Electrophoresis device with ambient light sensor and adaptive whiteness restoration and color balancing front light

Abstract

An electrophoretic display apparatus includes an ambient light sensor and a front light system for adaptively restoring the whiteness on the display and balancing the colors. This specification discloses an improved electrophoretic device having an ambient light sensor and a front light system for adaptively restoring the whiteness on the display and balancing the colors. In a first embodiment, the present invention provides an electrophoretic display apparatus comprising an electrophoretic device having a viewing surface, a drive system coupled to the electrophoretic device for driving the electrophoretic device between a plurality of optical states, and one or more ambient light sensors on the viewing surface of the electrophoretic device for detecting the level of ambient light incident on the viewing surface.

Description

[Background technology]

[0001] (Related applications) This application claims priority to U.S. Provisional Patent Application No. 63,523,487, filed on 27 June 2023, entitled “Electrophobic Device with Ambient Light Sensor and Adaptive Whiteness Restoring and Color Balancing Frontlight,” which is incorporated herein by reference in its entirety.

[0002] Electrophoretic displays (EPDs) change color by altering the position of charged colored particles on a light-transmitting viewing surface. Such EPDs are commonly called "electronic paper" or "ePaper" because the resulting display has high contrast, similar to ink on paper, and is readable even in sunlight. Electrophoretic displays are widely used in e-readers such as Amazon Kindle® because they provide a book-like reading experience, consume less power, and allow users to carry libraries of hundreds of books in a lightweight handheld device.

[0003] For many years, EPDs have contained only two types of charged color particles: black and white. (As used herein, “color” includes both black and white.) White particles are often light-scattering and may include, for example, titanium dioxide, while black particles are absorbent across the entire visible spectrum and may include carbon black or absorbent metal oxides such as copper chromite. In its simplest sense, black and white EPDs require only a light-transmitting electrode on the viewing surface, a back electrode, and an electrophoretic medium containing positively and negatively 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 contains controllable regions (pixels) (either segmented electrodes or an active matrix of transistor-controlled pixel electrodes), a pattern can be electronically displayed on the viewing surface. The pattern could be, for example, text from a book.

[0004] Recently, a variety of color options for EPDs have become commercially available, including tricolor displays (black, white, red; black, white, yellow), quadcolor displays (black, white, red, yellow), and color filter displays that rely on the aforementioned black / white particles. EPDs using three or four reflective particles operate similarly to conventional monochrome displays, as they drive the desired color particles onto the viewing surface. While the driving mechanism is far more complex than simple monochrome, the optical function of the particles is ultimately the same, reflecting incident light to the viewer in the correct color.

[0005] Advanced Color Electronic Paper (ACeP) TMAlthough it also contains four particles, the cyan, yellow, and magenta particles are subtractive rather than reflective, making it possible to generate thousands of colors in each pixel. The color process is functionally equivalent to the printing methods that have been used for many years in offset printing and inkjet printers. A specific color is produced by using the appropriate ratio of cyan, yellow, and magenta on a bright white paper background. In the ACeP example, the relative positions of the cyan, yellow, magenta, and white particles to the viewing surface determine the color of each pixel. In this type of EPD, thousands of colors are envisioned in each pixel, but it is crucial to carefully control the position of each pigment (50-500 nanometers in size) within a working space of approximately 10-20 micrometers in thickness. Naturally, variations in particle position will cause the wrong color to be displayed in a particular pixel. Therefore, precise voltage control is required in such systems. Further details of this system are described in the following U.S. patents, all incorporated together by reference: U.S. Patents No. 9,361,836, No. 9,921,451, No. 10,276,109, No. 10,353,266, No. 10,467,984, and No. 10,593,272.

[0006] The term "gray state" is used herein in its conventional sense in imaging techniques to refer to an intermediate state between two extreme optical states of a pixel, and does not necessarily mean a black-and-white transition between these two extreme optical states. For example, some of the E Ink patents and published applications cited below describe EPDs where the extreme states are white and deep blue, so the intermediate gray state would actually be light blue. In fact, as already mentioned, a change in optical state may not be a change in color at all. The terms black and white may be used below to refer to two extreme optical states of a display, and should be understood to usually include extreme optical states that are not strictly black and white (e.g., the white and deep blue states mentioned above).

[0007] The terms bistable and bistable are used herein in their conventional sense in imaging skills to refer to a display having a display element having a first and second display state of at least one distinct optical property, wherein after a given element is driven to take either the first or second display state using addressing pulses at finite intervals, the state persists for at least several times, e.g., at least four times, the minimum interval of addressing pulses required to change the state of the display element after the addressing pulses have finished. U.S. Patent No. 7,170,670 shows that some grayscale-capable particle-based EPDs 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-optical displays. While this type of display device is more accurately called polystable rather than bistable, for convenience, the term bistable may be used herein to encompass both bistable and polystable displays.

[0008] When the term "impulse" refers to the driving of an EPD, it 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.

[0009] Particles that absorb, scatter, or reflect light across a broadband or selected wavelength are referred to herein as colored particles or pigment particles. In the electrophoretic media and displays of the present invention, a variety of materials other than pigments (strictly speaking, meaning insoluble coloring materials), such as dyes and photonic crystals, may also be used to absorb or reflect light.

[0010] Particle-based EPDs have been the subject of intense research and development for many years. In such displays, multiple charged particles (sometimes referred to as pigment particles) move through a fluid under the influence of an electric field. Compared to liquid crystal displays, EPDs can offer advantages such as better brightness and contrast, wide viewing angles, state bistability, and low power consumption. Nevertheless, problems with the long-term image quality of these displays have hindered their widespread use. For example, the particles that make up EPDs tend to settle, resulting in insufficient lifespan for these displays.

[0011] As described 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 produced using a gaseous fluid. 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. Patents 7,321,459 and 7,236,291. Such gas-based electrophoretic media are considered susceptible to the same types of particle sedimentation problems as liquid-based electrophoretic media when the medium is used in a label where the medium is positioned in a vertical plane, for example, in an orientation that allows such sedimentation. In fact, particle sedimentation is considered a more serious problem in gas-based electrophoretic media than in liquid-based electrophoretic media, due to the lower viscosity of gaseous suspension fluids compared to the viscosity of liquids, which allows for faster sedimentation of electrophoretic particles.

[0012] 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 media and other electro-optical media. Such encapsulated media comprise numerous small capsules, each comprising an internal phase containing, in itself, particles movable by electrophoresis in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules themselves form a coherent layer, held within a polymer binder and positioned between two electrodes. The techniques described in these patents and applications include: (a) Electrophoretic particles, fluids, and fluid additives, see, for example, U.S. Patent Nos. 7,002,728 and 7,679,814. (b) Capsules, binders, and encapsulation processes, see, for example, U.S. Patent Nos. 6,922,276 and 7,411,719. (c) Microcell structures, wall materials, and methods for forming microcells, see, for example, U.S. Patents 7,072,095 and 9,279,906. (d) Methods for filling and sealing microcells, see, for example, U.S. Patent Nos. 7,144,942 and 7,715,088. (e) Films and subassemblies containing electro-optical materials, see, for example, U.S. Patent Nos. 6,982,178 and 7,839,564. (f) Backplanes, adhesive layers, other auxiliary layers, and methods used in displays, see, for example, U.S. Patents 7,116,318 and 7,535,624. (g) Color formation and color adjustment (e.g., U.S. Patent) [ka] and publication of U.S. patent applications [ka] [ka] See below. (h) A method for driving a display, e.g., a U.S. patent. [ka] and publication of U.S. patent applications [ka] See also. (These patents and applications may hereafter be referred to as MEDEOD (Method for Driving Electro-Optical Displays) applications.) (i) Application of the display (see, for example, U.S. Patent Nos. 7,312,784 and 8,009,348) (j) Non-electrophoretic displays as described in U.S. Patent No. 6,241,921 and U.S. Patent Publication No. 2015 / 0277160, and U.S. Patent Publication Nos. 2015 / 0005720 and 2016 / 0012710.

[0013] Many of the aforementioned patents and applications recognize that the wall surrounding discrete microcapsules in an encapsulated electrophoretic medium may be replaced by a continuous phase, thus producing a so-called polymer-dispersed EPD, in which the electrophoretic medium comprises a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymer material, and that discrete droplets of electrophoretic fluid in such a polymer-dispersed EPD may be considered capsules or microcapsules even if no discrete capsule membrane is associated with each individual droplet. See, for example, U.S. Patent No. 6,866,760. Therefore, for the purposes of this application, such polymer-dispersed electrophoretic media are considered a variant of encapsulated electrophoretic media.

[0014] A related type of EPD is the so-called microcell EPD. In microcell EPDs, charged particles and suspension fluids are not encapsulated within microcapsules, but instead are retained within multiple cavities formed within a carrier medium, usually a polymer film. See, for example, U.S. Patent Nos. 6,672,921 and 6,788,449.

[0015] Electrophoretic media are often impermeable (for example, in many electrophoretic media, the particles substantially block the transmission of visible light through the display) and operate in reflective mode, but many EPDs can be made to operate in a so-called shutter mode, where one display state is substantially impermeable and the other is light-transmitting. See, for example, U.S. Patents 5,872,552, 6,130,774, 6,144,361, 6,172,798, 6,271,823, 6,225,971, and 6,184,856. Dielectric displays, similar to EPDs but relying on variations in electric field strength, can operate in a similar mode. See, for example, U.S. Patent 4,418,346. Other types of electro-optical displays may also be able to operate in shielding mode. An electro-optical medium operating in shutter mode can be used in a multilayer structure for a full-color display, in which at least one layer adjacent to the viewing surface of the display operates in shutter mode, exposing or concealing a second layer further away from the viewing surface.

[0016] Encapsulated EPDs typically offer further advantages, such as the ability to print or coat displays on a wide variety of flexible and rigid substrates, without being plagued by the clustering and sedimentation failure modes of conventional electrophoretic devices. (The use of the term "printing" is intended to include all forms of printing and coating, including but not limited to: pre-measured 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; silkscreen printing process; electrostatic printing process; thermal printing process; inkjet printing process; electrophoretic deposition (see U.S. Patent No. 7,339,715); and other similar techniques.) Thus, the resulting displays can be flexible. Furthermore, since the display medium can be printed (using various methods), the displays themselves can be manufactured inexpensively.

[0017] As described above, the simplest prior art electrophoretic media essentially display only two colors. Such electrophoretic media use either a single type of electrophoretic particle having a first color in a colored fluid having a second distinct color (in which case the first color is displayed when the particle is adjacent to the viewing surface of the display, and the second color is displayed when the particle is separated from the viewing surface), or first and second types of electrophoretic particles having different first and second colors in an uncolored fluid (in which case the first color is displayed when the first type of particle is adjacent to the viewing surface of the display, and the second color is displayed when the second type of particle is adjacent to the viewing surface). Typically, the two colors are black and white. If a full-color display is desired, a color filter array (CFA) may be deposited on the screen of a monochrome (black and white) display. (For example, U.S. Patent No. 6,862,128 discloses an EPD having a CFA as shown in Figure 6, reproduced from that patent.) Displays having a CFA rely on area sharing and color mixing to create a color stimulus. 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 a one-dimensional (striped) or two-dimensional (2x2) repeating pattern. Other primary colors or selections of three or more primary colors are also known in the art. Three (in the case of an RGB display) or four (in the case of an RGBW display) subpixels are selected to be small enough that, at the intended viewing distance, they visually blend together into a single pixel with a uniform color stimulus ("color mixing"). An inherent disadvantage of area sharing is that a colorant is always present and 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 primary colors each occupy one-quarter of the display area (one of the four subpixels), the white subpixel is as bright as the white of the underlying monochromatic display, and each of the colored subpixels is no brighter than one-third of the white of the monochromatic display.The brightness of white displayed by the display cannot, in whole, exceed half the brightness of the white subpixels (the white area of ​​the display is generated by displaying one of each of four white subpixels, and in addition, each colored subpixel in its colored form is equivalent to one-third of a white subpixel; therefore, three combined colored subpixels contribute to one or fewer white subpixels). The brightness and saturation of a color are reduced by sharing space with color pixels switched to black. This sharing space is particularly problematic when mixing yellow, because yellow is brighter than any other color of equal brightness, and saturated yellow is almost as bright as white. Switching blue pixels (one-quarter of the display area) to black makes yellow too dark.

[0018] U.S. Patents 8,576,476 and 8,797,634 describe a multicolor electrophoretic display having a single backplane with independently addressable pixel electrodes and a common light-transmitting front electrode. Multiple electrophoretic layers are positioned between the backplane and the front electrode. The displays described in these patents are capable of rendering any primary color (red, green, blue, cyan, magenta, yellow, white, and black) at any pixel position. However, the use of multiple electrophoretic layers positioned between a set of address electrodes has drawbacks. The electric field experienced by particles in a particular layer is lower than that of a single electrophoretic layer addressed at the same voltage. In addition, optical losses in the electrophoretic layer closest to the viewing surface (e.g., caused by light scattering or undesirable absorption) can affect the appearance of the image formed in the underlying electrophoretic layers.

[0019] The other two types of electrophoresis systems provide a single electrophoretic medium capable of rendering any color at any pixel position. Specifically, U.S. Patent No. 9,697,778 describes a display combining a stained solvent with white (light-scattering) particles, which move in one direction when driven by a low applied voltage and in the opposite direction when driven by a higher voltage. By combining the white particles and the staining solvent with two additional particles that have the opposite charge to the white particles, a full-color display can be rendered. However, the color state described in Patent No. 778 is not suitable for applications such as text readers. In particular, there is always some portion of the staining fluid that separates the white-scattering particles from the viewing surface, which leads to a tint in the white state of the display.

[0020] A second embodiment of an electrophoretic medium capable of rendering any color at any pixel position is described in U.S. Patent No. 9,921,451. In Patent No. 451, the electrophoretic medium comprises four particles, namely white, cyan, magenta, and yellow, two of which are positively charged and two are negatively charged. However, the display of Patent No. 451 also suffers from color mixing with the white state. Since one of the particles has the same charge as the white particles, when a white state is desired, a certain amount of the same-charged particles move toward the viewing surface along with the white. It is possible to overcome this undesirable coloring using complex waveforms to drive the display, but such waveforms significantly increase the display refresh time and, in some cases, result in unacceptable "flickering" between images.

[0021] Reflective displays, such as EPDs, convey information by modulating reflected light. A reflective display comprises at least two basic optical elements: a reflector (such as a mirror, retroreflector, or diffuse white reflector) and a reflection modulator (e.g., pigment particles). The optical properties of the reflector determine the appearance of the “white” background. A subgroup of reflective displays is described as having a paper-like appearance and therefore “paper-like” or a high degree of “paper similarity.” To appear paper-like, the reflection in the white state must be as high as possible, spectrally uniform to appear colorless and neutral, and diffuse with nearly Lambertian scattering characteristics, thereby making its reflected brightness independent of the viewing direction. The spectral properties of the modulator determine whether the reflected light appears colorless or colored.

[0022] The optical properties of printed materials on paper provide a suitable standard for measuring the similarity of reflective displays to paper. Current industry specifications for printing on paper include SNAP (Newspaper Advertisement Printing Specification) for newspaper inserts and SWOP (Web Offset Publication Specification) for magazines and other high-quality printing. They specify white (determined by paper grade), contrast, and print primary colors (cyan / magenta / yellow (CMY)), overprinting of two primary colors (RGB), and black. The lightness of white paper (1976 CIELA B units) is 85L for SNAP. * And for SWOP, 95L * That is the case.

[0023] Current EPDs, especially color EPDs, do not reach the level of white reflectivity of paper. EPDs use charged pigments. White pigments with roughly Lambertian scattering properties constitute the "paper" of electronic paper, forming an opaque white background for image-forming pigments that absorb or spectrally modulate reflected light in the same way that ink does on conventional paper. Two of the most practical ways to produce color EPDs are displays that use CFA in front of an achromatic (black and white) backplane, or ACePs that use four pigments, namely one scattering white pigment and three subtractively mixed transparent color pigments, e.g., cyan, magenta, and yellow.

[0024] The Lambertian reflectivity of the white pigment ensures the paper-like appearance and wide viewing angle of e-paper, but its brightness (CIE1976L) is not directly comparable to that of printed paper. * -CIEL * ,a * ,b * There are fundamental limitations that reduce the color space. Unlike ink-coated paper, which is dry and open to the viewer, the pigments in EPDs are suspended in a liquid and contained in small compartments such as microcapsules or microcups. On the viewing side, these compartments are topped with functional transparent optical layers such as adhesives, electrodes, protective sheets, integrated illumination units (ILUs), and touchscreens. The appearance of the EPD surface can be glossy or matte. Surfaces, optical interfaces, and scattering within the stack of optical layers reflect some of the incident ambient light before it reaches the pigment. Total internal reflection (TIR) ​​captures some of the light diffusely reflected by the pigment in a direction beyond its critical angle. Inefficient incoupling, reflection, and outcoupling reduce the overall optical efficiency of electronic paper, resulting in darker and less vibrant colors compared to paper.

[0025] Compared to achromatic B&W EPDs or prints on paper, the CFA further restricts the brightness in the white state, because unlike prints or B&W EPDs where "white" simply means "no ink" or "no black pigment", the CFA is always present. In a CFA with three RGB sub-pixels having optimal filter characteristics with 100% filter transmittance over the corresponding one-third of the visible spectrum for red, green, or blue, the white reflectance in the state where all three sub-pixels are switched to the white state (WS) cannot exceed (1 / 3 + 1 / 3 + 1 / 3) / 3 = 1 / 3. This limits its white-state brightness to the theoretical maximum value of only 64L * . The brightness of a CFA display can be increased by reducing the CFA fill factor, by adding a fourth filter-free W sub-pixel, or by using filter primary colors that transmit 2 / 3 instead of 1 / 3 of the visible spectrum, e.g., CMY. This increases the brightness to the theoretical maximum value of 76L * but at the expense of a reduction in the color saturation. The above-mentioned optical loss factor reduces the brightness of a CFA display to slightly above 50L * . In addition, variations in CFA deposition (printing) can lead to an undesirable hue in the white state.

[0026] The above-mentioned optical loss factor also applies to ACeP displays. In contrast to CFA displays, ACeP displays do not require any sub-pixels and can thus be described as "full-color". Each pixel can be switched between full-area white, full-area color, and full-area black. Although the reflection of the white pigment alone can reach 75 - 80L * , the optical losses from the partitioning and functional optical layers further reduce the white state to less than 70L * . Furthermore, the mixing of color pigments into the white state causes an undesirable hue in the white and can further reduce its L * value to 63.

[0027] Therefore, the brightness of color EPD is generally about 50-70 L. * It is limited to the range, which is the printing specification (85-95L) * It is considerably lower than that.

[0028] Ambient lighting is necessary to view information displayed on an EPD. Ambient lighting originates from numerous light sources, each with its own spectrum, angular distribution, and direction of incidence. In principle, each lighting environment consists of light from directional sources (e.g., the sun, luminaires) and hemispherical diffuse background lighting (e.g., light scattered from a blue or cloudy sky outdoors, or from white walls and ceilings indoors). Ambient lighting is beyond the control of the display designer, but it significantly impacts the perception of the displayed information. The contrast ratio (CR) and color gamut volume (GV) of an EPD change with the lighting geometry, being highest in purely directional lighting and lowest in purely hemispherical diffuse lighting, where surface reflection, TIR, and scattering effects are most pronounced. The spectral distribution of incident light alters how displayed colors appear when viewing the display under daylight, incandescent, or fluorescent lighting. For EPDs and printing on paper, insufficient ambient light levels affect the visibility of information. Very low levels of illumination accentuate the lower contrast (Stevens effect) or lower saturation (Hunt effect) appearance of EPDs compared to color prints.

[0029] Front lighting of EPDs implemented as ILUs is effective in controlling EPD illumination and extending their use to low-light environments where luminescent displays were previously the only viable option. ILUs include a light guide sheet laminated on the front-viewing side of the EPD. Light from edge-mounted light-emitting diodes (LEDs) is coupled to the light guide sheet and propagates parallel to the EPD surface by total internal reflection. The distribution of microstructures that redirect light (scattering, reflection, or refraction) directs the light towards the electrophoretic layer. In most e-readers, the brightness (luminance) of the ILU is controlled by the user, typically 50 cd / m². 2 From 150 cd / m² 2 During that time, sometimes 300 cd / m²2 It can be freely changed across a wide range of white state brightness levels.

[0030] Therefore, using front lighting reduces the limited white state brightness (L) of EPD in ambient lighting compared to the brightness of paper. * ) has the potential to effectively compensate for this. However, ILUs with user-controlled brightness settings can negatively impact the overall performance of the EPD. Users often have the "brightness" (cd / m²) set to a certain degree. 2 There is a tendency to raise the brightness (specified as a unit of luminance) to a level higher than is actually needed for comfortable reading. An unnecessarily high brightness level drains the EPD's battery, negating or reducing its power-saving advantages compared to backlit LCDs. In addition, ILUs operating at high brightness increase the potential health risks of blue light exposure from the ILU's LED spectrum. Of the LED's blue emission peaks, radiation between 415nm and 45nm has an exposure level of 0.5 J / cm². 2 It is only potentially harmful to retinal cells if the dose limit is exceeded. Adaptive ILUs are needed to conserve battery power and protect the user's eye health by limiting brightness to the level necessary for reading. [Prior art documents] [Patent Documents]

[0031] [Patent Document 1] U.S. Patent No. 9,361,836 [Non-patent literature]

[0032] [Non-Patent Document 1] Kitamura,T.,et al.,Electrical toner movement for electronic paper-like display, IDW Japan, 2001, Paper HCS1-1 [Overview of the project] [Means for solving the problem]

[0033] This specification discloses an improved electrophoretic device having an ambient light sensor and a front light system for adaptively restoring whiteness on a display and balancing colors.

[0034] In a first embodiment, the present invention provides an electrophoresis display apparatus comprising an electrophoresis device having a viewing surface, a drive system coupled to the electrophoresis device for driving the electrophoresis device between a plurality of optical states, and one or more ambient light sensors on the viewing surface of the electrophoresis device for detecting the level of ambient light incident on the viewing surface. The electrophoresis display apparatus also comprises a front light unit positioned above the viewing surface of the electrophoresis device for illuminating the viewing surface. The front light control system is coupled to one or more ambient light sensors and the front light unit, and (a) receives one or more signals from the one or more ambient light sensors indicating a detected level of ambient illuminance incident on the viewing surface of the electrophoretic device; (b) compares the detected level of ambient illuminance with a predetermined threshold level; (c) when the detected level of ambient illuminance is equal to or less than the predetermined threshold level, controls the front light illuminance incident on the viewing surface from the front light unit to adaptively maintain a constant viewing surface brightness including the front light illuminance and light reflected by the viewing surface from the ambient illuminance, regardless of the detected level of ambient illuminance; and (d) when the detected level of ambient illuminance is greater than the predetermined threshold level, controls the front light illuminance incident on the viewing surface from the front light unit to maintain the viewing surface brightness at generally the same level as a white diffuse reflector under the same detected level of ambient illuminance, wherein the white diffuse reflector is L * (e) The system is configured to have a Lambertian reflecting surface having a value of =100, and to repeat steps (a) through (d) multiple times.

[0035] In a second embodiment, the present invention provides an electrophoretic display apparatus comprising: an electrophoretic device having a viewing surface; a drive system coupled to the electrophoretic device for driving the electrophoretic device between a plurality of optical states; and one or more ambient light sensors on the viewing surface of the electrophoretic device, including at least one tricolor sensor for detecting ambient tricolor irradiance in red, green, and blue channels incident on the viewing surface. A front light unit is positioned above the viewing surface of the electrophoretic device to illuminate the viewing surface and includes at least one tricolor light source having independently controllable red, green, and blue channels. A front light control system is coupled to the one or more ambient light sensors and the front light unit to control the chrominance of illumination from the at least one tricolor light source to compensate for the white state of the electrophoretic device, which is off-white.The front light control system (a) receives one or more signals from one or more ambient light sensors indicating the detected level of ambient trichromatic irradiance in the red, green, and blue channels incident on the viewing surface; (b) compares the detected level of ambient trichromatic irradiance with a predetermined threshold level in each of the red, green, and blue channels; and (c) controls the front light irradiance incident on the viewing surface from the front light unit in the channel when the detected level of ambient trichromatic irradiance is equal to or less than the predetermined threshold level in any of the red, green, and blue channels. The system is configured to: (d) adaptively maintain a constant level of trichromatic irradiance, including the front light irradiance and the trichromatic irradiance reflected by the viewing surface from the ambient trichromatic irradiance, regardless of the detected level of irradiance; (d) when the detected level of ambient trichromatic irradiance is greater than a predetermined threshold level in any of the red, green, and blue channels, control the front light irradiance incident on the viewing surface from the front light unit in that channel, thereby maintaining the trichromatic irradiance of the viewing surface at a level generally the same as that of an ideal Lambertian reflector under the same detected level of ambient trichromatic irradiance; and (e) repeat steps (a) through (d) multiple times.

[0036] In a third embodiment, the present invention provides an electrophoretic display apparatus comprising an electrophoretic device having a viewing surface, a drive system coupled to the electrophoretic device for driving the electrophoretic device between a plurality of optical states, and one or more ambient light sensors on the viewing surface of the electrophoretic device, including at least one multispectral sensor having three or more spectral channels. A front light unit is positioned above the viewing surface of the electrophoretic device to illuminate the viewing surface and includes at least one multispectral front light having three or more independently controlled spectral channels. The front light control system is coupled to one or more ambient light sensors and the front light unit and controls the spectral irradiance from at least one multispectral front light, and the front light control system (a) receives one or more signals from the one or more ambient light sensors indicating the detected level of ambient spectral irradiance in the spectral channel incident on the viewing surface, (b) compares the detected level of ambient spectral irradiance in each of the spectral channels with a predetermined threshold level, and (c) when the detected level of ambient spectral irradiance in any of the spectral channels is equal to or less than the predetermined threshold level, the front light unit in that channel is transmitted to the viewing surface (d) controlling the front light illuminance incident on the viewing surface, adaptively maintaining a constant level of spectral irradiance including the front light illuminance and the spectral irradiance reflected by the viewing surface from the ambient spectral irradiance, regardless of the detected level of the ambient spectral irradiance; (d) when the detected level of the ambient spectral irradiance is greater than a predetermined threshold level in any of the spectral channels, controlling the front light illuminance incident on the viewing surface from the front light unit in that channel, maintaining the spectral irradiance of the viewing surface at a level generally the same as that of an ideal Lambertian reflector under the same detected level of ambient spectral irradiance; and (e) repeating steps (a) through (d) multiple times. [Brief explanation of the drawing]

[0037] This patent or application file includes at least one color drawing. A copy of this patent or patent application publication accompanied by the color drawing will be provided by the Patent Office upon request and payment of the necessary fees.

[0038] [Figure 1] Figure 1 is a schematic cross-sectional view showing the positions of various colored particles in an electrophoretic medium when displaying the three primary colors of subtractive and additive color mixing: black, white, and white.

[0039] [Figure 2A] Figure 2A is a simplified diagram of an exemplary EPD, which has four types of particles (white, yellow, magenta, and cyan) in a nonpolar fluid capable of rendering the full range of colors at each pixel electrode.

[0040] [Figure 2B] Figure 2B illustrates the transition between a first optical state, in which all particles of the first charge polarity are present on the viewing surface, and a second optical state, in which particles with the second (opposite) polarity are present on the viewing surface.

[0041] [Figure 2C] Figure 2C illustrates the transition between a first optical state, in which all particles of the first charge polarity are present on the viewing surface, and a third optical state, in which particles with the second (opposite) polarity are present behind intermediate charged particles of the first polarity located on the viewing surface.

[0042] [Figure 2D] Figure 2D illustrates the transition between a first optical state, which has all the particles of the first charge polarity on the viewing surface, and a fourth optical state, which has particles with the second (opposite) polarity behind the low-charge particles of the first polarity located on the viewing surface.

[0043] [Figure 2E]Figure 2E illustrates the transition between a first optical state, which has all the particles of the first charge polarity on the viewing surface, and a fifth optical state, which has a combination of low-charged and medium-charged particles of the first polarity located on the viewing surface, with particles of the second (opposite) polarity behind them.

[0044] [Figure 3] Figure 3 illustrates an exemplary equivalent circuit of a single pixel of an EPD.

[0045] [Figure 4] Figure 4 is a simplified diagram showing the layers of an exemplary EPD.

[0046] [Figure 5] Figure 5 shows an exemplary push-pull drive waveform with five voltage levels for addressing a four-particle electrophoresis medium containing white, yellow, magenta, and cyan particles.

[0047] [Figure 6] Figure 6 is a simplified cross-sectional view of an EPD with a CFA.

[0048] [Figure 7] Figure 7 is a simplified cross-sectional view of an EPD with an ILU.

[0049] [Figure 8] Figure 8 is a simplified block diagram of an exemplary EPD with an adaptive front lighting system according to one or more embodiments.

[0050] [Figure 9] Figure 9 is a flowchart illustrating an exemplary process for adaptive front light control of an EPD according to one or more embodiments.

[0051] [Figure 10]Figure 10A is a graph showing the ambient illuminance against the white state luminance (without front lighting) of an exemplary EPD with 53 L* compared to a 100% white Lambert reflector, and the luminance loss ΔL compensated by front lighting. Figure 10B shows the front lighting illuminance and the required luminous flux required to compensate for the lower EPD white state.

[0052] [Figure 11] Figure 11A is a graph showing the total white state luminance against ambient illuminance for an exemplary EPD according to one or more embodiments. Figure 11B is a graph showing the forward illuminance against ambient illuminance for an EPD.

[0053] [Figure 12] Figure 12A is a graph showing the measured spectral reflectance of an exemplary EPD in the white state. Figure 12B shows the reflected display color without front light. Figure 12C shows the display color under ambient D50 illumination. Figure 12D is a graph showing the spectral radiance of the white state under ambient D50 illumination.

[0054] [Figure 13] Figure 13A is a graph showing the ambient spectral radiance in the EPD white state with the front light switched on. Figure 13B shows the display color under ambient D50 lighting without front lighting. Figure 13C shows the display color with front lighting on. Figure 13D is a graph showing the a*b* color gamut area under ambient D50 lighting with the front light switched off and on. [Modes for carrying out the invention]

[0055] Various embodiments of the present invention disclosed herein relate to improved electrophoretic devices having ambient light sensors and front light systems for adaptively restoring whiteness on a display and balancing colors.

[0056] As background, U.S. Patent Application Publication No. 20220082896, whose entire content is incorporated herein by reference, discloses an exemplary electrophoretic medium, specifically a four-particle electrophoretic medium comprising a first particle of a first polarity and three other particles having opposite polarities and different charge amounts. Typically, such a system includes a negatively charged white particle and positively charged yellow, magenta, and cyan particles having subtractive primary colors. Furthermore, some particles may be manipulated so that their electrophoretic mobility is nonlinear with respect to the intensity of the applied electric field. Thus, one or more particles will suffer a decrease in electrophoretic mobility upon application of a high electric field (e.g., 20V or higher) of the correct polarity. Such a four-particle system is schematically shown in Figure 1 and can provide white, yellow, red, magenta, blue, cyan, green, and black at each pixel.

[0057] As shown in Figure 1, the eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) each correspond to different arrangements of four particles, so that the viewer sees only their colored particles on the viewing side of the white particles (i.e., the only particles that scatter light). To achieve a wide range of colors, additional voltage levels are used for finer control of the particles. In the described formulation, the first (typically negative) particle is reflective (typically white), while the other three particles, charged oppositely (typically positive), include three substantially non-light-scattering ("SNLS") particles. The use of SNLS particles allows for color mixing and provides more color results than can be achieved with the same number of scattering particles. These thresholds must be sufficiently separated to avoid crosstalk, and this separation requires the use of higher address voltages for some colors. Four-particle electrophoretic display media can also be updated more quickly, require "less flashing" transitions, and can produce a more pleasing (and therefore more commercially valuable) color spectrum for the viewer. In addition, the disclosed formulation provides fast (e.g., less than 500ms, e.g. less than 300ms, e.g. less than 200ms, e.g. less than 100ms) updates between black and white pixels, thereby enabling fast page turning of black text over white text.

[0058] In Figure 1, the viewing surface of the display is at the top (as shown in the figure), meaning the user views the display from this direction, and light is assumed to be incident from this direction. As already mentioned, only one of the four particles used in the electrophoretic medium substantially scatters light, and in Figure 1, this particle is assumed to be a white pigment. This light-scattering white particle forms a white reflector, and any particles above the white particle are visible relative to this reflector (as shown in Figure 1). Light incident on the viewing surface of the display passes through these particles, is reflected by the white particle, passes through these particles again, and exits the display. Therefore, particles above the white particle may absorb various colors, and the colors that appear to the user result from the combination of particles above the white particle. Any particles positioned below the white particle (behind the user's viewpoint) are masked by the white particle and do not affect the displayed colors. The second, third, and fourth particles are substantially non-light-scattering, so their order or arrangement relative to each other is not important; however, for the reasons already mentioned, their order or arrangement relative to the white (light-scattering) particles is extremely important.

[0059] More specifically, when cyan, magenta, and yellow particles are below the white particle (situation [A] in Figure 1), 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 that single particle is displayed as yellow, magenta, and cyan in situations [B], [D], and [F] in Figure 1, respectively. When two particles are positioned above the white particle, the displayed color is a combination of the colors of these two particles; in Figure 1, in situation [C], 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 1), all incident light is absorbed by the three subtractive primary color particles, and the pixel displays black.

[0060] It is possible that a primary color of subtractive color mixing can be rendered by light-scattering particles, and therefore a display could have two types of light-scattering particles, one of which is white and the other is colored. However, in this case, the position of the light-scattering colored particles relative to the other colored particles that cover the white particles will be important. For example, when rendering a color as black (when all three colored particles are on top of the white particles), the scattering colored particles cannot be on top of the non-scattering colored particles (otherwise they would be partially or completely hidden behind the scattering particles, and the rendered color would be the color of the scattering colored particles, not black).

[0061] Figure 1 shows an ideal situation where color is not contaminated (i.e., white particles that scatter light completely mask the particles behind them). In reality, masking by white particles can be imperfect, as there may be some slight absorption of light by particles that would ideally be completely masked. Such contamination typically reduces both the brightness and saturation of the rendered color. Such color contamination should be minimized to a level where the resulting color meets industry-standard color reproduction. A particularly preferred standard is SNAP (a standard for newspaper advertising production), which specifies L for each of the eight primary colors mentioned above. * a * , and b * Specify the value. (Hereafter, "primary colors" will be used to refer to the eight colors shown in Figure 1: black, white, the three subtractive primary colors, and the three additive primary colors.)

[0062] Figures 2A–2E, also disclosed in U.S. Patent Application Publication No. 20220082896, show schematic cross-sectional views of four particle types. The display layer utilizing the improved electrophoretic medium includes a first (visible) surface 13 on the viewing side and a second surface 14 opposite the first surface 13. The electrophoretic medium is positioned between the two surfaces. Each space between the two vertical dotted lines represents a pixel. Within each pixel, the electrophoretic medium can be addressed, and the visible surface 13 of each pixel can achieve the color state shown in Figure 1 without the need for an additional layer and without a color filter array.

[0063] As is standard with respect to EPDs, the first surface 13 includes a common electrode 11, which is light-transmitting and consists of, for example, a sheet of PET with indium tin oxide (ITO) placed on top thereof. On the second surface 14 is an electrode layer 12 including a plurality of pixel electrodes 15. Such pixel electrodes are described in U.S. Patent No. 7,046,228, the contents of which are incorporated herein by reference in their entirety. While active matrix driving using a thin-film transistor (TFT) backplane is mentioned with respect to the layer of pixel electrodes, it should be noted that other types of pixel electrode addressing may be used as long as the pixel electrodes perform the desired function. For example, the upper and lower electrodes may be continuous or segmented. Furthermore, different pixel electrode backplanes than those described in Patent No. 228 are also suitable and may include an active matrix backplane capable of providing a higher drive voltage than those typically found in amorphous silicon thin-film transistor backplanes.

[0064] Newly developed active matrix backplanes include thin-film transistors incorporating metal oxide materials such as tungsten oxide, tin oxide, indium oxide, zinc oxide, or more complex metal oxides such as indium gallium zirconium oxide. In these applications, a channel-forming region is formed using such metal oxide materials for each transistor, enabling high-speed switching at higher voltages. Such metal oxide transistors also allow for less leakage in the "off" state of the thin-film transistor (TFT) than can be achieved by amorphous silicon TFTs, for example. In a typical scanning TFT backplane with n lines, the transistors are in the "off" state for about (n-1) / n of the time required to refresh each line of the display. Leakage of charge from the storage capacitor associated with each pixel will result in degradation of the electro-optical performance of the display. A TFT typically includes a gate electrode, a gate insulating film (typically SiO2), a metal source electrode, a metal drain electrode, and a metal oxide semiconductor film on the gate insulating film that at least partially overlaps the gate electrode, source electrode, and drain electrode. Such backplanes are available from manufacturers such as Sharp / Foxconn, LG, and BOE. Such backplanes are capable of providing drive voltages of ±30V (or higher). Intermediate voltage drivers may be included so that the resulting drive waveform can include five levels, seven levels, nine levels, or more.

[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 the active matrix backplane, it is possible to provide voltages exceeding 30V via a suitable display driver. This allows for the use of a source driver (a switch within the EPD that determines which voltage to apply to each column electrode of a given selected row of the display) that can supply at least five, and possibly seven, drive voltage levels. In one example, there are two positive voltages, two negative voltages, and zero volts. In another example, there are three positive voltages, three negative voltages, and zero volts. In yet another example, there are four positive voltages, four negative voltages, and zero volts. These levels can be selected within a range of approximately -27V to +27V without the limitations imposed by top-plane switching as described above.

[0066] The electrophoretic media shown in Figures 2A-2E contain four types of electrophoretic particles in a nonpolar fluid 17. The first particle (W- * The particles (white circle) can be negatively charged and surface-treated such that the electrophoretic mobility of the first particle depends on the strength of the driving electric field (discussed in more detail below). In such cases, the electrophoretic mobility of the particles actually decreases in the presence of a stronger electric field, which is somewhat counterintuitive. The second particle (M++ *The particles (black circles) are positively charged and may be surface-treated (or intentionally left untreated) so that the electrophoretic mobility of the second particles depends on the strength of the driving electric field, or so that the unpacking rate of the aggregate of second particles after being driven to one side of the cavity containing the particles is slower than the unpacking rate of the aggregates of third and fourth particles when the direction of the electric field is reversed, or so that the particles form Coulomb aggregates with the first particles (in this case, W-) which are separable by a high applied electric field rather than a low applied electric field. The third particles (Y+; checkered circles) are positive but have a smaller charge magnitude than the second particles. Furthermore, the third particles may be surface-treated, but not in such a way that the electrophoretic mobility of the third particles depends on the strength of the driving electric field. That is, the third particles may have a surface treatment, but such a surface treatment does not result in the aforementioned decrease in electrophoretic mobility due to an increased electric field. The fourth particle (C+++; gray circle) has the largest positive charge and the same type of surface treatment as the third particle. As shown in Figure 2A, the particles are nominally white, magenta, yellow, and cyan in color, producing the colors shown in Figure 1. However, the system is not limited to this particular set of colors, nor is it limited to one reflective particle and three absorbing particles. For example, the system may include one black absorbing particle and three reflective particles of red, yellow, and blue with suitably matched reflectance spectra, and when all three reflective particles are mixed and visible on the surface, the process may produce a white state.

[0067] The first particle (negative) is white and scattering. The second particle (positive, medium charge) is magenta and absorbent. The third particle (positive, low charge) is yellow and absorbent. The fourth particle (positive, high charge) is cyan and absorbent. Table 1 below shows the diffuse reflectance of representative yellow, magenta, cyan, and white particles useful in the electrophoretic medium of the present invention, along with the ratio of their absorption and scattering coefficients determined by Kuberka-Munk analysis of these substances dispersed in a poly(isobutylene) matrix. [Table 1]

[0068] The electrophoretic medium may be any of the forms described above. Therefore, the electrophoretic medium may be unencapsulated, encapsulated in discrete capsules surrounded by a capsule wall, encapsulated in sealed microcells, or in the form of a polymer dispersion medium. The pigments are described in more detail in other literature, such as U.S. Patent Nos. 9,697,778 and 9,921,451. Briefly, the white particles W1 are silanol-functionalized light-scattering pigments (titanium dioxide) bound to a polymer material containing lauryl methacrylate (LMA) monomer, as described in U.S. Patent No. 7,002,728. The white particles W2 are polymer-coated titania produced substantially as described in Example 1 of U.S. Patent No. 5,852,196, where the polymer coating contains lauryl methacrylate and 2,2,2-trifluoroethyl methacrylate in a ratio of about 99:1. Yellow particle Y1 is CI Pigment Yellow 180, as commonly described in U.S. Patent No. 9,697,778, used without coating and dispersed by friction in the presence of Solsperse 19000. Yellow particle Y2 is CI Pigment Yellow 155, as commonly described in U.S. Patent No. 9,697,778, used without coating and dispersed by friction in the presence of Solsperse 19000. Yellow particle Y3 is CI Pigment Yellow 139, as commonly described in U.S. Patent No. 9,697,778, used without coating and dispersed by friction in the presence of Solsperse 19000. Yellow particle Y4 is CI Pigment Yellow 139, which is coated by dispersion polymerization incorporating trifluoroethyl methacrylate, methyl methacrylate, and dimethylsiloxane-containing monomers, as described in Example 4 of U.S. Patent No. 9,921,451.The magenta particles M1 are positively charged magenta material (dimethylquinacridone, CI Pigment Red 122) coated with vinyl benzyl chloride and LMA, as described in Example 5 of U.S. Patent No. 9,697,778 and U.S. Patent No. 9,921,451.

[0069] Magenta particles M2 are CI Pigment Red 122, which is coated by dispersion polymerization with methyl methacrylate and dimethylsiloxane-containing monomers, as described in Example 6 of U.S. Patent No. 9,921,451. Cyan particles C1 are copper phthalocyanine material (CI Pigment Blue 15:3) coated by dispersion polymerization incorporating methyl methacrylate and dimethylsiloxane-containing monomers, as described in Example 7 of U.S. Patent No. 9,921,451. In some embodiments, the color gamut has been found to be improved by using Ink Jet Yellow 4 GC (Clariant) as the core yellow pigment and incorporating a methyl methacrylate surface polymer. The zeta potential of this yellow pigment can be adjusted by the addition of 2,2,2-trifluoroethyl methacrylate (TFEM) monomer and monomethacrylate-terminated poly(dimethylsiloxane).

[0070] Mechanisms proposed for electrophoretic medium additives and surface treatments to facilitate differential electrophoretic mobility, as well as for the interaction between the surface treatment and the surrounding charge control agent and / or free polymer, are discussed in detail in U.S. Patent No. 9,697,778, which is incorporated in whole as a reference. In such electrophoretic media, one way to control the interaction between different types of particles is by controlling the type, amount, and thickness of the polymer coating on the particles. For example, to control particle properties such that particle-particle interactions are less between a second type of particle and third and fourth type particles than, for example, between a third type of particle of a third species and a fourth type of particle, the second type of particle may have a polymer surface treatment, while the third and fourth type particles may have no polymer surface treatment or carry a polymer surface treatment having a lower mass coverage per unit area of ​​particle surface than the second type of particle. More generally, the Hamaker constant (a measure of the strength of the van der Waals interaction between two particles, where the pair potential is proportional to the Hamaker constant and inversely proportional to the sixth power of the distance between the two particles) and / or the interparticle spacing must be adjusted by a sensible selection of polymer coatings on the three types of particles.

[0071] As described in U.S. Patent No. 9,921,451, different types of polymers may involve different types of polymer surface treatments. For example, Coulomb interactions can be weakened when the closest approach distance of oppositely charged particles is maximized by a steric barrier (typically a polymer grafted or adsorbed on the surface of one or both particles). The polymer shell may be a covalent polymer fabricated by a grafting process or chemisorption, as is well known in the art, or it may be physically adsorbed on the particle surface. For example, the polymer may be a block copolymer containing insoluble and soluble segments. Alternatively, the polymer shell may be dynamic in that it is a loose network of free polymers from the electrophoretic medium that are complexed with pigment particles in the presence of an electric field and a sufficient amount and type of charge control agent (CCA - discussed below). Thus, depending on the strength and polarity of the electric field, the particles may have more associated polymers, which causes the particles to interact differently with the container (e.g., microcapsules or microcells) and other particles. The degree of polymer shelling is conveniently assessed by thermogravimetric analysis (TGA), a technique that involves increasing the temperature of a dry sample of particles and measuring the mass loss due to thermal decomposition as a function of temperature. Using TGA, the percentage of the particle's mass that is polymer can be measured, which can be converted to a volume fraction using the known densities of the core pigment and the polymer adhering to them. Conditions can be found in which the polymer coating is lost but the core pigment remains (these conditions depend on the exact core pigment particles used). Various polymer combinations can be made functional, as described below with respect to Figures 2A-2E. For example, in some embodiments, particles (typically first and / or second particles) may have a covalently bonded polymer shell that strongly interacts with the container (e.g., a microcell or microcapsule). Other particles of the same charge, on the other hand, do not have a polymer coating or form complexes with free polymer in the solution, and therefore these particles hardly interact with the container.In other embodiments, the particles (typically the first and / or second particles) have no surface coating so that they are more readily able to form a charge bilayer and experience a decrease in electrophoretic mobility in the presence of a strong electric field.

[0072] The fluid 17, in which four types of particles are dispersed, is colorless and transparent. The fluid contains charged electrophoretic particles that move through the fluid under the influence of an electric field. A preferred suspension fluid, when a conventional aqueous encapsulation method is used, has a low dielectric constant (about 2) and a high volume resistivity (about 10). 15 It has a viscosity of less than 5 mPas (Ohm-cm), low toxicity and environmental impact, and low water solubility (less than 10 ppm). However, it should be noted that this requirement may be relaxed for non-encapsulation or some microcell display, high boiling point (above about 90°C), and low refractive index (less than 1.5). The last requirement arises from the use of high refractive index scattering (typically white) pigments, whose scattering efficiency depends on the refractive index mismatch between the particles and the fluid.

[0073] Saturated linear or branched hydrocarbons, silicone oils, halogenated organic solvents, and organic solvents such as low molecular weight halogen-containing polymers are some useful fluids. The fluid may contain a single component or a blend of one or more components to adjust its chemical and physical properties. Reactants or solvents (if used) for microencapsulation processes, such as oil-soluble monomers, may be contained in this fluid.

[0074] The fluid preferably has low viscosity and a dielectric constant in the range of about 2 to about 30, preferably about 2 to about 15, for high particle mobility. Examples of suitable dielectric fluids include hydrocarbons such as Isopar®, decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oils, and silicon fluids; aromatic hydrocarbons such as toluene, xylene, phenylxylethane, dodecylbenzene, or alkylnaphthalenes; halogenated solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride, chloropentafluorobenzene, dichlorononane, or pentachlorobenzene; perfluorinated solvents such as FC-43, FC-70, or FC-5060 (manufactured by 3M Corporation, St. Paul MN); low molecular weight halogen-containing polymers such as poly(perfluoropropylene oxide) (manufactured by TCI America, Portland, Oregon); poly(chlorotrifluoroethylene) such as Halocarbon Oil (manufactured by Halocarbon Product Corp., River Edge, NJ); and Galden or DuPont products from Ausimont. Examples include perfluoropolyalkyl ethers such as Krytox Oils and Greases K-Fluid Series from Delaware, and polydimethylsiloxane-based silicone oils from Dow-corning (DC-200).

[0075] Electrophoretic media also typically contain one or more charge control agents (CCAs) and may also contain charge directors. CCAs and charge directors typically contain low molecular weight surfactants, polymers, or blends of one or more components which help to stabilize, or otherwise alter, the sign and / or magnitude of the charge on the electrophoretic particles. CCAs are typically molecules containing ionic or other polar groups (hereinafter referred to as heads). At least one of the positive or negative ionic heads is preferably bonded to a nonpolar chain (typically a hydrocarbon chain) referred to below as a tail. CCAs form inverse micelles in the inner phase, and it is thought that small collections of charged inverse micelles are responsible for the conductivity in the highly nonpolar fluids typically used as electrophoretic fluids.

[0076] The addition of CCA provides the generation of inverse micelles containing a highly polar core, which can vary in size from 1 nm to tens of nanometers (and may have spherical, cylindrical, or other shapes), surrounded by the nonpolar tails of the CCA molecules. In electrophoretic media, three phases can typically be distinguished: a solid particle with a surface, a highly polar phase distributed in the form of extremely small droplets (inverse micelles), and a continuous phase containing fluid. Both charged particles and charged inverse micelles can move through the fluid when an electric field is applied, and therefore there are two parallel paths for electrical conduction through the fluid (typically having negligibly small electrical conductivity themselves).

[0077] The polar core of CCA is thought to influence the surface charge through adsorption onto the surface. In EPD, such adsorption may occur on the surface of the electrophoretic particle or on the inner wall of the microcapsule (or other solid phase such as the wall of the microcell), forming a structure similar to an inverse micelle, which will be referred to as hemimicelle hereafter. If one ion of an ion pair is more strongly attached to the surface than the other (e.g., by covalent bonding), ion exchange between the hemimicelle and the unbonded inverse micelle may result in charge separation, where the more strongly bonded ion remains associated with the particle, while the less strongly bonded ion is incorporated into the core of the free inverse micelle.

[0078] The ionic material forming the head of the CCA may also induce ion pair formation on the particle (or other) surface. Therefore, CCAs can perform two fundamental functions: charge generation at the surface and charge separation from the surface. Charge generation can arise from acid-base or ion exchange reactions between a portion present in the CCA molecule, or otherwise incorporated into the inverse micelle core or fluid, and the particle surface. Thus, a useful CCA material is one that can participate in such reactions, or any other charging reactions known in the art.

[0079] A non-limiting class of charge control agents useful in electrophoretic media includes organic sulfates or sulfonates, metal soaps, block copolymers or comb copolymers, organic amides, organic zwitterions, and organic phosphates and phosphonates. Useful organic sulfates and sulfonates include, but are not limited to, sodium bis(2-ethylhexyl) sulfosuccinate, calcium dodecylbenzenesulfonate, calcium petrolium sulfonate, neutral or basic barium dinonylnaphthalene sulfonate, neutral or basic calcium dinonylnaphthalene sulfonate, sodium dodecylbenzenesulfonate, and ammonium lauryl sulfate. Useful metal soaps include, but are not limited to, basic or neutral barium petronates, calcium petronates, cobalt, calcium, copper, manganese, magnesium, nickel, zinc, aluminum, and iron salts of carboxylic acids such as naphthenic acid, octanoic acid, oleic acid, palmitic acid, stearic acid, and myristic acid. Useful block or comb copolymers include, but are not limited to, AB diblock copolymers of (A) a polymer of 2-(N,N-dimethylamino)ethyl methacrylate quaternized with methyl p-toluenesulfonate and (B) poly(2-ethylhexyl methacrylate), and comb graft copolymers having an oil-soluble tail of poly(12-hydroxystearic acid), a molecular weight of about 1800, and suspended on an oil-soluble anchor group of poly(methyl methacrylate-methacrylic acid). Useful organic amides / amines include, but are not limited to, polyisobutylene succinimide, e.g., OLOA 371 or 1200 (available from Chevron Oronite Company LLC, Houston, TX), or SOLSPERSE 17000 or 19000 (available from Lubrizol, Wickliffe, OH; Solsperse is a registered trademark), and N-vinylpyrrolidone polymers. Lecithin is one example of a useful organic zwitterion, but it is not limited to this.Useful organic phosphates and phosphonates include, but are not limited to, sodium salts of phosphorylated mono- and di-glycerides having saturated and unsaturated acid substituents. Useful tails for CCAs include olefin polymers such as poly(isobutylene) with molecular weights ranging from 200 to 10,000. The head may be a sulfonic acid, phosphoric acid, or carboxylic acid or amide, or an amino group such as a primary, secondary, tertiary, or quaternary ammonium group. One class of CCAs useful in the disclosed four-particle electrophoretic media is disclosed in U.S. Patent Publication 2017 / 0097556 (incorporated herein in whole by reference). Such a CCA typically comprises a quaternary amine head and an unsaturated polymer tail, i.e., at least one CCA double bond. The polymer tail is typically a fatty acid tail. A variety of CCA molecular weights can be used. In some embodiments, the molecular weight of CCA is 12,000 grams / mol or more, for example, between 14,000 grams / mol and 22,000 grams / mol.

[0080] Charge enhancers used in the medium can bias the charge on the electrophoretic particle surface, as described in more detail below. Such charge enhancers may be Brønsted acids or bases or Lewis acids or bases. Exemplary charge enhancers are disclosed in U.S. Patents 9,765,015, 10,233,339, and 10,782,586, all of which are incorporated in their entirety by reference. Exemplary enhancers include, but are not limited to, polyhydroxy compounds containing at least two hydroxyl groups, such as ethylene glycol, 2,4,7,9-tetramethyldecine-4,7-diol, poly(propylene glycol), pentaethylene glycol, tripropylene glycol, triethylene glycol, glycerol, pentaerythritol, glycerol tris(12-hydroxystearate), propyleneglycerol monohydroxystearate, and ethylene glycol monohydroxystearate. Examples of amino alcohol compounds containing at least one alcohol functional group and one amine functional group in the same molecule include, but are not limited to, triisopropanolamine, triethanolamine, ethanolamine, 3-amino-1-propanol, o-aminophenol, 5-amino-1-pentanol, and tetrakis(2-hydroxyethyl)ethylenediamine. In some embodiments, the charge booster is present in the EPD medium in an amount of about 1 to about 500 milligrams ("mg / g"), more preferably about 50 to about 200 mg / g per gram of particle mass.

[0081] Particle dispersion stabilizers may be added to prevent particle aggregation or adhesion to capsules or other walls or surfaces. For typical high-resistivity liquids used as fluids in EPDs, non-aqueous surfactants may be used. These include, but are not limited to, glycol ethers, acetylene glycols, alkanolamides, sorbitol derivatives, alkylamines, quaternary amines, imidazolines, dialkyl oxides, and sulfosuccinates.

[0082] As described in U.S. Patent No. 7,170,670, the bistability of an electrophoretic medium can be improved by including a polymer having a number-average molecular weight greater than approximately 20,000 in the fluid, which is essentially non-absorbent to the electrophoretic particles, and poly(isobutylene) is a preferred polymer for this purpose. Also, as described in U.S. Patent No. 6,693,620, for example, particles with fixed charges on their surface form an electrical bilayer of opposite charges in the surrounding fluid. The ionic heads of CCA can form ion pairs with charged groups on the surface of the electrophoretic particles, forming a layer of immobilized or partially immobilized charged species. Outside this layer is a diffusion layer containing charged (reverse) micelles containing CCA molecules in the fluid. In conventional DC electrophoresis, a sliding surface occurs within the diffusion layer, and the applied electric field exerts a force on the fixed surface charges and an opposite force on the mobile pair charges, causing the particles to move relative to the fluid. The potential at the sliding surface is known as the zeta potential.

[0083] As a result, some particle types within the electrophoretic medium have different electrophoretic mobilities depending on the strength of the electric field across the medium. For example, when a first electric field (low intensity, i.e., approximately ±10V or less) is applied to the electrophoretic medium, particles of the first type move in one direction relative to the electric field. However, when a second electric field (high intensity, i.e., approximately ±20V or more) with the same polarity as the first electric field is applied, particles of the first type begin to move in the opposite direction relative to the electric field. This behavior is theorized to arise from conduction in a highly nonpolar fluid mediated by charged inverse micelles or inversely charged electrophoretic particles. Thus, any electrochemically generated protons (or other ions) are likely transported through the nonpolar fluid within the micelle core or adsorbed onto the electrophoretic particles. For example, as illustrated in Figure 5B of U.S. Patent No. 9,697,778, a positively charged inverse micelle can approach a negatively electrophoretic particle moving in the opposite direction, where the inverse micelle is incorporated into an electric bilayer around the negatively charged particle. (The electric bilayer includes both a charge diffusion layer with an enhanced counterion concentration and a semi-micelle surface adsorption coating on the particle. In the latter case, the inverse micelle charge will associate with the particle in a slip envelope that defines the particle's zeta potential, as described above.) Through this mechanism, an electrochemical current of positively charged ions flows through the electrophoretic fluid, and negatively charged particles can be biased toward more positive charges. As a result, for example, the electrophoretic mobility of a first negative type of particle is a function of the magnitude of the electrochemical current and the residence time of the positive charge near the particle surface, which is a function of the electric field strength.

[0084] Furthermore, as described in U.S. Patent No. 9,697,778, positively charged particles exhibiting different electrophoretic mobilities depending on the applied electric field can be prepared. In some embodiments, secondary (or co)CCA can be added to the electrophoretic medium to adjust the zeta potentials of various particles. Careful selection of co-CCA may allow modification of the zeta potential of one particle without essentially altering the zeta potentials of other particles, enabling precise control of both the electrophoretic velocities and interparticle interactions of various particles during switching.

[0085] In some embodiments, a portion of the charge control agents intended for the final formulation are added during the synthesis of electrophoretic particles to manipulate the desired zeta potential and influence the reduction of electrophoretic mobility under a strong electric field. For example, it has been observed that when quaternary amine charge control agents are added during polymer grafting, some amount of CCA is complexed onto the particles. (This can be confirmed by removing the particles from the electrophoretic fluid and then stripping the surface species from the pigment using THF to remove all adsorbed species. Evaluation of the THF extract by 1H NMR reveals that a considerable amount of CCA was adsorbed onto the pigment particles or formed complexes with the surface polymers.) Experiments suggest that a high CCA loading between the surface polymers of the particles promotes the formation of a charge bilayer around the particles in the presence of a strong electric field. For example, magenta particles with more than 200 mg of charge control agent (CCA) per gram of finished magenta particles exhibit excellent retention properties in the presence of a high positive electric field. (See, for example, Figure 2C and the description above.) In some embodiments, the CCA includes a quaternary amine head and a fatty acid tail. In some embodiments, the fatty acid tail is unsaturated. When some of the particles in the electrophoretic medium contain high CCA loadings, it is important that particles for which consistent electrophoretic mobility is desired do not have a substantial CCA loading, e.g., less than 50 mg of charge control agent (CCA) per gram of finished particle, e.g., less than 10 mg of charge control agent (CCA) per gram of finished particle.

[0086] Alternatively, an electrophoretic medium containing four types of particles in the presence of Solsperse 17000 in Isopar E can be enhanced by adding a small amount of an acidic substance, such as aluminum salt of di-t-butylsalicylic acid (Bontron E-88, available from Orient Corporation in Kenilworth, New Jersey). The addition of an acidic substance shifts the zeta potential of many (but not all) particles to a more positive value. In some cases, about 1% of the acidic substance and 99% of Solsperse 17000 (based on the total weight of the two materials) shifts the zeta potential of a third type of particle (Y+) from -5mV to about +20mV. Whether the zeta potential of a particular particle is altered by a Lewis acidic substance such as aluminum salt depends on the details of the particle's surface chemistry.

[0087] Table 2 shows exemplary relative zeta potentials of three types of colored and single-white particles in a preferred embodiment. [Table 2-1] [Table 2-2]

[0088] The negative (white) particle has a zeta potential of -30mV, while the remaining three particles are all positive relative to the white particle. Therefore, a display with positively charged cyan, magenta, and yellow particles can switch between a black state (all colored particles are in front of the white particle relative to the viewing surface) and a white state, with the white particle being closest to the viewer and preventing the viewer from perceiving the other three particles. In contrast, when the white particle has a zeta potential of 0V, the negatively charged yellow particle is the most negative of all particles, and therefore a display with this particle will switch between a yellow state and a blue state. This would also occur if the white particle were positively charged. However, the positively charged yellow particle would be more positive than the white particle unless its zeta potential exceeds +20mV.

[0089] The behavior of the electrophoretic medium is consistent with the mobility of the white particles (shown as zeta potential in Table 2), which depends on the applied electric field. Therefore, in the embodiments illustrated in Table 2, when addressed at a low voltage, the white particles may behave as if their zeta potential is -30mV, but when addressed at a higher voltage, they may behave as if their zeta potential is more positive, possibly even as high as +20mV (matching the zeta potential of the yellow particles). Consequently, when addressed at a low voltage, the display will switch between a black state and a white state, but when addressed at a higher voltage, it will switch between a blue state and a yellow state.

[0090] Particles with opposite charges can also form Coulomb aggregates. In aggregate form, the mobility of the particles may differ from that which can be measured for the components of the aggregate. Therefore, for example, aggregates may form between negatively charged white particles and either of the oppositely charged pigments. In certain embodiments, the electric field required to separate the Coulomb aggregates may preferably be higher for some pigment pairs than for others. In this example, the electric field required to separate the aggregates formed between magenta and white pigments is likely to be greater than the electric field required to separate the aggregates formed between cyan and white or yellow and white particles.

[0091] The charge of various pigment particles can be influenced by the presence of other charged pigments in their environment. Therefore, it is not necessarily true that the charge (or zeta potential) of pigments in the final formulation is the same as that measured for the pigment itself dispersed in the solvent in the presence of CCA.

[0092] The motion of various particles, according to one possible hypothesis, in the presence of high (e.g., "±H", e.g., ±20V, e.g., ±25V) and low (e.g., "±L", e.g., ±5V, e.g., ±10V) electric fields, is shown in Figures 2B-2E. For illustrative purposes, each box enclosed by a dashed line represents a pixel enclosed by an upper light-transmitting electrode 21 and a lower electrode 22, which may be an active matrix pixel electrode, but may also be a light-transmitting electrode or a segmented electrode, etc. Starting from a first state (nominally black) where all positive particles are present on the viewing surface, the electrophoretic medium can be driven to four different optical states, as shown in Figures 2B-2E. In a preferred embodiment, this results in a white optical state (Figure 2B), a magenta optical state (Figure 2C), a yellow optical state (Figure 2D), and a red optical state (Figure 2E). It is clear that the remaining four optical states in Figure 1 can be achieved by reversing the order of the initial state and the driving electric field, as briefly shown in Figure 5.

[0093] As shown in Figure 2B, when addressed at a low voltage, the particles behave according to their relative zeta potential, with the relative velocity when a negative voltage is applied to the backplane indicated by the arrows. Thus, in this embodiment, cyan particles move faster than magenta particles, and magenta particles move faster than yellow particles. The first (positive) pulse does not change the position of the particles because their movement is already restricted by the enclosure walls. The second (negative) pulse swaps the positions of the colored and white particles, and the display switches between a black state and a white state, with transient colors reflecting the relative mobility of the colored particles. Reversing the starting position and polarity of the pulses allows for a transition from white to black. Thus, this embodiment provides a black-and-white update that requires lower voltages (and consumes less power) compared to other black-and-white formulations achieved with multiple colors via either process black or process white.

[0094] In Figure 2C, the first (positive) pulse is a sufficiently high positive voltage to reduce the mobility of the magenta particles (i.e., particles with intermediate mobility among the three positively charged colored particles). Due to the reduced mobility, the magenta particles remain essentially frozen in place, and subsequent pulses in the opposite direction with a lower voltage move more cyan, white, and yellow particles than magenta particles, thereby producing magenta on the viewing surface with negative white particles behind the magenta particles. Importantly, if the starting position and pulse polarity are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., through electrode 22), this pulse sequence will produce green (i.e., a mixture of yellow and cyan particles).

[0095] In Figure 2D, the first pulse is a low voltage that does not significantly reduce the mobility of magenta or white particles. However, the second pulse is a high negative voltage that reduces the mobility of white particles. This allows for more effective racing among the three positive particles, and thus the slowest type of particle (yellow in this embodiment) remains visible in front of the white particles, its movement reduced by the earlier negative pulse. In particular, the yellow particles do not reach the top surface of the cavity containing the particles. Importantly, if the starting position and pulse polarity are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., through electrode 22), this pulse sequence will produce blue (i.e., a mixture of magenta and cyan particles).

[0096] Figure 2E shows that when both pulses are high voltage, the magenta particle mobility will be reduced by the first high positive pulse, and the race between cyan and yellow will be improved by the reduction in white mobility caused by the second high negative pulse, which produces red. Importantly, if the starting position and pulse polarity are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., through electrode 22), this pulse sequence produces cyan.

[0097] To obtain a high-resolution display, each pixel of the display should be addressable without interference from adjacent pixels. One way to achieve this objective is to provide an array of nonlinear elements, such as transistors or diodes, where at least one nonlinear element is associated with each pixel, producing an "active matrix" display. The addressing or pixel electrode that addresses a single pixel is connected to a suitable voltage source through the associated nonlinear element. Typically, when the nonlinear element is a transistor, the pixel electrode is connected to the drain of the transistor, and this array, though assumed in the following description, is essentially arbitrary, and the pixel electrode may also be connected to the source of the transistor. Conventionally, in high-resolution arrays, pixels are arranged in a two-dimensional array of rows and columns such that any particular pixel is uniquely defined by the intersection of one designated row and one designated column. The sources of all transistors in each column are connected to a single column electrode, while the gates of all transistors in each row are connected to a single row electrode. Again, the assignment of sources to rows and gates to columns is conventional but essentially arbitrary and can be reversed as needed. The row electrodes are connected to a row driver, which essentially ensures that at any given moment, only one row is selected, i.e., a selection voltage is applied to the selected row electrodes such that all transistors in the selected row are conductive, while a deselection voltage is applied to all other rows such that all transistors in those unselected rows remain non-conductive. The column electrodes are connected to a column driver, which applies a selected voltage to various column electrodes to drive the pixels in the selected row to their desired optical state. (The aforementioned voltages are for a common front electrode, conventionally supplied from a nonlinear array to the opposite side of the electro-optical medium and extending throughout the display.) After a pre-selected interval known as the "line address time," the selected row is deselected, the next row is selected, and the voltage on the column driver is changed so that the next line of the display is written.This process is repeated so that the entire display is written line by line.

[0098] Conventionally, each pixel electrode has an associated capacitor electrode such that the pixel electrode and the capacitor electrode form a capacitor. See, for example, International Patent Application No. WO2001007961. In some embodiments, an N-type semiconductor (e.g., amorphous silicon) may be used to form a transistor, and the “selective” and “deselective” voltages applied to the gate electrode can be positive and negative, respectively.

[0099] Figure 3 of the attached drawings depicts an exemplary equivalent circuit of a single pixel of an EPD. As shown, this circuit includes a capacitor 10 formed between the pixel electrode and the capacitor electrode. The electrophoretic medium 20 is represented as a capacitor and resistor in parallel. In some cases, direct or indirect coupling capacitance 30 (commonly referred to as “parasitic capacitance”) between the gate electrode of the transistor associated with the pixel and the pixel electrode can generate unwanted noise in the display. Typically, the parasitic capacitance 30 is much smaller than that of the storage capacitor 10, and when a pixel row of the display is selected or deselected, the parasitic capacitance 30 can result in a small negative offset voltage to the pixel electrode, also known as “kickback voltage”, which is typically less than 2 volts. In some embodiments, to compensate for the unwanted “kickback voltage”, a common potential V com However, it may also be supplied to the top electrode and capacitor electrode associated with each pixel, and therefore, V com The kickback voltage (V KB When set to a value equal to ), all voltages supplied to the display are offset by the same amount, and net DC imbalance does not need to be present.

[0100] However, V comProblems can arise when the voltage is set to a level that is not compensated for kickback voltage. This can occur when it is desired to apply a higher voltage to the display than is available from the backplane alone. For example, the maximum voltage applied to the display is, for example, V com It is well known in the art that if -V is supplied while the backplane is supplied with nominal options of +V, 0, or -V, it can be doubled. In this case, the maximum voltage experienced is +2V (i.e., at the backplane relative to the top surface), while the minimum voltage is zero. If a negative voltage is required, V com The potential must be raised to at least zero. Therefore, the waveforms used to address the display with positive and negative voltages using top-plane switching must be one more V com Each voltage setting must have a specific frame assigned to it.

[0101] A set of waveforms for driving a color EPD having four particles is described in U.S. Patent No. 9,921,451, which is incorporated herein by reference. In U.S. Patent No. 9,921,451, seven different voltages, namely three positive voltages, three negative voltages, and a zero voltage, are applied to the pixel electrodes. However, in some cases, the maximum voltage used with these waveforms is higher than the voltage that amorphous silicon thin-film transistors can handle. In such cases, a suitable high voltage can be obtained by using top-plane switching. (As described above) V com But intentionally V KB If set to this, a separate power supply may be used. However, when top-plane switching is used, V com Using the same number of individual power supplies as the number of settings is costly and inconvenient. Furthermore, top-plane switching is known to increase kickback, thereby degrading the stability of color states.

[0102] Display devices can be constructed using electrophoretic fluids in several ways known in the prior art. The electrophoretic fluid may be encapsulated in microcapsules or incorporated into microcell structures, which are then sealed with a polymer layer. The microcapsules or microcell layers may be coated or embossed onto a plastic substrate or film having a transparent coating of conductive material. This assembly may be laminated onto a backplane having pixel electrodes using a conductive adhesive. Alternatively, the electrophoretic fluid may be dispensed directly onto a thin open-cell grid arranged on a backplane containing the active matrix of pixel electrodes. The filled grid can then be sealed on top with an integrated protective sheet / light-transmitting electrode.

[0103] Figure 4 shows a schematic cross-sectional view (not to exact scale) of a display structure 200 including an electrophoretic medium. In the display 200, the electrophoretic fluid is shown as being confined in a microcell, but other structures, e.g., microcapsules, may also be used. A substrate 202, which may be glass or plastic, supports pixel electrodes 204 that are individually addressed segments or associated with thin-film transistors in an active matrix array. (The combination of the substrate 202 and electrodes 204 is conventionally called the backplane of the display.) Layer 206 is an optional dielectric layer according to the present invention applied to the backplane. (A method for depositing a suitable dielectric layer is described in U.S. Patent Application Publication No. 2020 / 0348576, incorporated by reference.) The front of the display includes a transparent substrate 222 supporting a transparent conductive coating 220. Above the electrode layer 220 is an optional dielectric layer 218. Layer (or more layers) 216 is a polymer layer which may include a primer layer for bonding the microcell to the transparent electrode layer 220 and some residual polymer including the bottom of the microcell. The walls of the microcell 212 are used to contain the electrophoretic fluid 214. The microcell is sealed with layer 210 and the entire front plane structure is bonded to the back plane using a conductive adhesive layer 208. The process for forming the microcell is described in the prior art, e.g., U.S. Patent No. 6,930,818. In some examples, the microcell is less than 20 μm deep, e.g., less than 15 μm deep, e.g., less than 12 μm deep, e.g., about 10 μm deep, e.g., about 8 μm deep.

[0104] Most commercially available EPDs use amorphous silicon-based thin-film transistors (TFTs) in the construction of the active matrix backplane (202 / 024) due to the wider availability of manufacturing equipment and the cost of various starting materials. Unfortunately, amorphous silicon thin-film transistors become unstable when supplied with gate voltages that would allow switching of voltages higher than approximately + / -15V. Nevertheless, as described below, the performance of ACePs is improved when the magnitude of high positive and negative voltages exceeding + / -15V is tolerated. Thus, as described in previous disclosures, the improved performance is achieved by further varying the bias of the upper light-transmitting electrode relative to the bias on the backplane pixel electrode, which is also known as top-plane switching. Thus, if a voltage of +30V (relative to the backplane) is required, the top plane can be switched to -15V while the appropriate backplane pixels are switched to +15V. A method for driving a four-particle electrophoresis system using top-plane switching is described in detail, for example, U.S. Patent No. 9,921,451.

[0105] These waveforms illustrate each pixel of the display as 30V, 15V, 0, -15V, and -30V, +V high , +V low , 0, -V low , and -V high It is required that it can be driven by five different addressing voltages, as specified. In practice, it may be preferable to use more addressing voltages. Three voltages (i.e., +V high , 0, and -V high If only the voltage V is available, high However, by addressing using pulses with a duty cycle of 1 / n, it is possible to use lower voltages (e.g., V high It may be possible to achieve the same result as addressing in / n (where n is a positive integer > 1).

[0106] Figure 5 shows a typical waveform (in a simplified form) used to drive the aforementioned four-particle color electrophoresis display system. Such waveforms have a “push-pull” structure, meaning they consist of a dipole with two pulses of opposite polarity. The magnitude and length of these pulses determine the resulting color. The waveform has five such voltage levels. Figure 5 shows positive and negative voltages for high and low voltages, as well as zero volts. Typically, “low” (L) refers to a range of approximately 5–15V, and “high” (H) refers to a range of approximately 15–30V. Generally, the higher the magnitude of the “high” voltage, the better the color gamut achieved by the display. An additional “medium” (M) level, typically about 15V, may also be used, although the value of M will depend to some extent on the particle composition and the environment of the electrophoretic medium.

[0107] Figure 5 shows the simplest dipole required to form a color, but it should be understood that actual waveforms may use multiple repetitions of these patterns or other non-periodic patterns.

[0108] Naturally, achieving the desired color using the drive pulses in Figure 5 is conditional on starting the process from a known state where the particle is unlikely to be the last color displayed on the pixel. Therefore, a series of reset pulses precede the drive pulses, which increase the amount of time required to update the pixel from the first color to the second color. The reset pulses are described in detail in U.S. Patent No. 10,593,272 and are taken by reference. The length of these pulses (refresh and address) and any pauses (i.e., periods of zero voltage between them) may be chosen so that the entire waveform (i.e., the integral of voltage over time over the entire waveform) is DC balanced (i.e., the integral of voltage over time is substantially zero). DC balance can be achieved by adjusting the length of the pulses and pauses in the reset phase so that the net impulse supplied in the reset phase is equal in magnitude and opposite in sign to the net impulse supplied in the address phase, when the display is switched to a particular desired color. However, as shown in Figures 2B-2E, the starting state for the eight primary colors is either black or white, which can be achieved using a sustained low-voltage drive pulse. The ease of achieving this starting state further reduces the update time between states, which is more comfortable for the user and also reduces the amount of power consumed (and thus increases battery life).

[0109] Furthermore, the aforementioned discussion of waveforms, specifically the discussion of DC equilibrium, ignores the kickback voltage problem. In reality, as mentioned above, all backplane voltages are affected by the kickback voltage V KB It is offset by an amount equal to the voltage supplied by the power supply. Therefore, if the power supply used provides three voltages, +V, 0, and -V, the backplane is actually offset by the voltage V+V KB , V KB , and -V+V KB You will receive (V KBNote that in the case of amorphous silicon TFTs, this is usually a negative number. However, the same power supply will supply +V, 0, and -V to the front electrode without any kickback voltage offset. Therefore, for example, if the front electrode is supplied with -V, the display will have a maximum voltage of 2V+V KB and minimum V KB It will incur costs and inconvenience. KB Instead of using a separate power supply to provide voltage to the front electrode, the waveform is such that the front electrode receives positive voltage, negative voltage, and V KB It may be divided into supplied portions.

[0110] (EPD with front lighting system) Various embodiments disclosed herein relate to an EPD comprising an ambient light sensor and a front light for adaptively restoring whiteness and balancing colors on the display.

[0111] U.S. Patent No. 8,690,408, which is incorporated herein by reference in its entirety, discloses an exemplary front illumination system for an EPD, implemented as an ILU, for illuminating the EPD viewing surface in a low ambient light environment. Figure 7, reproduced from that patent, shows a simplified cross-sectional view of an illuminated display device 120. A light source 122 injects or emits light 124 into a waveguide 126. The light 124 may be injected at an angle such that total internal reflection (TIR) ​​is obtained. Since the injected light 124 undergoes total internal reflection within the waveguide 126, a frustrator 128 is provided to cause frustrated light 130 to exit from the bottom surface 132 of the waveguide 126. The frustrated light 130 is directed downward onto the EPD 180. Since the waveguide 126 is positioned above the array of pixel elements 140 of the EPD 180, obstructed light 130 is incident on the top surface 142 of the EPD, and thus illuminates the array of pixel elements 140 from above. The reflected light 144 is reflected off the top surface 142 of the array of pixel elements 140 and returns into the waveguide 126. The reflected light 144 propagates through the waveguide 126 and exits from the outer surface 146 of the waveguide 126. Thus, the reflected light 144 presents the image generated by the array of pixel elements 140 to the viewer's eye 150. Therefore, Figure 7 illustrates a top-illumination arrangement in which the array of pixel elements 140 is illuminated from above.

[0112] The waveguide 126 of the ILU can be a light guide sheet laminated on the front-viewing side of the EPD 180. The light source 122 can be one or more edge-mounted light-emitting diodes (LEDs) coupled to the light guide sheet, where light propagates parallel to the EPD surface by total internal reflection. The frustrator 128 can be a distribution of light-shifting microstructures (scattering, reflection, or refraction) that direct light towards the reflective electrophoretic layer.

[0113] Figure 8 is a simplified block diagram showing an exemplary electrophoretic display apparatus 300 according to one or more embodiments. The apparatus 300 includes an EPD unit 302, which can be any type of EPD known in the art, including ACeP, EPDs having CFA, and other EPDs as described above.

[0114] The device 300 also includes one or more ambient light sensors 304 embedded in the EPD unit housing on the viewing surface 320 to detect the level of ambient light incident on the viewing surface 320 of the EPD unit 302.

[0115] The device 300 also includes a front light unit 306 (which may be implemented as an ILU) positioned on the viewing surface 320 of the EPD unit 302. The front light unit 306 illuminates the viewing surface 320, for example, similar to the ILUs disclosed above.

[0116] A control system 308, including one or more processors 310, controls the operation of the EPD unit 302 and the front light unit 306. One or more processors 310 can execute a display driver process 312 for driving the EPD unit 302 between multiple optical states. Similar to the EPD described above, the control system 308 can thus control the source driver of the EPD unit 302 to apply a selected voltage from the power supply 318 to selected pixel electrodes to render the display output.

[0117] One or more processors 310 also execute a front light controller process 314 for controlling the operation of the front light unit 306 in response to the output received from the ambient light sensor 304, as will be described in more detail below. The processes of the display driver 312 and the front light controller 314 can be stored in memory 316 within the control system 308. A power supply (e.g., a battery) 318 provides power to the various components of the device.

[0118] One or more processors 310 may include a microcontroller, a microprocessor, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or any general-purpose or dedicated circuit that can be programmed or configured to perform the functions described herein.

[0119] The ambient light sensor 304 is a surface light sensor having the optical and spectral characteristics of an illuminometer or lux meter.

[0120] The ambient light sensor 304 detects the illuminance E of the ambient light incident on the viewing surface 320 of the EPD 302. AMB The white pigment displayed on the EPD viewing surface 320 reflects not only the light emitted by the front light unit 306 but also ambient light. From the physical model of light reflection from the display, the white state (L W The total brightness of the display in ) is due to reflected front light illumination E FL Brightness L W , FL And reflected ambient light E AMB Brightness L W , AMB It has been determined that it is the sum of the following. Illuminance E incident on the white pigment of the EPD from the front light unit 306 FL This is the luminous flux Φ emitted from the front LED lights. FL Area A of the front light seat FL It is calculated by dividing by (equal to the area of ​​the display or viewing surface). Light loss is the front light efficiency η FL It will be taken into consideration.

[0121] Assuming diffuse illumination from the front light and ambient light, the luminance of the EPD display for each of these illumination components is given by its diffuse reflectance R in the white state. W It is the product of and illuminance E divided by π. Total display luminance L in the white state under combined forward and ambient lighting. W The following applies: [ka]

[0122] Ambient illuminance E AMB This is not controllable because it can change at any time due to factors such as changes in the display's position, the time of day, or when the interior lights are switched on or off.

[0123] To account for the effects of changing lighting, the ambient illuminance E incident on the EPD is considered. AMB (and changes thereto) are measured using the ambient light sensor 304. The measured ambient illuminance E AMB Next, the luminous flux output Φ of the LEDs in the front light unit 306 FL Used in an automatic control loop to change the luminous flux output Φ FL It is directly proportional to the front light illuminance EFL incident on the EPD. FL The control is, 1. EPD's full white state brightness L W To increase the value and match the total white state luminance of a 100% white diffuse reflector (R=1, 100% white Lambert reflector) under the same ambient lighting as an EPD, 2. Recommended minimum level, e.g., 30 cd / m² 2 (Brightness of white paper under approximately 100 lx lighting) A constant minimum total display brightness L in low-light or dark viewing environments. W , min Maintaining The following two conditions must be met.

[0124] To satisfy condition 1, the difference ΔL between the total luminance of the EPD in a white state and the luminance of a 100% white diffuse reflector must be the same for both ambient illuminance E. AMB Under these conditions, it is compensated for by the front light illumination. [ka]

[0125] Front light brightness L FL The illuminance EFL and the measured white state reflectance R W By replacing with, the lower white state R of the EPD compared to an ideal Lambert reflector (R = 1) W To compensate for, the front light has an ambient illuminance E at any level​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​ From darkness to threshold E AMB,min Up to a certain point, the adaptive front light maintains a constant overall display brightness regardless of ambient lighting. Once the threshold is exceeded, it maintains the overall ambient display brightness at the same level as a 100% white diffuse reflector would have under the same ambient lighting.

[0129] Figure 9 is a simplified flowchart illustrating an exemplary process 400 for adaptively controlling the brightness of an EPD display according to one or more embodiments. In step 402, one or more signals indicating a detected level of ambient illuminance incident on the viewing surface 320 of the electrophoretic device 302 are received from the ambient light sensor 304. In step 404, the detected level of ambient illuminance is compared to a predetermined threshold level. In step 406, if the detected level of ambient illuminance is less than or equal to the predetermined threshold level, the front light unit 306 is controlled to adaptively maintain a generally constant viewing surface brightness (including front light illuminance and light reflected by the viewing surface from ambient illuminance), regardless of the detected level of ambient illuminance. In step 408, if the detected level of ambient illuminance is greater than the predetermined threshold level, the front light unit is controlled to maintain the viewing surface illuminance at generally the same level as a white diffuse reflector under the same detected level of ambient illuminance. This process is repeated. [Examples]

[0130] Figure 10A shows a 100% white Lambertian reflector (L 100%W ) compared to 53L * White (L W Figure 10B is a graph showing the white state luminance without front light relative to ambient illuminance and the luminance difference that needs to be added by front light reflection to compensate for the luminance loss ΔL due to white state reflection below 100% for an exemplary 6-inch CFA color EPD having ).

[0131] This example shows a density of only 75 cd / m². 2 The maximum front light brightness is sufficient to illuminate the display under ambient light levels of up to 300 lx, typical for indoor or office lighting. This can be achieved with a 6-inch CFA display with LEDs having a total output of 12 lm. Front lights commonly used in EPDs are at least 125 cd / m². 2 This achieves a white state brightness, which will enable white state compensation in an additional range up to 500 lx ambient illuminance with a 20 lm LED.

[0132] Figure 11A shows the front light off L with and without minimum threshold switching. W (FL off) and front lights on L W This shows the total white state luminance against ambient illuminance for EPDs with (FL on). Figure 11B shows that the front light illuminance below the threshold is not constant. AMB Starting from its highest level of 448 lx at =0 (darkness), E reaches its minimum of 355 lx at the threshold. AMB As the threshold increases, it decreases. This explains that the luminance contribution from ambient light reflection increases when the total display brightness is kept constant. Above the threshold, the front light illuminance increases with the ambient illuminance so that the display matches a 100% white standard.

[0133] (EPD with adaptive color balancing) In one or more further embodiments, an EPD is disclosed having an ambient light sensor and a front light for adaptively balancing the colors of the EPD viewing surface. The ambient light sensor in this embodiment comprises a tricolor sensor having spectral sensitivity in three RGB color channels. (The wavelengths of the RGB channels are approximately 620–750 nm for red, approximately 450–495 nm for blue, and approximately 495–570 nm for green.) The sensor can detect illuminance from the G channel using standard colorimetric analysis and can also detect the chrominance and color temperature of the ambient light from all three RGB channels.

[0134] In this embodiment, the front light is a three-color system including RGB LEDs instead of white LEDs. Each of the three RGB color channels of the front light can be controlled independently, for example, by changing the current applied to each LED to change the color temperature of the front lighting.

[0135] The combination of a tricolor sensor for detecting ambient illuminance and chrominance with a tricolor front light allows for control of the chrominance of the front light illumination, thereby compensating for the off-white white state of the EPD.

[0136] The control algorithm is applied independently to each of the three color channels (R, G, B). This is based on the ambient tricolor irradiance (E(R), E(G), E(B)) measured by the tricolor sensor. AMB The three-color irradiance (E(R), E(G), E(B)) that the front light must provide at any level. FL Calculate the non-neutral white state (R(R), R(G), R(B)) with an EPD lower than 1. W To compensate for this, the overall brightness and color of the display's white state in the combined front light and ambient light are optimized for an ideal Lambertian reflector (R(R), R(G), R(B)). WEquivalent to the value =1. The threshold for switching from adaptive display brightness and color to constant display brightness and color is (E(R), E(G), E(B)). AMB,min That is the case. [ka]

[0137] Each front light color channel (E(R), E(G), E(B)) FL The irradiance can be controlled by changing the current applied to each individual RGB LED in the front light unit.

[0138] In one or more embodiments, the color channels of the tricolor sensor and the front light are realized with spectral characteristics that approximate the CIE 1931 (X, Y, Z) color channels.

[0139] In one or more embodiments, the ambient light sensor is a spectral sensor having more than three spectral channels. Furthermore, the front light has more than three color channels that can be independently controlled to change the chrominance of the front light illumination.

[0140] The combination of a spectral sensor for detecting ambient illuminance and chrominance, along with a tricolor or multispectral front light, allows for finer control of the chrominance of the front light illumination, better compensating when the white state of the EPD is off-white.

[0141] The control algorithm is applied independently to each spectral channel λ. This is based on the ambient spectral irradiance E measured by the spectral sensor. AMB The tricolor or spectral irradiance E that the front light must provide at any level of (λ) FL Calculate (λ) and determine the non-neutral white state R of EPD that is lower than 1. W(λ) is compensated for, making the overall brightness and color of the display's white state in the combined front light and ambient light equal to that of an ideal Lambert reflector R(λ)=1. The threshold for switching from adaptive display brightness and color to constant display brightness and color is E AMB,min (λ) [ka]

[0142] (Examples) The following examples utilized an experimental ACeP display that was off-white with a low white state and an undesirable yellowish tint. Ambient illumination, measured with a tricolor or spectroscopic sensor, was approximately CIE D50. The display device included a front light with three independently controllable LEDs having R, G, and B peak wavelengths and full widths at half maximum of 630±16, 540±37, and 460±18 nm, respectively.

[0143] The diffuse spectral reflectance in the white state was measured using a spectrophotometer. The 1976 CIELAB color calculated from its off-white characteristics and spectral reflectance is L * =70, a * = -5.4, b * = 2.3. Figure 12A shows the measured spectral reflectance in the white state, and Figure 12B shows the reflected color. Figure 12C shows the color under ambient D50 illumination. Ambient illumination with CIE D50 spectral characteristics amplifies the yellow tint, and its CIELAB color, calculated from the spectrally displayed radiance measured in the white state shown in Figure 12D, is L * =70, a * = -9.7, b * It had a value of =10.2 and was even more colorful.

[0144] 900 lx front lighting was used in combination with 300 lx D50 ambient lighting. Applying the control algorithm to this combined lighting and the white state reflectance spectrum shown in Figure 12A, the relative current to be applied to the RGB LEDs was determined to be I R =100%, I G =76%, and I B =73% was obtained. The resulting white state in the combined front light and ambient lighting shown in Figure 13A is 100% white diffuse reflector (L * Not only is the brightness equal to (a =100), but the color balance is also good. * =b * (=0). Figures 12B and 12C show the display colors in ambient D50 lighting with and without front light illumination, respectively. A further advantage of the brighter, more balanced white is the visible improvement in the brightness and saturation of other display colors. The color gamut with the adaptive color balancing front light switched on, as shown in Figure 13D, is not only expanded but also L * =100, a * =b * It centers around neutral white, where =0.

[0145] Adaptive EPD front lighting offers significant technical advantages over conventional technologies. The model-based process improves the paper-like appearance of the white state by increasing the white state of the EPD across a wide range of levels, from ambient illumination levels to the level of a 100% white diffuse reflector. Furthermore, adaptive front lighting removes hues from the white state. This compensates for technically related shortcomings of EPDs, such as low white state brightness and undesirable hues in the white state. Front lighting adaptively increases the white state and removes hues when the illuminance and color temperature of ambient lighting change. Front lighting provides minimal, constant display brightness and chrominance in dim and dark viewing environments. This minimizes battery consumption due to front light use, extending battery life. Adaptive front lighting also minimizes exposure to potentially harmful blue light by keeping front light radiance at the minimum necessary level, providing users with a more comfortable and eye-safe viewing experience.

[0146] The front light control system processes described above can be implemented in software, hardware, firmware, or any combination thereof. These processes are preferably implemented by one or more computer programs running on one or more processors. Each computer program may be a set of instructions (program code) in a code module residing in the control system's random access memory. Until requested by the controller, the set of instructions may be stored in another computer memory or on another computer system and downloaded via the Internet or other networks.

[0147] While several exemplary embodiments have been described, it should be understood that various changes, modifications, and improvements will be readily conceivable to those skilled in the art. Such changes, modifications, and improvements are intended to form part of this disclosure and to be within the spirit and scope of this disclosure. While some embodiments presented herein involve specific combinations of functional or structural elements, it should be understood that those functions and elements may be combined in other ways according to this disclosure to achieve the same or different purposes. In particular, the actions, elements, and features described in relation to one embodiment are not intended to be excluded from similar or other roles in other embodiments.

[0148] Furthermore, the elements and components described herein may be further divided into additional components to perform the same function, or they may be combined together to form fewer components.

[0149] Therefore, the above description and accompanying drawings are merely examples and are not intended to be limiting.

Claims

1. Electrophoresis display device, An electrophoresis device having a visible surface, A drive system coupled to the electrophoresis device for driving the electrophoresis device between multiple optical states, One or more ambient light sensors on the viewing surface of the electrophoretic device for detecting the level of ambient illuminance incident on the viewing surface, A front light unit positioned above the viewing surface of the electrophoresis device to illuminate the viewing surface, A front light control system coupled to one or more ambient light sensors and the front light unit. Equipped with, The aforementioned front light control system is (a) Receiving one or more signals from one or more ambient light sensors that indicate the detected level of ambient illuminance incident on the viewing surface of the electrophoresis device, (b) Comparing the detected ambient illuminance level with a predetermined threshold level, (c) When the detected level of ambient illuminance is below the predetermined threshold level, the front light illuminance incident from the front light unit onto the viewing surface is controlled, and a constant viewing surface brightness is adaptively maintained, including the front light illuminance and the light reflected by the viewing surface from the ambient illuminance, regardless of the detected level of ambient illuminance. (d) When the detected ambient illuminance level is greater than the predetermined threshold level, the front light illuminance incident from the front light unit onto the viewing surface is controlled to maintain the viewing surface luminance at generally the same level as that of the white diffuse reflector under the same detected ambient illuminance level, wherein the white diffuse reflector is L ＊ It has a Lambertian reflecting surface having a value of 100, (e) Repeat steps (a) through (d) multiple times An electrophoretic display device configured to perform the following.

2. The illuminance of the front light incident on the viewing surface from the front light unit is controlled in step (c) according to the following formula: [Math 1] Here, E ＦＬ This is the illuminance incident on the viewing surface from the front light unit, E ＡＭＢ This is the detected level of ambient illuminance incident on the viewing surface, and E ＡＭＢ，ＭＩＮ R is the predetermined threshold level. Ｗ The electrophoretic display apparatus according to claim 1, wherein is the diffuse reflectance of the viewing surface in a white state.

3. The illuminance of the front light incident on the viewing surface from the front light unit is controlled in step (d) according to the following formula: [Math 2] Here, E ＦＬ is the illuminance incident on the viewing surface from the front light unit, and E ＡＭＢ is the detected level of the ambient illuminance incident on the viewing surface, and E ＡＭＢ，ＭＩＮ is the predetermined threshold level, and R Ｗ is the diffuse reflectance of the viewing surface in the white state. The electrophoretic display device according to claim 1.

4. The electrophoretic display apparatus according to claim 1, wherein the threshold level is between 3 lx and 500 lx.

5. The electrophoretic display apparatus according to claim 1, wherein the threshold level is approximately 94 lx.

6. The one or more ambient light sensors include at least one tricolor sensor for detecting ambient tricolor irradiance in the red channel, green channel, and blue channel incident on the viewing surface. The front light unit includes at least one tricolor light source having independently controllable red, green, and blue channels. The electrophoretic display apparatus according to claim 1, wherein the front light control system is further configured to control the chrominance of illumination from the at least one tricolor light source to compensate for the off-white white state of the electrophoretic device.

7. The one or more ambient light sensors include at least one tricolor sensor for detecting ambient tricolor irradiance in the red channel, green channel, and blue channel incident on the viewing surface. The front light unit includes at least one tricolor light source having independently controllable red, green, and blue channels. The aforementioned front light control system further, (a) Receiving one or more signals from one or more ambient light sensors that indicate the detected level of ambient tricolor irradiance in the red channel, the green channel, and the blue channel incident on the viewing surface, (b) Comparing the detected level of ambient tricolor irradiance with a predetermined threshold level in each of the red channel, the green channel, and the blue channel, (c) When the detected level of ambient tricolor irradiance is below the predetermined threshold level in any of the red channel, the green channel, and the blue channel, the front light irradiance incident on the viewing surface from the front light unit in that channel is controlled, and a constant level of tricolor irradiance, including the front light irradiance and the tricolor irradiance reflected by the viewing surface from the ambient tricolor irradiance, is adaptively maintained regardless of the detected level of ambient tricolor irradiance. (d) When the detected level of ambient tricolor irradiance is greater than the predetermined threshold level in any of the red channel, the green channel, and the blue channel, the front light irradiance incident on the viewing surface from the front light unit in that channel is controlled to maintain the tricolor irradiance of the viewing surface at a level generally the same as that of an ideal Lambertian reflector under the same detected level of ambient tricolor irradiance, (e) Repeat steps (a) through (d) multiple times The electrophoretic display apparatus according to claim 1, configured to perform the following:

8. The illuminance of the front light incident on the viewing surface from the front light unit is controlled in step (c) according to the following formula: [Math 3] Here, E(R) ＦＬ , E(G) ＦＬ , and E(B) ＦＬ These are the levels of tricolor irradiance provided by the front light unit in the red channel, the green channel, and the blue channel, respectively, and E(R) ＡＭＢ , E(G) ＡＭＢ , and E(B) ＡＭＢ These are the detected levels of ambient tricolor irradiance incident on the viewing surface in the red channel, the green channel, and the blue channel, respectively, and E(R) ＡＭＢ，ＭＩＮ , E(G) ＡＭＢ，ＭＩＮ , and E(B) ＡＭＢ，ＭＩＮ These are the predetermined threshold levels in the red channel, the green channel, and the blue channel, respectively, and R(R) Ｗ , R(G) Ｗ , and R(B) Ｗ The electrophoretic display apparatus according to claim 7, wherein is the diffuse reflectance coefficient of the viewing surface in the white state in the red channel, the green channel, and the blue channel, respectively.

9. The illuminance of the front light incident on the viewing surface from the front light unit is controlled in step (d) according to the following formula: [Math 4] Here, E(R) ＦＬ , E(G) ＦＬ , and E(B) ＦＬ These are the levels of tricolor irradiance provided by the front light unit in the red channel, the green channel, and the blue channel, respectively, and E(R) ＡＭＢ , E(G) ＡＭＢ , and E(B) ＡＭＢ These are the detected levels of ambient tricolor irradiance incident on the viewing surface in the red channel, the green channel, and the blue channel, respectively, and E(R) ＡＭＢ，ＭＩＮ , E(G) ＡＭＢ，ＭＩＮ , and E(B) ＡＭＢ，ＭＩＮ These are the predetermined threshold levels in the red channel, the green channel, and the blue channel, respectively, and R(R) Ｗ , R(G) Ｗ , and R(B) Ｗ The electrophoretic display apparatus according to claim 7, wherein is the diffuse reflectance coefficient of the viewing surface in the white state in the red channel, the green channel, and the blue channel, respectively.

10. E(R) ＡＭＢ，ＭＩＮ , E(G) ＡＭＢ，ＭＩＮ , and E(B) ＡＭＢ，ＭＩＮ is E(G) ＡＭＢ，ｍｉｎ The electrophoretic display apparatus according to claim 6, determined from the above.

11. The one or more ambient light sensors include at least one multispectral sensor having three or more spectral channels. The front light unit includes at least one multispectral front light having three or more independently controlled spectral channels for changing the chrominance of the front light illumination. The electrophoretic display apparatus according to claim 1.

12. The aforementioned front light control system further, (a) Receiving one or more signals from one or more ambient light sensors that indicate the detected level of ambient spectral irradiance in the spectral channel incident on the viewing surface, (b) In each of the spectral channels, the detected level of ambient spectral irradiance is compared with a predetermined threshold level, (c) When the detected level of ambient spectral irradiance is below the predetermined threshold level in any of the spectral channels, the front light irradiance incident on the viewing surface from the front light unit in that channel is controlled, and a constant level of spectral irradiance, including the front light irradiance and the spectral irradiance reflected by the viewing surface from the ambient spectral irradiance, is adaptively maintained regardless of the detected level of ambient spectral irradiance. (d) When the detected level of ambient spectral irradiance is greater than the predetermined threshold level in any of the spectral channels, the front light irradiance incident on the viewing surface from the front light unit in that channel is controlled to maintain the spectral irradiance of the viewing surface at a level generally the same as that of an ideal Lambertian reflector at the same detected level of ambient spectral irradiance, (e) Repeat steps (a) through (d) multiple times The electrophoretic display apparatus according to claim 11, configured to perform the following:

13. The illuminance of the front light incident on the viewing surface from the front light unit is controlled in step (c) according to the following formula: [Math 5] Here, E ＦＬ (λ) is the illuminance incident on the viewing surface from the front light unit in the spectral channel, and E ＡＭＢ (λ) is the detected level of ambient spectral irradiance incident on the viewing surface in the spectral channel, and R Ｗ The electrophoretic display apparatus according to claim 12, wherein (λ) is the diffuse reflectance coefficient of the viewing surface in the white state in the spectral channel.

14. E ＡＭＢ，ｍｉｎ This is the photopic luminescence efficiency function V(λ) [Math 6] Using E ＡＭＢ，ｍｉｎ An electrophoretic display apparatus according to claim 13, determined from (λ).

15. E ＡＭＢ，ｍｉｎ (λ) is the ambient illuminance threshold level E of approximately 94 lx. ＡＭＢ，ｍｉｎ The electrophoretic display apparatus according to claim 13, as determined in [the specified location].

16. E ＡＭＢ，ｍｉｎ (λ) is the ambient illuminance threshold level E between 3 lx and 500 lx. ＡＭＢ，ｍｉｎ The electrophoretic display apparatus according to claim 13, as determined in [the specified location].

17. The electrophoresis device is Light-transmitting electrodes on the viewing surface, Back electrode and An electrophoretic medium disposed between the light-transmitting electrode and the back electrode. Includes, The electrophoretic medium is Nonpolar fluids and A multiply pigment particle system dispersed in the aforementioned nonpolar fluid and The electrophoretic display apparatus according to claim 1, including the following:

18. The electrophoretic display apparatus according to claim 1, wherein the front light unit comprises a waveguide, a light source for injecting light into the waveguide, and a frustrator for dispersing the light from the waveguide onto the viewing surface.

19. It is a method, (a) Receiving one or more signals from one or more ambient light sensors in the electrophoretic display that indicate the detected level of ambient illuminance incident on the viewing surface of the electrophoretic display, (b) Comparing the detected ambient illuminance level with a predetermined threshold level, (c) When the detected level of ambient illuminance is below the predetermined threshold level, the front light illuminance incident from the front light unit onto the viewing surface is controlled, and a constant viewing surface brightness is adaptively maintained, including the front light illuminance and the light reflected by the viewing surface from the ambient illuminance, regardless of the detected level of ambient illuminance. (d) When the detected ambient illuminance level is greater than the predetermined threshold level, the front light illuminance incident from the front light unit onto the viewing surface is controlled to maintain the viewing surface luminance at generally the same level as that of the white diffuse reflector under the same detected ambient illuminance level, wherein the white diffuse reflector is L ＊ It has a Lambertian reflecting surface having a value of 100, (e) Repeat steps (a) through (d) multiple times Methods that include...

20. A control system for controlling the operation of a front light unit of an electrophoretic display, wherein the control system comprises one or more controllers, and the one or more controllers (a) Receiving one or more signals from one or more ambient light sensors in the electrophoretic display that indicate the detected level of ambient illuminance incident on the viewing surface of the electrophoretic display, (b) Comparing the detected ambient illuminance level with a predetermined threshold level, (c) When the detected level of ambient illuminance is below the predetermined threshold level, the front light illuminance incident from the front light unit onto the viewing surface is controlled, and a constant viewing surface brightness is adaptively maintained, including the front light illuminance and the light reflected by the viewing surface from the ambient illuminance, regardless of the detected level of ambient illuminance. (d) When the detected ambient illuminance level is greater than the predetermined threshold level, the front light illuminance incident from the front light unit onto the viewing surface is controlled to maintain the viewing surface luminance at generally the same level as that of the white diffuse reflector under the same detected ambient illuminance level, wherein the white diffuse reflector is L ＊ It has a Lambertian reflecting surface having a value of 100, (e) Repeat steps (a) through (d) multiple times A control system configured to perform the following actions.

21. A control system, At least one processor, Memory associated with the at least one processor, A program stored in the memory for controlling the operation of the front light unit of an electrophoretic display, wherein the program includes a plurality of instructions, and when the plurality of instructions are executed by the at least one processor, the at least one processor, (a) Receiving one or more signals from one or more ambient light sensors in the electrophoretic display that indicate the detected level of ambient illuminance incident on the viewing surface of the electrophoretic display, (b) Comparing the detected ambient illuminance level with a predetermined threshold level, (c) When the detected level of ambient illuminance is below the predetermined threshold level, the front light illuminance incident from the front light unit onto the viewing surface is controlled, and a constant viewing surface brightness is adaptively maintained, including the front light illuminance and the light reflected by the viewing surface from the ambient illuminance, regardless of the detected level of ambient illuminance. (d) When the detected ambient illuminance level is greater than the predetermined threshold level, the front light illuminance incident from the front light unit onto the viewing surface is controlled to maintain the viewing surface luminance at generally the same level as that of the white diffuse reflector under the same detected ambient illuminance level, wherein the white diffuse reflector is L ＊ It has a Lambertian reflecting surface having a value of 100, (e) Repeat steps (a) through (d) multiple times The program and A control system including...

22. Electrophoresis display device, An electrophoresis device having a visible surface, A drive system coupled to the electrophoresis device for driving the electrophoresis device between multiple optical states, The electrophoretic device includes one or more ambient light sensors on the viewing surface, which include at least one tricolor sensor for detecting ambient tricolor irradiance in the red channel, green channel, and blue channel incident on the viewing surface, A front light unit positioned above the viewing surface of the electrophoretic device to illuminate the viewing surface, comprising at least one tricolor light source having independently controllable red, green, and blue channels, A front light control system coupled to one or more ambient light sensors and the front light unit controls the chrominance of illumination from at least one tricolor light source and compensates for the off-white white state of the electrophoresis device. Equipped with, The aforementioned front light control system is (a) Receiving one or more signals from one or more ambient light sensors that indicate the detected level of ambient tricolor irradiance in the red channel, the green channel, and the blue channel incident on the viewing surface, (b) Comparing the detected level of ambient tricolor irradiance with a predetermined threshold level in each of the red channel, the green channel, and the blue channel, (c) When the detected level of ambient tricolor irradiance is below the predetermined threshold level in any of the red channel, the green channel, and the blue channel, the front light irradiance incident on the viewing surface from the front light unit in that channel is controlled, and a constant level of tricolor irradiance, including the front light irradiance and the tricolor irradiance reflected by the viewing surface from the ambient tricolor irradiance, is adaptively maintained regardless of the detected level of ambient tricolor irradiance. (d) When the detected level of ambient tricolor irradiance is greater than the predetermined threshold level in any of the red channel, the green channel, and the blue channel, the front light irradiance incident on the viewing surface from the front light unit in that channel is controlled to maintain the tricolor irradiance of the viewing surface at a level generally the same as that of an ideal Lambertian reflector under the same detected level of ambient tricolor irradiance, (e) Repeat steps (a) through (d) multiple times An electrophoretic display device configured to perform the following.

23. Electrophoresis display device, An electrophoresis device having a visible surface, A drive system coupled to the electrophoresis device for driving the electrophoresis device between multiple optical states, The electrophoretic device includes one or more ambient light sensors on the viewing surface, each including at least one multispectral sensor having three or more spectral channels, A front light unit positioned above the viewing surface of the electrophoresis device to illuminate the viewing surface, comprising at least one multispectral front light having three or more independently controlled spectral channels, A front light control system coupled to one or more ambient light sensors and the front light unit controls the spectral irradiance from the at least one multispectral front light. Equipped with, The aforementioned front light control system is (a) Receiving one or more signals from one or more ambient light sensors that indicate the detected level of ambient spectral irradiance in the spectral channel incident on the viewing surface, (b) In each of the spectral channels, the detected level of ambient spectral irradiance is compared with a predetermined threshold level, (c) When the detected level of ambient spectral irradiance is below the predetermined threshold level in any of the spectral channels, the front light irradiance incident on the viewing surface from the front light unit in that channel is controlled, and a constant level of spectral irradiance, including the front light irradiance and the spectral irradiance reflected by the viewing surface from the ambient spectral irradiance, is adaptively maintained regardless of the detected level of ambient spectral irradiance. (d) When the detected level of ambient spectral irradiance is greater than the predetermined threshold level in any of the spectral channels, the front light irradiance incident on the viewing surface from the front light unit in that channel is controlled to maintain the spectral irradiance of the viewing surface at a level generally the same as that of an ideal Lambertian reflector at the same detected level of ambient spectral irradiance, (e) Repeat steps (a) through (d) multiple times An electrophoretic display device configured to perform the following.

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