WO2005050604A1

WO2005050604A1 - Display device

Info

Publication number: WO2005050604A1
Application number: PCT/IB2004/052371
Authority: WO
Inventors: Ramon P. Gorkom
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-11-20
Filing date: 2004-11-10
Publication date: 2005-06-02
Also published as: TW200533954A

Abstract

A display device (30) having a light guide (32) optically coupled to a light source (46), a passive plate (34), a first electrode layer (40) arranged on the passive plate, a second electrode layer (38; 38') arranged on the light guide, and an electromechanically operable foil (36) provided with a third electrode layer (42) arranged between the light guide and the passive plate. The third electrode layer (42) is arranged on the side of the foil (36) facing said passive plate (34). The foil (36) is further provided with a fourth electrode layer (44; 44') on the opposite side of the foil (36) with respect to the third electrode layer (42). The electrode layers (38; 38', 40, 42, 44; 44') are arranged to induce electrostatic forces on the foil (36) and to bring selected portions of the foil into contact with the light guide (32), thereby extracting light from the light guide. Because of the fourth electrode layer on the foil, more freedom is given to locally vary the force towards the light guide and the force towards the passive plate, whereby a more effective display device may be achieved.

Description

Display device

The present invention relates to a display device having a light guide optically coupled to a light source, a passive plate facing the light guide, and a movable element arranged between the light guide and the passive plate. The display device further comprises a first electrode layer arranged on the passive plate, a second electrode layer arranged on said light guide, and a third electrode layer arranged on the side of said movable element facing the passive plate.

. A display of the above kind is normally referred to as a foil display, and is described in for example WO 00/38163. Such a display is shown in fig. 1, and comprises a light guide plate 10 and a non-lit plate 12, with a scattering foil 14 clamped in between. On both plates there are respective sets of parallel electrodes 16, 18 which are arranged perpendicularly with respect to each other. Also the foil is provided with an electrode layer 20. By application of voltages to appropriate electrodes on the light guide, the non-lit plate and the foil, the foil may be attracted to the light guide. When the foil is brought into contact with the light guide, light is extracted from the light guide. In order to minimize absorption, the light guide is made relatively thick, so as to reduce the number of reflections by the light guide surfaces. This means that the amount of light that can be extracted from the light guide per unit length, which is proportional to the number of times each light ray is reflected, is relatively small. Therefore, in order to reach sufficient brightness, a sub-frame addressing scheme is used, making use of the bi-stability of the foil. However, the control of the bi-stable switching in a conventional foil display may be difficult, as the switching curves are not everywhere the same. This is shown in fig. 2, wherein the ON-switching curve is denoted 22, the OFF-switching curve is denoted 24, and the bi-stable region is denoted 26. The lines along the switching curves represents one time the standard deviation. Certain pixels will have switching curves that are shifted even more, and these pixels will consequently not switch correctly. This may cause certain pixels to remain ON and certain pixels to remain OFF, thus causing the display to malfunction. An object of the present invention is to provide a foil display which makes more efficient use of the bi-stable region.

This and other objects are achieved by a device of the kind mentioned by way of introduction, wherein the movable element further is provided with a fourth electrode layer on the opposite side of the movable element with respect to the third electrode layer. Thus, the movable element is provided with two separate electrode layers, one layer on each side of the element. The electrode layers of the display are arranged to induce electrostatic forces on the movable element and to bring selected portions of the element into contact with the light guide, thereby extracting light from the light guide. The invention is based on the understanding that by providing the movable member, for example a foil, with a fourth electrode layer, more freedom is given to locally vary the force towards the light guide, and the force towards the passive plate. This allows a more flexible positioning of the switching points, whereby the switching points may be positioned in order to avoid that certain pixels remains ON and certain pixels remains OFF because of spread in the switching curves. According to a first embodiment of the invention, two of the electrode layers are structured into a first and a second set of parallel electrodes respectively. Preferably, the electrodes of the first set are orthogonal to the electrodes of the second set, thus defining intersections used to address individual pixels of the display device. Both of the structured electrode layers are preferably arranged on one side of the display with respect to the movable element. For example the electrode layer on the passive plate, and the electrode layer on the side of the movable element facing the passive plate, i.e. the first and third electrode layers, may be structured into sets of electrodes. In this case, the electrode layer on the light guide and the electrode layer on the side of the movable element facing the light guide, i.e. the second and fourth electrode layers, may be unstructured. This allows a constant voltage difference between the unstructured second electrode layer of the light guide and the unstructured fourth electrode layer of the foil. An advantage with this electrode arrangement is that it enables a constant force towards the active plate, whereby the switching is carried out using the structured electrodes to change the force towards the passive plate. Thus, all four switching points may be placed on a horizontal line, whereby it becomes easier to position the switching points inside or outside of the bi-stable region. Also, switching voltages may be increased to ensure that each pixel acts correctly upon the switching instructions. . Alternatively, the electrode layer layout may be reversed, i.e. the first and third electrode layers are unstructured, and the second and fourth electrodes are structured into sets of parallel electrodes. In this case, a constant voltage level is set between the unstructured passive plate electrode layer and the unstructured electrode layer on the side of the movable element facing the passive plate, i.e. there is a constant force towards the passive plate. The addressing is carried out using the electrodes on the other side of the movable element and the electrodes on the light guide in order to change the force towards the light guide and thus bring selected parts of the movable element into contact with the light guide. According to a second embodiment of the present invention, each of the first, second, third and fourth electrode layer is structured into sets of parallel electrodes. Thus, in this embodiment, structured electrode layers are provided on both sides on the foil. The sets of electrodes are preferably arranged so that the electrodes of the first and third layers, and the electrodes of the second and fourth layers, are orthogonal in pairs. Thus, the electrodes of the electrode layers on one side of the display in relation to the movable element are orthogonal to each other, and the electrodes of the layers on the other side of the display are orthogonal to each other. Preferably, the electrodes of the two foil electrode layers may be arranged in one direction, and the electrodes of the passive plate and the light guide layers may be arranged in another direction, which is perpendicular to the direction of the foil electrodes. An advantage with this arrangement is that the controllability of the positioning of the switching points is increased. The four switching points may be placed on the four corners of an arbitrary oriented parallelogram. Preferably, all switching points are positioned on a single line. This is a very flexible way to use the bi-stable region, and the switching voltages may further be increased to ensure that each pixel acts correctly upon the switching instructions and not incorrectly remains ON or OFF. According to a third embodiment of the invention, any three of the electrode layers are structured into sets of electrodes. Thus, one electrode layer may be unstructured. This embodiment also enables increased freedom in selecting forces compared to known foil displays. In a further embodiment of the invention, the electrode layer of the light guide, i.e. the second electrode layer, is arranged on the opposite side of the light guide with respect to the passive plate. This may enhance the contrast and light decoupling properties of the display.

Currently preferred embodiments of the invention will now be further described in reference to the accompanying drawings wherein: Fig. 1 is a schematic cross section of a display according to prior art. Fig. 2 illustrates the switching curves according to prior art. Fig. 3 is an exploded view of a display according to a first embodiment to the invention. Fig. 4 is an exploded view of a display according to a second embodiment to the invention. Fig. 5a and 5b illustrate the switching curves according to the embodiments in fig. 3 and fig. 4 respectively.

Figs. 3-4 schematically show a foil display according to two different embodiments of the present invention. Identical reference numerals have been used for corresponding elements of the device. The display 30 in figs. 3-4 comprises a light guide (active plate) 32 and a passive plate 34. The active plate 32 and the passive plate 34 are preferably made by glass. An electromechanically operated foil 36 is further clamed in between the active plate 32 and the passive plate 34. The foil 36 may for example be of a flexible light scattering material, such as parylene. Spacers 39, 41 are arranged on each side of the foil 36 to separate it from the active plate 32 and the passive plate 34. The display device further comprises two electrode layers, one layer 38 on the active plate 32, and one layer 40 on the passive plate 34. Also, the foil 36 according to the invention is provided with two electrode layers 42, 44, one layer on each side of the foil. All electrode layers may be formed by ITO layers disposed on the mentioned surfaces. Upon operation of the display, light from a light source, such as a LED 46, is coupled into the active plate 32. The light is confined inside the active plate by total internal reflection. Light may be extracted from the active plate 32 by bringing the foil 36 into contact with the active plate by means of applying appropriate voltages to the electrode layers 38, 40, 42, and 44, as will be further described below. Fig. 3 schematically shows the electrode arrangement according to a first embodiment of the invention. In fig. 3, the electrode layer 40 on the passive plate 34 contains a first set of parallel electrodes 48 (column electrodes), and the electrode layer 42 on the foil facing the passive plate contains a second set of parallel electrodes 50 (row electrodes), perpendicular with respect to the first set. The crossings of the electrodes of each set define the pixels of the display. The electrodes may for example be formed as strips, but may also be structured at the pixel level, e.g. become wider at a pixel and smaller in between pixels. The electrode layer 38 on the active plate 32, and the electrode layer 44 on the other side of the foil 36 are unstructured, i.e. not divided into smaller electrodes. Fig. 5a show the switching curves corresponding to the electrode arrangement in fig. 3. V_ac_ti_ve is the voltage difference between the active plate electrode layer 38 and the unstructured electrode 44 of the foil 36, and V_passive is the voltage difference between the foil electrode layer 42 and the passive plate electrode layer 40. All pixels are initially put in an OFF state by an robust OFF action. The robust OFF action may for example be obtained by putting V_active = 0 V and V_passive = 60 V (position 72 in fig. 5a) so that a switching point is obtained in the OFF region 71. The voltage difference V_active is then kept at a constant potential 70 (for example 35 V) during the addressing, resulting in a constant force towards the active plate. Next, all row electrodes 50 are held at a raised potential (for example 12 V), and all column electrodes 48 are held at a lowered potential (for example -28 V). The voltage difference Vpassive between the foil and the passive plate electrode layers 42, 40 (position 74 in fig. 5a) is in the bi-stable region 76, whereby the pixels remain in their current state, hence OFF. The row electrode 50 of a row to be addressed is then set to zero potential during a row pulse, which decreases the voltage difference V_passive (position 78 in fig. 5a). Thus, the force towards the passive plate exerted on this row is decreased. Next, the column electrodes 48 of pixels that should emit light are set to an increased voltage potential (for example -16 V), whereby the voltage difference V_paSsive is further decreased to a position 80 in the ON region 81 outside of the bi-stable region 76. This results in that the foil in these places due to the constant force towards the active plate will be attracted to the active plate, whereby the pixels are switched ON, i.e. light is emitted from the active plate at these selected pixels. After this ON-switching of selected pixels, the row electrodes 50 are again set to a raised potential (for example 12 V), thereby again increasing the voltage difference V_passi_ve (position 82 in fig. 5a). Position 82 is in the bi-stable region, whereby pixels switched ON remain in this state, hence ON. The pixels will remain in the ON-state for as long as light generation is desired. Finally, a robust OFF action may again be applied and all pixels are then put in the OFF -state. As mentioned above, the voltage difference V_acti_Ve is constant, whereby the positions 74, 78, 80, 72 are located on a horizontal line in fig. 5a. Fig. 4 schematically show the electrode arrangement according to a second embodiment of the invention. In fig. 4, the electrode layer 40 on the passive plate 34 again contains a first set of parallel electrodes 48, and the electrode layer 42 on the foil facing the passive plate contains a second set of parallel electrodes 50. Additionally, the electrode layer 38' on the active plate 32 contains a third set of parallel electrodes 52, and the electrode layer 44' on the other side of the foil 36 contains a fourth set of parallel electrodes 54. Thus, in this second embodiment, all electrode layers are structured. The electrodes 48, 52 of the first and the third set are further arranged in the same direction, and the electrodes of the second and the fourth set 50, 54, i.e. the foil electrodes, are arranged perpendicular with respect to the electrodes of the first and third set. Fig. 5b shows the switching curves corresponding to the electrode arrangement in fig. 4. All pixels are initially put in an OFF state (position 90 in fig. 5b) by an robust OFF action . Then, all electrodes 50 are held at a raised potential (for example 14 V), and all passive plate electrodes 48 are held at a lowered potential (for example -28 V). Also, all electrodes 54 are held at a raised potential (for example 4 V), and all active plate electrodes 52 are held at a lowered potential (for example -35 V). The voltage difference Vp_assive between the first foil electrode layer 42 and the passive plate electrode layer 40, and the voltage difference V_aotive between the second foil electrode layer 44' and the active plate electrode layer 38' corresponds to a position 92 in fig. 5b. Position 92 is in the bi-stable region 94, whereby the pixels remain in their current state, hence OFF. A row may then be selected by setting a corresponding electrode 50 to zero potential, which decreases the voltage difference V_passivej here by 14 V. At the same time, the voltage difference V_acti_Ve is decreased, here by 4 V, by setting the electrodes 54 to zero potential. Thus, the force towards the passive plate is decreased more than the force towards the active plate, whereby the pixels of the selected row due to the resulting change of force are attracted more towards the light guide although still remaining in the bi-stable region (position 96 in fig. 5b). Next, pixels in a selected row may be switched ON, i.e. moved to the position 98 outside the bi-stable region in fig. 5b, by increasing the voltage levels of the electrodes 48 and 44' (to a voltage level of for example -14 V and -31 V respectively), thereby further decreasing the voltage differences V_paSsive and Vactive for selected pixels. Again, Vpassive decreases more than V_aotive. After this ON-switching of selected pixels, the electrodes 50, 54 are again set to a raised potential (for example 14 V and 4 V respectively), thereby increasing the voltage differences V_paSsive and V_active (position 100 in fig. 5b). Position 100 is in the bi-stable region, whereby pixels switched ON remain in this state, hence ON. The pixels may remain in the ON-state for as long as light generation is desired. Finally, a robust OFF action may again be applied in order to put all pixels in the OFF-state. As may be seen in fig. 5b, according to this embodiment of the inventive electrode arrangement the switching points may be positioned on one line which tilts downwards to the left. This enables an increased distance between switching points 96, 100 in the bi-stable region, and switching point 98 in the ON region, thus reducing the risk of erroneous switching. In the above example, the switching voltages was increased compared to the switching voltages of the first embodiment (from 12 V to 14 V), and increased even more compared to the switching voltages of prior art displays. These higher switching voltages indicate larger movements in the switching curve diagrams which increases the likelihood that each pixel acts correctly upon the switching instructions. It should be noted that the switching voltages that should be applied in the above examples also are dependant on for example pixel dimension, layer thickness, and the foil material and thickness. The invention is not limited to the embodiments described above. Those skilled in the art will recognize that variations and modifications can be made without departing from the scope of the invention as claimed in the accompanying claims. For example, it may be possible to perform OFF switching of single rows or single pixels as an alternative to the robust OFF action discussed above.

Claims

CLAIMS:

1. A display device (30) comprising: a light guide (32) optically coupled to a light source (46), a passive plate (34) facing the light guide, a movable element (36) arranged between said light guide and said passive plate, a first electrode layer (40) arranged on said passive plate (34), a second electrode layer (38; 38')arranged on said light guide (32), and a third electrode layer (42) arranged on the side of said movable element (36) facing said passive plate (34), characterized in that said movable element (36) further is provided with a fourth electrode layer

(44; 44') on the opposite side of the movable element with respect to said third electrode layer (42), said electrode layers (38; 38', 40, 42, 44; 44') being arranged to induce electrostatic forces on the element (36) and to bring selected portions of the element into contact with the light guide (32), thereby extracting light from the light guide.

2. A display device according to claim 1, wherein at least two of said electrode layers (40, 42) are structured into first and second sets of parallel electrodes (48, 50) respectively.

3. A display device according to claim 2, wherein the electrodes of said first set (48) are orthogonal to the electrodes of said second set (50).

4. A display device according to claim 3, wherein said structured electrode layers (40, 42) are arranged on the same side of the display device (30) with respect to the movable element (36).

5. A display device according to claim 4, wherein said first set of parallel electrodes (48) is formed by said first electrode layer (40), and said second set of parallel electrodes (50) is formed by said third electrode layer (42).

6. A display device according to any one of claims 1 -5, wherein at least three of said electrode layers are structure into sets of parallel electrodes.

7. A display device according to any one of claims 1-6, wherein all electrode layers (38', 40, 42, 44') are structured into sets of parallel electrodes.

8. A display device according to claim 7, wherein the electrodes (48, 50) of said first and third structured electrode layers (40, 42), and the electrodes (52, 54) of said second and fourth structured electrode layers (38', 44'), are orthogonal in pairs.

9. A display device according to any one of the preceding claims, wherein said second electrode layer (38, 38') is arranged on the opposite side of the light guide with respect to the passive plate.