WO2007072360A1

WO2007072360A1 - Backlight arrangement for highlighting a display panel

Info

Publication number: WO2007072360A1
Application number: PCT/IB2006/054860
Authority: WO
Inventors: Willem L. Ijzerman; Wouter Oepts; Michel C. J. M. Vissenberg; Dirk K. G. De Boer; Marcellinus P. C. M. Krijn
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-12-21
Filing date: 2006-12-14
Publication date: 2007-06-28
Also published as: TW200734763A

Abstract

A backlight assembly (100) comprising at least one light source (101) and an optically transparent wave guide plate (102), which wave guide plate comprises a front surface (103), an opposing back surface (104) and a surface (105) for receiving light from said at least one light source (101), connecting said front and back surfaces, is provided. The thickness of the wave guide plate (102) decreases with the distance from said surface (105) for receiving light, and the backlight assembly (100) comprises means (106) for controlling the angle of incidence of light from said at least one light source on said surface for receiving light. By varying the angle of incidence on the surface for receiving light, the location on the wave guide where the light is extracted from the wave guide can easily be controlled.

Description

Backlight arrangement for highlighting a display panel

The present invention relates to a backlight assembly comprising at least one light source and an optically transparent wave guide plate, which wave guide plate comprises a planar front surface, an opposing planar back surface and a surface for receiving light from said at least one light source, connecting said front and back surfaces. The present invention also provides a display device comprising a backlight assembly of the present invention. In addition, the present invention provides a method for illuminating a defined area of a display surface.

For displays such as LCD displays it is in certain applications desirable that there is the possibility to make one area, such as a pixel or a group of pixels, much brighter than its surrounding. This highlighting can be performed by a dedicated backlight that is able to locally create a higher brightness. In such a dedicated backlight, it is not necessary that the light is created in a very small area of one or two pixels only (like is done in a CRT). The human eye is not able to distinguish darker areas that are located next to bright areas due to intra-ocular scattering. Therefore, it suffices to create a backlight that is able to create a high brightness in a restricted area of for instance 10 x 10 pixels. For instance for a television application, it is desired that the enlightened area has a typical size of 3 x 3mm, (assuming a pixel size of 300 x 300 μm). Although the size of the highlighted part can be larger than a few pixels, it is desired that the positioning is arbitrary (at pixel level). It is desired that the highlighted pixel can be positioned at every pixel location and it is favorable that this pixel forms the center of the highlighted part.

One approach for highlighting portions of a display panel is disclosed in US patent application no 2004/0047141 Al, to An, disclosing a backlight unit structure comprising a lamp disposed under the display panel and a reflection plate rotatably disposed around the lamp having an opening through which light can be concentrated on the display panel. By rotating the reflection plate so that the opening in the reflection plate is located between the display panel and the lamp, the part of the display paned immediately above the lamp will be highlighted.

However, this approach requires a plurality of such lamps arranged on the backside of the display panel, or alternatively, one such lamp movably arranged on the backside of the display panel. Both of these alternatives require a quite complicated arrangement to provide highlighting of arbitrary positions of the display panel.

If such an arrangement would be used as a supplement to an ordinary backlight, i.e. not providing the ordinary lighting, but only the highlighting of arbitrary positions, the arrangement according to 2004/0047141 would present a light blocking obstruction, lowering the light efficiency of the backlight.

Thus, there still is a need for a highlighting assembly, which can highlight arbitrarily positions of the display panel, and which optionally can be arranged between a display panel and a backlight, without drastically impairing the light utilization efficiency of the backlight.

It is an object of the present invention to overcome these problems, and to provide a high lighting arrangement, which can highlight arbitrarily positions of a display panel and optionally be arranged between a display panel and a backlight, without drastically impairing the light utilization efficiency of the backlight.

The present inventors have surprisingly found that this and other objects may be achieved with the use of a wave guide plate, which for example can be arranged on the backside of a display panel.

Such a wave guide plate may in addition be arranged between the display panel and a backlight in order to provide highlighting of the display panel.

Alternatively, such wave guide plate may also be used in a display, for use as a backlight.

Thus, according to a first aspect, the present invention provides a backlight assembly comprising at least one light source for providing light to an optically transparent wave guide plate, which wave guide plate comprises a front surface, an opposing back surface and a surface for receiving light from said at least one light source, connecting said front and back surfaces.

According to the present invention, the thickness of the wave guide plate decreases with the distance from said surface for receiving light. Further, according to the present invention, the backlight assembly comprises means for controlling the angle of incidence of light from said at least one light source on said surface for receiving light.

The distance between the front and back surfaces (the thickness of the wave guide plate) decreases with the distance from the surface for receiving light, thus forming a wedge-like shape, such as a wedge or a truncated wedge.

Further, the light guide is made of an optically transparent material having a refractive index higher than the surrounding material.

Light is received into the wave-guide via the surface for receiving light, is alternately reflected on the front and the back surfaces. Due to the wedge-like shape, the angle of incidence of the light encountering the front and back surfaces respectively will successively increase, until the angle of incidence supersedes the critical angle for total internal reflection. At this location, at least part of the light will be extracted from the waveguide. The location of this extraction position depends on the angle of the incoming light, and thus on the angle of incidence of the light from the light source on the surface for receiving light.

Thus, by varying the angle of incidence of the light on the surface for receiving light, the extraction point will vary accordingly.

In embodiments of the present invention, a reflecting surface may be arranged at said back surface of the wave guide plate.

For part of the light introduces into the wave guide, the first surface that light will encounter with an angle superseding the critical angle for total internal reflection will be the back surface. In order to utilize this light, even though it is extracted through the back surface of the wave guide, this light is reflected on this reflecting surface towards the front surface for extraction through the front surface.

In embodiments of the present invention, an optical diffuser may be arranged at said front side of said wave guide plate.

Due to Fresnel reflections and an angular range of the received light, not all light will be extracted in the exact same point. A diffuser arranged at the front side of the wave guide will at least partly eliminate the periodical pattern that otherwise would appear.

The angle between the front surface and the back surface may typically be below 10°, such as below 5°, for example below 2°.

The top angle of the wave guide is typically rather small as this leads to a number of advantages, such as for example a thin wave guide even for a large display, to be able to use small light sources even for large displays, and as multiple light extraction positions that appear due to e.g. Fresnel-reflections, will be close to each other.

In embodiments of the present invention, a collimator may be arranged in the beam path between said light source and said surface for receiving light. In order to provide a well-defined highlighted point, the incoming light should be well collimated. In case the light source is not enough collimated, a collimator may be needed to provide this collimation of light.

In embodiments of the present invention the means for controlling the angle of incidence may comprise the light source, which light source in such embodiments is movably arranged between at least a first position providing a first angle of incidence on the surface for receiving light and a second position providing a second angle of incidence on the surface for receiving light.

By moving the light source from a first position, giving rise to a first angle of incidence, to a second position, giving rise to a second angle of incidence, the location on the wave guide at which the light is extracted from the wave guide also moves from a first position to a second position.

In embodiments of the present invention, the means for controlling the angle of incidence may comprise at least one optically active element selected from the group consisting of a collimator, a reflecting surface, a refractive element and an electro-optical element, which is arranged in the beam path between the light source and the surface for receiving light, and is controllably arranged between at least a first state providing a first angle of incidence on said surface for receiving light and a second state providing a second angle of incidence on said surface for receiving light.

By changing the state (such as for example the position, angle and/or electrical current through the element) of the optically active element from a first state, giving rise to a first angle of incidence, to a second state, giving rise to a second angle of incidence, the location on the wave guide at which the light is extracted from the wave guide also moves from a first position to a second position.

In embodiments of the present invention, the optically active element comprises a reflecting surface, which is rotatably arranged between at least a first position providing a first angle of incidence on the surface for receiving light and a second position providing a second angle of incidence on the surface for receiving light. It is advantageous to incorporate a reflecting surface in the means for controlling the angle of incidence, as this can be used to obtain a more compact backlight assembly.

In addition, a refractive element may be arranged in the beam path between the reflecting surface and said surface for receiving light, as this may increase the controllability of the extraction location as well as leading to a more compact backlight assembly.

In embodiments of the present invention, a backlight assembly may comprise at least a first means for controlling the angle of incidence on a first portion of the surface for receiving light and second means for controlling the angle of incidence on a second portion of the surface for receiving light.

According to a second aspect, the present invention provides a display device comprising a backlight assembly of the present invention.

According to a third aspect, the present invention provides a method for illuminating a defined area of a display surface of a display device, comprising: providing a display device comprising a backlight assembly of the present invention, and adjusting the means for controlling the angle of incidence to highlight said defined area.

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention.

Fig. Ia illustrates, in cross-sectional view, one embodiment of a backlight assembly of the present invention.

Fig. 2 illustrates, in cross-sectional view, another embodiment of a backlight assembly of the present invention.

Fig. 3 illustrates, in top view, yet another embodiment of a backlight assembly of the present invention.

Fig. 4 represents test results from one of the examples below.

As used herein, "Total internal reflection", herein also abbreviated "TIR", refers to the phenomenon where a light beam is totally reflected in the interface between two medias, e.g. that no light passes the interface. The passage of a light beam through a surface is bound to Snell's law: ni * sin(θi) = n₂ * Sm(B₂).

In this formula ni is the refractive index in the first media (wave guide) and G₁ is the angle of incidence on the interface in the first media, and n₂ is the refractive index in the second media (surroundings) and θ₂ is the angle of incidence on the interface in the second media. If ni>n₂, there does not exist any solution to Snell's law in case G₁ is large. Above a critical angle Gc (where Gc= arcsin(n₂/ni)), this means that a light beam encountering the interface from the first medium, is fully reflected, without any light passing the surface.

As used herein, the term "angle of incidence" relates to the angle between light beam encountering a surface and the normal to that surface. The angle of incidence for a not perfectly collimated light beam, i.e. a beam having a collimation angle above 0°, is approximated with the angle of incidence of the center of that beam. As used herein, the term "collimation angle" refers to an angular range within which 98% of the light intensity is contained.

A first exemplary embodiment of the present invention is illustrated in fig 1, showing an LCD display device comprising a backlight arrangement 100 arranged on the back side (opposite to the viewer) of the liquid crystal cell 111 of the display. The backlight arrangement 100 comprises a light source 101, which is arranged to provide light into a wedge-shaped wave guide 102 via a surface 105 for receiving light. The wave guide 102 has a front surface 103 facing the LC-cell 111 and a back surface opposite to the front surface.

The front surface 103 and back surface 104 are connected by the surface for receiving light 105, forming an angle β between the front and the back surfaces.

The light source emits light onto the surface 105 for receiving light, and at least part of this light is introduced into the wave guide. The angle with which the light is introduced depends according to Snell's law on the angle of incidence of the light from the light source 101 on the surface for receiving light. As an introduced light beam travels in the wave guide 102, it will, once it encounters the front 103 or the back surface 104 at an angle of incidence exceeding the critical angle for total internal reflection, be reflected thereon and redirected towards the opposite surface. Further, due to the wedge shape the angle of incidence will successively decrease, until it eventually supersedes the critical angle for total internal reflection. At this position, the extraction position, at least part of this light will be extracted from the light guide to the surroundings.

The location of this extraction position thus depends on the angle of incidence for the first reflection in the front or back surface and on the position of this reflection. In a particular case, the angle of incidence supersedes the critical angle already the first time the light beam encounters the front or back surface, respectively.

The angle of incidence at the first encounter on the back or front surface is dependent on the shape of the wave-guide and the angle of light introduced via the surface for receiving light, and consequently, thus depends on the angle of incidence of light from the light source on the surface for receiving light.

The location for the first encounter is dependent on the angle of the light beam introduced and on the position along the height of the surface for receiving light, at which position the light beam is introduced.

The light extracted at an extraction position located on the side facing the LC- cell 111 will highlight a certain area of the LC-cell corresponding to the extraction position. Thus, by varying the angle of incidence of light on the surface for receiving light 105, the highlighted area of the LC-cell will vary accordingly.

As discussed above, the location of the extraction position depends on the angle of incidence of the light from the light source 101 on the surface 105 for receiving light. Thus, in order to obtain a well- focused extraction position, it is desirable to provide collimated light for this purpose.

Thus, the size of the highlighted area of the LC-cell 111 is correlated to the angular spread, i.e. the collimation angle, of the light introduced into the wave guide via the surface 105 for receiving light. However, the size of the highlighted area depends in addition on other factors, such as the length and height of the wave guide as well as on the distance between the front surface 103 of the wave guide 102 and the LC-cell 111.

Typically, the collimation angle used is below about 20°, such as below about 10°, for example below 5°, or even lower, such as below about 1°. However, when a larger area of the LC-cell 111 is to be highlighted, light being less collimated may also be used. In general, any light source of any color may be used as light source for providing light to the wave guide in a backlight assembly of the present invention. Typically, though, light emitting diodes (LEDs) are used for this purpose.

The term "light emitting diode" as used herein refers to all types of light emitting diodes, including inorganic based LEDs, polymeric based LEDs (polyLED) and small organic molecule based LEDs (smOLED), emitting light of any color, such as in the range from UV to IR. The term "light emitting diodes" is also taken to encompass laser diodes.

For example, in case the light source it self is not capable of producing well collimated light, a collimator may be arranged in the beam path between the light source and the surface for receiving light. Several types of collimators that may be suitable for this purpose are known to those skilled in the art.

Typically, the wave guide is made of an optically transparent material having a refractive index higher than the surroundings. This allows for light in the wave guide to be subject to total internal reflection. Materials suitable for use in a wave guide of the present invention include those commonly used as wave guide materials and include, but are not limited to, transparent polymers, such as PolyMethylMethacrylate (PMMA), PolyCarbonate (PC) or PolyStyrene (PS), glass materials and transparent ceramic materials.

As is illustrated in Fig. 1, the back and front surfaces are shown as flat surfaces. However, the present invention is not limited to flat front and/or back surfaces. Thus, the front and/or back surfaces may be convex or concave, as long as the thickness of the wave guide decreases with the distance from the surface for receiving light.

The angle between the front surface and the back surface (measured at the same distance from the surface for receiving light may typically be below 10°, even though the angle locally may be above 10°, such as at the distant end of the wave guide.

In order to reduce unwanted reflections on the surfaces, for example Fresnel reflections, the surfaces, such as the surface for receiving light, the front and the back surface may be provided with an antireflective coating, as is well known to those skilled in the art.

In some cases, depending on the angle of incidence, the shape of the wave guide and the refractive index of the wave guide material, the first surface that a light will encounter under an angle of incidence superseding the critical angle for total internal reflection, and thus, through which surface at least part of the light beam will be the extracted from the wave guide, will be the back surface 104. This light will, unless otherwise provided, be lost for the purposes of highlighting the LC-cell 111, which is located on the front side of the wave guide. Thus, for that reason it may be advantageous to arrange a reflecting layer at the back surface of the wave guide, at least on such portions thereof where extraction through the back surface is likely to occur. The light subject to extraction through the back surface will be reflected by the reflective layer 107 and redirected towards the front surface 103, where it will be extracted if it fulfills the extraction criteria, or alternatively reflected towards the back surface 104.

Due to optical effects, such as Fresnel reflections, optionally only a part of the light beam will be extracted at the extraction position. Part of the light beam may be reflected back into the wave guide again, being subject to extraction at the following encountered surfaces. This effect provides for double (or triple or more) extraction points possible being present for each received light beam.

In terms of highlighting of the LC-cell, this may lead to a plurality of highlighted regions, which may be unwanted. Due to that the reflection angle successively increases with the number of reflections against the front and back surfaces, as discussed above, the light extracted at the first extraction position and the light extracted at the second extraction position will be converging.

To reduce the detrimental effects of these multiple extraction points, an optical diffuser 108 may be arranged in the beam path between the wave guide and the LC-cell.

For example, this optical diffuser may be arranged at a distance from the wave guide which approximately corresponds to the location where light beam extracted at the first extraction position converges with the light beam extracted at the second extraction position.

In general, the distance between the first and the second extraction point is correlated to the angle between the front and the back surface of the wave guide.

The diffuser 108 may for example be arranged parallel or near parallel to the front surface 103 of the wave guide 102.

Optical diffusers suitable for use in the present invention includes optical diffusers know to those skilled in the art. Typically, the optical diffusers are provided in the form of a thin diffusing film, foil or sheet.

Non- limiting examples of diffusers suitable for use in the present invention includes BEF and DBEF foils, commercially available from 3M.

Diffusing the light at or near the above mentioned converging point thus reduces the extent to which a single light beam give rise to several simultaneously highlighted areas on the LC-cell.

As illustrated in Fig. 1, a backlight assembly of the present invention for a transmissive (or transflective) LCD-display may typically further comprise an ordinary backlight plate 110, for providing the continuous backlighting of the LC-cell. Typically, this backlight plate 110 is arranged on the back side of the wave guide 102, such that the light from the wave guide complements the backlighting light from the backlight plate 110.

When a backlight plate 110 is used in a backlight assembly of the present invention, there is preferably no reflective layer 107 arranged between the backlight plate 110 and the wave guide 102, since such a reflective layer 107 would block light emanating from the backlight plate 110.

It may in such cases be desirable to use a backlight plate 110 providing a slightly collimated light, since light with oblique angles may be reflected by the wave guide 102 back towards the backlight plate 110.

As discussed above, the location of the extraction position is a function of the angle of incidence on the surface for receiving light 105 and the position on that surface at which the light is introduced in the wave guide.

For a wave guide having a small height : length ratio, i.e. a wave guide having a small angle between the front surface 103 and the back surface 104, the position of light introduction will only have a very minor effect on the extraction position. The main determinative factor for the location of the extraction position is the angle of incidence of the light beam and thus also the collimation angle.

Thus, in order to obtain highlighting of arbitrary positions of the LC-cell 111, there is a need for a means for obtaining arbitrary extraction positions on the front surface 103 of the wave guide. Thus, according to the discussion above, there is a need for a means for controlling the angle of incidence of light from the light source 101 on the surface for receiving light 105.

One arrangement for such means for controlling the angle of incidence 106 is illustrated in Fig. 1 , where the light source is pivotably arranged. By rotating the light source, the angle of incidence can be varied according to the rotation angle.

Thus, the expression "movable between at least a first position and a second position" as used herein, refers to that an object, here for example the light source, is movable between a first and a second end position, but can also be positioned in essentially any position between these end positions. The same applies for the similar expression

"controllable between at least a first state and a second state", where the controllable object also can have essentially any state between the first and the second state. Typically, the means for controlling the angle of incidence may typically further comprise a controller device (not shown), such as a user or computer controllable motor for rotating the pivotably arranged light source.

An alternative arrangement for controlling the angle of incidence is illustrated in Fig. 2, showing a detail of a backlight arrangement 200 comprising a wave guide 202 essentially as illustrated in Fig. 1, and further comprising a mirror 210 rotatably arranged in the beam path between the light source 201 and the surface for receiving light 205. By controlling the rotational angle of the mirror 210, the angle of incidence is controlled accordingly. As is illustrated in Fig. 2, a lens 211 may advantageously, but optionally, be arranged between the mirror 210 and the wave guide 202. The strength (focal length) of the lens may advantageously be such that the reflection point on the mirror 210 is imaged on the surface 205 for receiving light. By arranging a lens 211 between the mirror and the wave guide, the distance between the mirror 211 and the surface 205 for receiving light may be longer, still providing the same range of angles of incidence.

The backlight arrangement 200 also typically comprises a controller device (not shown), such as user or computer controllable motor for rotating the mirror 210.

As will be appreciated by those skilled in the art, many different such means for controlling the angle of incidence is possible and are encompassed by the scope of the present invention. Controlling the angle of incidence may generally be accomplished by controlling the light source itself or by controlling an optically active element arranged in the light beam between the light source and the surface for receiving light, so that the angle of incidence on the surface for receiving light may be controlled accordingly. Any such means for controlling the angle of incidence is encompassed by the scope of the present invention in its broadest sense.

Alternative examples of such means for controlling the angle of incidence includes any controllable optically active element arranged in the beam path between the light source and the surface for receiving light, which optically active element operatively may be controlled to vary the angle of incidence of the light on the surface for receiving light. Controllable optically active elements suitable for use in the present invention include, but are not limited to, controllable lenses, prisms, electro wetting lenses, electro wetting prisms, liquid crystal lenses, liquid crystal prisms, GRIN lenses, etc.

If the means for controlling the angle of incidence is sufficiently fast, it may be possible to highlight several areas of the LC-cell time sequentially. If the refresh rate is sufficiently high (for example above 20 Hz, such as 50 or 100 Hz or higher), a human viewer will not notice the refreshment.

The above description of means for controlling the angle of incidence only provides a description for the control of the location of the extraction point along the length of the wave guide.

For controlling the location of the extraction position along the width of the wave guide, it is desirable to provide control of the position along the width of the surface for receiving light at which position the light from the light source in introduced into the wave guide. This may for example be performed by a set of controllable mirrors or other optically active elements, for example as described above, which is capable of directing the light from a light source to the desired position along the width of the surface for receiving light. This set of controllable optically active elements may also be capable of determining the angle of incidence of the light on the surface for receiving light. Alternatively, as is illustrated in Fig. 3, a backlight assembly 300 of the present invention may comprise of a wave guide 300, essentially as described above, further comprising a plurality of means 316, 326, 336 for controlling the angle of incidence of light on the surface 305 for receiving light. Here, the means for controlling the angle of incidence may be of any type, for example of any of the types described herein above. In this embodiment, each means 316, 326, 336 is arranged to control the angle of incidence on a separate portion 315, 325, 335, respectively of the surface 305 for receiving light. Thus, each means for controlling the angle of incidence is responsible for a certain portion of the wave guide, controlling the location of the extraction position within this portion.

One light source may be used to provide light to one, several or all of the separate means 316, 326, 336.

In an alternative embodiment, a backlight assembly of the present invention may comprise a single means for controlling the angle of incidence and a plurality of light sources arranged along the width of the wave guide, providing light that is well collimated in the direction of the width of the wave guide. The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

Thus, to summarize, the present invention relates a backlight assembly comprising at least one light source and an optically transparent wave guide plate, which wave guide plate comprises a front surface, an opposing back surface and a surface for receiving light from said at least one light source, connecting said front and back surfaces. The thickness of the wave guide plate decreases with the distance from said surface for receiving light, and the backlight assembly comprises means for controlling the angle of incidence of light from said at least one light source on said surface for receiving light. By varying the angle of incidence on the surface for receiving light, the location on the wave guide where the light is extracted from the wave guide can easily be controlled.

The present invention will now be described more in detail with regard to the following non-limiting example, which is provided for illustrative purposes only.

Determination of the effects of point of light introduction and angle of incidence on the location of the extraction position.

The following results are obtained by performing a computer (ray tracing) simulation. The simulated wave guide was in the shape of a right angled wedge having a 2 mm high surface for receiving light and a 100 mm long back surface formed at a right angle to the surface for receiving light. The angle between the front surface and the backsurface was thus arctan (2/100) - 1,15°.

The refractive index of the wave guide was set to 1,5 with a surrounding media of air (refractive index 1). The value of the extraction position was defined as the distance from the top of the wedge (distant from the surface for receiving light) along the plane of the front surface. Three different positions for introducing light into the wave guide was simulated: y = 0, 1 and 2 mm (0 mm is the bottom of the surface for receiving light, 2 mm is the top of the same surface). The angle of the introduced light, δ, was varied between 0 and 50 °.

The result from this simulation is shown in Fig. 4.

As is apparent from these results, the position for introducing light did influence the location of the extraction position only to a very small extent.

The angle at which the light was introduced into the wave guide (which correlates to the angle of incidence through Snells law) did however have a big influence on the location of the extraction point, following an almost linear relationship between the extraction position and light angle.

These results show that the collimation angle of the introduced light will be a determinative factor in the focusing of the extraction position, and thus on the size of the highlighted area of an LC-cell, and that the location of the extraction point is unambiguously predictable from the angle of incidence on the surface for receiving light.

Claims

CLAIMS:

1. A backlight assembly (100) comprising at least one light source (101) and an optically transparent wave guide plate (102), which wave guide plate comprises a front surface (103), an opposing back surface (104) and a surface (105) for receiving light from said at least one light source (101), connecting said front and back surfaces, said backlight assembly being characterized in that the thickness of the wave guide plate (102) decreases with the distance from said surface for receiving light (105), and the backlight assembly (100) comprises means (106) for controlling the angle of incidence of light from said at least one light source on said surface for receiving light.

2. A backlight assembly according to 1, wherein a reflecting surface (107) is arranged at said back surface of said wave guide plate (102).

3. A backlight assembly according to claim 1 or 2, wherein a optical diffuser (108) is arranged at said front surface of said wave guide plate (102).

4. A backlight assembly according to any of the preceding claims, wherein the angle (β) between the front surface and the back surface is below 10°.

5. A backlight assembly according to any of the preceding claims, wherein a collimator is arranged in the beam path between said light source (101) and said surface (105) for receiving light.

6. A backlight assembly according to any of the preceding claims, wherein said means for controlling the angle of incidence (106) comprises said light source (101), which light source is movably arranged between at least a first position providing a first angle of incidence on said surface (105) for receiving light and a second position providing a second angle of incidence on said surface (105) for receiving light.

7. A backlight assembly according to any of the preceding claims, wherein said means for controlling the angle of incidence (106) comprises at least one optically active element selected from the group consisting of a collimator, a reflecting surface, a refractive element and an electro-optical element, said at least one optically active element is arranged in the beam path between said light source and said surface for receiving light, and is controllably arranged between at least a first state providing a first angle of incidence on said surface (105) for receiving light and a second state providing a second angle of incidence on said surface (105) for receiving light.

8. A backlight assembly (200) according to claim 7, wherein said optically active element comprises a reflecting surface (210), which is rotatably arranged between at least a first position providing a first angle of incidence on said surface for receiving light and a second position providing a second angle of incidence on said surface for receiving light.

9. A backlight assembly according to claim 8, comprising a refractive element (211) arranged in the beam path between said reflecting surface (210) and said surface (205) for receiving light.

10. A backlight assembly (300) according to any of the preceding claims comprising at least a first means (316) for controlling the angle of incidence on a first portion (315) of said surface (305) for receiving light and second means (326) for controlling the angle of incidence on a second portion (325) of said surface (305) for receiving light.

11. A backlight assembly according to any of the preceding claims, further comprising a backlight plate (110) arranged at said back surface of said wave guide plate (102).

12. A display device comprising a backlight assembly according to any of the claims 1 to 11.

13. A method for illuminating a defined area of a display surface of a display device, comprising: providing a display device according to claim 12; adjusting said means for controlling the angle of incidence to high- light said defined area.