The present invention relates to driving means for an electrowetting display device having a picture element. The driving means is arranged to apply, prior to a data write signal, a voltage signal to said picture element.
Electrowetting displays are becoming attractive to an ever increasing extent, mainly because of a combination of high brightness, a high contrast ratio, a large viewing angle and a fast switching speed. These properties make electrowetting displays suitable for video applications. In principle, electrowetting display can be made transmissive or reflective. For reflective electrowetting displays the power consumption can be relatively low, because no backlight is required.
An electrowetting display typically comprises a closed electrowetting cell, a polar and non-polar liquid, such as water and oil, having different optical properties and being contained in the cell, a number of electrodes for controlling the liquids contained in the cell, a front layer and a rear reflective layer. The liquids, which are immiscible, may be displaced by means of applying voltages to the electrodes. In an equilibrium-state (in which no voltages are applied to the electrodes) the polar and non-polar liquids are naturally layered in the closed cell, whereby a thin film is created. In this state, a colored state, the film covers the reflective area and the cell or pixel appears dark or black. By applying a voltage across the electrodes, the layered, colored state is no longer energetically favorable and the cell or pixel may lower its energy by moving the polar liquid towards one of the comers of a pixel. As a result the non-polar liquid is displaced and the underlying reflective or white surface is exposed. Consequently, in this state, a white state, the cell or pixel appears white or bright. The interaction between electrostatic and capillary forces determines how far the non-polar liquid is displaced to the side. In this manner, the optical properties of the layered composition may be adjusted such that intermediate color states, i.e. states lying between the colored state and the white state, are achieved.
Patent application WO 2005/036517 A1 discloses a display device comprising picture elements having at least a first fluid and a second fluid immiscible with each other above a first transparent support plate, the second fluid being electro-conductive or polar. Further, driving means of a display, providing pre-pulses, is disclosed. The pre-pulses bring the picture elements of the display device into electro-optical states associated with the voltage levels of the pre-pulses. When driving an electrowetting display device of this type, each row must accordingly be selected twice (two times for each frame). A first selection signal is setting the picture elements in an optical state associated with the applied voltage level, and a second selection signal is writing data to the pixels. A problem of the prior art disclosed in the abovementioned patent document is that the pre-pulses produce an optical response, which may become visible when longer pre-pulses are used.
An object of the present invention is, inter alia, to solve this problem in the prior art, and to this end, there is provided driving means as set forth in the appended independent claim 1 and a method as defined by independent claim 11. Specific embodiments are defined in the dependent claims.
The inventors have found that the a desired optical state can be better maintained for its desired period of time if a voltage signal is applied to the picture element which is different from the writing signal used for bringing the picture element into and maintaining the desired optical state, which voltage signal has a voltage level and duration such that the optical state is substantially maintained between for and after application of such voltage signal. In particular, the voltage signal prevents backflow of the oil.
The optical state of a picture element refers to the optical appearance of the picture element. For instance, the picture element of a grey scale display may attain any one of the extreme optical states black or white or different intermediate optical states in-between the extreme states, i.e. different levels of grey. As should be understood, the optical state of a picture element may further refer to different color levels (in case of a color display), luminance levels, levels of reflectivity, etc.
Normally, when a picture element is switched to an open, white state, wherein the oil of the electrowetting cell is displaced, the oil slowly returns to a fully closed, colored state, in which the oil creates a layer over the entire electrowetting cell. In this state the picture element appears dark. Depending on the characteristics of the oil, this may take from about only half a second to tenths of seconds. This phenomenon is referred to as back flow and seems to originate from electrical charging of the picture element. It has been found that the application of a voltage signal reduces the charging of the picture element. The duration of the voltage signal is such that the optical state of the picture element substantially does not change when the signal is applied. This implies that the optical state of the picture element in principal is maintained. If the optical state is slightly altered by the voltage signal such that the picture element attains a new optical state, this new state is preferably close to the state that the picture element had when the signal was applied, such that a viewer does not perceive any substantive change in optical state. However, electrical properties of the picture element, such as voltage charged across the picture element or capacitance of the picture element, may very well change when the voltage signal is applied.
In a first embodiment of the invention, there is provided driving means, which is arranged such that a plurality of data write signals can be applied to the picture element within a frame, whereby the frame time of the display device may be decreased.
In an embodiment of the invention, the driving means is arranged to apply the voltage signal to a picture element having an optical state in a first frame such that the picture element has the same optical state in a second frame. Hence, the grey level of a picture element may advantageously be employed over time and over several frames.
In a first embodiment of the invention, the voltage level of the voltage signal is set to zero volt. Advantageously, a voltage level of zero volt provides a minimal charge on the picture element. A voltage level of less than one sixth of the data write signaling voltage may be used to approximate a zero-volt signal, i.e. if the driving voltage is −30 V, the approximated value would be from −5 V to 5 V.
Moreover, the voltage signal may have a duration of 2 to 15% of the frame time, more specifically about 10% of the frame time. The frame time is the duration of a frame. For a frame updating frequency of e.g. 50 Hz, the frame time is 20 ms. An advantage with applying a voltage signal of a short duration as specified above is that it allows electrical properties of the picture element to be changed without changing the optical state of the picture element.
In another embodiment of the present invention, the driving means is arranged to apply at least one voltage signal for each picture element within one frame. In this manner, consistent and frequent discharging of the picture elements is ensured. Advantageously, the optical state may be maintained for a plurality of picture elements over time.
Furthermore, there is provided an electrowetting display device comprising the driving means according to an embodiment of the invention. The display device comprises, according to another embodiment of the present invention, an active matrix display device. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following.
The various aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which:
a shows a side view of a typical electrowetting display picture element in a colored state;
b shows a side view of a typical electrowetting display picture element in a white state;
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In general, a display device having electrowetting cells as above comprises an active matrix plane, which may be addressed using column and row drivers. The column drivers set the voltage levels of the picture elements and the row drivers select (or activate) a specific row, such that the voltage levels of the column drivers set the selected picture elements in the desired state. When writing data to a picture element of the display, the row of the picture element must be selected and an appropriate voltage level must be applied to the picture element column driver, in order for the picture element to be selected and written in accordance with the voltage level applied to the column driver. This addressing technique is usually known as matrix addressing.
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Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. The described embodiments are therefore not intended to limit the scope of the invention, as defined by the appended claims.
Number | Date | Country | Kind |
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05110756.3 | Nov 2005 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB06/53966 | 10/27/2006 | WO | 00 | 5/14/2008 |