Color display configured to convert RGB image data for display on advanced color electronic paper

Abstract

A preferred color display is provided that is configured to convert RGB image data for display on advanced color electronic paper. A color display including an electrophoretic medium includes a processor configured to convert image source colors, typically standard RGB values, into electrophoretic display device colors, such as ACeP device colors, in order to display the image on the color display in its best possible color. 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-transmitting 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 widespread adoption in e-readers, such as the AMAZON KINDLE®, because they offer a book-like reading experience, use low power, 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). White particles are often light-scattering and include, for example, titanium dioxide, while black particles are absorbing 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-transmitting electrode at the viewing surface, a back electrode, and an electrophoretic medium containing oppositely charged white and black particles. When a voltage of one polarity is applied, the white particles migrate to the viewing surface, and when a voltage of the opposite polarity is applied, the black particles migrate to the viewing surface. If the back electrode contains controllable regions (pixels), either segmented 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 could 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. While this driving scheme is much more complex than black-and-white only, 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, allowing thousands of colors to be produced at each pixel. This color process is functionally equivalent to the printing method long 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 for thousands of colors at each pixel, but careful control of the position of each pigment (50 to 500 nanometers in size) within a working space approximately 10 to 20 microns thick is crucial. Obviously, variations in pigment position will result in the wrong color being displayed at a given pixel. Therefore, precise voltage control is required for such systems. Further details of this 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 entirety.

[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 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, several of the E INK patents and published applications referenced below describe electrophoretic displays in which the extreme states are white and dark blue, so that an intermediate gray state would actually be light blue. In fact, as already noted, a 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 a display and should be understood to include extreme optical states that are not typically strictly black and white, such as the white and dark blue states discussed above.

[0008] The terms "bistable" and "bistable" are used herein in their conventional sense in the art to refer to a display comprising display elements having first and second display states that differ in at least one optical property, such that after any given element is driven with a finite-duration addressing pulse to assume either its first or second display state, that state will persist after the addressing pulse is terminated 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. U.S. Pat. No. 7,170,670 indicates that some grayscale-capable particle-based electrophoretic displays are stable not only in their extreme black-and-white states but also in their intermediate gray states, and the same is true for several other types of electro-optic displays. Displays of this type are properly referred to as "multistable" rather than "bistable," although for convenience the term "bistable" may be used herein to encompass 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 broadly 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) migrates 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 for these displays.

[0012] As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can also be generated 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. Such gas-based electrophoretic media are believed to be susceptible to the same types of problems as liquid-based electrophoretic media due to particle settling when used in an orientation that allows such settling, such as in a sign where the medium is positioned in a vertical plane. In fact, particle settling is believed to be a more serious 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 the 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 multitude of small capsules, each of which comprises an internal phase containing electrophoretically movable particles in a fluid medium and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymer binder to form a coherent layer positioned between two electrodes. Techniques described in these patents and applications include the following: (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. Patent Nos. 6,922,276 and 7,411,719) (c) Microcell structures, wall materials, and methods of forming the microcells (see, e.g., U.S. Patent Nos. 7,072,095 and 9,279,906) (d) Methods for filling and sealing microcells (see, e.g., U.S. Patent Nos. 7,144,942 and 7,715,088) (e) Films and subassemblies containing electro-optical materials (see, e.g., U.S. Patent 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-Optic Display) Applications) (i) Display applications (see, e.g., U.S. Patent Nos. 7,312,784 and 8,009,348) (j) Non-electrophoretic displays such as those described in U.S. Patent 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 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 polymer 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. Therefore, 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 plurality of cavities formed in a carrier medium, typically a polymer film. See, e.g., 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 can operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "obscured mode," in which one display state is substantially opaque and one is light transmissive. See, e.g., U.S. Patent 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. Patent No. 4,418,346. Other types of electro-optic displays may also be capable of operating in a obscured mode. Electro-optic media operating in the shielding mode can be used in multilayer structures for full-color displays, where at least one layer adjacent to the viewing surface of the display operates in the shielding mode to expose or obscure a second layer 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 the display 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 coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating, roll coatings 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.) The resulting display can therefore be flexible. Furthermore, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.

[0018] As noted 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 (where 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 (where 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 among 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 (striped) or two-dimensional (2x2) repeating patterns. Other primary color alternatives 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 corresponding pixels of the underlying monochrome display to white or black (turning the corresponding primary colors on or off). For example, in an ideal RGBW display, the red, green, blue, and white primary colors 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, while each colored subpixel is no brighter than one-third of the white of the monochrome display. The brightness of the white displayed by the display as a whole cannot exceed one-half the brightness of a white subpixel (the white area of ​​the display is generated by displaying one white subpixel out of four plus each colored subpixel in its colored form equal to one-third of the white subpixel, so the combined three colored subpixels contribute no more than one white subpixel). Color brightness and saturation 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 the blue pixels (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 ) corresponding to the 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 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. Multiple 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 using multiple electrophoretic layers positioned 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 (e.g., caused 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, 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 of opposite charge polarities and 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 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. U.S. 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 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, different levels of mobility, or both. Neither of these patent applications discloses 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 environmental conditions and the selected driving waveform. See, for example, US Pat. No. 10,467,984 (incorporated by reference in its entirety).

[0023] The vast majority of electronic color images worldwide are generally formatted within the RGB color space, which corresponds to the red, green, and blue subpixels used in liquid crystal displays (LCDs), 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 sets of numbers corresponding to pixels in the image. Once those color levels are applied to the assigned pixels, the image appears on the display. The next image file, corresponding to the next frame of a new photograph or video, will have a new set of numbers for each pixel.

[0024] Unfortunately, RGB numbers do not map directly to the color space used with electrophoretic displays, so it is necessary to convert RGB image files to a new format. Additionally, 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, using the barycentric coordinate method, it is possible to define a smooth mapping between them. 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 employed to generate the tetrahedral decomposition. By mapping the source RGB color angles to the associated device palette colors, 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 tetrahedra. 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 section connecting device black and white. The Kuhn decomposition therefore does not map colors that lie 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 dithered pattern consists of a palette of colors with brightness similar to that of RGB space with reduced granularity.

[0028] In one aspect, a color display comprising an electrophoretic display includes: 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 lookup 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: receive RGB image data for each pixel in an image from the non-transitory memory; convert the RGB image data for each pixel in the image into electrophoretic display image data using the lookup 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 lookup 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 comprises 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, for each pixel in the image, the electrophoretic display image data 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 tetrahedron-to-tetrahedron mapping 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 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 with 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 backplane including, at a viewing surface, a light-transmitting electrode and an array of thin-film transistors coupled to pixel electrodes, each thin-film transistor including a layer of metal oxide semiconductor, and a color electrophoretic medium disposed between the light-transmitting electrode and the backplane. The color electrophoretic medium includes: (a) a fluid; (b) a plurality of first and second particles dispersed in the fluid, the first and second particles having charges of opposite polarities, 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 fourth particles dispersed in the fluid, the third and fourth particles having charges of opposite polarities, 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 of the 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, 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, the yellow, magenta, and cyan pigments exhibit diffuse reflectance at 650, 550, and 450 nm, respectively, measured against a black background of less than 2.5%. In some embodiments, the liquid is a nonpolar liquid having a dielectric constant less than about 5. In some embodiments, the fluid has dissolved or dispersed therein a polymer having an average molecular weight greater than about 20,000 and that is essentially non-adsorbing onto 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-transmitting 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-transmitting electrode and the backplane and includes: (a) a fluid; (b) a plurality of first and second particles dispersed in the fluid, the first and second particles having charges of opposite polarities, 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 fourth particles dispersed in the fluid, the third and fourth particles having charges of opposite polarities, the third and fourth particles having subtractive primary colors different 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-transmitting 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 25 V and 0 V and a voltage between −25 V and 0 V. 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 second particles dispersed in the fluid, the first and second particles having charges of opposite polarities, 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 fourth particles dispersed in the fluid, the third and fourth particles having charges of opposite polarities, and the third and fourth particles having subtractive primary colors different 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 on the viewing surface, and the magnitudes of at least one of the first high voltage, the first low voltage, the second high voltage, and the second low voltage are not the same. In some embodiments, the magnitudes of the first high voltage and the second high voltage are the same. In some embodiments, the magnitudes of the first low voltage and the second low voltage are the same, and the magnitudes 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 second particles dispersed in the fluid, the first and second particles having charges of opposite polarities, 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 fourth particles dispersed in the fluid, the third and fourth particles having charges of opposite polarities, and the third and fourth particles having subtractive primary colors different 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 on 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 voltage and the negative voltage 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-transmitting 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 polarities, the third and fourth sets of particles have charges of opposite polarities, 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 first and second sets of particles have charges of opposite polarities, 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 different subtractive primary colors. 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 +15 V and the negative voltage is −9 V. In one embodiment, the positive voltage is +9 V and the negative voltage is −15 V.

[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 voltage and the negative voltage 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-transmitting 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 polarities, the third and fourth sets of particles have charges of opposite polarities, 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 first and second sets of particles have charges of opposite polarities, 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 different subtractive primary colors. 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 +15 V and the negative voltage is −9 V. In one embodiment, the positive voltage is +9 V and the negative voltage is −15 V. The present specification also provides, for example, the following: (Item 1) A color display, an electrophoretic display (101, 102) comprising a light-transmitting 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-transmitting 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 that maps 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 for each pixel in an image from the non-transitory memory (70); 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 (Item 2) Item 1. The color display of item 1, wherein the lookup table (LUT) incorporates a mapping between tetrahedra that incorporates the black-white axis in an RGB color space and the black-white axis in an electrophoretic display color space. (Item 3) 3. The color display of claim 1, wherein for each pixel in the image, 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). (Item 4) 4. The color display of claim 3, wherein the processor (50) is further configured to compare the color separation accumulation value with a threshold array prior to transmitting the electrophoretic display image data to the controller (60). (Item 5) Item 5. The color display of item 4, wherein the threshold array is a blue noise mask (BNM). (Item 6) Item 5. The color display of item 4, wherein the processor (50) compares the color separation accumulation value with a threshold array by using a quantization function. (Item 7) Item 1, a color display according to item 1, wherein the electrophoretic medium (120) is confined within a plurality of microcapsules (126) or microcells (127). (Item 8) Item 2. The color display of item 1, wherein the processor (50) is further configured to resize the RGB image data. (Item 9) Item 10. The color display of item 1, wherein the color display further comprises a temperature sensor, and the look-up table (LUT) is indexed to temperature. (Item 10) Item 10. The color display of item 1, wherein the active matrix of pixel electrodes (130) comprises thin film transistors (TFTs) comprising metal oxide semiconductors. (Item 11) 1. A method for converting RGB (red, green, blue) image data into electrophoretic display image data, wherein the electrophoretic display (101, 102) comprises four types of electrophoretic particles (121, 122, 123, 124), and the electrophoretic display (101, 102) is capable of generating eight primary colors at each pixel electrode (130) of an active matrix of pixel electrodes (130), the method comprising: receiving RGB image data for each pixel in the 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 the electrophoretic display image data for each pixel in the image 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: (Item 12) Item 12. The method of item 11, wherein the look-up table (LUT) incorporates a mapping between tetrahedra that incorporates the black-white axis in an RGB color space and the black-white axis in an electrophoretic display color space. (Item 13) Item 13. The method of item 11 or 12, wherein, for each pixel, 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). (Item 14) Item 14. The method of item 13, wherein the processor (50) is further configured to compare the color separation accumulation value with a threshold array prior to transmitting the electrophoretic display image data to the controller (60). (Item 15) Item 15. The method of item 14, wherein the threshold array is a blue noise mask (BNM). [Brief explanation 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 method of the present invention.

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

[0039] [Figure 3A]3A illustrates an exemplary equivalent circuit of a single pixel of an electrophoretic display, where the voltage across the pixel is controlled using a transistor. The circuit of FIG. 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, 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 three subtractive primary colors, and the three additive primary colors.

[0042] [Figure 5] FIG. 5 shows an exemplary push-pull driving 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 a 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 series 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 the second step of mapping color data from the decomposed RGB tetrahedron to a decomposed device color space (i.e., 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 INVENTION

[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 palette colors are chosen, which will typically have the 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 those names. These colors can be dithered to provide the sensation of a continuous 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 from a few centimeters to several meters (or more).

[0050] In a typical 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 Figure 6). These colors are the most chromatic source space colors, so it makes sense to map these colors directly to palette colors. That is, if a pixel in source space has (R, G, B) = 255, 0, 0, that pixel should be mapped to a palette color associated with red, etc. One way to ensure this property is to dither the source image in source space using only the source space colors associated with the palette. Then, after dithering, the associated device colors are replaced 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 when mapping from a cube to a distorted polygon. Color hue shifts can be unstable, especially in neutral areas. For example, dithering can assume that neutral colors can be formed by dithering equal amounts of red, green, and blue, as is possible in source space. However, if the green color in 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 using positive and negative voltage sources, the voltage sources having different magnitudes, and a controller that coordinates the driving of at least two drive electrodes opposite the top electrode while cycling the top electrode between two voltage sources and ground. The resulting system can achieve nearly identical color states compared to providing six independent drive levels and ground to each drive electrode. Thus, the system simplifies the required electronics with only a small loss in color gamut. The system is particularly useful for addressing electrophoretic media containing 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, which is 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 cell grid arranged on a backplane, containing the active matrix of pixel electrodes. The filled grid may then be top-sealed with an integrated protective sheet / light-transmitting electrode.

[0052] Referring to Figures 1 and 2, an electrophoretic display (101, 102) typically includes a top light-transmitting electrode 110, an electrophoretic medium 120, and bottom drive electrodes 130 / 135, which are often pixel electrodes of an active matrix of pixels controlled using thin-film transistors (TFTs). Alternatively, the bottom drive electrodes 130 / 135 may be wired directly to a controller or some other switch, which 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 divided 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), rather than the pattern of the containers, will be apparent when the display is viewed from the viewing surface. Electrophoretic medium 120 contains at least one electrophoretic particle 121; however, second electrophoretic particle 122, 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 a 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 needed. 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. (Electrophoretic display and component part structures, 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 electrode 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. WO 01 / 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 shown 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 a 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 therefore the voltage on the scan line is transmitted 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; other pixels are not affected by the voltage on the scan line when addressed (i.e., Vg is "off" or "closed"). Ideally, therefore, 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 therefore 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 reservoir capacitor Cs reduces crosstalk, but at the cost of making it more difficult to charge the pixel, increasing the charging time. As shown in Figure 3A, a separate voltage (V TOP ) is provided to the top electrode, and therefore an electric field (V FPL ) which ultimately determines the optical state of the associated electro-optic medium, V FPL The first side of the reservoir capacitor is coupled to the pixel electrode, while the second side of the reservoir capacitor is connected 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, many different designs exist 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 their operating voltage is limited to roughly ±15 V, at which point the transistors begin to leak current and eventually fail. While a ±15 V 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 upper (common) electrode is varied as a function of time. Another solution is to use advanced TFT materials, such as metal oxides, to enable higher voltage switching, i.e., an operating range of roughly ±28 V.

[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 then coupled to an electro-optic medium, as shown in FIGS. 1 and 2, and typically sealed to create a display module 55, as shown in FIG. 3B. Such a display module 55 is the focus of color displays 100. Color displays 100 typically include a processor 50, which coordinates many functions related to displaying content on the display module 55 and is configured to convert "standard" images, such as sRGB images, into a color regime that best reproduces the image on the display module 55. The processor is typically a mobile processor chip, such as one manufactured 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 lookup tables and performs the color image conversion described below. Color displays 100 may have more than one non-transitory memory chip. Memory 70 may be flash memory. Once the desired image has been converted for display on display module 55, specific image instructions are sent to controller 60, which prompts voltage sequences to be sent to 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). Color display 100 may additionally include communications 85, which may be, for example, a WIFI protocol or BLUETOOTH®, enabling color display 100 to receive images and instructions, which may also be stored in memory 70.Color display 100 may additionally include one or more sensors 90, which may include temperature and / or light sensors; such information may be fed to processor 50, enabling the processor to select an optimal lookup table when such lookup 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 perform 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 four pigments, so that the viewer sees only those colored pigments (i.e., only the 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 Figure 4), there are no particles above the white particle, and the pixel simply displays white. When a single particle is present above the white particle, the color of that single particle is displayed as yellow, magenta, and cyan, respectively, in situations [B], [D], and [F] in Figure 4. 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 could 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 particle 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 produce these colors are best achieved using at least seven voltage levels (high positive, medium positive, low positive, zero, low negative, medium negative, and 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; that is, they consist of a dipole with two pulses of opposite polarity. The magnitude and length of these pulses determine the color obtained. Generally, 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, and L will depend somewhat on the composition of the particles and the environment of the electrophoretic medium. In some applications, H, M, and L may be set by the cost of the components to generate and control these voltage levels.

[0061] 5, when the top electrode is held at a constant voltage (i.e., not top-plane switched), even a "simple" waveform for an ACeP® system requires the drive electronics to provide seven different voltages (+H, +M, +L, 0, -L, -M, -H) to the data lines during an 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 (typically a positive voltage, zero, and a negative voltage) to be delivered during a single frame.

[0062] Of course, achieving a desired color using the drive pulses of FIG. 5 depends on starting the process from a known state, which is unlikely to be the final color displayed on the pixel. Therefore, a series of reset pulses precedes the drive pulse, which increases 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 rest periods (i.e., periods of zero voltage between them) may be selected so 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 rest periods in the reset phase so 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 switches to a 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 using 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 complexity 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, the channel-forming region is formed for each transistor using such metal oxide materials, allowing for 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, source, and drain electrodes. 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 electron mobility 20 to 50 times greater than that of amorphous silicon. By using IGZO TFTs in an active matrix backplane, it is possible to provide voltages greater than 30 V 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 another embodiment, there will be four positive voltages, four negative voltages, and zero volts. These levels can be selected within the range of approximately -27 V to +27 V, without the limitations imposed by top-plane switching as described above.

[0066] With the use of advanced backplanes, such as metal oxide backplanes, it is possible to directly address each pixel using a suitable push-pull waveform, 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 occur more quickly at higher voltages. Additionally, in four-color electrophoretic displays with a reduced color set, it is possible to drive directly 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] While it is possible to generate eight primary colors simply as illustrated in Figures 4 and 5, 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; the size and shape of the reflective color space depend 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 shown 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 image color data for a device tetrahedron, as shown in Figure 7C. However, as illustrated in Figure 7C, the shape of each tetrahedron need not 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 and generates the required voltages to achieve the desired color at each pixel.

[0069] During device image data reconstruction, 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 (in which the “error” introduced by printing one pixel with a particular color different from the theoretically desired color at that pixel is distributed among neighboring 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,” in which the electric field formed by a pixel electrode affects an area of ​​the electro-optic medium that is larger than the area of ​​the pixel electrode itself, effectively diffusing the optical state of one pixel into part of the area of ​​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 color gamut by using linear combinations of multi-primary colors (known as separate accumulation values). Such accumulation values ​​are simply dithered by establishing device primary thresholds. 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 Figure 8, with all steps 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 The image data may first be fed through several 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., 3x3, 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 barycentric coordinate methods. 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 input image color values ​​and 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 accumulation data 608 is used in conjunction with a threshold array 610 to generate 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 step is not performed by finding barycentric coordinates in a tetrahedralization of the multi-primary colors, but may be implemented by a lookup table, which allows for more flexibility. In addition, the outputs calculated by the methods illustrated herein are calculated completely independently of other outputs. Furthermore, the threshold array T(x) used herein may be a blue noise mask (BNM).

[0074] A complete sequence 900 for converting RGB image data, i.e., data such as that contained in a .jpeg, .png, or bitmap file, to an image on an electrophoretic color display is shown in FIG. 9. Starting in step 910, an image file is provided as RGB data. The RGB data may be adjusted, resized, smoothed, sharpened, brightened, etc., in 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 performed using a lookup table 935, which maps 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 cases, 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 lookup tables 935 that are indexed with respect to device performance 933 and environmental data 938.

[0075] The resulting device image data 940 may be dithered in step 950, for example, using the 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 delivered to 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 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 present technology 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, those 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 considered 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. Accordingly, the foregoing embodiments are presented by way of example only, and it should be understood that, within the scope of the appended claims and their equivalents, embodiments of the present invention may be practiced otherwise than as specifically described. Additionally, 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

[Claim 1] The invention described in this specification.

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