Disaggregation driving sequences for four-particle electrophoretic displays

Abstract

To provide improved driving methods for a color electrophoretic display device in which each pixel can display at least four high-quality color states.SOLUTION: The present invention provides improved driving methods for four-particle electrophoretic displays that improve the performance of such displays when the displays are deployed in low temperature environments and when the displays are required to be updated when positioned vertically (i.e., the driving electric fields are substantially perpendicular to the direction of Earth's gravity). Methods are provided for displaying each of colors at each pixel, as desired, with minimal interference (contamination) from the other particles.SELECTED DRAWING: None

Description

Technical Field

[0001] (Reference to Related Applications) This application claims the benefit of priority of U.S. Provisional Patent Application No. 63 / 181,514, filed Apr. 29, 2021. The entire contents of all patents and published documents described below are hereby incorporated by reference in their entirety herein.

[0002] (Field of the Invention) The present invention is directed to an improved driving method for a color electrophoretic display device in which each pixel can display at least four high-quality color states.

Background Art

[0003] Electrophoretic displays (electronic paper, e-paper, etc.), such as those commercially available from E Ink Holdings (Hsinchu, Taiwan), have the advantages of being lightweight, durable, and environmentally friendly because they consume very little power. The technology is incorporated into electronic book readers (e.g., e-books, e-books) and other display environments (e.g., phones, tablets, electronic shelf tags, hospital signs, road signs, public transportation schedules). The combination of low power consumption and readability in sunlight enables rapid growth in so-called "plug-and-play" operation (where an electronic sign system is simply mounted on a surface, interfaces with an existing communication network, and provides periodic updates of information or images). Since the display is powered by a battery or a solar heat collector, there is no need for wiring and not even a need to have a plug hanging from the display.

[0004] A variety of color options for electrophoretic displays (ranging from improved color filter arrays to complex subtractive pigment sets to high-fidelity color options relying on multiple sets of reflective color particles) have recently become available. This latest system has been widely accepted for commercial signage such as grocery stores, clothing stores, and electronics retailers. In particular, three-color electrophoretic displays of the type described in U.S. Patent Application No. 2020 / 0379312 (Patent Document 1) are being rapidly adopted for outdoor and indoor signage and for room temperature and refrigerated food sections. U.S. Patent Application No. 2020 / 0379312 is hereby incorporated by reference in its entirety.

[0005] U.S. Patent Application No. 2020 / 0379312, and U.S. Patents No. 8,717,664 (Patent Document 2), No. 10,162,242, and No. 10,339,876 for three-particle electrophoretic displays have been deployed worldwide in millions of individual displays. In contrast, there is a strong demand for adding a fourth particle with a fourth color, such as those described in U.S. Patents No. 9,285,649, No. 9,513,527, and No. 9,812,073. Such four-color displays are not currently commercially available. It is desirable that such four-particle electrophoretic displays can be "thrown" into the same retail environment. However, initial tests show that four-particle electrophoretic systems of the above type have unique peculiarities different from three-particle systems depending not only on the operating temperature but also on the orientation of the display, i.e., horizontal (where charged pigments are driven up and down along the Earth's gravitational field) versus vertical (where charged pigments are driven back and forth across the Earth's gravitational field). One surprising effect observed is that when such four-particle electrophoretic displays are used in a cold environment (e.g., the refrigerated or frozen food section), the particles aggregate in an unexpected way, resulting in black pixels with intermittent contamination by other colors (e.g., white, yellow, and red). Interestingly, this phenomenon cannot be fully reproduced when the display is driven horizontally at low temperatures. Specifically, there is a need for an improved drive sequence to disaggregate the pigments prior to addressing to achieve the desired color performance and meet the customer's demand for pure and vivid colors in electronic digital signage.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

[0007] The driving method disclosed in this specification overcomes the drawbacks described above to address 4-particle electrophoresis displays at lower temperatures in a typical environment, i.e., an environment where the display panel is oriented vertically. In a first aspect, a method of driving a display layer, the display layer being disposed between a viewing surface including a light-transmissive electrode and a second surface on the opposite side of the display layer from the viewing surface, the second surface including a driving electrode, the display layer including an electrophoretic medium comprising a fluid and first, second, third, and fourth types of particles dispersed in the fluid, the first, second, third, and fourth types of particles having first, second, third, and fourth optical properties that are different from each other, the first and third types of particles having a charge of a first polarity, the second and fourth types of particles having a charge of a second polarity opposite to the first polarity, the first type of particle and the third type of particle not having the same charge magnitude, and the second type of particle and the fourth type of particle not having the same charge magnitude, the method comprising, in sequence, the following steps: (i) Applying a first electric field having a large scale and a first or second polarity to drive the first or second type of particle toward the viewing surface, thereby causing the display layer to display a first or second optical property at the viewing surface; (ii) Applying a second electric field having a large scale and a negative polarity; (iii) Applying a vibration pulse including at least four periods of a large-scale electric field in a first polarity and at least four periods of a large-scale electric field in a second polarity; (iv) Applying a second electric field having a large scale and the same polarity as in step (i) to drive the first or second type of particle toward the viewing surface again, thereby causing the display layer to display a first or second optical property at the viewing surface again; (v) Apply a third electric field having a polarity opposite to that of step (iv) on a small scale, and drive the particles of the fourth or third type toward the viewing surface, thereby causing the display layer to display the fourth or third optical property at the viewing surface; comprising.

[0008] In some embodiments, the first electric field is applied for a longer time than the second electric field, and the third electric field is applied for a longer time than the second electric field. In some embodiments, each of steps (i)-(v) is repeated. In some embodiments, the scale of the third electric field is less than 50 percent of the scale of the second electric field. In some embodiments, only the fourth or third optical property is displayed after completion of step (v). In some embodiments, the first electric field is applied for a time longer than 400 ms. In some embodiments, the second electric field is applied for a time longer than 100 ms. In some embodiments, the vibration pulse is applied for a time shorter than 80 ms. In some embodiments, the vibration pulse is applied for a time of about 40 ms. In some embodiments, a rest period without an electric field is implemented after step (iii), and steps (i)-(iii) are repeated twice before completing steps (iv) and (v). In some embodiments, each electric field is applied in a direction substantially perpendicular to the direction of the earth's gravity.

[0009] In a second aspect, the present invention is a method of driving a display layer, the display layer being disposed between a viewing surface including a light transmissive electrode and a second surface on the opposite side of the display layer from the viewing surface, the second surface including drive electrodes, the display layer including an electrophoretic medium comprising a fluid and first, second, third, and fourth types of particles dispersed in the fluid, the first, second, third, and fourth types of particles having first, second, third, and fourth optical properties that are different from each other, the first and third types of particles having a charge of a first polarity, the second and fourth types of particles having a charge of a second polarity opposite to the first polarity, the first and third types of particles not having the same charge magnitude, and the second and fourth types of particles not having the same charge magnitude, The method comprises, in order, the following steps: (i) applying a first electric field having a large scale and a first or second polarity to drive the first or second type of particles towards the viewing surface, thereby causing the display layer to display a first or second optical property at the viewing surface; (ii) applying a second electric field having a large scale and a negative polarity; (iii) applying a vibration pulse including at least four periods of the large scale electric field in the first polarity and at least four periods of the large scale electric field in the second polarity; (iv) applying a third electric field having a large scale and a polarity opposite to that of step (i) to drive the second or first type of particles towards the viewing surface, thereby causing the display layer to display a second or first optical property at the viewing surface and provides a method.

[0010] In some embodiments, the first electric field is applied for the same period of time as the third electric field. In some embodiments, each of steps (i)-(iv) is repeated. In some embodiments, only the second or the first optical property is displayed after completion of step (iv). In some embodiments, the first electric field is applied for a period longer than 400 ms. In some embodiments, the second electric field is applied for a period longer than 100 ms. In some embodiments, each period of the vibration pulse is applied for a period shorter than 80 ms. In some embodiments, each period of the vibration pulse is applied for a period of about 40 ms. In some embodiments, each electric field is applied in a direction substantially perpendicular to the direction of the earth's gravity. The present invention provides, for example, the following items. (Item 1) A method of driving a display layer, the display layer being disposed between a viewing surface including a light-transmissive electrode and a second surface on the opposite side of the display layer from the viewing surface, the second surface including a driving electrode, the display layer including an electrophoretic medium including a fluid and first, second, third, and fourth types of particles dispersed in the fluid, the first, second, third, and fourth types of particles each having different first, second, third, and fourth optical properties, the first and third types of particles having a charge of a first polarity, the second and fourth types of particles having a charge of a second polarity opposite to the first polarity, the first and third types of particles not having the same charge magnitude, and the second and fourth types of particles not having the same charge magnitude, the method comprising (vi) applying a first electric field having a large-scale first or second polarity to drive the first or second type of particles toward the viewing surface, thereby causing the first or second optical property to be displayed on the display layer at the viewing surface; and (vii) applying a second electric field having a large-scale negative polarity. (viii) Applying a vibration pulse including at least four periods of the large-scale electric field in the first polarity and at least four periods of the large-scale electric field in the second polarity; (ix) Applying a second electric field having the same polarity as in step (i) on a large scale, and driving the particles of the first or second type towards the viewing surface again, thereby causing the first or second optical property to be displayed on the display layer at the viewing surface again; (x) Applying a third electric field having a polarity opposite to that in step (iv) on a small scale, and driving the particles of the fourth or third type towards the viewing surface, thereby causing the fourth or third optical property to be displayed on the display layer at the viewing surface; A method comprising the steps in order. (Item 2) The method according to item 1, wherein the first electric field is applied for a longer time than the second electric field, and the third electric field is applied for a longer time than the second electric field. (Item 3) The method according to item 1, wherein each of steps (i)-(v) is repeated. (Item 4) The method according to item 1, wherein the scale of the third electric field is smaller than 50 percent of the scale of the second electric field. (Item 5) The method according to item 1, wherein only the fourth or third optical property is displayed after the completion of step (v). (Item 6) The method according to item 1, wherein the first electric field is applied for a time longer than 400 ms. (Item 7) The method according to item 1, wherein the second electric field is applied for a time longer than 100 ms. (Item 8) The method according to item 1, wherein each period of the vibration pulse is applied for a time shorter than 80 ms. (Item 9) The method according to item 8, wherein each period of the vibration pulse is applied for a time of about 40 ms. (Item 10) The method according to item 1, wherein a rest period without an electric field is carried out after step (iii), and steps (i)-(iii) are repeated twice before steps (iv) and (v) are completed. (Item 11) The method according to item 1, wherein each electric field is applied in a direction substantially perpendicular to the direction of the earth's gravity. (Item 12) A method of driving a display layer, wherein the display layer is disposed between a viewing surface including a light-transmissive electrode and a second surface on the opposite side of the display layer from the viewing surface, the second surface including a driving electrode, and the display layer includes an electrophoretic medium including a fluid and first, second, third, and fourth types of particles dispersed in the fluid. The first, second, third, and fourth types of particles each have different first, second, third, and fourth optical properties, the first and third types of particles have a charge of a first polarity, the second and fourth types of particles have a charge of a second polarity opposite to the first polarity, the first and third types of particles do not have the same charge magnitude, and the second and fourth types of particles do not have the same charge magnitude. The method comprises: (v) applying a first electric field having a large-scale first or second polarity to drive the first or second type of particles toward the viewing surface, thereby causing the display layer to display the first or second optical property at the viewing surface; (vi) applying a second electric field having a large-scale negative polarity; (vii) applying a vibration pulse including at least four periods of the large-scale electric field in the first polarity and at least four periods of the large-scale electric field in the second polarity; (viii) Applying a third electric field having a polarity opposite to that of step (i) on a large scale to drive the particles of the second or first type toward the viewing surface, thereby causing the second or first optical property to be displayed on the display layer at the viewing surface; A method comprising the steps in sequence. (Item 13) The method according to item 12, wherein the first electric field is applied for the same period of time as the third electric field. (Item 14) The method according to item 12, wherein each of steps (i)-(iv) is repeated. (Item 15) The method according to item 12, wherein only the second or the first optical property is displayed after the completion of step (iv). (Item 16) The method according to item 12, wherein the first electric field is applied for a period longer than 400 ms. (Item 17) The method according to item 12, wherein the second electric field is applied for a period longer than 100 ms. (Item 18) The method according to item 12, wherein each period of the vibration pulse is applied for a period shorter than 80 ms. (Item 19) The method according to item 18, wherein each period of the vibration pulse is applied for a period of about 40 ms. (Item 20) The method according to item 12, wherein each electric field is applied in a direction substantially perpendicular to the direction of the earth's gravity.

Brief Description of the Drawings

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DETAILED DESCRIPTION OF THE INVENTION

[0024] As already described, the present invention relates to a driving method for a display layer comprising an electrophoretic medium containing first, second, third, and fourth types of particles, all dispersed in a fluid and all having different optical properties. These optical properties are typically colors perceptible to the human eye, but can also be other optical properties (such as light transmittance, reflectance, luminescence, or in the case of a display intended for machine reading, pseudo-colors in the sense of changes in reflectance of electromagnetic wavelengths outside the visible range, etc.). The present invention broadly encompasses particles of any color as long as the multiple types of particles are visually distinguishable.

[0025] The four types of particles present in the electrophoretic medium can be regarded as comprising two pairs of oppositely charged particles. The first pair (the first and second types of particles) consists of positively charged particles of the first type and negatively charged particles of the first type. Similarly, the second pair (the third and fourth types of particles) consists of positively charged particles of the second type and negatively charged particles of the second type. Of the two pairs of oppositely charged particles, one pair (the first and second particles) is more strongly charged than the other pair (the third and fourth particles). Thus, the four types of particles can also be referred to as highly positively charged particles, highly negatively charged particles, lowly positively charged particles, and lowly negatively charged particles.

[0026] In the context of this application, the term "charge potential" can be used synonymously with "zeta potential" or electrophoretic mobility. The charge polarity and the level of the charge potential of the particles can be varied (and / or measured in terms of the zeta potential) by the method described in U.S. Patent Application Publication No. 2014 / 0011913. In one embodiment, the zeta potential is determined by a Colloidal Dynamics AcoustoSizer IIM with a CSPU-100 signal processing unit, an ESA EN#Attn flow-through cell (K:127). Instrument constants such as the density of the solvent used in the sample, the dielectric constant of the solvent, the speed of sound in the solvent, the viscosity of the solvent, etc. (all of which are at the test temperature (25 °C)) are input before the test. The pigment sample is dispersed in a solvent (which is usually a hydrocarbon fluid having less than 12 carbon atoms) and diluted to a weight ratio of 5-10%. The sample also contains a charge control agent (Solsperse TM 17000 available from Lubrizol Corporation, a subsidiary of Berkshire Hathaway company) with a weight ratio of charge control agent to particles of 1:10. The mass of the diluted sample is determined and the sample is then filled into the flow-through cell for the determination of the zeta potential. Methods and apparatuses for the measurement of electrophoretic mobility are well known to those skilled in the art of electrophoretic display technology.

[0027] As an example shown in FIG. 1, the first black particle (K) and the second yellow particle (Y) are a pair of oppositely charged particles, in which the black particle is a highly positive particle and the yellow particle is a highly negative particle. The third red particle (R) and the fourth white particle (W) are a pair of oppositely charged particles, in which the red particle is a low positive particle and the white particle is a low negative particle.

[0028] In another example not shown, the black particles can be high positive particles, the yellow particles can be low positive particles, the white particles can be low negative particles, and the red particles can be high negative particles. In another example not shown, the black particles can be high positive particles, the yellow particles can be low positive particles, the white particles can be high negative particles, and the red particles can be low negative particles. In another example not shown, the black particles can be high positive particles, the red particles can be low positive particles, the white particles can be high negative particles, and the yellow particles can be high negative particles. Of course, any particular color can be replaced with another color according to the requirements of the application. For example, if a specific combination of black, white, green, and red particles is desired, the high negative yellow particles shown in FIG. 1 can be replaced with high negative green particles.

[0029] In addition, the color states of the four types of particles can be intentionally mixed. For example, yellow pigments essentially often have a greenish tint, and if a better yellow state is desired, yellow particles and red particles can be used, in which case both types of particles have the same charge polarity and the yellow particles are more highly charged than the red particles. As a result, in the yellow state, a small amount of red particles will be mixed with the greenish yellow particles, resulting in a better color purity in the yellow state.

[0030] The particles are preferably opaque in the sense that they should be light - reflective rather than light - transmissive. It will be apparent to those skilled in the art of color science that if the particles are light - transmissive, some of the color states that appear in the following description of specific embodiments of the present invention will be significantly distorted or not obtained. White particles, of course, are not reflective but light - scattering, but care should be taken to ensure that an excessive amount of light does not pass through the layer of white particles. For example, in the white state shown in FIG. 2F, discussed below, the layer of white particles allows a significant amount of light to pass through and be reflected from the black and yellow particles behind it, and the luminance of the white state can be significantly reduced.

[0031] In some embodiments, the particles are primary particles without a polymer shell. Alternatively, each particle may comprise an insoluble core with a polymer shell. The core can be either an organic or inorganic pigment, which can be a single core particle or an aggregate of multiple core particles. The particles can also be hollow particles.

[0032] White particles can be formed from inorganic pigments such as TiO 2 、ZrO 2 、ZnO、Al 2 O 3 、Sb 2 O 3 、BaSO 4 、PbSO 4 and so on. Black particles can be formed from pigments such as Pigment Black 26 or 28 (e.g., manganese ferrite black spinel or copper chromite black spinel), or carbon black. Other colored particles (which are non-white and non-black) can be red, green, blue, magenta, cyan, yellow, or any other desired color, and can be formed, for example, from CI Pigment PR254, PR122, PR149, PG36, PG58, PG7, PB28, PB15:3, PY83, PY138, PY150, PY155, or PY20. These are commonly used organic pigments described in the Color Index Handbook "New Pigment Application Technology" (CMC Publishing Co, Ltd, 1986) and "Printing Ink Technology" (CMC Publishing Co, Ltd, 1984). Specific examples are Clariant Hostaperm Red D3G 70-EDS, Hostaperm Pink E-EDS, PV It contains fast red D3G, Hostaperm red D3G 70, Hostaperm Blue B2G-EDS, Hostaperm Yellow H4G-EDS, Novoperm Yellow HR-70-EDS, Hostaperm Green GNX, BASF Irgazine red L 3630, Cinquasia Red L 4100 HD, and Irgazin Red L 3660 HD, Sun Chemical phthalocyanine blue, phthalocyanine green, diarylide yellow, or diarylide AAOT yellow. The coloring particles can also be inorganic pigments such as red, green, blue, and yellow. Examples can include, but are not limited to, CI pigment blue 28, CI pigment green 50, and CI pigment yellow 227.

[0033] A fluid in which four types of particles are dispersed can be clear and colorless. It preferably has a low viscosity and a dielectric constant in the range of about 2 to about 30, preferably about 2 to about 15, due to high particle mobility. Examples of suitable dielectric solvents are hydrocarbons such as isoparaffin, decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oil, silicone solutions, aromatic hydrocarbons such as toluene, xylene, phenylxylylethane, dodecylbenzene or alkylnaphthalene, halogenated solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride, chloropentafluorobenzene, dichlorononane or pentachlorobenzene, and perfluorinated solvents such as FC-43, FC-70, or FC-5060 manufactured by 3M Company (St. Paul MN), low molecular weight halogen-containing polymers such as poly(perfluoropropylene oxide) manufactured by TCI America (Portland, Oregon), poly(chlorotrifluoroethylene) such as halocarbon oil manufactured by Halocarbon Product Corp. (River Edge, NJ), perfluoropolyalkyl ethers such as Galden manufactured by Ausimont or Krytox Oils, DuPont (Delaware) Greases K-Fluid Series, Dow-corning polydimethylsiloxane-based silicone oil (DC-200).

[0034] The ratio of the different types of particles in the fluid can vary. For example, one type of particle can occupy a volume ratio of 0.1% to 10%, preferably 0.5% to 5% of the electrophoretic fluid, another type of particle can occupy a volume ratio of 1% to 50%, preferably 5% to 20% of the fluid, and each of the remaining types of particles can occupy a volume ratio of 2% to 20%, preferably 4% to 10% of the fluid.

[0035] Particles of various types can have different particle sizes. For example, smaller particles can have sizes ranging from about 50 nm to about 800 nm. Larger particles can have sizes that are about 2 to about 50 times, more preferably about 2 to about 10 times, the size of the smaller particles.

[0036] An electrophoretic display typically includes a layer of electrophoretic material and at least two other layers disposed on opposite sides of the electrophoretic material, one of these two layers being an electrode layer. In most such displays, both layers are electrode layers, and one or both of the electrode layers are patterned to define the pixels of the display. For example, one electrode layer can be patterned into elongated row electrodes, and the other can be patterned into elongated column electrodes extending perpendicular to the row electrodes, and the pixels can be defined by the intersections of the row and column electrodes. Alternatively, and more generally, one electrode layer has the form of a single continuous electrode, and the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines one pixel of the display. In another type of electrophoretic display intended for use with a stylus, print head, or similar movable electrode separate from the display, only one of the layers adjacent to the electrophoretic layer includes electrodes, and the layer on the opposite side of the electrophoretic layer is typically a protective layer intended to prevent damage to the electrophoretic layer by the movable electrode.

[0037] Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT), E Ink Corporation, E Ink (California), LLC, E Ink Holdings, Prime View International, and related companies describe various technologies used in encapsulated electrophoretic media, microcell electrophoretic media, and other electro-optic media. Encapsulated electrophoretic media comprises a number of small capsules, each of which itself consists of an internal phase containing particles movable by electrophoresis in a fluid medium and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. In a microcell electrophoretic display, charged particles and fluid are not encapsulated within microcapsules, but instead are held within a plurality of cavities formed within a carrier medium (typically a polymeric film). The technologies described in these patents and applications include: (a) Electrophoretic particles, fluids, and fluid additives (see, e.g., U.S. Pat. Nos. 7,002,728 and 7,679,814); (b) Capsules, binders, and encapsulation processes (see, e.g., U.S. Pat. Nos. 6,922,276 and 7,411,719); (c) Microcell structures, wall materials, and methods of forming microcells (see, e.g., U.S. Pat. Nos. 7,072,095 and 9,279,906); (d) Methods for filling and sealing microcells (see, e.g., U.S. Pat. Nos. 7,144,942 and 7,715,088); (e) Films and subassemblies containing electro-optic materials (see, e.g., U.S. Pat. Nos. 6,982,178 and 7,839,564); (f) Backplanes, adhesive layers and other auxiliary layers, and methods of use within a display (see, e.g., U.S. Pat. Nos. 7,116,318 and 7,535,624) (g) Color formation and color adjustment (see, e.g., U.S. Pat. Nos. 7,075,502 and 7,839,564); (h) Methods for driving a display (see, e.g., U.S. Pat. Nos. 7,012,600 and 7,453,445); (i) Applications of displays (see, e.g., U.S. Pat. Nos. 7,312,784 and 8,009,348); (j) Non - electrophoretic displays as described in U.S. Pat. No. 6,241,921 and U.S. Patent Application Publication No. 2015 / 0277160, and applications of encapsulation and microcell technologies other than displays (see, e.g., U.S. Patent Application Publication Nos. 2015 / 0005720 and 2016 / / 0012710);

[0038] Many of the aforementioned patents and applications recognize that the walls surrounding separate microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thus producing a so - called "polymer - dispersed electrophoretic display" in which the electrophoretic medium comprises a plurality of separate droplets of an electrophoretic fluid and a continuous phase of a polymer material, and the separate droplets of the electrophoretic fluid within such a polymer - dispersed electrophoretic display can be regarded as capsules or microcapsules even when no separate capsule membranes are associated with each individual droplet (see, e.g., the aforementioned 2002 / 0131147). Thus, for the purposes of this application, such a polymer - dispersed electrophoretic medium is regarded as a subspecies of the encapsulated electrophoretic medium.

[0039] A related type of electrophoretic display is the so - called "microcell electrophoretic display". In a microcell electrophoretic display, charged particles and a suspending fluid are not encapsulated within microcapsules but are instead held within a plurality of cavities formed within a carrier medium, typically a polymer film (see, e.g., International Application Publication No. WO02 / 01281 and U.S. Pat. No. 6,788,449).

[0040] Preferred embodiments of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings.

[0041] Figure 1 is a schematic cross-section through a display layer that can be driven by the method of the present invention. The display layer has two major surfaces, namely, a first viewing surface 13 (the upper surface as shown in Figure 1) through which the user views the display, and a second surface 14 on the opposite side of the display layer from the first surface 13. The display layer comprises an electrophoretic medium comprising a fluid and first black particles (K) having a high positive charge, second yellow particles (Y) having a high negative charge, third red particles (R) having a low positive charge, and fourth white particles (W) having a low negative charge. The display layer comprises electrodes as are known in the art for applying an electric field across the display layer, i.e., it includes two electrode layers, of which the first electrode is a light-transmissive or transparent common electrode layer 11 that extends across the viewing surface 13 of the display layer. This electrode layer 11 can be formed from indium tin oxide (ITO) or a similar light-transmissive conductor. The other electrode layer 12 is a layer of separate pixel electrodes 12a on the second surface 14, these electrodes 12a defining the individual pixels of the display, which pixels are indicated by the dotted vertical lines in Figure 1. Alternatively, the other electrode layer 12 can be a solid electrode, for example, a metal foil or a graphite plane, or a conductive polymer. Alternatively, the electrode layer 12 can also be a light-transmissive or transparent electrode layer, similar to the transparent common electrode layer 11. The electric field is generated for a pixel by the potential difference between the voltage applied to the common electrode and the voltage applied to the corresponding pixel electrode. The pixel electrodes 12a can form part of an active matrix drive system, for example, with a thin film transistor (TFT) backplane, but other types of electrode addressing can also be used, provided that the electrodes provide the electric field required across the display layer.

[0042] The pixel electrodes can be described in U.S. Patent No. 7,046,228. The pixel electrode 12a can form part of an active matrix thin film transistor (TFT) backplane, although other types of electrode addressing can also be used, provided that the electrodes provide the electric field required across the display layer.

[0043] In one embodiment, the charge carried by the "low charge" particles can be less than about 50% of the charge carried by the "high charge" particles, preferably about 5% to about 30%. In another embodiment, the "low charge" particles can be less than about 75% of the charge carried by the "high charge" particles, or about 15% to about 55%. In a further embodiment, the comparison of charge levels as shown is applied to two types of particles having the same charge polarity. The charge on the "high positive" particles can be the same as or different from the charge on the "high negative" particles. Similarly, the amplitude of the "low positive" particles and the amplitude of the "low negative" particles can be the same or different. In any specific electrophoretic fluid, the two pairs of high - low charge particles can have different levels of charge difference. For example, in one pair, the low positively charged particles can have a charge intensity that is 30% of the charge intensity of the high positively charged particles, and in another pair, the low negatively charged particles can have a charge intensity that is 50% of the charge intensity of the high negatively charged particles.

[0044] Figures 2A - 2F illustrate four color states that can be displayed on the viewing surface of each pixel of the display layer shown in FIG. 1, and the transitions between them. As previously mentioned, the high positive particles are black (K), the high negative particles are yellow (Y), the low positive particles are red (R), and the low negative particles are white (W).

[0045] In FIGS. 2A and 2B, the high negative drive voltage (V H2and the following (e.g., -15V, e.g., -30V) is applied to the pixel electrode 22a (hereinafter, the common electrode 21 is maintained at 0V, and thus, in this case, it will be assumed that the common electrode is strongly positive with respect to the pixel electrode) for a sufficient period of time, an electric field is generated, causing highly negatively charged yellow particles to be driven adjacent to the common electrode 21 and highly positively charged black particles to be driven adjacent to the pixel electrode 22a, resulting in the state of FIG. 2A.

[0046] Low positively charged red and low negatively charged white particles move more slowly than the more highly charged black and yellow particles because they are less charged. As a result, they remain in the center of the pixel, with the white particles above the red particles, and both are masked by the yellow particles and thus not visible on the viewing surface. Accordingly, yellow is displayed on the viewing surface.

[0047] Conversely, when a high positive driving voltage (V H1 and the following (e.g., +15V, e.g., +30V) is applied to the pixel electrode 22a (such that the common electrode 21 is strongly negative with respect to the pixel electrode) for a sufficient period of time, an electric field is generated, causing highly positively charged black particles to be driven adjacent to the common electrode 21 and highly negatively charged yellow particles to be driven adjacent to the pixel electrode 22a. The resulting state in FIG. 2B is the exact opposite of FIG. 2A, and black is displayed on the viewing surface.

[0048] FIGS. 2C and 2D illustrate the manner in which low positive (red) particles are displayed on the viewing surface of the display layer shown in FIG. 1. This process starts from the (yellow) state shown in FIG. 2A and is repeated as FIG. 2C. A low positive voltage (V L1, for example, +3V, for example, +5V, for example, +10V) is applied to the pixel electrode 22a for a sufficient period of time (i.e., the common electrode 21 is made slightly negative with respect to the pixel electrode), causing the high-negative yellow particles to move towards the pixel electrode 22a while causing the high-positive black particles to move towards the common electrode 21. However, as shown in FIG. 2D, when the yellow and black particles meet in the middle between the pixel and the common electrode, they remain in the middle position because the electric field generated by the low driving voltage is not strong enough to cancel the attractive force between them. As shown, the yellow and black particles remain in the middle between the pixel electrode and the common electrode in the mixed state.

[0049] The term "attractive force" as used herein encompasses electrostatic interactions and depends linearly on the particle charge potential, and the attractive force can be further enhanced by other forces such as van der Waals forces, hydrophobic interactions, etc.

[0050] Obviously, attractive forces also exist between low-positive red particles and high-negative yellow particles, and between low-negative white particles and high-positive black particles. However, these attractive forces are not as strong as the attractive force between black particles and yellow particles. Therefore, the weak attractive forces on red and white particles can be canceled by the electric field generated by the low driving voltage, whereby low-charged particles and high-charged particles of opposite polarity can be separated. The electric field generated by the low driving voltage is also sufficient to separate the low-negative white particles and the low-positive red particles, thereby moving the red particles to be adjacent to the common electrode 21 and moving the white particles to be adjacent to the pixel electrode 22a. As a result, as shown in FIG. 2D, the pixel displays red while the white particles are present in the vicinity of the pixel electrode.

[0051] FIGS. 2E and 2F illustrate the manner in which low-negative (white) particles are displayed on the viewing surface of the display layer shown in FIG. 1. This process starts from the (black) state of FIG. 2B and is repeated as FIG. 2E. The low negative voltage (V L2, for example, -3V, for example, -5V, for example, -10V) is applied to the pixel electrode for a sufficient period of time (i.e., the common electrode is made slightly positive with respect to the pixel electrode), causing the high positive black particles to move towards the pixel electrode 22a while causing the high negative yellow particles to move towards the common electrode 21. However, when the yellow and black particles meet in the middle between the pixel and the common electrode as shown in FIG. 2F, they remain in the middle position because the electric field generated by the low driving voltage is not strong enough to cancel the gravitational force between them. Therefore, as discussed earlier with reference to FIG. 2D, the yellow and black particles remain in the middle between the pixel and the common electrode in the mixed state.

[0052] As discussed above with reference to FIGS. 2C and 2D, gravitational forces also exist between the low positive red particles and the high negative yellow particles, and between the low negative white particles and the high positive black particles. However, these gravitational forces are not as strong as the gravitational force between the black particles and the yellow particles. Therefore, the weak gravitational forces on the red and white particles can be canceled by the electric field generated by the low driving voltage, whereby the low-charged particles and the high-charged particles of the opposite polarity can be separated. The electric field generated by the low driving voltage is sufficient to separate the low negative white particles and the low positive red particles, thereby moving the white particles to be adjacent to the common electrode 21 and moving the red particles to be adjacent to the pixel electrode 22a. As a result, as shown in FIG. 2F, the pixel displays white while the red particles are present in the vicinity of the pixel electrode.

[0053] In the display layer shown in FIGS. 1 and 2A - 2F, the black particles (K) are positively charged, the yellow particles (Y) are negatively charged, the red particles (R) are positively charged, and the white particles (W) are negatively charged. However, in principle, the particles with high positive charge, or high negative charge, or low positive charge, or low negative charge can be of any color. All these variations are intended to be within the scope of the present application.

[0054] Note also that the low potential difference applied to reach the color states of FIGS. 2D and 2F can be about 5% to about 50% of the high potential difference required to drive the pixel from the color state of the high positive particles to the color state of the high negative particles, or vice versa (i.e., as shown in FIGS. 2A and 2B).

[0055] For ease of illustration, FIGS. 1 and 2A-2F show the display layer as being non-encapsulated, but the electrophoretic fluid can be filled within the display cell, which can be a cup-like microcell as described in U.S. Patent No. 6,930,818. The display cell can also be other types of microcontainers such as microcapsules, microchannels, or equivalents, regardless of their shape or size. All of these are within the scope of the present application.

[0056] It will be readily apparent to those skilled in the art of imaging science that all non-black and non-white particles used within the electrophoretic medium should be light-reflective rather than light-transmissive when "pure" fully saturated colors, as illustrated in FIGS. 2A-2F, are obtained in the various color states (white particles are essentially light-scattering while black particles are essentially light-absorbing). For example, in the red state of FIG. 2D, if the red particles are substantially light-transmissive, a significant proportion of the light entering the electrophoretic layer through the viewing surface will pass through the red particles, and a certain proportion of this transmitted light will be reflected back from the yellow particles "behind" (i.e., below as shown in FIG. 2D) the red particles. The overall effect would be a severe "contamination" of the desired red with a yellow hue, i.e., a highly undesirable result.

[0057] To ensure both the color luminance and the color purity, the vibration waveform can be applied prior to driving the display layer from one color state to another. FIG. 3 is a voltage-versus-time graph of such a vibration waveform. The vibration waveform can consist of repeating pairs of opposing drive pulses over many cycles. When used with an active matrix display, each positive or negative pulse is at least the frame width of an update. For example, each pulse width can be about 16 milliseconds when the display is updated at 60 Hz. However, in practice, the frame time is typically slightly longer due to various charges and decay times for the capacitive elements of the backplane. For example, as shown in FIG. 3, the vibration waveform consists of a +15V pulse over 20 milliseconds and a -15V pulse over 20 milliseconds, and this pair of pulses can be repeated 50 times. The total duration of such a vibration waveform would be 2,000 milliseconds. For ease of illustration, FIG. 3 shows only seven pairs of pulses.

[0058] The pulse width need not be limited to the frame time, and each pulse may include a plurality of frames, for example, a pulse width of 40 milliseconds, for example, a pulse width of 60 milliseconds, for example, a pulse width of 80 milliseconds, for example, a pulse width of 100 milliseconds. In some embodiments, the pulse width of each element of the vibration pulse may be 80 milliseconds or less, for example, 60 milliseconds or less, for example, 40 milliseconds or less, for example, 20 milliseconds or less. In practice, there may be at least 4 repetitions (i.e., 4 pairs of positive and negative pulses), for example, at least 6 repetitions, for example, at least 8 repetitions, for example, at least 10 repetitions, for example, at least 12 repetitions, for example, at least 15 repetitions. Similarly, all subsequent drawings showing the vibration waveform simplify the vibration waveform in the same manner. The vibration waveform can be applied regardless of the optical state prior to the application of the driving voltage. After the vibration waveform is applied, the optical state will be a mixture of the colors of various types of pigment particles rather than a pure color (either on the viewing surface or, if visible, on the second surface). In certain cases, a plurality of vibration pulses are delivered with a 0V pause between the vibration pulses, which will allow the electrophoretic medium to equilibrate and / or the charge accumulated on the electrodes to dissipate.

[0059] Each of the drive pulses in the vibration waveform is applied for a time that does not exceed 50% (or does not exceed 30%, 10%, or 5%) of the drive time required to drive from the color state of the high positive particles to the color state of the high negative particles, or vice versa. For example, if it takes 300 milliseconds to drive the display device from the color state of Figure 2B to the high positive particles, i.e., to the color state of Figure 2A, or vice versa, the vibration waveform consists of positive and negative pulses, each of which can be applied for a time not exceeding 150 milliseconds. In practice, it is preferred that the pulses be rather short.

[0060] For the purposes of this object, the high driving voltage (V H1 or V H2 ) is defined as the driving voltage sufficient to drive the pixel from the color state of the high positive particles to the color state of the high negative particles, or vice versa (see Figures 2A and 2B). The low driving voltage (VL1 or V L2 ) is defined as a driving voltage that may be sufficient to drive the pixel from the color state of the high-charge particle to the color state of the low-charge particle (see FIGS. 2D and 2F). Generally, V L 's magnitude (e.g., V L1 or V L2 ) is less than 50%, preferably less than 40%, of the amplitude of V H (e.g., V H1 or V H2 ).

[0061] As described in the background art, the orientation of the electrophoretic medium with respect to gravity affects the purity of the resulting color state, especially when the display is operated at a lower temperature, e.g., less than 5°C, e.g., less than 0°C, e.g., less than -5°C, e.g., less than -10°C, e.g., less than -15°C. As shown in FIG. 4A, horizontal driving occurs when the electric field gradient provided by the electrodes (11 and 12a) is along the direction of gravity (G). In contrast, vertical driving occurs when the electric field gradient provided by the electrodes (11 and 12a) is transverse to the direction of gravity (G).

[0062] Experimental measurements of the black state using the CIELAB color space (e.g., L*, a*, b*) show that, as explained, for example, in FIGS. 2A and 2B, driven black pixels consistently have a higher L* for those driven in the vertical orientation compared to the same display driven in the horizontal orientation (for the black state, a lower L* is better, i.e., the reflectance is less). Additionally, using a magnifying glass or similar magnification, extra spots of white, yellow, and red pigments that contaminate the black state may be visible to the viewer. Using a predetermined test pattern, the L* value for black is typically about 3 L* higher for a vertically driven 4-particle panel driven at 0° C. compared to a horizontally driven 4-particle panel driven at 0° C. This is not particularly noticeable, but it has been observed that all color states have increased contamination, especially at low temperatures, when driven in the vertical orientation. The cause of this color contamination is not fully understood, but it can result from the differential separation of various components in the electrophoretic medium, including pigments, charge control agents, and other additives.

[0063] FIG. 5A illustrates a standard waveform that can be used to effect the transition from yellow to red (from high negative to low positive) of FIGS. 2C and 2D. In the waveform of FIG. 5A, a high negative drive voltage (V H2 , e.g., -15 V) is applied for a period of t1 to drive the pixel towards the yellow state (see FIG. 2C). This initial application of the high negative drive voltage is known as an equilibrium phase included to ensure that the entire waveform of FIG. 5A is DC balanced (the term "DC balanced" is used herein to mean that the integral of the drive voltage applied to the pixel over the time required for the entire waveform is substantially zero). The equilibrium pulse of t1 can last for 500 milliseconds or more, e.g., longer than 1 second. An oscillatory waveform (alternatively, a mixed waveform) is then applied, with the high negative drive voltage (V H2) is applied continuously over a period of t2, which places the pixel in the yellow state shown in Figure 2C. The width of period t2 is typically smaller than t1, for example, half the length, for example, about 200 milliseconds, or about 250 milliseconds, or about 500 milliseconds. In some embodiments of Figure 5A, each pulse of the vibration pulse can be about 80 ms wide, however, longer or shorter pulse widths are acceptable. From this yellow state, the pixel is driven to the red state by applying a low positive drive voltage (V L1 , for example, +3V), resulting in a transition from yellow to red (shown as progressing from Figure 2C to Figure 2D). Period t2 is sufficient to drive the pixel to the yellow state when V H2 is applied, and period t3 is sufficient to drive the pixel from the yellow state to the red state when V L1 is applied. Period t3 is typically longer than t2, for example, about 300 milliseconds, for example, about 400 milliseconds, for example, about 600 milliseconds. It should be understood that the waveform of Figure 5A is the "base" waveform for the preparation of red on the viewing surface. A portion of the waveform can be repeated, for example, the balance pulse and the vibration pulse can be repeated before the first drive pulse is applied. In some embodiments, there can be a 0V pause between repeated portions of the waveform (i.e., balance, vibration, pause, balance, vibration). Additionally, a contamination removal pulse can be added to the waveform as described in U.S. Patent No. 10,586,499, which is incorporated by reference in its entirety.

[0064] However, as discussed above, the waveform of FIG. 5A does not provide sufficient initial separation of the aggregated pigments to achieve a pure optical state, particularly when driven at low temperatures (e.g., 0° C.) and in the vertical orientation. That is, after driving with the waveform of FIG. 5A, black, yellow, and white pigment contamination can be seen within the red pixels. Surprisingly, it has been found that this contamination can be canceled by adding a simple high negative aggregation release pulse at time t1' as shown in FIG. 5B. This extra high negative time appears to be an extension of the equilibrium pulse t1, but the aggregation release pulse t1' has been found to be effective in the preparation of all color states for the described four-particle electrophoresis display system of the type described above with respect to FIGS. 1 and 2A-2F. The period t1' is typically from 100 milliseconds to 700 milliseconds, such as about 400 milliseconds, or about 500 milliseconds, or from 400 to 500 milliseconds.

[0065] Although the inventors do not wish to be bound by the proposed mechanism below, it is speculated that the positively charged black and red particles are spreading out their aggregates after sustained driving (particularly at lower temperatures). Particle aggregates can be promoted by charge control agents in the electrophoretic medium, however, the effect does not appear to be sensitive to a particular type of charge control agent. When a negative aggregation release pulse, i.e., t1' of FIG. 5B, is added, the red and black particles are driven closer to the drive electrode (22a), which results in a higher dispersive force (i.e., a sharper "kick") on the positive particle aggregates when the vibration pulse starts. Thus, the positive particles are better separated and respond better to subsequent drive (i.e., address) pulses. When the aggregation release pulse is added, there is less color mixing and the resulting colors are more consistent when evaluated using electro-optical metrology (see example).

[0066] In a similar manner, FIGS. 6A and 6B illustrate waveforms that can be used to effect the transition from black to white (from high positive to low negative) as shown in FIGS. 2E through 2F. The waveform of FIG. 6A is a standard waveform, while the waveform of FIG. 6B is modified to include a disaggregation pulse t4’, which reduces contamination in the resulting white state. In the waveform of FIG. 6A, which is essentially an inverted version of the waveform of FIG. 5A, a high positive drive voltage (V H1 , e.g., +15V) is applied as a balancing pulse over the period t4. The oscillating waveform is then applied, and the application of the high positive drive voltage (V H1 ) continues over the period t5, thus ensuring that the pixel is in the black state as shown in FIG. 2E. From this black state, the pixel is driven to the white state by applying a low negative drive voltage (V L2 , e.g., -3V) over the period t6, effecting the transition from black to white as shown in FIGS. 2E through 2F. Period t5 is sufficient to drive the pixel to the black state when V H1 is applied, and period t6 is sufficient to drive the pixel from the black state to the white state when V L2 is applied. The disaggregation pulse t4’ shown in FIG. 6B improves the purity of the final white state, particularly when the display is driven in the vertical orientation at low temperatures. Period t4’ is typically from 100 milliseconds to 700 milliseconds, e.g., about 400 milliseconds, or about 500 milliseconds, or from 400 milliseconds to 500 milliseconds.

[0067] FIG. 7A illustrates a standard waveform that can be used to effect the transition from yellow to black (from high negative to high positive) as shown in FIGS. 2A through 2B. A balancing pulse having a width t7 and a high negative voltage is delivered prior to the oscillating waveform. The balancing pulse achieves DC balance for the overall waveform, and the oscillating pulse is included to ensure color luminance and purity. Following the balancing and oscillating pulses, as shown in FIG. 7A, a high positive drive voltage (V H1 , e.g., +15V, +30V) is applied during the period t8 to drive the pixel towards the black state after the oscillating waveform.

[0068] As detailed above and as explained in the following examples, the waveform of FIG. 7A does not achieve the desired black purity, especially with respect to low-temperature driving, when the display is in the vertical orientation. Thus, in a manner similar to the waveforms of FIGS. 5B and 6B, the addition of a high negative pulse over an intermediate time t7' has been found to achieve particle flocculation release and result in improved electro-optical performance of the black state. Similar to FIGS. 5B and 6B, the period t7' is typically from 100 milliseconds to 700 milliseconds, for example, about 400 milliseconds, or about 500 milliseconds, or from 400 milliseconds to 500 milliseconds.

[0069] FIG. 8A illustrates a standard waveform that can be used to effect a transition from black to yellow (from high positive to high negative) from FIG. 2B to 2A. An equilibrium pulse having a width t9 and a high negative voltage is delivered prior to the oscillatory waveform. The equilibrium pulse achieves DC equilibrium for the overall waveform, and the oscillatory pulse is included to ensure color luminance and purity. Following the equilibrium and oscillatory pulses, as shown in FIG. 8A, a high negative drive voltage (V H1 , for example, +15V, +30V) is applied for a period t10 to drive the pixel towards the yellow state after the oscillatory waveform.

[0070] As detailed above, the waveform of FIG. 8A does not achieve the desired yellow purity, especially with respect to low-temperature driving, when the display is in the vertical orientation. Thus, in a manner similar to the waveform of FIG. 7B, the addition of a high negative pulse over an intermediate time t9' has been found to achieve particle flocculation release and result in improved electro-optical performance of the black state. Similar to FIG. 7B, the period t9' is typically from 100 milliseconds to 700 milliseconds, for example, about 400 milliseconds, or about 500 milliseconds, or from 400 milliseconds to 500 milliseconds.

[0071] The waveforms described heretofore are intended to display one of the four optical states shown in FIGS. 2A-2F, and essentially, one of the four types of particles present in the display layer. It will be understood from the foregoing that while the embodiments of the present invention described above enable the display of any one of the four colors at each pixel, they do not provide an easy way to reproducibly control the gray level or saturation of each color. Therefore, if it is desired to use the present invention to provide a grayscale color image, it will be necessary to dither (modulate in area) the pixels of the display in order to provide the required grayscale. For example, a non-saturated red (pink) color can be displayed by setting alternating pixels of the display to red and white. Area modulation effectively trades an increased number of gray levels for a reduction in display resolution (since individual pixels are used as sub-pixels of a large pixel that can effectively display gray levels), and the loss in resolution can be limited by increasing the number of reproducible color states (primary colors) that can be displayed at each pixel. In the method of the present invention, it has been found that the number of primary colors available from each pixel can be increased by driving each pixel with a color presented by a mixture of low-positive (red) particles and high-negative (yellow) particles (orange in the embodiment shown in the drawings), and / or a color presented by a mixture of low-negative (white) particles and high-positive (black) particles (gray).

[0072] It has been found that reproducible mixed colors can be obtained only by first driving the display to the color of the low-charged particles required in the mixed color and then applying a high driving voltage of the opposite polarity to mix the low-charged particles with the appropriate high-charged particles to form the desired mixed color. More specifically, it is necessary to start from the red state to provide reproducible orange. To transition from this red state 2 to the orange state (i.e., the mixed red and yellow), a high negative driving voltage (V H2, for example, -15V) is applied to the pixel electrode (22a) for a short period of time (i.e., the common electrode is made strongly positive with respect to the pixel electrode). The high driving voltage is sufficient to cancel the interaction between the black particles and the yellow particles that were previously aggregated between the pixel electrode and the front electrode, whereby the negatively charged yellow particles start to move rapidly towards the front electrode (21), while the positively charged black particles start to move towards the pixel electrode (22a). At the same time, the positively charged red particles start to move towards the pixel electrode (22a) away from the front electrode (21), while the negatively charged white particles start to move towards the front electrode (21) away from the pixel electrode (22a). However, since the electrophoretic mobilities of the low-charged red and white particles are smaller than those of the high-charged black and yellow particles, the red and white particles move at a lower speed than the black and yellow particles. The length of the driving pulse is adjusted so that the mixture of red and yellow particles is present adjacent to the front electrode (21), whereby orange is seen on the viewing surface. When the mixture of black and white particles is present adjacent to the pixel electrode (22a) and thus this surface is visible, gray will be seen through the second surface of the display.

[0073] (Example) A 4-particle electrophoresis medium containing black, white, yellow, and red particles of the type described above with reference to FIG. 1 was prepared, filled into an array of transparent microcells, and sealed using an acrylate seal layer. The array of microcells was laminated to a front transparent electrode (PET-ITO) and subsequently bonded to a thin-film transistor (TFT) backplane. The resulting display was placed on an optical bench using a temperature-controlled chuck that allowed for the horizontal and vertical positioning of the test display. As shown in FIG. 9, the panel was initially driven horizontally through various patterns, with little or no dwell time between successive patterns. The test patterns for the horizontal patterns were recorded using video to ensure reliable switching between states and to check for "dead pixels" or other defects that could arise due to improper filling or sealing. To ensure that the panel was functioning correctly, after being driven in the horizontal pattern, the panel was reoriented to the vertical position and activated with multiple updates with long dwell times between updates. This position and test sequence were intended to mimic real-world conditions where the panel is typically installed in the vertical state and updated only occasionally. In this test, the dwell time was 30 minutes, although it could be 60 minutes or longer. The total time for the evaluation in the vertical orientation was 3 days. After 3 days of vertical driving, the display was evaluated for electro-optical performance using a spectrophotometric detector that measured L* and b* values at multiple measurement locations on the display, as shown in the schematic at the right end of FIG. 9.

[0074] As shown in Table 1 below, the test update was performed at 0 °C using waveforms of the type shown in FIGS. 5A, 6A, 7A, and 8A, and the black measurement points have a large amount of dispersion after the extended vertical drive at low temperature. When viewed through an eyeloop or similar magnification, it is clear that this variability is mainly due to the black state being inappropriately contaminated (tinged with light colors) with white, yellow, and red pigments. However, when the panel is driven using the waveforms of the type shown in FIGS. 5B, 6B, 7B, and 8B, the resulting L* values of the black measurement locations are lower and the final L* values have less dispersion (from high to low). In addition, the b* values are closer to zero and have far less dispersion. This data suggests that the waveforms of FIGS. 5B, 6B, 7B, and 8B are superior to the waveforms of FIGS. 5A, 6A, 7A, and 8A for driving a four-particle electrophoretic display in the vertical direction at low temperature.

Table 1

[0075] Although the present invention has been described with reference to its specific embodiments, it should be understood by those skilled in the art that various changes can be made without departing from the scope of the present invention and equivalents can be substituted. In addition, many modifications can be made to adapt a particular situation, material, composition, process, process step, or step to the purpose and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.

Claims

[Claim 1] The invention described in this specification.

Images