Patent Description:
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.

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.

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.

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 <CIT>, and similarly in <CIT> on which the two-part form of claim <NUM> 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.

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.

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.

Said part of each side surface along which the additional light sources are disposed may be at least <NUM>% 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 <NUM>% of the side surface from the input end.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

Embodiments are illustrated by way of example in the accompanying FIGURES, in which like reference numbers indicate similar parts, and in which:.

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.

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.

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 <NUM> 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.

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 <NUM> or greater. However, the increased size of the illuminator elements means that the size of the directional optical elements increases proportionately. For example, a <NUM> wide illuminator imaged to a <NUM> wide viewing window may require a <NUM> back working distance. Thus, the increased thickness of the optical elements can prevent useful application, for example, to mobile displays, or large area displays.

Addressing the aforementioned shortcomings, optical valves as described in commonly-owned <CIT>, 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.

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.

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.

In contradistinction, conventional non-imaging backlights or light guiding plates (LGPs) are used for illumination of 2D displays. See, e.g., <NPL>). 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 <NUM>. 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.

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 <CIT> (<CIT>).

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.

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.

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.

Thin imaging directional backlight implementations for use with LCDs have been proposed and demonstrated by <NUM>, for example <CIT>, for example <CIT> which may be referred to herein as a "wedge type directional backlight;" by RealD, for example <CIT> which may be referred to herein as an "optical valve" or "optical valve directional backlight".

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.

<FIG> 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 <FIG> is a schematic diagram illustrating a side view of light propagation in the directional display device of <FIG>.

<FIG> illustrates a front view in the xy plane of a directional backlight of a directional display device, and includes an illuminator array <NUM> which may be used to illuminate a stepped waveguide <NUM>. Illuminator array <NUM> includes illuminator elements 15a through illuminator element 15n (where n is an integer greater than one). In one example, the stepped waveguide <NUM> of <FIG> may be a stepped, display sized waveguide <NUM>. 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, <FIG> illustrates a side view in the xz plane, and includes illuminator array <NUM>, SLM <NUM>, extraction features <NUM>, guiding features <NUM>, and stepped waveguide <NUM>, arranged as shown. The side view provided in <FIG> is an alternative view of the front view shown in <FIG>. Accordingly, the illuminator array <NUM> of <FIG> and <FIG> corresponds to one another and the stepped waveguide <NUM> of <FIG> and <FIG> may correspond to one another.

Further, in <FIG>, the stepped waveguide <NUM> may have an input end <NUM> that is thin and a reflective end <NUM> that is thick. Thus the waveguide <NUM> extends between the input end <NUM> that receives input light and the reflective end <NUM> that reflects the input light back through the waveguide <NUM>. The length of the input end <NUM> in a lateral direction across the waveguide is greater than the height of the input end <NUM>. The illuminator elements 15a - 15n are disposed at different input positions in a lateral direction across the input end <NUM>.

The waveguide <NUM> has first and second, opposed guide surfaces extending between the input end <NUM> and the reflective end <NUM> for guiding light forwards and back along the waveguide <NUM>. The second guide surface has a plurality of light extraction features <NUM> facing the reflective end <NUM> and arranged to reflect at least some of the light guided back through the waveguide <NUM> 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.

In this example, the light extraction features <NUM> are reflective facets, although other reflective features could be used. The light extraction features <NUM> do not guide light through the waveguide, whereas the intermediate regions of the second guide surface intermediate the light extraction features <NUM> 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 <NUM> extend laterally to those regions so that the second guide surface has a stepped shape which may include the light extraction features <NUM> and intermediate regions. The light extraction features <NUM> are oriented to reflect light from the light sources, after reflection from the reflective end <NUM>, through the first guide surface.

The light extraction features <NUM> 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 <NUM> 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 <NUM> and with regard to the output light remain parallel in this embodiment where the deflections at the reflective end <NUM> 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.

The SLM <NUM> extends across the waveguide and modulates the light output therefrom. Although the SLM <NUM> 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 <NUM> 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 <NUM>.

The operation of a directional display device that may provide a one dimensional array of viewing windows is illustrated in front view in <FIG>, with its side profile shown in <FIG>. In operation, in <FIG> and <FIG>, light may be emitted from an illuminator array <NUM>, such as an array of illuminator elements 15a through 15n, located at different positions, y, along the surface of thin end side <NUM>, x=<NUM>, of the stepped waveguide <NUM>. The light may propagate along +x in a first direction, within the stepped waveguide <NUM>, while at the same time, the light may fan out in the xy plane and upon reaching the far curved end side <NUM>, may substantially or entirely fill the curved end side <NUM>. 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 <NUM> that link the guiding features <NUM> of the bottom side of the stepped waveguide <NUM> 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.

Continuing the discussion of <FIG> and <FIG>, the curved end side <NUM> of the stepped waveguide <NUM> 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 <NUM> degree tilted extraction features <NUM>, 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 <NUM> through refraction, but may be decreased somewhat dependent on the reflective properties of the extraction features <NUM>.

In some embodiments with uncoated extraction features <NUM>, 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 <NUM> approximately collimated and may be directed off normal in proportion to the y-position of the respective illuminator element 15a - 15n in illuminator array <NUM> from the input edge center. Having independent illuminator elements 15a - 15n along the input edge <NUM> then enables light to exit from the entire first light directing side <NUM> and propagate at different external angles, as illustrated in <FIG>.

Illuminating a spatial light modulator (SLM) <NUM> 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 <NUM> end in <FIG>, front view in <FIG> and side view in <FIG>. <FIG> 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, <FIG> is a schematic diagram illustrating in a front view, propagation of light in a directional display device, and <FIG> is a schematic diagram illustrating in side view propagation of light in a directional display device. As illustrated in <FIG>, <FIG>, and <FIG>, a stepped waveguide <NUM> may be located behind a fast (e.g., greater than <NUM>) LCD panel SLM <NUM> that displays sequential right and left eye images. In synchronization, specific illuminator elements 15a through 15n of illuminator array <NUM> (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 <NUM> are turned on together, providing a one dimensional viewing window <NUM> 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 <NUM> 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.

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

<FIG> 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, <FIG> illustrates additional detail of a side view of the operation of a stepped waveguide <NUM>, which may be a transparent material. The stepped waveguide <NUM> may include an illuminator input side <NUM>, a reflective side <NUM>, a first light directing side <NUM> which may be substantially planar, and a second light directing side <NUM> which includes guiding features <NUM> and light extraction features <NUM>. In operation, light rays <NUM> from an illuminator element 15c of an illuminator array <NUM> (not shown in <FIG>), that may be an addressable array of LEDs for example, may be guided in the stepped waveguide <NUM> by means of total internal reflection by the first light directing side <NUM> and total internal reflection by the guiding feature <NUM>, to the reflective side <NUM>, which may be a mirrored surface. Although reflective side <NUM> may be a mirrored surface and may reflect light, it may in some embodiments also be possible for light to pass through reflective side <NUM>.

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

<FIG> 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, <FIG> shows in front view further guiding of light rays from illuminator element 15c of illuminator array <NUM>, in the stepped waveguide <NUM>. Each of the output rays are directed towards the same viewing window <NUM> from the respective illuminator <NUM>. Thus light ray <NUM> may intersect the ray <NUM> in the window <NUM>, or may have a different height in the window as shown by ray <NUM>. Additionally, in various embodiments, sides <NUM>, <NUM> of the waveguide <NUM> may be transparent, mirrored, or blackened surfaces. Continuing the discussion of <FIG>, light extraction features <NUM> may be elongate, and the orientation of light extraction features <NUM> in a first region <NUM> of the light directing side <NUM> (light directing side <NUM> shown in <FIG>, but not shown in <FIG>) may be different to the orientation of light extraction features <NUM> in a second region <NUM> of the light directing side <NUM>.

<FIG> 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, <FIG> shows the light rays <NUM>, <NUM> from a second illuminator element <NUM> of the illuminator array <NUM>. The curvature of the reflective end on the side <NUM> and the light extraction features <NUM> cooperatively produce a second viewing window <NUM> laterally separated from the viewing window <NUM> with light rays from the illuminator element <NUM>.

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

<FIG> 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, <FIG> shows a similar arrangement of components to <FIG> (with corresponding elements being similar), with one of the differences being that the light extraction features <NUM> 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 <FIG> and <FIG>.

<FIG> 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, <FIG> 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 <FIG> 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, <FIG> shows schematically the generation of illumination window <NUM> from stepped waveguide <NUM>. Illuminator element group <NUM> in illuminator array <NUM> may provide a light cone <NUM> directed towards a viewing window <NUM>. <FIG> shows schematically the generation of illumination window <NUM>. Illuminator element group <NUM> in illuminator array <NUM> may provide a light cone <NUM> directed towards viewing window <NUM>. In cooperation with a time multiplexed display, windows <NUM> and <NUM> may be provided in sequence as shown in <FIG>. If the image on a spatial light modulator <NUM> (not shown in <FIG>, <FIG>, <FIG>) 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 <NUM>, <NUM> each include one or more illumination elements from illumination elements 15a to 15n, where n is an integer greater than one.

<FIG> 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 <FIG>, selectively turning on and off illuminator elements 15a to 15n along axis <NUM> provides for directional control of viewing windows. The head <NUM> 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 <NUM> may be turned on and off to provide substantially independent images to each eye irrespective of the head <NUM> position. The head tracking system (or a second head tracking system) may provide monitoring of more than one head <NUM>, <NUM> (head <NUM> not shown in <FIG>) 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.

<FIG> 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 <FIG>, at least two 2D images may be directed towards a pair of viewers <NUM>, <NUM> so that each viewer may watch a different image on the spatial light modulator <NUM>. The two 2D images of <FIG> may be generated in a similar manner as described with respect to <FIG> 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 <NUM> in a first phase, and a second image is presented on the spatial light modulator <NUM> 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 <NUM>, <NUM> respectively. An observer with both eyes in window <NUM> will perceive a first image while an observer with both eyes in window <NUM> will perceive a second image.

<FIG> 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 <NUM> as shown in <FIG>. Further, as illustrated in <FIG>, although first viewer <NUM> may be able to view an image on device <NUM>, light is not directed towards second viewer <NUM>. Thus second viewer <NUM> is prevented from viewing an image on device <NUM>. Each of the embodiments of the present disclosure may advantageously provide autostereoscopic, dual image or privacy display functions.

<FIG> 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, <FIG> shows in side view an autostereoscopic directional display device, which may include the stepped waveguide <NUM> and a Fresnel lens <NUM> arranged to provide the viewing window <NUM> in a window plane <NUM> at a nominal viewing distance from the spatial light modulator for a substantially collimated output across the stepped waveguide <NUM> output surface. A vertical diffuser <NUM> may be arranged to extend the height of the window <NUM> further. The light may then be imaged through the spatial light modulator <NUM>. The illuminator array <NUM> 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 <NUM> 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 <NUM> to provide a substantially uniform light source with the appropriate output angle, and further to provide reduction in speckle. Alternatively, the illuminator array <NUM> 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.

A further wedge type directional backlight is generally discussed by <CIT>. 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.

There will now be described some waveguides, directional backlights and directional display devices that are based on and incorporate the structures of <FIG> 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.

<FIG> 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 <NUM> and an array <NUM> 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.

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

The control system may comprise a sensor system arranged to detect the position of the observer <NUM> relative to the display device <NUM>. The sensor system comprises a position sensor <NUM>, such as a camera arranged to determine the position of an observer <NUM>; and a head position measurement system <NUM> that may for example comprise a computer vision image processing system. The position sensor <NUM> may comprise known sensors including those comprising cameras and image processing units arranged to detect the position of observer faces. Position sensor <NUM> may further comprise a stereo sensor arranged to improve the measure of longitudinal position compared to a monoscopic camera. Alternatively position sensor <NUM> 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.

The control system may further comprise an illumination controller and an image controller <NUM> that are both supplied with the detected position of the observer supplied from the head position measurement system <NUM>.

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

The image controller <NUM> is arranged to control the SLM <NUM> to display images. To provide an autostereoscopic display, the image controller <NUM> and the illumination controller may operate as follows. The image controller <NUM> controls the SLM <NUM> to display temporally multiplexed left and right eye images and the LED controller <NUM> operates the light sources <NUM> 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 <NUM> (which may comprise one or more LEDs) by means of drive line <NUM> wherein other drive lines are not driven as described elsewhere.

The head position measurement system <NUM> detects the position of an observer relative to the display device <NUM>. The LED controller <NUM> selects the light sources <NUM> to be operated in dependence on the position of the observer detected by the head position measurement system <NUM>, 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 <NUM> 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.

<FIG> 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 <NUM> with faceted mirror end <NUM>. The first guide surface <NUM> of the waveguide <NUM> is arranged to guide light by total internal reflection and the second guide surface <NUM> 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 <NUM>. The display device further comprises a deflection element <NUM> extending across the first guide surface <NUM> of the waveguide <NUM> for deflecting light from array <NUM> of light sources towards the normal to the first guide surface <NUM>. Further the waveguide <NUM> may further comprise a reflective end <NUM> for reflecting input light back through the waveguide <NUM>, the second guide <NUM> surface being arranged to deflect light as output light through the first guide surface <NUM> after reflection from the reflective end <NUM>. The reflective end has positive optical power in the lateral direction (y-axis) in a similar manner to the reflective end shown in <FIG> for example. Further facets in the reflective end <NUM> deflect the reflected light cones within the waveguide <NUM> to achieve output coupling on the return path. Thus viewing windows are produced in a similar manner to that shown in <FIG>. Further the directional display may comprise a spatial light modulator <NUM> and parallax element <NUM> aligned to the spatial light modulator <NUM> that is further arranged to provide optical windows. A control system <NUM> similar to that shown in <FIG> may be arranged to provide control of directional illumination providing viewing windows <NUM> and windows <NUM> from the parallax element and aligned spatial light modulator.

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.

<FIG> 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 <NUM>, those close to the optical axis <NUM> 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, <NUM> is provided by light that is outside a cone angle subtended by the light source and adjacent edge of the Fresnel reflector. Boundary <NUM> separates void A from the main illumination region. Void B, <NUM> 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 <NUM> separates void B from the main illumination region. Both voids create undesirable non-uniformities for off-axis viewing positions.

<FIG> and <FIG> 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 <FIG>, void A that is provided for example by input light source 15e on the right side of the optical axis <NUM> can be compensated by light source array <NUM> arranged on the left side <NUM> of the waveguide <NUM>. Void B can be compensated by modification of the structure of the input end. Thus a directional backlight may comprise a waveguide <NUM> comprising an input end <NUM>; an array <NUM> of input light sources arranged at different input positions in a lateral direction across the input end <NUM> of the waveguide <NUM> and arranged to input input light into the waveguide <NUM>, the waveguide <NUM> further comprising first and second opposed, laterally extending guide surfaces <NUM>,<NUM> for guiding light along the waveguide <NUM>, side surfaces <NUM>, <NUM> extending between the first and second guide surfaces <NUM>,<NUM> , and a reflective end <NUM> having positive optical power facing the input end <NUM> for reflecting the input light back along the waveguide <NUM>, the second guide surface <NUM> being arranged to deflect the reflected input light through the first guide surface <NUM> as output light, and the waveguide <NUM> 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 <NUM> in a direction in which the additional light is reflected by the reflective end <NUM> onto the opposite side surface <NUM> and by the opposite side surface <NUM> into a segment of the waveguide <NUM> adjacent the opposite side surface <NUM> extending from a corner between the reflective surface <NUM> and the side surface <NUM>.

Similarly the additional light sources 19a-n may be arranged to direct additional light into the waveguide <NUM> through one of the side surfaces <NUM> in a direction in which the additional light is reflected by the reflective end <NUM> onto the opposite side surface <NUM> and by the opposite side surface <NUM> into a segment of the waveguide <NUM> adjacent the opposite side surface <NUM> extending from a corner between the reflective surface <NUM> and the side surface <NUM>.

Region <NUM> between lines <NUM>, <NUM> is illuminated by light source <NUM>. Advantageously void A, <NUM> may be filled in a controllable manner by adjustment of the flux from the light sources <NUM>, 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 <NUM>.

As illustrated in <FIG>, which is in accordance with the present invention, light source 15e may be arranged to provide optical window 26e from region <NUM> of the waveguide <NUM>. Light source 15e may be arranged to the right of the optical axis <NUM> 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 <NUM> of the waveguide <NUM>. Void correcting light source 17e is to the left of the optical axis <NUM> 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.

<FIG> 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 <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> 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.

It would be desirable to reduce cost and complexity by minimizing the number of light sources <NUM>, <NUM> on the sides <NUM>, <NUM> of the waveguide <NUM>.

<FIG> are schematic diagrams illustrating in front view arrangements of LEDs arranged to achieve void A filling, in accordance with the present invention. <FIG> is a detail of the corner of <FIG>.

The additional light sources 17a-n, 19a-n may be disposed along at least a part <NUM>, <NUM> of each side surface <NUM>, <NUM> adjacent the input end <NUM>, the additional light sources 17a-n, 19a-n being arranged to direct additional light into the waveguide <NUM> through one of the side surfaces <NUM>, <NUM>.

Typically it is desirable to provide void A, <NUM> filling for optical windows <NUM> that are up to approximately <NUM> degrees off-axis in a lateral direction. At much higher angles, general diffusion and stray light in the waveguide <NUM> and diffuser elements may be provided to achieve void A, <NUM> filling and window <NUM> provision. Such a window direction requires the uppermost light source 17n to create a ray <NUM> that has an angle θ to the side <NUM> that may be approximately <NUM> degrees inside the waveguide in the x-y plane. In an illustrative embodiment, a waveguide <NUM> with <NUM>:<NUM> aspect ratio may be provided with an edge facet <NUM> facet angle θef of approximately 20degrees. The angle θ1 is given by: <MAT>.

To continue the illustrative embodiment θ1 may be <NUM> degrees. The relative size of the region of the side <NUM> that is free of light sources 17a-n may then be determined as <MAT>.

Where W is the waveguide <NUM> width (in y direction) and L is its height (in x direction). Thus for the <NUM>:<NUM> arrangement of waveguide <NUM>, Δ = <NUM>% and the light sources <NUM> are required for <NUM>% of the height of the waveguide <NUM> from the input side. When θ = θc then the light exits the waveguide parallel to the surface <NUM>. Thus the minimum Δ is approximately <NUM>%.

Thus said part <NUM>, <NUM> of each side surface along which the additional light sources are disposed is at least <NUM>% of the side surface from the input end <NUM> and preferably at least <NUM>% of the side surface. Said part <NUM>, <NUM> of each side surface along which the additional light sources are disposed is at most <NUM>% of the side surface from the input end <NUM> and preferably at most <NUM>% of the side surface.

Advantageously void A may be filled and the number of light sources <NUM>, <NUM> may be reduced, minimizing cost and optimizing uniformity. In operation, light sources <NUM>, <NUM> 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 <NUM>.

It would be desirable to provide filling of Void B, <NUM> to optimize uniformity in an efficient manner.

<FIG> 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.

<FIG> illustrates that the input facets <NUM> 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 <NUM>, <NUM> may be provided. Thus for optical windows <NUM> in a particular angular direction, the amount or light directed may comprise a mixture of cones <NUM> and <NUM>, enabling tuning of actual light distribution in said direction. The axis of said direction may be determined by the slope <NUM> of the P or S surface.

<FIG> illustrates one half of the input side <NUM> of a waveguide <NUM>. The central facets in region <NUM>, corresponding to central LEDs may be planar (P) type, thus achieving high efficiency of illumination of the reflective end <NUM>. In the outer region <NUM>, higher angular distributions of light may be desirable so that S type surfaces may be used. In intermediate region <NUM>, 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 <NUM> positions.

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.

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

<FIG> 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 <NUM> while the input side <NUM> may be reflective in region <NUM>. Further portion <NUM> of the sides <NUM>, <NUM> may be reflective. Thus light rays <NUM> that are incident on the input side <NUM> are reflected by the reflective regions <NUM> with a profile arranged to partially illuminate side regions <NUM> and re-reflect to the mirror end <NUM>. Rays <NUM> may be provided that fill voids for off axis position and advantageously uniform illumination is provided for reduced cost and complexity/.

<FIG> 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 <NUM> and side mounted light source arrays 17a-n, 19a-n. Advantageously improved uniformity control can be achieved compared to the embodiment of <FIG>.

<FIG> is a schematic diagram illustrating in front view arrangements of LED for use in cooperation with the waveguide input of <FIG> arranged to achieve void B filling, in accordance with the present invention. Light source arrays <NUM>, <NUM> connected by means of electrodes <NUM> may be provided by LEDs that are arranged in multiple groups comprising pairs 250a-250c in region <NUM>, pairs 250d-<NUM> in region <NUM> and pairs <NUM>-<NUM> in region <NUM>. Similarly light source array <NUM> may comprise LEDs 19a-19n arranged in pairs 252a-252d that extend approximately <NUM>/<NUM> of the height of the waveguide <NUM> from in the input side.

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 <NUM>.

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.

<FIG> is a schematic diagram illustrating an arrangement of LED connections to achieve correction of luminance uniformity, in accordance with the present invention. <FIG> illustrates an arrangement similar to <FIG> wherein the complete array of <NUM> LEDs is shown, using <NUM> connections with LEDs arranged in pairs. Advantageously the number of connections is reduced compared to individual LED control.

<FIG> 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 <NUM> LEDs such an arrangement takes advantage of the lateral symmetry of the waveguide <NUM>.

Advantageously such an arrangement reduces the number of connections from <NUM> to <NUM>, advantageously reducing size and cost of the control circuit.

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.

<FIG> 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 <NUM>'a-n first group <NUM> may comprise dual die LEDs, that is LEDs comprising two separate gallium nitride emitting dies, whereas group <NUM> may comprise single die LEDs. Thus the addressing voltage for each input is the same for the first and second groups <NUM>, <NUM> but the total number of connections is reduced. Further, the light output at the edges of the array, for example in group <NUM> is typically more uniform and lower luminous flux than for group <NUM>. 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.

<FIG> 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 <FIG>, LEDs <NUM>, <NUM>, <NUM> 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 <NUM>, <NUM> to remove stray light artifacts. Such an arrangement advantageously improves privacy performance by reducing reflection from the input side.

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

<FIG> is a schematic diagram illustrating a graph of optical window luminous intensity <NUM> against viewing position <NUM> in the window plane of a directional waveguide <NUM>, corresponding to the angle of the output direction, and <FIG> is a schematic diagram illustrating a graph of luminous flux <NUM> distribution for an array <NUM> 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 <NUM>. Such errors can be corrected as shown by distribution <NUM> of scaled luminous fluxes from the respective illuminator elements in comparison to a reference level <NUM>. Advantageously, the luminance distribution of the display may be maintained throughout the device lifetime.

<CIT>, generally describes a luminance calibration apparatus and method wherein photodetectors are arranged at the input side <NUM> of directional waveguide <NUM>, for example as part of the lightbar comprising the array <NUM> and electrical connections. It would be desirable to reduce the cost and complexity of the photodetector and control apparatus.

<FIG> 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 <NUM> may be arranged within a display unit within hingeup <NUM> further comprising camera <NUM>. Hinge <NUM> may be used to connect base unit <NUM> which may comprise a keyboard, photodetector <NUM> and reflective region <NUM>. Camera <NUM> may be arranged to receive light from the waveguide <NUM>, for example after reflection from region <NUM>.

<FIG> 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. <FIG> shows that photodetectors <NUM>, <NUM>, <NUM> may be arranged in parts of the base unit <NUM> including under keys of keyboard <NUM> or other part of the base unit <NUM>, 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, <FIG> shows the relative position of detectors <NUM>, <NUM>, <NUM> with respect to the waveguide <NUM> of the display in the hingeup unit <NUM>. 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 <NUM>. Further when the lid is not fully closed ambient light sensor <NUM> or camera <NUM> may observer reflected light from the keyboard or reflector <NUM>. Advantageously such light can be used to calibrate the light source array <NUM> to compensate for ageing or loss of single LEDs.

<FIG> 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 <NUM> and array <NUM> 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.

Light rays <NUM> from illuminator element <NUM> are directed to reflective end <NUM>, reflected and directed back towards the input end <NUM>. Some of the light from source <NUM> will be extracted by means of light extraction features <NUM>, while some of the light will be incident on at least a portion of the input end <NUM>. Sensor elements that may include <NUM>, <NUM> may be arranged as described above.

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

Array luminous flux distribution controller <NUM> may include for example a stored reference grey level profile <NUM> 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.

Data from sensors <NUM>, <NUM> may be supplied for example to calibration measurement system <NUM> that may provide data to a look up table <NUM> within the luminous flux distribution controller <NUM>. Further selection of luminous intensity distribution (for example to select between luminous intensity distributions <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) may be provided by selection controller <NUM>. 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.

In device manufacture, the output of the sensors <NUM>, <NUM> in response to each of the light sources of the array <NUM> 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.

In operation of a calibration mode, a single illuminator element of the array <NUM> is illuminated and sensors <NUM>, <NUM> 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 <NUM> and the illuminator element controller <NUM> appropriately configured to achieve the desired luminous flux distribution.

Advantageously the light from the whole array <NUM> may be measured by a combination of sensors <NUM>, <NUM> and a desired luminous intensity distribution may be achieved.

The sensor system may be arranged with the waveguide <NUM> 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 <NUM> 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 <FIG>.

<FIG> is a schematic diagram illustrating a further graph of optical window luminous intensity <NUM> against viewing position <NUM> 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 <NUM> of the array of light sources may have failed in region <NUM> compared to the desired output <NUM>.

<FIG> 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 <FIG>. Such an output may occur in the symmetric drive scheme of <FIG> for example. Thus the calibration system illustrated in <FIG> 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 <NUM>. To compensate for the drop, adjacent LEDs on both sides of the optical axis <NUM> may be illuminated with higher outputs <NUM>, <NUM> such that after diffusion the desired output <NUM> may be provided. Advantageously LED compensation may be achieved in symmetric drive systems.

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

<FIG> is a schematic diagram illustrating a graph of nominal LED output against time. Luminous flux <NUM> of a nominal light source of the array <NUM> may degrade with time <NUM> as shown by profiles <NUM>, <NUM> for different drive conditions.

<FIG> is a schematic diagram illustrating a flowchart for compensation of LED ageing. In a first step <NUM> for the nth LED, the peak current, Ipeakn(t) and average current, Iavn(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 <FIG>. In a third step <NUM> the corrected current after PWM control Iavn may be computed and applied to the respective LED. Advantageously LED ageing effects may be compensated and cost and complexity reduced.

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

Spatial light modulator <NUM> may comprise a liquid crystal display that may comprise an input polarizer <NUM>, TFT glass substrate <NUM>, liquid crystal layer <NUM>, color filter glass substrate <NUM> and output polarizer <NUM>. Red pixels <NUM>, green pixels <NUM> and blue pixels <NUM> may be arranged in an array at the liquid crystal layer <NUM>. 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.

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 <NUM> and scatter from waveguide and other surfaces near the input end <NUM> 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.

<FIG> 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.

<FIG> 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. <FIG> similarly illustrates in side view the propagation of rays from light source array 1317a-n. <FIG> illustrates in perspective view the formation of optical windows by light source array 1317a-n. <FIG> illustrates in perspective view a display apparatus comprising an optical stack comprising a waveguide as illustrated in <FIG>.

As described in <CIT>, to which this application claims priority, a directional display device may comprise a waveguide <NUM> that further comprises a reflective end <NUM> that is elongated in a lateral direction (y-axis), the first and second guide surfaces <NUM>,<NUM> extending from laterally extending edges of the reflective end <NUM>, the waveguide <NUM> further comprising side surfaces <NUM>, <NUM> extending between the first and second guide surfaces <NUM>,<NUM>, and wherein the light sources include an array <NUM> of light sources 1317a-n arranged along a side surface <NUM> to provide said input light through that side surface <NUM>, and the reflective end <NUM>, comprises first and second facets <NUM>, <NUM> alternating with each other in the lateral direction, the first facets <NUM> being reflective and forming reflective facets of a Fresnel reflector having positive optical power in the lateral direction, the second facets <NUM> forming draft facets of the Fresnel reflector, the Fresnel reflector <NUM> having an optical axis <NUM> that is inclined towards the side surface <NUM> in a direction in which the Fresnel reflector <NUM> deflects input light from the array of light sources <NUM> into the waveguide <NUM>. Thus angle <NUM> is non-zero. Similarly the second facets <NUM> may be reflective and form reflective facets of a Fresnel reflector having positive optical power in the lateral direction, the Fresnel reflector <NUM> having an optical axis <NUM> that is inclined towards the side surface <NUM> in a direction in which the Fresnel reflector <NUM> deflects input light from the array of light sources <NUM> into the waveguide <NUM>.

Illustrative light ray <NUM> from source 1317a may be arranged to provide optical window 1326a and light ray <NUM> 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 <NUM> in a similar manner to that described for waveguide <NUM> in the arrangement of <FIG> for example.

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 <NUM> and thus may have small form factor. Further light sources <NUM> and <NUM> may be arranged with overlapping optical windows, and thus display luminance may be increased.

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 <FIG> or the short side light source array input of <FIG>. 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.

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

Claim 1:
A directional backlight comprising:
a waveguide (<NUM>) comprising an input end (<NUM>); and
an array of input light sources (<NUM>) arranged at different input positions in a lateral direction across the input end (<NUM>) of the waveguide (<NUM>) and arranged to input input light into the waveguide (<NUM>),
the waveguide (<NUM>) further comprising first and second opposed, laterally extending guide surfaces (<NUM>, <NUM>) for guiding light along the waveguide (<NUM>), side surfaces extending between the first and second guide surfaces (<NUM>, <NUM>), and a reflective end (<NUM>) facing the input end (<NUM>) for reflecting the input light back along the waveguide (<NUM>) and having positive optical power laterally, the second guide surface (<NUM>) being arranged to deflect the reflected input light through the first guide surface (<NUM>) as output light, and the waveguide (<NUM>) 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 (<NUM>) disposed along only a part of each side surface (<NUM>) adjacent the input end (<NUM>), the additional light sources (<NUM>) being arranged to direct additional light into the waveguide (<NUM>) through a respective one of the side surfaces (<NUM>) in a direction in which the additional light is reflected by the reflective end (<NUM>) onto the opposite side surface (<NUM>) and by the opposite side surface (<NUM>) into a segment of the waveguide (<NUM>) adjacent the opposite side surface (<NUM>) extending from a corner between the reflective end (<NUM>) and the opposite side surface (<NUM>).