstract: A light source (1) comprises an array of contiguous transparent blocks (2), each of which contains a light emitter such as a cold cathode fluorescent tube (4). The blocks (2) have a transparent front surface which may be optically diffusing, the other surfaces being coated with a thin optically reflective layer so that each block (2) acts as a light guide. The tubes (4) are independently controllable, for instance so as to simulate a moving light source which may be controlled so as to track the position of an observer of a 3D display.

Description: The present invention relates to displays. EP 0540 140 describes a back light which is intended to provide a substantially constant level of illumination, providing a high illumination intensity for day time viewing and a relatively low intensity for night time viewing. EP 0491 662 discloses a display including a back light having a face plate to produce a magnified image of the display in order to hide the gaps between adjacent parts of the display. The face plate uses fibre optic light guides in order to provide the magnified image. US 5057 974 describes a back light for producing uniform illumination of a liquid crystal display. The back light comprises an array of fluorescent tubes disposed within a flat acrylic plate, the rear surface of which is provided with a reflector including minute concavities and convexities. The arrangement of concavities and convexities varies with the distance from the fluorescent tubes. A flat light guide is described in US 4954 891 which is arranged to conduct light from fluorescent tubes to form a strip illuminator. US 4948 214 describes an illumination arrangement for an optical scanner, the arrangement comprising an array of light sources which are coupled to light guides. The ends of the light guides are not contiguous, an array of lenses being provided and arranged to produce rays of light which may intersect. A projector is described in EP 0372 568 which employs curved reflectors. A stereoscopic projector is described in US 5121 983 in which curved reflectors are used in the light condenser of the projector. The curved reflectors in these projectors are not used to image the light source at a viewer location. GB 2284 487 and GB 2273 577 both describe directional display arrangements including beam combiners. EP 0316 465 describes an autostereoscopic display arrangement. The GB 2273 577 fails to disclose the matching of asymmetric contrast and viewing angle for two or more spatial light modulators. According to a first aspect of the invention there is provided a three dimensional observer tracking display having a light source comprising a plurality of individually controllable light emitting means and a plurality of optical waveguides, each of which is arranged to receive light from a respective one of the light emitting means, the optical waveguides being arranged to substantially prevent the passage of light between adjacent waveguides the optical waveguides having light outputs arranged as an array, each of the light emitting means comprising at least one light emitter, characterised in that adjacent ones of the light outputs are contiguous with each other and in that the light emitters comprise fluorescent light tubes and characterised by a heater for maintaining the fluorescent tubes substantially at normal operating temperature. Preferred embodiments of the invention are defined in the dependent claims. The invention will be further described by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a light source for a 3D display; Figure 2 shows part of the light source of Figure 1 in more detail; Figure 3 shows another light source for a 3D display; Figure 4 shows part of the light source of Figure 3 in more detail; Figure 5 shows part of the light source of Figure 1 with the addition of an optical diffuser; Figure 6 shows a further light source for a 3D display; Figure 7 shows part of a light source, for instance of the type shown in Figure 6; Figure 8 shows yet another light source for a 3D display; Figure 9 shows yet a further light source for a 3D display; Figure 10 is a diagram showing a 3D display constituting a first embodiment of the invention and including a light source as shown in Figure 3; Figure 11 is a diagram showing a 3D display constituting a second embodiment of the invention and including a light source as shown in Figure 3; Figure 12 is a diagram showing a 3D display constituting a third embodiment of the invention and including two light sources as shown in Figure 1; Figure 13 is a diagram showing a 3D display constituting a fourth embodiment of the invention and including a light source as shown in Figure 1; and Figure 14 is a diagram showing a 3D display constituting an fifth embodiment of the invention and including two light sources as shown in Figure 1. Figure 1 shows a light source which may be used as part of a three dimensional (3D) display, for instance of the autostereoscopic type. The light source 1 comprises a plurality of optical waveguides 2, one of which is shown in more detail in Figure 2. Each optical waveguide 2 comprises a cuboidal block of optically transmissive material. The material may comprise glass or a transparent plastics, such as perspex (RTM). Each block 2 has formed therein by drilling or moulding a cylindrical cavity 3 which contains an elongate light source 4 in the form of a cold cathode fluorescent tube. Each block 2 has a light-emitting surface 5 shown as an optically diffusing surface formed by roughening the block surface, for instance by sand blasting, although it may comprise a smooth surface covered by a thin optical diffusing layer. The remaining surfaces 6 of the block 2 are optically reflective, for instance as a result of being coated with a thin film of reflective material. The film is preferably less than 100 micrometres thick so that, as shown in Figure 1, the blocks 2 can be arranged as a linear array with adjacent pairs of blocks abutting against each other with a minimal gap between the surfaces 5 thereof. Thus, the light source 1 comprises a linear array of contiguous light-emitting surfaces. The arrows 7 in Figure 1 illustrate various light paths for light generated by the florescent tubes 4 in two of the blocks 2. The reflective surfaces 6 contain the light within each block 2 so that it acts as a waveguide, the light being emitted only from the surface 5. The reflective surfaces 6 therefore cause the light output from each block to be maximised and to prevent light from passing from each block 2 to an adjacent block so as to prevent "optical cross talk" which would be disadvantageous, when the light source 1 is used in an autostereoscopic 3D display. It is desirable for variations in light output from the blocks 2 to be minimised. Cold cathode florescent tubes provide high brightness and efficiency with fast switching speeds but exhibit brightness variations because of their long warm-up time, which can amount to several minutes. As a result, there will be brightness differences between those tubes which have recently been switched on and those tubes which have been recently switched off. A heater pad 8 provided with a suitable temperature controller (not shown) is therefore disposed adjacent the florescent tubes 4 so as to maintain the temperatures of all of the tubes 4 at their normal operating temperature, which is typically about 55°C. Thus, tubes which have been switched off for a substantial time will emit substantially their full intensity immediately on being switched on. Figures 3 and 4 show a modified form of light source 1 in which the cuboidal blocks 2 are replaced by wedge-shaped blocks 12. The surfaces 15 correspond to the diffusing surfaces 5 of the blocks 2 and provide the light output of the blocks 12. The remaining surfaces are coated with reflective material as in the case of the blocks 2. It is thus possible to provide a curved one dimensional light source in which the surfaces 15 of the wedge blocks 12 form or approximate part of a cylindrical or spherical surface. The surfaces 15 may be narrower than the width of the tubes 4 so as to increase the spatial resolution of the light source. Such a curved light source may be useful in overcoming field curvature aberration, for instance associated with off-axis performance of Fresnel lenses used in 3D display systems, thus increasing the field of view of such a display. In many applications such as autostereoscopic 3D displays, it is important for any residual gap between adjacent surfaces 5 or 15 of the blocks to be substantially invisible to the viewer. This may be at least partly achieved by carefully machining the blocks to have sharp edges at the surfaces 5 and 15 which are pressed together with only a thin reflective film separating adjacent blocks. However, in order to reduce the visibility of the gaps still further, an additional thin diffuser 20 may be disposed across the surfaces 5 of the blocks so as to allow a small amount of controlled cross-diffusion of light between adjacent blocks, as shown in Figure 5. In addition, a layer of 3M Brightness Enhancing Film (BEF) may be used on the surface of the diffuser 20 to enhance the brightness of the source in the normal direction. Figure 6 shows a two dimensional light source 1 comprising a plurality of blocks 2 arranged as a two dimensional contiguous array. The light sources of the individual blocks are independently controllable so as to permit any pattern of illumination to be provided. For instance, associated control circuitry may be arranged to illuminate one block at a time so that all of the blocks are illuminated in a repeating sequence. Such a light source may, for instance, be used in a 3D display to provide both vertical and horizontal parallax. Figure 7 illustrates a modified type of block 2 in which the fluorescent tube 4 is located within a slot 3', extending inwardly from the rear surface of the block 2, instead of in the cylindrical cavity 3 shown in Figure 2. The surface defining the slot 3' is optically transmissive so that light from the fluorescent tube 4 is coupled into the waveguide formed by the block 2. Figure 8 illustrates a light source formed from blocks 2 which differ from the type shown in Figure 2 in that the rear surface of each block is curved. Such an arrangement may be used to improve the uniformity of output brightness of the waveguide element. The light source shown in Figure 9 is of a similar configuration to that shown in Figure 8 but differs in that the essentially solid perspex (RTM) blocks 2 are replaced by "air filled waveguides" enclosing cavities 16. The cavities are defined by opaque barriers 17 having knife-edges at the diffuser 20 and by opaque end barriers (not shown). The barriers have reflective surfaces and the rear of each cavity 16 is defined by a cylindrically or parabolically curved reflector 18. The perspex blocks 2 and 12 shown in Figures 1 to 5 may likewise be replaced by air filled waveguides. Figure 10 shows an application of the light source 1 of Figures 3 and 4 in a 3D autostereoscopic display of the type disclosed in Figure 3 of British Patent Application No.9324703.9. The light source 1 supplies light to a beam splitter 22 which transmits substantially half of the light to a mirror 23 and reflects substantially half of the light to a mirror 24. The mirrors 23 and 24 reflect the light through Fresnel lenses 25 and 26 and spatial light modulators 27 and 28, respectively. The light rays modulated by the spatial light modulators 27 and 28 are then combined by a beam combiner 29 such that an image formed on the spatial light modulator 27 is visible to the right eye of an observer and an image formed on the modulator 28 is visible to a left eye of the observer. The Fresnel lenses 25 and 26 form images of the light source 1 at the right and left eye positions, respectively, of the observer. The individual florescent tubes of the light source 1 are controlled by an illuminator controller 30 which in turn is controlled by an observer tracking system 31. The observer tracking system 31 is arranged to track the position of the observer and to cause the illuminator controller 30 to switch on those florescent tubes which result in images of the surfaces 15 of the corresponding blocks 12 being formed at the positions of the eyes of the observer. For instance, with the observer located at the position 32, the florescent tubes 4a are illuminated and typical ray paths for the light from the corresponding blocks is shown by the continuous lines. When the observer moves to position 33, the observer tracking system 31 detects the change in position and causes the illuminator controller 30 to extinguish the tubes 4a and to light the tubes 4b. Typical ray paths are illustrated by the chain dot lines in Figure 10. Alternatively, in order to allow two observers to see autostereoscopic images simultaneously, for instance at the positions 32 and 33, both sets of tubes 4a and 4b may be illuminated simultaneously. The observer tracking system 31 may further be arranged to track the positions of both observers and to cause the illuminator controller 30 to illuminate those tubes 4 which cause autostereoscopic images to be observable by both observers within the range of movement permitted by the display. Figure 11 shows another autostereoscopic 3D display which differs from the display shown in Figure 10 in that the two views making up the autostereoscopic image are provided by temporal multiplexing. With the observer at the position 32, light from the tubes 4c is imaged by a Fresnel lens 35 through a spatial light modulator in the form of a fast liquid crystal device panel 36 at the left eye of the observer (as shown by continuous lines) whereas light from the tubes 4d is imaged at the right eye (as shown by the chain dot lines). When the observer is at the position 33, the tubes 4e and 4f are illuminated. With the observer at the position 32, the tubes 4c are first illuminated and other tubes are switched off. A left eye image is presented to the LCD panel 36 and is seen by the left eye of the observer. The tubes 4c are then extinguished and the tubes 4d are illuminated while the image data have been changed to represent a right eye image which is seen by the right eye of the observer at the position 32. This sequence is repeated with a repetition rate sufficiently high to prevent the visibility of flicker so that the observer sees an autostereoscopic 3D image. As the observer moves, for instance from the position 32 to the position 33, the active tubes 4 change from 4c and 4d to 4e and 4f continuously so that, within the range of permissible viewing positions, the observer sees the same autostereoscopic 3D image. Alternatively, as in the case of Figure 10, two sets of tubes 4c, 4d and 4e, 4f may be active simultaneously so as to permit two observers to see the same autostereoscopic 3D image at different observer locations. For instance, when the observers are at the positions 32 and 33, the tubes 4c and 4e are switched in synchronism and the tubes 4d and 4f are switched in synchronism. Both observers can be independently tracked and the appropriate tubes 4 activated in synchronism with the image data presented to the LCD panel 36. Figure 12 shows an autostereoscopic 3D display of a type similar to that shown in Figure 10 but in which two light sources 1a and 1b are provided. The light from the light sources 1a and 1b is divided by the beam splitter 22 with the path of light from the source 1a being shown by continuous lines and that from the source 1b shown by broken lines. The light sources 1a and 1b are disposed in positions such that the images, shown at 38, overlap each other by half the pitch of the blocks 2 or 12 of the light sources. Thus, such an arrangement provides a higher light intensity for the display and doubles the effective resolution of the individually illuminatable elements of the light source. Further, any gap between adjacent surfaces 2 or 12 of one light source will be covered by the light from one of the surfaces 2 or 12 of the other light source so that such gaps are less visible. The tubes 4 of the light sources are individually controllable and may be switched on and off in order to represent any desired type of light source. In the case of a single tracked observer, there are always several tubes 4 in adjacent blocks which are simultaneously illuminated. As an observer moves, for instance to the left as shown by an arrow 39 in Figure 10, it is necessary for the light source effectively to move in the direction of the arrow 40. This is achieved by switching off the tube at one end of the group of illuminated tubes and switching on the tube adjacent the other end. Thus, as an observer moves, an autostereoscopic 3D image is continuously visible. The light source 1 therefore effectively simulates a movable light source but requires no moving parts. As described hereinbefore, in order for two or more observers to see simultaneously the 3D image, two or more groups of tubes 4 may be simultaneously illuminated or controlled with the illuminated groups tracking the observers independently of each other. In a typical example of a light source of the type shown in Figure 1, the fluorescent tubes 4 are 4mm in diameter and the blocks 2 are 8mm wide. An array of 24 blocks therefore provides a light source having a total width of 192mm. When used in a beam combiner display of the type shown in Figure 10, it is required to produce two images of the light source 64mm wide separated by 64mm at the observer. For Fresnel lenses 25 and 26 arranged to provide a magnification of 2:1, four tubes can be illuminated so that the illumination width at the light source is 32mm. The maximum range of movement of the observer is then 384mm and the tubes must be switched progressively for approximately each 16mm of movement of the observer in order to obtain smooth viewing freedom of movement of the observer. Figure 13 shows an autostereoscopic 3D display of a type similar to that shown in Figure 10. However, the mirrors 23 and 24 and the lenses 25 and 26 are replaced by mirrors 40 and 41. Each of the mirrors 40 and 41 comprises a spherical or aspherical mirror which deflects the light from the light source 1 through the corresponding SLM 27 or 28 and forms an image of the light source 1 at an observer location 42. The reflecting surfaces of the mirrors 40 and 41 may additionally incorporate a diffractive pattern so as to produce a hybrid reflective/diffractive element with the focusing power shared between the reflection and the diffraction. This allows an improved optical performance to be obtained together with a larger effective aperture. In addition, the back working distance of the display may be reduced so as to make the display more compact. Figure 14 shows an autostereoscopic 3D display similar to that shown in Figure 13 but in which the light source 1 and the beam splitter 22 are replaced by two light sources 1a and 1b. As described in British Patent Application No.9324703.9, in which the position of an observer is tracked and the display is adjusted so as to present an autostereoscopic 3D image to the observer within the range of permissible positions, the relative positions of one or more sources of illumination and an imaging device such as a lens are varied so as to track the observer. It is believed desirable to limit the fluctuations of brightness.

Claims: A three dimensional observer tracking display having a light source (1) comprising a plurality of individually controllable light emitting means (4) and a plurality of optical waveguides (2, 12), each of which is arranged to receive light from a respective one of the light emitting means (4), the optical waveguides (2, 12) being arranged to substantially prevent the passage of light between adjacent waveguides (2, 12) the optical waveguides (2, 12) having light outputs (5, 15) arranged as an array, each of the light emitting means (4) comprising at least one light emitter (4), characterised in that adjacent ones of the light outputs (5, 15) are contiguous with each other and in that the light emitters (4) comprise fluorescent light tubes (4) and characterised by a heater (8) for maintaining the fluorescent tubes (4) substantially at normal operating temperature. A display as claimed in Claim 1, in which the light outputs (5, 15) are arranged as a one dimensional array. A display as claimed in Claim 1, in which the light outputs (5, 15) are arranged as a two dimensional array. A display as claimed in any one of Claims 1 to 3, in which each of the optical waveguides (2, 12) comprises a block of optically transparent material having a cavity (3, 3') containing the respective one of the light emitters (4). A display as claimed in Claim 4, in which each of the blocks has a first face (5, 15) constituting the light output (5, 15) and at least one further face (6), the or each further face (6) being optically internally reflective or optically absorptive. A display as claimed in Claim 5, in which the or each further face (6) is coated with a reflective layer. A display as claimed in Claim 5 or 6, in which the first face (5, 15) is optically diffusing. A display as claimed in any one of Claims 5 to 7, in which each of the blocks is cuboidal and the first faces (5) of the blocks are disposed in a common plane. A display as claimed in any one of Claims 5 to 7, in which each of the blocks is wedged-shaped and the first faces (15) of the blocks are disposed in a substantially arcuate surface. A display as claimed in Claim 1, 2 or 3, in which each of the optical waveguides (2, 12) comprises a cavity (16) defined by at least one opaque barrier (17). A display as claimed in Claim 10, in which the or each barrier (17) is optically reflective. A display as claimed in any one of the preceding claims, in which a diffuser (20) is disposed in front of the light outputs (5, 15). A display as claimed in any one of the preceding claims, characterised by at least one spatial light modulator (27, 28), and at least one curved reflector (40, 41) arranged to deflect light from the light source (1) through the at least one spatial light modulator (27, 28) and to form an image of the light source (1) at a viewer location (42). A display as claimed in Claim 13, in which the at least one spatial light modulator (27, 28) comprises first and second spatial light modulators (27, 28), the at least one curved reflector (40, 41) comprises first and second curved reflectors (40, 41) for deflecting light through the first and second spatial light modulators (27, 28), respectively, and there is provided a beam combiner (29) for combining light from the first and second spatial light modulators (27, 28). A display as claimed in Claim 14, comprising a beam splitter (22) for directing light to the first and second curved reflectors (40, 41) from the light source (1). A display as claimed in any one of Claims 13 to 15, in which the or each curved reflector (40, 41) has a diffractive pattern formed thereon for focusing light from the at least one light source (1).

Inventors: EZRA DAVID, OMAR BASIL, WOODGATE GRAHAM JOHN, EZRA, DAVID, OMAR, BASIL, WOODGATE, GRAHAM JOHN
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1995
Main classification: G02B27
IPC codes: G02F1, G02B27, H04N13