WO2006134552A2

WO2006134552A2 - Flexible displays and user input means therefor

Info

Publication number: WO2006134552A2
Application number: PCT/IB2006/051878
Authority: WO
Inventors: Cornelis Van Berkel; James Bateman
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-06-14
Filing date: 2006-06-13
Publication date: 2006-12-21
Also published as: WO2006134552A3

Abstract

A flexible display device (1 ), and method of sensing user input, comprising: a flexible display substrate (20); a light emitter (34) positioned to emit visible and/or infra-red light (38) into the flexible display substrate (20) via an edge of the flexible display substrate (20); and a light sensor (36) positioned to sense light (38) emitted from the light emitter (34) that exits the flexible display substrate (20) at an edge of the flexible display substrate (20) having been internally reflected in the flexible display substrate (20); wherein the amount of the light (38) emitted from the light emitter (34) that reaches the light sensor (36) is dependent upon how much of the light (38) escapes from the flexible display substrate (20) dependent upon the physical deformation of the flexible display substrate (20), the user input being provided by a user physically deforming the flexible display substrate (20). In other embodiments, a flexible lightguide (50), e.g. a fibre-optic cable, is attached to the flexible display substrate (20).

Description

DESCRIPTION

FLEXIBLE DISPLAYS AND USER INPUT MEANS THEREFOR

The present invention relates to flexible displays, and to user input apparatus and means for flexible displays.

Flexible displays, in which the substrates of the display are flexible, e.g. deformable, are known. See for example WO 02/082555 A2; "Roll-up Displays: Fact or Fiction?", L. Collins, IEE Review, February 2003, pages 42-45; and two papers by N. D. Young et al, the first of these being "AMLCDs and Electronics on Polymer Substrates", SID Proceedings Euro Display 1996, pages 555 to 558, the second of these being "Thin-film-transistor- and diode- addressed AMLCDs on polymer substrates", Journal of the SID 5/3, 1997, pages 275-281 ; the contents of both of these papers being incorporated herein by reference.

Flexible displays have primarily been pursued for the purpose of ease of use, e.g. as an extreme example it has been envisaged to roll-up a flexible display for storage/portability purposes. Either in addition or alternate thereto, display flexibility has been pursued for the benefit of stress relief.

In an article entitled "The Design and use of Squeezable Computers: A Exploration of Manipulative User Interfaces" by B. L. Harrison et al., available at http://nano.xerox.com/want/papers/squeezy-chi-apr98.pdf, it is suggested to use squeezable interfaces as user input devices. Pressure sensors are used to detect a user squeezing a device housing. The user input devices are included in devices which also have a display, but the user input device is not part of the display, nor vice versa.

Entirely separate from the field of displays, it is known to attach fibre- optic cables to objects, and to detect levels of light passing along the fibre- optic cable, as a way of sensing physical deformation of the objects. See, for example, US 6,389,187 B2. The present inventors have realised it would be desirable to provide a user input for a flexible display, and preferably integrated in a display, that operates by sensing physical deformation of the flexible display, for example by sensing a change in shape or curvature of a substrate of the flexible display, where that change is produced by a user pressing, squeezing or otherwise mechanically interacting with the flexible display.

The present inventors have further realised that, preferably, it would be advantageous to avoid attaching items to the display for the purpose of sensing the physical deformation of the flexible display. This would be the case, for example, were such items required to be positioned over some or all of the viewing area of the flexible display, thereby tending to impair the visual appearance of the image displayed, and/or were such items required to be positioned at the region outside the viewing area, which region is usually already in demand for other display operating components and circuitry. Furthermore, preferably, it would be advantageous to avoid the fabrication and operational (wear and tear) implications of attaching external physical sensors over large parts of a flexible display.

In a first aspect, the present invention provides a flexible display device, comprising: a flexible display substrate; a light emitter positioned to emit light, of one or more wavelengths in the wavelength range of visible and infra-red, into the flexible display substrate via an edge of the flexible display substrate; and a light sensor positioned to sense light emitted from the light emitter that exits the flexible display substrate at an edge of the flexible display substrate having been internally reflected in the flexible display substrate; wherein the amount of the light emitted from the light emitter that reaches the light sensor is dependent upon how much of the light escapes from the flexible display substrate dependent upon the physical deformation of the flexible display substrate.

In a further aspect, the present invention provides a flexible display device, comprising: a flexible display substrate; a flexible lightguide attached to a surface of the flexible display substrate; a light emitter positioned to emit light, of one or more wavelengths in the wavelength range of visible and infra- red, into the flexible lightguide via an edge of the flexible lightguide; and a light sensor positioned to sense light emitted from the light emitter that exits the flexible lightguide at an edge of the flexible lightguide having been internally reflected in the flexible lightguide; wherein the amount of the light emitted from the light emitter that reaches the light sensor is dependent upon how much of the light escapes from the flexible lightguide dependent upon the physical deformation of the flexible display substrate.

In a further aspect, the present invention provides a method of sensing user input to a flexible display device, wherein the flexible display device comprises a flexible display substrate; the method comprising: emitting light, of one or more wavelengths in the wavelength range of visible and infra-red, into the flexible display substrate via an edge of the flexible display substrate; and sensing light emitted from the light emitter exiting the flexible display substrate at an edge of the flexible display substrate having been internally reflected in the flexible display substrate; wherein the amount of the light emitted from the light emitter that reaches the light sensor is dependent upon how much of the light escapes from the flexible display substrate dependent upon the physical deformation of the flexible display substrate.

In a further aspect, the present invention provides a method of sensing user input to a flexible display device, wherein the flexible display device comprises a flexible display substrate; the method comprising: emitting light, of one or more wavelengths in the wavelength range of visible and infra-red, into a flexible lightguide attached to a surface of the flexible display substrate, via an edge of the flexible lightguide; and sensing light emitted from the light emitter exiting the flexible lightguide at an edge of the flexible lightguide having been internally reflected in the flexible lightguide; wherein the amount of the light emitted from the light emitter that reaches the light sensor is dependent upon how much of the light escapes from the flexible lightguide dependent upon the physical deformation of the flexible display substrate. The light emitter and the photodiode may be arranged such that the light emitted from the light emitter travels by means of the internal reflection across the substrate/I ightguide over the course of plural circuits across the substrate area prior to being sensed by the light sensor. This enables, for example, increased sensitivity and/or a larger number of different parts of the substrate area to be sensed for user input deformation.

One or more further light emitters and one or more further light sensors may be used thereby providing a plurality of light emitters and a corresponding plurality of light sensors arranged such that the light emitters are positioned to emit light, of one or more wavelengths in the wavelength range of visible and infra-red, into the flexible display substrate/I ightguide via one or more edges of the flexible display substrate/lightguide, and the light sensors are positioned to sense light emitted from respective light emitters that exits the flexible display substrate/lightguide at a respective edge of the flexible display substrate/lightguide having been internally reflected in the flexible display substrate/lightguide. This enhances the capability to process spatially dependent inputs. The light emitter may emit light of one or more wavelengths only in the infrared range. This tends to alleviate or avoids the light from the emitter affecting the display image quality (visible light) seen by the user.

The flexible display device may further comprise an input processor coupled to the light sensor for processing the level of light sensed by the light sensor or sensors to determine a user input provided by a user physically deforming the flexible display substrate.

All of the above aspects tend to provide a user input means or process that alleviates or overcomes some or all of the above identified disadvantages with known approaches. Furthermore, at least those aspects of the present invention where the sensing light is internally reflected in the display substrate itself tend to provide a means of detecting user produced physical deformation in the flexible display that does not require a sensor item to be placed over large parts of the display, e.g. over some or all of the viewing area of the display, and which tends to provide detection of physical deformation with little addition to the bulk of the overall flexible display device, with low levels of wear and tear as large items do not need to be attached (in such certain aspects of the invention). Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic illustration of part of a flexible display device; Figure 2 is a schematic illustration showing certain elements of the flexible display device of Figure 1 in cross-section and a connection to an input processor;

Figure 3a schematically illustrates a substrate of the flexible display device of Figures 1 and 2 in a substantially non-deformed, i.e. substantially flat, condition;

Figure 3b schematically illustrates the substrate of Figure 3a in a deformed or curved condition;

Figure 4 is a schematic illustration of a layout of an LED and a photodiode relative to the area of a substrate of the flexible display device of Figures 1 and 2 as viewed from above relative to the cross-sectional representation of Figures 2and 3;

Figure 5 is a schematic illustration of a further layout of the LED and the photodiode relative to the area of the substrate of the flexible display device of Figures 1 and 2 as viewed from above relative to the cross-sectional representation of Figures 1 and 2;

Figure 6 is a schematic illustration of a flexible display device comprising plural LEDs and plural corresponding photodiodes;

Figure 7 is a schematic illustration of a flexible display device comprising a fibre-optic waveguide; and Figures 8-12 are schematic illustrations providing details of factors to be taken into account by the skilled person with respect to the internal reflection process.

The embodiments described below are implemented in an active matrix liquid crystal display device (AMLCD), with a driving circuit as described in more detail below with reference to Figures 1 and 2. Nevertheless, it is to be appreciated that the invention may also be implemented with other active matrix circuitry compared to that shown in Figures 1 and 2, or indeed in other types of active matrix display such as electrophoretic displays or organic LED (OLED) displays which may use different forms of active matrix circuitry compared to that shown in Figures 1 and 2. Furthermore, the invention may be implemented in passive display devices which do not employ an active matrix circuit.

Figure 1 is a schematic illustration of part of a first embodiment of a flexible display device 1. The flexible display device 1 is an active matrix liquid crystal display device (AMLCD) comprising a row and column, regular, array of display pixels 10, each comprising a liquid crystal display element 11 and an associated thin film transistor, TFT, 12. Each pixel is arranged adjacent the intersection of respective ones of sets of row and column address conductors 14 and 16 to which, in use, selection (gating) and data signals are supplied respectively by a peripheral drive circuit (not shown) to drive the pixels and cause their display elements to produce desired display outputs.

Figure 2 is a schematic illustration of the flexible display device 1 , in particular showing certain elements of the flexible display device 1 in cross- section, and a connection to an input processor 30. Referring to Figure 2, the sets of address conductors 14 and 16, the TFTs 12 and individual pixel (display element) electrodes, 18, together form a thin film active matrix circuit which is carried on the flat surface of a transparent flexible substrate 20 of flexible and compliant polymer material, and formed by depositing and patterning appropriate layers of conductive, insulating and semiconductive materials. In the particular section shown the row conductors 14 are not visible. Usually, a metal is used for the address conductors 14 and 16 and the electrodes 18 may be of metal or a transparent conducting material such as ITO or PEDOT polymer depending on whether the pixels are reflective or transmissive respectively. An opposing transparent flexible substrate 22, similarly of flexible polymer material, carries a continuous, transparent, electrode 23 common to all display elements in the array and is spaced by spacers (not shown) from the substrate 20. LC orientation films 26 and 27 are provided as continuous layers covering the structures carried on the substrates 22 and 23 respectively.

The two substrates are sealed together around their periphery and liquid crystal material 24 is contained between them. The aforementioned papers by N. Young et al describe typical processes and materials for fabricating an AMLCD using polymer substrates which may be employed for fabricating the structure shown in Figure 2, reference is invited to those papers for further details in these respects and the subject matter of those papers is incorporated herein by reference. As is described in those papers, the substrate carrying the active matrix circuit may comprise polymer materials such as polyimide, polyethersulphone, polyarylate, high temperature polyestercarbonate, polyethylenenapthalta andpolyethyleneterephtalate, and of which can have a film thickness of around 100-200um. As is also described in those papers the semiconductor layers for the distributed TFTs are provided as discrete islands formed by patterning a continuous layer, with each island occupying a comparatively small area adjacent the intersection of the associated row and column address conductors.

In this embodiment, additionally, an LED 34 is positioned at one edge of the substrate 20, and a photodiode 36 is positioned at the opposite edge of the substrate 20. The LED 34 is positioned such that, in operation, light 38 emitted therefrom enters into the body of the substrate 20. Photodiode 36 is positioned such that light 38 originating from the LED 34 and internally reflected across (i.e. along) the body of the substrate 20 enters the photodiode 36 when exiting the substrate 20 through the edge of the substrate 20. The LED 34 an photodiode 36 are each coupled to an input processor 30, whose operation will be described later below.

Figures 3a and 3b are schematic representations in cross-section of a portion of the substrate 20. Figure 3a schematically illustrates the substrate 20 in a substantially non-deformed, i.e. substantially flat, condition. In this condition, the light 38 from the LED 34 is totally internally reflected as it passes along the substrate 20, i.e. this is shown in Figure 3a, by way of example, as totally internally reflected light 38. Broadly speaking, total internal reflection occurs since the top and bottom internal surfaces of the substrate are substantially parallel, and hence the light is reflected at an angle above the critical angle at both surfaces. Figure 3b schematically illustrates the substrate 20 in a deformed or curved condition. In particular, the substrate is bent at the portion of the substrate shown schematically in Figure 3b. In this condition, some of the light from the LED 34 is internally reflected as it passes along the substrate 20, as again represented by internally reflected light 38, nevertheless at each reflection some light 40 is lost from the substrate. Broadly speaking, light reflected above the critical angle from one internal surface of the substrate hits the other internal surface of the substrate (in Figure 3b, the bottom surface) at an angle below the critical angle. The greater the physical deformation or curvature of the substrate, i.e. the extent of bend, the greater will be the light loss, hence the lower will be the amount of light reaching the photodiode 36.

Further details of factors to be taken into account by the skilled person with respect to the above described internal reflection process are described below with reference to Figures 8-12.

The photodiode 36 senses the different respective levels of light in the situation of Figure 3b compared to Figure 3a, and these different levels are processed by the input processor 30 to provide a user input signal as required.

Figure 4 is a schematic illustration of a first embodiment of the layout of the LED 34 and the photodiode 36 relative to the area of the substrate 20, as viewed from above relative to the cross-sectional representation of Figures 1 and 2. In this embodiment, the LED 34 is positioned approximately at the centre of one edge of the substrate 20, and the photodiode 36 is positioned approximately at the centre of the opposite edge of the substrate 20. In this relatively simple embodiment, the internally reflected light 38 is directed from LED 34 to photodiode 36 across a central region of the substrate 20, and physical deformation of the substrate 20 at any point along the path of the light 38 is sensed by the photodiode 36 and input processor 30 as described above. Thus, in this embodiment, relatively simple user input interactions can be provided, relying on the user providing the physical deformation of the substrate 20 along the centre of the substrate 20. In variations of the arrangement shown in Figure 4, the LED 34 and photodiode mat be positioned at different locations on their respective substrate edges. For example, both the LED 34 and the photodiode 36 may be positioned toward the top of their respective substrate edges, such that deformation of the substrate along the top of the substrate is sensed. Another possibility is for the LED 34 to be placed near one corner of the substrate 20 and the photodiode 36 placed near the opposite corner of the substrate 20, such that the light passes approximately along a diagonal of the substrate 20, which region is thus sensed for deformation. Clearly, a wide range of respective positions for the LED 34 and photodiode 36 are possible and will be selected by the skilled person according to the requirements of the particular apparatus or process being implemented. In this embodiment, the input processor 30 processes the varying received light levels, as detected by the photodiode 36, to determine a level of user input for a given function. For example, the user input may be to zoom in or zoom out an image being displayed by the flexible display device 1. That is, the more the user pushes the substrate 20, the more the amount of light lost 40 from the substrate increases, hence the lower the light level received by the photodiode 36 becomes. The input processor processes this light level signal, and sends a resultant input signal to the display drivers (not shown) of the flexible display device 1 , which adapt the image driving signal accordingly to show a zoomed view of the image being displayed. The extent to which the image is zoomed (e.g. level of magnification) may be made dependent upon the extent to which the substrate 20 is deformed by the user, i.e. dependent upon how hard the user presses or squeezes the substrate 20.

In this embodiment, any desired user input can be implemented by suitable programming or adaptation of the input processor 30 and other display driving or other image processing components. For example, the apparatus can be arranged for the user input to control, say, brightness of the flexible display image, and so on. In each case, the extent to which the parameter being controlled is varied (e.g. brightness level) may be made dependent upon the extent to which the substrate 20 is deformed by the user, i.e. dependent upon how hard the user presses or squeezes the substrate 20.

Alternatively, the apparatus can be arranged for the user input to control any conventional graphical user interface input, in particular one suitable for

"touchscreen" type arrangements, e.g. pressing a given user button displayed on the flexible display device 1. In this type of situation, the apparatus may be arranged such that the more the user deforms the substrate 20, the stronger a version of input is e.g. if selecting an item from a list, a first given amount of deformation may select one off of the item, whereas a larger deformation (i.e. the user pressing or squeezing harder) selects two units of the item, whereas a third yet larger amount of deformation corresponds to selecting three units of the item, and so on.

Other, more complicated, user input functions will be described below following a description of other embodiments which offer more complicated input signal possibilities compared to the above described simpler embodiment.

Figure 5 is a schematic illustration of a further embodiment of the layout of the LED 34 and the photodiode 36 relative to the area of the substrate 20, as viewed from above relative to the cross-sectional representation of Figures 1 and 2. In this embodiment, the LED 34 is at one edge of the substrate 20, and the photodiode 36 is positioned further along the same edge of the substrate 20. The photodiode 36 is orientated at an angle to the edge of the substrate 20 such that in the internally reflected light 38 rebounds from edge to edge to travel two "circuits" over the area of the substrate 20, as shown schematically in Figure 2, before arriving at the photodiode 36. Physical deformation of the substrate 20 at any point along either or both of the circuits of travel of the path of the light 38 is sensed by the photodiode 36 and processed by input processor 30 (not shown) as described above. This arrangement of Figure 5 provides various advantages. For example, by virtue of the light travelling two circuits, the loss of light produced by a given deformation of the substrate will be increased compared to if the light only travelled one circuit, if the area of the substrate deformed is large enough to encompass a given portion of each circuit of travel of the light. This effect may be increased by arranging the LED 34 and photodiode 36 such that the two circuits of travel of the light are closely positioned relative to each other. Another advantage is that deformation of a larger area of the substrate may be sensed despite the use of only one LED and one photodiode, by virtue of the two circuits of the travel of the light being positioned relatively far apart from each other. This effect may be increased by arranging the LED 34 and photodiode 36 such that the two circuits of travel of the light are positioned further apart from each. Each of the advantages described in this paragraph may be enhanced by arranging, in other embodiments, the LED 34 and photodiode 36 such that more than two circuits of travel of the light are performed, for example ten circuits.

However, yet another possibility is to arrange the LED 34 and photodiode 36 such that only one circuit of light takes place, or even less than one, e.g. the photodiode 36 may be positioned on the bottom edge (as viewed in Figure 5) such that the light 38 travels from the left edge to the top edge then to the right hand edge and then to the photodiode at the bottom edge.

Figure 6 is a schematic illustration of a further embodiment. In this embodiment, plural LEDs 34 and plural corresponding photodiodes 36 are arranged at the edges of the substrate 20. Figure 6 shows schematically the positions of these LEDs 34 and photodiodes 36 relative to the area of the substrate 20, as viewed from above relative to the cross-sectional representation of Figures 1 and 2. In particular, three LEDs 34 are evenly spaced along a first edge of the substrate 20, and a further three LEDs 34 are evenly spaced along an adjoining second edge of the substrate 20. Three photodiodes 36 are positioned in corresponding spaced fashion along the edge of the substrate opposite the first edge, and three further photodiodes 36 are positioned in spaced fashion along the edge of the substrate opposite the second edge so as to correspond to the three LEDs 34 spaced along the second edge. Thus, overall, three LED/photodiode pairs direct internally reflected light 38 across the substrate from left to right as shown in Figure 6, each LED/photodiode pair directing the light across the substrate 20 at a respective "y-cc-c-rcHnate" of the substrate 20. Similarly, three further LED/photodiode pairs direct internally reflected light 38 down the substrate 20 from top to bottom as shown in Figure 6, each directing the light down the substrate at a respective "x-coordinate" of the substrate 20.

For each LED/photodiode pair, the physical deformation of the substrate 20 at any point along the path of the light 38 is sensed by the respective photodiode 36 and processed by input processor 30. Thus independent deformation values are achieved for each LED/photodiode pair. These values can be processed as required, to achieve an overall input. For example, the six inputs may be processed as six separate inputs, e.g. six separate input functions may be prompted or displayed to the user at appropriate positions on an image being displayed by the flexible display device 1. Another possibility is that two or more of the inputs may be processed in combination to produce a combined input giving a spatially-dependent user input, using any appropriate algorithm implemented by the skilled person according to the desired operation of the overall display device.

In other embodiments, other numbers or arrangements of LEDs and photodiodes may be used. Also, spacings between different LEDs or photodiodes need not be even; generally the layouts need not be symmetrical.

The embodiments described above with reference to Figures 5 and 6 may be used for the types of user input described above in relation to the

Figure 4 arrangement. However, additionally or alternatively, the embodiments described above with reference to Figures 5 and 6 may be used for types of user inputs making particular use of the more wide ranging ambit of the response of the overall surface of the substrate 20. The skilled person implementing the flexible display device 1 may make use of existing graphical user interface processes, for example by using the arrangement of Figure 6 to determine user control of movement of a cursor over the display area of the flexible display device 1. However, another possibility is that the skilled person implementing the flexible display device 1 may derive new graphical user interface possibilities by making use of the new design and interaction possibilities offered by the above described input arrangements.

For example, when the flexible display device 1 is displaying a computer or console game, a ball or other item can be physically manipulated by, as it were, changing the shape of the surface the ball is moving on. This is just one example of the many possibilities for new user interface concepts that are opened up by the present invention. The tactile aspect of the user input mechanism (pressing or squeezing or other such activities of the surface of the display, i.e. in effect pressing the content of the image being displayed) also offers possibilities for new types of graphical interface designed for artistic input purposes e.g. the user may be given the feel of moving paint or other material around the image displayed.

Figure 7 is a schematic illustration of a further embodiment of a flexible display device 1. The details of the flexible display device 1 of this embodiment are the same as those of the embodiments described above, and the same reference numerals are used for the same items, except where stated below.

In this embodiment, a flexible lightguide 50, more particularly a fibre- optic waveguide 50, is attached to the outside surface of the flexible transparent substrate 20. In this embodiment, the LED 34 and photodiode 36 are attached to respective ends of the fibre-optic waveguide 50, such that light output from the LED 34 passes into the fibre-optic waveguide 50, and totally internally reflected light 38 exits therefrom into the photodiode 36. The LED 34 and photodiode 36 are again coupled to an input processor 30. The fibre-optic waveguide is deformed physically when the substrate 20 is deformed physically, due to these two items being attached. Hence, in this embodiment, the amount of light from the LED 34 being internally reflected along the fibre- optic waveguide 50 to the photodiode 36 varies according to the physical deformation of the flexible substrate 20 as some light is lost when deformation or curvature takes place for the same reasons as explained above with reference to Figures 3a and 3b.

The processing options and different LED/photodiode layout options mentioned for the embodiments described above in which the LED light passes along inside the substrate 20 are equally possible to provide further embodiments based upon the fibre-optic waveguide approach shown in Figure 7. For example, using a single LED 34 and photodiode 36, different areas of the substrate can be monitored for deflection by virtue of the fibre-optic waveguide being laid out accordingly across the area of the substrate 20, e.g. such as to make multiple-passes over the area of the substrate. Another possibility is for multiple LED/photodiode pairs to be employed, each feeding a respective one of a plurality of fibre-optic waveguides laid out over the area of the substrate 20. The fibre-optic waveguide may be protected by one or more protection layers/structures positioned over or around the fibre-optic waveguide. In other embodiments, the fibre-optic waveguide may be positioned on the surface of the substrate facing into the display device, which may for example provide protection from physical damage in use. In the case of the above described embodiments employing one or more fibre-optic waveguides, certain advantages of the earlier embodiments in which the LED light passes along inside the substrate 20, in particular advantages related to avoiding the need to attach items across the substrate, no longer apply. Nevertheless, these embodiments using an attached fibre- optic waveguide (or other example of flexible lightguide) nevertheless still tend to provide display devices with at least some of the other advantages described earlier, e.g. ease of user input.

In the embodiments described above with reference to Figure 7, the lightguide is a fibre-optic waveguide, more particularly a solid optical fibre. In other embodiments, other forms of flexible lightguide, e.g. other forms of flexible fibre-optic waveguide, and more generally, other forms of flexible waveguide, may be employed. For example, a plate, e.g. made of polymer, or glass, and for example approximately corresponding in area to the area of the substrate, may be attached to the substrate and used as the lightguide. Further details of factors to be taken into account by the skilled person with respect to the above described internal reflection process will now be described with reference to Figures 8-12. It will be appreciated that further details may readily be determined by the skilled person, employing application of conventional optics calculations and/or trial and error depending on which types and dimensions of substrates/waveguides, materials, shapes, layouts, wavelengths and so on are being employed or are desired to be employed. Light escapes through the edges of the waveguide when the guide is bent due to changes introduced to the angle of incidence.

For the purposes of this discussion, the waveguide is treated in two dimensions as being composed of a number of straight sections, as shown schematically in Figure 8. As can be seen from Figure 8, the bend in the guide introduces a change in angle of α. In the above example, B₂ =θ_x -α . However, the light may strike the upper and lower surfaces in the opposite order, leading to an increase in θ. Hence, in general,

θ,₊₁ =θ, ±α

Many straight sections, each at a constant angle to the next, will have an effective radius, as shown schematically in Figure 9.

To find this effective radius, it is sufficient to find the distance at which two lines, extending perpendicularly from the centre of adjacent straight sections, intersect, as shown in Figure 10, and as follows:

tan(α) = ^ y+x . tan i(α \) = — ^2δ

/ cos(α ) = cos(α ) = — r + δ 2x

Hence, δ = -tan(α) / X = -

2 cos(α )

Therefore, r =

After every reflection, a spread of α will be introduced to the angle of incidence θ. Only light incident on the boundary at less than the critical angle θ_c will be reflected; the remainder will escape and be lost from the waveguide. We wish to know the fraction of light which will remain within the fibre after each reflection.

Assuming circular symmetry, we can consider the area defined by the cross section of the cone associated with each angle, see Figure 11.

The ratio of light retained is given by the ratio of these two areas. The area of each surface is proportional to the square of the tangent of the corresponding angle. Hence, after one reflection,

where Ii is the intensity of light in section i of the bent waveguide.

All light outside of the cone defined by θ_c is lost and hence the maximum value of θ after light has been lost from the waveguide will be θ_c- This allows us to make the following replacement in the above equation: θ →θ_c +α which yields

As α is constant, this reduces to:

J₁₊I = w_here β = ( *5&i_ = constant tan(θ_c +α)

Observing that this describes of a constant proportion per section the above can be rearranged to give a simple differential equation.

I₁₊₁ = ^, => I₁₊₁ = I, -SI, where δ/, = /,(l - β )

Let c be the effective circumference of the collection of straight sections with effective radius r. c will be the sum of the length all straight sections and, for an increase of one complete section, δc = /

Hence, dl = l{l - $ )dc

The solution of which is

I = I₀ eχρ((l - β )c) = I₀ expf ^{~ c}/ c, I where c₀ = - — is the scale length o β - i for attenuation.

Given the refractive index for the materials involved it is trivial to calculate the critical angle for total internal reflection, θ_c. Hence, the only unknown in the above equation is the angle between straight sections, α. In order to calculate this we find the length I over which the bent waveguide appears straight to the propagating light. This can be estimated by considering the thickness of the waveguide and the critical angle, see Figure 12.

Hence, cos(θ ) = - but here we assume θ ~ a

. / » cos (α)

t(cθs(α ) + l) u- i- i i. ■ -ι- r - -

^{→ r =} — A ,\ which relates a given radius of curvature, r, to an 2cos(α)sin(α) incremental angle, α.

The following equations can be rearranged to give the characteristic attenuation length, C₀, as a function of radius of curvature, waveguide thickness and relevant refractive indices:

t(cos(α) + l) ^co = - r = β - i 2cos(α)sin(α)

(

^Kθ_c ^c )' = \ -n waveguide

It will be appreciated that regardless of whether the above geometrical model represents the most dominant form of attenuation, the approach of measuring deformation by attenuation will still be practicable.

In the above embodiments, the LED transmits light at infra-red wavelengths (i.e. not including visible wavelengths) and the photodiode is one that senses only infra-red wavelengths (i.e. not visible wavelengths). This provides an advantage that none of this light being used for sensing of the deformation of the substrate is visible to a user of the display, hence it does not impair the visual appearance of the image being displayed by the display; likewise any image changes do not affect the photodiode. However, in other embodiments, other LEDs and photodiodes may be used such that the deformation-sensing light may be entirely in the visible range, and/or partly visible and partly infra-red. As such, the term "light" as employed herein is to be understood to include visible radiation, infra-red radiation and indeed any other wavelength of radiation that can be employed to provide the above described measurement process based on bending of a waveguide through which the radiation can pass in a substantially totally internally reflected manner and subject to loss of the radiation from the totally internally reflected guided mode as a function of, or as an effect of, physical deformation of the waveguide.

This means that in some embodiments, where one of the substrates of the flexible display device is opaque to visible wavelength light (e.g. when in a reflective display device), the substrate can nevertheless be employed as described above for deformation sensing for user input by virtue of the substrate, for example, being transparent to infra-red wavelengths.

Also, any standard modulation techniques may be used for the LED and photodiode, for example the light being sensed may be pulsed.

Also, instead of LEDs and photodiodes as such, any other appropriate light emitting and/or light sensing components may be used. In the above embodiments the LED and the photodiode are both coupled to the input processor. However, in other embodiments, the LED need not be coupled to the processor that processes the photodiode output, i.e. the LED output may be separately controlled to be constant, or compensation or calibration may be performed by the processor based on the output of the photodiode without feedback from, or specific control of, the driving of the LED.

In the above embodiments, substantially all the light is totally internally reflected in the substrate/lightguide when the substrate is flat. However, in other embodiments, light may be lost from the substrate/lightguide when the substrate is flat, nevertheless the variations in amount lost when deformed compared to when flat may be sensed and used as user input. Furthermore, the present invention may be employed for displays where the substrate is never, or not necessarily ever, in a flat condition, rather the invention is employed by sensing different light levels for different deformations compared to a default, albeit non-flat, deformation condition, e.g. the display may typically be used in a curved or other non-flat condition.

Although in the above embodiments TFTs are used as the active devices of the matrix array, it will be appreciated that other active devices, for example semiconductor thin film diodes, may be used instead as is also described in the aforementioned papers by N D Young et al.

While in the above described embodiments polymer materials have been used to provide the substrates, a flexible metal foil could be used instead, for example.

In the above embodiments, the internally reflected light passes along the bottom substrate of the device (in the sense of the viewer viewing the display from above), or in the case of an attached lightguide, the lightguide is essentially attached to the bottom substrate of the device (in the sense of the viewer viewing the display from above). However, this need not be the case, and in other embodiments the LED(s) and photodiode(s) may be positioned at the top substrate of the device such that the internally reflected light passes along the top substrate of the device (in the sense of the viewer viewing the display from above), or in the case of an attached lightguide, the lightguide may be essentially attached to the top substrate of the device (in the sense of the viewer viewing the display from above); i.e. the physical deformation of the upper substrate is detected rather than that of the bottom substrate. In this case, the bottom substrate may be opaque. Furthermore, if desired, the physical deformation of each substrate may be detected separately or in a combined fashion by applying and sensing internally reflected light to both substrates.

Although the invention has been described with reference to active matrix liquid crystal display devices in particular, it will be appreciated that it can be applied to similar advantage in matrix display devices using different display materials, for example electroluminescent materials, and in other types of matrix array devices of the kind having an array of matrix elements that include an active semiconductor device. Examples of the latter include large area image sensor, using photosensitive pixels comprising a photodiode or the like, active matrix touch sensing arrays, thin film data stores and memory devices. These different types of thin film electronic array devices need only use one substrate. The embodiments described above are implemented in an active matrix liquid crystal display device (AMLCD), with a driving circuit as described in more detail below with reference to Figures 1 and 2. Nevertheless, it is to be appreciated that the invention may also be implemented with other active matrix circuitry compared to that shown in Figures 1 and 2, or indeed in other types of active matrix display such as electrophoretic displays or organic LED (OLED) displays which may use different forms of active matrix circuitry compared to that shown in Figures 1 and 2. Furthermore, the invention may be implemented in passive display devices which do not employ an active matrix circuit.

Claims

1. A flexible display device (1 ), comprising: a flexible display substrate (20); a light emitter (34) positioned to emit light (38), of one or more wavelengths in the wavelength range of visible and infra-red, into the flexible display substrate (20) via an edge of the flexible display substrate; and a light sensor (36) positioned to sense light (38) emitted from the light emitter (34) that exits the flexible display substrate (20) at an edge of the flexible display substrate having been internally reflected in the flexible display substrate (20); wherein the amount of the light (38) emitted from the light emitter (34) that reaches the light sensor (36) is dependent upon how much of the light (38) escapes from the flexible display substrate (20) dependent upon the physical deformation of the flexible display substrate (20).

2. A flexible display device according to claim 1 , wherein the light emitter (34) and the light sensor (36) are arranged such that the light (38) emitted from the light emitter (34) travels by means of the internal reflection across the substrate (20) over the course of plural circuits across the substrate prior to being sensed by the light sensor (36).

3. A flexible display device(1 ), comprising: a flexible display substrate (20); a flexible lightguide (50) attached to a surface of the flexible display substrate (20); a light emitter (34) positioned to emit light (38), of one or more wavelengths in the wavelength range of visible and infra-red, into the flexible lightguide (50) via an edge of the flexible lightguide (50); and a light sensor (36) positioned to sense light (38) emitted from the light emitter (34) that exits the flexible lightguide (50) at an edge of the flexible lightguide (50) having been internally reflected in the flexible lightguide (50); wherein the amount of the light (38) emitted from the light emitter (34) that reaches the light sensor (36) is dependent upon how much of the light (38) escapes from the flexible lightguide (50) dependent upon the physical deformation of the flexible display substrate (20).

4. A flexible display device according to claim 3, wherein the light emitter (34) and the light sensor (36) are arranged such that the light (38) emitted from the light emitter (34) travels by means of the internal reflection through the lightguide (50) over the course of plural circuits of the substrate (20) area prior to being sensed by the light sensor (36).

5. A flexible display device according to any of claims 1 to 4, further comprising one or more further light emitters (34) and one or more further light sensors (36) thereby providing a plurality of light emitters (34) and a corresponding plurality of light sensors (36) arranged such that: the light emitters (34) are positioned to emit light, of one or more wavelengths in the wavelength range of visible and infra-red, into the flexible display substrate (20) via one or more edges of the flexible display substrate (20), and the light sensors (36) are positioned to sense light emitted from respective light emitters (34) that exits the flexible display substrate (20) at a respective edge of the flexible display substrate (20) having been internally reflected in the flexible display substrate (20).

6. A flexible display device according to any of claims 1 to 5, wherein the light emitter (34) emits light of one or more wavelengths only in the infrared range.

7. A flexible display device according to any of claims 1 to 6, further comprising an input processor (30) coupled to the light sensor (36) for processing the level of light sensed by the light sensor or sensors to determine a user input provided by a user physically deforming the flexible display substrate (20).

8. A method of sensing user input to a flexible display device (1 ), wherein the flexible display device (1 ) comprises a flexible display substrate

(20); the method comprising: a light emitter (34) emitting light (38), of one or more wavelengths in the wavelength range of visible and infra-red, into the flexible display substrate

(20) via an edge of the flexible display substrate (20); and a light sensor (36) sensing light (38) emitted from the light emitter (34) exiting the flexible display substrate (20) at an edge of the flexible display substrate (20) having been internally reflected in the flexible display substrate

(20); wherein the amount of the light (38) emitted from the light emitter (34) that reaches the light sensor (36) is dependent upon how much of the light escapes from the flexible display substrate (20) dependent upon the physical deformation of the flexible display substrate (20).

9. A method according to claim 8, wherein the light emitter (34) and the light sensor (36) are arranged such that the light (38) emitted from the light emitter (34) travels by means of the internal reflection across the flexible display substrate (20) over the course of plural circuits across the flexible display substrate (20) prior to being sensed by the light sensor (36).

10. A method of sensing user input to a flexible display device (1 ), wherein the flexible display device (1 ) comprises a flexible display substrate

(20); the method comprising: a light emitter (34) emitting light (38), of one or more wavelengths in the wavelength range of visible and infra-red, into a flexible lightguide (50) attached to a surface of the flexible display substrate (20), via an edge of the flexible lightguide (50); and a light sensor (36) sensing light (38) emitted from the light emitter (34) exiting the flexible lightguide (50) at an edge of the flexible lightguide (50) having been internally reflected in the flexible lightguide (50); wherein the amount of the light (38) emitted from the light emitter (34) that reaches the light sensor (36) is dependent upon how much of the light (38) escapes from the flexible lightguide (50) dependent upon the physical deformation of the flexible display substrate (20).

11. A method according to claim 10, wherein the light emitter (34) and the light sensor (36) are arranged such that the light (38) emitted from the light emitter (34) travels by means of the internal reflection through the lightguide (50) over the course of plural circuits of the substrate area prior to being sensed by the light sensor (36).

12. A method according to any of claims 8 to 11 , further comprising using one or more further light emitters (34) and one or more further light sensors (36) thereby using a plurality of light emitters (34) and a corresponding plurality of light sensors (36) arranged such that: the light emitters (34) are positioned to emit light (38), of one or more wavelengths in the wavelength range of visible and infra-red, into the flexible display substrate (20) via one or more edges of the flexible display substrate

(20), and the light sensors (36) are positioned to sense light (38) emitted from respective light emitters (34) that exits the flexible display substrate (20) at a respective edge of the flexible display substrate (20) having been internally reflected in the flexible display substrate (20).

13. A method according to any of claims 8 to 12, wherein the light emitter (34) emits light (38) of one or more wavelengths only in the infrared range.

14. A method according to any of claims 8 to 13, further comprising using an input processor (30) coupled to the light sensor (36) for processing the level of light sensed by the light sensor (36) or sensors to determine a user input provided by a user physically deforming the flexible display substrate (20).