Light coupling structures for optical touch panels

A coupling element (14) for use in a touch-sensitive apparatus is arranged to transfer light between an electro-optical device (2) and a panel (4) for light transmission. The electro-optical device (2) is an emitter or a detector and has an operative solid angle given by orthogonal device divergence angles. The coupling element (14) is an optical component with a first light transmission surface (21) for facing the electro-optical device (2), and a second light transmission surface (22) for mounting on the panel (4). The coupling element (14) has an optical structure (23) that directs the light between the first and second light transmission surfaces (21, 22) by one or more reflections while expanding one device divergence angle (αx) into a component divergence angle at the second light transmission surface (22). Thereby, the component divergence angle defines a divergence (φp) in the plane of the panel (4) with respect to light propagating by internal reflections inside the light transmissive panel (4).

TECHNICAL FIELD

The present invention relates to touch-sensitive systems that operate by light transmission inside light transmissive panels, and in particular to optical solutions for coupling light into and out of such panels.

BACKGROUND ART

The prior art comprises different types of touch-sensitive systems that operate by transmitting light inside a solid light transmissive panel, which defines two parallel boundary surfaces connected by a peripheral edge surface. Specifically, light is injected into the panel so as to propagate by total internal reflection (TIR) between the boundary surfaces. An object that touches one of the boundary surfaces (“the touch surface”) causes a change in the propagating light that is detected by one or more light detectors. In one implementation, e.g. as disclosed in WO2008/017077, US2009/267919 and WO2010/056177, light detectors are arranged behind the panel to detect light which scatters off the touching object and escapes the panel via the boundary surface opposite to the touch surface. In another implementation, e.g. as disclosed in WO2007/003196 and U.S. Pat. No. 7,435,940, light detectors are arranged at the periphery of the panel to detect light which scatters off the touching object and is confined within the panel by total internal reflection. In yet another implementation, e.g. as disclosed in U.S. Pat. No. 7,432,893 and WO2010/064983, light detectors are arranged at the periphery of the panel to sense the attenuation of the light transmitted through the panel.

There are different approaches for injecting the light into the panel. It is known in the art to inject light along an elongate portion of the peripheral edge surface. For example, US2006/0114237, U.S. Pat. No. 6,972,753 and US2007/0075648 propose injecting directional light into the panel via the edge surface. Incoupling via the edge surface is simple but requires the edge surface to be highly planar and free of defects, at least if the light source is not attached to the edge surface. It may be undesirable to attach the light source to the edge surface, since this may impose mechanical load on soldering seams between the light source and a connecting substrate such as a PCB (Printed Circuit Board). Defect free edge surfaces may be difficult and/or costly to achieve, especially if the panel is thin and/or manufactured of a comparatively brittle material such as glass. In order to improve the strength of the panel, the edge surface may be provided with a bevel, which may further limit or obscure the incoupling of light. It may also be difficult to optically access the edge surface if the panel is attached to a mounting structure, such as a frame or bracket, and the mounting structure may cause strain in the edge surface, affecting the optical quality of the edge surface and resulting in reduced incoupling performance.

An alternative approach is to inject the light via a coupling element attached to one of the boundary surfaces of the panel to define an incoupling site.

Depending of touch-sensing technique, it may be desirable to inject light into the panel such that the light diverges in the plane of the panel as it propagates away from the incoupling site, to form a so-called “fan beam”.

Aforesaid U.S. Pat. No. 7,432,893 proposes incoupling of diverging light from a point source by means of a revolved prism which is attached to the top boundary surface of the panel. The revolved prism is designed to receive the diverging light and refract the incoming light in a direction transverse to the panel to generate reflection angles in the panel that sustain propagation by TIR, while retaining the direction of the incoming light in the plane of the panel. The revolved prism is a bulky component which may add significant weight and size to the touch system. The size of the prism also limits the number and density of the incoupling sites. To reduce weight and cost, the wedge may be made of plastic material. On the other hand, the panel is often made of glass, e.g. to attain required bulk material properties (e.g. index of refraction, transmission, homogeneity, isotropy, durability, stability, etc) and surface evenness of the top and bottom surfaces. The present Applicant has found that the difference in thermal expansion between the plastic material and the glass may cause such a prism to come loose from the panel as a result of temperature variations during operation and storage of the touch system. Even a small or local detachment of the prism may cause a significant decrease in the performance of the system.

The above discussion is equally applicable to techniques for coupling of light out of the panel. The light may be detected by light detectors directly attached to the edge surface, but this may cause the light detectors to also act as mirrors to the light in the panel at certain angles of incidence, potentially causing uncontrolled and undesirable reflections inside the panel. Alternatively, the light detectors may be directly attached to one of the boundary surfaces. In either case, this may lead to mechanical load on soldering seams between the detector and a connecting PCB. Furthermore, light detectors directly attached to the panel may be exposed to ambient light, i.e. light that originates from sources outside of the panel.

The prior art also comprises U.S. Pat. No. 7,995,039.

SUMMARY

It is an objective of the invention to at least partly overcome one or more of the above-identified limitations of the prior art.

Another objective is to provide an optical component that is compact and suited for transferring light between a light transmissive panel and an electro-optical device, such that the optical component when attached to the panel defines a divergence inside the light transmissive panel in a direction parallel to the boundary surfaces of the panel.

Yet another objective is to provide a compact optical component for coupling light from an emitter into a light transmissive panel such that the light diverges while propagating by internal reflections in the panel.

A further objective is to provide a compact optical component for coupling light propagating by internal reflections out of a light transmissive panel onto a detector in an efficient way.

A still further objective is to enable low levels of ambient light on the light sensor.

These and other objectives, which may appear from the description below, are at least partly achieved by way of optical components, an optical system, an optical film and a touch-sensitive apparatus according to the independent claims, embodiments thereof being defined by the dependent claims.

A first aspect of the invention is an optical component for transferring light between an electro-optical device and a light transmissive panel which defines two opposing boundary surfaces, the electro-optical device having an operative solid angle given by first and second device divergence angles in orthogonal first and second device planes. The optical component comprises: a first light transmission surface configured to face the electro-optical device, a second light transmission surface for mounting on one of the boundary surfaces, and a control surface arrangement configured to direct the light between the first and second light transmission surfaces by one or more reflections so as to expand the first device divergence angle into a first component divergence angle at the second light transmission surface, the first component divergence angle being arranged to define, with respect to light propagating by internal reflections in the light transmissive panel, a divergence inside the light transmissive panel in a direction parallel to the boundary surfaces.

The optical component is thus designed to convert a first device divergence angle of the electro-optical device into an expanded divergence angle (denoted first component divergence angle) at the second light transmission surface, which is adapted for mounting on the boundary surface of panel, and also to orient the expanded divergence angle such that it defines a divergence or spread in a direction parallel to the boundary surfaces inside the panel. The expansion and orientation is achieved by means by one or more reflections in the control surface arrangement that directs the light inside the optical component between the first and second light transmission surfaces. In different embodiments, the divergence inside the panel spans at least 100°, 120°, 140°, 160° and 170°. It is to be noted that, at least in certain embodiments, the divergence may exceed 180° inside the panel, if desired.

The optical component is primarily intended to be made of solid light transmissive material, which thus defines the first and second light transmissive surfaces and the light path between the first and second light transmissive surfaces, including the control surface arrangement.

As used herein, “a divergence in a direction parallel to the boundary surfaces” refers to a divergence in the plane of the panel in a direction away from an incoupling/outcoupling site which is defined by the optical component on the panel. If the electro-optical component is a light emitter, the divergence defines the outer limits of the “fan beam” that propagates away from the incoupling site by internal reflections between the boundary surfaces. If the electro-optical component is a light detector, the divergence defines the outer limits of field of view of the light detector inside the panel for receiving light that propagates by internal reflections between the boundary surfaces, as seen through the optical component. In other words, the divergence refers to the apparent spreading of limiting rays away from the optical component as attached to the panel, as seen in a plan view of the panel.

In one application, the optical component is thus used for coupling of light from an electro-optical light emitter into the panel, by the optical component being arranged to operate on the first device divergence angle, which is a beam spread given by the cone of light emitted by the emitter, to fold the emitted light for propagation by internal reflections in the panel, while expanding the beam spread of the transferred light that is emitted from the second light transmissive surface, resulting in an increased beam divergence in a direction parallel to the boundary surfaces inside the panel. Thus, the inventive optical component enables generation of a wider fan beam inside the panel for a given emitter. It thus has the advantage of enabling use of emitters with smaller cones of light. The expansion of the beam spread by reflection(s) in the control surface arrangement also enables a size reduction of the component compared to conventional incoupling elements which, at best, preserve the beam spread of the emitter. This size reduction is, inter alia, enabled by the fact that the angle expansion is caused by reflection(s) in the control surface arrangement which is optically located between first and second light transmissive surface. Thereby, the origin of the expanded beam spread (the control surface arrangement) is located closer to the second light transmissive surface than the origin of the light (the emitter). It is realized that this enables a size reduction of the second light transmissive surface for a given beam spread of the transferred light.

The inventive design will enable an optical component with smaller dimensions compared to prior art solutions, and thereby also lower weight. Furthermore, reducing the extent of the second light transmissive surface, i.e. the surface used for attaching the component to the panel, may result in lower stress in the bond between the component and the panel due to differences in thermal expansion. Thereby, an improved robustness may be achieved, even if the component and the panel are made of different materials.

As noted in the Background section, it may be desirable to arrange the optical component with an air gap between the first light transmission surface and the electro-optical device. However, the use of an air gap results in a step change in index of refraction at the first light transmission surface, which may cause the emitted cone of light to be compressed when it enters the component through the first light transmissive surface. It is realized that the inventive component, by its ability of expanding the first device divergence angle, may be designed to generate a desired beam divergence in the panel even in the presence of an air gap.

Furthermore, since the optical component allows coupling of light into the transmissive panel through the boundary surfaces, it is possible to provide all or part of the edge surface that joins the boundary surfaces of the panel with a coating or surface structure that reduces or prevents light that propagates towards the edge surface inside the panel from being reflected by the edge surface back into the panel. Such reflected light, which may originate from ambient light or from light coupled into the panel, may result in an uncontrolled and undesirable light distribution inside the panel.

In another application, the optical component is used for coupling of light out of the panel onto an electro-optical light detector, by the optical component defining an enlarged field of view at the second light transmissive surface compared to the field of view of the detector. The enlargement of the field of view enables an efficient collection of light from the panel at the detector. It is realized that the above-mentioned advantages and effects of the optical component when used for incoupling of light are equally applicable when it is used for outcoupling of light.

The panel may have any configuration allowing light to propagate by internal reflections inside the panel while allowing the light to interact with one or more objects in contact with at least one of the boundary surfaces. The interaction is achieved when the object locally couples the propagating light out of the panel to be scattered against the object. In certain touch systems, one of the boundary surfaces may be provided with a reflective coating if only the other boundary surface is to be used for touch interaction. The reference to “boundary surfaces” is intended to indicate the surfaces that have the same extent as the front and back pane of the panel, i.e. the surfaces that extend in the larger extension of the panel. Each boundary surface is typically planar, but may be curved. The periphery of the panel is defined by a circumferential edge surface that joins the boundary surfaces. A touch-sensitivity region, also denoted “touch surface”, is defined on at least one of the boundary surfaces.

It is to be understood that the second light transmission surface may be configured for mounting on the boundary surface either directly or via a light transmissive spacer.

Generally, both the panel and the optical component (and the spacer, if any) may be made of any material that transmits a sufficient amount of radiation in the relevant wavelength range to permit a sensible measurement of transmitted energy. Such material includes glass, polymethyl methacrylate (PMMA) and polycarbonates (PC). It is to be understood that the panel and the optical component may be composed of different materials, compositions or layers. For example, at least one of the first and second light transmissive surfaces may be provided with a filter coating designed to only pass certain wavelengths, so as to suppress ambient light. Alternatively or additionally, a suitable wavelength filter compound may be dispersed in the bulk material of the optical component.

As used herein, the “operative solid angle” of an electro-optical device refers to a solid angle in which the device operates on light. Thus, an emitter generates light within a specific solid angle, and a light detector detects light within a specific solid angle. The solid angle is typically defined by divergence angles (also denoted first and second device divergence angles) on both sides of a nominal centerline (beam axis) in each of two orthogonal planes (also denoted first and second device planes). The divergence angles may be defined according to any suitable criterion. One commonly used criterion (beam angle criterion) sets the limits of the divergence angle to where the intensity is 50% of the maximum intensity within the solid angle. Another criterion (field angle criterion) sets the limits of the divergence angle to where the intensity is 10% of the maximum intensity within the solid angle. Other criteria are conceivable, but the limits of the divergence angle are normally set at 10-90% of the maximum intensity. Irrespective of criterion, it is used consistently to define all divergence angles of light in the system.

In one embodiment, the control surface arrangement is further configured to convert the second device divergence angle into a second component divergence angle at the second light transmission surface, the second component divergence angle defining a confined range of angles of incidence inside the light transmissive panel with respect to the boundary surfaces. Thus, the control surface arrangement operates to orient and possibly re-scale the second device divergence angle to form the second component divergence angle such that it matches the confined range of angles of incidence inside the panel. Thus, the second component divergence angle may be equal to, or larger/smaller than, the second device divergence angle.

When the optical component is used for incoupling, it may ensure that all or a desired portion of the light generated by the emitter is transferred into the panel with a direction that matches a desired range of angles of incidence, also denoted reflection angles, against the boundary surfaces.

When the optical component is used for outcoupling, it may ensure that the detector only receives light within the desired range of angles of incidence. In other words, the combination of optical component and light detector will provide an angular selectivity or an angular filter to light rays in the transverse direction of the panel, i.e. perpendicular to the boundary surfaces. Such an angular selectivity may reduce the amount of ambient light that reaches the detector, since the ambient light generally is distributed over a large range of angles, even ambient light that enters the panel via contaminations on the boundary surfaces and then propagates inside the panel by internal reflections.

In one embodiment, the confined range of angles of incidence inside the light transmissive panel extends from a minimum angle to a maximum angle, which are given relative to a normal of the boundary surfaces, and wherein the minimum angle is in the approximate range of 42°-54° and the maximum angle is in the approximate range of 56°-85°. The minimum angle may be selected to exceed a critical angle for total internal reflection inside the light transmissive panel. Since the critical angle depends on the transition in index of reflection at each boundary surface, it is realized that the critical angle and thus the minimum angle may be significantly larger if, e.g., one of the boundary surfaces is defined by an interface between two material layers. The minimum and maximum angles may also be selected to optimize the interaction between the light and the touching objects, which may differ between different types of touching objects, such as animate and inanimate objects.

In one embodiment, the panel has a first index of refraction and the optical component has a second index of refraction that exceeds the first index of refraction. Since light is refracted away from the normal at transition from a higher to a lower index of refraction, such a difference in index of refraction will cause the second component divergence to be refracted towards larger angles of incidence inside the panel. Thereby, the difference in index of refraction may allow a larger second component divergence angle to fit within a given range of angles of incidence inside the panel.

The first component divergence angle may be seen to represent a divergence in an azimuth angle with respect to the second light transmissive surface, and the second component divergence angle may be seen to represent a divergence in an elevation angle with respect to the second light transmissive surface. With this definition, one embodiment of the optical component may be designed to provide essentially the same second component divergence angle for all azimuth angles within the first component divergence angle. This will allow the touching object(s) to be illuminated in approximately the same way (same confined range of angles of incidence inside the panel with respect to the boundary surfaces) at all locations on the touch surface and thereby ensure a consistent interaction (touch sensitivity) between the light and touching object(s) across the touch surface. It will also enable a uniform touch sensitivity within the field of view inside the panel when the optical component is used for outcoupling.

In one embodiment, the control surface arrangement comprises a three-dimensional control surface which is configured to reflect the light and which comprises a first and a second two-dimensional shape feature that at least partly defines the first and second component divergence angle, respectively. The three-dimensional control surface may be configured to reflect the light by either total internal reflection or by means of a reflective coating applied to the outside of the optical component, or a combination thereof.

By using separate shape features for defining the first and second component divergence angles, it is possible to optimize each of these component divergence angles independent of each other when designing the optical component. The two-dimensional shape features may be seen as mutually independent shape elements of the three-dimensional control surface, where each shape feature is defined by a separate set of design parameters.

In one embodiment, the optical component is an elongated light guide which is configured to taper in a direction from the first light transmission surface towards the second light transmission surface in a first geometric plane, and which has a given inclination between a center line of the light guide and the second light transmissive surface in a second geometric plane orthogonal to the first geometric plane. The taper corresponds to the first shape-feature and will serve to increase the divergence of light that enters the light guide to propagate by one or more reflections between opposing control surfaces that converge to the propagating light. Further, the inclination between the center line and the second light transmissive surface corresponds to the second shape feature and will serve to orient the incoming light with respect to the boundary surfaces, so as to match a confined range of angles of incidence with respect to the boundary surfaces. The light guide may also be designed with a taper towards or away from the second light transmissive surface in the second geometric plane, in order to increase or decrease the divergence of the light in the second geometric plane, as desired. The light guide may have a rectangular or elliptical cross-section. It is realized that by aligning the first and second geometric planes with the first and second device planes of the electro-optical device, the light guide will serve to generate desired first and second component divergence angles at the second light transmissive surface.

In another embodiment, the first two-dimensional shape feature is a peripheral curvature in a first geometric plane and the second two-dimensional shape feature is an inclination with respect to a second geometric plane orthogonal to the first geometric plane. Thereby, the peripheral curvature of the three-dimensional control surface controls the angular expansion to generate the first component divergence angle, and the inclination controls the orientation of the light to generate the second component divergence angle. Such an embodiment has the ability of being designed to yield a desired illumination of the boundary surfaces, in terms of the divergence in the plane of the panel and the range of reflection angles inside the panel, by separately optimizing the peripheral curvature and the inclination with respect to the operative solid angle of the electro-optical device and the configuration of the panel.

In one embodiment, the three-dimensional control surface further comprises a curvature in a third geometric plane orthogonal to the first and second geometric planes. This curvature may be of any shape that is optimized or suitable to control the extent of the second component divergence angle (i.e. the range of reflection angles inside the panel) or to achieve a desired light distribution within the second component divergence angle. For example, the curvature may be designed to expand or compress the second component divergence angle in relation to the second device divergence angle.

In one embodiment, the first geometric plane is parallel to the second light transmission surface.

In one embodiment, the control surface has an extent in a projection direction away from the second light transmission surface, and wherein the peripheral curvature is essentially invariant in the projection direction. This means that the shape of the peripheral curvature is essentially the same along the control surface in the projection direction.

In one embodiment, the peripheral curvature is part of an ellipse. As used herein, an “ellipse” is intended to also comprise a circle. In such an embodiment, the first device divergence angle may have a point of origin which is given an optical placement relative to a focal point of the ellipse to generate the first component divergence angle. In other words, the optical component is designed with respect to a specific placement of the electro-optical device to achieve the desired angular expansion.

The foregoing embodiments with a peripheral curvature and an inclination may be implemented by an optical component in which the control surface arrangement is based on a cone defined by a directrix generating a base, a vertex and a generatrix generating a lateral surface, wherein the second light transmission surface is formed by at least part of the base, and the three-dimensional control surface is formed by at least part of the lateral surface. As is well-known in the field of geometry, the directrix is the perimeter of the base of a cone, the vertex is the tip of the cone, and the generatrix represents the line segments between the directrix and the vertex along the lateral surface of the cone.

In one embodiment, the control surface arrangement further comprises a conic section formed in the lateral surface to extend parallel to the base. The conic section is thus formed on the opposite side of the second light transmissive surface and may serve as a pick-up surface to use by an automatic or manually operated tool for gripping the optical component for mounting it on the panel.

In one embodiment, the conic section is provided with a reflective coating. This has been found to increase the effeciency of the electro-optical device, if the electro-optical device is arranged such that part of the light in the second device plane is reflected in the conic section.

A second aspect of the invention is an optical component for coupling light into a light transmissive panel which defines two opposing boundary surfaces, wherein the optical component comprises: a light exit surface for mounting on one of the boundary surfaces; a light entry surface for receiving diverging light with an angular distribution; and an optical structure configured to expand the angular distribution of the diverging light while redirecting the diverging light from the light entry surface to the light exit surface such that the thus-expanded angular distribution defines a divergence in a direction parallel to the boundary surfaces of light that enters the light transmissive panel via the light exit surface for propagation by internal reflections between the opposing boundary surfaces.

In one embodiment, the optical structure is further configured to convert the angular distribution of the diverging light so as to define a confined range of angles of incidence inside the light transmissive panel with respect to the boundary surfaces.

The optical structure is defined by a first shape feature that controls the ability of the optical structure to expand the angular distribution, and a second shape feature that controls, independently of the first shape feature, the ability of the optical structure to convert the angular distribution. This may facilitate the task of designing the optical component with respect to the diverging light since the expansion of the angular distribution (with respect to the divergence inside the panel) may be set independently of the conversion of the angular distribution (with respect to the angles of incidence inside the panel) by separate optimization of the first and second shape features.

A third aspect of the invention is an optical component for coupling light out of a light transmissive panel which defines two opposing boundary surfaces and directing the light onto a light detector having an operative solid angle given by first and second device divergence angles in orthogonal first and second device planes, wherein the optical component comprises: a light entry surface for mounting on one of the boundary surfaces; a light exit surface configured to face the light detector; and an optical structure configured to expand the first device divergence angle into a first component divergence angle at the light entry surface, the first component divergence angle being arranged to define, with respect to light propagating by internal reflections inside the light transmissive panel, a divergent field of view inside the light transmissive panel in a direction parallel to the boundary surfaces.

In one embodiment, the optical structure is further configured to convert the second device divergence angle into a second component divergence angle at the light entry surface, the second component divergence angle being arranged to define a confined range of angles of incidence inside the light transmissive panel with respect to the boundary surfaces.

Like in the second aspect, the optical structure may be defined by first and second shape features that can be independently optimized.

A fourth aspect of the invention is an optical component for transferring light between an electro-optical device and a light transmissive panel which defines two opposing boundary surfaces, wherein the optical component comprises a light transmissive surface and a control surface arrangement which is based on a cone defined by a directrix generating a base, a vertex and a generatrix generating a lateral surface, wherein the optical component is operable to transfer light by mounting the base to one of the boundary surfaces and arranging the electro-optical device to face the light transmissive surface, such that the light is transferred through the optical component by at least one internal reflection in the lateral surface.

In one embodiment, the directrix is configured such that, when the base is mounted to the light transmissive panel, a first device divergence angle of the electro-optical device in a first device plane parallel to the base corresponds to a divergence inside the light transmissive panel in a direction parallel to the boundary surfaces.

In one embodiment, the divergence inside the light transmissive panel is larger than the first device divergence angle.

In one embodiment, the generatrix is configured such that, when the base is mounted to the light transmissive panel, a second device divergence angle of the electro-optical device in a second device plane perpendicular to the first device plane corresponds to a confined range of angles of incidence inside the light transmissive panel with respect to the boundary surfaces.

In one embodiment, the electro-optical device is a light source, and the optical component is configured to couple light emitted by the light source into the light transmissive panel for propagation by internal reflections inside the light transmissive panel.

In another embodiment, the electro-optical device is a light detector, and the optical component is configured to couple light propagating by internal reflections inside the light transmissive panel out of the light transmissive panel for receipt by the light detector.

A fifth aspect of the invention is an optical system comprising: a light transmissive panel which defines two opposing boundary surfaces; an electro-optical device having an operative solid angle given by first and second device divergence angles in orthogonal first and second device planes; and an optical component for transferring light between the electro-optical device and the light transmissive panel, wherein the optical component comprises a first light transmission surface facing the electro-optical device, a second light transmission surface arranged on one of the boundary surfaces, and a control surface arrangement configured to direct the light between the first and second light transmission surfaces by one or more reflections so as to expand the first device divergence angle into a first component divergence angle at the second light transmission surface, the first component divergence angle being arranged to define, with respect to light propagating by internal reflections inside the light transmissive panel, a divergence inside the light transmissive panel in a direction parallel to the boundary surfaces.

In one embodiment, the optical component is attached to the light transmissive panel.

In one embodiment, the optical component is integrated with the light transmissive panel.

A sixth aspect of the invention is an optical film for transferring light between one or more electro-optical devices and a light transmissive panel which defines two opposing boundary surfaces, each electro-optical device having an operative solid angle given by first and second device divergence angles in orthogonal first and second device planes, wherein the optical film is adapted for attachment to one of the boundary surfaces and comprises a micro-structured surface portion that implements at least one optical component according to any one of the first to fourth aspects.

A seventh aspect of the invention is a touch-sensitive apparatus, comprising: a light transmissive panel which defines two opposing boundary surfaces; an illumination arrangement configured to couple light into the panel such that the light propagates by total internal reflection in at least one of the boundary surfaces and such that an object touching said at least one of the boundary surfaces causes a change in the propagating light; a detection arrangement comprising a light detector arranged to detect said change in the propagating light; wherein at least one of the illumination arrangement and the detection arrangement comprises at least one electro-optical device having an operative solid angle given by first and second device divergence angles in orthogonal first and second device planes, and at least one optical component of any one of the first to fourth aspects.

Any one of the above-identified embodiments of the first aspect may be adapted and implemented as an embodiment of any one of the above-identified second to fourth aspects.

Still other objectives, features, aspects and advantages of the present invention will appear from the following detailed description, from the attached claims as well as from the drawings.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The present invention relates to optical components (“coupling elements”) for coupling light into and out of light transmissive panels in touch-sensitive systems. The description starts out by presenting an example of a touch-sensitive system, especially an apparatus operating by frustrated total internal reflection (FTIR) of light. The description continues to present different embodiments of coupling elements for use in such an apparatus.

Throughout the description, the same reference numerals are used to identify corresponding elements.

FIG. 1illustrates a touch-sensitive apparatus100which is based on the concept of transmitting energy across a touch surface1, such that an object that is brought into close vicinity of, or in contact with, the touch surface1causes a local decrease in the transmitted energy. As will be explained in more detail below, the apparatus100is configured to permit transmission of energy in the form of light that propagates by internal reflections inside a light transmissive panel.

The touch-sensitive apparatus100includes an arrangement of emitters and detectors, which are distributed along the periphery of the touch surface1. Each pair of an emitter and a detector defines a detection line, which corresponds to the propagation path for a transmitted signal from the emitter to the detector. InFIG. 1, only one such detection line D is illustrated to extend from emitter2to detector3, although it should be understood that the arrangement typically defines a dense grid of intersecting detection lines, each corresponding to a signal being emitted by an emitter and detected by a detector. Any object that touches the touch surface along the extent of the detection line D will thus decrease its energy, as measured by the detector3.

The arrangement of detectors3is electrically connected to a signal processor10, which samples and processes an output signal from the arrangement. The output signal is indicative of the received power at each detector3. The signal processor10may be configured to process the output signal for extraction of touch data, such as a position (e.g. x, y coordinates), a shape or an area of each touching object.

Generally, the touch surface1may be of any shape, such as circular, elliptical or polygonal, including rectangular. The apparatus100may be used as an overlay to a display device or monitor, as is well-known in the art.

In the example ofFIG. 1, the apparatus100also includes a controller12which is connected to selectively control the activation of the emitters2and, possibly, the readout of data from the detectors3. The signal processor10and the controller12may be configured as separate units, or they may be incorporated in a single unit. One or both of the signal processor10and the controller12may be at least partially implemented by software executed by a processing unit.

In the following, an example embodiment will be described in more detail.FIG. 2Ais a side view of a touch-sensitive apparatus100which includes a light transmissive panel4, a set of light emitters2(one shown) and a set of light detectors3(one shown). The panel4defines two opposing and generally parallel boundary surfaces5,6and may be planar or curved. A radiation propagation channel is provided between the boundary surfaces5,6, wherein at least one of the boundary surfaces allows the propagating light to interact with a touching object7. Typically, the light from the emitter(s)2propagates by total internal reflection (TIR) in the radiation propagation channel. The detectors3are arranged to receive the propagating light and generate a respective measurement signal which is indicative of the power (or equivalently, energy or intensity) of received light.

InFIG. 2A, light is coupled into the panel4via an incoupling element14attached to the bottom surface6, and the light is coupled out of the panel to impinge on the detector3at the edge portion that connects the top and bottom surfaces5,6of the panel4. When the object7is brought sufficiently close to the boundary surface, part of the light may be scattered by the object7, part of the light may be absorbed by the object7, and part of the light may continue to propagate by TIR in its original direction in the panel4. Thus, when the object7touches a boundary surface of the panel (e.g. the top surface5), the total internal reflection is frustrated and the energy of the transmitted light is decreased. This type of touch-sensitive apparatus is denoted “FTIR system” (FTIR—Frustrated Total Internal Reflection) in the following.

The FTIR system100may be operated to measure the power of the light transmitted through the panel4on a plurality of detection lines. This may, e.g., be done by activating a set of spaced-apart emitters2to generate a corresponding number of light beams inside the panel4, and by operating a set of detectors3to measure the transmitted power of each light beam. Such an embodiment is illustrated inFIG. 2B, where each emitter2generates a beam of light that expands in the plane of the panel4while propagating away from the emitter2. Such a beam is denoted a “fan beam” herein. Thus, each fan beam diverges from an entry or incoupling site, as seen on a top plan view. Arrays of light detectors3are located around the perimeter of the panel4to receive the light from the emitters2at a number of spaced-apart outcoupling sites on the panel4.

As used herein, each emitter2may be any type of device capable of emitting divergent radiation in a desired wavelength range, for example a diode laser, a VCSEL (vertical-cavity surface-emitting laser), an LED (light-emitting diode), an incandescent lamp, a halogen lamp, etc. The emitter may also be formed by the end of an optical fiber. Analogously, the detector3may be any device capable of converting light into an electrical signal, such as a photo-detector, a CCD device, a CMOS device, etc.

In the example ofFIG. 2, the emitters2are attached to PCBs15which are designed to supply power and transmit control signals to the individual emitters2, as is well-known in the art. Likewise, the detectors3are attached to PCBs16which are designed to supply power and transmit measurement data from the individual detectors3. The PCBs15, the emitters2and the incoupling elements14form an illumination arrangement for illuminating the boundary surfaces5,6from within the panel4. The PCBs16and the detectors3form a light detection arrangement for detecting the transmitted power of light on the different detection lines D.

The following description will now focus on the design of the incoupling elements14. To facilitate the understanding of the design and functionality of the incoupling element14,FIG. 3illustrates the operative solid angle Ω of an emitter2given by the cone of light generated by the emitter. The cone of light has a point of origin O and is emitted with respect to a centerline, known as beam axis BA, which defines a main direction ze. The cone of light may be defined by a divergence angle in each of two mutually orthogonal planes (“device planes”), which are orthogonal to the beam axis BA. InFIG. 3, these device planes are given by the xe-ze-plane and the ye-ze-plane. In the following, the divergence angle in the xe-ze-plane is denoted αx, and the divergence angle in the ye-ze-plane is denoted αy. These divergence angles are also denoted “device divergence angles” or “device angles”. It should be understood that the device planes may have any orientation in the xe-ye-plane and they may, but need not, coincide with the symmetry planes normally used for characterizing the emission of light sources, e.g. in polar diagrams.

FIGS. 4A-4Bare plots of two exemplifying illumination patterns for two different types of Light Emitting Diodes (LEDs), as measured in the xe-ye-plane ofFIG. 3. The plots are represented as iso-intensity curves, which indicate a decreasing intensity away from the center (beam axis BA). The light distribution is asymmetric inFIG. 4Aand symmetric inFIG. 4B. Each type of light emitter has a nominal illumination pattern, as well as nominal divergence angles αx, αy. The nominal divergence angles αx, αyare defined as the angle included between maximum angles on both sides of the beam axis BA. In the following, the maximum angles are set to where the intensity has decreased to 50% of the maximum intensity. The corresponding intensity curve is indicated by I50inFIGS. 4A-4B. A typical LED has nominal divergence angles αx, αyin the range of 10°-120°. For comparison, a Lambertian source has nominal divergence angles αx=αy=120° (i.e., 2·cos−1(0.5)). The type of criterion used for defining the divergence angles is not essential for the invention. However, given that the incoupling element14achieves a certain divergence in the plane of the panel (φp, below), a stricter criterion (e.g. with the maximum angles set at a higher percentage of the maximum intensity) generally results in a more uniform distribution of light in the panel4.

The incoupling element14operates on the cone of light, or part thereof, to direct the light into the panel4such that it propagates by internal reflections between the boundary surfaces5,6. This operation is schematically indicated inFIG. 2A, albeit only for a center ray (corresponding to an “optical axis” of the incoupling element14). Thus, the incoupling element14is designed such that light propagating on the optical axis is re-directed and injected into the panel with a nominal angle of incidence θp(also denoted “nominal reflection angle” or “nominal bounce angle” herein) to the normal of the boundary surfaces5,6. In order to sustain total internal reflection (TIR), the reflection angle should not be less than the critical angle at each boundary surface5,6that should reflect light by TIR. The critical angle is given by Snell's law and is well-known to the skilled person.

The incoupling element14also operates on the cone of light, or part thereof, to direct the light into the panel4such that it forms a fan beam with a given divergence φpin the plane of the panel4.FIG. 5is a top plan view of such a fan beam, where the solid and dashed arcs indicate the locations, given for the nominal reflection angle θp, where the beam hits the upper and lower boundary surfaces5,6, respectively. Also indicated is the main direction zpof the propagating fan beam in the plane of the panel4. InFIG. 5, the arcs are mutually parallel, which means that the incoupling element14generates essentially the same nominal reflection angle θpfor all angles within the divergence φp. However, in other embodiments, the nominal reflection angle θpmay vary within the divergence φp.

FIG. 6is a perspective view of a first embodiment of the incoupling element14as attached to the bottom boundary surface6of the panel4, e.g. by means of an optical adhesive. The incoupling element14is made of an optically transparent material and comprises a planar light entry face21arranged to project from the panel4, a planar light exit face or attachment surface22arranged to face and be affixed to the boundary surface6, and a three-dimensional control surface23which is designed with respect to the solid angle Ω (FIG. 3) of the emitter2so as to achieve the desired panel divergence φp, main direction zpand bounce angle θpof the injected light. A coordinate system is defined at the center of the exit face, with zcextending normal to the exit face, and xcextending parallel to the main direction (zp) of the light coupled into the panel4.

FIG. 7is a top plan view of the first embodiment as attached to the panel4and combined with an emitter2.FIG. 7illustrate defining light paths from the emitter2through the incoupling element14and into the panel4. The emitter2generates a cone of light with a device angle αx. As the light enters the solid element14via the entry face21, the cone of light is compressed, i.e. the divergence is decreased (indicated by α′x), as a result of the light entering a medium of higher index of refraction. It can in fact be shown that, for a planar entry face21, the divergence after entry cannot exceed twice the critical angle at the entry face21. For example, if the incoupling element14has an index of refraction of 1.6, the maximum divergence after entry is approximately 77°. The cone of light then hits the control surface23, which reflects the cone so as to redirect it and expand its divergence significantly with respect to the device angle αx. The reflected cone has a component divergence φc(not indicated), which in turn controls the panel divergence φpinside the panel (typically φp=φc).

It is understood that the shape (“peripheral curvature”) of the control surface23inFIG. 7, i.e. as seen in a first geometric plane parallel to the exit face22, defines the component divergence φc. In the illustrated embodiment, the shape is elliptical. As used herein, “elliptical” is intended to also include “circular”. It is to be understood that other shapes may be used to attain a desired distribution of light in the panel4. The elliptical shape of the control surface23may optimize the performance of the coupling element14, whereas the use of a circular shape may simplify design, modeling and manufacture of the coupling element14. Although the control of the component divergence φcvia the shape of the control surface23is essentially independent of the control of the nominal reflection angle θcvia the inclination of the control surface23(see below), a small interdependence remains which may lead to a slight aberration of the light leaving the coupling element14. It is possible to design the control surface23with a varying shape in the transverse direction, i.e. such that the shape of the control surface differs in cross-sections taken through the coupling element14at different distances from the exit face22, to reduce this aberration and/or other artifacts.

FIG. 8is a section view along the beam axis BA inFIG. 7, to illustrate how the generated cone of light is transferred through the incoupling element14in the transverse direction of the panel4. In the illustrated example, the light on the beam axis is reflected in the control surface23to form the nominal reflection angle θpat the boundary surface5. For the same reason as inFIG. 7, the cone of light is compressed on entry through the entry face (going from αyto α′y), whereupon the limiting rays hit the inclined control surface23to be reflected into the panel4, in which they define a minimum and a maximum reflection angle θp,min, θp,max, respectively. In the illustrated embodiment, the device divergence αyis redirected by the inclination of the control surface23(and the compression), so as to form a desired range of reflection angles (Δθp=θp,max−θp,min) inside the panel4. In the illustrated embodiment, the entire cone of light emitted by emitter2is re-shaped and re-directed into the range of reflection angles (Δθp). However, it is conceivable that the emitted cone of light expands beyond the entry face21and/or the compressed cone of light expands beyond the control surface23.

Thus, as indicated inFIG. 9B, the control surface23is inclined by an angle γ with respect to a second geometric plane that is orthogonal to the above-mentioned first geometric plane. In the example ofFIG. 8andFIG. 9B, the control surface23is planar in a third geometric plane, which is orthogonal to the above-mentioned first and second geometric planes and which coincides with the plane of the paper inFIG. 8andFIG. 9B. In another example, the control surface23is curved in the third geometric plane so as to expand or compress the divergence of the incoming light. The inclination of the control surface23, and possibly its curvature (which may be of any non-planar shape), in the transverse direction of the panel4may thus be designed to confine the light to a desired range of reflection angles (Δθp) inside the panel4. Again it should be remembered that the limits of the range of reflection angles are defined according to the same criterion as the limits of the device divergence αy(e.g. I50as indicated inFIG. 4).

The minimum reflection angle θp,min(FIG. 8) may be set to exceed the critical angle. This may minimize the leakage of light through the top boundary surface5. The maximum bounce angle θp,maxmay be set with respect to a sensitivity criterion for the touch interaction. For example, it may be desirable to confine the light to such internal reflection angles that interact significantly with the touching objects, i.e. reflection angles with significant frustration of the total internal reflection.

It is to be understood that the design of the incoupling element14needs to take any refraction between the exit face22and the panel4into account, e.g. refraction caused by a difference in index of refraction. Such refraction is e.g. seen inFIG. 8. Thus, the control surface23is designed to generate a component divergence θc, which in turn results in the desired range of reflection angles (Δθp) inside the panel4.

FIGS. 9A-9Bis a top plan view and a section view, respectively, of the coupling element14according to the first embodiment, which is designed starting from a cone. The section view is taken along the symmetry line B-B inFIG. 9A. Solid lines designate the contours of the coupling element14, and dashed lines indicate portions of the cone that have been “cut away” when designing the coupling element14. It should be emphasized that these sections are generally cut away in the design process, not in the actual manufacturing of the coupling element14. The cone is defined by a directrix D which defines the base of the cone and a generatrix G which extends between the vertex V and the directrix D. Thus, the generatrix G is “swept” along the directrix D to generate the lateral surface of the cone. The planar base of the cone forms the exit face22of the coupling element14, whereas part of the lateral surface forms the control surface23. The inclination of the control surface23is given by the cone angle γ, and the shape of the control surface23is given by the radii a, b of an ellipse (the directrix D). A lateral cut-out portion is designed, by intersecting the cone with a plane with an angle δ to the center line of the cone, to form the planar entry face21. The angle δ may be set to generate a desired refraction of the cone of light generated by the emitter2(cf.FIG. 8) and/or for manufacturing purposes, e.g. to provide a draft angle in injection molding. A bottom cut-out portion is designed to form a planar surface24facing away from the base. The planar surface24may be denoted a “conic section”, which is a conventional term for a surface formed by an intersection between a geometric plane and a cone. The conic section24(also denoted “bottom surface” and “pick-up surface”), which may be omitted, may be implemented to facilitate gripping of the coupling element14during mounting of coupling element14to the panel4. The pick-up surface24may, but need not, be parallel to the base.FIGS. 9A-9Bindicate various parameters that may be collectively optimized when designing the coupling element14for a specific emitter2to achieve desired properties of the light coupling into the panel4. These parameters include the angle δ, the radii a, b, the cone angle γ, as well as the distance s from the exit surface22to the entry point of the optical axis OA on the entry face21, the height h, the spacing d between the entry point and the emitter2, and the distance c from the axis of the cone to the entry point. The optical axis OA of the coupling element14is the light path that defines the nominal reflection angle θpand the nominal main direction zpin the panel4.

The outer surface of the coupling element14is partially coated by a reflective material, at least on the portion of the lateral surface that is internally illuminated by the cone of light from the emitter2.

It is possible to optimize the design of the coupling element14such that the light at the exit surface22has an essentially rectangular distribution of intensity with respect to an elevation angle θ and an azimuth angle φ.FIG. 10Ais a plot of such a light distribution at the exit face23of the coupling element14, with respect to the angles θ, φ.FIG. 10Billustrates the definition of the elevation and azimuth angles θ, φ in relation to the coordinate system inFIG. 6. The illustrated light distribution is illustrated for a coupling element14designed for a Lambertian emitter placed with a spacing from the entry face21. In this example, the distribution is essentially symmetric with respect to the main direction (φ=0°), and the outer limits at 50% of maximum intensity are approximately located at φ=±75° (i.e. φc=150°). It is also seen that the distribution is essentially symmetric with respect to θ=57.5° and that the outer limits at 50% of maximum intensity are approximately located within 45°-70° (i.e. θc=25°). The angle θ=57.5° sets the nominal reflection angle θpinside the panel, and the component divergence θcsets the range of reflection angles Δθparound the nominal reflection angle θp. Thus, as illustrated by the dashed rectangle inFIG. 10A, the coupling element defines as essentially rectangular distribution of light in the (θ, φ)-plane.

The rectangular distribution inFIG. 10Ameans that the component divergence θcis essentially invariant as a function of the component divergence φc. Thereby, the injected light will propagate similarly in all directions away from the incoupling site (i.e. for all φ) within the panel divergence φp(cf.FIG. 5), which may be desirable for achieving a consistent interaction across the panel4, i.e. that the light that impinges on a touching object from within the panel4has approximately the same intensity distribution as function of angle of incidence across the panel. In other words, the touching object is illuminated in approximately the same way across the entire touch surface, which results in approximately the same interaction between the touching object and the light across the touch surface.

FIG. 11is a plot of total intensity as a function of azimuth angle φ (solid line), which may be obtained by a summation of the data inFIG. 10Ain the θ direction. For comparison,FIG. 11also includes a corresponding intensity profile (dashed line) obtained for a fictive Lambertian emitter directly attached to the peripheral edge surface of the panel4(similarly to sensor3inFIG. 2A). As shown, the coupling element14operates to both increase the overall intensity and improve the uniformity of the generated light in the plane of the panel4, i.e. within the panel divergence φp.

The above-described design principles are equally applicable when designing a coupling element for transferring the light from the panel onto a light detector. For example, in the context ofFIGS. 6-9, the base of the cone (i.e. the attachment surface22) will form a light entry face, the reflective portion of the lateral surface will form the control surface, and the lateral cut-out portion will form the light exit surface, which may be spaced from the light detector by an air gap. The above-described cone of light corresponds to a field of view of the detector, where the field of view may be defined by the same criterion as the cone of light (e.g. 50% of maximum intensity), and the device angles αx, αy(FIG. 3) corresponds to viewing angles in two mutually orthogonal planes. In analogy with the incoupling element described in the foregoing, the outcoupling element will expand the device angle αxinto a component divergence or view angle φcat the light entry face, and orient (and possibly expand/compress) the device angle αyinto a component divergence or view angle θcat the light entry face.

Reverting toFIG. 2, such outcoupling elements may be used for coupling the light out of the panel4for receipt by the light detectors3. The outcoupling elements may be attached to either of the boundary surfaces5,6. These outcoupling elements may be designed in complete analogy with the incoupling elements as described in the foregoing, except that the light emitters2are replaced by light detectors3. Depending on implementation, it may be desirable to make the outcoupling elements larger than the incoupling elements, in order to allow the detector to have a larger light-sensitive surface area to receive the light from the exit face of the outcoupling element. This may e.g. increase the signal-to-noise ratio (SNR) of the output signal. In one implementation, the incoupling elements and the outcoupling elements have a width in the ycdirection (FIG. 6) of about 3-7 mm and about 5-10 mm, respectively.

In a variant of the outcoupling element, the pick-up surface24may be made reflective, e.g. by applying a reflective coating on the outside of the pick-up surface24.FIG. 12is a section view of such an outcoupling element14and illustrates how light rays are reflected in the pick-up surface24onto the detector3. The dotted lines inFIG. 12illustrate that the effective light-sensitive surface area of the detector3may be increased by the reflections in the pick-up surface24causing more light to be directed onto the detector. A corresponding advantage may be obtained by a reflective pick-up surface in an incoupling element14, where the pick-up surface24may be designed relative to the cone of light so as to increase the amount of light coupled into the panel4and/or modify the distribution of light within the range of reflection angles in the panel4.

FIG. 13is a side view of another variant of the incoupling element14described in the foregoing. In this variant, the incoupling element14includes a reflective folding surface25that defines a folded beam path from the entry face21which is arranged to be parallel with the attachment surface22. It should be noted that the folding surface25inFIG. 13may be planar to merely redirect the incoming cone of light onto the control surface23. Alternatively, the folding surface25may have a shape that modifies the distribution of the incoming cone of light. The coupling element14inFIG. 13may also be used in coupling light out of the panel4.

The embodiment inFIG. 13, and alternative configurations that allow the light emitter2to be arranged beneath the incoupling element14as seen from the panel4, has certain attractive properties. For one, the thickness of the touch-sensitive apparatus may be reduced, e.g. if the emitter2is attached to a PCB (e.g.15inFIG. 2), since the PCB may be arranged parallel with the panel4. Further, a group of incoupling elements14may be arranged to operate on the cone of light from a single emitter2, whereby each incoupling element14operates to couple a separate portion of the cone of light into the panel4. The incoupling elements within such a group may (but need not) be mutually identical. Corresponding advantages and effects may be obtained if the incoupling element14inFIG. 13(including equivalent structures) is used and designed for coupling light out of the panel.

In the description above, it is presumed that the control surface23is provided with a reflective coating. However, it is conceivable that the reflection(s) in the control surface23occurs via total internal reflection in the coupling element14, be it used for incoupling or outcoupling, provided that the coupling element14is designed to ensure that light hits the control surface23at an angle that exceeds the critical angle.

FIGS. 14A-14Bare elevated side views, taken from mutually orthogonal sides, of an incoupling element14according to a second embodiment. The incoupling element14, which is attached to the bottom boundary surface6of the panel4, is designed as an elongate light guide of light transmissive material that defines an entry face21at one end and an exit face22at the other end. The light guide14comprises a first pair of elongate opposing control surfaces30,31that expand the emitter divergence αxinto a component divergence φcat the exit face22, resulting in a desired panel divergence φpinside the panel4. As shown inFIG. 14A, the control surfaces30,31taper from the entry face21to the exit face22, as seen in a first geometric plane which extends along the light guide14and coincides with the xe-ze-plane of the emitter. Thereby, the reflection angle of the light propagating between the control surfaces30,31is decreased for every reflection inside the light guide14, whereby the light is caused to leave the exit face22at an increased divergence.

As shown inFIG. 14B, the exit face22is inclined with respect to a center line or optical axis OA of the light guide14, as seen in a second geometric plane which extends along the light guide14and coincides with the ye-ze-plane of the emitter. Thereby, the optical axis OA has a given inclination to the panel4when the light guide14is attached thereto. The inclination defines the nominal bounce angle θpinside the panel. The light guide14also comprises a second pair of elongate opposing control surfaces32,33that control the component divergence θcat the exit face22. In the illustrated example, the control surfaces flare32,33from the entry face21to the exit face22, in order to confine the emitter divergence αyto a desired range of reflection angles Δθpinside the panel4. In other variants, depending on the emitter divergence αy, in relation to the desired range of reflection angles Δθp, the control surfaces32,33may be parallel or tapering towards the exit face22.

The light guide14is at least partially coated by a reflective material. However, it is possible that certain parts of one or more control surfaces30,31,32,33may be uncoated, provided that the guided light undergoes total internal reflection in these parts.

The second embodiment may also be designed as an outcoupling element for transferring the light from the panel onto a light detector.

In yet another alternative, schematically depicted in plan view inFIG. 15, one or more coupling elements14are defined by a sheet-like micro-structured surface portion50which is fixedly arranged on one of boundary surfaces (6inFIG. 15). As used herein, a “microstructured surface” contains surface structures having at least one dimension in the range of 0.1-1000 μm. For example, the microstructured surface portion50may comprise a plurality of microreplicated prismatic elements51that collectively form one or more coupling elements14. In one such embodiment, each prismatic element51has a design similar to the coupling elements described in the foregoing, whereby each prismatic element operates on a small part of the operative solid angle of the associated electro-optical device. In another embodiment, the coupling element14is implemented by a microstructured surface portion50defining a Fresnel lens or a diffractive optical element (DOE), which both are compact and well-proven components that may be designed with desired re-directing and re-shaping properties. The diffractive optical component (DOE) may be a grating, such as a holographic grating. The microstructured surface portion50may have an essentially flat configuration on the boundary surface, e.g. projecting 1 mm or less from the boundary surface. It is to be understood that that the microstructured surface portion50may be either integrated in the boundary surface5,6or provided on a substrate, such as a thin film, with an adhesive backing for attachment to the boundary surface5,6.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope and spirit of the invention, which is defined and limited only by the appended patent claims.

For example, although the boundary surfaces have been illustrated as external surfaces of the panel, it is conceivable that one of the boundary surfaces is formed by an internal interface in the panel, e.g. a reflective layer or a step change in index of refraction, against which the light is reflected as is propagates through the panel.

In all embodiments, the entry face of the incoupling element and the exit face of the outcoupling element may be designed to refract the transmitted light, e.g. by means of a curvature or inclination of the entry/exit face, so as to modify the transmitted cone of light in any direction.

For avoidance of doubt, it should be emphasized that the inventive incoupling elements may, but need not, be used in combination with the inventive outcoupling elements. Thus, the touch-sensitive apparatus may include at least one of the inventive incoupling and outcoupling elements.

It is also to be understood thatFIG. 2merely illustrates one example of a fan beam based FTIR system. The inventive coupling elements may be used in other types of FTIR systems as well, including any of the systems referenced in the Background section. Generally, the inventive incoupling element may be used in all types of touch systems that operate by generating fan beams that propagate by internal reflections in a light transmissive panel, and the inventive outcoupling element may be used in all types of touch systems that operate by locally coupling light out of a light transmissive panel.