FIELD OF THE INVENTION

The invention relates to a backlight for a display and in particular, but not exclusively to a backlight for a Liquid Crystal Display (LCD). BACKGROUND OF THE INVENTION

In the last decade, flat panel displays have almost completely replaced traditional Cathode Ray Tube (CRT) displays in almost all applications including as computer monitors, televisions etc.

A popular type of flat panel displays are backlight displays wherein a backlight is generated and modulated by variable and controllable light attenuating layers to provide the desired image. A typical example of such types of display are LCD displays where a liquid crystal layer with individually controllable pixel elements is used to modulate a backlight in accordance with the image to be presented.

However, a problem with many backlight displays, including LCD displays, is that they tend to provide relatively poor contrast ratios and further tend to be relatively power inefficient.

In order to address such disadvantages it has been proposed to use a variable control of the backlight intensity. In particular, it has been proposed to use local backlight dimming wherein different segments of the backlight may be individually controlled dependent on the image characteristics of the area. Local dimming may for example be achieved by generating a backlight from Light Emitting Diodes (LEDs) spread over the area of the display.

However, although such local dimming backlight solutions can improve contrast and reduce power consumption, they tend to require a substantial implementation depth or thickness of the display. In addition, such LED based backlights tend to be relatively costly and difficult to manufacture. There is accordingly a trend to have less and less LEDs in order to reduce the cost of display. This has a direct influence on the thickness of the display that typically scales with the LED pitch (in the direct lit configuration). In order to achieve very thin displays, edge backlight solutions have been proposed where the backlight is produced by edge mounted light sources which inject light into a planar optical element that distributes the light over the display area. Such solutions may provide very thin backlights (e.g. around 1 mm) thereby allowing very thin displays to be manufactured. However, they do not allow for local backlight dimming and thus suffer from the disadvantages associated with conventional backlight displays.

Hence, an improved display backlight solution would be advantageous and in particular a solution allowing increased flexibility, reduced depth, improved local dimming/control/ adaptation, reduced power consumption, improved contrast, reduced cost, facilitated implementation, and/or improved performance would be advantageous.

SUMMARY OF THE INVENTION

Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to an aspect of the invention there is provided a backlight for a display comprising: a group of elongated light guides arranged to illuminate an elongated strip of the display, each light guide of the group of elongated light guides comprising a light in-coupling for receiving light and at least one light out-coupling for radiating light, the at least one light out-coupling having a restricted extension in a longitudinal direction of the light guide; a plurality of light sources, each of the plurality of light sources being arranged to feed light to a light in-coupling of one light guide of the group of elongated light guides; wherein a light out-coupling for at least one light guide of the group of elongated light guides is offset along a longitudinal direction of the elongated strip relative to a light out-coupling of at least one other light guide of the group of elongated light guides.

The invention may allow improved and/or facilitated backlight in many embodiments. In particular, the approach may in many scenarios allow facilitated

implementation and/or reduced cost. The approach may allow for very thin backlights and thus for thin displays.

In particular, the invention may in many embodiments allow low cost and thin backlight displays with local backlight control thereby allowing increased contrast and reduced power consumption.

The approach may allow substantially one dimensional light guides to provide a two-dimensional backlight segmentation allowing for each segment to be individually controlled simply by controlling a light source. The light guides may be simple, low cost light guides e.g. manufactured from inexpensive transparent plastic.

The elongated strip may be a rectangular plane parallel to a plane formed by at least some of the elongated light guides. The length of the strip may be substantially longer than the width of the strip, e.g. it may be no less than 2, 4 or 10 times longer than it is wide. The group of light guides may have a combined width corresponding to the width of the strip or may e.g. be substantially narrower.

The elongated light guides of the group may be arranged substantially parallel to each other and may be substantially parallel to the strip. The elongated light guides may have the same length or may be of different lengths.

The light in-coupling for a light guide may be at an end of the light guide. A light out-coupling may e.g. be generated as one of a (fully or partly) painted surface section, a roughed surface section (done by means of e.g. sand blasting of laser ablation), a wedge shape, a reflecting slanted half-way ending, a micro-structured optical local eat-away pattern (maybe with graded index) etc.

The light out coupling may specifically correspond to a light out-coupling area. The light in-coupling may specifically correspond to an in-coupling facet or side of the light guide.

In many embodiments, the extension of the light out-coupling in the longitudinal direction is no more than 30%, 20%, 10% or even 5% of the display active area dimension in that direction.

In many embodiments, no less than 70%>, 80%>, 90%> or even 95% of light entering a light guide exits the light through the light out-couplings.

In some embodiments, the light guide may specifically be a hollow light pipe. The display (area) may be divided into a plurality of elongated strips (which e.g. may be horizontal or vertical strips). Such an approach combined with the offset light out-couplings may provide a two-dimensional backlight segmentation with individually controllable back light. Backlight for each of the elongated strips may be provided by a group of elongated light guides. The groups of elongated light guides may e.g. be substantially identical arrangements providing substantially identical backlight segmentation.

In some embodiments, the light out-couplings of the group of elongated light guides are arranged to substantially uniformly illuminate the strip in the longitudinal direction. In some embodiments the group of light guides may further be arranged to substantially uniformly illuminate the strip in a direction perpendicular to the longitudinal direction. A uniform illumination may specifically be one where the maximum backlight intensity varies by less than 20% or even 10% in some scenarios.

In accordance with an optional feature of the invention, the group of elongated light guides comprises at least two layers of overlapping light guides offset relative to each other in a direction perpendicular to the strip.

This may in many embodiments provide an efficient arrangement and allow finer longitudinal backlight segmentation while maintaining a low width of the strip.

The strip may define a planar surface of the display and the arrangement of the two layers may be at different distances to the planar surface in the perpendicular direction to the surface.

In accordance with an optional feature of the invention, at least a first layer of the at least two layers is arranged to radiate backlight for the display through at least a second layer of the at least two layers.

This may in many embodiments provide an improved performance and/or facilitated implementation.

In some embodiments the light guides may be arranged such that light of light out-couplings from one layer is projected at least partially through gaps between light guides of the other layer.

In some embodiments the light guides may be arranged such that light of light out-couplings from one layer is projected at least partially through light guides of the other layer.

In accordance with an optional feature of the invention, at least one light source of the plurality of light sources is a variable light source having a variable colour spectrum.

This may provide improved performance in many embodiments while allowing a low complexity and typically low cost implementation. The approach may provide an improved local adaptation of the backlight. In particular, the backlight may be adapted to reflect local color characteristics while still maintaining low cost, high contrast, high power efficiency, thin dimensions etc.

In accordance with an optional feature of the invention, the variable light source comprises a plurality of differently coloured light sub-sources, and a light guide of the group of elongated light guides associated with the variable light source is arranged to mix light outputs from the differently coloured light sub-sources. This may provide improved performance in many embodiments while allowing a low complexity and typically a low cost implementation. The differently coloured light sub-sources may e.g. be separate light sources, such as differently coloured LEDs, or may be single light sources arranged to provide a plurality of different spectra. The differently coloured light sub-sources may comprise a white light sub-source.

In accordance with an optional feature of the invention, at least some of the light guides of the group of elongated light guides have light out-couplings with different offsets along the longitudinal direction relative to the light in-coupling.

This may provide a particularly advantageous implementation in many embodiments and may e.g. simplify the mounting and fixation of the light guides and may provide a uniform illumination in the longitudinal direction of the display.

In accordance with an optional feature of the invention, at least a first light guide of the group of elongated light guides comprises at least two light out-couplings.

This may be particularly advantageous in many embodiments. For example, it may in many embodiments increase the available number of backlight segments along the horizontal direction.

In accordance with an optional feature of the invention, the first light guide comprises two light in-couplings and the plurality of light sources comprises a first light source for a first of the at least two light in-couplings and a second light source for a second of the at least two light in-couplings.

This may be particularly advantageous in many embodiments. For example, it may in many embodiments increase the available number of backlight segments along the horizontal direction while providing a low complexity approach (e.g. using less components) for individually controlling the backlight for each segment.

The light in-couplings may specifically correspond to opposite ends of the elongated light guides and the two light sources may be located at opposite ends of the light guide. This may e.g. improve the uniformity by reducing the differences (between segments with different offsets to the in coupling) induced by light absorption or unwanted scattering during the transport.

In accordance with an optional feature of the invention, the backlight further comprises a driver for individually controlling the first light source and the second light source to provide individual backlight control for a first backlight segment of the strip associated with a first of the two light out-couplings and a second backlight segment of the strip associated with a second of the two light out-couplings. This may provide improved performance in many scenarios.

In accordance with an optional feature of the invention, the first light guide comprises a light throughput attenuation section between the two light out-couplings.

This may allow improved separation between the backlight segments formed by the two light out-couplings. In particular, it may reduce the cross-talk between the two segments and may e.g. allow an increased backlight dynamic range to be achieved for each segment.

The light throughput attenuation section may be an integrated part of the light guide or may be a separate feature providing the light throughput attenuation. Specifically, the light throughput attenuation section may be a section arranged to absorb, reflect or out- couple light reaching the section from one or both directions of the light guide.

In accordance with an optional feature of the invention, the backlight further comprises a driver for individually driving the plurality of light sources to provide local backlight control.

This may provide improved performance. The local backlight control allows the backlight for different segments of the strip to be set differently.

In accordance with an optional feature of the invention, at least one light guide of the group of elongated light guides is a controllable light guide arranged to control a light output from at least one light out-coupling in response to a control signal.

This may provide additional flexibility and improved performance in many scenarios. The control functionality of the controllable light guide may e.g. be an integral part of the light guide or may be part of a light out-coupling arrangement e.g. comprising a variable attenuation of out-coupled light.

In accordance with an optional feature of the invention, the strip comprises no more than 10000 pixels per light out-coupling.

The invention may allow a fine granularity backlight segmentation to be provided with low complexity, cost, etc.

In accordance with an optional feature of the invention, a light out-coupling of at least one light guide of the group of elongated light guides has an extension no less than twice a largest cross section dimension of the at least one light guide of the group of elongated light guides.

This may allow an extended and distributed light out-coupling over a large interval in the longitudinal direction resulting in a more uniform backlight in the longitudinal direction. According to an aspect of the invention there is provided a display comprising: a light modulating layer; a group of elongated light guides arranged to illuminate an elongated strip of the light modulating layer, each light guide of the group of elongated light guides comprising a light in-coupling for receiving light and at least one light out-coupling for radiating light, the at least one light out-coupling having a restricted extension in a longitudinal direction of the light guide; a plurality of light sources, each of the plurality of light sources being arranged to feed light to a light in-coupling of one light guide of the group of elongated light guides; wherein a light out-coupling for at least one light guide of the group of elongated light guides is offset along a longitudinal direction of the elongated strip relative to a light out-coupling of at least one other light guide of the group of elongated light guides.

These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

FIG. 1 illustrates an example of a backlight display in accordance with some embodiments of the invention;

FIG. 2 illustrates a view of an exemplary display in accordance with some embodiments of the invention;

FIG. 3 illustrates an example of a layer of light guides for a display in accordance with some embodiments of the invention; and

FIG. 4 illustrates an example of a light guide for a display in accordance with some embodiments of the invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

The following description focuses on embodiments of the invention applicable to a backlit LCD display. However, it will be appreciated that the invention is not limited to this application but may be applied to many other displays using backlights.

FIG. 1 illustrates an example of a backlight display in accordance with some embodiments of the invention. The backlight display comprises a backlight 101 that creates light which is projected towards a light modulating layer 103. The light modulating layer 103 comprises a typically large number of picture elements that can be individually controlled to provide a variable attenuation of the light from the backlight 101. Thus, the light modulating layer 103 comprises controllable light attenuating elements that form the individual pixels of the display. For clarity and brevity FIG. 1 illustrates only the backlight 101 and the light modulating element 103 but it will be appreciated that practical displays may comprise additional layers such as light diffusing layers, reflective layers, protective screens etc.

The display of FIG. 1 comprises a receiver 105 which receives an image signal comprising image data for an image to be rendered by the display. The input signal may for example be a video signal comprising a temporal sequence of images to be displayed.

The receiver 105 is coupled to a backlight controller 107 and an LCD controller 109. As will be described in the following, the backlight 101 is arranged to provide a plurality of backlight segments that may be individually controlled. Thus, the backlight may be set differently in the different backlight segments thereby allowing local control of the backlight in order to adapt the backlight to the specific image characteristics in the locality of the image.

The backlight controller 107 is coupled to the backlight 101 and is arranged to generate a drive signal that controls the luminance level of each of the different backlight segments. As a low complexity example, the backlight 101 may be a white backlight with an intensity of each segment being determined from the input signal, i.e. with the light intensity being set to the minimum intensity value that for a fully open LCD element (of the light modulating layer 103) will result in a light output from the display corresponding to the maximum intensity indicated by the image data for that segment.

The backlight controller 107 is further coupled to the LCD controller 109 which is coupled to the light modulating layer 103. The LCD controller 109 generates drive signals for each LCD element of the light modulating layer 103 corresponding to each pixel of the display. The drive signal for a given pixel may specifically be determined such that the light attenuation of the LCD element results in the backlight incident on the LCD element being attenuated to provide the appropriate light output for the image data value for the pixel.

Thus, the setting of the attenuation of the LCD element depends on the backlight level. In simple embodiments, the LCD controller 109 may simply assume that the incident light on an LCD element is the backlight provided by the backlight segment to which the LCD element belongs. However, in many practical implementations, the cross talk between different backlight segments may be significant and the LCD controller 109 may determine the total amount of backlight incident on the LCD element from a plurality of backlight segments before determining the appropriate attenuation level. FIG. 2 illustrates a view of an exemplary display in accordance with some embodiments of the invention. The view correspond to a frontal view of the display with the frontal layers (including the light modulating layer 103) being transparent such that the backlight 101 is visible. In the example, the display area 201 is divided into different backlight areas. In particular, the display area 201 is divided into elongated strips 203, 205, 207. In the example of FIG. 2, the display area 201 is divided into three strips 203, 205, 207 but it will be appreciated that in other embodiments more or fewer strips may be used.

Indeed, in some embodiments only one strip may be used which may cover the entire display area.

Each of the strips is illuminated by a backlight created by a group of elongated light guides 209, 211, 213. In the example, each strip is illuminated by an identical group of light guides in an identical arrangement. Accordingly, the following description will focus on the backlight illumination of the first strip 203.

The example of FIG. 2 illustrates the backlight illumination of the first strip 203 being provided by three elongated light guides 209, 211, 213 but it will be appreciated that in other embodiments fewer or more light guides may be used for each strip.

The light guides 209, 211, 213 comprise a light in-coupling which is arranged to receive incident light. In the example, the light in-coupling is simply an end facet of the light guide which may receive light from a light source 215.

The light source 215 may for example be a LED which radiates light into the end of the light guide 209. In the example of FIG. 2, each light guide 209, 211, 213 is associated with only a single light source 215 located at one end of the light guide 209, 211, 213. However, in other embodiments, each light guide 209, 211, 213 may be associated with more than one light source. E.g. in the example of FIG. 2, a LED may be located at each end of one or more of the light guides 209, 211, 213 such that light is fed into the light guide from both ends of the light guide. This may in many scenarios provide a more homogenous light distribution and a reduced sensitivity to the position of the light out-coupling along the light guide.

In the example, the light guide may for example be transparent plastic light guides which maintain the light within it due to total internal reflections (TIR) at the edges of the light guide. Examples of such transparent plastic materials are Polymethyl methacrylate (PMMA) or polycarbonate (PC). When light enters such a material from air, it undergoes a refraction process and the beam will be collimated within the critical angle of the material (from the normal to the surface). For the PMMA the critical angle is approx. 42°. If the side facets are normal to the in-coupling facets, the TIR condition will always be fulfilled (light will impinge on the surface under an angle of e.g. 48° for PMMA, larger than the critical angle).

Each of the light guides is furthermore arranged with a light out-coupling 217, 219, 221 for out-coupling of the light from the light guide. In the ideal case all the light from the light source 215 will thus exit through the light out-coupling 217. However, in practical implementations it will be appreciated that there may be some light leakage and light absorption due to various imperfections. However, in most practical embodiments no less than 80%, 90% or even 95% of the light entering the light guide leaves through a light out- coupling 217, 219, 221.

The light out-couplings 217, 219, 221 are restricted in the longitudinal direction (i.e. along the length direction of the light guide) and thus light radiation from the light guides 209, 211, 213 are restricted to specific intervals along the longitudinal direction.

The preferred extension of the light out-coupling along the longitudinal direction of the light guide depends on the specific preferences and requirements of the individual embodiment. However, in many embodiments, the extension of each light out- coupling is advantageously no more than 50%, 30%, 20%, 10%> or even 5% of the length of the strip (and in the specific case thus of the width of the display area 201).

The different light guides 209, 211, 213 of the group of light guides that illuminate the strip 203 are in the example not identical but have substantially the same dimensions, i.e. substantially the same cross section and length. However, the light out- couplings 217, 219, 221 are positioned differently along the longitudinal direction. In the example of FIG. 2, the first light guide 209 has a light out-coupling 217 substantially centered around 5/6 ^th along the light guide, the second light guide 211 has a light out- coupling 219 substantially at the middle of the light guide, and the third light guide 213 has a light out-coupling 221 substantially l/6 ^th along the light guide.

Thus, the light out-couplings 217, 219, 221 for the different light guides 209, 211, 213 are offset relative to each other along the longitudinal direction of the elongated strip 203. E.g. the first light out-coupling 217 is offset relative to the second light out- coupling 219 (and to the third light out-coupling 221) along the longitudinal direction of the strip 203. As a result, the light from each light guide 209, 211, 213 is radiated at a different position along the longitudinal strip direction, i.e. at different horizontal positions in the example of FIG. 2. As a consequence, each light guide 209, 211, 213 provides backlight for a backlight segment of the strip 203 and thus the approach allows for the strip to be divided into a plurality of different backlight segments. This combined with the use of a plurality of strips provides for a two dimensional backlight segmentation. For example, in FIG. 2, the display area is divided into nine different backlight segments.

Furthermore, such a system is achieved using very simple and low cost light guides which are essentially one dimensional. In addition, the approach uses a low complexity edge lighting of the light guides and may substantially facilitate manufacturing and reduce cost. Furthermore, as the backlight is provided by typically very low cross section light guides, a very narrow backlight layer can be implemented thereby allowing very thin displays to be manufactured.

In addition, the display can easily be manufactured with individual light sources, such as LEDs, for each light guide. This allows the light for each light guide to be individually controlled and as each light guide corresponds to one backlight segment (i.e. illuminates a limited area) the approach allows for an efficient and easy to implement local backlight control. This individual backlight control is furthermore a two dimensional backlight control despite using substantially one dimensional light guides and the light sources possibly being arranged in a simple one dimensional row. Thus, the approach allows for substantially improved two-dimensional backlight control using one dimensional functional elements.

In different embodiments, different types of light out-couplings may be used. For example, in some embodiments the light out-coupling may simply be generated by painting a part of the surface of the light guide by a white colour. This will result in the light hitting the surface being scattered and partially refracted out of the light guide rather than being reflected back into it.

A typical example of an out-coupling structure is the above mentioned white paint. Small dots of white paint in optical contact with one surface of the light guide will scatter the incident light in a Lambertian manner with the result that light within the critical angle cone from the normal to the light guide surface will not fulfill the TIR condition and be out-coupled.

Another example is roughening of the surface by means of e.g. sand blasting or laser ablation. The light scattering in this case results in a more narrow light distribution close to the specular direction of the incident light.

In the example, the light guide may specifically be a transparent plastic element such as polycarbonate (PC) or Polymethyl methacrylate (PMMA).. In other embodiments, the light guide may for example be a hollow light guide (a light pipe). The light guides may often have a maximum cross section dimension of between 1 mm and 2 cm. Typically the cross section may advantageously have an area between 1 mm ² and 2 cm ² .

It will be appreciated that the dimensions of the light out-couplings may depend on the specific preferences of the individual embodiment and design. In some embodiments the extension of the light out-couplings of the light guides may be at least twice the largest cross section dimension of the light guide. E.g. for a square light guide the longitudinal extension of the light out-coupling may be at least twice the diagonal of the cross-section, and the for a circular light guide the longitudinal extension of the light out- coupling may be at least twice the diameter of the cross-section.

Thus, in some embodiments the light out-couplings may be e.g. rectangular with a much longer extension in the longitudinal direction than possible in the perpendicular direction. Such an approach may provide an improved masking of the locally out-coupled light and may specifically provide a more uniform illumination of the backlight segment.

In the example of FIG. 2 the elongated light guides are arranged in parallel along the longitudinal direction of both the light guides and the strip. However, it will be appreciated that in other embodiments, the light guides need not be arranged parallel to each other or to the strip as long as the offset of the light out-coupling differs between (at least some) of the light guides. Also, in some embodiments, some of the light guides supporting a strip may have light out-couplings with the same offset. For example, two light guides may in some embodiments be used to provide backlight for one backlight segment.

In many embodiments the light guides may be arranged to substantially uniformly illuminate the strip in the longitudinal direction. For example, the light out- couplings are arranged equidistantly such that they (e.g. in combination with a diffusion element) achieves a substantially uniform illumination across the entire segment covered by the light out-coupling and between segments covered by different light out-couplings. In many embodiments a substantially uniform illumination may also be achieved in the direction perpendicular to the longitudinal direction. This may e.g. be achieved by including a light diffusion layer between the light guides and the light modulating layer 103 or by creating an emission pattern such that the final illumination is uniform (e.g. by means of a curved top surface of the light guide). A substantially uniform illumination may specifically be one where the maximum intensity illumination varies by less than 20% or even 10%.

In the example of FIG. 2, the different offsets of the light out-couplings are achieved using light guides for which the light out-couplings are positioned differently along the light guides. However, it will be appreciated that other options are possible for achieving the different offsets. For example, in some embodiments identical light guides may be used but the individual light guides may be arranged with an offset relative to each other. In other embodiments this may e.g. be achieved by using light guides with different lengths.

In the described system the backlight thus comprises a plurality of substantially one dimensional light emitters formed by side light sources and associated light guides. Each of the light emitters comprises at least one light out-coupling (with restricted extension in the one dimensional direction). The light out-couplings of at least some of the one dimensional light sources are offset relative to each other along the one dimensional direction.

The side light sources may be individually controlled and the arrangement accordingly provides a two dimensional array of backlight segments that may be individually controlled simply by controlling the light output of the side light sources.

The specific example combines edge lit (LEDs on the side) with local dimming using a long, but thin and narrow light guides. The light that enters the light guide remains in the light guide due to total internal reflection. An out-coupling structure on the light guide regulates the out coupling of light in the area of the backlight. Because that area is typically much larger than the LED surface, it is much easier to make the total backlight uniform. Together the light guides form a grid of individually controllable backlight sources, just like an area lit by a backlight with a matrix of LEDs.

The approach may thus provide the advantages of local dimming/ backlight control while at the same time allowing low cost, low complexity implementation, simple backlight control, and in particular while still allowing a very thin display to be

manufactured.

It will be appreciated that the number of individually controllable backlight segments may vary depending on the specific embodiment. However, the approach typically allows each backlight segment to correspond to no more than 10000 pixels, or in many embodiments advantageously no more than 5000 or even 1000 pixels. Thus, the display area may be divided into strips wherein there are no more than 10000 pixels, (or in many embodiments advantageously no more than 5000 or even 1000 pixels) per light out-coupling. This may provide improved local backlight control thus resulting in reduced power consumption, increased contrast etc.

In some embodiments, the light guides may be arranged in a single

substantially planar layer. However, this may in some embodiments be restrictive as it may limit the number of light guides that can be used for a given size of the strip. This may again restrict the number of light out-couplings and thus may restrict the longitudinal division of the strip into different backlight segments.

In some embodiments such considerations may be addressed by the light guides being offset relative to each other in the direction which is perpendicular to the plane of the strip 203, i.e. which is perpendicular to the display surface. Thus some light guides may be placed at least partially behind other light guides with reference to the display area

(and thus the backlight propagation direction).

The light guides may be arranged in at least two layers of overlapping elongated light guides offset relative to each other in a direction perpendicular to the strip. A layer may in some scenarios correspond to a single light guide but will typically comprise a plurality of light guides arranged substantially at the same relative depth to the display front/ light modulating layer 103.

For example, a first layer of light guides may be positioned closely together to form a substantially planar light guide layer at a given depth. The light guides of this layer may have light out-couplings in the left half of the strip (using the exemplary orientation of

FIG. 2 for reference) and may thus divide the left half of the strip into N backlight segments where N is the number of light guides in the first layer.

Behind the first layer a second layer of light guides may be positioned close together to also form a substantially planar light guide layer. This second layer of light guides may have light out-couplings in the right half of the strip (for brevity using the exemplary orientation of FIG. 2 for reference) and may thus divide the left half of the strip into M backlight segments where M is the number of light guides in the first layer. Typically, N and

M may be the same and indeed in some embodiments, the first and second layer may use substantially identical light guides which are simply turned in opposite directions in the two layers.

Thus, the approach may allow a horizontal segmentation of the strip into M+N segments while maintaining a width of the light guide arrangement corresponding to the width of only N (or M) segments.

A consideration facing such an arrangement is that the light guide of the second layer must project its backlight through the first layer. In some embodiments, the light from the out-couplings of the second layer may propagate at least partially through the light guides of the first layer and this may result in refraction and/or diffraction which may shift the effective position of the backlight provided. In some embodiments, such position shifts may be pre-calculated and compensated for. For example, the positions of the light out- couplings may be moved to result in an illumination of a desired area of the strip following the optical effect of the propagation through the first layer. Alternatively or additionally, the modified position of the illuminated area may be determined and the local backlight control may be modified to be based on the image characteristics in that area, i.e. the system may simply use the shifted position of the backlight segment.

In some embodiments, gaps may be present between the light guides of the first layer, and the light out-couplings of the second layer may be arranged to propagate at least partially through the gaps of the first layer. Indeed, in some embodiments, the light guides may be arranged to have a shape that increases the gap between light guides. An example of such is shown in FIG. 3 wherein three light guides 301 of a first layer are illustrated. The light guides 301 have light out-couplings 303 on the left side of the strip and then narrows to a much smaller cross sectional area (while maintaining the same length to allow e.g. fixation at both ends of the display). The narrowing of the light guides 301 provides substantially increased gaps wherein light out-couplings 305 of a second underlying layer may be positioned.

In some embodiments the light guides may e.g. be arranged to have a variation in the dimension perpendicular to the longitudinal direction of the strip. For example, the light guides may be arranged to have a bending between the layers. For example, the sections of a light guide which transports the light may be arranged to reside in the lower layer with the sections corresponding to the light out-couplings being arranged to be closer to the screen, i.e. the light guides may be arranged to be shaped such that the transport sections of the light guides are predominantly in the lower layer whereas the sections corresponding to the light out-couplings are in the upper or top layer.

In some embodiments light guides from the different layers may have different lengths in such a way that the light from the lower layer(s) will not need to travel through the upper layers. For example, rather than narrowing, the light guides 301 of FIG. 3 may simply be truncated and end at the onset of the tapering.

Indeed, light guides may also have different lengths in scenarios wherein only a single layer is utilized. For example, shorter light guides may be used to illuminate close to the edge and longer light guides may be used to illuminate further away from the edge (e.g. towards the middle or perhaps towards the other edge of the display). Thus, the light guides may be disparate and have differing lengths which for example may correspond to the distance from the edge to the backlight segment, and thus the light out-coupling, of the specific light guide. In some embodiments, the width/cross section of the light guide may vary along the individual light guide. For example, the cross sectional area of the light guide may be higher for a section corresponding to a light guide than for a section corresponding to a light transport section (i.e. outside the light out-coupling).

Specifically, a relatively narrow light guide (say with a diameter of l-2mm) may widen at the specific point of the light out-coupling (say to a diameter of 5- 10mm). This may specifically combine narrow physical characteristics of most sections of the light guide with a broad and better dispersed light radiation from the light out-coupling. This may for example be advantageous in some multi-layer arrangements as the thin sections of the light guides may provide significant gaps through which lower layers can illuminate.

The approach may also be advantageous in many single layer

implementations. For example, the widening of the light out-coupling section relative to transport sections may allow it to extend into the space allocated to the neighboring light guide which has a corresponding narrow transport section. For example, light guides may be arranged in a single layer with a pitch of 2 mm. Each light guide may have a diameter of 1 mm during transport sections and 3 mm during light out-coupling sections. Thus, for a given section wherein a first light guide has a light out-coupling, the neighboring light guides have transport sections and thus only extend 0.5 mm from the pitch center. Accordingly, a space of 3 mm is available for the light guide out-coupling. This increased light out-coupling dimension may for example allow a better illumination of the entire backlight segment associated with the light out-coupling.

In some embodiments, the light sources 215 may advantageously be light sources with a variable colour output. Thus, rather than merely providing a white light output, the light sources 210 may be coloured light sources and may be variable not only in intensity but also in the colour output which is generated. Thus, the light source may be controlled to provide a variable spectrum.

This may for example be achieved by the light source comprising a plurality of differently coloured light sub-sources where the intensity of each of the light sub-sources may be individually controlled.

Thus, in some embodiments the white LED can be replaced by e.g. a three-in- one RGB LED package or with an R, G, and B LED package arranged close together.

Such an approach may be particularly advantageous as the elongated light guides are particularly suitable for color mixing. Indeed, due to the relatively long light guides or light pipes a very efficient color mixing is often achieved resulting in the colored light output from the light out-couplings appearing as a single colored light source even if the incident light is generated by a plurality of differently colored light sources.

The use of colored light sources provides for improved local backlight adaptation. For example, it may allow the spectra of the backlight to be adjusted to match the dominant color in the image area supported by the backlight segment. This may reduce power efficiency and increase the image quality.

In the previous examples, each light guide has only had one light out-coupling and thus there has been a direct correspondence between light guides and backlight segments. However, in some embodiments, one or more of the light guides may advantageously comprise more than one light out-coupling. As a simple example, a plurality of light guides may e.g. be used to provide a more homogeneous illumination of the backlight or may e.g. allow a light guide to support a plurality of backlight segments. In some embodiments, the display may comprise individual light sources for the two light out-couplings.

An example of such a light guide arrangement is illustrated in FIG. 4. In the example, a light guide 401 comprises a first and second light out-coupling 403, 405 which are displaced relative to each other in the longitudinal direction. The light out-couplings 403, 405 may preferably be designed to have a relatively large distance between them thereby reducing light coupling from one light out-coupling to the other.

The arrangement furthermore comprises a first light source 407 and a second light source 409 arranged at each end of the light guide 401, i.e. at opposite ends of the light guide 401. Thus, the two light sources 407, 409 inject light into the opposite ends of the light guide 401. Accordingly, the light of the first light source 407 will first reach the first light out-coupling 403 and a substantially part of this light will be radiated from the first light out- coupling 403. Similarly, the light of the second light source 409 will first reach the second light out-coupling 405 and a substantially part of this light will be radiated from the second light out-coupling 405.

Such an approach may allow an improved flexibility and may in particular provide a plurality of backlight segments from each light guide thereby allowing smaller backlight segments for the same number of light guides. In many practical implementations, the number of light guides for each strip may be restricted e.g. due to the size of the light guides relative to the strip or due to practical considerations such as fixation or mechanical strength considerations for the light guides. However, by providing the light guides with a plurality of light out-couplings and associated independently controlled light sources, the number of horizontal segments is not restricted to the number of light guides but rather it may be substantially increased. E.g. using the light guide arrangement of FIG. 4, the number of horizontal segments may be doubled for a given number of light guides. This approach may further be used to reduce the width of the strip (as fewer light guides are necessary) thereby also allowing a reduced vertical extension of each segment. Thus, the display area may be divided into a larger number of smaller backlight segments thereby allowing improved local backlight control and adaptation.

In such embodiments, the backlight controller 107 may thus be arranged to drive the two light sources 407, 409 differently. This may allow the backlight of the two light out-couplings to be different. However, in standard light guides the optical isolation between the two light out-couplings 403, 405 may not be very high as the absorption and light leakage is relatively low. Thus, the cross-talk between the two backlight segments may be high and the resulting correlation between the generated backlight may accordingly be relatively high.

In order to reduce the correlation between the backlight segments, the optical isolation between the two light out-couplings 403, 405 may be increased by the light guide 401 comprising a light throughput attenuation section 411 between the two light out- couplings 403, 405. The light throughput attenuation section 411 may thus increase the optical isolation between the light out-couplings and thus increase the independence and decrease the correlation between the backlight segments associated with the light out- couplings.

The light throughput attenuation section 411 may be at least one of an increased absorption section, a light reflecting section and a light out coupling section.

For example, the light guide may be generated to comprise a section 411 which has an increased light absorption and which thus attenuates the light from either direction. Such an increased light absorption may e.g. be achieved by including various imperfections in the light guide material. In some embodiments, the light throughput attenuation section 411 may specifically be a light throughput blocking section such that light from one side of the section 411 is substantially prevented from reaching the other side. Such a blocking section may for example be achieved by coloring/painting a section of the light guide substantially black.

In some embodiments a light reflecting section may be used to provide improved isolation. For example, a mirror material may be integrated in the light guide to reflect light waves from the two directions.

In some embodiments, the light throughput attenuation section 411 may be formed by a light out-coupling. For example, a light out coupling may be generated by painting the light guide surface in a white color. The light out-coupling should in many embodiments be arranged to radiate the light such that it does not reach the light modulating layer 103 and thus does not contribute to the illumination of the light modulating layer 103. For example, the light out-coupling may be arranged to radiate light to the back or side of the display area.

The light throughput attenuation can be formed by a wedge shape at the edge of the segment. This will squeeze the remaining light out

In some embodiments, one or more of the light guides may be a controllable light guide wherein the light output from the light out-coupling is not only given by static characteristics and the incident light but is also dynamically dependent on a controllable characteristic of the light guides.

The controllable characteristics may be an integral characteristic of the light guide or may be provided by an external or internally positioned controllable element.

For example, in some embodiments a light attenuating element may be positioned in front of the out-coupling area of the light guide. For example, an LCD element may be positioned in front of this area with the opacity of the LCD element being controllable by an electrical signal.

As another example, the light guide may comprise an element that can change optical characteristics in response to the application of an electrical or magnetic signal.

As yet another example, the light guide may be a hollow light guide comprising an electromechanical element. For example, the light guide may comprise a mechanically moveable micro-mirror which can be turned in response to an electrical or magnetic signal. When turning the micro-mirror the amount of light reflected out through the light out-coupling may be varied.

Such controllable features may allow an improved backlight control and may e.g. allow increased flexibility or dynamic range for the local backlight control.

In the example of FIG. 2, the light was injected from the same end of the light guide for all light guides. However, in some embodiments, the light may be injected from the nearest end to the out-coupling. This may in many embodiments reduce light leakage and absorption. Indeed, in some embodiments, light may be injected from both ends.

In some embodiments, the light sources may be arranged to have a light emitting area less than a surface of the light in-coupling. In particular, a LED having an efficient light radiation surface which is less than the cross-sectional area of the light guide may be used. It will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional circuits, units and processors. However, it will be apparent that any suitable distribution of functionality between different functional circuits, units or processors may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controllers. Hence, references to specific functional units or circuits are only to be seen as references to suitable means for providing the described functionality rather than indicative of a strict logical or physical structure or organization.

The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be

implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units, circuits and processors.

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements, circuits or method steps may be implemented by e.g. a single circuit, unit or processor.

Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate.

Furthermore, the order of features in the claims do not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus references to "a", "an", "first", "second" etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example shall not be construed as limiting the scope of the claims in any way.