WO2008030740A2

WO2008030740A2 - Methods for driving electrophoretic displays

Info

Publication number: WO2008030740A2
Application number: PCT/US2007/077091
Authority: WO
Inventors: Thomas H. Whitesides; Joanna F. Au; Karl R. Amundson; Robert W. Zehner; Ara N. Knaian; Benjamin Zion
Original assignee: E Ink Corporation
Priority date: 2006-09-05
Filing date: 2007-08-29
Publication date: 2008-03-13
Also published as: WO2008030740A3

Abstract

A pixel of an electrophoretic display is driven from one extreme optical state to a second optical state different from the one extreme optical state by applying to the pixel a first drive pulse of one polarity; and thereafter applying to the pixel a second drive pulse of the opposite polarity, the second drive pulse being effective to drive the pixel to the second optical state.

Description

METHODS FOR DRIVING ELECTROPHORETIC DISPLAYS

[Para 1 ] This application is related to a series of patents and applications assigned to E Ink Corporation, this series of patents and applications being directed to MEthods for Driving Electro-Optic Displays, and hereinafter collectively referred to as the "MEDEOD" applications. This series of patents and applications comprises:

(a) U.S. Patent No. 6,504,524;

(b) U.S. Patent No. 6,531,997;

(c) U.S. Patent No. 7,012,600;

(d) U.S. Patent Publication No. 2005/0219184;

(e) U.S. Patent Publication No. 2006/0139310;

(f) U.S. Patent Publication No. 2006/0139311;

(g) U.S. Patent No. 7,193,625;

(h) U.S. Patent Publication No. 2007/0091418;

(i) U.S. Patent No. 7,119,772;

G) U.S. Patent Publication No. 2006/0232531;

(k) U.S. Patent No. 7,170,670;

(1) U.S. Patent Publication No. 2005/0179642;

(m) U.S. Patent Publication No. 2005/0212747;

(n) U.S. Patent Publication No. 2005/0213191;

(o) U.S. Patent Publication No. 2005/0280626;

(p) U.S. Patent Publication No. 2006/0038772;

(q) U.S. Patent No. 7,230,751;

(r) U.S. Patent Publication No. 2006/0262060; and

(s) International Application No. PCT/US2007/069529, filed May 26, 2007.

[Para 2] This invention relates to methods for driving electrophoretic displays. [Para 3] The terms "electro-optic", "gray state", and impulse are used herein in exactly the same manner as in the aforementioned International Application No. PCT/US2007/069529. This International Application also gives a general introduction to the technology of electrophoretic displays, to which the reader is referred for background information. The term "monochrome" may be used hereinafter to denote a drive scheme which only drives pixels to their two extreme optical states with no intervening gray states. [Para 4] The terms "bistable" and "bistability" are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in U.S. Patent No. 7,170,670 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called "multi-stable" rather than bistable, although for convenience the term "bistable" may be used herein to cover both bistable and multi-stable displays.

[Para 5] The term "drive pulse" is used herein to mean any application of a voltage for a time which can potentially change the optical state of an electrophoretic medium. The term "waveform" is used herein to refer to a series of one or more drive pulses effective to cause an electrophoretic medium to change from an initial gray level to a final gray level. The term "drive scheme" is used herein to refer to a set of waveforms covering all possible transitions between all gray levels desired in an electrophoretic medium.

[Para 6] Particle-based electrophoretic displays exhibit bistable or multi-stable behavior, 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. [Para 7] A further complication in driving electrophoretic displays is the need for so- called "DC balance". As discussed in the aforementioned U.S. Patents Nos. 6,531,997 and 6,504,524, problems may be encountered, and the working lifetime of a display reduced, if the method used to drive the display does not result in zero, or near zero, net time-averaged applied electric field across the electro-optic medium. A drive method which does result in zero net time-averaged applied electric field across the electro-optic medium is conveniently referred to a "direct current balanced" or "DC balanced".

[Para 8] It is, of course, also desirable to obtain the greatest possible dynamic range (the difference between the two extreme optical states, usually measured in units of L*, where L* has the usual CIE definition:

L* = 116(R/Ro)^1/3 - 16 where R is the reflectance and Ro is a standard reflectance value) and contrast ratio, when driving electrophoretic displays. As discussed in some of the aforementioned patents and applications, the extreme optical states of electrophoretic displays are to some extent "soft" and the exact optical state achieved can vary with the driving method used. It should be noted that simply increasing the length of a drive pulse does not always produce the most desirable extreme optical states.

[Para 9] It is also desirable to obtain stable optical states from an electrophoretic display. Although electrophoretic displays are typically bistable, this bistability is not unlimited, and the optical state of an electrophoretic display gradually changes over time when the display is allowed to remain undriven. It is desirable to reduce as far as possible the "drift" of the optical state of an electrophoretic display with time, and in particular it is desirable to reduce such drift during the first few minutes after a display is driven, which is the period which a user typically keeps a single image on a display used as an E-book reader or similar device.

[Para 1 0] It has now been found that these problems may be reduced or eliminated by modification of the method used to drive an electrophoretic display.

[Para 1 1 ] As noted in Paragraphs 269 et seq. of Publication No. 2005/0024353, complications in determining the optimum waveform for driving an electrophoretic medium arise from a phenomenon which may be called "impulse hysteresis". Except in rare situations of extreme overdrive at the optical rails, electro-optic media driven with voltage of one polarity always get blacker, and electro-optic media driven with voltage of the opposite polarity always get whiter. However, for some electro-optic media, and in particular some encapsulated electro-optic media, the variation of optical state with impulse displays hysteresis; as the medium is driven further toward white, the optical change per unit of applied impulse decreases, but if the polarity of the applied voltage is abruptly reversed so that the display is driven in the opposed direction, the optical change per impulse unit abruptly increases. In other words the magnitude of the optical change per impulse unit is strongly dependent not only upon the current optical state but also upon the direction of change of the optical state.

[Para 1 2] This impulse hysteresis produces an inherent "restoring force" tending to bring the electro-optic medium towards middle gray levels, and confounds efforts to drive the medium from state to state with unipolar pulses (as in general gray scale image flow) while still maintaining DC balance. As pulses are applied, the medium rides the impulse hysteresis surface until it reaches an equilibrium. This equilibrium is fixed for each pulse length and is generally in the center of the optical range. For example, it has been found empirically that driving one encapsulated four gray level electro-optic medium from black to dark gray required a l00 ms x -15 V unipolar impulse, but driving it back from dark gray to black required a 300 ms x 15 V unipolar impulse. This waveform was not DC balanced, for obvious reasons.

[Para 1 3] A solution to the impulse hysteresis problem is to use a bipolar drive, that is to say to drive the electro-optic medium on a (potentially) non-direct path from one gray level to the next, first applying an impulse to drive the pixel into either optical rail as required to maintain DC balance and then applying a second impulse to reach the desired optical state. For example, in the above situation, one could go from black to dark gray by applying 100 ms x -15 V of impulse, but go back from dark gray to white by first applying additional negative voltage, then positive voltage, riding the impulse curve down to the black state. Such indirect transitions also avoid the problem of accumulation of errors by rail stabilization of gray scale.

[Para 1 4] It has now been found that impulse hysteresis can usefully be exploited to provide various advantages in driving electrophoretic media, in particular improved DC balance, shortened switching times, improved extreme optical states and improved image stability. [Para 1 5] Accordingly, this invention provides a method of driving a pixel of an electrophoretic display from one extreme optical state to a second optical state different from the one extreme optical state, the method comprising applying to the pixel a first drive pulse of one polarity; and thereafter applying to the pixel a second drive pulse of the opposite polarity, the second drive pulse being effective to drive the pixel to the second optical state. This method may hereinafter for convenience be referred to as the "reverse pre-pulse method" or "RPP method", while the first drive pulse may be referred to as the "reverse pre-pulse" or simply "pre-pulse" while the second drive pulse may be referred to as the "main" drive pulse. [Para 1 6] In one form of this method, the second optical state is the opposed extreme optical state of the pixel. In another form of this method, the impulse of the first drive pulse is from about 15 to about 50, and preferably from about 20 to about 45, per cent of the sum of the absolute values of the first and second drive pulses. In the common situation where the first and second drive pulses are simple rectangular pulses with a constant voltage (of either sign) applied for a predetermined time, the first drive pulse may occupy from about 15 to about 50, and preferably from about 20 to about 45, per cent of the total time occupied by the first and second drive pulses. Either or both of the drive pulses used in the present method may include periods of zero voltage or (to put it another way) each of the drive pulses may actually comprise at least two sub-pulses separated by a period of zero voltage. There may be a pause (i.e., a period of zero voltage) between the RPP and the main pulse. [Para 1 7] It should be noted that the RPP method of the present invention need not be symmetric, in the sense that one may choose to use a reverse pre-pulse for a transition in one direction but not use a reverse pre-pulse for a transition in the opposite direction. Thus, a transition from a first extreme optical state to a second extreme optical state may be effected using a RPP and a main pulse, but the reverse transition from the second extreme optical state to the first extreme optical state may be effected using only a main pulse. For example, there is described below with reference to Figure 4 a specific preferred drive method for a monochrome display in which a RPP is used for a black-to-white transition but not for the reverse white-to-black transition.

[Para 1 8] The use of a RPP in accordance with the present invention need not increase the total time required for a transition between the two relevant optical states. It has been found that the use of a RPP enables the main drive pulse needed for a transition to be substantially shortened. Indeed, as illustrated in detail below, it has been found that, for example, it may be possible to replace a single conventional 250 millisecond 15 V drive pulse used for a black-to-white transition with a 60 millisecond -15V RPP followed by a 190 millisecond +15 V main pulse, with no increase in transition time but with an improved resulting white state.

[Para 1 9] The present invention is not, of course, confined to drive methods which use only a reverse pre-pulse and a main drive pulse; the present method may include additional drive pulses, as described in the aforementioned MEDEOD applications. In particular, the present method may include the use of reinforcing pulses after the main drive pulse, as described in the aforementioned Application Serial No. 11/751,879. Thus, when a first pixel is driven by a method of the present invention to one extreme optical state and a second pixel is already in that extreme optical state, there may be applied to the second pixel a reinforcing pulse of the same polarity as the second drive pulse applied to the first pixel, the reinforcing pulse being applied either simultaneously with the second drive pulse or within a predetermined period after the end of the second drive pulse.

[Para 20] The RPP method of the present invention can provide several advantages. Firstly, the method can reduce the DC imbalance for a given transition. For example, the aforementioned case in which a single 250 millisecond 15 V drive pulse is replaced by a 60 millisecond -15V RPP followed by a 190 millisecond +15 V main pulse reduces the DC imbalance for the transition by almost 50 per cent. Reducing the DC imbalance of a transition tends to make it easier to DC balance, or at least reduce the DC imbalance of, a drive scheme. (The term "drive scheme" is used herein the mean a set of all waveforms capable of effecting all transitions between gray levels of an electro-optic medium.) Secondly, the present invention enables improvement in the extreme optical states of at least some displays (i.e., it enables one to obtain whiter whites and blacker blacks) with consequent improvements in dynamic range and contrast ratio of the displays. Thirdly, the present invention can result in improvements in image stability.

[Para 21 ] The electrophoretic display used in the present invention may be of any of the types previously described. Thus, the electrophoretic display may comprise an electrophoretic medium having a single type of electrically charged particle disposed in a colored fluid. Alternatively, the electrophoretic display may comprise an electrophoretic medium having two types of electrically charged particles with different optical characteristics disposed in a fluid. In either case, the electrically charged particles and the fluid may be confined within a plurality of capsules or microcells, or may be present as a plurality of discrete droplets surrounded by a continuous phase comprising a polymeric material, so that the electrophoretic medium is of the polymer-dispersed type. The fluid may be liquid or gaseous.

[Para 22] This invention also provides an electrophoretic display comprising an electrophoretic medium having at least two different optical states, voltage supply means for applying a voltage to the electrophoretic medium, and a controller for controlling the voltage applied by the voltage supply means, the controller being arranged to drive the electrophoretic medium from one extreme optical state to a second optical state different from the one extreme optical state, by applying to the electrophoretic medium a first drive pulse of one polarity; and thereafter applying to the electrophoretic medium a second drive pulse of the opposite polarity, the second drive pulse being effective to drive the electrophoretic medium to the second optical state.

[Para 23] The present invention extends to a bistable electro-optic display, display controller or application specific integrated circuit (ASIC) arranged to carry out the method of the invention.

[Para 24] 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.

[Para 25] Figure 1 of the accompanying drawings is a graph showing the white state reflectivity (converted to L* units) as a function of pre-pulse length measured during the experiments described in Example 1 below.

[Para 26] Figure 2 is a graph showing the dynamic range as a function of pre-pulse length measured during the same series of experiments as in Figure 1.

[Para 27] Figure 3 is a graph showing the image stability of the black and white states of an electrophoretic medium as a function of pre-pulse length during a series of experiments described in Example 2 below.

[Para 28] Figure 4 shows the waveforms of a drive scheme employing the method of the present invention, as used in Example 10 below.

[Para 29] As already indicated, this invention relates to a method of driving an electrophoretic display in which a reverse pre-pulse is applied to a pixel which is in one of its extreme optical states, the reverse pre-pulse having a polarity which is normally used to drive the pixel towards the extreme optical state in which it already resides. The pre-pulse "drives the pixel into the optical rail" in effect trying to make an already-black pixel blacker or an already-white pixel whiter. The reverse pre-pulse is followed by a main drive pulse of the opposite polarity, which drives the pixel to a desired optical state different from its previous optical state, the desired optical state typically being the other extreme optical state of the pixel.

[Para 30] Although the MEDEOD applications and patents mentioned above describe many more complex drive schemes, one common technique for driving an electrophoretic display, especially if only monochrome driving is required, is to use a "square wave drive scheme" in which a drive pulse of constant voltage is applied to a pixel for a predetermined period, the polarity of the drive pulse varying of course with the direction of the transition being effected. One form of the present method modifies such a square wave drive scheme by inserting into one or more waveforms thereof a short pre-pulse of the opposite polarity before the main drive pulse. The total drive time in this process can remain unchanged. For example, if a 250 millisecond drive pulse at 15 V gives a good electro-optic response in a given display, it has been found that a waveform of the form (x) milliseconds at -15 V and (250-x) milliseconds at +15 V will, with the appropriate choice of the pre-pulse length x, gives a response that is improved in several or all of its important parameters. These include the optical states (White State, WS, and Dark State, DS, and therefore the dynamic range (DR) and contrast ratio (CR)), the image stability (IS), and the dwell time dependence (DTD); the last two parameters are defined below. The pre-pulse drive pulse length (PPPL) is a variable parameter, and has an optimal value for a given display. If the PPPL is zero, the drive is the conventional square wave drive scheme; if (reductio ad absurdum) the PPPL is equal to the total pulse length, then no drive to a second optical state will occur, and the dynamic range will be small (and probably in the wrong direction). The present invention thus gives a device designer an additional parameter (the PPPL) for use in the construction and operation of new electrophoretic display products and display media.

[Para 31 ] It has been found that, typically, reverse pre-pulses occupying about 15 to about 50, and preferably about 20 to about 45, per cent of the total drive time are most useful in the present invention. The reverse pre-pulse can therefore occupy a substantial part of the total drive time. It is thus very surprising that the advantages demonstrated below can be achieved without sacrificing (and even with improving) the dynamic range of a display, since the "right-way" drive time (i.e., the time during which a voltage of the polarity tending to drive the display toward the desired optical state) is, in the present method, substantially shortened by the partitioning of the total drive time between the pre-pulse and the main drive pulse.

[Para 32] While this invention may be used in gray scale displays, as already noted it is believed to be particularly useful in monochrome displays, especially the so-called "direct drive" displays having a backplane comprising a plurality of pixel electrodes each of which is provided with a separate conductor connected to drive circuitry arranged to control the voltage on the associated pixel electrode. Typically, such a display will have a single ("common") front electrode, on the opposed side of the electrophoretic medium from the pixel electrodes, and extending over a large number of pixel electrodes and typically the whole display. Accordingly, the following discussion will focus on such direct drive monochrome displays, since the necessary modifications for use with other types of display will readily be apparent to those skilled in the technology of electro-optic displays. The following discussion will also focus on driving such displays so as to achieve the brightest white state and darkest dark state possible, with good image stability and dwell time dependence. The following discussion also focuses on improvements achieved at constant total drive times, although of course total drive time is a parameter subject to optimization, taking into account the properties of the electrophoretic medium used and the intended application of the display; for example, a total drive time that might be unacceptable in an E- book reader might be perfectly acceptable in a sign, such as a railroad station sign, that might be updated only about once an hour.

[Para 33] The Examples below use the following abbreviated nomenclature. A waveform (reverse pre-pulse and subsequent main drive pulse) is indicated in the format:

Voltage x (PPPL/total drive time-PPPL).

Thus, a 15 V waveform with total length of 250 milliseconds (ms), using a pre-pulse of 60 ms, would be described as 15 V x (60/190 ms). As already noted, the present invention can use a pre-pulse and a main pulse having different voltage magnitudes; such a waveform is indicated by:

(Vl x PPPL/V2 x (Total drive time - PPPL)) .The voltages are of course always chosen so that the pre-pulse voltage is a wrong-way drive pulse (i.e., so that it drives the display into the relevant optical rail), and the main drive pulse is right-way.

[Para 34] Example 1 : White state reflectivity and dynamic range [Para 35] Experimental single-pixel electrophoretic displays having an encapsulated electrophoretic medium comprising polymer-coated titania and polymer-coated copper chromite were prepared substantially as described in Example 4 of the aforementioned U.S. Patent No. 7,002,728, except that heptane was used as the fluid instead of Isopar E. These experimental displays were driven using drive schemes of the present invention with a voltage of 15 V and a total drive time of 250 milliseconds, the pre-pulse length varying from 0 to 60 milliseconds (the zero pre-pulse length of course provides a control example). Thus, the waveforms used varied from 15 x (0/250) to 15 x (60/190). In a first series of experiments, the displays were driven to their black and white states and the reflectivities of these states measured 2 minutes after the end of the waveform. Figure 1 of the accompanying drawings shows the white state reflectivity (converted to L* units) as a function of pre-pulse length, while Figure 2 shows the dynamic range (white state reflectivity - dark state reflectivity, both expressed in L* units) also as a function of pre-pulse length. [Para 36] From Figure 1, it will be seen that the brightness of the white state increased monotonically with pre-pulse length over the range tested, increasing from 77.7 L* at zero pre-pulse length to 80.5 L* at 60 millisecond pre-pulse length. The latter, corresponding to a reflectivity of 57.4 per cent, is the brightest white state ever recorded for this type of electrophoretic medium. From Figure 2, it will be seen that the dynamic range peaked at around 20 to 40 millisecond pre-pulse length. [Para 37] Example 2 : Image stability

[Para 38] In a further series of experiments, the same displays as in Example 1 were tested for image stability using the same drive schemes as in Example 1 above. Experimentally, image stability is measured by driving the displays to their black or white state, measuring their reflectivity 3 seconds after the end of the waveform (this 3 second delay being used to avoid certain very short term effects which take place immediately after the end of the waveform) and again 2 minutes after the end of the waveform, the difference between the two readings, both expressed in units of L*, being the image stability. The image stability of the black and white states can of course differ, and the image stabilities of both states are plotted in Figure 3 as a function of pre-pulse length.

[Para 39] From Figure 3, it will be seen that increase in pre-pulse length caused a monotonic improvement (decrease) in the image stability values of both the black and white states with pre-pulse length within the range tested, although the improvement is much greater for the black state than for the white state. The black image stability at zero pre-pulse length was almost 7 L* units, which would be totally unacceptable in many applications. Using a 60 millisecond pre-pulse reduced the image stability to about 3 L* units, with a white state reflectivity greater than 56 per cent, a dynamic range of 53 L* units, and a contrast ratio of 12.5, all substantially better than the values of 53 per cent white state reflectivity, 52 L* units dynamic range and 10.7 contrast ratio at zero pre-pulse length. [Para 40] Examples 3-9 : Various electrophoretic media

[Para 41 ] To show that the advantageous results produced in Examples 1 and 2 above were not peculiar to the particular electrophoretic medium used, the experiments were repeated using differing electrophoretic media. Examples 3 and 4 were essentially repetitions of the formulation used in Examples 1 and 2 above. Example 5 increased the concentration of the Solsperse 17K charge control agent by approximately 50 per cent, while Example 6 was essentially similar to the composition used in Examples 1 and 2. Example 7 retained the original level of the Solsperse 17K but increased the level of polyisobutylene from 0.7 to 0.95 per cent, while Example 8 used the increased concentrations of both Solsperse 17K and polyisobutylene. Example 9 was a composition using polymer-coated carbon black as the black pigment and was prepared substantially as described in Examples 27-29 of the aforementioned U.S. Patent No. 6,822,782. A total driving time of 500 milliseconds was used in this Example because this medium switches more slowly than the copper chromite-based media. The results are shown in the Table below, in which bold face indicates improved performance with the reverse pre-pulse drive scheme of the present invention. [Para 42] Table

[Para 43] From the data in the Table, it will be seen that the performance of the copper chromite and carbon black-containing media was improved by the present driving methods (compare last column with the rest) and in most cases the modified performance is preferable to that obtained with a simple square wave. In the case of copper chromite media generally, the white state brightness is improved by 1-3 L* and in all of the cases shown, the dark state is either improved or increased by a negligible amount, so that the dynamic range is also increased. In the carbon black medium, the dark state is improved (in the case shown, by more than 4 L*) with a modest decrease in the white state, with the contrast ratio improving from 9.5 to 12.5. In almost all cases, the overall image stability and dwell time dependence are improved as well, in many cases from unacceptable to acceptable (less than about 3 L*) levels. Examples 5-8 constitute a designed experiment in Solsperse 17K and poly(isobutylene) levels. When operated using 15 V (0/250 ms) (square-wave) drive, many of these formulations show clearly unacceptable image stability. The use of the present drive methods improves image stability, while at the same time yielding distinctly improved electro-optic properties, particularly white state and dynamic range. Thus the present method can enable the use of lower Solsperse levels, which in turn (in practice) improves encapsulation yields.

[Para 44] Example 10 : Exemplary monochrome drive scheme [Para 45] An exemplary monochrome drive scheme using a reverse pre-pulse in accordance with the present invention is shown in Figure 4 of the accompanying drawings. [Para 46] This drive scheme is designed for use with a simple, low cost monochrome display (useful, for example, in a digital watch updated once every minute) having a plurality of pixel electrodes on one side of the electrophoretic medium and a single common front (or "top plane") electrode on the opposed side of the electrophoretic medium and extending across the entire display, each of the pixel electrodes and the front electrode being provided with a separate conductor which enables the relevant electrode to be held at one of only two voltages, 0 or +V, where V is a driving voltage. To enable electric fields of both polarities to be applied to the electrophoretic medium, the front electrode is periodically switched between 0 and +V.

[Para 47] Trace (a) in Figure 4 shows the voltages actually applied to the front electrode. These are, in order:

(i) 0 for 500 milliseconds (period AB in Figure 4);

(ii) +V for 500 milliseconds (period BC);

(iii) 0 for 100 milliseconds (period CDE);

(iv) +V for 250 milliseconds (period EFG);

(v) 0 for 750 milliseconds (period GHI); and

(vi) +V for 500 milliseconds (period IJK).

[Para 48] Trace (b) in Figure 4 shows the voltages actually applied to a pixel electrode for a pixel which is undergoing a black-to-black "transition", i.e., which is black in both the initial and final images, while Trace (c) shows the voltage difference between the pixel electrode and the front electrode and thus represents the electric field actually applied to the electrophoretic medium. As shown in Trace (b), the pixel electrode is held at 0 for the first 1350 milliseconds (period ABCDEFG), then held at +V for the final 1250 milliseconds (period GHIJK). The variation of the actual applied field is more complex, however. As shown in Trace (c), for the first 500 milliseconds (period AB), since both the pixel electrode and the front electrode are at 0, no field is applied. For the next 500 milliseconds (period BC), with the pixel electrode at 0 and the front electrode at +V, a field of -V is applied to the electrophoretic medium, which drives the relevant pixel white. For the next 100 milliseconds (period CDE), no field is applied, while for the following 250 milliseconds (period EFG) a field of -V is applied to the electrophoretic medium, which drives the relevant pixel white. At this point G, the pixel is white. For the next 750 milliseconds (period GHI), with the pixel electrode at +V and the front electrode at 0, a field of +V is applied, which drives the pixel black; by point I the pixel is back to the desired black state. Over the period IJK, no field is applied to the pixel, which remains black.

[Para 49] Trace (d) in Figure 4 shows the voltages applied to a pixel electrode for a pixel undergoing a black-to-white transition while Trace (e) shows the voltage difference between the pixel electrode and the front electrode. For the first 500 milliseconds (period AB), with the pixel electrode at +V and the front electrode at 0, a field of +V is applied to the pixel, which is thus driven black, i.e., a reverse pre-pulse is applied in accordance with the present invention. For the remainder of the transition period, the pixel electrode is held at 0. Accordingly, for the 500 millisecond period BC, a field of -V is applied to the pixel, which is thus driven white. For the period CDE, no field is applied to the pixel, for the period EFG the pixel is again driven white, for the period GHI, no field is applied to the pixel, and for the period IJK, the pixel is again driven white. The next result is that the pixel is driven black for 500 milliseconds and white for 1250 milliseconds, and ends up white. Note that, at point G, the pixel is already white.

[Para 50] Trace (f) in Figure 4 shows the voltages applied to a pixel electrode for a pixel undergoing a white-to-black transition while Trace (g) shows the voltage difference between the pixel electrode and the front electrode. For the entire period ABCDEFG, the pixel electrode is held at the same voltage as the front electrode, so that no field is applied to the pixel. Note that there is no reverse pre-pulse used in this white-to-black transition, so that the illustrated drive scheme is asymmetric in the sense used above. Note also that at point G the pixel is still in its original white state. The pixel electrode is held at +V over the 750 millisecond period GHI, while the front electrode is at 0, so that a voltage of +V is applied across the pixel, which is thus driven black. Finally, over the period IJK, no voltage is applied across the pixel.

[Para 51 ] It will be noted that the net effect of the white-to-black waveform shown in Figure 4 is a 750 millisecond +V pulse, while the net effect of the black-to-white waveform shown in this Figure is a 500 millisecond +V pulse followed by a 1250 millisecond -V pulse. Thus, the drive scheme shown in Figure 4 is DC balanced for white-black-white or black- white-black loops.

[Para 52] Finally, Trace (h) in Figure 4 shows the voltages applied to a pixel electrode for a pixel undergoing a white-to-white "transition" while Trace (i) shows the voltage difference between the pixel electrode and the front electrode. Over the entire period ABCD, the pixel electrode and the front electrode are held at the same voltage and no field is applied to the pixel. Over the 20 millisecond period DE, the pixel electrode is at +V and the front electrode at 0, while for the 80 millisecond period EF these potentials are reversed. Thus, the pixel experiences a 20 millisecond black-going pulse during period DE followed by an 100 millisecond white-going pulse during period EF. These two pulses together constitute a "double reinforcing pulse" as described in the aforementioned Application Serial No. 11/751,879, and are provided to ensure that the white color of the pixel undergoing the white- to-white transition matches the white color of the pixels undergoing the black-black and black-white transitions as described in this copending application. Over the period FG no field is applied to the pixel, so that at point G, the pixel is in its white state, newly "refreshed" by the double reinforcing pulse. Over period GH no field is again applied to the pixel. However, the period HIJ repeats the period DEF, thus applying a second double reinforcing pulse to the pixel to ensure that the color of the pixel matches the final white color of the pixel undergoing a black-to-white transition. Finally, over the period JK no field is applied to the pixel. The net effect of the waveform shown in Trace (i) is a 160 millisecond white-going pulse, which causes a small but tolerable DC imbalance in the drive scheme. [Para 53] Although the drive scheme shown in Figure 4 has a total length of 2600 milliseconds, the apparent length of the transition seen by an observer is only 2100 milliseconds since the only action taken during the first 500 millisecond period AB is the application of a black-going pulse to a black pixel, and such a pulse is not normally visible to an observer. At point G of the drive scheme all the pixels are white; hence, the drive scheme produces a visually pleasing transition, with the originally black pixels fading until the display is a uniform white, from which the black pixels of the new image then re-emerge. The drive scheme shown in Figure 4 has been found to give good results with an electrophoretic medium generally similar to that used in Examples 1 and 2 above but using Isopar E as the suspending fluid; the Figure 4 drive scheme produced a white state of 70 L* (40 per cent reflectivity) and a dark state of 28 L* (5.5 per cent reflectivity), and exhibited minimal ghosting.

[Para 54] The present invention may be useful with non-electrophoretic electro-optic media which exhibit behavior similar to electrophoretic media.

Claims

1. A method of driving a pixel of an electrophoretic display from one extreme optical state to a second optical state different from the one extreme optical state, the method comprising applying to the pixel a first drive pulse of one polarity; and thereafter applying to the pixel a second drive pulse of the opposite polarity, the second drive pulse being effective to drive the pixel to the second optical state.

2. A method according to claim 1 wherein the second optical state is the opposed extreme optical state.

3. A method according to claim 1 wherein the impulse of the first drive pulse is from 15 to 50 per cent of the sum of the absolute values of the first and second drive pulses.

4. A method according to claim 3 wherein the impulse of the first drive pulse is from 20 to 45 per cent of the sum of the absolute values of the first and second drive pulses.

5. A method according to claim 3 wherein the first and second drive pulses are simple rectangular pulses with a constant voltage of either sign applied for a predetermined time.

6. A method according to claim 1 wherein at least one of the first and second drive pulses comprises at least two sub-pulses separated by a period of zero voltage.

7. A method according to claim 1 wherein the first and second drive pulses are separated by a period of zero voltage.

8. A method according to claim 1 wherein a transition from a first extreme optical state to a second extreme optical state is effected using the first and second drive pulses, but a transition from the second extreme optical state to the first extreme optical state is effected using one or more pulses of a single polarity.

9. A method according to claim 1 wherein a first pixel is driven to one extreme optical state using the first and second drive pulses, and a second pixel is already in that extreme optical state, and there is applied to the second pixel a reinforcing pulse of the same polarity as the second drive pulse applied to the first pixel, the reinforcing pulse being applied either simultaneously with the second drive pulse or within a predetermined period after the end of the second drive pulse.

10. A method according to claim 1 wherein the electrophoretic display comprises an electrophoretic medium having a single type of electrically charged particle disposed in a colored fluid.

11. A method according to claim 1 wherein the electrophoretic display comprises an electrophoretic medium having two types of electrically charged particles with different optical characteristics disposed in a fluid.

12. A method according to claim 10 or 11 wherein the electrically charged particle and the fluid are confined within a plurality of capsules or microcells.

13. A method according to claim 10 or 11 wherein the electrically charged particles and the fluid are present as a plurality of discrete droplets surrounded by a continuous phase comprising a polymeric material.

14. A method according to claim 1 wherein the electrophoretic display comprises an electrophoretic medium comprising at least one type of electrically charged particle disposed in a gaseous fluid.

15. An electrophoretic display comprising an electrophoretic medium having at least two different optical states, voltage supply means for applying a voltage to the electrophoretic medium, and a controller for controlling the voltage applied by the voltage supply means, the controller being arranged to drive the electrophoretic medium from one extreme optical state to a second optical state different from the one extreme optical state, by applying to the electrophoretic medium a first drive pulse of one polarity; and thereafter applying to the electrophoretic medium a second drive pulse of the opposite polarity, the second drive pulse being effective to drive the electrophoretic medium to the second optical state.

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

17. An electronic book reader, portable computer, tablet computer, cellular telephone, smart card, sign, watch, shelf label or flash drive comprising a display arranged to carry out a method according to claim 1.