EP1911016A2

EP1911016A2 - Methods for driving electro-optic displays

Info

Publication number: EP1911016A2
Application number: EP06789193A
Authority: EP
Inventors: Karl R. Amundson
Original assignee: E Ink Corp
Current assignee: E Ink Corp
Priority date: 2005-08-01
Filing date: 2006-08-01
Publication date: 2008-04-16
Anticipated expiration: 2026-08-01
Also published as: JP2013047855A; EP1911016A4; CN101233557B; HK1118371A1; JP2009516856A; CN101233557A; EP1911016B1; WO2007016627A9; JP2015111307A; WO2007016627A2; JP5765875B2; WO2007016627A3

Abstract

An electro-optic display is driven using a plurality of different drive schemes. The waveforms of the drive schemes are chosen such that the absolute value of the net impulse applied to a pixel for all homogeneous and heterogeneous irreducible loops divided by the number of transitions in the loop is less than 20 per cent of the characteristic impulse (i.e., the average of the absolute values of the impulses required to drive a pixel between its two extreme optical states).

Description

METHODS FOR DRIVING ELECTRO-OPTIC DISPLAYS

[Para 1] This application is related to U.S. Patent Application Publication No. 2006/0280626); U.S. Patent No. 7,012,600; U.S. Patent No. 6,531,997; U.S. Patent No. 6,504,524); U.S Patent Application Publication No. 2005/0001812; U.S. Patent Application Publication No. 2005/0024353; U.S. Patent Application Publication No. 2005/0270261; U.S Patent Application Publication No. 2005/0179642; and U.S. Patent Application Publication No. 2002/0180687.

[Para 2] This invention relates to methods for driving electro-optic displays, especially bistable electro-optic displays, and to apparatus for use in such methods. More specifically, this invention relates to driving methods which are intended to enable a plurality of drive schemes to be used simultaneously to update 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.

[Para 3] 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. [Para 4] 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. [Para 5] 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 the aforementioned 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.

[Para 6] 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. [Para 7] 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, such a waveform will comprise a plurality of waveform elements; where these elements are essentially rectangular (i.e., where a given element comprises application of a constant voltage for a period of time); the elements may be called "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.

[Para 8] 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. [Para 9] 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. Patents Nos. 6,301,038 and 6,870,657, and in U.S. Patent Application Publication No. 2003/0214695. This type of medium is also typically bistable.

[Para 10] Another type of electro-optic display is an electro-wetting display developed by Philips and described in an article in the September 25, 2003 issue of the Journal "Nature" and entitled "Performing Pixels: Moving Images on Electronic Paper", Hayes, R.A., et al., "Video-Speed Electronic Paper Based on Electrowetting", Nature, 425, 383-385 (2003). It is shown in U.S. Patent Application Publication No. 2005/0151709, that such electro-wetting displays can be made bistable. [Para 11] 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.

[Para 12] 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 HCSl-I, and Yamaguchi, Y., et al., "Toner display using insulative particles charged triboelectrically", IDW Japan, 2001, Paper AMD4-4). See also U.S. Patent Application Publication No. 2005/0001810; European Patent Applications 1,462,847; 1,482,354; 1,484,635; 1,500,971; 1,501,194; 1,536,271; 1,542,067; 1,577,702; 1,577,703; and 1,598,694; and International Applications WO 2004/090626; WO 2004/079442; and WO 2004/001498. 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.

[Para 13] 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 liquid suspending medium, 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; 6,922,276; 6,950,200; 6,958,848; 6,967,640; 6,982,178; 6,987,603; 6,995,550; 7,002,728; 7,012,600; 7,012,735; 7,023,430; 7,030,412; 7,030,854; 7,034,783; 7,038,655; 7,061,663; 7,071,913; 7,075,502; 7,075,703; and 7,079,305; and U.S. Patent Applications Publication Nos. 2002/0060321; 2002/0090980; 2002/0113770; 2002/0180687; 2003/0011560; 2003/0102858; 2003/0151702; 2003/0222315; 2004/0014265; 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; 2005/0012980; 2005/0017944; 2005/0018273; 2005/0024353; 2005/0062714; 2005/0067656; 2005/0078099; 2005/0099672; 2005/0105159; 2005/0122284; 2005/0122306; 2005/0122563; 2005/0122564; 2005/0122565; 2005/0134554; 2005/0146774; 2005/0151709; 2005/0152018; 2005/0152022; 2005/0156340; 2005/0168799; 2005/0179642; 2005/0190137; 2005/0212747; 2005/0213191; 2005/0219184; 2005/0253777; 2005/0270261; 2005/0280626; 2006/0007527; 2006/0023296; 2006/0024437; and 2006/0038772; and International Applications Publication Nos. WO 00/38000; WO 00/36560; WO 00/67110; and WO 01/07961; and European Patents Nos. 1,099,207 Bl; and 1,145,072 Bl.

[Para 14] 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 U.S. Patent No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as subspecies of encapsulated electrophoretic media.

[Para 15] 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. [Para 16] 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. [Para 17] 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.

[Para 18] 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.

[Para 19] 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 nonlinear 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 preselected 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 so 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.

[Para 20] 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.

[Para 21] 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:

L* = 116(R/R₀)^1/3 - 16, where R is the reflectance and Ro 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.

[Para 22] 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 U.S. Patent No. 7,012,600, 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 multi-dimensional 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. [Para 23] 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. [Para 24] Under some circumstances, it may be desirable for a single display to make use of multiple drive schemes. For example, a display capable of more than two gray levels may make use of a gray scale drive scheme ("GSDS") which can effect transitions between all possible gray levels, and a monochrome drive scheme {"MDS") which effects transitions only between two gray levels, typically the two extreme optical states of each pixel, the MDS providing quicker rewriting of the display that the GSDS. The MDS is used when all the pixels which are being changed during a rewriting of the display are effecting transitions only between the two gray levels used by the MDS. For example, the aforementioned 2005/0001812 describes a display in the form of an electronic book or similar device capable of displaying gray scale images and also capable of displaying a monochrome dialogue box which permits a user to enter text relating to the displayed images. When the user is entering text, a rapid MDS is used for quick updating of the dialogue box, thus providing the user with rapid confirmation of the text being entered. On the other hand, when the entire gray scale image shown on the display is being changed, a slower GSDS is used.

[Para 25] A display may usefully use more than two drive schemes. For example, a display may have one GSDS which is used for updating small areas of the display and a second GSDS which is used when the entire image on the display needs to be changed or refreshed. For example, a user editing small portions of a drawing shown on a display might use a first GSDS (which does not require flashing of the display) to view the results of the edits, but might use a second "clearing" GSDS (which does involve flashing of the display) to show more accurately the final edited drawing, or to display a new drawing on the display. In such a scheme, the second GSDS may be referred to a "gray scale clear" drive scheme or "GSCDS".

[Para 26] As discussed in detail in the aforementioned 2005/0001812, for at least some types of electro-optic displays it is desirable that the drive scheme used be DC balanced, in the sense that, for any series of transitions beginning and ending at the same gray level, the algebraic sum of the impulses applied during the series of transitions is bounded. DC balanced drive schemes have been found to provide more stable display performance and reduced image artifacts. Desirably all individual waveforms within a drive scheme are DC balanced, but in practice it is difficult to make all waveforms DC balanced, so that drive schemes are usually a mixture of DC balanced and DC imbalanced waveforms, even though the drive scheme as a whole is DC balanced. [Para 27] Use of two such mixed DC balanced drive schemes in the same display may result in a DC imbalanced overall drive scheme because of transition loops using transitions from both drive schemes. For example, consider a display using a MDS and a GSDS, and a simple transition loop, white-black-white. The GSDS may have a net impulse of I₁ for the white-black (W-^B) transition and (since it is DC balanced) a net impulse of -Ii for the B->W transition. Similarly, the MDS may have a net impulse of I₂ (not equal to Ii) for the white-black (W->B) transition and (since it is DC balanced) a net impulse of -I₂ for the B-^W transition. If a pixel is driven from white to black using the GSDS and then from black to white using the MDS, the net impulse for the loop is Ii -I₂, which is not equal to zero. Furthermore, since this same loop can be repeated indefinitely, the net impulses for the loop can accumulate, so that the net impulse is unbounded and the overall drive scheme is no longer DC balanced. [Para 28] The present invention provides an electro-optic display, and a method for operating such a display, which allows two different drive schemes to be used simultaneously in a manner which ensures that the overall drive scheme is DC balanced, or very close to DC balanced.

[Para 29] This invention provides a method of driving an electro-optic display using a plurality of different drive schemes, the waveforms of the drive schemes being chosen such that the absolute value of the net impulse applied to a pixel for all homogeneous and heterogeneous irreducible loops divided by the number of transitions in the loop is less than 20 per cent of the characteristic impulse, wherein: a homogeneous irreducible loop is a sequence of gray levels, starting at a first gray level, passing through zero or more gray levels, and ending at the first gray level, wherein all transitions are effected using the same drive scheme, and wherein the loop does not visit any gray level except the first gray level more than once; a heterogeneous irreducible loop is a sequence of gray levels, starting at a first gray level, passing through one or more gray levels and ending at the first gray level, wherein the loop comprises transitions using at least two different drive schemes, the drive scheme used to effect the last transition in the loop is the same as the drive scheme used to effect the transition to the first gray level immediately prior to the start of the loop, and the loop comprises no shorter irreducible loops; and the characteristic impulse is the average of the absolute values of the impulses required to drive a pixel between its two extreme optical states. [Para 30] Desirably, the net impulse applied to a pixel for all homogeneous and heterogeneous irreducible loops (as defined below) divided by the number of transitions in the loop is less than 10 per cent, and preferably less than 5 per cent, of the characteristic impulse. Most desirably, the net impulse for all homogeneous and heterogeneous irreducible loops is essentially zero, i.e., all such loops are DC balanced. [Para 31] In the present method, the plurality of drive schemes may comprise a gray scale drive scheme and a monochrome drive scheme, or two gray scale drive schemes and a monochrome drive scheme. In the latter case, one of the two gray scale drive schemes may use local updating of the image and the other may use global updating. Alternatively, one of the two gray scale drive schemes may provide more accurate gray levels than the other but cause more flashing of the display.

[Para 32] The present method may make use of any of the types of electro-optic medium discussed above. Thus, for example, the electro-optic display may comprise a rotating bichromal member, electrochromic or electrowetting display medium. Alternatively, the electro-optic display may comprise a particle-based electrophoretic medium in which a plurality of charged particles move through a fluid under the influence of an electric field. The charged particles and the fluid may be encapsulated within a plurality of capsules or microcells, or may be present as a plurality of discrete droplets within a continuous phase comprising a polymeric binder. The fluid may be gaseous.

[Para 33] This invention extends to an electro-optic display comprising a layer of electro-optic medium, at least one electrode arranged to apply an electric field to the layer of electro-optic medium, and a controller arranged to control the electric field applied to the electro-optic medium by the at least one electrode, the controller being arranged to carry out a method of the present invention.

[Para 34] The displays of the present invention may be used in essentially any application in which electro-optic displays have previously been used, for example electronic book readers, portable computers, tablet computers, cellular telephones, smart cards, signs, watches, shelf labels and flash drives.

[Para 35] As already mentioned, this invention provides a method of driving an electro-optic display using a plurality of different drive schemes, the waveforms of the drive schemes being chosen such that the absolute value of the net impulse applied to a pixel for all homogeneous and heterogeneous irreducible loops divided by the number of transitions in the loop is less than about 20 per cent of the characteristic impulse. [Para 36] The present invention is based upon the concepts of homogeneous and heterogeneous irreducible loops. For present purposes, a gray level loop is a sequence of gray levels where the first and last gray levels are the same. For example, assuming a four gray level (two-bit) gray scale, with the gray levels being denoted, from darkest to lightest, 1, 2, 3 and 4, examples of such gray level loops are:

2->3^2 l-»4-»3-»2-»l.

[Para 37] Homogeneous irreducible loops are sequences of gray levels, starting at a first gray level, passing through zero or more gray levels to end up at the first gray level, in which all the transitions are effected using the same drive scheme (typically a gray scale drive scheme or "GSDS"). While in general gray level loops can visit any gray level multiple times, a homogeneous irreducible loop does not visit any gray level more than once, except for the final gray level, which as already noted must be the same as the first gray level. For example, assuming the same four gray level (two-bit) gray scale, homogeneous irreducible loops are:

2^2 l-»2-»l

3^2-^1^3

1^2->3^4->l

The first loop simply transitions from gray level 1 to gray level 1, and the second from gray level 2 to gray level 2. The third example starts at gray level 1, transitions to gray level 2, and then transitions back to gray level 1. [Para 38] Gray level loops can be homogeneous (i.e., having all transitions effected using the same drive scheme) but not irreducible. Examples of homogeneous loops that are not irreducible are:

1->2^3^2^1

1^2^2^1

3^2^3^2->3

All of these loops are not irreducible because they contain repeated visits to the same gray level other than the first and last gray level, and all can be reduced to a plurality of irreducible loops.

[Para 39] It will readily be apparent that, for any number of gray levels within a gray scale, there are a finite number of homogeneous irreducible loops. [Para 40] Heterogeneous loops are similar to homogeneous loops except that heterogeneous loops include transitions using at least two different drive schemes. In heterogeneous loops, as in homogeneous ones, the first and last gray levels must be the same; also, in heterogeneous loops, the drive scheme used to effect the last transition of the loop must be the same as the drive scheme previously used to effect the transition to the first gray level. By way of example, consider the transition, in the aforementioned four gray level scale, from gray level 1 to gray level 4 using drive scheme A, denoted symbolically as: l-»(a)-»4

A reverse transition from gray level 4 to gray level 1 using drive scheme B is denoted symbolically as:

4-»(b)-»l A heterogeneous loop can be constructed from these two transitions, thus: l-»(a)-»4-»(b)-M where the original gray level 1 state was achieved using drive scheme B as indicated at the end of the loop.

[Para 41] It will readily be apparent that various other heterogeneous loops can be constructed each using a plurality of drive schemes. Irreducible heterogeneous loops can be defined as having the following two properties: (a) the first and last gray levels are the same, and the drive scheme used to achieve the last gray level is the same as that used to achieve the first gray level; and

(b) the heterogeneous loop itself contains no irreducible loops. [Para 42] Examples of irreducible heterogeneous loops are: l-»(a)-»4-»(b)-M-»(b)-»2-»(a)->l l-»(a)-»4-»(b)-»l-»(c)-»4-»(d)-»l [Para 43] Examples of heterogeneous loops that are not irreducible are: l->(a)-»4-»(a)-»l-»(b)->4->(a)-M l-»(a)-»2-»(b)-»3-»(b)->2-»(a)-M because they contain irreducible loops; the first loop comprises two successive l->(a)->4->(a)->l irreducible loops, while the second contains two nested irreducible loops.

[Para 44] It will be appreciated that complex homogeneous loops can be "deconstructed" in a similar manner into finite sets of irreducible loops and irreducible loops embedded within other irreducible loops. Thus, for example, the homogeneous loop:

1^4-*3->2^3→^>2^3^2^1^2^1 can be decomposed into two consecutive 2->θ->2 loops embedded within a l-»4-»3-»2-»l, loop, and followed by the loop l->2-M.

[Para 45] Since both homogeneous and heterogeneous loops can be deconstructed in this manner to combinations of irreducible loops, it follows that if all irreducible loops are DC balanced, all possible loops (i.e., all possible sequences that start and end at the same gray level) are DC balanced.

[Para 46] As already mentioned, where a single display makes use of a plurality of drive schemes, it is advantageous for the overall drive scheme as well as the individual drive schemes to be DC balanced (or, less desirably, substantially DC balanced, in the sense that the algebraic sum of the impulses in any given loop is small). In accordance with the present invention, the drive schemes are chosen so that all homogeneous and heterogeneous irreducible loops are DC balanced, or, in a less preferred form of the invention, all homogeneous and heterogeneous irreducible loops are substantially DC balanced. Substantial DC-balance allows for small DC imbalance in some or all of the homogeneous and heterogeneous loops.

[Para 47] As already mentioned, one preferred form of the present method uses as the plurality of drive schemes a monochrome drive scheme and at least one gray scale drive scheme. As is well known to those skilled in the technology of electro-optic displays, a gray scale drive scheme (GSDS) can be used to make transitions from any gray level to any other gray level in a gray scale. An example of a gray level sequence achieved through the action of a GSDS grayscale update is:

2 →(G) 3 →(G) 1 -KG) 4 →(G) 3 →(G) 1 →(G) 3 →(G) 3 ->(G) 3 →(G) 2 where "-KG)" denotes that the relevant transition is effected by the GSDS. This example assumes the aforementioned four gray level (two-bit) gray scale, with the gray levels denoted, from darkest to lightest, 1, 2, 3 and 4.

[Para 48] A monochrome drive scheme (MDS) can be used to effect transitions between gray levels belonging to a monochrome subset of gray levels, the monochrome subset containing two of the gray levels in the aforementioned gray scale. In this example, the monochrome subset is {1,4}, that is, the darkest and lightest gray levels (typically black and white respectively). In any given sequence of gray levels, some of the transitions may be effected by the MDS, while others may be effected by the GSDS. For example, a gray level sequence could be:

2 →(G) 3 →(G) 1 -KM) 4 →(M) 1 →(M) 4 →(G) 3 -»(G) 1 →(M) 4 →(G) 2 where "-KM)" denotes that the relevant transition is effected by the MDS. This sequence illustrates heterogeneous updating, that is, updating using combinations of GSDS and MDS.

[Para 49] A particularly preferred embodiment of the present invention uses three different drive schemes, namely a gray scale drive scheme, a gray scale clear drive scheme, and a monochrome drive scheme. The gray scale drive scheme and the gray scale clear drive scheme may differ in various ways; for example, the gray scale drive scheme may use local updating (i.e., only the pixels which need to be changed are rewritten), while the gray scale clear drive scheme may use global updating (i.e., all pixels are rewritten whether or not their gray levels are to change). Alternatively, the gray scale clear drive scheme may provide more accurate gray levels than the gray scale drive scheme but at the cost of more flashing during transitions.

[Para 50] Adjustment of the individual waveforms of the drive schemes used in the present invention to substantially or completely DC balance all irreducible homogeneous and heterogeneous irreducible loops may be effected by any of the techniques described in the various patents and applications referred to in the first paragraph of this application. These techniques including varying the waveform depending upon various prior states of the display (so that, for example, the homogeneous loops 1~>2->1 and l->3-^2-M both end with a 2->l transition, the waveform used for this 2->l transition can be different in the two cases), and insert of balanced pulse pairs and other waveform elements which can effect some change in gray level but have zero net impulse.

Claims

1. A method of driving an electro-optic display using a plurality of different drive schemes, the method being characterized in that that waveforms of the drive schemes are chosen such that the absolute value of the net impulse applied to a pixel for all homogeneous and heterogeneous irreducible loops divided by the number of transitions in the loop is less than 20 per cent of the characteristic impulse, wherein: a homogeneous irreducible loop is a sequence of gray levels, starting at a first gray level, passing through zero or more gray levels, and ending at the first gray level, wherein all transitions are effected using the same drive scheme, and wherein the loop does not visit any gray level except the first gray level more than once; a heterogeneous irreducible loop is a sequence of gray levels, starting at a first gray level, passing through one or more gray levels and ending at the first gray level, wherein the loop comprises transitions using at least two different drive schemes, the drive scheme used to effect the last transition in the loop is the same as the drive scheme used to effect the transition to the first gray level immediately prior to the start of the loop, and the loop comprises no shorter irreducible loops; and the characteristic impulse is the average of the absolute values of the impulses required to drive a pixel between its two extreme optical states.

2. A method according to claim 1 wherein the net impulse applied to a pixel for all homogeneous and heterogeneous irreducible loops divided by the number of transitions in the loop is less than 10 per cent of the characteristic impulse.

3. A method according to claim 2 wherein the net impulse applied to a pixel for all homogeneous and heterogeneous irreducible loops divided by the number of transitions in the loop is less than 5 per cent of the characteristic impulse.

4. A method according to claim 3 wherein the net impulse applied to a pixel for all homogeneous and heterogeneous irreducible loops is essentially zero.

5. A method according to claim 1 wherein the drive schemes comprise a gray scale drive scheme and a monochrome drive scheme.

6. A method according to claim 1 wherein the drive schemes comprise two gray scale drive schemes and a monochrome drive scheme.

7. A method according to claim 6 wherein one of the two gray scale drive schemes uses local updating of the image and the other uses global updating.

8. A method according to claim 6 wherein one of the two gray scale drive schemes provides more accurate gray levels than the other but causes more flashing of the display.

9. A method according to claim 1 wherein the electro-optic display comprises a rotating bichromal member, electrochromic or electrowetting display medium.

10. A method according to claim 1 wherein the electro-optic display comprises a particle-based electrophoretic medium in which a plurality of charged particles move through a fluid under the influence of an electric field.

11. A method according to claim 10 wherein the charged particles and the fluid are encapsulated within a plurality of capsules or microcells.

12. A method according to claim 10 wherein the charged particles and the fluid are present as a plurality of discrete droplets within a continuous phase comprising a polymeric binder.

13. A method according to claim 10 wherein the fluid is gaseous.

14. An electro-optic display comprising a layer of electro-optic medium, at least one electrode arranged to apply an electric field to the layer of electro- optic medium, and a controller arranged to control the electric field applied to the electro-optic medium by the at least one electrode, the controller being arranged to carry out a method according to any one of the preceding claims.

15. An electronic book reader, portable computer, tablet computer, cellular telephone, smart card, sign, watch, shelf label or flash drive comprising a display according to claim 14.