WO2006075289A2

WO2006075289A2 - Optically addressable display

Info

Publication number: WO2006075289A2
Application number: PCT/IB2006/050085
Authority: WO
Inventors: Murray F. Gillies; Mark T. Johnson
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-01-17
Filing date: 2006-01-10
Publication date: 2006-07-20
Also published as: TW200702868A; WO2006075289A3

Abstract

An optically addressable display comprises a pixels (Pi) which has a pixel volume (PVi) filled with a material (BMi) which comprises moveable charged particles (PAi). The pixel volume (PVi) comprises: a reservoir volume (RVi) in which particles are invisible to a viewer, and a display volume (DVi) in which particles are visible to the viewer. The pixel (Pi) comprises a second electrode (DEi; REi) and a third electrode (REi; DEi), both being associated with the pixel volume (PVi) to generate an in-plane electric field (EFi) in the pixel volume (PVi) to move the particles (PAi) between the reservoir volume (RVi) and the display volume (DVi). The pixel (Pi) further comprises a first electrode (BEi) which has a first surface area (SA1i) smaller than a pixel area (PIAi), and a photoconductor (PCi) arranged between the first electrode (BEi) and the third electrode (REi; DEi). The optically addressable display further comprises: a driver (DR) which supplies a first voltage (VIi) to the first electrode (BEi), and a second voltage (V2i) to the second electrode (DEi; REi). The third electrode (REi; DEi) is floating. A light source (LS) selectively illuminates the photoconductor (PCi).

Description

Optically addressable display

The invention relates to an optically addressable display comprising a plurality of pixels, a controller for use in such a display, and a method of displaying.

US-A-2003/0011868 discloses a card which includes a photoconductive layer and an electrophoretic layer. The impedance of the photoconductive layer is lowered when struck by light from a light emitting layer. Where the impedance of the photoconductive layer is lower, the electrophoretic layer may be addressed by an applied electrical field to update an image on the card. In an embodiment, the light emitting layer is open from the rear, and is addressed via direct drive or active matrix drive schemes. An electrical change in the light- emitting layer either causes an optical response across a corresponding sub-pixel of the display or, by electrical connection, causes an optical response across the entire pixel. By creating sub-pixel regions in which the photoconductive layer differs, varying optical effects are achieved. The different photoconductive layers of the sub-pixels may be sensitive to different wavelengths, intensities, or duration of the impinging light.

In this prior art the electrophoretic layer is preferably an encapsulated electrophoretic layer which contains particles in a suspending fluid. The particles are moved from one side to the other side of the display by applying a suitable voltage between electrodes sandwiching the electrophoretic layer. Once the particles are on one side of the display, they will remain in a stable position near the wall of the electrophoretic layer, i.e. the display is bi-stable. The voltage is then reversed by ramping very slowly to the opposite polarity. With the voltage remaining reversed, the photoconductive layer is illuminated image-wise. The particles then move to the opposite electrode in regions of the electrophoretic layer that is adjacent to the illuminated regions of the photoconductive layer. This driving method has the disadvantage that during the ramping, which has to be very slow, no image is visible. This dead-time is for today's electrophoretic displays in the order of 30 seconds. Consequently, an update period during which the image is updated has a considerable duration. It is an object of the invention to provide an optically addressable display with a reduced update period.

A first aspect of the invention provides an optically addressable display as claimed in claim 1. A second aspect of the invention provides a controller for use in an optically addressable display as claimed in claim 25. A third aspect of the invention provides a method of displaying as claimed in claim 28. Advantageous embodiments are defined in the dependent claims.

The optically addressable display in accordance with the first aspect comprises a pixel which has a pixel volume which comprises material with moveable charged particles. The pixel volume comprises a reservoir volume in which particles are invisible to a viewer and a display volume in which particles are visible to the viewer. The optical state of the pixel is determined by the number of particles present in the display volume. A second electrode and a third electrode are associated with the pixel volume to generate an in-plane electric field in the pixel volume to move the particles between the in-plane arranged reservoir volume and the display volume. A photoconductor is arranged between the third electrode and a first electrode which has a first surface area which is smaller than the pixel area.

The third electrode may be a display electrode which is associated with the display volume and covers a surface area referred to as the display area. The second electrode is then the reservoir electrode which is associated with the reservoir volume and covers a surface which is referred to as the reservoir area. Now, the photoconductor is present within the display area. Alternatively, the third electrode may be the reservoir electrode and the second electrode is then the display electrode. Now, the photoconductor is present within the reservoir area. Usually, the reservoir area is smaller than the display area. Preferably, the reservoir area is at least 5 times smaller than the display area. When the photoconductor is present in the reservoir area, it has no influence on the optical state of the display volume.

A driver supplies a first voltage to the first electrode and a second voltage to the second electrode. The third electrode is floating. When a light source selectively illuminates the photoconductor of the pixel its impedance is relatively low and it effectively forms an electrical connection between the first and the third electrode. Thus, the voltage present across the first and the second electrode is now present across the second and third electrode and an electrical field is generated in the pixel volume. If the photoconductor is not illuminated, its impedance is relatively high and it effectively forms an electrical separation between the first and the third electrode. Thus the voltage present across the first and second electrode is substantially present across the photoconductor and not between the second and the third electrode. Consequently, the optical state of the pixel is not changed.

The dead time is shortened with respect to the prior art because the photoconductor which is associated with either the second or the third electrode only covers part of the pixel area and thus has a lower capacitance than in the prior art where this layer covers the complete pixel area.

In the prior art, to be able to erase the electrophoretic material in the absence of light, the capacitance of the photoconductor layer must be much larger than the capacitance of the electrophoretic layer. A rapid change of the voltage across the stack of the electrophoretic layer and the photoconductor layer capacitively splits across these capacitances. The majority of the voltage will be present over the electrophoretic layer and erase this layer. In the erased state, all pixels have the same limit optical state. The drawback of this strategy is that the erasure without having light impinging on the photoconductor layer occurs for every last polarity change of the applied voltage. Therefore, during the transition from the erase phase to the address phase, a required polarity change has to be performed sufficient slowly such that the capacitive split is not relevant and the voltage division of the layers is determined by the resistance of the layers. For example, in a practical application, about 30 seconds are required to change the voltage from -15V to +15 V without causing a visible change of the optical state of the electrophoretic layer. If the capacitance of the photoconductive layer were lowered, the required ramp time (or dead time) would be reduced. However, then the image cannot anymore be erased by pulses in the absence of light, because the voltage across the electrophoretic material decreases. Anyhow, an acceptable capacitance of the photoconductive layer, which allows this type of erasure often results in a very thick layer which easily cracks.

In the cell structure in accordance with the present invention, a separate reservoir volume and display volume are present. The particles are moved between the reservoir volume and the display volume by generating an electric field between these two volumes. Therefore, a reservoir electrode is associated with the reservoir volume and a display electrode is associated with the display volume. Now, two options exist. The photoconductive element (or layer with a restricted area), which now, per pixel, is present locally only, may be present between the display electrode and a further electrode, or the local photoconductive layer may be present between the reservoir electrode and the further electrode. The driver supplies the voltage between the further electrode and the electrode to which the local photoconductive layer is not associated. If light impinges on the photoconductive layer the two electrodes sandwiching the photoconductive layer are electrically connected. Otherwise the one of the electrodes associated with the photoconductive layer which is not the further electrode is floating. As the surface area of the photoconductive layer is smaller than the surface area of the pixel, its capacitance is lower than in the prior art and thus the dead time is shorter. In fact, the operation of the pixel in accordance with the invention is largely independent of the surface area of the photoconductive layer. It is thus possible to select this surface area much smaller than the surface area of either the display electrode or the reservoir electrode. This allows obtaining a capacitance which is much lower than in the prior art and thus a much shorter ramp time.

In a display with a plurality of pixels, each one of the pixels may have the same structure as elucidated hereinabove. The optical state of each pixel separately can be determined by selectively supplying light to their associated photoconductors. Preferably, the surface area of the photoconductive layer is selected to obtain a capacitance of this layer which is smaller than the capacitance formed by the electrophoretic material such that fast changing voltages are predominantly present across the photoconductive layer and do not disturb the optical state of the electrophoretic material. Now, it is not anymore required to have a slow changing voltage at all because the state of the electrophoretic material is not influenced. Consequently, the image update period is considerably shortened.

The material may be electrophoretic material, or cholesteric LC material. In an embodiment in accordance with the invention, the first surface area is selected such that a capacitance of the photoconductor is obtained which is smaller than a capacitance of the material. Consequently, fast changing voltages are predominantly present across the photoconductor and not across the material. It is thus allowed to rapidly change the voltage without influencing the optical state of the pixel.

In an embodiment in accordance with the invention, the controller controls the light source and the driver in the following sequence. Firstly, the controller activates the light source to illuminate the photoconductor, and controls the driver to supply a voltage between the first electrode and second electrode to move the particles into the reservoir volume. Now, the pixel is reset or erased. Secondly, the controller deactivates the light source and controls the driver to supply a voltage between the first electrode and the second electrode suitable for moving the particles from the reservoir volume towards the display volume. Finally, the controller selectively activates the light source to illuminate the photoconductor to determine a number of the particles to be actually moved from the reservoir volume into the display volume. The light source is not activated if the optical state of the pixel should not change. Thus, the light source only illuminates the photoconductors of pixels which should change their optical state by moving particles into the display volume.

The amount of particles moved from the reservoir volume to the display volume can be controlled by one or more of the following actions: varying the intensity of the light source, varying the duration of the light pulse generated by the light source, varying the level of the voltage between the first and the second electrode, or varying the duration of voltage pulse between the first and the second electrode. The image data can be written via pulses applied in either one (sub-)frame or divided over more sub- frames.

In an embodiment in accordance with the invention, the controller controls a back-light, the light source, and the driver in the following sequence. Usually, a back-light is present if the display comprises a plurality of pixels. However, although the operation in the now following is elucidated with respect to a plurality of pixels, a single pixel may be driven in a similar manner. Firstly, the back- light unit is activated to illuminate the photoconductors, and the driver is controlled to supply a voltage between the first and the second electrodes to move the particles from the display volumes into the reservoir volumes. Secondly, the back light unit is deactivated, and the driver is controlled to supply a voltage between the first and second electrodes suitable for moving the particles from the reservoir volumes towards the display volumes. Finally, the light sources are selectively activated to illuminate the corresponding photoconductors to determine a number of the particles to be moved into the corresponding display volumes and to change the optical state of the corresponding pixels.

In an embodiment in accordance with the invention, the light source comprises a scanning light emitting device. The light beam of the scanning light emitting device, such as for example a laser or a light emitting diode, is scanning along the pixels to sequentially illuminate the photoconductors of the pixels. The light emitting device itself may be moved to obtain the scanning light beam, or the light emitting device may be stationary and a movable optical unit deflects the light beam. In an embodiment in accordance with the invention, the light source comprises a plurality of light sources. Each one of the plurality of light sources is associated with one of the plurality of pixels. Thus, each photoconductor of each pixel is illuminated by an associated light source. Preferably, the array of light sources comprises light emitting diodes. In an embodiment in accordance with the invention, the second electrodes are display electrodes associated with the display volumes, and the third electrodes are reservoir electrodes associated with the reservoir volumes. Now, the photoconductors, which are present between the first and third electrodes are associated with the reservoir electrodes which occupy a smaller area of the pixel than the display electrodes. Consequently, the capacitance of the photoconductor will be small. Further, as the reservoir volumes are shielded from the viewer, the area covered by the photoconductor does not influence the optical state of the pixel.

In an embodiment in accordance with the invention, the third electrodes are display electrodes associated with the display volumes, and the second electrodes are reservoir electrodes associated with the reservoir volumes.

In an embodiment in accordance with the invention, the first surface areas are smaller than surface areas of the display electrodes. This has the advantage that the capacitance of the photoconductor will be small. Further, the area covered by the photoconductor only minimally influences the optical state of the pixel.

In an embodiment in accordance with the invention, the optically addressable display is a matrix display wherein the pixels are arranged in a common plane. The pixels comprise different materials which have different optical properties. The pixels with the different optical properties are grouped to form color display elements. The second electrodes of the pixels are interconnected. The first electrodes are interconnected for pixels with the same material. The driver supplies first voltages to the different groups of interconnected first electrodes, and a single second voltage to the second electrodes.

For example, in a full color matrix display, each color display element comprises three kinds of pixels with red, green, blue colored particles, respectively. Each pixel color may be driven in the same manner. For example, each color can be addressed sequentially with corresponding voltages and light pulses. It is also common practice to use the term "display pixel" instead of the display elements mentioned above, and to use the term "sub-pixels" instead of the pixels mentioned above.

In an embodiment in accordance with the invention, the light source comprises a plurality of light sources each arranged for illuminating at least two pixels of a same one of the color display elements. Preferably, each light source illuminates all the pixels of the same color display element. This means that the light beam covers all the photoconductors of the pixels of the same display element. The sensitivity to intensity and/or duration of the light pulse may be different for different pixels of the same display element. Firstly, the controller activates the light sources or a back- light unit to illuminate the photoconductors, and controls the driver to sequentially supply a voltage between the first and the second electrodes to move the different particles into the reservoir volumes. Secondly, the controller deactivates the light sources or the back-light unit. Thirdly, the controller controls the driver to sequentially supply a drive voltage between the first and the second electrodes of the different pixels of the same color display elements. The drive voltage is suitable to move the different particles towards the display volumes. Further, the controller sequentially and selectively activates the light sources to only illuminate the photoconductors associated with the pixels which receive the drive voltage. Thus, the different pixels of the same display elements receive a drive voltage one by one. The light source illuminating all the photoconductors of these different pixels of the same display element is also activated sequentially in synchronism with the drive voltages.

For example, it is assumed that the display elements comprise red, green and blue pixels. First, when the light source is deactivated, the drive voltage is supplied to the red pixels, the light source is activated and illuminates the photoconductors of the red, green and blue pixels. But only the red pixel may change its state because no drive voltage is present across the green and blue pixels. Then, the light source is deactivated, the green pixel is driven, and the light source is activated. Now only the green pixel may change state. Finally, again the light source is deactivated, the blue pixel is driven, and the light source is activated again. Preferably, the amount of colored particles which move from the reservoir volume to the display volume is determined by the intensity or duration of the light pulse generated by the light source, such that the same drive voltage can be used.

In an embodiment in accordance with the invention, the optically addressable display is a matrix display which comprises a stack of at least a first layer of first pixels and a second layer of second pixels. The first and the second pixels are aligned to obtain groups of stacked pixels which form color display elements. In a full color display, three pixels may be stacked. The first pixels have interconnected first electrodes, interconnected second electrodes, and separate floating third electrodes. The second pixels have interconnected first electrodes, interconnected second electrodes, and separate floating third electrodes. The driver supplies first voltages and second voltages between the first electrodes and the second electrodes, respectively of the different layers. This construction makes it possible to separately update the first and the second pixels. For example, by firstly applying a drive voltage between the pixels of the first layer, then activating the light sources which may illuminate both the first and the second pixels. If the optical state is determined by the intensity of the light sources, the intensity of the light sources is selected in accordance with the optical state to be reached by the pixels in the first layer. After a predetermined period in time the light sources are deactivated. Now, a drive voltage is applied to the second pixels, and again the light sources are activated. In this example, it is assumed that the light source is illuminating both the photoconductor of the first and the second pixel of a display element. In an embodiment in accordance with the invention, during a reset phase, the controller controls the driver to supply the first voltages and the single second voltage or the plurality of second voltages to all the pixels. Now, all the pixels are erased during a same single reset or erase period. This has the advantage that the duration of the reset phase is relatively short.

In an embodiment in accordance with the invention, the photoconductors are spatially displaced for the pixels of a same one the color display elements. This enables the light source to separately illuminate the photoconductors of the pixels of a same color display element. In an embodiment in accordance with the invention, the light beam generated by the light source is focused on different depths where the different ones of the photoconductors of a same one of the color display elements are present. Now, the light source is able to separately illuminate the different photoconductors of the same display element and it is possible to provide the drive voltages in parallel to the different pixels. In an embodiment in accordance with the invention, the light source comprises different light sources having different spectra. Photoconductors associated with each color of pixels of a same one of the color display elements are sensitive to different spectra only. Now, the different pixels of a color display element can be driven at the same time by applying suitable voltages and then activating the light sources during a same period in time. In an embodiment in accordance with the invention, the second electrode is a display electrode associated with the display volume, the third electrode is a reservoir electrode associated with the reservoir volume. The pixels comprise a further reservoir volume and a further reservoir electrode associated with the further reservoir volume. A further photoconductor is arranged between the further reservoir electrode and a further first electrode. The driver further supplies a further first voltage to the further first electrode. In a starting state, first and second particles having different optical properties are present in the first mentioned reservoir volume and the further reservoir volume, respectively. Such a construction provides a multi-color display. In an embodiment in accordance with the invention, the first mentioned reservoir electrode and the further reservoir electrode are interconnected. Firstly, the controller supplies a voltage between the display electrode and the interconnected reservoir electrodes suitable for moving the particles of either the first mentioned reservoir volume or the further reservoir volume to the display volume. Secondly, the controller controls the light source to illuminate either the first mentioned or the further photoconductor, respectively. Thirdly, the controller controls the driver to supply a voltage between the display electrode and the interconnected reservoir electrodes suitable for moving the particles out of the display volume back into the first mentioned reservoir volume or the further reservoir volume (RViI), respectively. Finally, the controller controls the light source to illuminate either the first mentioned or the further photoconductor, respectively, for moving the particles out of the display volume back into the same one of the first mentioned reservoir volume or the further reservoir volume wherefrom they were moved to the display volume earlier. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawings:

Fig. 1 shows schematically a pixel structure and its drivers in accordance with an embodiment of the invention,

Fig. 2 shows schematically a pixel structure and its drivers in accordance with another embodiment of the invention,

Fig. 3 shows schematically surface areas of a pixel in accordance with an embodiment of the invention, Figs. 4A-4B show schematically a color display element with three adjacent in-plane pixels in accordance with an embodiment of the invention,

Figs. 5A-5B show schematically a color display element with three stacked pixels in accordance with an embodiment of the invention,

Figs. 6A-6D shows different optical states of a pixel with multiple reservoir volumes in accordance with an embodiment of the invention, and

Figs. 7A-7C show schematically, in three steps, how a multiple reservoir pixel can be manufactured. The same references in different Figures refer to the same items having the same function.

Fig. 1 shows schematically a pixel structure and its drivers in accordance with an embodiment of the invention. It has to be noted that only a single pixel Pi is shown. In a practical implementation, many pixels Pi may be present. The pixel Pi comprises a pixel volume PVi which is filled with a material which for example, is an electrophoretic material. The electrophoretic material comprises, for example charged particles PAi in a suspension. A display electrode DEi is associated with a display volume DVi of the pixel volume PVi. A reservoir electrode REi is associated with a reservoir volume RVi of the pixel volume PVi. The optical state of the pixel Pi as observed by a viewer depends on the number of particles PAi present in the display volume DVi. The particles present in the reservoir volume RVi are invisible to the viewer and thus do not influence the optical state of the pixel Pi. Preferably, the reservoir volume RVi is shielded from the viewer by a black layer BMEi associated with the reservoir volume RVi and present between the reservoir volume RVi and the viewer. A voltage between the display electrode DEi and the reservoir electrode REi causes an electrical field EFi in the pixel volume PVi between the reservoir volume RVl and the display volume DVi. This field EFi may be used to move the particles PAi into the reservoir volume RVi during a reset or erase phase, or to move the particles PAi into the display volume DVi during an update phase. A photoconductor PCi is present between the reservoir electrode REi and a bottom electrode BEi. A driver DR supplies a first voltage VIi to the bottom electrode BEi and a second voltage V2i to the display electrode DEi. A light source LS may illuminate the photoconductor PCi. A controller CO supplies a control signal Cl to the light source LS and a control signal C2 to the driver DR. The operation of this pixel structure is elucidated in the now following. In the starting situation, it is assumed that all the particles PAi are in the reservoir volume RVi. Now, the driver DR supplies the first and second voltage VIi and V2i such that if connected between the reservoir electrode REi and display electrode DEi, an electrical field EFi is obtained which would move the particles PAi towards the display volume DVi. Then, for those pixels Pi which should change their optical state, the light source LS illuminates the associated photoconductors PCi to connect the voltages VIi on the bottom electrodes BEi to the corresponding reservoir electrodes REi. Consequently, the voltage difference Vli-V2i is present between the reservoir electrode REi and the display electrode DEi and the particles PAi start moving from the reservoir volume RVi towards the display volume DVi. For the pixels Pi for which the optical state should not change, the light source LS does not illuminate the associated photoconductors PCi.

The number of particles PAi moved into the display volume DVi may be controlled with the voltage difference Vli-V2i, or with the duration of a predetermined voltage difference, or with an intensity or duration of the light pulse supplied by the light source LS to the photoconductor PCi. Combinations of these control mechanisms are possible. The voltage difference Vli-V2i is also referred to as the drive voltage.

The light source LS may be a single light source which scans along the display to sequentially illuminate the photoconductors PCi of pixels Pi which should change their optical state. Preferably, the light source LS has an intensity which is modulated to obtain different optical states for different pixels Pi. The light source LS itself may be moved to obtain the scanning light beam. Alternatively, the light source LS may be stationary and an optical unit moves to deflect the stationary light beam generated by the light source to obtain the scanning light beam. Preferably, such a single light source is a laser or a high power light emitting diode. The light source LS may also comprise multiple light generating elements, such as for example light emitting diodes, preferably one for each pixel.

Fig. 2 shows schematically a pixel structure and its drivers in accordance with another embodiment of the invention. In fact this pixel structure is largely the same as that shown in Fig. 1. The only difference is that the photoconductor is now arranged between the bottom electrode BEi and the display electrode DEi instead of the reservoir electrode REi. The operation of this structure is the same as that of the pixel structure shown in Fig. 1. Fig. 3 shows schematically surface areas of a pixel in accordance with an embodiment of the invention. The surface areas are indicated in a top view of the pixel structure of Fig. 2. The total surface area of the pixel Pi is PIAi. The display electrode DEi covers the surface area DAi, the reservoir electrode REi covers the surface area RAi, and the photoconductor PCi covers the area SAIi. Usually, the display area DAi is much larger than the reservoir area RAi to optimize the visible part of the pixel area PIAi. For example, the display area DAi is at least ten times larger than the reservoir area RAi. In Fig. 3, the photoconductor PCi is present below the display area DAi. The photoconductor area SAIi is relatively small with respect to the pixel area PIAi. This allows obtaining a capacitance of the photoconductor PCi which is much smaller than the capacitance of the material BMi. Consequently, fast changing voltages VIi and V2i will be predominantly present across the photoconductor PCi, and thus do not anymore influence the optical state of the pixels Pi. This in contrast to the prior art where the photoconductor PCl covers the complete pixel area PIAi and has a relatively large capacitance. Such a large capacitance requires the voltages VIi and V2i to change very slowly between the reset phase and the update phase.

The photoconductor area SAIi may cover the complete display area DAi, still its capacitance is smaller than in the prior art. But, then the photoconductor PCi should be transparent or the backlight color point should be compensated for the absorption of the photoconductor. If the photoconductor is present below the reservoir electrode REi, the photoconductor area SAIi may be selected equal to or smaller than the reservoir area RAi. Preferably, the photoconductor PCi is present below the reservoir electrode REi because then the display area DAi is not partly obstructed by the photo conductor PCi. Further, preferably, the photoconductor area SAIi is selected to optimize the capacitance of the photoconductor PCi such that it is substantially smaller than the capacitance of the material BMi present in the pixel Pi. In the structure shown in Fig. 1 and Fig. 2, it is easily possible to tailor the capacitance of the photoconductor PCi by selecting the appropriate photoconductor area SAIi. In contrast, in the prior art the ratio of the capacitance of the photoconductor layer and the electrophoretic layer can only be optimized by either decreasing the thickness of the electrophoretic layer or increasing the thickness of the photoconductor layer.

Figs. 4A-4B show schematically a color display element with three adjacent in-plane pixels in accordance with an embodiment of the invention. By way of example only, the display element CDEi shown comprises three pixels Pi, PiI, Pi2. Each of the three pixels Pi, PiI, Pi2 may have a structure as shown in Fig. 1 or Fig. 2. In Figs. 4A-4B, the pixels Pi, PiI, Pi2 have the pixel structure shown in Fig. 2: the photoconductors PCi are present between the reservoir electrodes REi (REl, RE2 and RE3) and the bottom electrodes BEi.

Fig 4A shows a top view of the display element CDEi. The display electrodes DEl to DE3, and the reservoir electrodes REl to RE3 are shown as white areas. The bottom electrodes BEl to BE3 are shown as black areas. The reservoir volumes RVi and the display volumes DVi are indicated at the bottom. Different particles PAi, PAiI and PAi2 with different optical properties are present in the pixels Pi, PiI, Pi2, respectively. Preferably, the different particles PAi, PAiI and PAi2 have different colors. The different colors are, for example, red, green and blue. The driver DR supplies the voltages VIi to the bottom electrodes BEl to BE3.

Fig. 4B shows a side view of the display element CDEi. The display electrodes DEl, DE2, DE3 shown in Fig. 4A are now referred to as DEi, the reservoir electrodes REl, RE2, RE3 are now referred to as REi, and the bottom electrodes BEl, BE2, BE3 are now referred to as BEi. In fact, the pixels Pi, PiI, Pi2 of the display element CDEi, have a structure which is very similar to the construction shown in Fig. 2. Only, their aspect ratio is different, and the display electrode DEi and the reservoir electrode REi are now present inside the pixel volume PVi. Further, a backlight unit BL is present.

The pixels Pi, PiI, Pi2 of the display element CDEi may be operated sequentially, wherein each one of the pixels Pi, PiI, Pi2 is operated in the same manner as is elucidated with respect to Fig. 1. However, it is possible to reset all the pixels Pi, PiI, Pi2 during a common reset period wherein a same drive voltage is supplied to all the pixels Pi, PiI, Pi2 while all the photoconductors PCi are illuminated. This significantly shortens the reset period wherein all the particles PIAi are moved into the reservoir volumes REi. If the different photoconductors PCi are illuminated by a single light beam, the updates of the different pixels Pi have to be performed sequentially. The update of the pixels Pi may be performed in parallel if the different photoconductors are illuminated by different light beams which can be separately modulated, or if the different photoconductors PCi are sensitive to different spectra of associated light sources. For example, the light sources may provide three different spectra, and the photoconductors PCi are sensitive to only one of these different spectra.

This pixel layout is relatively simple to make with roll-to-roll processing. The display electrodes DEi are all connected together, whilst there are now three separate sets of bottom electrodes BEl to BE3. Each set of bottom electrodes BEl to BE3 is common for all pixels of a given color. The reservoir electrodes REi are left floating. There are only four external connections required for this display (one for each group of electrodes). The display electrode can be made from ITO (as is the bottom electrode) so that the display can be used in the transmissive mode. Alternatively, the display could be used in a reflective mode by adding a reflective layer, such as a reflective display electrode. Preferably, the dimensions of the bottom electrode are selected to tune the capacitance of the photoconductor PCi to be substantially smaller than that of the cell.

Figs. 5A-5B show schematically a color display element with three stacked pixels in accordance with an embodiment of the invention. By way of example only, the display element CDEi shown comprises three pixels Pi, PiI, Pi2. Each of the three pixels Pi, PiI, Pi2 may have a structure as shown in Fig. 1 or Fig. 2. In Figs. 5A-5B, the pixels Pi, PiI, Pi2 have the pixel structure shown in Fig. 2: the photoconductors PCi are present between the reservoir electrodes REi (REl to RE3) and the bottom electrodes BEi (BEl to BE3).

Fig 5 A shows a top view of the display element CDEi. The display electrode DEl and the reservoir electrode REl of the top-pixel Pi are shown as white areas. The bottom electrode BEl of the top-pixel Pi is shown as a black area. The voltages VIi are supplied to the bottom electrodes BEi.

Fig. 5B shows a side view of the display element CDEi. In fact, the pixels Pi, PiI, Pi2 of the display element CDEi, have a structure which is very similar to the construction shown in Fig. 2. Again, the display electrodes DEi and the reservoir electrodes REi are now present inside the pixel volume PVi. Further, a backlight unit BL is present. The pixels Pi, PiI, Pi2 have the reservoir volumes RVi (RVl to RV3) and the display volumes DVi (DVl to DV2), respectively. Different particles PAi, PAiI and PAi2 with different optical properties are present in the pixels Pi, PiI, Pi2, respectively. Preferably, the different particles PAi, PAiI and PAi2 have different colors. The different colors are, for example, cyan, magenta and yellow. The driver DR supplies the voltages VIi (not shown) to the bottom electrodes BEi.

The pixels Pi, PiI, Pi2 of the display element CDEi may be operated sequentially, wherein each one of the pixels Pi, PiI, Pi2 is operated in the same manner as is elucidated with respect to Fig. 1. However, it is possible to reset all the pixels Pi, PiI , Pi2 during a common reset period wherein a same drive voltage is supplied to all the pixels Pi, PiI, Pi2 while all the photoconductors PCi are illuminated. This significantly shortens the reset period wherein all the particles PIAi are moved into the reservoir volumes REi.

If the different photoconductors PCi are illuminated by a single light beam, the updates of the different pixels Pi have to be performed sequentially. The update of the pixels Pi may be performed in parallel if the different photoconductors are illuminated by different light beams which can be separately modulated, or if the different photoconductors PCi are sensitive to different spectra of associated light sources. For example, the light beam may be focused on the photoconductor PCi which should be illuminated. In another example, the light sources may provide three different spectra, and the photoconductors PCi are sensitive to only one of these different spectra. Now, the backlight should contain all the three spectra to be able to simultaneously reset all the pixels Pi.

This stacked pixel layout is relatively simple to make with roll-to-roll processing. For each layer, the display electrodes DEi are all connected together as are the bottom electrodes BEi. The reservoir electrodes REi are left floating. There are only two external connections required for each layer of this display (one for each group of electrodes). The display electrode is made from ITO (as is the bottom electrode) so that the display can be used in the stacked format, in either the transmissive or reflective modes. Preferably, the dimension of the bottom electrode is selected to tune the capacitance of the photoconductor PCi to be substantially smaller than that of the electrophoretic material. The display is completed by stacking 2 or more of such layers on top of each other. In Fig. 5B, three layers with subtractive colored magenta, cyan and yellow particles (which absorb green, red and blue wavelengths, respectively) are illustrated. In a roll-to-roll process, these layers will typically be obtained by lamination.

Figs. 6A-6D show a pixel with multiple reservoir volumes in accordance with an embodiment of the invention. In each of the Figs. 6 A to 6D, by way of example only, the pixel Pi comprises four reservoir volumes RVi, RViI, RVi2, and RVi3, collectively also referred to as RVi, per display volume DVi. Preferable, these four reservoir volumes RVi are positioned at the corners of the pixel Pi. Further, in a starting situation shown in Fig. 6A, again by way of example only, four different positively charged particles are present in the reservoir volumes RVi. The different particles have different optical properties, which are, for example, different colors. A photoconductor PCi (not shown) is present below each reservoir electrode REi. The photoconductors PCi are sandwiched between the associated reservoir electrodes REi and bottom electrodes BEi (not shown).

In Fig. 6A, a short pulse of +15V is supplied to the reservoir electrode REi while the other reservoir electrodes REiI, REi2, REi3 are on a level of +5 V. This pulse causes the particles in the top left corner to move into the display volume DVi as is shown in Fig. 6B. In Fig. 6C, a short pulse of -15V is supplied to the reservoir electrode REi while the other reservoir electrodes REiI, REi2, REi3 are on a level of +5 V. This pulse causes the particles in the display volume DVi to move back to the top left corner reservoir volume RVi as is shown in Fig. 6D. Now the pixel Pi is ready for a next cycle wherein none or one of the four particles is moved into the display volume DVi. Thus in fact four successive cycles are present during each of which another one of the particles can be selected to be moved into the display volume DVi and to be moved back into the reservoir volume RVi. The amount of particles moved again can be controlled by the drive voltage amplitude and/or duration or the light beam intensity and/or duration. If during a cycle no particles have to move into the display volume DVi, the photoconductor associated with the corresponding reservoir volume RVi is not illuminated.

The voltage on the reservoir electrodes REi is obtained by supplying this voltage to the corresponding bottom electrodes BEi and illuminating the associated photoconductors PCi.

It is possible to interconnect the bottom electrodes as is shown in Figs. 7A-7C. Figs. 7A-7C show schematically, in three steps, how a multiple reservoir pixel can be manufactured.

It is assumed that it is possible to create an initial condition wherein four separately colored types of particles are deposited at the four corner electrodes. The bottom electrode is patterned to obtain four interconnected bottom electrodes BEi, BEiI, BEi2, BEi3 as is shown schematically in Fig. 7A. The bottom electrodes are made from ITO. Metallic shunts can be deposited at any position on the electrode apart from that which is located under the four corner electrodes. Photoconductor material PCi, PCiI, PCi2, PCi3 is now deposited over the areas that will become the corner electrodes see Fig. 7B. The top floating corner electrodes, which are the reservoir electrodes REi, and the central electrodes, which are the display electrodes DEi are now deposited and finally the particles are placed at the corner electrodes in the reservoir volumes RVi, RViI, RVi2, RVi3, see Fig. 7C. It is necessary to have two crossovers for this design but it could be considered to also use photoconductor material at crossover points which would act as insulator if not illuminated. Only two connections are required for the display as all the bottom electrodes are connected together and all the central electrodes are connected together.

The operation of the pixel structure shown in Fig. 7C is elucidated in the following description. By way of example only, it is assumed that the particles are positively charged. First, a voltage pulse (for example +15V) is supplied between the display electrode DEi and the bottom electrode BEi which is positive at the bottom electrode. It is further assumed that the capacitance of the photoconductors PCi is significantly lower than that of the optical material in the pixel Pi. The voltage pulse of +15 V is capacitively divided over the capacitance of the photoconductors and the material, and thus the vast majority of this voltage drops over the photoconductor and not over the volume containing the particles PAi. This voltage therefore has no effect on the position of the particles PAi.

The pixel can now be addressed by scanning either one light source (LED or laser beam) twice over each pixel or by two separate light sources. If a particular color is required then the photoconductor under the reservoir volume RVi holding such particles is illuminated. The voltage across this reservoir volume RVi then no longer drops over the photoconductor but is present between the reservoir electrode REi and the display electrode DEi. This results in particles moving into the display volume DVi. Since the photoconductor PCi has a relatively slow closing time, only short light pulses (for example, with a duration less than 1 ms) are required at each pixel Pi while the voltage remains for a much longer period of time (several hundreds of ms). For each pixel Pi, one of the four colors can be addressed. The alignment of the addressing light and the photoconductor islands is not crucial but care should be taken that neighboring islands are not accidentally illuminated. The addressing of the pixels Pi is also referred to as the writing or updating of the pixels Pi. The erasing of the image is performed per pixel Pi. First a voltage of the opposite polarity to that used for writing (in this example -15 V (not necessarily the same magnitude)) is applied to the bottom electrode BEi associated with the reservoir volume RVi from which the particles were moved into the display volume DVi. These particles have to be moved back into the same reservoir volume RVi from which they originate to prevent intermingling of the different particles in the different reservoir volumes RVi. Then the same image is re-written. In practice, however, it is only the color of the pixel that has to be correctly selected for resetting and the grey scale is irrelevant. It is in fact better to use the maximum intensity of the laser beam for the resetting. In essence, resetting is writing an inverse image to the pixel at the correct reservoir volume RVi. This of course requires that the last image which is written to the pixels of the display is stored in a memory to be used later for erasing. Due to the response of the photoconductor, which only needs a relatively short light pulse, just as in the case of writing an image, the reset can be performed in a short period of time.

It has to be noted that the backlight cannot be used for erasing the image as this would not guarantee that the colors do not mix. It has further to be noted that the pixel structures can be manufactured with the roll-to-roll process, but other manufacturing technologies, such as photolithography or printing, may be used also.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. Electrophoretic display panels can form the basis of a variety of applications where information may be displayed, for example in the form of information signs, public transport signs, advertising posters, pricing labels, billboards etc. In addition, they may be used where a changing non- information surface is required, such as wallpaper with a changing pattern or colour, especially if the surface requires a paper like appearance. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. An optically addressable display comprising a pixel (Pi) having a pixel volume (PVi) comprising material (BMi) with moveable charged particles (PAi), the pixel volume (PVi) comprises: a reservoir volume (RVi) in which particles are invisible to a viewer, and a display volume (DVi) in which particles are visible to the viewer, the pixel (Pi) comprises a second electrode (DEi; REi) and a third electrode (REi; DEi), both being associated with the pixel volume (PVi) for generating an in-plane electric field (EFi) in the pixel volume (PVi) to move the particles (PAi) between the reservoir volume (RVi) and the display volume (DVi), a first electrode (BEi) having a first surface area (SAIi) being smaller than a pixel area (PIAi), and a photoconductor (PCi) arranged between the first electrode (BEi) and the third electrode (REi; DEi), the optically addressable display further comprises: a driver (DR) for supplying a first voltage (VIi) to the first electrode (BEi), and a second voltage (V2i) to the second electrode (DEi; REi), wherein the third electrode (REi; DEi) is floating.

2. An optically addressable display as claimed in claim 1, further comprising a light source (LS) for selectively illuminating the photoconductor (PCi).

3. An optically addressable display as claimed in claim 1, wherein the first surface area (SAIi) is selected to obtain a capacitance of the photoconductor (PCi) being smaller than a capacitance of the material (BMi) such that voltage pulses are predominantly present across the photoconductor (PCi).

4. An optically addressable display as claimed in claim 2, further comprising a controller (CO) for, in the following order: (i) activating the light source (LS) to illuminate the photoconductor (PCi), and controlling the driver (DR) to supply a voltage between the first electrode (BEi) and second electrode (DEi; REi) for moving the particles (PAi) into the reservoir volume (RVi),

(ii) deactivating the light source (LS) and controlling the driver (DR) to supply a voltage between the first electrode (BEi) and the second electrode (DEi; REi) suitable for moving the particles towards the display volume (DVi), and

(iii) selectively activating the light source (LS) to illuminate the photoconductor (PCi) for determining a number of the particles (PAi) to be moved into the display volume (DVi).

5. An optically addressable display as claimed in claim 2, further comprising a back light unit (BL), and a controller (CO), for in the following order:

(i) activating the back light unit (BL) to illuminate the photoconductor (PCi), and controlling the driver (DR) to supply a voltage between the first electrode (BEi) and second electrode (DEi; REi) for moving the particles (PAi) into the reservoir volume (RVi),

(ii) deactivating the back light unit (BL) and controlling the driver (DR) to supply a voltage between the first electrode (BEi) and the second electrode (DEi; REi) suitable for moving the particles towards the display volume (DVi), and

6. An optically addressable display as claimed in claim 2 comprising a plurality of pixels (Pi), wherein the light source (LS) comprises a scanning light emitting device, being arranged for scanning along said pixels (Pi).

7. An optically addressable display as claimed in claim 2 comprising a plurality of pixels (Pi), wherein the light source (LS) comprises a plurality of light sources, each one being associated with one of said pixels (Pi).

8. An optically addressable display as claimed in claim 1, wherein the second electrode (DEi; REi) is a display electrode (DEi) associated with the display volume (DVi), and the third electrode (REi; DEi) is a reservoir electrode (REi) associated with the reservoir volume (RVi).

9. An optically addressable display as claimed in claim 1, wherein the third electrode (REi; DEi) is a display electrode (DEi) associated with the display volume (DVi), and the second electrode (DEi; REi) is a reservoir electrode (REi) associated with the reservoir volume (RVi).

10. An optically addressable display as claimed in claim 8, wherein the first surface area (SAIi) is smaller than a surface area (DAi) of the display electrode (DEi).

11. An optically addressable display as claimed in claim 9, wherein the first surface area (SAIi) is smaller than a surface area (RAi) of the reservoir electrode (REi).

12. An optically addressable display as claimed in claim 1, forming a matrix display wherein adjacent ones of pixels (Pi) are arranged in a same plane and comprise different material (BMi) having different optical properties for obtaining groups of different pixels (Pi) forming color display elements (CDEi), the different pixels (Pi) having interconnected second electrodes (DEi; REi) and separate first electrodes (BEi), wherein the separate first electrodes (BEi) for pixels (Pi) comprising a same material (BMi) are interconnected to obtain different groups of first electrodes (BEi), the driver (DR) being arranged for supplying first voltages (VIi) to the different groups of first electrodes (BEi), and a single second voltage (V2i) to the second electrodes (DEi; REi).

13. An optically addressable display as claimed in claim 12, wherein the third electrodes (REi; DEi) are not electrically interconnected.

14. An optically addressable display as claimed in claim 12, further comprising a plurality of light sources (LS) each being arranged for illuminating at least two pixels (Pi) of a same one of the color display elements (CDEi), the optically addressable display further comprises a controller (CO) for, in the following order: (i) activating the light sources (LS) or a back-light unit (BL) to illuminate the photoconductors (PCi), and controlling the driver (DR) to supply a voltage between all the different groups of first electrodes (BEi) and the second electrodes (DEi; REi) for moving the particles (PAi) into the reservoir volumes (RVi),

(ii) deactivating the light sources (LS) or the back-light unit (BL), (iii) controlling the driver (DR) to sequentially supply a drive voltage between the different groups of first electrodes (BEi) and the second electrodes (DEi; REi) of the different pixels (Pi) of the color display elements (CDEi), the drive voltage being suitable for moving the particles (PAi) towards the display volumes (DVi), and selectively activating the light sources (LSi) to only illuminate the photoconductors (PCi) being associated with the pixels (Pi) receiving the drive voltage.

15. An optically addressable display as claimed in claim 12, further comprising a scanning light emitting element (LS) for scanning light along the pixels (Pi) of the color display elements (CDEi) one by one.

16. An optically addressable display as claimed in claim 1 forming a matrix display comprising a stack of at least a first layer (Ll) of first pixels (PIi) and a second layer (L2) of second pixels (P2i) being aligned with the first pixels (PIi) for obtaining groups of stacked pixels (PIi, P2i) forming color display elements (CDEi), wherein the first electrodes (BEi) of the first pixels (PIi) are interconnected, the second electrodes (DEi; REi) of the first pixels(Pli) are interconnected, the first electrodes (BEi) of the second pixels (PIi) are interconnected, the second electrodes (DEi; REi) of the second pixels (PIi) are interconnected, the driver (DR) being arranged for supplying associated first voltages (VIi) and second voltages (V2i) between the first electrodes (BEi) and the second electrodes (DEi; REi) of the first layer (Ll) and the second layer (L2).

17. An optically addressable display as claimed in claim 12 or 16, further comprising a controller (CO) for controlling the driver (DR), during a reset phase, to supply the first voltages (VIi) and the single second voltage or the plurality of second voltages (V2i) to all the pixels (Pi).

18. An optically addressable display as claimed in claim 12 or 16, wherein the photoconductors (PCi) of the pixels (Pi) of a same one the color display elements (CDEi) are spatially displaced for allowing a light source (LS) to separately illuminate said spatially displaced photoconductors (PCi).

19. An optically addressable display as claimed in claim 12 or 16, further comprising means for focusing light generated by a light source (LS) on different positions of different ones of the photoconductors (PCi) of a same one of the color display elements (CDEi), for allowing the light source (LS) to separately illuminate the different ones of the photoconductors (PCi).

20. An optically addressable display as claimed in claim 12 or 16, wherein different ones of the photoconductors (PCi) of a same one of the color display elements (CDEi) are sensitive to only an associated one of different spectra.

21. An optically addressable display as claimed in claim 20, further comprising a light source (LS) comprising different light emitting devices having the different spectra.

22. An optically addressable display as claimed in claim 1, wherein the second electrode (DEi; REi) is a display electrode (DEi) associated with the display volume (DVi), the third electrode (REi; DEi) is a reservoir electrode (REi) associated with the reservoir volume (RVi), the pixel (Pi) comprises a further reservoir volume (RViI) and a further reservoir electrode (REiI) being associated with the further reservoir volume (RViI), a further photoconductor (PCiI) being arranged between the further reservoir electrode (REiI) and a further first electrode (BEiI), and wherein the driver (DR) is adapted to further supply a further first voltage (Vlil) to the further first electrode (BEiI), wherein, in a starting state, first and second particles having different optical properties are present in the first mentioned reservoir volume (RVi) and the further reservoir volume (RViI), respectively.

23. An optically addressable display as claimed in claim 22, wherein the first mentioned reservoir electrode (REi) and the further reservoir electrode (REiI) are interconnected, the optically addressable display further comprising a controller (CO) for controlling

(i) the driver to supply a voltage between the display electrode (DEi) and the interconnected reservoir electrodes (REi, REiI) suitable for moving the particles of either the first mentioned reservoir volume (RVi) or the further reservoir volume (RViI) to the display volume (DVi),

(ii) a light source (LS) to illuminate either the first mentioned or the further photoconductor (PCi, PCiI), respectively

(iii) the driver to supply a voltage between the display electrode (DEi) and the interconnected reservoir electrodes (REi, REiI) suitable for moving the particles out of the display volume (DEi) back into the first mentioned reservoir volume (RVi) or the further reservoir volume (RViI), and

(iv) the light source (LS) to illuminate either the first mentioned or the further photoconductor (PCi, PCiI), respectively, for moving the particles out of the display volume (DEi) back into the same one of the first mentioned reservoir volume (RVi) or the further reservoir volume (RViI) wherefrom they were moved to the display volume (DVi) earlier.

24. An optically addressable display as claimed in claim 1, wherein the material is electrophoretic material.

25. A controller for use in an optically addressable display as claimed in claim 1, the controller being arranged for, in the following order:

(i) activating the light source (LS) to illuminate the photoconductor (PCi), and controlling the driver (DR) to supply a voltage between the first electrode (BEi) and second electrode (DEi; REi) for moving the particles (PAi) into the reservoir volume (RVi),

26. A controller for use in an optically addressable display as claimed in claim 1, further comprising a back light unit (BL), the controller being arranged for, in the following order:

27. A controller for use in optically addressable display as claimed in claim 22, wherein the first mentioned reservoir electrode (REi) and the further reservoir electrode (REiI) are interconnected, the controller (CO) being arranged for controlling (i) the driver to supply a voltage between the display electrode (DEi) and the interconnected reservoir electrodes (REi, REiI) suitable for moving the particles of either the first mentioned reservoir volume (RVi) or the further reservoir volume (RViI) to the display volume (DVi),

(iii) the driver to supply a voltage between the display electrode (DEi) and the interconnected reservoir electrodes (REi, REiI) suitable for moving the particles out of the display volume (DEi) back into the first mentioned reservoir volume (RVi) or the further reservoir volume (RViI), and (iv) the light source (LS) to illuminate either the first mentioned or the further photoconductor (PCi, PCiI), respectively, for moving the particles out of the display volume (DEi) back into the same one of the first mentioned reservoir volume (RVi) or the further reservoir volume (RViI) wherefrom they were moved to the display volume (DVi) earlier.

28. A method of displaying in an optically addressable display comprising: a pixel (Pi) having a pixel volume (PVi) comprising material (BMi) with moveable charged particles (PAi), the pixel volume (PVi) comprises: a reservoir volume (RVi) in which particles are invisible to a viewer, and a display volume (DVi) in which particles are visible to the viewer, the pixel (Pi) comprises a second electrode (DEi; REi) and a third electrode (REi; DEi), both being associated with the pixel volume (PVi) for generating an in-plane electric field (EFi) in the pixel volume (PVi) to move the particles (PAi) between the reservoir volume (RVi) and the display volume (DVi), a first electrode (BEi) having a first surface area (SAIi) being smaller than a pixel area (PIAi), and a photoconductor (PCi) arranged between the first electrode (BEi) and the third electrode (REi; DEi), the method comprises: supplying (DR) a first voltage (VIi) to the first electrode (BEi), and a second voltage (V2i) to the second electrode (DEi; REi), wherein the third electrode (REi; DEi) is floating.