WO2005071652A1

WO2005071652A1 - Optical activation of an electronic paint

Info

Publication number: WO2005071652A1
Application number: PCT/IB2005/050135
Authority: WO
Inventors: Murray F Gillies
Original assignee: Koninklijke Philips Electronics, N.V.; U.S. Philips Corporation
Priority date: 2004-01-15
Filing date: 2005-01-12
Publication date: 2005-08-04
Also published as: TW200535541A; JP2007522494A; CN1910644A; KR20070104207A; EP1709621A1

Abstract

A method of activating an electronic paint comprises the steps of applying an activation voltage (30) to the electronic paint and receiving radiation (36) on a portion of an electrophoretic ink (22) in the electronic paint. The electrophoretic ink is activated based on the received radiation (36) and the applied activation voltage (30).

Description

OPTICAL ACTIVATION OF AN ELECTRONIC PAINT

This invention relates generally to electrophoretic displays, and more specifically to optical addressing an electronic paint. Electrophoretic display media are non-volatile systems used to store digital information in the form of text or images. They are generally characterized by the movement of particles in an applied electric field, and can be bi-stable with display elements having first and second display states that differ in at least one optical property such as lightness or darkness of a color. In recently developed electrophoretic displays, the display states occur after microencapsulated particles in the electronic ink have been driven to one state or another by an electronic pulse of a finite duration, and the driven state persists after the activation voltage has been removed. Such displays can have attributes of good brightness and contrast, wide-viewing angles, state stability for two or more states, and low power consumption when compared with liquid crystal displays (LCDs). An exemplary electrophoretic display with microcapsules containing either a cellulosic or gellike phase and a liquid phase, or containing two or more immiscible fluids are described in "Process for Creating an Encapsulated Electrophoretic Display," Albert et al., U.S. Patent No. 6,067,185 issued May 23, 2000 and "Multi-Color Electrophoretic Displays and Materials for Making the Same," Albert et al., U.S. Patent No. 6,017,584 issued January 25, 2000. Most currently available electrophoretic displays receive data and are addressed by driving an active matrix, which may be located on the frontside or backside of the display. Active-matrix driving, however, is not an attractive option for inexpensive billboard- like displays, which require only a low to extremely low refresh rate. Electronic-ink systems have been proposed for large electrophoretic displays that have no intrinsic addressing schemes such as fixed coordinates on a pixel-by-pixel grid to accurately write text and graphics. Researchers are also working on applying this digital- or electronic-ink technology to a large electronic wall display of a so-called electronic wallpaper, poster or wall screen, which could consist of a thin electrophoretic film placed on a wall. Electrophoretic displays are often designed with various layers of electrophoretic and protective materials. Some passively addressed displays sandwich a layer of electrophoretic microcapsules between two electrodes. One such electrophoretic display having a protective transparent electrode is described in "Protective Electrodes for Electrophoretic Displays," Drzaic et al., International Patent Application No. WO0038001 published June 29, 2000. The protective electrode can be a vapor-permeable electrode with a reticulated electrically conductive structure, such as a metal screen or wire mesh, or a reticulated structure coated or impregnated with a conductive material. A method for addressing an electrophoretic display with a photoconductive layer is proposed in "Electrophoretic Displays in Portable Devices and Systems for Addressing such Displays," Zehner et al., U.S. Patent Application No. 2003/0011868 published January 16, 2003. Where the photoconductive layer is struck by light from the light-emitting layer of the display, the impedance of the photoconductive layer is lowered and the electrophoretic layer may be addressed by an applied electric field to write an image. While smaller electrophoretic displays often receive data and are addressed by driving an active matrix of the display, large electrophoretic displays may have no intrinsic addressing schemes to accurately write text and graphics. Various methods, systems and related devices have been proposed for externally addressing electrophoretic displays, yet their slow addressing speeds continue to be a challenge. The relatively slow switching speeds of many electrophoretic displays result in an external addressing device being able to transfer image data to the electrophoretic display much more quickly than the time that is necessary for the electrophoretic material to be switched to the correct display state. The switching and addressing speed of the electrophoretic material may be affected by temperature. For example, a paper-like electrophoretic display has been designed with material that is non-fluid at room temperature and fluidic at higher temperatures, as described in "Image Recording Medium, Image Recording Erasing Device, and Image Recording Method," Kino et al, International Patent Application WO0043835 published July 27, 2000. Heat has been used to write to selected regions of an electrophoretic display, as described in "Modifiable Display Having Fixed Image Patterns," Howard et al., U.S. Patent No. 6,340,965 issued January 22, 2002; and in "Electrophoretic Device, Driving Method of Electrophoretic Device, and Electronic Apparatus," Kawai et al., U.S. Patent Publication No. 2002/0150827 published October 17, 2002. Heat and light, as well as electromagnetic fields, have been used in methods for addressing an electrophoretic display. A laser beam, for example, is used for heating in "Image Recording Medium, Image Recording/Erasing Device, and Image Recording Method," Miwa et al., International Patent EP1162496 published December 12, 2001. The recording medium is made from a large number of fine coloring particles, which are responsive to an electromagnetic field, and a supporting medium in which the particles are dispersed. The supporting medium is preferably fluidic at higher temperatures and non- fluid at room temperature, allowing a formed image to be frozen at room temperature. While additional materials such as a photoconductive layer can be disposed on the electrophoretic material to aid the processes of addressing electrophoretic displays, the added layers and materials increase the complexity and cost of manufacture. Thus, passively addressed electrophoretic systems need alternative approaches to locally change the speed at which the electrophoretic material switches without requiring additional parts or complexity. An improved system also avoids affecting the display state of the surrounding areas near the surface where the display is currently being addressed, and provides faster switching times for electrophoretic displays that are addressed by handheld activation devices. One form of the present invention is a method of activating an electronic paint. An activation voltage is applied to an electronic paint. Radiation is received on a portion of an electrophoretic ink in the electronic paint. The electrophoretic ink is activated based on the received radiation and the applied activation voltage. Another form of the present invention is a system for activating an electronic paint. The electronic paint includes a lower conductive layer, a layer of electrophoretic ink disposed on the lower conductive layer, an upper conductive layer disposed on the electrophoretic ink, a controller electrically coupled to the lower conductive layer and the upper conductive layer, and an electronic brush coupled to the controller. The controller sends a command signal to optically address the electrophoretic ink with thermal radiation from the electronic brush. An activation voltage is applied between the upper conductive layer and the lower conductive layer by the controller to allow activation of the optically addressed electrophoretic ink. Another form of the present invention is a system for activating an electronic paint, including a lower conductive layer, a layer of electrophoretic ink disposed on the lower conductive layer, an upper conductive layer disposed on the layer of electrophoretic ink, a controller electrically coupled to the lower conductive layer and the upper conductive layer, and an electronic brush coupled to the controller. A conditioning voltage is applied between the upper conductive layer and the lower conductive layer to precondition the electrophoretic ink. The controller sends a command signal to optically address the preconditioned electrophoretic ink with illuminative radiation from the electronic brush. An activation voltage is applied between the upper conductive layer and the lower conductive layer by the controller to activate the optically addressed electrophoretic ink. The aforementioned forms as well as other forms and features and advantages of the present invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the present invention rather than limiting, the scope of the present invention being defined by the appended claims and equivalents thereof. Various embodiments of the present invention are illustrated by the accompanying figures, wherein: FIG. 1 is an illustration of an electronic paint with a backing layer in accordance with one embodiment of the present invention; FIG.2 illustrates a cross-sectional view of an electronic paint with a backing layer in accordance with one embodiment of the present invention; FIG.3 illustrates a system for activating an electronic paint in accordance with one embodiment of the present invention; FIG.4 shows a plot of switching time versus temperature for an electrophoretic ink in accordance with one embodiment of the present invention; FIG. 5A, FIG. 5B, FIG. 5C, FIG. 5D, and FIG. 5E are illustrations of a method for activating an electronic paint in accordance with one embodiment of the present invention; FIG. 6 is a flow diagram of a method for activating an electronic paint in accordance with one embodiment of the present invention. FIG. 7 shows a plot of switching time for an electrophoretic ink with and without preconditioning, in accordance with one embodiment of the present invention; FIG. 8A, FIG. 8B, FIG. 8C, FIG. 8D, FIG. 8E and FIG. 8F are illustrations of a method for activating an electronic paint in accordance with another embodiment of the present invention; and FIG. 9 is a flow diagram of a method for activating an electronic paint in accordance with another embodiment of the present invention. FIGS. 1 and 2 illustrate an electronic paint 10 employing a lower conductive layer 20, a layer of electrophoretic ink 22, and an upper conductive layer 24, each layer disposed upon the previous layer. Referring to FIG. 1, activation of electrophoretic ink 22 is based on optical addressing and absorption of radiation in a portion of electrophoretic ink 22 and an activation voltage applied between upper conductive layer 24 and lower conductive layer 20. Shining or directing thermal or illuminative radiation onto selected portions of electrophoretic ink 22 adjusts the optical state of the ink. The electric field resulting from the applied activation voltage generates a force to rotate and re-orient and/or displace particles of electrophoretic ink 22, providing a black and white or variable color display from which text, graphics, images, photographs and other image data can be presented. Gray tones or specific colors of electrophoretic ink 22 can be achieved, for example, by controlling the intensity, location and timing of the incident radiation, along with controlling the timing, polarity and magnitude of the activation voltage. A backing layer 26 with or without an array of recessed regions 28 may be coupled to lower conductive layer 20. In one mode, optical addressing of electrophoretic ink 22 is accomplished by applying an activation voltage, locally heating electrophoretic ink 22 with the selective application of thermal radiation, switching the electrophoretic ink to the desired optical state, and quenching or freezing the electrophoretic ink by allowing the electrophoretic ink to cool or by removing the applied activation voltage. In another mode, optical addressing of electrophoretic ink 22 is accomplished by preconditioning the ink, illuminating selected portions of the ink with selective application of illuminative radiation, then applying an activation voltage to switch the electrophoretic ink to the desired optical state. Electrophoretic ink 22 may be quenched or frozen, for example, by removing the activation voltage. Alternatively, the activation voltage may be applied for a time longer than is required to fully switch the optically addressed portions of the ink, and then removed. With optical addressing of electrophoretic ink 22, short pulses or scanned beams of light can be used to control the activation of electrophoretic ink 22 to a desired optical state, even if activation occurs at a slower time scale than the scanning process. In one example, local heating of electrophoretic ink 22 provides a short-term storage effect that allows the scanned beam of light to be moved elsewhere while the image continues to form into the ink. In another example, a latent effect of illumination on a preconditioned layer of electrophoretic ink 22 allows the desired image to be written and subsequently activated with application of an activation voltage. Optical addressing of electronic paint 10 is used to write an image onto an electrophoretic display having electronic paint 10 with, for example, a portable brush or handheld device that locally heats up or selectively heats or illuminates portions of electronic paint 10 as it moves over electronic paint 10. The areas where electrophoretic ink 22 is locally heated or locally illuminated have a shorter switching time. Thus, when an activation voltage is applied between upper conductive layer 24 and lower conductive layer 20, an electric field is generated across the layer of electrophoretic ink 22 and the heated or preconditioned and illuminated regions of electrophoretic ink 22 switch faster than surrounding cooler areas or areas that have not been illuminated. The electric field causes transitions from one optical state to another of electrophoretic ink 22. In one example, while the activation voltage is applied and portions of electrophoretic ink 22 are warm, pixel segments of electronic paint 10 switch to the desired optical state. In another example, when the activation voltage is applied after preconditioning and selective illumination of electrophoretic ink 22, pixel segments of electronic paint 10 switch to the desired optical state. For example, electrophoretic ink 22 may be switched from white to black. In another example, an initially black optical state is switched controllably to a gray or white state. In another example, a white optical state is switched to a gray-scale optical state. In yet another example, colored electrophoretic ink switches from one color to another based on the activation voltage and the thermal or illuminative radiation. After optical addressing and switching has been completed, electrophoretic displays incorporating electronic paint 10 continue to be viewable with no additional power consumption. Referring to FIG.2, electronic paint 10 again includes lower conductive layer 20, a layer of electrophoretic ink 22 disposed on lower conductive layer 20, and upper conductive layer 24 disposed on the layer of electrophoretic ink 22. Layers in the stack may be formed sequentially where, for example, electrophoretic ink 22 is deposited or applied to lower conductive layer 20 and then upper conductive layer 24 is deposited or otherwise applied to electrophoretic ink 22. For example, a layer of electrophoretic ink 22 may be formed on lower conductive layer 20, and then coated with a thin transparent electrode material such as indium tin oxide (ITO). In another example, a layer of electrophoretic ink 22 sandwiched between two conductive layers 20, 24 that has an adhesive material on one conductive layer is adhered to a plastic, glass, or metal backing layer. Since no patterning or masking is required, electronic paint 10 may be formed in other sequences with process steps such as rolling, screening, or depositions in any suitable order. Sections or tiles of electronic paint 10 of various sizes may be assembled together or placed side-by-side to form nearly any desired size of electrophoretic display that can be mounted, for example, on walls or other large surfaces. Electronic paint 10 may be formed with a size, for example, of a few centimeters on a side to as large as one meter by one meter or larger. In an exemplary embodiment of electronic paint 10, images are viewed through transparent upper conductive layer 24, although other embodiments allow backside viewing of or transmissive viewing through electronic paint 10. Reflective displays comprising electronic paint 10 with a metallic backing layer 26 are viewed from the top. Alternatively, electronic paint 10 may be viewed through lower conductive layer 20, and can be optically addressed from its backside. In configurations such as a transmissive display, lower conductive layer 20 is transparent over the visible light range and electrophoretic ink 22 is selectively absorbent, allowing backside viewing of written images or optional backlighting of the display. Image data including text, graphics, drawings or photos may be written onto electronic paint 10 by scanning thermal or illuminative radiation from a scanned laser beam onto a surface of electronic paint 10. In an exemplary electronic-paint display, incident thermal radiation is transmitted through upper conductive layer 24 and is absorbed by the layer of electrophoretic ink 22. Activation of electrophoretic ink 22 is based on thermal absorption of thermal radiation 36 in a portion 38 of electrophoretic ink 22 and on an activation voltage 30 applied between upper conductive layer 24 and lower conductive layer 20. Local temperature increases within the layer of electrophoretic ink 22 may be generated with focused thermal radiation from a suitable source. Thermal radiation 36 includes, for example, infrared radiation, visible light, ultraviolet light, or a combination thereof. Thermal radiation 36 may be generated, for example, with a laser within a handheld electronic brush, and directed towards selected portions 38 of electrophoretic ink 22 with an optical scanner coupled to the electronic brush. As electrophoretic ink 22 heats up, the elevated temperature of electrophoretic ink 22 increases the rate at which the ink will switch, allowing pixel segments of electronic paint 10 to be written in a prescribed manner. As layer of electrophoretic ink 22 cools, electrophoretic ink 22 continues to transition to an intended display state as long as activation voltage 30 is applied. The desired optical state of electrophoretic ink 22 can be locked in or frozen by cooling electrophoretic ink 22, by removing activation voltage 30, or both. In another exemplary electronic-paint display, incident illuminative radiation is transmitted through upper conductive layer 24 and is absorbed by a preconditioned layer of electrophoretic ink 22. Illuminative radiation 36 includes, for example, infrared radiation, visible light, ultraviolet light, or a combination thereof. Illuminative radiation 36 may be generated, for example, with a laser within a handheld electronic brush, and directed towards selected portions 38 of electrophoretic ink 22 with an optical scanner coupled to the electronic brush. When an activation voltage 30 is applied, the illuminated regions of electrophoretic ink 22 switch according to the degree of illumination and the level of activation voltage 30. Electrophoretic ink 22 is preconditioned, for example, with the application of a conditioning voltage 32 applied between upper conductive layer 24 and lower conductive layer 20. Before an image is written, electronic ink 22 of the display material may need to be reset to a well-defined state, such as an all-white surface with white particles moved to the top of the microcapsules. Electrophoretic ink 22 can be forced into an initialized or reset optical state through an applied electric field with, for example, the sustained application of relatively high initialization voltage 34 applied between upper conductive layer 24 and lower conductive layer 20 of electronic paint 10; the application of one or more rapid transitions of activation voltage 30; the application of thermal radiation to heat the layer of electrophoretic ink 22 while applying a relatively large supply voltage; or the flooded exposure of light onto a preconditioned or unconditioned electrophoretic ink 22 with a subsequently applied activation voltage 30. Lower conductive layer 20 comprises, for example, a reflective metal such as aluminum, platinum or chrome, or a transparent electrode material such as indium tin oxide (ITO), a conductive polymer including polyethylenedioxythiophene (PEDOT) doped with polyphenylene sulfide (PPS), or other suitably conductive transparent material. With their concomitant higher thermal conductivity, metals tend to disperse heat more rapidly and to locally spread the image unless they are thin. Electrophoretic ink 22 comprises an electrophoretic material such as encapsulated electrophoretic particles that can be rotated by application of an electric field into a desired orientation. The electrophoretic particles orient themselves along the field lines of the applied electric field and can be switched from one optical state to another based on the direction and intensity of the electric field and the time allowed to switch states. Electrophoretic ink 22 may comprise one of several commercially available electrophoretic inks, commonly referred to as electronic inks or e-ink. The layer of electrophoretic ink 22 comprises, for example, a thin electrophoretic film with millions of tiny microcapsules in which positively charged white particles and negatively charged black particles are suspended in a clear fluid. When a negative electric field is applied to the display, the white particles move to the top of the microcapsules where they become visible to the user. This makes the surface appear white at the top position or outer surface of the microcapsules. At the same time, the electric field pulls the black particles to the bottom of the microcapsules where they are hidden. When the process is reversed, the black particles appear at the top of the microcapsules, which makes the surface appear dark at the surface of the microcapsules. When the activation voltage is removed, a fixed image remains on the display surface. Electrophoretic ink 22 may contain an array of colored electrophoretic materials to allow the generation and display of colored images. Upper conductive layer 24 comprises, for example, a transparent electrode material such as indium tin oxide that provides topside viewing. It should be observed that upper conductive layer 24 and lower conductive layer 20 do not need to be patterned or have any active matrix addressing capability. Upper conductive layer 24 is at least transparent to the wavelength of the thermal or illuminative laser light. A backing layer 26 may optionally be coupled to lower conductive layer 20 to increase the strength or protection of the display while retaining the desired flexibility of the display surface. Backing layer 26 comprises, for example, a sheet of plastic, a sheet of glass, a sheet of metal such as aluminum, copper or a metal alloy, or a ceramic substrate. Backing layer 26 may contain an array of recessed regions 28 to thermally isolate pixel segments in the layer of electrophoretic ink 22. As electronic paint 10 is optically addressed, portions of the layer of electrophoretic ink 22 are locally heated above one or more recessed regions 28 and electrophoretic particles within electrophoretic ink 22 are switched to the desired optical state accordingly. Thermal isolation of pixel segments allows faster switching, higher contrast, and less bleeding of an image into neighboring regions. Recessed regions 28 and the perimeter region may be sized to provide a desired time constant for heating and cooling layer of electrophoretic ink 22 and to provide the desired latency time for switching electrophoretic ink 22. Backing layer 26 may be glued, adhered, or otherwise attached to lower conductive layer 20 of electronic paint 10. Recessed regions 28 may be configured with small, locally isolated points or regions. In one example, the size of recessed regions 28 is on the order of the pixel size for the display. In another example, the size of recessed regions 28 is appreciably smaller than the pixel size for the display, such that more than one recessed region 28 is irradiated with thermal radiation from an applied laser beam to activate electrophoretic ink 22. The array of recessed regions may be configured to encompass, for examples: an array of magenta, yellow, and cyan electrophoretic materials; an array of magenta, yellow, cyan and black electrophoretic materials; or an array of red, green and blue electrophoretic materials for transmissive displays. FIG. 3 illustrates a system for activating an electronic paint including a controller

40, an electronic brush 50 and an electronic paint 10. Electronic brush 50 includes a laser scanner 52 and a position detector 54. Electronic paint 10 includes a lower conductive layer 20, a layer of electrophoretic ink 22, and an upper conductive layer 24. Lower conductive layer 20 comprises, for example, a reflective metal or a transparent electrode material. Upper conductive layer 24 comprises, for example, a transparent electrode material. Controller 40 is electrically coupled to lower conductive layer 20 and upper conductive layer 24. Controller 40 sends command signals to optically address electrophoretic ink 22 with thermal or illuminative radiation from electronic brush 50. Activation of electrophoretic ink 22 is based on thermal or illuminative radiation 36 from electronic brush 50 into a portion 38 of electrophoretic ink 22 and an activation voltage 30 applied by controller 40 between upper conductive layer 24 and lower conductive layer 20 of electronic paint 10. An activation voltage 30 is applied between upper conductive layer 24 and lower conductive layer 20 by controller 40 to activate optically addressed electrophoretic ink 22. With applied activation voltage 30 and incident thermal or illuminative radiation 36 directed onto a portion 38 of electrophoretic ink 22, one or more pixels can be written onto electronic paint 10 as desired. An initialization voltage 32 may be applied to electronic paint 10 to put electrophoretic ink 22 into an initialized optical state. A conditioning voltage 34 may be applied between upper conductive layer 24 and lower conductive layer 20 to precondition electrophoretic ink 22. Thermal or illuminative radiation 36 may be generated, for example, from a laser source within electronic brush 50 and directed by laser scanner 52 onto desired portions of electronic paint 10. Position detector 54 provides position input such as location and rotation to accurately write the desired image. The exemplary electronic paint activation system includes controller 40 that is electrically coupled to electronic brush 50 and controls thermal and illuminative radiation 36 from electronic brush 50 along with other initialization and writing functions. Controller 40, such as a microprocessor, a microcontroller, a field-programmable gate array (FPGA), or other digital device may receive and execute microcoded instructions to write a desired image onto electronic paint 10. Controller 40 controls laser scanner 52 and the light striking electrophoretic ink 22 based on a determined position of electronic brush 50. Controller 40 may be wired to or wirelessly connected to electronic brush 50 with a suitable serial or parallel interface. For example, controller 40 may be contained within a personal computer (PC), a laptop computer, or a personal digital assistant (PDA) and connected to electronic brush 50 via a cable or a short-range wireless link such as Bluetooth™ or 802.11 protocols. Alternatively, controller 40 is contained within electronic brush 50, and image data is provided to electronic brush 50 and controller 40 via a memory device such as a memory stick, or an uplink from a PC, laptop computer or PDA that is optionally connected to the communication network 42. Controller 40 may be connected to a communications network 42 such as a local area network (LAN), a wide-area network (WAN), or the Internet to receive and send information to activate and transfer images onto electronic paint 10. As electronic brush 50 is stroked or swept across the surface of electronic paint 10, thermal or illuminative radiation 36 from laser scanner 52 is directed preferentially at portions of electrophoretic ink 22 to write the image data. Activation voltage 30 may be set to a fixed level as laser scanner 52 optically addresses electronic paint 10. Alternatively, activation voltage 30 may be continuously varied as thermal or illuminative radiation 36 from laser scanner 52 is scanned across the surface of electronic paint 10, while position detector 54 provides sensor information that allows controller 40 to determine the location and rotation of electronic brush 50. The image data can be provided in real time as the image is written with electronic brush 50, or stored within electronic brush 50 until written. In one embodiment, a backing layer 26 such as a sheet of plastic or a sheet of glass is coupled to lower conductive layer 20, offering desirable rigidity and ruggedness, and helping to thermally insulate image pixels and pixel segments from neighboring pixels. Backing layer 26 may include an array of recessed regions that thermally isolates pixel segments in the layer of electrophoretic ink 22. FIG. 4 shows a plot of switching time versus temperature for an electrophoretic ink. Switching response curves 60 and 62 represent two samples of electrophoretic ink that are heated to predetermined temperatures and then activated by an activation voltage. Using a photodetector and a threshold detector, the switching time at each temperature was measured. Switching response curve 60 indicates that an increase in temperature from 20 degrees centigrade to 60 degrees centigrade results in a decrease in switching time from about 150 milliseconds to less than 50 milliseconds. Switching response curve 62 shows a reduction in switching time from about 70 milliseconds at 25 degrees centigrade to about 30 milliseconds at 58 degrees centigrade. In one embodiment of the present invention, the electrophoretic ink layer is locally heated to a temperature of 60 degrees centigrade with thermal radiation from a laser while at the same time a short voltage pulse of about 50 milliseconds is applied. In the area that is locally heated, the electrophoretic ink has sufficient time to switch. In other areas that are close to room temperature, the electrophoretic ink requires a pulse period of 150 milliseconds or more in order to switch, and the short period of 50 milliseconds is not sufficient to have an effect on the electrophoretic ink. In this manner, the electrophoretic ink is optically addressed and activated with application of the activation voltage. FIG. 5A, FIG. 5B, FIG. 5C, FIG. 5D, and FIG. 5E are illustrations of a method for activating an electronic paint 10. Electronic paint 10 is exposed to focused thermal radiation that can control and switch portions of electrophoretic ink 22 within electronic paint 10. These cross-sectional views pictorially illustrate the orientation of particles of electrophoretic ink 22 between a lower conductive layer 20 and an upper conductive layer 24 as thermal radiation is applied at selected portions along with the application of various initialization voltages and activation voltages. The spheres show electrophoretic particles that appear black or white when facing upwards, depending on their orientation. A continuum of gray levels can be achieved by controlling the average rotation of particles in a region. These concepts of switching black and white particles can be readily extended to switching colored particles. The polarity of initialization and activation voltages, the color of electronic ink, the thickness of the various layers, and the aspect ratio for writing an individual pixel have been chosen to be illustrative and instructive. The voltages, the color of electronic ink, the scale and relative thickness of the included layers, and the pixel size may vary appreciably from that shown without departing from the spirit and scope of the claimed invention. A random optical state is illustrated in FIG. 5A, where the aggregate particles appear gray or medium colored to the viewer. This initial optical state could alternatively be represented by a previously written image. An initialization voltage 32 is applied across electrophoretic ink 22 for an extended period of time and at an elevated level to initialize electrophoretic ink 22 into an initialization state corresponding to an all-white surface as shown in FIG. 5B. Initialization voltage 32 may comprise, for example, a relatively large DC voltage or a pulsed voltage. After electrophoretic ink 22 has been initialized, initialization voltage 32 is removed, and the electrophoretic ink remains in the initialized state. When an image is to be transferred to electrophoretic ink 22, an activation voltage 30 is applied between upper conductive layer 24 and lower conductive layer 20, as seen in FIG. 5C. Activation voltage 30 has an opposite polarity compared to initialization voltage 32 to switch selected portions of electrophoretic ink 22 from the initialization state to a predetermined optical state. Activation voltage 30 is set at a lower level to prevent electrophoretic ink 22 from switching until optically addressed. A beam of thermal radiation 36 is applied to a portion of electrophoretic ink 22, as seen in FIG. 5D. Electrophoretic ink 22 heats and switches accordingly. The pixel segment switches, for example, from an initially white optical state to a black optical state with white electrophoretic ink 22 in neighboring areas, in accordance with the levels of activation voltage 30 and applied thermal radiation 36. When thermal radiation 36 and activation voltage 30 are removed, electrophoretic ink 22 remains in the switched optical state, as seen in FIG. 5E, until subsequently rewritten. FIG. 6 is a flow diagram of a method for activating an exemplary electronic paint, as shown in FIG.2 and described with the cross-sectional views of FIG. 5. This flow diagram illustrates the optical addressing of a layer of electrophoretic ink sandwiched between two conductive layers by using thermal radiation. An electronic paint including an electrophoretic ink is initialized, as seen at block 70. The electrophoretic ink may be initialized, for example, to an all-white, an all-black optical state, or to a colored optical state depending on the type of electrophoretic ink and the applied supply voltage. Initialization of the electrophoretic ink is accomplished, for example, with application of a negative supply voltage and flooding or sweeping the electronic paint with thermal radiation to switch the electrophoretic particles within the electrophoretic ink to the initialized state. From this first optical state, the electrophoretic can be adjusted in one common direction based on the driving forces applied to the electrophoretic ink. The electronic paint may be stored in the initialized state for an indeterminate period of time or written upon forthwith. Alternatively, the electrophoretic ink may be initialized with the application of one or more rapid transitions of the supply voltage, or with the application of a sustained, high-level supply voltage. When writing to the electrophoretic ink, an activation voltage is applied as seen at block 72. The activation voltage is applied between an upper conductive layer and a lower conductive layer with the electrophoretic ink disposed therebetween. In one example, the activation voltage is fixed at a positive voltage or a negative voltage depending on the initialized state of the electrophoretic ink. The voltages may be applied by ramping to the desired level. In another example, the activation voltage is varied based on the image data and the position of a scanned beam of laser light so that the driving force on the electrophoretic ink is controlled. Thermal radiation is received at selected portions of the electrophoretic ink, as seen at block 74. In combination with the applied activation voltage, the electrophoretic ink is activated and switches to the desired optical state. As light energy is absorbed, the local temperature of the electrophoretic ink decreases and the electrophoretic ink is activated and switches to the desired optical state. In one example, the activation voltage is applied and sustained while selectively heating the electrophoretic ink. In another example, the electrophoretic ink layer is locally heated to an elevated temperature of about 60 degrees centigrade while a short activation voltage pulse of approximately 50 milliseconds is applied. The electrophoretic ink has sufficient time to switch in the areas that are locally heated, while in other areas, the time of the short pulse is insufficient to switch the electrophoretic ink. In another example, the voltage pulse is kept constant and the local temperature is modulated via the laser power. In another example, the electrophoretic ink is locally heated and the activation voltage pulse is applied for a period of less than 50 milliseconds using pulse width modulation to write gray levels onto the electrophoretic ink. Thermal radiation is received, for example, from a laser scanner that projects and directs thermal radiation such as infrared, visible, or ultraviolet light to locally heat portions of the electrophoretic ink layer. The thermal radiation may be received from a scanned beam of laser light from an electronic brush. The electronic brush includes, for example, a laser scanner and one or more position detectors. The location and rotation of the electronic brush is determined with detector signals from the position detectors. The laser scanner is actuated to direct laser light from the electronic brush onto the electronic paint so that an image may be transferred. The electrophoretic ink is cooled, as seen at block 76. As the electrophoretic ink cools, the written image is frozen or locked in, and the electrophoretic ink is stabilized in the desired optical state. Even as the electronic brush or other thermal activator moves away from the heated portion of the thermal addressing layer, the heated portion of the thermal addressing layer may continue to switch the electrophoretic ink as it cools. If the cooling is too rapid, heat may be dissipated too quickly and the electrophoretic ink incompletely switched. To assist in controlled cooling of the thermal addressing layer, the backing layer of the electronic paint may include an array of recessed regions to thermally isolate pixel segments in the layer of electrophoretic ink. Removal of the activation voltage also freezes or locks in the optical state, even when the electrophoretic ink is still warm. In one embodiment, brief exposure of the electrophoretic ink layer to incident thermal radiation switches rapidly and fully the electrophoretic ink in the vicinity of the heating. In another embodiment, the degree of incident thermal radiation and the cooling rate are controlled to allow the electrophoretic ink to reach an intermediate state even after the source of the incident thermal radiation has moved away from the heated area. Other portions of the electrophoretic ink may be optically addressed and written to by locally heating the desired pixel segments and activating the electrophoretic ink accordingly. To write image data to all portions of the electronic paint, the steps for activating one portion can be performed in series, in parallel, or some combination thereof with the steps for activating another portion of the electronic paint, so that the optical state of each portion is set at the desired level. In an electronic-paint system having an electronic brush, for example, the image data is written onto additional portions of the electronic paint as the electronic brush is moved across the surface of the electronic paint or is lifted from the surface and new strokes are started. After the desired image has been written to the electronic paint, the image may be viewed. Further refreshing or writing of new images may occur as desired within, for example, minutes, hours, days, weeks or even months after writing previous images. In an alternative embodiment for optically addressing an electrophoretic ink display, the electrophoretic ink is preconditioned, written to with illuminative radiation, and then activated with the application of an activation voltage applied across the electrophoretic ink. FIG. 7 shows a plot of switching time for an electrophoretic ink with and without preconditioning. Response curves are shown for unconditioned electrophoretic ink and for preconditioned electrophoretic ink, as measured with a photodetector and shown with arbitrary units. Unconditioned response curve 64 shows typical switching time of an electrophoretic ink from a first state to a second state, and then back again. Switching times from the first state to the second state are on the order of 50 to 150 milliseconds for unconditioned electrophoretic ink. Preconditioned response curve 66 shows appreciably lengthened switching times for a preconditioned electrophoretic ink. Preconditioning of an electrophoretic ink is achieved, for example, with sustained application of a relatively high conditioning voltage. During the preconditioning step, light need not be applied. The conditioning voltage alters the response time of the electrophoretic ink, such that switching times become appreciably longer for preconditioned electrophoretic ink. The preconditioned electrophoretic ink has a switching time as long as two seconds, as shown by the preconditioned response curve 66. The extended switching time of preconditioned electrophoretic ink can be reversed, however, with the application of light. This characteristic can be used to optically address the electrophoretic ink using illuminative radiation. FIG. 8A, FIG. 8B, FIG. 8C, FIG. 8D, FIG. 8E, and FIG. 8F are illustrations of a method for activating an electronic paint 10, in accordance with another embodiment of the present invention. These cross-sectional illustrative views illustrate the orientation of particles of electrophoretic ink 22 between a lower conductive layer 20 and an upper conductive layer 24 as conditioning voltages, initialization voltages, and activation voltages are applied with illuminative radiation at selected portions of electrophoretic ink 22. As described with respect to FIG.5, the spheres represent electrophoretic particles that appear black or white when facing upwards depending on their orientation. A continuum of gray levels can be achieved by controlling the average rotation of the particles. These concepts of switching black and white particles can be readily extended to switching colored particles. A random optical state is illustrated in FIG. 8A, where the aggregate particles appear gray to the viewer. This initial optical state could alternatively be represented by a previously written image. An initialization voltage 32 is applied across electrophoretic ink 22 for an extended period of time and at an elevated voltage level to initialize electrophoretic ink 22 into an initialization state corresponding to an all-white surface as shown in FIG. 8B. After electrophoretic ink 22 has been initialized, initialization voltage 32 is removed, and electrophoretic ink 22 remains in the initialized state. When writing or transferring an image to electrophoretic ink 22, a conditioning voltage 34 is first applied between upper conductive layer 24 and lower conductive layer 20, as seen in FIG. 8C. Conditioning voltage 34 has the same polarity as initialization voltage 32, and the optical state of electrophoretic ink 22 is retained. Conditioning voltage 34 is set at, for example, 30 volts for approximately three minutes. Once preconditioned, electrophoretic ink 22 can be written to using optical addressing. A beam of illuminative radiation 36 from an electronic brush or other optical addressing device is applied to a portion of electrophoretic ink 22, as seen in FIG. 8D. Illuminative radiation 36 is absorbed by electrophoretic ink 22 and the preconditioning effect is effectively reversed, shorting the switching time of the illuminated pixels. Pixels that have not been illuminated retain the extended switching time. During optical addressing with illuminative radiation, initialization voltages, conditioning voltages, or activation voltages need not be applied. An activation voltage 30 is applied between upper conductive layer 24 and lower conductive layer 20, as seen in FIG. 8E. As activation voltage 30 is applied, the area where electrophoretic ink 22 has been illuminated previously switches to the desired optical state. The amount of time for applying activation voltage 30 is selected so that the previously illuminated regions of electrophoretic ink 22 are switched while the preconditioned regions that have not been illuminated have insufficient time to switch or change optical states appreciably. The optically addressed pixel segments switch from the initially white optical state to a predetermined optical state, in accordance with the level of activation voltage 30 and applied illuminative radiation 36. Activation voltage 30 is then removed to lock in or freeze the optical state of electrophoretic ink 22. Electrophoretic ink 22 remains in the switched optical state, as seen in FIG. 8F, until subsequently rewritten. FIG. 9 is a flow diagram of a method for activating an electronic paint in accordance with another embodiment of the present invention. This flow diagram illustrates optical addressing of an electrophoretic ink layer between two conductive layers in an electronic paint using illuminative radiation. An electronic paint including a layer of electrophoretic ink is initialized, as seen at block 80. The electrophoretic ink is initialized, for example, with a sustained negative voltage applied across the electrophoretic ink to switch the electrophoretic ink to an initialized optical state, such as all white or all black. The electrophoretic ink is preconditioned, as seen at block 82. The electrophoretic ink is preconditioned, for example, with application of a conditioning voltage for a sustained period of time and at a higher level to slow the switching time of the electrophoretic ink. When the electrophoretic ink has been preconditioned, the conditioning voltage is removed and the electrophoretic ink is ready to be written to. In one example, a conditioning voltage of 30 volts for three minutes preconditions the electrophoretic ink. Illuminative radiation from, for example, a scanned beam of laser light from a handheld electronic brush, is applied to selected portions of the electrophoretic ink, as seen at block 84. Illuminative radiation may comprise infrared, visible or ultraviolet radiation to optically address the electrophoretic ink. In this step, voltages across the electrophoretic ink are removed. All portions of the electrophoretic ink are optically addressed as desired to locally reduce the switching time. Once the electronic paint has been written to, the electrophoretic ink may be activated. An activation voltage is applied between the upper conductive layer and the lower conductive layer to activate the electrophoretic ink, as seen at block 86. The electrophoretic ink is activated based on the received illuminative radiation and the applied activation voltage. The electrophoretic ink switches to the desired optical state when the activation voltage is applied, with optically addressed regions of electrophoretic ink switching at a faster rate than preconditioned, unaddressed regions. The activation voltage is removed when the electrophoretic ink has reached the desired optical state, as seen at block 88. The written image is frozen or locked, and the electrophoretic ink is stabilized in the desired optical state. After the desired image has been written and activated within the electronic paint, the image may be viewed. Further refreshing or writing of new images may occur as desired within, for example, minutes, hours, days, weeks or even months after writing the previous image. While the embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.

Claims

A method of activating an electronic paint, the method comprising: applying an activation voltage to the electronic paint; receiving radiation on a portion of an electrophoretic ink in the electronic paint; and activating the electrophoretic ink based on the received radiation and the applied activation voltage.

2. The method of claim 1, wherein the electronic paint includes a lower conductive layer, a layer of electrophoretic ink disposed on the lower conductive layer, and an upper conductive layer disposed on the layer of electrophoretic ink; and wherein the activation voltage is applied between the upper conductive layer and the lower conductive layer.

3. The method of claim 1, wherein receiving radiation on the portion of the electrophoretic ink comprises receiving radiation from a scanned beam of laser light from an electronic brush.

4. The method of claim 1, wherein the received radiation comprises thermal radiation, and wherein the thermal radiation is received in the electrophoretic ink when the activation voltage is applied.

5. The method of claim 4, further comprising: cooling the electrophoretic ink; and stabilizing the electrophoretic ink in a predetermined optical state based on the cooling of the electrophoretic ink.

6. The method of claim 1, wherein the received radiation comprises illuminative radiation, and wherein the illuminative radiation is received in the electrophoretic ink prior to applying the activation voltage.

7. The method of claim 6, further comprising: preconditioning the electrophoretic ink, wherein the electrophoretic ink is preconditioned with a conditioning voltage applied to the electronic paint, and wherein the conditioning voltage is applied and removed prior to applying the activation voltage and receiving the illuminative radiation.

8. The method of claim 1, further comprising: removing the activation voltage; and stabilizing the electrophoretic ink in a predetermined optical state responsive to the removal of the activation voltage.

9. The method of claim 1, further comprising: initializing the electrophoretic ink to an initialized optical state.

10. A system for activating an electronic paint, comprising: a lower conductive layer; a layer of electrophoretic ink disposed on the lower conductive layer; an upper conductive layer disposed on the electrophoretic ink; a controller electrically coupled to the lower conductive layer and the upper conductive layer; and an electronic brush coupled to the controller; wherein the controller sends a command signal to optically address the electrophoretic ink with thermal radiation from the electronic brush, and wherein an activation voltage is applied between the upper conductive layer and the lower conductive layer by the controller to allow activation of the optically addressed electrophoretic ink.

11. The system of claim 10, wherein the lower conductive layer comprises one of a reflective metal or a transparent electrode material.

12. The system of claim 10, wherein the upper conductive layer comprises a transparent electrode material.

13. The system of claim 10, wherein the controller is one of wire connected or wirelessly connected to the electronic brush.

14. The system of claim 10, further comprising: a backing layer coupled to the lower conductive layer.

15. The system of claim 14, wherein the backing layer contains an array of recessed regions to thermally isolate pixel segments in the layer of electrophoretic ink.

16. A system for activating an electronic paint, the system comprising: a lower conductive layer; a layer of electrophoretic ink disposed on the lower conductive layer; an upper conductive layer disposed on the layer of electrophoretic ink; a controller electrically coupled to the lower conductive layer and the upper conductive layer; and an electronic brush coupled to the controller; wherein a conditioning voltage is applied between the upper conductive layer and the lower conductive layer to precondition the electrophoretic ink, wherein the controller sends a command signal to optically address the preconditioned electrophoretic ink with illuminative radiation from the electronic brush, and wherein an activation voltage is applied between the upper conductive layer and the lower conductive layer by the controller to activate the optically addressed electrophoretic ink.

17. The system of claim 16, wherein the lower conductive layer comprises one of a reflective metal or a transparent electrode material.

18. The system of claim 16, wherein the upper conductive layer comprises a transparent electrode material.

19. The system of claim 16, wherein the controller is one of wire connected or wirelessly connected to the electronic brush.

20. The system of claim 16, further comprising: a backing layer coupled to the lower conductive layer.