FIELD OF THE INVENTION

The invention disclosed herein generally relates to optical devices and more particularly relates to a light guide adjustable by means of a dielectric polymer actuator, so that the light guide has locally adjustable outcoupling properties. The invention can also be embodied as a wave guide for non-visible electromagnetic radiation.

BACKGROUND OF THE INVENTION

Distribution of light from a light source for illumination purposes or for producing a luminous pattern are two important applications of light guides. As a first example, light guides can be used to provide backlight in liquid-crystal displays. In simple displays, the backlight intensity may be constant throughout the screen, while more sophisticated products direct the backlight (possibly one color component at a time) to specific areas in accordance with the screen pattern to be produced at a given instant.

As a second example, light guides play a central role in foil displays, wherein light is coupled out (extracted) in regions of contact between the light guide and an adjacent, accurately controllable optical foil. WO- 1999/28890 discloses a display of this type, wherein a foil is suspended between electric selection means in the form of pairs of fixed, opposed electrodes. When a switching voltage is applied between the electrodes of a pair, the foil is locally depressed, makes contact with the light guide and frustrates total internal reflection, so that light is coupled out. Required switching voltages are typically of the order of hundreds of volts, and the foil display is suitably operated at very low pressure, e.g., in an evacuated space.

SUMMARY OF THE INVENTION

It is an object of the present invention to propose a simpler, and possibly more economical, light guide with accurately controllable outcoupling. It is a particular object to propose a light guide for use as an alternative to available foil displays.

According to an aspect of the invention, at least one of these objects is achieved by an adjustable light guide including a light-conducting element in which light travels by total internal reflection, an outcoupling surface adapted to couple light out from the light-conducting element, and an actuator comprising an electroactive polymer (EAP) layer and electrode structures disposed on each side thereof.

According to the invention, the local outcoupling coefficient of the outcoupling surface is variable by deformation of the actuator. To make this statement more precise, the "outcoupling coefficient" may be defined as the ratio between light transmitted out of the light-conducting element and the intensity of the incident field at a given area segment. A local outcoupling coefficient for a point may be defined as the outcoupling coefficient for a small area segment around this point. The invention achieves its first purpose by allowing accurate control of the local outcoupling coefficient as a consequence of deformation of the dielectric polymer actuator of the light guide. In particular, the

deformation of the actuator may give rise to one or more areas in which total internal reflection is reduced, so that travelling light is coupled out.

It is to be noted that the terms "outcoupling surface" and "emission surface" are used interchangeably in this disclosure. Light may leave the outcoupling surface either to be emitted for illumination purposes (upon which it may propagate in an optically less dense medium, e.g., air) or to be carried further by a further light-conducting element.

In one embodiment, the actuator is operable to alter a surface structure of the outcoupling surface, or at least a region of the outcoupling surface. As will become apparent below, the surface structure may relate to both microscopic features of the surface (e.g., degree of ruggedness) and its macroscopic geometry. Advantageously, the electroactive polymer layer forms the light-conducting element.

In one embodiment, the light-conducting element has planar shape and the outcoupling surface is an external surface thereof. Here, the actuator is adapted to alter the orientation of the outcoupling surface and the orientation of the light-conducting element. In other words, the actuator deforms a portion of the external surface of the light-conducting element (which surface serves as outcoupling surface) so that its orientation is varied with respect to the relaxed state of the actuator. Accordingly, an outcoupling area is created by changing the angle of a portion of the outcoupling surface in relation to the general direction of light propagation inside the light-conducting element, particularly in relation to the general direction of light travelling along the light-conducting element. By virtue of electrode structures (which preferably comprise separately addressable areas controllable by digital or analogue, electric or magnetic signals), the actuator is operable to deform so as to alter parallelity between an area of the outcoupling surface and the actuator, so that total internal reflection is locally frustrated. In a condition where the outcoupling surface and the plane of an EAP layer forming the light-conducting element are substantially parallel, it will be possible for light to travel in the EAP layer by total internal reflection (parallelity of the local tangent planes of the outcoupling surface and the EAP layer, respectively, may in fact be sufficient). As an example, the EAP layer may be shaped as a straight plate in its energy-less state, wherein light rays introduced at suitable angles will substantially undergo total internal reflection, i.e., will impinge at relatively large angles of incidence throughout the light- conducting element. In this example, the EAP layer can deform when energized to have local depressions on its external surface, causing a light ray from within the EAP layer to impinge on a portion of a local depression at a smaller angle of incidence and thus, to leave the EAP layer.

In one embodiment, the actuator is arranged so that it is operable to establish optical contact with the outcoupling surface. The actuator may be in optical contact with the outcoupling surface either in an energized (compressed) or energyless (relaxed) condition. In the portion of the outcoupling surface to which the actuator is applied, the local ratio of the refractive indices on each side changes, so that total internal reflection is either reduced or favored. This makes it possible to control the (local) outcoupling coefficient of the outcoupling surface.

In one embodiment, the actuator is arranged adjacent to the light-conducting element and separated from this by a distance. The outcoupling surface is the external surface of the light-conducting element facing the actuator. Further, the actuator is operable to locally bulge towards the light-conducting element in order to make optical contact. When optical contact is established, the ratio of refractive indices across the outcoupling surface may change in such manner that total internal reflection is frustrated. The light-conducting element may be a plate.

In one embodiment, the invention provides a system of at least two selectively connected optical elements. To this effect, the actuator is arranged between the light- conducting element and an additional light-conducting element. The actuator is adapted to deform so that it is either separated from one or both elements by the surrounding medium or makes (optical) contact with both elements, enabling light to travel along a direct light path between the elements. Still within the same inventive concept, the outcoupling coefficient of the outcoupling surface on the light-conducting element can be varied by deforming the actuator, so that it is either in contact with or separated from the outcoupling surface. In the connected condition, the actuator acts as an optical intermediary element, optically bridging the two light-conducting elements. The light path is direct in the sense that it is does substantially not include any segment where the light travels in the medium surrounding the elements and the actuator, e.g., air. There may be further light-conducting elements and further actuators for providing selective optical connections to these. Each actuator may be adapted to make optical contact with the light-conducting elements in its active or its energyless (relaxed) condition.

In a further development of the preceding embodiment, the actuator is a Z- mode actuator. A Z-mode actuator is an actuator arranged to have a wide dynamic

deformation range in the axial (Z) direction. The actuator is preferably arranged in such manner that the Z-direction extends between the two light-conducting elements which the actuator is adapted to connect optically in a selective fashion. Such an actuator may have a structure similar to those shown in Figs. 1 and 2 of US-2008/289952. Unlike the actuators disclosed in that document, an actuator is more suitable for use with the present embodiment if the electrode structures are provided in a (tangentially or transversally) peripheral region of the actuator, so that a (tangentially) central region remains substantially unobscured, allowing light rays to pass through the actuator regardless of the transparency of the electrode material. Typically, the thickness of the central portion of the actuator increases on activation, and hence the activated state of the actuator corresponds to the optically connected state.

As an alternative to the further development described above, the actuator may have electrode structures extending along a light path between the light-conducting elements. In particular, there may be a plurality of actuators forming a stack with layers extending from one light-conducting element to the other. Consequently, since the layers will approach one another on activation, incompressibility of the EAP material will cause the actuator (or actuator stack) to expand towards each of the light-conducting elements; the activated state of the actuator will therefore correspond to the optically connected state.

The most recently described embodiments can be arranged to form larger systems, either by being coupled sequentially (whereby a surface of luminous tiles arranged in chains or matrices may be created) or in a star-shaped fashion (wherein selectable

'secondary' light-conductors receive light from a common 'primary' light-conductor, so that a light switch for routing light is formed).

One embodiment comprises a common light-conducting element for selectively feeding a plurality of additional light-conducting elements. The common element is optically connectable to each of the additional light-conducting elements via associated actuators. Advantageously, each additional light-conducting element is associated with one actuator which is operable to connect the former to the common light-conducting element. The additional light-conducting elements may be illuminators or may be light-guides (e.g., optical fibers) for routing the light to locations where it is used for illumination, indication, irradiation or other purposes.

In another embodiment, a modular illumination system comprises two or more light-conducting elements arranged adjacent to one another. The additional light-conducting elements are optically connectable by actuators arranged between the additional light- conducting elements. As already discussed above, the actuators are adapted to deform so that they either make optical contact with the additional light-conducting elements or are separated from these. Thus, it is possible to form a chain from a first light-conducting element to a first additional light-conducting element and further to a second additional light- conducting element. The light-conducting elements allow light to travel along them by total internal reflection. One or more of the light-conducting elements also comprise an

illumination surface, through which a portion of the light travelling in the element is coupled out. The illumination surface may be a transparent surface, possibly including optical features for scattering light (e.g., frosted or rugged finish) and possibly supplemented by a reflective surface on the opposite side. Together with a light source, the modular illumination system provides an adaptable way of illuminating a spatially extended area, since portions (one or more light-conducting elements) can be selectively connected or disconnected by operating the actuators. When a portion is disconnected, the light introduced into the illumination system becomes more concentrated and vice versa.

In one embodiment, an adjustable light guide includes a light-conducting element of substantially planar shape, in which element light travels by total internal reflection, and an emission surface adapted to emit light coupled out of the light-conducting element.

According to the invention, the light guide further comprises a dielectric polymer actuator extending along the emission surface and comprising at least a layer of electroactive polymer (EAP) and electrode structures disposed on each side thereof. Further, a local outcoupling coefficient of the emission surface is variable by deformation of the dielectric polymer actuator.

The light-conducting element may be plate-shaped but may also be a curved body, such as a deformable leaf, sheet or blanket. The light-conducting element may have an even or variable thickness. To avoid frustrating total-internal reflection, thickness variations are preferably smooth, not abrupt. The in-plane shape of the light-conducting element is not expected to limit or appreciably influence the intended technical effect of the invention. In other respects, the light-conducting element has properties favoring total internal reflection, so that light properly introduced - e.g., via incoupling means - for the most part does not leave the light-conducting element except at a (conceptual) emission surface of the light guide, or at a portion thereof. Hence, the local outcoupling coefficient of the light-conducting element is generally low except for portions of the emission surface where it is made higher.

In one embodiment, light is extracted from the light-conducting element at at least one outcoupling area, which is a region of the emission surface that has an appreciably higher outcoupling coefficient than the rest of the emission surface. The increase in outcoupling coefficient may be achieved by reducing total internal reflection. Put differently, a portion of the light impinging at the outcoupling area from within the light-conducting element may not undergo total internal reflection but may be transmitted across the outcoupling surface, at least partially. When the dielectric actuator deforms, at least one outcoupling area is created. On reverse movement of the actuator, at least one outcoupling area is removed. Firstly, an outcoupling area can be created by changing the angle of a portion of the emission surface in relation to the general direction of light propagation of light inside the light-conducting element, particularly in relation to the general direction of light travelling in the light-conducting element. Generally speaking, total internal reflection is less likely to take place if the angle of incidence is made smaller. Secondly, an outcoupling area may be created by bringing the emission surface into contact with a piece of material, whereby the numerical relation between refractive indices across that portion of the emission surface changes so that the critical angle for total internal reflection changes. The movement which causes the emission surface to contact the other piece of material may be carried out by a dielectric actuator. Thirdly, an outcoupling area may be created by altering the scattering properties of a portion of the emission surface. This may be achieved by a macroscopic deformation, such as can be effected by a dielectric actuator.

In one embodiment, an electric field is applied across the EAP layer in the actuator by electrode structures that are stretchable. Stretchable electrode structures may be made of a material having a similar or smaller mechanical stiffness than the EAP layer itself. Either the mechanical stiffness of the electrode material as such is lower, or the overall stiffness of structure, which may possibly be of open-work type or contain through holes or be perforated, is lower. From a materials point of view, the elastic modulus (or Young's modulus) of the electrode material should be approximately equal to or less than that of the EAP. In the case of an anisotropic electrode material, the tangential modulus may be the relevant parameter to whether the electrode structure can be expected to be sufficiently stretchable.

In one embodiment, the light-conducting element is the EAP layer of the actuator and the emission surface is an external surface of the actuator. It is the relation between the EAP layer and its external surface (acting as the emission surface in this embodiment) that determines the degree of outcoupling. By virtue of electrode structures (which preferably comprise separately addressable areas controllable by digital or analogue, electric or magnetic signals), the actuator is operable to deform so as to alter parallelity between an area of the emission surface and the actuator, so that total internal reflection is locally frustrated. In a condition where the emission surface and the plane of the EAP layer are substantially parallel, it will be possible for light to travel in the EAP layer by total internal reflection (parallelity of the local tangent planes of the emission surface and the EAP layer, respectively, may in fact be sufficient). As an example, the EAP layer may be shaped as a straight plate in its energy-less state, wherein light rays introduced at suitable angles will substantially undergo total internal reflection, i.e., will impinge at relatively large angles of incidence throughout the light-conducting element. In this example, the EAP layer can deform when energized to have local depressions on its external surface, causing a light ray from within the EAP layer to impinge on a portion of a local depression at a smaller angle of incidence and thus, to leave the EAP layer.

In a further development of the preceding embodiment, the actuator further comprises a reflective layer opposite the external surface acting as emission surface. By thereby providing an optical 'sealing', this feature helps enhance the energy efficacy of the light guide, in that total internal reflection is not relied upon entirely to keep the light inside the light-conducting element. The reflective surface, which recycles outcoupled light back into the EAP layer, may be provided between the stretchable electrode structure and the EAP layer or outside the stretchable electrode structure. The reflective surface may be provided by metallization (e.g., vapor deposition) or by attachment of a reflective foil.

In one embodiment, the light-conducting element is a plate configured to enable light to travel in it by total internal reflection. The plate is arranged adjacent to the dielectric actuator and separated from this by a distance. Further, the emission surface is an external surface of the plate, namely a surface facing away from the actuator. If the actuator is separated from the plate in its energy-less condition, then the actuator can establish an outcoupling area by bulging towards the plate and make optical contact with it, so that total internal reflection is frustrated. The outcoupling area will cause light to leave the light- conducting element, and may have a brighter visual appearance than the rest of the emission surface if the outcoupled light is scattered or, by some other means, is directed towards an observer. As an alternative to this setup, the actuator may be arranged proximate to the plate in its energy-less condition and may be adapted to selectively bulge off the plate to create a local ratio of refractive indices that causes travelling light to undergo total internal reflection. Such an area may appear darker than the non-actuated areas.

In a further development of the preceding embodiment, the electrode structures are secured to the EAP layer and movable with the EAP layer as one actuator unit. An advantage over the prior art, in particular over foil displays, lies in that the electrodes are not in contact with the light-conducting element, which could lead to undesired leakage of light.

With an appropriate electrode layout, the actuator will perform a bulging movement in relation to the plate, as is suitable in this embodiment.

In another further development, the EAP layer contains scattering centers, so that the outcoupling areas will emit a diffuse, non-directive flow of light. This is

advantageous for producing a luminous visual pattern on the light guide, notably for an intended application as a visual display. The scattering centers may be constituted of an additive to the EAP, including one or more of polymer particles, ceramic particles, Ti0 ₂ particles and organic phosphor particles.

In yet another further development of the embodiment wherein the light- conducting element is a plate, an anti-adhesive composition, such as electrically non- conductive oil, is applied at locations of potential contact between the plate and the dielectric actuator. This feature facilitates detachment of the actuator from the plate, and so decreases the required switching time for actuating the light guide between two dissimilar outcoupling patterns. This is particularly advantageous in embodiments where the light guide acts as a visual display for producing visual luminous patterns.

All embodiments wherein the light-conducting element is a plate may be varied by the use, instead of this, of a curved plate or a plurality of light-conducting (curved) plates joined by appropriate optical coupling elements, such as prisms, to avoid sharp bends and accompanying undesired outcoupling of light (leakage). The advisable range of curvature is to some extent determined by the refractive index of the plate material, from which the critical angle for total internal reflection can readily be computed.

In one embodiment, an outcoupling element is arranged on the dielectric actuator or is formed integrally with this, wherein the outcoupling element has a rugged (or ragged or jagged or rippled) surface facing away from the actuator. Unless the outcoupling element is formed integrally with the actuator, these elements are in both optical and mechanical contact, so that firstly, light may propagate between the outcoupling element and the actuator, and secondly, the outcoupling element is deformed by shear forces exerted on it by the actuator when this deforms. In this embodiment, the EAP layer acts as the light- conducting element, and the rugged surface of the outcoupling element acts as the emission surface (outcoupling surface). In the relaxed state of the actuator, a portion of the light propagating in the EAP layer will leave this layer via the emission surface (outcoupling surface). This may be due mainly to scattering or mainly to light rays impinging at sub- critical angles or to a combination. When the actuator stretches the outcoupling element, the ruggedness of the external surface will be decreased, so that its outcoupling coefficient decreases. The external surface will in general have a darker appearance.

In a further development of the preceding embodiment, the electrode structures are addressable in such manner that it is possible to stretch portions of the outcoupling element independently.

As far as manufacture is concerned, the rugged surface of the outcoupling element of the preceding embodiments may be obtained by coating the actuator in an elastically pre-stretched condition. The pre-stretched condition may approximately correspond to the maximal stretching deformation offered by the actuator. On relaxation from the pre-stretched condition, the coating will contract into a rugged shape, the length scale of which may be microscopic or macroscopic depending on the properties of the coating material. Suitably, the coating material is chosen to deform elastically between the pre- stretched and the relaxed condition of the actuator, so as to minimize wear during use.

In one embodiment, the electrode structures of the dielectric polymer actuator are optically transparent. This makes them well suited for optical devices, wherein an overall transparency of the actuator may be desirable. This feature may be readily combined with any other embodiment outlined above.

In one embodiment, the electrode structures of the dielectric polymer actuator are of an open-work type, that is, they comprise through holes allowing light to pass. Thus, even though the electrode material itself is not entirely transparent, a large portion of a transversal light beam will be let through the actuator.

In another aspect, the invention provides a lighting device with a light source, the light from which is conducted by an adjustable light guide in accordance with one of the above embodiments. The adjustability of the light guide will allow light to be coupled out as desired by a user, thereby ensuring a high degree of adaptability to given use situations.

In yet another aspect, the invention provides a visual display comprising an adjustable light guide in accordance with any of the above embodiments or a combination of these. The light guide is controllable by digital or analogue, electric or magnetic signals. The visual display further comprises a control section, which is communicatively coupled to the light guide for causing it to emit light from the emission surface in an area forming the visual image. As an example, by causing a combination of suitably shaped outcoupling areas to appear, the control section may produce a time-varying luminous pattern to appear on the emission surface of the light guide.

It is noted that the invention relates to all possible combinations of features recited in the claims.

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 embodiments of the invention, on which:

Fig. 1 illustrates the mechanism for coupling out light by altering the parallelity of two internally reflecting surfaces, according to the invention;

Fig. 2 are two cross-sectional views of a light guide in accordance with an embodiment of the invention for coupling out light by altering parallelity of two reflecting surfaces by means of a dielectric polymer actuator;

Fig. 3 shows, in accordance with another embodiment of the present invention, a light guide in which outcoupling areas are created by causing two optical bodies to make contact locally;

Fig. 4 shows, in accordance with a third exemplifying embodiment, a light guide where stretching of a rugged surface into a smoother state operationally decreases an outcoupling coefficient;

Fig. 5 is a side view of a system comprising two selectively connected optical elements, Fig. 5a showing the disconnected position and Fig. 5b the connected position;

Fig. 6 is a cross-sectional side view of the central portion of a particular embodiment of the system shown in Fig. 5;

Fig. 7 is a side view of the central portion of another particular embodiment of the system shown in Fig. 5; Fig. 8 shows a common light-conducting element for selectively feeding a plurality of additional light-conducting elements, which are connectable via deformable dielectric polymer actuators; and

Fig. 9 shows a modular lighting system in accordance with an embodiment of the invention.

It is pointed out that the Figs, are generally not to scale; frequently the physical dimensions of the dielectric actuators and their movements have been exaggerated for clarity. Unless otherwise indicated, the upward and downward directions on a drawing do not necessarily correspond to the orientation of the gravitational field.

DETAILED DESCRIPTION OF THE EMBODIMENT

Fig. 1 is a side view showing the paths of light rays in a light guide 110 consisting of an EAP layer with affixed electrodes (not shown) on the top and bottom surfaces. On the one hand, the figure shows the path of a light ray 151 propagating through the light guide 110 in an undeformed (or energy-less) state, when the top surface 111

(emission surface) in the figure is parallel with the bottom surface. On the other hand, the figure shows the path of a light ray 152 in a deformed (energized) state of the light guide 110, when the top surface 112 is curved. Items referring to the undeformed configuration are generally drawn by solid lines, and those referring to the deformed configuration are drawn by dashed lines. The light ray 151 travelling through the undeformed light guide 110 will hit both the top 111 and bottom surfaces at angles greater than the critical angle arcsin(nVn), n' < n, so that it undergoes total internal reflection. As can be seen more clearly in the enlarged portion of the drawing, the light ray 151 impinges from within the light guide on the internal top surface at a critical or supercritical angle of incidence, ¾ > arcsin(nVn) with respect to the optical axis 121, and is reflected off the internal top surface under the same angle ¾. The next angle of incidence of the light ray 151 onto the internal bottom surface of the light guide 110 will also be io, provided this surface is parallel to the top surface. In contrast to this, when the top and bottom surfaces are made non-parallel by depressing the top surface 112, then light will impinge at a smaller angle of incidence in portions where the top surface 112 has a downward slope, and will impinge at a greater angle of incidence in portions with an upward slope. In a portion with a downward slope, as shown in the enlarged area, the optical axis 122 will be located closer to the incident ray, so that the angle of incidence is smaller. Thus, the light ray 152 will impinge at a subcritical angle i < arcsin(nVn) and will be transmitted through the surface, out of the light guide, under an angle r, which is in general governed by Snell's law.

The deformation of the electrode EAP layer constituting the light guide 110 may take place as follows. The electrodes apply an electrostatic force to the EAP layer and thus cause an elastic deformation of the polymer layer in the tangential and/or transversal direction. More precisely, a contractive strain in the transversal direction will, by virtue of the near-incompressibility of the EAP, be accompanied by a dilative strain in the tangential direction. The electrodes disposed on each side of the EAP layer need not be made of the same material, nor have corresponding stiffness. In one embodiment, one electrode is stiffer than the other, which makes the actuator prone to relax into a curved shape, or a shape where the top and bottom surfaces are non-parallel, when actuated. In a variation to this

embodiment, one of the electrodes is relatively rigid to bending, whereas the other is relatively compliant; this is expected to lead to a shape similar to that shown in Fig. 1 when an electric field is applied to the middle portion. Further, duck-mode actuation may be used. In duck mode, by virtue of the mechanical properties of the electrodes and EAP layer materials, actuation shifts the active region, in which an electric field is applied, out of plane while maintaining the two sides of the EAP layer substantially parallel in the active region. At the boundary of the active region, the EAP layer will extend in a direction not parallel with the actuator plane; the two sides of the EAP layer will be non-parallel in this boundary region, so that light outcoupling is likely to occur. The curvature of the EAP-layer will further be relatively large in the boundary region. Consequently, each EAP side will be non- parallel to the actuator plane, so that the angle of incidence is locally reduced and

outcoupling will take place.

The EAP layer may be made of a composition containing at least one material chosen from the following list:

acrylic,

poly[styrene-b-(ethylene-co-butylene)-b-styrene] ,

polyurethane,

polyvinyl chloride,

- silicone,

silicone rubber.

These materials have demonstrated advantageous properties as EAP materials. For one thing, they have Poisson ratios equal to or close to 0.5, which ensures near- incompressible behavior, by which tangential contraction takes place jointly with transversal expansion and vice versa.

A stretchable and/or compliant electrode may be made of a composition containing at least one material chosen from the following list:

- carbon black,

carbon nanotubes,

graphene,

poly-aniline (PANI), and

poly(3,4-ethylenedioxythiophene) (PEDOT), e.g., poly(3,4- ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS).

These layers have a low elastic stiffness and cooperate well with a typical EAP layer. Graphene, PANI, PEDOT and PEDOT:PSS are mostly transparent (have low absorption coefficient) and therefore suitable for optical (e.g., refractive) applications.

The non-emitting bottom surface of the light guide 110 may be secured to a substrate (not shown) serving as a stiffener and/or as a bonding layer and/or as an optical reflector to avoid outcoupling through the bottom surface. When serving as a stiffener, the substrate preferably is more rigid than the opposite electrode; this structure may create a tendency to deform as shown in Fig. 1.

The principle explained with reference to Fig. 1 may be utilized in a light guide with a plurality of independently addressable electrode areas, as shown by the cross- sectional views in Fig. 2. Fig. 2a shows such a light guide in a non-actuated (thus, substantially non-outcoupling) state, while Fig. 2b shows the same light guide in an actuated (thus, partially outcoupling) state. The light guide comprises an EAP layer 210 with electrodes 213, 214 arranged pairwise opposed to one another on the top and bottom surfaces of the EAP layer 210. The top surface 219 may be considered to be an outcoupling surface of the device. The EAP material may be one of those mentioned above, and preferably a soft polymer, such as silicone, which has an elastic modulus of about 1 MPa. The electrodes may be transparent, reflective or colored. Each pair of electrodes 213, 214 is separately connectable to a voltage source 212 by means of a switch 211; the drawing is simplified for the sake of clarity, at least regarding the electric wiring. Light enters the light guide from a first 217 and/or a second 218 light source (either being optional). Each of the light sources 217, 218 may be a natural or artificial light source. Light from the first source 217 is directed into the EAP layer 210 via a prism 215. Light from the second source 218 either propagates directly into the EAP layer 218 or is reflected against a curved mirror 216. In either case, a substantial part of the light enters the EAP layer 210 at an angle such that the light will stay in the layer by virtue of a sequence of total internal reflections and travel along the layer. The outcoupling coefficient in areas at or around each top-side electrode 213 is independently controllable. As suggested by the drawing, the bottom side is stiffer than the top side, so that no appreciable depressions will arise around bottom-side electrodes 214, thereby limiting the amount of light coupled out through the bottom surface. This is not an essential feature of this embodiment; instead, a light guide in which both sides are equally deformable and/or have comparable transparency will show double-sided outcoupling properties.

In a variation to this embodiment, the invention provides a combined tactile and luminous display. Indeed, because light outcoupling takes place as a consequence of a local depression, luminous and non-luminous portions will be located at different levels. The tactile effect can be further enhanced by operating the actuator in a vibrating fashion. Indeed, the human tactile system is able to detect vibrations with smaller amplitude than it is able to detect static level differences. Typically a human finger tip is most sensitive to vibrations around 250 Hz, at which amplitudes of the order of 10 um can be detected.

In a further variation to the embodiment shown in Fig. 2, the electrodes are light-transparent. This is likely to improve the energy efficiency of the light guide, in that a smaller percentage of light introduced into the light guide is absorbed by the electrodes. If a one-sided outcoupling behavior is desired, it may be advantageous to arrange a reflective plate or layer on the opposite side with respect to the outcoupling side.

As to the control of the electrode voltages for the light guide shown in Fig. 2, it is noted that intensity of light travelling in the EAP layer 210 will decrease with the distance to the light source(s) 217 and/or 218. The decrease is due both to outcoupling and to absorption in the EAP layer 210 and the electrodes 213, 214. Therefore, if all active areas are depressed to an equal extent, their perceived luminosity will be largest in the vicinity of the light source(s) 217, 218. To compensate this effect, distant active areas are advantageously actuated using a greater voltage amplitude than areas nearby.

Fig. 3a is a side view of a light guide with locally adjustable outcoupling, according to an embodiment of the invention. The light guide comprises a light-conducting element 301 and a dielectric actuator layer 302 disposed at some distance from the conducting element 301. The actuator 302 and the conducting element 301 are substantially parallel and are separated by a plurality of supports 303, which are preferably neutral from an optical point of view - e.g., they are substantially transparent (or small in size) and do not cause outcoupling of light from the light-conducting element 301 - so as not to interfere appreciably with the visual image produced by the lighting device. For example, the supports 303 may consist of a material with a low refractive index. The light-conducting element 301 may be plate-shaped. It may be made of any suitable transparent material, preferably one with a relatively large refractive index. The light-conducting element 301 may be made of a polymeric material or glass. Specifically, it may be made of acrylic (e.g., polymethylmethacrylate), PMMA, or polycarbonate. The actuator 302 may be manufactured from similar materials as the actuator shown in Fig. 2, but preferably has transparent electrode structures. Further, it is adapted to deform in a bulging, buckling or bending fashion. Duck- mode deformation, as outlined above, may also be suitable. This deformation may be achieved by selectively applying an electric voltage across different regions of the actuator surface. It may be further enhanced by means of stiffening elements, pre-stretched or pre- strained layers, or by choosing electrode materials having mechanical properties that may induce a tendency to deform in this manner. Then, as shown in Fig. 3b, when an electric field is applied, the actuator 302 will approach the conducting element 301 and make (optical) contact with this, so that internal reflection is frustrated and light is coupled out. Thus, the upper surface 304 of the conducting element 301 has variable outcoupling coefficient and acts as the outcoupling surface in this embodiment.

With the setup described so far, the outcoupled light will propagate substantially in the direction (transmission angle) it is transmitted out of the light-conducting layer 301. However, in a variation to the embodiment described so far, the actuator 302 may contain scattering particles, such as transparent particles of optional shape, notably polymer particles, ceramic particles, Ti0 ₂ particles and organic phosphor particles. Elastomer particles may also be used, such as particles of Silastic®, a white polymer. By scattering, the outcoupled light will propagate substantially in all directions, as suggested by Fig. 3b.

Preferably, the bottom side of the light-conducting element 301 is reflective, so that light scattered in the downward direction will be reflected out of the device and contribute to the luminosity of the active region.

As already noted in connection with the embodiment shown in Fig. 2, the intensity of light travelling in the light-conducting element will decrease with the distance to the light source(s), that is, leftwards in Fig. 3. To achieve a more uniform luminosity, this effect may be compensated by modifying the scattering properties of the actuator 302, or by adding small amounts of a light-absorbing material to the rightmost portion of the actuator 302, or by operating the actuator 302 in such manner that its contact surface against the light- conducting element 301 has greater area for pixels farther from the light source, or by carrying out a combination.

Fig. 4 is a cross-sectional view of a light guide wherein the outcoupling coefficient is varied by stretching an element with a rugged surface. The ruggedness may have a macroscopic or microscopic length scale. In the latter case, where a microstructure is provided on the surface, the ruggedness may have the appearance of a whitish, light- scattering area. The light guide of Fig. 4 comprises an EAP layer 401 serving as light- conducting element, electrodes 402, 403 disposed on each side of this, and an outcoupling element 405 arranged on the top side of the EAP layer 401 and secured to the latter in order for them to be in both optical and mechanical (shearing) contact. Light from a light source

404 enters the EAP layer 401, and will be coupled out of the device, to an extent that depends on the present actuation state, through the outcoupling element 405. In a relaxed state of the device, as shown in Fig. 4a, wherein the EAP layer 401 is not compressed by the electrodes 402, 403, this layer has maximal thickness and thus, by its near-incompressibility, minimal tangential extent. The top surface 406 of the outcoupling element 405 will therefore have a markedly rugged surface, so that light rays at numerous places will impinge on the top surface 406 of the outcoupling element 405 at sub-critical angles, so that they leave the device. Fig. 4b shows an actuated state of the light guide, wherein the EAP layer 401 is stretched tangentially as a consequence of the substantially compressive force applied by the electrodes 402, 403. As the drawing shows, light rays from within the EAP layer 401 will impinge on the top surface 406 of the outcoupling element 405 under super-critical angles, so that total internal reflection is favored and a greater portion of the light stays inside the light guide. Clearly, the top surface 406 is an outcoupling surface in this embodiment.

In the exemplifying embodiment illustrated in Figs. 4a and 4b, the lower electrode 403 is reflective but the upper electrode 402 is transparent. This is advantageous from an energy-efficiency point of view, as less leakage through the bottom surface can be expected. A reflective electrode surface may be achieved by metal coating, e.g., by vapor deposition, or may itself be constituted of a reflective conductive film. The metal coating may comprise two layers: a thicker chromium layer acting as binding material, and a thinner aluminum layer acting as reflector. As one alternative to coating, a thin polyethylene terephthalate film with aluminum, such as Mylar®, can be draped onto the EAP.

Alternatively, the lower electrode 403 is also optically transmissive and the light guide comprises a reflector arranged next to (and possibly, though not necessarily, bonded to) the non-outcoupling side of the electroded EAP layer, that is, below the electrode 403 which appears at the bottom of Fig. 4b.

A method of manufacturing the outcoupling element 405, including coating the actuator in an elastically pre-stretched condition, has been discussed in a preceding section. Advantageous properties of the outcoupling element 405 include high elasticity and resistance to the wear caused by the numerous stretching and relaxing movements over a life cycle.

In accordance with the teachings of the invention, it is possible to devise a visual display including an arrangement of variably outcoupling light-guide portions of the kind illustrated in Fig. 4. By such an arrangement, it is possible to direct light from a light source so that it forms a luminous image composed of pixels.

An arrangement of variable light guides according to Fig. 4 may also be used as a light source. For example, this aspect of the invention may be embodied as a luminous ceiling with independently controllable segments.

Fig. 5 shows a light-conducting element 501 and an additional light- conducting element 503, between which optical contact can be established by expanding an interposed actuator 502 laterally. Fig. 5a shows the disconnected position, wherein the actuator is physically separated from an outcoupling surface 504 on the right side of the light- conducting element 501. An optically thinner surrounding medium separates the elements when these are not in mechanical contact or sufficiently close in order to behave as a monolithic optical system. On the drawing, the actuator 502 is separated by a distance from the additional light-conducting element 503 as well, thereby interrupting the light path at two locations; this is clearly not strictly necessary to achieve decoupling of the element. The light-conducting element 501 receives incoming light 510 from the left, allows this to travel by internal reflections and transmits emitted light 511 through the front surface (facing downward on the drawing). In particular, there is substantially no transmission through the outcoupling surface 504 in view of the ratio of refractive indices and the typical angle of incidence onto this surface 504. Transmission through the front surface may be provided for by coating the surface with one or more antireflective layers or by adding scattering centers near the interface.

In the connected position, as shown in Fig. 5b, the actuator 502 establishes an optical connection between the two light-conducting elements 501, 503. In this position, there is substantially no total internal reflection at the outcoupling surface 504, and a substantial portion 512 of the light travelling in the light-conducting element 501 leaves the latter through the actuator 502 and enters the additional light-conducting element 503. In this embodiment, the additional light-conducting element also has an emitting (downward) surface, which may be structurally similar to that of the light-conducting element 501 and through which further emitted light 513 enters the surrounding medium.

To prevent sticking of an EAP actuator to a surface of a light-conducting element, one or both of these may be treated (e.g., coated) by halogenated hydrocarbons, in particular fluorinated hydrocarbons.

The (additional) light-conducting elements in Fig. 5 may be provided in the form of light-conducting tiles, which are joinable to form a modular illumination system comprising two or more tiles. Light is transmitted from the outcoupling surface of each tile into the next tile, that is, parallel to the surface of the joined tiles, and is partially emitted in directions from the tiles through their front surfaces. In this application, the light-conducting elements (tiles) may have a thickness of the order of millimeters. The in-plane dimensions of the tiles can vary within wide limits if the tile material has good conductivity. The actuators may typically have a width of between 50 and 100 μιη, corresponding to the gap size between adjacent tiles.

Fig. 6 illustrates an exemplifying embodiment of the system shown in Fig. 5. Here, the actuator 502 is a Z-mode actuator as described in US-2008/289952, and comprises electrode structures 513, 514 extending substantially parallel to each end surface of the light- conducting elements. The electrode structures 513, 514 are arranged in a peripheral region and thus leave the central portion of the actuator 502 transparent to light even if a non- transparent electrode material is chosen. The actuator 502 is drawn in solid line in its disconnected position, whereas dashed line is used to draw the actuator 502' in its laterally expanded, connected position.

Fig. 7 illustrates a second exemplifying embodiment of the system shown in

Fig. 5. Instead of a Z-mode actuator, this embodiment comprises a plurality of actuators stacked with respect to a direction extending parallel with the end surfaces of the light- conducting elements and hence, extending substantially parallel with a potential light path between the elements. Consequently, each of the stacked actuators with its electrode structures 515 extends in a plane substantially normal to each end surface. By comparing the disconnected 502 and connected 502' positions of the actuator, it is clear that the change in optical connectivity of the system is achieved by virtue of a lateral expansion of the actuator. Similarly, if a single actuator is used, the lateral expansion may be achieved by applying a compressive electric field vertically, by virtue of incompressibility of the EAP material. Fig. 8 shows a system which may be used as a light switch for routing light from a first light-conducting element 501 (or light pipe), in which light travels by internal reflections as long as this is in contact with a surrounding, optically less dense medium such as air or vacuum. The first light-conducting element makes optical contact with each of a plurality of additional light-conducting element 503 via actuators 502. Each actuator 502 may connect the first light-conducting element to one or more additional light-conducting elements 503, and each additional light-conducting element 503 may receive light from the first light-conducting element 501 via one or more actuators 502. An additional light- conducting element 503 may act as a pure light-transporting means (possibly forwarding the light to a dedicated light emission structure), as a pure light emitter or as a combination of these. The additional light-conducting elements 503 may be geometrically shaped and geometrically arranged with respect to each other in many different configurations, not only the one indicated on the drawing. Fig. 8 shows six actuators 502a, b, c, d, f, g in a

disconnected state and one actuator 502e in a connected position, in which it makes optical contact with both the first light-conducting element 501 and one of the additional light- conducting elements 503e. The arrangement shown in Fig. 8 may be used as a segmented, energy-economical backlight, e.g., in a visual display. It may also be used to control the outcoupling area of a lamp, including the size and shape of the active emission area. One particular application is as a luminous ceiling or wall in a large room, which would otherwise be difficult to illuminate by point sources.

Fig. 9 shows a modular illumination system comprising a surface formed by adjacent tiles 901, 903. At least some of the tiles comprise an emitting front surface, through which a portion of light travelling in the tile 901, 903 is emitted in off-plane directions. Light from a light source 217 is fed into an initial tile 901 of the system. A plurality of actuators 902 are arranged at positions between pairs of tiles, so that optical contact between a pair of tiles may be established by causing the interposed actuator to expand so that it touches or comes close to both tiles and acts as an intermediary optical bridge. In the momentary situation shown on the drawing, the tiles 901, 903 topo logically belong to one of four connected tile components: two single tiles 903c, 903 e; two tiles 903 f, 903 g connected via extended actuator 902fg; and a four-tile component including the initial tile 901, tiles 903a, 903b, 903d and extended actuators 902a, 902ab, 902bd. Since the four-tile component receives light from the light source 217, the corresponding tiles are luminous, as opposed to the tiles which lack a connection to the initial tile 901 and which have been indicated by "X" symbols. Since the deformation state of the actuators 902 can be controlled by electric signals - e.g., the voltage currently applied between opposite electrode structures - the topology of the system can quite easily be changed during use. For instance, if only a portion close to the initial tile 901 needs to be illuminated, the distal tiles may be cut off from the initial tile 901 by causing relevant actuators 902 to contract so that optical contact is interrupted. Since the available light then is distributed to connected tiles only, the light source 217 can be operated at less power while preserving the emitted light intensity. In particular, the appropriate power reduction can be determined on the basis of the number of tiles that are switched off.

It is recalled that the separation of the tiles 901, 903 has been considerably upsized for clarity, as have the axial dimensions of the actuators 902. In a realistic embodiment, the tiles themselves occupy the majority of the area and the actuators only a relatively minor portion.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, two or more light guides of one of the types described previously may be stacked in a structure, where each is used to distribute light of a given color. By varying the local outcoupling coefficients of the respective layers, possibly allowing light of different colors to mix, multicolored luminous patterns can be produced.