DIRECTIONAL BACKLIGHTS FOR WIDE-ANGLE IMAGERY

Description

[0001] This disclosure generally relates to illumination of light modulation devices, and more specifically relates to light guides for providing large area illumination from localized light sources for use in 2D, 3D, and / or autostereoscopic display devices.

[0002] Spatially multiplexed autostereoscopic displays typically align a parallax component such as a lenticular screen or parallax barrier with an array of images arranged as at least first and second sets of pixels on a spatial light modulator, for example an LCD. The parallax component directs light from each of the sets of pixels into different respective directions to provide first and second viewing windows in front of the display. An observer with an eye placed in the first viewing window can see a first image with light from the first set of pixels; and with an eye placed in the second viewing window can see a second image, with light from the second set of pixels.

[0003] Such displays have reduced spatial resolution compared to the native resolution of the spatial light modulator and further, the structure of the viewing windows is determined by the pixel aperture shape and parallax component imaging function. Gaps between the pixels, for example for electrodes, typically produce non-uniform viewing windows. Undesirably such displays exhibit image flicker as an observer moves laterally with respect to the display and so limit the viewing freedom of the display. Such flicker can be reduced by defocusing the optical elements; however such defocusing results in increased levels of image cross talk and increases visual strain for an observer. Such flicker can be reduced by adjusting the shape of the pixel aperture, however such changes can reduce display brightness and can comprise addressing electronics in the spatial light modulator.

[0004] According to the present disclosure, a directional illumination apparatus may include an imaging directional backlight for directing light, an illuminator array for providing light to the imaging directional backlight. The imaging directional backlight may include a waveguide for guiding light. The waveguide may include a first light guiding surface and a second light guiding surface, opposite the first light guiding surface.

[0005] Display backlights in general employ waveguides and edge emitting sources. Certain imaging directional backlights have the additional capability of directing the illumination through a display panel into viewing windows. An imaging system may be formed between multiple sources and the respective window images. One example of an imaging directional backlight is an optical valve that may employ a folded optical system and hence may also be an example of a folded imaging directional backlight. Light may propagate substantially without loss in one direction through the optical valve while counter-propagating light may be extracted by reflection off tilted facets as described in US-2012 / 0127573, and similarly in US-2013 / 307831 on which the two-part form of claim 1 is based. Directional backlights provide illumination through a waveguide with directions within the waveguide imaged to viewing windows. Diverging light from light sources at the input end and propagating within the waveguide is provided with reduced divergence, and typically collimated, by a curved reflecting mirror at a reflecting end of the waveguide and is imaged towards a viewing window by means of curved light extraction features or a lens such as a Fresnel lens. For the on-axis viewing window, the collimated light is substantially parallel to the edges of a rectangular shaped waveguide and so light is output across the entire area of the waveguide towards the viewing window. For off-axis positions, the direction of the collimated light is not parallel to the edges of a rectangular waveguide but is inclined at a non-zero angle. Thus a non-illuminated (or void) outer portion (that may be triangular in shape) is formed between one edge of the collimated beam and the respective edge of the waveguide. Ideally, no light is directed to the respective viewing window from within the outer portion and the display will appear dark in this region. It would be desirable to reduce the appearance of the dark outer portions for off-axis viewing positions so that more of the area of the waveguide can be used to illuminate a spatial light modulator, advantageously reducing system size and cost.

[0006] According to the present invention, there is provided a directional backlight comprising: a waveguide comprising an input end; an array of input light sources arranged at different input positions in a lateral direction across the input end of the waveguide and arranged to input input light into the waveguide, the waveguide further comprising first and second opposed, laterally extending guide surfaces for guiding light along the waveguide, side surfaces extending between the first and second guide surfaces, and a reflective end facing the input end for reflecting the input light back along the waveguide and having positive optical power laterally, the second guide surface being arranged to deflect the reflected input light through the first guide surface as output light, and the waveguide being arranged to direct the output light into optical windows in output directions that are distributed in a lateral direction in dependence on the input position of the input light; and additional light sources disposed along only a part of each side surface adjacent the input end, the additional light sources being arranged to direct additional light through a respective one of the side surfaces into the waveguide in a direction in which the additional light is reflected by the reflective end onto the opposite side surface and by the opposite side surface into a segment of the waveguide adjacent the opposite side surface extending from a corner between the reflective surface and the opposite side surface.

[0007] Advantageously the spatial uniformity of the output of the backlight can be improved for off-axis viewing positions by means of filling of illumination voids. Advantageously the efficiency of filling of illumination voids may be optimized, reducing power consumption while maintaining high spatial uniformity.

[0008] Said part of each side surface along which the additional light sources are disposed may be at least 20% of the side surface from the input end. Said part of each side surface along which the additional light sources are disposed may be at most 80% of the side surface from the input end.

[0009] The first guide surface may be arranged to guide light by total internal reflection and the second guide surface may comprise a plurality of light extraction features oriented to direct light guided along the waveguide in directions allowing exit through the first guide surface as the output light and intermediate regions between the light extraction features that are arranged to guide light along the waveguide. The second guide surface may have a stepped shape in which said light extraction features are facets between the intermediate regions. The light extraction features may have positive optical power in the lateral direction. The reflective end may be a Fresnel reflector comprising alternating reflective facets and draft facets, the reflective facets may provide the Fresnel reflector with positive optical power.

[0010] Advantageously the number of additional light sources provided may be minimized, reducing cost and complexity.

[0011] A directional display device may comprise a directional backlight according to the present invention; and a transmissive spatial light modulator arranged to receive the output light from the waveguide and to modulate it to display an image.

[0012] A directional display apparatus may comprise such a directional display device and a control system arranged to control the light sources. Advantageously an array of optical windows can be formed, to provide a controllable directionality of optical output. The optical windows can be arranged to provide modes of operation that may be switched between (i) wide viewing angle mode that has similar spatial and angular uniformity to conventional non-imaging backlights, (ii) autostereoscopic 3D mode, (iii) privacy mode, (iv) dual view mode, (v) power savings mode, and (vi) efficient high luminance mode for outdoors operation.

[0013] The control system may be arranged to control input light sources selected to direct output light into desired optical windows, and may be further arranged to control at least one additional light source selected to provide additional light that is output from the directional backlight in the same output directions as the desired optical windows. The control system may be arranged, when a selected input light source is off-center of the array of input light surfaces, to control at least one additional light source that is on the opposite side of the directional backlight from the selected input light source.

[0014] Embodiments herein may provide an autostereoscopic display that provides wide angle viewing which may allow for directional viewing and conventional 2D compatibility. The wide angle viewing mode may be for observer tracked autostereoscopic 3D display, observer tracked 2D display (for example for privacy or power saving applications), for wide viewing angle 2D display or for wide viewing angle stereoscopic 3D display. Further, embodiments may provide a controlled illuminator for the purposes of an efficient autostereoscopic display. Such components can be used in directional backlights, to provide directional displays including autostereoscopic displays. Additionally, embodiments may relate to a directional backlight apparatus and a directional display which may incorporate the directional backlight apparatus. Such an apparatus may be used for autostereoscopic displays, privacy displays, multi-user displays and other directional display applications that may achieve for example power savings operation and / or high luminance operation.

[0015] Embodiments herein may provide an autostereoscopic display with large area and thin structure. Further, as will be described, the optical valves of the present disclosure may achieve thin optical components with large back working distances. Such components can be used in directional backlights, to provide directional displays including autostereoscopic displays. Further, embodiments may provide a controlled illuminator for the purposes of an efficient autostereoscopic display.

[0016] Embodiments of the present disclosure may be used in a variety of optical systems. The embodiment may include or work with a variety of projectors, projection systems, optical components, displays, microdisplays, computer systems, processors, self-contained projector systems, visual and / or audiovisual systems and electrical and / or optical devices. Aspects of the present disclosure may be used with practically any apparatus related to optical and electrical devices, optical systems, presentation systems or any apparatus that may contain any type of optical system. Accordingly, embodiments of the present disclosure may be employed in optical systems, devices used in visual and / or optical presentations, visual peripherals and so on and in a number of computing environments.

[0017] Before proceeding to the disclosed embodiments in detail, it should be understood that the disclosure is not limited in its application or creation to the details of the particular arrangements shown, because the disclosure is capable of other embodiments. Moreover, aspects of the disclosure may be set forth in different combinations and arrangements to define embodiments unique in their own right. Also, the terminology used herein is for the purpose of description and not of limitation.

[0018] Directional backlights offer control over the illumination emanating from substantially the entire output surface controlled typically through modulation of independent LED light sources arranged at the input aperture side of an optical waveguide. Controlling the emitted light directional distribution can achieve single person viewing for a security function, where the display can only be seen by a single viewer from a limited range of angles; high electrical efficiency, where illumination is primarily provided over a small angular directional distribution; alternating left and right eye viewing for time sequential stereoscopic and autostereoscopic display; and low cost.

[0019] These and other advantages and features of the present disclosure will become apparent to those of ordinary skill in the art upon reading this disclosure in its entirety.

[0020] Embodiments are illustrated by way of example in the accompanying FIGURES, in which like reference numbers indicate similar parts, and in which: FIGURE 1A is a schematic diagram illustrating a front view of light propagation in one example of a directional display device, which does not include all the features of the present invention but is useful for understanding; FIGURE 1B is a schematic diagram illustrating a side view of light propagation in the directional display device of FIGURE 1A; FIGURE 2A is a schematic diagram illustrating in a top view of light propagation in another example of a directional display device, which does not include all the features of the present invention but is useful for understanding; FIGURE 2B is a schematic diagram illustrating light propagation in a front view of the directional display device of FIGURE 2A; FIGURE 2C is a schematic diagram illustrating light propagation in a side view of the directional display device of FIGURE 2A; FIGURE 3 is a schematic diagram illustrating in a side view of a directional display device, which does not include all the features of the present invention but is useful for understanding; FIGURE 4A is a schematic diagram illustrating in a front view, generation of a viewing window in a directional display device including curved light extraction features, which does not include all the features of the present invention but is useful for understanding; FIGURE 4B is a schematic diagram illustrating in a front view, generation of a first and a second viewing window in a directional display device including curved light extraction features, which does not include all the features of the present invention but is useful for understanding; FIGURE 5 is a schematic diagram illustrating generation of a first viewing window in a directional display device including linear light extraction features, which does not include all the features of the present invention but is useful for understanding; FIGURE 6A is a schematic diagram illustrating one example of the generation of a first viewing window in a time multiplexed directional display device in a first time slot, which does not include all the features of the present invention but is useful for understanding; FIGURE 6B is a schematic diagram illustrating another example of the generation of a second viewing window in a time multiplexed directional display device in a second time slot, which does not include all the features of the present invention but is useful for understanding; FIGURE 6C is a schematic diagram illustrating another embodiment of the generation of a first and a second viewing window in a time multiplexed directional display device, which does not include all the features of the present invention but is useful for understanding; FIGURE 7 is a schematic diagram illustrating an observer tracking autostereoscopic directional display device, which does not include all the features of the present invention but is useful for understanding; FIGURE 8 is a schematic diagram illustrating a multi-viewer directional display device, which does not include all the features of the present invention but is useful for understanding; FIGURE 9 is a schematic diagram illustrating a privacy directional display device, which is not an embodiment of the invention but is useful for understanding; FIGURE 10 is a schematic diagram illustrating in side view, the structure of a time multiplexed directional display device, which does not include all the features of the present invention but is useful for understanding; FIGURE 11 is a schematic diagram illustrating a directional display apparatus comprising a directional display device and a control system, which does not include all the features of the present invention but is useful for understanding; FIGURE 12 is a schematic diagram illustrating in side view, the structure of a directional display device comprising a wedge waveguide, which does not include all the features of the present invention but is useful for understanding; FIGURE 13 is a schematic diagram illustrating in front view the origin of illumination void non-uniformities in a directional waveguide; FIGURE 14 is a schematic diagram illustrating in front view correction of illumination void non-uniformities in a directional waveguide, in accordance with the present invention; FIGURE 15 is a schematic diagram illustrating in perspective view correction of illumination void non-uniformities in a directional waveguide, in accordance with the present invention; FIGURE 16 is a schematic diagram illustrating a control method arranged to provide correction of void non-uniformities that can be used with the directional backlight according to the claimed invention; FIGURE 17 and FIGURE 18 are schematic diagrams illustrating in front view arrangements of LEDs arranged to achieve void A filling, in accordance with the present invention; FIGURE 19 and FIGURE 20 are schematic diagrams illustrating in front view arrangements of input aperture cross section comprising planar and structured surfaces arranged to achieve void B filling for a wide angular range; FIGURE 21 is a schematic diagram illustrating uniformity control using a partially reflective input side, which does not include all the features of the present invention but is presented by way of comparison; FIGURE 22 is a schematic diagram illustrating uniformity control using a partially reflective input side and side LEDs, in accordance with the present invention; FIGURE 23, FIGURE 24, FIGURE 25, and FIGURE 27 are schematic diagrams illustrating arrangements of LED connections to achieve correction of luminance uniformity, in accordance with the present invention; FIGURE 26 is a schematic diagram illustrating arrangements of LED connections to achieve correction of luminance uniformity, which does not include all the features of the present invention but is useful for understanding; FIGURE 28 is a schematic diagram illustrating a further graph of optical window luminous intensity against viewing position in the window plane of a waveguide; FIGURE 29 is a schematic diagram illustrating a graph of luminous flux distribution for an array of light sources to compensate for light source degradation; FIGURE 30 is a schematic diagram illustrating in side view a display and integrated camera arranged to provide in-field LED array calibration that can be used with the directional backlight according to the claimed invention; FIGURE 31 and FIGURE 32 are schematic diagrams illustrating in front view a waveguide and keyboard mounted detectors arranged to provide in-field LED calibration, that can be used with the directional backlight according to the claimed invention; FIGURE 33 is a schematic diagram illustrating a control system and front view of a directional backlight apparatus, in accordance with the present invention; FIGURE 34 is a schematic diagram illustrating a further graph of optical window luminous intensity against viewing position in the window plane of a waveguide comprising a luminous intensity defect; FIGURE 35 is a schematic diagram illustrating a graph of optical window luminous intensity against viewing position in the window plane of a waveguide further illustrating a correction of the defect of FIGURE 34; FIGURE 36 is a schematic diagram illustrating a graph of nominal LED output against time; FIGURE 37 is a schematic diagram illustrating a flowchart for compensation of LED ageing; FIGURE 38 is a schematic diagram illustrating in perspective view, the structure of a directional display device comprising an directional backlight arranged with a spatial light modulator, which does not include all the features of the present invention but is useful for understanding; FIGURE 39 is a schematic diagram illustrating in front view, an optical valve comprising a side light source arranged to achieve an on-axis optical window, which does not include all the features of the present invention but is useful for understanding; FIGURE 40 is a schematic diagram illustrating in side view, an optical valve comprising a side light source arranged to achieve an on-axis optical window, which does not include all the features of the present invention but is useful for understanding; FIGURE 41 is a schematic diagram illustrating in perspective view, the formation of first and second optical windows by edge and side light sources with a valve with arrangement similar to that shown in FIGURES39 and 40; and FIGURE 42 is a schematic diagram illustrating in perspective view, the structure of a directional display device comprising a directional backlight comprising a side light source arranged with a spatial light modulator, which does not include all the features of the present invention but is useful for understanding.

[0021] Time multiplexed autostereoscopic displays can advantageously improve the spatial resolution of autostereoscopic display by directing light from all of the pixels of a spatial light modulator to a first viewing window in a first time slot, and all of the pixels to a second viewing window in a second time slot. Thus an observer with eyes arranged to receive light in first and second viewing windows will see a full resolution image across the whole of the display over multiple time slots. Time multiplexed displays can advantageously achieve directional illumination by directing an illuminator array through a substantially transparent time multiplexed spatial light modulator using directional optical elements, wherein the directional optical elements substantially form an image of the illuminator array in the window plane.

[0022] The uniformity of the viewing windows may be advantageously independent of the arrangement of pixels in the spatial light modulator. Advantageously, such displays can provide observer tracking displays which have low flicker, with low levels of cross talk for a moving observer.

[0023] To achieve high uniformity in the window plane, it is desirable to provide an array of illumination elements that have a high spatial uniformity. The illuminator elements of the time sequential illumination system may be provided, for example, by pixels of a spatial light modulator with size approximately 100 micrometers in combination with a lens array. However, such pixels suffer from similar difficulties as for spatially multiplexed displays. Further, such devices may have low efficiency and higher cost, requiring additional display components.

[0024] High window plane uniformity can be conveniently achieved with macroscopic illuminators, for example, an array of LEDs in combination with homogenizing and diffusing optical elements that are typically of size 1 mm or greater. However, the increased size of the illuminator elements means that the size of the directional optical elements increases proportionately. For example, a 16 mm wide illuminator imaged to a 65 mm wide viewing window may require a 200 mm back working distance. Thus, the increased thickness of the optical elements can prevent useful application, for example, to mobile displays, or large area displays.

[0025] Addressing the aforementioned shortcomings, optical valves as described in commonly-owned US-2012 / 0127573, advantageously can be arranged in combination with fast switching transmissive spatial light modulators to achieve time multiplexed autostereoscopic illumination in a thin package while providing high resolution images with flicker free observer tracking and low levels of cross talk. Described is a one dimensional array of viewing positions, or windows, that can display different images in a first, typically horizontal, direction, but contain the same images when moving in a second, typically vertical, direction.

[0026] Conventional non-imaging display backlights commonly employ optical waveguides and have edge illumination from light sources such as LEDs. However, it should be appreciated that there are many fundamental differences in the function, design, structure, and operation between such conventional non-imaging display backlights and the imaging directional backlights discussed in the present disclosure.

[0027] Generally, for example, in accordance with the present disclosure, imaging directional backlights are arranged to direct the illumination from multiple light sources through a display panel to respective multiple viewing windows in at least one axis. Each viewing window is substantially formed as an image in at least one axis of a light source by the imaging system of the imaging directional backlight. An imaging system may be formed between multiple light sources and the respective window images. In this manner, the light from each of the multiple light sources is substantially not visible for an observer's eye outside of the respective viewing window.

[0028] In contradistinction, conventional non-imaging backlights or light guiding plates (LGPs) are used for illumination of 2D displays. See, e.g., Kälil Käläntär et al., Backlight Unit With Double Surface Light Emission, J. Soc. Inf. Display, Vol. 12, Issue 4, pp. 379-387 (Dec. 2004). Non-imaging backlights are typically arranged to direct the illumination from multiple light sources through a display panel into a substantially common viewing zone for each of the multiple light sources to achieve wide viewing angle and high display uniformity. Thus non-imaging backlights do not form viewing windows. In this manner, the light from each of the multiple light sources may be visible for an observer's eye at substantially all positions across the viewing zone. Such conventional non-imaging backlights may have some directionality, for example, to increase screen gain compared to Lambertian illumination, which may be provided by brightness enhancement films such as BEF ™< from 3M. However, such directionality may be substantially the same for each of the respective light sources. Thus, for these reasons and others that should be apparent to persons of ordinary skill, conventional non-imaging backlights are different to imaging directional backlights. Edge lit non-imaging backlight illumination structures may be used in liquid crystal display systems such as those seen in 2D Laptops, Monitors and TVs. Light propagates from the edge of a lossy waveguide which may include sparse features; typically local indentations in the surface of the guide which cause light to be lost regardless of the propagation direction of the light.

[0029] As used herein, an optical valve is an optical structure that may be a type of light guiding structure or device referred to as, for example, a light valve, an optical valve directional backlight, and a valve directional backlight ("v-DBL"). In the present disclosure, optical valve is different to a spatial light modulator (even though spatial light modulators may be sometimes generally referred to as a "light valve" in the art). One example of an imaging directional backlight is an optical valve that may employ a folded optical system. Light may propagate substantially without loss in one direction through the optical valve, may be incident on an imaging reflector, and may counter-propagate such that the light may be extracted by reflection off tilted light extraction features, and directed to viewing windows as described in US Pat. App. Ser. No. 13 / 300,293 (U.S. Pat. Publ. No. 2012 / 0127573).

[0030] Additionally, as used herein, a stepped waveguide imaging directional backlight may be at least one of an optical valve. A stepped waveguide is a waveguide for an imaging directional backlight comprising a waveguide for guiding light, further comprising: a first light guiding surface; and a second light guiding surface, opposite the first light guiding surface, further comprising a plurality of light guiding features interspersed with a plurality of extraction features arranged as steps.

[0031] In operation, light may propagate within an exemplary optical valve in a first direction from an input side to a reflective side and may be transmitted substantially without loss. Light may be reflected at the reflective side and propagates in a second direction substantially opposite the first direction. As the light propagates in the second direction, the light may be incident on light extraction features, which are operable to redirect the light outside the optical valve. Stated differently, the optical valve generally allows light to propagate in the first direction and may allow light to be extracted while propagating in the second direction.

[0032] The optical valve may achieve time sequential directional illumination of large display areas. Additionally, optical elements may be employed that are thinner than the back working distance of the optical elements to direct light from macroscopic illuminators to a window plane. Such displays may use an array of light extraction features arranged to extract light counter propagating in a substantially parallel waveguide.

[0033] Thin imaging directional backlight implementations for use with LCDs have been proposed and demonstrated by 3M, for example US-7,528,893; by Microsoft, for example US-7,970,246 which may be referred to herein as a "wedge type directional backlight;" by RealD, for example US-2012 / 0127573 which may be referred to herein as an "optical valve" or "optical valve directional backlight".

[0034] The present disclosure provides stepped waveguide imaging directional backlights in which light may reflect back and forth between the internal faces of, for example, a stepped waveguide which may include a first side and a first set of features. As the light travels along the length of the stepped waveguide, the light may not substantially change angle of incidence with respect to the first side and first set of surfaces and so may not reach the critical angle of the medium at these internal faces. Light extraction may be advantageously achieved by a second set of surfaces (the step "risers") that are inclined to the first set of surfaces (the step "treads"). Note that the second set of surfaces may not be part of the light guiding operation of the stepped waveguide, but may be arranged to provide light extraction from the structure. By contrast, a wedge type imaging directional backlight may allow light to guide within a wedge profiled waveguide having continuous internal surfaces. The optical valve is thus not a wedge type imaging directional backlight.

[0035] FIGURE 1A is a schematic diagram illustrating a front view of light propagation in one example of a directional display device, which does not include all the features of the present invention but is useful for understanding, and FIGURE 1B is a schematic diagram illustrating a side view of light propagation in the directional display device of FIGURE 1A.

[0036] FIGURE 1A illustrates a front view in the xy plane of a directional backlight of a directional display device, and includes an illuminator array 15 which may be used to illuminate a stepped waveguide 1. Illuminator array 15 includes illuminator elements 15a through illuminator element 15n (where n is an integer greater than one). In one example, the stepped waveguide 1 of FIGURE 1A may be a stepped, display sized waveguide 1. Illumination elements 15a through 15n are light sources that may be light emitting diodes (LEDs). Although LEDs are discussed herein as illuminator elements 15a - 15n, other light sources may be used such as, but not limited to, diode sources, semiconductor sources, laser sources, local field emission sources, organic emitter arrays, and so forth. Additionally, FIGURE 1B illustrates a side view in the xz plane, and includes illuminator array 15, SLM 48, extraction features 12, guiding features 10, and stepped waveguide 1, arranged as shown. The side view provided in FIGURE 1B is an alternative view of the front view shown in FIGURE 1A. Accordingly, the illuminator array 15 of FIGURES 1A and 1B corresponds to one another and the stepped waveguide 1 of FIGURES 1A and 1B may correspond to one another.

[0037] Further, in FIGURE 1B, the stepped waveguide 1 may have an input end 2 that is thin and a reflective end 4 that is thick. Thus the waveguide 1 extends between the input end 2 that receives input light and the reflective end 4 that reflects the input light back through the waveguide 1. The length of the input end 2 in a lateral direction across the waveguide is greater than the height of the input end 2. The illuminator elements 15a - 15n are disposed at different input positions in a lateral direction across the input end 2.

[0038] The waveguide 1 has first and second, opposed guide surfaces extending between the input end 2 and the reflective end 4 for guiding light forwards and back along the waveguide 1. The second guide surface has a plurality of light extraction features 12 facing the reflective end 4 and arranged to reflect at least some of the light guided back through the waveguide 1 from the reflective end from different input positions across the input end in different directions through the first guide surface that are dependent on the input position.

[0039] In this example, the light extraction features 12 are reflective facets, although other reflective features could be used. The light extraction features 12 do not guide light through the waveguide, whereas the intermediate regions of the second guide surface intermediate the light extraction features 12 guide light without extracting it. Those regions of the second guide surface are planar and may extend parallel to the first guide surface, or at a relatively low inclination. The light extraction features 12 extend laterally to those regions so that the second guide surface has a stepped shape which may include the light extraction features 12 and intermediate regions. The light extraction features 12 are oriented to reflect light from the light sources, after reflection from the reflective end 4, through the first guide surface.

[0040] The light extraction features 12 are arranged to direct input light from different input positions in the lateral direction across the input end in different directions relative to the first guide surface that are dependent on the input position. As the illumination elements 15a-15n are arranged at different input positions, the light from respective illumination elements 15a-15n is reflected in those different directions. In this manner, each of the illumination elements 15a-15n directs light into a respective optical window in output directions distributed in the lateral direction in dependence on the input positions. The lateral direction across the input end 2 in which the input positions are distributed corresponds with regard to the output light to a lateral direction to the normal to the first guide surface. The lateral directions as defined at the input end 2 and with regard to the output light remain parallel in this embodiment where the deflections at the reflective end 4 and the first guide surface are generally orthogonal to the lateral direction.. Under the control of a control system, the illuminator elements 15a - 15n may be selectively operated to direct light into a selectable optical window. The optical windows may be used individually or in groups as viewing windows.

[0041] The SLM 48 extends across the waveguide and modulates the light output therefrom. Although the SLM 48 may a liquid crystal display (LCD), this is merely by way of example and other spatial light modulators or displays may be used including LCOS, DLP devices, and so forth, as this illuminator may work in reflection. In this example, the SLM 48 is disposed across the first guide surface of the waveguide and modulates the light output through the first guide surface after reflection from the light extraction features 12.

[0042] The operation of a directional display device that may provide a one dimensional array of viewing windows is illustrated in front view in FIGURE 1A, with its side profile shown in FIGURE 1B. In operation, in FIGURES 1A and 1B, light may be emitted from an illuminator array 15, such as an array of illuminator elements 15a through 15n, located at different positions, y, along the surface of thin end side 2, x=0, of the stepped waveguide 1. The light may propagate along +x in a first direction, within the stepped waveguide 1, while at the same time, the light may fan out in the xy plane and upon reaching the far curved end side 4, may substantially or entirely fill the curved end side 4. While propagating, the light may spread out to a set of angles in the xz plane up to, but not exceeding the critical angle of the guide material. The extraction features 12 that link the guiding features 10 of the bottom side of the stepped waveguide 1 may have a tilt angle greater than the critical angle and hence may be missed by substantially all light propagating along +x in the first direction, ensuring the substantially lossless forward propagation.

[0043] Continuing the discussion of FIGURES 1A and 1B, the curved end side 4 of the stepped waveguide 1 may be made reflective, typically by being coated with a reflective material such as, for example, silver, although other reflective techniques may be employed. Light may therefore be redirected in a second direction, back down the guide in the direction of -x and may be substantially collimated in the xy or display plane. The angular spread may be substantially preserved in the xz plane about the principal propagation direction, which may allow light to hit the riser edges and reflect out of the guide. In an embodiment with approximately 45 degree tilted extraction features 12, light may be effectively directed approximately normal to the xy display plane with the xz angular spread substantially maintained relative to the propagation direction. This angular spread may be increased when light exits the stepped waveguide 1 through refraction, but may be decreased somewhat dependent on the reflective properties of the extraction features 12.

[0044] In some embodiments with uncoated extraction features 12, reflection may be reduced when total internal reflection (TIR) fails, squeezing the xz angular profile and shifting off normal. However, in other embodiments having silver coated or metallized extraction features, the increased angular spread and central normal direction may be preserved. Continuing the description of the embodiment with silver coated extraction features, in the xz plane, light may exit the stepped waveguide 1 approximately collimated and may be directed off normal in proportion to the y-position of the respective illuminator element 15a - 15n in illuminator array 15 from the input edge center. Having independent illuminator elements 15a - 15n along the input edge 2 then enables light to exit from the entire first light directing side 6 and propagate at different external angles, as illustrated in FIGURE 1A.

[0045] Illuminating a spatial light modulator (SLM) 48 such as a fast liquid crystal display (LCD) panel with such a device may achieve autostereoscopic 3D as shown in top view oryz-plane viewed from the illuminator array 15 end in FIGURE 2A, front view in FIGURE 2B and side view in FIGURE 2C. FIGURE 2A is a schematic diagram illustrating in a top view, propagation of light in a directional display device, which does not include all the features of the present invention but is useful for understanding, FIGURE 2B is a schematic diagram illustrating in a front view, propagation of light in a directional display device, and FIGURE 2C is a schematic diagram illustrating in side view propagation of light in a directional display device. As illustrated in FIGURES 2A, 2B, and 2C, a stepped waveguide 1 may be located behind a fast (e.g., greater than 100Hz) LCD panel SLM 48 that displays sequential right and left eye images. In synchronization, specific illuminator elements 15a through 15n of illuminator array 15 (where n is an integer greater than one) may be selectively turned on and off, providing illuminating light that enters right and left eyes substantially independently by virtue of the system's directionality. In the simplest case, sets of illuminator elements of illuminator array 15 are turned on together, providing a one dimensional viewing window 26 or an optical pupil with limited width in the horizontal direction, but extended in the vertical direction, in which both eyes horizontally separated may view a left eye image, and another viewing window 44 in which a right eye image may primarily be viewed by both eyes, and a central position in which both the eyes may view different images. In this way, 3D may be viewed when the head of a viewer is approximately centrally aligned. Movement to the side away from the central position may result in the scene collapsing onto a 2D image.

[0046] The reflective end 4 may have positive optical power in the lateral direction across the waveguide. In embodiments in which typically the reflective end 4 has positive optical power, the optical axis may be defined with reference to the shape of the reflective end 4, for example being a line that passes through the center of curvature of the reflective end 4 and coincides with the axis of reflective symmetry of the end 4 about the x-axis. In the case that the reflecting surface 4 is flat, the optical axis may be similarly defined with respect to other components having optical power, for example the light extraction features 12 if they are curved, or the Fresnel lens 62 described below. The optical axis 238 is typically coincident with the mechanical axis of the waveguide 1. In the present embodiments that typically comprise a substantially cylindrical reflecting surface at end 4, the optical axis 238 is a line that passes through the center of curvature of the surface at end 4 and coincides with the axis of reflective symmetry of the side 4 about the x-axis. The optical axis 238 is typically coincident with the mechanical axis of the waveguide 1. The cylindrical reflecting surface at end 4 may typically comprise a spherical profile to optimize performance for on-axis and off-axis viewing positions. Other profiles may be used.

[0047] FIGURE 3 is a schematic diagram illustrating in side view a directional display device, which does not include all the features of the present invention but is useful for understanding. Further, FIGURE 3 illustrates additional detail of a side view of the operation of a stepped waveguide 1, which may be a transparent material. The stepped waveguide 1 may include an illuminator input side 2, a reflective side 4, a first light directing side 6 which may be substantially planar, and a second light directing side 8 which includes guiding features 10 and light extraction features 12. In operation, light rays 16 from an illuminator element 15c of an illuminator array 15 (not shown in FIGURE 3), that may be an addressable array of LEDs for example, may be guided in the stepped waveguide 1 by means of total internal reflection by the first light directing side 6 and total internal reflection by the guiding feature 10, to the reflective side 4, which may be a mirrored surface. Although reflective side 4 may be a mirrored surface and may reflect light, it may in some embodiments also be possible for light to pass through reflective side 4.

[0048] Continuing the discussion of FIGURE 3, light ray 18 reflected by the reflective side 4 may be further guided in the stepped waveguide 1 by total internal reflection at the reflective side 4 and may be reflected by extraction features 12. Light rays 18 that are incident on extraction features 12 may be substantially deflected away from guiding modes of the stepped waveguide 1 and may be directed, as shown by ray 20, through the side 6 to an optical pupil that may form a viewing window 26 of an autostereoscopic display. The width of the viewing window 26 may be determined by at least the size of the illuminator, output design distance and optical power in the side 4 and extraction features 12. The height of the viewing window may be primarily determined by the reflection cone angle of the extraction features 12 and the illumination cone angle input at the input side 2. Thus each viewing window 26 represents a range of separate output directions with respect to the surface normal direction of the spatial light modulator 48 that intersect with a plane at the nominal viewing distance.

[0049] FIGURE 4A is a schematic diagram illustrating in front view a directional display device which may be illuminated by a first illuminator element and including curved light extraction features, which does not include all the features of the present invention but is useful for understanding. Further, FIGURE 4A shows in front view further guiding of light rays from illuminator element 15c of illuminator array 15, in the stepped waveguide 1. Each of the output rays are directed towards the same viewing window 26 from the respective illuminator 14. Thus light ray 30 may intersect the ray 20 in the window 26, or may have a different height in the window as shown by ray 32. Additionally, in various embodiments, sides 22, 24 of the waveguide 1 may be transparent, mirrored, or blackened surfaces. Continuing the discussion of FIGURE 4A, light extraction features 12 may be elongate, and the orientation of light extraction features 12 in a first region 34 of the light directing side 8 (light directing side 8 shown in FIGURE 3, but not shown in FIGURE 4A) may be different to the orientation of light extraction features 12 in a second region 36 of the light directing side 8.

[0050] FIGURE 4B is a schematic diagram illustrating in front view an optical valve which may illuminated by a second illuminator element, which does not include all the features of the present invention but is useful for understanding. Further, FIGURE 4B shows the light rays 40, 42 from a second illuminator element 15h of the illuminator array 15. The curvature of the reflective end on the side 4 and the light extraction features 12 cooperatively produce a second viewing window 44 laterally separated from the viewing window 26 with light rays from the illuminator element 15h.

[0051] Advantageously, the arrangement illustrated in FIGURE 4B may provide a real image of the illuminator element 15c at a viewing window 26 in which the real image may be formed by cooperation of optical power in reflective side 4 and optical power which may arise from different orientations of elongate light extraction features 12 between regions 34 and 36, as shown in FIGURE 4A. The arrangement of FIGURE 4B may achieve improved aberrations of the imaging of illuminator element 15c to lateral positions in viewing window 26. Improved aberrations may achieve an extended viewing freedom for an autostereoscopic display while achieving low cross talk levels.

[0052] FIGURE 5 is a schematic diagram illustrating in front view an embodiment of a directional display device having substantially linear light extraction features, which does not include all the features of the present invention but is useful for understanding. Further, FIGURE 5 shows a similar arrangement of components to FIGURE 1 (with corresponding elements being similar), with one of the differences being that the light extraction features 12 are substantially linear and parallel to each other. Advantageously, such an arrangement may provide substantially uniform illumination across a display surface and may be more convenient to manufacture than the curved extraction features of FIGURE 4A and FIGURE 4B.

[0053] FIGURE 6A is a schematic diagram illustrating one embodiment of the generation of a first viewing window in a time multiplexed imaging directional display device in a first time slot, FIGURE 6B is a schematic diagram illustrating another embodiment of the generation of a second viewing window in a time multiplexed imaging directional backlight apparatus in a second time slot, and FIGURE 6C is a schematic diagram illustrating another embodiment of the generation of a first and a second viewing window in a time multiplexed imaging directional display device, which do not include all the features of the present invention but are useful for understanding. Further, FIGURE 6A shows schematically the generation of illumination window 26 from stepped waveguide 1. Illuminator element group 31 in illuminator array 15 may provide a light cone 17 directed towards a viewing window 26. FIGURE 6B shows schematically the generation of illumination window 44. Illuminator element group 33 in illuminator array 15 may provide a light cone 19 directed towards viewing window 44. In cooperation with a time multiplexed display, windows 26 and 44 may be provided in sequence as shown in FIGURE 6C. If the image on a spatial light modulator 48 (not shown in FIGURES 6A, 6B, 6C) is adjusted in correspondence with the light direction output, then an autostereoscopic image may be achieved for a suitably placed viewer. Similar operation can be achieved with all the directional backlights described herein. Note that illuminator element groups 31, 33 each include one or more illumination elements from illumination elements 15a to 15n, where n is an integer greater than one.

[0054] FIGURE 7 is a schematic diagram illustrating one embodiment of an observer tracking autostereoscopic directional display device, which does not include all the features of the present invention but is useful for understanding. As shown in FIGURE 7, selectively turning on and off illuminator elements 15a to 15n along axis 29 provides for directional control of viewing windows. The head 45 position may be monitored with a camera, motion sensor, motion detector, or any other appropriate optical, mechanical or electrical means, and the appropriate illuminator elements of illuminator array 15 may be turned on and off to provide substantially independent images to each eye irrespective of the head 45 position. The head tracking system (or a second head tracking system) may provide monitoring of more than one head 45, 47 (head 47 not shown in FIGURE 7) and may supply the same left and right eye images to each viewers' left and right eyes providing 3D to all viewers. Again similar operation can be achieved with all the directional backlights described herein.

[0055] FIGURE 8 is a schematic diagram illustrating one embodiment of a multi-viewer directional display device as an example including an imaging directional backlight, which does not include all the features of the present invention but is useful for understanding. As shown in FIGURE 8, at least two 2D images may be directed towards a pair of viewers 45, 47 so that each viewer may watch a different image on the spatial light modulator 48. The two 2D images of FIGURE 8 may be generated in a similar manner as described with respect to FIGURE 7 in that the two images would be displayed in sequence and in synchronization with sources whose light is directed toward the two viewers. One image is presented on the spatial light modulator 48 in a first phase, and a second image is presented on the spatial light modulator 48 in a second phase different from the first phase. In correspondence with the first and second phases, the output illumination is adjusted to provide first and second viewing windows 26, 44 respectively. An observer with both eyes in window 26 will perceive a first image while an observer with both eyes in window 44 will perceive a second image.

[0056] FIGURE 9 is a schematic diagram illustrating a privacy directional display device which includes an imaging directional backlight, which does not include all the features of the present invention but is useful for understanding. 2D display systems may also utilize directional backlighting for security and efficiency purposes in which light may be primarily directed at the eyes of a first viewer 45 as shown in FIGURE 9. Further, as illustrated in FIGURE 9, although first viewer 45 may be able to view an image on device 50, light is not directed towards second viewer 47. Thus second viewer 47 is prevented from viewing an image on device 50. Each of the embodiments of the present disclosure may advantageously provide autostereoscopic, dual image or privacy display functions.

[0057] FIGURE 10 is a schematic diagram illustrating in side view the structure of a time multiplexed directional display device as an example including an imaging directional backlight, which does not include all the features of the present invention but is useful for understanding. Further, FIGURE 10 shows in side view an autostereoscopic directional display device, which may include the stepped waveguide 1 and a Fresnel lens 62 arranged to provide the viewing window 26 in a window plane 106 at a nominal viewing distance from the spatial light modulator for a substantially collimated output across the stepped waveguide 1 output surface. A vertical diffuser 68 may be arranged to extend the height of the window 26 further. The light may then be imaged through the spatial light modulator 48. The illuminator array 15 may include light emitting diodes (LEDs) that may, for example, be phosphor converted blue LEDs, or may be separate RGB LEDs. Alternatively, the illuminator elements in illuminator array 15 may include a uniform light source and spatial light modulator arranged to provide separate illumination regions. Alternatively the illuminator elements may include laser light source(s). The laser output may be directed onto a diffuser by means of scanning, for example, using a galvo or MEMS scanner. In one example, laser light may thus be used to provide the appropriate illuminator elements in illuminator array 15 to provide a substantially uniform light source with the appropriate output angle, and further to provide reduction in speckle. Alternatively, the illuminator array 15 may be an array of laser light emitting elements. Additionally in one example, the diffuser may be a wavelength converting phosphor, so that illumination may be at a different wavelength to the visible output light.

[0058] A further wedge type directional backlight is generally discussed by US-7,660,047. The wedge type directional backlight and optical valve further process light beams in different ways. In the wedge type waveguide, light input at an appropriate angle will output at a defined position on a major surface, but light rays will exit at substantially the same angle and substantially parallel to the major surface. By comparison, light input to a stepped waveguide of an optical valve at a certain angle may output from points across the first side, with output angle determined by input angle. Advantageously, the stepped waveguide of the optical valve may not require further light re-direction films to extract light towards an observer and angular non-uniformities of input may not provide non-uniformities across the display surface.

[0059] There will now be described some waveguides, directional backlights and directional display devices that are based on and incorporate the structures of FIGURES 1 to 10 above. Except for the modifications and / or additional features which will now be described, the above description applies equally to the following waveguides, directional backlights and display devices, but for brevity will not be repeated. The waveguides described below may be incorporated into a directional backlight or a directional display device as described above. Similarly, the directional backlights described below may be incorporated into a directional display device as described above.

[0060] FIGURE 11 is a schematic diagram illustrating a directional display apparatus comprising a directional display device and a control system, which does not include all the features of the present invention but is useful for understanding. The arrangement and operation of the control system will now be described and may be applied, with changes as necessary, to each of the display devices disclosed herein. The directional backlight comprises a waveguide 1 and an array 15 of illumination elements 15a-15n arranged as described above. The control system is arranged to selectively operate the illumination elements 15a-15n to direct light into selectable viewing windows.

[0061] The reflective end 4 converges the reflected light. Fresnel lens 62 may be arranged to cooperate with reflective end 4 to achieve viewing windows at a viewing plane. Transmissive spatial light modulator 48 may be arranged to receive the light from the directional backlight. The image displayed on the SLM 48 may be presented in synchronization with the illumination of the light sources of the array 15.

[0062] The control system may comprise a sensor system arranged to detect the position of the observer 99 relative to the display device 100. The sensor system comprises a position sensor 406, such as a camera arranged to determine the position of an observer 408; and a head position measurement system 404 that may for example comprise a computer vision image processing system. The position sensor 406 may comprise known sensors including those comprising cameras and image processing units arranged to detect the position of observer faces. Position sensor 406 may further comprise a stereo sensor arranged to improve the measure of longitudinal position compared to a monoscopic camera. Alternatively position sensor 406 may comprise measurement of eye spacing to give a measure of required placement of respective arrays of viewing windows from tiles of the directional display.

[0063] The control system may further comprise an illumination controller and an image controller 403 that are both supplied with the detected position of the observer supplied from the head position measurement system 404.

[0064] The illumination controller comprises an LED controller 402 arranged to determine which light sources of array 15 should be switched to direct light to respective eyes of observer 408 in cooperation with waveguide 1; and an LED driver 400 arranged to control the operation of light sources of light source array 15 by means of drive lines 407. The illumination controller 74 selects the illuminator elements 15 to be operated in dependence on the position of the observer detected by the head position measurement system 72, so that the viewing windows 26 into which light is directed are in positions corresponding to the left and right eyes of the observer 99. In this manner, the lateral output directionality of the waveguide 1 corresponds with the observer position.

[0065] The image controller 403 is arranged to control the SLM 48 to display images. To provide an autostereoscopic display, the image controller 403 and the illumination controller may operate as follows. The image controller 403 controls the SLM 48 to display temporally multiplexed left and right eye images and the LED controller 402 operates the light sources 15 to direct light into viewing windows in positions corresponding to the left and right eyes of an observer synchronously with the display of left and right eye images. In this manner, an autostereoscopic effect is achieved using a time division multiplexing technique. In one example, a single viewing window may be illuminated by operation of light source 409 (which may comprise one or more LEDs) by means of drive line 410 wherein other drive lines are not driven as described elsewhere.

[0066] The head position measurement system 404 detects the position of an observer relative to the display device 100. The LED controller 402 selects the light sources 15 to be operated in dependence on the position of the observer detected by the head position measurement system 404, so that the viewing windows into which light is directed are in positions corresponding to the left and right eyes of the observer. In this manner, the output directionality of the waveguide 1 may be achieved to correspond with the viewer position so that a first image may be directed to the observer's right eye in a first phase and directed to the observer's left eye in a second phase.

[0067] FIGURE 12 is a schematic diagram illustrating in side view, the structure of a directional display device, which does not include all the features of the present invention but is useful for understanding. The directional display device comprises a wedge directional backlight comprising a wedge waveguide 1104 with faceted mirror end 1102. The first guide surface 1105 of the waveguide 1104 is arranged to guide light by total internal reflection and the second guide surface 1106 is substantially planar and inclined at an angle to direct light in directions that break the total internal reflection for outputting light through the first guide surface 1105. The display device further comprises a deflection element 1108 extending across the first guide surface 1105 of the waveguide 1104 for deflecting light from array 1101 of light sources towards the normal to the first guide surface 1105. Further the waveguide 1104 may further comprise a reflective end 1102 for reflecting input light back through the waveguide 1104, the second guide 1106 surface being arranged to deflect light as output light through the first guide surface 1105 after reflection from the reflective end 1102. The reflective end has positive optical power in the lateral direction (y-axis) in a similar manner to the reflective end shown in FIGURE 5 for example. Further facets in the reflective end 1102 deflect the reflected light cones within the waveguide 1104 to achieve output coupling on the return path. Thus viewing windows are produced in a similar manner to that shown in FIGURE 11. Further the directional display may comprise a spatial light modulator 1110 and parallax element 1100 aligned to the spatial light modulator 1110 that is further arranged to provide optical windows. A control system 72 similar to that shown in FIGURE 11 may be arranged to provide control of directional illumination providing viewing windows 26 and windows 109 from the parallax element and aligned spatial light modulator.

[0068] Thus a first guide surface may be arranged to guide light by total internal reflection and the second guide surface may be substantially planar and inclined at an angle to direct light in directions that break that total internal reflection for outputting light through the first guide surface, and the display device may further comprise a deflection element extending across the first guide surface of the waveguide for deflecting light towards the normal to the first guide surface.

[0069] FIGURE 13 is a schematic diagram illustrating in front view the origin of illumination void non-uniformities in a directional waveguide. As illustrated above for on-axis light sources of array 15, those close to the optical axis 199 of the Fresnel reflector, then uniformity can be controlled by means of compensation of Fresnel mirror efficiency roll-off. For off-axis sources, then additional void regions are provided. Void A, 300 is provided by light that is outside a cone angle subtended by the light source and adjacent edge of the Fresnel reflector. Boundary 301 separates void A from the main illumination region. Void B, 302 is provided by light rays that are outside the critical angle θc of the light entering the waveguide for a light source in air. Boundary 303 separates void B from the main illumination region. Both voids create undesirable non-uniformities for off-axis viewing positions.

[0070] FIGURES 14 and 15 are schematic diagrams illustrating in front view and perspective view, respectively, correction of illumination void non-uniformities in a directional waveguide, in accordance with the present invention. As will be described, in FIGURE 14, void A that is provided for example by input light source 15e on the right side of the optical axis 199 can be compensated by light source array 17 arranged on the left side 22 of the waveguide 1. Void B can be compensated by modification of the structure of the input end. Thus a directional backlight may comprise a waveguide 1 comprising an input end 2; an array 15 of input light sources arranged at different input positions in a lateral direction across the input end 2 of the waveguide 1 and arranged to input input light into the waveguide 1, the waveguide 1 further comprising first and second opposed, laterally extending guide surfaces 6,8 for guiding light along the waveguide 1, side surfaces 22, 24 extending between the first and second guide surfaces 6,8 , and a reflective end 4 having positive optical power facing the input end 2 for reflecting the input light back along the waveguide 1, the second guide surface 8 being arranged to deflect the reflected input light through the first guide surface 6 as output light, and the waveguide 1 being arranged to direct the output light into optical windows 26a-n in output directions that are distributed in a lateral direction in dependence on the input position of the input light; and additional light sources 17a-n arranged to direct additional light into the waveguide 1 in a direction in which the additional light is reflected by the reflective end 4 onto the opposite side surface 24 and by the opposite side surface 24 into a segment of the waveguide 1 adjacent the opposite side surface 24 extending from a corner between the reflective surface 4 and the side surface 24.

[0071] Similarly the additional light sources 19a-n may be arranged to direct additional light into the waveguide 1 through one of the side surfaces 24 in a direction in which the additional light is reflected by the reflective end 4 onto the opposite side surface 222 and by the opposite side surface 22 into a segment of the waveguide 1 adjacent the opposite side surface 22 extending from a corner between the reflective surface 4 and the side surface 22.

[0072] Region 305 between lines 301, 303 is illuminated by light source 15. Advantageously void A, 300 may be filled in a controllable manner by adjustment of the flux from the light sources 17, for a wide range of viewing positions. Further the angular illumination profile of the output windows may be controlled to provide a wide angle mode of similar or better performance compared to conventional waveguides 1.

[0073] As illustrated in FIGURE 15, which is in accordance with the present invention, light source 15e may be arranged to provide optical window 26e from region 305 of the waveguide 1. Light source 15e may be arranged to the right of the optical axis 199 when viewed from the position of the optical window 26e. Further light source 17e may be arranged to substantially illuminate optical window 26e from void A regions 300 of the waveguide 1. Void correcting light source 17e is to the left of the optical axis 199 when viewed from the front. In practice, optical aberrations will create an overlap of optical windows 26a-n from sources 15a-n and sources 17a-n, 19a-n that is not identical but similar. Illumination of multiple sources provides a final uniform illuminated waveguide with desirable angular and spatial uniformity characteristics.

[0074] FIGURE 16 is a schematic diagram illustrating a control method arranged to provide correction of void non-uniformities in a directional waveguide, that can be used with the directional backlight according to the claimed invention. The control system illustrated by steps 890, 892, 894, 896, 897, 898 is arranged to control input light sources 15e selected to direct output light into desired optical windows 26a-n, and is further arranged to control at least one additional light source 17a-n, 19a-n selected to provide additional light that is output from the directional backlight in substantially the same output directions as the desired optical windows 26a-n. Thus the control system is arranged, when a selected input light source is off-center of the array of input light surfaces, to control at least one additional light source that is on the opposite side of the directional backlight from the selected input light source. The control system may be used to determine look up tables for LED flux control in various modes of operation such as wide angle, privacy, 3D and power savings.

[0075] It would be desirable to reduce cost and complexity by minimizing the number of light sources 17, 19 on the sides 22, 24 of the waveguide 1.

[0076] FIGURES 17 and 18 are schematic diagrams illustrating in front view arrangements of LEDs arranged to achieve void A filling, in accordance with the present invention. FIGURE 18 is a detail of the corner of FIGURE 17.

[0077] The additional light sources 17a-n, 19a-n may be disposed along at least a part 877, 879 of each side surface 22, 24 adjacent the input end 2, the additional light sources 17a-n, 19a-n being arranged to direct additional light into the waveguide 1 through one of the side surfaces 22, 24.

[0078] Typically it is desirable to provide void A, 300 filling for optical windows 26 that are up to approximately 45 degrees off-axis in a lateral direction. At much higher angles, general diffusion and stray light in the waveguide 1 and diffuser elements may be provided to achieve void A, 300 filling and window 26 provision. Such a window direction requires the uppermost light source 17n to create a ray 307 that has an angle θ to the side 24 that may be approximately 30 degrees inside the waveguide in the x-y plane. In an illustrative embodiment, a waveguide 1 with 16:9 aspect ratio may be provided with an edge facet 220 facet angle θef of approximately 20degrees. The angle θ1 is given by: θ 1 = θ + 2 . θ ef

[0079] To continue the illustrative embodiment θ1 may be 70 degrees. The relative size of the region of the side 22 that is free of light sources 17a-n may then be determined as Δ = W / L . tan θ 1

[0080] Where W is the waveguide 1 width (in y direction) and L is its height (in x direction). Thus for the 16:9 arrangement of waveguide 1, Δ = 65% and the light sources 17 are required for 35% of the height of the waveguide 1 from the input side. When θ = θc then the light exits the waveguide parallel to the surface 6. Thus the minimum Δ is approximately 25%.

[0081] Thus said part 877, 879 of each side surface along which the additional light sources are disposed is at least 20% of the side surface from the input end 2 and preferably at least 40% of the side surface. Said part 877, 879 of each side surface along which the additional light sources are disposed is at most 80% of the side surface from the input end 2 and preferably at most 60% of the side surface.

[0082] Advantageously void A may be filled and the number of light sources 17, 19 may be reduced, minimizing cost and optimizing uniformity. In operation, light sources 17, 19 are arranged to provide illumination for off-axis optical windows in wide angle operation. Advantageously, the flux provided by such LEDs may be minimized due to the relatively low illumination levels at high angles compared to the on-axis LEDs of array 15.

[0083] It would be desirable to provide filling of Void B, 332 to optimize uniformity in an efficient manner.

[0084] FIGURES 19 and 20 are schematic diagrams illustrating in front view arrangements of input side cross section comprising planar and structured surfaces arranged to achieve illumination void B filling for a wide angular range, which do not include all the features of the present invention but are useful for understanding.

[0085] FIGURE 19 illustrates that the input facets 326 may be provided with either planar (P) with polar profiles limited by the critical angle or structured (S) surfaces with polar profiles that may be greater than the critical angle. The P and S surfaces may be aligned with respective addressable light sources 15x and 15y. The angular profiles in a particular direction may be controlled by the surface profile, thus polar profiles 324, 330 may be provided. Thus for optical windows 26 in a particular angular direction, the amount or light directed may comprise a mixture of cones 324 and 330, enabling tuning of actual light distribution in said direction. The axis of said direction may be determined by the slope 325 of the P or S surface.

[0086] FIGURE 20 illustrates one half of the input side 2 of a waveguide 1. The central facets in region 331, corresponding to central LEDs may be planar (P) type, thus achieving high efficiency of illumination of the reflective end 4. In the outer region 335, higher angular distributions of light may be desirable so that S type surfaces may be used. In intermediate region 333, some control of illumination may be desirable so that interlaced pairs of S and P surfaces, with aligned addressable light sources 15x, 15y may be provided to advantageously achieve control of uniformity at mid optical window 26 positions.

[0087] Advantageously the filling of Void B may be controlled for a wide range of optical window positions in the lateral direction. Both angular profile uniformity and image uniformity from a given angular direction may be achieved.

[0088] It would be desirable to reduce the number of light sources on the input side 2 to reduced cost and physical size of the backlight unit while maintaining desirable uniformity by means of void filling.

[0089] FIGURE 21 is a schematic diagram illustrating uniformity control using a partially reflective input side, which does not include all the features of the present invention but is presented by way of comparison. Light source array 15a-n may be arranged in a first region 905 while the input side 2 may be reflective in region 900. Further portion 901 of the sides 22, 24 may be reflective. Thus light rays 902 that are incident on the input side 2 are reflected by the reflective regions 900 with a profile arranged to partially illuminate side regions 901 and re-reflect to the mirror end 4. Rays 906 may be provided that fill voids for off axis position and advantageously uniform illumination is provided for reduced cost and complexity /

[0090] FIGURE 22 is a schematic diagram illustrating uniformity control using a partially reflective input side and side LEDs, in accordance with the present invention. Increased illumination output can be achieved in void regions using a combination of reflective input side 2 and side mounted light source arrays 17a-n, 19a-n. Advantageously improved uniformity control can be achieved compared to the embodiment of FIGURE 21.

[0091] FIGURE 23 is a schematic diagram illustrating in front view arrangements of LED for use in cooperation with the waveguide input of FIGURE 19 arranged to achieve void B filling, in accordance with the present invention. Light source arrays 15, 19 connected by means of electrodes 251 may be provided by LEDs that are arranged in multiple groups comprising pairs 250a-250c in region 331, pairs 250d-250g in region 333 and pairs 250h-250l in region 335. Similarly light source array 19 may comprise LEDs 19a-19n arranged in pairs 252a-252d that extend approximately 1 / 3 of the height of the waveguide 1 from in the input side.

[0092] The separation of the LEDs 15a-n and 19a-n may advantageously be non-uniform. The diffusion properties of elements in the backlight may be angularly dependent, thus higher separations may be provided at higher angular input positions. Further, higher lumens per millimeters can desirably be provided in the central region of the waveguide 1.

[0093] Advantageously the number of LEDs and the number of addressable channels of the control system may be reduced, reducing cost while maintaining angular and spatial uniformity performance.

[0094] FIGURE 24 is a schematic diagram illustrating an arrangement of LED connections to achieve correction of luminance uniformity, in accordance with the present invention. FIGURE 24 illustrates an arrangement similar to FIGURE 23 wherein the complete array of 64 LEDs is shown, using 32 connections with LEDs arranged in pairs. Advantageously the number of connections is reduced compared to individual LED control.

[0095] FIGURE 25 is a schematic diagram illustrating an arrangement of LED connections to further reduce the number of connections, in accordance with the present invention. By means of providing matching symmetric strings of 4 LEDs such an arrangement takes advantage of the lateral symmetry of the waveguide 1.

[0096] Advantageously such an arrangement reduces the number of connections from 64 to 16, advantageously reducing size and cost of the control circuit.

[0097] Light in the waveguide that is reflected from LED elements after reflection from the Fresnel reflector undesirably can increase the level of stray light and reduce the stray light performance of the display. It would be desirable to reduce stray light levels in privacy mode of operation.

[0098] FIGURE 26 is a schematic diagram illustrating an arrangement of LED connections to further reduce the number of connections, which does not include all the features of the present invention but is useful for understanding. In array 15'a-n first group 241 may comprise dual die LEDs, that is LEDs comprising two separate gallium nitride emitting dies, whereas group 243 may comprise single die LEDs. Thus the addressing voltage for each input is the same for the first and second groups 241, 243 but the total number of connections is reduced. Further, the light output at the edges of the array, for example in group 243 is typically more uniform and lower luminous flux than for group 241. Advantageously desirable luminous flux variation across the array can be achieved while reducing the total number of connections. Such an arrangement can be used in combination with the symmetric designs described elsewhere herein.

[0099] FIGURE 27 is a schematic diagram illustrating a further arrangement of LED connections and input surfaces to further improve privacy performance by reducing the amount of stray light in off-axis positions. In comparison to FIGURE 23, LEDs 291, 293, 295 are removed, thus reducing the stray light resulting from LED reflections. Further the optical surface in the region between the remaining LEDs can be arranged to transmit light or direct returning light to the sides 22, 24 to remove stray light artifacts. Such an arrangement advantageously improves privacy performance by reducing reflection from the input side.

[0100] It would be desirable to provide automatic control of image uniformity to compensate light source degradation mechanisms.

[0101] FIGURE 28 is a schematic diagram illustrating a graph of optical window luminous intensity 1264 against viewing position 1262 in the window plane of a directional waveguide 1, corresponding to the angle of the output direction, and FIGURE 29 is a schematic diagram illustrating a graph of luminous flux 1263 distribution for an array 15 of illuminator elements to compensate for light source degradation. Illuminator elements such as LEDs including gallium nitride blue emitters and yellow phosphors may undergo ageing wherein the scaled luminous fluxes and chromaticity may vary with usage. In particular, on-axis illuminator elements may be used more frequently or at higher average currents and thus junction temperatures than off-axis illuminator elements that may provide a non-uniform degradation in luminous emittance across the array 15. Such errors can be corrected as shown by distribution 1279 of scaled luminous fluxes from the respective illuminator elements in comparison to a reference level 1269. Advantageously, the luminance distribution of the display may be maintained throughout the device lifetime.

[0102] US-2014 / 0009508, generally describes a luminance calibration apparatus and method wherein photodetectors are arranged at the input side 2 of directional waveguide 1, for example as part of the lightbar comprising the array 15 and electrical connections. It would be desirable to reduce the cost and complexity of the photodetector and control apparatus.

[0103] FIGURE 30 is a schematic diagram illustrating in side view a display and integrated camera arranged to provide in-field LED array calibration that can be used with the directional backlight according to the claimed invention. Waveguide 1 may be arranged within a display unit within hingeup 1420 further comprising camera 1410. Hinge 1424 may be used to connect base unit 1422 which may comprise a keyboard, photodetector 1400 and reflective region 1423. Camera 1410 may be arranged to receive light from the waveguide 1, for example after reflection from region 1423.

[0104] FIGURES 31 and 32 are schematic diagrams illustrating in front view a waveguide and keyboard mounted detectors arranged to provide in-field LED calibration, that can be used with the directional backlight according to the claimed invention. FIGURE 31 shows that photodetectors 1400, 1402, 1406 may be arranged in parts of the base unit 1422 including under keys of keyboard 1421 or other part of the base unit 1422, so that when the lid of the notebook is closed or substantially closed then light can be received from the waveguide unit. The photodetectors may be integrated with the illuminated keyboard control where provided. When the notebook is closed, FIGURE 33 shows the relative position of detectors 1400, 1402, 1406 with respect to the waveguide 1 of the display in the hingeup unit 1420. Advantageously the detectors can measure the output of the display across various different areas of the waveguide. Voids A and B may otherwise mean that some parts of the waveguide do not illuminate a photodetector for certain light sources in the array 15. Further when the lid is not fully closed ambient light sensor 1430 or camera 1410 may observer reflected light from the keyboard or reflector 1423. Advantageously such light can be used to calibrate the light source array 15 to compensate for ageing or loss of single LEDs.

[0105] FIGURE 33 is a schematic diagram illustrating a control system and front view of a directional backlight apparatus comprising a directional backlight as described above including a waveguide 1 and array 15 of illuminator elements in accordance with the present invention. The directional backlight apparatus includes a control system, as described above, that implements a method of controlling the illuminator elements 15n making a calibration of the drive signals, as follows.

[0106] Light rays 1210 from illuminator element 1216 are directed to reflective end 4, reflected and directed back towards the input end 2. Some of the light from source 1216 will be extracted by means of light extraction features 12, while some of the light will be incident on at least a portion of the input end 2. Sensor elements that may include 1400, 1402 may be arranged as described above.

[0107] Measured signals from sensors 1400, 1402 may be passed to illumination controller 1274 which drives illuminator elements of array 15 using an illuminator element driver 1233 which may be a current driver with grey level control to drive lines 1244 to provide drive signals to the array of illuminator elements. The illumination controller 1274 calibrates the drive signals supplied to the illuminator elements 15n in response the measured signals representing the sensed light, as follows.

[0108] Array luminous flux distribution controller 1224 may include for example a stored reference grey level profile 1230 from front of screen measurements that may be provided at the time of manufacture. This enables the control system to output scaled luminous fluxes that have a predetermined distribution across the array of light sources, for example to vary the scaled luminous fluxes as described above.

[0109] Data from sensors 1400, 1402 may be supplied for example to calibration measurement system 1222 that may provide data to a look up table 1226 within the luminous flux distribution controller 1224. Further selection of luminous intensity distribution (for example to select between luminous intensity distributions 1266, 1272, 1274, 1276, 1294) may be provided by selection controller 1228. Selection controller may have user input or an automatic input that is determined by sensing of display viewing conditions. For example the number of viewers, the room brightness, display orientation, the image quality settings and / or the power savings mode settings may be used to vary the selected distribution.

[0110] In device manufacture, the output of the sensors 1400, 1402 in response to each of the light sources of the array 15 may be compared to the signal from a camera or detector placed in the window plane of the display. This achieves an initial calibration or referencing of the internal sensors with respect to light in the window plane. Such calibration may be stored in a look up table or similar.

[0111] In operation of a calibration mode, a single illuminator element of the array 15 is illuminated and sensors 1400, 1402 may measure a signal for the said illuminator element. The said illuminator element is extinguished and the next source of the array operated and a measurement taken. The output of the array of measurements is compared with a factory calibration so that the output luminous intensity for the given luminous flux distribution can be interpolated. The appropriate luminous flux distribution for the required luminous intensity distribution is then derived by the controller 1224 and the illuminator element controller 1233 appropriately configured to achieve the desired luminous flux distribution.

[0112] Advantageously the light from the whole array 15 may be measured by a combination of sensors 1400, 1402 and a desired luminous intensity distribution may be achieved.

[0113] The sensor system may be arranged with the waveguide 1 during the fabrication of the display for characterization purposes and removed after completion of product fabrication. Preferably the sensor system may be arranged with the waveguide 1 during normal operation. The in-field calibration phase may be applied during display switch-on. The spatial light modulator may be arranged with a black image during calibration to remove visibility to the user of the calibration phase. The calibration phase may be repeated on a daily, weekly or monthly basis for example to compensated for ageing artefacts as shown in FIGURES 28 and 29.

[0114] FIGURE 34 is a schematic diagram illustrating a further graph of optical window luminous intensity 1264 against viewing position 1262 in the window plane of a waveguide comprising a luminous intensity defect, when cycling through illumination of each LED. The diffusion properties of the optical system mean that such a plot may be different to the output when luminous intensity outputs from the light sources are combined in normal operation. Thus a single LED 15 of the array of light sources may have failed in region 1272 compared to the desired output 1288.

[0115] FIGURE 35 is a schematic diagram illustrating a graph of optical window luminous intensity against viewing position in the window plane of a waveguide further illustrating a correction of the defect of FIGURE 34. Such an output may occur in the symmetric drive scheme of FIGURE 25 for example. Thus the calibration system illustrated in FIGURE 33 may detect the loss of angular output when a particular LED pair is illuminated. To compensate for the loss of a particular LED, the failed channel may be undriven creating a symmetric drop region 1277. To compensate for the drop, adjacent LEDs on both sides of the optical axis 199 may be illuminated with higher outputs 1273, 1275 such that after diffusion the desired output 1288 may be provided. Advantageously LED compensation may be achieved in symmetric drive systems.

[0116] It would be desirable to further reduce the cost and complexity of the LED ageing compensation system by eliminating the photodetection and control system.

[0117] FIGURE 36 is a schematic diagram illustrating a graph of nominal LED output against time. Luminous flux 1500 of a nominal light source of the array 15 may degrade with time 1502 as shown by profiles 1504, 1506 for different drive conditions.

[0118] FIGURE 37 is a schematic diagram illustrating a flowchart for compensation of LED ageing. In a first step 1508 for the nth LED, the peak current, Ipeak n (t) and average current, Iav n (t) may be recorded. In pulse width modulation schemes, the peak current may be significantly higher than the average current. In a second step the average expected luminance may be computed using data similar to FIGURE 36. In a third step 1512 the corrected current after PWM control Iav n may be computed and applied to the respective LED. Advantageously LED ageing effects may be compensated and cost and complexity reduced.

[0119] FIGURE 38 is a schematic diagram illustrating in perspective view, the structure of a directional display device comprising a waveguide 1 arranged with a spatial light modulator 48, which does not include all the features of the present invention but is useful for understanding. Reflective end 4 may be provided by a Fresnel mirror. Taper region 2204 may be arranged at the input to the waveguide 1 to increase input coupling efficiency from the light sources 15a-15n of the array of illuminator elements 15 and to increase illumination uniformity. Shading layer 2206 with aperture 2203 may be arranged to hide light scattering regions at the edge of the waveguide 1. Rear reflector 2200 may comprise facets 2202 that are curved and arranged to provide viewing windows 26 from groups of optical windows provided by imaging light sources of the array 15 to the window plane 106. Optical stack 2208 may comprise reflective polarizers, retarder layers and diffusers. Rear reflectors 2200 and optical stack 2208 are described further inUS-2014 / 0240828.

[0120] Spatial light modulator 48 may comprise a liquid crystal display that may comprise an input polarizer 2210, TFT glass substrate 2212, liquid crystal layer 2214, color filter glass substrate 2216 and output polarizer 2218. Red pixels 2220, green pixels 2222 and blue pixels 2224 may be arranged in an array at the liquid crystal layer 2214. White, yellow, additional green or other color pixels (not shown) may be further arranged in the liquid crystal layer to increase transmission efficiency, color gamut or perceived image resolution.

[0121] In the embodiment of FIGURE 59A, injection of input light into the waveguide is along the long edge. The physical size of the LED packages of the array 15 and scatter from waveguide and other surfaces near the input end 2 limit the minimum bezel width that can be achieved. It would be desirable to reduce the width of the side bezel along the long edges of the waveguide.

[0122] FIGURES 39 to 42 are schematic diagrams illustrating in perspective, front, side and perspective views respectively, an optical valve comprising a light source 1317a arranged to achieve an on-axis optical window, that can be used with the directional backlight according to the claimed invention.

[0123] FIGURE 39 illustrates in top view the propagation of light rays from light source arrays 1319a-n and 1317a-n arranged on the short side of a directional waveguide. FIGURE 40 similarly illustrates in side view the propagation of rays from light source array 1317a-n. FIGURE 41 illustrates in perspective view the formation of optical windows by light source array 1317a-n. FIGURE 42 illustrates in perspective view a display apparatus comprising an optical stack comprising a waveguide as illustrated in FIGURES39 to 41.

[0124] As described in U.S. Provisional Patent Application No. 62 / 167,203, to which this application claims priority, a directional display device may comprise a waveguide 1301 that further comprises a reflective end 1304 that is elongated in a lateral direction (y-axis), the first and second guide surfaces 6,8 extending from laterally extending edges of the reflective end 1304, the waveguide 1301 further comprising side surfaces 1322, 1324 extending between the first and second guide surfaces 6,8, and wherein the light sources include an array 1317 of light sources 1317a-n arranged along a side surface 1322 to provide said input light through that side surface 1322, and the reflective end 1304, comprises first and second facets 1327, 1329 alternating with each other in the lateral direction, the first facets 1327 being reflective and forming reflective facets of a Fresnel reflector having positive optical power in the lateral direction, the second facets 1329 forming draft facets of the Fresnel reflector, the Fresnel reflector 1304 having an optical axis 1287 that is inclined towards the side surface 1322 in a direction in which the Fresnel reflector 1304 deflects input light from the array of light sources 1317 into the waveguide 1301. Thus angle 1277 is non-zero. Similarly the second facets 1329 may be reflective and form reflective facets of a Fresnel reflector having positive optical power in the lateral direction, the Fresnel reflector 1304 having an optical axis 1289 that is inclined towards the side surface 1324 in a direction in which the Fresnel reflector 1304 deflects input light from the array of light sources 1319 into the waveguide 1301.

[0125] Illustrative light ray 1363 from source 1317a may be arranged to provide optical window 1326a and light ray 1365 from source 1317b may be arranged to provide optical window 1326b. Other layers such as diffusers, prismatic reflection films, retarders and spatial light modulators may be arranged in series with the waveguide 1301 in a similar manner to that described for waveguide 1 in the arrangement of FIGURE 38 for example.

[0126] Advantageously a thin backlight with low bezel size may be achieved. Such an arrangement has light sources that are not arranged on the long sides of the waveguide 1301 and thus may have small form factor. Further light sources 1317 and 1319 may be arranged with overlapping optical windows, and thus display luminance may be increased.

[0127] It would be further desirable to achieve uniform illumination of a waveguide with a narrow bezel along the edges of the waveguide in wide angle mode of operation. The embodiments described elsewhere herein may be applied to either the long side light source array input of FIGURE 38 or the short side light source array input of FIGURES 39 to 42. Advantageously uniform display appearance may be achieved in directional displays with a narrow long side bezel. Such displays may be used in mobile displays such as cell phones or tablets as well as laptops, TV and monitors.

[0128] The embodiments related to stepped waveguide directional backlights may be applied with changes as necessary to the wedge directional backlight as described herein.

[0129] As may be used herein, the terms "substantially" and "approximately" provide an industry-accepted tolerance for its corresponding term and / or relativity between items. Such an industry-accepted tolerance ranges from zero percent to ten percent and corresponds to, but is not limited to, component values, angles, et cetera. Such relativity between items ranges between approximately zero percent to ten percent.

Claims

1. A directional backlight comprising: a waveguide (1) comprising an input end (2); and an array of input light sources (15) arranged at different input positions in a lateral direction across the input end (2) of the waveguide (1) and arranged to input input light into the waveguide (1), the waveguide (1) further comprising first and second opposed, laterally extending guide surfaces (6, 8) for guiding light along the waveguide (1), side surfaces extending between the first and second guide surfaces (6, 8), and a reflective end (4) facing the input end (2) for reflecting the input light back along the waveguide (1) and having positive optical power laterally, the second guide surface (8) being arranged to deflect the reflected input light through the first guide surface (6) as output light, and the waveguide (1) being arranged to direct the output light into optical windows in output directions that are distributed in a lateral direction in dependence on the input position of the input light, characterized by further comprising additional light sources (19) disposed along only a part of each side surface (24) adjacent the input end (2), the additional light sources (19) being arranged to direct additional light into the waveguide (1) through a respective one of the side surfaces (24) in a direction in which the additional light is reflected by the reflective end (4) onto the opposite side surface (24) and by the opposite side surface (24) into a segment of the waveguide (1) adjacent the opposite side surface (24) extending from a corner between the reflective end (4) and the opposite side surface (24).

2. A directional backlight according to claim 1, wherein said part of each side surface (24) along which the additional light sources are disposed is at least 20% of the side surface (24) from the input end.

3. A directional backlight according claim 1 or 2, wherein said part of each side surface (24) along which the additional light sources are disposed is at most 80% of the side surface (24) from the input end.

4. A directional backlight according to any one of the preceding claims, wherein the first guide surface (6) is arranged to guide light by total internal reflection and the second guide surface (8) comprises a plurality of light extraction features (12) oriented to direct light guided along the waveguide (1) in directions allowing exit through the first guide surface (6) as the output light and intermediate regions between the light extraction features (12) that are arranged to guide light along the waveguide (1).

5. A directional backlight according to claim 4, wherein the second guide surface (8) has a stepped shape in which said light extraction features (12) are facets between the intermediate regions.

6. A directional backlight according to claim 4 or 5, wherein the light extraction features (12) have positive optical power in the lateral direction.

7. A directional backlight according to any one of the preceding claims, wherein the reflective end (4) is a Fresnel reflector comprising alternating reflective facets and draft facets, the reflective facets provide the Fresnel reflector with positive optical power.

8. A directional display device comprising: a directional backlight according to any one of the preceding claims; and a transmissive spatial light modulator (48) arranged to receive the output light from the waveguide (1) and to modulate it to display an image.

9. A directional illumination apparatus comprising: a directional waveguide according to any one of claims 1 to 7; and a control system (402) arranged to control the light sources.

10. A directional illumination apparatus according to claim 9, wherein the control system (402) is arranged to control input light sources (15) selected to direct output light into desired optical windows, and is further arranged to control at least one additional light source (19) selected to provide additional light that is output from the directional backlight in the same output directions as the desired optical windows.

11. A directional illumination apparatus according to claim 10, wherein the control system (402) is arranged, when a selected input light source (15) is off-center of the array of input light surfaces, to control at least one additional light source (19) that is on an opposite side of the directional backlight from the selected input light source (15).

12. A directional display device comprising: a directional illumination apparatus according to any one of claims 9 to 11; and a transmissive spatial light modulator (48) arranged to receive the output light from the waveguide and to modulate it to display an image.