CN113168005B - Illumination system for reflective display - Google Patents

Abstract

A display device (30) includes: a reflective display (38) arranged to provide a first image visible through the viewing surface; and projection means (31-37) arranged to provide a second image visible in reflection on the viewing surface; the reflective display (38) and the projection means (31-37) are mounted on a common frame.

Description

Illumination system for reflective display

Citation of related application

The present application claims the benefit of provisional application sequence No.61/636,070 filed on month 4 and 20 of 2012, provisional application sequence No.61/654,405 filed on month 6 and 1 of 2012, and co-pending non-provisional application sequence No.13/867,633 filed on month 4 and 22 of 2013. However, applicants state herein that the present application also encompasses the rights to the claimed invention at or after date of effective filing of 2013, 3, 16 days according to 37cfr 1.78 (a) (6).

The foregoing co-pending applications, as well as all patents, co-pending applications, and published applications referred to below, are incorporated herein by reference in their entirety.

Technical Field

The present invention relates to an illumination system for a reflective display. More particularly, the present invention relates to a display and apparatus for projecting patterned light onto at least a portion of the display. The invention also relates to a method for directing spatially and spectrally modulated illumination onto a reflective display to improve contrast and colour-to-view (colour) under ambient illumination conditions. Certain embodiments of the present invention relate to a display that is primarily intended for outdoor use, but which may also have some indoor applications (in terms of use in buildings, tents, and other similar structures); these embodiments of the display of the invention utilize a reflective bistable electro-optic display in combination with a light source arranged to illuminate the reflective electro-optic display.

Background

As the term "electro-optic" is applied to a material or display, it is used herein in its conventional sense in the imaging arts to refer to a material having first and second display states that differ in at least one optical property, the material being changed from its first display state to its second display state by application of an electric field to the material. Although the optical property is typically a color perceptible to the human eye, it may be another optical property, such as light transmission, reflection, luminescence, or, in the case of a display for machine reading, a false color in the sense of a change in reflectivity of electromagnetic wavelengths outside the visible range.

The term "gray state" is used herein in its conventional sense in the imaging arts to refer to a state intermediate between the two extreme optical states of a pixel, but does not necessarily mean a black-and-white transition between the two extreme states. For example, several patents and published applications of the Yingk corporation referred to hereinafter describe electrophoretic displays in which the extreme states are white and dark blue such that the intermediate "gray state" is effectively pale blue. In fact, as already mentioned, the change in optical state may not be a color change at all. The terms "black" and "white" may be used hereinafter to refer to the two extreme optical states of the display and should be understood to generally include extreme optical states that are not strictly black and white, such as the white and deep blue states mentioned above. The term "monochrome" may be used hereinafter to refer to a driving scheme that drives a pixel to only its two extreme optical states, without an intermediate gray state.

The terms "bistable" and "bistable" are used herein in their conventional sense in the art to refer to displays comprising display elements having first and second display states, at least one optical property of which is different, such that after any given element is driven to assume its first or second display state with an addressing pulse of finite duration, that state will last at least several times (e.g. at least 4 times) the minimum duration of the addressing pulse required to change the state of that display element after the addressing pulse has terminated. Some particle-based electrophoretic displays supporting gray scale are shown in U.S. Pat. No.7,170,670 to be stable not only in their extreme black and white states, but also in their intermediate gray states, as well as in some other types of electro-optic displays. This type of display is properly referred to as "multi-stable" rather than bistable, but for convenience the term "bistable" may be used herein to encompass both bistable and multi-stable displays.

Several types of electro-optic displays are known. One type of electro-optic display is a rotating bi-color member type, as described in, for example, U.S. Pat. nos. 5,808,783, 5,777,782, 5,760,761, 6,054,071, 6,055,091, 6,097,531, 6,128,124, 6,137,467, and 6,147,791 (although this type of display is commonly referred to as a "rotating bi-color ball" display, the term "rotating bi-color member" is preferably more accurate because in some of the patents mentioned above the rotating member is not spherical). Such displays use a number of small bodies (generally spherical or cylindrical) comprising two or more portions with different optical properties and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, which are filled with liquid to allow the bodies to freely rotate. The appearance of the display is changed by: an electric field is applied to the display, thereby rotating the body to various positions and changing the portion of the body that is seen through the viewing surface. Electro-optic media of this type are typically bistable.

Another type of electro-optic display uses electrochromic media, for example in the form of a nanochromic (nanochromic) film comprising an electrode formed at least in part of a semiconducting metal oxide and a plurality of dye molecules attached to the electrode that are capable of reversible color change; see, e.g., O' Regan, b. Et al, nature 1991,353,737; and Wood, d., information Display,18 (3), 24 (month 3 of 2002). See also Bach, u. Et al, adv. Nanochromic films of this type are described, for example, in U.S. patent No.6,301,038;6,870,657; and 6,950,220. This type of medium is also typically bistable.

Another type of electro-optic display is the electrowetting display developed by philips, which is described in Hayes, r.a. et al, "Video-Speed Electronic Paper Based on Electrowetting", nature,425,383-385 (2003). Such an electrowetting display is shown in us patent No.7,420,549 to be manufacturable in bistable.

One type of electro-optic display that has been the subject of intensive research and development for many years is a particle-based electrophoretic display in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have good brightness and contrast, wide viewing angle, state bistable, and low power consumption properties compared to liquid crystal displays. However, the problem of long-term image quality of these displays has prevented their widespread use. For example, particles that make up electrophoretic displays tend to settle, resulting in an insufficient lifetime of these displays.

As mentioned above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, the fluid is a liquid, but the electrophoretic media may be created using a gaseous fluid; see, e.g., kitamura, T.et al, "Electronic toner movement for electronic paper-like display", IDW Japan,2001, paper HCS 1-1, and Yamaguchi, Y.et al, "Toner display using insulative particles charged triboelectrically", IDW Japan,2001, paper AMD4-4). See also U.S. patent nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic media are susceptible to the same type of problems due to the same particle settling as liquid-based electrophoretic media when used in a direction that allows the particles to settle, such as in a sign where the media are arranged in a vertical plane. In fact, the problem of particle sedimentation in gas-based electrophoretic media is more serious than liquid-based electrophoretic media, because the lower viscosity of gaseous suspension fluids allows faster sedimentation of the electrophoretic particles compared to liquids.

Numerous patents and applications assigned to or in the name of the institute of technology (MIT) and the company eikon of the bureau of technology describe various techniques for electrophoresis of encapsulation and other electro-optic media. Such encapsulated media comprise a plurality of capsules, each capsule itself comprising an internal phase and a wall surrounding the internal phase, wherein the internal phase contains electrophoretically mobile particles in a fluid medium. Typically, the capsules themselves are held in a polymeric binder to form a coherent layer between the two electrodes. The techniques described in these patents and applications include:

(a) Electrophoretic particles, fluids, and fluid additives; see, for example, U.S. Pat. nos. 7,002,728 and 7,679,814;

(b) A capsule body, an adhesive and a packaging process; see, for example, U.S. patent nos. 6,922,276 and 7,411,719;

(c) Films and subassemblies comprising electro-optic materials; see, for example, U.S. Pat. nos. 6,982,178 and 7,839,564;

(d) Backsheets, adhesive layers, and other auxiliary layers and methods for use in displays; see, for example, U.S. patent nos. d485,294;6,124,851;6,130,773;6,177,921;6,232,950;6,252,564;6,312,304;6,312,971;6,376,828;6,392,786;6,413,790;6,422,687;6,445,374;6,480,182;6,498,114;6,506,438;6,518,949;6,521,489;6,535,197;6,545,291;6,639,578;6,657,772;6,664,944;6,680,725;6,683,333;6,724,519;6,750,473;6,816,147;6,819,471;6,825,068;6,831,769;6,842,167;6,842,279;6,842,657;6,865,010;6,967,640;6,980,196;7,012,735;7,030,412;7,075,703;7,106,296;7,110,163;7,116,318;7,148,128;7,167,155;7,173,752;7,176,880;7,190,008;7,206,119;7,223,672;7,230,751;7,256,766;7,259,744;7,280,094;7,327,511;7,349,148;7,352,353;7,365,394;7,365,733;7,382,363;7,388,572;7,442,587;7,492,497;7,535,624;7,551,346;7,554,712;7,583,427;7,598,173;7,605,799;7,636,191;7,649,674;7,667,886;7,672,040;7,688,497;7,733,335;7,785,988;7,843,626;7,859,637;7,893,435;7,898,717;7,957,053;7,986,450;8,009,344;8,027,081;8,049,947;8,077,141;8,089,453;8,208,193 and 8,373,211; U.S. patent application publication No.2002/0060321;2004/0105036; 2005/012306; 2005/012563; 2007/0052757;2007/0097489;2007/0109219;2007/0211002; 2009/012389; 2009/0315044;2010/0265239;2011/0026101;2011/0140744;2011/0187683;2011/0187689;2011/0286082;2011/0286086; 2011/0292321; 2011/0292493;2011/0292494;2011/0297309;2011/0310459; and 2012/0182599; international application publication No. WO 00/38000; european patent Nos. 1,099,207B1 and 1,145,072B1;

(e) Color formation and color adjustment; see, for example, U.S. Pat. nos. 6,017,584;6,664,944;6,864,875;7,075,502;7,167,155;7,667,684;7,791,789;7,956,841;8,040,594;8,054,526;8,098,418;8,213,076 and 8,363,299; U.S. patent application publication No.2004/0263947;2007/0109219;2007/0223079;2008/0023332;2008/0043318;2008/0048970;2009/0004442;2009/0225398;2010/0103502;2010/0156780;2011/0164307;2011/0195629;2011/0310461;2012/0008188;2012/0019898;2012/0075687;2012/0081779; 2012/013009; 2012/0182597;2012/0212462;2012/0157269 and 2012/0326957;

(f) A method for driving a display; see, for example, U.S. Pat. nos. 7,012,600 and 7,453,445;

(g) Application of the display; see, for example, U.S. patent No.6,118,426;6,473,072;6,704,133;6,710,540;6,738,050;6,825,829;7,030,854;7,119,759;7,312,784; and 8,009,348;7,705,824; and 8,064,962; U.S. patent application publication No.2002/0090980; 2004/019681; 2007/0285385 and 2010/0201651; international application publication No. WO 00/36560; and

(h) Non-electrophoretic displays, as in U.S. patent No.6,241,921;6,950,220;7,420,549 and 8,319,759; as described in U.S. patent application publication No. 2012/0293858.

Many of the foregoing patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium may be replaced by a continuous phase, thereby creating a so-called polymer-dispersed electrophoretic display, wherein the electrophoretic medium comprises a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymeric material, and the discrete droplets of electrophoretic fluid within such polymer-dispersed electrophoretic display may be considered capsules or microcapsules, even if no discrete capsule film is associated with each individual droplet; see, for example, the aforementioned U.S. patent No.6,866,760. Thus, for the purposes of this application, such polymer-dispersed electrophoretic media are considered a subclass of encapsulated electrophoretic media.

One related type of electrophoretic display is the so-called "microcell electrophoretic display". In microcell electrophoretic displays, charged particles and fluid are not encapsulated within microcapsules, but rather are held in a plurality of cavities formed within a carrier medium (typically a polymer film). See, for example, U.S. Pat. nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging corporation.

Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media, the particles substantially block the transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays may be made to operate in a so-called "shutter mode" in which one display state is substantially opaque and one display state is light transmissive. See, for example, U.S. Pat. nos. 5,872,552, 6,130,774, 6,144,361, 6,172,798, 6,271,823, 6,225,971 and 6,184,856. Dielectrophoretic displays similar to electrophoretic displays but which rely on variations in the strength of the electric field may operate in a similar mode; see U.S. patent No.4,418,346. Other types of electro-optic displays are also capable of operating in a shutter mode. Electro-optic media operating in shutter mode may be used in multi-layer structures for full color displays; in this configuration, at least one layer adjacent to the viewing surface of the display operates in a shutter mode to expose or hide a second layer further from the viewing surface.

Packaged electrophoretic displays are generally free of the trouble of aggregation and sedimentation failure modes of conventional electrophoretic devices and offer further benefits, such as the ability to print or coat displays on a variety of flexible and rigid substrates. (use of the word "printing" is intended to include all forms of printing and coating including, but not limited to, pre-metered coating such as repair die coating, slot or extrusion coating, slide or laminate coating, curtain coating, roll coating such as roller blade coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, screen printing processes, xerographic processes, thermal printing processes, ink jet printing processes, electrophoretic deposition (see U.S. patent No.7,339,715), and other similar techniques.) thus, the resulting display may be flexible. In addition, because the display medium can be printed (using a variety of methods), the display itself can be manufactured cheaply.

Other types of electro-optic media may also be used in the displays of the present invention.

Regardless of the exact technique used to display data thereon, electro-optic displays can be functionally divided into two broad categories, namely emissive displays where light is emitted from or transmitted through the active layer, and reflective displays where light is reflected from the active layer. The emissive display conveys information by a change in its brightness and can be viewed in the absence of ambient light, while the reflective display conveys information by a change in its reflectivity and cannot be viewed in the absence of ambient light. The emissive display may comprise a material that is electroluminescent in nature (e.g., an organic light emitting diode, OLED), or may be constructed by combining a transmissive or reflective light modulator with a light source; for example, liquid Crystal Displays (LCDs) typically combine a non-emissive light valve layer with a backlight. Digital projectors can be considered emissive displays that include a high intensity light source and light modulator, and appropriate lenses to transfer an image to a remote reflective surface. A disadvantage of all emissive displays is that their contrast and viewing color depend on the intensity of the ambient light. In very bright environments, such as daylight, the emitted light may be submerged and the displayed information is difficult to see. The advantage of a reflective display is that its contrast and viewing color are not affected by the ambient light level, and indeed, in very bright light, the contrast and viewing color of the reflective display may even improve. However, reflective displays are clearly difficult to see in dim light.

Other difficulties arise with reflective displays intended to provide color images. For example, as described in U.S. patent No.8,054,526, a color filter array may be placed such that an appropriate black and white image is observed through the color filter array. Although a color image is thus provided to the viewer, the color filter necessarily reduces the amount of light reflected from the display in the white state, and the available display surface area between the different color primaries must be shared to limit the visual chroma and color gamut.

Many attempts have been made to construct hybrid emissive/reflective displays that are visible under any ambient light. For example, U.S. patent No.7,170,506 describes a hybrid emissive/reflective display in which the intensity of the emitted light is adjustable. As previously mentioned, legibility of information on emissive displays is compromised in very bright ambient light, and the internal brightness of the display must be increased to offset the resulting loss of contrast and visual chroma. Conversely, under very low illumination conditions, the legibility of the information on the reflective display is compromised, and therefore the illumination of the display must be increased accordingly to counteract the loss of contrast and apparent color.

Technical solutions have been developed to increase the contrast of emissive displays, so that such displays can display High Dynamic Range (HDR) still images and moving images. Some of these so-called HDR displays combine an LCD color panel with a bright backlight and modulate the backlight with image information, typically in the form of low-pass filtered luminance channels (this approach is sometimes referred to as "local dimming"). More generally, HDR display may be achieved by combining two low dynamic range (or low contrast) devices, and generating two low dynamic range images from an HDR image for each display device. Examples of modulated backlights include digital projectors or LED arrays described in U.S. patent application publication nos. 2008/0137490 and 2008/013976. However, no equivalent solution currently exists to increase the contrast and the viewing color of reflective displays.

It is well known that the contrast of a spatially uniform combination of a projected image and a reflected image may exceed the contrast of either projected image and reflected image alone. One solution aimed at increasing the contrast of the projected image is described in U.S. patent No.6,853,486 (the' 486 patent). This patent describes an active projection screen that is registered with the projected image and is used to increase the contrast of the projected image under bright ambient light that would reduce the contrast of the same image projected onto a uniformly reflective screen. The main disadvantage of this system is the need to maintain accurate registration between the active projection screen and the projector. Without a rigid mechanical coupling between the projector and the screen, it is difficult and expensive to achieve and maintain the required precise alignment and registration, as well as the solution described in the' 486 patent, i.e., the electronic coupling between the projector and the screen via the "reflection processor" and the "display controller".

The system described in the' 486 patent is a large emissive (projection) display and does not disclose any means of combining a color projection device image and a means for providing a reflected image into the same device so that both images can be provided in registry with each other. In one aspect, the present invention seeks to improve contrast and visual color of reflective displays, particularly hand-held devices, over a wide range of ambient lighting conditions by combining a projected image with a reflected image.

One particular application where bistable electro-optic displays may be useful is outdoor signage, particularly traffic control devices. In the past, traffic signals and other traffic control signals have relied on incandescent bulbs to generate light; recently, light Emitting Diodes (LEDs) have begun to be used for this purpose. Incandescent bulbs and LEDs (and virtually all other emitting light sources) require a continuous power source, typically mains ac power, and any interruption in power due to equipment failure, weather conditions, or traffic accidents will result in traffic signal failure, traffic accidents, and a large interruption in traffic flow. Conventional traffic signals have other drawbacks including:

(a) False signals may be generated due to sunlight and sun glare; conventional traffic signals must overcome ambient light conditions, including specular reflection from various surfaces of the sign, which can make it difficult to clearly and effectively discern the on and off state of a particular sign; even the usual use of light baffles and high power LEDs or incandescent lamps in the range of 25-100W do not completely overcome these problems;

(b) Traffic lights are located outdoors and are therefore subject to harsh mechanical and environmental conditions; they must withstand mechanical damage and remain operable even if subjected to vandalism, mechanical shock and impact, extreme temperatures, and exposure to ultraviolet radiation;

(c) The total cost of ownership, particularly the cost of operation, is a very important factor in the use of traffic lights; in new york city alone, there are 11,871 traffic lights, and great efforts have been made to reduce power consumption, including converting incandescent light signals to LEDs;

(d) There are severe weight restrictions on street lamps to prevent overload of the signage support structure, and therefore, the industrial design and weight distribution of the signage must be carefully managed; and

(e) The need to increase traffic signal size (in some cases) may sacrifice performance in terms of power consumption, weight, and cost.

The present invention therefore seeks to provide a form of information display (which may be in the form of a traffic light or other traffic control device) which overcomes the above-mentioned problems with prior art devices.

Similar problems are encountered when the "traffic control device" is an indicator on an automobile or other vehicle. Even relatively high power bulbs do not guarantee adequate visibility. For example, if a collision is to be avoided during high speed driving, the driver of a car may have to react to a change in the brightness of the front brake lights in a fraction of a second. Most brake lights on automobiles have parabolic reflectors to concentrate light from the bulb. While such a parabolic reflector does help concentrate light from the stop lamp into a narrow beam, it also concentrates all light incident on the reflector (e.g., light from sunlight or from a subsequent vehicle headlamp) through the colored plastic cover, thus creating a background reflection that tends to obscure the state of the stop lamp. In the worst case, the combination of specular reflection from the parabolic reflector, reflection from the gloss paint on the car and reflection from the rear windshield significantly reduces the visibility of the brake light when the sun is facing the rear of the car.

In another aspect, the present invention provides a hybrid emissive-reflective display that can improve the visibility of an on-board sign under high ambient light conditions, thereby providing additional safety margin and providing the possibility of reduced power consumption.

Disclosure of Invention

Accordingly, in one aspect, the present invention provides a display device comprising: a reflective display arranged to provide a first image visible through the viewing surface; and projection means arranged to provide a second image visible in reflection on the viewing surface; the reflective display and the projection device are mounted on a common frame. Hereinafter, this display device will be referred to as a "projection display" of the present invention.

The present invention also provides a display device comprising a digital projector and a reflective surface mounted on a common frame, the digital projector comprising a light source, a projection lens and at least one additional optical element adapted to form a projected image on the reflective surface, wherein light passing from the light source through the projection lens is folded more than 180 degrees in a plane containing the principal axis of the projection lens and the plane of symmetry of the at least one additional optical element before being reflected from the reflective surface.

The invention also provides a reflective display arranged to provide a first image visible through a viewing surface; and a projection device comprising a light modulator arranged to provide a second image visible in reflection on the viewing surface; the first image and the second image are overlapping and have substantially the same width and height, the first image having w1 pixels in a width dimension and h1 pixels in a height dimension, wherein

h1>w1

The second image has w2 pixels in the width dimension and h2 pixels in the height dimension, wherein

h2<w2

The present invention also provides a display device including: a reflective display arranged to provide a first image visible through the viewing surface; and projection means arranged to provide a second image visible in reflection on the viewing surface; wherein the frame rate of the reflective display differs from the frame rate of the projection device by at least 10%.

The foregoing display devices may be collectively referred to hereinafter as a "projection display" of the present invention.

The present invention also provides a method for providing an image using a reflective display arranged to provide a first image visible through a viewing surface; and projection means arranged to provide a second image visible in reflection on the viewing surface; the method comprises the following steps:

(a) Dividing the image information into at least two components, a first component comprising at least luminance information and a second component comprising chrominance information; and

(b) The first image is directed using the first component and the second image is directed using the second component.

In another aspect, the present invention provides an information display comprising a bi-stable reflective display having a display surface and at least one light emitter arranged to direct light onto the display surface of the reflective display. In one form of the invention there are at least two separate reflective displays having display surfaces arranged to display different colours and independently controllable light emitters arranged to direct light onto the two display surfaces. This form of the invention may take the form of a traffic signal having three separate reflective displays with display surfaces arranged to display red, amber and green respectively, and three light emitters arranged to direct light onto the three display surfaces. Alternatively, this form of the invention may, for example, have the form of a crosswalk sign with two separate display surfaces, one forming a red "DON' T WALK" sign and the other forming a white "WALK" sign, and two light emitters arranged to direct light onto the two display surfaces.

It will be appreciated that in any display surface of the present invention where colour is to be displayed rather than simply black or white, the necessary chromaticity may be provided by the surface itself or by the light emitters. Thus, in a display designed to simulate a conventional traffic signal, three different colored electro-optic display areas (using either an inherent colored medium or a single colored medium behind a color filter) illuminated by a white light emitter, or three single colored electro-optic medium areas illuminated by three different colored light emitters, may be used, if desired. In general, however, the latter is preferred because it provides a higher contrast. The "on" or color state of such a monochrome electro-optic medium/color light emitter display requires setting the electro-optic medium to its reflective (white) state and turning on the light emitters so that colored light from the emitters is reflected from the electro-optic medium. In the "unlit" or dark state of such a display, the electro-optic medium is set to its dark, non-reflective state, and the light emitters are turned off, thus producing a very dark display surface and a high contrast between the two states of the display. While this type of display does require the constant use of light emitters, its power requirements can be made relatively modest; for example, the number of LEDs required as light emitters may be reduced compared to conventional LED traffic signals, since each LED may illuminate a major area of the display surface, in contrast to the fact that the entire display surface of a conventional LED traffic signal must cover the LEDs. The LEDs in such displays of the present invention will typically be arranged in a "light ring" around the periphery of a circular display surface, and providing LEDs in such a light ring may be technically simpler than providing a larger number of LEDs in a compact array on the display surface of a conventional LED traffic signal. Furthermore, since the light from the LEDs is spread relatively evenly across the reflective display surface, the appearance of traffic signals tends to be more attractive than that of conventional LED traffic signals, which suffer from pixelation due to the significantly separated LEDs on their display surfaces.

In another aspect, the present invention provides an electrophoretic display comprising:

at least one front electrode through which a viewer may view the display;

a layer of electrophoretic medium comprising a fluid and two types of charged particles disposed in the fluid and capable of moving through the fluid upon application of an electric field thereto, one of the two types of particles being dark and the other being reflective and having a color different from that of the dark particles;

at least one rear electrode disposed on a side of the electrophoretic medium layer opposite the front electrode, the rear electrode having a plurality of holes extending therethrough; and

a light source arranged on a side of the rear electrode opposite to the electrophoretic medium layer and configured to direct light through the electrophoretic medium layer,

the display has a first optical state in which dark particles are adjacent the front electrode such that a viewer sees a dark color; a second optical state in which the reflective particles are adjacent to the front electrode such that the viewer sees the color of the reflective particles; and a third optical state in which the dark particles are adjacent the rear electrode, the reflective particle light source generates light, and the viewer can see the color of the reflective particles.

The invention extends to a vehicle carrying an electrophoretic display of the invention.

In another aspect, the present invention provides an electrophoretic display using grid electrodes. The grid electrode may be used as a rear electrode of an electrophoretic display. The grid electrode may be used to aggregate black pigment into a relatively small area corresponding to the grid lines of the grid electrode. In addition, a reflective backing may be used in combination with the grid electrode. Thus, the backing may reflect a relatively high degree of light, providing an enhanced white state of the electrophoretic display. Enhanced brightness can be obtained by simply modifying the front plane laminate structure of an electrophoretic display, which can be implemented into electrophoretic displays with Thin Film Transistor (TFT) technology as well as segmented electrophoretic displays.

It will be appreciated that in the electrophoretic display of the present invention, the so-called "reflective particles" should not be totally reflective, since in the third optical state the totally reflective particles will not allow any light to pass through the electrophoretic medium. The reflective particles must be sufficiently reflective to reflect a substantial portion of the light incident on the front electrode when the display is in its second optical state, but still have sufficient transmissivity to allow light to pass through the reflective particles when the display is in its third optical state. As will be seen from the detailed examples below, there is no difficulty in finding commercial pigments that meet these requirements.

The display of the present invention may utilize any of the types of bistable electro-optic media previously described. Thus, the electro-optic medium may comprise a rotating bi-color member, electrochromic or electrowetting material. Alternatively, the electro-optic medium may comprise an electrophoretic material comprising a plurality of charged particles disposed in a fluid and capable of moving through the fluid under the influence of an electric field. The charged particles and fluid may be confined within a plurality of capsules or microcells. Alternatively, the charged particles and fluid may be present in a plurality of discrete droplets surrounded by a continuous phase comprising the polymeric material. The fluid may be a liquid or a gas.

Drawings

1A-1C of the drawings are schematic diagrams illustrating artifacts associated with glancing angle projection in a projection display of the present invention.

Fig. 2A-2C are schematic diagrams showing the arrangement of a projected image and a reflected image in a projection display of the present invention.

Fig. 3 is a side view of a projection engine for use in the projection display of the present invention.

Fig. 4 is a block diagram illustrating a method of controlling a projection display of the present invention.

Fig. 5 is a side view of a portion of an information display of the present invention in the form of a traffic light with a portion of its housing broken away to show a light source.

Fig. 6 is a front view of the same portion of the information display as shown in fig. 5.

Fig. 7 is a three-quarter side view of the sight source assembly of the information display shown in fig. 5 and 6 from the front and to one side.

Fig. 8 is a schematic cross-sectional view through an electrophoretic display of the present invention in the form of a brake light for an automobile.

Fig. 9 is a front view of a rear electrode of the electrophoretic display shown in fig. 8.

Fig. 10 is a graph of values of L x a x b versus time obtained by the electrophoretic displays shown in fig. 8 and 9 in some of the experiments described below.

Fig. 11 shows an exploded view of an electrophoretic display with rear grid electrodes according to a non-limiting embodiment of the present application.

Fig. 12A and 12B are photomicrographs showing the black pigment gathering near the grid lines of the grid electrode according to a non-limiting embodiment of the present application.

Detailed Description

As described above, in one aspect, the present invention provides a projection display in which a means for projecting a color image and a means for providing a reflected image are incorporated into a single unit so that the two images can be superimposed in registration with each other, a composite image with improved color quality that is visible under a wider range of ambient lighting conditions can be obtained than can be provided using either the reflective display or the projector alone. In low light, the projected image is easily seen, and its contrast is enhanced by the reflected image superimposed thereon. Under intense light, the projected image will fade away, but the reflected image will be well lit and have good advantages. In order to save power, it is desirable to adjust the intensity of the projected image in accordance with ambient illumination (the intensity of which may be measured by means known in the art, such as a photodiode or the like). In very bright light, the projector may be completely turned off.

The reflective display used in the projection display of the present invention may be of any of the types previously described, including but not limited to electrophoretic, electrowetting, electrochromic, rotating bichromal, and reflective liquid crystals; the electrophoretic display may for example be of the magnetophoretic and/or frustrated total internal reflection subtype. Other types of reflective displays known in the art may also be employed, such as electronic liquid powders, micro-mechanical (interferometry), photonic crystals (structural colors), electrofluidic and light valves/reflectors.

In a preferred form of the invention, the reflective display comprises an electrophoretic material comprising a plurality of charged particles disposed in a fluid and capable of moving through the fluid under the influence of an electric field. The charged particles and fluid may be confined within a plurality of capsules or microcells. Alternatively, the charged particles and fluid may be present in the form of a plurality of discrete droplets surrounded by a continuous phase comprising the polymeric material. The fluid may be a liquid or a gas.

The projection device ("engine") used in the projection display of the present invention may use a variety of technologies including light sources such as color Light Emitting Diodes (LEDs) and solid state color laser sources in combination with light modulators such as micro-displays made using technologies including liquid crystal on silicon (LCoS), deformable Mirror Displays (DMD), or scanning mirrors (a type of microelectromechanical system, MEMS). Combinations of light sources, light modulators, and associated optical elements (e.g., beam splitters and projection lenses) are well known in the art and can be packaged in very small sizes such that they are currently known as miniature projectors. In a preferred form of the invention, the micro projector is embedded in a mobile reflective display device such as an electronic document reader (E reader) or an electronic book (E-book) to form a hybrid display.

The technical challenges of combining a micro projector with a mobile reflective display are very difficult. As a mobile device, the hybrid display must be compact. Embedded miniature projection systems do not greatly increase the size and weight of electronic document readers. The projector must not obscure the view of the screen and the range of viewing angles from which the user reads the display. In order to provide a compact and unobstructed view, an angled projection is required. Such projections are used in "short-range" and "ultra-short-range" projectors, and are well known in the art; see, for example, U.S. patent No.7,239,360. Some of the artifacts associated with short-range projection will be described in more detail below.

Fig. 1A shows a projector 10 arranged to provide an image onto a reflective screen 12 using an angled projection, the term angled projection being used to denote that the angle of incidence of the central chief ray with respect to the normal of the display surface is greater than zero (and preferably in the present invention at least about 60 deg.). A large projection angle is desirable because it allows for a more compact package and increases the angular range of the viewable display where the projector assembly does not obstruct the view. As described in more detail below, folded optics may be used so that the projector may be located below the plane of the reflective screen for maximum compactness. These folded optical elements are omitted from fig. 1A for clarity.

As shown in fig. 1B and 1C, the angled projection introduces image defects that increase dramatically with projection angle: such defects include keystone distortion (see fig. 1B) and anamorphic distortion (see fig. 1C). Not shown in fig. 1A-1C, but still apparent are visible blur (when the size of the angled screen exceeds the depth of focus of the projector), decrease in light intensity with projection distance (and hence non-uniformity in brightness of the projected image), and other well known optical aberrations such as chromatic aberration and astigmatism.

In the projection display of the present invention, some of these artifacts may be digitally corrected. For example, keystone and anamorphic distortion, as well as a drop in light intensity, may be corrected by projecting an image that has been pre-distorted in space and/or brightness, as is known in the art. This digital correction comes at the cost of some other attribute of the projected image. For example, spatial correction of keystone and/or anamorphic distortion will reduce the overall resolution of the projected image, while light intensity correction will reduce the image brightness. As discussed in more detail below, some artifacts (e.g., blurring) are not suitable for digital correction, but must be corrected by appropriate optical element selection.

The screen of most reflective displays, such as E-book readers (E-readers), is rectangular. For reading books, the screen is typically used in a portrait orientation (i.e., a longer dimension oriented toward and away from the user, while a shorter dimension is oriented horizontally). Most commercially available miniature projection engines are also designed to project rectangular images. However, when such an engine is projected at an angle, the most compact package size (i.e., the smallest "throw ratio", which is the distance between the projector and the screen divided by the diagonal dimension of the screen) is achieved when the lateral orientation of the projector is projected onto the longitudinal orientation of the reflective display, as shown in fig. 2A-2C.

Fig. 2A shows a light source 20 modulated by a light modulator 22 to form an image that is projected onto a screen 24. The modulator is in landscape orientation but its image is projected onto a screen that is oriented in portrait orientation.

Fig. 2B shows light source 20 modulated by light modulator 22 to form an image that is projected onto screen 28. The modulator and the screen are both in a landscape orientation.

Fig. 2C shows light source 20 modulated by light modulator 26 to form an image that is projected onto screen 24. The modulator is in a portrait orientation and its image is projected onto a landscape oriented screen.

As will be apparent from fig. 2A-2C, the throw ratio is minimal when the long axis of the modulator is projected onto the short axis of the screen as shown in fig. 2A, and thus the compactness of the device incorporating the screen and projector is maximized. Thus, in the projection display of the present invention, it is preferable that the projection image and the reflection image are registered as a whole and have substantially the same width and height, the reflection display has w1 pixels in the width dimension and h1 pixels in the height dimension, wherein

h1>w1

The light modulator of the projector has w2 pixels in the width dimension and h2 pixels in the height dimension, wherein

h2<w2

A disadvantage of the arrangement of fig. 2A is that square pixels in the light modulator 22 become anisotropic in the image projected onto the screen 24, resulting in a mismatch between the size of the projected image pixels and the size of the (square) pixels on the reflective display screen 24. For example, if the reflective screen is 600x 800 pixels (SVGA) and each pixel is square with a width of about 150 μm, and the light modulator of the projection engine is 848x 480 pixels (WVGA), the resolution of the projected image cannot exceed about half the resolution of the image on the reflective display (i.e., the projected pixel size is approximately 300 μm and not square after the above-mentioned distortion correction).

In the projection display of the present invention, if achromatic (brightness) information of a combined image is carried by only one of the mixed display components (either the projected image or the reflected image), or if the spatial frequency content of one of the achromatic image components is reduced so that the effect of misalignment is no longer visible, the pixels of the projected image can be relaxed by 1:1 to the pixel of the reflective display. The different sensitivities of the human visual system to achromatic color (brightness) and color image components to position and motion make it possible for the visibility of mismatches in resolution to be reduced. The color acuity of the human visual system is significantly lower than its brightness acuity, such that perception of clarity, fineness and readability of text in a displayed image is dominated by achromatic image components.

In a preferred embodiment of the projection display of the invention, the achromatic image component is displayed on a reflective display, because the display maintains its contrast over a wide range of ambient light levels, and the perceived contrast is even improved at very high ambient light levels (e.g. sunlight). In order to provide luminance and chrominance (chroma) information correctly using a projection display, the input image is divided into achromatic (black and white) and chromatic (color only) components. Examples of color image coding systems that perform such separation into one luminance component and two chrominance components include, but are not limited to YCbCr, YIQ, YCC, CIELab and ohgb. With the projection display of the present invention, these achromatic and chromatic components can be displayed using the following method.

The achromatic component may be displayed on the reflected image; a color projector projects color components onto the image. The viewer's eye recombines (fuses) the displayed achromatic and chromatic image components into a full color image. Since the color acuity of the human eye is significantly lower than its achromatic color acuity, the perception of clarity, fineness and readability of text will be dominated by the achromatic color component. If the resolution of the color components projected onto the reflected image is lower than the resolution of the displayed achromatic components and/or the color and achromatic components are not registered, this will not interfere with the perception of sharpness, finesse and readability of the combined image. In addition, the lower motion of the human visual sensitivity to the color components ensures that subtle changes in the relative positions of the color and achromatic image components over time (e.g., subtle changes caused by vibration if the alignment of the projector to the reflected image is not perfectly rigid) will not significantly interfere with the perception of detail in the combined image.

In addition to color image information, the projector engine may be provided with achromatic (luminance) image information that has been spatially filtered to improve overall image quality without reintroducing registration requirements. For example, achromatic (luminance) channels may be low pass filtered (blurred) to keep the effects of misalignment and motion (vibration) between the projected image and the reflected image invisible, but the contrast of the combined image increases compared to an image where only the color component is projected onto the reflected image.

These methods can be applied if the reflected image is a color image carrying luminance information and chrominance information, and the projected image carries only chrominance information or carries chrominance information and luminance information modified as described above.

Fig. 3 shows the optical design of a projection display (generally designated 30) for use with the present invention. The projection display 30 comprises a projector module 31 (which itself comprises a light source and a spatial light modulator as described above) and associated optical elements, such as a beam splitter, required to produce the modulated images of the three primary colors (red, green and blue). The image is projected onto the viewing surface 38 of the reflective display using the following optical elements:

(a) A projection lens or combination of lenses 32. The lens plane of the projection lens 32 intersects the plane of the light modulator (in the projector module 31) and the plane of the reflective display 38 (the folding of the light path is corrected by the mirrors 36 and 37) at a common line (i.e., the modulator, the projection lens 32, and the reflective display 38 are set to satisfy scheimpflug condition);

(b) An aspheric achromatic lens combination 33 designed to provide additional focusing of the projected image and minimize chromatic aberration (this combination may alternatively be combined with projection lens 32);

(c) Annular lens 34, which minimizes astigmatism caused by cone lens 37 (described below). Lens 34 may be omitted if mirror 37 has a more complex curvature than a simple conical shape;

(d) A non-rotationally symmetric element 34 that provides a variable focusing power with field position;

(e) Fold mirror 36, which may be a flat mirror (preferred) or curved (e.g., conical); and

(f) Cone mirror 37, which steers the beam to the appropriate position on display viewing surface 38.

The optical elements in fig. 3 are shown in positions to project an image from projector 31 onto viewing surface 38. When not in use they may be folded down to save space or the entire assembly including the elements 31-37 may be detachable from the reflective display. Alternatively, fold mirror 36 may be provided removable from the optical path, or replaced with another optical element, so that projector 31 may project an image onto a remote surface instead of onto viewing surface 38.

The optical element shown in fig. 3 may be replaced by a diffractive or holographic element or by an element using nano-optical phase discontinuity techniques.

The projection engine may be positioned on the top, bottom, or side of the viewing surface 38 when the reader views in portrait mode. If the projection engine is located at the bottom of the viewing surface 38, other elements of the display, such as the keyboard, may be located above the mirror 37 such that the mirror is not visible to the reader. Although viewing surface 38 is shown as planar in fig. 3, this is not required as flexible reflective displays are well known in the art and the curvature of the display surface may be used to simplify or improve the optical design described above. Additional elements such as light blocking sheets may be incorporated to reduce stray light specularly reflected from any surface of the display.

The projection display of the present invention may also include the elements necessary to drive the projector apparatus and the reflective display. It is not always necessary to drive both displays simultaneously. Thus, for example, it may be desirable to switch a full or partial area of a reflective display to its white state (or possibly a gray state) and project an image onto that white area using an embedded projector. This is desirable, for example, if video rate content is to be viewed. In the current state of the art, the switching rate of some reflective display technologies is not as high as that of projection engines.

When video addressing, the frame rate of the reflected image and the frame rate of the projected image do not have to match. For example, the projection engine may run at 60 frames/second, but the reflective display may run at 15 frames/second, enhancing the contrast of the video in a subset of the frames.

FIG. 4 is a block diagram of one possible controller architecture for a projection display of the present invention. The information to be displayed is loaded onto the controller by the input/output unit 4 and temporarily stored in a memory 44 which also holds the necessary software and firmware. The user interface 41 allows the viewer to control all necessary functions such as loading, saving, selecting and deleting information, and starting and proceeding by viewing information. The image and video signal processing unit 45 conditions the stored information for display and performs image and video processing algorithms including, but not limited to, brightness and color correction, separation into components for reflective and projection display, providing images, resolution, dithering, geometric predistortion, and uniformity correction to the display and projector. The processed image signal is then transmitted to a necessary hardware controller. Electrophoretic display controller 48 controls the pixels of reflective display 51; the micro display controller 47 controls the pixels of the projector's image modulator 50. The light engine controller 46 operates the light source 49 so that the light source 49 may be adapted to the projected color image or video and ambient lighting level and is turned off when not needed to save power from the battery 42, the battery 42 being provided by the power controller 43.

The information display of the present invention will now be described in more detail. The display may be used as an outdoor information display, which is adaptable to all lighting conditions, spans a wide temperature range, and is configurable to operate without a mains supply. The information display may comprise an electrophoretic display for daylight conditions, with the aid of LEDs or similar light emitters for dim light or other conditions. Light emitters may also be required to aid in dim daylight conditions, or may be used at all times to provide a desired color. The light emitters may be integrated into the front baffle or front lighting cavity to best suit the environment.

It is desirable that the information display of the present invention requires very low power to operate and that the display can incorporate solar charging elements for self-sustaining operation without the need for an external power source. Since some types of electro-optic materials do not function well at low temperatures, it may be necessary to incorporate a layer of transparent material in the front or thermally insulating material in the back of the electro-optic portion of the display. Alternatively, the light emitter may be capable of generating sufficient heat to keep the electro-optic material within its operating range. Other forms of electro-optic material heating may be activated as needed to ensure proper switching.

The information display of the present invention is an ideal alternative to existing street signs (e.g., traffic lights and crosswalk signs) because it has the advantages of lighter weight, lower power, more visible under sun glare, better tamper resistance, being able to be easily deployed in emergency environments, a bill of materials cost similar to prior art signs.

A simple traffic light system may consist of three separate electro-optic segmentation units, each behind a complementary color filter. Such a system may comprise a plurality of light emitters for each cell, which are directed towards the electro-optic display in a pattern to maximize color visibility in low light conditions.

A simple crosswalk sign may have a similar design to a traffic light, except that it is not a single segmented electro-optic unit, which may include multi-segmented icons or information.

These information displays may be made compatible with the processor and detection system so that the appropriate display information needs to be synchronized with the situation. Control system options may be provided to use low power wireless management information and solar charging elements may be included to enable self-sustaining operation without mains power.

Compared to prior art information displays, the information display of the present invention provides the following advantages:

the weight of the system is reduced;

the electricity utilization efficiency is improved;

improving visibility under sun glare;

the damage resistance is enhanced;

easier deployment in emergency situations;

a plurality of improvements are made with a competitive bill of materials cost; and

the scale can be extended with minimal design tradeoff.

FIG. 5 of the drawings is a side view of one unit (generally designated 100) of the three-unit information display of the present invention in the form of a traffic light; some of the figures are broken up to show internal details of the cell. The unit includes a substantially semi-cylindrical sun visor 102 (best shown in fig. 6) and a lens 104, the sun visor 102 being in a form similar to that of prior art traffic lights. A circular monochromatic electrophoretic display 106 is arranged at the rear surface of the cell 100 and is provided with a single electrode (not shown) on each of its main surfaces to enable the display to be used as a single pixel display. A plurality of Light Emitting Diodes (LEDs) 108 are arranged at uniform intervals around the inner surface of the ring 110, which surrounds the display 106, such that light from the LEDs 108 is directed onto the surface of the display 106. The circle 110 and the LED 108 are shown to a larger scale in fig. 7. The LED 108 has a single color, either red, amber or green depending on the particular unit of the traffic light.

In normal operation, the LEDs are continuously driven and the phase of the traffic signal is controlled by switching the display 106 between its bright and dark states.

The electrophoretic display of the invention will now be described in more detail. As already mentioned, a third aspect of the invention provides an electrophoretic display comprising: at least one front electrode through which a viewer may view the display; an electrophoretic medium layer comprising a fluid and two types of charged particles disposed in the fluid, one of the two types of particles being dark and the other being reflective and having a color different from that of the dark particles; at least one rear electrode disposed on a side of the electrophoretic medium layer opposite the front electrode, the rear electrode having a plurality of holes extending therethrough; and a light source disposed on a side of the rear electrode opposite the electrophoretic medium layer and configured to guide light through the electrophoretic medium layer. The display has a first optical state in which dark particles are adjacent the front electrode such that a viewer sees a dark color; a second optical state in which the reflective particles are adjacent to the front electrode such that the viewer sees the color of the reflective particles; and a third optical state in which the dark particles are adjacent the rear electrode, the reflective particle light source generates light, and the viewer can see the color of the reflective particles.

This aspect of the invention will be described below primarily in its application as a brake light on a vehicle. However, the electrophoretic display of the present invention is not limited to this application and may be used as any form of vehicle or traffic sign, or in other applications, such as warning lights on a control panel. Basically, the electrophoretic display is designed to have a normal dark state and a color state of a conventional dual particle electrophoretic display, and an additional emission state (particularly useful in low light conditions) in which light from the light source passes through the electrophoretic medium and displays the color of the color particles.

An electrophoretic display (generally designated 200) of the present invention is shown in figures 8 and 9 of the drawings. The display 200 is in the form of a brake light for a vehicle and includes a moisture barrier film 202 for protecting the remaining components of the display 200 from ambient moisture, including spray mist when the vehicle is traveling in wet conditions, a substantially transparent front electrode 204, an adhesive layer 206, and an electrophoretic medium layer, generally designated 208. The electrophoretic medium layer 208 is an encapsulated electrophoretic medium comprising a plurality of capsules having capsule walls 210 surrounding a dielectric fluid 212, in which dielectric fluid 212 red particles 214 and black particles 216 are dispersed, the red and black particles being charged with opposite polarities. Behind layer 208 (i.e., to the left as shown in fig. 8) is rear grid electrode 218; as best shown in fig. 9, the grid electrode 218 includes wires 220 arranged in a hexagonal pattern such that hexagonal holes 222 occupy a majority of the area of the electrode 218. Finally, a light source is arranged on the side of the electrode 218 opposite the medium 208, which light source is in the form of an incandescent bulb 224, which incandescent bulb 224 is provided with a parabolic reflector 226. Although not shown in fig. 8, the electrophoretic display including components 202-218 may be secured to reflector 226 by an optically clear adhesive layer; the bulb 224 and reflector 226 may form part of a prior art vehicle brake light.

The display 200 is provided with a voltage source (not shown) for establishing a potential difference between the electrodes 204 and 218. When it is not desired to display a stoplight, the potential difference between electrodes 204 and 218 is set to attract black particles 216 adjacent electrode 204 and red particles 214 adjacent electrode 218, such that the display assumes a first optical state in which the surface of the stoplight appears dark. Note that in this state, there is no relation whether the bulb 224 is lit or unlit, as no light is emitted from the display 200; however, to save power and increase bulb life, the bulb 224 will typically be turned off.

When it is desired to turn on the stoplight, the potential difference between electrodes 204 and 218 is reversed such that red particles 214 are adjacent to electrode 204 and black particles 216 are adjacent to electrode 218. The display thus assumes a second optical state in which the red particles 214 reflect light incident on the display, while the brake light shows red and "lights up".

So far, the description of the operation of the display 200 has assumed a high ambient lighting condition. In dim light conditions, to turn on the stoplight, the potential difference between electrodes 204 and 218 is set such that red particles 214 are adjacent to electrode 204 and black particles 216 are adjacent to electrode 218, and bulb 224 lights up such that the display assumes a third optical state, wherein light from bulb 224 is formed into a narrow beam by parabolic reflector 226, passing through red particles 214 adjacent to electrode 204, causing red light to be emitted from the display, and the stoplight display lights up. Thus, the display 200 may achieve significantly improved contrast under both low and high light conditions.

A suitable driving scheme involving voltage or pulse width modulation may be used in the display 200 to produce a defined state of visibility. Such a driving scheme may be synchronized with a clock or light sensor or temperature sensor to produce a desired level of visibility at any time of the day.

To provide experimental testing of the electrophoretic display of the present invention, red and black pigment dispersions comprising Solsperse 17k as a charging agent were prepared. Red pigment Paliotan Red L3745 was treated with silane Z6030 and coated with poly (lauryl methacrylate) substantially as described in example 28 of us patent No.8,822,782. An electrophoretic medium containing 50 wt% pigment (red/black ratio 10:1) and 25mg/gm of Solsperse 17k in Isopar E was prepared and tested in a liquid test unit. As shown in fig. 10, the medium obtained a 45a red state and a 10L (0.01% reflectance) dark state.

After replacing the back plate of the test cell with a transparent grid electrode, it was observed that the black pigment closed the shutter in response to the applied electric field. The film transmitted through the test unit is acquired at the camera and the variable transmission through the device is clearly visible.

Of course, the electrophoretic medium of the present invention is not limited to the use of red particles. If one of the particles is highly absorptive, the other particle may be reflective (white), colored, retro-reflective or transparent. One or more dyes may also be included in the fluid to achieve a desired color state in the display.

As previously mentioned with respect to fig. 9, some embodiments of the invention may include an electro-optic display having grid electrodes. In one embodiment, the electro-optic display may be an electrophoretic display comprising two or more pigments, for example a white pigment and a black pigment, each comprising electrophoretic particles. The grid electrode may be configured as the rear electrode of the display and may be used in combination with a white or reflective background. One example of a reflective background is a mirror. Conventionally, electrophoretic displays with white and black pigments suffer from loss of transmitted light in the white state, with the black pigment being located behind the white pigment. In accordance with aspects of the present application, a highly reflective surface may be placed behind an electro-optic layer (e.g., an electrophoretic medium), and a suitably configured electrode operated to draw the black pigment into a narrow line, exposing a substantial portion of the reflective surface. The exposed reflective surface directs the transmitted light back toward and through the lambertian white pigment layer, thereby improving the white state. In at least some embodiments, the electrodes are configured as a grid, although not all embodiments are limited in this manner.

Fig. 11 shows an embodiment in which the electrophoretic display comprises rear grid electrodes. Display 1000 includes front electrode 1020, back electrode 1040, and backing layer 1060. An electro-optic layer 1080 is disposed between the front electrode and the back electrode.

The front electrode 1020 may be an Indium Tin Oxide (ITO) layer, and may optionally be on a transparent substrate, such as a substrate of polyethylene terephthalate (PET) (not separately shown).

The electro-optic layer 1080 may be an electrophoretic layer that includes a continuous (binder) phase and a discontinuous phase. For ease of illustration, the continuous phase is omitted. The discontinuous phase is shown as comprising a plurality of capsules 1100, but may also be encapsulated in polymer dispersed droplets or microcells. The capsules or droplets preferably have a mass of about 1:3 to 3:1, more preferably about 1:2 to 2: aspect ratio of 1, i.e., aspect ratio. One or more capsules 1100, and in some cases, each capsule 1100 may include black and white pigments formed from suitable electrophoretic particles that may be controlled by front and rear electrodes. Five bladders 1100 are shown, but any suitable number may be used.

Rear electrode 1040 may be a wire grid electrode as shown and may be formed of any suitable material, such as an electroformed web of conductive material, such as Al, crMo, niB, ag and CNT, etc. The spacing of the grids and the width of the wires can be adjusted to maximize the open area while maintaining sufficient electric field uniformity to achieve a smooth appearance of the front surface. In some embodiments, the spacing of the grid lines is selected to generally correspond to the size of bladder 1100. To ensure that sufficient light is transmitted to the reflective backing layer through the wire grid electrode, the wire grid electrode preferably has a light transmission of at least 85%. Moreover, if the electro-optic layer includes an encapsulated medium, it is preferable that the area of each open space between the plurality of grid lines forming the rear electrode (for example, the area of one of squares constituting the grid in fig. 12A and 12B) is substantially the same as the cross-sectional area of the capsule, microcell, or droplet containing the electrophoretic particle, and it is preferable that the ratio between the area of the open space between the grid lines and the cross-sectional area of the capsule, microcell, or droplet is about 1:5 to 5:1, more preferably about 1:4 to 4:1.

Backing layer 1060 may be reflective, such as a mirror, or may be a white layer. Although the display 1000 is shown as "exploded" for clarity, in actual practice the mirror back plate should be placed as close to the rear of the rear electrode 1040 as possible.

When the electro-optic layer 1080 is switched to a white state, a uniform white pigment layer will fill up at the front surface of the balloon 1100, causing the electro-optic layer to appear as a smooth white color. As further described below in connection with fig. 12A and 12B, black pigment will collect on the grid lines of rear electrode 1040 exposing a majority of backing layer 1060. Light transmitted through the electro-optic layer in this state will reflect back from the exposed portions of the backing layer 1060 and increase the brightness in the white state.

Fig. 12A and 12B show images of an electrophoretic display with grid electrodes, and made using a microscope. The imaged display contains only black pigment. These figures show how the black pigment 2040 within the capsule 2020 is pulled to the grid lines (or wires) 2000 of the rear grid electrode. Thus, these figures show that charged black electrophoretic particles are concentrated on the micro-wires of the grid electrode, rather than being uniformly distributed in the capsules, exposing most of the open areas between the micro-wires. This operation effectively enables shutter mode operation of the display without the need for a conventional shutter mode waveform. As described herein, this action serves to attract the black pigment onto a small portion of the back plate surface of the capsule, thereby exposing the highly reflective back plate surface to regain transmitted light. In another embodiment, a single black pigment with a grid creates a black and white display with the presence of a white backing. When the black pigment moves to the front of the display (i.e., disperses), the display is in a dark state. When the black pigment moves to the grid, the display is in a white state.

It will be apparent to those skilled in the art that many changes and modifications can be made in the specific embodiments of the invention described above without departing from the scope of the invention. The entire foregoing description is therefore to be construed in an illustrative and not a limiting sense.

Claims (10)

1. An electro-optic display comprising:

a light-transmitting front electrode;

an electro-optic layer;

a rear electrode; and

a planar white or reflective backing,

wherein the rear electrode is in the shape of a grid having a plurality of grid lines and is disposed between the planar white or reflective backing and the electro-optic layer, an

Wherein the electro-optic layer comprises a dispersion comprising a plurality of charged particles in a fluid, the charged particles being capable of moving through the fluid upon application of an electric field, wherein the charged particles and the fluid are encapsulated within a plurality of capsules, and wherein the grid has a plurality of open spaces, and the ratio between the area of the open spaces and the cross-sectional area of the capsules is 1:5 to 5:1.

2. an electro-optic display according to claim 1 wherein the capsules have an aspect ratio of 1:3 to 3:1.

3. an electro-optic display comprising:

A light-transmitting front electrode;

an electro-optic layer;

a rear electrode; and

a planar white or reflective backing,

wherein the rear electrode is in the shape of a grid having a plurality of grid lines and is disposed between the planar white or reflective backing and the electro-optic layer, an

Wherein the electro-optic layer comprises a dispersion comprising a plurality of charged particles in a fluid, the charged particles being capable of moving through the fluid upon application of an electric field, wherein the charged particles and the fluid are encapsulated within a plurality of microcells, and wherein the grid has a plurality of open spaces, and the ratio between the area of the open spaces and the cross-sectional area of the microcells is 1:5 to 5:1.

4. an electro-optic display comprising:

a light-transmitting front electrode;

an electro-optic layer;

a rear electrode; and

a planar white or reflective backing,

wherein the rear electrode is in the shape of a grid having a plurality of grid lines and is disposed between the planar white or reflective backing and the electro-optic layer, an

Wherein the electro-optic layer comprises a dispersion comprising a plurality of charged particles in a fluid, the charged particles being capable of moving through the fluid upon application of an electric field, wherein the charged particles and the fluid are encapsulated within a plurality of discrete droplets surrounded by a continuous phase comprising a polymeric material, and wherein the grid has a plurality of open spaces, and the ratio between the area of the open spaces and the cross-sectional area of the droplets is 1:5 to 5:1.

5. An electro-optic display according to claim 4 wherein the aspect ratio of the plurality of droplets is 1:3 to 3:1.

6. an electro-optic display according to claim 1 wherein the rear electrode has a light transmission of at least 85%.

7. An electro-optic display according to claim 1 wherein the rear electrode is disposed adjacent the white or reflective backing.

8. An electro-optic display according to claim 1 wherein the plurality of charged particles comprises black particles.

9. An electro-optic display according to claim 8 wherein the plurality of charged particles further comprises white particles.

10. An electro-optic display according to claim 1 wherein the gridlines comprise micro-wires.

Images