WO2005050608A1

WO2005050608A1 - Improved addressing of a foil display device

Info

Publication number: WO2005050608A1
Application number: PCT/IB2004/052445
Authority: WO
Inventors: Ruediger J. Lange; Fransiscus J. Vossen
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: TW200527341A

Abstract

A method for addressing a display of the kind having a light guide optically connected to a light source, an electrostatically operable foil and means for controlling electrostatic forces on said foil, for locally bringing it into contact with the light guide and extract light from the display, the method comprising during a reset period, controlling said forces to move a section of the foil to a first position, during an idle period, maintaining said forces, and during an addressing period, controlling said forces to move selected pixels of said section of the foil into a second position. According to the inventive method, the dynamic behavior of the foil is avoided by introducing an idle period immediately before the addressing pulses. The foil will thus have time to stabilize in the first position before being switched to the second position, and any dynamic behavior such as wave motions will fade away. As a consequence, when the addressing of individual pixels to the second position starts, the foil will be stable and behave as desired.

Description

IMPROVED ADDRESSING OF A FOIL DISPLAY DEVICE

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 electrostatically operable foil and means for controlling electrostatic forces on said foil, for locally bringing it into contact with the light guide and extract light from the display. The invention also relates to a display device adapted to be addressed in this way.

Displays of the above-mentioned kind are referred to as foil displays, known from e.g. WO 00/38163. In a dynamic foil display a foil is switched electrostatically between two glass plates, one being a light guide and optically connected to a light source. The position of the foil is controlled by the potential difference between electrodes, arranged on each of the plates and on the foil. Typically, the electrodes on the light guide and the passive plate are structured, and define the columns and rows of the display, while the electrode on the foil is unstructured. However, alternatives are possible and sometimes advantageous, including e.g. structured electrodes on the foil. A pixel is defined by the intersection of a row and a column electrode. 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 corresponding pixel is in the ON state. Otherwise the pixel will be in the OFF state, which is dark. Since the dynamic foil display is bi-stable, it is normally operated using passive matrix addressing. Conventionally, passive matrix addressing of the display can be divided into three phases: erase, address, and light generation. In the erase phase a number of rows (or the entire display) is erased. The panel is then addressed one row at a time. Pixels on the row that is currently being addressed can be selectively switched ON. Otherwise they will remain in the OFF state. The addressing cycle is followed by a phase during which the light source is activated, and light is emitted from the display. Alternatively, the light source is always activated, and light is emitted immediately when a pixel is switched ON. Greyscales can be generated using so called sub-field addressing, i.e. the erase and addressing phases are repeated several times per frame period, and the different light generation phases vary in length. A pixel can be switched ON during a selected number of such weighted sub-fields to thereby generate the desired light intensity. It has been found that conventional addressing is not reliable for the first rows in each group of rows addressed together, causing edge effects in these locations of the display. Several rows along the upper edge of such a location cannot be addressed correctly, i.e. remain dark or show a very irregular pattern. An object of the present invention is therefore to improve the addressing of a foil display, in order to avoid or improve problems of the above-mentioned kind. This and other objects are achieved by a method of the kind mentioned by way of introduction, comprising during a reset period, controlling said forces to move a section of the foil to a first position, during an idle period, maintaining said forces, and during an addressing period, controlling said forces to move selected pixels of said section of the foil into a second position. The present invention is based on the realization that the problematic effect is related to the behavior of the foil when it is switched first to one plate, and then, without having had time to stabilize, an attempt is made to switch it to the other plate. This is typically the case in conventional addressing, where the foil is first brought into contact with the passive plate during the erase phase, and then moved into contact with the light guide during the addressing phase. The effect is probably the result of a wave motion in the foil, caused by the first switching. The time it takes for the foil to stabilize depends on the mechanical properties of the foil (thickness, elasticity, etc) as well as on the amplitude of the pulse that is applied to switch the foil, and this time is in the order of several microseconds (typically 1-50 μs). The effect could also be related to the residual gas pressure in the panel, as air must be moved when the foil is switched from one plate to the other. According to the inventive method, the adverse effects of the dynamic behavior of the foil are avoided by introducing an idle period immediately before the addressing pulses. The foil will thus have time to stabilize in the first position before being switched to the second position, and any dynamic behavior such as wave motions will fade away. As a consequence, when the addressing of individual pixels to the second position starts, the foil will be stable and behave as desired. As mentioned, the method can advantageously be applied to any addressing scheme that employs an erase-address sequence, i.e. first switches all pixels OFF, and then selectively switches pixels ON. However, it may also advantageously be applied to schemes employing a write-address sequence, i.e. first switches all pixels ON, and then selectively switches pixels OFF. Such schemes are the subject of a copending application.

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 schematic view of a foil display. Fig 2a shows a foil display addressing scheme according to prior art. Fig 2b shows a foil display addressing scheme according to an embodiment of the present invention. Fig 3 is a flow chart of a method according to an embodiment of the present invention.

Fig 1 shows schematically a foil display to which the present invention is applicable. The display comprises a light guide 1 1, 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 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. an 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. 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. As mentioned, conventional addressing is performed as illustrated in the scheme in fig 2a, showing a group of rows (here eight rows) of a display addressed together. Such a group comprises the entire display, or just a sub-section of a display. First, the entire group of rows is erased (period 24), preferably by applying a positive voltage to the row electrodes while connecting the column electrodes to ground, hence attracting the foil to the passive plate. The skilled person will realize that any potential difference applied between the passive plate and the foil will create a force towards the passive plate. Then, the rows are addressed one by one (period 25) by reducing the voltage on the corresponding row electrode, and for each selected row the corresponding column data (ON or OFF) is applied to the column electrodes. When an ON voltage is applied to a column in a selected row, the foil in the intersection area (i.e. the pixel) will be attracted to the light guide, and the pixel is thus switched ON. When all rows have been addressed, the light source is activated in period 26, and the group of rows emits light from all pixels switched ON. Alternatively, the light source is always active, and light is generated immediately when a pixel is switched ON. According to the invention, an additional delay period 27 is introduced between the erase period 24 and the addressing period 25, as is illustrated in fig 2b. The process is illustrated in fig 3. The first step (SI) takes place during the period 24, and the foil is switched to a first position. The second step (S2) is the idle period 27. The foil is then addressed in step S3, during the addressing period 25, and finally the light source is activated in step S4. As mentioned above, the light source can alternatively be activated all along. During the delay period, the row and/or column voltages are slightly changed in order to weaken the force pressing the foil against the passive plate, thereby enabling the foil to relax and preparing it for a switch to the light guide. The idle period is in the order of a few μs, i.e. roughly in the same order as the switching time slots. A period 27 of 5 μs has been found to be enough to let the foil relax and stabilize after the erase period 24.

Alternatively, it may be possible to simply maintain the voltages applied during the reset period 24, i.e. to let the idle period 27 be a prolongation of the reset period 24, so that periods 24 and 27 in fig 2b simply represent a longer reset period 24. Many variations of the present invention as defined in the appended claims may be realized by the skilled person. For example, and as was mentioned above, the addressing scheme may alternatively be of a "select OFF" type. In this case, the foil is first attracted to the light guide during the reset period 24, and then selected pixels are switched OFF during the addressing period 25, before the light source is activated in period 26. Again, according to the invention, an idle period 27 is introduced between the two switching actions, so that the foil is stable before the second switch is performed.

Claims

CLAIMS:

1. A display device comprising a light guide (1 1) optically connected to a light source (12), an electrostatically operable foil (14), and means (18, 19) for controlling electrostatic forces on said foil, for locally bringing it into contact with the light guide and extract light from the display, said means being adapted to: during a reset period (24), controlling said forces to move a section of the foil to a first position, during an idle period (27), maintaining said forces, and during an addressing period (27) maintaining said forces, and during an addressing period (25), controlling said forces to move selected pixels of said section of the foil into a second position.

2. A display device according to claim 1, wherein the first position is out of contact with the light guide and the second position is in contact with the light guide.

3. A display device according to claim 2, wherein the first position is in contact with a passive plate (13), arranged in parallel with the light guide (11), on the other side of the foil (14).

4. A display device according to claim 1 , wherein the first position is in contact with the light guide (11) and the second position is out of contact with the light guide (11).

5. A method for addressing a display of the kind having a light guide (11) optically connected to a light source (12), an electrostatically operable foil (14) and means for controlling electrostatic forces on said foil, for locally bringing it into contact with the light guide and extract light from the display, comprising during a reset period (24), controlling (SI) said forces to move a section of the foil to a first position, during an idle period (27), maintaining (S2) said forces, and during an addressing period (25), controlling (S3) said forces to move selected pixels of said section of the foil into a second position.

6. A method according to claim 5, wherein the first position is out of contact with the light guide and the second position is in contact with the light guide.

7. A method according to claim 6, wherein the first position is in contact with a passive plate (13), arranged in parallel with the light guide (1 1), on the other side of the foil (14).

8. A method according to claim 5, wherein the first position is in contact with the light guide (11) and the second position is out of contact with the light guide (11).