EP1889104A2

EP1889104A2 - Lighting device

Info

Publication number: EP1889104A2
Application number: EP06765694A
Authority: EP
Inventors: Johan Marra; Dirk K. G. De Boer
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2005-05-27
Filing date: 2006-05-19
Publication date: 2008-02-20
Also published as: JP2008546139A; WO2006126155A3; WO2006126155A2; TW200706935A; KR20080021059A; CN101185015A

Abstract

The present invention relates to a lighting device comprising a light-guide arrangement in which inputted light from at least one light source is propagated through and at least partly confined within the light-guide arrangement by means of total internal reflection. The light-guide arrangement comprises first (6) and second (7) light-guide plates and an intermediate layer (8) having a refractive index which is lower than the refractive index of said first light-guide plate (6) , said first light-guide plate (6) being arranged to receive light from said at least one light source. The second (7) light-guide plate comprises an out-coupling structure (10) to output and collimate light from the lighting device. This light-guide arrangement provides an improved lateral light-intensity averaging effect, such that light is more uniformly emitted from the output surface of the lighting device, even if said at least one light source is point or line-shaped.

Description

Lighting device

FIELD OF THE INVENTION

The present invention relates to a lighting device comprising a flat light-guide arrangement having first and second main surfaces and being arranged to receive light from at least one light source and to at least partly constrain light therein by total internal reflection, and, at the second main surface, an out-coupling arrangement out-coupling light from the light-guide arrangement.

BACKGROUND OF THE INVENTION

Such a device is disclosed in WO 2004/027467 Al and can be used as a backlighting arrangement in e.g. an LCD-TV. The use of a planar light guide serves to laterally smoothen, to some extent, the intensity of the emitted light from the light-emitting surface of the device. Even though only a few point or line-shaped light sources are used, the lighting device may emit a laterally relatively uniform light flow from its entire light-emitting surface area. However, there may still be a need to provide an even more uniform light flow from the entire second main surface of the lighting device.

OBJECT AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to improve a lighting device of the type mentioned in the opening paragraph with regard to light flow variations across the light-emitting surface of the lighting device.

This object is achieved by means of a lighting device as defined in claim 1. In this case, the light-guide arrangement comprises first and second parallel light-guide plates, the first light-guide plate being arranged to receive light from a light source and the second light-guide plate comprising said out-coupling arrangement, and at least one intermediate layer between the first and second light-guide plates, the intermediate layer having a lower index of refraction than the first light-guide plate.

This provides an even more laterally uniform light flow from the light- emitting surface of the lighting device, because at least some light rays will undergo total internal reflection at the interface between the first light-guide plate and the intermediate layer. Light rays will thus, on average, travel over a longer distance in a lateral direction through the first light-guide plate before exiting the light-guide arrangement.

The device may comprise at least one light source, arranged to feed light through one edge of the first light-guide plate. This provides a very thin device.

Alternatively, the device may comprise at least one light source, arranged to feed light through a main surface of the first light-guide plate, constituting the first main surface of the light-guide arrangement. In this case, the main surface preferably comprises an in-coupling structure. This allows a greater amount of light to enter the first light-guide plate. A reflector is preferably arranged, which, together with the first main surface of the light- guide arrangement, surrounds the light source. This allows light rays that are (initially) unable to enter the first light-guide plate to be recycled within the space bounded by the reflector and the first main surface until the moment when they manage to enter the first light-guide plate. The out-coupling structure may comprise a birefringent material, such that the lighting arrangement produces and emits polarized light.

The first and second light-guide plates may be spaced apart to form at least one gap therebetween, which gap may thus constitute said intermediate layer, and may be composed of at least one segment, which may be positioned between adjacent spacer elements. The height of the spacer elements is preferably substantially the same as the spacing of the gap.

Said segment may be filled with at least one member of the group of at least partly transparent media comprising gases, liquids, and solids. Suitable provisions may be made that enable said member, which is optionally colored, to become moveable, thereby enabling its in-situ exchange for another member.

A number of segments may be arranged, e.g. in an array that fills at least partly the entire gap volume between the first and second light-guide plates of the lighting arrangement. Different segments may be filled with one or more different members of the above group of at least partly transparent media. Such lighting devices may be arranged to provide, as a backlighting arrangement, light to a transmissive LCD panel. Alternatively, they may be used for general lighting purposes, e.g. as switchable light tiles.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates a lighting device in accordance with a first embodiment of the invention. Fig. 2 illustrates a lighting device in accordance with an alternative embodiment of the invention.

Fig. 3 illustrates a first example of an in-coupling structure.

Fig. 4 illustrates a second example of an in-coupling structure.

Fig. 5 illustrates a first example of an out-coupling structure. Fig. 6 illustrates a second example of an out-coupling structure.

Fig. 7 illustrates a display system using the lighting device according to the invention.

DESCRIPTION OF EMBODIMENTS Fig. 1 illustrates schematically and in a cross-section a lighting device in accordance with an embodiment of the invention. The device comprises a number of light sources 1, 2, 3, such as fluorescent lamps. The light sources are partly enclosed by a reflector 4, such that the light emitted from the light sources will be directed towards a flat light-guide arrangement 5, either directly or indirectly, via one or more reflections from the reflector 4. The flat light-guide arrangement 5 has a first main surface facing the light sources 1, 2, 3, and a second main surface facing in the opposite direction.

The light-guide arrangement 5 comprises a first and a second light-guide plate 6, 7, respectively, which are arranged in a sandwiched structure. The light-guide arrangement further comprises a flat intermediate layer 8 between the first and second light-guide plates. The first light-guide plate 6 is arranged to receive light from the light sources

1, 2, 3, either directly or indirectly via the reflector 4. At its surface facing the light sources and constituting the first main surface of the light-guide arrangement 5, the first light-guide plate 6 comprises an in-coupling structure 9, which will be described in greater detail hereinafter. Briefly, this in-coupling structure 9 ensures that the light entering the first light- guide plate 6 will propagate through this light-guide plate within a limited angular range that supports the occurrence of at least some degree of light containment within this first light- guide plate through the occurrence of total internal reflection. Thus, light is, at least partly, constrained in the light-guide arrangement by total internal reflection. The refractive index of the first light-guide plate 6 is n_ls the refractive index of the intermediate layer 8 is n₂, and the refractive index of the second light-guide plate 7 is n₃; n₃ may be equal to or different from U₁. The refractive index n₂ of the intermediate layer 8 is, however, substantially lower than n_ls such that total internal reflection will occur for light rays within the first light-guide plate 6 that are incident upon the ^- >n₂ interface at sufficiently oblique angles. Suitable light-guide materials include glass, PMMA (polymethylmethacrylate), silicone resin, and polycarbonate. The intermediate layer may comprise one or more at least partly transparent materials such as air, water, an organic oil, or a solid transparent material. The materials contained within the intermediate layer are preferably substantially non-scattering materials.

At its surface facing away from the intermediate layer 8 and also constituting the second main surface of the light-guide arrangement 5, the second light-guide plate 7 comprises an out-coupling structure 10 which serves to collimate light exiting the second light-guide plate 7 at this surface, i.e. to direct exiting light such that, on average, the light is emitted within a limited angular range with respect to the normal of the light-emitting main surface of the second light-guide plate 7. The light-guide arrangement 5 is preferably provided with specularly reflective mirrors 11, 12 on its edges. Alternatively, the mirrors 11, 12 may comprise diffusely reflective materials, such as white powders, provided that the diffusely reflective materials are not in a substantial degree of optical contact with the light propagating through the interior of the light-guide plates 6, 7.

As compared with the case in which the light-guide arrangement 5 comprises only a single, solid, light-guide material of refractive index n_ls the intermediate layer 8 with its refractive index n₂ < ni has the effect that the light entering the light-guide arrangement will, on average, undergo more internal reflections inside the light-guide arrangement. This provides the effect that the emitted light flow from the light-guide arrangement will become laterally more uniform in intensity due to the improved intensity-smoothing function of the light-guide arrangement.

Moreover, since only a part of the light rays propagating through the first light-guide plate 6 will be able to cross the interface between the first light-guide plate 6 and the intermediate layer 8, i.e. only those light rays can cross that do not undergo total internal reflection at the latter interface, the transmitted rays propagate within a relatively more limited angular range than within the first light-guide plate 6. Hence, this interlace provides an effective limitation to the angular range of possible directions of propagation, the degree of limitation having a proportionality with the difference ni - n₂. When light rays subsequently pass from the intermediate layer into the second light-guide plate 7, which preferably has a refractive index n₃ > n₂, their more limited angular propagation range persists within the second light-guide plate 7. This characteristic feature facilitates the emitted light from the second main surface of the second light-guide plate 7 to also have a limited angular emission range when said second main surface is provided with a suitable out-coupling structure. The limited angular emission range associated with the light emitted from the light-guide arrangement 5 can be used for collimation purposes.

In its simplest form, the intermediate layer 8 may be formed by separating the first and second light-guide plates 6, 7 by means of preferably reflective spacers (not shown), such that a gap between the first and second plates is obtained, constituting the intermediate layer 8. This gap may be filled with a gas, such as e.g. air, or a liquid. If a liquid is used, this liquid may be colored to enable a colored light output to be obtained from the light-guide arrangement 5, also when the used light sources 1, 2, 3, or 14 (see Fig. 2) emit white light.

If a fluid medium is used, it is also possible to provide means, such as a pump, for moving this medium in and out of the gap so as to change the optical properties of the lighting arrangement, in particular the refractive index n₂ and/or the color of the medium inside the gap. It is even possible to divide the gap between the first and second light-guide plates into different segments, i.e. sub-areas, each sub-area having separately controllable properties. It is therefore possible to change e.g. the intensity of light outputted from a specific area of light-emitting surface of the lighting device, as well as the color and/or the collimation characteristics of this light.

Furthermore, it is possible to increase the lateral light-intensity homogenizing effect by providing a large gap and providing, in this large gap, a third light-guide plate (not shown). Then a sandwiched construction having three light-guide plates and two interpositioned low refractive index layers can be obtained.

Fig. 2 illustrates an alternative embodiment of the present invention. This embodiment also comprises first and second light-guide plates enclosing an intermediate layer having a relatively low refractive index n₂ < U₁. In this case, however, the first light- guide plate is edge-lit. A light source 14 is arranged to feed light into the first light-guide plate 6 through an edge 13 of the same. A reflector 15 is arranged at the light source 14 in order to direct light therefrom towards the edge 13 of the light-guide plate 6.

Fig. 3 illustrates a first example of an in-coupling structure which can be used as the in-coupling structure 9 on the first light-guide plate 6 in Fig. 1. Such a structure is known per se from WO Al 2004/027467, and comprises a structured reflective foil 25 positioned in between adjacent upstanding optical elements 20. The optical elements 20 may be in the form of upstanding cubes, cylinders or ribs 20, facing the light sources 1, and being in optical contact with the light-guide plate 6.

The top surfaces 22 of these elements 20 are preferably substantially parallel to the light-guide plate main surface and are covered by a highly reflective coating which may also have light-diffusing properties. Light rays 23, incident on the surfaces 22 will thus be reflected. The side surfaces 21, however, are transmissive so as to transmit all light rays 24a that are incident upon the surfaces 21. At least part of the transmitted light rays 24a may subsequently propagate through the light-guide plate 6 by total internal reflection and eventually exit the light-guide plate 6 back through the transmissive surfaces 21 of the optical elements 20 in order to be recycled inside the space bounded by the first light-guide plate 6 and the reflector 4 before re-entering the light-guide plate 6 at a possibly different angle. Another part of the light rays transmitted into the light-guide plate 6 will exit this first light- guide plate via the intermediate layer 8, either or not after having undergone one or more Fresnel reflections that also serve to smoothen the lateral light intensity. In Fig. 3, reflecting foil elements 25 are provided between the optical elements 20, reflecting light rays 26 onto the side surfaces 21. These foil elements 25 do not need to be in optical contact with the light-guide plate but serve to prohibit a direct entry of light into the first light-guide plate 6 via surfaces other than the upstanding transmissive surfaces 21 of the optical elements 20. This embodiment has the advantage that said other surfaces in between adjacent optical elements 20 do not need to be provided with a separate reflective coating. Via the highly reflective surfaces of the foil elements 25, light rays 24b may be reflected directly onto the transmissive side surfaces 21 of the optical elements 20 from where they can enter the light- guide plate 6. Fig. 4 illustrates a second example of an in-coupling structure. In this case, optical elements 26, such as wedge-shaped ribs, possibly with flat tops, or e.g. cones, possibly truncated, are provided. The optical elements 26 have upstanding transmissive side surfaces 29. If ribs are used, upstanding transmissive surfaces 29 are provided primarily in one direction along the light-guide plate, whereas the transmissive surfaces 29 may be provided in two dimensions if e.g. cones are used. The optical elements 26 have highly reflective top surfaces 27 reflecting and possibly diffusing light rays 28 incident thereon. The side surfaces 29 of the optical elements 26 are transmissive and may be somewhat oblique, thus forming a trapezoid cross-section together with the top surfaces and the spaces in between the optical elements. These side surfaces allow light rays 30 to enter the light-guide plate in a similar way as described with reference to Fig. 3. The optical elements 26 are provided with some spacing in between them, the interpositioned surfaces 31 associated with the spacing between adjacent optical elements 26 being provided with a reflective coating reflecting light rays 32 incident thereon. In one embodiment with reference to Fig.. 4, the in-coupling arrangement comprise upstanding ribs having transparent surfaces 29 that are oriented at the angle βin= 18° with respect to the normal of the first main surface of the light-guide plate. The rib height h^ = 4 mm, and the width of the reflecting top area din = 0 mm (i.e. no top area 27). The spacing Sin between adjacent ribs may vary depending on the desired light input capacity, but Sj_n > 10 mm is a convenient value. When polycarbonate is used as the material from which the in- coupling elements 26 and the light-guide plate 6 are composed, the refractive index ni = 1.59.

It should be noted that the in-coupling structures described with reference to Figs. 3 and 4 may also be used independently of the light-guide arrangement having first and second light-guide plates and an interpositioned low refractive index layer. These in-coupling structures may thus be used together also with e.g. a light-guide plate arrangement that is composed from only a single solid material.

In general, the concept disclosed with reference to Fig. 4 thus comprises a lighting arrangement having a light-guide plate 6 and an in-coupling structure on a first main surface of said light-guide plate, for inputting light into said light-guide plate, the inputted light being at least partly confined therein by total internal reflection, wherein the in-coupling structure comprises optical elements 26 upstanding from said first main surface and having transparent side surfaces 29, the width of said optical elements in between said side surfaces being reduced in the direction facing away from said first main surface. The optical elements are thus tapering in the direction facing away from the main surface. Such in-coupling structures are generally less complex than the structures described with reference to Fig. 3. Any areas in between the optical elements as well as any areas parallel to the main surface on top of the optical elements are preferably provided with a reflective coating.

The general idea with the in-coupling structures is to enable a large amount of light to enter the light-guide plate 6 in such a way that this entered light is at least partly constrained inside the light-guide by means of total internal reflection and, to a lesser degree, by means of Fresnel reflections. Of course, structures other than those shown in Figs. 3 and 4 may also provide such an effect.

The function of a selective light input that occurs only through said transparent side surfaces of the optical elements 20, 26 does not need to be perfect. Some leakage of light into the light-guide plate through surfaces other than the upstanding transparent surfaces 21, 29 can be allowed in some applications. In addition, light that is initially reflected by the reflective surface elements 22, 28, or 31 can be subsequently back-reflected by e.g. the reflector 4. Since this reflector may have diffusing properties, this back-reflected light is recycled in such a way that it may subsequently still succeed in entering the light-guide plate 6 and can thus be prevented from becoming lost.

Fig. 5 illustrates a first example of a possible out-coupling structure on the second main surface of light-guide plate 7. This structure can be provided in a similar way as the in-coupling structure described above with reference to Fig. 3. The out-coupling structure in Fig. 5 features upstanding optical elements 33 having transparent upstanding side surfaces 35a, in optical contact with the light-guide 7, and oblique specularly reflective foil elements 34 positioned in between adjacent optical elements 33. The top surfaces 35 of the optical elements 33 and the surface areas of the light-guide plate 7 that are covered by the foil elements 34 can remain transmissive, hence no reflective coatings need to be applied to the second main surface of the light-guide plate 7 of the lighting device or to any of the surfaces associated with the optical elements 33. This out-coupling structure has the effect of drawing light from the light-guide plate 7 through the transparent side surfaces 35a of the optical elements 33 into air, after which the withdrawn light can possibly undergo one or more specular reflections from the structured foil elements 34 before being emitted into a direction away from the lighting device within a limited angular range of directions with respect to the normal of the emitting surface of the light-guide plate 7.

Fig. 6 illustrates a second example of an out-coupling structure, which is similar to the in-coupling structure in Fig. 4. The structure now comprises upstanding optical elements 36 (wedge-shaped ribs or e.g. cones) with side walls 37 having an angle β_out > 0° with respect to the normal of the second main surface of the light-guide plate 7. The top surfaces 27 may remain transmissive. The same holds for the surfaces in between adjacent elements 36. However, in cases wherein the light within the light-guide plate 7 does not have angular propagation characteristics that support the occurrence of total internal reflection from the surfaces in between adjacent elements 36, it may become a preferred option to provide these surfaces with a specularly reflective coating in order to prevent light escaping therethrough.

In a further embodiment, the out-coupling arrangement in Fig. 6 comprises upstanding ribs having transparent side surfaces 37 at an angle β_out= 21° from the light-guide plate normal. The height of the ribs h_out = 2 mm, and their width of the top surface d_out = 0 mm. The distance between adjacent ribs s_out = 0 mm, such that a sawtooth shape is obtained, the out-coupling ribs 36 thus being positioned side by side on the second main surface of the light-guide plate 7.

In applications wherein propagating light within the light-guide plate 7 does not have angular propagation characteristics that support the occurrence of total internal reflection from surface areas in the light-guide plate 7 in between adjacent upstanding ribs, it may be preferred to provide these surface areas with a specularly reflective coating in order to prevent light escaping therethrough. In addition, the rib height h_oUt should be sufficiently high to prevent propagating light rays within the light-guide plate 7 from reaching the top surface, if provided, of the optical elements 36. The latter measure ensures that light will exit the light-guide plate 7 only through the transparent side surfaces 37 and will become refracted there so as to be emitted away from the light-guide plate 7 within a limited angular range of directions.

It should be noted that the out-coupling structure described with reference to Fig. 6 may be used independently of the light-guide arrangement having first and second light-guide plates and an interpositioned low refractive index layer. This out-coupling structure may thus be used also together with e.g. a monolithic single-plate light-guide arrangement.

In general, the concept disclosed with reference to Fig. 6 thus comprises a lighting arrangement having a light-guide plate 7 and an out-coupling structure on a light- guide plate main surface, for outputting light from the lighting arrangement and limiting the directions of the emitted light to within a preferred angular range with respect to the normal of the light-guide plate main surface, wherein the out-coupling structure comprises optical elements 36, upstanding from said main surface and having transparent side surfaces 37, the width of said optical elements in between said side surfaces being reduced in the direction facing away from said main surface. The optical elements are thus tapering in the direction facing away from the main surface. Such out-coupling structures are generally less complex than the structures described with reference to Fig 5.

The general idea with the out-coupling structures in Figs. 5 and 6 is to provide a limited angular range of directions wherein light is emitted from the second light-guide plate 7. In many cases of practical interest, the angular distribution of the emitted light is preferably cone-shaped, i.e. it should provide light emission within a limited angular range with respect to the normal of the light-emitting main surface of the light guide. A cone- shaped angular light distribution around the normal of the main surface corresponds to that of an ordinarily collimated light output.

Two cases will now be described by way of example. In case 1, isotropic light, emitted from one or more light sources, is laterally homogenized in intensity and collimated to a certain degree through the presence of the light-guide arrangement 5.

The following parameters may be used for the in-coupling structure, when devised as indicated in Fig. 4: 18°, din=0 (no flat top on the optical elements 26 of the in- coupling structure), 4 mm. Sj_n can in principle be chosen arbitrarily, but preferably Si_n ≥ 10 mm in order to prevent interference from adjacent in-coupling optical elements. The first light-guide plate 6 ni = 1.59 (polycarbonate) and the intermediate layer 8 has a refractive index n₂ = 1.0 (air in the intermediate low-n layer), while the second light-guide plate 7 has a refractive index n₃ = 1.59 (polycarbonate).

In the out-coupling structure, which may be similar to the one indicated in Fig. 6, one chooses β_out = 21°, s_out = 0 mm (no space being present between adjacent out-coupling elements), dout = 0 mm and ho_Ut = 2 mm.

The above parameters result in an emitted collimated light beam at a collimation angle θ_c = 16°. Light is thus emitted from the light-guide arrangement 5 within a 16° angular cone around the normal of the emitting second main surface of the light-guide arrangement 5. In case 2, an adjustable collimation and/or an adjustable light intensity output is achieved by exchanging, in an intermediate layer 8 constituted by a gap, an oil (n₂ = 1.50) for water (n₂ = 1.33) and/or air (n₂ = 1.0), e.g. by pumping different fluids and/or air into and out of the gap as mentioned above.

In the in-coupling structure, one may choose βin= 0°, din=5 mm, and h^ > 5 mm. Si_n can in principle be chosen arbitrarily, but preferably exceeds 10 mm.

The refractive index of the first and second light-guide plates 6, 7 is chosen to be ni = n₃ = 1.50 (e.g. polymethylmethacrylate, PMMA). When the intermediate layer 8 contains water, one has n₂ = 1.33.

In the out-coupling structure, one may choose β_out = 0°, s_out= 4 mm, h_out = 2

The above parameters, with water as the intermediate layer 8, result in an emitted collimated light beam at a collimation angle θ_c = 46°, such that light is emitted within an angle of 46° from the normal of the emitting second main surface of the light-guide arrangement 5. If, with the above parameters, the water (n = 1.33) is replaced by an oil (n = 1.50), the emitted light becomes Lambertian diffuse (but laterally homogenized with respect to intensity) and is thus emitted within an angle of 90° as measured from the normal of the emitting second main surface of the light-guide arrangement (selectable collimation). If, with the above parameters, the oil or water is replaced by air (n = 1.0), no light is emitted from the light-guide arrangement (selectable emission intensity).

The media in the intermediate layer can be replaced throughout the surface of the light-guide arrangement 5 or in segmented parts thereof as mentioned hereinbefore. Media may have a color so that (white) light passing through these media also becomes colored.

The out-coupling structure may also comprise a birefringent material which allows, at least to some degree, a polarization of the emitted light flow, i.e. light of one polarization is more easily emitted from the light-guide plate than light of another polarization. This may be useful in LCD applications. Light of the "wrong" polarization, which is not emitted from the light guide, may subsequently be allowed to be reflected from a diffusing reflector, resulting in a different polarization. Such out-coupling layers are disclosed per se e.g. in WO 2003/027568 and WO 2003/078892.

Fig. 7 illustrates a display system using the lighting device according to the invention. The lighting device 38 is used as a backlighting arrangement for a transmissive LCD panel 39. This may be applied in e.g. a computer monitor or an LCD-TV. Of course, other applications are possible, such as general room lighting.

In summary, the invention relates to a lighting device having a light-guide arrangement in which inputted light from at least one light source is propagated through and at least partly confined within the light-guide arrangement by means of total internal reflection. The light-guide arrangement comprises first and second light-guide plates and an intermediate layer having a refractive index which is lower than the refractive index of said first light-guide plate, said first light-guide plate being arranged to receive light from said at least one light source. This light-guide arrangement provides an improved lateral light- intensity averaging effect, such that light is more uniformly emitted from the output surface of the lighting device, even if said at least one light source is point or line-shaped.

The invention is not limited to the embodiments described hereinbefore. It can be altered in different ways within the scope of the appended claims.

Claims

CLAIMS:

1. A lighting device comprising: a flat light-guide arrangement (5) having first and second main surfaces and being arranged to receive light from at least one light source and to at least partly constrain light therein by total internal reflection, and at the second main surface, an out-coupling structure (10) out-coupling light from the light-guide arrangement (5), wherein said light-guide arrangement comprises: first (6) and second (7) parallel light-guide plates, the first light-guide plate (6) being arranged to receive light from the light source and the second light-guide plate (7) comprising said out-coupling structure (10), and at least one intermediate layer (8) between the first and second light-guide plates, said intermediate layer having a lower index of refraction than the first light-guide plate.

2. A lighting device according to claim 1, comprising at least one light source

(14), which is arranged to feed light through one edge of the first light-guide plate (6).

3. A lighting device according to claim 1, comprising at least one light source (1,

2, 3) which is arranged to feed light through a main surface of the first light-guide plate (6), wherein said main surface comprises an in-coupling structure (9) and constitutes said first main surface of said flat light-guide arrangement.

4. A lighting device according to claim 3, comprising a reflector (4), wherein said first main surface and said reflector (4) substantially surround said at least one light source (1, 2, 3).

5. A lighting device according to any one of the preceding claims, wherein the out-coupling structure (10) comprises a birefringent material.

6. A lighting device according to any one of the preceding claims, wherein the first and second light-guide plates (6, 7) are spaced apart to form at least one gap therebetween, said gap constituting said intermediate layer and being composed of at least one segment, said segment being positioned between adjacent spacer elements which have substantially the same height as the spacing of said gap.

7. A lighting device according to claim 6, wherein said segment is filled with at least one member of the group of at least partly transparent media comprising gases, liquids, and solids.

8. A lighting device according to claim 7, wherein said member is moveable and can be exchanged for another member.

9. A lighting device according to claim 7 or 8, wherein said member is colored.

10. A lighting device according to claim 7, 8, or 9, comprising a plurality of segments, wherein different segments are filled with different members of said group of at least partly transparent media.

11. A lighting device according to any one of the preceding claims, wherein the lighting device is arranged to provide, as a backlighting arrangement, light to a transmissive LCD panel (39).

12. A lighting device according to any one of claims 1 to 10, wherein the lighting device is arranged to provide, as a light tile, light for general lighting purposes.

13. A lighting arrangement comprising a light-guide plate (6) and an in-coupling structure on a first main surface of said light-guide plate for inputting light into said light- guide plate, the inputted light being at least partly confined therein by total internal reflection, wherein the in-coupling structure comprises optical elements (26) upstanding from said first main surface and having transparent side surfaces (29), the width of said optical elements in between said side surfaces being reduced in the direction facing away from said first main surface.

14. A lighting arrangement comprising a light-guide plate (7) and an out-coupling structure on a light-guide plate main surface for outputting and collimating light from said light-guide plate, wherein the out-coupling structure comprises optical elements (36) upstanding from said main surface and having transparent side surfaces (37), the width of said optical elements in between said side surfaces being reduced in the direction facing away from said main surface.