WO2005071644A1

WO2005071644A1 - Foil display

Info

Publication number: WO2005071644A1
Application number: PCT/IB2005/050206
Authority: WO
Inventors: Johannes Marra
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-01-21
Filing date: 2005-01-18
Publication date: 2005-08-04
Also published as: TW200539074A

Abstract

A dynamic foil display device comprises a back plate (4), a front plate (2), a flexible element (3). Row and column electrode means (5,6) are arranged to bring one or more portions of the flexible element (3) either in contact with the front plate (2) or out of contact with the front plate (2). The flexible element (3) comprises a light-reflecting layer (7). Thus, incoming light is reflected from the portions of the display device where the flexible element is in contact with the front plate.

Description

Foil display

TECHNICAL FIELD OF THE INVENTION The present invention relates to a dynamic foil display according to the preamble of claim 1.

BACKGROUND ART A conventional emissive foil display comprises a light guide in the form of an edge-lit glass plate, also known as a light guide plate, an active plate, or a back plate. It also comprises a non-lit front plate, also known as a passive plate. A light-scattering foil, also known as a flexible element, is clamped in between spacer elements disposed on these plates. The distribution of the spacer elements across the plates defines the size and location of the individual picture elements (pixels) of the foil display. A foil electrode layer is disposed on at least one side of the light-scattering foil for the purpose of enabling the application of a substantially uniform voltage to the foil, thereby making the foil electrode to serve as a common display electrode. On both plates there are respective sets of parallel electrodes which are arranged perpendicularly with respect to each other such as to constitute a set of row electrodes on the front plate and a set of column electrodes on the light guide plate. By application of suitable voltages to the foil electrode and to appropriate electrodes on the light guide plate and the front plate it is possible to generate two opposing electrostatic forces onto the foil with the force vectors directed towards the light guide plate and towards the front plate, respectively. The balancing of these two opposing electrostatic forces in combination with the elastic force existing in the foil is used to attract a portion of the foil associated with a single picture element towards either the light guide plate or towards the front plate. Typically, the foil can be locally attracted towards the light guide plate by applying a larger voltage difference between the foil electrode and the local column electrode on the light guide than between the foil electrode and the local row electrode on the front plate. Similarly, the foil can be locally attracted towards the front plate by applying a larger voltage difference between the foil electrode and the local row electrode on the front plate than between the foil electrode and the local column electrode on the light guide plate. When a portion of the light-scattering foil associated with a single picture element gets locally into contact with the light guide, light is locally decoupled from the light guide into the foil and is subsequently scattered away into all directions. The light that is locally scattered towards the front plate can reach the eye of a display viewer, thereby creating a localised bright picture element in the display. The light that is scattered towards the light guide is partially re-fed into the light guide, the remainder being directed away from the display viewer and therefore lost from visibility. When a portion of the foil associated with a single picture element on the other hand is in local contact with the front plate, no light is locally decoupled from the light guide and that portion of the foil therefore appears as a localised dark picture element. A dynamic foil display of this general emissive type is known from WO00/38163.

SUMMARY OF THE INVENTION However, there are cases when it is advantageous to have a dynamic foil display with reflective properties instead of emissive. This provides a reflective dynamic foil display comprising a plurality of localised picture elements that either reflect or absorb the ambient light that is incident upon the respective localised picture elements of the display. A reflective foil display does not rely on the presence of a light guide plate or on light sources that couple light into the light guide. Instead, only a thin back plate and the presence of ambient light suffice. A reflective foil display can therefore be made thinner and reduced in weight, and consumes significantly less electrical power than an emissive foil display. The latter characteristics enhance the portability of the display, allow more design freedom for creating a flexible display, and increase the applicability of the display under a variety of circumstances. It is an object of the present invention to fulfil the need for a reflective foil display, by disclosing a display device having the characterising features of independent claim 1, with preferred embodiments indicated in the dependent claims 2-14. In a display device according to the invention the flexible element comprises a light- reflecting layer. This has the effect that when a portion of the flexible element associated with a single picture element is in local contact with the front plate, the incoming light through the front plate is locally reflected back towards a viewer by virtue of this light- reflecting layer, and the portion appears illuminated to the viewer, thereby creating a bright picture element. The illuminated portion appears bright-white in case no light-absorbing materials and/or colour filters are present in the path of the incoming and reflected light. The illuminated portion appears coloured in case a colour filter is present in the path of the incoming light. The colour filter may be disposed on either the side of front plate facing the flexible element or on the side of the flexible element facing the front plate. Preferably, use is made of an absorptive non-scattering colour filter comprising a two-dimensional geometrical pattern of red, green and blue patches, the size of the patches corresponding with the size of the picture elements of the reflective foil display. By disposing light-scattering means between the said light-reflecting layer comprised in the flexible element and the side of the flexible element facing the front plate, the reflected light from the said light-reflecting layer becomes scattered across a wide angular range, thereby creating a wide viewing angle. By omitting the incorporation of light-scattering means in the flexible element and by substantially avoiding the presence of light-scattering means in the path of the incoming and reflected light, and by using a flexible element that comprises a specular- reflective light-reflecting layer, the locally reflected light from the flexible element becomes mirror-reflective. The inventive foil display can then be locally or entirely turned into a mirror by allowing mirror reflection of incident light to occur from all locally-present picture elements or from all display picture elements, respectively. Further, the flexible element is preferably surrounded by an essentially electrically-insulating light-absorbing fluid. This allows electric fields to be applied across the fluid and creates good optical contrast properties. When a portion of the flexible element associated with a picture element is in contact with the back plate, this picture element appears dark, since substantially none of the incoming light is reflected back from the portion towards the viewer. Instead, the incoming light is absorbed by the dark-coloured fluid that is present between the front plate and the flexible element. The light-absorbing fluid can comprise a dark-coloured dye that either constitutes the fluid or that is dissolved in the fluid. This is a simple and efficient way of providing a dark-coloured light-absorbing fluid. As an alternative, this solution can either be combined with or exchanged for a solution where the light-absorbing fluid comprises substantially non-scattering dark-coloured pigment particles. This also has the above advantages of simply and efficiently providing a dark-coloured light-absorbing fluid. If the light-absorbing fluid comprises dark-coloured pigment particles, these particles preferably have a diameter of approximately 5 - 50 nm, which gives a substantially non-scattering fluid with good light-absorbing properties. Preferred dark-coloured pigment particles comprise carbon-black particles. The flexible element is preferably fluid pervious, which makes it possible for the fluid to travel through the flexible element when portions thereof are moved into and out of contact with the front plate and the back plate, respectively. This gives the advantage that the fluid can quickly move from one side of the flexible element to the other and, thus, that local portions of the flexible element can be switched quickly into and out of contact with the front plate. The pervious property of the flexible element is preferably obtained by way of providing pores in the flexible element. This is a simple and efficient manner of providing a fluid pervious flexible element. The pores preferably have a diameter of at least 0.5 μm and are preferably present at a volume density that constitutes at least 0.5% of the volume of the flexible element so that the above mentioned particles do not clog the pores and that the fluid can easily flow through the flexible element. The pores are preferably straight which further enhances the above advantages of an efficient and fast fluid flow through the flexible element so that a fast switching of the individual picture elements can be accomplished. Further, the light-reflecting layer is preferably arranged on the side of the flexible element facing away from the front plate. This provides efficient back-reflection and scattering of the reflected light towards a display viewer by the light-scattering means provided to the flexible element. The light-reflecting layer preferably comprises a coating of aluminium which is an inexpensive and simple way of providing good reflective properties. By omitting the incorporation of light-scattering means in the flexible element, the presence of a specular- reflective coating of aluminium disposed on the flexible element allows the inventive foil display to be used as a mirror display. Preferably, the light-reflecting coating of aluminium on the flexible element also comprises the flexible element electrode with which an electric potential can be applied to the flexible element. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic cross-sectional view of a prior art display device, Fig. 2 is a schematic plan view of a display device according to the invention, Fig. 3a is a schematic cross-sectional view of a portion of the display in a first state, Fig. 3b is a schematic cross-sectional view of the portion in Fig. 1 in a second state, and Fig 4 is a cross-sectional, perspective, enlarged view of a flexible element.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION A reflective Dynamic Foil Display (DFD) of a general type is shown in Fig. 1. The display device 1 comprises a front plate 2, a flexible element or light-scattering foil 3 and a back plate 4. Electrode systems 5 and 6 are arranged, respectively, on the surfaces of the front plate 2 and the back plate 4 facing the flexible element 3. The electrodes 5 and 6 form two perpendicularly-oriented sets of electrodes which cross each other at an angle of preferably 90°, c.f. Fig. 2. Picture elements are present at each crossing of the electrodes 5 and 6 and are laterally bounded by adjacent spacer elements 11 disposed on each of the plates 2 and 4. By locally generating a potential difference between the electrodes 5 and the flexible element 3 on the one hand, and between the electrodes 6 and the flexible element 3 on the other hand, through the application of electric voltages to the electrodes 5, 6 and the flexible element 3, opposing attractive electrostatic forces are locally exerted on the flexible element 3 towards both the front plate 2 and the back plate 4, the balance of which will pull the flexible element 3 either against the front plate 2 or against the back plate 4. As shown in Fig. 2 the electrodes 5 and 6 form a passive matrix structure.

From a control unit 12, which comprises selection means, selection signals (electric voltages) are supplied to the electrodes. This set of selection signals determines the set of potentials on the electrodes 5 and 6. Furthermore, a separate voltage is applied to the flexible element 3.

By applying suitable potentials to the electrodes 5, 6 and the flexible element 3, the flexible element 3 can be actuated, in operation, to move towards and away from the electrodes 5 and

6, respectively, or vice-versa, at the location of the selected crossings of the electrodes 5 and 6. Electrodes 5 form column electrodes, i.e. electrodes extending in the "short" direction of a rectangular display, while electrodes 6 form the row or line electrodes, i.e. electrodes extending in the "long" direction of such a display. Referring now to Fig. 3a and 3b, showing a portion of the display device according to the invention that represents a single picture element, the flexible element 3 is provided with a light-reflecting coating 7, preferably of aluminium, which also serves as a flexible element electrode, on the side of the flexible element facing away from the front plate 2. Further, referring to Fig. 3 a and 3b, the flexible element 3 is surrounded by a dark-coloured, light-absorbing, electrically insulating fluid 8. The fluid 8 can be obtained by composing it from at least one electrically insulating dark-coloured dye, by dissolving at least one dark-coloured dye in the fluid 8 and/or by dispersing e.g. 5 - 50 nm-sized, non-scattering dark-coloured pigment particles, for instance carbon-black particles of high jetness, in the liquid 8. The fluid 8 must be electrically insulating in nature in order to allow electric fields to be imposed across the fluid with which the flexible element 3 can be moved either to or from contact with the plates 2 and/or 4, respectively. The flexible element 3 is pervious to the dark-coloured fluid 8. According to the shown embodiment, the flexible element 3 is provided with pores 9, c.f. Fig. 4. When the flexible element 3 is switched between the states shown in Figs 3 a and 3b, the pores 9 enable the fluid 8 to flow through the flexible element 3 and quickly be moved from one side of the flexible element 3 to the other side as the flexible element 3 moves from the front plate 2 to the back plate 4 and vice versa under influence of the voltages imposed on the electrodes 5 and 6 and on the flexible element electrode (the reflective layer) 7. The pores 9 in the flexible element 3 should have a diameter of at least 0.5 μm so that the above described carbon-black particles or any aggregates of these do not clog the pores 9. The pores 9 are preferably straight and are preferably present at a volume density that constitutes at least 0.5% of the volume of the flexible element 3 so that the fluid 8 can easily flow through them from one side of the flexible element 3 to the other side of the flexible element 3. By straight is in this case meant that the pores 9 extend through the flexible element substantially perpendicularly to the surface of the foil, as is shown in Fig. 4, which is a cross-sectional, perspective view of the pores 9 in the flexible element 3. In Fig. 3 a, the shown portion of the flexible element 3 is associated with a single picture element of the inventive dynamic foil display and is in a first state in contact with the back plate 4. Here, a maximum amount of dark-coloured fluid 8 is present between the flexible element 3 and the front plate 2. A light ray 10 incident on the front plate 2 traverses the front plate 2 and is absorbed in the light-absorbing fluid 8. Thus, a display viewer observing the front plate 2 sees the picture element associated with the portion of the flexible element 3 depicted in Fig. 3a as dark, as substantially no light is able to escape from this picture element of the dynamic foil display and can therefore not reach the eye of the display viewer. In Fig. 3b the shown portion of the flexible element 3 is associated with a single picture element of the inventive dynamic foil display and is now in a second state in which it is in contact with the front plate 2. A minimum amount, if any, of fluid 8 is in this case present between the flexible element 3 and the front plate 2. A light ray 10 incident on the front plate 2 traverses the front plate 2 and is scattered or reflected back towards a viewer observing the front plate 2 by the reflective coating 7 on the flexible element 3. Thus, the picture element of the dynamic foil display associated with the portion of the flexible element 3 shown in Fig. 3b appears bright to a viewer. Hence, a dynamic foil display device comprises a back plate (4), a front plate (2), a flexible element (3). Row and column electrode means (5, 6) are arranged to bring one or more portions of the flexible element (3) either in contact with the front plate (2) or out of contact with the front plate (2). The flexible element (3) comprises a light-reflecting layer (7). Thus, incoming light is reflected from the portions of the display device where the flexible element is in contact with the front plate. The protective scope of the invention is not limited to the embodiments shown. The invention resides in each and every novel characteristic and each and every combination of characteristic features. Moreover, reference numerals in the claims are not to be construed as limiting their protective scope.

Claims

CLAIMS:

1. A dynamic foil display device comprising a back plate (4), a front plate (2), a flexible element (3), and row and column electrode means (5, 6) to bring one or more portions of the flexible element (3) either in contact with the front plate (2) or out of contact with the front plate (2), characterized in that the flexible element (3) comprises a light- reflecting layer (7).

2. A dynamic foil display device according to claim 1, wherein the flexible element (3) is surrounded by an electrically insulating light-absorbing fluid (8).

3. A dynamic foil display device according to claim 2, wherein the light- absorbing fluid (8) comprises a dark-coloured dye.

4. A dynamic foil display device according to claim 2 or 3, wherein the light- absorbing fluid (8) comprises dispersed substantially non-scattering dark-coloured pigment particles.

5. A dynamic foil display device according to claim 4, wherein the pigment particles have a diameter of approximately 5 - 50 nm.

6. A dynamic foil display device according to claim 5, wherein the pigment particles comprise carbon-black particles.

7. A dynamic foil display device according to any of the preceding claims, wherein the flexible element (3) is fluid pervious.

8. A dynamic foil display device according to claim 7, wherein the flexible element (3) comprises pores (9).

9. A dynamic foil display device according to claim 8, wherein the pores (9) have a diameter of at least 0.5 μm.

10. A dynamic foil display device according to claim 8 or 9 wherein the pores constitute at least 0.5% of the volume of the flexible element (3).

11. A dynamic foil display device according to any of claims 8-10, wherein the pores (9) are straight.

12. A dynamic foil display device according to any of the preceding claims, wherein the light-reflecting layer (7) is arranged on the side of the flexible element (3) facing away from the front plate (2).

13. A dynamic foil display device according to any of the preceding claims, wherein the light-reflecting layer (7) comprises a coating of aluminium.

14. A dynamic foil display device according to any of the preceding claims, wherein the light-reflecting layer (7) comprises a flexible element electrode layer.