Classifications

• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
  • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
  • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
  • G09G3/3433—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using light modulating elements actuated by an electric field and being other than liquid crystal devices and electrochromic devices
  • G09G3/344—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using light modulating elements actuated by an electric field and being other than liquid crystal devices and electrochromic devices based on particles moving in a fluid or in a gas, e.g. electrophoretic devices
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2300/00—Aspects of the constitution of display devices
  • G09G2300/04—Structural and physical details of display devices
  • G09G2300/0439—Pixel structures
  • G09G2300/0443—Pixel structures with several sub-pixels for the same colour in a pixel, not specifically used to display gradations
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2310/00—Command of the display device
  • G09G2310/02—Addressing, scanning or driving the display screen or processing steps related thereto
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2310/00—Command of the display device
  • G09G2310/06—Details of flat display driving waveforms
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2310/00—Command of the display device
  • G09G2310/06—Details of flat display driving waveforms
  • G09G2310/061—Details of flat display driving waveforms for resetting or blanking
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2310/00—Command of the display device
  • G09G2310/06—Details of flat display driving waveforms
  • G09G2310/065—Waveforms comprising zero voltage phase or pause
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2320/00—Control of display operating conditions
  • G09G2320/02—Improving the quality of display appearance
  • G09G2320/0204—Compensation of DC component across the pixels in flat panels
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2320/00—Control of display operating conditions
  • G09G2320/02—Improving the quality of display appearance
  • G09G2320/0247—Flicker reduction other than flicker reduction circuits used for single beam cathode-ray tubes
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2320/00—Control of display operating conditions
  • G09G2320/04—Maintaining the quality of display appearance
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
  • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
  • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
  • G09G3/38—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using electrochromic devices

Definitions

• This invention relates to methods and apparatus for driving electro- optic displays, particularly bistable electro-optic displays.
• the methods and apparatus of the present invention are especially, though not exclusively, intended for use in driving bistable electrophoretic displays.
• optical-optic as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material.
• the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
• gray state is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states.
• extreme states are white and deep blue, so that an intermediate "gray state” would actually be pale blue. Indeed, as already mentioned the transition between the two extreme states may not be a color change at all.
• bistable and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in the aforementioned copending Application Serial No. 10/063,236 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays.
• bistable This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
• impulse is used herein in its conventional meaning of the integral of voltage with respect to time.
• bistable electro-optic media act as charge transducers, and with such media an alternative definition of impulse, namely the integral of current over time (which is equal to the total charge applied) may be used.
• the appropriate definition of impulse should be used, depending on whether the medium acts as a voltage-time impulse transducer or a charge impulse transducer.
• bistable electro-optic displays are known.
• One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Patents Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a "rotating bichromal ball" display, the term "rotating bichromal member" is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical).
• Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed to applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface.
• bodies typically spherical or cylindrical
• These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate.
• the appearance of the display is changed to applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface.
• electro-optic medium uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood,
• Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays.
• encapsulated electrophoretic media comprise numerous small 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.
• the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Patents Nos.
• polymer-dispersed electrophoretic media are regarded as subspecies of encapsulated electrophoretic media.
• An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates.
• the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.
• pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating
• roll coating such as knife over roll coating, forward and reverse roll coating
• gravure coating dip coating
• spray coating meniscus coating
• spin coating spin coating
• brush coating air knife coating
• silk screen printing processes electrostatic printing processes
• thermal printing processes ink jet printing processes; and other similar techniques.
• microcell electrophoretic display A related type of electrophoretic display is a so-called "microcell electrophoretic display".
• the charged particles and the suspending fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film.
• a carrier medium typically a polymeric film.
• electrophoretic displays are often opaque (since the particles substantially block transmission of visible light through the display) and operate in a reflective mode
• electrophoretic displays can be made to operate in a so-called "shutter mode" in which the particles are arranged to move laterally within the display so that the display has one display state which is substantially opaque and one which is light-transmissive.
• shutter mode in which the particles are arranged to move laterally within the display so that the display has one display state which is substantially opaque and one which is light-transmissive.
• Twisted nematic liquid crystals act are not bi- or multi-stable but act as voltage transducers, so that applying a given electric field to a pixel of such a display produces a specific gray level at the pixel, regardless of the gray level previously present at the pixel.
• LC displays are only driven in one direction (from non-transmissive or "dark” to fransmissive or “light”), the reverse transition from a lighter state to a darker one being effected by reducing or eliminating the electric field.
• the gray level of a pixel of an LC display is not sensitive to the polarity of the electric field, only to its magnitude, and indeed for technical reasons commercial LC displays usually reverse the polarity of the driving field at frequent intervals.
• bistable electro-optic displays act, to a first approximation, as impulse transducers, so that the final state of a pixel depends not only upon the electric field applied and the time for which this field is applied, but also upon the state of the pixel prior to the application of the electric field.
• electrophoretic and some other electro-optic displays exhibit bistability, this bistability is not unlimited, and images on the display slowly fade with time, so that if an image is to be maintained for extended periods, the image may have to be refreshed periodically, so as to restore the image to the optical state which it has when first written.
• these pulses need to be of the same polarity as the addressing pulse originally used to drive the relevant pixel of the display to the optical state being maintained, which results in a DC unbalanced drive scheme.
• a further aspect of the present invention relates to dealing with the problem that the aforementioned drive requirements of bistable electro-optic displays render conventional driving methods used for driving LCD's unsuitable for such bistable electro-optic displays. Furthermore, as discussed in the aforementioned U.S. Patents Nos. 6,531,997 and 6,504,524, problems may be encountered, and the working lifetime of a display reduced, if the method used to drive the display does not result in zero, or near zero, net time-averaged applied electric field across the electro-optic medium.
• a drive method which does result in zero net time-averaged applied electric field across the electro-optic medium is conveniently referred to a "direct current balanced” or “DC balanced”. Similar problems could be encountered with LCD's, but the insensitivity of such displays to the polarity of the applied electric field, and the consequent ability to reverse polarity at will, renders DC balance problems unimportant in LCD's. However, the need for DC balance is an important consideration in devising drive schemes for bistable electro-optic displays in which the electro-optic medium is sensitive to the polarity of the applied electric field.
• a further aspect of the present invention relates to methods and apparatus for driving electro-optic displays which meet the particular requirements of bistable displays already discussed. Certain methods and apparatus of the present invention are especially intended for producing accurate gray scale rendition in bistable displays.
• this 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; leaving the pixel undriven for a period of time, thereby permitting the pixel to assume a second optical state different from the first optical state; and applying to the pixel a refresh pulse which substantially restores the pixel to the first optical state, the refresh pulse being short relative to the addressing pulse.
• This aspect of the invention may hereinafter for convenience be referred to as the "refresh pulse" method of the invention.
• the refresh pulse will typically have an impulse not greater than about 20 per cent of the impulse of the addressing pulse, desirably not greater than about 10 per cent of this impulse, and preferably not greater than about 5 per cent of this impulse.
• typically the difference between the first and second optical states does not exceed about 1 unit of L* (where L* has the usual CIE definition); desirably this difference does not exceed about 0.5 unit of L*, and preferably does not exceed about 0.2 unit of L*.
• a plurality of refresh pulses may be applied to the pixel at regular intervals.
• the refresh pulse method after application of the refresh pulse, there is applied to the display a second addressing pulse which drives the pixel to a third optical state different from the first and second optical states, and wherein the impulse applied by the second addressing pulse is the sum of (a) the impulse needed to drive the pixel from the first to the third optical state; and (b) an impulse equal in magnitude but opposite in 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 of constant voltage but variable duration.
• the second addressing pulse may be a blanking pulse which drives all the pixels of the display to one extreme optical state.
• the display comprises a plurality of pixels, the first addressing pulse is applied to each pixel so as to drive a first group of pixels white and a second group of pixels black, at least one refresh pulse is applied to each pixel, and there are thereafter applied to the display a first blanking pulse which turns all the pixels black and a second blanking pulse which drives all the pixels white, the two blanking pulses being applied in either order.
• the impulse applied to each of the first group of pixels during the first blanking pulse is the sum of (a) the impulse needed to drive the pixel from white to black; and (b) an impulse equal in magnitude but opposite in polarity to the algebraic sum of the refresh pulses applied to the pixel between the first addressing pulse and the first blanking pulse.
• the impulse applied to each of the second group of pixels during the second blanking pulse is the sum of (c) the impulse needed to drive the pixel from black to white; and (d) an impulse equal in magnitude but opposite in polarity to the algebraic sum of the refresh pulses applied to the pixel between the first addressing pulse and the first blanking pulse.
• the refresh pulse method of the invention may be used with any of the types of electro-optic medium previously described.
• the display may be a rotating bichromal member or electrochromic display, or an electrophoretic display, desirably an encapsulated electrophoretic display.
• this invention provides a method for addressing a bistable electro-optic medium which comprises applying to the medium an alternating current pulse having a direct current offset.
• this invention provides a method for addressing a bistable electro-optic medium which comprises applying to the medium an alternating current pulse, and varying at least one of the duty cycle and the frequency of the pulse to change the optical state of the electro-optic medium following the alternating current pulse.
• this invention provides 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 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 to the column electrodes during the selection of any given row electrode voltages chosen so as to address the pixels in the row associated with the selected row electrode and write one row of a desired image on to the display.
• the method comprises: writing a 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 set, in which at least one pixel of the row differs between the first and second images, and a second set, in which no pixel of the row differs between the first and second images; and writing the second image 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.
• this invention provides an electro-optic display having a plurality of pixels, at least one of the pixels comprising a plurality of sub- pixels differing from each other in area, the display comprising drive means arranged to change the optical state of the sub-pixels independently of one another.
• Figure 1 is a graph showing variations of gray level with time in a display addressed using a direct current pulse with pulse length modulation
• Figure 2 is a graph similar to Figure 1 for a display addressed using a direct current pulse with pulse height modulation
• Figure 3 is a graph similar to Figure 1 for a display addressed using an alternating current pulse with a direct current offset in accordance with the present invention
• Figure 4 is a graph similar to Figure 1 for a display addressed using an alternating current pulse with duty cycle modulation in accordance with the present invention
• Figure 5 is a graph showing variations of gray level with time in a display addressed using a- double-prepulse slideshow waveform
• Figure 6 is a graph showing variations of gray level with time in a display addressed using a single-prepulse slideshow waveform
• Figures 7A and 7B show possible arrangements of sub-pixels within a single pixel of a display of the present invention.
• the present invention provides a number of improvements in methods for addressing electro-optic media and displays, and in the construction of such displays.
• the various aspects of the invention will now be described sequentially, but it should be recognized that a single electro-optic medium or display may make use of more than one aspect of the invention.
• a single electro-optic display might use AC pulse with DC offset driving and also use refresh pulses. Refresh pulse method of the invention
• the present invention provides a method for refreshing the image on an electro-optic display by applying to the display a short refresh pulse.
• a short refresh pulse which is sufficient to change the optical state of that pixel.
• the impulse applied by the refresh pulse is not greater than about 20 (desirably not greater than 10, and preferably not greater than 5) per cent of the impulse applied by the addressing pulse. For example, if a pixel requires an addressing pulse of 15 N for 500 msec, a refresh pulse could be 15 N for 10 msec, with an impulse of 2 per cent of that of the addressing pulse.
• the timing of the refresh pulses in this method should be adjusted to take account of the sensitivity of the human eye to abrupt small changes in optical state.
• the human eye is relatively tolerant of gradual fading of an image so that, for example, the bistability of an electro-optic medium of often measured as the time necessary for the lightness L* (defined by the usual CIE definition; see, for example, Hunt, R.W.G. Measuring Color, 3rd edition, Fountain Press, guitarist-upon-Tha es, England (1998). (ISBN 0 86343 387 1)) to change by 2 units from the maximum for white optical states (or minimum for black states) observed after the conclusion of the addressing pulse.
• the change in L* caused by a single refresh pulse should be less than about 1 unit of L*, desirably less than about 0.5 unit, and preferably less than about 0.2 unit.
• refresh pulses in the present method introduces some DC imbalance into the drive scheme during the period in which the refresh pulses are being applied, it does not preclude achieving long term DC balance in the drive scheme, and it has been found that it is the long term rather than short term DC balance which is of major importance in determining the operating life of electro-optic displays.
• the pixel which has received the refresh pulses may be driven to its opposed optical state by a "switching" or second addressing pulse, and the impulse applied in this switching addressing pulse may be adjusted to provide DC balance (or at least minimal DC imbalance) over the whole period since the first addressing pulse, by adjusting the impulse of this second addressing pulse by an amount substantially equal in magnitude, but 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 which can be switched between white and black optical states by applying an impulse of ⁇ 15 N for 500 msec.
• a pixel of this display is first switched from black to white by applying an impulse of + 15 N for 500 msec, and the white state of the pixel is subsequently maintained by applying at intervals ten refresh pulses each of + 15 V for 10 msec. If after these ten refresh pulses, it is desired to return the pixel to its black optical state, this may be achieved by applying an addressing pulse of - 15 N for 600 (rather than 500) msec, thereby achieving overall DC balance over the whole black- white-black transition of the pixel.
• This type of adjustment of the switching addressing pulse may be effected when a new image is to be written on the display and it is thus necessary to change the optical states of certain pixels.
• the adjustment may be carried out during the application of "blanking pulses" to the display.
• blanking pulses As discussed in the aforementioned PCT/US02/37241, it is often necessary or desirable to apply at regular intervals to an electro-optic display so-called “blanking pulses"; such blanking pulses involve first driving all the pixels of the display to one extreme optical state (for example, a white state), then driving all the pixels to the opposite optical state (for example, black), and then writing the desired image.
• Effecting the adjustment during blanking pulses has the advantage that all of the pixels may be DC balanced at substantially the same time; the pixels which were black in the prior image (the image present immediately prior to the blanking pulse) can be DC balanced during the blanking pulse which drives all pixels white, while the pixels which were white in the prior image can be DC balanced during the blanking pulse which drives all pixels black, using the technique already described in detail above.
• effecting the adjustment during blanking pulses can avoid the need to keep track of how many refresh pulses each individual pixel has received since its previous addressing pulse; assuming that black and white pixels are refreshed at the same intervals (as will usually be the case), and that a blanking pulse is inserted at each image transition, each pixel will need the same adjustment (except for polarity) during the blanking pulse, this adjustment being determined by the number of refresh pulses which have been applied to the display since the previous blanking pulse.
• effecting DC balancing during blanking pulses provides a way to apply the refresh pulse method to electro-optic displays having more than two gray levels, since obviously adjusting the impulse applied during a gray-to- gray transition in such a display may lead to undesirable errors in gray levels.
• the refresh pulse method of the present invention may be used as an alternative to, or in combination with, additives for increasing the bistability of an electro-optic medium.
• the present invention may be used with the electrophoretic media described in aforementioned 2002/0180687, which media have a suspending fluid having dissolved or dispersed therein a polymer which increases the bistability of the medium.
• Example 1 The following Example is now given, though by way of illustration only, to show one embodiment of the refresh pulse method of the present invention.
• This Example uses displays containing an encapsulated dual particle opposite charge type medium comprising a polymer-coated titania white particle and a polymer-coated black particle with an uncolored suspending fluid.
• the displays were prepared substantially according to "Method B" described in Paragraphs [0061]-[0068] of the aforementioned 2002/0180687.
• the displays had only limited bistability, the time necessary for the white optical state to change by 2 L* units being only about 15 sec. at ambient temperature.
• the display was addressed with the standard image, and this image was maintained using the aforementioned refresh pulses for 480 minutes. A series of blanking pulses were then applied, and the cycle of addressing and refresh pulses repeated. No DC balancing pulse was applied at any time. After 83 hours of operation, a series of blanking pulses were applied and then separate areas of the display, which had been white and black respectively in the standard image were tested. The area of the display which had been held white during the testing period is denoted by "480W” in the Table below, while the area which has been held black is denoted by "480D”. Each tested area was driven to its white optical state by a standard 500 msec addressing pulse, and its percentage reflectance value measured; this value is denoted by "w %" in the Table.
• each tested area was then allowed to stand for 15 sec without any refresh pulses being applied, and the change in L* measured after this 15 second interval; the resultant change in L*, known as the "bright holding difference", is denoted by "bhdl” in the Table.
• each tested area was driven to its black optical state by a standard 500 msec addressing pulse, and its percentage reflectance value measured; this value is denoted by "d %" in the Table.
• Each tested area was then allowed to stand for 15 sec without any refresh pulses being applied, and the change in L* measured after this 15 second interval; the resultant change in L*, known as the "dark holding difference", is denoted by "dhdl” in the Table.
• Electrophoretic media used in the refresh pulse method of the present invention may employ the same components and manufacturing techniques as in the aforementioned E Ink and MIT patents and applications, to which the reader is referred for further information.
• a voltage waveform or drive scheme capable of achieving grayscale in a bistable electro-optic display may hereinafter be called a "grayscale waveform” or “grayscale drive scheme” respectively.
• grayscale waveform element is used to mean a voltage pulse, or series of voltage pulses, that is capable of producing a change in optical state of an electro-optic display.
• a grayscale waveform element is itself capable of generating grayscale, and one or more grayscale waveform elements arranged in a particular sequence together form a grayscale drive waveform.
• a grayscale drive waveform is capable of switching a pixel of a display from one gray state to another.
• a sequence of one or more drive waveforms makes up a drive scheme, which is capable of displaying any series of grayscale images on a display.
• Drive waveform elements fall into two basic categories, namely direct current (DC) voltage pulses and alternating current (AC) voltage pulses.
• DC direct current
• AC alternating current
• the parameters of the pulse that can be varied are the pulse height and the pulse length.
• grayscale addressing schemes will primarily be discussed with reference to encapsulated particle-based electrophoretic media, but it is considered that the necessary modifications of such schemes to allow for the properties of other types of bistable electro-optic media will readily be apparent to those skilled in the technology of such media.
• the fundamental elements of grayscale drive waveforms are as follows: DC pulse with pulse length modulation
• the display tested responded immediately to the end of the applied voltage pulse, and its optical state ceased to evolve. On the microscopic level, it may be presumed that the electrophoretic particles instantly stop in transit from one electrode to the other and remain suspended at an intermediate position within the capsule.
• An advantage of a DC grayscale drive pulse with pulse length modulation is the speed with which the desired gray state is achieved.
• the inset illustrates the DC pulse height modulated waveform elements used to produce the grayscale transitions in an 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 switch the medium at the maximum voltage level.
• the three pulses used were 5, 10 and 15 N for 500 msec respectively, and the three curves produced are labelled accordingly; note that the time scale in the inset is not the same as that in the main Figure.
• the pulse length was fixed, while the height of the pulse was varied for different changes in reflectivity.
• Grayscale driving of the aforementioned encapsulated electrophoretic medium has been effected with oscillating (AC) electric fields; the switching mechanism with such AC fields is presumed to be entirely different from that effected in the DC driving of the same medium discussed above.
• the inset illustrates the AC pulse with DC offset modulation waveform elements used to produce the grayscale transitions in an encapsulated electrophoretic medium shown in the main part of the Figure.
• the frequency of the AC component (approximately 10 Hz) was set at a value that allowed the particles to respond to the oscillating field, while the magnitude and direction of the DC offset (which was 0, -1 or -2.5 N, as indicated for the three curves in Figure 3) determined the gray state that the pixel eventually attained.
• the time scale used in the inset is not the same as that in the main Figure.
• the reflectivity of a pixel, whose reflective state was changed from black to different levels of gray upon application of these voltage pulses, is plotted against time; it will be seen that greater DC offsets yielded greater changes in reflectivity.
• the electrophoretic particles oscillate in the suspending fluid and this oscillation is observed motion as a cyclic variation in reflectivity, superimposed upon the overall change in reflectivity, as is readily seen on the left hand side of Figure 3.
• the DC offset was applied.
• the reflectivity approaches a constant value after the waveform has been applied for some time. It appears that there must be a restoring force that opposes the force on the particles due to the DC offset voltage, otherwise, the particles would continue to flow to the cell wall. This restoring force may be due to the motion of fluid in between the capsule wall and the particles and/or to the interaction of the particles directly with the cell wall.
• AC waveform elements are the ability to reach a particular reflectivity state by specifying the parameters of the waveform element, while DC waveform elements enable only a change in reflectivity.
• An advantage of an AC waveform element with DC offset over other AC waveform elements is that precise timing of the addressing pulse is not required.
• the inset illustrates the AC pulses with duty cycle modulation used to produce the grayscale transitions shown in the main part of the Figure.
• the voltage is set to a maximum value, and the duty cycle (the percentage of time that the voltage is in the positive or negative direction) determines the reflectivity.
• the three duty cycles used were 50, 47 and 40 per cent, as indicated in Figure 4.
• the time scale used in the inset is not the same as that in the main Figure.
• the reflectivity of a pixel, whose reflective state was changed from black to different levels of gray upon application of the voltage pulses, is plotted against time.
• AC grayscale switching is to apply to an electro-optic medium an AC field which causes the optical state of the medium to oscillate and then to terminate the AC field in mid-cycle at the point having the desired reflectivity.
• the voltage may be set at a maximum value and the AC frequency varied in order to achieve a greater or lesser reflectivity range.
• the frequency determines the amplitude of the oscillation in reflectivity.
• the electrophoretic particles When such an approach is applied to an encapsulated particle-based electrophoretic medium, the electrophoretic particles respond to the AC field by oscillating around their initial positions. Since the reflectivity typically does not reach either the extreme black or white optical state, interactions with the cell wall are minimized and the response of the reflectivity is relatively linear with the applied voltage. An advantage of AC pulses with frequency modulation is that voltage modulation is not required.
• pulse width modulation and AC pulses are used to achieve an intermediate gray state in an electro-optic display which otherwise would be capable of achieving only black and white states.
• the ability to achieve gray scale is highly desirable in electro-optic displays.
• providing a large number of gray levels requires either pulse width modulation with a high frame rate driver (the high frame rate being needed to "slice” the pulse width into a large number of intervals, thereby enabling pulse width and hence gray scale to be controlled very accurately) or a driver capable of voltage modulation.
• Either type of driver is substantially more costly than the simple tri-level drivers, which enable individual pixels of a display to be set only to + N -N and 0 potential (where V is an arbitrary operating potential) relative to the potential of a common front plane electrode, and which are commonly used to drive displays capable of only black and white states.
• This invention provides a drive scheme which enables a tri-level driver to produce a gray level intermediate the black and white levels of a bistable electro-optic display.
• the drive scheme is most easily appreciated from the following Table 2, which shows the voltage applied during the successive frames of various types of transitions in such a display of the present invention:
• the transitions from black to white or vice versa are the same as in a binary (black/white only) display.
• the transitions to gray have two parts.
• the first part is a square wave pulse (i.e., a plurality of frames at the same potential) of the proper polarity and length to bring the reflectivity of the electro-optic medium as close as possible to the desired middle gray lightness.
• the accuracy possible with this step will be limited by the frame rate of the display.
• the second part of the addressing pulse consists of an equal number of positive and negative voltage pulses, each one frame in width.
• this invention provides a method to produce a single gray level in an otherwise binary electro-optic display using only a simply tri-level driver rather than without the use of a complex and costly voltage modulated driver.
• this invention provides a collection of two-dimensional transition matrices, wherein each element in the matrix specifies how to get from an initial optical state (denoted "the row index” herein, although obviously the allocation of the initial optical states to rows is arbitrary) to a final optical state (denoted “the column index” herein).
• Each element of this matrix is constructed from a series of waveform elements (as defined above), and in general, for an n-bit grayscale display, this matrix will contain 2 (2N) elements.
• the matrices of the present invention take account of such considerations as the need for DC balancing of the drive scheme (as discussed above), minimizing "memory" effects in certain electro-optic media (i.e., effects whereby the result of applying a particular pulse to a pixel depends not only upon the current state of the pixel but also upon certain prior states), thereby producing uniform optical states and maximizing the switching speed of the display, while working within the constraints of an active matrix drive scheme.
• the present invention also provides a method for determining the optimal values of each term of the elements in such a matrix for any specific electro-optic medium.
• PCT/US02/37241 For further discussion of such matrices and their use in driving electro-optic displays, the reader is referred to the aforementioned PCT/US02/37241.
• the presently preferred waveforms of the invention are described below in terms of pulse width modulation (PWM) as discussed above.
• PWM pulse width modulation
• the same or similar results may be achieved using pulse height modulation, or the various hybrid types of AC modulation discussed above, and various different types of modulation may be employed within a single waveform, for example, pulse width modulation for all but the last section of the pulse, followed by voltage modulation on the last section of the pulse.
• the first two waveforms of the present invention described below are "slide show" waveforms, which return from one gray state to the black state before addressing out to the next gray state. Such waveforms are most compatible with a display update scheme where the entire screen blanks at once, as in a slide projector.
• a pixel of the electro-optic medium is initially driven (as indicated at 100) from black to an initial (first) gray state using a partial pulse.
• the pixel is first driven (at 102) from the first gray state to white, and then (at 104) from white to black.
• the proper pulse to reach the second gray state is applied at 106.
• This waveform requires a maximum of three times the switching time of the medium (i.e., the time necessary for a single pixel to switch from its black optical state to its white optical state, or vice versa) to effect a transition between any two arbitrary gray states, and is therefore referred to as a 3X waveform.
• a pixel of the electro-optic medium is initially driven (as indicated at 110) from black to an initial (first) gray state using a partial pulse, in the same way as in the double-prepulse waveform discussed in Section 6 above.
• a partial pulse in the same way as in the double-prepulse waveform discussed in Section 6 above.
• a display may be updated by addressing it directly from one gray state to another without passing through a black or white state. Since there are no obvious artifacts (i.e., black and/or white "flashes") associated with such a transition, it may be referred to as "gray-to-gray" addressing.
• gray-to-gray waveform There are two main forms of gray-to-gray waveform, namely DC-balanced and DC-imbalanced.
• the transition between any two gray states is effected by applying a modulated pulse of the precise length necessary to shift between the two states.
• the electro-optic medium does not pass through any intermediate black or white states.
• Another aspect of the present invention relates to improving the performance of an active matrix bistable electro-optic display by selective driving of the rows of the display.
• the continuous refreshment is effected by using a row driver to turn on the gates of the transistors associated with one row of pixels of the display, placing on column drivers (connected to the source electrodes of the transistors in each column of the display) the potentials needed to write to the pixels in the selected row the relevant portion of the desired image on the display, and thus writing the selected row of the display.
• the row driver selects the next row of the display and the process is repeated, with the rows thus being refreshed cyclically.
• bistable electro-optic displays do not need continuous refreshing, and indeed such continuous refreshing is undesirable, since it unnecessarily increases the energy consumption of the display. Furthermore, during such refreshing, the gate (row) lines may deliver capacitative voltage spikes to pixel electrodes, and any driver voltage errors or uncompensated gate feedthrough bias errors can accumulate; all these factors can result in undesirable shifts in the optical states of the pixels of the display. Accordingly, in bistable electro-optic displays it is desirable to provide some means for updating a portion of an image without the need to rewrite the whole image on the display, and one aspect of this invention relates to a bistable electro-optic display provided with such "partial updating" means.
• this is done by comparing successive images to be written to the display, identifying the rows which differ in the two images, and addressing only the rows thus identified.
• a display controller cf. the aforementioned PCT/US02/37241 examines all of the desired pixel electrode output voltages.
• the controller If all of the output voltages for that line are equal to the potential N COm of the common front electrode of the display (i.e., if no pixel in that row needs to be rewritten), then the controller outputs a synchronizing (N s ync) pulse without loading data values into the column drivers, and without issuing a corresponding output enable (OE) command.
• N s ync synchronizing
• OE output enable
• This invention gives two distinct types of advantages. Firstly, many sources of spurious voltage are eliminated for pixels that are not rewritten. There is no capacitative gate spike for these pixels, and errors in the column driver voltage will not be passed on to a pixel in frames where it is not addressed. Because of the relatively lower resistivity of many electro-optic media, especially electrophoretic media, as compared with liquid crystals, the pixel electrode will tend to relax to the actual front plane voltage, thus maintaining the hold state of the electro-optic medium. Secondly, power consumption of the display is minimized. For every row that is not rewritten, the corresponding gate line does not have to be charged. In addition, when data is not loaded into the column drivers of the display, the additional power consumption of moving that data across the display interface is also eliminated. Spatial area dithering
• the aspects of the present invention previously discussed relate to the waveforms used to drive electro-optic displays. However, the behavior of such displays can also be changed by varying the structure of the backplane, and this aspect of the invention relates to dividing one or more pixel, and preferably each pixel, of a display into a plurality of sub-pixels having differing areas.
• grayscale in an electro-optic display.
• This gray scale may be achieved either by driving a pixel of the display to a gray state intermediate its two extreme states.
• the medium is not capable of achieving the desired number of intermediate states, or if the display is being driven by drivers which are not capable of providing the desired number of intermediate states, other methods must be used to achieve the desired number of states, and this aspect of the invention relates to the use of spatial dithering for this purpose.
• a display may be divided into a plurality of "logical" pixels, each of which is capable of displaying the desired number of gray or other optical states.
• logical pixels each of which comprises three sub-pixels of primary colors, for example red, green and blue; see, for example, the aforementioned 2002/0180688.
• full color logical pixels each of which comprises three sub-pixels of primary colors, for example red, green and blue; see, for example, the aforementioned 2002/0180688.
• a logical pixel comprising four independently controllable sub-pixels of equal area could be used to provide two-bit gray scale.
• the present invention provides an electro-optic display having at least one pixel which comprises a plurality of sub-pixels, these sub-pixels being of differing areas. In a preferred embodiment of this invention, at least two sub-pixels differ in area by substantially a factor of two.
• a logical pixel might have sub-pixels with areas of IX, 2X and 4X, where X is an arbitrary area.
• a logical pixel of this type is illustrated schematically in Figure 7A of the accompanying drawings. This logical pixel achieves three-bit grayscale using only three electrodes, whereas achieving the same three-bit grayscale with sub-pixels of equal area would require eight sub-pixels.
• each sub-pixel When driven, each sub-pixel reflects or transmits a portion of the incoming light, and the fractional amount is dictated by the area of the sub-pixel. If the reflectance/transmission is averaged over the area of the logical pixel, then a binary weighting of driven area is achieved, and hence spatially dithered grayscale.
• the areas of the sub-pixels are arbitrary. The ones shown in Figure
• Figure 7A is driven in each logical pixel), a viewer still see a line or grating pattern arising from the pattern of sub-pixels.
• individual pixels could have, at random, each of the four possible orientations of the arrangement shown in Figure 7A.
• Such "randomization" of the sub-pixels helps to break up patterns and render them less noticeable to an observer.
• the electro-optic medium itself does not need to be capable of gray scale; essentially the display can be a black/white display, and sub-pixels turned on and off to produce gray scale.
• the necessary control of the sub-pixels can be achieved by providing additional column drivers for the same number of rows, as in color sub-pixel arrays. This reduces demands upon the electro-optic medium used; for example, one does not need to worry about possible drift of gray levels of the medium over its operating lifetime.
• TFT thin film transistor
• the voltages of the pixel electrodes on the display backplane are varied in order to impose desired voltages across pixels.
• the top plane is typically held at a particular voltage deemed advantageous for addressing the pixels. For example, if the data line voltage supplied to the pixel electrodes varies between zero volts and a voltage Vo, the top plane may be held at No/2 in order to permit voltage drops across the pixel to be as large as No/2 in both directions.
• the voltage of the top plane may be varied to enhance the addressing of the electro-optic medium.
• the top plane voltage could be held at zero volts in order to permit the total pixel voltage drop (top plane minus pixel voltage) to be as low at -No. Raising the top plane up to No permits a pixel voltage drop as large as N o . These larger voltage drops permit faster addressing of the electro-optic medium.
• the top plane voltage may be able to set the top plane voltage not only at voltages zero and No, but to other voltages as well.
• this invention provides an electro-optic display having a storage capacitor formed between a pixel electrode and a (second) electrode that has a voltage that can be varied independently from the select lines of the display.
• the second electrode follows the top plane voltage, that is, its voltage differs from the top plane only by a time- independent constant.
• This type of capacitor greatly reduces the capacitative voltage spikes experienced by the pixel, as compared with a storage capacitor is created by an overlap between a pixel electrode and a select line that controls the adjacent (previous) row of the display.
• Another aspect of the present invention relates to reducing or eliminating unwanted switching of the electro-optic medium by select and data lines.
• select and data lines are essential elements of an active matrix panel in that they provide the voltages required for charging pixel electrodes to desired values.
• the select and data lines can have the unwanted effect of switching the electro-optic medium adjacent the data lines.
• the undesirable optical artifacts caused by such switching can be eliminated by using a black mask to hide the regions switched by the data and/or select lines from a viewer.
• providing such a black mask requires registration of the front plane of the display with its back plane and a reduction in the fraction of the electro-optic medium that is exposed to the viewer. The result is a display darker and lower in contrast than one could achieve without the black mask.
• the need for a black mask is avoided by making the data lines to have a small lateral extent in one direction so that they do not appreciably address the adjacent electro-optic medium during normal display operation. This obviates the need for a black mask.
• a related aspect of the present invention relates to the use of passivated electrodes and modification of the drive scheme used to drive the electro-optic medium.
• An impulse-driven electro-optic medium can be electronically addressed when it is in a thin film form between two electrodes.
• the electrodes make contact with the electro-optic medium.
• a voltage impulse can still be applied to the medium and the medium can be addressed through these voltage impulses if the dielectric layer is properly engineered.
• the optical state of an electro-optic medium is of course achieved by changing the voltage on a pixel electrode.
• This voltage change results in a voltage across the electro-optic medium that decays as charge leaks through the medium. If an external dielectric layer (i.e., a dielectric layer between the medium and one electrode) is sufficiently thin and the electro-optic medium is sufficiently resistive, the voltage impulse across the medium will be sufficient to cause a desirable shift in the optical state of the medium. Electronic addressing of an electro-optic medium through a dielectric layer is therefore possible.
• the addressing scheme is different, however, from addressing an electro-optic medium with electrodes in direct contact with the medium since, in the latter case, the medium is addressed by applying voltages across the pixel, whereas, in the former case, addressing is achieved by causing a change in the pixel voltage. At every change, a voltage impulse is experienced by the electro-optic medium.
• this invention provides drive schemes for reducing crosstalk in active matrix electro-optic displays.
• Inter-pixel cross-talk where addressing one pixel affects the optical state of other pixels, is undesirable but has many causes.
• One cause is the finite current flow through transistors in the off state. Bringing a data line to a voltage intended for charging one pixel can charge up transistors in unselected rows because of off state current leakage.
• a solution is to use transistors with a low off- state current.
• electro-optic medium for use in the present invention is an encapsulated particle-based electrophoretic medium.
• electrophoretic media used in the methods and apparatus of the present invention may employ the same components and manufacturing techniques as in the aforementioned E Ink and MIT patents and applications, to which the reader is referred for further information.

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