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
  • 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
  • G09G2320/00—Control of display operating conditions
  • G09G2320/02—Improving the quality of display appearance
  • G09G2320/0209—Crosstalk reduction, i.e. to reduce direct or indirect influences of signals directed to a certain pixel of the displayed image on other pixels of said image, inclusive of influences affecting pixels in different frames or fields or sub-images which constitute a same image, e.g. left and right images of a stereoscopic display
• • 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
  • G09G2320/06—Adjustment of display parameters
  • G09G2320/066—Adjustment of display parameters for control of contrast

Definitions

• This invention relates to methods for driving electro-optic displays, particularly bistable electro-optic displays, and to displays using such methods.
• the methods and displays of the present invention are especially, though not exclusively, intended for use in driving bistable electrophoretic displays.
• 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.
• black and
• white may be used hereinafter to refer to the two extreme optical states of a display, and should be understood as normally including extreme optical states which are not strictly black and white.
• the term “monochrome” may be used hereinafter to denote a drive scheme which only drives pixels to their two extreme optical states with no intervening gray states.
• 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.
• Several types of electro-optic displays are known, for example:
• rotating bichromal member displays see, for example, 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);
• Electrophoretic media can use liquid or gaseous fluids; for gaseous fluids see, for example, Kitamura, T., et al, "Electrical toner movement for electronic paper-like display", IDW Japan, 2001, Paper HCSl-I, and Yamaguchi, Y., et al., "Toner display using insulative particles charged triboelectrically", IDW Japan, 2001, Paper AMD4-4); U.S. Patent Publication No.
• the media may be encapsulated, comprising numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspending medium, and a capsule wall surrounding the internal phase.
• the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes; see the aforementioned MIT and E Ink patents and applications.
• the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium may be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material; see for example, U.S. Patent No. 6,866,760.
• such polymer- dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
• microcell electrophoretic display in which the charged particles and the fluid are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film; see, for example, U.S. Patents Nos. 6,672,921 and 6,788,449.
• Electrophoretic media can operate in a "shutter mode" in which one display state is substantially opaque and one is light-transmissive. See, for example, U.S. Patents Nos. 6,130,774 and 6,172,798, and U.S. Patents Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays can operate in a similar mode; see U.S. Patent No. 4,418,346. Other types of electro-optic displays may also be capable of operating in shutter mode.
• 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.
• 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.
• the difference between the first and second optical states does not exceed about 1 unit of L* (where L* has the usual CIE definition:
• L* 116(R/Ro) 1/3 - 16, where R is the reflectance and Ro is a standard reflectance value); 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. [Para 1 7] This refresh pulse method does allow refreshing of images on bistable electro- optic displays, thus improving the appearance of the images, but necessarily introduces some DC imbalance. The aforementioned 2005/0270261 also describes various methods for reducing the DC imbalance produced.
• Electro-optic displays typically have a backplane provided with a plurality of pixel electrodes each of which defines one pixel of the display; conventionally, a single common electrode extending over a large number of pixels, and normally the whole display is provided on the opposed side of the electro-optic medium.
• the individual pixel electrodes may be driven directly (i.e., a separate conductor may be provided to each pixel electrode) or the pixel electrodes may be driven in an active matrix manner which will be familiar to the those skilled in backplane technology.
• Blooming refers to the tendency for application of a drive voltage to a pixel electrode to cause a change in the optical state of the electro-optic medium over an area larger than the physical size of the pixel electrode. Although excessive blooming should be avoided (for example, in a high resolution active matrix display one does not wish application of a drive voltage to a single pixel to cause switching over an area covering several adjacent pixels, since this would reduce the effective resolution of the display) a controlled amount of blooming is often useful. For example, consider a black-on-white electro-optic display which displays numbers using a conventional seven-segment array of seven directly driven pixel electrodes for each digit. When, for example, a zero is displayed, six segments are turned black.
• This electric field is primarily directed perpendicular to the layer of electro-optic medium and is of approximately uniform intensity (ignoring for present purposes the distorting effects of space charges and polarization of the electro-optic medium itself).
• fringe fields form, and these fringe fields extend into the interpixel-regions between the pixel subjected to the electric field and the adjacent pixels.
• the electric field lines are curved, going from the pixel to the top plane or the neighboring electrodes.
• the electric field in this region is generally weaker than in the central part of the pixel.
• Waveforms are typically designed to achieve correct transitions to desired optical states in central parts of pixels, i.e., far from pixel edges, and thus away from the fringe fields.
• the first voltage pulse does not drive the electro-optic medium from black to white, but instead from black to an intermediate gray.
• the second voltage pulse drives the electro-optic medium from this intermediate gray towards black.
• the response of many electro-optic media to electric fields is not necessarily linear with the magnitude of the applied field, nor is it necessarily symmetric with respect to the direction of the electric field. Consequently, after the two transitions previously discussed, the inter-pixel region does not necessarily return to exactly its original black state.
• the optical state of the fringe field region will be slightly less black than the original state, i.e., it will be a very dark gray rather than a true black, thus leaving the fringe field region lighter in color than the black central part of the pixel.
• This is referred to as an edge ghost, which is one class of edge artifact.
• Such edge ghosts if sufficiently large, are readily detected by the human eye and highly objectionable.
• An area of blooming is not a uniform white or black but is typically a transition zone where, as one moves across the area of blooming, the color of the medium transitions from white through various shades of gray to black. Accordingly, an edge ghost will typically be an area of varying shades of gray rather than a uniform gray area, but can still be visible and objectionable, especially since the human eye is well equipped to detect areas of gray in monochrome images where each pixel is supposed to be pure black or pure white.)
• asymmetric blooming may contribute to edge ghosting.
• Asymmetric blooming refers to a phenomenon whereby in some electro-optic media (for example, the copper chromite/titania encapsulated electrophoretic media described in U.S. Patent No. 7,002,728) the blooming is "asymmetric" in the sense that more blooming occurs during a transition from one extreme optical state of a pixel to the other extreme optical state than during a transition in the reverse direction; in the media described in this patent, typically the blooming during a black-to-white transition is greater than that during a white- to-black one.
• the present invention provides methods for driving bistable electro-optic displays which can reduce or eliminate edge ghosting.
• Another problem associated with blooming is that one pixel may in effect control an area of the display which is intended to be controlled by an adjacent pixel. Ideally, one might desire that blooming be controlled so that the fringe field from one pixel causes a change in optical state extending half-way across gap between it and an adjacent pixel. In practice, however, blooming varies with numerous factors, including particularly the temperature of the electro-optic medium, so that it may not always be possible to control the blooming to the optimum value. Furthermore, as already noted, because of fringe fields varying with distance, an area of blooming does not have a sharp edge between the two extreme optical states, but rather an area of finite width where a transition between the two optical states occurs.
• the white area of a white pixel extends half-way across the inter-pixel gap (so that when the two adjacent pixels are both driven white, the inter-pixel gap is the same white color as the pixels themselves), it may be necessary to tolerate a gray area extending more than half-way across the inter-pixel gap when one pixel is driven white while the other is still black.
• the presence of such a gray area may be problematic in some circumstances.
• the display is a color display provided with a color filter array
• the two pixels involved underlie different colored areas of the color filter array
• the presence of the gray area when the first pixel is white and the second black may result in "contamination" of the color of the first pixel with some amount of the color of the second, thus adversely affecting the color rendering of the display
• the present invention may be regarded as comprising various modifications of the refresh pulse driving method of the aforementioned 2005/0270261.
• "refresh" pulses are not generated at arbitrary intervals but are keyed to transitions occurring at other pixels.
• the refresh pulses may not be applied to all pixels or all pixels having a given optical state, but may be confined to certain pixels adjacent a pixel undergoing a transition.
• the corrective pulses applied in accordance with the method of the present invention will be referred to as "reinforcing pulses” while the term “refresh pulses” will be used to denote pulses applied in the method of the aforementioned 2005/0270261.
• this invention provides a first method of driving a bistable electro- optic display having at least first and second pixels each of which can display first and second extreme optical states (the pixels may or may not have other intermediate optical states), the first and second pixels having adjacent edges separated by an inter-pixel gap.
• the first method comprises applying to the first pixel a drive pulse effective to cause the first pixel to change its optical state to one of its extreme optical states, and applying to the second pixel, which is in the said one extreme optical state, a reinforcing pulse of the same polarity as the drive pulse, the reinforcing pulse being applied either simultaneously with the drive pulse or within a predetermined period after the end of the drive pulse.
• This first method of the present invention (which may hereinafter be referred to as the "reinforcing pulse method") may be applied to a monochrome display in which each pixel is intended to display only first and second (typically white and black) optical states.
• first and second optical states typically white and black
• the first pixel is given a pulse of one polarity to effect the desired transition.
• the second pixel is given a reinforcing pulse of the same polarity as the drive pulse given to the first pixel (i.e., a black-going pulse). Since the second pixel is already black, the reinforcing pulse does not effect a gross change in the black color of the second pixel. However, if the second pixel has been in its black state for some time, so that its color has "drifted" from a true black to a dark gray, the reinforcing pulse serves to drive the second pixel back to a true black, thus avoiding having a dark gray second pixel immediately adjacent a true black first pixel, a situation which is readily apparent to the human eye.
• a reinforcing pulse of the same polarity as the drive pulse given to the first pixel (i.e., a black-going pulse). Since the second pixel is already black, the reinforcing pulse does not effect a gross change in the black color of the second pixel. However, if the second pixel has been in its black state for some time
• the reinforcing pulse also serves to reduce or eliminate edge ghosting in the inter-pixel gap between the first and second pixels.
• waveforms used to drive pixels of electro-optic displays from one optical state to another may be quite complex and may include drive pulses of both polarities.
• the "drive pulse" applied to the first pixel in the reinforcing pulse method of the present invention may in fact be a complex waveform including multiple individual drive pulses some of which may have opposing polarities.
• the polarity of such a complex waveform may be defined as the polarity of a single drive pulse of constant magnitude which effects the same optical transition of the first pixel as the complex waveform.
• the "reinforcing pulse” used in the present method may itself comprise more than a single pulse.
• the single and double reinforcing pulse variants there are two principal variants of the present reinforcing pulse method, which may be termed the single and double reinforcing pulse variants.
• the single reinforcing pulse method uses only one reinforcing pulse.
• the double reinforcing pulse method is more complex. Consider the situation discussed above where a display has first and second pixels sharing a common edge, with the first pixel undergoing a transition from white to black, while the second pixel remains in its black state.
• a single reinforcing method there is applied to the second pixel a single pulse of the same polarity as the pulse applied to the first pixel, i.e., a black-going pulse.
• the double reinforcing pulse method there is first applied to the second pixel a pulse of the opposite polarity to that applied to the first pixel (i.e., a white-going pulse), and thereafter there is applied to the second pixel a pulse of the same polarity as that applied to the first pixel (i.e., a black-going pulse).
• the two successive pulses applied to the second pixel may hereinafter be referred to a the "reverse reinforcing pulse” and the "forward reinforcing pulse” respectively.
• the reinforcing pulse may be applied only to a pixel that shares an edge with the pixel being driven (hereinafter for convenience pixels having this relationship may be called “edge-adjacent pixels” to distinguish them from “corner-adjacent pixels” which only have one corner in common).
• edge-adjacent pixels pixels having this relationship may be called “edge-adjacent pixels” to distinguish them from “corner-adjacent pixels” which only have one corner in common.
• Variant (c) may require further explanation.
• a series of images in which a single object, or small number of objects, are moving against an essentially static background More specifically, consider a series of monochrome images in which a black plane is moving against a white sky. To effect a transition between successive images, certain pixels around the periphery of the plane must be rewritten. To avoid visually-distracting effects, it may be desirable to apply the reinforcing pulses to black pixels representing the plane, but not to black pixels representing the ground beneath. Similarly, if a display is configured as a series of essentially independent windows, it may be desirable to use a drive scheme in which a "global" updating method is applied to each window independently.
• the reinforcing pulse is applied either simultaneously with the drive pulse or within a predetermined period after the end of the drive pulse. It is desirable that the reinforcing pulse appear to a user of the display to be part of the same transition as that of the driven pixel, rather than there appearing to be a first transition for the driven pixel and a second one for the reinforcing pulse.
• the reinforcing pulse should be applied either simultaneously with the drive pulse or within a subsequent period equal to the length of the drive pulse, and preferably not exceeding about 400 milliseconds. In most cases, it is conveniently to effect the reinforcing pulse simultaneously with the last part of the drive pulse (i.e., simultaneously with a terminal portion of the drive pulse).
• the impulse applied by the reinforcing pulse may vary over a wide range, depending, of course, upon the specific electro-optic medium employed and other parameters of a specific display.
• the reinforcing pulse may have the same impulse as the drive pulse (assuming that the drive pulse is that needed to drive the first pixel from its first to its second extreme optical state) but usually the impulse of the reinforcing pulse will be smaller.
• the impulse of the reinforcing pulse may be from about 10 to about 70, more usually from about 20 to about 50, per cent of the impulse of the drive pulse. It appears that in many cases a reinforcing pulse having an impulse of about 25 per cent of the impulse of the drive pulse gives good results.
• the reinforcing pulse need not have the form of a single continuous pulse but may be in form of a plurality of discrete sub-pulses separated by one or more pauses (i.e., periods of zero applied voltage); for example, if the reinforcing pulse is to have a impulse of 20 per cent of the impulse of the drive pulse, the reinforcing pulse could be in the form of two discrete sub-pulses each having an impulse of 10 per cent of the drive pulse impulse, with one sub-pulse being applied simultaneously with the last part of the drive pulse and the second (say) 100 milliseconds after the end of the drive pulse, with a period of zero voltage between the two sub-pulses.
• the voltage of the reinforcing pulse may be the same or different from that of the drive pulse; in some cases, it appears that it may be advantageous for the reinforcing pulse to be of a lower voltage than the drive pulse.
• the present invention extends to a bistable electro-optic display, controller or application specific integrated circuit arranged to carry out the reinforcing pulse method of the present invention.
• the invention extends to an electro-optic display comprising a layer of bistable electro-optic medium which can display first and second extreme optical states, first and second pixel electrodes disposed adjacent the layer of bistable electro-optic medium and capable of applying electric fields to the medium, the first and second pixel electrodes having adjacent edges separated by an inter-pixel gap, and a controller for controlling the voltages applied to the first and second pixel electrodes, wherein the controller is arranged to carry out a drive method comprising: applying to the first pixel electrode a drive pulse effective to cause the electro- optic medium adjacent the first pixel electrode to change its optical state to one of its extreme optical states, and applying to the second pixel electrode, while the electro-optic medium adjacent the second pixel electrode is in the said one extreme optical state, a reinforcing pulse of the same polarity as the drive pulse, the
• the present invention also provides a second drive method, which may hereinafter be referred to as the "inverse reinforcing pulse method" of the present invention.
• the inverse reinforcing pulse method of the present invention is applied where one of an edge-adjacent pair of pixels is transitioning from one optical state while the other of the pair is remaining in that optical state.
• the inverse reinforcing pulse method may be applied where a first pixel is transitioning from white to black, while an adjacent pixel is staying white.
• the non-transitioning pixel is given a pulse of the opposite polarity to that applied to the transitioning pixel, i.e., in the foregoing example, the adjacent pixel would be given a white- going pulse.
• the present invention provides a method of driving a bistable electro- optic display having at least first and second pixels each of which can display first and second extreme optical states (the pixels may or may not have other intermediate optical states), the first and second pixels having adjacent edges separated by an inter-pixel gap.
• the present inverse reinforcing pulse method comprises applying to the first pixel a drive pulse effective to cause the first pixel to change its optical state from one of its extreme optical states, and applying to the second pixel, which is in the said one extreme optical state, an inverse reinforcing pulse of the opposite polarity to the drive pulse, the inverse reinforcing pulse being applied either simultaneously with the drive pulse or within a predetermined period after the end of the drive pulse.
• the inverse reinforcing pulse method of the present invention can make use of any of the variants of the main reinforcing pulse method already described; thus, for example, the inverse reinforcing method may be applied on an edge-adjacent, contiguous area or global basis, although typically it will be applied on an edge-adjacent basis.
• the inverse reinforcing pulse method may also comprise more than one pulse, and in particular may comprise two pulses of opposite polarity as described for the first method of the present invention.
• the inverse reinforcing pulse method of the present invention is intended to reduce to eliminate the problems caused by excessive blooming as described above.
• the present invention extends to a bistable electro-optic display, display controller or application specific integrated circuit (ASIC) arranged to carry out the second method of the invention.
• the invention extends to an electro-optic display comprising a layer of bistable electro-optic medium which can display first and second extreme optical states, first and second pixel electrodes disposed adjacent the layer of bistable electro-optic medium and capable of applying electric fields to the medium, the first and second pixel electrodes having adjacent edges separated by an inter-pixel gap, and a controller for controlling the voltages applied to the first and second pixel electrodes, wherein the controller is arranged to carry out a drive method comprising: applying to the first pixel a drive pulse effective to cause the first pixel to change its optical state from one of its extreme optical states, and applying to the second pixel, which is in the said one extreme optical state, an inverse reinforcing pulse of the opposite polarity to the drive pulse, the inverse reinforcing pulse being applied either simultaneously with the drive pulse or within a
• the displays of the present invention may make use of any of the types of bistable electro-optic media described above.
• the displays of the present invention may comprise a rotating bichromal member or electrochromic medium.
• the displays may comprise an electrophoretic medium comprising a plurality of electrically charged particles disposed in a fluid and capable of moving through the fluid under the influence of an electric field.
• the electrically charged particles and the fluid may be confined within a plurality of capsules or microcells.
• the electrically charged particles and the fluid may be present as a plurality of discrete droplets surrounded by a continuous phase comprising a polymeric material, so that the electrophoretic medium is of the polymer- dispersed type.
• the fluid may be liquid or gaseous.
• the displays of the present invention may be used in any application in which prior art electro-optic displays have been used.
• the present displays may be used in electronic book readers, portable computers, tablet computers, cellular telephones, smart cards, signs, watches, shelf labels and flash drives.
• Figure 1 of the accompanying drawings is a graph showing the variation with time of the reflectivities of two pixels during the refresh pulse method of the aforementioned 2005/0270261.
• Figure 2 is a graph similar to Figure 1 but showing the reflectivities of two pixels during a first, reinforcing pulse, method of the present invention.
• Figure 3 is a graph showing the variation with time of the reflectivities of a "flashing" pixel which is repeatedly cycled between its black and white extreme optical states, and of a second pixel which is subjected to a single reinforcing pulse method of the present invention.
• Figure 4 is a graph similar to Figure 3 but showing the effect of driving the flashing pixel with longer drive pulses.
• Figure 5 is a graph similar to those of Figures 3 and 4 but showing the effect of using a modified waveform for the flashing pixel.
• a first aspect of this invention relates to a method of driving a bistable electro-optic display in which a reinforcing pulse, which does not effect a gross change in the optical state of a pixel, is applied to one or more pixels during or shortly after the application to another pixel of a drive pulse which does change the optical state of that pixel.
• the reinforcing pulse serves to match the color of the pixel receiving the reinforcing pulse to that of the pixel receiving the drive pulse and, if the two pixels are edge-adjacent, reduces edge ghosting between the two pixels.
• the present method may reduce the number of reinforcing pulses needed, since if the display is not updated for a long period, no reinforcing pulses will be applied. (It is of course possible to combine the present methods with the refresh pulse method by ensuring that, if any pixel does not receive a drive pulse or a reinforcing pulse for a long period, that pixel will receive a refresh pulse at the end of the long period.) Also, the reinforcing pulses need not be applied to every pixel of a display in a given extreme optical state, which further reduces the number of reinforcing pulses needed. Accordingly, the present invention can reduce the amount of DC imbalance in a refresh pulse driving method while still avoiding undesirable visual artifacts.
• the DC imbalance introduced by the present reinforcing pulse method may be compensated in a manner similar to that described in the aforementioned 2005/0270261; the display controller may keep track of the DC imbalance of each pixel and adjust the impulse of a drive pulse or waveform used to drive the pixel from one extreme optical state to the other to compensate for accumulated DC imbalance of the pixel. If the drive scheme employed makes use of blanking pulses (i.e., pulses which drive all pixels of the display, or of a particular area thereof, to the same optical state) DC balancing is conveniently effected during application of the blanking pulse.
• blanking pulses i.e., pulses which drive all pixels of the display, or of a particular area thereof, to the same optical state
• Figure 1 of the accompanying drawings is a graph of the optical states of two pixels of a monochrome bistable electro-optic display plotted against time. As shown in Figure 1, both pixels (designated Pixel 1 and Pixel 2) are originally in their dark extreme optical state. Pixel 2 receives a drive pulse which drives it to its white optical state (represented near the upper end of reflectivity (R) ordinate in Figure 1). Thereafter, this white state slowly decays, as indicated by the gradual downward slope of the Pixel 2 curve in Figure 1. Although not shown in Figure 1, a pixel driven to its black optical state will also decay in an analogous manner, so that after some time a black-and-white image written on the display becomes a dark gray and light gray image, with reduced contrast ratio.
• Pixel 1 (assumed for present purposes to be edge-adjacent to Pixel 2) is driven to its white state, there is a significant difference between the white, freshly rewritten Pixel 1 and the partially- decayed, light gray Pixel 2, as shown by the vertical spacing between the two curves in Figure 1, which also shows that this difference will persist for some time as the optical states of both pixels gradually decay.
• the resultant difference in color between adjacent pixels tends to be very obvious to the human eye, depending of course upon the extent of the decay in white state.
• An exactly analogous situation occurs if Pixel 1 and Pixel 2 are driven to their black optical states at different times.
• FIG. 55 Figure 2 shows the same sequence of events as Figure 1 but with the addition of a reinforcing pulse in accordance with the present invention.
• a reinforcing pulse is applied to Pixel 2 at essentially the same time as Pixel 1 is driven to its white state, so that after the reinforcing pulse, both Pixel 1 and Pixel 2 are in the same freshly rewritten white state. Thereafter, the white states of Pixels 1 and 2 decay together, so Pixels 1 and 2 maintain the same color at all later times shown in Figure 2.
• the reinforcing pulse also helps to reduce or eliminate edge ghosting effects between the edge- adjacent Pixels 1 and 2.
• Figure 6 shows traces similar to those of Figures 3 to 5 but in which the unchanging pixel was driven in accordance with a double reinforcing pulse method of the present invention, there being applied to this unchanging pixel a -10 V, 15 millisecond (white-going) reverse reinforcing pulse, immediately followed by a +10 V, 25 millisecond (black-going) forward reinforcing pulse.
• the slight flash experienced in the Figure 6 drive scheme can be minimized by careful choice of when the reinforcing pulses are applied relative to the drive pulse applied to the flashing pixel; the eye will be less sensitive to the slight flash of the unchanging pixel is this is timed to coincide with the last part of the much more noticeable white-to-black transition of the flashing pixel.
• the double reinforcing pulse method of Figure 6 not only produces more accurate matching of the final states of the flashing and unchanging pixels than the single reinforcing pulse method of Figure 3 but also reduces the amount of DC imbalance introduced by the reinforcing pulses by a factor of 4; the algebraic sum of the forward and reverse reinforcing pulses in the Figure 6 method is a 10 V, 10 millisecond pulse, as compared with the 10 V, 40 millisecond pulse used in the Figure 3 method.
• the reinforcing pulse lengths are not necessarily the same for the black and white states.
• the 15/25 millisecond combination used in Figure 6 was found to be optimal for transitions to the black extreme optical state of the medium.
• the white extreme optical state of the same medium at the same temperature it was found that a 20/25 millisecond combination gave optimal results.
• a single unchanging black pixel may have one edge-adjacent neighbor which is changing from black to white (suggesting the use of an inverse reinforcing pulse method) and another neighbor which is changing from white to black (suggesting the use of a "regular" reinforcing pulse method) and the optimum reinforcing pulse lengths may not be the same in the two cases.
• artifacts due to mismatches between the optical states of entire pixels are more objectionable than artifacts due to mismatches within the inter-pixel region. Accordingly, where the demands of the reinforcing pulse method are in conflict with those of the inverse reinforcing pulse method, in general the demands of the former should prevail.

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• 2007
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