Apparatus and methods for reversible imaging of nonemissive display systems

A writing head capable of rapidly and efficiently generating high field gradients while remaining amenable to low-voltage control utilizes a piezoelectric or Rosen transformer. A one- or two-dimensional array of such writing heads may be used to address a substrate bearing an arrangement of electrically responsive, nonemissive display elements, applying an image pattern thereto. Similarly, the array of writing heads can be used to remove an image, returning the substrate to its original, unimaged state.

FIELD OF THE INVENTION
 The present invention relates to nonemissive display and
 information-bearing elements, and in particular to methods and apparatus
 for creating patterns and images in arrays of such elements.
 BACKGROUND OF THE INVENTION
 Nonemissive displays convey information using contrast differences, which
 are achieved by varying the reflectance or transmission of light; they are
 thus distinct from traditional emissive displays, which stimulate the eye
 by emitting light. One type of nonemissive display is an electrophoretic
 display, which utilizes the phenomenon of electrophoresis to achieve
 contrast. Electrophoresis refers to movement of charged particles in an
 applied electric field. When electrophoresis occurs in a liquid, the
 particles move with a velocity determined primarily by the viscous drag
 experienced by the particles, their charge (either permanent or induced),
 and the magnitude of the applied field.
 An electrophoretic display utilizes charged particles of one color
 suspended in a dielectric liquid medium of a different color (that is,
 light reflected by the particles) is absorbed by the liquid. The
 suspension is housed in a cell located between (or partly defined by) a
 pair of oppositely disposed electrodes, one of which is transparent. When
 the electrodes are operated to apply a DC or pulsed field across the
 medium, the particles migrate toward the electrode of opposite sign. The
 result is a visually observable color change. In particular, when a
 sufficient number of the particles reach the transparent electrode, their
 color dominates the display; if the particles are drawn to the other
 electrode, however, they are obscured by the color of the liquid medium,
 which dominates instead.
 Ideally, the particles maintain a strong uniform charge throughout the
 lifetime of the device and move as rapidly as possible under the influence
 of a relatively small electric field. The "switching time" t of suspended
 particles located between two electrodes, i.e., the time required for the
 population of particles to migrate from one of the electrodes to the
 other, is given by
 ##EQU1##
 where d is the spacing between electrodes, .eta. is the viscosity of the
 liquid medium, .di-elect cons. is its dielectric constant, V is the
 potential difference between the electrodes, and .zeta. is the zeta
 potential of the particles. Thus, the system is usually selected to
 minimize t. For example, the spacing between electrodes is only as large
 as is necessary to ensure that the particles are completely obscured
 following migration away from the transparent electrode.
 Useful electrophoretic displays are bistable: their state persists even
 after the activating electric field is removed. This is generally achieved
 via residual charge on the electrodes and van der Waals interactions
 between the particles and the walls of the electrophoretic cell. As
 disclosed in U.S. Ser. Nos. 08/738,260, 08/819,320 and 08/935,800, and PCT
 application Ser. No. US96/13469, the entire disclosures of which are
 hereby incorporated by reference, electrophoretic displays may be
 fabricated from discrete, microencapsulated electrophoretic 10 elements.
 This approach eliminates the effects of agglomeration on a scale larger
 than the size of the capsule, which preferably is sufficiently small to be
 individually unnoticeable. Thus, the capsules function in a manner similar
 to pixels (although typically they are not individually addressable); even
 if agglomeration occurs, its effect is confined to a very small area.
 Furthermore, by setting an upper limit to the possible size of an
 agglomeration--that is, by preventing accumulations larger than the
 particle content of a capsule--the bulk effects of diminished field
 responsiveness and vulnerability to gravity are likewise limited.
 Electrophoretic displays in accordance with the '260 application are based
 on microcapsunes each having therein an electrophoretic composition of a
 dielectric fluid and a suspension of particles that visually contrast with
 the dielectric liquid and also exhibit surface charges. A pair of
 electrodes, at least one of which is visually transparent, covers opposite
 sides of a two-dimensional arrangement of such microcapsules. A potential
 difference between the two electrodes causes the particles to migrate
 toward one of the electrodes, thereby altering what is seen through the
 transparent electrode. When attracted to this electrode, the particles are
 visible and their color predominates; when they are attracted to the
 opposite electrode, however, the particles are obscured by the dielectric
 liquid.
 This approach is well-suited to applications involving contiguous arrays of
 electrophoretic elements intended to change state in unison. More
 difficult are applications requiring imposition of a visible pattern by
 selective activation of elements in the array. Imaging, in this sense,
 requires the ability to selectively apply electric fields of small spatial
 extent and high magnitude. The dimensions of the field effectively
 determine the resolution of the applied pattern, while the field magnitude
 dictates the switching time of the display and, therefore, the speed at
 which imaging can occur. Of course, the imaging speed is also limited by
 the rate at which the field itself can be toggled between high and low
 states.
 Printer-type applications capable of imaging, at realistic rates,
 substrates bearing a multitude of small electrophoretic display elements
 may require fields on he order of 1 V/.mu.m. Generating such fields
 rapidly, and controlling them with conventional digital logic devices that
 operate at low voltages, represents a significant design challenge.
 DESCRIPTION OF THE INVENTION
 BRIEF SUMMARY OF THE INVENTION
 In accordance with the invention, a writing head capable of rapidly and
 efficiently generating high field gradients while remaining amenable to
 low-voltage control utilizes a piezoelectric or Rosen transformer. A one-
 or two-dimensional array of such writing heads may be used to separately
 address a small portion of a substrate bearing an arrangement of
 electrically responsive, nonemissive microcapsule display elements, and to
 apply an image pattern thereto. Similarly, the array of writing heads can
 be used to remove an image, returning the substrate to its original,
 unimaged state. The microcapsule arrangement can be flat or curved;
 applied to such arrangements, the term "two-dimensional" herein refers to
 configurations that may be fully planar, distorted or curved, and does not
 exclude some third-dimensional thickness. The arrangement can involve
 packing the microcapsules against one another to form a planar display,
 dispersing the microcapsules in a transparent matrix, or forming cavities
 or voids within such a matrix that themselves constitute the
 microcapsules.
 Thus, in accordance with a first aspect of the invention, an arrangement of
 nonemissive, bistable display elements are selectively addressed by at
 least one piezoelectric transformer, the output of which is rectified and
 scanned over the display elements to transfer a predetermined pattern to
 the display.
 In a second aspect, the pattern is transferred by means of a charge
 receptor which may be, for example, associated with a rotating drum. An
 imagewise electrostatic charge pattern is established on the charge
 receptor, which passes the display elements so as to activate the
 display--i.e., alter its visual appearance--in accordance with the
 pattern. The charge receptor may comprise a photoconductor, the imagewise
 electrostatic pattern being established by depositing a substantially
 uniform charge over at least a portion of the receptor, and subsequently
 exposing the charged receptor to an image pattern so as to cancel the
 charge in accordance with the pattern.
 In a third aspect, a piezoelectric transformer is used to sense a voltage
 rather than to generate an electric field; an array of such sensors may
 therefore operate as a scanner. In this way, an electrophotographic charge
 pattern can be sensed and replicated digitally. Indeed, the same sensor
 array can be alternatively employed in a write mode to apply, to a
 nonemissive display sheet, the very image just scanned.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Refer first to FIG. A, which depicts the components of a multi-element
 writing head in accordance with the invention. One element, indicated
 generally at 100, is shown in greater detail and illustrates the mode of
 operation. First and second primary electrodes 105, 107 are disposed at
 one end, and on opposite faces, of a parallelpiped piezoelectric element
 110. At least the portion of element 110 residing between electrodes 105,
 107 is polarized along the thickness of the element, i.e., in the
 direction between the electrodes as indicated by the arrow P.sub.p. A
 secondary electrode 115, shaped to terminate into a tip 117, is disposed
 on the other end of piezoelectric element 110. At least a portion of
 element 110 residing between secondary electrode 115 and the primary
 electrodes 105, 107 is polarized along the longitudinal extent of the
 element, as indicated by the arrow P.sub.s One terminal of an AC power
 source 120 is connected to primary electrode 107, and the other terminal
 of the power source is connected, via a low-voltage switch 122.sub.1, to
 primary electrode 105.
 When switch 122, is closed, power source 120 applies an AC voltage to
 piezoelectric element 110, stimulating mechanical vibration along the
 thickness of element 110--i.e., the axis passing through primary
 electrodes 105, 107--in the region between the primary electrodes. This
 vibration results in a complementary distortion along the length of
 element 110; for example, a rapid compression C in the region of primary
 electrodes 105, 107 induces a transitory longitudinal expansion E along
 the remainder of element 110 in accordance with Poisson's ratio. Owing to
 the longitudinal polarization between primary electrodes 105, 107 and
 secondary electrode 115, mechanical distortion along the length of
 piezoelectric 15 element 110 creates a voltage at secondary electrode 115.
 The magnitude of that voltage depends on the changes in length undergone
 by the longitudinally polarized segment as a result of transverse
 mechanical stimulation; those length changes depend, in turn, on the
 overall length of the longitudinally polarized segment, since mechanical
 force operating over a longer segment will induce a larger change in
 length. The frequency of the induced vibrations--and, hence, the frequency
 of the voltage observed at electrode 115--is the same as the driving
 frequency of power source 120; and if that frequency matches the resonant
 frequency of the piezoelectric element 110, the maximum voltage step-up at
 electrode 115 is obtained.
 Writing-head element 100 may be used to image nonemissive display elements.
 As shown in FIG. 1A, a two-dimensional arrangement of microencapsulated
 electrophoretic display elements 125 is disposed on a substrate 127, which
 may be, for example, paper or plastic. Substrate 127 is itself disposed on
 an electrode 130 dimensionally contiguous (or substantially so) therewith.
 Application of an electric field across elements 125 causes the
 electrophoretic particles therein to migrate along the field in a
 direction determined by the sign of the particles' zeta potential. The tip
 117 of electrode 115 is shaped to concentrate the field between electrode
 115 and planar electrode 130 so that most of the field passes through one
 or a very few display elements 125. A field varying in polarity is clearly
 unsuitable for setting the optical state of an electrophoretic display;
 accordingly, electrode 115 contains a rectifier element 132 that restricts
 the voltage between electrodes 115, 130 to a single polarity.
 The operation of writing-head element 100 is governed by a controller 135,
 which is capable of operating a plurality of writing-head elements by
 selective activation of their switches (representatively indicated at
 122.sub.1, 122.sub.2, 122.sub.3, 122.sub.4). Controller 135 receives image
 data from a source 140 and controls the operation of switches 122 in
 accordance therewith. Switches 122 are low-voltage devices, such as
 transistors, that are actuated by conventional digital signals (generally
 about 5 V); ideally, the controlled voltage is of a similar order. Because
 of the high output voltages required of the transformer (on the order of 1
 V/.mu.m), however, achieving the necessary step-up from digital voltage
 levels may require special transformer designs such as, for example, using
 the multiple-stage approach described in U.S. Pat. No. 5,701,049, the
 entire disclosure of which is hereby incorporated by reference. Source 140
 may be a computer, a scanner, or other device generating and/or storing
 image data.
 FIG. 1B illustrates this operation in greater detail. A writing head 150
 includes a row of elements 100 as described above, all controlled by the
 controller 135. Writing head 150 is positioned adjacent to the substrate
 127, which is coated with electrophoretic elements 125 (not shown in FIG.
 1B), and relative motion is caused to occur between writing head 150 and
 substrate 127. For example, substrate 127 may be affixed to a drum that
 serves as electrode 130, and which rotates past writing head 150. The drum
 may be equipped with an angular encoder that registers movement of the
 drum.
 Controller 135 keeps track of the position of writing head 150 (and, hence,
 each of the elements 100) relative to substrate 127, e.g., by means of
 signals received from the angular encoder. At the same time, controller
 135 receives from source 140 data representative of the image to be
 applied to substrate 127. The image data is typically in a rasterized or
 "bitmap" format; each location in the bitmap corresponds to an imageable
 location on substrate 127, and the contents of each bitmap location
 determine whether the corresponding point on substrate 127 is to be
 "imaged"--i.e., to receive an imaging pulse that alters the optical state
 of the electrophoretic element(s) at that point-or to remain unchanged.
 Controller 135 coordinates the bitmap data with the instantaneous relative
 positions of elements 100 as writing head 150 scans over substrate 127,
 actuating the various elements 100 at appropriate times to reproduce the
 image onto substrate 127. Suitable circuitry for implementing these
 functions is conventional in the scanning, plotting, and printing arts.
 If elements 100 are spaced closely enough together, the fringing fields 155
 emanating from the associated electrode tips 117 spread sufficiently to
 cover the space between the electrode tips; the resolution of the writing
 head, in this case, corresponds to the inter-electrode spacing. If writing
 head 150 extends across the entirety of substrate 127, only a single pass
 thereover is necessary. Otherwise, writing head 150 passes over substrate
 127 multiple times, and is indexed after each pass.
 The maximum speed of relative motion between writing head 150 and substrate
 127 depends on the switching time of the electrophoretic material, given
 the magnitude of the imposed electric field, and the frequency of the
 driving voltage applied to the electrodes. The applied voltage must reach
 its maximum level while the electrode tip remains adjacent to an image
 point, and must also decay to a non-imaging level before the electrode
 reaches the next image location.
 If a resolution finer than the inter-electrode spacing is desired, it is
 possible to utilize multiple, staggered rows of elements 100, all
 controlled by the same controller. In effect, each row of writing elements
 scans over a different series of laterally offset image columns. Indeed,
 it is possible to go still further, utilizing a non-moving,
 two-dimensional array of writing elements. In this way, an image can be
 "stamped" onto a substrate by activating the elements in an imagewise
 pattern and bringing the element array into proximity with substrate 127.
 Conversely, the writing head may consist of as few as one electrode, e.g.,
 contained within a handheld wand that may be wiped over the nonemissive
 display.
 It should also be noted that one may dispense with electrode 130 by
 utilizing complementary electrodes, each of which is connected in the
 manner of electrode 130 and positioned proximate each electrode 115. So
 long as the spacing between electrode 115 and its electrode is
 sufficiently small, the field therebetween can be used to draw
 electrophoretic particles display elements 125 toward the electrodes in an
 imagewise fashion. In this case, the resolution is determined by the
 spacing between the individual electrodes 115, and between each electrode
 115 and its complementary electrode.
 Refer now to FIG. 2, which illustrates a reversible, electrophotographic
 application of the present invention. A rotating drum 200 includes a
 photoconductive surface layer 205 and a grounded metallic backing 210.
 Photoconductive layer 205 is a conventional electrophotographic material
 that is an insulator in the dark but becomes capable of conducting
 electric current when exposed to light. A charging element 215, such as a
 corona device, applies a positive (as shown in the figure) or negative
 charge to photoconductive surface 205, which induces an equal and opposite
 charge at the interface between layers 205, 210. The charge is of
 sufficient overall magnitude (e.g., 1000 V) to facilitate operation as
 discussed below.
 An imaging element 220, located (rotationally) downstream of charging
 element 215, optically focuses an image to be reproduced onto
 photoconductive surface 205. A substrate 225 to be imaged includes an
 arrangement of nonemissive display elements 227, which are disposed on a
 grounding plane 230. Substrate 227 translates at a linear velocity equal
 to the peripheral velocity of the rotating drum 200, so the surfaces of
 substrate 227 and drum 200 pass each other at the same speed; for example,
 the surfaces may be in rolling contact.
 In operation, the reflection of an image to be applied to drum 200 is
 focused onto surface 205 by imaging element 220, scanning along the
 rotating surface to produce thereon an electrostatic charge replica of the
 image on surface 205. The electric field between the charged surface 205
 and ground plane 225 reaches its maximum level when the surfaces are
 closest to each other. Accordingly, as segments of the charge pattern
 rotate into adjacency with substrate 225, they alter the visual appearance
 of display elements 227 in accordance with that pattern. The magnitude of
 the applied charge, the velocity of drum 200 and substrate 225, and the
 switching time of the nonemissive display elements 227 are matched so that
 the image is effectively transferred at an acceptable rate. The image may
 be erased by applying an opposite charge to the entire surface of
 substrate 225, and re-imaged in the manner described above.
 An array of piezoelectric transformers may also be used as a scanner to
 detect patterns of charge deposition. FIGS. 3A and 3B illustrate an
 application of this approach in a high-speed electronic camera. The camera
 300 includes a roll of photoconductive film 305, an optical imaging
 element 310, a charging element, and a reader 320. A motor 322 advances
 the film 305 past imaging element 310.
 Photoconductive film 305 is a three-layer structure that includes a
 photoconductive surface layer 325, a grounded metallic layer 327, and an
 insulating layer 330. As film 305 is advanced, charging element 315
 applies a positive (as shown in the figure) or negative charge to
 photoconductive surface 325, which induces an equal and opposite charge at
 the interface between layers 325, 330. Imaging element 310 optically
 focuses the image to be recorded onto photoconductive surface 325,
 creating a charge replica of the image on surface 325. As film 305 moves
 past reader 320, the reader detects the charge pattern and records it in a
 computer storage device, which may comprise a volatile computer memory
 and/or a nonvolatile mass storage device such as a miniature hard disk.
 It is not necessary, however, for reader 320 to scan each picture
 immediately after it is recorded and film 305 advanced. Because of
 insulating layer 330, the patterns of successive charge "pictures" remain
 undisturbed on layer 325 notwithstanding advancement and re-rolling of
 film 305. Consequently, the images may be recorded and film 305 advanced
 at high speed, with reader 320 retracted or simply inactive. At the
 photographer's convenience, film 305 is wound in the reverse direction at
 the normal operating speed of reader 320, so that recorded images are read
 and successively stored in storage 335. The charge patterns are removed
 from film 305 by applying an opposite charge to the entire surface of
 photoconductive layer 325 as it is re-rolled in the opposite direction.
 The details of a suitable scanning device are shown in FIG. 3B. Each of a
 linear (or other) array of charge-detecting elements 350 includes a
 piezoelectric transformer as shown in FIG. 1A. Instead of being driven by
 an AC power supply, however, a primary electrode of each transformer are
 instead connected to a comparator, representatively shown at 355, and the
 other primary electrode is grounded. The transformer is thus used to step
 down the potential on layer 325 sensed by electrode tips 357 to a voltage
 level suitable for the digital comparator device 355 (see, e.g., Miyauchi
 et al., "Step-down transformer utilizing the piezoelectric transversal
 effect," Transactions of the Institute of Electronics, Information and
 Communication Engineers A, J80-A: 1699-1704, the entire disclosure of
 which is hereby incorporated by reference). The stepped-down sensed
 voltage is compared against a reference voltage V.sub.r corresponding to
 the minimum sensed (stepped-down) voltage that would be produced by a
 deposited charge. A clock circuit (not shown) places and locks the output
 voltages of the comparators onto an output bus, for transmission to
 storage 335, at preset intervals. The frequency of the clock circuit
 determines the longitudinal resolution of scanner 300.
 The lateral resolution of the scanner 300 depends, once again, on the
 proximity of the detecting elements 350. These may, therefore, be arranged
 in multiple staggered rows to improve resolution.
 It should be noted that each charge pattern can be viewed as an image by
 placing it into proximity with a sheet bearing an arrangement of
 nonemissive display elements as previously described. Alternatively, the
 stored images can be applied by elements 350, with transformers configured
 to switchably connect to an AC power supply in accordance with the
 configuration shown in FIG. 1A, so that the elements 350 behave as writing
 elements.
 It will therefore be seen that the foregoing approaches to reversible image
 generation and recording are both versatile in application and
 conveniently practiced using conventional digital circuitry. The terms and
 expressions employed herein are used as terms of description and not of
 limitation, and there is no intention, in the use of such terms and
 expressions, of excluding any equivalents of the features shown and
 described or portions thereof, but it is recognized that various
 modifications are possible within the scope of the invention claimed.