WO2008125926A1

WO2008125926A1 - Controllable light-guide and display device

Info

Publication number: WO2008125926A1
Application number: PCT/IB2007/054503
Authority: WO
Inventors: Giovanni Cennini; Roberto Caputo; Fetze Pijlman; Gabriel-Eugen Onac; Hugo J. Cornelissen; Jan F. STRÖMER; Maarten Sluijter; Thomas C. Kraan
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-04-17
Filing date: 2007-11-06
Publication date: 2008-10-23

Abstract

A controllable light-guide (2; 21; 51; 62) comprising first (7) and second (8) optically transparent substrates and a controllable light-modulating member (6; 22; 55) sandwiched therebetween. The light-guide is configured to guide light, through internal reflections, between first and second oppositely located outer boundaries (14, 15) of the light- guide, and the light-modulating member (6; 22; 55) is controllable to exhibit a refractive index gradient in a selected portion thereof, thereby enabling bending of a guided light-beam (10a-c; 30; 40) passing through the selected portion, such that the guided light beam hits one of the first and second outer boundaries (14, 15) at a sufficiently small angle (Θ) with respect to a normal (13) of the boundary to escape from the light-guide. Utilizing the light-guide according to the present invention, not only the position of outcoupling can be controlled, but also the direction thereof. This enables a higher resolution and precision of the controlled outcoupling.

Description

CONTROLLABLE LIGHT-GUIDE AND DISPLAY DEVICE

TECHNICAL FIELD OF THE INVENTION

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

TECHNICAL BACKGROUND

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 kinds of flat panel displays, such as so-called plasma displays, are comprised of arrays of light emitting pixels, the majority of flat-panel displays have arrays of pixels which can be switched between states but which are unable to independently emit light. Such flat-panel displays include the ubiquitously found 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 so-called backlight, in the case of a transmissive-type pixel array, or, in the case of a reflective-type pixel array, by ambient light or a so-called frontlight. A conventional backlight (and frontlight) is comprised of 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 to enable 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, as is often the case, only a very small proportion of the pixels are bright (in their transmissive state), 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. By modulating a portion of the light-management member between a transparent state and a scattering state, the outcoupling of light from the optical waveguide can be controlled. However, the light emitted by this illumination system is scattered, which, for some applications, is a disadvantage.

SUMMARY OF THE INVENTION In view of the above-mentioned and other drawbacks of the prior art, a general object of the present invention is to provide an improved controllable light-guide, in particular a light-guide capable of coherently outcoupling light.

According to the present invention, these and other objects are achieved through a controllable light-guide comprising first and second optically transparent substrates, and a controllable light-modulating member sandwiched therebetween, the light- guide being configured to guide light, through internal reflections, between first and second oppositely located outer boundaries of the light-guide, with the light-modulating member being controllable to exhibit a refractive index gradient in a selected portion thereof, thereby enabling bending of a guided light-beam passing through the selected portion, such that the guided light beam hits one of the first and second outer boundaries at a sufficiently small angle with respect to a normal of the boundary to escape from the light-guide.

The light-guide may advantageously be a planar light-guide, which guides light, through internal reflection, between oppositely located, essentially parallel faces thereof. The planar light-guide may be made of 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, or polymers, for example poly-methyl methacrylate (PMMA) etc.

The light-guide may be essentially flat or curved, depending on the application. It may, furthermore, be either substantially rigid, or flexible. Furthermore, by an "optically transparent" medium is, in the present context, meant a medium which permits passage of at least a fraction of the light (electromagnetic radiation in the visible spectrum) impinging on it.

The present invention is based upon the realization that controlled outcoupling of coherent light from a light-guide can be achieved by changing the direction of a guided light beam at some point during its passage through the light-guide. The present inventors have further realized that this can be achieved in an advantageous manner by including in the light-guide a light-modulating member which can be controlled to exhibit a refractive index gradient at a selected location. Utilizing the light-guide according to the present invention, not only the position of outcoupling can be controlled, but also the direction thereof. This enables a higher resolution and precision of the controlled outcoupling.

A further advantageous effect obtainable through the present invention is that light can be outcoupled with an essentially unchanged spectral distribution, which is typically not the case for outcoupling by scattering.

According to the theory related to the interaction between light and an inhomogeneous transparent medium, a light beam bends toward regions of higher refractive index. By controlling the light-modulating member in such a way that a refractive index gradient is created, the direction of light can consequently be controlled correspondingly.

Starting from a homogeneous medium having a refractive index no, a refractive index gradient is created by locally modifying the medium to have another refractive index ni . Depending on the medium, such a local change in refractive index and the accompanying creation of a refractive index gradient can be brought about by various kinds of external stimuli, such as, for example, heat, pressure, an electric field or a magnetic field.

A spatially controllable refractive index can be obtained in a variety of ways. For example, the light-modulating member may include a controllable birefringent material, such as a liquid crystal layer. As is well known to one skilled in the relevant art, a birefringent material has an anisotropic refractive index, with an ordinary refractive index no for a ray of light (an ordinary ray) which is polarized perpendicularly to the optical axis of the material, and an extraordinary refractive index n_e for a ray of light (an extraordinary ray) which is polarized parallel to the optical axis. For a liquid crystal layer comprising a plurality of elongated liquid crystal molecules, the optical axis is usually parallel to the long axis of the liquid crystal molecules.

By subjecting a portion of the liquid crystal (LC) layer to an electric field, a local reorientation of the liquid crystal molecules in that portion can be achieved. Thereby, one linearly polarized component of an unpolarized guided light beam having an electric field which oscillates in the plane in which the reorientation takes place (the extraordinary component) will encounter a refractive index that gradually varies from the ordinary refractive index no to the extraordinary refractive index ne or vice versa. During its passage through the LC-layer having reoriented LC-molecules, this extraordinary component will experience a refractive index gradient, and be bent towards an area with a higher refractive index.

The other component, i.e the orthogonal linearly polarized component (the ordinary component) typically experiences no change in refractive index since its electric field oscillates in a plane perpendicular to the long axis of the LC-molecules. Consequently, the ordinary component passes through the LC-layer having reoriented LC-molecules without having its direction changed.

Other examples of methods for achieving a controllable refractive index gradient include controlled displacement of particles and/or fluids, through, for example electrophoresis, magnetophoresis, or electrowetting.

As discussed above, the light-modulating member may be controllable to exhibit a first refractive index gradient with respect to a first polarization component of an unpolarized light beam, and a second refractive index gradient with respect to a second polarization component of the light beam, thereby enabling different bending of the po larization components .

Thus, outcoupling of polarized light can be achieved. This is especially advantageous for applications where only one polarization component is required, such as when the light-guide according to the present invention is used in a backlight for a liquid crystal display (LCD). For conventional backlights, which are capable of emitting unpolarized light only, 50% of the emitted light is lost at the first polarizer of the liquid crystal panel.

As discussed above, a liquid crystal layer is one example of a suitable light- modulating member which is controllable to bend light in a polarization-dependent manner. The liquid crystal molecules can, through a proper configuration of the electric field, all be made to reorient in a plane perpendicular to the light-guide. The polarization component perpendicular to this plane, and hence perpendicular to the elongated liquid crystal molecules, will experience no change in the refractive index resulting from the reorientation, while the polarization component in the plane of reorientation will be bent when passing through the region with reoriented liquid crystal molecules. At an interface between two optical members, refraction and/or reflection of light will generally take place unless the two optical members are matched to each other at the interface. When a ray of light traveling in a first optical member hits an interface with a second optical member, total internal reflection may occur. In this case, the ray of light is completely reflected at the interface and none of its energy enters the second optical member. For the controllable light-guide according to the invention, total internal reflection in either of the substrates should preferably be avoided or at least minimized.

In case both a substrate and the light-modulating member are isotropic at an interface therebetween, total internal reflection can be avoided by providing a substrate having a refractive index which is lower than or equal to the refractive index of the light- modulating member.

In various embodiments thereof, however, the light-modulating member is anisotropic, which makes it more complicated to select parameters for avoiding, or at least minimizing, the occurrence of total internal reflection in a substrate. According to one embodiment, this can be achieved by matching at least one of the first and second substrates, at least at a boundary between the substrate and the light- modulating member, to the controllable light-modulating member with respect to refractive index and optical axis direction.

An anisotropic optical member generally has an ordinary refractive index H₀, an extraordinary refractive index n_eo and an optical axis having a certain direction d. When the substrate is matched to the light-modulating member with respect to refractive index and optical axis direction, the ordinary refractive index n^ of the substrate equals the ordinary refractive index rio,i of the light-modulating member; the extraordinary refractive index n_eo,_s of the substrate equals the extraordinary refractive index ne_O,i of the light-modulating member; and the direction d_s of the optical axis of the substrate equals the direction di of the light-modulating member. This matching, furthermore, takes place at the interface between the substrate and the light-modulating member.

In the case described above, there is a perfect match between the substrate and the light-modulating member, and no reflection will occur. According to an alternative embodiment, total internal reflection in a substrate can be avoided or at least minimized by configuring the controllable light-guide in such a way that at least one of the first and second substrates has an effective refractive index which is lower than or equal to an effective refractive index of the controllable light-modulating member, at least at a boundary between the substrate and the light-modulating member. By "effective" refractive index n_e,_eff should be understood the index of refraction which an extraordinary ray or light experiences inside an anisotropic material for certain values of the ordinary refractive index H₀, the extraordinary refractive index n_eo and the direction d of the optical axis of the material, and the direction θ of the extraordinary ray of light. By way of example, the effective refractive index for a simple two- dimensional situation with a vertically oriented optical axis, i.e. d = (0,0,1), the effective index of refraction n_e,_eff is given by:

n_nn_n ne,eff = ^■ (1),

■^ sπ^ θ + ^ cos^

from which can be seen that

depends on n₀, n_eo, and θ. In three dimensions, the determination of n_e,_eff becomes more difficult, but, of course the same laws of physics apply. The desired relation between the refractive indices of the substrate and the light-modulating member respectively may be achieved by adding a refractive index matching layer therebetween. In the event that the substrate is an isotropic material, such as ordinary glass, and the light-modulating member is an anisotropic material, such as a liquid crystal layer, the refractive index matching layer should have isotropic characteristics on the side facing the substrate and anisotropic characteristics on the side facing the anisotropic light-modulating member. In particular, the refractive index matching layer may have a refractive index transition from, at a side thereof facing the base layer, a first effective refractive index being essentially equal to an effective refractive index of the base layer to, at a side thereof facing the light-modulating member, a second effective refractive index being lower than or equal to the effective refractive index of the light-modulating member. This may, for example, be achieved by providing the refractive index matching layer in a material having similar characteristics as the anisotropic layer, which at the side thereof facing the substrate layer is configured to match the refractive index of the substrate layer. As a practical example, the refractive index matching layer may be provided in the form of a liquid crystal layer having a pre-tilt at the side thereof facing the substrate. Moreover, the light-guide may additionally comprise light-recycling means configured to alter a polarization state of light exiting from the light-guide after having been guided therethrough, and re-introduce the altered light into the light-guide.

Such light-recycling means may, for example, be provided in the form of a suitable retardation plate in combination with a mirror to re-introduce the altered light into the light-guide.

By providing such light-recycling means, practically all of the (unpolarized) light coupled into the light-guide can be outcoupled as polarized light. According to one embodiment, the light-guide according to the present invention may comprise a plurality of light-guide cells, each having individually controllable control means for controlling the light-modulating member to exhibit a refractive index gradient in the light-guide cell According to one embodiment, the light-modulating member may include a liquid crystal layer, and the control means may comprise a first and a second electrode arranged in such a way that a voltage across the first and second electrodes results in an electric field being formed in the liquid crystal layer.

As discussed above, the direction of the liquid crystal molecules, and hence the refractive index of the liquid crystal layer, can be locally controlled through subjecting the liquid crystal layer in the light-guide cell to an appropriate electric field. Such an electric field can be formed by applying a voltage over the electrode pair.

The electrode pair may be arranged on opposite sides of the liquid crystal layer, or on the same side thereof. In the latter case, the liquid crystal layer may be controlled through so-called in-plane switching.

Each light-guide cell may comprise several electrodes, which may be arranged on either side or both sides of the liquid crystal layer. Through individual control of these electrodes, the electric field, and hence the refractive index gradient of the liquid crystal layer in the light-guide cell, can be controlled to bend a light-beam traveling through the liquid crystal layer practically at will.

In particular, by providing electrodes on opposing sides of the liquid crystal layer, a larger refractive index gradient can be obtained than when using in plane switching, which results in an increased bending of a beam of light passing through the liquid crystal layer and thus enables a smaller angle of exit from the light-guide with respect to a normal to the light-guide.

Moreover, at least one of the electrodes provided in the light-guide may have a first set of electrode portions each extending in a first direction in a plane parallel to the light- guide and a second set of electrode portions each extending in a second direction, different from the first direction, in the plane. In this manner, the liquid crystal molecules comprised in the liquid crystal layer can be reoriented in more than one plane perpendicular to the light-guide, which enables outcoupling of light entering a controlled portion of the liquid crystal layer from more than one direction in a plane parallel to the light guide. By providing a plurality of electrode portions, this capability may be enhanced.

The at least one electrode may, for example, be provided in the form of a zigzag pattern. The liquid crystal layer may advantageously be homeotropically aligned in a state where no electric field is applied.

When a liquid crystal layer is homeotropically aligned, the liquid crystal molecules are arranged perpendicularly to the liquid crystal layer, so that molecule ends are facing the substrates between which the liquid crystal layer is sandwiched. Using this kind of alignment, the liquid crystal molecules can be controlled to be reoriented in any direction without any anomalies. Alternatively, the liquid crystal molecules may be oriented essentially in parallel with at least one of the substrates when no voltage is applied. In this case, the liquid crystal molecules may preferably further be oriented essentially in perpendicular to a general direction of extension of the electrodes in order to facilitate reorientation of the molecules through the application of a voltage across a pair of electrodes.

The light-guide may, additionally, comprise a light-modifying member for modifying at least one property of light having been outcoupled from the light-guide.

Examples of such properties include, for example, the spatial, angular, and spectral distributions, and the polarization state of the outcoupled light.

In order to bring about such a modification of the outcoupled light, one optical element or a combination of optical elements may be used. Examples of suitable optical elements include mirrors, lenses, lenticular plates, retardation plates, prisms, in-cell retarder layers, reactive mesogen (RM) cured in LC material, light scattering elements, diffractive gratings, layers of anisotropic media or phosphor layers or polarization layers.

According to another embodiment, the controllable planar light-guide according to the present invention may comprise a plurality of individually controllable light- guide cells, each having first and second optically transparent substrates; a liquid crystal layer comprising a plurality of liquid crystal molecules sandwiched between said first and second optically transparent substrates, and first and second electrodes arranged in such a way that an application of a voltage across the electrodes results in a localized reorientation of the liquid crystal molecules.

As already discussed above, the outcoupling of light from such a light-guide can be controlled by individually controlling the voltages applied to the light-guide cells comprised in the light-guide. Through proper control of the voltage over the electrodes in a particular light-guide cell, the amount of outcoupled light and/or the direction of outcoupled light from that light-guide cell can be controlled. Furthermore, the outcoupled light is coherent and polarized. Advantageously, these first and second electrodes may be arranged in a plane parallel to the light-guide.

Thus, so-called in-plane switching/control can be achieved, which enables the formation of a large refractive index gradient in the portion of the light-modulating member/LC-layer corresponding to the light-guide cell. The controllable planar light-guide according to the present invention may, furthermore, advantageously be comprised in a controllable light output device, further comprising a light-source arranged in such a way that light emitted by the light-source is coupled into the light-guide.

Such a controllable light output device may be utilized in a wide variety of applications, including as a backlight in a flat-panel display device, as a luminaire for providing illumination in various settings, such as an office or home environment, and as an ambience creating device which emits light for decorative purposes rather than for illumination.

According to one embodiment, the controllable light output device may be included in a display device, further comprising an image-forming member, and be arranged to illuminate the image-forming member.

By including a controllable light output device according to the present invention in a display device in this manner, the energy efficiency and the display performance, including brightness and contrast, of the display device can be improved. The energy efficiency can be improved by coordinating the spatial output of the controllable light output device with the image data fed to the image-forming member.

By virtue thereof, areas of the display device intended to be dark can be illuminated with a very low intensity, or not at all, while areas intended to be bright can be illuminated with a higher intensity. Based on the same principle, the display performance may be enhanced through the accompanying increased dynamic range and/or local highlighting.

By configuring the light-guide in such a way that the light-output device in which it is included becomes controllable to alternatingly illuminate the image forming member with light having a first direction and light having a second direction, the first direction being different from the second direction, a viewer can be provided with a 3D image. For example, the same image as determined by the image forming member can first be illuminated by light having a first direction, corresponding to the viewer's, say, left eye, and then be illuminated by light having a second direction corresponding to the viewer's right eye. The resulting impression will be that of a 3D image.

Of course, the light-output device may be configured to successively illuminate the image forming member with light having a larger number of directions than the two directions discussed above. A larger viewing angle can thus be achieved.

Furthermore, the controllable light-guide according to the present invention may be included in a display device further comprising an image-forming member, wherein the controllable light-guide may be arranged in front of the image-forming member in relation to a viewer and be controllable to alternatingly direct at least a portion of an image output by the image-forming member in a first direction, and in a second direction different from the first direction. Also for this embodiment, the impression of a 3D image can be created.

According to another embodiment, the controllable light output device of the invention can be included in a display device, further comprising control circuitry for controlling the lighting device, to emit light in response to image data supplied to the display device. In this embodiment, the controllable output device consequently is an emissive image-forming member. In order to achieve color capabilities, the light output device may be provided with conventional color filters, and light beams passing through the light-guide may be bent to exit from the light-guide through a selected color filter to display the desired color at the particular location.

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 a currently preferred embodiment of the invention, wherein: Figs la-b schematically illustrate a controllable illumination device comprising a controllable light-guide according to an embodiment of the present invention;

Fig 2a schematically illustrates possible paths of light beams in the light-guide in figs la-b when the refractive indices of the substrates is not matched with that of the light- modulating member sandwiched therebetween; Fig 2b schematically illustrates the controllable light-guide in fig 2a with refractive index matching layers being inserted between the respective substrates and the light-modulating member;

Fig 3 schematically illustrates a controllable light guide having a refractive index matching layer with an index matching that is dependent on the distance from the light- source;

Figs 4a-b schematically illustrate a controllable backlight including a controllable light-guide according to an embodiment of the present invention;

Figs 5a-b schematically show a portion of the controllable light-guide in Figs 4a-b controlled to achieve different degrees of bending of a passing light beam;

Fig 6 schematically illustrates exemplary paths of light-beams having different polarization states in the light-guide in fig 4b for the case when the lower substrate and the liquid crystal layer are not refractive index matched to each other;

Fig 7a schematically illustrates one exemplary way of implementing a refractive index matching layer between a substrate and the light-modulating member;

Fig 7b is a graph schematically illustrating the reflection of light at the interface between the substrate and the refractive index matching layer in fig 7a;

Fig 8 schematically illustrates one exemplary way of achieving refractive index match between the substrates and the light-modulating member; Figs 9a-c schematically illustrate the exit angle for a beam of light for different electrode configurations;

Fig 10 schematically illustrates outcoupling of light from the controllable light-guide in fig 4a for a first electrode configuration;

Fig 11 schematically illustrates outcoupling of light from the controllable light-guide in fig 4a for a second electrode configuration;

Fig 12 is a cross-sectional view of an exemplary display device comprising a controllable light-guide according to the present invention;

Fig 13 is a cross-sectional view of another exemplary, stereoscopic display device comprising a controllable light-guide according to the present invention; and Fig 14 is a cross-sectional view of a further exemplary, stereoscopic display device comprising a controllable light-guide according to the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In the following description, the present invention is mainly described with reference to a planar controllable light-guide in which a controllable refractive index gradient is achieved by controlling the orientation of liquid crystal molecules in a liquid crystal layer sandwiched between two substrates. It should be noted that this by no means limits the scope of the present invention, which is equally applicable to controllable light-guides in which a light-modulating member other than a liquid crystal layer is used. Such a light-modulating member could, for example, include an electrophoretic or magnetophoretic cell, in which a refractive index gradient is achieved by locally controlling the concentration of particles, having a first refractive index, suspended in a fluid having a second refractive index, or an electrowetting cell containing two immiscible fluids having different refractive indices.

Figs la-b schematically illustrate a controllable illumination device, or luminaire, which is one exemplary application for a controllable light-guide according to the present invention. In Fig Ia, an illumination device 1 is shown comprising a controllable planar light-guide 2 and a light-source 3, here in the form of a fluorescent lamp 3 and a reflector 4 arranged to couple light into the controllable light-guide 2 at an incoupling edge 5 thereof.

The controllable light-guide 2 comprises a light-modulating member 6 which is sandwiched between first 7 and second 8 transparent substrates. As illustrated in Fig Ia, the light-guide 2 is controllable in nine square segments 9a-i, of which the centrally located one 9e emits light (or rather permits light to escape) in both directions as indicated by the arrows (Fig. Ib). Of course, the light-guide 2 may have virtually any number of controllable segments having practically any shape, which may be different from application to application. The nine segments 9a-i chosen here are for illustration purposes only. With reference to Fig Ib, which is a cross-sectional view of a portion of the light-guide 2 in Fig Ia, taken along the line A-A', the mechanism behind the controllable out- coupling of light illustrated in Fig Ia will now be explained.

In Fig Ib, four different light beams 10a-d emanating from the light-source 3 and being guided by the light-guide 2 are followed as they pass through the light-guide 2. As illustrated in Fig Ib, the light-modulating member 6 has a first refractive index no in the dark segments 9d, 9f flanking the bright segment 9e. In the bright segment 9e, the refractive index is modified to repeatedly vary between no and the higher value ni . This is illustrated in Fig Ib by the refractive index curve 11 in the portion of the light-modulating member 6 corresponding to the bright segment 9e. When the light beams 10a-d pass through this portion of the light-modulating member 6, they will each encounter a refractive index gradient, and will be bent there towards regions with a higher refractive index, which is a well-known property of light passing through an inhomogeneous medium. Through this bending in the light-modulating member 6, each of the light beams 10a-d is redirected to hit the boundary between either of the substrates 7, 8 and a respective ambient substance 12, in this case air on both sides of the light-guide 2, at a sufficiently small angle Θ with respect to a normal 13 to the light-guide 2 to no longer fulfill the condition for total internal reflection (TIR) and be outcoupled from the light-guide 2. Depending on the direction in which each of the light beams 10a-d travels when passing through the light-modulating member 6 in the central segment 9e, it will be outcoupled on the first 14 or second 15 side of the light-guide 2.

In the presently illustrated example, the first 7 and second 8 substrate each have the same refractive index no as the light-modulating member 6 in its "uncontrolled" state. It should be noted that this selection has been made for illustration purposes only, and that a different selection, such as each of the substrates having a substantially lower refractive index than the light-modulating member or the substrates 7, 8 having mutually different refractive indices, may be advantageous depending on the application.

For the description above and for the beams of light shown in fig Ib, it has been assumed that the refractive indices of the substrates 7, 8 and the light-modulating member 6 match, and that there consequently is not refraction at the interfaces therebetween.

With reference to fig 2a, a situation will now be described in which the substrates 7, 8 and the light-modulating member 6 are not matched with respect to refractive index and/or direction of their respective optical axes. In the situation illustrated in fig 2a, both substrates 7, 8 have higher effective refractive indices that the light-modulating member 6. As can be seen in fig 2a, a first beam of light 20a having a first angle βi of incidence at the interface between the first substrate 7 and the light-modulating member 6 will pass through the light-modulating member 6 - and possibly be redirected during its passage therethrough - and the second substrate 8 to either be outcoupled or returned through TIR at the interface between the second substrate 8 and the ambient atmosphere 12. For a second beam of light 20b having a second angle β 2 of incidence, which larger than the first angle β 1, there will be TIR already at the interface between the first substrate 7 and the light-modulating member 6. This second beam of light 20b will then not be controllable by the light-modulating member 6. As is schematically illustrated by the light beam 23 in fig 2b, this can be mitigated through the inclusion of refractive index matching layers 21, 22 inserted between the light-modulating member 6 and the first 7 and second 8 substrates, respectively.

It will now be briefly described with reference to fig 3 how the uniformity across the surface of the controllable light-guide 2 in fig Ia can be improved by providing a position dependent refractive index matching between the first 7 and/or the second 8 substrate and the light-modulating member 6 sandwiched therebetween.

If light from the light-source 3 is outcoupled close to the light-source, less light will be available for outcoupling further away from the light-source 3. By providing a refractive index matching layer 25 having a refractive index matching between, say, the first substrate 7 and the refractive index matching layer, which varies in the x-direction as indicated in the graph in fig 3, the amount of light trapped in the first substrate 7 through TIR can be controlled as is indicated in the graph 26 in fig 3.

With reference to Figs 4a-b, a controllable backlight for a display device, which is another application for the controllable light-guide according to the present invention, will now be described.

In Fig 4a, the controllable backlight 40 having essentially the same configuration including nine individually controllable segments 9a-e as the controllable illumination device 1 in Figs la-b, is schematically shown. In contrast to the illumination device 1 of Figs la-b, the controllable backlight

40 in Fig 4a emits light in one direction only, as indicated by the arrows in Fig 4a. Furthermore, the controllable backlight 40 comprises a controllable light-guide 41 which is configured to controllably outcouple polarized light.

Similarly to the light-guide 2 comprised in the illumination device shown in Figs la-b, the light-guide 41 in Fig 4a includes a light-modulating member 42 sandwiched between first 7 and second 8 transparent substrates. However, as will be described in more detail below in connection with Fig 4b, the light-modulating member 42 is configured to controllably bend one polarization component of the guided light only.

In order to emit light through the first side 14 of the light-guide 41 only, the light-guide 41 is provided with a mirror foil 43 covering the second side 15 of the light-guide 41.

Furthermore, in order to enable outcoupling of all the incoupled light, the backlight 40 further comprises light-recycling means 44 in the form of a λ/4 retardation plate 45 and a mirror 46 for reversing the polarization state of the light having passed through the light-guide 41 and re-introducing the light back into the light-guide 41 through the opposite edge 47 thereof with respect to the in-coupling edge 5.

Turning now to Fig 4b, which is a cross-sectional view of the light-guide 41 in fig 4a taken along the line B-B', a beam 50 of unpolarized light will be followed as it passes through the light-guide 41.

In the exemplary light-guide 41, schematically illustrated in Fig 4b, the light- modulating member 42 is provided in the form of a liquid crystal (LC) layer having a plurality of elongated liquid crystal molecules 51 which are aligned in parallel to the first 7 and second 8 substrates in the absence of an electric field acting on the LC molecules 51. Alternatively, the liquid crystal molecules may be ho meo tropically oriented, that is, oriented perpendicular to the substrates 7, 8.

The light-guide 41 further comprises a number of individually controllable electrodes 52a-b (only two of these are indicated in Fig 4b for the sake of clarity of the drawing). By applying a voltage V over these electrodes 52a-b, the liquid crystal molecules 51 in the LC layer 42 reorient so as to follow the resulting electric field lines. It should be noted that the degree of reorientation typically depends on the strength of the applied electric field, which implies that the strength of the electric field (equivalent to the magnitude of the applied voltage V) can be used to analogously vary the bending of the light, as will be further elucidated below in connection with Figs 5a-b. With continued reference to Fig 4b, the reorientation of the LC molecules 51 in the "bright" segment 9e, results in the areas with varying refractive index and, consequently, in the development of refractive index gradients. Due to the electrode configuration in the backlight in Figs 4a-b, the LC molecules 51 are reoriented in a plane perpendicular to the light-guide 41. Therefore, only the polarization component 53 of the unpolarized light beam 50, which is polarized in the plane of reorientation of the LC molecules 51, experiences the refractive index gradient(s) and is bent. The other polarization component 54 which is polarized in a plane perpendicular to the reorientation plane of the LC molecules 51 will pass through the LC layer 42 without seeing a refractive index gradient, and will accordingly not be bent. Due to the provision of the mirror 43 on the second side 15 of the light-guide

41, all the outcoupled light exits the light-guide 41 on the first side 14 thereof, as indicated in Fig 4b.

As can also be seen in Fig 4b, the perpendicular polarization component 54 passes through the light-guide 41 from the incoupling edge 5 and exits through the opposite edge 47. After exiting the light-guide 41 through this edge 47, the light beam 54 passes through the λ /4 retardation plate 45 a first time, is reflected in the mirror 46, and then passes the λ /4 retardation plate 45 a second time before again entering the light-guide 41. Due to the resulting polarization reversal, the light beam 54 will have been transformed to a parallel- polarized light beam 55 traveling in the opposite direction. When passing through the LC- layer 42 in the bright segment 9e, this beam 54 is bent by the refractive index gradient and is outcoupled, following reflection in the mirror 43, on the second side 15 as indicated in Fig 4b.

With reference to Figs 5a-b, schematically showing a portion of the light- guide 41 in Fig 4b, the analog beam-bending capability previously touched upon will be described in more detail.

In Fig 5a, a voltage Vi is applied across the electrodes 52a-b. This voltage Vi is low and produces a relatively weak electric field which is only able to slightly reorient the LC molecules 51, as schematically illustrated in Fig 5a. As a result, a light beam 60 passing through the LC layer will only experience a small refractive index gradient and be bent only slightly. In the presently illustrated example, the light beam 60 is bent sufficiently to allow it to escape only just from the light-guide on the first side 14 thereof at a large angle Θi with respect to the normal 13 to the light-guide 41.

In Fig 5b, a higher voltage V₂ has been applied to the electrodes 52a-b, resulting in a larger electric field capable of providing a larger degree of reorientation (practically complete reorientation to the electric field lines) of the LC molecules 51. Therefore, the beam 60 will experience a larger refractive index gradient than was the case in Fig 5 a, and will be bent more, enabling it to exit the light-guide at a smaller angle Θ₂ with respect to the normal 13 to the light-guide 41. The situation with a non-match of the refractive indices between the substrates

7, 8 and the light-modulating member, here in the form of the liquid crystal layer 42 will now be described with reference to fig 6.

For the light-guide 65 in fig 6, the first substrate 7 is assumed to be made of glass (n_gi_ass=1.5), and the liquid crystal layer, in this exemplary case has an ordinary refractive index U₀=I .45 and an extraordinary refractive index n_eo=l .89. In the case of a beam 61 of unpolarized light that is incident on the interface between the first substrate 7 and the liquid crystal layer 42 at an angle that is larger then the critical angle for total internal reflection (TIR) with respect to the effective refractive index (which is a function of the ordinary refractive index, the extraordinary refractive index, the position-dependent direction of the optical axis and the angle of incidence of the beam of light), the extraordinary polarization component 62a of the unpolarized beam 61 of light will be contained in the first substrate 7 as is indicated in fig 6. The ordinary polarization component 62b will pass the interface between the substrate 7 and the liquid crystal layer 42 practically without refraction or reflection since the refractive indices n_gi_ass and n₀ essentially match. Consequently, there will be no controllable outcoupling of light from the light-guide 65 in this case as is also indicated in fig 6.

In fig 7a an exemplary light-guide configuration for achieving a simultaneous match between the first substrate 7 and the liquid crystal layer 42 is schematically shown, having a refractive index matching layer 64 provided between the first substrate 7 and the controllable liquid crystal layer 42.

The refractive index matching layer 64 is in this exemplary embodiment made of the same or similar material as the controllable liquid crystal layer 42. In order to achieve the desired match of the effective refractive index on both sides of the refractive index matching layer 64, the liquid crystal molecules 65 (only one is indicated here) have a pre-tilt of 18.5° at the interface between the first substrate 7 and the refractive index matching layer 64. At the interface between the refractive index matching layer 64 and the controllable liquid crystal layer 42, the liquid crystal molecules 66 (only one is indicated here) are aligned with the interface. The thickness D of the refractive index matching layer 64 should preferably be large enough for it to be continuous, i.e. the material properties of the refractive index matching layer 64 should change slowly over a distance corresponding to one wavelength of the light. This means that the thickness of the refractive index matching layer 64 should prefereably be of the same order of magnitude as the thickness of the controllable liquid crystal layer 42, which may typically be about 5 μm.

In fig 7b, the intensity reflectance coefficient R calculated as a function of the angle of incidence is schematically shown for the light-guides in figs 6 and 7a, respectively. The intensity reflectance coefficient is an indication of the percentage of the incident energy flux that is reflected. The simulations were carried out for an isotropic-anisotropic interface, where the isotropic medium represents the bottom glass substrate (n_gi_ass=l .5) and the anisotropic medium represents liquid crystal (in this embodiment: U₀=I.45 and n_eo=1.89). In fig 7b, the situation of fig 6 is illustrated by the solid line 70. As can be seen, total internal reflection (TIR) occurs for angles of incidence of approximately 75° and higher. The situation of fig 7a, with the director of the liquid crystal molecules 65 pre-tilted under an angle of approximately 18.5 degrees, is illustrated in fig 7b by the dashed line 71. In this specific example, referring to fig 7b, it can be concluded that R approaches 1 and TIR occurs only for incident angles above 89 degrees.

The pre-tilt referred to above can, for example, be achieved using a polymer network that stabilizes the director profile of the liquid crystal layer forming the refractive index matching layer 64. When forming such a refractive index matching layer 64, the first step would be to make a cell with a cell gap of the desired thickness of the refractive index matching layer 64. This cell is then filled with a mixture of the liquid crystal material and a a reactive mesogen material. The top surface of the cell would be planar alignment and the opposite surface would have the required 18.5° pre-tilt. A UV exposure step will freeze this alignment, so that an electric field will only insignificantly change the director profile. Then the top substrate is removed in such a way that the polymer network remains on the bottom surface. In a subsequent step, another top substrate, having a planar alignment layer, is applied to the cell. After assembling the preprocessed bottom substrate and the new top plate, the new cell is filled with the same liquid crystal material as in the previous cell but without the reactive mesogen material.

Fig 8 schematically shows a further embodiment of a controllable light-guide 80 having a match of the effective refractive index between the respective substrates 81, 82 and the light-modulating member in the form of a controllable liquid crystal layer 42. According to this embodiment, each of the first 81 and the second 82 substrates is provided in the form of a homogeneous polymerized liquid crystal layer. If the same of similar material is used for the substrates 81, 82 and the controllable liquid crystal layer 42 sandwiched therebetween, there will be practically no refractive index transition at either of the interfaces between the substrates 81, 82 and the layer 42 sandwiched therebetween. There will consequently be no TIR at the internal interfaces, but only at the interfaces between the ambient medium 12 and the substrates 81, 82, respectively.

In the various liquid crystal based embodiments of the invention described above, the control electrodes have been provided on one side of the liquid crystal layer in a so-called in-plane switching (IPS) configuration. As will be described below in connection with figs 9a-c, other kinds of electrode configurations may advantageously be used, especially in cases when smaller outcoupling angles are desired.

In fig 9a, a light-guide 90 with electrodes 91a-b configured in an IPS configuration is again schematically illustrated. The directions of the liquid crystal molecules comprised in the liquid crystal layer 42 resulting from the application of a voltage across the electrodes 91a-b are illustrated by director profiles 92. Furthermore, an exemplary beam of light 93 is shown to be bent through the refractive index gradient in the liquid crystal layer 42 and to then exit the light-guide through the second substrate at an angle αi .

In fig 9b, the light-guide 90 has another electrode configuration, with electrodes 95a-b on opposite sides of the liquid crystal layer 42. Application of a voltage across the electrodes 95a-b as indicated in fig 9b results in a liquid crystal molecule orientation that is different from that in fig 9a, as is illustrated by the directors 96 in fig 9b. The redirection in fig 9b results in a larger refractive index gradient than the redirection in fig 9a, which results in an increased degree of bending of the beam of light 97 in fig 9b as compared to the beam of light 93 in fig 9a. The increased bending results in a smaller exit angle α₂.

In the light-guide 90 schematically illustrated in fig 9c, electrodes are again provided on opposite sides of the liquid crystal layer 42. According to this embodiment, however, the electrodes are provided in a double-sided IPS-configuration, with a first pair of electrodes 101a-b on a side of the liquid crystal layer 42 facing the first substrate 7 and a second pair of electrodes 102a-b on the opposite side. This configuration results in the directors 103 shown in fig 9c and, as illustrated, an even more increased bending of the beam of light 104, which is outcoupled at an angle Ct₃ that is smaller than α₂.

For the embodiments described above, the light has been shown to be traveling inside the light-guide in a direction perpendicular to the electrodes. The light has then encountered refractive index gradients induced by reorientation of liquid crystal molecules and been bent. If the electrodes are provided as mutually parallel interleaved finger electrodes, then light traveling in a direction parallel to the electrodes would not encounter any refractive index gradient and would thus not be outcoupled. This situation is schematically illustrated in fig 10, where parallel finger electrodes 110a-b are provided in an IPS-configuration on one side of the light-guide 111. As can be seen in fig 10, the outcoupling of light is in this case dependent on the direction of the incoupled light.

By providing the electrodes as having several portions with different directions, such as the zig-zag-shaped electrodes 113a-b in the light-guide 114 in fig 11, outcoupling of light can be achieved regardless of from which edge of the light-guide that the light is incoupled. With reference to Fig 12, a display device 120, which is a further application for the controllable light-guide according to the present invention, will now be described. The display device 120 schematically illustrated in Fig 12 comprises a controllable light-guide 121, an array of micro-lenses 122a-c, a color filter 123 having red (R) 124a, green (G) 124b, and blue (B) 124c portions, and a diffuser 128.

Similar to the previously described light-guides, the light-guide 121 comprised in the display device 120 in Fig 12 comprises a light-modulating member 125 which is sandwiched between transparent substrates 7, 8. Again, the refractive index of the light- modulating member 125 is controllable in such a way that it can be locally controlled between, say, no and ni. By controlling the light-modulating member 125 to have a refractive index profile 126 as indicated in Fig 12, light beams 127a-c passing through the light- modulating member 125 are bent in varying degrees.

Depending on the degree of bending, a particular light beam 127a-c will exit the light-guide 120 at a certain angle Φi_3. Depending on this angle Φi_3, each of the respective light beams 127a-c will hit its associated micro-lens 122a-c at a given angle of incidence. This angle of incidence will, as indicated in Fig 12, be translated by the micro-lens 122a-c to a corresponding position on the color filter 123. After having passed through the color filter 123, each light beam 127a-c will hit the diffuser to produce a colored spot 129a-c visible to a viewer.

This enables the use of a controllable light-guide 121 having a relatively low spatial resolution to realize a scanning display 120 (or backlight) having a considerably higher resolution by controlling the direction of the outcoupled light.

Turning now to Fig 13, a stereoscopic display device with a backlight comprising a light-guide according to an embodiment of the present invention will be described.

In Fig 13, a stereoscopic display device 130 is schematically shown in cross- section, comprising an image forming member 131 in the form of a transmissive liquid crystal (LC) panel, and a controllable backlight 132. The controllable backlight 132 includes a light-source 133 and a controllable light-guide 134 according to an embodiment of the present invention.

The controllable light-guide 134 has a plurality of first segments 135 and a plurality of second segments 136 interleaved with the first segments 135. As is indicated for representative segments 135, 136 in Fig 13, each of the first segments 135 is configured to emit light in a first direction r_{l s} here perpendicular to the LC-panel, and each of the second segments 136 is configured to emit light in a second direction r₂. By controlling the light-guide 134 in such a way that the LC panel 131 is alternatingly illuminated by light in the first ri and second r₂ directions, two images having mutually different directions are formed and displayed to a viewer 137. By virtue thereof, as is schematically indicated in Fig 13, a first image adapted for the viewer's right eye 138, and a second image adapted for the viewer's left eye 139 can be presented to the viewer, giving the impression of a 3D image.

In addition to the first 135 and second 136 segments described above and shown in Fig 13, the light-guide 134 may include further segments for displaying images in further directions to thereby increase the viewing angle of the stereoscopic display device. Finally, as is schematically illustrated in fig 14, another sterescopic display device 140 is provided which comprises a display 141 and a controllable light-guide 142 provided on a viewer side of the display device. Through controlled bending of light by the controllable light-guide 142, which may, for example, have one of the electrode configurations shown in figs 9a-c, the image output by the display 141 can alternatingly be directed in first ri and second r₂ directions as described above, giving the impression of a 3D image

The person skilled in the art will realize that the present invention is by no means limited to the preferred embodiments. For example, many other configurations of electrodes, or control means other than those described herein, are feasible, such as the electrodes or other control means being provided on opposite sides of the light-modulating member or as a combination of a transversal and an in-plane electrode configuration. Furthermore, the light-source can be provided in the form of any other suitable light-source configuration, such as an array of light-emitting diodes (LEDs). Furthermore, a spatially varying degree of total internal reflection between inside a substrate can be achieved by other means than through the inclusion of a refractive index matching layer, such as by configuring the substrate such that a spatially varying effective refractive index is obtained at the interface to the light-modulating layer. Moreover, the variation across the light-guide in the degree of total internal reflection is not limited to the example shown in fig 3. Various other variation profiles may be desirable to, for example, a certain outcoupling pattern.

Claims

CLAIMS:

1. A controllable light-guide (2; 21; 51; 62) comprising first (7) and second (8) optically transparent substrates, and a controllable light-modulating member (6; 22; 55) sandwiched therebetween, said light-guide being configured to guide light, through internal reflections, between first and second oppositely located outer boundaries (14, 15) of said light-guide, characterized in that said light-modulating member (6; 22; 55) is controllable to exhibit a refractive index gradient in a selected portion thereof, thereby enabling bending of a guided light-beam (lOa-c; 30; 40) passing through said selected portion, such that said guided light beam hits one of said first and second outer boundaries (14, 15) at a sufficiently small angle (Θ) with respect to a normal (13) of said boundary to escape from the light-guide.

2. A light-guide (21) according to claim 1, wherein said light-modulating member (22) is controllable to exhibit a first refractive index gradient with respect to a first polarization component (33) of an unpolarized light beam (30), and a second refractive index gradient with respect to a second polarization component (34) of said light beam, thereby enabling different bending of said polarization components.

3. A light-guide according to claim 1 or 2, wherein at least one of said first and second substrates, at least at a boundary between said substrate and said light-modulating member, is matched to said controllable light-modulating member with respect to refractive index and optical axis direction.

4. A light-guide according to claim 1 or 2, wherein at least one of said first and second substrates has an effective refractive index which is lower than or equal to an effective refractive index of said controllable light-modulating member, at least at a boundary between said substrate and said light-modulating member.

5. A light-guide according to claim 1 or 2, wherein a difference, at a boundary between at least one of said first and second substrates and said light-modulating member, between an effective refractive index of said substrate and an effective refractive index of said controllable light-modulating member varies along the light-guide, to enable uniform outcoupling of light from said light-guide.

6. A light-guide according to any one of claims 3 to 5, wherein at least one of said first and second substrates comprises a base layer and a refractive index matching layer facing said controllable light-modulating member, said refractive index matching layer having a refractive index transition from, at a side thereof facing said base layer, a first effective refractive index being essentially equal to an effective refractive index of said base layer to, at a side thereof facing said light-modulating member, a second effective refractive index being lower than or equal to said effective refractive index of the light-modulating member.

7. A light-guide according to claim 6, wherein said refractive index matching layer comprises a substance having essentially the same composition as the controllable light- modulating member.

8. A light-guide (21) according to any one of claims 2 to 7, further comprising light-recycling means (24) configured to alter a polarization state of light (34) exiting from said light-guide (21) after having been guided therethrough, and re-introduce said altered light (35) into the light-guide.

9. A light-guide (2; 21; 51; 62) according to any one of the preceding claims, comprising a plurality of light-guide cells (9a-i), each having individually controllable control means (32a-b) for controlling said light-modulating member (22) to exhibit a refractive index gradient in said light-guide cell.

10. A light-guide (21) according to claim 9, wherein said said light-modulating member (22) includes a liquid crystal layer and said control means comprises a first and a second electrode (32a-b) arranged in such a way that a voltage across said first and second electrodes results in an electric field being formed in said liquid crystal layer.

11. A light-guide according to claim 10, wherein said first and second electrodes are arranged on a side of said light-modulating member facing said first substrate.

12. A light-guide according to claim 11, further comprising a third and a fourth electrode arranged on a side of said light-modulating member facing said second substrate.

13. A light-guide according to claim 12, wherein said third and fourth electrodes are arranged opposite said first and second electrodes with respect to said light-modulating member.

14. A light-guide according to claim 10, wherein said first electrode is arranged on a side of said light-modulating member facing said first substrate, and said second electrode is arranged on a side of said light-modulating member facing said second substrate.

15. A light-guide according to any one of claims 10 to 14, wherein at least one of said electrodes has a first set of electrode portions each extending in a first direction in a plane parallel to said light-guide and a second set of electrode portions each extending in a second direction, different from said first direction, in said plane.

16. A light-guide according to claim 15, wherein each of said sets includes a plurality of electrode portions.

17. A light-guide (21) according to any one of claims 9 to 16, wherein said liquid crystal layer (22) is homeotropically oriented in the absence of an electric field therein.

18. A light-guide (2; 21; 51; 62) according to any one of the preceding claims, further comprising a light-modifying member (23; 52a-c) for modifying at least one property of light having been outcoupled from said light-guide.

19. A light-guide (21) according to claim 1, comprising a plurality of individually controllable light-guide cells (9a-i), each having first (7) and second (8) optically transparent substrates; a liquid crystal layer (22) comprising a plurality of liquid crystal molecules (31) sandwiched between said first (7) and second (8) optically transparent substrates, and first

(32a) and second (32b) electrodes arranged in such a way that an application of a voltage (V) across said electrodes results in a localized reorientation of said liquid crystal molecules.

20. A light-guide (21) according to claim 19, wherein said first (32a) and second (32b) electrodes are arranged in a plane parallel to the light-guide.

21. A controllable light output device (1; 20, 50) comprising a light-guide (2; 21; 51; 62) according to any one of the preceding claims, and a light-source (3; 63) arranged in such a way that light emitted by said light-source is coupled into the light-guide.

22. A display device (60) comprising an image-forming member (61) and a controllable light output device (62) according to claim 21, arranged to illuminate said image-forming member.

23. A display device (60) according to claim 22, wherein said light output device (62) is controllable to alternatingly illuminate said image-forming member (61) with light having a first direction (ri) and light having a second direction (r₂), said first direction being different from said second direction.

24. A display device comprising an image-forming member and a controllable light-guide according to any one of claims 1 to 20 being arranged in front of said image- forming member in relation to a viewer and controllable to alternatingly direct at least a portion of an image output by said image-forming member in a first direction, and in a second direction different from said first direction.

25. A display device (50) comprising a controllable light output device according to claim 21, and control circuitry for controlling said lighting device to emit light in response to image data supplied to said display device.