WO2008032248A1

WO2008032248A1 - Controllable light guide

Info

Publication number: WO2008032248A1
Application number: PCT/IB2007/053609
Authority: WO
Inventors: Dirk K. G. De Boer; Mark T. Johnson; Sander J. Roosendaal; Patrick J. Beasjou; Hugo J. Cornelissen
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2006-09-12
Filing date: 2007-09-07
Publication date: 2008-03-20
Anticipated expiration: 2009-03-12
Also published as: CN101517466A; KR20090058558A; EP2074474A1; JP2010503950A

Abstract

A controllable light guide (101; 405) comprising a light guide (103) configured to guide in-coupled light through reflections of the light against a guiding boundary (109) of the light guide (103), and at least one light-modifying member (102; 201-202; 303-304; 414-416) arranged adjacent to the guiding boundary (109) of the light guide (103). The light-modifying member (102; 201-202; 303-304; 414-416) comprises a fluid (104) and a plurality of particles (105) distributed in the fluid. The light-modifying member is controllable between at least a first state having a first particle distribution resulting in at least a portion (106; 209, 210) of the light-modifying member (102; 201-202; 303-304; 414-416) having a first compound refractive index (rid), and a second state having a second particle distribution resulting in the portion (106; 209, 210) of the light-modifying member having a second compound refractive index (n^). At least the second compound refractive index (nc2) is sufficiently high in relation to the refractive index of the light guide (103) at the guiding boundary (109) so as to allow at least partial outcoupling of light through a corresponding portion of the guiding boundary of the light guide.

Description

Controllable light guide

FIELD OF THE INVENTION

The present invention relates to a controllable light guide, a lighting device and a display device comprising such a controllable light guide.

BACKGROUND OF THE INVENTION

Today, various types of flat-panel displays are used in a wide variety of applications, from mobile phone displays to large-screen television sets. While some types of flat-panel dislays, such as plasma displays, comprise arrays of light-emitting pixels, the majority of flat-panel displays has arrays of pixels which can be switched between states but are unable to independently emit light. Such flat-panel displays include the ubiquitous LCD- displays. In order for such flat-panel displays to be able to display an image to a user, the pixel array must be illuminated by either a backlight, in the case of a pixel array of the transmissive type, or, in the case of a pixel array of the reflective type, by ambient light or a frontlight. A conventional backlight (and frontlight) comprises a planar light guide into which light is coupled from a light source. One face of the planar light guide is typically modified through structuring or modification, for example, surface roughening, so as to allow outcoupling of light through that face. The outcoupled light then passes through pixels in the pixel array, which are in a transmissive state, and a corresponding image becomes visible to a viewer.

When, however, only a very small proportion of the pixels is bright (in their transmissive state), as is often the case, a correspondingly large fraction of the light emitted by the backlight is prevented from reaching the viewer and precious energy is thus wasted. In order to overcome this problem, backlights having spatially controllable outcoupling of light have been proposed. For example, WO 2004079437 discloses an illumination system comprising an optical waveguide and a matrix-addressable light- management member. The outcoupling of light from the optical waveguide can be controlled by modulating a portion of the light-management member between a transparent state and a scattering state. Although the illumination system disclosed in WO 2004079437 provides controllable illumination of, for example, a liquid crystal panel, the outcoupling efficiency is limited because the light-management member is sensitive to only one state of polarization. There is thus a need for an improved controllable light guide allowing a more energy-efficient outcoupling of light.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved controllable light guide. It is a further object of the invention to provide energy-efficient controlled illumination of a pixel array in a flat-panel display.

According to the present invention, these and other objects are achieved by a controllable light guide comprising a light guide configured to contain light coupled into the light guide and to guide the light along a principal extension of the light guide through reflections of the light against a guiding boundary of the light guide, the light guide having a first refractive index at the guiding boundary; and at least one light-modifying member arranged adjacent to the guiding boundary of the light guide, the light-modifying member comprising a fluid having a second refractive index; and a plurality of particles having a third refractive index different from the second refractive index, the plurality of particles being distributed in the fluid, wherein the light-modifying member is controllable between at least a first state having a first particle distribution resulting in at least a portion of the light- modifying member having a first compound refractive index, and a second state having a second particle distribution resulting in the portion of the light-modifying member having a second compound refractive index, at least the second compound refractive index being sufficiently high in relation to the first refractive index so as to allow at least partial outcoupling of light through a corresponding portion of the guiding boundary of the light guide.

The light guide may be, for example, a planar optical waveguide or an optical fiber. If the light guide is a planar optical waveguide, it may be made of, for example, a slab of a single dielectric material or combinations of dielectric materials. Suitable dielectric materials include different transparent materials, such as various types of glass, polymethyl methacrylate (PMMA), etc. Such a planar waveguide may be flat or curved. A slab-type planar waveguide typically relies upon total internal reflection (TIR) in order to contain light coupled into the waveguide.

In the present application, "fluid" is understood to be a substance, which alters its shape in response to any force and tends to flow or to conform to the outline of the chamber in which it may be contained. The term "fluid" thus includes gases, liquids, vapors and mixtures of solids and liquids, when these mixtures are capable of flow.

The term "particles" is not limited to solid particles, but also includes liquid droplets and fluid-filled capsules.

The present invention is based on the recognition that outcoupling of light from a light guide can be modified by altering the refractive index of a light-modifying member positioned adjacent to a guiding boundary of the light guide, and that such an alteration of the refractive index is achievable by controlling the distribution of particles with a certain refractive index in a fluid having a different refractive index.

Since the controllable light guide according to the present invention primarily relies upon outcoupling due to refraction, it is ensured that practically all the light intended for outcoupling is actually permitted to leave the light guide, regardless of the state of polarization.

The particles distributed in the fluid are preferably small enough to limit scattering to a level at which a particle-fluid suspension mainly acts as a fluid having a variable refractive index.

Particles having a median diameter of around 100 nm or smaller and a size distribution which is not too wide are suitably used, because (back)scattering becomes dominant when the particles are significantly larger than 100 nm, i.e. their size is of the order of the wavelength of visible light or larger. The light-modifying member may be controllable between two states, or between a plurality of states. In the first-mentioned case, the light-modifying member may function as a light switch, locally permitting or preventing outcoupling of light from the light guide, and in the last-mentioned case, the light in the light guide may be partially outcoupled, in which case gray scales may be achieved. Which light is outcoupled and which remains in the light guide then depends on the angle of incidence at the guiding boundary, where rays of light having small angles of incidence are outcoupled while rays of light having large angles of incidence are totally reflected. Advantageously, the first compound refractive index may be sufficiently low to allow total internal reflection at the guiding boundary of the light guide, thereby preventing outcoupling of light through the corresponding portion of the guiding boundary.

For the light guide to totally internally reflect light entering the light guide at all angles, the refractive index of any element adjacent to a boundary of the light guide should fulfill the following relation:

wherein n_lg is the refractive index of the light guide at the guiding boundary of the light guide, and n_ci denotes the refractive index of the adjacent element, which may be, for example, a cladding layer arranged to cover the light guide core or central slab portion, air, and/or, as in the case of the present invention, a controllable light-modifying member.

For example, if the light guide has a refractive index of 1.67, the first compound refractive index may be smaller than 1.34 in order to achieve total internal reflection (TIR) for incoupled light of all angles.

In this way, light coupled into the light guide is effectively prevented from being coupled out of the light guide through a portion of the guiding boundary corresponding to a portion of an adjacent light-modifying member being in its first state. The second refractive index (fluid) may be lower than the third refractive index (particles).

In this case, for a given portion of the light-modifying member containing a fluid having an essentially uniform distribution of particles, a higher concentration of particles means a higher compound refractive index of that portion. Consequently, TIR can be achieved at a low particle concentration in the portion of the light-modifying member and outcoupling may be obtained at a higher particle concentration.

Alternatively, the third refractive index may be lower than the second refractive index, in which case, obviously, a higher concentration of particles leads to a lower compound refractive index in the above-described given portion of the light-modifying member.

In accordance with one embodiment of the present invention, the particles may be susceptible to the influence of an electric field, and the light-modifying member may be controllable between the at least first and second states by altering an electric field in the above-mentioned portion of the light-modifying member, thereby controlling the distribution ofparticles.

In the present embodiment, the particles may or may not be charged. For uncharged particles, the particles are caused to move in response to the application of an electric field through dielectrophoresis, which is described in detail in "Dielectrophoresis; the behaviour of neutral matter in non-uniform electric fields", by H. A. Pohl, University Press,

Cambridge, 1978.

In the case of charged particles, the majority of the particles advantageously has the same sign charge so as to prevent clustering of oppositely charged particles. (Electrical neutrality of the fluid is ensured by the presence of ions of opposite charge.) With an arrangement in accordance with the present embodiment of the invention, the particles may be essentially uniformly distributed in the abscence of an electric field. When a field is applied, the particles may be re-distributed. Either the particles move until the field is removed or a state is entered in which there is an equilibrium between the forces exerted on the particles through their own charges (in the case of electrophoresis) or dipoles (in the case of dielectrophoresis) and the applied electric field. For a more detailed description of electrophoresis, the following document is referred to:

"Principles of Colloid and Surface Chemistry", by P. C. Hiemenz and R.

Rajagopalan, 3^rd edition, Marcel Dekker Inc., New York, 1997, pp. 534-574. In accordance with another embodiment, the particles are susceptible to the influence of a magnetic field, and the light-modifying member is controllable between the at least first and second states by applying a magnetic field in the above-mentioned portion of the light-modifying member, thereby controlling the distribution ofparticles

(magnetophoresis) . In this embodiment, the particles may be, for example, magnetic and have such properties that magnetic dipoles can be induced in the particles. The particle distribution is then controllable through the application of a magnetic field.

As an alternative or complement to the two above-mentioned embodiments of the present invention, the distribution of the particles in the fluid may also be controlled by mechanical means (as in a MEMS device), which may lead to a spatially varying density of the fluid and, consequently, a varying concentration ofparticles in the fluid-particle suspension. In order to realize control of the particle distribution in accordance with, for example, any of the above-mentioned mechanisms, the controllable light guide may further comprise control means.

These control means may be provided, for example, in the form of patterns, such as electrode patterns, coil patterns or mechanical actuators, which may be positioned adjacent to the light-modifying member, or elsewhere in the controllable light guide. To prevent unnecessary blocking of outcoupled light, these patterns, depending on their position, may be advantageously formed by using one or several transparent materials, such as ITO (Indium-Tin-Oxide) or similar well-known materials. At locations where outcoupling should be prevented, electrodes that reflect light (e.g. made of aluminum or silver) are used advantageously.

The control means may be provided on opposite sides of the light-modifying member, in which case the particle distribution is controlled by causing the particles to selectively move towards either side of the light-modifying member (in a direction which is substantially perpendicular to the light guide).

Alternatively, the control means may be provided to achieve so-called "in- plane switching". In this case, the particle distribution is controlled by causing the particles to selectively move laterally in the light-modifying member. Combinations of in-plane and perpendicular switching are also possible. As an alternative to control means comprised in the controllable light guide, the electric field and/or magnetic field may be generated externally of the controllable light guide.

The controllable light guide according to the present invention may advantageously comprise a plurality of light-modifying members, each being individually controllable.

More or less complex patterns of outcoupling and light-containing portions of the controllable light guide may thus be formed.

In accordance with a further embodiment, the controllable light guide may further comprise a light-altering member arranged adjacent to the light-modifying member on a far side of the light-modifying member in relation to the light guide and adapted to alter at least one property of the at least partly outcoupled light.

This light-altering member may be, for example, a layer which is added to the controllable light guide on the light-exiting face of the light-modifying member, and the light-altering member may alter, for example, at least one of an angular distribution, a spatial distribution and a state of polarization of the outcoupled light.

In order to direct the outcoupled light towards a direction normal to an outcoupling surface of the controllable light guide, a light-altering member which may be provided in the form of a layer having a higher refractive index than the light-modifying element may be used.

Furthermore, the light-altering member may have outcoupling structures so as to enhance the directing effect. Such outcoupling structures may be provided separately or in combination with the above-mentioned higher refractive index. These outcoupling structures may be advantageously formed by substantially parallel grooves.

In order to outcouple polarized light, the light-altering member may be made of a birefringent material.

In particular, the light-altering member may be birefringent and have grooves which are substantially parallel. These grooves may be provided in such a way that a refractive index component of the birefringent light-altering member is perpendicular to the grooves. This refractive index component may be substantially equal to that of the fluid with a low concentration of suspended particles, or, alternatively, substantially equal to that of the fluid with a high concentration of suspended particles. Furthermore, the controllable light guide according to the present invention may be advantageously comprised in a controllable lighting device, further comprising a light source arranged to allow incoupling of light emitted by the light source into the controllable light guide.

The above-described controllable lighting device may be advantageously included in a display device, further comprising an image-forming member having an at least partly controllable transmittance, wherein the controllable lighting device is positioned behind the image-forming member in relation to a viewer and configured to illuminate the image-forming member so as to act as a controllable backlight

Alternatively, the controllable lighting device may be included in a display device, further comprising an image-forming member having an at least partly controllable reflectance, wherein the controllable lighting device is positioned in front of the image- forming member in relation to a viewer and configured to illuminate the image- forming member so as to act as a controllable frontlight. BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention, wherein: Figs, la-c schematically illustrate a portion of the controllable light guide according to the present invention in three different states;

Fig. 2 schematically shows a portion of a controllable light guide in accordance with a first embodiment of the present invention;

Fig. 3 schematically shows a portion of a controllable light guide in accordance with a second embodiment of the present invention, comprising a light-altering member arranged adjacent to the light-modifying member;

Figs. 4a-b schematically illustrate an example of a display device according to the present invention.

DESCRIPTION OF EMBODIMENTS

The present invention will be mainly described hereinafter with reference to a planar controllable light guide comprising a light guide and a light-modifying member having a plurality of positively charged particles suspended in a fluid which has a refractive index which is lower than that of the light guide as well as that of the particles. It should be noted that this by no means limits the scope of the invention, which is equally applicable to controllable light guides of other configurations, such as optical fibers and curved planar light guides. Furthermore, the particles suspended in the fluid may be negatively charged, uncharged, magnetic or non-magnetic. Of course, the fluid may have a higher refractive index than the light guide and the particles. Additionally, an example of application of an embodiment of the controllable light guide according to the present invention is illustrated, in which the controllable light guide is included in a backlight for a transmissive flat-panel display. In this context, it should be noted that a lighting device comprising a controllable light guide according to the present invention is equally applicable as an ambience-creating lighting device or as a frontlight for various types of reflective displays, such as reflective liquid-crystal displays, electrowetting displays, electrophoretic displays, or electrochromic displays.

Figs, la-c show a portion of a controllable light guide 101 comprising a light- modifying member 102, which is arranged adjacent to a light guide 103. The light-modifying member 102 is here provided in the form of a compartment containing a fluid 104 and a plurality of particles, collectively denoted by the reference numeral 105. In this case, the light guide 103 has a single refractive index n_lg, the fluid 104 has a refractive index nfi which is lower than the refractive index n_lg of the light guide 103, and a particle 105 has a refractive index n_p which is higher than the refractive index n_lg of the light guide 103. In each Fig. la-c, a portion 106 of the light-modifying member 102 is indicated, in which a concentration of particles 105 in the fluid 104 is altered between the three example states illustrated in Figs, la-c. Light is coupled into the light guide by a suitably positioned light source (not shown).

Fig. Ia shows the controllable light guide 101 with the light-modifying member 102 being in a first state, in which the particles 105 are distributed in the fluid 104 so that the concentration of particles in the central portion 106 of the light-modifying member 102 is very low. This results in a low compound refractive index rid of the fluid-particle suspension present in the central portion 106. In the illustrated example, the compound refractive index nd in this first state is sufficiently low to fulfill the requirement for total internal reflection (TIR) in the light guide as indicated by the light ray 107. Note that the central portion 106 of the light-modifying member 102 is preferably as large as possible (in the currently available technology, the central portion 106 may occupy, for example, over 90% of the surface of the light-modifying member 102). In order to render the light guide 103 as efficient as possible, the area of the light-modifying member 102 outside the central portion 106 is preferably made reflective, which may be achieved by covering this area with a mirror. Alternatively, particles that are reflective when densely packed may be used.

Fig. Ib shows the controllable light guide 101 with the light-modifying member 102 being in a second state, in which the particles 105 are essentially uniformly distributed in the fluid 104. This results in a high compound refractive index n_c2 of the central portion 106. In the illustrated example, the compound refractive index n_c2 is approximately equal to the refractive index of the light guide, leading to essentially complete outcoupling of the light in the light guide through the portion 106, as indicated by the light ray 108. The state of Fig. Ib is preferably the zero-power state.

Fig. Ic shows the controllable light guide 101 with the light-modifying member 102 being in a third state which is intermediate to the first and second states. In this third state, the particles 105 are distributed in the fluid 104 so that the concentration of particles 105 in the central portion 106 is higher than is the case in the first state and lower than in the second state. This results in an intermediate compound refractive index n_C3 of the central portion 106. In the illustrated example, the compound refractive index n_C3 allows outcoupling of a proportion of the light incident on the guiding boundary 109 between the light guide 103 and the light-modifying member 102, as indicated by the transmitted and reflected rays of light 110a and b, respectively.

Fig. 2 schematically illustrates a first embodiment of the controllable light guide according to the present invention, in which two light-modifying members 201 and 202 are shown, one of the light-modifying members, i.e. 201, being in a non-outcoupling state and the other, i.e. 202, being in an outcoupling state.

Each light-modifying member 201, 202 shown is formed by a compartment containing an essentially insulating fluid 203 and a plurality of electrically charged particles 204. In the present example, the particles hold positive charges. Each light-modifying member 201, 202 further comprises three electrodes 207a-c and 208a-c, respectively, at least some of which are preferably formed by an optically transparent electrically conductive material, such as ITO or an equivalent material. The peripheral electrodes 207a,c and 208a,c may be advantageously reflective in order to prevent outcoupling of light through densely packed particles in the non-outcoupling state illustrated by the light-modifying member 201 in Fig. 2. In order to limit the angular spread of outcoupled light, each light-modifying member 201, 202 further has a structured outcoupling face 205, 206.

As is illustrated in Fig. 2, the light-modifying member 201 is controlled to a first, non-outcoupling state by applying a negative voltage to the two peripheral electrodes 207a,c and a positive voltage to the central electrode 207b. By selecting suitable voltages, the particles 204 are concentrated in the vicinity of the peripheral electrodes 207a,c, while the concentration of particles in the central portion 209 of the light-modifying member 201 becomes very low. The compound refractive index in this portion 209 thereby becomes sufficiently low to prevent outcoupling of light as described with reference to Fig. Ia. As is also illustrated in Fig. 2, the light-modifying member 202 is controlled to a second, outcoupling state by not applying any voltages or connecting all of the three electrodes 208a-c to the same potential, in which case the charged particles minimize the energy in the system by forming an essentially uniform suspension. The concentration of particles in the central portion 210 of the light-modifying member 202 thereby becomes high, and outcoupling is permitted as indicated by the outcoupled ray of light 211.

It should be emphasized that Fig. 2 is a schematic illustration, in which, for the sake of simplicity, no measures to prevent cross-talk between adjacent light-modifying members have been included. Measures to prevent or at least reduce cross-talk include spacing apart of adjacent fluid-particle filled compartments and/or introducing shielding electrodes between adjacent light-modifying members.

Fig. 3 schematically illustrates a second embodiment of the controllable light guide according to the present invention. Here, control electrodes 301a-c and 302a-c have been formed on the two adjacent light-modifying members 303 and 304, respectively.

Furthermore, a light-altering member 305 in the form of a structured layer has been added on the side of the light-modifying members 303, 304 facing away from the light guide 103.

The operation of the controllable light guide illustrated in Fig. 3 is identical to that described above with reference to Fig. 2, the main difference between the two illustrated embodiments being that various properties of the light which is outcoupled from the light guide 103 by means of the light-modifying member 304 are changeable by means of the light-altering member 305. In the illustrated example, the light-altering member 305 is a structured layer having a refractive index which is higher than the compound refractive index of the outcoupling light-modifying member 304. As indicated by the outcoupled ray of light 306, the outcoupled light has been directed towards the normal of the light guide 103.

Figs. 4a-b schematically illustrate an example of application of an embodiment of the controllable light guide according to the present invention, in the form of a flat-panel display 401 comprising a 3x3 pixel array 402 of controllably tranmissive pixels 403a-i and a backlight 404. In Fig. 4a, the backlight 404 is positioned behind the pixel array 402 from the viewpoint of a viewer of the flat-panel display. In the present example, one of the pixels 403e in the pixel array 402 is in its transmissive state, while the remaining pixels 403a-d, 403f-i are in their non-transmissive state.

Fig. 4b shows the backlight 404 unobscured by the pixel array 402. As illustrated, the backlight comprises a controllable light guide 405, the outcoupling of light from which is controllable in three segments 406, 407, and 408. The backlight 404 further comprises at least one light source (not shown) which is arranged to achieve incoupling of light into the controllable light guide 405.

In order to allow more efficient energy usage and/or achieve local high- lighting of the transmissive pixel 403e, the segments 406 and 408 are controlled to a non- outcoupling state, while the segment 407 corresponding to the transmissive pixel 403 e is controlled to an outcoupling state as indicated by the rays of light 409, 410 in Fig. 4b.

The outcoupling states of the segments 406-408 are controlled by applying voltages to electrodes 41 la-c, 412a-c and 413a-c comprised in light-modifying members 414, 415 and 416, respectively. By applying voltages to these electrodes, the particle distributions and hence the compound refractive indices in the light-modifying members 414-416 are controlled in the manner described above with reference to Figs. 2 and 3.

It will be evident to the person skilled in the art that the present invention is by no means limited to the preferred embodiments described hereinbefore. For example, many configurations of electrodes or control means other than those described hereinbefore are feasible, such as electrodes or other control means being formed at boundaries between adjacent light-modifying members and/or protruding into the light-modifying member. Furthermore, any electrodes or other control means may be made of a non-transparent material, especially if their extensions are very small as compared with the remainder of the light-modifying member.

Claims

CLAIMS:

1. A controllable light guide (101; 405) comprising: a light guide (103) configured to contain light coupled into said light guide and to guide said light along a principal extension of said light guide through reflections of said light against a guiding boundary (109) of said light guide (103), said light guide having a first refractive index at said guiding boundary (109); and at least one light-modifying member (102; 201-202; 303-304; 414-416) arranged adjacent to said guiding boundary (109) of the light guide (103), said light- modifying member comprising: a fluid (104) having a second refractive index; and - a plurality of particles (105) having a third refractive index different from said second refractive index, said plurality of particles being distributed in said fluid, wherein said light-modifying member (102; 201, 202; 303-304; 414-416) is controllable between at least a first state having a first particle distribution resulting in at least a portion (106; 209, 210) of said light-modifying member (102; 201-202; 303-304; 414-416) having a first compound refractive index (n_ci), and a second state having a second particle distribution resulting in said portion (106; 209, 210) of said light-modifying member having a second compound refractive index (n_c2), at least said second compound refractive index being sufficiently high in relation to said first refractive index so as to allow at least partial outcoupling of light through a corresponding portion of said guiding boundary (109) of the light guide (103).

2. A controllable light guide (101; 405) according to claim 1, wherein said first compound refractive index (n_ci) is sufficiently low to allow total internal reflection at said guiding boundary (109) of the light guide (103), thereby preventing outcoupling of light through said corresponding portion of said guiding boundary.

3. A controllable light guide (101; 405) according to claim 1 or 2, wherein said third refractive index is higher than said second refractive index.

4. A controllable light guide (101; 405) according to any one of the preceding claims, wherein: said particles (105) are susceptible to the influence of an electric field; and said light-modifying member (102; 201-202; 303-304; 414-416) is controllable between said at least first and second states by altering an electric field in said portion (106; 209, 210) of said light-modifying member, thereby controlling said distribution ofparticles.

5. A controllable light guide (101; 405) according to any one of claims 1 to 3, wherein: said particles are susceptible to the influence of a magnetic field; and said light-modifying member (102; 201-202; 303-304; 414-416) is controllable between said at least first and second states by applying a magnetic field in said portion (106; 209, 210) of said light-modifying member, thereby controlling said distribution ofparticles.

6. A controllable light guide (101; 405) according to any one of the preceding claims, further comprising control means (207a-c, 208a-c; 301a-c, 302a-c;

41 la-c, 412a-c, 413a-c) for controlling said distribution ofparticles (105) in said fluid (104).

7. A controllable light guide (101; 405) according to any one of the preceding claims, comprising a plurality of said light-modifying members (201-202;

303-304; 414-416), each being individually controllable.

8. A controllable light guide (101; 405) according to any one of the preceding claims, further comprising a light-altering member (305) arranged adjacent to said light- modifying member (303-304) on a far side of said light-modifying member in relation to said light guide (103) and adapted to alter at least one property of said at least partly outcoupled light.

9. A controllable light guide (101; 405) according to claim 8, wherein said light- altering member (305) is a structured element having a refractive index which is higher than said first refractive index.

10. A controllable light guide (101; 405) according to claim 8 or 9, wherein said light-altering member (305) is birefringent, whereby said outcoupled light passing through said light-altering member becomes polarized.

11. A controllable lighting device comprising: a controllable light guide (101; 405) according to any one of the preceding claims; and a light source arranged to allow incoupling of light emitted by said light source into said controllable light guide (101; 405).

12. A display device (401) comprising: an image-forming member (402) having an at least partly controllable transmittance; and a controllable lighting device (404) according to claim 11 , said controllable lighting device being positioned behind said image-forming member in relation to a viewer and being configured to illuminate said image-forming member so as to act as a controllable backlight.

13. A display device comprising: - an image-forming member having an at least partly controllable reflectance; and a controllable lighting device according to claim 11 , said controllable lighting device being positioned in front of said image-forming member in relation to a viewer and being configured to illuminate said image-forming member so as to act as a controllable frontlight.