Color display configured to convert RGB image data for display on advanced color electronic paper - Patents.com

Abstract

A color display including an electrophoretic medium, the display including a processor configured to convert image source colors, typically standard RGB values, to electrophoretic display device colors, e.g., ACeP device colors, in order to display the image on the color display in its best possible colors. The processor uses a look-up table that depends on the eight primary colors produced by the electrophoretic medium.

Description

[Technical field]

[0001] (Related Applications) This application claims priority to U.S. Provisional Patent Application No. 63 / 335,677, filed April 27, 2022. All patents and publications disclosed herein are incorporated by reference in their entirety. [Background technology]

[0002] Electrophoretic displays (EPDs) change color by modifying the position of electrically charged colored particles relative to a light-transmissive viewing surface. Such electrophoretic displays are typically referred to as "electronic paper" or "e-paper" because the resulting display has high contrast and is sunlight readable, much like ink on paper. Electrophoretic displays have enjoyed wide adoption in e-book readers, such as the AMAZON KINDLE®, because they provide a book-like reading experience, use low power consumption, and allow users to carry a library of hundreds of books in a lightweight, handheld device.

[0003] For many years, electrophoretic displays have included only two types of charged color particles: black and white (for clarity, "color," as used herein, includes black and white). The white particles are often light scattering and include, for example, titanium dioxide, while the black particles are absorptive across the visible spectrum and may include carbon black, or an absorbing metal oxide such as copper chromite. In the simplest sense, a black and white electrophoretic display requires only a light-transmissive electrode at the viewing surface, a back electrode, and an electrophoretic medium that includes oppositely charged white and black particles. When a voltage of one polarity is applied, the white particles move to the viewing surface, and when a voltage of the opposite polarity is applied, the black particles move to the viewing surface. If the back electrode includes controllable regions (pixels), either split electrodes or an active matrix of pixel electrodes controlled by transistors, a pattern can be made to appear electronically at the viewing surface. This pattern can be, for example, text for a book.

[0004] More recently, a variety of color options have become commercially available for electrophoretic displays, including three-color displays (black, white, red, and black, white, yellow) and four-color displays (black, white, red, yellow). Similar to the operation of black-and-white electrophoretic displays, electrophoretic displays with three or four reflective pigments operate similarly to simple black-and-white displays, as the desired color particles are driven relative to the viewing surface. This driving scheme is much more complicated than black-and-white only, but ultimately the optical function of the particles is the same.

[0005] Advanced Color Electronic Paper (ACeP®) also contains four particles, but the cyan, yellow, and magenta particles are subtractive rather than reflective, thereby allowing thousands of colors to be produced at each pixel. This color process is functionally equivalent to the printing method that has long been used in offset and inkjet printers. A given color is produced by using the correct ratios of cyan, yellow, and magenta on a bright white paper background. In the case of ACeP, the relative positions of the cyan, yellow, magenta, and white particles with respect to the viewing surface will determine the color at each pixel. This type of electrophoretic display allows thousands of colors at each pixel, but careful control of the position of each of the pigments (50-500 nanometers in size) within a working space about 10-20 microns thick is crucial. Obviously, variations in the position of the pigments will result in the wrong color being displayed at a given pixel. Hence, precise voltage control is required for such systems. Further details of the present system are available in the following U.S. patents: U.S. Patent Nos. 9,361,836, 9,921,451, 10,276,109, 10,353,266, 10,467,984, and 10,593,272, all of which are incorporated by reference in their entireties.

[0006] The present invention relates to color electrophoretic displays, particularly, but not exclusively, to electrophoretic displays capable of rendering more than two colors using a single layer of electrophoretic material comprising a plurality of colored particles, e.g., white, cyan, yellow, and magenta particles. In some cases, two of the particles will be positively charged and two particles will be negatively charged. In some cases, three of the particles will be positively charged and one particle will be negatively charged. In some cases, one positively charged particle has a thick polymer shell and one negatively charged particle has a thick polymer shell.

[0007] The term "gray state" is used herein in its conventional sense in the imaging arts to refer to a state intermediate between two extreme pixel optical states, and does not necessarily imply a black / white transition between these two extreme states. For example, some of the E INK patents and published applications referenced below describe electrophoretic displays in which the extreme states are white and dark blue, such that an intermediate gray state would actually be light blue. In fact, as already described, the change in optical state may not be a change in color at all. The term "black and white" may hereinafter be used herein to refer to the two extreme optical states of the display, and should be understood to include extreme optical states that are not strictly black and white, such as the white and dark blue states mentioned above.

[0008] The terms "bistable" and "bi-stable" are used herein in their conventional sense in the art to refer to displays that have display elements with first and second display states that differ in at least one optical property, such that after any given element is driven with an addressing pulse of finite duration to assume either its first or second display state, that state will persist after the addressing pulse has ended for at least several times, e.g., at least four times, the minimum duration of the addressing pulse required to change the state of the display element. In U.S. Pat. No. 7,170,670, it is shown that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states, but also in their intermediate gray states, and the same is true for some other types of electro-optic displays. Displays of this type are properly referred to as "multistable" rather than "bistable", but for convenience the term "bistable" may be used herein to cover both bistable and multistable displays.

[0009] The term "impulse", when used to refer to driving an electrophoretic display, is used herein to refer to the integral of the applied voltage with respect to time during the period in which the display is driven.

[0010] Particles that absorb, scatter, or reflect light, either broadband or at selected wavelengths, are referred to herein as colored or pigment particles. A variety of materials other than pigments (in the strict sense of that term as meaning insoluble colored materials), that absorb or reflect light, such as dyes or photonic crystals, may also be used in the electrophoretic media and displays of the present invention.

[0011] Particle-based electrophoretic displays have been the subject of intense research and development for many years. In such displays, a plurality of charged particles (sometimes referred to as pigment particles) move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared to liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in an inadequate usable lifespan of these displays.

[0012] As mentioned above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, the fluid is a liquid, but electrophoretic media can also be produced using gaseous fluids. See, for example, Kitamura, T., et al., Electrical toner movement for electronic paper-like display, IDW Japan, 2001, Paper HCS1-1 and Yamaguchi, Y,. et al. Toner display using insulative particles charged triboelectrically, IDW Japan, 2001, Paper AMD4-4. See also U.S. Patent Nos. 7,321,459 and 7,236,291. It is believed that such gas-based electrophoretic media are susceptible to the same types of problems as liquid-based electrophoretic media due to particle settling when the media is used in an orientation that allows such settling, for example in a sign where the media is placed in a vertical plane. In fact, particle settling is believed to be a more severe problem in gas-based electrophoretic media than in liquid-based electrophoretic media due to the lower viscosity of the gaseous suspending fluid compared to the viscosity of a liquid, which allows for faster settling of the electrophoretic particles.

[0013] Numerous patents and applications assigned to or in the name of Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various techniques used in encapsulated electrophoretic and other electro-optic media. Such encapsulated media comprise a number of small capsules, each of which itself comprises an internal phase that contains electrophoretically mobile particles 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 that is positioned between two electrodes. Techniques 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 the 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-optical materials (see, e.g., U.S. Pat. Nos. 6,982,178 and 7,839,564) (f) Backplanes, adhesive layers, other auxiliary layers, and methods used in displays (see, e.g., U.S. Pat. Nos. 7,116,318 and 7,535,624) (g) Color formation and color adjustment [ka] (h) A method for driving a display [ka] [ka] (These patents and applications may hereinafter be referred to as the MEDEOD (Method for Driving an Electro-Optical Display) Applications.) (i) Display applications (see, e.g., U.S. Pat. Nos. 7,312,784 and 8,009,348) (j) Non-electrophoretic displays, such as those described in U.S. Pat. No. 6,241,921, and U.S. Patent Application Publication Nos. 2015 / 0277160, and U.S. Patent Application Publication Nos. 2015 / 0005720 and 2016 / 0012710.

[0014] Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium may be replaced by a continuous phase, thus producing so-called "polymer-dispersed electrophoretic displays" in which the electrophoretic medium comprises a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymeric material, and that the discrete droplets of electrophoretic fluid in such polymer-dispersed electrophoretic displays may be considered capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet. See, e.g., U.S. Pat. No. 6,866,760. Thus, for purposes of this application, such polymer-dispersed electrophoretic media are considered a subspecies of encapsulated electrophoretic media.

[0015] A related type of electrophoretic display is the so-called "microcell electrophoretic display." In a microcell electrophoretic display, the charged particles and fluid are not encapsulated in microcapsules, but instead are held within a number of cavities formed in a carrier medium, typically a polymer film. See, for example, U.S. Patent Nos. 6,672,921 and 6,788,449.

[0016] Although electrophoretic media are often opaque (e.g., because in many electrophoretic media the particles substantially block the transmission of visible light through the display) and may operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "obstructed mode" in which one display state is substantially opaque and one is light-transmitting. See, for example, U.S. Pat. Nos. 5,872,552, 6,130,774, 6,144,361, 6,172,798, 6,271,823, 6,225,971, and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely on variations in electric field strength, can operate in a similar mode. See U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in the obscured mode. Electro-optic media operating in the shielding mode can be used in multi-layer structures for full color displays, where at least one layer adjacent the viewing surface of the display operates in the shielding mode to reveal or obscure a second layer that is more remote from the viewing surface.

[0017] Encapsulated electrophoretic displays typically do not suffer from the clustering and settling failure modes of conventional electrophoretic devices, and offer additional advantages such as the ability to print or coat displays on a wide variety of flexible and rigid substrates. (The use of the term "printing" is intended to include, but is not limited to, all forms of printing and coating, including pre-metered coating such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating, roll coating such as knife-over-roll coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, silk screen printing processes, electrostatic printing processes, thermal printing processes, inkjet printing processes, electrophoretic deposition (see U.S. Pat. No. 7,339,715), and other similar techniques.) Thus, the resulting display can be flexible. Furthermore, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.

[0018] As shown above, the simplest prior art electrophoretic media essentially display only two colors. Such electrophoretic media use either a single type of electrophoretic particles having a first color in a colored fluid having a second, different color (wherein the first color is displayed when the particles are adjacent to the viewing surface of the display and the second color is displayed when the particles are spaced from the viewing surface), or first and second types of electrophoretic particles having different first and second colors in a non-colored fluid (wherein the first color is displayed when the first type of particles are adjacent to the viewing surface of the display and the second color is displayed when the second type of particles are adjacent to the viewing surface). Typically, the two colors are black and white. If a full color display is desired, a color filter array may be deposited across the viewing surface of a monochrome (black and white) display.

[0019] Displays with color filter arrays rely on area sharing and color blending to create color stimuli. The available display area is shared between three or four primary colors, such as red / green / blue (RGB) or red / green / blue / white (RGBW), and the filters can be arranged in one-dimensional (stripes) or two-dimensional (2×2) repeating patterns. Other alternative primary colors or more than three primary colors are also known in the art. Three (for RGB displays) or four (for RGBW displays) subpixels are chosen to be small enough that at the intended viewing distance, they visually blend together into a single pixel with a uniform color stimulus ("color blending"). An inherent disadvantage of area sharing is that the colorants are always present, and the color can only be modulated by switching the corresponding pixels of the underlying monochrome display to white or black (switching the corresponding primary colors on or off). For example, in an ideal RGBW display, the red, green, blue, and white primaries each occupy one-quarter of the display area (one subpixel out of four), and the white subpixel is as bright as the white of the underlying monochrome display, but each colored subpixel is no brighter than one-third of the white of the monochrome display. The brightness of the white color shown by the display as a whole cannot exceed one-half the brightness of the white subpixel (the white area of ​​the display is generated by displaying one-out-of-four white subpixels plus each colored subpixel in its colored form equal to one-third of the white subpixel, so the three colored subpixels combined do not contribute more than one white subpixel). The brightness and saturation of the color are reduced by area sharing with color pixels switched to black. Area sharing is particularly problematic when mixing yellow, because it is brighter than any other color of equal brightness, and saturated yellow is nearly as bright as white. Switching a blue pixel (one quarter of the display area) to black causes the yellow to darken significantly.

[0020] A commonly used system for quantifying the color characteristics of displays, including both luminance and hue, is the CIELAB system, which defines color coordinate values ​​(i.e., L, λ, λ′) corresponding to colors displayed by a typical color reflective display device under the CIE standard illuminant D65 (e.g., with a color temperature of 6,500 K). * , a * , b * ) is assigned. * represents the brightness from black to white on a scale of 0 to 100, while a * and b * represents a chromaticity without any specific numerical limits. * corresponds to green, and positive a * corresponds to red, and negative b * corresponds to blue, and positive b * corresponds to yellow. * is expressed as follows: L * = 116(R / R0)1 / 3-16, where R is the reflectance and R0 is the standard reflectance value.

[0021] U.S. Patent Nos. 8,576,476 and 8,797,634 describe multicolor electrophoretic displays having a single backplane with independently addressable pixel electrodes and a common light-transmitting front electrode. The common light-transmitting front electrode is also known as the top electrode. A number of electrophoretic layers are disposed between the backplane and the front electrode. The displays described in these applications are capable of rendering any of the primary colors (red, green, blue, cyan, magenta, yellow, white, and black) at any pixel location. However, there are disadvantages to the use of multiple electrophoretic layers located between a single set of addressing electrodes. The electric field experienced by particles in a particular layer is lower than would be the case for a single electrophoretic layer addressed with the same voltage. In addition, optical losses (caused, for example, by light scattering or unwanted light absorption) in the electrophoretic layers closest to the viewing surface can affect the appearance of the image formed in the underlying electrophoretic layers.

[0022] Attempts have been made to provide a full-color electrophoretic display using a single electrophoretic layer. For example, U.S. Patent No. 8,917,439 describes a color display comprising an electrophoretic fluid containing one or two types of pigment particles dispersed in a clear and colorless or colored solvent, the electrophoretic fluid being disposed between a common electrode and a plurality of pixel or drive electrodes. The drive electrodes are arranged to expose a background layer. U.S. Patent No. 9,116,412 describes a method for driving a display cell filled with an electrophoretic fluid containing two types of charged particles that carry opposite charge polarities and are two contrasting colors. The two types of pigment particles are dispersed in a colored solvent or in a solvent with uncharged or weakly charged colored particles dispersed therein. The method includes the steps of driving the display cell by applying a drive voltage that is about 1 to about 20% of the total drive voltage to display the color of the solvent or the color of the uncharged or weakly charged colored particles. US Patent Nos. 8,717,664 and 8,964,282 describe electrophoretic fluids and methods for driving electrophoretic displays. The fluid comprises first, second, and third types of pigment particles, all of which are dispersed in a solvent or solvent mixture. The first and second types of pigment particles carry opposite charge polarities, and the third type of pigment particles has a charge level that is less than about 50% of the charge level of the first or second types. The three types of pigment particles have different levels of threshold voltage, or different levels of mobility, or both. None of these patent applications disclose a full-color display, as that term is used below, that is capable of achieving at least eight independent colors (white, red, green, blue, cyan, yellow, magenta, and black). As mentioned above, the color gamut (color space) resulting from an electrophoretic display system, such as advanced color electronic paper, can be variable, depending on the environmental conditions and the driving waveforms selected. See, for example, US Pat. No. 10,467,984, which is incorporated by reference in its entirety.

[0023] The vast majority of electronic color images around the world are generally formatted in the RGB color space, which corresponds to the red, green, and blue subpixels used in liquid crystal displays (LCD), light emitting diode (LED) displays, or cathode ray tube (CRT) displays. A common format is 8-bit RGB, which assigns red, green, and blue subpixel values ​​to each pixel in the image as a set of three numbers, each number ranging from 0 to 255. Thus, a standard RGB image file consists of a set of numbers that correspond to the pixels in the image. Once those color levels are provided to the assigned pixels, the image appears on the display. The next image file, which corresponds to the next frame of a new photo or video, will have a new set of numbers at each pixel.

[0024] Unfortunately, RGB numbers do not map directly to the color space used with electrophoretic displays, so it is necessary to convert the RGB image file to a new format. In addition, the shape of the RGB gamut is very different from the shape of, for example, the ACeP gamut, so there is no simple conversion that will convert an RGB file to an ACeP file. [Prior art documents] [Patent documents]

[0025] [Patent Document 1] U.S. Patent No. 7,321,459 [Patent Document 2] U.S. Patent No. 9,361,836 Summary of the Invention [Means for solving the problem]

[0026] Disclosed herein is a system for converting RGB image data into image data for advanced color electronic paper. In the first stage, the RGB source space is mapped to the ACeP device space using a tetrahedral decomposition of the RGB source space. The source tetrahedron is then associated with the tetrahedral decomposition of the device color space using the same set of vertices with associated color names. For example, if one tetrahedron is R, Y, W, K in the source space, the associated palette colors R, Y, W, K in the device space define a tetrahedron in the device space, which may be distorted in shape compared to the source space tetrahedron. However, it is possible to define a smooth mapping between them using the method of barycentric coordinates. Using this method, each color point in any source space tetrahedron can be mapped to a device space tetrahedron. In particular, the barycentric quantization method is adopted to generate the tetrahedral decomposition. By mapping the source RGB color angle to the associated device palette color, it is possible to maintain the neutrality of the neutral axis (i.e., the black-white axis). This is done by using a Kuhn decomposition into six tetrahedrons. In this decomposition, the black-white (KW) edge is a member of every tetrahedron. This means that any gray source color will be mapped to a partition that connects device black and white. The Kuhn decomposition therefore does not map colors that are between two adjacent hues outside its hue range. The system can be implemented in real time using lookup tables and on-device processing, or the processing can be done remotely, i.e., via cloud computing, which would allow the use of more sophisticated color maps.

[0027] In a second step, the system dithers the image set in the device color space to generate a larger number of perceived colors using a limited set of primary colors (typically red, green, blue, cyan, yellow, magenta, white, and black). The system includes a quantizer that implements the dithering step and generates separations separated across the neutral axis. The resulting dithering pattern consists of a palette of colors with similar brightness as the RGB space with reduced granularity.

[0028] In one aspect, a color display comprising an electrophoretic display comprises: a light-transmissive electrode; an active matrix of pixel electrodes; an electrophoretic medium comprising four types of electrophoretic particles, the electrophoretic medium being disposed between the light-transmissive electrode and the active matrix of pixel electrodes, the electrophoretic display being capable of generating eight primary colors at each pixel electrode; a non-transitory memory for storing a look-up table that maps RGB (Red, Green, Blue) colors to colors generated by the electrophoretic display; a processor coupled to the non-transitory memory; and a controller coupled to the processor and configured to provide electrophoretic display pixel color instructions to the active matrix of pixel electrodes. The processor is configured to perform the following steps: for each pixel in an image, receive RGB image data from the non-transitory memory; convert the RGB image data for each pixel in the image into electrophoretic display image data using a look-up table (LUT) stored in the non-transitory memory; and transmit the electrophoretic display image data for each pixel to the controller. In some embodiments, the look-up table (LUT) incorporates a mapping between tetrahedra that incorporates a black-white axis in the RGB color space and a black-white axis in the electrophoretic display color space. In some embodiments, for each pixel in the image, converting the RGB image data to electrophoretic display image data further includes assigning a color separation accumulation value to the electrophoretic display image data based on a linear combination of primary colors produced by the electrophoretic display. In some embodiments, the processor is further configured to compare the color separation accumulation value to a threshold array prior to transmitting the electrophoretic display image data to the controller. In some embodiments, the threshold array is a blue noise mask (BNM). In some embodiments, the processor compares the color separation accumulation value to the threshold array by using a quantization function. In some embodiments, the electrophoretic medium is confined within a plurality of microcapsules or microcells.In some embodiments, the processor is further configured to resize the RGB image data. In some embodiments, the color display additionally comprises a temperature sensor, and the look-up table (LUT) is indexed to temperature. In some embodiments, the active matrix of pixel electrodes includes thin film transistors (TFTs) comprising metal oxide semiconductors.

[0029] In one aspect, a method for converting RGB (red, green, blue) image data into electrophoretic display image data, the electrophoretic display comprising four types of electrophoretic particles, the electrophoretic display capable of generating eight primary colors at each pixel electrode of an active matrix of pixel electrodes. The method includes receiving RGB image data for each pixel in an image, converting, with a processor, the RGB image data for each pixel into electrophoretic display image data using a look-up table (LUT) stored in a non-transitory memory coupled to the processor, transmitting the electrophoretic display image data for each pixel in the image to a controller coupled to the processor, and transmitting voltage commands from the controller to the active matrix of pixel electrodes. In some embodiments, the look-up table (LUT) incorporates a mapping between tetrahedra that incorporates a black-white axis in the RGB color space and a black-white axis in the electrophoretic display color space. In some embodiments, converting, for each pixel, the RGB image data into electrophoretic display image data further comprises assigning a color separation accumulation value to the electrophoretic display image data based on a linear combination of primary colors produced by the electrophoretic display. In some embodiments, the processor is further configured to compare the color separation accumulation value to a threshold array prior to transmitting the electrophoretic display image data to the controller. In some embodiments, the threshold array is a blue noise mask (BNM).

[0030] In one aspect, the system involves a color electrophoretic display including a light-transmissive electrode at a viewing surface, a backplane including an array of thin film transistors coupled to pixel electrodes, each thin film transistor including a layer of a metal oxide semiconductor, and a color electrophoretic medium disposed between the light-transmissive electrode and the backplane, the color electrophoretic medium including (a) a fluid, (b) a plurality of first and a plurality of second particles dispersed in the fluid, the first and second particles having charges of opposite polarity, the first particles being light scattering particles, and the second particles having one of the subtractive primary colors, and (c) a plurality of third and a plurality of fourth particles dispersed in the fluid, the third and fourth particles having charges of opposite polarity, the third and fourth particles having subtractive primary colors different from each other and from the second particles, respectively.

[0031] In some embodiments, the first electric field required to separate the aggregates formed by the third and fourth types of particles is greater than the second electric field required to separate the aggregates formed by any other two types of particles. In some embodiments, at least two of the second, third, and fourth particles are non-light scattering. In some embodiments, the first particle is white and the second, third, and fourth particles are non-light scattering. In some embodiments, the first and third particles are negatively charged and the second and fourth particles are positively charged. In some embodiments, the first, second, third, and fourth particles are white, cyan, yellow, and magenta, respectively, and the white and yellow particles are negatively charged and the magenta and cyan particles are positively charged. In some embodiments, the yellow, magenta, and cyan pigments exhibit diffuse reflectance at 650, 550, and 450 nm, respectively, measured over a black background of less than 2.5% when the pigments are distributed approximately isotropically at 15% by volume in a 1 μm thick layer comprising the pigment and a liquid with a refractive index less than 1.55. In some embodiments, the liquid is a non-polar liquid having a dielectric constant less than about 5. In some embodiments, the fluid has dissolved or dispersed therein a polymer that has an average molecular weight value greater than about 20,000 and is essentially non-adsorbing on the particles. In some embodiments, the metal oxide semiconductor is indium gallium zinc oxide (IGZO). The above invention may be incorporated into an e-book reader, a portable computer, a tablet computer, a mobile phone, a smart card, a sign, a watch, a shelf sign, or a flash drive.

[0032] In another aspect, a color electrophoretic display includes a controller, a light-transmissive electrode at a viewing surface, and a backplane including an array of thin film transistors coupled to the pixel electrodes, each thin film transistor including a layer of a metal oxide semiconductor. A color electrophoretic medium is disposed between the light-transmissive electrode and the backplane and includes (a) a fluid, (b) a plurality of first and a plurality of second particles dispersed in the fluid, the first and second particles having charges of opposite polarity, the first particles being light scattering particles, and the second particles having one of the subtractive primary colors, and (c) a plurality of third and a plurality of fourth particles dispersed in the fluid, the third and fourth particles having charges of opposite polarity, the third and fourth particles having different subtractive primary colors from each other and from the second particles, respectively. The controller is configured to provide a plurality of drive voltages to the pixel electrodes such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed at each pixel electrode while maintaining the light-transmissive electrode at a constant voltage. In some embodiments, the controller is configured to provide a voltage greater than 25 volts and a voltage less than -25 volts to the pixel electrodes. In some embodiments, the controller is configured to additionally provide a voltage between 25V and 0V and a voltage between -25V and 0V. In some embodiments, the metal oxide semiconductor is indium gallium zinc oxide (IGZO).

[0033] In another aspect, a color electrophoretic display includes a controller, a light-transmissive electrode at a viewing surface, a backplane electrode, and a color electrophoretic medium disposed between the light-transmissive electrode and the backplane electrode, the color electrophoretic medium including (a) a fluid, (b) a plurality of first and a plurality of second particles dispersed in the fluid, the first and second particles having charges of opposite polarity, the first particles being light scattering particles, and the second particles having one of the subtractive primary colors, and (c) a plurality of third and a plurality of fourth particles dispersed in the fluid, the third and fourth particles having charges of opposite polarity, and the third and fourth particles having different subtractive primary colors from each other and from the second particles, respectively. The controller is configured to provide a first high voltage and a first low voltage to the light-transmissive electrode and a second high voltage, zero voltage, and a second low voltage to the backplane electrode, whereby white, yellow, red, magenta, blue, cyan, green, and black can be displayed at the viewing surface, and the magnitude of at least one of the first high voltage, the first low voltage, the second high voltage, and the second low voltage is not the same. In some embodiments, the magnitude of the first high voltage and the magnitude of the second high voltage are the same. In some embodiments, the magnitude of the first low voltage and the magnitude of the second low voltage are the same, and the magnitude of the first high voltage and the first low voltage are not the same.

[0034] In another aspect, a color electrophoretic display includes a controller, a light-transmissive electrode at a viewing surface, a backplane electrode, and a color electrophoretic medium disposed between the light-transmissive electrode and the backplane electrode, the color electrophoretic medium including (a) a fluid, (b) a plurality of first and a plurality of second particles dispersed in the fluid, the first and second particles having charges of opposite polarity, the first particles being light scattering particles, and the second particles having one of the subtractive primary colors, and (c) a plurality of third and a plurality of fourth particles dispersed in the fluid, the third and fourth particles having charges of opposite polarity, and the third and fourth particles having different subtractive primary colors from each other and from the second particles, respectively. The controller is configured to cause white, yellow, red, magenta, blue, cyan, green, and black to be displayed at the viewing surface by providing one of a plurality of time-dependent drive voltages to the backplane electrode while providing one of the following drive voltages to the light-transmissive electrode: 1) a high voltage for a first time, a low voltage for a second time, and a high voltage for a third time; or 2) a low voltage for the first time, a high voltage for a second time, and a low voltage for a third time.

[0035] In another aspect, a system for driving an electrophoretic medium includes an electrophoretic display, a power supply capable of providing a positive voltage and a negative voltage, the positive and negative voltages having different magnitudes, and a controller coupled to the top electrode driver, the first drive electrode driver, and the second drive electrode driver. The electrophoretic medium includes a light-transmissive top electrode at a viewing surface, the first drive electrode, the second drive electrode, and an electrophoretic medium disposed between the top electrode and the first and second drive electrodes. The controller is configured to: A) provide a positive voltage to the top electrode, a negative voltage to the first drive electrode, and a positive voltage to the second drive electrode in the first frame, B) provide a negative voltage to the top electrode, a negative voltage to the first drive electrode, and a negative voltage to the second drive electrode in the second frame, c) provide a ground voltage to the top electrode, a ground voltage to the first drive electrode, and a positive voltage to the second drive electrode in the third frame, and D) provide a positive voltage to the top electrode, a positive voltage to the first drive electrode, and a positive voltage to the second drive electrode in the fourth frame. In one embodiment, the controller is further configured to: E) provide a negative voltage to the top electrode, a ground voltage to the first drive electrode, and a negative voltage to the second drive electrode in the fifth frame, and F) provide a ground voltage to the top electrode, a ground voltage to the first drive electrode, and a ground voltage to the second drive electrode in the sixth frame. In one embodiment, the electrophoretic medium is encapsulated in a plurality of microcapsules, the microcapsules being dispersed in a polymer binder between the top electrode and the first and second drive electrodes. In one embodiment, the electrophoretic medium is encapsulated in an array of microcells having openings, the openings being sealed with a polymer binder, and the array of microcells being disposed between the top electrode and the first and second drive electrodes. In one embodiment, the electrophoretic medium comprises a non-polar fluid and four sets of particles having different optical properties. In one embodiment, the first and second sets of particles have charges of opposite polarity, the third and fourth sets of particles have charges of opposite polarity, the first particles are light scattering particles, and the second, third, and fourth sets of particles are each a subtractive primary color different from one another.In one embodiment, the controller is configured to provide a combination of positive, negative, and ground voltages to the top electrode and the first drive electrode such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed on the viewing surface. In one embodiment, the first and second sets of particles have charges of opposite polarity, the third and fourth sets of particles have the same charge as the second particles, the first particles are light scattering particles, and the second, third, and fourth sets of particles are each a different subtractive primary color. In one embodiment, the controller is configured to provide a combination of positive, negative, and ground voltages to the top electrode and the first drive electrode such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed on the viewing surface. In one embodiment, the positive voltage is +15V and the negative voltage is -9V. In one embodiment, the positive voltage is +9V and the negative voltage is -15V.

[0036] In another aspect, a system for driving an electrophoretic medium includes an electrophoretic display, a power supply capable of providing a positive voltage and a negative voltage, the positive and negative voltages having different magnitudes, and a controller coupled to the top electrode driver, the first drive electrode driver, and the second drive electrode driver. The electrophoretic medium includes a light-transmissive top electrode at a viewing surface, the first drive electrode, the second drive electrode, and an electrophoretic medium disposed between the top electrode and the first and second drive electrodes. The controller is configured to: A) provide a positive voltage to the top electrode, a negative voltage to the first drive electrode, and a positive voltage to the second drive electrode in the first frame, B) provide a negative voltage to the top electrode, a negative voltage to the first drive electrode, and a negative voltage to the second drive electrode in the second frame, c) provide a ground voltage to the top electrode, a ground voltage to the first drive electrode, and a positive voltage to the second drive electrode in the third frame, and D) provide a positive voltage to the top electrode, a positive voltage to the first drive electrode, and a positive voltage to the second drive electrode in the fourth frame. In one embodiment, the controller is further configured to: E) provide a negative voltage to the top electrode, a ground voltage to the first drive electrode, and a negative voltage to the second drive electrode in the fifth frame, and F) provide a ground voltage to the top electrode, a ground voltage to the first drive electrode, and a ground voltage to the second drive electrode in the sixth frame. In one embodiment, the electrophoretic medium is encapsulated in a plurality of microcapsules, the microcapsules being dispersed in a polymer binder between the top electrode and the first and second drive electrodes. In one embodiment, the electrophoretic medium is encapsulated in an array of microcells having openings, the openings being sealed with a polymer binder, and the array of microcells being disposed between the top electrode and the first and second drive electrodes. In one embodiment, the electrophoretic medium comprises a non-polar fluid and four sets of particles having different optical properties. In one embodiment, the first and second sets of particles have charges of opposite polarity, the third and fourth sets of particles have charges of opposite polarity, the first particles are light scattering particles, and the second, third, and fourth sets of particles are each a subtractive primary color different from one another.In one embodiment, the controller is configured to provide a combination of positive, negative, and ground voltages to the top electrode and the first drive electrode such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed on the viewing surface. In one embodiment, the first and second sets of particles have charges of opposite polarity, the third and fourth sets of particles have the same charge as the second particles, the first particles are light scattering particles, and the second, third, and fourth sets of particles are each a different subtractive primary color. In one embodiment, the controller is configured to provide a combination of positive, negative, and ground voltages to the top electrode and the first drive electrode such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed on the viewing surface. In one embodiment, the positive voltage is +15V and the negative voltage is -9V. In one embodiment, the positive voltage is +9V and the negative voltage is -15V. [Brief description of the drawings]

[0037] [Figure 1] FIG. 1 is a schematic cross-sectional view illustrating an embodiment of an encapsulated electrophoretic display suitable for use with the methods of the present invention.

[0038] [Diagram 2] FIG. 2 is a schematic cross-sectional view illustrating an embodiment of an encapsulated electrophoretic display suitable for use with the methods of the present invention.

[0039] [Figure 3A] Figure 3A illustrates an exemplary equivalent circuit of a single pixel of an electrophoretic display, where the voltage across the single pixel is controlled using a transistor. The circuit of Figure 3A is typically used in active matrix backplanes.

[0040] [Figure 3B]3B illustrates an exemplary color display including a display module, which may be any electro-optical display module, but is preferably a color electrophoretic display module. The color display also includes a processor, a memory, one or more power supplies, and a controller.

[0041] [Figure 4] FIG. 4 is a schematic cross-sectional view showing the positions of various colored particles in a colored electrophoretic medium when displaying black, white, the subtractive primary colors, and the additive primary colors.

[0042] [Diagram 5] FIG. 5 shows an exemplary push-pull drive scheme for addressing an electrophoretic medium containing three subtractive particles and a scattering (white) particle.

[0043] [Figure 6] Figure 6 depicts an idealized transformation between the standard RGB color space and an idealized device color space, where the two color spaces are similar in shape and size. In practice, the standard RGB color space and the ACeP color space are not similar in shape and size, necessitating the use of tetrahedral decomposition, mapping, and reconstruction.

[0044] [Figure 7A] FIG. 7A shows the first step in decomposing the RGB color space into a series of tetrahedrons.

[0045] [Figure 7B] 7B shows the second step of decomposing the device color space (ACeP color space) into a set of tetrahedrons. The shape and size of the device color space are merely exemplary and may vary depending on environmental factors such as the temperature of the display and the spectrum of the incident light.

[0046] [Figure 7C]FIG. 7C illustrates a second step of mapping color data from the decomposed RGB tetrahedron to a decomposed device color space (ie, ACeP color space) tetrahedron.

[0047] [Figure 8] FIG. 8 illustrates that the mapped colors are then dithered to generate an ACeP image file for display on the device.

[0048] [Figure 9] FIG. 9 is an exemplary flow chart used by the system of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] Detailed Description Here we are interested in mapping source (input) colors, typically standard RGB values, to device colors, e.g., ACeP device colors, at each pixel of a color electrophoretic display. Such displays typically have a short list of possible colors that can be created, called a palette. In a typical situation, eight of the palette colors are chosen, which will typically have color names black, red, green, blue, cyan, magenta, yellow, and white, although the actual colors will not be identical to the source space colors with their names. These colors can be dithered to provide the sensation of a sustained range of tones when viewed from a sufficient distance. Depending on the number of pixels in the device and the size of the individual pixels, a sufficient distance can be a few centimeters to a few meters (or more).

[0050] In a standard situation, eight palette colors are associated with the basic colors K, R, G, B, C, M, Y, and W, which are the corners of an R, G, B cube (see FIG. 6). These colors are the most chromatic source space colors, so it makes sense to map these colors directly to the palette colors. That is, if a pixel in the source space has (R, G, B)=255, 0, 0, the pixel should be mapped to a palette color associated with red, etc. Now, one way to ensure this property is to dither the source image in the source space using only the source space colors associated with the palette. Then, after dithering, replace the associated device colors with each of the source colors in the actual pixel. This works well in some cases, but large color hue shifts can occur when the device colors are not well balanced, such as mapping from a cube to a distorted polygon. The color hue shifts can be unstable, especially in neutral areas. For example, it may be assumed that by dithering, neutral colors can be formed by dithering equal amounts of red, green, and blue, as is possible in the source space. However, if the green color in the device space is particularly weak, the target gray tone will take on a purplish hue in the new device. Undesirable color hue shifts can be minimized by using the systems and methods described herein. In some cases, the system includes an electrophoretic medium that uses positive and negative voltage sources, where the voltage sources have different magnitudes, and a controller that cycles the top electrode between two voltage sources and ground while coordinating the driving of at least two drive electrodes opposite the top electrode. The resulting system can achieve roughly the same color state compared to providing six independent drive levels and ground to each drive electrode. Thus, the system simplifies the electronics required with only a small loss in color gamut. The system is particularly useful for addressing an electrophoretic medium that contains four sets of different particles, for example, three of the particles are colored and subtractive and one of the particles is light scattering.

[0051] Display devices may be constructed using the electrophoretic fluid of the present invention in several ways known in the prior art. The electrophoretic fluid may be encapsulated in microcapsules or incorporated into a microcell structure and then sealed with a polymer layer. The microcapsule or microcell layer may be coated or embossed onto a plastic substrate or film with a transparent coating of conductive material. The assembly may be laminated to a backplane with pixel electrodes using a conductive adhesive. Alternatively, the electrophoretic fluid may be dispensed directly onto a thin continuous grid of cells arranged on a backplane, containing an active matrix of pixel electrodes. The filled grid may then be top-sealed with an integrated protective sheet / light-transparent electrode.

[0052] 1 and 2, an electrophoretic display (101, 102) typically includes a top light-transmissive electrode 110, an electrophoretic medium 120, and bottom drive electrodes 130 / 135, which are often pixel electrodes of an active matrix of pixels controlled with thin film transistors (TFTs). Alternatively, the bottom drive electrodes 130 / 135 may be wired directly to a controller or some other switch that provides a voltage to the bottom drive electrodes 130 / 135, causing a change in the optical state of the electrophoretic medium 120, i.e., the split electrodes. Importantly, it is not required that the junctions between the drive electrodes 130 / 135 coincide with the intersections of the microcapsules or the walls 127 of the microcells. The electrophoretic medium 120 is sufficiently thin and the capsules or microcells are sufficiently wide so that the pattern of the drive electrodes (square, circle, hexagon, wave, string, or otherwise) will be shown when the display is viewed from a viewing surface, rather than the pattern of the containers. Electrophoretic medium 120 contains at least one electrophoretic particle 121, however, second electrophoretic particle 122 or third electrophoretic particle 123, fourth electrophoretic particle 124, or more particles are feasible [note that third electrophoretic particle 123 and fourth electrophoretic particle 124 may be contained within microcapsules 126 of FIG. 1, but are omitted for clarity]. Electrophoretic medium 120 typically includes a solvent such as isoparaffin, and may also include a dispersed polymer and charge control agent to promote state stability, e.g., bistability, i.e., the ability to maintain an electro-optical state without any additional energy input.

[0053] The electrophoretic medium 120 is typically compartmentalized, such as by walls of microcapsules 126 or microcells 127. The entire display stack is typically disposed on a substrate 150, which can be rigid or flexible. The displays (101, 102) also typically include a protective layer 160, which may simply protect the top electrode 110 from damage, or which may surround the entire display (101, 102) to prevent water ingress, etc. The electrophoretic displays (101, 102) may also include one or more adhesive layers 140, 170 and / or a sealing layer 180, as required. In some embodiments, the adhesive layer may include a primer component to improve adhesion to the electrode layer 110, or a separate primer layer (not shown in FIGS. 1 or 2) may be used. (The structure of electrophoretic displays and component parts, pigments, adhesives, electrode materials, etc. are described in many patents and patent applications published by E Ink Corporation, such as U.S. Pat. Nos. 6,922,276, 7,002,728, 7,072,095, 7,116,318, 7,715,088, and 7,839,564, all of which are incorporated herein by reference in their entireties.)

[0054] Thin film transistor (TFT) backplanes typically have only one transistor per pixel or drive electrode. Conventionally, each pixel electrode has a capacitor electrode associated with it, such that the pixel electrode and the capacitor electrode form a capacitor. See, for example, International Patent Application No. WO01 / 07961. In some embodiments, N-type semiconductors (e.g., amorphous silicon) may be used to form the transistors, and the "select" and "unselect" voltages applied to the gate electrodes can be positive and negative, respectively.

[0055] As illustrated in FIG. 3A, each transistor (TFT) is connected to a gate line, a data line, and a pixel electrode (drive electrode). When there is a sufficient positive voltage (or negative voltage, depending on the type of transistor) on the TFT gate, there is a low impedance between the scan line and the pixel electrode coupled to the TFT drain (i.e., Vg is in the "on" or "open" state), and thus the voltage on the scan line is transferred to the pixel's electrode. However, when there is a negative voltage on the TFT gate, there is a high impedance, and the voltage is stored on the pixel storage capacitor, and other pixels are not affected by the voltage on the scan line when they are addressed (i.e., Vg is "off" or "closed"). Thus, ideally, the TFT should act as a digital switch. In practice, when the TFT is in the "on" setting, there is still a certain amount of resistance, and thus the pixel takes some time to charge. In addition, when the TFT is in the "off" setting, the voltage is V S From V pix Increasing the capacitance of the storage capacitor Cs reduces crosstalk, but at the cost of making it more difficult to charge the pixel, increasing the charging time. As shown in FIG. 3A, a separate voltage (V TOP ) is provided to the top electrode, so that an electric field (V FPL ) which ultimately determines the optical state of the associated electro-optic medium, V FPL A first side of the storage capacitor is coupled to the pixel electrode, while a second side of the storage capacitor is coupled to a separate line (V COM ). See, e.g., U.S. Pat. No. 7,176,880, which is incorporated by reference in its entirety. [In some embodiments, N-type semiconductors (e.g., amorphous silicon) may be used to form the transistors, and the "select" and "unselect" voltages applied to the gate electrodes can be positive and negative, respectively.] In some embodiments, V COMcan be grounded, however, there are many different designs for draining charge from the charge capacitor, as described, for example, in U.S. Pat. No. 10,037,735 (incorporated by reference in its entirety).

[0056] One problem with conventional amorphous silicon TFTs is that the operating voltage is limited to roughly ±15V, in response to which the transistors begin to leak current and eventually fail. While a ±15V operating range is suitable for many two-particle electrophoretic systems, it has been found that having an increased voltage range makes it easier to separate particles with different zeta potentials, resulting in advanced electrophoretic displays that update faster and have more reproducible colors. One solution for increasing the voltage range for the pixel electrodes is to use top-plane switching, whereby the voltage on the top (common) electrode is varied as a function of time. Another solution is to use advanced TFT materials, such as metal oxides, to allow for higher voltage switching, i.e., an operating range of roughly ±28V.

[0057] Typically, the TFTs are arranged in a matrix with gate and signal lines to each TFT, and the drain electrode is typically coupled to a pixel electrode. This active matrix backplane is coupled to an electro-optic medium, for example as illustrated in Figures 1 and 2, and is typically sealed to create a display module 55, as illustrated in Figure 3B. Such a display module 55 is the focus of color display 100. Color display 100 typically includes a processor 50, which is configured to coordinate many functions related to displaying content on display module 55 and convert "standard" images, such as sRGB images, into a color regime that best reproduces the image on display module 55. The processor is typically a mobile processor chip, typically made by Freescale or Qualcomm, although other manufacturers are known. The processor frequently communicates with non-transitory memory 70, from which it retrieves image files and / or look-up tables and performs the color image conversion described below. Color display 100 may have more than one non-transitory memory chip. The memory 70 may be a flash memory. Once a desired image has been converted for display on the display module 55, specific image instructions are sent to the controller 60, which facilitates sending voltage sequences to the individual thin film transistors (described above). Such voltages typically come from one or more power supplies 80, which may include, for example, a power management integrated chip (PMIC). The color display 100 may additionally include communications 85, which may be, for example, a WIFI protocol or BLUETOOTH, enabling the color display 100 to receive images and instructions, which may also be stored in the memory 70.Color display 100 may additionally include one or more sensors 90, which may include a temperature sensor and / or a light sensor, and such information may be fed to processor 50 to enable the processor to select an optimal look-up table when such look-up table is indexed with respect to ambient temperature or incident illumination intensity or spectrum. In some cases, multiple components of color display 100 may be embedded within a single integrated circuit. For example, a specialized integrated circuit may fulfill the functions of processor 50 and controller 60.

[0058] In the case of ACeP, each of the eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) corresponds to a different arrangement of the four pigments, so that the viewer sees only those colored pigments (i.e., only light-scattering pigments) that are on the viewing side of the white pigment. More specifically, when cyan, magenta, and yellow particles are present below the white particle (situation [A] in FIG. 4), there are no particles above the white particle, and the pixel simply displays white. When a single particle is above the white particle, the color of the single particle is displayed as yellow, magenta, and cyan in situations [B], [D], and [F] in FIG. 4, respectively. When two particles are present above the white particle, the displayed color is a combination of those two particles. That is, in situation [C] in Figure 4, the magenta and yellow particles display red, in situation [E] the cyan and magenta particles display blue, and in situation [G] the yellow and cyan particles display green. Finally, when all three colored particles are above the white particle (situation [H] in Figure 4), all incident light is absorbed by the subtractive primary colored particles and the pixel displays black.

[0059] It is possible that one subtractive primary color can be rendered by light-scattering particles, so that the display would have two types of light-scattering particles, one of which would be white and the other colored. However, in this case, the position of the light-scattering colored particles relative to the other colored particles that cover the white particles would be important. For example, when rendering black (when all three colored particles are present over the white particles), the scattering colored particles cannot be present over the non-scattering colored particles (otherwise they would be partially or completely hidden behind the scattering particles, and the color rendered would be that of the scattering colored particles, not black). If more than one type of colored particles scatters light, it would not be easy to render black.

[0060] It has been found that waveforms for arranging the four pigments in the appropriate configuration to create these colors are best achieved with at least seven voltage levels (high positive, medium positive, low positive, zero, low negative, medium negative, high negative). Figure 5 shows a typical waveform (in simplified form) used to drive the four-particle color electrophoretic display system described above. Such waveforms have a "push-pull" structure, i.e. they consist of a dipole with two pulses of opposite polarity. The magnitude and length of these pulses determine the color obtained. In general, the higher the magnitude of the "high" voltage, the better the color gamut achieved by the display. The "high" voltage is typically 20V to 30V, more typically about 25V, e.g. 24V. The "medium" (M) level is typically 10V to 20V, more typically about 15V, e.g. 15V or 12V. The "low" (L) level is typically 3V to 10V, more typically about 7V, e.g., 9V or 5V. Of course, the values ​​of H, M, L will depend somewhat on the composition of the particles and the environment of the electrophoretic medium. In some applications, H, M, L may be set by the cost of the components to generate and control these voltage levels.

[0061] As shown in Figure 5, when the top electrode is held at a constant voltage (i.e., not top-plane switched), even the "simple" waveform for the ACeP® system requires the drive electronics to provide seven different voltages (+H, +M, +L, 0, -L, -M, -H) to the data lines during the update of a selected pixel of the display. Although multi-level source drivers capable of delivering seven different voltages are available, many commercially available source drivers for electrophoretic displays allow only three different voltages to be delivered during a single frame (typically a positive voltage, zero, and a negative voltage).

[0062] Of course, achieving a desired color using the drive pulses of FIG. 5 is dependent on the particle starting the process from a known state, which is unlikely to be the final color displayed on the pixel. Thus, a series of reset pulses precede the drive pulses, which increase the amount of time required to update the pixel from a first color to a second color. Reset pulses are described in more detail in U.S. Pat. No. 10,593,272, which is incorporated by reference. The lengths of these pulses (refresh and address) and any rests (i.e., periods of zero voltage between them) may be selected such that the entire waveform (i.e., the integral of the voltage with respect to time over the entire waveform) is DC balanced (i.e., the integral of the voltage over time is substantially zero). DC balance can be achieved by adjusting the lengths of the pulses and rests in the reset phase such that the net impulse delivered in the reset phase is equal in magnitude and opposite in sign to the net impulse delivered in the address phase during which the display is switched to the particular desired color.

[0063] While modifying the rail voltages offers some flexibility in achieving different electro-optical performance than four-particle electrophoretic systems, there are many limitations introduced by top-plane switching. For example, typically a lower negative voltage V is required to produce a white state with the displays of this invention. M- However, the maximum negative voltage V H- It is preferable that the ratio is less than half of the above.

[0064] An alternative solution to the complexities of top-plane switching can be provided by fabricating the control transistors from less common materials with higher electron mobility, thereby allowing the transistors to directly switch larger control voltages, e.g., + / - 30V. Newly developed active matrix backplanes can include thin film transistors incorporating metal oxide materials such as tungsten oxide, tin oxide, indium oxide, and zinc oxide. In these applications, channel forming regions are formed for each transistor using such metal oxide materials, allowing faster switching of higher voltages. Such transistors typically include a gate electrode, a gate insulating film (typically SiO2), a metal source electrode, a metal drain electrode, and a metal oxide semiconductor film overlying the gate insulating film, which at least partially polymerizes with the gate electrode, source electrode, and drain electrode. Such backplanes are available from manufacturers such as Sharp / Foxconn, LG, and BOE.

[0065] One preferred metal oxide material for such applications is indium gallium zinc oxide (IGZO). IGZO-TFTs have 20-50 times the electron mobility of amorphous silicon. By using IGZO-TFTs in an active matrix backplane, it is possible to provide voltages greater than 30V via a suitable display driver. Furthermore, a source driver capable of providing at least five, and preferably seven, levels provides different driving paradigms for a four-particle electrophoretic display system. In one embodiment, there will be two positive voltages, two negative voltages, and zero volts. In another embodiment, there will be three positive voltages, three negative voltages, and zero volts. In one embodiment, there will be four positive voltages, four negative voltages, and zero volts. These levels may be selected within the range of about -27V to +27V, without the limitations imposed by the switching of the top plane as described above.

[0066] With advanced backplanes such as metal oxide backplanes, it is possible to directly address each pixel using a suitable push-pull waveform, i.e., as illustrated in FIG. 5. This significantly reduces the time required to update each pixel, in some cases converting a 6 second update to less than 1 second. In some cases it may be necessary to use a reset pulse to establish a starting point for addressing, but the reset can be done more quickly at higher voltages. Additionally, in a four-color electrophoretic display with a reduced color set, it is possible to directly drive from a first color to a second color using a specific waveform that is only slightly longer than the push-pull waveform shown in FIG. 5.

[0067] It is possible to generate eight primary colors simply as illustrated in Figures 4 and 5, but the resulting color space is not compatible with standard RGB image data, e.g., 8-bit RGB color image data. Ideally, the range of eight primary colors in a reflective color device would be roughly cubic, as shown in Figure 6. In such a case, a simple transformation f(p i ) can be used to convert each pixel color assignment to a new pixel assignment in the new device. In this idealized example, it would be trivial to convert any of the trillions of existing RGB images into a new image suitable for use on a new device, such as a reflective color electrophoretic display. Unfortunately, commercially available reflective color devices typically do not have a cubic color space, and the size and shape of the reflective color space depends on the illumination source. Furthermore, in the case of an electrophoretic display, the color response may depend on other environmental factors such as temperature and device performance such as TFT performance and frame rate.

[0068] Therefore, the present invention uses a three step process to convert RGB image data to device image data. One method for converting RGB image data to ACeP image data is illustrated in Figures 7A-7C. In the first step, the RGB color space and the device color space are deconvolved into a set of tetrahedrons, each of which includes two other primary color vertices, such as the KW axis and RY, as illustrated in Figures 7A and 7B. To map the color space, it is only necessary to define six tetrahedrons, however, additional tetrahedrons can be created. In the second step, the portion of the image color data that resides within a particular RGB tetrahedron is mapped to the image color data for the device tetrahedron, as illustrated in Figure 7C. However, as illustrated in Figure 7C, the shape of each tetrahedron does not have to be uniform; typically, device tetrahedrons vary in shape and size, while RGB tetrahedrons are more uniform in shape and size. In the third step, the device image data is reconstructed, ultimately producing an image file that is provided to a controller, which provides instructions to the device backplane to generate the required voltages to achieve the desired color at each pixel.

[0069] During reconstruction of the device image data, the image data can undergo several additional steps to improve the perceived quality of the image when displayed on the device. For example, standard dithering algorithms such as error diffusion algorithms (where the "error" introduced by printing one pixel with a particular color different from the color theoretically desired at that pixel is distributed among adjacent pixels so that an overall correct color sensation is produced) can be employed with limited palette displays. See, for example, Pappas, Thrasyvoulos N. "Model-based halftoning of color images," IEEE Transactions on Image Processing 6.7 (1997): 1014-1024 (incorporated by reference in its entirety). Reconstruction can also compensate for errors in the device such as "blooming," where the electric field formed by a pixel electrode affects an area of ​​the electro-optic medium larger than the area of ​​the pixel electrode itself, in effect spreading the optical state of one pixel into a portion of the area of ​​the adjacent pixels.

[0070] In another embodiment, when implementing color mapping, it is assumed that the input color can be represented as a linear combination of multi-primary colors. In the system described herein, this is achieved by gamut mapping the input to the device space gamut by using a linear combination of multi-primary colors (known as a separated accumulation value). Such accumulation values ​​are simply dithered by establishing a device primary threshold. In other words, each color C in the device image can be defined as follows: [ka]

[0071] where Pi is the color of a given primary i in La*b* space. The partial sum of these weights is the separated cumulative value Λ k (C), where: [ka]

[0072] In advanced embodiments, the multi-color rendering algorithm is integrated into the color mapping process, as illustrated in FIG. 8, with all steps being performed by one or more processors. Such processors are typically built specifically for use with portable (mobile) displays, efficiently distributing the computation steps and saving energy. See, for example, processors from Freescale or Qualcomm. As shown, standard RGB image data im i, j may first be fed through some cleanup steps, which may include a sharpening filter 602, which may be optional in some embodiments. This sharpening filter 602 may be useful in some cases where the threshold array T(x) or filter is not as sharp as an error diffusion system. This sharpening filter 602 may be a simple finite impulse response (FIR) filter, e.g., 3×3, which may be easily calculated. Additionally, although not shown in FIG. 8, the RGB image data may be resized, e.g., from 16 bits to 8 bits, or the actual image may be resized to accommodate the presence of more pixels in the RGB image than are available on the target device.

[0073] Subsequently, the color data may be mapped in a color mapping step 604 as discussed above with respect to Figures 7A-7C, and color separations may be generated in a separation generation step 606 by methods commonly available in the art, such as using a barycentric coordinate method, and this color data may be used to index a CSC_LUT lookup table, which may have N entries per index that provide the desired separation information in a form directly required by the mask-based dithering step (e.g., step 612). In some embodiments, this CSC_LUT lookup table may be constructed by combining both the desired color enhancement and / or color gamut mapping and the selected separation algorithm, and is configured to include a mapping between the color values ​​of the input image and the color separation accumulation values. In this manner, the lookup table (e.g., CSC_LUT) may be designed to provide the desired separation accumulation value information quickly and in a form directly required by the mask-based dithering step (e.g., step 612 using a quantizer). Finally, the separated accumulated value data 608 is used in conjunction with a threshold array 610 to quantize the device image data y using a quantizer 612. i,j , generating multiple colors. The quantizer may be a separate integrated circuit, however, this function is typically incorporated into the processor 50. In some embodiments, the color mapping 604, separation generation 606, and accumulation value 608 steps may be implemented as a single interpolated CSC_LUT lookup table. In this configuration, the separation stage is not performed by finding the barycentric coordinates in the tetrahedralization of the multi-primary colors, but may be implemented by a lookup table, which allows for more flexibility. In addition, the output calculated by the method illustrated herein is calculated completely independently from the other outputs. Furthermore, the threshold array T(x) used herein may be a blue noise mask (BNM).

[0074] A complete sequence 900 of conversion from RGB image data, i.e., data as contained in a .jpeg, .png, or bitmap file, to an image on an electrophoretic color display is shown in FIG. 9. Beginning at step 910, an image file is provided as RGB data. The RGB data may be scaled, resized, smoothed, sharpened, brightened, etc., at step 920. The resulting RGB data is mapped onto a device color space, as discussed above with respect to FIGS. 7A-7C. In practice, the color mapping step is typically accomplished using a lookup table 935, which maps the RGB data to device data based on existing measurements of device performance, e.g., using calibrated test patterns and a color optical bench. In many instances, the lookup table will be dynamic and change in response to measurements of device performance 933 (battery, front light, frame rate) and environmental data 938, such as temperature. In some embodiments, the device will have non-transitory memory to store multiple look-up tables 935 that are indexed with respect to device capabilities 933 and environmental data 938.

[0075] The resulting device image data 940 may be dithered in step 950, for example, using a barycentric coordinate method. The device image data may also undergo error diffusion to compensate for blooming. Once the final device image data has been transformed, it is stored in memory until it is delivered to the controller 950, which ultimately commands the gate and source drivers to deliver suitable voltages to the front electrodes and display pixels to display the desired image on the device 970.

[0076] Thus, the present invention provides a full-color electrophoretic display capable of receiving standard RGB data and displaying it on a color electrophoretic display such as an Advanced Color Electronic Paper (ACeP®) device. While several aspects and embodiments of the technology of the present application have been described above, it should be understood that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the technology described herein. For example, one skilled in the art will readily envision various other means and / or structures for performing the functions and / or obtaining one or more of the results and / or advantages described herein, and each such alterations and / or modifications are deemed to be within the scope of the embodiments described herein. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments described herein. Thus, the foregoing embodiments are presented by way of example only, and it should be understood that within the scope of the appended claims and equivalents thereof, the embodiments of the present invention may be practiced otherwise than as specifically described. In addition, any combination of two or more features, systems, articles, materials, kits, and / or methods described herein is also included within the scope of the present disclosure, if such features, systems, articles, materials, kits, and / or methods are not mutually inconsistent.

Claims

1. A color display, an electrophoretic display (101, 102) comprising a light-transmissive electrode (110), an active matrix of pixel electrodes (130) and an electrophoretic medium (120) comprising four types of electrophoretic particles (121, 122, 123, 124), the electrophoretic medium (120) being disposed between the light-transmissive electrode (110) and the active matrix of pixel electrodes (130), the electrophoretic display (101, 102) being capable of generating eight primary colors at each pixel electrode (130); a non-transitory memory (70) for storing a look-up table mapping RGB (Red, Green, Blue) colors to colors produced by said electrophoretic display (101, 102); a processor (50) coupled to the non-transitory memory (70); a controller (60) coupled to the processor (50) and configured to provide electrophoretic display pixel color instructions to the active matrix of pixel electrodes (130); wherein the processor (50) performs the following steps: receiving RGB image data from said non-transitory memory (70) for each pixel in an image; converting the RGB image data into electrophoretic display image data for each pixel in the image using a look-up table (LUT) stored in the non-transitory memory (70); transmitting the electrophoretic display image data to the controller (60) on a pixel-by-pixel basis; a color display configured to implement

2. 2. The color display of claim 1, wherein the look-up table (LUT) incorporates a mapping between tetrahedra incorporating a black-white axis in an RGB color space and a black-white axis in an electrophoretic display color space.

3. 3. The color display of claim 1 or 2, wherein for each pixel in the image, converting the RGB image data to electrophoretic display image data further comprises assigning a color separation accumulation value to the electrophoretic display image data based on a linear combination of primary colors produced by the electrophoretic display (101, 102).

4. 4. The color display of claim 3, wherein the processor (50) is further configured to compare the color separation accumulation value to a threshold array prior to transmitting the electrophoretic display image data to the controller (60).

5. The color display of claim 4 , wherein the threshold array is a blue noise mask (BNM).

6. 5. The color display of claim 4, wherein the processor (50) compares the color separation accumulation value with a threshold array by using a quantization function.

7. 10. The color display of claim 1, wherein the electrophoretic medium (120) is confined within a plurality of microcapsules (126) or microcells (127).

8. The color display of claim 1 , wherein the processor (50) is further configured to resize the RGB image data.

9. 10. The color display of claim 1, further comprising a temperature sensor, and wherein the look-up table (LUT) is indexed to temperature.

10. 2. The color display of claim 1, wherein the active matrix of pixel electrodes (130) comprises thin film transistors (TFTs) comprising metal oxide semiconductors.

11. 1. A method for converting RGB (Red, Green, Blue) image data into electrophoretic display image data, said electrophoretic display (101, 102) comprising four types of electrophoretic particles (121, 122, 123, 124), said electrophoretic display (101, 102) being capable of generating eight primary colors at each pixel electrode (130) of an active matrix of pixel electrodes (130), said method comprising: receiving RGB image data for each pixel in an image; converting, with a processor (50), on a pixel-by-pixel basis, the RGB image data into electrophoretic display image data using a look-up table (LUT) stored in a non-transitory memory (70) coupled to the processor (50); transmitting, for each pixel in the image, the electrophoretic display image data to a controller (60) coupled to the processor (50); sending voltage commands from said controller (60) to said active matrix of pixel electrodes (130); A method comprising:

12. 12. The method of claim 11, wherein the look-up table (LUT) incorporates a mapping between tetrahedra that incorporate a black-white axis in an RGB color space and a black-white axis in an electrophoretic display color space.

13. 13. The method of claim 11 or 12, wherein for each pixel, converting the RGB image data into electrophoretic display image data further comprises assigning a color separation accumulation value to the electrophoretic display image data based on a linear combination of primary colors produced by the electrophoretic display (101, 102).

14. 14. The method of claim 13, wherein the processor (50) is further configured to compare the color separation accumulation value to a threshold array prior to transmitting the electrophoretic display image data to the controller (60).

15. The method of claim 14 , wherein the threshold array is a blue noise mask (BNM).

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