METHODS AND APPARATUS FOR DRIVING ELECTRO-OPTIC DISPLAYS

Description

[0001] This application is related to United States Patents Nos. 7,170,670; 6,657,772; 6,831,769; and 6,982,178.

[0002] This application is also related to U.S. Patent Applications Serial Nos. 10/904,707 (filed November 24, 2004, now U.S. Patent No. 8,558,783, issued October 15, 2013); 10/879,335 (filed June 29, 2004, now United States Patent No. 7,528,822, issued May 5, 2009); 10/814,205 (filed March 31, 2004, now U.S. Patent No. 7,119,772, issued October 10, 2006); 10/249,973 (filed May 23, 2003, now United States Patent No. 7,193,625, issued March 20, 2007); and 10/065,795 (filed November 20, 2002, now U.S. Patent No. 7,012,600, issued March 14, 2006), which may hereinafter for convenience collectively be referred to as the "MEDEOD" (MEthods for Driving Electro-Optic Displays) applications.

[0003] This invention relates to methods for driving electro-optic displays, especially bistable electro-optic displays. More specifically, this invention relates to driving methods which are intended to enable more accurate control of gray states of the pixels of an electro-optic display. This invention is especially, but not exclusively, intended for use with particle-based electrophoretic displays in which one or more types of electrically charged particles are suspended in a liquid and are moved through the liquid under the influence of an electric field to change the appearance of the display.

[0004] The electro-optic displays in which the methods of the present invention are used often contain an electro-optic material which is a solid in the sense that the electro-optic material has solid external surfaces, although the material may, and often does, have internal liquid- or gas-filled space. Such displays using solid electro-optic materials may hereinafter for convenience be referred to as "solid electro-optic displays".

[0005] The term "electro-optic" as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.

[0006] The term "gray state" is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, several of the patents and published applications referred to below describe electrophoretic displays in which the extreme states are white and deep blue, so that an intermediate "gray state" would actually be pale blue. Indeed, as already mentioned the transition between the two extreme states may not be a color change at all. The term "gray level" is used herein to denote the possible optical states of a pixel, including the two extreme optical states.

[0007] The terms "bistable" and "bistability" are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in published U.S. Patent Application No. 2002/0180687 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called "multi-stable" rather than bistable, although for convenience the term "bistable" may be used herein to cover both bistable and multi-stable displays.

[0008] The term "impulse" is used herein in its conventional meaning of the integral of voltage with respect to time. However, some bistable electro-optic media act as charge transducers, and with such media an alternative definition of impulse, namely the integral of current over time (which is equal to the total charge applied) may be used. The appropriate definition of impulse should be used, depending on whether the medium acts as a voltage-time impulse transducer or a charge impulse transducer.

[0009] Much of the discussion below will focus on methods for driving one or more pixels of an electro-optic display through a transition from an initial gray level to a final gray level (which may or may not be different from the initial gray level). The term "waveform" will be used to denote the entire voltage against time curve used to effect the transition from one specific initial gray level to a specific final gray level. Typically, as illustrated below, such a waveform will comprise a plurality of waveform elements; where these elements are essentially rectangular (i.e., there a given element comprises application of a constant voltage for a period of time), the elements may be called "voltage pulses" or "drive pulses". The term "drive scheme" denotes a set of waveforms sufficient to effect all possible transitions between gray levels for a specific display.

[0010] Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Patents Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a "rotating bichromal ball" display, the term "rotating bichromal member" is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed to applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface. This type of electro-optic medium is typically bistable.

[0011] Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O' Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Patent No. 6,301,038, International Application Publication No. WO 01/27690, and in U.S. Patent Application 2003/0214695. This type of medium is also typically bistable.

[0012] Another type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.

[0013] As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see, for example, Kitamura, T., et al., "Electrical toner movement for electronic paper-like display", IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., "Toner display using insulative particles charged triboelectrically", IDW Japan, 2001, Paper AMD4-4). See also European Patent Publication Nos. EP1429178; EP1462847; and EP1482354; and International Applications WO 2004/090626; WO 2004/079442; WO 2004/077140; WO 2004/059379; WO 2004/055586; WO 2004/008239; WO 2004/006006; WO 2004/001498; WO 03/091799; and WO 03/088495. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media, when the media are used in an orientation which permits such settling, for example in a sign where the medium is disposed in a vertical plane. Indeed, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based ones, since the lower viscosity of gaseous fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles.

[0014] Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a fluid, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Patents Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545; 6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,704,133; 6,710,540; 6,721,083; 6,724,519; 6,727,881; 6,738,050; 6,750,473; 6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068; 6,825,829; 6,825,970; 6,831,769; 6,839,158; 6,842,167; 6,842,279; 6,842,657; 6,864,875; 6,865,010; 6,866,760; 6,870,661; 6,900,851; and 6,922,276; and U.S. Patent Applications Publication Nos. 2002/0060321; 2002/0063661; 2002/0090980; 2002/0113770; 2002/0130832; 2002/0180687; 2003/0011560; 2003/0020844; 2003/0025855; 2003/0102858; 2003/0132908; 2003/0137521; 2003/0214695; 2003/0222315; 2004/0012839; 2004/0014265; 2004/0027327; 2004/0075634; 2004/0094422; 2004/0105036; 2004/0112750; 2004/0119681; 2004/0136048; 2004/0155857; 2004/0180476; 2004/0190114; 2004/0196215; 2004/0226820; 2004/0239614; 2004/0252360; 2004/0257635; 2004/0263947; 2005/0000813; 2005/0001812; 2005/0007336; 2005/0007653; 2005/0012980; 2005/0017944; 2005/0018273; 2005/0024353; 2005/0035941; 2005/0041004; 2005/0062714; 2005/0067656; 2005/0078099; 2005/0105159; 2005/0122284; 2005/0122306; 2005/0122563; 2005/0122564; 2005/0122565; 2005/0151709; and 2005/0152022; and International Applications Publication Nos. WO 99/67678; WO 00/05704; WO 00/38000; WO 00/36560; WO 00/67110; WO 00/67327; WO 01/07961; and WO 03/107,315.

[0015] Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called "polymer-dispersed electrophoretic display" in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned United States Patent No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.

[0016] An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word "printing" is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.

[0017] A related type of electrophoretic display is a so-called "microcell electrophoretic display". In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within capsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Application Publication No. WO 02/01281, and U.S. Patent Application Publication No. 2002/0075556, both assigned to Sipix Imaging, Inc.

[0018] Other types of electro-optic media may also be used in the displays of the present invention.

[0019] Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "shutter mode" in which one display state is substantially opaque and one is light-transmissive. See, for example, the aforementioned U.S. Patents Nos. 6,130,774 and 6,172,798, and U.S. Patents Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Patent No. 4,418,346.

[0020] The bistable or multi-stable behavior of particle-based electrophoretic displays, and other electro-optic displays displaying similar behavior (such displays may hereinafter for convenience be referred to as "impulse driven displays"), is in marked contrast to that of conventional liquid crystal ("LC") displays. Twisted nematic liquid crystals are not bi- or multi-stable but act as voltage transducers, so that applying a given electric field to a pixel of such a display produces a specific gray level at the pixel, regardless of the gray level previously present at the pixel. Furthermore, LC displays are only driven in one direction (from non-transmissive or "dark" to transmissive or "light"), the reverse transition from a lighter state to a darker one being effected by reducing or eliminating the electric field. Finally, the gray level of a pixel of an LC display is not sensitive to the polarity of the electric field, only to its magnitude, and indeed for technical reasons commercial LC displays usually reverse the polarity of the driving field at frequent intervals. In contrast, bistable electro-optic displays act, to a first approximation, as impulse transducers, so that the final state of a pixel depends not only upon the electric field applied and the time for which this field is applied, but also upon the state of the pixel prior to the application of the electric field.

[0021] Whether or not the electro-optic medium used is bistable, to obtain a high-resolution display, individual pixels of a display must be addressable without interference from adjacent pixels. One way to achieve this objective is to provide an array of non-linear elements, such as transistors or diodes, with at least one non-linear element associated with each pixel, to produce an "active matrix" display. An addressing or pixel electrode, which addresses one pixel, is connected to an appropriate voltage source through the associated non-linear element. Typically, when the non-linear element is a transistor, the pixel electrode is connected to the drain of the transistor, and this arrangement will be assumed in the following description, although it is essentially arbitrary and the pixel electrode could be connected to the source of the transistor. Conventionally, in high resolution arrays, the pixels are arranged in a two-dimensional array of rows and columns, such that any specific pixel is uniquely defined by the intersection of one specified row and one specified column. The sources of all the transistors in each column are connected to a single column electrode, while the gates of all the transistors in each row are connected to a single row electrode; again the assignment of sources to rows and gates to columns is conventional but essentially arbitrary, and could be reversed if desired. The row electrodes are connected to a row driver, which essentially ensures that at any given moment only one row is selected, i.e., that there is applied to the selected row electrode a voltage such as to ensure that all the transistors in the selected row are conductive, while there is applied to all other rows a voltage such as to ensure that all the transistors in these non-selected rows remain non-conductive. The column electrodes are connected to column drivers, which place upon the various column electrodes voltages selected to drive the pixels in the selected row to their desired optical states. (The aforementioned voltages are relative to a common front electrode which is conventionally provided on the opposed side of the electro-optic medium from the non-linear array and extends across the whole display.) After a pre-selected interval known as the "line address time" the selected row is deselected, the next row is selected, and the voltages on the column drivers are changed to that the next line of the display is written. This process is repeated so that the entire display is written in a row-by-row manner.

[0022] It might at first appear that the ideal method for addressing such an impulse-driven electro-optic display would be so-called "general grayscale image flow" in which a controller arranges each writing of an image so that each pixel transitions directly from its initial gray level to its final gray level. However, inevitably there is some error in writing images on an impulse-driven display. Some such errors encountered in practice include:

(a) Prior State Dependence; With at least some electro-optic media, the impulse required to switch a pixel to a new optical state depends not only on the current and desired optical state, but also on the previous optical states of the pixel.

(b) Dwell Time Dependence; With at least some electro-optic media, the impulse required to switch a pixel to a new optical state depends on the time that the pixel has spent in its various optical states. The precise nature of this dependence is not well understood, but in general, more impulse is required that longer the pixel has been in its current optical state.

(c) Temperature Dependence; The impulse required to switch a pixel to a new optical state depends heavily on temperature.

(d) Humidity Dependence; The impulse required to switch a pixel to a new optical state depends, with at least some types of electro-optic media, on the ambient humidity.

(e) Mechanical Uniformity; The impulse required to switch a pixel to a new optical state may be affected by mechanical variations in the display, for example variations in the thickness of an electro-optic medium or an associated lamination adhesive. Other types of mechanical non-uniformity may arise from inevitable variations between different manufacturing batches of medium, manufacturing tolerances and materials variations.

(f) Voltage Errors; The actual impulse applied to a pixel will inevitably differ slightly from that theoretically applied because of unavoidable slight errors in the voltages delivered by drivers.

[0023] General grayscale image flow suffers from an "accumulation of errors" phenomenon. For example, imagine that temperature dependence results in a 0.2 L* (where L* has the usual CIE definition:

where R is the reflectance and R₀ is a standard reflectance value) error in the positive direction on each transition. After fifty transitions, this error will accumulate to 10 L*. Perhaps more realistically, suppose that the average error on each transition, expressed in terms of the difference between the theoretical and the actual reflectance of the display is ± 0.2 L*. After 100 successive transitions, the pixels will display an average deviation from their expected state of 2 L*; such deviations are apparent to the average observer on certain types of images.

[0024] This accumulation of errors phenomenon applies not only to errors due to temperature, but also to errors of all the types listed above. As described in the aforementioned 2003/0137521, compensating for such errors is possible, but only to a limited degree of precision. For example, temperature errors can be compensated by using a temperature sensor and a lookup table, but the temperature sensor has a limited resolution and may read a temperature slightly different from that of the electro-optic medium. Similarly, prior state dependence can be compensated by storing the prior states and using a multidimensional transition matrix, but controller memory limits the number of states that can be recorded and the size of the transition matrix that can be stored, placing a limit on the precision of this type of compensation.

[0025] Thus, general grayscale image flow requires very precise control of applied impulse to give good results, and empirically it has been found that, in the present state of the technology of electro-optic displays, general grayscale image flow is infeasible in a commercial display.

[0026] Almost all electro-optic medium have a built-in resetting (error limiting) mechanism, namely their extreme (typically black and white) optical states, which function as "optical rails". After a specific impulse has been applied to a pixel of an electro-optic display, that pixel cannot get any whiter (or blacker). For example, in an encapsulated electrophoretic display, after a specific impulse has been applied, all the electrophoretic particles are forced against one another or against the capsule wall, and cannot move further, thus producing a limiting optical state or optical rail. Because there is a distribution of electrophoretic particle sizes and charges in such a medium, some particles hit the rails before others, creating a "soft rails" phenomenon, whereby the impulse precision required is reduced when the final optical state of a transition approaches the extreme black and white states, whereas the optical precision required increases dramatically in transitions ending near the middle of the optical range of the pixel.

[0027] Various types of drive schemes for electro-optic displays are known which take advantage of optical rails. For example, Figures 9 and 10 of the aforementioned U.S. Patent Application No. 2003/0137521, and the related description at Paragraphs [0177] to [0180], describe a "slide show" drive scheme in which the entire display is driven to at least one optical rail before any new image is written. Obviously, a pure general grayscale image flow drive scheme cannot rely upon using the optical rails to prevent errors in gray levels since in such a drive scheme any given pixel can undergo an infinitely large number of changes in gray level without ever touching either optical rail.

[0028] Before proceeding further, it is desirable to define slideshow drive schemes more precisely. The fundamental slideshow drive scheme is that a transition from an initial optical state (gray level) to a final (desired) optical state (gray level) is achieved by making transitions to a finite number of intermediate states, where the minimum number of intermediate states is one. Preferably, the intermediate states are at or near the extreme states of the electro-optic medium used. The transitions will differ from pixel to pixel in a display, because they depend upon the initial and final optical states. The waveform for a specific transition for a given pixel of a display may be expressed as:

        R₂ ⇒ goal₁ ⇒ goal₂ ⇒ ... ⇒ goal_n ⇒ R₁     (Scheme 1)

where there is at least one intermediate or goal state between the initial state R₂ and the final state R₁. The goal states are, in general, functions of the initial and final optical states. The presently preferred number of intermediate states is two, but more or fewer intermediate states may be used. Each of the individual transitions within the overall transition is achieved using a waveform element (typically a voltage pulse) sufficient to drive the pixel from one state of the sequence to the next state. For example, in the waveform indicated symbolically above, the transition from R₂ to goal₁ is typically achieved with a waveform element or voltage pulse. This waveform element may be of a single voltage for a finite time (i.e., a single voltage pulse), or may include a variety of voltages so that a precise goal₁ state is achieved. This waveform element is followed by a second waveform element to achieve the transition from goal₁ to goal₂. If only two goal states are used, the second waveform element is followed by a third waveform element that drives the pixel from the goal₂ state to the final optical state R₁. The goal states may be independent of both R₂ and R₁, or may depend upon one or both.

[0029] This invention seeks to provide improved slide show drive schemes for electro-optic displays which achieve improved control of gray levels.

[0030] This invention relates to a method and apparatus for driving an electro-optic display in which the data used to define the drive scheme is compressed in a specific manner. The aforementioned MEDEOD applications describe methods and apparatus for driving electro-optic displays in which the data defining the drive scheme (or plurality of drive schemes) used are stored in one or more look-up tables ("LUT's"). Such LUT's must of course contain data defining the waveform for each waveform of the or each drive scheme, and a single waveform will typically require multiple bytes. As described in the MEDEOD applications, the LUT may have to take account of more than two optical states, together with adjustments for such factors as temperature, humidity, operating time of the medium etc. Thus, the amount of memory necessary for holding the waveform information can be substantial. It is desirable to reduce the amount of memory allocated to waveform information in order to reduce the cost of the display controller. A simple compression scheme that can be realistically accommodated in a display controller or host computer would be helpful in reducing the display controller cost. This invention relates to a simple compression scheme that appears particularly advantageous for electro-optic displays.

[0031] US 2011/0187684 A1 and US 2005/0280626 A1 (the former is a divisional of the latter) both describe methods for driving an electro-optic display having a plurality of pixels, each of which is capable of achieving at least two different gray levels. The driving method uses waveform compression to reduce the storage requirements needed to store the large variety of drive schemes required to cope with changes in temperature and other environmental factors, and the effects of dwell time on individual pixels. The method comprises storing a base waveform and a multiplication factor. The display controller applies to a pixel the sequence of voltages defined by the base waveform for periods dependent upon the multiplication factor. In a preferred form of the method, a bit set is used to represent the base waveform but the voltage defined by each bit set is applied to the pixel for n time segments (frames in an active matrix display), where n is the multiplication factor associated with the waveform.

[0032] The invention provides for a method of improving performance of an electro-optic display, e.g., an electrophoretic display, over a range of temperatures by adjusting the frame rate of the display to accommodate for changes in the electrophoretic medium due to temperature. This method involves storing a base waveform defining a sequence of voltages to be applied to a pixel during a specific transition by the pixel between gray levels at a first temperature and a base frame rate, and also storing a temperature-dependent multiplication factor, n, where n is a positive number. The specific transition is then effected by applying to the pixel the base waveform at a frame rate that that is n times the base frame rate. The new frame rate may be faster or slower than the base frame rate, for example, a higher temperature will allow operation at a faster frame rate. The temperature-dependent multiplication factor, n, may be stored in a look-up table (LUT), whereby a temperature measurement is obtained and value of n matching that temperature is obtained from the LUT. In some embodiments, the method additionally comprises adjusting the amplitude of the base waveform by a second temperature-dependent factor, p, which may also be stored in a LUT. By adjusting the frame rate, the overall performance of the electro-optic medium is improved, e.g., as indicated by a reduction in the intensity of residual images after a pixel has been changed from a first image to a second image, a phenomenon known as "ghosting."

[0033] Accordingly, this invention provides a method for driving an electro-optic display having a plurality of pixels, each of which is capable of achieving at least two different gray levels, the method comprising: storing a base waveform defining a sequence of voltages to be applied to a pixel during a specific transition by the pixel between gray levels at a first temperature, a base frame rate and base amplitude.

[0034] The method of the present invention is characterized by;

storing temperature-dependent multiplication factors, n and p, where n and p are positive numbers; and
effecting the specific transition by applying to the pixel the base waveform at a frame rate that that is n times the base frame rate and at an amplitude equal to p times the base amplitude.

[0035] The Figure shows a comparison of ghosting in graytone transitions between a standard frame rate (solid line) and a temperature-adjusted frame rate (dashed line) at several temperatures.

[0036] The invention provides methods for adjusting driving waveforms for electrophoretic displays to improve performance over a range of temperatures. In particular, a base waveform comprising a sequence of voltages and a base frame rate may be stored for a specific transition, along with temperature-dependent multiplication factors. A specific transition at a specific transition is thus driven by applying the base waveform at a framerate equivalent to the base framerate adjusted by a temperature-dependent multiplication factor.

[0037] The invention provides a method of improving the performance of an electro-optic display, e.g., a bistable electrophoretic display, over a range of temperatures by adjusting the frame rate of the display to accommodate for changes in the electro-optic medium due to temperature. For example, in an electrophoretic display, decreased temperature results in decreased electrophoretic mobility because the viscosity of the internal phase increases. As a consequence, temperature fluctuations can result in slow updates and/or image effects when the display is driven with a waveform that was optimized at a temperature different than the current operating temperature. To overcome this problem, some display controllers include complete sets (gray_m(T) → gray_n(T)) of waveforms for a select group of temperatures (T₁, T₂, T₃ ...). For a given operating temperature, the set of gray scale transitions (gray_m(T) → gray_n(T)) closest to a measured temperature is used to effect a grayscale transition. Nonetheless, at intermediate temperatures, e.g., between T₁ and T₂, the performance of the display may be unacceptable because of higher order effects of the temperature change.

[0038] The method of the present invention can dramatically reduce the amount of memory needed to store waveforms for a given grayscale transition over a range of temperatures. The method involves storing a base waveform defining a sequence of voltages to be applied to a pixel during a specific transition by the pixel between gray levels at a first temperature and a base frame rate, and also storing a temperature-dependent multiplication factor, n, where n is a positive number. The temperature-dependent multiplication factor, n, may be between 0.1 and 100, for example between 0.5 and 10, for example between 0.8 and 3. In some embodiments n is about 0.9, about 0.95, about 1.05, about 1.1, about 1.15, about 1.2, about 1.25, or about 2. The specific transition is then effected by applying to the pixel the base waveform at a frame rate that that is n times the base frame rate. The new frame rate may be faster or slower than the base frame rate, for example, a higher temperature will require operation at a faster frame rate. The temperature-dependent multiplication factor, n, may be stored in a look-up table (LUT), whereby a temperature measurement is obtained and value of n matching that temperature is obtained from the LUT. In some embodiments, the method additionally comprises adjusting the amplitude of the base waveform by a second temperature-dependent factor, p, which may also be stored in a LUT. By adjusting the frame rate, the overall performance of the electro-optic medium is improved, e.g., as indicated by a reduction in the intensity of residual images after a pixel has been changed from a first image to a second image, a phenomenon known as "ghosting." The frame rate can be adjusted using techniques known in the art and described in a number of the patents and patent applications listed in the Background section.

[0039] Because each row of an active matrix needs to be individually selected during each frame, in practice the base frame rate does exceed about 50 to 100 Hz. In some instances, frames of this length lead to difficulties in fine control of gray scale with many fast switching electro-optic medium. For example, some encapsulated electrophoretic media substantially complete a switch between their extreme optical states (a transition of about 30 L* units) within about 100 ms, and with such a medium a 20 ms frame corresponds to a gray scale shift of about 6 L* units. Such a shift is too large for accurate control of gray scale; the human eye is sensitive to differences in gray levels of about 1 L* unit, and controlling the impulse only in graduations equivalent to about 6 L* units is likely to give rise to visible artifacts. Such artifacts include "ghosting" due to prior state dependence of the electro-optic medium, that is, if the transition is under-driven, or not completely cleared, the second image will have remnants of the first image, i.e., "ghosts." The base frame rate is typically on the order of 50 Hz, however, in theory, the base frame rate could be anything reasonable, e.g., between 1 Hz and 200 Hz, e.g., between 40 Hz and 80 Hz.

[0040] The variation in ghosting due to temperature, and the ability to correct it using the methods of the invention is illustrated in the Figure. A standard waveform, optimized for 26 °C is assessed for ghosting by driving an electrophoretic test panel between first and second gray states multiple times, and then measuring the amount of residual reflectance that resides in the second darker state using a standardized optical bench having a calibrated light source and photodiode. When this standard waveform is applied at the same frame rate to the electrophoretic test panel at temperatures different from 26 °C, however, the ghosting worsens because the transition is either under-driven (lower temperature) or over-driven (higher temperature). See the solid line in the Figure. In contrast, using the technique of the invention, the frame rate is modified by a temperature-dependent factor, n, and the ghosting is dramatically improved using the same standard waveform. See the dashed line in the Figure. (Note that the solid and dashed lines intersect at 26 °C because they are both using the same, i.e., 26 °C-optimized, frame rate.) Accordingly, it is not necessary to store complete transition sets for 22 °C, 26 °C, and 30 °C. Rather, the same 26 °C base waveform can be used with a slightly different frame rate at 22 °C and 30 °C.

[0041] The temperature-dependent multiplication factors, n, can be stored in a look-up table (LUT) that is, for example, stored in flash memory. The display may include a temperature sensor to allow the display to monitor the temperature of the display in real time. Once the temperature is obtained, the corresponding factor, n, can be matched from the look-up table. In principle, an n could be measured for each unit of °C over the operating range, or even for each tenth of °C over the operating range. Overall, this accumulation of n's takes up very little memory as compared to storing complete wave sets for each temperature.

[0042] It is also beneficial to modify the amplitude of the waveform as a function of temperature. The amplitude of the base waveform is altered by a second temperature-dependent factor, p. The second temperature-dependent multiplication factor, p, may be between 0.1 and 100, for example between 0.5 and 10, for example between 0.8 and 3. In some embodiments p is about 0.75, about 0.8, about 0.9, about 1.1, about 1.5, about 2, about 3, about 4, or about 5. Thus, the invention allows for the simultaneous adjustment of both the frame rate and the amplitude of the base waveform to counteract performance changes due to environmental conditions, e.g., temperature. It is to be understood that "amplitude" means the magnitude of the voltage of the waveform compared to ground or some other floating voltage. By altering both the frame rate and the amplitude of the waveform, it is possible to maintain (or decrease) the overall energy consumption of the electro-optic display with time, without sacrificing performance. The second temperature-dependent factor, p, may also be stored in the same or a different LUT, thus the display controller can adjust the amplitude of the base waveform to optimize performance.

Claims

1. A method for driving an electro-optic display having a plurality of pixels, each of which is capable of achieving at least two different gray levels, the method comprising:

storing a base waveform defining a sequence of voltages to be applied to a pixel during a specific transition by the pixel between gray levels at a first temperature, a base frame rate and base amplitude,

storing temperature-dependent multiplication factors, n and p, where n and p are positive numbers;

obtaining a temperature measurement, and matching a value of n and a value of p to the measured temperature; and

effecting the specific transition by applying to the pixel the base waveform at a frame rate that is n times the base frame rate and at an amplitude equal to p times the base amplitude.

2. The method of claim 1, wherein the base frame rate is between 1 Hz and 200 Hz.

3. The method of claim 2, wherein the base frame rate is between 40 Hz and 80 Hz.

4. The method of claim 3, wherein the base frame rate is about 50 Hz.

5. The method of claim 1, wherein the base waveform comprises a set of bits.

6. The method of claim 1, wherein the base waveform is DC balanced.

7. The method of claim 1, wherein the temperature-dependent multiplication factor, n, is stored in a look-up table.

8. The method of claim 1, wherein the base amplitude is between 2 Volts and 60 Volts.

9. The method of claim 8, wherein the base amplitude is between 4 Volts and 21 Volts.

10. The method of claim 9, wherein the base amplitude is about 15 Volts.

11. The method of claim 8, wherein the temperature-dependent multiplication factor, p, is stored in a look-up table.

12. The method of any of claims 1-11, wherein the electro-optic display comprises an electrophoretic medium.

Ansprüche

1. Verfahren zum Ansteuern einer elektrooptischen Anzeige mit einer Vielzahl von Pixeln, von denen jedes in der Lage ist, mindestens zwei unterschiedliche Graustufen zu erreichen, wobei das Verfahren umfasst:

Speichern einer Basiswellenform, die eine Sequenz von Spannungen definiert, die während eines spezifischen Übergangs des Pixels zwischen Graustufen bei einer ersten Temperatur an ein Pixel anzulegen sind, einer Basisbildrate und Basisamplitude,

Speichern von temperaturabhängigen Multiplikationsfaktoren, n und p, wobei n und p positive Zahlen sind;

Erhalten einer Temperaturmessung, und Anpassen eines Wertes für n und eines Wertes für p an die gemessene Temperatur; und

Bewirken des spezifischen Übergangs, indem die Basiswellenform mit einer Bildrate, die das n-fache der Basisbildrate ist, und einer Amplitude angelegt wird, die gleich dem p-fachen der Basisamplitude ist.

2. Verfahren nach Anspruch 1, wobei die Basisbildrate zwischen 1 Hz und 200 Hz liegt.

3. Verfahren nach Anspruch 2, wobei die Basisbildrate zwischen 40 Hz und 80 Hz liegt.

4. Verfahren nach Anspruch 3, wobei die Basisbildrate etwa 50 Hz beträgt.

5. Verfahren nach Anspruch 1, wobei die Basiswellenform einen Satz von Bits umfasst.

6. Verfahren nach Anspruch 1, wobei die Basiswellenform gleichstromfrei ist.

7. Verfahren nach Anspruch 1, wobei der temperaturabhängige Multiplikationsfaktor, n, in einer Nachschlagtabelle gespeichert ist.

8. Verfahren nach Anspruch 1, wobei die Basisamplitude zwischen 2 Volt und 60 Volt liegt.

9. Verfahren nach Anspruch 8, wobei die Basisamplitude zwischen 4 Volt und 21 Volt liegt.

10. Verfahren nach Anspruch 9, wobei die Basisamplitude etwa 15 Volt beträgt.

11. Verfahren nach Anspruch 8, wobei der temperaturabhängige Multiplikationsfaktor, p, in einer Nachschlagtabelle gespeichert ist.

12. Verfahren nach einem der Ansprüche 1 bis 11, wobei die elektrooptische Anzeige ein elektrophoretisches Medium umfasst.

Revendications

1. Procédé de pilotage d'un écran électro-optique ayant une pluralité de pixels, dont chacun peut atteindre au moins deux niveaux de gris différents, le procédé comprenant :

le stockage d'une forme d'onde de base définissant une séquence de tensions devant être appliquées à un pixel pendant une transition spécifique réalisée par le pixel entre niveaux de gris à une première température, d'une fréquence de trame de base et d'une amplitude de base,

le stockage de facteurs de multiplication dépendants de la température, n et p, n et p étant des entiers positifs ;

l'obtention d'une mesure de température, et la mise en correspondance d'une valeur de n et d'une valeur de p avec la température mesurée ; et

la réalisation de la transition spécifique par application au pixel de la forme d'onde de base à une fréquence de trame qui vaut n fois la fréquence de trame de base et à une amplitude égale à p fois l'amplitude de base.

2. Procédé de la revendication 1, dans lequel la fréquence de trame de base se situe entre 1 Hz et 200 Hz.

3. Procédé de la revendication 2, dans lequel la fréquence de trame de base se situe entre 40 Hz et 80 Hz.

4. Procédé de la revendication 3, dans lequel la fréquence de trame de base est d'environ 50 Hz.

5. Procédé de la revendication 1, dans lequel la forme d'onde de base comprend un ensemble de bits.

6. Procédé de la revendication 1, dans lequel la forme d'onde de base est équilibrée en CC.

7. Procédé de la revendication 1, dans lequel le facteur de multiplication dépendant de la température, n, est stocké dans une table de conversion.

8. Procédé de la revendication 1, dans lequel l'amplitude de base se situe entre 2 volts et 60 volts.

9. Procédé de la revendication 8, dans lequel l'amplitude de base se situe entre 4 volts et 21 volts.

10. Procédé de la revendication 9, dans lequel l'amplitude de base est d'environ 15 volts.

11. Procédé de la revendication 8, dans lequel le facteur de multiplication dépendant de la température, p, est stocké dans une table de conversion.

12. Procédé de l'une quelconque des revendications 1 à 11, dans lequel l'écran électro-optique comprend un milieu électrophorétique.

Drawing