WO2005050609A2

WO2005050609A2 - Foil display addressing

Info

Publication number: WO2005050609A2
Application number: PCT/IB2004/052446
Authority: WO
Inventors: Volker Schoellmann; Peter A. Duine
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-11-20
Filing date: 2004-11-16
Publication date: 2005-06-02
Also published as: TW200519806A; WO2005050609A3

Abstract

A method for addressing a foil display, comprising applying (S2) voltages to all column electrodes to bring the entire foil into contact with the light guide, applying (S3) voltages to all row electrodes without bringing the foil out of contact with the light guide for each line of said display, providing (S4) a corresponding row electrode with a voltage increasing the electrostatic force towards the light guide, and applying (S5) to selected column electrodes a voltage that reduces the electrostatic force towards the passive plate, thereby bringing selected pixels along this line out of contact with said light guide and then activating (S6) the light source. According to the inventive method, the ON-state of a pixel is reached by switching the foil from an intermediate position, between the light guide and the passive plate, to the active plate.

Description

Foil display addressing

The present invention relates to a method for addressing a display of the kind having a light guide optically connected to a light source, an electrically conducting movable foil adapted to locally be brought into contact with the light guide to thereby extract light from the display, a first set of electrodes for creating a first electrostatic force, forcing said foil towards the light guide, and a second set of electrodes, for creating a second electrostatic force, forcing said foil away from the light guide. The invention also relates to such a display being adapted to be addressed in this way.

Such a display is referred to as a foil display, known from e.g. WO 00/38163, and its operation is based on the local extraction of light from the light guide by means of the scattering foil clamped between the light guide and the passive plate. The movement of the foil within each pixel can be controlled by means of voltages applied to the electrodes, which typically comprise column electrodes on the light guide, row electrodes on the passive plate and a conducting electrode layer on the foil. The electrodes create two electrostatic fields that exert forces on the foil, one towards the light guide, and one toward the passive plate. When the foil is brought into contact with the light guide, it will frustrate the total internal reflection of the light guide, and light will be extracted. The switching curves of a pixel element of such a display are shown in fig 1 , where the x-axis represents the potential difference between the foil electrode and the row electrodes on the passive plate, and the y-axis represents the potential difference between the foil electrode and the column electrodes on the light guide. Indirectly, these potential differences represent the forces exerted on the foil away from and towards the light guide respectively. The bi-stable region 1, between the ON-curve 2 and the OFF-curve 3, creates a memory effect in the pixel element. This memory effect makes it possible to use a passive matrix addressing method to drive the display. Conventionally, the foil display is addressed by first switching all pixels OFF (position 4), and then the force towards the passive plate is reduced by lowering the row voltage on all rows (position 5). Then, one row at a time is selected by further reducing the row voltage (position 6). From this position it is now possible to selectively turn ON pixels in the selected row by choosing a larger column voltage, i.e. increasing the force towards the light guide (position 7). Depending on the column data during the selection pulses the pixels in the selected row are switched ON, or remain in the OFF state. Due to the bistability of the pixels, the state of pixels in unselected rows (in position 5) is not changed during application of column data. In this way it is possible to write row by row information to the foil display, and then, when addressing is completed, activate the light source connected to the light guide. There are various addressing schemes for the foil display that in principle follow the above addressing strategy. In these schemes the entire display is ON addressed* row by row, and after an ON period the display is then OFF addressed. Grey scales are achieved through suitable pulse width modulation of the ON periods. Note that with this technique, the switching of the foil from the passive plate to the active plate occurs in the presence of electrostatic forces both towards the light guide and towards the passive plate (from position 6 to 7 in fig. 1). As a result the foil is initially attached to the passive plate, is subsequently released from the passive plate as a result of the voltage pulses on column and row electrode, and finally lands on the light guide. As a result of this switching there are complex movements and oscillations of the foil on the active plate, which lead to the accumulation of local tribo-electrically induced charges. Local charges have a dramatic effect on the switching characteristics of a foil display pixel. The presence of local charge acts to decrease the addressing margins for the passive matrix addressing approach and consequently cause pixel errors and inhomogenieties in the overall display performance. These local charging phenomena have been primarily observed on the light guide, and experiments have shown that the charge accumulation is closely linked to the foil dynamics during the "landing" of the foil on the active plate.

It is therefore an object with the present invention to provide an improved method for addressing a foil display, eliminating, or at least reducing the above mentioned charge build-up.

This and other objects are achieved by a method of the kind mentioned by way of introduction, comprising applying voltages to said first set to exert said first electrostatic force, thereby bringing the entire foil into contact with the light guide, applying voltages to said second set to exert said second electrostatic force, without bringing the foil out of contact with the light guide, for each line of said display, providing a corresponding electrode in said second set with a voltage increasing said second electrostatic force along this line, and applying to selected electrodes in said first set a voltage that reduces said first electrostatic force, thereby bringing selected pixels along this line out of contact with said light guide, and activating said light source. According to the inventive method, the ON-state of a pixel is reached by switching the foil from an intermediate position, between the light guide and the passive plate, to the active plate. This is accomplished by first applying an essentially zero potential difference between the foil and the second set of electrodes (typically the row electrodes), thus canceling any force towards the passive plate, and then switching the foil to the light guide by applying a voltage difference between the first set of electrodes (typically column electrodes) and the foil electrode. When all pixels thus have reached the ON state, pixels are selectively switched OFF, by addressing the first and second sets of electrodes. Experiments have shown that the accumulation of local charge in foil display pixels is avoided when switching the foil from the intermediate foil position to the light guide. Neither does the selective OFF switching generate any accumulation of charge. The method can further comprise applying voltages to the first set essentially removing the first electrostatic force, thereby bringing the entire foil out of contact with the light guide. This represents a robust OFF action, to switch all pixels OFF at the end of a light generation period. The method can further comprise applying voltages to the second set of electrodes for essentially removing also the second electrostatic force. This effectively places all pixels in an intermediate state, between the light guide and a passive plate on its other side. The method can be implemented by a driver in a display device, thereby providing an improved foil display. The first and second electrode sets are preferably arranged on the light guide and a passive plate on the other side of the foil. In this case, the foil is provided with an unstructured electrode (i.e. common for all pixels), which can be grounded. Alternatively, the foil is provided with a structured electrode, forming one of the two electrode sets. In a preferred embodiment, the method is applied to a foil display having column electrodes on the light guide and row electrodes on the passive plate, and the foil electrode grounded. The method can then be described as applying voltages to all column electrodes to bring the entire foil into contact with the light guide, applying voltages to all row electrodes, without bringing the foil out of contact with the light guide, for each line of said display, providing a corresponding row electrode with a voltage increasing the electrostatic force towards the light guide, and applying to selected column electrodes a voltage that reduces the electrostatic force towards the passive plate, thereby bringing selected pixels along this line out of contact with said light guide, and then activating the light source.

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention. Fig. 1 is a diagram of switching curves of a pixel in a foil display. Fig. 2 is a schematic expanded perspective view of a foil display. Fig. 3 is a diagram of switching curves similar to fig 1. Fig. 4 is a flow chart of the method according to the invention. Fig. 5a is a diagram showing the accumulation of charge according to prior art. Fig. 5b is a diagram showing the absence of charge accumulation according to the invention. Fig. 6 is a timeline showing a sub-field addressing scheme implementing the method according to the invention.

Fig. 2 shows schematically a foil display to which the present invention is applicable. The display comprises a light guide 11, optically connected to a light source 12, a passive plate 13 (as opposed to the active light guide), and a flexible element 14, referred to as a foil, sandwiched in between the two plates. The light guide and passive plate are typically made of glass, while the foil can be made of parylene. The light guide is provided with a first set of parallel electrodes 15, here arranged as column electrodes, and the passive plate is provided with a second set of electrodes 16, here arranged as row electrodes. Further, the foil is provided, on one of its sides, with an unstructured electrode layer 17, i.e. a ITO layer covering the entire foil surface. A column driver 18 is connected to the column electrodes 15, and a row driver 19 is connected to the row electrodes 16, while the foil electrode 17 is connected to ground. Spacers 20, 21, are arranged between the light guide and the foil, and the passive plate and the foil, respectively. Typically, a colour filter (not shown) is arranged on the passive plate 13. The spacers are adapted to hold the foil at a distance from the light guide and the passive plate, while the drivers 18, 19 are used to apply voltages to the row and column electrodes in such a way as to exert electrostatic forces on the foil, and to thereby move selected portions of the foil in and out of contact with the light guide. Every time the foil is brought into contact with the light guide, light is extracted from the light guide, and allowed to exit from the display. The light guide can be placed as a front plate, in which case the light extracted from the light guide exits directly out of the display, or be placed as a back plate behind the passive plate, in which case the light extracted from the light guide exits the display via the foil and the passive plate, which thus both must be transparent. Fig. 3 shows the switching curves of a pixel of the display in fig. 2, similar to fig. 1. Again, the x-axis represents the potential difference between the foil electrode 17 and the row electrodes 16 (on the passive plate), while the y-axis represents the potential difference between the foil electrode 17 and the column electrodes 15 (on the light guide). In the first region 24 (the ON region), the foil is brought into contact with the light guide, and light is extracted. In the second region 25 (the OFF region), the foil is brought out of contact with the light guide (typically brought into contact with the passive plate), and no light is extracted. In the first intermediate region 26, the bi-stable region, the foil will remain in its previous state, i.e. in contact with either the light guide or the passive plate. Finally, in the second intermediate region 27, the foil will be held in between the two plates. According to the invention, a foil display, for example the one shown in fig. 2, is addressed as illustrated in fig. 4. The process begins in step SI by essentially removing all potential differences between the electrodes, i.e. applying a voltage close to the foil electrode voltage (here grounded) to the first and second electrode sets. This places all pixels in a position 31 located in the second intermediate region 27. As a result, the entire foil 14 is held between the plates 11, 13, without making contact with any plate. The light source 12 is inactive. Then, in step S2, a negative voltage, V_COI,_ON, is applied to all electrodes in the first set 15 (the column electrodes), thereby moving all pixels to a position 32 located in the ON region 24. This step is referred to as a "robust-ON" step. In step S3, a negative voltage, V_r0 ,unsei, is applied to all electrodes in the second set 16 (the row electrodes), thereby placing all pixels in a position 33 in the by-stable region 26, while maintaining the foil 14 in contact with the light guide 11. In step S4, one of the row electrodes is provided with an even lower voltage, V_row,sei_OFF, moving the pixels in the corresponding display line to a position 34 in fig. 3. Then, in step S5, the column electrodes are provided with addressing data, either V^_ON, as previously applied, or V_COI,O_FF, which is chosen to bring a pixel to position 35 in the OFF region 25, thus switching this pixel OFF. Note the relationship between positions 33 and 34. When the voltages on the column electrodes are changed in step S5, to switch the pixels in the selected row (in position 33) OFF, pixels in other rows (in position 34) only move to position 36 and remain in the bi-stable region 27. The steps S4 and S5 are repeated for each line of the display, so that the entire display is addressed. Then, in step S6, the light source is activated, and the display emits light in accordance with the addressing data. After a suitable light emitting period, which depends on the applied addressing scheme (e.g. binary weighted sub-fields), the light source is turned off. In step S7, the voltage of the column electrodes 15 is set essentially equal to the foil electrode voltage (here grounded), to thereby place all pixels in position 37 in fig. 3, located in the OFF region 25. This step is referred to as a "robust OFF" action. From position 37, the pixels are returned to position 31 in step SI by essentially removing also the potential difference between the foil and the row electrodes. In fig. 5 the charge build up in a foil display 6" demo panel is shown to demonstrate the charge accumulation for two different addressing schemes. The plot shown in fig. 5a illustrates the build up of the charge as a result of conventional switching, i.e. where the foil is switched from the passive to the active plate. The average charge for about 200 pixels is plotted as a function of time. It can be seen that the absolute charge increases continuously, and after 16 hours reaches a value of about -15 V. In fig. 5b the charge build up is shown for an addressing scheme according to the invention, where the foil is switched from the intermediate position to the active plate. It is apparent that the charge does not increase as a result of these switching events. The method according to the invention can be implemented in conventional sub-field addressing schemes, which is illustrated in fig. 6. The top time line relates to the addressing of a number of rows, and the different steps of fig. 4 are indicated. The bottom time line shows how the activation of the light source 12 is synchronized with the addressing. The different lengths of the light emitting periods 41 and 42 indicate different weights of the sub-fields. The above description is related to a dynamic foil display of the kind where structured electrodes are arranged on the light guide and passive plate, while the foil is provided with an unstructured electrode layer. Further, the electrodes on the light guide are oriented vertically, and have thus been referred to as column electrodes. The electrodes on the passive plate are oriented horizontally, and have consequently been referred to as row electrodes. This design should by no means be considered to be limiting for the present invention. On the contrary, several alternative designs are possible, for example by letting the row and column electrodes trade places. Further, it is possible, and sometimes preferred, to provide the foil with a structured electrode, and/or to have several electrode layers on the foil. The light guide and/or passive plate may also be provided with several electrode layers (structured or unstructured), in order to provide improved or alternative addressing possibilities.

Claims

CLAIMS:

1. A display device comprising: a light guide (11) optically connected to a light source (12), an electrically conducting movable foil (14) adapted to locally be brought into contact with the light guide to thereby extract light from the display, a first set of electrodes (15) for creating a first electrostatic force (FI), forcing said foil towards the light guide, and a second set of electrodes (16), for creating a second electrostatic force (F2), forcing said foil away from the light guide, and driver means (18, 19) adapted to: apply voltages to said first set (15) to exert said first electrostatic force (FI), thereby bringing the entire foil (14) into contact with the light guide (1 1), apply voltages to said second set (16) to exert said second electrostatic force (F2), without bringing the foil (14) out of contact with the light guide (11), for each line of said display, providing a corresponding electrode in said second set (15) with a voltage increasing said second electrostatic force (F2) along this line, and providing to selected electrodes in said first set (16) a voltage reducing said first electrostatic force (FI), thereby bringing selected pixels along this line out of contact with said light guide, and activate said light source (12)

2. A display device according to claim 1 , wherein said driver is further adapted to apply voltages to said first set (15) essentially removing said first electrostatic force (FI), thereby bringing the entire foil (14) out of contact with the light guide (1 1).

3. A display device according to claim 2, wherein said driver is further adapted to apply voltages to said second set of electrodes for essentially removing said second electrostatic force (F2).

4. A display device according to any one of the preceding claims, wherein said first set of electrodes (15) is arranged on the light guide (11).

5. A display device according to any one of the preceding claims, wherein said second set of electrodes is arranged on a passive plate (13) arranged on the other side of said foil (14) in relation to the light guide (11).

6. A display device according to any one of the preceding claims, wherein said sets (15, 16) comprise parallel electrodes, and wherein said first set is orthogonal to said second set.

7. A display device according to claim 6, wherein the electrodes in said first set of electrodes (15) are oriented vertically (column electrodes), and the electrodes in said second set of electrodes (16) are oriented horizontally (row electrodes).

8. A display device according to any one of the preceding claims, wherein the foil (14) is provided with an unstructured electrode (17).

9. A display device according to claim 8, wherein said foil electrode (17) is grounded.

10. A method for addressing a display of the kind having a light guide (11) optically connected to a light source (12), an electrically conducting movable foil (14) adapted to locally be brought into contact with the light guide to thereby extract light from the display, a first set of electrodes (15) for creating a first electrostatic force (FI), forcing said foil towards the light guide, and a second set of electrodes (16), for creating a second electrostatic force (F2), forcing said foil away from the light guide, said method comprising: applying (S2) voltages to said first set (15) to exert said first electrostatic force (FI), thereby bringing the entire foil (14) into contact with the light guide (1 1), applying (S3) voltages to said second set (16) to exert said second electrostatic force (F2), without bringing the foil (14) out of contact with the light guide (1 1), for each line of said display, providing (S4) a corresponding electrode in said second set (15) with a voltage increasing said second electrostatic force (F2) along this line, and applying (S5) to selected electrodes in said first set (16) a voltage that reduces said first electrostatic force (FI), thereby bringing selected pixels along this line out of contact with said light guide, and activating (S6) said light source (12).

11. A method according to claim 10, further comprising: applying (S7) voltages to said first set (15) essentially removing said first electrostatic force (FI), thereby bringing the entire foil (14) out of contact with the light guide (11).

12. A method according to any one of claims 10-1 1, further comprising: applying (SI) voltages to said second set of electrodes for essentially removing said second electrostatic force (F2).

13. A method according to any one of claims 10-12, wherein said first set of electrodes (15) is arranged on the light guide (1 1).

14. A method according to any one of claims 10-13, wherein said second set of electrodes is arranged on a passive plate (13) arranged on the other side of said foil (14) in relation to the light guide (11).

15. A method according to any one of claims 10-14, wherein said sets (15, 16) comprise parallel electrodes, and wherein said first set is orthogonal to said second set.

16. A method according to claim 15, wherein the electrodes in said first set of electrodes (15) are oriented vertically (column electrodes), and the electrodes in said second set of electrodes (16) are oriented horizontally (row electrodes).

17. A method according to any one of claims 10-16, wherein the foil (14) is provided with an unstructured electrode (17).

18. A method according to claim 17, wherein said foil electrode (17) is grounded.