Patent Publication Number: US-8125501-B2

Title: Voltage modulated driver circuits for electro-optic displays

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of copending application Ser. No. 10/609,119, filed Jun. 27, 2003 (Publication No. 2004/0075634, now U.S. Pat. No. 7,202,847, issued Apr. 10, 2007), which claims the benefit of U.S. Provisional Patent application Ser. No. 60/392,245, filed Jun. 28, 2002. 
     This application is also a continuation-in-part of copending application Ser. No. 11/425,408, filed Jun. 21, 2006 (Publication No. 2006/0232531, now U.S. Pat. No. 7,733,311, issued Jun. 8, 2010), which is itself a divisional of application Ser. No. 10/814,205, filed Mar. 31, 2004 (now U.S. Pat. No. 7,119,772, issued Oct. 10, 2006) which itself claims benefit of the following Provisional Applications: (a) Ser. No. 60/320,070, filed Mar. 31, 2003; (b) Ser. No. 60/320,207, filed May 5, 2003; (c) Ser. No. 60/481,669, filed Nov. 19, 2003; (d) Ser. No. 60/481,675, filed Nov. 20, 2003; and (e) Ser. No. 60/557,094, filed Mar. 26, 2004. 
     This application is also a continuation-in-part of copending application Ser. No. 11/160,455, filed Jun. 24, 2005 (Publication No. 2005/0219184, now U.S. Pat. No. 7,312,794, issued Dec. 25, 2007), which is a divisional of application Ser. No. 10/065,795, filed Nov. 20, 2002 (now U.S. Pat. No. 7,012,600, issued Mar. 14, 2006), which itself claims benefit of the following Provisional Applications: (f) Ser. No. 60/319,007, filed Nov. 20, 2001; (g) Ser. No. 60/319,010, filed Nov. 21, 2001; (h) Ser. No. 60/319,034, filed Dec. 18, 2001; (i) Ser. No. 60/319,037, filed Dec. 20, 2001; and (j) Ser. No. 60/319,040, filed Dec. 21, 2001. 
     The entire contents of all the preceding patents and applications, and of all U.S. patents, and published and copending applications mentioned below, are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to electro-optic displays, and, more particularly, to methods and apparatus for addressing of such displays. The methods of the present invention are especially, though not exclusively, intended for use in driving bistable electrophoretic displays. 
     BACKGROUND OF THE INVENTION 
     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. 
     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 terms “bistable” and “bistability” are used herein in their conventional meaning in the imaging 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. 
     The term “impulse” is used herein in its conventional meaning in the imaging art 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. 
     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. Pat. 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. 
     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&#39;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. Pat. Nos. 6,301,038; 6,870.657; and 6,950,220. This type of medium is also typically bistable. 
     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. 
     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 U.S. Patent 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 suspending fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles. 
     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. Pat. 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; 7,023,420; 7,030,412; 7,030,854; 7,034,783; 7,038,655; 7,061,663; 7,071,913; 7,075,502; 7,075,703; 7,079,305; 7,106,296; 7,109,968; 7,110,163; 7,110,164; 7,116,318; 7,116,466; 7,119,759; 7,119,772; 7,148,128; 7,167,155; 7,170,670; 7,173,752; 7,176,880; and 7,180,649; and U.S. Patent Applications Publication Nos. 2002/0060321; 2002/0090980; 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; 2004/0257635; 2004/0263947; 2005/0000813; 2005/0007336; 2005/0012980; 2005/0017944; 2005/0018273; 2005/0024353; 2005/0062714; 2005/0067656; 2005/0078099; 2005/0099672; 2005/0122284; 2005/0122306; 2005/0122563; 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; 2006/0024437; 2006/0038772; 2006/0139308; 2006/0139310; 2006/0139311; 2006/0176267; 2006/0181492; 2006/0181504; 2006/0194619; 2006/0197736; 2006/0197737; 2006/0197738; 2006/0198014; 2006/0202949; and 2006/0209388; 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 B1; and 1,145,072 B1. 
     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. Pat. 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. 
     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 microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, Inc. 
     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. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. 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. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in shutter mode. 
     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. 
     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 act 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. 
     A method of controlling and applying well defined voltage impulses to an electro-optic medium is required to produce desired optical states in the medium. There are several ways of providing a particular voltage impulse, e.g., a particular ∫vdt value, to a display medium. Two common methods entail modulation of the length of a constant voltage pulse, and modulation of the amplitude of a constant length pulse. 
     Amplitude modulation methods are commonly employed because such methods can provide, for example, reduced power consumption and reduced controller complexity. When an insufficient range of impulse control is possible using only amplitude modulation, amplitude modulation can be combined with time modulation to produce a more precise modulation of the total impulse applied to a display medium. 
     To control amplitude modulation at the pixel level in an active matrix display, a column driver circuit is typically required to adjust the amplitude of the driver circuit&#39;s output based on display signal data received from a display controller. A row driver circuit sequentially selects each row of pixels, temporarily connecting a selected row of pixel electrodes to the column driver circuits. In this way, the voltage of applied to each pixel electrode in the display can be set once per scan by the column and row drivers. 
     A column driver circuit commonly includes a resistive digital-to-analog converter (R-DAC) system with output buffers and offset trimming. Although a DAC-based architecture has many benefits, it typically requires a large number of transistors for implementation. This can lead to two problems: 1) the implementation of the circuit can be complex with care required to insure proper functionality and accuracy; and 2) a large area of active circuit can be required, which can lead to higher cost (especially at higher voltages). 
     For example, a LCD having 256 gray levels may include a separate 256-level DAC for each column of display elements. Each DAC converts digital image data supplied to a column driver into a voltage to be applied to a pixel electrode. The cost of a large number of DACs in a high-resolution display may increase the manufacturing cost of a display. 
     Further, fabrication of an R-DAC-based design may require specialized process provisions, such as a floating polycrystalline silicon capacitor layer, to enable design features which improve accuracy. Specialized processes may reduce the number of vendors available with a suitable manufacturing capability and may increase final cost as well as the complexity and cost of designing the architecture. 
     Another aspect of the present invention relates to a method for addressing a bistable electro-optic medium which improves the appearance of the display as the display is being updated. Electro-optic displays are driven from an initial state to a final state by means of a waveform comprising a series of voltage pulses, which may include pulses of zero voltage. Each voltage pulse induces part of a transition from an initial gray level to a final gray level. The optical nature of the transition from one image to another is an important attribute of the display performance. Also, the time of a transition (here, referred to as the update time) is a second important attribute of the display performance. The update time of many electro-optic displays is sufficiently long (typically of the order of several hundred milliseconds) that a user can observe intermediate states of the display between the initial and final states. For example, the aforementioned U.S. Pat. No. 7,012,600 describes so-called “slide show” waveforms in which each pixel is driven from its initial gray level to one extreme optical state (for example, white), then to the opposite extreme optical state (for example, black) and finally to the desired final gray level; the “excursions” to the two extreme optical states may be repeated. Although such slide show waveforms can produce accurate gray levels in the final image, they have the disadvantage that if all the pixels of the display are driven simultaneously to white and then to black, the user sees at least one “flash” between the initial and final images on the display. Most users find such flashes distracting and annoying. 
     Another problem in updating bistable displays is that, in practice, because it normally necessary to use drivers with only a limited number of voltage levels, the greater the number of gray levels which have to be written, the greater the update time. For this reason, it has been suggested that when rapid updating of a display is desirable, for example when a user is entering text in a dialogue box, the display should make use of two discrete drive schemes (the term “drive scheme” being used herein to denote a collection of waveforms sufficient to effect all possible transitions between the desired gray levels), one drive scheme typically being a monochrome drive scheme with a short update time and the other a non-monochrome gray level drive scheme; see for example, the aforementioned U.S. Pat. No. 7,119,772. However, such a “double drive scheme” approach can give rise to additional problems. The gray levels of the monochrome drive scheme may not correspond exactly to gray levels of the non-monochrome gray level drive scheme and, in view of the need to consider such factors as DC balance (again, see the aforementioned U.S. Pat. No. 7,119,772), special arrangements may be needed when a given pixel of the display switches between the two drive schemes. 
     One aspect of the present invention relates to a method of driving a bistable display which reduces or eliminates flashing and produces a more pleasing transition for a user. The method also eliminates the problems associated with switching between different drive schemes. 
     A third aspect of the present invention relates to optimizing gray levels using drivers having only a limited number of voltage levels. As already indicated, it is normally necessary to drive bistable electro-optic displays using drivers capable of providing only a limited number of voltage levels, because drivers capable of applying large numbers of voltage levels are considerably more expensive. Such drivers are normally arranged to operate with a series of timing or clock pulses with the driver applying the same voltage to a pixel during the interval between successive clock pulses, i.e., a graph of applied voltage against time essentially consists of a series of rectangles with the time dimension of each rectangle being an integral multiple of a predetermined clock period. Since only a limited number of driving voltages are available, the impulses which can be applied to the pixel (these impulses being proportional to the areas of the aforementioned rectangles) are quantized, and it may be difficult to combine such quantized impulses to reproduce accurately the impulse needed for a particular gray level transition, and there may be some inaccuracy in the final gray level resulting from the transition. 
     Such inaccuracy in final gray level can give rise to a “areal ghosting” problem. “Areal ghosting” refers to a phenomenon whereby, when a display bearing a first image is rewritten to display a second image, a vague “ghost” of the first image can be seen in the second image. One cause of such ghost images is inaccuracy in gray levels during the rewriting of the display. For example, consider a situation where a first image comprises a white shape on a black background, whereas the second image comprises a uniform field at an intermediate gray level. If, because of the limitations of the drivers and waveforms employed, the actual gray level resulting from the rewriting of the originally white pixels to the intermediate gray level differs visibly from the actual gray level resulting from the rewriting of the originally black pixels to the intermediate gray level, a “ghost” of the originally white shape will be visible in the second image. (Note that it does not matter whether it is the originally white pixels or the originally black pixels which are darker in the second image, a ghost image will still be visible; the ghost image may be similar to or an inverse of the first image.) Such ghost images are frequently objectionable to users of electro-optic displays, and the third aspect of the present invention seeks to reduce or eliminate such ghost effects. 
     SUMMARY OF THE INVENTION 
     In broad overview, the first aspect of the present invention involves display addressing architectures that utilize numerical data signals to apply addressing voltage impulses to pixels in the display without digital-to-analog conversion of the numerical data into a voltage impulse. According to principles of the invention, a display signal is used, for example, by a driver and related circuitry, to select voltage sources to provide a desired voltage impulse. The voltage sources can be, for example, one or more voltage rails. 
     A voltage impulse can be applied to a portion of a display medium defined by a pixel electrode, in part, by selecting one or more voltage sources, such as voltage rails, to provide a voltage of an amplitude indicated by the display signal, and by applying the voltage amplitude to the pixel electrode for a pre-selected period of time. The pre-selected period of time may be, for example, a refresh cycle or portion of a refresh cycle. The selected voltage sources may be connected to a pixel electrode simultaneously, sequentially, or some combination thereof. 
     One refresh cycle of a display, i.e., a “frame”, can be divided into multiple sub-cycles, i.e., “sub-frames”. In some embodiments, the voltage source or sources connected to a pixel electrode can be changed from one sub-frame to the next, to provide a total impulse that corresponds to a total impulse indicated by the display signal. In some embodiments, one or more voltage sources charge a pixel capacitor during one or more sub-frames until the capacitor attains a desired addressing voltage, which can be less than the voltage of the voltage sources. 
     Various embodiments of the invention utilize display image numerical data signals that are known to one having ordinary skill in the electronic display arts. For example, a display can include a controller, e.g., a video card, that processes image bitmap data and forwards image data to logic circuitry. The logic circuitry, as known in the art, can receive numerical voltage impulse data that characterizes a voltage signal, horizontal timing data, and vertical timing data. The logic circuitry can then provide numerical signals to row and column drivers. 
     The invention features, in part, addressing architectures in which driver circuitry need not include DACs. According to principles of the invention, a digital data signal, which includes data that identifies addressing impulses, can be used to select voltage sources having preexisting voltage amplitudes to provide the addressing impulses. Thus, a display need not utilize a digital-to-analog conversion process to produce a voltage impulse from a display signal. 
     Impulse duration information may be explicitly or implicitly included in a display signal. In the latter case, for example, the display signal may include a series of numbers that identify voltage magnitudes of addressing impulses while the duration of an impulse is implicitly indicated by the period of a display refresh cycle and/or sub-cycles. 
     Each digit of a number associated with a voltage impulse, for example, each bit of a binary number, can be used to select a related voltage source having a unique voltage amplitude for application during an associated sub-cycle of an addressing cycle. Thus, for example, a column driver can select different preexisting voltage amplitudes for application to each pixel electrode during each sub-cycle to obtain a total voltage impulse for each pixel electrode as indicated by a received display signal. 
     In other embodiments, an addressing voltage impulse is created, in part, by charging a pixel capacitor to a voltage amplitude that is less than the voltage amplitude of a voltage source. For example, voltage impulses can have a fixed duration and a variable voltage amplitude that is controlled by limiting the length of time a voltage source is connected to a pixel capacitor. In one embodiment, the resistance-capacitance (RC) time delay of a charging circuit is utilized to control the charging of the pixel capacitor. Thus, without use of digital-to-analog conversion of addressing impulse data, the data can be used by a source driver to control the impulse applied to a pixel electrode. 
     The first aspect of the present invention can provide, for example, lower cost of implementation for column driver circuits, faster design time and lower complexity, to decrease time-to-market and development risk. Smaller die size of integrated circuits (ICs) can decrease cost and increase yield. Smaller dice on a polycrystalline silicon panel can permit, for example, fabrication of more panels on a glass substrate or increase the fraction of panel footprint available for pixels. 
     In one embodiment of the first aspect of the invention, the number of transistors required to implement a power rail switching scheme is less than the number of transistors required to implement a conventional R-DAC system. The number of transistors can be further reduced when the number of voltage levels provided by the driver circuit is relatively low (for example, approximately 16 or fewer.) When the transistors are operated only in saturation mode, and no sensitive analog nodes exist in the circuit, a driver design can produce more accurate output levels, and can be less complex and easier to design, analyze, fabricate, and test. 
     Any of the above features can be used in an electro-optic display with a variety of display media, for example, an electrophoretic display medium, a rotating ball medium or an electrochromic medium. For example, such display media can include nonemissive display elements such as particles, particle-containing capsules (e.g., microencapsulated electrophoretic display elements), bichromal spheres or cylinders, or rotating round balls, dispersed in a binder. As a further example, an electrochromic medium can be used as a nonemissive display medium. 
     In embodiments that utilize a bistable medium, information regarding a present optical state of a pixel can be stored, for example, in a lookup table. If the pixel display medium optical state must be updated to accommodate a change in the displayed image, an addressing voltage impulse can then be applied to yield a change from a present optical state to the new optical state. 
     Thus, a voltage impulse applied to a display medium to obtain a new optical state is determined via a comparison of the desired optical state to the previous optical state. The required addressing voltage impulse is determined by calculating the voltage impulse required to drive the display medium from its present state to the desired optical state. 
     In a second aspect, the present invention provides a method for writing a final gray scale image on a bistable electro-optic display having a plurality of pixels each of which is capable of displaying at least four gray levels (including the two extreme optical states of each pixel), the method comprising applying a first set of waveforms to the display, thereby producing an intermediate image, and thereafter applying a second set of waveforms to the display, thereby producing the final image, wherein the first set of waveforms are chosen such that the intermediate image is a projection of the final image on to a subset of the gray levels of the display. In a preferred form of this method, each of the pixels of the display is capable of displaying 2 n  gray levels (where n is an integer greater than 1) and the intermediate image is a projection of the final image on to a subset of 2 m  gray levels (where m is an integer less than n). Accordingly, this second aspect of the present invention may hereinafter for convenience be called the “lower bit depth intermediate image” or “LBDII” method of the invention, although as noted above in the most general form of this method it is not essential that the ratio between the number of gray levels in the final image and the number of gray levels in the subset be an integral power of 2. For example, if the final image is a 16 gray level (4 bit, i.e., n=4) image, the intermediate image could be a 4 gray level (2 bit, i.e., m=2) image. However, more generally for the same 16 gray level final image, the intermediate image could make use of (say) 3 or 6 gray levels, even though 16/3 and 16/6 are not integral powers of 2. 
     The term “projection” is used herein in accordance with its conventional meaning in the imaging art to refer to a rendering of a gray scale image into a similar image using a smaller number of gray levels, such that the relationships between the gray levels of the various pixels are substantially preserved. More formally, such a projection requires that:
         (a) all pixels in the final image which are at the same gray level have the same gray level in the projection;   (b) for at least one gray level in the projection, all pixels in the final image which are at one of a group of contiguous gray levels are mapped to the same gray levels in the projection; and   (c) there are no inversions of relative gray levels in the projection (i.e., if in the final image pixel A is darker than pixel B, in the projection either pixels A and B have the same gray level—if the relevant gray levels are two of a contiguous group of gray levels in the final image which are mapped to the same gray level in the intermediate image—or pixel A is darker than pixel B in the intermediate image).
 
For example, if the final image has 16 gray levels denoted 0 (black) to 15 (white), the intermediate image could be a 4 gray level projection of the final image using only gray levels 2, 6, 10 and 14, with the mapping of the final image gray levels (shown within [ ]) to the intermediate gray levels (shown within { }) being as follows:
       [0, 1,2,3]→{2}   [4, 5,6,7]→{6}   [8, 9,10,11]→{10}   [12, 13, 14, 15]→{14}.   

     In a third aspect, the present invention provides a method of driving a bistable electro-optic display having a plurality of pixels each of which is capable of displaying at least two different optical states, which method comprises applying to at least one pixel of the display a waveform comprising a first drive pulse followed by a second drive pulse, wherein the absolute value of the voltage of the second drive pulse is less than the absolute value of the voltage of the first drive pulse. This method may for convenience be called the “Reducing voltage drive method” or RVD method. 
     The term “absolute value” is used herein in its normal algebraic sense to denote the magnitude of a number without regard to the sign thereof. In other words, in the method of the third aspect of the present invention, the voltage of the second drive pulse is less than the voltage of the first drive pulse, but the two voltages may be of opposite sign. 
     In one preferred form of the third aspect of the present invention, there is applied to at least one pixel of the display a first drive pulse of one polarity followed by a second drive pulse of the same polarity but lower voltage. The method may further comprise applying a third drive pulse of the same polarity as the first or second drive pulse but of lower voltage than the second drive pulse. 
     In another preferred form of the third aspect of the present invention, there is applied to at least one pixel of the display a first drive pulse of one polarity followed by a second drive pulse of the opposite polarity but lower voltage, followed in turn by a third drive pulse of the same polarity as the first drive pulse but of lower voltage than the second drive pulse. 
     Any of the methods of the present invention can be used to drive any of the types of electro-optic medium previously described. Thus, for example, the display used in the present processes may comprise a rotating bichromal member or electrochromic material. Alternatively, the display may comprise an electrophoretic material comprising a plurality of electrically charged particles disposed in a fluid and capable of moving through the fluid under the influence of an electric field. The electrically charged particles and the fluid may be confined within a plurality of capsules or microcells. Alternatively, the electrically charged particles and the fluid may be present as a plurality of discrete droplets surrounded by a continuous phase comprising a polymeric material. The fluid may be liquid or gaseous. 
     This invention extends to a display controller arranged to carry out any of the methods of the present invention. 
     The displays of the present invention may be used in any application in which prior art electro-optic displays have been used. Thus, for example, the present displays may be used in electronic book readers, portable computers, tablet computers, cellular telephones, smart cards, signs, watches, shelf labels and flash drives. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is pointed out with particularity in the appended claims. The advantages of the invention described above, together with further advantages, may be better understood by referring to the following description taken in conjunction with the accompanying drawings. In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. 
         FIG. 1   a  is a schematic diagram of an embodiment of an addressing structure of a display. 
         FIG. 1   b  is a flowchart of an embodiment of a method for addressing a display, as can be implemented, for example, by the structure shown in  FIG. 1   a.    
         FIG. 2   a  is a block diagram of an embodiment of a addressing structure. 
         FIG. 2   b  is a more detailed schematic diagram of a portion of the structure illustrated in  FIG. 2   a.    
         FIG. 3   a  is a block diagram of a sequential voltage frame addressing structure, which is an alternative detailed embodiment of the addressing structure shown in  FIG. 1   a.    
         FIG. 3   b  is a schematic diagram of one embodiment of a voltage supply. 
         FIGS. 3   c  to  3   e , are bar graphs that illustrate some addressing voltage impulses that can be applied to a pixel electrode by the addressing structure shown in  FIG. 3   a.    
         FIG. 4   a  is a schematic diagram of an embodiment of an addressing structure for a display. 
         FIG. 4   b  is a flowchart of an embodiment of a method for addressing a display, as can be implemented, for example, by the addressing structure shown in  FIG. 4   a.    
         FIGS. 5   a  to  5   e  show graphs of voltage versus time, which illustrate the functioning of one embodiment of the invention. 
         FIGS. 6   a  and  6   b  show respectively a prior art waveform for driving a bistable display and a modified waveform in accordance with the third aspect of the present invention. 
         FIG. 7  shows a second waveform in accordance with the third aspect of the present invention. 
         FIG. 8  is a graph showing reflectance of an electrophoretic medium as a function of time for various applied drive voltages. 
     
    
    
     DETAILED DESCRIPTION 
     As discussed above, this invention has three principal aspects, and these three principal aspects will primarily be described separately below. However, it should be understood that a single display may make use of more than one aspect of the present invention. For example, a display having an addressing architecture in accordance with the first aspect of the present invention may be used to carry out a drive method in accordance with the second or third aspect of the present invention. 
     Part A: Addressing Architecture 
       FIG. 1   a  is a schematic diagram of an embodiment of an addressing structure  10  of a display, according to principles of the invention. The display includes one or pairs of switch circuits  22  and pixel electrodes  23 , which may be arranged in one or more columns and one or more rows. The addressing structure  10  includes a switch unit  12  that responds to a display signal, and one or more voltage sources  14  that are each associated with a voltage level V 1 , V 2 , V 3 —at least two of which are preferably different—and are in electrical communication with the switch unit  12 . If the display includes a column of switch circuits  22 , the structure  10  may include a column electrode  18  electrically connecting the switch unit  12  to each of the switch circuits  22  in the column. 
     The addressing structure  10  may also include a display signal generator  16  that provides the display signal. The structure may include a column voltage selector  13  to control the switch unit  12  in response to the display signal, received, for example, from the display signal generator  16 . 
     The addressing structure  10  may include a data storage unit  17  that stores optical state information for portions of a display medium defined by the pixel electrodes  23 . The voltage selector  13  may then appropriately operate the switch unit  12  in response to a desired change in the optical state of one or more portions of display medium. The voltage selector  13 , for example, or another component, may compare a desired new optical state to a present optical state and determining an impulse that will change the optical state from the present state to desired new state. 
       FIG. 1   b  is a flowchart of an embodiment of a method  20  for addressing a display, as can be implemented, for example, by the addressing structure  10  shown in  FIG. 1   a . The method  20  includes providing a plurality of voltage sources, e.g., sources  14 , that preferably have different voltage levels from each other (Step  21 ), receiving a display signal that indicates an addressing impulse to be applied to a pixel electrode (Step  22 ), e.g., the pixel electrode  23 , and selecting a portion of the plurality of voltage sources responsive to the display signal (Step  23 ). The step of selecting (Step  23 ) may include comparing a new optical state to the present optical state of the pixel unit. 
     The method  20  further includes connecting the selected voltage sources to a switch circuit that is connected to the pixel electrode (Step  24 ). A selection signal may be applied to the switch circuit to activate it (Step  25 ) in cooperation with the step of connecting the selected voltage sources (Step  24 ). Thus, the selected voltage sources may be electrically connected to the pixel electrode. 
     The method  20  may further include storing data that identifies the present optical state of the pixel unit (Step  26 ). The stored data may then support a comparison of the new optical state with the present optical state. 
     Referring next to  FIGS. 2   a  and  2   b , voltage sources, such as the voltage sources  10  of the embodiment described above, may be provided via a set of voltage rails, each supplying a predetermined voltage. Thus, according to general principles illustrated by the method  20 , various combinations of the rails can be selected to obtain a voltage of a desired amplitude and sign to apply to a pixel unit. 
       FIG. 2   a  is a block diagram of an embodiment of a driver circuit  100 , i.e., an addressing structure. The driver circuit  100  illustrates a particular detailed implementation of the addressing structure  10  of  FIG. 1   a . The circuit  100  is configured to connect voltages to 324 column electrodes, each electrode connected to a column of switch circuits, such as pixel transistors. The driver circuit  100  includes a voltage rail switch unit  110  (related to the switch unit  12  shown in  FIG. 1   a ), voltage rail sources  150  in electrical communication with the voltage rail switch unit  110 , and a data latch  120 , a data register  130  and a 162-bit shift register  140  (the last three being related to the voltage selector  13  shown in  FIG. 1   a ). 
     The driver circuit  100  receives a display signal that identifies addressing impulses by a four-bit binary number and a sign bit. The binary number and sign bit indicate a voltage amplitude and a voltage sign. Depending on a particular implementation of the circuit  100 , the magnitude of the four-bit number can correspond to a voltage magnitude that is to be applied to a pixel electrode for one frame of a predetermined length of time. 
     The driver circuit  100  provides 324 pixel addressing voltage outputs, one for each of the 324 column electrodes. Each column electrode permits addressing of pixel electrodes attached to the column. The voltage rail sources  150  include 31 rails each providing different voltage levels. The driver circuit  100  is thus capable of applying 31 different output voltage levels to each of the 324 outputs, by selecting a voltage rail  150  having a desired voltage. The voltage levels are a reference voltage Vcom, 15 voltages greater than Vcom (positive voltages), and 15 voltages less than Vcom (negative voltages.) Each voltage level is provided by a corresponding power rail  150  that is in communication with the voltage rail switch unit  110 . 
     A numerical display signal, which includes data indicating a desired addressing impulse, is used by the driver  100  to select one or more of the voltage rails  150 . The selected rails have voltages associated with the impulse identified by the display signal. The driver circuit  100  thus provides a voltage impulse to be applied to a pixel electrode without use of conventional conversion of a numerical display signal data into an analog voltage impulse signal. 
     The driver circuit  100  can select output voltages without reliance on a DAC-based architecture. The driver circuit  100  uses display signal data to select none, one or more than one of the voltage rails  150 . The switch unit  110  may include transistors operating as switches to either connect or disconnect each power rail  150  to output lines, depending on the status of the data loaded for that output line. The driver circuit  100  can be implemented, for example, in any suitable semiconductor technology. 
     Some of the signals shown in  FIGS. 2   a  and  2   b  may be provided by a display controller. Signal indicia shown in  FIG. 2   a , which are familiar to one having ordinary skill in the electronic display circuitry arts, are as follows: 
     VDD—logic power supply, for example, supply a voltage of 3 V; 
     AVDD—driver power supply, for example, supply a voltage of 15 V; and 
     VSS—ground. 
     Signals associated with the shift register  140  are as follows: 
     SHL—shift direction control input, a parity bit which controls the direction in which the shift register  140  shifts; 
     DIO 1 -DIO 7 —seven start pulse inputs, set high to reset the shift register  140  and begin an image refresh cycle, only one of these seven inputs need be used in any specific configuration, but seven, or more or fewer, may be provided to enable the same circuitry to be used with displays having differing numbers of columns; and 
     Clock  1 —a fast clock signal, set to one half of the cycle scan rate. The vertical lines extending upwardly from the shift register  140  in  FIG. 2   a  carry enable signals (as also shown in  FIG. 2   b .) There are  162  enable lines, although only four are shown in  FIG. 2   a.    
     Signals associated with the data register  130 : 
     D 0 ( 0 : 3 )—a 4-bit data value specifying an impulse for an “odd” pixel electrode, i.e. a pixel electrode in an odd-numbered column; 
     D 1 ( 0 : 3 )—Similar to D 0 ( 0 : 3 ), but identifying an impulse for an even pixel electrode; 
     D 0 pol—polarity signal for odd pixel electrode; and 
     D 1 pol—polarity signal for even pixel electrode. 
     Signal associated with the data latch  120 : 
     Clock  2 —a slow clock signal that identifies a row scan rate. 
     Signals associated with the voltage rail switch unit  110 : 
     Y 001 -Y 324 —column voltage outputs, fed to the  324  column electrodes  18  of the driver circuit  100 ; and 
     BL—a blanking signal used to set all voltage outputs of the driver circuit  100 , i.e., Y 001 -Y 324 , to Vcom. Setting all voltage outputs to Vcom need not blank a bistable display; rather, it may stop the driver circuit  100  writing to the display, thus allowing a present image to remain. 
       FIG. 2   b  is a more detailed schematic diagram of a portion of the driver circuit  100  illustrated in  FIG. 2   a .  FIG. 2   b , in particular, shows more detail of the circuitry of the voltage rail switch unit  110 . The voltage rail switch unit  110  includes multiplexing units  111  (DMUX), for each pixel unit column of the display, and individual switches  112 . Each multiplexing unit  111  operates switches  112  to connect a portion of the voltage rails  150  to obtain a desired voltage for application to an associated column electrode, e.g., the column electrode  18 . 
     The switches  112  may include one or more transistors. Each of the voltage sources  150  can be connected to each of the outputs Y 001 -Y 324  via one of the switches  112 . A switch  112  may be, for example, a high voltage field-effect transistor (FET) capable of handling +/−15 V. The outputs of the multiplexing units  111  can be connected to the gates of these transistors. 
     The data register  130  includes  324  individual column data registers  131 , one for each corresponding column electrode. The data latch  120  includes  324  individual column data latches  121 , again, one for each corresponding column. 
     The circuit  100  operates in the following manner. First, a start pulse is provided by setting, for example, DIO 1  high to reset the shift register  140  to a starting location. The shift register may now operate in a conventional manner. For example, at each pulse of Clock  1 , one of the  162  outputs of the shift register shifts high, the other outputs are held low, and the high output is then shifted one place at each subsequent pulse of Clock  1 . 
     A display controller may provide the two four-bit impulse data values D 0 ( 0 : 3 ) and D 1 ( 0 : 3 ) and the two polarity signals D 0 pol and D 1 pol to inputs of the data registers  131  (see  FIG. 1B ). At the rising edge of each clock pulse Clock  1 , the two data registers  131  that are connected to the selected (high) enable output of shift register  140  write five-bit values, provided by combining the appropriate one-bit polarity signal D 0 pol or D 1 pol with the appropriate four-bit impulse values D 0 ( 0 : 3 ) or D 1 ( 0 : 3 ). The five-bit values are provided to the data Latches  121 , where the five-bit values may be Latched in a conventional fashion. After 162 cycles of Clock  1 , an array of 324 five-bit values will have been written into the data latches  121 , and this array of five-bit values represents the impulses to be applied to one row of pixel electrodes of the display. 
     When the data latches  121  receive the rising edge of a slow clock pulse, Clock  2 , the latches  121  write the latched values into the multiplexing units  111 . Each of the multiplexing units  111  has a single output (designated “ 1 ,” in  FIG. 2B , and held permanently high during typical operation of the circuit  100 ) and  32  outputs, one of which is held high and all the others low. The selected high output depends on the value written into the multiplexing unit  111  by the data latch  121 . 
     Each multiplexing unit  111  controls to which of the  31  possible input voltages sources  150 , V+(1-15), Vcom and V−(1-15), its associated output line, i.e., Y 001 -Y 324 , is connected. The different between the  32  possible outputs of each multiplexing unit  111  and the  31  possible input voltages is accommodated by permitting two different outputs of a multiplexing unit to both select the Vcom voltage rail  150 . 
     The whole of the foregoing procedure is repeated for each row of the display, the selected row being chosen by a row controller not shown in these drawings. The driver circuit  100  may be employed in a display having a common front electrode configuration, as suitable for active matrix electro-optic displays. In such a configuration, the viewing surface of the display (i.e., the surface through which an observer views the display) includes a transparent substrate bearing a single transparent common electrode which extends across all the pixels of the display. The electro-optic medium is disposed between this common electrode and a matrix of pixel electrodes disposed on an active matrix backplane. The voltage applied to the common front electrode is designated “Vcom”. For example, Vcom may be set to ground. Thus, to apply a non-zero voltage across a pixel capacitor, a voltage other than Vcom is placed on the pixel electrode. 
       FIG. 3   a  is a block diagram of a sequential voltage frame addressing structure  200 , which is an alternative detailed embodiment of the addressing structure  10  of  FIG. 1   a . The structure  200  includes a voltage supply  240  providing voltage sources  214 , column electrode  18   a , pixel transistors  22   a  connected to the column electrode  18   a , a source driver circuit  210  for each column of pixel transistors  22   a , a sequencer  220 , a switch unit  212 , and three voltage rails  215   a ,  215   b ,  215   c . The functionality of the switching unit  12  of the structure  10  shown in  FIG. 1   a  is associated with portions of multiple components of the addressing structure  200 , i.e., with portions of the source driver circuit  210 , the sequencer  220 , and the switch unit  212 . 
     The switch unit  212  communicates with nine voltage sources  214  as inputs to receive nine voltage levels having values of +V, +V/2, +V/4, +V/8, Vcom, −V/8, −V/4, V/2, and −V. The switch unit  212  communicates with the three voltage rails  215   a ,  215   b ,  215   c  as outputs to receive voltage levels provided via the switch unit  212 . One rail  215   b  is provided with a voltage level of Vcom, while the other two rails  215   a ,  215   c , responsive to control of the switch unit  212  by the sequencer  220 , sequentially are provided with voltage levels of V/8, V/4, V/2, and V, one rail  215   a  being positive and the other rail  215   c  being negative. 
       FIG. 3   b  is a schematic diagram of one embodiment  240   a  of the voltage supply  240 . The supply  240   a  includes a series of eight resistors  244  and eight buffers  242  that output the nine voltage Levels described above. The eight resistors act as voltage dividers, to supply each of the nine buffers  242  with an appropriate voltage. 
     The eight resistors  244  provide a potentiometer that defines nine voltages. The nine buffers  242  assert on the nine voltage sources  214  voltages that correspond to those defined by the potentiometer, while allowing circuitry attached to the nine voltage sources  214  to draw from these nine voltage sources  214  currents substantially in excess of the currents which could be drawn directly from the potentiometer outputs without significant distortion of the voltages on these outputs. 
     The addressing structure  200  can provide addressing impulses by providing different voltage levels during each sub-cycle of a refresh cycle. The embodiment can permit finer impulse modulation than possible for architectures that utilize pure time modulation, yet does not require the use of a DAC-based architecture in a column driver circuit. 
     Each source driver  210  is in communication with all three rails  215   a ,  215   b ,  215   c . The source driver  210  connects one of three rails  215   a ,  215   b ,  215   c  to a column electrode  18   a  as required to provide an addressing voltage to a pixel transistor  22   a  connected to the column electrode  18   a.    
     The addressing structure  200  utilizes an address cycle (one frame) of 160 ms, which is divided into  4  sub-frames of 40 ms each. The source drivers  210  select one of the three voltage rails  215   a ,  215   b ,  215   c  in response to a magnitude bit and a sign bit loaded from the sequencer  220  for each output line. The addressing structure  200  sequences the voltage supplied to the voltage rails  215   a ,  215   b ,  215   c , via switching instructions from the sequencer  220 , as each sub-frame of addressing occurs. One power rail  215   a  provides a voltage of Vcom, and the positive rail  215   b  and the negative rail  215   c  are switched from sub-frame to sub-frame, e.g., switched from V to V/2 to V/4 to V/8. 
     The sequencer  220  causes the switch unit  212  to connect voltage sources  214  to the rails  215   a ,  215   b ,  215   c  in an appropriate sequence with each sub-frame of a frame. The sequencer  220  also receives display signal input data (4 bits and a sign bit per pixel update) regarding a desired addressing voltage impulse. A source driver circuit  210  may then complete the connection of the appropriate voltage sources  214  to a pixel transistor  22   a  by connecting one of the three voltage rails  215   a ,  215   b ,  215   c  to the associated column electrode  18   a . Two bits are clocked into the driver  210  from the sequencer  220  in sequence with each sub-frame; a sign bit and a magnitude bit. The two bits identify the rail  215   a ,  215   b ,  215   c  to be connected to the pixel unit. 
     If the magnitude bit is set, either the positive or negative rail  215   a ,  215   c  is selected, depending on the value of the sign bit. If the magnitude bit is clear, the common plane supply rail  215   b  is selected. The drivers  210  function as one-bit voltage rail switch unit circuits. 
     Desired voltage impulses can be constructed by appropriately connecting rails  215   a ,  215   b ,  215   c  to a column electrode  18   a  during a cycle of the display. Thus, an effectively fine degree of impulse control is possible. 
     In more detail, the addressing structure  200  receives a display signal that identifies an addressing impulse, in part, via the four-bit binary number described above. The 4 bits of addressing voltage impulse data are mapped to the four sub-frames, one bit per sub-frame. For example, the first bit of data (i.e. the 8&#39;s bit for a 4-bit binary number) is mapped to the first sub-frame, the second bit of data (the 4&#39;s bit) is mapped to the second sub-frame, and so on. 
     The mapping provides that a bit of value zero will cause a voltage amplitude of Vcom to be applied during the corresponding sub-frame. A bit of value one will cause a voltage of V, V/2, V4 or V/8 to be applied, depending on the position of the bit. As described above, a sign bit determines whether the positive or negative rail  215   a ,  215   c  is selected. 
     The addressing structure  200  updates the image presented by the display once per cycle, herein referred to as a “frame”. A frame can be divided into more than one “sub-frame”, where all rows of pixels of the display may be partially addressed once per sub-frame. As described above, the present illustrative embodiment includes four sub-frames per frame. Other embodiments of the invention utilize, for example, one, two, three, five, six, seven, eight or more sub-frames. 
     The voltage amplitude of an addressing impulse can thus change from sub-frame to sub-frame. Further, a sub-frame may be used to refresh the charge of a pixel capacitor, if, for example, an applied voltage amplitude is to remain the same as during a previous frame. Thus, refreshing of capacitors during each sub-frame scan may be used to compensate for capacitor charge loss. 
     Numerous voltage impulses can be provided by the addressing structure  200  by making use of different combinations of sub-frame voltages. During the first of the four sub-frames, a voltage amplitude of +V, Vcom or −V can be applied. During the second sub-frame, a voltage amplitude of +V/2, Vcom or −V/2 can be applied. During the third sub-frame, a voltage amplitude of +V/4, Vcom or −V/4 can be applied. During the fourth sub-frame, a voltage amplitude of +V/8, Vcom or −V/8 can be applied. 
       FIGS. 3   c  to  3   e , are bar graphs that illustrate some addressing voltage impulses that can be applied to a pixel unit by the addressing structure  200  of  FIG. 3   a .  FIGS. 3   c ,  3   d  and  3   e  illustrate the application of a voltage impulse associated respectively with 4-bit voltage impulse data of +0001, −0010 and −1011. Each bit identifies whether or not the voltage amplitude of the associated sub-frame is to be applied during that sub-frame. For example, the impulse data of −0010 causes a voltage amplitude of −V/4 to be applied during the third sub-frame of a frame. 
     More generally, the first sub-frame may be used to provide a +/−V amplitude portion of an impulse, if required. The second sub-frame may provide +/−V/2, the third may provide +/−V/4, and so on such that, in an n th  sub-frame (with the first sub-frame being identified as the zeroth sub-frame), two of the voltage rails  215   a    215   c  are powered by +/−V/2^n). Integrated over all sub-frames, 2 n  distinct voltage impulses can be provided to pixel electrodes of the display. 
     The addressing structures  10 ,  100 ,  200  may permit delivery of deterministic and discrete impulses in a repeatable manner. The impulse values coded for a particular optical transition may be pre-determined according to a model or by empirical analyses. Thus, addressing structures according to principles of the invention are well suited to electro-optic display media that have a non-linear response to voltage changes and a response that varies during a voltage impulse. 
     The amount of time required for a full display image update may be greater than obtained via a conventional time or voltage modulation scheme. For the addressing structure  200 , the impulse integral of a maximal impulse is 2Vt, where t is the duration of application of the voltage. In other words, 2Vt is the integral of the voltage curve if a maximal impulse is commanded with the input data (i.e. +/−1111 in the above example.) If 2Vt is the impulse needed for the full scale optical transition of the display medium, the same response could be achieved in 2 frames rather than n frames (i.e., 4 frames in the above example) in a system with pure voltage or time modulation. Thus, a tradeoff may be required between more sub-frames for finer impulse control and longer response speed due to the need to sequence voltages across sub-frames. 
     The possible tradeoff can be mitigated in several ways. For example, a display may be operated in a hybrid mode, in which each update may be either a black/white update or a gray scale update. The nature of the update may be communicated from a controller to a sequencer via a dedicated signal line or a special command sequence. If a gray scale update is selected, a sequencer may process the update as described above. A gray scale update may be selected to either display a gray scale image or to change a gray scale image to a black and white image. 
     If a black and white update is selected, as required, for example, to switch between two pages of text, a sequencer may conduct the update as two sub-frames both of which are done with +/−V switches activated. The sequencer may then determine whether each pixel electrode should receive a positive full voltage for two frames, negative full voltage for two frames, or no impulse, and would send the appropriate data to the drivers during the two frames. In this way, a black and white update may occur at full speed while a gray scale update would require more time. 
     Other embodiments of an addressing architecture vary the particular features of the illustrative embodiment described above. For example, the order in which voltage amplitudes are applied can be varied, i.e., the order need not correspond to the bit order. Also, for example, the voltage amplitudes associated with more than one bit may be combined and applied during the same sub-frame. 
     Referring to  FIGS. 4   a ,  4   b , and  5   a  to  5   e , some embodiments of a method and architecture for addressing a display feature a time delay in the charging of a capacitive element to address a pixel electrode with a voltage of a desired amplitude. The voltage amplitude may be less than that of a voltage source applied to the capacitive element. 
       FIG. 4   a  is a schematic diagram of an embodiment of an addressing structure  300  for a display that includes one or more pixels. The structure  300  includes a resistive switch  320  and a capacitive element  335  associated with each pixel. The capacitive element  335  may be, for example, a capacitor formed in part from a pixel electrode. The resistive switch  320  may be, for example, a transistor. The addressing structure  300  also includes a voltage source  314 , an addressing voltage controller  330  in communication with the voltage source  314 , and the resistive switch  320 . 
     The resistive circuit  320  may be, for example, a FET. When activated by a selection signal, the FET provides a resistive link between the voltage output from the addressing voltage controller  330  and the capacitive element  335 . 
     The addressing voltage controller  330  provides a voltage Vd(t) to the resistive circuit  320  in response to an addressing impulse identified by a display signal. The voltage Vd(t) may be a column drive voltage directed to a column of pixels. A selection voltage Vg(t) may be applied to the resistive circuit  320  if it is, for example, a FET, to switch the resistive circuit  320  to an active, i.e., on, state; the voltage Vd(t) is then applied to the capacitive element  335  to cause it to gradually charge. 
     The capacitive element  335 , in turn, applies a voltage Vp(t) to the pixel unit. The capacitive element  335  and resistive circuit  320  cooperate to provide a RC delay time constant. The addressing structure  300  thus permits use of a single voltage source  314  to provide addressing impulses by permitting the gradual charging of the capacitive element  335  until it provides a desired addressing voltage for the pixel. 
     The addressing voltage controller  330  can include a digital pulse width modulation (PWM) driver, e.g., similar to drivers used in some gray-scale super-twisted nematic (STN) LCDs. Thus, an addressing voltage impulse can be controlled by controlling an amplitude of the impulse, even though a fixed voltage source may be employed. For example, a variable voltage impulse having an essentially constant pulse length can be produced while using a fixed amplitude voltage source  314  to produce a variable pulse voltage amplitude. The charging of the capacitive element  335  can be accomplished during a length of time that is brief in comparison to the duration of an addressing impulse. Thus, the capacitive element  335  may continue to apply a voltage impulse after an addressing voltage is disconnected from the element  335 . 
       FIG. 4   b  is a flowchart of an embodiment of a method  400  for addressing a display, as can be implemented, for example, by the addressing structure  300  described above with reference to  FIG. 4   a . The method  400  includes providing a capacitive element, e.g., element  335 , to apply an addressing voltage to a portion of a display medium (Step  410 ), providing a voltage source having a voltage greater than the addressing voltage (Step  420 ), and charging the capacitive element with the voltage source until the capacitive element applies the addressing voltage (Step  430 ). 
     Each row of capacitive elements of a display may be addressed once per scan of the entire display. For example, a gate driver for each row of pixel FETs activates the TFTs in that row once per scan. The addressing structure may be configured so that one line time, e.g., the amount of time a gate driver activates a single gate row, is enough to charge the capacitive element to a high percentage of its final value, e.g., 5RC. Thus, the final value effectively is a fully charged pixel capacitive element, i.e., the voltage of the capacitive element is effectively equal to the addressing voltage. 
     Alternatively, the line time and components of an addressing structure may be selected such that a line time is only, e.g., 2RC, and a capacitive element can be left partially charged at the completion of the line time. One way to implement partial charging of the pixel electrode is through use of a digital PWM source driver. The driver can utilize addressing impulse data to begin application of an addressing voltage at different times within a line time. At the end of the line time, e.g., when a source driver ceases to charge the capacitive element, the capacitive element can thus have a voltage that is controlled by the length of time the source driver allowed the capacitive element to be charged. 
     The level to which the capacitive element is charged is determined by the duration of the charging pulse delivered by the source driver during the line time. The capacitive element of the charging pulse can be controlled by use of the input data loaded into a PWM source driver. 
       FIGS. 5   a  to  5   c  shows graphs of voltage versus time that illustrate the functioning of one embodiment of the invention. The graphs illustrate how a gate driver voltage Vg ( FIG. 5   a ), a source driver addressing voltage Vd 1 , Vd 2 , Vd 3 , and Vd 4  (respectively,  FIGS. 5   b ,  5   c ,  5   d  and  5   e ) and a capacitive element voltage Vp 1 , Vp 2 , Vp 3 , and Vp 4  (respectively,  FIGS. 5   b ,  5   c ,  5   d  and  5   e ) can appear for an addressing structure having 2-bit PWM column drivers. The 2-bit PWM drivers may utilize 2-bit addressing impulse data to control the length of charging of capacitive elements. 
     The graphs illustrate the effect of input data of 11 ( FIG. 5   b ),  01  ( FIG. 5   c ),  10  ( FIG. 5   d ) and 00 ( FIG. 5   e ). For example, no addressing voltage is applied for input data of 00, and thus no charging of the capacitive element occurs. For data of 01, charging occurs for one portion of a full line time, and the final capacitive element voltage is thus less than the addressing voltage. The specific mapping between input data and Length of time of charging a capacitive element can be selected as desired for a particular application. 
     Preferably, the drivers are constructed such that they can output either a common plane voltage, or a positive or negative rail voltage (with respect to the common plane voltage.) Note that no net addressing voltage is applied to a capacitive element when the capacitive element is addressed with the common plane voltage. 
     The drivers preferably can modulate the portion of one line time that a column electrode is connected to a positive or negative rail. During the portion of one line time that the electrode is not connected to the positive or negative rail, it is preferably connected to the common voltage rail. 
     If multiple scans are required for a single update of an image presented by a display, voltage waveforms, e.g., may be produced continuously or may be used only to modify a capacitive element voltage during the last frame of an otherwise time-modulated impulse. 
     If used continuously, a capacitive element, which experiences the same PWM waveform during each line time of a series of scans during an update, will approach a steady state voltage related to the PWM value that is used during the pixel&#39;s line time. This addressing technique may be combined with other addressing techniques, for example, with frame-based time modulation. 
     Part B: Lower Bit Depth Intermediate Image Method 
     As already mentioned, the second aspect of the present invention relates to a method for writing a final gray scale image on a bistable electro-optic display having a plurality of pixels each of which is capable of displaying at least four gray levels, the method comprising applying a first set of waveforms to the display, thereby producing an intermediate image, and thereafter applying a second set of waveforms to the display, thereby producing the final image, wherein the first set of waveforms are chosen such that the intermediate image is a projection of the final image on to a subset of the gray levels of the display. In a preferred form of this method, each of the pixels of the display is capable of displaying 2 n  gray levels (where n is an integer greater than 1) and the intermediate image is a projection of the final image on to a subset of 2 m  gray levels (where m is an integer less than n). In other words, this aspect of the invention relates to rendering images in a “higher bit depth” through an intermediate image rendering at a “lower bit depth”, with the understanding that higher and lower bit depth do not necessarily refer to an image having exactly 2 x  gray levels, where x is a positive integer. Such an LBDII method can be advantageous in two ways: 
     (a) It may be more pleasing to the viewer for an image to be quickly rendered in a lower bit depth and then refined to a higher bit depth. This could be seen as more pleasing than a transition to a new image that does not first render the image in a lower bit depth; and 
     (b) With proper controller functionality, a drive scheme (a term which is used herein to refer to a set of waveforms capable of effecting all possible transitions between gray levels of a display) of this sort could be used to support updates to a variety of bit depths in a manner that affords the greatest uniformity in update appearance. 
     A standard transition from one image to another on a display can be represented schematically as:
 
{initial image; n-bit}→{final image; n-bit}
 
     Here, within each bracket the text after the semicolon indicates the bit depth of the image. The aforementioned E Ink and MIT patents and applications describe numerous drive schemes for achieving transitions between images of various bit depth, including 1-bit, 2-bit, and 4-bit. A 1-bit drive scheme makes transitions between images where the images are all rendered using a 1-bit grayscale, that is monochrome images:
 
{initial image; 1-bit}→{final image; 1-bit}
 
A 4-bit gray level drive scheme makes transitions between gray levels of a 4-bit gray scale, allowing rendering images using a 4-bit gray scale:
 
{initial image; 4-bit}→{final image; 4-bit}
 
     The LBDII drive method may be represented schematically as:
 
{initial image; n-bit}→{intermediate image; m-bit}→{final image; n-bit}  (A)
 
and
 
{initial image; m-bit}→{intermediate image; m-bit}→{final image; n-bit}  (B)
 
where m is less than n, and both m and n are greater than unity.
 
     The LBDII drive method has the inherent property that any transition of Type (A) can be halted at an intermediate point to achieve a transition:
 
{initial image; n-bit}→{“final” image; m-bit}  (C)
 
so that what was originally the intermediate m-bit image becomes the “final” image of the truncated transition. A single transition of Type (C) may be followed by one or more transitions of the type:
 
{initial image; m-bit}→{final image; m-bit}  (D)
 
before a final transition of Type (B) restores the display to “normal” n-bit operation allowing later transitions to be of Type (A). Note that while transitions of Type (D) are occurring, the display is in effect using a lower bit depth, and typically more rapid drive scheme, without the need to use a completely separate drive scheme, as in the prior art.
 
     The LBDII drive method has the advantages that: 
     (a) rendering the image in a lower bit depth on the way to a higher bit depth rendering will typically be more pleasing to a viewer than the use of a drive scheme where there is no clear low-bit-depth rendering on the way to a final bit depth rendering; and 
     (b) the drive scheme allow the display to make faster transitions to a lower bit depth image rendering and slower transition to a higher bit depth rendering. This flexibility allows one to select one of these options for one application and another for another application. For example, when a fast update is important, the rendering at a lower bit depth can be used, and when a higher bit depth rendering is more important, the full update can be used. Having one drive scheme be able to achieve two bit depths adds coherence to all the transitions. Such coherence would not be as great when two separate drive schemes are used to render images to two distinct bit depths. Furthermore, such issues as DC balance are more easily handled within a single transition than in two separate drive schemes. 
     Part C: Reducing Voltage Drive Method 
     As already mentioned, the third aspect of the present invention is a method of driving a bistable electro-optic display by applying to at least one pixel thereof a waveform comprising a first drive pulse followed by a second drive pulse, wherein the absolute value of the voltage of the second drive pulse is less than the absolute value of the voltage of the first drive pulse. 
     As discussed in the introductory part of this application, bistable electro-optic imaging media undergo changes in optical state when an impulse is applied, and an image can be retained for a substantial period without application of voltage. Because of this characteristic, the methods used to apply voltage impulses to such displays need to be well designed. For example, one prior art drive scheme for a monochrome (black and white) display comprises the following four waveforms: 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 transition 
                 waveform 
               
               
                   
                   
               
             
            
               
                   
                 black to black 
                    0 V for 420 ms 
               
               
                   
                 black to white 
                 −15 V for 400 ms, then 0 V for 20 ms 
               
               
                   
                 white to black 
                 +15 V for 400 ms, then 0 V for 20 ms 
               
               
                   
                 white to white 
                    0 V for 420 ms 
               
               
                   
                   
               
            
           
         
       
     
     For a black-to-white transition, the voltage impulse is:
 
−15V*400 milliseconds=−6 Volt seconds
 
Numerous different waveforms can be used to provide this impulse to a pixel of the display; to a first approximation, the impulse for this particular transition should be maintained constant to ensure that the final white state of the transition is also maintained constant. (As discussed in some of the E Ink and MIT patents and applications discussed above, the “white state” of a pixel may vary slightly depending upon the impulse and waveform applied during a transition.) Table 2 below list three possible waveforms which could be used for the same black-to-white transition:
 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 transition 
                 waveform 
               
               
                   
                   
               
             
            
               
                   
                 black to white 
                 −15 V for 400 ms, then 0 V for 20 ms 
               
               
                   
                   
                 −12 V for 500 ms, then 0 V for 20 ms 
               
               
                   
                   
                 −10 V for 600 ms, then 0 V for 20 ms 
               
               
                   
                   
               
            
           
         
       
     
     From Table 2, it will be seen that one advantage of using varying drive voltages is control of update time. A low driving voltage can be used for a long update time or a high driving voltage for a fast update. Another advantage of variable driving voltages is fine tuning of optical states. Consider a driver that provides a 50 Hz maximum refresh rate, which is a 20 ms minimum update time. A single driving voltage, −15V, for example, limits the minimum voltage impulse of −15 V*20 ms=−0.3 V-s; this also defines the minimum change of optical state. Because of this minimum impulse, the ability to achieve higher bit-depth in gray levels is limited. However, variable drive voltages allow fine tuning of gray levels when the minimum update time is limited by the driver, and thus help to achieve higher bit-depth in gray levels. The following Table 3 lists minimum voltage impulses for several different drive voltages, assuming a 20 ms minimum update time. 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 driving voltage, V 
                 update time, ms 
                 voltage impulse, V-s 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 −15 
                 20 
                 −0.3 
               
               
                 −12 
                 20 
                 −0.24 
               
               
                 −10 
                 20 
                 −0.2 
               
               
                   
               
            
           
         
       
     
     The reducing voltage method of the present invention makes use of a first relatively high drive voltage to provide a fast change in optical state, followed by a second lower drive voltages for fine-tuning of the final optical state. In contrast to prior art driving methods, in which often only a single drive voltage is employed, the RVD method of the present invention uses multiple drive voltages for each transition. 
     For example,  FIG. 6   a . is a prior art waveform using only a single drive voltage. In this waveform, a drive voltage of 12 V is applied for 600 milliseconds to apply an impulse of 12V*600 ms=7.2 V-s. In accordance with the RVD method of the present invention, the waveform shown in  FIG. 6   a  can be replaced by that shown in  FIG. 6   b , in which a first drive voltage of 15 V is applied for 300 milliseconds, a second drive voltage of 12 V is applied for 100 milliseconds and a third drive voltage of 10 V is applied for 200 milliseconds, to deliver the same 7.2 V-s impulse as the waveform shown in  FIG. 6   a.    
     The RVD method of the present invention may also be employed in so-called “slide show” waveforms as described in the aforementioned U.S. Pat. No. 7,012,600. As mentioned above, in such slide show waveforms, each pixel is driven from its initial gray level to one extreme optical state (for example, white), then to the opposite extreme optical state (for example, black) and finally to the desired final gray level; the “excursions” to the two extreme optical states may be repeated. Such waveforms are effective in securing accurate gray levels but distracting to the user because of the black and white flashes which they produce during image transitions. 
       FIG. 7  illustrates a slide show RVD waveform of the present invention. This waveform comprises a first drive pulse of +15 V, a second drive pulse of −12 V, and a third drive pulse of +12 V. All of these drive pulses are used to effect the slide show transitions to the extreme optical states, although it will be appreciated that the second and third drive pulses may not drive the display completely to an extreme optical state. The slide show RVD waveform shown in  FIG. 7  further comprises a fourth drive pulse of +10 V which drives the pixel to the desired gray level. 
       FIG. 8  illustrates the effect of applying, to a typical electrophoretic medium of the type described in the aforementioned U.S. Pat. No. 7,002,728, 5, 10 and 15 V pulses for various numbers of 20 millisecond frames; the pulses are applied starting from the extreme white optical state of the medium and drive the medium towards its black optical state. It will be seen that, during the first few five to ten frames, the rate of change of the reflectance is very high at both 10 and 15 V; in several cases, a single frame produces a change in reflectance of about 6 L* units or more. Since a typical display controller only allows one to apply drive pulses comprising an integral number of frames, a change in reflectance of 6 L* units per frame means that the gray level produced by the controller may be in error by about ±3 L* units. This is unacceptable, since in many images even casual observers can detect errors of 2 L* units. However, such errors can be greatly reduced using the RVD method of the present invention. 
     To take one extreme example,  FIG. 8  shows that a 2-frame 15 V drive pulse produces a reflectance of about 57 L*, whereas a 3-frame 15 V drive pulse produces a reflectance of about 49 L*. Hence, if one desired a gray level of 53 L*, use of 15 V drive pulses only would result in an unacceptable error of 4 L* units in this gray level. However,  FIG. 8  also shows that it take 13 frames of 5 V drive voltage to change reflectance from 57 L* to 49 L*, the former being reached after 8 frames and the latter only after 21 frames. Accordingly, any desired gray level within the range of 57 L* to 49 L* can be achieved with high precision by first applying 15 V for two frames, and then 5 V for from 1 to 12 additional frames. More specifically, if one desires to achieve the aforementioned gray level of 53 L*, this may be done by first applying 15 V for two frames, and then 5 V for 5 or 6 additional frames, with the final error in gray level being less than 0.5 L*, an error which is acceptable for most uses of electro-optic displays. 
     It should be noted that  FIG. 8  also illustrates why it is important to have high drive voltages available, namely that such high drive voltages increase the dynamic range of the medium, that is to say the maximum difference between the extreme optical states. It will be seen from  FIG. 8  that a long drive pulse at 15 V produces a final black state having a value of about 32 L*, whereas a long drive pulse at 10 V produces a final black state having a value of about 34 L*. Similarly, as appears at the left hand side of  FIG. 8 , driving at the extreme white optical state using a 15 V drive pulse produces a white state having a value of about 72 L*, whereas using a 10 V drive pulse produces a white state having a value of about 70 L*. Accordingly, it is desirable to have 15 V drive pulses available, since they provide a wider dynamic range, and hence a greater contrast ratio, than are available from the use of 5 and 10 V drive pulses alone. 
     The RVD method of the present invention necessarily uses at least two different (non-zero) voltages. Since electro-optic media are sensitive to the polarity of the drive voltage as well as its magnitude, the drive method must have the capability to drive the medium in both directions, and hence voltages are normally used as matched pairs of +V and −V, and the RVD method requires a minimum of five voltage levels (two negative, two positive and zero). More voltage levels can of course be used; for example, the drive method shown in  FIG. 6   b  uses three different positive voltages (10, 12 and 15 V) so that a total of seven voltage levels are required. However, since the cost and complexity of the circuitry needed to supply the various voltage levels increases with the number of voltage levels, it is desirable to limit the number of levels used. Typically, an RVD method of the present invention will use 5, 7 or 9 total voltage levels, which use 3- or 4-bit bandwidth in the driver. In summary, limited total voltage levels should be used, and the voltage levels should be chosen based on the characteristics of the electro-optic medium used. 
     The RVD method of the present invention can be used with both monochrome and gray scale displays. The method has the advantages that it can provide improved control of gray levels (i.e., the gray level achieved by a transition can very closely approach the theoretically desired level) and the display typically shows a reduced level of ghosting. 
     Part D: Illustrative Examples of Display Media that may be Used in Conjunction with Addressing Structures and Drive Methods of the Invention 
     Any of the types of display media described above may be included in a display according to the present invention. For example, such display media can include nonemissive display elements such as particles, bichromal spheres or cylinders, or rotating round balls. Such display media also include electrochromic display media and electrophoretic display media, which may be of the unencapsulated, encapsulated, polymer-dispersed or microcell types. 
     When the display medium includes particle-containing capsules, the capsules may be of any size or shape. In one embodiment of the invention, the capsules are spherical and have diameters in the millimeter or micron range. In a preferred embodiment, the capsule diameters are from about ten to about a few hundred microns. The capsules may be formed by an encapsulation technique and, in one embodiment, include two or more different types of electrophoretically mobile particles. 
     Some useful materials for constructing encapsulated electrophoretic displays are discussed below. 
     A. Particles 
     There is much flexibility in the choice of particles for use in electrophoretic displays, as described above. For purposes of this invention, a particle is any component that is charged or capable of acquiring a charge (i.e., has or is capable of acquiring electrophoretic mobility), and, in some cases, this mobility may be zero or close to zero (i.e., the particles will not move). The particles may be neat pigments, dyed (laked) pigments or pigment/polymer composites, or any other component that is charged or capable of acquiring a charge. Typical considerations for the electrophoretic particle are its optical properties, electrical properties, and surface chemistry. The particles may be organic or inorganic compounds, and they may either absorb light or scatter light. The particles for use in the invention may further include scattering pigments, absorbing pigments and luminescent particles. The particles may be retroreflective, such as corner cubes, or they may be electroluminescent, such as zinc sulfide particles, which emit light when excited by an AC field, or they may be photoluminescent. Finally, the particles may be surface treated so as to improve charging or interaction with a charging agent, or to improve dispersibility. 
     A preferred particle for use in electrophoretic displays of the invention is titania. The titania particles may be coated with a metal oxide, such as aluminum oxide or silicon oxide, for example. The titania particles may have one, two, or more layers of metal-oxide coating. For example, a titania particle for use in electrophoretic displays of the invention may have a coating of aluminum oxide and a coating of silicon oxide. The coatings may be added to the particle in any order. 
     The electrophoretic particle is usually a pigment, a polymer, a laked pigment, or some combination of the above. A neat pigment can be any pigment, and, usually for a light colored particle, pigments such as, for example, rutile (titania), anatase (titania), barium sulfate, kaolin, or zinc oxide are useful. Some typical particles have high refractive indices, high scattering coefficients, and low absorption coefficients. Other particles are absorptive, such as carbon black or colored pigments used in paints and inks. The pigment should also be insoluble in the suspending fluid. Yellow pigments such as diarylide yellow, hansa yellow, and benzidin yellow have also found use in similar displays. Any other reflective material can be employed for a light colored particle, including non-pigment materials, such as metallic particles. 
     Useful neat pigments include, but are not limited to, PbCrO 4 , Cyan blue GT 553295 (American Cyanamid Company, Wayne, N.J.), Cibacron Black BG (Ciba Company, Inc., Newport, Del.), Cibacron Turquoise Blue G (Ciba), Cibalon Black BGL (Ciba), Orasol Black BRG (Ciba), Orasol Black RBL (Ciba), Acetamine Blac, CBS (E. I. du Pont de Nemours and Company, Inc., Wilmington, Del.), Crocein Scarlet N Ex (du Pont) (27290), Fiber Black VF (DuPont) (30235), Luxol Fast Black L (DuPont) (Solv. Black 17), Nirosine Base No. 424 (DuPont) (50415 B), Oil Black BG (DuPont) (Solv. Black 16), Rotalin Black RM (DuPont), Sevron Brilliant Red 3 B (DuPont); Basic Black DSC (Dye Specialties, Inc.), Hectolene Black (Dye Specialties, Inc.), Azosol Brilliant Blue B (GAF, Dyestuff and Chemical Division, Wayne, N.J.) (Solv. Blue 9), Azosol Brilliant Green BA (GAF) (Solv. Green 2), Azosol Fast Brilliant Red B (GAF), Azosol Fast Orange RA Conc. (GAF) (Solv. Orange 20), Azosol Fast Yellow GRA Conc. (GAF) (13900 A), Basic Black KMPA (GAF), Benzofix Black CW-CF (GAF) (35435), Cellitazol BNFV Ex Soluble CF (GAF) (Disp. Black 9), Celliton Fast Blue AF Ex Conc (GAF) (Disp. Blue 9), Cyper Black IA (GAF) (Basic Blk. 3), Diamine Black CAP Ex Conc (GAF) (30235), Diamond Black EAN Hi Con. CF (GAF) (15710), Diamond Black PBBA Ex (GAF) (16505); Direct Deep Black EA Ex CF (GAF) (30235), Hansa Yellow G (GAF) (11680); Indanthrene Black BBK Powd. (GAF) (59850), Indocarbon CLGS Conc. CF (GAF) (53295), Katigen Deep Black NND Hi Conc. CF (GAF) (15711), Rapidogen Black 3 G (GAF) (Azoic Blk. 4); Sulphone Cyanine Black BA-CF (GAF) (26370), Zambezi Black VD Ex Conc. (GAF) (30015); Rubanox Red CP-1495 (The Sherwin-Williams Company, Cleveland, Ohio) (15630); Raven 11 (Columbian Carbon Company, Atlanta, Ga.), (carbon black aggregates with a particle size of about 25 μm), Statex B-12 (Columbian Carbon Co.) (a furnace black of 33 μm average particle size), and chrome green. 
     Particles may also include laked, or dyed, pigments. Laked pigments are particles that have a dye precipitated on them or which are stained. Lakes are metal salts of readily soluble anionic dyes. These are dyes of azo, triphenylmethane or anthraquinone structure containing one or more sulphonic or carboxylic acid groupings. They are usually precipitated by a calcium, barium or aluminum salt onto a substrate. Typical examples are peacock blue lake (CI Pigment Blue 24) and Persian orange (lake of CI Acid Orange 7), Black M Toner (GAF) (a mixture of carbon black and black dye precipitated on a lake). 
     A dark particle of the dyed type may be constructed from any light absorbing material, such as carbon black, or inorganic black materials. The dark material may also be selectively absorbing. For example, a dark green pigment may be used. Black particles may also be formed by staining latices with metal oxides, such latex copolymers consisting of any of butadiene, styrene, isoprene, methacrylic acid, methyl methacrylate, acrylonitrile, vinyl chloride, acrylic acid, sodium styrene sulfonate, vinyl acetate, chlorostyrene, dimethylaminopropylmethacrylamide, isocyanoethyl methacrylate and N-isobutoxymethacryl-amide), and optionally including conjugated diene compounds such as diacrylate, triacrylate, dimethylacrylate and trimethacrylate. Black particles may also be formed by a dispersion polymerization technique. 
     In the systems containing pigments and polymers, the pigments and polymers may form multiple domains within the electrophoretic particle, or be aggregates of smaller pigment/polymer combined particles. Alternatively, a central pigment core may be surrounded by a polymer shell. The pigment, polymer, or both can contain a dye. The optical purpose of the particle may be to scatter light, absorb light, or both. Useful sizes may range from 1 nm up to about 100 μm, as long as the particles are smaller than the bounding capsule. In a preferred embodiment, the density of the electrophoretic particle may be substantially matched to that of the suspending (i.e., electrophoretic) fluid. As defined herein, a suspending fluid has a density that is “substantially matched” to the density of the particle if the difference in their respective densities is between about zero and about two g/ml. This difference is preferably between about zero and about 0.5 g/ml. 
     Useful polymers for the particles include, but are not limited to: polystyrene, polyethylene, polypropylene, phenolic resins, Du Pont Elvax resins (ethylene-vinyl acetate copolymers), polyesters, polyacrylates, polymethacrylates, ethylene acrylic acid or methacrylic acid copolymers (Nucrel Resins—DuPont, Primacor Resins—Dow Chemical), acrylic copolymers and terpolymers (Elvacite Resins, DuPont) and PMMA. Useful materials for homopolymer/pigment phase separation in high shear melt include, but are not limited to, polyethylene, polypropylene, polymethylmethacrylate, polyisobutylmethacrylate, polystyrene, polybutadiene, polyisoprene, polyisobutylene, polylauryl methacrylate, polystearyl methacrylate, polyisobornyl methacrylate, poly-t-butyl methacrylate, polyethyl methacrylate, polymethyl acrylate, polyethyl acrylate, polyacrylonitrile, and copolymers of two or more of these materials. Some useful pigment/polymer complexes that are commercially available include, but are not limited to, Process Magenta PM 1776 (Magruder Color Company, Inc., Elizabeth, N.J.), Methyl Violet PMA VM6223 (Magruder Color Company, Inc., Elizabeth, N.J.), and Naphthol FGR RF6257 (Magruder Color Company, Inc., Elizabeth, N.J.). 
     The pigment-polymer composite may be formed by a physical process, (e.g., attrition or ball milling), a chemical process (e.g., microencapsulation or dispersion polymerization), or any other process known in the art of particle production. From the following non-limiting examples, it may be seen that the processes and materials for both the fabrication of particles and the charging thereof are generally derived from the art of liquid toner, or liquid immersion development. Thus any of the known processes from liquid development are particularly, but not exclusively, relevant. 
     New and useful electrophoretic particles may still be discovered, but a number of particles already known to those skilled in the art of electrophoretic displays and liquid toners can also prove useful. In general, the polymer requirements for liquid toners and encapsulated electrophoretic inks are similar, in that the pigment or dye must be easily incorporated therein, either by a physical, chemical, or physicochemical process, may aid in the colloidal stability, and may contain charging sites or may be able to incorporate materials which contain charging sites. One general requirement from the liquid toner industry that is not shared by encapsulated electrophoretic inks is that the toner must be capable of “fixing” the image, i.e., heat fusing together to create a uniform film after the deposition of the toner particles. 
     Typical manufacturing techniques for particles are drawn from the liquid toner and other arts and include ball milling, attrition, jet milling, etc. The process will be illustrated for the case of a pigmented polymeric particle. In such a case the pigment is compounded in the polymer, usually in some kind of high shear mechanism such as a screw extruder. The composite material is then (wet or dry) ground to a starting size of around 10 μm. It is then dispersed in a carrier liquid, for example ISOPAR® (Exxon, Houston, Tex.), optionally with some charge control agent(s), and milled under high shear for several hours down to a final particle size and/or size distribution. 
     Another manufacturing technique for particles drawn from the liquid toner field is to add the polymer, pigment, and suspending fluid to a media mill. The mill is started and simultaneously heated to temperature at which the polymer swells substantially with the solvent. This temperature is typically near 100.degree. C. In this state, the pigment is easily encapsulated into the swollen polymer. After a suitable time, typically a few hours, the mill is gradually cooled back to ambient temperature while stirring. The milling may be continued for some time to achieve a small enough particle size, typically a few microns in diameter. The charging agents may be added at this time. Optionally, more suspending fluid may be added. 
     Chemical processes such as dispersion polymerization, mini- or micro-emulsion polymerization, suspension polymerization precipitation, phase separation, solvent evaporation, in situ polymerization, seeded emulsion polymerization, or any process which falls under the general category of microencapsulation may be used. A typical process of this type is a phase separation process wherein a dissolved polymeric material is precipitated out of solution onto a dispersed pigment surface through solvent dilution, evaporation, or a thermal change. Other processes include chemical means for staining polymeric latices, for example with metal oxides or dyes. 
     B. Suspending Fluid 
     The suspending fluid containing the particles can be chosen based on properties such as density, refractive index, and solubility. A preferred suspending fluid has a low dielectric constant (about 2), high volume resistivity (about 10^15 ohm-cm), low viscosity (less than 5 cst), low toxicity and environmental impact, low water solubility (less than 10 ppm), high specific gravity (greater than 1.5), a high boiling point (greater than 90° C.), and a low refractive index (less than 1.2). 
     The choice of suspending fluid may be based on concerns of chemical inertness, density matching to the electrophoretic particle, or chemical compatibility with both the electrophoretic particle and bounding capsule. The viscosity of the fluid should be low when you want the particles to move. The refractive index of the suspending fluid may also be substantially matched to that of the particles. As used herein, the refractive index of a suspending fluid “is substantially matched” to that of a particle if the difference between their respective refractive indices is between about zero and about 0.3, and is preferably between about 0.05 and about 0.2. 
     Additionally, the fluid may be chosen to be a poor solvent for some polymers, which is advantageous for use in the fabrication of microparticles because it increases the range of polymeric materials useful in fabricating particles of polymers and pigments. Organic solvents, such as halogenated organic solvents, saturated linear or branched hydrocarbons, silicone oils, and low molecular weight halogen-containing polymers are some useful suspending fluids. The suspending fluid may comprise a single fluid. The fluid will, however, often be a blend of more than one fluid in order to tune its chemical and physical properties. Furthermore, the fluid may contain surface modifiers to modify the surface energy or charge of the electrophoretic particle or bounding capsule. Reactants or solvents for the microencapsulation process (oil soluble monomers, for example) can also be contained in the suspending fluid. Charge control agents can also be added to the suspending fluid. 
     Useful organic solvents include, but are not limited to, epoxides, such as, for example, decane epoxide and dodecane epoxide; vinyl ethers, such as, for example, cyclohexyl vinyl ether and Decave® (International Flavors &amp; Fragrances, Inc., New York, N.Y.); and aromatic hydrocarbons, such as, for example, toluene and naphthalene. Useful halogenated organic solvents include, but are not limited to, tetrafluorodibromoethylene, tetrachloroethylene, trifluorochloroethylene, 1,2,4-trichlorobenzene, carbon tetrachloride. These materials have high densities. Useful hydrocarbons include, but are not limited to, dodecane, tetradecane, the aliphatic hydrocarbons in the Isopar® series (Exxon, Houston, Tex.), Norpar® (series of normal paraffinic liquids), Shell-Sol® (Shell, Houston, Tex.), and Sol-Trol® (Shell), naphtha, and other petroleum solvents. These materials usually have low densities. Useful examples of silicone oils include, but are not limited to, octamethyl cyclosiloxane and higher molecular weight cyclic siloxanes, poly (methyl phenyl siloxane), hexamethyldisiloxane, and polydimethylsiloxane. These materials usually have low densities. Useful low molecular weight halogen-containing polymers include, but are not limited to, poly(chlorotrifluoroethylene) polymer (Halogenated hydrocarbon Inc., River Edge, N.J.), Galden® (a perfluorinated ether from Ausimont, Morristown, N.J.), or Krytox® from DuPont (Wilmington, Del.). In a preferred embodiment, the suspending fluid is a poly(chlorotrifluoroethylene) polymer. In a particularly preferred embodiment, this polymer has a degree of polymerization from about 2 to about 10. Many of the above materials are available in a range of viscosities, densities, and boiling points. 
     The fluid must be capable of being formed into small droplets prior to a capsule being formed. Processes for forming small droplets include flow-through jets, membranes, nozzles, or orifices, as well as shear-based emulsifying schemes. The formation of small drops may be assisted by electrical or sonic fields. Surfactants and polymers can be used to aid in the stabilization and emulsification of the droplets in the case of an emulsion type encapsulation. A preferred surfactant for use in displays of the invention is sodium dodecylsulfate. 
     It can be advantageous in some displays for the suspending fluid to contain an optically absorbing dye. This dye must be soluble in the fluid, but will generally be insoluble in the other components of the capsule. There is much flexibility in the choice of dye material. The dye can be a pure compound, or blends of dyes to achieve a particular color, including black. The dyes can be fluorescent, which would produce a display in which the fluorescence properties depend on the position of the particles. The dyes can be photoactive, changing to another color or becoming colorless upon irradiation with either visible or ultraviolet light, providing another means for obtaining an optical response. Dyes could also be polymerizable, forming a solid absorbing polymer inside the bounding shell. 
     There are many dyes that can be chosen for use in encapsulated electrophoretic display. Properties important here include light fastness, solubility in the suspending liquid, color, and cost. These are generally from the class of azo, anthraquinone, and triphenylmethane type dyes and may be chemically modified so as to increase the solubility in the oil phase and reduce the adsorption by the particle surface. 
     A number of dyes already known to those skilled in the art of electrophoretic displays will prove useful. Useful azo dyes include, but are not limited to: the Oil Red dyes, and the Sudan Red and Sudan Black series of dyes. Useful anthraquinone dyes include, but are not limited to: the Oil Blue dyes, and the Macrolex Blue series of dyes. Useful triphenylmethane dyes include, but are not limited to, Michter&#39;s hydrol, Malachite Green, Crystal Violet, and Auramine O. 
     C. Charge Control Agents and Particle Stabilizers 
     Charge control agents are used to provide good electrophoretic mobility to the electrophoretic particles. Stabilizers are used to prevent agglomeration of the electrophoretic particles, as well as prevent the electrophoretic particles from irreversibly depositing onto the capsule wall. Either component can be constructed from materials across a wide range of molecular weights (low molecular weight, oligomeric, or polymeric), and may be pure or a mixture. In particular, suitable charge control agents are generally adapted from the liquid toner art. The charge control agent used to modify and/or stabilize the particle surface charge is applied as generally known in the arts of liquid toners, electrophoretic displays, non-aqueous paint dispersions, and engine-oil additives. In all of these arts, charging species may be added to non-aqueous media in order to increase electrophoretic mobility or increase electrostatic stabilization. The materials can improve steric stabilization as well. Different theories of charging are postulated, including selective ion adsorption, proton transfer, and contact electrification. 
     An optional charge control agent or charge director may be used. These constituents typically consist of low molecular weight surfactants, polymeric agents, or blends of one or more components and serve to stabilize or otherwise modify the sign and/or magnitude of the charge on the electrophoretic particles. The charging properties of the pigment itself may be accounted for by taking into account the acidic or basic surface properties of the pigment, or the charging sites may take place on the carrier resin surface (if present), or a combination of the two. 
     Additional pigment properties which may be relevant are the particle size distribution, the chemical composition, and the lightfastness. The charge control agent used to modify and/or stabilize the particle surface charge is applied as generally known in the arts of liquid toners, electrophoretic displays, non-aqueous paint dispersions, and engine-oil additives. In all of these arts, charging species may be added to non-aqueous media in order to increase electrophoretic mobility or increase electrostatic stabilization. The materials can improve steric stabilization as well. Different theories of charging are postulated, including selective ion adsorption, proton transfer, and contact electrification. 
     Charge adjuvants may also be added. These materials increase the effectiveness of the charge control agents or charge directors. The charge adjuvant may be a polyhydroxy compound or an aminoalcohol compound, which are preferably soluble in the suspending fluid in an amount of at least 2% by weight. Examples of polyhydroxy compounds which contain at least two hydroxyl groups include, but are not limited to, ethylene glycol, 2,4,7,9-tetramethyl-decyne-4,7-diol, poly(propylene glycol), pentaethylene glycol, tripropylene glycol, triethylene glycol, glycerol, pentaerythritol, glycerol tris(12-hydroxystearate), propylene glycerol monohydroxystearate, and ethylene glycol monohydroxystrearate. Examples of aminoalcohol compounds which contain at least one alcohol function and one amine function in the same molecule include, but are not limited to, triisopropanolamine, triethanolamine, ethanolamine, 3-amino-1-propanol, o-aminophenol, 5-amino-1-pentanol, and tetrakis(2-hydroxyethyl)ethylene-diamine. The charge adjuvant is preferably present in the suspending fluid in an amount of about 1 to about 100 mg/g of the particle mass, and more preferably about 50 to about 200 mg/g. 
     The surface of the particle may also be chemically modified to aid dispersion, to improve surface charge, and to improve the stability of the dispersion, for example. Surface modifiers include organic siloxanes, organohalogen silanes and other functional silane coupling agents (Dow Corning® Z-6070, Z-6124, and 3 additive, Midland, Mich.); organic titanates and zirconates (Tyzor® TOT, TBT, and TE Series, DuPont, Wilmington, Del.); hydrophobing agents, such as long chain (C12 to C50) alkyl and alkyl benzene sulphonic acids, fatty amines or diamines and their salts or quaternary derivatives; and amphipathic polymers which can be covalently bonded to the particle surface. 
     In general, it is believed that charging results as an acid-base reaction between some moiety present in the continuous phase and the particle surface. Thus useful materials are those which are capable of participating in such a reaction, or any other charging reaction as known in the art. 
     Different non-limiting classes of charge control agents which are useful include organic sulfates or sulfonates, metal soaps, block or comb copolymers, organic amides, organic zwitterions, and organic phosphates and phosphonates. Useful organic sulfates and sulfonates include, but are not limited to, sodium bis(2-ethyl hexyl) sulfosuccinate, calcium dodecyl benzene sulfonate, calcium petroleum sulfonate, neutral or basic barium dinonylnaphthalene sulfonate, neutral or basic calcium dinonylnaphthalene sulfonate, dodecylbenzenesulfonic acid sodium salt, and ammonium lauryl sulphate. Useful metal soaps include, but are not limited to, basic or neutral barium petronate, calcium petronate, Co-, Ca-, Cu-, Mn-, Ni-, Zn-, and Fe salts of naphthenic acid, Ba-, Al-, Zn-, Cu-, Pb-, and Fe-salts of stearic acid, divalent and trivalent metal carboxylates, such as aluminum tristearate, aluminum octanoate, lithium heptanoate, iron stearate, iron distearate, barium stearate, chromium stearate, magnesium octanoate, calcium stearate, iron naphthenate, and zinc naphthenate, Mn- and Zn-heptanoate, and Ba-, Al-, Co-, Mn-, and Zn-octanoate. Useful block or comb copolymers include, but are not limited to, AB diblock copolymers of (A) polymers of 2-(N,N)-dimethylaminoethyl methacrylate quaternized with methyl-p-toluenesulfonate and (B) poly-2-ethylhexyl methacrylate, and comb graft copolymers with oil soluble tails of poly (12-hydroxystearic acid) and having a molecular weight of about 1800, pendant on an oil-soluble anchor group of poly (methyl methacrylate-methacrylic acid). Useful organic amides include, but are not limited to, polyisobutylene succinimides such as OLOA 1200 and 3700, and N-vinyl pyrrolidone polymers. Useful organic zwitterions include, but are not limited to, lecithin. Useful organic phosphates and phosphonates include, but are not limited to, the sodium salts of phosphated mono- and di-glycerides with saturated and unsaturated acid substituents. 
     Particle dispersion stabilizers may be added to prevent particle flocculation or attachment to the capsule walls. For the typical high resistivity liquids used as suspending fluids in electrophoretic displays, nonaqueous surfactants may be used. These include, but are not limited to, glycol ethers, acetylenic glycols, alkanolamides, sorbitol derivatives, alkyl amines, quaternary amines, imidazolines, dialkyl oxides, and sulfosuccinates. 
     D. Encapsulation 
     Liquids and particles can be encapsulated, for example, within a membrane or in a binder material. Moreover, there is a long and rich history to encapsulation, with numerous processes and polymers having proven useful in creating capsules. Encapsulation of the internal phase may be accomplished in a number of different ways. Numerous suitable procedures for microencapsulation are detailed in both  Microencapsulation, Processes and Applications , (I. E. Vandegaer, ed.), Plenum Press, New York, N.Y. (1974) and Gutcho,  Microcapsules and Microencapsulation Techniques , Nuyes Data Corp., Park Ridge, N.J. (1976). The processes fall into several general categories, all of which can be applied to the present invention: interfacial polymerization, in situ polymerization, physical processes, such as coextrusion and other phase separation processes, in-liquid curing, and simple/complex coacervation. 
     Numerous materials and processes should prove useful in formulating displays of the present invention. Useful materials for simple coacervation processes include, but are not limited to, gelatin, polyvinyl alcohol, polyvinyl acetate, and cellulosic derivatives, such as, for example, carboxymethylcellulose. Useful materials for complex coacervation processes include, but are not Limited to, gelatin, acacia, carageenan, carboxymethylcellulose, hydrolyzed styrene anhydride copolymers, agar, alginate, casein, albumin, methyl vinyl ether co-maleic anhydride, and cellulose phthalate. Useful materials for phase separation processes include, but are not limited to, polystyrene, PMMA, polyethyl methacrylate, polybutyl methacrylate, ethyl cellulose, polyvinyl pyridine, and poly acrylonitrile. Useful materials for in situ polymerization processes include, but are not limited to, polyhydroxyamides, with aldehydes, melamine, or urea and formaldehyde; water-soluble oligomers of the condensate of melamine, or urea and formaldehyde; and vinyl monomers, such as, for example, styrene, MMA and acrylonitrile. Finally, useful materials for interfacial polymerization processes include, but are not limited to, diacyl chlorides, such as, for example, sebacoyl, adipoyl, and di- or poly-amines or alcohols, and isocyanates. Useful emulsion polymerization materials may include, but are not limited to, styrene, vinyl acetate, acrylic acid, butyl acrylate, t-butyl acrylate, methyl methacrylate, and butyl methacrylate. 
     Capsules produced may be dispersed into a curable carrier, resulting in an ink which may be printed or coated on large and arbitrarily shaped or curved surfaces using conventional printing and coating techniques. 
     In the context of the present invention, one skilled in the art will select an encapsulation procedure and wall material based on the desired capsule properties. These properties include the distribution of capsule radii; electrical, mechanical, diffusion, and optical properties of the capsule wall; and chemical compatibility with the internal phase of the capsule. 
     The capsule wall generally has a high electrical resistivity. Although it is possible to use walls with relatively low resistivities, this may limit performance in requiring relatively higher addressing voltages. The capsule wall should also be mechanically strong (although if the finished capsule powder is to be dispersed in a curable polymeric binder for coating, mechanical strength is not as critical). The capsule wall should generally not be porous. If, however, it is desired to use an encapsulation procedure that produces porous capsules, these can be overcoated in a post-processing step (i.e., a second encapsulation). Moreover, if the capsules are to be dispersed in a curable binder, the binder will serve to close the pores. The capsule walls should be optically clear. The wall material may, however, be chosen to match the refractive index of the internal phase of the capsule (i.e., the suspending fluid) or a binder in which the capsules are to be dispersed. For some applications (e.g., interposition between two fixed electrodes), monodispersed capsule radii are desirable. 
     An encapsulation procedure involves a polymerization between urea and formaldehyde in an aqueous phase of an oil/water emulsion in the presence of a negatively charged, carboxyl-substituted, linear hydrocarbon polyelectrolyte material. The resulting capsule wall is a urea/formaldehyde copolymer, which discretely encloses the internal phase. The capsule is clear, mechanically strong, and has good resistivity properties. 
     The related technique of in situ polymerization utilizes an oil/water emulsion, which is formed by dispersing the electrophoretic composition (i.e., the dielectric liquid containing a suspension of the pigment particles) in an aqueous environment. The monomers polymerize to form a polymer with higher affinity for the internal phase than for the aqueous phase, thus condensing around the emulsified oily droplets. In one especially useful in situ polymerization processes, urea and formaldehyde condense in the presence of poly(acrylic acid) (See, e.g., U.S. Pat. No. 4,001,140). In other useful process, any of a variety of cross-linking agents borne in aqueous solution is deposited around microscopic oil droplets. Such cross-linking agents include aldehydes, especially formaldehyde, glyoxal, or glutaraldehyde; alum; zirconium salts; and poly isocyanates. The entire disclosures of the U.S. Pat. Nos. 4,001,140 and 4,273,672 are hereby incorporated by reference herein. 
     The coacervation approach also utilizes an oil/water emulsion. One or more colloids are coacervated (i.e., agglomerated) out of the aqueous phase and deposited as shells around the oily droplets through control of temperature, pH and/or relative concentrations, thereby creating the microcapsule. Materials suitable for coacervation include gelatins and gum arabic. 
     The interfacial polymerization approach relies on the presence of an oil-soluble monomer in the electrophoretic composition, which once again is present as an emulsion in an aqueous phase. The monomers in the minute hydrophobic droplets react with a monomer introduced into the aqueous phase, polymerizing at the interface between the droplets and the surrounding aqueous medium and forming shells around the droplets. Although the resulting walls are relatively thin and may be permeable, this process does not require the elevated temperatures characteristic of some other processes, and therefore affords greater flexibility in terms of choosing the dielectric liquid. 
     Coating aids can be used to improve the uniformity and quality of the coated or printed electrophoretic ink material. Wetting agents are typically added to adjust the interfacial tension at the coating/substrate interface and to adjust the liquid/air surface tension. Wetting agents include, but are not limited to, anionic and cationic surfactants, and nonionic species, such as silicone or fluoropolymer based materials. Dispersing agents may be used to modify the interfacial tension between the capsules and binder, providing control over flocculation and particle settling. 
     Surface tension modifiers can be added to adjust the air/ink interfacial tension. Polysiloxanes are typically used in such an application to improve surface Leveling while minimizing other defects within the coating. Surface tension modifiers include, but are not limited to, fluorinated surfactants, such as, for example, the Zonyl series from DuPont (Wilmington, Del.), the Fluorod® series from 3M (St. Paul, Minn.), and the fluoroalkyl series from Autochem (Glen Rock, N.J.); siloxanes, such as, for example, Silwet from Union Carbide (Danbury, Conn.); and polyethoxy and polypropoxy alcohols. Antifoams, such as silicone and silicone-free polymeric materials, may be added to enhance the movement of air from within the ink to the surface and to facilitate the rupture of bubbles at the coating surface. Other useful antifoams include, but are not limited to, glyceryl esters, polyhydric alcohols, compounded antifoams, such as oil solutions of alkyl benzenes, natural fats, fatty acids, and metallic soaps, and silicone antifoaming agents made from the combination of dimethyl siloxane polymers and silica. Stabilizers such as uv-absorbers and antioxidants may also be added to improve the lifetime of the ink. 
     Other additives to control properties like coating viscosity and foaming can also be used in the coating fluid. Stabilizers (UV-absorbers, antioxidants) and other additives which could prove useful in practical materials. 
     E. Binder Material 
     The binder is used as a non-conducting, adhesive medium supporting and protecting the capsules, as well as binding the electrode materials to the capsule dispersion. Binders are available in many forms and chemical types. Among these are water-soluble polymers, water-borne polymers, oil-soluble polymers, thermoset and thermoplastic polymers, and radiation-cured polymers. 
     Among the water-soluble polymers are the various polysaccharides, the polyvinyl alcohols, N-methylpyrrolidone, N-vinylpyrrolidone, the various Carbowax® species (Union Carbide, Danbury, Conn.), and poly-2-hydroxyethylacrylate. 
     The water-dispersed or water-borne systems are generally latex compositions, typified by the Neorez® and Neocryl® resins (Zeneca Resins, Wilmington, Mass.), Acrysol® (Rohm and Haas, Philadelphia, Pa.), Bayhydrol® (Bayer, Pittsburgh, Pa.), and the Cytec Industries (West Paterson, N.J.) HP line. These are generally latices of polyurethanes, occasionally compounded with one or more of the acrylics, polyesters, polycarbonates or silicones, each lending the final cured resin in a specific set of properties defined by glass transition temperature, degree of “tack,” softness, clarity, flexibility, water permeability and solvent resistance, elongation modulus and tensile strength, thermoplastic flow, and solids level. Some water-borne systems can be mixed with reactive monomers and catalyzed to form more complex resins. Some can be further cross-linked by the use of a crosslinking reagent, such as an aziridine, for example, which reacts with carboxyl groups. 
     A typical application of a water-borne resin and aqueous capsules follows. A volume of particles is centrifuged at low speed to separate excess water. After a given centrifugation process, for example 10 minutes at 60×G, the capsules are found at the bottom of the centrifuge tube, while the water portion is at the top. The water portion is carefully removed (by decanting or pipetting). The mass of the remaining capsules is measured, and a mass of resin is added such that the mass of resin is between one eighth and one tenth of the weight of the capsules. This mixture is gently mixed on an oscillating mixer for approximately one half hour. After about one half hour, the mixture is ready to be coated onto the appropriate substrate. 
     The thermoset systems are exemplified by the family of epoxies. These binary systems can vary greatly in viscosity, and the reactivity of the pair determines the “pot life” of the mixture. If the pot life is long enough to allow a coating operation, capsules may be coated in an ordered arrangement in a coating process prior to the resin curing and hardening. 
     Thermoplastic polymers, which are often polyesters, are molten at high temperatures. A typical application of this type of product is hot-melt glue. A dispersion of heat-resistant capsules could be coated in such a medium. The solidification process begins during cooling, and the final hardness, clarity and flexibility are affected by the branching and molecular weight of the polymer. 
     Oil or solvent-soluble polymers are often similar in composition to the water-borne system, with the obvious exception of the water itself. The latitude in formulation for solvent systems is enormous, limited only by solvent choices and polymer solubility. Of considerable concern in solvent-based systems is the viability of the capsule itself—the integrity of the capsule wall cannot be compromised in any way by the solvent. 
     Radiation cure resins are generally found among the solvent-based systems. Capsules may be dispersed in such a medium and coated, and the resin may then be cured by a timed exposure to a threshold level of very violet radiation, either long or short wavelength. As in all cases of curing polymer resins, final properties are determined by the branching and molecular weights of the monomers, oligomers and crosslinkers. 
     A number of “water-reducible” monomers and oligomers are, however, marketed. In the strictest sense, they are not water soluble, but water is an acceptable diluent at low concentrations and can be dispersed relatively easily in the mixture. Under these circumstances, water is used to reduce the viscosity (initially from thousands to hundreds of thousands centipoise). Water-based capsules, such as those made from a protein or polysaccharide material, for example, could be dispersed in such a medium and coated, provided the viscosity could be sufficiently lowered. Curing in such systems is generally by ultraviolet radiation. 
     While the invention has been particularly shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as described throughout this specification. For example, addressing architectures of the invention can be used in a variety of displays, for example, displays with electrophoretic or rotating ball media, and with encapsulated or unencapsulated media. For example, the number of sub-frames of a frame may be greater or fewer than described in the illustrative examples, and the number of digits of an addressing impulse data unit may be greater or fewer than described in the illustrative examples.