WO2009130637A1

WO2009130637A1 - Direction-dependent control of light guide

Info

Publication number: WO2009130637A1
Application number: PCT/IB2009/051582
Authority: WO
Inventors: Jan F. STRÖMER; Erno H. A. Langendijk; Giovanni Cennini; Maarten Sluijter
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2008-04-23
Filing date: 2009-04-16
Publication date: 2009-10-29

Abstract

A light-guide (2; 41; 81; 91) configured to guide light within the light-guide through reflections between opposite faces (14, 15) thereof. The light-guide comprises a first light-guide portion (9e) controllable to selectively outcouple light traveling in a first direction of propagation within the light-guide (2; 81; 91) through at least one of the faces (14, 15); and a second light-guide portion (9d, 9f) controllable to selectively outcouple light traveling in a second direction of propagation, different from the first direction of propagation, within the light-guide (2; 41; 81; 91) through at least one of the faces (14, 15). Hereby, output of light that has been injected into the light-guide can be controlled not only through the outcoupling state of a selected portion of the light-guide, but also through the direction of propagation within the light-guide of the injected light.

Description

Direction-dependent control of light guide

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, planar light-guides are used in various applications such as flat-panel displays and light-output devices, for example luminaires. When used in flat-panel displays, the planar light-guides are typically used in backlights or frontlights for illuminating a pixel array to make an image formed by the pixel array visible to a viewer. In a conventional backlight, light is typically coupled into an edge of the light-guide. Furthermore, one face of the planar light-guide is typically modified through structuring or other modification to enable outcoupling of light through that face and/or the opposite 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.

In this illumination system, however, the controllability is limited to a spatial control of the outcoupling of any light present in the light-guide.

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 enabling an increased level of controllability of a light-output device including the light-guide.

According to the present invention, these and other objects are achieved by means of a light-guide configured to guide light within the light-guide through reflections between opposite faces thereof, the light-guide comprising a first light-guide portion controllable to selectively outcouple light traveling in a first direction of propagation within the light-guide through at least one of the faces; and a second light-guide portion controllable to selectively outcouple light traveling in a second direction of propagation, different from the first direction of propagation, within the light-guide through at least one of the faces. The light-guide may advantageously be a planar light-guide, which guides light, through internal reflection, between oppositely located, essentially parallel faces thereof. A planar light-guide may include different optically 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 meant, in the present context, a medium which permits passage of at least a fraction of the light (electromagnetic radiation in the visible spectrum) impinging on it.

By selective outcoupling of light traveling in a particular direction within the light-guide should, in the context of the present application, be understood that outcoupling of light traveling in that particular direction is considerably more efficient than outcoupling of light traveling in another direction.

The present invention is based upon the realization that the controllability of a light-output device including a controllable light-guide can be increased by implementing a controllable direction-dependent out-coupling mechanism in the light-guide. Hereby, output of light that has been injected into the light-guide can be controlled not only through the outcoupling state of a selected portion of the light-guide, but also through the direction of propagation within the light-guide of the injected light. In particular, by providing a light-guide having at least two portions that are controllable to selectively outcouple light traveling in different directions of propagation within the light-guide, the controllability of a light-output device including the light-guide is increased through the added variables associated with a light-source or light-sources injecting light traveling within the light-guide in different directions of propagation. Such variables may, for example, include intensity modulation, such as switching or dimming, polarization state, color etc. In this manner, the number of states accessible by a light-output device including the light-guide according to the present invention can be greatly increased as compared to light-output devices based on conventional, controllable light-guides.

According to one embodiment of the present invention, the light-guide may comprise a controllable light-modulating member sandwiched between first and second optically transparent substrates, 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 faces of the light-guide at a sufficiently small angle with respect to a normal to the face to escape from the light-guide; first control means arranged to control the light-modulating member in a portion thereof corresponding to the first light-guide portion to exhibit a refractive index gradient component, in the first direction of propagation, sufficient to bend light-beams traveling in the first direction of propagation so as to make them escape from the light-guide, and a refractive index gradient component ,in the second direction of propagation, insufficient to bend light-beams traveling in the second direction of propagation so as to make them escape from the light-guide; and second control means arranged to control the light-modulating member in a portion thereof corresponding to the second light- guide portion to exhibit a refractive index gradient component, in the second direction of propagation, sufficient to bend light-beams traveling in the second direction of propagation so as to make them escape from the light-guide, and a refractive index gradient component, in the first direction of propagation, insufficient to bend light-beams traveling in the first direction of propagation so as to make them escape from the light-guide.

Since the refractive index gradient is a vector, it can be divided into refractive index components which may or may not be orthogonal. By controlling the light-modulating member so as to achieve a refractive index gradient having different magnitudes in the first and second directions of propagation (different magnitudes of the first and second refractive index gradient components), selective, direction-dependent outcoupling of light from the light-guide can be achieved.

Hence, according to the present embodiment, the desired, selective, direction- dependent outcoupling may be achieved by arranging the first control means so as to induce a refractive index gradient that is sufficiently high to enable outcoupling of light beams traveling in the first direction of propagation within the light-guide, but not sufficiently high to enable outcoupling of light beams traveling in the second direction of propagation, and conversely for the second control means. A further effect obtainable through the light-guide according to the present embodiment is that the light outcoupled from the light-guide is coherent, which may be advantageous depending on the field of application of the light-guide.

Additionally, not only the position of outcoupling and the direction of propagation within the light-guide for which outcoupling takes place can be controlled, but also the direction of the outcoupled light. This enables a higher resolution and precision of the controlled outcoupling.

An additional effect obtainable through the light-guide according to the present embodiment 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 a light beam 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.

Hence, the first and second control means may be any means for controllably subjecting the light-modulating member to such various kinds of external stimuli. In particular, the first and second control means may or may not be provided in contact with the light-guide.

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 rie or conversely. 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 of achieving a controllable refractive index gradient include controlled displacement of particles and/or fluids, through, for example, electrophoresis, magnetophoresis, or electro wetting. 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 polarization 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. Through a proper configuration of the electric field, the liquid crystal molecules can 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. Accordingly, the light-modulating member may advantageously comprise a plurality of liquid crystal molecules, the first control means may include a first electrode pair arranged in such a way that liquid crystal molecules in the first light-guide portion are redirected through application of a voltage across the first electrode pair, and the second control means may include a second electrode pair arranged in such a way that liquid crystal molecules in the second light-guide portion are redirected through application of a voltage across the second electrode pair.

To achieve the desired, selective, direction-dependent (dependent on direction of propagation within the light-guide) outcoupling of light from the light-guide, at least one of the electrodes in the first electrode pair may be arranged to extend essentially perpendicularly to the first direction of propagation, and at least one of the electrodes in the second electrode pair may be arranged to extend essentially perpendicularly to the second direction of propagation.

Arranging at least one of the electrodes in each electrode pair so as to extend in a direction that is essentially perpendicular to the respective direction of propagation is one way of redirecting the liquid crystal molecules in the respective portions in such a way that selective, direction-dependent outcoupling is achieved.

It should, in this context, be noted that it is not necessary for the electrode(s) to extend exactly perpendicularly to the direction of propagation of the light within the light-guide. It is expected that outcoupling with a sufficient degree of direction-dependence (depending on application) can be achieved using electrodes that are arranged to form an angle of, say, 90°±10° with respect to the direction of propagation within the light-guide of the light to be outcoupled.

Furthermore, the electrode lines need not be completely straight, but may deviate from a straight line without substantially influencing the desired direction-dependent outcoupling.

Moreover, both electrodes in one or both electrode pairs may be provided as essentially parallel lines extending perpendicularly to the respective directions of propagation. It should be noted that the desired selective and direction-dependent outcoupling can be achieved for electrode lines that are not exactly parallel, but are provided at some angle with respect to each other.

Moreover, both electrodes comprised in at least one of the first and second electrode pair may be arranged on a side of the light-modulating member facing the first substrate.

Through such a so-called in plane switching configuration, manufacturing of the light-guide may be facilitated.

According to one embodiment, each of the first and second electrode pairs may, furthermore, comprise a plurality of interleaved electrode segments.

Each light-guide portion may, furthermore, 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 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, the light-guide according to the present invention may include a plurality of first light-guide portions, each being controllable to selectively outcouple light traveling in the first direction of propagation within the light-guide through at least one of the faces; and a plurality of second light-guide portions controllable to selectively outcouple light traveling in a second direction of propagation, different from the first direction of propagation, within the light-guide through at least one of the faces.

Depending on the intended use of the light-guide, the first and second light- guide portions may be arranged in different configurations, such as, for example, in a striped arrangement for a scanning backlight or in a checkerboard type arrangement for a color controllable light-output device.

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 including a light-modulating member sandwiched between substrates, 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 between them, 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 of light experiences inside an anisotropic material at 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, as regards 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:

from which it 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 between them. In the event that the substrate is made of 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, on 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, on 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 manufacturing the refractive index-matching layer of a material having similar characteristics as the anisotropic layer, which, on 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 on the side thereof facing the substrate.

Moreover, the light-guide may additionally comprise light-recycling means configured to alter the polarization state of light exiting 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.

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.

The light-guide according to the present invention may, furthermore, advantageously be comprised in a controllable light-output device, further comprising first and second light-sources arranged to inject light into the planar light-guide in the first and second directions of propagation, respectively.

The first and second light-sources may be provided in the form of two individually controllable light emitting devices, such as LEDs, or may be provided in the form of a single light emitting device and means for directing light emitted by the light emitting device in such a way that the light is injected into the planar light-guide in the first and second directions of propagation. Such means may, for example, include various optical components, such as optical fibers, mirrors, optical switches etc.

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 first and second light-sources may be adapted to emit differently colored light.

Hereby, the color output by the light-output device can be controlled by controlling the outcoupling of light from the first and second light-guide portions in combination with the differently colored light-sources.

By providing the light-guide with a plurality of alternating first and second light-guide portions that are sufficiently small, the color of the light output by the light-output device can be controlled between the first and the second color by controlling the first and second light-guide portions and the emitted (average) intensities of the light-sources. Of course, pulse-width modulation is a possible way of controlling the respective, emitted, average intensities.

As is well known to the person skilled in the art, "sufficiently small", in the present context, depends on various parameters such as, for example, viewing distance and arrangement of the light-guide portions.

Moreover, the first and second light-sources may be adapted to emit polarized light with major axes of polarization that are essentially perpendicular to the light-guide, which increases the selectivity with respect to the direction of propagation of the light outcoupled from the light-guide in cases where controllable birefringence is used to achieve controllable outcoupling of light.

A particularly favorable case is one in which the first and second light-sources emit linearly polarized light.

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

By thus including a controllable light output device according to the present invention in a display device, the image quality of the display device can be improved because of the increased controllability of the backlight or frontlight constituted by the light- output device according to the present invention.

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-c schematically illustrate a controllable light-output 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-c when the refractive indices of the substrates are 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; Figs 3a-c schematically illustrate a controllable light-output device including a controllable light-guide according to another embodiment of the present invention;

Fig 4 schematically illustrates exemplary paths of light-beams having different polarization states in the light-guide in fig 3b for the case when the lower substrate and the liquid crystal layer are not refractive index-matched to each other;

Fig 5a schematically illustrates one exemplary way of implementing a refractive index-matching layer between a substrate and the light-modulating member;

Fig 5b is a graph schematically illustrating the reflection of light at the interface between the substrate and the refractive index matching layer in fig 5a; Fig 6a schematically illustrates a fast scanning backlight comprising a light- guide according to an embodiment of the present invention;

Fig 6b shows an addressing scheme for the fast scanning backlight in fig 6a; and

Figs 7 schematically illustrates a color controllable light-output device including a light-guide according to an embodiment of 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 any other light-guide that is susceptible to selective, direction-dependent outcoupling of light. In particular, a light- modulating member other than a liquid crystal layer may be 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-c schematically illustrate a controllable light-output device, which is one exemplary application for a controllable light-guide according to the present invention. In Fig Ia, a light-output device 1 is shown comprising a controllable planar light-guide 2 with edges 3a-d and oppositely located faces 4a-b. As is schematically illustrated by the arrows in fig Ia, light is injected into the light-guide 2 at two of its edges 3a-b. Furthermore, 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 segments 9d-f in the center row emit light (or rather permit light to escape) through the upper face of the light-guide 2 as indicated by the arrows in Fig Ia. 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.

As is also indicated in fig Ia, two of the segments 9d and 9f outcouple light that was injected at the top left edge 3a and that travels within the light-guide in a direction of propagation from the top left edge 3a to the opposite edge 3c, and one of the segments, i.e. 9e, outcouples light that was injected at the top right edge 3b and that travels within the light- guide in a direction of propagation from the top right edge 3b to the opposite edge 3d.

Through the injection of light in two different directions of propagation and the provision of segments that are controllable to selectively outcouple light depending on the direction of propagation of the light within the light-guide 2, the illumination of, say, segment 9e requires that the segment 9e is controlled to outcouple light and that light travels in the appropriate direction of propagation within the light-guide 2, that is, from the top right edge 3b to the bottom left edge 3d, or in the opposite direction. This increases the controllability of the light-output device 1, because the illumination of the segment 9e can now be controlled either through control of its associated light-source or through control of the segment itself, depending on what is more suitable in the particular application. Moreover, the properties of two different light-sources can be "mixed" in a controlled fashion by activating selected segments. 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', one exemplary mechanism behind the controllable outcoupling of light illustrated in Fig Ia will now be explained.

In Fig Ib, four different light beams 10a-d having a direction of propagation within the light-guide 2 from the top right edge 3b to the bottom left edge 3d thereof, with reference to fig Ia, are followed as they pass through the light-guide 2.

As illustrated in Fig Ib, the light-modulating member 6 has a first, constant refractive index nc_Onst in the segments 9d, 9f flanking the center segment 9e, as encountered by the light beams 10a-d. Accordingly, the light beams 10a-d do not experience a refractive index gradient when passing through the light-modulating member 6 in the segments 9d, f. In the center segment 9e, the refractive index is modified to repeatedly vary between a first value no and a second, 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 center 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 one 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.

As explained above in connection with Fig Ib, the light beams 10a-d injected at the top right edge 3b and traveling in the indicated direction of propagation within the light-guide, can only be controllably outcoupled in the center segment 9e. In Fig Ic, which is a view of a cross-section of the light-output device 1 of Fig Ia taken along the line B-B', light beams 17a-c injected at the top left edge 3a and traveling within the light-guide 2 in a direction of propagation that is essentially perpendicular to that of the light beams 10a-d in Fig Ib are followed through the light-guide 2.

In analogy to what was described above in connection with Fig Ib, the light beams 17a-c encounter refractive index gradients when passing through the light-modulating member 6 in segments 9d and f, and are bent as is schematically indicated in Fig Ic. As for the center segment 9e, the light beams 17a-c experience, because of their direction of propagation within the light-guide 2, a constant refractive index and are thus not bent when passing through the light-modulating member 6 in the center segment 9e. It should be noted that the light-guide 2 is in the same state for Fig Ic as for

Fig Ib, and that the difference therebetween results from the viewing direction.

Although not explicitly indicated in Figs la-c, the light-output device 1 includes control means that are arranged to control the light-modulating member 6 to exhibit the refractive index gradient components indicated in Fig Ib and Fig Ic. 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 Figs lb-c, it has been assumed that the refractive indices of the substrates 7, 8 and the light-modulating member 6 match, and that consequently there is no refraction at the interfaces therebetween. With reference to fig 2a, a situation will 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 than 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 - and through the second substrate 8 and will 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 is 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 not be controllable then 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.

With reference to Figs 3a-c, a controllable light-output device 40 including a light-guide 41 according to another embodiment of the present invention will now be described.

In Fig 3 a, a controllable light-output device 40 having essentially the same configuration, including nine individually controllable segments 9a-e, as the controllable light-output device 1 in Figs la-c, is schematically shown.

In contrast to the light-output device 1 of Figs la-c, the controllable light-output device 40 in Fig 3 a emits light in one direction only, as indicated by the arrows in Fig 3 a. Furthermore, the light-output device 40 comprises a controllable light-guide 41 which is configured to contra llab Iy outcouple polarized light.

Similarly to the light-guide 2 comprised in the light-output device shown in Figs la-c, the light-guide 41 in Fig 3a 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 Figs 3b-c, the light-modulating member 42 is configured to controllably bend only one polarization component of the guided light.

In order to emit light through only the first side 14 of the light-guide 41, 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 44a-b in the form of a pair of λ/4 retardation plates 45a-b and mirrors 46a-b for reversing the polarization state of the light having traveled through the light-guide 41 and re-introducing the light back into the light- guide 41 through the opposite edges 3c,d thereof with respect to the respective in-coupling edges 3a,b.

Moreover, each of the segments 9a-i of the light-guide 41 includes individually controllable control means in the form of an electrode pair 47a-i having a plurality of interleaved electrode segments. As is schematically indicated in Fig 3a, the electrode segments are provided in the form of essentially parallel electrode lines, that are arranged perpendicularly to either the direction of propagation of the light that is injected at the top left edge 3 a and is guided by the light-guide 41 towards the opposite edge 3 c or the direction of propagation of the light that is injected at the top right edge 3b and is guided by the light-guide 41 towards the opposite edge 3d.

Turning first to Fig 3b, which is a cross-sectional view of the light-guide 41 in fig 3a taken along the line A-A', a beam 50 of unpolarized light will be followed as it passes through the light-guide 41 in the first direction of propagation from the top right edge 3b towards the opposite edge 3d and back. In the exemplary light-guide 41, schematically illustrated in Fig 3b, 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 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 perpendicularly to the substrates 7, 8. In the light-guide 41 of Figs 3a-c, the liquid crystal molecules are aligned to have a direction in the plane of the light-guide 41 that is at an angle of about 45° to the electrode segments. Such an alignment can, for example, be achieved by rubbing the substrate in the desired alignment direction. Alternatively, the different segments 9a-i may have different alignment directions, depending on the direction of the electrode segments in the respective light-guide segments 9a-i. The preferred alignment direction would then be approximately perpendicular to the electrode segments.

In Figs 3a-c, for the sake of clarity of the drawing, only the two electrodes 47a-b included in the electrode pair arranged in the center segment 9e have been assigned reference numerals. By applying a voltage V across these electrodes 47a-b, the liquid crystal molecules 51 in the LC layer 42 are reoriented 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 for a given geometry) can be used to vary the bending of the light in an analogue fashion.

With continued reference to Fig 3b, the reorientation of the LC molecules 51 in the center segment 9e, results in areas with varying refractive index in the section plane and, consequently, in the formation of refractive index gradients. Due to the electrode configuration in the light-output device 40 in Figs 3a-c, 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 encountering 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 3b.

As can also be seen in Fig 3b, the perpendicular polarization component 54 passes through the light-guide 41 from the incoupling edge 3b and exits through the opposite edge 3d. After exiting the light-guide 41 through this edge 3d, the light beam 54 passes through the λ /4 retardation plate 45a a first time, is reflected in the mirror 46a, and then passes the λ /4 retardation plate 45 a 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 center 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 3b. Furthermore, since the electrode segments 47a-b in the center segment 9e are parallel to the light injected at the top left edge 3a of the light-guide 41 and perpendicular to the light injected at the top right edge 3b, only light-beams having a direction of propagation from the top right edge 3b towards the bottom left edge 3d will experience a refractive index gradient and be bent when passing through the light-modulating member 6 in the center segment 9e. The light-beams having a direction of propagation from the top left edge 3a towards the bottom right edge 3 c will experience an essentially constant refractive index (zero gradient) and will thus not be bent. This effect is schematically illustrated in Fig 3 c, which shows a section of the light-guide 41 taken along the line B-B' in Fig 3 a.

The situation of 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 4.

For the light-guide 65 in fig 4, the first substrate 7 is assumed to be made of glass (n_gi_ass=1.5), and the liquid crystal layer has, in this exemplary case, an ordinary refractive index U₀=I.45 and an extraordinary refractive index n_eo=1.89. 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 than 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 4. 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 4.

In fig 5 a, an exemplary light-guide configuration for achieving a simultaneous match between the first substrate 7 and the liquid crystal layer 42 is schematically shown, said light-guide configuration 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 the layer 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 preferably 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 5b, the intensity reflectance coefficient R calculated as a function of the angle of incidence is schematically shown for the light-guides in Figs 4 and 5 a, 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=1.5) and the anisotropic medium represents liquid crystal (in this embodiment: U₀=I .45 and n_eo=l .89). In fig 5b, the situation of fig 4 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 5a, with the director of the liquid crystal molecules 65 pre-tilted at an angle of approximately 18.5 degrees, is illustrated in fig 5b by the dashed line 71. In this specific example, referring to fig 5b, 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 reactive mesogen material. The top surface of the cell would be a planar alignment layer 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.

In the following, a light-output device in the form of a fast scanning backlight 80 comprising a light-guide 81 according to an embodiment of the present invention will be described with reference to Figs 6a-b.

As described above in connection with Figs la-c, the light-guide 81 in Fig 6a has a number of individually controllable segments 82a-g by means of which selective, direction-dependent outcoupling of light from the light-guide 81 can be achieved. The fast scanning backlight 80 further comprises a first set 83 of LEDs arranged to inject light at the left edge 3a of the light-guide 81 and a second set 84 of LEDs arranged to inject light at the top edge 3b of the light-guide 81. In the fast scanning backlight 80 of Fig 6a, the segments 82a-g are provided in the form of segments extending across the entire width of the light-guide. Furthermore, every other segment, i.e. 82 a, c, e, g, is adapted to outcouple light having a direction of propagation within the light-guide 81 from the left edge 3 a to the right edge 3 c (or the opposite direction), while the remaining segments 82b, d, f are adapted to outcouple light having a direction of propagation within the light-guide 81 from the top edge 3b to the bottom edge 3d (or the opposite direction).

In principle, any controlled outcoupling of light from a light-guide can be used to implement a scanning backlight. Through the present invention, however, the scanning frequency can be increased, because the switching time from one scanning state to the next scanning state becomes limited to the switching time of a light-source rather than to the switching time of the light-guide.

This will now be explained further with reference to Fig 6b, which schematically shows an exemplary addressing scheme for the fast scanning backlight in Fig 6a. As is indicated in Fig 6b, the first set 83 of LEDs (Left LEDs) and the second set 84 of LEDs (Top LEDs) are switched in an alternating fashion. This leads to continuous light incoupling into the backlight (from alternating sides). For a first sub-frame SFi, however, the backlight will be OFF. During this sub-frame SFi, the first 82a and the last 82g row, seen from the top, are addressed and switched ON. Then, during the second sub-frame SF₂, the top LEDs 84 are switched on and the first 82a and the second 82b rows, seen from the top, are addressed, but only the first row 82a outcouples light, since the second row 82b can only couple out light that is injected from the left edge 3a. In the third sub-frame SF3, the left LEDs 83 are switched on and the second 82b and the third 82c row, seen from the top, are addressed, but only the second row 82b outcouples light. This cycle continuous until the last backlight segment has coupled out light and one full backlight frame is completed.

Turning now to Fig 7, a further embodiment of the present invention, in the form of a color-controllable light-output device will be described.

The direction-dependent, selective outcoupling properties of the light-guide according to the present invention can be used to achieve a color-controllable light-output device 90 as schematically shown in Fig 7. In the color-controllable light-output device 90, differently colored light-sources (not shown in Fig 7) are provided to inject differently colored light at the top left edge 3a and the top right edge 3b, respectively, of the light-guide 91, as indicated by the different arrows in Fig 7. In the same way as for the light-output device 1 described in connection with

Figs la-c, the light-output device 90 in Fig 7 includes a number of segments 9a-i. Furthermore, each segment 9a-i comprises a number of sub-segments. The 36 sub-segments comprised in the center segment 9e are schematically illustrated in the enlarged portion of the light-guide 91 shown in Fig 7. As is schematically indicated in Fig 7, half of the number of sub-segments have control electrodes that are perpendicular to the direction of propagation of light injected into the light-guide 91 at the top left edge 3 a, while the remaining electrodes are perpendicular to the direction of propagation of light injected into the light-guide 91 at the top right edge 3b. Accordingly, half of the number of sub-segments can be individually controlled to outcouple the color of light that is injected at the top left edge 3a, while the remaining half of the number of sub-segments can be individually controlled to outcouple the color of light that is injected at the top right edge 3b. Hereby, the color of the center segment 9e as perceived by a user can be controlled by controlling which sub-segments outcouple light and/or how much light is outcoupled by each "active" sub-segment. To improve the color separation of the light-output device 90, the light-guide

91 may be configured to outcouple linearly polarized light, for example through an embodiment similar to that described in connection with Figs 3a-c, and the light injected into the light-guide 91 may have different linear polarizations such that the direction-dependent outcoupling is supported polarization-dependent outcoupling. In the simplest embodiment of a color-controllable light-output device using the selective direction-dependent outcoupling of light through the light-guide 91, light of two different colors is mixed by controlling the sub-segments as discussed above. The accessible colors are then limited to a line in a color space between the first and the second colors. By providing differently colored light-sources at one or more of the in- coupling edges 3a-d of the light-guide, as is indicated in Fig 7, and sequentially switching between the colors and between sub-segments that are "active", a considerably larger number of points in a color space can be accessed. In this case, however, the frame rate of the light- output device 90 should then be at least 120 Hz (for switching between two colors, using the configuration of Fig 7) to ensure flicker- free color mixing.

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 transverse 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 electroluminescent (EL) light-source. Moreover, many other segment shapes and configurations are conceivable, and may be advantageous, depending on application. For example, the segments may be hexagonally shaped.

Claims

CLAIMS:

1. A light-guide (2; 41; 81; 91) configured to guide light within said light-guide through reflections between opposite faces (14, 15) thereof, said light-guide comprising: a first light-guide portion (9e) controllable to selectively outcouple light traveling in a first direction of propagation within said light-guide (2; 81; 91) through at least one of said faces (14, 15); and a second light-guide portion (9d, 9f) controllable to selectively outcouple light traveling in a second direction of propagation, different from said first direction of propagation, within said light-guide (2; 41; 81; 91) through at least one of said faces (14, 15).

2. The light-guide (2; 41; 81; 91) according to claim 1, comprising: a controllable light-modulating member (6; 42) sandwiched between first (7) and second (8) optically transparent substrates, said light-modulating member (6; 42) being controllable to exhibit a refractive index gradient in a selected portion thereof, thereby enabling bending of a guided light-beam (10a-d; 17a-c) passing through said selected portion, such that said guided light beam (10a-d; 17a-c) hits one of said faces of the light-guide at a sufficiently small angle with respect to a normal to said face to escape from the light-guide; first control means (47a-b) arranged to control said light-modulating member (6; 42) in a portion thereof corresponding to said first light-guide portion to exhibit a refractive index gradient component in the first direction of propagation, sufficient to bend light-beams traveling in said first direction of propagation so as to make them escape from said light-guide (2; 41; 81; 91), and a refractive index gradient component in the second direction of propagation, insufficient to bend light-beams traveling in said second direction of propagation so as to make them escape from said light-guide (2; 41; 81; 91); and second control means arranged to control said light-modulating member (6; 42) in a portion thereof corresponding to said second light-guide portion so as to cause said light-modulating member to exhibit a refractive index gradient component in the second direction of propagation, sufficient to bend light-beams traveling in said second direction of propagation so as to make them escape from said light-guide (2; 41; 81; 91), and a refractive index gradient component in the first direction of propagation, insufficient to bend light- beams traveling in said first direction of propagation so as tomake them escape from said light-guide (2; 41; 81; 91).

3. The light-guide (2; 41; 81; 91) according to claim 2, wherein said light- modulating member (6; 42) comprises a plurality of liquid crystal molecules (51), said first control means include a first electrode pair (47a-b) arranged in such a way that liquid crystal molecules (51) in said first light-guide portion are redirected through application of a voltage across said first electrode pair, and said second control means include a second electrode pair arranged in such a way that liquid crystal molecules in said second light-guide portion are redirected through application of a voltage across said second electrode pair.

4. The light-guide (2; 41; 81; 91) according to claim 3, wherein at least one of the electrodes (47a-b) in said first electrode pair is arranged to extend essentially perpendicularly to said first direction of propagation, and at least one of the electrodes in said second electrode pair is arranged to extend essentially perpendicularly to said second direction of propagation.

5. The light-guide (2; 41; 81; 91) according to claim 4, wherein both electrodes (47a-b) comprised in at least one of the first and second electrode pair include essentially parallel segments extending perpendicularly to the respective direction of propagation.

6. The light-guide (2; 41; 81; 91) according to claim 4 or 5, wherein both electrodes (47a-b) comprised in at least one of the first and second electrode pair are arranged on a side of said light-modulating member (6; 42) facing said first substrate (8).

7. The light-guide (2; 41; 81; 91) according to claim 5 or 6, wherein each of said first and second electrode pairs comprises a plurality of interleaved electrode segments.

8. The light-guide (2; 41; 81; 91) according to any one of the preceding claims, comprising: a plurality of first light-guide portions (9a, 9c, 9e, 9g) each being controllable to selectively outcouple light traveling in said first direction of propagation within said light- guide through at least one of said faces; and a plurality of second light-guide portions (9b, 9d, 9f, 9i) controllable to selectively outcouple light traveling in a second direction of propagation, different from said first direction of propagation, within said light-guide through at least one of said faces.

9. A light-guide (2; 41; 81; 91) according to any one of claims 2 to 8, wherein at least one of said first (7) and second (8) substrate is matched, at least at a boundary between said substrate and said light-modulating member (6; 42), to said controllable light-modulating member with respect to refractive index and optical axis direction.

10. A controllable light-output device (1; 40; 80; 90) comprising a light-guide (2; 41; 81; 91) according to any one of the preceding claims, and first (83) and second (84) light-sources arranged to inject light into said planar light-guide (2; 41; 81; 91) in said first and second directions of propagation, respectively.

11. The light-output device (1; 40; 80; 90) according to claim 10, wherein said first and second light-sources are adapted to emit differently colored light.

12. The light-output device (1; 40; 80; 90) according to claim 10 or 11, wherein said first and second light-sources are adapted to emit polarized light with major axes of polarization that are essentially perpendicular to said light-guide (2; 41; 81; 91).

13. A display device comprising an image-forming member and a controllable light output device (1; 40; 80; 90) according to any one of claims 10 to 12, arranged to illuminate said image-forming member.

14. Use of the light-output device (1; 40; 80; 90) according to any one of claims

10 to 12 as a scanning backlight.