WO2006111879A2

WO2006111879A2 - Optically addressable bi-stable display

Info

Publication number: WO2006111879A2
Application number: PCT/IB2006/051058
Authority: WO
Inventors: Murray F. Gillies; Mark T. Johnson
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-04-21
Filing date: 2006-04-06
Publication date: 2006-10-26
Also published as: TW200707053A; WO2006111879A3

Abstract

An optically addressable bi-stable display module comprises a stack of a photoconductor and a bi-stable material which is sandwiched between a first electrode and a second electrode to obtain a display device. A light unit illuminates the display device. A light driver drives the light unit, and a data driver supplies a drive voltage between the first electrode and the second electrode. A controller controls the light driver and the data driver to obtain a sequence comprising at least a first phase and a second phase succeeding the first phase. The sequence comprises at least a first address phase (TA), and an erase phase (TR) and/or a hold phase (TO), and/or a second address phase (TA). The controller controls, during a transition (TU, TD) from the first phase to the second phase, (i) the data driver to change the drive voltage (VD) from one level to a next level, and (ii) the light driver to substantially coincidently switch off the light unit. In this approach an optical state of the display device is kept substantially unaltered during the transition (TU, TD).

Description

Optically addressable bi-stable display

The invention relates to an optically addressable bi-stable display module, a display apparatus comprising such a display module, and a method of driving such an optically addressable bi-stable display module.

WO-2004/090624 Al discloses a display for displaying and storing images and comprises an optically addressable electrophoretic display with a stack of a photoconductive layer and an electrophoretic layer being sandwiched between electrodes. An optical addressing circuit supplies addressing light to the photoconductive layer. A controller controls a driver to supply a drive voltage between the electrodes with a value enabling a change of the optical state of the electrophoretic layer in response to the addressing light impinging on the photoconductive layer. Then, the driver changes the drive voltage to a value enabling storage of the optical state of the electrophoretic layer independent on the amount of addressing light impinging on the photoconductive layer. Finally, the power consumption of the optical addressing means is minimized and the image displayed by the electrophoretic layer is kept without requiring a voltage over the electrophoretic layer.

An important characteristic of electrophoretic displays is that once an image is written into its pixels, this image can be retained for a long period of time without requiring any drive pulses. As both the photoconductive layer and the electrophoretic layer have a capacitance, the drive voltage applied to the electrodes will be capacitively tapped during level changes. Therefore, if the optical state reached should be kept during the level change from the level at the end of the address period to the hold level, the drive voltage has to change sufficiently slowly such that the voltage across the electrophoretic layer stays low. If the drive voltage changes too steeply, due to the capacitive division, the voltage across the electrophoretic layer may become too large and influence its optical state. This slow change of the drive voltage has the drawback that it takes a relatively long time to refresh the image on the display. It is an object of the invention to provide a bi- stable display which requires less time to present a next image.

A first aspect of the invention provides an optically addressable bi-stable display module as claimed in claim 1. A second aspect of the invention provides a display apparatus as claimed in claim 14. A third aspect of the invention provides a method of driving an optically addressable bi-stable display module as claimed in claim 15. Advantageous embodiments are defined in the dependent claims.

In accordance with the first aspect of the invention, the optically addressable bi-stable display module comprises a stack of a photoconductor and bi-stable material layer which is sandwiched between first and second electrodes to obtain a display device. The bistable material, which for example is an electrophoretic material, has an optical state which is substantially stable in the absence of an electrical field. The bi-stable material may be any material of which the relaxation time is longer than the duration of the hold period during which the image is displayed such that the optical state of the material is substantially constant during the hold period. A light unit is arranged to illuminate the display device. The amount of light generated by the light unit, which locally travels through the display device or is locally reflected by the display device towards the viewer, depends on the local optical state of the display device. A data driver supplies a drive voltage between the first electrode and the second electrode.

A controller controls the light driver and the data driver to obtain a sequence comprising at least a first and a second phase. The sequence comprises at least one address phase and further an erase phase and a hold phase. The sequence may also comprise two address phases. Usually, if present, the erase phase precedes the address phase, and the address phase precedes the hold phase.

In one embodiment each sequence comprises successively an erase phase, an address phase, and a hold phase. Preferably, during the erase phase the data driver supplies several pulses with opposite polarity such that at the end of the erase period the bi-stable display is in one of its two extreme optical states. During the transition in-between the erase phase and the address phase, the drive voltage has to change level to enable a change of the optical state during the address phase. However the level change of the drive voltage has to take place such that the optical state reached at the end of the erase period is still present unaltered at the start of the address phase. If, during the addressing phase, the bi-stable material is locally illuminated at a particular position, the optical state changes at this particular position towards the other one of the two extreme optical states. The amount of change depends on the intensity and duration of the illumination. If the bi-stable material is not illuminated at a particular position, the optical state at this position does not change. During the hold phase the optical state of the display is held and the desired image is displayed. Again, during the transition from the address phase to the hold phase the drive voltage level has to change such that the optical state at reached at the end of the address phase is substantially not altered.

In another embodiment, no erase phase need to be present, and each sequence comprises successively a first address phase, a second address phase and a hold phase. During the first address phase, the positions where the optical state has to change towards the first one of the optical limit states are illuminated. During the second address phase, the positions where the optical state has to change towards the second one of the optical limit states are illuminated. This approach requires a memory to store the previous state at the positions. Again, during the transition phases, now between the first and the second address phase and between the second address phase and the hold phase, the optical state should be kept substantially unaltered.

During a transition from the first phase to the second phase, the controller controls the data driver to change the drive voltage starting from one level to a next level, and the light driver to substantially coincidently switch off the light unit such that the display is not illuminated anymore. In this manner, the optical state of the display device is kept substantially unaltered during the transition. If the light unit is switched off too early, the drive voltage is capacitively coupled to the bi-stable material and the changing drive voltage will occur for a part across the bi-stable material and thus change its optical state. If the light unit is switched off too late, due to the low impedance of the photoconductor the changing drive voltage will be present too long across the bi-stable material layer and change the optical state of the bi-stable material layer too much. Because the capacitive tap is not relevant anymore, the transition can be performed in a short time period.

In an embodiment as claimed in claim 2, the bi-stable display device is transmissive and the light unit comprises a backlight unit. This transmissive structure is advantageous in applications such as, for example, electronic billboards.

In an embodiment as claimed in claim 3, an erase phase precedes an addressing phase. During the erase phase, the data driver supplies the drive voltage as an alternating sequence of two opposite polarity levels. The light driver controls the light unit to be active to illuminate the display device. Thus, during the erase phase of the display device, the light unit is on and the photoconductor has a low impedance for the complete display or the part of the display which should be erased. Consequently, the majority of the drive voltage is present across the bi-stable material and is able to change its optical state. At the end of the erase phase the optical state of the complete display or the part of the display which should be erased is in one of the two limit optical states. Now, substantially coincident, the level of the drive voltage is changed, preferably to the opposite polarity, and the light unit is switched off. In contrast, in the prior art, during the erase phase, the light unit does not illuminate the display and the capacitive tap is used to obtain the majority of the drive voltage across the bi-stable material. In an embodiment as claimed in claim 4, during the addressing phase the optical state reached at the end of the erase phase should be kept at locations where the light unit does not provide addressing light. While, on the other hand, the addressing voltage has to change polarity to be able to change the optical state at locations where the light unit provides addressing light. During the transition from the erase phase to the address phase, the data driver changes the level of the drive voltage from the level at the end of the erase phase to the opposite polarity level. The light driver substantially coincidently switches off the light unit.

Because the simultaneously changing drive voltage and switching off of the light, the drive voltage polarity is changed when the photoconductor has a low resistance. Thus, initially, the full drive voltage is dropped across the bi-stable material, but due to the rapidly increasing resistance of the photoconductor results in a rapid decrease of the voltage across the bi-stable material. If the increase of the resistance of the photoconductor is sufficiently rapid, the bi-stable material does not have the time to change its optical state due to fast decreasing voltage.

In an embodiment as claimed in claim 5, the light is switched off substantially coincidently while changing from one of the polarities of the drive voltage to the other one during a transition period from the first to the second address phase.

In an embodiment as claimed in claim 6, during the address phase, the driver keeps the level of the drive voltage occurring at an end of the transition from the erase phase to the address phase stable, and the light driver controls the light unit to selectively illuminate the display device at positions where an optical state of the display device has to change. It has to be noted that at the end of the erase phase and also at the end of the transition period from the erase phase to the address phase, the bi-stable material is in one of its limit optical states. On the other hand, at the end of the transition phase, the drive voltage has the opposite level with respect to the level at the end of the erase phase. Thus, at positions where no light from the light unit impinges on the photoconductor, the photoconductor has a high impedance and the majority of the drive voltage is present across the photoconductor, and the bi-stable material keeps its optical state. At positions where light from the light unit illuminates the photoconductor, the photoconductor has a low impedance and the majority of the drive voltage is present across the bi-stable material which changes its optical state towards the other limit state. The amount of change of the optical state depends on the time of application and on the intensity of the light.

In an embodiment as claimed in claim 7, during the transition from the address phase to the hold phase, the driver changes the level of the drive voltage from its limit value during the address phase to a hold level at which the optical state is kept. The light driver substantially coincidently switches off the light unit. Again, because the simultaneously changing drive voltage and switching off of the light, the drive voltage polarity is changed when the photoconductor has a low resistance. Thus, initially, the full drive voltage is dropped across the bi-stable material, but the rapidly increasing resistance of the photoconductor results in a rapid decrease of the voltage across the bi-stable material. If the increase of the resistance of the photoconductor is sufficiently rapid, the bi-stable material does not have the time to change its optical state due to fast decreasing voltage. During the hold phase, the optical state reached at the end of the addressing phase should be kept.

It has to be noted that in the prior art, first the addressing light is switched off and then the drive voltage level has to be changed slowly such that the resistive tap overrules the capacitive tap.

In an embodiment as claimed in claim 8, the controller takes care that, during a transition period, a time delay between a first instant the data driver has to change the drive voltage from one level to another level, and a second instant the light driver has to switch off the light unit is limited such that the optical state of the bi-stable material does not change more than five percent. A zero delay is optimal, however in a particular embodiment, in which the bi-stable material is a particular E-ink material, a time delay within the range of - 100 to +100 milliseconds provided very acceptable results.

In an embodiment as claimed in claim 9, the transition is performed within a time period lasting shorter than one second. This in contrast to the prior art wherein the slow ramp lasts about 20 to 30 seconds for a comparable display. Now, due to the substantially simultaneously switching off of the light unit and the change of the drive voltage, both within a short period of time, the refresh period required to refresh the image on the display is much shorter. Although the short period is preferably shorter than 1 second, what matters is that the short period is smaller than the period in time covered by the slow ramp which is required in usual drive algorithms.

In an embodiment as claimed in claim 10, the light unit comprises a scanning laser or LED bar for scanning along the display device during the address phase. Such a scanning light unit has the advantage that the display can be addressed line by line. The laser or LED bar is able to separately address the pixels in the line.

In an embodiment as claimed in claim 11, the light unit comprises light sources forming a pixelated structure for locally addressing the display device. The pixelated structure is preferably a matrix of light sources which are individually switchable. Preferably, during the erase phase, all the light sources are producing light such that the complete display is erased, while during the address phase the light sources are individually controlled to individually control the optical state of the bi-stable material at positions associated with the light sources. These positions are also referred to as pixels.

In an embodiment as claimed in claim 12, the pixelated structure comprises an electroluminscent device (e.g. OLED) array.

In an embodiment as claimed in claim 13, the bi-stable material comprises E- ink material.

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 a prior art optically addressable bi-stable display,

Figs. 2A and 2B show prior art drive waveforms, Fig. 2C shows a drive waveform in accordance with an embodiment of the invention,

Fig. 3 shows an optically addressable electrophoretic display with lateral movement of particles,

Fig. 4 shows the effect of a time delay between the instant the drive waveform changes its level and the instant the light unit is switched off, Fig. 5 shows a laser scanned bi-stable display, and

Fig. 6 shows drive waveforms for a drive algorithm in which two address phases are present. The same references in different Figures refer to the same entities. Fig. 1 shows a prior art optically addressable bi-stable display. The light unit LS comprises light sources Dl to DN which generate light AL. Alternatively, the light unit LS may comprise a single light source. The bi-stable display RD comprises a stack of layers, which seen from the light unit LS occur in the order: a top electrode El, a bi-stable display substance DL, a photoconductive layer PL, and a bottom electrode E2. Alternatively, the photoconductive layer PL may be sandwiched between the top electrode El and the display substance DL.

Preferably, the top electrode El is a transparent conductive ITO layer. The display substance DL may be any substance suitable to be operated as a bi-stable display. A bi-stable display is a display of which the optical state does not change when no voltage is applied across it. Examples of bi-stable displays are electrophoretic displays, electrochromic, and cholesteric texture LCD's. The photoconductive layer PL comprises a material of which the resistance at a particular location depends on the amount of light impinging at this particular location. The bottom electrode is a conductive layer, which preferably is a transparent ITO layer.

When a drive voltage VD is supplied between the top electrode El and the bottom electrode E2, the optical state of the display RD is sensitive to the light AL. If the light AL impinges at a particular location on the photoconductive layer PL, its conductivity locally increases. At this particular location, a major part of the drive voltage VD supplied between the top and the bottom conductive layers El and E2 will be present across the display substance DL and will influence its optical state. If no light impinges on the photoconductive layer PL, its resistance is very high with respect to the resistance of the display substance DL. The drive voltage VD between the top electrode El and the bottom electrode E2 occurs substantially across the photoconductive layer PL and substantially no voltage occurs across the display substance DL, and the optical state of the display substance DL does not change.

It is thus possible to selectively change the optical state of the display substance DL by selectively illuminating the bi-stable material. The light unit LS may comprise a line or a matrix of light sources Dl to DN. The set of light sources Dl to DN is driven to address a corresponding set of pixels on the display RD. The light of the light sources Dl to DN may extend over the complete area of the display RD to be able to address all the pixels. Alternatively, the light unit LS or the light AL generated by the light sources Dl to DN may move across the display RD and then needs to address a small area of the display RD only. The complete display RD will be addressed because the light AL generated by the light unit LS moves across the display RD. The light unit LS may be a line or a two- dimensional matrix of light sources such as LED's which preferably are OLED's. Again, the matrix may cover the complete area of the display or may comprise a row of light sources which are scanned along the display RD. Alternatively, the light unit may comprise a scanning laser LAD which scans along the display RD as is shown in Fig. 5. Usually, besides the scanning laser LAD a further light source may be present to illuminate the complete display during the hold phase and preferably also during the erase phase, if present. Instead of the laser, a high power LED may be used. It has to be noted that in a reflective display, the layers need not be pixelated.

However, if such a reflective display comprises an elecrophoretic layer DL with charged particles, the particles would move in the layer DL between the electrodes El and E2 upwards and downwards, but this would not change the optical response for light transferred through the layer DL. Thus, in the transmissive display, it should be possible to concentrate the particles, preferably laterally, to be able to obtain different transmission states. Usually, the layers need to be pixelated to be able to concentrate the particles within pixels.

Usually the thickness of the photoconductive layer PL is much smaller than the thickness of the layer of the display substance DL. This thickness of the photoconductive layer PL prevents a build up of excessive stress and fracturing of the layer. Consequently, the capacitance of the photoconductive layer PL is much larger than the capacitance of the display substance layer DL. This capacitance difference is used in the prior art to erase the display in the absence of light AL by applying pulses between the electrodes El and E2 which have fast changing levels. These fast changing levels are predominantly present across the small capacitance which represents the display substance layer DL. However, now level transitions between the erase phase and the address phase, between the address phase and the hold phase, or between two successive address phases have to change sufficiently slowly, such that the voltage across the display substance layer DL due to the capacitive division is negligible. The drive of the prior art display is elucidated in more detail in Fig. 3 A. The same reasoning holds for a display RD in which the electrodes are laterally displaced. An embodiment of a display RD with laterally displayed electrodes wherein the display substance DL is an electrophoretic material is shown in Fig. 2. Fig. 3 C shows a drive voltage between the electrodes in accordance with an embodiment of the invention.

Figs. 2 A and 2B show prior art drive waveforms, and Fig. 2C shows a drive waveform in accordance with an embodiment of the invention. Fig. 2 A shows the voltage waveform VWl of the drive voltage VD supplied between the electrodes El and E2 shown in Fig. 1. Fig. 2B shows the data voltage DV supplied to the light unit LS. If the data voltage DV has a high level, the light AL is generated, if the data voltage has a low level, no light AL is generated. Fig. 2C shows the voltage waveform VW2 of the drive voltage VD supplied in accordance with an embodiment in accordance with the invention.

The voltage waveform VWl has an erase phase TR which lasts from the instant tO to the instant tl. The erase phase comprises a sequence of erase pulses which have alternating polarity. During the erase phase the previous image is erased. In the prior art, the erase pulses are supplied between the electrodes El and E2 while the light unit LS does not generate light AL. Because the capacitance of the relatively thin photoconductor layer PL is much higher than the capacitance of the relatively thick display substance layer DL, these relatively fast pulses are capacitively divided or tapped and thus are substantially completely present over the display substance layer DL, which in this example is an electrophoretic material. Due to the series of pulses with extreme levels (-20V and +20V) which have opposite polarity, the particles of the electrophoretic material DL have a high mobility which allows to reach a particular one of the two limit optical states at the end of the erase phase TR. This particular limit optical state corresponds to the polarity of the last pulse (-20V) during the erase phase TR.

Now, during a transition period TU, before the address phase TA starts, the voltage waveform VWl has to change polarity to the other extreme value (+20V) without changing the optical state of the electrophoretic material DL. During the address phase TA, the portions of the electrophoretic material DL which do not receive light keep the particular limit optical state, and portions of the electrophoretic material DL which receive light change their optical state towards the other limit optical state. The voltage waveform VWl during the transition period TU, which lasts from the instant tl to the instant t3, has to change sufficiently slowly such that the voltage across the electrophoretic layer DL is determined by the ratio of the resistances of the photoconductive layer PL and the electrophoretic layer DL and not by the capacitive tap. Now, the majority of the drive voltage VWl occurs across the photoconductive layer PL which has a resistance much higher than the resistance of the electrophoretic layer because no light impinges on the photoconductive layer.

During the transition period TU of the voltage waveform VWl the level at the end tl of the erase phase TR is slowly changed into an addressing voltage level ADL which allows the electrophoretic material DL to be selectively optically addressed with the light unit LS. This addressing voltage level ADL must have the opposite polarity with respect to the polarity of the last erase pulse level. The last erase pulse level changes the display to one of the limit optical states. During the addressing phase TA, it should be possible to change the optical state of the pixels towards the other limit optical state. Preferably, if the electrophoretic layer is an E-ink layer with negatively charged white and positively charged black particles, the erase pulse ends with a negative voltage such that the display is black. Now, the addressing voltage level ADL should be positive to allow the selected pixels to change their optical state towards white during the addressing phase. The change from the level of the last reset pulse to the addressing level must be slow enough to avoid a too large voltage drop over the electrophoretic layer DL due to the capacitive coupling of the capacitance of the electrophoretic layer DL and the photoconductor PL.

For example only, in a practical embodiment, the electrophoretic layer DL has a thickness of 50μm. The thickness of the photoconductor layer PL is a factor 10 less than the thickness of the electrophoretic layer. The resistance area product of the photoconductor is lOMΩm² in the dark state and lOkΩm² in the illuminated state. The resistance area product of the E-ink is 200kΩm². More in general, preferably, the capacitance of the electrophoretic is substantially lower than that of the photoconductor, the resistance of both the electrophoretic and the photoconductor is very high to obtain large time constants, and the resistance of the photoconductor should be higher that that of the electrophoretic when not illuminated and lower when illuminated. In such a display, it is found that the gradient of the voltage change must not be larger than 0.75V/S. Thus, for a swing of 30V, the total ramp time is 40s. In this prior art approach this means that a pause of 40s is present during which a blank (one of the limit optical states) image is presented to a viewer.

As shown in Fig. 2B, during the address phase TA, the light unit LS optically addresses a single or a group of pixels. A pixel is defined as a particular area of the electrophoretic layer DL which can be individually illuminated. A pixel which should keep its optical state obtained after the erase phase TR must not be illuminated. A pixel which should change its optical state obtained after the erase phase TR should be illuminated. It has to be noted that bi-stable displays itself do not necessarily have a pixel structure. The dimensions of the impinging light spots determine the pixel areas. If the optical addressing is performed by a light unit LS which comprises a line of light sources Dl to DN, the update section can be addressed line by line. The pixels of each line are addressed in parallel during a line period TL. The last update period TA ends at the instant t4, and the light unit LS does not anymore illuminate the display. The period between the instants t5 and t6 is the transition period TD from the addressing phase TA to the hold phase TO. The ramp down has to be performed sufficiently slowly such that the resistive tap prevails above the capacitive tap and the optical state of the electrophoretic layer DL does not change. Once the zero volts level has been reached at instant t6, the optical state reached at the end of the address phase TA is kept until a next erase phase TR starts at the instant t7. Thus, the hold phase lasts from the instant t6 to the instant t7.

The total update or refresh period comprises the erase phase TR, the transition period TU, the address phase TA, the transition period TD and the hold phase TO. At the instant t7 a next refresh period starts.

It is clear from Fig. 2 A that the duration of the refresh period is lengthened considerably by the relatively long duration of the transition periods TU and TD. In the practical embodiment described above, the total time consumed by these two transition periods TU, TD is 60 seconds. Fig. 2C shows the voltage waveform VW2 supplied in accordance with an embodiment of the invention. The address phase TA is kept identical to that shown in Fig. 2A and still starts at the instant t3 and runs until the instant t5. Now, the transition periods TU and TD are very short and are shown to occur at the instant t3 and t5. Consequently, the erase phase TR, which occurs from the instant tθ' to the instant t3, immediately precedes the address phase TA. The hold phase TO immediately succeeds the address period TA and lasts from the instant t5' to the instant t7'. The total refresh period now lasts from the instant tO'to the instant t7', and thus is considerably shorter than the prior art refresh period which lasts from the instant tO to the instant t7.

During both the transition periods TU and TD, the drive voltage present between the electrodes El and E2, changes, starting from one of two opposite polarity levels to a next level, and the light unit LS substantially coincidently switches off the light AL such that the display is not illuminated anymore. In this manner, the optical state of the display device RD is kept substantially unaltered during the transition. This allows performing the transition in a short time period. If the light unit LS is switched off too early, the drive voltage is capacitively coupled to the bi- stable material DL and the changing drive voltage will occur for a part across the bi-stable material DL and thus change its optical state. If the light unit LS is switched off too late, due to the low impedance of the photoconductor PL the changing drive voltage will be present too long across the bi-stable material DL and change its optical state too much.

In Figs. 2A and 2C a drive algorithm is shown which comprises refresh periods which comprise successively an erase phase TR to erase the previous image on the display RD, an address phase TA to write the present image on the display, and a hold phase TO during which the present image is displayed. However, the present invention can also be advantageously used during a transition between two successive address phases which may be present if no erase phase is used.

Fig. 3 shows schematically a pixel structure and its drivers in accordance with an embodiment of the invention wherein the particles move laterally. 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 bi-stable 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. Preferably, the particles present in the reservoir volume RVi are invisible to the viewer and 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 address phase. A photoconductor PCi is present between the reservoir electrode REi and a bottom electrode BEi. A data driver DR supplies a first voltage VIi to the bottom electrode BEi and a second voltage V2i to the display electrode DEi. A light unit LS may illuminate the photoconductor PCi. A controller CO supplies a control signal Cl to the light unit LS and a control signal C2 to the data 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 data 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 unit LS selectively 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 unit 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 unit 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. This approach generates grey scales or different colors. Alternatively, the photoconductor PCi may be present below part of or the complete area of the display volume DVi. The photoconductor PCi should then be transmissive. Again, alternatively, the photoconductor PCi may be a photoconductive layer which is present below substantially the complete pixel volume PVi, or even below all pixel volumes PVi. If the thickness of the photoconductive layer PCi is selected to be much smaller than the distance in-between the opposing ends of the reservoir electrode REi and the display electrode DEi, even when the complete photoconductive layer PCi is illuminated, its resistance between the reservoir electrode REi and the display electrode DEi is much higher than its resistance across the thickness of the photoconductive layer PCi.

In the same manner as discussed with respect to Fig. 1, usually the thickness of the photoconductors or the photoconductive layer PCi is much smaller than the thickness of the electrophoretic layer DL. And thus, again, the pixels Pi are in the prior art erased with fast changing levels while no light impinges on the photoconductors or the photoconductive layer PCi. After the erase phase TR, all particles are present in the reservoir volume RVi. Before the addressing phase TA can be started first the drive voltage has to slowly change polarity which takes a considerable amount of time.

In contrast, in accordance with the present invention, during a transition period, the controller CO controls the data driver DR with a control signal C2 to change the drive voltage ViI -Vi2 starting from one of two opposite polarity levels to a next level. The controller CO to substantially coincidently controls the light driver LD with the control signal Cl to switch off the light unit LS such that the display device RD is not illuminated anymore. In this manner, the optical state of the display device RD is kept substantially unaltered during the transition. This allows performing the transition in a short time period.

If the light unit LS is switched off too early, the drive voltage ViI -Vi2 is capacitively coupled to the bi-stable material and the changing drive voltage will occur for a significant part across the electrophoretic layer DL which thus changes its optical state too much. If the light unit LS is switched off too late, due to the low impedance of the photoconductor PCi, the changing drive voltage Vil-Vi2 will be present too long across the electrophoretic layer DL which thus changes its optical state too much. The light unit LS may comprise a single light source DJ which scans along the display device RD to sequentially illuminate the photoconductors PCi of pixels Pi which should change their optical state. Preferably, the light unit LS produces an intensity which is modulated to obtain different optical states for different pixels Pi. Now, all the electrodes of the all the pixels Pi receive the same voltage levels and the electrophorectic layer need not be mechanically pixelated. The light unit LS may be moved to obtain the scanning light beam AL. Alternatively, the light source DJ may be stationary and an optical unit moves to deflect the stationary light beam generated by the light source DJ to obtain the scanning light beam. Preferably, such a single light source DJ is a laser LAD or a high power light emitting diode. The light unit LS may also comprise multiple light generating elements DJ, such as for example light emitting diodes, preferably one for each pixel.

A signal processor SP receives an input signal IV which represents an image to be displayed. The signal processor SP generates a data signal DA which is supplied to the light driver LD to determine for the individual pixels Pi the amount and/or duration of addressing light during the addressing period TA. The signal processor SP further generates a synchronization signal or signals SY which enable the controller CO to generate the control signals Cl and C2 in coordination with the timing of the input signal IV.

Fig. 4 shows the effect of a time delay between the instant the drive waveform changes its level and the instant the light is switched off. The time delay dt is depicted along the horizontal axis in milliseconds. A negative time delay indicates that the light unit LS is switched off before the drive voltage changes, and a positive time delay indicates that the light unit LS is switched off after the drive voltage changed level. The vertical axis shows the gray level GL obtained if before the transition the gray level was full black BL. It becomes clear that for the particular electrophoretic display tested, a time delay of ± 100 milliseconds is acceptable without causing a noticeable change of the black level DL. The black level DL is one of the two limit optical states, the other limit optical state is depicted by the white level WL.

If the light unit LS is switched off too early, for example at -500 ms, the low impedance of the photoconductor PL or PCi is already increasing towards it high level when the changing drive voltage Vil-Vi2 occurs. Consequently, an increasing part of the drive voltage is present across the bi-stable material DL due to the capacitive tap formed by the capacitances of the photoconductor PL and the bi-stable material DL. The bi-stable material changes its optical state considerably towards the white level. If the light unit LS is switched off too late, for example at +500 ms, due to the low impedance of the photoconductor PCi, the changing drive voltage Vil-Vi2 will be present too long across the electrophoretic layer DL and thus changes its optical state too much towards the white level WL.

Fig. 5 shows a laser scanned bi-stable display. The light unit LS comprises a scanning laser LAD. The scanning laser LAD scans a laser beam LB along the optically addressable bi-stable display RD. The intensity of the laser beam LB is controlled in accordance with the image to be written on the photoconductive layer PL. The operation of the laser addressed display RD is similar to the operation of the optically addressed display RD which is addressed by a line of light sources Dl to DN. First the display RD is brought in a state wherein the local conductivity of the photoconductive layer PL determines the optical state of the bi-stable material DL. Then, the scanning laser LAD is activated to scan the laser beam along the display RD to transfer the image to the photoconductive layer PL and thus to the bi-stable material DL. Now, the display RD is brought in a state wherein the optical state of the bi-stable material DL is stored independent on the local conductivity of the photoconductive layer PL. Preferably, the scanning laser LAD scans the laser beam LB in a frame of lines. Instead of a laser, any other high power point light source can be used. It should be noted that if it is referred to pixels of or on the display RD, it is not meant that hardware cells must be present in the display RD. The display RD may have a homogeneous construction. Then, the pixels Pi are only referred to as areas of the display RD which are present due to the addressing of the display RD with the discrete light sources DJ. Fig. 6 shows drive waveforms for a drive algorithm in which two address phases are present. The drive voltage VD during the first address period TAl, which lasts from the instant tlO to the instant tl 1 has a first polarity which in Fig. 6 is positive. The drive voltage VD during the second address period TA2, which lasts from the instant tl 1 to the instant tl2 has a second polarity which in Fig. 6 is negative. The pixels which should change their present optical state towards the limit optical state corresponding to the first polarity should be illuminated during the first address period TAl. The pixels which should change their present optical state towards the limit optical state corresponding to the second polarity should be illuminated during the second address period TA2. After the second address period TA2, the complete image is written to the display RD and is displayed during the hold period TO. A next cycle starts at the instant tl3.

With the expression "the limit optical state corresponding to the polarity" is meant, the optical state reached if the particular polarity is applied sufficiently long and the optical state not longer changes.

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.

The bi-stable display may be any other display then an electrophoretic display. For example, the bi-stable display may be the rotating ball display of Gyricon. A bi-stable display is any display where the pixels maintain their brightness level after the voltage to the pixel is removed. It has to be noted that a bi-stable display may have more than 2 brightness levels.

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 color, 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 bi- stable display module comprising a stack of a photoconductor (PL) and a bi-stable material (DL) being sandwiched between a first electrode (El) and a second electrode (E2) to obtain a display device (RD), a light unit (LS) for supplying light (AL) to illuminate said display device

(RD), a light driver (LD) for driving the light unit (LS), a data driver (DR) for supplying a drive voltage (VD) between the first electrode (El) and the second electrode (E2), a controller (CO) for controlling the light driver (LD) and the data driver (DR) to obtain a sequence comprising at least a first phase and a second phase succeeding the first phase, the sequence comprising at least a first address phase (TA; TAl), and an erase phase (TR) and/or a hold phase (TO), and/or a second address phase (T A2), wherein the controller (CO) is constructed for, during a transition (TU, TD) from the first phase to the second phase, controlling (i) the data driver (DR) to change the drive voltage (VD) from one level to a next level, and (ii) the light driver (LD) for substantially coincidently decreasing an intensity of the light (AL) to obtain an impedance of the photoconductor (PL) being substantially larger than an impedance of the bi-stable material (DL) for keeping an optical state of the display device (RD) substantially unaltered during the transition (TU, TD).

2. An optically addressable bi-stable display module as claimed in claim 1, wherein the light unit (LS) comprises a backlight unit, and the display device is transmissive.

3. An optically addressable bi-stable display module as claimed in claim 1, wherein the first phase is the erase phase (TR) and the second phase is the first address phase (TA), and wherein the controller (CO) is constructed for, during the erase phase (TR), controlling (i) the data driver (DR) to supply the drive voltage (VD) having an alternating sequence of two opposite polarity levels, and (ii) the light driver (LD) to activate the light unit (LS) for illuminating the display device (RD).

4. An optically addressable bi-stable display module as claimed in claim 3, wherein the controller (CO) is constructed for, during the transition from the erase phase (TR) to the first address phase (TA), controlling (i) the data driver (DR) to supply the drive voltage (VD) to change from one of the two opposite polarity levels occurring at an end of the erase phase (TR) to the other one of the two opposite polarity levels, and (ii) the light driver (LD) to substantially coincidently switch off the light unit (LS).

5. An optically addressable bi-stable display module as claimed in claim 1, wherein the first phase is a first address phase (TAl) and the second phase is a second address phase (T A2), wherein the controller (CO) is constructed for, during the transition from the first address phase (TAl) to the second address phase (T A2), controlling (i) the data driver (DR) to supply the drive voltage (VD) to change from one of the two opposite polarity levels occurring at an end of the first address phase (TAl) to the other one of the two opposite polarity levels at a start of the second address phase (T A2), and (ii) the light driver (LD) to substantially coincidently switch off the light unit (LS).

6. An optically addressable bi-stable display module as claimed in claim 1, wherein the first phase is the erase phase (TR) and the second phase is the first address phase (TA), and wherein the controller (CO) is constructed for, during the second phase, controlling (i) the data driver (DR) to keep the drive voltage (VD) occurring at an end of the transition from the first phase to the second phase unaltered, and (ii) the light driver (DR) to control the light unit (LS) to selectively illuminate the display device (RD) at positions where an optical state of the display device (RD) has to change.

7. An optically addressable bi-stable display module as claimed in claim 1, wherein the first phase is the first address phase (TA) and the second phase is the hold phase (TO), and wherein the controller (CO) is constructed for, during the transition from the first address phase (TA) to the hold phase (TO), controlling (i) the data driver (DR) to supply the drive voltage (VD) to change from one of the two opposite polarity levels occurring at an end of the first address phase (TA) to a hold level at which the optical state is kept, and (ii) the light driver (LD) to substantially coincidently switch off the light unit (LS).

8. An optically addressable bi-stable display module as claimed in claim 1, wherein the controller (CO) is constructed for obtaining a time delay (dt) between a first instant the data driver (DR) has to change the drive voltage (VD) from the one level to the another level, and a second instant the light driver (LD) has to switch off the light unit (LS), being limited to not change the optical state more than five percent.

9. An optically addressable bi-stable display module as claimed in claim 1, wherein the controller (CO) is constructed for having the transition performed within a time period (TU, TD) lasting shorter than one second.

10. An optically addressable bi-stable display module as claimed in claim 1, wherein the light unit (LS) comprises an illumination device for illuminating all pixels of the display device during the hold phase (TO) and/or erase phase (TR), and a scanning laser (LAD) or LED bar (Dl, ...,DN) for scanning along the display device (RD) during the first addressing phase (TA).

11. An optically addressable bi-stable display module as claimed in claim 1 , wherein the light unit (LS) comprises light sources (DJ) comprising a set of electroluminescent elements forming a pixelated structure for locally addressing the display device (RD).

12. An optically addressable bi-stable display module as claimed in claim 11, wherein the light unit comprises a passive matrix OLED array.

13. An optically addressable bi-stable display module as claimed in claim 1, wherein the bi-stable material (DL) is an electrophoretic layer comprising E-ink material.

14. A display apparatus comprising an optically addressable bi-stable display module (LS, RD) as claimed in claim 1, and a signal processing circuit (SP) for supplying data to be displayed to the light driver (LD).

15. A method of driving an optically addressable bi- stable display module comprising a stack of a photoconductor (PL) and a bi-stable material (DL) being sandwiched between first electrode (El) and second electrode (E2) to obtain a display device (RD), and a light unit (LS) arranged for illuminating said display device (RD), the method comprises driving (LD) the light unit (LS), supplying (DR) a drive voltage (VD) between the first electrode (El) and the second electrode (E2), controlling (CO) the driving (LD) of the light unit (LS) and the supplying (DR) of the drive voltage (VD) to obtain a sequence comprising at least a first phase and a second phase succeeding the first phase, the sequence comprising at least a first address phase (TA; TAl), and an erase phase (TR) and/or a hold phase (TO), and/or a second address phase (TA2), wherein, during a transition (TU, TD) from the first phase to the second phase, the drive voltage (VD) is changed from one of two opposite polarity levels to a next level, and substantially coincidently the light unit is switched off, to keep the optical state of the display device substantially unaltered during the transition (TU, TD).