WO2008018016A1

WO2008018016A1 - Electrophoretic display devices

Info

Publication number: WO2008018016A1
Application number: PCT/IB2007/053104
Authority: WO
Inventors: David A. Fish
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-08-11
Filing date: 2007-08-07
Publication date: 2008-02-14
Also published as: TW200816113A

Abstract

An active matrix electrophoretic display device comprises an array of rows and columns of display pixels. Each pixel comprises a plurality of sensors for detecting movement of the electrophoretic display particles, different sensors detecting particle movement which reaches different regions within the pixel. The invention uses multiple in- pixel sensors, and determines the pixel optical state by using a relationship between the sensor outputs which is independent of ambient light. In this way ambient light compensation is carried out simultaneously with the addressing of the pixel, and without requiring dedicated ambient light sensors.

Description

Electrophoretic display devices

This invention relates to electrophoretic display devices.

Electrophoretic display devices are one example of bistable display technology, which use the movement of particles within an electric field to provide a selective light scattering or absorption function.

In one example, white particles are suspended in an absorptive liquid, and the electric field can be used to bring the particles to the surface of the device. In this position, they may perform a light scattering function, so that the display appears white. Movement away from the top surface enables the colour of the liquid to be seen, for example black. In another example, there may be two types of particle, for example black negatively charged particles and white positively charged particles, suspended in a transparent fluid. There are a number of different possible configurations.

It has been recognised that electrophoretic display devices enable low power consumption as a result of their bistability (an image is retained with no voltage applied), and they can enable thin display devices to be formed as there is no need for a backlight or polariser. They may also be made from plastics materials, and there is also the possibility of low cost reel-to-reel processing in the manufacture of such displays.

If costs are to be kept as low as possible, passive addressing schemes are employed. The most simple configuration of display device is a segmented reflective display, and there are a number of applications where this type of display is sufficient. A segmented reflective electrophoretic display has low power consumption, good brightness and is also bistable in operation, and therefore able to display information even when the display is turned off. However, improved performance and versatility is provided using a matrix addressing scheme. An electrophoretic display using passive matrix addressing typically comprises a lower electrode layer, a display medium layer, and an upper electrode layer. Biasing voltages are applied selectively to electrodes in the upper and/or lower electrode layers to control the state of the portion(s) of the display medium associated with the electrodes being biased.

Another type of electrophoretic display device uses so-called "in plane switching". This type of device uses movement of the particles selectively laterally in the display material layer. When the particles are moved towards lateral electrodes, an opening appears between the particles, through which an underlying surface can be seen. When the particles are randomly dispersed, they block the passage of light to the underlying surface and the particle colour is seen. The particles may be coloured and the underlying surface black or white, or else the particles can be black or white, and the underlying surface coloured.

An advantage of in-plane switching is that the device can be adapted for transmissive operation, or trans flective operation. In particular, the movement of the particles creates a passageway for light, so that both reflective and transmissive operation can be implemented through the material. This enables illumination using a backlight rather than reflective operation. The in-plane electrodes may all be provided on one substrate, or else both substrates may be provided with electrodes.

This invention relates in particular to in-plane switching devices, in which there is lateral movement of particles.

Active matrix addressing schemes are also used for electrophoretic displays, and these are generally required when bright full colour displays with high resolution greyscale are required. Such devices are being developed for signage and billboard display applications, and as (pixellated) light sources in electronic window and ambient lighting applications. Colours can be implemented using colour filters, and the display pixels then function simply as greyscale devices. This invention also relates in particular to active matrix devices.

The description below refers to greyscales and grey levels, but it will be understood that this does not in any way suggest only monochrome display operation.

Electrophoretic displays are typically driven by complex driving signals. For a pixel to be switched from one grey level to another, often it is first switched to white or black as a reset phase and to then to the final grey level. Grey level to grey level transitions and black/white to grey level transitions are slower and more complicated than black to white, white to black, grey to white or grey to black transitions.

This reset operation is used in order to achieve satisfactory grey level accuracy. Typically, the display pixels are reset to either the positive or the negative rail depending on the final image (i.e. to either black or white). The pixel is reset to black if the target grey level is closer to black than to white, and vice versa. This results in a visually more attractive image transition compared to the more simply example of always resetting to black or to white, because the result of such a reset sequence is to produce momentarily a black and white image of the final greyscale image.

It has also been proposed to provide additional control signals before the transition to the final grey level, to implement a so-called shaking phase, which functions as a preparatory drive phase before the particles are moved to implement new grey levels. This is used in order to speed up the subsequent grey level transition phase, and to reduce the dependency of the response of the display to the previous history. Further discussion of known drive schemes can be found in WO 2005/071651 and WO 2004/066253.

It has been recognised that an alternative way to eliminate the dependency on previous history is to use a feedback scheme, in which the optical state of the pixel is used as a control parameter in a feedback control loop. WO 2003/100514 discloses the use of an optical sensor to detect the optical state of each pixel, and this is used to control the pixel drive signals. This approach requires compensation for the ambient light level. This can be achieved with further ambient light sensors, or else the pixel light sensors can be used as ambient light sensors before the display is operated. This means that significant additional circuitry is required or a complicated drive scheme has to be implemented.

According to the invention, there is provided an active matrix electrophoretic display device, comprising: an array of rows and columns of display pixels; and - control means for supplying drive signals to the pixels to drive the pixels to optical states corresponding to an image to be displayed, wherein each pixel comprises a plurality of sensors for detecting movement of the electrophoretic display particles, different sensors detecting particle movement which reaches different regions within the pixel, wherein the control means is adapted to monitor the sensor outputs a plurality of times during addressing of a pixel, and determine that a desired optical state has been reached when a relationship between the sensor outputs is satisfied which corresponds to the desired optical state and which is substantially independent of the ambient light level. The invention uses multiple in-pixel sensors, and determines the pixel optical state by using a relationship between the sensor outputs which is independent of ambient light. In this way ambient light compensation is carried out simultaneously with the addressing of the pixel, and without requiring dedicated ambient light sensors. The relationship preferably comprises a ratio between the sensor outputs, and different ratios correspond to different optical states, but are independent of the ambient light level.

Each pixel preferably comprises three sensors, and they may comprise photodiodes. The active matrix switching devices may also comprise diodes, for example amorphous silicon PIN diodes.

Each pixel may comprise a storage electrode and a drive electrode, and the electrophoretic display particles are adapted to be collected at the storage electrode before the pixel is driven to a selected optical state. The sensors then detect the movement of particles out of the storage electrode. For this purpose, a first sensor of each pixel can be for detecting the presence of particles at a location adjacent the storage electrode. A second sensor can be for detecting the presence of particles at a location substantially mid way between the storage electrode and the drive electrode, and a third sensor can be for detecting the presence of particles at a location adjacent the drive electrode.

The invention also provides a method of driving an electrophoretic display device, comprising an array of rows and columns of display pixels, the method comprising, for each pixel: supplying a drive signal to the pixel to drive the pixel to a desired optical state corresponding to an image to be displayed; monitoring the movement of the electrophoretic display particles using a plurality of sensors, different sensors detecting particle movement which reaches different regions within the pixel; determining that a desired optical state has been reached when a relationship between the sensor outputs is satisfied which corresponds to the desired optical state and which is substantially independent of the ambient light level; and - when the desired optical state has been reached, ceasing supply of the drive signal.

The invention also provides a control circuit for controlling the addressing of an active matrix electrophoretic display device to drive the pixels to optical states corresponding to an image to be displayed, the device comprising an array of rows and columns of display pixels, the control circuit being adapted to: supply a drive signal to the pixel to drive the pixel to a desired optical state corresponding to an image to be displayed; - monitor the movement of the electrophoretic display particles using a plurality of sensors, different sensors detecting particle movement which reaches different regions within the pixel; determine that a desired optical state has been reached when a relationship between the sensor outputs is satisfied which corresponds to the desired optical state and which is substantially independent of the ambient light level; and when the desired optical state has been reached, cease supply of the drive signal.

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Fig. 1 shows schematically one known type of device to explain the basic technology;

Fig. 2 shows in schematic form the electric circuit control for the device of Fig. 1;

Fig. 3 shows another known example of display device; Fig. 4 shows a first example of display device of the invention; Fig. 4 is used to explain when particle movement is detected within the device of Fig. 3; Fig. 5 shows an example of pixel circuit and sensor circuit for a pixel of a display device of the invention;

Fig. 6 is a timing diagram to explain the operation of the circuit of Fig. 5; Fig. 7 is a sensor output diagram to explain the operation of the circuit of Fig.

5; Fig. 8 shows a display device including control circuit of the invention; and

Fig. 9 is one example of timing diagram to explain the method of the invention. The same references are used in different Figures to denote the same layers or components, and description is not repeated.

The invention provides an active matrix electrophoretic display device and drive method in which pixel sensors are used for detecting movement of the electrophoretic display particles, different sensors detecting particle movement which reaches different regions within the pixel. The in-pixel sensor signals are used during addressing to provide an optical feedback drive scheme which compensates for ambient light levels, in addition to removing the dependency on historical data.

Before describing the invention in more detail, one example of the type of display device to which the invention relates will be described briefly.

Fig. 1 diagrammatically shows a cross section of a portion of an electrophoretic display device 1, for example showing only a few display elements, comprising a base substrate 2, an electrophoretic film with an electronic ink which is present between two transparent substrates 3,4 for example PET (polyethylenenapthalate). One of the substrates 3 is provided with transparent picture electrodes 5 and the other substrate 4 with a transparent counter electrode 6.

The electronic ink comprises multiple micro capsules 7, of about 10 to 50 microns. Each micro capsule 7 comprises positively charged white particles 8 and negatively charged black particles 9 suspended in a fluid F. When a positive field is applied to the picture electrode 5, the white particles 8 move to the side of the micro capsule 7 directed to the counter electrode 6 and the display element become visible to a viewer.

Simultaneously, the black particles 9 move to the opposite side of the microcapsule 7 where they are hidden to the viewer. By applying a negative field to the picture electrodes 5, the black particles 9 move to the side of the micro capsule 7 directed to the counter electrode 6 and the display element becomes dark to a viewer (not shown). When the electric field is removed, the particles 8,9 remain in the acquired state and the display exhibits a bi-stable character and consumes substantially no power.

Fig. 2 shows diagrammatically an equivalent circuit of a display device 1 incorporating the display pixels of Fig. 1, and comprising an electrophoretic film laminated on the base substrate 2 provided with active switching elements, a row driver 16 and a column driver 10. Preferably, the counter electrode 6 is provided on the film comprising the encapsulated electrophoretic ink, but it could be alternatively provided on a base substrate in the case of operation using in-plane electric fields. The display device 1 is driven by active switching elements, in this example thin film transistors 19. The display thus comprises a matrix of display elements 18 at the area of crossing of row (selection) electrodes 17 and column (data) electrodes 11.

The row driver 16 consecutively selects the row electrodes 17, while a column driver 10 provides a data signal to the column electrode 11. Preferably, a processor 15 firstly processes incoming data 13 into the data signals. Mutual synchronisation between the column driver 10 and the row driver 16 takes place via drive lines 12. Select signals from the row driver 16 select the pixel electrodes 22 via the thin film transistors 19 whose gate electrodes 20 are electrically connected to the row electrodes 17 and the source electrodes 21 are electrically connected to the column electrodes 11.

A data signal present at the column electrode 11 is transferred to the pixel electrode 22 of the display element coupled to the drain electrode via the TFT. In the embodiment shown, an additional capacitor 23 is provided at the location at each display element 18, and is connected to one or more storage capacitor lines 24. Instead of TFTs, other switching elements can be applied such as diodes,

MIMs devices, etc.

This is an example of transverse field active matrix device, but the invention will be described with reference to its preferred implementation in an in-plane switching transmissive display device. Fig. 3 shows an in-plane pixel structure. The pixel cell is bounded by side walls 36 to define a cell volume in which the electrophoretic ink particles 34 are housed. The pixel can be reflective or transmissive, and a reflective pixel is shown which achieves a required colour or greyscale via ambient light reflection from the particles and a back reflector. A transmissive pixel will also have illumination from a light source, for example through a colour filter.

The particle position within the cell is controlled by an electrode arrangement comprising a data (drive) electrode 30 and a storage electrode 32 which can be a common electrode to all pixels. The relative voltages on the electrodes 30 and 32 determine whether the particles move under electrostatic forces to the storage electrode 32 or the drive electrode 30.

The storage electrode 32 defines a region in which the particles are hidden from view, by a light shield 38. With the particles over the storage electrode 32, the pixel is in an optically transmissive state allowing reflection to show the reflector colour, or allowing illumination from a backlight to pass to the viewer on the opposite side of the display. The pixel aperture is defined by the size of the light transmission opening relative to the overall pixel dimension.

An optical feedback scheme detects the optical state of the pixel, which is dependent on the particle distribution, and uses this as a control parameter for the drive method. Optical sensors can be formed as hydrogenated amorphous silicon PIN diodes, and can be used to sense the greyscale or colour within a pixel.

Fig. 4 shows a pixel layout of the invention, and uses the same reference numerals as Fig. 3. The pixel circuit of the invention includes a plurality of sensors 4OA, 4OB, 4OC. There may be only two sensors, but the preferred example shown has three. The sensors detect movement of the electrophoretic display particles 34, in particular they detect when a particle front has reached the location of the sensor.

The use of a number of discrete light sensors means that the area available for the reflector (or else the pixel aperture for a transmissive pixel design) is not significantly reduced.

Holes are formed in the reflector 42 over the photo-sensors to enable light to fall upon the sensors. The sensors 4OA, 4OB, 4OC are all in-pixel photosensors and none are dedicated to ambient light sensing. In a reset phase, the particles are collected at the storage electrode 32, and at this time, the sensor 4OC will be illuminated substantially only by ambient light. However, rather than simply using the output of the sensor 4OC as an ambient light sensor, (for example during an initial reset phase) the relationship between the sensor outputs is analysed during pixel addressing in order to compensate for the ambient light level.

Fig. 4 shows the display pixel shortly after the application of a drive voltage following the reset phase. As schematically shown, particles have started to be driven towards the drive electrode 30 and start blocking the passage of light to the sensor 4OA, thereby reducing the photodiode output, and this change in photodiode current is used as a particle detection mechanism.

The pixel of the invention is particularly suitable for implementation with an active matrix array which uses diode based switching devices, as the same technology can then be used to define the diode switches and the photosensors.

Fig. 5 shows a pixel circuit in which the electrophoretic pixel is represented as a capacitance 50, and two diodes 52,54 are provided in series (with the same polarity) between a column data line 56 and a reset line 58. The pixel 50 connects between the diode junction and a row line 59. The pixel is charged through the diode 52 to the voltage on the data line 56. The capacitance 50 can comprise an additional storage capacitor for holding a voltage applied to the pixel.

A timing diagram for the pixel operation is also shown as the top timing plot in Fig. 5. During the addressing cycle, a row of pixels is selected by pulsing the row line 59 low, in particular below the lowest column data voltage level. The column data swing is shown in Fig. 5 as 64 and the row pulse as 66. The row pulse forward biases the diode 52 so that it can be charged from the column conductor 56 to the desired data voltage. The diode 52 in the pixels of other rows remains reverse biased. The reset diodes 54 of all pixels are reverse biased by a high voltage on the reset line 58, which is sufficiently high that even with a fully charged pixel the diode 54 remains reverse biased.

The reset pulse is shown as 68. A separate reset diode 54 is required so that pixels which had a higher voltage in the last frame can be written with a new lower voltage. The diode 52 does not allow discharging of the pixel, and a low reset pulse performs this operation. The reset can be performed with a reset line 58 per row, or with a common reset for the whole display at the end of the field period.

Fig. 5 also shows a read bus 60 for reading out the photodiode currents from the sensors A,B,C. Diode switches 62A,62B,62C are used to couple the photodiode charge flow from a row line 64, through the sensors A,B,C to the readout bus, so that charge sensitive amplifiers can measure the charge flow. This can be a continuous charge measurement, with the photodiode charges providing a reverse bias current flow, or else the charge required to recharge the photodiode self-capacitance may be measured periodically, by periodically switching the voltage on the row line 64 so that the diodes 62A,62B,62C are selectively switched on and off.

The sensors do not require a reset operation as the column lines of the bus 60 are always reset to the voltage of the charge sensitive amplifier. Fig. 5 shows as the bottom timing plot the way the sensor voltage 70 evolves over time, between reset pulses 69 applied to the row 64. The reset pulse causes the sensors A,B,C to be charged to the charge sensitive amplifier voltage, and light illumination between the reset pulses causes the sensor voltage to discharge its self-capacitance (or an additional capacitor which can be provided). During the next reset pulse, the measured charge flow to recharge the sensors provides a measure of the illumination during the preceding period.

In operation, a data voltage is written into the storage capacitor (after the reset) and this causes the charged particles to start to move across the pixel cell shielding the photosensors from the ambient light as they progress. Therefore, the change in charge stored in each photosensor (or else the charge flow measured) will vary across the cell as shown in Fig. 6.

Plot 72A is for the photosensor nearest the storage electrode, plot 72B is for the photosensor in the middle of the pixel aperture, and plot 72C is for the photosensor nearest the drive electrode.

The sensor 4OC receives ambient light for longest as the particles take longer to reach the sensor, and accordingly the sensor charge reaches a peak later, as seen in plot 72C.

In a preferred implementation, the charge is not continuously monitored. Instead, at given intervals, the charge is read out of the sensors using charge sensitive amplifiers connected to the read-out bus 60, as explained above.

The number of driver stages will correspond to the number of columns, and a data shift register for supplying data to the columns is correspondingly wide. The number of charge amplifier stages can however be reduced by multiplexing. To provide periodic feedback information, the photosensor charge can be reset so that subsequent readings of the pixel state can be performed. Alternatively, a simpler technique is to store in memory the charge read-out values, and add subsequent read out values to this to obtain a total cumulative charge.

In either case, the sensor outputs are monitored a plurality of times (or continuously) during addressing of a pixel, and it can be determined that a desired optical state has been reached when an expected relationship between the sensor outputs is reached.

To do this, the charge profile across the pixel is compared with a profile expected for the required grey-level or colour.

The length of time during which the particles are driven of course dicates the particle positions and therefore the optical state of the pixel. As can be seen from Fig. 6, the ratio of the three sensor outputs is different for each time value (at least from the point in time when the signal for sensor A starts to tail off until the time when sensor C reaches a plateau). The ambient light level will essentially scale the graph of Fig. 6 up and down the y- axis, and the analysis of the relative sensor values therefore provides a way of compensating for ambient light levels. Some examples of ratio are shown in Fig. 6.

This process is explained with reference to Fig. 7, which shows the cumulative charge for each sensor A,B,C at a particular point in time. This enables the ambient light level to be compensated because the measurement technique becomes relative. In particular, the profiles are normalised to take out the effect of the ambient light level. The profile matching algorithm includes error bounds, for example to take account of ambient light changes during the addressing period, so that Fig. 7 shows a match within given error bounds. Thus, the pixel is determined to have reached the required grey level. Fig. 8 shows an example of control circuit for implementing the method outlined above coupled to a display 80.

As shown, the display 80 has a write line 56 to each pixel 82 and a read bus 60 from each pixel. The write line is a shared column conductor, and the read bus can also be implemented as shared column conductors. A column driver 84 has a driver stage 86 in the form of a DAC 89 providing a signal to the column through a buffer 90.

Pixel drive data is provided to the driver stage 86 using a shift register 88. However, a multiplexer 91 in the driver stage 86 selects between the drive data and a feedback signal 92 may be considered as an override signal. The feedback signal 92 is the result of the analysis of the photosensor signals received by the charge sensitive amplifier circuit 94. A controller 96 performs the analysis and controls the timing of the pixel drive signal by means of the feedback signal 92.

The use of an optical feedback system means that the data applied to the pixel can simply be a drive signal which is data-independent, and the data is then simply used to control the time when the signal is ceased. Alternatively, normal data may be applied to the pixel, but this can be corrected based on the analysis carried out.

A look-up-table in the controller 96 includes stored profiles for a given grey level. If the pixel is not within the error bound then a data voltage with a correction can be written to the pixel to push it towards the required grey level and this may be done several times per frame for every pixel. Thus, the feedback can be used to maintain static images, after the initial addressing stage.

When the required profile is achieved (or re-established) the voltage on the storage capacitor 50 is zeroed to prevent further movement of the charged particles.

The method requires multiple charge reads and correctional data writes to the pixel. As the electrophoretic display effect is slow, non-video applications are anticipated, and such a scheme is possible.

For displays of this type, it is not yet practical to achieve an update rate of less than 1 second. Assuming this update rate, and a 1000 line display, there will be a line time of lms. The optical feedback needs to control the state of all pixels in the array over the field period. For example, the photosensors for 10 rows of pixels can interrogated per line time. This gives a reading time for interrogating the photosensors of 0.1ms. This means the full frame of pixels are being interrogated 10 times every field period. The optical feedback is used to enable correctional data to be written to the pixel, as well as writing the original data to it. Therefore the original line time of lms is divided by 11 for the pixel addressing time periods. These 11 line pulses for a particular row are spaced evenly in time along the frame time. Thus, each line pulse is of duration 0.09ms, and they are every 0.09s. Thus, during the line time, the row of pixels is addressed once with the original data, and then ten more times.

As the physical speed of response of the pixel gives the limitation to the addressing speed, the charging of the pixel capacitance to the initial voltage and to the corrected voltage during only 1/11 of the total available line time is not a problem. The column driver buffer can be used both as the source of column data signals and as the charge sensor (by switching state), so as to reduce the IC size. In this case, it must perform 10 reading operations and 11 writing operations within each line time. Thus, each individual write operation or read operation has available a time period of 1/21 ms (0.05ms). This line time is still relatively modest for the charging of the pixel capacitance and the recharging of the sensor capacitances.

Fig. 9 shows the row pulses for two adjacent rows for the drive scheme and timings outlined above. For simplicity, the read and write pulses are shown in one pulse train, but in fact they will be applied to different row conductors 58 and 64.

Each row signal starts with an initial row addressing pulse during which pixel data is written to the pixel.

The next row pulse is the first sensor read (SR) pulse during which the sensors are reset and the charge measurement made. The next row pulse is the first corrective write (CW) pulse. The read and write pulses alternate throughout the field period of Is, as shown.

The row pulse train for one row is staggered by 1/21 ms compared to the previous row pulse train, as shown. This gives 10 corrective write operations and 10 sensor read operations per frame time.

The invention has been described in connection with an in-plane switching arrangement, but the concepts can be extended to other configurations. Essentially, optical sensing is used to detect the change in optical state of the pixel, and additionally to compensate for ambient light, by normalising out the effect of the ambient light.

The invention has also been described in connection with a diode based active matrix. However, the invention can also be applied to TFT active matrix displays, and indeed phototransistors can be used as optical sensing elements.

One example of display has been given with row and columns in a particular orientation. The orientation is however somewhat arbitrary. These may be switched around, and it should therefore be understood that a "row" may run from top to bottom, and a "column" may run from side to side. The scope of the claims should be understood accordingly.

The materials used and manufacturing process for the display has not been described in detail, as these implementation details will be apparent to those skilled in the art. In the example above, the ratio between the multiple sensor outputs is used as a measure of the pixel state which is independent of the ambient light level. However, more complicated analysis of the sensor levels and their evolution over time may be performed to provide a more accurate determination of the pixel optical state and the ambient light level. The ambient light compensation can be performed with two sensors only, but three or more sensor will enable more accurate determination of the pixel optical state. The pixel state is read a plurality of times during addressing so that the evolution of the sensor signals is monitored. It should be understood that this is intended to include the possibility of continuous sensor monitoring during addressing.

It has been described that when the desired optical sensor ratio is obtained, the drive signal is halted (because the display device is bistable). However, it will be appreciated from the above description that the drive voltage can also be adjusted at each of the correction time points, before the pixel has reached its desired optical state.

It will be apparent to those skilled in the art that diodes which are not performing a light sensing function should be shielded from light.

Various modifications will be apparent to those skilled in the art.

Claims

CLAIMS:

1. An active matrix electrophoretic display device, comprising: an array of rows and columns of display pixels; and control means for supplying drive signals to the pixels to drive the pixels to optical states corresponding to an image to be displayed, wherein each pixel comprises a plurality of sensors (40A,40B,40C) for detecting movement of the electrophoretic display particles, different sensors detecting particle movement which reaches different regions within the pixel, wherein the control means is adapted to monitor the sensor outputs (72A,72B,72C) a plurality of times during addressing of a pixel, and determine that a desired optical state has been reached when a relationship between the sensor outputs is satisfied which corresponds to the desired optical state and which is substantially independent of the ambient light level.

2. A device as claimed in claim 1, wherein the relationship comprises a ratio between the sensor outputs (72A,72B,72C).

3. A device as claimed in any preceding claim, in which each pixel comprises three sensors (40A,40B,40C).

4. A device as claimed in any preceding claim wherein the sensors (40A,40B,40C) comprise photodiodes.

5. A device as claimed in any preceding claim, wherein each pixel comprises a diode switching device (52).

6. A device as claimed in claim 5, wherein the diode switching device (52) of each pixel comprises an amorphous silicon PIN diode.

7. A device as claimed in any preceding claim, wherein each pixel comprises a storage electrode (32) and a drive electrode (30), and wherein the electrophoretic display particles are adapted to be collected at the storage electrode (32) before the pixel is driven to a selected optical state.

8. A device as claimed in claim 7, wherein a first sensor (40A) of each pixel is for detecting the presence of particles at a location adjacent the storage electrode (32).

9. A device as claimed in claim 8, wherein a second sensor (40B) is for detecting the presence of particles at a location substantially mid way between the storage electrode

(32) and the drive electrode (30).

10. A device as claimed in any preceding claim, comprising an in-plane switching electrophoretic display device.

11. A method of driving an electrophoretic display device, comprising an array of rows and columns of display pixels, the method comprising, for each pixel: supplying a drive signal to the pixel to drive the pixel to a desired optical state corresponding to an image to be displayed; - monitoring the movement of the electrophoretic display particles using a plurality of sensors (40A,40B,40C), different sensors detecting particle movement which reaches different regions within the pixel; determining that a desired optical state has been reached when a relationship between the sensor outputs (72A,72B,72C) is satisfied which corresponds to the desired optical state and which is substantially independent of the ambient light level; and when the desired optical state has been reached, ceasing supply of the drive signal.

12. A method as claimed in claim 11, wherein the predetermined relationship comprises a particular ratio between the sensor outputs (72A,72B,72C).

13. A method as claimed in claim 11 or 12, in which each pixel comprises three sensors (40A,40B,40C).

14. A method as claimed in claim 11,12 or 13, comprising collecting the electrophoretic display particles at a storage electrode (32) before the pixel is driven to the desired optical state.

15. A method as claimed in claim 14, wherein monitoring the movement of the electrophoretic display particles comprises detecting the presence of particles at a location adjacent the storage electrode (32).

16. A method as claimed in claim 15, wherein monitoring the movement of the electrophoretic display particles comprises detecting the presence of particles at a location substantially mid way between the storage electrode (32) and the drive electrode (30).

17. A method as claimed in any one of claims 11 to 16, further comprising adapting the drive signal in response to the sensor outputs before the desired optical state is reached.

18. A control circuit for controlling the addressing of an active matrix electrophoretic display device to drive the pixels to optical states corresponding to an image to be displayed, the device comprising an array of rows and columns of display pixels, the control circuit being adapted to: supply a drive signal to the pixel to drive the pixel to a desired optical state corresponding to an image to be displayed; monitor the movement of the electrophoretic display particles using a plurality of sensors (40A,40B,40C), different sensors detecting particle movement which reaches different regions within the pixel; determine that a desired optical state has been reached when a relationship between the sensor outputs is satisfied which corresponds to the desired optical state and which is substantially independent of the ambient light level; and when the desired optical state has been reached, cease supply of the drive signal.