HK1147339B - Method for addressing a bistable electro-optic display - Google Patents

Description

Method for addressing bistable electro-optic displays

This application is a reissue application from the first divisional application of the following PCT international applications, which have been filed on 23/5/2003: international application No. PCT/US03/16434, national application No. 03813604.X entitled "method for driving electro-optic display", first divisional application No. 200810215240.8 entitled "method for driving electro-optic display".

The present invention relates to a method and apparatus for driving an electro-optic display, in particular a bistable electro-optic display. The method and apparatus of the present invention is primarily, although not exclusively, useful for driving a bi-stable electrophoretic display.

This application relates to U.S. Pat. Nos. 6,504,524 and 6,531,997. The present application also relates to co-pending international applications PCT/US02/10267 (publication No. WO02/079869) and PCT/US 02/37241.

The term "electro-optic" as applied to materials or displays is used herein in its conventional sense in imaging technology to refer to a material having first and second display states differing in at least one optical property, the material being transitioned from the first display state to the second display state by application of an electric field to the material. While this optical property is typically a color that is perceptible to the human eye, other optical properties are possible, such as optical transmission, reflectance, brightness, or a display read by a machine, a pseudocolor in the sense of a change in reflectance of an electromagnetic wave outside the visible range.

The term "grey scale state" as used herein is its conventional meaning in imaging technology and refers to a state intermediate two extreme optical states of a pixel and does not necessarily imply a black and white transition between the two extreme states. For example, electrophoretic displays in which the extreme states are white and deep blue, and thus the intermediate "grey state" will actually be pale blue, are described in the patents and published applications referenced below. In fact, as mentioned before, the transition between the two extreme states may also not be a change in color at all.

The terms "bistable" and "bistability" are used herein in their conventional sense in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property such that any given element is driven to assume either its first or second display state by an addressing pulse having a finite duration which, after the addressing pulse has terminated, will last for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of that display element. Shown in the aforementioned co-pending application serial No. 10/063236: some particle-based electrophoretic displays capable of displaying gray levels are stable not only in their extreme black and white states, but also in their intermediate gray states, as are other types of electro-optic displays. This type of display is properly referred to as "multi-stable" rather than bi-stable, but the term "bi-stable" as used herein covers both bi-and multi-stable displays for convenience.

The term "impact" as used herein is taken in its conventional sense: integration of voltage with respect to time. However, some bistable electro-optic media act as charge sensors, and another definition of impulse, i.e. the integral of current with respect to time (equal to the total charge applied), can be used for such media. Depending on whether the medium acts as a voltage-time impact sensor or a charge impact sensor, the appropriate definition for impact should be used.

Various types of bistable electro-optic displays are known. One type of electro-optic display is a rotating bichromal member of the type disclosed, for example, in U.S. patent 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 more accurate because the rotating member is not spherical in some of the above patents). Such displays use a large number of small bodies (typically spherical or cylindrical) having two or more portions with different optical properties, and an internal dipole. The bodies are suspended in liquid-filled vacuoles in a matrix, the vacuoles being filled with liquid so that the bodies can rotate freely. Applying an electric field to the display, the appearance of the display changes, thus rotating the bodies to various positions and changing the portions of the bodies seen through the viewing surface.

Another type of electro-optic medium uses an electrochromic medium, such as in the form of a nano-chrome (nanochromic) thin film, which includes an electrode formed at least in part of a semiconducting metal oxide and a plurality of reversibly color-changeable dye molecules attached to the electrode; see, e.g., O' Regan, B. et al Nature 1991, 353, 737; and Wood, d., Information Display, 18(3), 24 (3 months 2002). See also Bach, u., et al adv.mater., 2002, 14(11), 845. Nano-chromium films of this type are also described, for example, in U.S. patent No.6,301,038 and international application publication No. wo 01/27690.

Another type of electro-optic display that has been extensively studied and developed over the years is a particle-based electrophoretic display in which a plurality of charged particles move through a suspension under the influence of an electric field. Electrophoretic displays contribute to good brightness and contrast, wide viewing angles, state bistability, and low power consumption compared to liquid crystal displays. However, the long-term image quality issues of these displays have prevented their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in inadequate service life for these displays.

A number of patents assigned to or filed by the Massachusetts (MIT) and E Ink companies have recently been published which describe encapsulated electrophoretic media. Such encapsulated media comprise a plurality of capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held in a polymeric binder to form an adhesive layer between two electrodes. For example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185, respectively; 6,118,426, respectively; 6,120,588; 6,120,839, respectively; 6,124,851, respectively; 6,130,773, respectively; 6,130,774, respectively; 6,172,798; 6,177,921, respectively; 6,232,950, respectively; 6,249,721, respectively; 6,252,564, respectively; 6,262,706, respectively; 6,262,833; 6,300,932, respectively; 6,312,304, respectively; 6,312,971, respectively; 6,323,989, respectively; 6,327,072, respectively; 6,376,828, respectively; 6,377,387, respectively; 6,392,785, respectively; 6,392,786, respectively; 6,413,790, respectively; 6,422,687, respectively; 6,445,374, respectively; 6,445,489, respectively; 6,459,418, respectively; 6,473,072, respectively; 6,480,182, respectively; 6,498,114, respectively; 6,504,524; 6,506,438, respectively; 6,512,354, respectively; 6,515,649, respectively; 6,518,949, respectively; 6,521,489, respectively; 6,531,997, respectively; 6,535,197, respectively; 6,538,801, respectively; and 6,545,291 and U.S. patent application publication No. 200210019081; 2002/0021270, respectively; 2002/0053900, respectively; 2002/0060321, respectively; 2002/0063661, respectively; 2002/0063677, respectively; 2002/0090980, respectively; 2002/0106847, respectively; 2002/0113770, respectively; 200210130832, respectively; 2002/0131147, respectively; 2002/0145792, respectively; 2002/0154382, 2002/0171910; 2002/0180687, respectively; 2002/0180688, respectively; 2002/0185378, respectively; 2003/0011560, respectively; 2003/0011867, respectively; 2003/0011868, respectively; 2003/0020844, respectively; 2003/0025855, respectively; 2003/0034949, respectively; 2003/0038755, and international application publication No. wo 99/67678; WO 00/05704; WO 00/20922; WO 00/26761; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/67110; WO 00/67327; WO 01/07961; and WO 01/08241 describes a packaged medium of this type.

Many of the above patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium can be replaced with a continuous phase, thus producing a so-called polymer-dispersed (polymer-dispersed) electrophoretic display, wherein the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and the discrete droplets of the electrophoretic fluid within such a polymer-dispersed electrophoretic display can be considered as capsules or microcapsules, even if no discrete capsule membrane is associated with each individual droplet; see, e.g., 2002/0131147, supra. Thus, for the purposes of this application, such polymer-dispersed electrophoretic media are considered to be a subclass of encapsulated electrophoretic media.

Encapsulated electrophoretic displays typically do not suffer from the aggregation and settling failure modes of conventional electrophoretic display devices and provide additional advantages such as the ability to coat or print the display on a variety of flexible and rigid substrates. (use of the word "printing" is intended to include without limitation all forms of printing and coating: pre-measured coating such as patch di e coating, slot or bump coating, slide or cascade coating, curtain coating, adhesive rolls such as knife over roll coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscu (meniscus) coating, spin coating, brush coating, air knife coating, screen printing processes, electrostatic printing processes, thermal printing processes, ink jet printing processes, and other similar techniques.) thus, the manufactured display may be flexible. In addition, since the display medium can be printed (using various methods), the display itself can be manufactured inexpensively.

A related type of electrophoretic display is the so-called "microcell electrophoretic display". In microcell electrophoretic displays, the charged particles and suspending fluid are not encapsulated in microcapsules but are held within a plurality of cavities formed within a carrier medium, typically a polymer film. See, for example, International application publication No. WO 02/01281 and published U.S. application No.2002-0075556 (both assigned to Sipix Imaging, Inc.)

Although electrophoretic displays are typically opaque (since the particles substantially block visible light from passing through the display) and operate in a reflective mode, electrophoretic displays may operate in a so-called "shutter mode" in which the particles are arranged to move laterally within the display such that the display has a substantially opaque display state and a light-transmissive display state. 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. Similar to electrophoretic displays, but electrophoretic displays that rely on changes in electric field strength can also operate in a similar mode; see U.S. patent No.4,418,346. Other types of electro-optic displays can also operate in the shutter mode.

The bi-or multi-stable performance of particle-based electrophoretic displays, and the like of other electro-optic displays, is in sharp contrast to the performance of conventional Liquid Crystal (LC) displays. The properties of twisted nematic liquid crystals are not bistable or multistable but act as voltage sensors, so that the application of a given voltage to a pixel of such a display produces a particular grey level at that pixel, irrespective of the grey level previously present at that pixel. Furthermore, the LC display is driven in only one direction (from non-transmissive or "dark" to transmissive or "bright"), and the reverse transition from the lighter state to the darker state is achieved by reducing or eliminating the electric field. Finally, the grey levels of the pixels of an LC display are not sensitive to the polarity of the electric field, only to its magnitude, and indeed for technical reasons commercial LC displays typically reverse the polarity of the driving electric field at frequent intervals. In contrast to the first approximation, a bistable electro-optic display acts as an impact sensor, so that the final state of a pixel depends not only on the applied electric field and the time it takes to apply the electric field, but also on the state of the pixel before the electric field is applied.

Although it has been pointed out above that electrophoretic and other types of electro-optic displays exhibit bistability, this bistability is not without limit, and the image on such displays slowly fades over time, so that if the image is to remain for a long period, the image must be periodically refreshed in order to restore the image to the optical state it had when first written.

However, such refreshing of images may cause its own problems. As discussed in the aforementioned U.S. patent nos. 6,531,997 and 6,504,524, problems may be encountered and the operating life of the display reduced if the method of driving the display does not produce a net time-averaged applied electric field of zero (or near zero) across the electro-optic medium. The driving method that produces a net time-averaged applied electric field of zero across the electro-optic medium is commonly referred to as "DC-balancing" or "DC-balancing". If the image is to be held for a long time by applying refresh pulses, the polarity of these pulses needs to be the same as the polarity of the addressing pulses used to drive the relevant pixels of the display to the optical state being held, which results in a DC-unbalanced driving scheme.

In accordance with another aspect of the present invention, it has been found that if refresh is accomplished using short pulses, the image on the display can be refreshed while reducing the deleterious effects associated with DC imbalance drive schemes.

Another aspect of the invention relates to dealing with the problem of: the driving requirements of the aforementioned bistable electro-optic displays render conventional driving methods for driving LCDs unsuitable for such bistable electro-optic displays. Furthermore, as discussed in the aforementioned U.S. Pat. Nos. 6,531,997 and 6,504,524, problems may be encountered and the operating life of the display reduced if the method of driving the display does not produce a net time-averaged applied electric field of zero (or near zero) across the electro-optic medium. The driving method that produces a net time-averaged applied electric field of zero across the electro-optic medium is commonly referred to as "DC-balancing" or "DC-balancing". Similar problems are encountered with LCDs but the DC balance problem is less important in LCDs because such displays are not sensitive to the polarity of the applied electric field and, in turn, have the ability to reverse polarity at will. However, the need for DC balancing is an important consideration in designing drive schemes for bistable electro-optic displays (in which the electro-optic medium is sensitive to the polarity of the applied electric field).

Accordingly, another aspect of the invention relates to a method and apparatus for driving an electro-optic display which meets the special requirements of the bi-stable display discussed above. Certain methods and apparatus of the present invention are primarily directed to producing accurate gray scale reproduction in bi-stable displays.

Accordingly, in one aspect, the invention provides a method for addressing a bistable electro-optic display having at least one pixel, the method comprising: applying an addressing pulse to drive the pixel to a first optical state;

keeping the pixel undriven for a period of time, thereby allowing the pixel to assume a second optical state different from the first optical state;

a refresh pulse is applied to the pixel, which refresh pulse substantially restores the pixel to the first optical state, the refresh pulse being shorter with respect to the addressing pulse.

This aspect of the invention is hereinafter referred to for convenience as the "refresh pulse" method of the invention.

In the refresh pulse method, the impact of the refresh pulse is generally not more than about 20% of the impact of the address pulse, desirably not more than about 10% of the impact, and preferably not more than 5% of the impact. For reasons explained below, typically the difference between the first and second optical states does not exceed about one unit of L^＊(wherein L^＊With the usual CIE definition); desirably, the difference is no more than about 0.5 units of L^＊And preferably no more than about 0.2 units of L^＊. A plurality of refresh pulses may be applied to the pixel at regular intervals.

In one form of the refresh pulse method, after applying the refresh pulse, a second address pulse is applied to the display, the second address pulse driving the pixel to a third optical state different from the first and second optical states, and wherein the impulse applied by the second address pulse is the sum of: (a) the impulse required to drive the pixel from the first optical state to the third optical state, and (b) an impulse of equal and opposite magnitude and polarity to the algebraic sum of the refresh pulses applied to the pixel between the first and second addressing pulses. The second addressing pulse may be constant in voltage but varying in duration. In a display comprising a plurality of pixels, the second addressing pulse may be a blanking pulse which drives all pixels of the display to one extreme optical state. In one preferred form of this "blanking/refresh pulse" process, the display comprises a plurality of pixels, a first addressing pulse being applied to each pixel to drive a first group of pixels to white and a second group of pixels to black, at least one refresh pulse being applied to each pixel, and then a first blanking pulse to black all pixels, and a second blanking pulse to drive all pixels to white, the blanking pulses being applied in any order. The impact applied to the pixels of each first group during the first blanking pulse is the sum of: (a) the impulse required to drive the pixel from white to black, and (b) an impulse of equal algebraic and magnitude but opposite polarity to the refresh pulse applied to the pixel between the first address pulse and the first blanking pulse. Similarly, the impact applied to the pixels of each second group during the second blanking pulse is the sum of: (a) the impulse required to drive the pixel from black to white, and (b) an impulse of equal algebraic and magnitude but opposite polarity to the refresh pulse applied to the pixel between the first address pulse and the first blanking pulse.

The refresh pulse method of the present invention can be used with any of the types of electro-optic media previously described. Thus, in this approach, the display may be a rotating dichroic element or an electrochromic display, or an electrophoretic display, ideally an encapsulated electrophoretic display.

In another aspect, the invention provides a method for addressing a bistable electro-optic medium comprising applying an ac pulse having a dc offset (offset) to the medium.

In another aspect, the invention provides a method of addressing a bistable electro-optic medium comprising applying alternating pulses to the medium and varying at least one of the frequency and duty cycle of the pulses to vary the optical state of the electro-optic medium with the alternating pulses.

In another aspect, the invention provides a method of driving a bistable electro-optic display comprising a plurality of pixels arranged in a plurality of rows and a plurality of columns; a plurality of row electrodes, each row electrode associated with one of the plurality of rows; a plurality of column electrodes, each associated with one of the plurality of columns; and drive means arranged to select each of the row electrodes in turn and to apply selected voltages to the column electrodes during selection of any given row electrode, so as to address the pixels in the row associated with the selected row electrode and to write a row of a desired image on the display. The method comprises the following steps:

writing a first image to a display;

receiving data representing a second image to be written onto the display;

comparing the first and second images and dividing the rows of the display into a first group and a second group, at least one pixel between the first and second images being different in the rows of the first group and the pixel being the same between the first and second images in the rows of the second group; and

a second image is written by sequentially selecting only the row electrodes associated with the first set of rows and applying voltages to the column electrodes to write only the first set of rows, thereby forming a second image on the display.

In another aspect, the invention provides an electro-optic display having a plurality of pixels, at least one of the pixels comprising a plurality of sub-pixels of mutually different areas, the display comprising drive means arranged to change the optical states of said sub-pixels independently of each other. In such a display, it is desirable that at least two of the sub-pixels have areas that differ by a factor of substantially 2

Preferred embodiments of the present invention will now be described, but are by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a graph showing the change in gray level over time in a display addressed using direct current pulses with pulse length modulation;

FIG. 2 is a graph similar to FIG. 1 of the change in gray scale over time for a display addressed using DC pulses with pulse height modulation;

FIG. 3 is a graph similar to FIG. 1 of the gray scale level of a display addressed using AC pulses with DC offset as a function of time in accordance with the present invention;

FIG. 4 is a graph similar to FIG. 1 of the change in gray scale over time for a display addressed using AC pulses with duty cycle modulation according to the present invention;

FIG. 5 is a graph showing the variation of gray level over time in a display addressed using a double pre-pulse slide show (slider) waveform;

FIG. 6 is a graph showing the variation of gray level over time in a display addressed using a single pre-pulse slide show waveform;

fig. 7A and 7B show possible arrangements of sub-pixels within a single pixel of a display of the invention.

As has been indicated previously, the present invention provides methods of addressing electro-optic media and displays, as well as many improvements in the structure of such displays. Various aspects of the invention will now be described sequentially, but it should be recognized that a single electro-optic medium or display may utilize more than one aspect of the invention. For example, a single electro-optic display may be driven using AC pulses with a DC offset, and also using refresh pulses.

The refresh pulse method of the present invention

As previously mentioned, the present invention provides a method of refreshing an image on an electro-optic display by applying short refresh pulses to the display. Thus, in the method of the invention, an addressing pulse is first applied to a pixel of the bi-stable display, the addressing pulse being sufficient to change the optical state of the pixel. After keeping the display undriven for a while, a refresh pulse is applied to the pixels, the refresh pulse being short relative to the address pulse. Typically, the impact applied by the refresh pulse is no more than 20% (ideally no more than 10%, and preferably no more than 5%) of the impact applied by the address pulse. For example, if a pixel requires an address pulse of 15V lasting 500 milliseconds (msec), the refresh pulse may be 15V lasting 10 msec, with an impact of 2% of the impact of the address pulse.

The time of the refresh pulse in this method should be adjusted taking into account the sensitivity of the human eye to small changes in the sudden optical state. The human eye has a relative tolerance to a gradual attenuation of the image, so that, for example, it is usually measured as the luminance L^＊(defined by the usual CIE definition; see, e.g., Hunt, R.W.G.measuring Color, 3rd edition, fountain Press, Kingston-up-names, England (1998). (ISBN 0863433871)) the bistability of the electro-optic medium at the desired time varies by two units from the maximum value of the observed white optical state (or the minimum value of the black state) after the end of the addressing pulse. However, when a refresh pulse is applied to the display, the brightness of the associated pixel changes abruptly and is substantially less than 1 unit L^＊Is readily detectable by the human eye. Depending on the interval between refresh pulses, the changes in the image caused by these pulses may appear as "flicker" in the image, which is annoying to most viewers. To avoid such flicker or other perceptible changes in the image caused by the refresh pulses, it is desirable to select the interval between the addressing pulse and the first refresh pulse or the interval between successive refresh pulses such that each refresh pulse causes minimal changes in the image. Thus, L caused by a single refresh pulse^＊Should be less than about 1 unit L^＊Desirably less than about 0.5 units, and more preferably less than about 0.2 units.

Although the refresh pulses used in the present method may introduce some DC-imbalance in the drive scheme during the application of the refresh pulses, it is not excluded that long-term DC-balancing is achieved in the drive scheme, and it has been found that long-term rather than short-term DC-balancing is a major factor in determining the operational lifetime of an electro-optic display. To achieve such long-term DC-balancing, pixels that have received one or more refresh pulses may be driven to their opposite optical state by a "transition" or second addressing pulse after application of the refresh pulse, and the impulse applied in the transition addressing pulse may be adjusted to provide DC-balancing (or at least a minimum DC-imbalance) over the entire period since the first addressing pulse, by adjusting the impulse of the second addressing pulse by an amount equal to and opposite in polarity to the algebraic sum of the refresh pulses applied to the pixels between the two addressing pulses. For example, consider a display that can be transitioned between white and black optical states by applying an impact of ± 15V, 500 milliseconds (msec). Assume that a pixel of the display is first turned from black to white by applying a +15V shock of 500 milliseconds (msec) and that the white state of the pixel is subsequently maintained by applying 10 refresh pulses of +15V at intervals of 10 msec. If after these 10 refresh pulses it is desired to return the pixel to its black optical state, this can be achieved by applying a 600 (rather than 500) millisecond (msec) addressing pulse of-15V, thus achieving a DC balance throughout the black-white-black transition of the pixel.

This type of adjustment of the transition addressing pulses can be achieved when a new image is written on the display and therefore the optical state of some pixels has to be changed. Alternatively, the adjustment may be made during the application of a "blanking pulse" to the display. As discussed in the aforementioned PCT/US02/37241, it is often necessary or desirable to apply so-called "blanking pulses" to the electro-optic display at regular intervals; such blanking pulses include first driving all pixels of the display to one extreme optical state (e.g., white state), then driving all pixels to the opposite optical state (e.g., black), and then writing the desired image. An advantage of implementing this adjustment during the blanking pulse is that all pixels can be DC balanced at substantially the same time; using the techniques already detailed above, pixels that were black in the previous image (the image that appeared just prior to the blanking pulse) may be DC balanced during the blanking pulse that drives all pixels white, while pixels that were white in the previous image may be DC balanced during the blanking pulse that drives all pixels black. Also, an advantage of implementing this adjustment during the blanking pulse is that it is not necessary to know explicitly how many refresh pulses each individual pixel has received since its previous addressing pulse; assuming that the black and white pixels are refreshed at the same time interval (as is often the case), and that a blanking pulse is inserted at each image transition, each pixel will require the same adjustment (except for polarity) during the blanking pulse, as determined by the number of refresh pulses applied to the display since the previous blanking pulse. Also, achieving DC balance during the blanking pulse provides a way to apply the refresh pulse method to electro-optic displays having more than two gray levels, since adjusting the impact applied during the gray-to-gray transition in such displays obviously can lead to unwanted errors in the gray levels.

The refresh pulse method of the present invention may be used as an alternative to or in combination with additives that increase the bistability of the electro-optic medium. For example, the present invention may be used with an electrophoretic medium as described in the aforementioned 2002/0180687 having a suspension of a polymer dissolved or dispersed therein, wherein the polymer increases the bistability of the medium.

The following example is now given, showing by way of illustration only one embodiment of the refresh pulse method of the present invention.

Example 1

This example uses a display containing an encapsulated dual particle counter-charged medium comprising polymer-coated titanium oxide white particles and polymer-coated black particles, and the suspension is uncolored. The display was prepared essentially according to "method B" described in paragraphs [0061] - [0068] of 2002/0180687, supra.

A display prepared as described above, comprising a plurality of pixels, can be switched between black and white optical states using a ± 15V addressing pulse lasting 500 milliseconds (msec). The display has limited bistability and changes white optical state by 2L in ambient environment^＊The time required for the unit is only about 15 sec. However, it was empirically determined that the white and black optical states could be maintained indefinitely by applying a short refresh pulse of 15V for 4sec/min (with a duty cycle of approximately 6.7%). To provide realistic testing and to avoid flicker in the standard images used in these experiments, containing black and white regions, a refresh pulse of 15V of 7 millisecond (msec) duration was applied to the black and white pixels of the display every approximately 100 msec after the initial 500 millisecond (msec) addressing pulse.

To determine the effect of each cycle of the DC-unbalanced driving scheme on the display, 4 driving schemes were tested:

scheme 480:

the display was addressed using the standard image and the image was held for 480 minutes using the refresh pulse described above. A series of blanking pulses are then applied and the cycle of addressing and refreshing pulses is repeated. No DC balancing pulse is applied at any time. After 83 hours of operation, a series of blanking pulses were applied and then separate areas of the display were tested which had been white and black respectively. The area of the display that has been kept white during the test is indicated in the table below by "480W" and the area that has been black is indicated by "480D". Each tested area was driven to its white optical state by a standard 500 millisecond (msec) addressing pulse and its percent reflectance value was measured; this value is indicated by "w%" in the table. Each area under test is then allowed to remain without applying any refresh pulse for 15sec, after which 15 second interval L is measured^＊A change in (c); obtained L^＊Is called "bright-holdThe difference (bright holding difference) "is represented by" bhdl "in the table. After the application of the further blanking pulse, each tested area was driven to its black optical state by a standard 500 millisecond (msec) addressing pulse and its percent reflectance value was measured; this value is indicated by "d%" in the table. Each tested area is then allowed to remain without applying any refresh pulse for 15 seconds, after which 15 second interval L is measured^＊Of L obtained^＊The change in (c) is called "dark retention difference" and is represented by "dhdl" in the table.

Scheme 60:

this scheme is the same as scheme 480 except that the image is held for only 60 minutes before the blanking pulse is applied. The area of the display that remained white during the test period is indicated in the following table by "60W" and the area that remained black is indicated by "60D".

Scheme 10:

in this scheme, the image is written in the same manner as scheme 480, and is held for 10 minutes using the same refresh pulse as scheme 480. A 40sec pulse of opposite polarity is then applied to DC balance the display, the image is then rewritten, and the cycle is repeated. The area of the display that remained white during the test period is indicated in the following table by "10W" and the area that remained black is indicated by "10D".

Scheme 1:

the protocol is the same as protocol 10 except that the image is held for only 1 minute, then a second DC balance pulse of 4 seconds is applied and the cycle is repeated. The area of the display that remained white during the test period is indicated in the following table by "1W" and the area that remained black is indicated by "1D".

The results obtained in these experiments are shown in table 1 below.

TABLE 1

	480W	480D	60W	60D
					w％	37.90	30.63	38.21	38.47
d％	2.89	2.69	3.03	2.45
					dhdl	2.05	0.64	4.79	1.05
bhdl	-1.34	-4.06	-0.47	-2.72
						10W	10D	1W	1D
w％	37.31	37.39	37.20	37.20
					d％	2.75	2.75	3.14	3.13
dhdl	0.89	0.84	0.98	0.99
					bhdl	-2.24	-2.30	-2.02	-1.98

It can be seen from the data of table 1 that in the highly unbalanced scheme 480, the white state reflectivity is significantly different between the areas of the display that remain white and black during the test period, and the light and dark retention differences are also significantly different. Thus, the highly unbalanced drive scheme produces a substantial change in the optical state of the display, much less the other effects that may accompany such an unbalanced drive scheme, such as damage to the electrodes. Also, as shown by the difference in the light and dark retention differences, the unbalanced drive scheme introduces a "bias" to the display, i.e., areas that remain white for long periods of time tend to remain white thereafter, while areas that remain black for long periods of time tend to remain black thereafter. The results obtained from the unbalanced scenario 60 are similar, but not as significant (as expected). In contrast, the DC balance schemes 10 and 1 show substantially no difference between the regions where black and white are maintained.

Thus, these experiments show that as long-term DC-balancing is produced by the spaced-apart blanking pulses, the temporary DC-imbalance caused by the use of short refresh pulses does not have a negative effect on the properties of the display.

The electrophoretic media used in the refresh pulse method of the present invention may employ the same components and fabrication techniques as described in the aforementioned EInk and MIT patents and applications to which the reader is referred for further information.

Basic elements of the grey scale driving waveform (including the use of AC pulses)

As described in the aforementioned U.S. patent nos. 6,531,997 and 6,504,524, many displays currently transition from one extreme optical state to the other (e.g., from black to white, or vice versa) by applying a voltage pulse of sufficient duration to saturate the electro-optic medium; for example in particle-based electro-optic media, charged particles are moved all the way to a front or back electrode. The conventional need to address the electro-optic medium until the optical state becomes saturated does not allow intermediate grey states to exist. Electro-optic displays that achieve gray scale offer significant advantages in image capacity and image quality.

For convenience, the voltage waveform or driving scheme capable of achieving gray scale in a bistable electro-optic display is hereinafter referred to as the "gray scale waveform" or "gray scale driving scheme", respectively. There are 5 basic gray scale waveform elements that can be used in such a gray scale waveform or drive scheme; the term "gray scale waveform element" refers to a voltage pulse or sequence of voltage pulses capable of producing a change in the optical state of an electro-optic display. The gray scale waveform elements themselves are capable of generating gray scales, and one or more gray scale waveform elements arranged in a particular sequence together form a gray scale drive waveform. The gray scale drive waveforms are capable of transitioning pixels of the display from one gray state to another. The sequence of one or more drive waveforms constitutes a drive scheme that is capable of displaying any sequence of gray scale images on the display.

The driving waveform elements are classified into two types, i.e., Direct Current (DC) voltage pulses and Alternating Current (AC) voltage pulses. In both cases, the parameters of the pulses that can be varied are the pulse height and the pulse length.

Although the key to producing a grey scale optical state in an electro-optic medium depends on the way in which a voltage is applied to the medium, the ability of the medium to maintain said grey scale optical state once no voltage is applied is equally important in a grey scale addressing scheme and will depend on the properties of the medium, and indeed on all grey scale transition properties. In this application the grey scale addressing scheme will be discussed mainly with reference to encapsulated particle-based electrophoretic media, but it is believed that the necessary modifications to this scheme will be apparent to those skilled in the art of media in view of the other types of properties of bistable electro-optic media.

The basic elements of the grayscale driving waveform are as follows:

pulse length modulated DC pulse

One of the simplest ways to achieve the desired grey state is to stop addressing pixels that are in transition from one extreme optical state to another. In fig. 1 of the drawings, an inset shows a DC pulse length modulated waveform element for producing a grey scale transition in an encapsulated electrophoretic medium (as shown in the main part of the figure). (displays used here and in subsequent experiments described below were manufactured essentially according to "method B" described in paragraphs [0061] - [0068] of 2002/0180687 supra.) the three pulses used were 15V for 200, 400, and 600 milliseconds (msec), respectively, and the three curves generated were labeled accordingly; note that the time scale in the interpolated graph is different from that in the main graph. Thus, for different variations in reflectivity, the pulse height is fixed and the duration of the pulse varies. In fig. 1, the reflectance of a pixel (the reflective state of which changes from black to different levels of grey scale in accordance with the applied voltage pulses) is plotted over time; it can be seen that longer pulse lengths produce greater reflectivity variations.

The display being tested responds quickly to the end of the applied voltage pulse and its optical state stops evolving. On a microscopic level, it can be assumed that the electrophoretic particles stop migrating from one electrode to the other immediately and remain suspended in an intermediate position within the capsule.

The advantage of the pulse length modulated DC grayscale driving pulse is the speed with which the desired grayscale state is reached.

Pulse height modulated DC pulse

Another way of obtaining the desired grey state is to address the pixel with a lower voltage than required to cause a complete transition of one extreme optical state of the pixel to the other extreme optical state. In fig. 2 of the drawings, an interpolated graph shows DC pulse height modulated waveform elements for producing grey scale transitions in an encapsulated electrophoretic medium (as shown in the main part of the figure). The voltage pulse length is fixed at the length of time required to completely transition the medium at the maximum voltage level. The three pulses used were 5, 10 and 15V, respectively, lasting 500 milliseconds (msec), and the three curves generated were also labeled accordingly; note that the time scale in the interpolated graph is different from that in the main graph. Thus, for different variations in reflectivity, the pulse length is fixed and the height of the pulse varies. In fig. 2 of the drawings, the reflectance of a pixel (the reflective state of which changes from black to different levels of grey scale in accordance with the applied voltage pulses) is plotted over time; it can be seen that a larger pulse height produces a larger change in reflectivity.

It can be assumed that the electrophoretic particles pass through the suspension at a lower speed at a lower voltage and remain suspended when the application of the driving voltage is stopped.

An advantage of the pulse height modulated DC grayscale driving pulse is the precise control of the achieved grayscale state.

AC pulse with DC offset modulation

The aforementioned grey scale driving of encapsulated electrophoretic media has been affected by oscillating (AC) electric fields; the switching mechanism using such an AC field is assumed to be completely different from that implemented in the above-described DC drive of the same medium. In fig. 3 of the drawings, the interpolation diagram shows AC pulses of DC-offset modulated waveform elements for producing gray level transitions in the encapsulated electrophoretic medium shown in the main part of the figure. In any case, the frequency of the AC component (about 10Hz) is set at a value that allows the particles to respond to the oscillating field, while the magnitude and direction of the DC offset (0, -1 or-2.5V shown for the three curves in FIG. 3) determines the final achieved gray state of the pixel. As in the previous figures, the time scale in the interpolated figures is different from the time scale in the main figure. In fig. 3, the reflectance of a pixel (the reflective state of which changes from black to different levels of grey scale in accordance with the applied voltage pulses) is plotted over time; it can be seen that a larger DC offset produces a larger change in reflectivity.

Upon application of an AC field, the electrophoretic particles oscillate in the suspension, and this oscillation is a movement observed as a periodic variation of the reflectivity superimposed on the entire variation of the reflectivity, as can be easily seen on the left side of fig. 3. However, until a DC offset is applied, there is no net effect on the reflectivity. Under the influence of the DC offset, the reflectivity approaches a constant value after a period of waveform application. It appears that there is a restoring force opposing the force exerted on the particles due to the DC offset voltage, which would otherwise continue to flow towards the cell wall. This restoring force may be due to the movement of the fluid between the capsule wall and the particle and/or due to the action between the particle and the cell wall directly. Consistent with the other waveform elements, the stability of the optical state remains unchanged after the voltage is removed.

An AC waveform element has the advantage that a particular reflectivity state can be reached by specifying the parameters of the waveform element, whereas a DC waveform element can only change the reflectivity. An advantage of an AC waveform element with a DC offset over other AC waveform elements is that the addressing pulses do not need to be precisely timed.

Duty cycle modulated AC pulses

Another way to introduce DC bias using an oscillating field is to modulate the duty cycle. In fig. 4, the interpolation diagram shows duty cycle modulated AC pulses used to produce gray scale transitions in the main part of the diagram. In each of these pulses, the voltage is set to a maximum value and the duty cycle (the percentage of time the voltage is in either the positive or negative direction) determines the reflectivity. The three duty cycles used are 50%, 47% and 40% as shown in fig. 4. As in the previous figures, the timescale used in the interpolation is different from the timescale in the main graph. In the figure the reflectivity of a pixel (which changes its reflection state from black to different grey levels depending on the applied voltage pulses) is plotted over time.

As can be seen from fig. 4, the curve shown in fig. 4 reaches a constant value after the pulse has been applied for a period of time, as the AC/DC offset pulse used to generate the curve shown in fig. 3. Thus, consistent with the AC/DC offset, there appears to be a restoring force using duty cycle modulation that forces the particles away from the cell walls, keeping the grey state unchanged. The physical mechanism of this restoring force is similar to that discussed previously. Also, the change in gray state stops immediately after the application of the pulse is stopped.

An advantage of a duty cycle modulated AC waveform is that no voltage modulation is required.

Frequency modulated AC pulse

Another way of achieving an AC grey level transition is to apply an AC field to the electro-optical medium: the AC field causes the optical state of the medium to oscillate and then terminates the AC field at the point in the cycle where the desired reflectivity is achieved. The voltage may be set to a maximum value and the AC frequency varied to achieve a greater or lesser range of reflectivity. The frequency determines the amplitude of the reflectivity oscillations.

When this method is applied to encapsulated particle-based electrophoretic media, the electrophoretic particles respond to the AC field by oscillating around their initial position. Since reflectance does not typically reach an extreme black or white optical state, interaction with the cell walls is minimized and the response of reflectance to applied voltage is relatively linear.

The advantage of frequency modulated AC pulses is that no voltage modulation is required.

By combining pulses of the above type, a variety of waveform elements can be developed, each including a unique switching mechanism, thus providing various ways of driving different electro-optic media having different switching characteristics.

In one particular application of the above-described drive scheme principles, pulse width modulation and AC pulsing are used to achieve intermediate gray states in electro-optic displays that otherwise can only achieve black and white states.

For the reasons already discussed above, it is highly desirable to be able to achieve grey levels in electro-optical displays. However, given the large number of gray levels, any pulse width modulation with a high frame rate driver or voltage-modulation capable driver is required (a high frame rate is required to "slice" the pulse width into intervals, thus enabling precise control of the pulse width and hence the gray level). Either of these drivers is substantially more costly than a simple three-level driver which only enables the potentials of the individual pixels of the display to be set to + V, -V and 0 relative to the potential of the common front electrode (V being any operating potential) and which is typically used to drive displays which can only display black and white states.

The present invention provides a driving scheme that enables a three-level driver to produce intermediate gray levels between the black and white levels of a bistable electro-optic display. This driving scheme is most readily understood from table 2 below, which shows the voltages applied during successive frames for various types of transitions in such a display of the invention:

TABLE 2

0

1

2

3

4

5

6

...

N-1

N

White to black

+V

+V

+V

+V

+V

+V

+V

...

+V

+V

Black to white

-V

-V

-V

-V

-V

-V

-V

...

-V

-V

White to gray

+V

+V

+V

+V

-V

+V

-V

...

+V

-V

Black to gray

-V

-V

-V

-V

+V

-V

+V

...

-V

+V

Grey to black

+V

+V

+V

0

0

0

0

0

0

0

Gray to white

-V

-V

-V

0

0

0

0

0

0

0

As can be seen from table 2 above, the transition from black to white (and vice versa) is the same as in a binary (black/white only) display. On the other hand, there are two parts to the transition to gray scale. The first part is a square waveform pulse (i.e. a number of frames of the same potential) of the appropriate polarity and length so that the reflectivity of the electro-optic medium becomes as close as possible to the required intermediate grey scale brightness. The accuracy of having this step may be limited by the frame rate of the display. The second part of the addressing pulse comprises an equal number of positive and negative voltage pulses, each pulse having a width equal to one frame. As previously described with reference to fig. 3 and 4, it has been previously demonstrated that: the application of an AC square wave to an encapsulated particle-based surge medium causes the medium to "relax" to certain "mid-grey" states. Thus, regardless of the previous pulse history, the second portion of the pulse will cause all pixels to become the same uniform intermediate gray state. Addressing from the grey state to black or white is achieved using short pulses of appropriate polarity.

More generally, the AC part of the pulse does not change polarity every frame, but may change at a lower frequency, with alternating voltages every other frame (frequency frame/4) or generally every n frames (frequency frame/2 n).

The present invention thus provides a method of producing a single gray scale in an otherwise binary electro-optic display using only simple three-level drivers, rather than using complex and expensive voltage modulation drivers.

In a second particular application of the above-described drive scheme principles, the present invention provides a collection of two-dimensional transition matrices, wherein each element of the matrix specifies how to reach from an initial optical state (here denoted by a "row index", although it is apparent that the initial optical state assigned to a row is arbitrary) to a final optical state (here denoted by a "column index"). Each element of the matrix is built up from a series of waveform elements (as defined previously) and typically for an n-bit grey scale display the matrix will contain 2^(2N)And (4) each element. The matrix of the present invention takes into account considerations such as the need for DC balancing of the drive scheme (as discussed earlier), minimizing the "memory" effect in certain electro-optic media (i.e. the effect of the result of applying a particular pulse to a pixel depends not only on the current state of that pixel, but also on some previous state), thus producing a uniform optical state and maximizing the transition speed of the display, while operating under the limitations of an active matrix drive scheme. The invention also provides a method for determining the optimum of each of the elements in such a matrix for any particular electro-optic mediumThe value is obtained. For such matrices and their use in driving electro-optic displays, the reader is referred to the aforementioned PCTUS 02/37241.

In terms of the aforementioned Pulse Width Modulation (PWM), the presently preferred waveforms of the present invention are as follows. However, the same or similar results may be obtained using pulse height modulation or various mixed types of AC modulation as described above, and various different types of modulation may be used within a single waveform, e.g., pulse width modulation for all but the final portion of the pulse followed by voltage modulation for the final portion of the pulse.

The first two waveforms of the present invention, described below, are "slideshow" waveforms that return from one gray state to a black state before addressing to the next gray state. This waveform is most compatible with display update schemes where the entire screen is blanked once (as in a slide projector).

Double prepulse slide show waveform

In this waveform, a preferred form of which is shown in figure 5 of the accompanying drawings, a pixel of the electro-optic medium is initially driven (as indicated at 100) from black to an initial (first) grey state using a partial pulse. To change the pixel from this initial gray state to a different desired (second) gray state, the pixel is first driven from the first gray state to white (at 102) and then from white to black (at 104). Finally, an appropriate pulse is applied at 106 to achieve the second gray state. To ensure that this type of waveform maintains overall DC balance, the sum of the lengths of the address pulse at 106 and the white pulse at 102 must equal the length of the white-to-black pulse at 104. This waveform requires a maximum of three times the media transition time (i.e., the time required for a single pixel to transition from the black optical state to the white optical state, or vice versa) to achieve a transition between any two arbitrary grays, and is therefore referred to as a 3X waveform.

Single prepulse slide show waveform

In this waveform, a preferred form of which is shown in figure 6 of the accompanying drawings, the pixels of the electro-optic medium are initially driven (as indicated at 110) from black to an initial (first) grey state using a partial pulse in the same manner as the double pre-pulse waveform discussed in section 6 above. To change the pixel from the initial gray state to a different desired (second) gray state, the pixel is first driven from the first gray state to black (at 112), and then an appropriate pulse is applied to reach the second gray state at 114. It is apparent that the pixel will again go back to black at 116 before the second transition. This type of waveform maintains DC balance throughout the waveform because the impulses applied at 112 and 116 are equal (except for polarity) to the impulses applied at 110 and 114, respectively. This waveform requires twice the maximum value of the media transition time to achieve a transition between any two arbitrary gray states and is therefore referred to as a 2X waveform.

Grayscale-grayscale waveform

Instead of using the above-described slideshow waveform, the display can be updated by addressing it directly from one grey state to another without going through a black or white state. Since this transition is not accompanied by significant artifacts (i.e., black and/or white "flicker"), it is referred to as "gray-to-gray" addressing. There are two main forms of gray-to-gray waveforms, namely DC-balanced and DC-unbalanced.

In a DC-balanced greyscale waveform, the transition between two greyscale states is achieved by applying a modulating pulse of precise length necessary to switch between the two states. The electro-optic medium does not pass through any intermediate black or white states. This waveform is also referred to as a 1X waveform since the maximum pulse length is equal to the addressing time of the ink. To maintain DC balance, there are n-1 free parameters available in the optimization of the transformation matrix associated with any particular waveform for a display having n gray states. This results in an overly restricted system. For example, all transitions require equal and opposite pulses for opposite transitions (i.e., 2-3 must be the same as 3-2 except for polarity).

The DC unbalanced gray-scale waveform is substantially the same as the DC balanced case, except that the pulse length is notAgain subject to the DC balance limitation. Thus 2 in the transformation matrix^(2N)Each of the items may vary independently of all other items.

The various waveforms discussed above enable grey levels to be addressed in active matrix displays, which is crucial for the use of electro-optic media in Personal Digital Assistant (PDA) and electronic book applications. These waveforms minimize the effect of memory in the electro-optic medium, which can lead to image ghosting. By selecting the optimal pulse length and sequence, the desired grey scale optical state can be obtained in the fewest number of pulses.

Selective row drive

Another aspect of the invention relates to improving the performance of an active matrix bi-stable electro-optic display by selectively driving the rows of the display.

As previously mentioned, and as discussed in more detail in the aforementioned patents and applications, in order to maintain a desired image on a conventional LCD, the entire image area must be continuously refreshed, since liquid crystals are generally not bistable and the image on the LCD will diminish in a short period of time if the image on the LCD is not refreshed. As is well known to those skilled in the art of active matrix, in such displays, continuous refresh is achieved by: a row driver is used to turn on the gates of the transistors associated with a row of pixels of the display and a potential is applied across the column driver (connected to the source electrodes of the transistors in each column of the display) that writes the relevant part of the desired image on the display to the pixels in the selected row and, hence, the selected row of the display. The row driver then selects the next row of the display and repeats the process so that the rows are cyclically refreshed. (the assignment of the row drivers to the gate electrodes and the column drivers to the source electrodes is conventional, but is also essentially arbitrary and can of course be reversed if desired.)

Since the LCD requires continuous refreshing of the image, only partial changes in the displayed image are handled as part of the overall refresh process. In a continuously refreshed display, there is no need to provide updated portions of the image; since there are actually several new images written to the display (in the case of an LCD) per second, any change in the partial images fed to the display automatically appears on the display within a short interval. Therefore, conventional circuits for LCDs that have been developed do not provide for the updating of only a portion of the image.

In contrast, bistable electro-optic displays do not require continuous refreshing, and in fact such continuous refreshing is detrimental because it unnecessarily increases the power consumption of the display. Furthermore, during such refreshing, the gate (row) lines may deliver capacitive voltage spikes to the pixel electrodes, and any driver voltage errors or uncompensated gate feedthrough bias errors may accumulate; all of these factors lead to undesirable transitions in the optical state of the display pixels. Accordingly, in bistable electro-optic displays, it is desirable to provide means for updating a portion of an image without the need to overwrite the entire image on the display, and one aspect of the invention relates to bistable electro-optic displays fitted with such "partial update" means. According to the invention this is achieved by comparing successive images to be written to the display, identifying different rows in the two images and addressing only the identified rows.

In the method, only the rows of the display containing pixels whose optical state is to be changed are identified in order to achieve a partial update of the display. In a preferred form of the method, the display controller (see the aforementioned PCT/US02/37241) checks all of the ideal pixel electrode output voltages for each row of the display. If for that row all output voltages are equal to the potential V of the common front electrode of the display_com(i.e., if no pixels in that row need to be rewritten), the controller outputs synchronization (V)_sync) The pulse does not load the data value into the column driver and does not issue a corresponding Output Enable (OE) command. The net effect of this is that the token bits of the row driver are passed to the next row of the display without activating the current row. Data is only loaded into the column drivers and the output enable is only asserted for rows in which at least one pixel needs to be rewritten.

The present invention provides two distinct advantages. First, for pixels that are not overwritten, many stray voltage sources can be eliminated. There is no capacitive gate spike (gate spike) for these pixels and errors in the column driver voltage are not passed to the pixel in frames where the pixel is not addressed. Since many electro-optic media have a relatively low resistivity compared to liquid crystals, especially electrophoretic media, the pixel electrode will tend to relax to the actual front plane voltage, thus maintaining the hold state of the electro-optic medium. Second, the power consumption of the display is minimized. For each row that is not rewritten, the corresponding gate line does not need to be charged. In addition, the additional power consumption of moving data across the display interface is also eliminated when the output is not loaded into the column electrodes of the display.

Spatial zone jitter (diter)

Aspects of the invention described above relate to waveforms for driving electro-optic displays. The performance of such displays may also be varied by varying the structural variations of the backplane, and this aspect of the invention involves dividing one or more pixels (preferably each pixel) of the display into a plurality of sub-pixels having different areas.

As already mentioned, it is highly desirable to provide grey levels in electro-optic displays. The grey scale may also be obtained by driving a pixel of the display to a grey state between its two extreme states. However, if the medium is not able to achieve the desired number of intermediate states, or if the display is driven by a driver which is not able to provide the desired number of intermediate states, the desired number of states must be achieved in other ways, this aspect of the invention relating to the spatial dithering used for this purpose.

A display may be divided into a plurality of "logical" pixels, each of which is capable of displaying a desired number of gray levels or other optical states. However, it is clear that more than one physically separate area may be present per logical pixel, as is in fact common for color displays with "full-color" logical pixels, each of which comprises three sub-pixels with primary colors (e.g. red, green, blue); see, for example, 2002/0180688, supra. Similarly, gray levels may be obtained using a combination of sub-pixels as logical pixels, each of which is capable of binary transition. For example, a logical pixel comprising 4 independently controllable sub-pixels having the same area may be used to provide a 2-bit gray scale. However, for any case of more than 1 or 2 bit gray levels, the number of sub-pixels is inconveniently large because the number of sub-pixels required doubles for every 1 bit increase in gray level.

The invention provides an electro-optic display having at least one pixel comprising a plurality of sub-pixels having different areas. In a preferred embodiment of the invention the areas of at least two sub-pixels differ substantially by a factor of 2. Thus, for example, a logical pixel may have subpixels with areas of 1X, 2X, and 4X, where X is an arbitrary area. A logical pixel of this type is schematically illustrated in fig. 7A of the drawings. The logical pixel uses only three electrodes to achieve a 3-bit gray level, whereas using sub-pixels equal in area to achieve the same 3-bit gray level would require 8 sub-pixels.

When each subpixel is driven, it reflects or transmits a portion of the incident light, and the amount of the portion is determined by the area of the subpixel. If the reflection/transmission is averaged over the area of the logical pixel, a binary weighting of the driving area is obtained, thus obtaining a spatially dithered grey level.

The area of the sub-pixel is arbitrary. The sub-pixels shown in fig. 7A are weighted with reflectivity. If non-linear weighting is to be used (this for equally stepped L)^＊Or gamma corrected gray scale intervals are appropriate) the area is changed accordingly.

In addition to considering their relative areas, the shape of the sub-pixels should be carefully considered. Simple large blocks as shown in fig. 7A allow simple shaping of the sub-pixel array, but under certain conditions, these sub-pixels may be resolved by the viewer. Similarly, if mid-level gray (thus, say, only region 4 of fig. 7A is driven in each logical pixel) is displayed over a large area (covering many logical pixels), the viewer will see a line or grid pattern emerging from the sub-pixel pattern.

Increasing the resolution of logical pixels will reduce these problems, but requires a large number of additional pixels, since the number of pixels increases as the square of the resolution. Conversely, the visibility of the sub-pixels and/or problems with visible graphics may be reduced by interdigitating the sub-pixels (e.g., as shown in FIG. 7B); note that the figure is only intended to show the interdigitation and does not accurately represent the relative area of the sub-pixels. Many interdigitated patterns similar to those of fig. 7B may be used to improve image quality.

Another way to deal with the problem of sub-pixel visibility and/or visible graphics is to randomly orient the sub-pixels. For example, in a pixel array where each pixel is arranged by sub-pixels as shown in fig. 7A, a single pixel may have each of 4 possible orientations of the arrangement shown in fig. 7A at random. This "randomization" of the sub-pixels helps to segment the patterns and make them less visible to the viewer.

Although the embodiment of the invention shown in fig. 7A and 7B produces 3-bit gray levels, it will be appreciated that the invention can produce gray levels of any number of bits by simply adding additional sub-pixels.

The advantages of this aspect of the invention are as follows:

(a) the electro-optic medium itself need not have grey levels; basically the display may be a black/white display and the sub-pixels are switched on and off to produce grey levels. In a scanning array, the necessary control over the sub-pixels can be obtained by providing additional column drivers (for the same number of rows). This reduces the requirements on the electro-optic medium; for example, there is no need to worry about possible drift in the grey level of the electro-optic medium beyond its operating life.

(b) No complicated column driver is required; the present invention is compatible with the simple use of binary level drivers used in many conventional displays. Thus, the use of readily available electro-optic media, inexpensive "off-the-shelf" parts, is facilitated. Some methods of generating grey levels require the use of voltage modulated drivers for the column electrodes, which are not widely available and are more expensive/difficult to manufacture than binary level drivers.

(c) The design of Thin Film Transistors (TFTs) for use with active matrix arrays of the present invention is no more difficult than that required for full color, where there are three sub-pixels (e.g., RGB) per pixel, and the amount of data that needs to be provided to the various components is no greater. There is no need to develop new technologies in the active matrix backplane implementing the present invention.

Miscellaneous techniques

In the most conventional active matrix driving scheme for electro-optic displays, the voltage of the pixel electrodes on the backplane of the display is varied in order to apply the required voltage across the pixels. The top surface is usually held at a certain voltage, which is considered advantageous for addressing the pixels. For example, if the data line voltage supplied to the pixel electrode is at zero volts and a voltage V₀Will remain at V₀2 so as to allow the voltage drop across the pixel to have V in both directions₀The value of/2 is large.

In accordance with one aspect of the invention, the voltage at the top surface may be varied to enhance addressing of the electro-optic medium. For example, the top surface voltage may be held at zero volts to allow the total pixel voltage drop (top surface negative pixel voltage) to be as low as-V₀. Raising the top surface voltage to V₀Allowing the pixel voltage to drop by V₀Then it is large. These larger voltage drops allow the electro-optic medium to be addressed more quickly.

More generally, it is advantageous not only to be able to set the top surface voltage to the voltages zero and V₀Other voltages can be set. For example, it may be advantageous to apply a global time-varying voltage across the electro-optic medium in correspondence with the pixel-to-pixel voltage applied by the backplane.

It is known in electro-optic displays to provide a capacitor between the pixel electrode and the electrode formed by the extension of the select line, so as to charge the same voltage as the select line; the provision of such a capacitor reduces the decay rate of the electric field across the pixel after the drive voltage has been removed, as described in the aforementioned WO 01/07961. In another convenient aspect the invention provides an electro-optic display having a storage capacitor formed between a pixel electrode and a (second) electrode whose voltage can be varied independently of the selection lines of the display. In a preferred embodiment the second electrode follows the top surface voltage, i.e. its voltage differs from the top surface only by a constant that is not time dependent. The provision of this type of capacitor greatly reduces the capacitive voltage spikes experienced by the pixel compared to the storage capacitor formed by the overlap between the pixel electrode and the select line controlling the adjacent (previous) row of the display.

Another aspect of the invention relates to reducing or eliminating unwanted transitions of the electro-optic medium by the select and data lines.

As discussed above, the select and data lines are the basic elements of an active matrix panel, which provide the voltages required to charge the pixel electrodes to the desired values. However, the select and data lines may have the deleterious effect of transitioning the electro-optic medium adjacent to the data line. By hiding the areas that are converted by the data lines and/or the select lines from the viewer using black masking, the unwanted optical artifacts caused by such conversion can be eliminated. However, providing such a black mask requires aligning the front of the display with its rear and reduces the portion of the electro-optic medium exposed to the viewer. The result is a darker and lower contrast display than would be obtained without the black mask.

In another aspect of the invention, the use of black masking is avoided by making the lateral extent of the data lines in one direction small so that they do not significantly address the adjacent electro-optic medium during normal display operation. This avoids the need for black masking.

A related aspect of the invention relates to the use of passivated electrodes and modifications of the drive scheme for driving the electro-optic medium. The impact-driven electro-optic medium is electrically addressable when it is within the film between the two electrodes. Typically, the electrodes are in contact with the electro-optic medium. However, the electrodes can be addressed even if a dielectric material with a long electron relaxation time is present between one or both of the electrodes and the medium. Passivation of one or both electrodes may be required in order to avoid reverse chemical or electrochemical interactions at the back plane or front of the display device; see the aforementioned WO 00/38001. Although the presence of the dielectric layer greatly reduces the ability to maintain the voltage across the electro-optic medium, if the dielectric layer is properly designed, voltage surges can still be applied to the medium and the medium can be addressed by these voltage surges.

The change of the optical state of the electro-optical medium is of course achieved by changing the voltage on the pixel electrodes. This voltage change results in a voltage across the electro-optic medium and the voltage across the electro-optic medium decays due to charge leakage through the medium. If the outer dielectric layer, i.e. the dielectric layer between the medium and one of the electrodes, is thin enough and the resistivity of the electro-optic medium is large enough, the voltage impulse on the medium will be sufficient to cause the desired change in the optical state of the medium. Electronic addressing of the electro-optic medium through the dielectric layer is thus possible. This addressing scheme differs, however, from addressing an electro-optic medium whose electrodes are in direct contact with the medium, because in the latter case the medium is addressed by applying a voltage across the pixel, and in the former case addressing is achieved by causing a change in the pixel voltage. At each change the electro-optic medium experiences a voltage surge.

Finally, the present invention provides a drive scheme for reducing crosstalk in an active matrix electro-optic display.

Cross-talk between pixels (addressing one pixel affects the optical state of the other pixels) is detrimental for a number of reasons. One reason is that there is limited current flow through the transistor in the off state. Due to current leakage in the off-state, supplying a voltage to the data line (intended to charge one pixel) may charge the transistors in the non-selected row. The solution is to use transistors with low off-state current.

Another source of crosstalk is current leakage between adjacent pixels. Current may leak through the components of the backplane or through the electro-optic medium in contact with the backplane. The solution to this crosstalk is to design a backplane with large insulating gaps between the pixel electrodes. The larger the gap, the smaller the leakage current.

As already indicated above, a preferred type of electro-optical medium for use in the present invention is an encapsulated particle-based electrophoretic medium. Such electrophoretic media used in the methods and apparatus of the present invention may employ the same components and fabrication techniques as described in the aforementioned patents and applications for E Ink and MIT, to which the reader is referred for further information.

Claims (1)

1. A method for addressing a bistable electro-optic display having a plurality of pixels arranged in a plurality of rows and a plurality of columns; a plurality of row electrodes, each row electrode associated with one of the plurality of rows; a plurality of column electrodes, each associated with one of the plurality of columns; and drive means arranged to select each row electrode in turn and to apply a selected voltage to the column electrodes during selection of any given row electrode, so as to address the pixels in the row associated with the selected row electrode, and to write a row of a desired image on the display, the method being characterized by:

writing a first image to the display;

receiving data representing a second image to be written on the display;

comparing the first and second images and dividing the rows of the display into a first group and a second group, at least one pixel in the row being different between the first and second images of the first group, there being no difference in pixels in the row between the first and second images of the second group; and

a second image is written by sequentially selecting only the row electrodes associated with the first set of rows and applying voltages to the column electrodes to write only the first set of rows, thereby forming the second image on the display.