CN100437714C - Method for driving an electro-optic display - Google Patents

Abstract

A method of addressing a bistable electro-optic display having at least one pixel, 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.

Description

Method for driving an electro-optic display

The present invention relates to methods and devices for driving electro-optic displays, especially bistable electro-optic displays. The method and apparatus of the present invention are primarily, though not exclusively, useful for driving bistable electrophoretic displays.

This application is related to US Patent Nos. 6,504,524 and 6,531,997. This application is also related to co-pending International Applications PCT/US02/10267 (Publication No. WO02/079869) and PCT/US02/37241.

The term "electro-optic" as used herein as applied to materials or displays has its conventional meaning in imaging technology and refers to a material having a first and a second display state, at least one of the first and second display states The optical properties are different, and the material is transitioned from a first display state to a second display state by applying an electric field to the material. Although this optical property is usually a color perceivable by the human eye, it can be other optical properties such as optical transmission, reflectivity, brightness, or display read by a machine, changes in reflectivity of electromagnetic waves outside the visible range False color in sense.

The term "gray state" is used here in its conventional meaning in imaging technology to refer to a state between two extreme optical states of a pixel and does not necessarily imply a black-and-white transition between these two extreme states . For example, various patents and published applications cited below describe electrophoretic displays in which the extreme states are white and dark blue, so that the intermediate "grey state" would actually be light blue. In fact, as mentioned earlier, the transition between the two extreme states may not be a change in color at all.

The terms "bistable" and "bistable" are used herein with their conventional meaning in the art and refer to a display that includes a display element having a first and a second display state, the first and second display states The states differ in at least one optical property such that any given element is driven to assume its first or second display state by an addressing pulse of finite duration, which state will persist for at least the change after the addressing pulse has terminated. Several times, for example at least four times, the minimum duration of the addressing pulse required to display the state of the element. In the aforementioned co-pending application Serial No. 10/063236 it is shown that some particle based electrophoretic displays capable of displaying gray scale are stable not only in their extreme black and white states but also in their intermediate gray The same is true for other types of electro-optic displays. This type of display is properly called "multistable" rather than bistable, but for convenience the term "bistable" is used in this article to cover both bistable and multistable displays.

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

Various types of bistable electro-optic displays are known. One type of electro-optic display is the rotating dichroic element type (although this type of bichromal member) is disclosed, for example, in U.S. Pat. Displays are often referred to as "rotary dichroic" displays, but since the rotating element is not spherical in some of the above patents, the term "rotary dichroic element" is more accurate). Such displays use a large number of small bodies (typically spherical or cylindrical) having two or more parts with different optical properties, and internal dipoles. These bodies are suspended in the stroma in fluid-filled vacuoles that are filled with fluid so that the bodies can rotate freely. Applying an electric field to the display changes the appearance of the display, thus rotating the bodies to various positions and altering those parts of the bodies seen by viewing the surface.

Another type of electro-optic medium uses an electrochromic medium, for example in the form of a nanochromic thin film, comprising an electrode formed at least in part of a semiconducting metal oxide and a plurality of electrodes attached to the electrode capable of Reversible color reversible dye molecules; see, eg, O'Regan, B. et al. Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al. Adv. Mater., 2002, 14(11), 845. Nanochrome 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 researched and developed over the years is the particle-based electrophoretic display, in which multiple charged particles move through a suspension under the influence of an electric field. Compared with liquid crystal displays, electrophoretic displays contribute to good brightness and contrast, wide viewing angles, state bistability, and low power consumption. However, long-term image quality issues with these displays have prevented their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in insufficient service life of these displays.

Numerous patents assigned to or filed with the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise a plurality of vesicles, each of which itself contains an inner phase comprising electrophoretically movable particles suspended in a liquid suspending medium, and a wall surrounding the inner phase. Typically, the sacs themselves are held in a polymeric binder to form an adhesive layer between the two electrodes.例如，在美国专利N0.5,930,026；5,961,804；6,017,584；6,067,185；6,118,426；6,120,588；6,120,839；6,124,851；6,130,773；6,130,774；6,172,798；6,177,921；6,232,950；6,249,721；6,252,564；6,262,706；6,262,833；6,300,932；6,312,304；6,312,971；6,323,989；6,327,072 ；6,376,828；6,377,387；6,392,785；6,392,786；6,413,790；6,422,687；6,445,374；6,445,489；6,459,418；6,473,072；6,480,182；6,498,114；6,504,524；6,506,438；6,512,354；6,515,649；6,518,949；6,521,489；6,531,997；6,535,197；6,538,801；和6,545,291以及美国专利申请公布No.200210019081；2002/0021270；2002/0053900；2002/0060321；2002/0063661；2002/0063677；2002/0090980；2002/0106847；2002/0113770；200210130832；2002/0131147；2002/0145792；2002/0154382， 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378; 2003/0011560; 2003/0011867; 2003/0011868; 2003/0020844; 2003/0025855; 2003/0034949; .WO 99/67678; WO 00/05704; WO 00/20922; WO 00/26761; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/67110; ; and WO01/08241 describes this type of encapsulated media.

Many of the aforementioned patents and applications recognize that the walls surrounding the separating microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thus resulting in a so-called polymer-dispersed electrophoretic display in which the electrophoretic medium comprises multiple electrophoretic Separate droplets of fluid and a continuous phase of polymer material, and separate droplets of electrophoretic fluid within such a polymer dispersed electrophoretic display can be detected even if no separate capsule is associated with each individual droplet Considered to be capsules or microcapsules; see, eg, the aforementioned 2002/0131147. Thus, for the purposes of this application, such polymer dispersed electrophoretic media are considered a subclass of encapsulated electrophoretic media.

Encapsulated electrophoretic displays generally do not suffer from the aggregation and precipitation failure modes of conventional electrophoretic display devices, and offer additional advantages such as the ability to coat or print displays 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-metered coating such as patch die coating, slot or protrusion coating, slip or Cascade coating, curtain coating; stick roll such as knife over roll, forward and reverse roll coating; gravure coating; dip coating; spray coating; crescent (meniscus) coating; spin coating; brush coating; air knife coating; screen printing process; electrostatic printing process; thermal printing process, inkjet printing process; and other similar techniques.) Therefore, the manufactured display can be flexible . Furthermore, 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 microcellular electrophoretic displays, the charged particles and suspending fluid are not enclosed in microcapsules but are held within multiple cavities formed within a carrier medium, usually a polymer film. See, e.g., International Application Publication No. WO 02/01281 and Published U.S. Application No. 2002-0075556 (both assigned to Sipix Imaging, Inc.)

While electrophoretic displays are usually opaque (since the particles substantially block visible light from passing through the display) and operate in a reflective mode, electrophoretic displays can operate in a so-called "shutter mode" in which the particles are arranged in Lateral movement within the display causes the display to have a substantially opaque display state and a translucent display state. See, eg, aforementioned US Patent Nos. 6,130,774 and 6,172,798, and US Patent Nos. 5,872,552, 6,144,361, 6,271,823, 6,225,971 and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely on changes in electric field strength, can also operate in a similar mode; see US Patent No. 4,418,346. Other types of electro-optic displays are also capable of operating in shutter mode.

The bistable or multistable properties of particle-based electrophoretic displays, and similar properties of other electro-optic displays, are in stark contrast to the performance of conventional liquid crystal (LC) displays. Twisted nematic liquid crystals do not behave as bistable or multistable, but instead act as a voltage sensor such that applying a given voltage to a pixel of such a display produces a specific gray level at that pixel that differs from that previously present at that pixel. The gray level on the above is irrelevant. Furthermore, the LC display is driven in only one direction (from non-transmissive or "dark" to transmissive or "bright"), and the reverse transition from a brighter state to a darker state is achieved by reducing or eliminating the electric field. Finally, the gray 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 usually reverse the polarity of the driving electric field at frequent intervals . In contrast to the first approximation, a bistable electro-optic display acts as a shock sensor, so that the final state of a pixel depends not only on the applied electric field and when it is applied, but also on the state of the pixel before the electric field is applied.

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

However, such refreshing of the image may cause its own problems. As discussed in the aforementioned U.S. Patent Nos. 6,531,997 and 6,504,524, 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, problems may be encountered and the operation of the display Reduced lifespan. Driving methods that produce a net time-averaged applied electric field of zero across the electro-optic medium are often referred to as "direct current balance" or "DC balance". If the image is held for a long time by applying refresh pulses, these pulses need to be of the same polarity as the addressing pulses used to drive the associated pixels of that display to the optical state being held, which results in a DC unbalanced drive scheme .

According to another aspect of the invention, it has been found that if the refresh is achieved using short pulses, the image on the display can be refreshed while reducing the deleterious effects associated with DC unbalanced drive schemes.

Another aspect of the present invention relates to addressing the problem that the aforementioned drive requirements of bistable electro-optic displays render conventional drive methods for driving LCDs unsuitable for such bistable electro-optic displays. Furthermore, as discussed in the aforementioned U.S. Patent Nos. 6,531,997 and 6,504,524, problems may be encountered and the display's Reduced working life. Driving methods that produce a net time-averaged applied electric field of zero across the electro-optic medium are often referred to as "direct current balance" or "DC balance". LCDs suffer from similar problems, but DC balance issues are less important in LCDs due to the insensitivity of such displays to the polarity of the applied electric field and the consequent ability to reverse polarity at will. However, the need for DC balance 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 present invention relates to a method and apparatus for driving an electro-optic display, which method and apparatus meet the particular requirements of the bistable displays discussed above. Certain methods and apparatus of the present invention are primarily useful for producing accurate grayscale reproduction in bi-stable displays.

Accordingly, in one aspect, the present 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, the refresh pulse substantially restoring the pixel to the first optical state, the refresh pulse being short relative to the address 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 hit of the refresh pulse is typically no more than about 20% of the hit of the address pulse, desirably no more than about 10% of the hit, and preferably no more than 5% of the hit. For reasons explained below, usually the difference between the first and second optical states is no more than about one unit of L ^* (where L ^* has the usual CIE definition); ideally the difference is no more than about 0.5 unit L ^* , and preferably not more than about 0.2 units of L ^* . Multiple 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 addressing pulse is applied to the display, which drives the pixel to a third optical state different from the first and second optical states. state, and wherein the shock applied by the second addressing pulse is the sum of (a) the shock required to drive the pixel from the first optical state to the third optical state, and (b) the same as the A shock of equal magnitude and opposite polarity between the first and second addressing pulses of the refresh pulse applied to the pixel. The second addressing pulse may be constant in voltage but variable in duration. In a display comprising a plurality of pixels, the second addressing pulse may be a blanking pulse which drives all the pixels of the display to an extreme optical state. In a preferred form of this "blank pulse/refresh pulse" process, the display includes a plurality of pixels, a first addressing pulse is applied to each pixel to drive a first set of pixels to white and a second set of pixels to black , at least one refresh pulse is applied to each pixel, and then a first blanking pulse that turns all pixels black and a second blanking pulse that drives all pixels white are applied to the display, both blanking pulses at any Sequential application is acceptable. The shock applied to each pixel of the first group during the first blanking pulse is the sum of (a) the shock required to drive that pixel from white to black, and (b) the A shock of equal magnitude and opposite polarity to the refresh pulse applied to the pixel between the pulse and the first blanking pulse. Similarly, the shock applied to each pixel of the second group during the second blanking pulse is the sum of (a) the shock required to drive that pixel from black to white, and (b) the same as in the first A shock of equal magnitude and opposite polarity to the refresh pulse applied to the pixel between the address pulse and the first blanking pulse.

The refresh pulse method of the present invention can be used with any of the aforementioned types of electro-optic media. Thus, in this approach the display may be a roto-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, the method comprising applying an AC pulse with a DC offset to the medium.

In another aspect, the present invention provides a method of addressing a bistable electro-optic medium, the method comprising applying an AC pulse to the medium, and varying at least one of the frequency and duty cycle of the pulse so as to vary with the AC pulse. The optical state of the electro-optic medium.

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 being associated with one of the plurality of rows; a plurality of column electrodes, each column electrode being associated with one of the plurality of columns; and drive means arranged to select each of the row electrodes in turn and to apply the selected column electrode to the column electrode during selection of any given row electrode. to address the pixels in the row associated with the selected row electrode and to write the desired row of the image on the display. The method includes:

writing the first image to the display;

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

comparing the first and second images and dividing the rows of the display into a first group in which at least one pixel differs between the first and second images and a second group in the rows of the second group the same pixel between the first and second images in the row; and

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

In another aspect, the invention provides an electro-optic display having a plurality of pixels, at least one of which comprises a plurality of sub-pixels having mutually different areas, the display comprising drive means arranged for varying the optical properties of said sub-pixels independently of one another. state. In such displays, it is desirable that at least two of the sub-pixels differ in area by substantially a factor of two.

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

Figure 1 is a graph showing the variation of gray levels over time in a display addressed using DC pulses with pulse length modulation;

The graph of Fig. 2 is similar to Fig. 1, is the change of the gray level with time of the display addressed with the direct current pulse with pulse height modulation;

FIG. 3 is a graph similar to FIG. 1 of gray levels versus time for a display addressed according to the invention using AC pulses with a DC offset;

FIG. 4 is a graph similar to FIG. 1 of gray levels versus time for a display addressed according to the invention using AC pulses with duty cycle modulation;

Fig. 5 is a graph showing the variation of gray levels over time in a display addressed using a dual pre-pulse slideshow waveform;

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

Figures 7A and 7B illustrate possible arrangements of sub-pixels within a single pixel of a display of the present invention.

As already indicated, the present invention provides a number of improvements in the method of addressing electro-optic media and displays, and in the construction of these displays. The various aspects of the invention will now be described in sequence, but it should be appreciated that a single electro-optic medium or display may utilize more than one aspect of the invention. For example, a single electro-optic display can be driven using AC pulses with a DC offset, and also using refresh pulses.

Refresh pulse method of the present invention

As previously stated, 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 a bistable display, the addressing pulse being sufficient to change the optical state of the pixel. After keeping the display undriven for a period of time, a refresh pulse is applied to the pixel, which refresh pulse is short relative to the address pulse. Typically, the impact imposed by the refresh pulse is no greater than 20% (ideally no greater than 10%, and preferably no greater than 5%) of the impact imposed by the address pulse. For example, if a pixel requires a 15V address pulse for 500 milliseconds (msec), the refresh pulse could be 15V for 10 milliseconds (msec), which is 2% of the impact of the address pulse.

The timing of the refresh pulses in this method should be adjusted taking into account the sensitivity of the human eye to sudden small changes in optical state. The human eye is relatively tolerant of gradual attenuation of the image, so that, for example, it is usually measured as luminance L ^* (defined by the usual CIE definition; see, e.g. Hunt, RWG Measuring Color, 3rd edition, FountainPress, Kingston-upon-Thames , England(1998).(ISBN 0863433871)) Bistability of electro-optic media for the time required to change from the maximum value of the white optical state (or the minimum value of the black state) observed after the end of the addressing pulse by two units . However, when a refresh pulse is applied to the display, there is a sudden change in the brightness of the associated pixel, and abrupt changes of substantially less than 1 unit L ^* are readily detectable by the human eye. Depending on the interval between refresh pulses, changes in the image caused by these pulses may appear as "flicker" in the image, which is objectionable to most observers. To avoid such flickering or other perceptible changes in the image caused by refresh pulses, it is desirable to choose the interval between the address pulse and the first refresh pulse or the interval between successive refresh pulses such that each refresh pulse Induces minimal changes in the image. Therefore, the change in L ^* caused by a single refresh pulse should be less than about 1 unit L ^* , ideally less than about 0.5 units, and more preferably less than about 0.2 units.

While the refresh pulse used in this method introduces some DC imbalance in the drive scheme during application of the refresh pulse, it does not preclude obtaining a long-term DC balance in this drive scheme, and it has been found that long-term rather than short-term DC balance is important. The main factor that determines the working life of electro-optic displays. To achieve this long-term DC balance, after one or more refresh pulses are applied, pixels that have received these refresh pulses can be driven to their opposite optical state by a "switch" or second addressing pulse and can adjust The impact applied in the switching address pulse to provide DC balance (or at least minimal DC imbalance) over the entire period from the first address pulse is adjusted by adjusting the impact of the second address pulse, where The amount of adjustment is equal in magnitude and opposite in polarity to the algebraic sum of the refresh pulses applied to the pixel between the two addressing pulses. For example, consider a display capable of transitioning between white and black optical states by applying a ±15V, 500 millisecond (msec) shock. Assume that a pixel of the display is first changed from black to white by applying a +15V shock for 500 milliseconds (msec), and the pixel's white state is subsequently changed by applying 10 refresh pulses of +15V at 10 millisecond (msec) intervals. be kept. 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 -15V addressing pulse for 600 milliseconds (msec) instead of 500 - DC balance is fully achieved during the white-black transition.

This type of adjustment of switching addressing pulses can be achieved when a new image is written on the display and therefore the optical state of certain pixels has to be changed. Alternatively, the adjustment can be made during the application of a "blank 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 electro-optic displays at regular intervals; this blanking involves first driving all the pixels of the display to an extreme optical state (eg, white state), then drive all pixels to the opposite optical state (eg, black), and then write the desired image. The advantage of doing this adjustment during the blanking pulse is that all pixels can be DC balanced at substantially the same time; using the technique already detailed before, it was black in the previous image (the one just before the blanking pulse) A pixel may be DC balanced during a blanking pulse that drives all pixels to white, while a pixel that was white in the previous image may be DC balanced during a blanking pulse that drives all pixels to black. Again, the advantage of implementing this adjustment during the blanking pulse is that it does not need to know exactly how many refresh pulses each individual pixel has received since its previous addressing pulse; assuming black and white pixels are refreshed at equal intervals (which is often the case), and inserting a blanking pulse at each image transition, each pixel will require the same adjustment (except polarity) during that blanking pulse that has been made since the previous blanking pulse The pulse is determined by the number of refresh pulses applied to the display. Also, achieving DC balance during the blanking pulse provides a way to apply this refresh pulse method to electro-optic displays with more than two gray levels, since in such displays the shock applied during the gray-to-gray transition is adjusted Undesirable errors in the gray scale may obviously result.

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

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

example 1

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

Displays prepared as previously described, comprising a plurality of pixels, were able to be transitioned between black and white optical states using ±15V addressing pulses lasting 500 milliseconds (msec). The display has limited bistability, the time it takes for the white optical state to change 2L ^* units in ambient is only about 15sec. However, it was empirically determined that the white and black optical states could be maintained indefinitely by applying short refresh pulses of ±15V at 4 sec/min (duty cycle approximately 6.7%). To provide realistic testing and to avoid flicker in the standard images (containing black and white areas) used in these experiments, the display's A refresh pulse of ±15V of 7 millisecond (msec) duration is applied to the black and white pixels.

To determine the effect of individual cycles of the DC unbalanced drive scheme on the display, 4 drive schemes were tested:

Program 480:

The display was addressed using the standard image, and the image was maintained for 480 minutes using the aforementioned refresh pulse. A series of blanking pulses are then applied, and the cycle of addressing and refresh pulses is repeated. No DC balance pulses are applied at any time. After 83 hours of operation, a series of blanking pulses were applied, and then the separate areas of the display which had been white and black respectively were tested. Areas of the display that have been kept white during testing are indicated in the table below as "480W", while areas that have been black are indicated as "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 in the table as "w %". Each tested area was then allowed to hold for 15 sec without applying any refresh pulse, and the change in L ^* was measured after this 15 second interval; the resulting change in L ^* was called the "brightholding difference". ”, denoted by “bhdl” in said table. After applying an additional 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 in the table Indicated by "d%". Each tested area was then allowed to hold for 15 seconds without applying any refresh pulse, and the change in L ^* was measured after this 15-second interval. The resulting change in L ^* was called the "dark hold difference". It is indicated by "dhdl" in the table.

Scheme 60:

This scheme is the same as scheme 480 except that the image is only maintained for 60 minutes before a blanking pulse is applied. The areas of the display that remained white during the test cycle are denoted "60W" in the table below, while the areas that remained black were denoted "60D".

Option 10:

In this scheme, the image is written in the same manner as scheme 480 and 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 overwritten, and the cycle repeats. The areas of the display that remained white during the test cycle are denoted "10W" in the table below, while the areas that remained black were denoted "10D".

plan 1:

The protocol was the same as protocol 10, except that the image was only held for 1 minute, then a second DC balance pulse of 4 seconds was applied, and the cycle repeated. The areas of the display that remained white during the test cycle are indicated in the table below as "1W", while the areas that remained black are indicated as "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 bbdl -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

From the data in Table 1, it can be seen that in the highly unbalanced scheme 480, the white state reflectance differs significantly between regions of the display that remain white and black during the test period, and the light and dark hold differences also differ significantly. Thus, the highly unbalanced drive scheme produces a substantial change in the optical state of the display, far from other effects that may accompany such an unbalanced drive scheme, such as damage to the electrodes. Also, the unbalanced drive scheme introduces a "bias" to the display as shown by the difference in the light and dark hold differences, i.e. areas that stay white for a long time tend to also stay white afterwards, while areas that stay black for a long time tend to Remains black thereafter. The results obtained from the unbalanced scheme 60 are similar, but not as dramatic (as expected). In contrast, DC balancing schemes 10 and 1 showed little difference between areas that remained black and white.

Thus, these experiments show that the temporary DC imbalance caused by the use of short refresh pulses does not negatively affect the properties of the display, as long as the long-term DC balance is produced by spaced blanking pulses.

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

Basic elements of grayscale drive waveforms (including the use of AC pulses)

As described in the aforementioned U.S. Patent Nos. 6,531,997 and 6,504,524, many current displays transition from one extreme optical state to another by applying a voltage pulse of sufficient duration to saturate the electro-optic medium (e.g., from black to white and vice versa); for example in particle-based electro-optic media, moving charged particles all the way to the front or back of the electrode. The traditional requirement of not addressing the electro-optic medium until the optical state becomes saturated does not allow intermediate gray-scale 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 obtaining gray scale in a bistable electro-optic display is hereinafter referred to as "gray scale waveform" or "gray scale driving scheme", respectively. There are five basic grayscale waveform elements that can be used in this grayscale waveform or drive scheme; the term "grayscale waveform element" refers to a voltage pulse or voltage pulse capable of producing a change in the optical state of an electro-optic display sequence. The grayscale waveform element itself can generate grayscale, and one or more grayscale waveform elements arranged in a specific sequence together form a grayscale driving waveform. Grayscale drive waveforms are capable of transitioning a display's pixels from one grayscale state to another. A sequence of one or more drive waveforms constitutes a drive scheme capable of displaying any sequence of grayscale images on the display.

Drive waveform elements fall into two categories, direct current (DC) voltage pulses and alternating current (AC) voltage pulses. In both cases, the variable parameters of the pulse are pulse height and pulse length.

While the key to generating grayscale optical states in an electro-optic medium depends on the manner in which voltage is applied to the medium, the ability of the medium to maintain said grayscale optical state once no voltage is applied in a grayscale addressing scheme Equally important, and the capability will depend on the properties of the medium, in fact all gray scale transition properties. In this application, grayscale addressing schemes will be discussed primarily with reference to encapsulated particle-based electrophoretic media, but it is believed that it will be useful for those skilled in the art of said media to take into account the properties of other types of bistable electro-optic media. The necessary modifications to this scheme are obvious.

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

Pulse Length Modulated DC Pulses

One of the easiest ways to achieve a desired grayscale state is to stop addressing pixels that are in the midst of transitioning from one extreme optical state to another. In Figure 1 of the accompanying drawings, the inset shows the DC pulse length modulated waveform elements used to produce gray scale transitions in the encapsulated electrophoretic medium (shown in the main part of the figure). (The displays used here and in the following follow-up experiments are basically manufactured according to the "method B" described in paragraphs [0061]-[0068] of the aforementioned 2002/0180687.) The three pulses used were 15V for 200, 400 and 600 milliseconds (msec), and the three resulting curves are labeled accordingly; note that the time scale in the inset is different from that in the main figure. Thus, for different changes in reflectivity, the pulse height is fixed and the duration of the pulse varies. In Figure 1, the reflectivity of a pixel (whose reflective state changes from black to various levels of gray according to these applied voltage pulses) is plotted versus time; it can be seen that longer pulse lengths produce larger change in reflectivity.

The tested display responded rapidly to the end of the applied voltage pulse and its optical state ceased to evolve. On a microscopic scale, it can be assumed that the electrophoretic particles immediately stop migrating from one electrode to the other and remain suspended in an intermediate position within the capsule.

The advantage of pulse length modulated DC grayscale driving pulses is the speed at which the desired grayscale state can be achieved.

Pulse height modulated DC pulses

Another way to achieve the desired grayscale state is to address the pixel with a lower voltage than necessary to completely transition the pixel from one extreme optical state to the other extreme optical state. In Figure 2 of the accompanying drawings, the inset shows the DC pulse height modulated waveform elements used to produce gray scale transitions in the encapsulated electrophoretic medium (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 15 V for 500 milliseconds (msec), and the three resulting curves are labeled accordingly; note that the time scale in the inset is different from that in the main figure. Thus, for different changes in reflectivity, the pulse length is fixed and the height of the pulse varies. In Figure 2 of the accompanying drawings, the reflectivity of a pixel (whose reflective state changes from black to various levels of gray according to these applied voltage pulses) versus time is plotted; a larger pulse height can be seen produce large reflectivity changes.

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

An advantage of pulse-height modulated DC grayscale drive pulses is precise control over the achieved grayscale state.

AC Pulse with DC Offset Modulation

The previously described gray scale drive of encapsulated electrophoretic media has been influenced by an oscillating (AC) electric field; the transition mechanism using this AC field is assumed to be quite different from that realized in the above described DC drive of the same media. In Figure 3 of the accompanying drawings, the interpolation shows the DC offset modulating the AC pulses of the waveform elements used to produce gray scale transitions in the encapsulated electrophoretic medium shown in the main part of the figure. In any case, the frequency of the AC component (approximately 10 Hz) 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 is shown for the three curves in Figure 3. , -1 or -2.5V) determines the final gray state of the pixel. As in the previous figure, the time scale in the interpolated figure differs from that in the main figure. In Figure 3, the reflectivity of a pixel (whose reflective state changes from black to various levels of gray according to these applied voltage pulses) is plotted over time; it can be seen that a larger DC offset produces a larger change in reflectivity.

Once the AC field is applied, the electrophoretic particles oscillate in the suspension, and this oscillation is a motion observed as a periodic change in reflectivity superimposed on the overall change in reflectivity, which is easily seen on the left side of Fig. 3 out. However, there is no net effect on reflectivity until a DC offset is applied. Under the influence of DC offset, the reflectivity approaches a constant value after the waveform is applied for some time. There appears to be a restoring force against the force exerted on the particles due to the DC offset voltage, otherwise, the particles would 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 interaction between the particle and the cell wall directly. Consistent with other waveform elements, the stability of the optical state remains unchanged after the voltage is removed.

The advantage of the AC waveform element is that it can achieve a specific reflectivity state by specifying the parameters of the waveform element, while the 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 precise timing of the address pulse is not required.

Duty cycle modulated AC pulse

Another way to introduce a DC bias using an oscillating field is to modulate the duty cycle. In Figure 4, the inset shows the duty cycle modulated AC pulses used to generate the gray level transitions in the main part of the figure. 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 the positive or negative direction) determines the reflectivity. The three duty cycles used are 50%, 47% and 40%, as shown in Figure 4. As in the previous figures, the time scale used in the insets differs from that in the main figure. In this figure, the reflectivity of a pixel whose reflective state changes from black to different gray levels according to these applied voltage pulses is plotted as a function of time.

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

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

Frequency modulated AC pulse

Another way to achieve AC grayscale transitions is to apply an AC field to the electro-optic medium that causes the optical state of the medium to oscillate, then terminate the AC field at the point in the cycle at which the desired reflectivity is achieved. The voltage can be set to its maximum value, and the AC frequency varied for greater or lesser reflectivity ranges. The frequency determines the amplitude of the reflectivity oscillations.

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

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

By combining the above types of pulses, a variety of waveform elements can be developed, each including a unique switching mechanism, thus providing a variety of methods for driving different electro-optic media with different switching characteristics.

In one specific application of the principles of the drive scheme described above, pulse width modulation and AC pulses are used to obtain intermediate gray-scale states in electro-optic displays that would otherwise only be able to obtain black and white states.

For reasons already discussed above, it is highly desirable to be able to achieve gray scale in electro-optic displays. However, assuming a large number of gray levels requires pulse-width modulation of any driver with a high frame rate or a driver capable of voltage modulation (requires a high frame rate to "slice" the pulse width into multiple intervals, thus being able to precisely control the pulse width, and thus precisely control grayscale). Either of these drivers is substantially more costly than a simple tri-level driver that only enables the potential of a single pixel of the display to be set to +V, -V, and 0 (V is an arbitrary operating potential), and it is usually used to drive a display that can only display black and white states.

The present invention provides a drive scheme that enables a three-level driver to produce intermediate gray levels between the black and white levels of a bistable electro-optic display. The drive scheme is most easily understood from Table 2 below, which shows the voltages applied during successive frames of various types of transitions in this 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 gray 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 (only black/white) display. On the other hand, the transition to grayscale has two parts. The first part is a square wave pulse (ie, multiple frames of the same potential) of appropriate polarity and length to bring the reflectivity of the electro-optic medium as close as possible to the desired half-gray brightness. The accuracy with this step will probably be limited by the frame rate of the display. The second part of the address pulse includes an equal number of positive and negative voltage pulses, each pulse having a width equal to one frame. As previously described with reference to Figures 3 and 4, it has previously been demonstrated that applying an AC square wave to an encapsulated particle-based surge medium causes the medium to "relax" to certain "intermediate gray" states. Thus, regardless of previous pulse history, the second part of the pulse will bring all pixels to the same uniform intermediate gray state. Addressing from a gray state to black or white is achieved using short pulses of appropriate polarity.

More generally, instead of switching polarity every frame, the AC portion of the pulse can switch at a lower frequency, with every other frame (frequency = frame rate / 4) or usually every n frames (frequency = frame rate /2n) The voltage is alternated.

Thus, the present invention provides a method of producing a single gray scale in an otherwise binary electro-optic display using only simple three-level drivers without the use of complex and expensive voltage modulated drivers.

In a second specific application of the principles of the drive scheme described above, the present invention provides a collection of two-dimensional transition matrices, where each element in the matrix specifies how to get from an initial optical state (here denoted by "row index", although obviously is the initial optical state assigned to the row is arbitrary) to the final optical state (here denoted by "column index"). Each element of this matrix is built from a sequence of waveform elements (as defined previously), and typically for an n-bit grayscale display, this matrix will contain 2 ^(2N) elements. The matrix of the present invention takes into account considerations such as the need for DC balance of the drive scheme (as previously discussed), minimizing "memory" effects (i.e. The resulting effect depends not only on the current state of the pixel, but also on some previous state), thus producing a uniform optical state and maximizing the transition speed of the display while working within the constraints of the active matrix drive scheme. The present invention also provides a method for determining, for any particular electro-optic medium, the optimum value for each of the elements in such a matrix. The reader is referred to the aforementioned PCTUS02/37241 for such matrices and their use in driving electro-optic displays.

With respect to the aforementioned pulse width modulation (PWM), the presently preferred waveforms of the present invention are as follows. However, the same or similar results can be obtained using pulse height modulation or various mixed types of AC modulation as described above, and various different types of modulation can be used within a single waveform, e.g. for Pulse width modulation of all parts followed by voltage modulation of the last part of the pulse.

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

Dual pre-pulse slideshow waveform

In this waveform, a preferred form of which is shown in Figure 5 of the accompanying drawings, a partial pulse is used to initially drive a pixel of the electro-optic medium from black to (shown at 100) an initial (first) grayscale state. To change a pixel from this initial grayscale state to a different desired (second) grayscale state, the pixel is first driven from the first grayscale 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 an 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-black pulse at 104 . This waveform requires a maximum of three times the medium transition time (i.e., the time required for a single pixel to transition from a black optical state to a white optical state, or vice versa) to achieve a transition between any two arbitrary gray scales, Hence the name 3X waveform.

Single prepulse slide waveform

In this waveform, a preferred form of which is shown in Figure 6 of the accompanying drawings, a partial pulse is used to initially drive a pixel of the electro-optic medium from black to (shown at 110) an initial (first) grayscale state in a manner Same as the dual pre-pulse waveform discussed in section 6 above. To change the pixel from an initial grayscale state to a different desired (second) grayscale state, the pixel is first driven from the first grayscale state to black (at 112) and then an appropriate pulse is applied at 114 to reach the second grayscale state. Two grayscale states. Obviously, the pixel will go back to black again at 116 before the second transition. This type of waveform maintains DC balance across 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 medium transition time to transition between any two arbitrary gray-scale states, and is therefore called a 2X waveform.

grayscale-grayscale waveform

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

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

DC Unbalanced Grayscale - The grayscale waveform is essentially the same as in the DC balanced case, except that the pulse length is no longer constrained by the DC balance limitation. Thus each of the 2 ^(2N) entries in the transformation matrix can vary independently of all other entries.

The various waveforms discussed above enable addressing grayscale levels in active matrix displays, which is critical for the use of electro-optic media in personal digital assistant (PDA) and e-book applications. These waveforms minimize memory effects in electro-optic media that can cause image ghosting. By choosing the optimal pulse length and sequence, ideal grayscale optical states 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 bistable 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 the desired image on a conventional LCD, the entire image area must be refreshed continuously, because liquid crystals are generally not bistable and if not refreshed The image on the LCD fades away after a short period of time. As known to those skilled in the art of active matrices, in such displays, continuous refresh is accomplished by using row drivers to open the gates of the transistors associated with a row of pixels of the display, A potential is applied to the source electrodes of the transistors in each column of the display to write the relevant portion of the desired image on the display to the pixels in the selected row, and thus to 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 row drivers to gate electrodes and column drivers to source electrodes is conventional, but also essentially arbitrary, and can of course be reversed if desired.)

Since LCDs require continuous refreshing of images, only partial changes in the displayed image are handled as part of the overall refreshing process. In a continuously refreshed display, there is no need to provide updated portions of the image; since new images are actually written to the display several times a second (in the case of LCDs), any change in the portion of the image fed to the display occurs within short intervals Automatically display the effect on that monitor. Therefore, conventional circuits that have been developed for LCDs do not provide 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. In addition, during such a refresh, the gate (row) lines may pass capacitive voltage spikes to the pixel electrodes, and any driver voltage errors or uncompensated gate feedthrough bias errors may accumulate; all of these factors cause display pixel Undesired transition of the optical state. Therefore, in a bistable electro-optic display, it would be desirable to provide some means for updating a portion of the image without rewriting the entire image on the display. One aspect of the invention relates to a bistable electro-optic display equipped with such a "partial update" means monitor. According to the invention, this is achieved by comparing successive images to be written to the display, identifying lines that differ in the two images, and addressing only the identified lines.

In this method, in order to achieve a partial update of the display, only the row of the display containing the pixels whose optical state is to be changed is identified. In a preferred form of the method, for each row of the display, the display controller (see aforementioned PCT/US02/37241) checks all ideal pixel electrode output voltages. If, for that row, all output voltages are equal to the potential _Vcom of the display's common front electrode (i.e., if no pixels in that row need to be rewritten), the controller outputs a sync ( _Vsync ) pulse without loading a data value into column driver without issuing a corresponding output enable (OE) command. The net effect of this is that the row driver's token bit is passed on to the next row of the display without activating the current row. Data is only loaded to the column drivers, and output enables are only asserted for rows in which at least one pixel needs to be rewritten.

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

Spatial domain dithering (dither)

The foregoing aspects of the invention relate to waveforms for driving electro-optic displays. The performance of such a display can also be varied by varying the structure of the backplane. This aspect of the invention involves dividing one or more pixels (preferably each pixel) of the display into multiple sub-pixels with different areas.

As already mentioned, it is highly desirable to provide gray scale in electro-optic displays. This gray scale can also be achieved by driving the pixels of the display to a gray state between its two extreme states. However, if the medium cannot obtain the desired number of intermediate states, or if the display is driven by a driver that cannot provide the desired number of intermediate states, other methods must be used to obtain the desired number of states, and this aspect of the invention involves providing Spatial dithering used for this purpose.

A display can be divided into "logical" pixels, each capable of displaying a desired number of grayscales or other optical states. However, while it is clear that more than one physically separate region can appear in each logical pixel, it is in fact common for color displays to utilize "full-color" logical pixels, each of which includes eg three sub-pixels of red, green, blue); see eg aforementioned 2002/0180688. Similarly, gray scales can be obtained using a combination of sub-pixels, each capable of binary transitions, as logical pixels. For example, a logical pixel comprising 4 independently controllable sub-pixels with the same area can be used to provide 2-bit gray scale. However, for anything more than 1 or 2 bits of grayscale, the number of sub-pixels becomes inconveniently large, since every 1-bit increase in grayscale doubles the number of subpixels required.

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 present invention, the areas of at least two sub-pixels differ substantially by a factor of 2. Thus, for example, a logical pixel may have sub-pixels with areas IX, 2X, and 4X, where X is any area. This type of logical pixel is shown schematically in Figure 7A of the accompanying drawings. The logical pixel achieves a 3-bit gray scale using only three electrodes, whereas achieving the same 3-bit gray scale using sub-pixels equal in area would require 8 sub-pixels.

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

The area of the sub-pixel is arbitrary. The sub-pixels shown in Figure 7A are weighted with reflectivity. If non-linear weighting is to be used (which is appropriate for equal-stepped L ^* or gamma-corrected grayscale intervals), the area changes accordingly.

In addition to considering their relative areas, the shape of the subpixels should also be carefully considered. A simple bulk as shown in Figure 7A allows simple shaping of the sub-pixel array, but under certain conditions these sub-pixels may be resolved by the observer. Likewise, if intermediate grayscales (thus (say) only region 4 of FIG. 7A is driven in each logical pixel) are displayed over a large area (covering many logical pixels), the viewer will see A line or grid graphic appears.

Increasing the resolution of logical pixels will reduce these problems, but requires a lot of extra pixels, since the number of pixels increases as the square of the resolution. Conversely, subpixel visibility and/or visible pattern problems can be reduced by interdigitating subpixels, such as shown in FIG. 7B ; note that this figure is only intended to show intercrossing. Not an accurate representation of the relative area of the sub-pixels. Many intersecting patterns similar to those in Figure 7B can 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 Figure 7A, a single pixel may randomly have each of the 4 possible orientations of the arrangement shown in Figure 7A. This "randomization" of the subpixels helps segment the graphics and make them less visible to the viewer.

Although the embodiment of the present invention shown in FIGS. 7A and 7B produces 3-bit grayscales, it is understood that the present invention can produce grayscales of any number of bits by simply adding additional subpixels.

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

(a) The electro-optic medium itself need not have gray scale; basically the display can be a black/white display with sub-pixels switched on and off to produce gray scale. In a scanned 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 demands placed on the electro-optic medium; for example, there is no need to worry about a possible drift of the gray scale of the electro-optic medium beyond its working life.

(b) No complicated column drivers are required; the invention is compatible with simple use of the binary level drivers used in many conventional displays. Thus, it is advantageous to use readily available various electro-optical media, inexpensive "off-the-shelf" components. Some methods of generating gray scale 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 active matrix arrays using the present invention is no more difficult than that required for full color, where each pixel has three sub-pixels (e.g., RGB), and needs to provide The data volume of various components will not be larger. Therefore no new technology needs to be developed in an active matrix backplane implementing the present invention.

miscellaneous technology

In the most traditional active-matrix drive scheme for electro-optic displays, the voltage at the pixel electrodes on the display substrate is varied in order to apply the desired voltage across the pixels. The top surface is typically held at a particular voltage that is considered favorable for addressing the pixel. For example, if the data line voltage supplied to the pixel electrode varies between zero volts and a voltage of _V0 , the top surface will remain at _V0 /2 in order to allow the voltage drop across the pixel to have _V0 / in both directions 2 that big.

According to one aspect of the invention, the voltage on the top surface can be varied to enhance addressing of the electro-optic medium. For example, the top voltage can be kept at zero volts to allow the total pixel voltage drop (top negative pixel voltage) to be as low as -V ₀ . Raising the top surface voltage to V ₀ allows the pixel voltage drop to be as large as V ₀ . These larger voltage drops allow for faster addressing of the electro-optical medium.

More generally, it would be advantageous to be able to set the top surface voltage not only to voltage zero and V ₀ , but also to other voltages. For example, it may be advantageous to apply a global time-varying voltage across the electro-optic medium corresponding to 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 an extension of the selection line, so as to be charged with the same voltage as the selection line; as described in the aforementioned WO01/07961, the provision of this capacitance reduces The rate at which the electric field on a pixel decays after the drive voltage is removed. In another aspect, the present invention is an electro-optic display having a storage capacitor formed between a pixel electrode and a (second) electrode whose voltage can be varied independently of a select line of the display. In a preferred embodiment, the second electrode follows the top surface voltage, ie its voltage differs from the top surface only by a time-independent constant. Providing a capacitor of this type greatly reduces the capacitive voltage spike experienced by the pixel compared to a 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 through select and data lines.

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

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

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

The change of the optical state of the electro-optic medium is of course achieved by changing the voltage on the pixel electrode. This voltage change results in a voltage across the electro-optic medium, which decays due to charge leakage through the medium. If the outer dielectric layer (ie, the dielectric layer between the medium and an electrode) is thin enough and the resistivity of the electro-optic medium is high enough, a voltage surge across the medium will be sufficient to cause a desired change in the optical state of the medium. Electronic addressing of the electro-optical medium through the dielectric layer is thus possible. However, this addressing scheme differs 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 Addressing is achieved by inducing a change in the pixel voltage. At each transition, the electro-optic medium experiences a voltage surge.

Finally, the present invention provides a driving scheme for reducing crosstalk in active matrix electro-optic displays.

Pixel-to-pixel crosstalk (addressing one pixel affects the optical state of other pixels) is detrimental for a number of reasons. One reason is that there is limited current flowing through the transistor in the off state. Applying a voltage to the data line (intended to charge a pixel) may charge transistors in non-selected rows due to off-state current leakage. The solution is to use transistors with low off-state current.

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

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

Claims (15)

1, a kind ofly be used for the method that addressing has the bistable electro-optic displays of at least one pixel, this method comprises:

Apply addressing pulse to drive this pixel to first optical states; With

In a period of time, keep this pixel not to be driven, allow described pixel to present to be different from second optical states of first optical states thus;

This method is characterised in that, applies refresh pulse to this pixel, and this refresh pulse returns to this pixel its first optical states substantially, and this refresh pulse is shorter with respect to addressing pulse.

2, method according to claim 1 is characterized in that, refresh pulse has 20% impact of the impact that is not more than this addressing pulse.

3, method according to claim 1 is characterized in that, refresh pulse has 10% impact of the impact that is not more than this addressing pulse.

4, method according to claim 1 is characterized in that, refresh pulse has 5% impact of the impact that is not more than this addressing pulse.

5, according to each described method in the aforementioned claim, it is characterized in that the difference between first and second optical states is no more than the L of 1 unit ^*

6, according to each described method among the claim 1-4, it is characterized in that the difference between first and second optical states is no more than the L of 0.5 unit ^*

7, according to each described method among the claim 1-4, it is characterized in that the difference between first and second optical states is no more than the L of 0.2 unit ^*

8, according to each described method among the claim 1-4, it is characterized in that a plurality of refresh pulses are applied to this pixel at interval with rule.

9, according to each described method among the claim 1-4, it is characterized in that, after applying refresh pulse, apply second addressing pulse to this display, this second addressing pulse drives this pixel to the 3rd optical states that is different from first and second optical states, and the impact that is wherein applied by second addressing pulse is following two sums: (a) drive this pixel from the required impact of first optical states to the, three optical states; And (b) equate but opposite polarity impact with the algebraic sum amplitude of the refresh pulse that between first and second addressing pulses, is applied to pixel.

10, method according to claim 9 is characterized in that, the voltage constant of second addressing pulse but duration change.

11, method according to claim 9 is characterized in that, this display comprises a plurality of pixels, and second addressing pulse comprises the blanking pulse with all pixel drive to one extreme optical state of this display.

12, according to each described method among the claim 1-4, it is characterized in that, this display comprises a plurality of pixels, first addressing pulse is applied to each pixel so that with first group of pixel drive Cheng Bai, and second group of pixel drive become black, apply at least one refresh pulse to each pixel, and apply to this display afterwards and all pixels are become the first black blanking pulse and second blanking pulse of all pixel drive Cheng Bai, these two blanking pulses can apply with any order, and the impact that wherein during first blanking pulse, is applied to each first group of pixel be below two and: (a) with this pixel from being driven into black needed impact in vain, and (b) equate but opposite polarity impact with the algebraic sum amplitude that between first addressing pulse and first blanking pulse, is applied to the refresh pulse of this pixel, and the impact that during second blanking pulse, is applied to each second group of pixel be below two and: (c) this pixel is driven into white needed impact from black, and (d) equate but opposite polarity impact with the algebraic sum amplitude that between first addressing pulse and first blanking pulse, is applied to the refresh pulse of this pixel

According to each described method among the claim 1-4, it is characterized in that 13, this display is rotation dichromatism parts or electrochromic display device (ECD).

14, according to each described method among the claim 1-4, it is characterized in that this display is an electrophoretic display device (EPD).

15, method according to claim 14 is characterized in that, this display is the electrophoretic display device (EPD) of encapsulation.

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