WO2007031915A2

WO2007031915A2 - Electrophoretic display devices

Info

Publication number: WO2007031915A2
Application number: PCT/IB2006/053152
Authority: WO
Inventors: Sander J. Roosendaal; Mark T. Johnson
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-09-13
Filing date: 2006-09-07
Publication date: 2007-03-22
Also published as: WO2007031915A3

Abstract

An electrophoretic display device comprises an array (70) of rows and columns of display pixels and a backlight (72). The backlight (72) comprises an array of independently drivable backlight portions, each backlight portion being associated with a sub-array of a plurality of display pixels. The use of independently controlled backlight portions enables them to be driven to a brightness and/or color which takes account of the local pixel values. This can be used to increase the proportion of pixels driven fully transmissive or fully black. It also enables the power consumption of the backlight to be reduced, as the individual portions can be driven off or to levels selected in dependence on the local pixel drive values.

Description

Electrophoretic display devices

This invention relates to an electrophoretic display device, in particular an in- plane switching electrophoretic display device, a method of driving an electrophoretic display device, and a backlight driver.

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 color 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 recognized 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 polarizer. They may also be made from plastics materials, and there is also the possibility of low cost roll-to roll processing in the manufacture of such displays.

For example, the incorporation of an electrophoretic display device into a smart card has been proposed, taking advantage of the thin and intrinsically flexible nature of a plastic substrate, as well the low power consumption.

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. Figure 1 shows a known passive matrix display layout for generating perpendicular electric fields between the top column electrodes 10 and the bottom row electrodes 12. The electrodes are generally situated on two separate substrates. The passive matrix electrophoretic display comprises an array of electrophoretic cells arranged in rows and columns and sandwiched between the top and bottom electrode layers. The column electrodes 10 are transparent.

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 color is seen. The particles may be colored and the underlying surface black or white, or else the particles can be black or white, and the underlying surface colored. An advantage of in-plane switching is that the device can be adapted for transmissive operation, or transflective 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.

Monochrome electrophoretic display systems are used for electronic reading devices, whilst color versions are being developed for signage and billboard display applications, and as (pixellated) light sources in electronic window and ambient lighting applications.

Electrophoretic displays are driven by complex driving signals, particularly if grey scales are to be enabled. For a particle to be switched from one grey level to another, often it is first switched to white or black and to then to the final grey level. This can lead to visible artefacts of the image during transition, in particular a highly undesirable flashing of the image can occur. 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.

A monochrome electrophoretic display typically uses black and/or white particles within a transparent fluid. A number of ways are being explored to implement a color display. For reflective color displays, the use of color filters is not attractive, as there is an associated loss of brightness. For transmissive/transflective displays, the use of a backlight makes color filtering more appropriate.

One approach is therefore to provide a white backlight, and to use color filtering to convert a monochrome output into a color sub-pixellated display. One example of the use of a color filter is disclosed in WO 04/074921.

An alternative is to provide a backlight which flashes in three different colors, and to control each color output in sequence. This may use a monochrome pixel array, essentially functioning as a light valve for each color, although this requires higher speed operation which may not be suitable for existing electrophoretic display technology.

An alternative is to use different color particles to implement a color filtering operation within the pixel. For example, the black level can be made in a subtractive way by absorbing red, green and blue parts of the backlight spectrum by moving cyan, magenta and yellow electrophoretic particles in a transparent fluid into the light path. White is made by moving all of these colored particles out of the light path into a so-called "container".

This approach enables a white backlight to be used, and the pixel output color is obtained with one addressing phase. This approach does however require three different types of particle which can be moved independently between the container and the pixel aperture. This can be achieved by having particles which move with different speeds, and using these differences to devise a control scheme which enables selected particles to be moved to the pixel aperture. Such an approach is described in WO 2004/088409 and WO 04/066023. Different frequency responses of the particles has also been proposed as a way of providing independent driving of each color particle.

A further alternative approach is to stack multiple display modules on top of each other, each with pixels for a specific color. This enables a simple monochrome pixel design to be used to implement a color display without reducing the pixel resolution, but does introduce alignment issues. This invention relates to all of the above types of transmissive or transflective color or monochrome displays, and relates in particular to the backlight design and a driving method using the backlight as a controlled device.

According to the invention, there is provided an electrophoretic display device, comprising:

- a plurality of display pixels;

- a backlight; and - a backlight driver, wherein the backlight comprises an array of independently drivable backlight portions, each backlight portion being associated with a sub-array of the plurality of display pixels, and wherein the backlight driver is operated to control each backlight portion in dependence on desired pixel outputs of the pixels of the associated sub-array. The use of independently controlled backlight portions enables them to be driven to a brightness and/or color which takes account of the local pixel values. This can be used to increase the proportion of pixels driven fully transmissive (i.e. white) or fully black.

It also enables the power consumption of the backlight to be reduced, as the individual portions can be driven off or to levels selected in dependence on the local pixel drive values. Each pixel may comprise particles suspended in a fluid, with a reservoir for housing the particles outside the pixel aperture. This defines an in-plane arrangement, with lateral movement of particles into or out of the pixel aperture.

Each pixel may comprise three sets of colored particles suspended in a transparent fluid. These may be provided within one pixel structure, or may be provided as a multiple layer structure, effectively of three overlapping display panels.

The backlight may comprise a segmented white light source, and in this case, the intensity of each portion (segment) can be independently selectable in order to provide power savings and an improved drive response.

Alternatively, a color output of each backlight portion can be independently selectable. For example, each backlight portion may comprise a light emitting diode arrangement having a selectable output color.

The invention also provides a method of driving an electrophoretic display device, comprising a plurality of display pixels and a backlight, the method comprising independently driving a plurality of backlight portions forming the backlight, each backlight portion being associated with a sub-array of the plurality of display pixels, the drive of each backlight portion being selected in dependence on desired pixel outputs of the pixels of the associated sub-array.

In one drive scheme, drive of each backlight portion is selected such that, with the pixel with the brightest desired output of the associated sub-array driven to full brightness, the intensity of the backlight output is selected to provide the desired pixel output. This provides power saving, as each backlight is only driven as brightly as it needs to be for the brightest pixel of the associated sub-array.

In a modification, the brightness level of each backlight portion is selected such that, with the pixel with the brightest desired output of the associated sub-array driven to full brightness, the intensity of the backlight output is selected to be below that required to provide the desired pixel output.

This can provide further power savings. The number of pixels of the sub-array which are driven to a brightness below the desired pixel output can be counted, and the intensity of the backlight output can be selected such that the counted number of pixels is below a threshold.

The difference between the desired pixel output and the produced pixel output can also be determined for each pixel, and the backlight intensity and pixel drive levels can then be selected to maintain a total difference value below a threshold. This enables a trade off between accuracy and the transition performance.

If all pixels of the associated sub-array of pixels are to be driven to black, the backlight portion can be turned off, and furthermore, the pixels of the associated sub-array can then be driven to values based on the desired pixel output levels for the next image frame. This enables the speed of response to be improved. The invention also provides a backlight driver for controlling a backlight of an electrophoretic display device, in which the backlight comprises an array of backlight portions each associated with a sub-array of a plurality of display pixels, wherein the backlight driver is for independently controlling the operation of each backlight portion in dependence on the desired pixel outputs of the pixels of the associated sub-array.

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

Figure 1 shows a known passive matrix display layout; Figures 2 is used to explain a first method which can be implemented by the invention;

Figure 3 is used to explain a second method which can be implemented by the invention; Figure 4 is used to explain a further detail of the method explained with reference to Figure 3;

Figure 5 is used to explain the (known) principle of operation of a subtractive color system;

Figure 6 is used to explain an application of the invention to a color system which operates in the manner explained with reference to Figure 5; and

Figure 7 shows a display device 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 electrophoretic display device, comprising an array of rows and columns of display pixels and a backlight, in which the backlight comprises an array of independently drivable backlight portions, each backlight portion being associated with a sub-array of a plurality of display pixels. This combination of an in-plane electrophoretic LCD with a segmented backlight enables various advantages to be obtained. Firstly, it enables a backlight segment to be turned off to save power. Secondly, it enables the intensity of each segment of the backlight to be a function of the grey levels of the pixels it illuminates, in such a way that the light intensity can, for example, be set to the maximum grey level present in the segment. The pixel is then switched to fully transparent to save power in the backlight. Thirdly, the number of grey-to-grey transitions can be minimized, and fourthly the response speed can be optimized.

As mentioned above, a first application of the invention is to save power by switching off backlight segments that are not needed, and this is illustrated in Figure 2. The top part of Figure 2 shows the segmented backlight 20, with 16 segments by way of example, numbered 1 to 16. The lower part of Figure 2 shows an image 22 to be displayed, and this image only requires illumination by backlight segments 1,6,7,10 and 11, and all other backlight segments can be turned off to save power. In addition to the power saving associated with turning off backlight segments, there is also the possibility to drive the pixels associated with a non- illuminated backlight segment to any desired levels. In particular, the pixels associated with the turned off backlight segment can be driven with preparation pulses. During the frame with the non- illuminated backlight segment, the presence or absence of absorbing particles in the light path does not influence the color of the segment, and the particles can be "prepared" for an easy transition to the next image.

For example, a pixel can be switched to fully black or white, then switched to a grey scale level (for example 50%) which is the desired level in the next image. In this way, there is more time for the slower transition to an intermediate grey scale, and the transition artifacts associated with the preparatory driving fully to black or to white are not visible.

It is also possible to carry out multiple complete resets from one extreme optical state to another of all the pixels within the black segment. Such repeated resetting is extremely efficient in removing image retention by erasing any memory the particles may have of their position in a previous image, but is unacceptable in continuously backlit (or reflective) applications as it results in highly visible repeated flashing of the display for example from black to white.

Figure 2 thus illustrates a first way to use the ability to operate backlight segments independently. However, additional power saving advantages can be obtained by more intelligent use of the independent control of the backlight segments.

In particular, if the output intensity of each backlight segment is independently selectable, additional power savings can be made. Independent intensity control enables the drive of each backlight segment to be selected such that the pixel with the brightest desired output of the associated sub-array is driven to full brightness, and the intensity of the backlight output is selected to provide the desired pixel output.

This concept is illustrated in Figure 3, which shows values associated with the pixels illuminated by one backlight segment. In this example, each backlight segment is associated with a 4x4 sub-array of pixels. Figure 3 shows on the left pixel information for one frame "Frame 1" and on the right pixel information for a subsequent frame "Frame 2". For this example, it is assumed that each pixel can be driven to 8 grey levels,

1-8, and to black (grey level 0). The top tables 30 in Figure 3 show the pixel grey level data for each frame. If all the pixel grey levels are 0, this enables the backlight to be turned off, and the grey levels for the next frame can then be used to drive the pixels. This is not illustrated in Figure 3. The tables 32 show the backlight intensity, which is the same for all pixels of the sub-array, and is selected to correspond to the brightness of the maximum brightness pixel of the sub-array of pixels. In this example, the maximum brightness pixel is at level 4 for each frame, and the backlight segment is thus driven to an output intensity such that this desired pixel brightness is obtained with a pixel driven to a fully transmissive state.

To take account of the altered backlight intensity, all pixel drive levels need to be scaled, and tables 34 show the effect of this scaling on the pixel drive signals. In this simple example, the pixel drive levels are doubled.

Tables 36 show the resulting pixel outputs, which correspond to the desired grey level data of tables 30.

This drive scheme enables the backlight intensity to be reduced in dependence on the maximum brightness pixel within a pixel sub-array. This provides power savings. In addition, this drive scheme increases the number of pixels which are driven to a fully transmissive state, and thereby reduces image artifacts and reduces the time to obtain the desired output state.

Tables 38 show which pixels are driven to the extreme black or white state (with a "1"). For Frame 1, 75% of the pixels are in one of these extreme drive conditions and in Frame 2, 50% of the pixels are in one of these extreme drive conditions.

The reduction in light intensity also provides an opportunity to increase the number of grey levels. Additional grey levels can in particular be introduced by using all the original pixel drive levels in combination with the reduced light intensity. For example, if all original pixel drive levels 1 to 8 are used in combination with a 50% intensity backlight, the darker half of the pixel output range will have 8 possible drive levels.

Preferably, each backlight segment (other than a backlight segment which can be turned off) has at least one pixel in the associated sub-array set at the maximum brightness level, and this is the result of selecting the backlight output to correspond to the brightest pixel.

The intensity of each backlight segment can be a function not only of the current frame, but also of the previous frame and the next frame image. By taking into account multiple frames of pixel output data, the number of preferred transitions (i.e. black to white, white to black, grey to white, grey to black) can be maximized. This approach improves the accuracy of grey levels, reduces the occurrence of image retention, and can reduce the image update time. In the example above, the backlight is set to the luminance of the brightest pixel in the associated sub-array. An alternative is to set the backlight at a lower output level. This leads to clipping of a number of the brightest luminance levels to this lower output level. A constraint can be set that the number of clipped pixels (which end up with a lowered grey level) in this segment should be lower than a tolerance level. This approach can be especially useful in situations with a large number of grey levels (16 or more) and/or where the input data has a high number of grey scales but the display has a lower number of grey scales. For example, this approach may be of particular benefit when converting from high resolution, e.g. 12bit, input images to a low resolution, e.g. 2 or 3 bit, display. A problem with the use of a small number of grey levels is that grey level/color errors occur. An additional constraint in the algorithm can be that the total grey scale error (intended luminance - achieved luminance) should be smaller than another set value. This can give the possibility to set a pixel to a grey level close to that of the next frame even if the backlight is not switched off. In this example, grey scale errors are traded off against fast, smooth and accurate transitions.

In addition to taking into account the number of grey scale errors and the maximum percentage of clipped pixels, the information for additional frames of image data can be used for controlling the backlight segment outputs. In principle, complicated algorithms can be envisioned, taking into account all following frames until the segment reaches a black frame.

For example, if the calculated backlight output is zero (entire field black), the next value of the backlight output is calculated for the following frame. If that one is also zero, the pixels are left in the states they are in. If the next value of the backlight output does not equal zero, switching starts for the transition to the calculated grey levels for the next frame. In this case, the maximum response time improvement is one frame time. If a larger response time improvement is necessary, the algorithm can look at more than one future frame and start switching already if it is determined that the next two frames are black for this segment.

The backlight segment outputs can be calculated in isolation so that, for ease of computation, the algorithm ignores the grey levels of neighboring segments. However, this may lead to a "tiling effect" where a homogeneous desired grey field produces different grey levels for each segment, each with a small grey level error within the segment, but with a noticeable difference between the same grey level for adjacent segments. It may be possible to ignore the neighboring segments of the backlight luminance if the above-mentioned constraints on the number of clipped pixels and the total grey scale error are determined for the "present" frame only.

A more complicated algorithm can also be used which takes into account the grey level errors of the adjacent segments. A similar (but more complicated again) algorithm can be used taking into account the grey level error in the "present" frame and the grey level error in the "next" frame. The number of frames taken into account may of course be increased beyond only the next frame, providing a minimization of grey scale errors and clipping over the total number of frames.

Figure 4 shows an example in which a backlight can be turned off for Frame 1, and the pixel settings can be chosen based on the data for the next frame. Furthermore, reset operations may also be carried out during Frame 1. Figure 4 has the same structure as Figure

3 and shows the same information in the rows of tables 30-38.

The examples above have described only grey level pixel data, for ease of explanation. A color display can be produced using the same principles. In particular, if a backlight is associated with a pixellated color filter array, there will effectively be a set of pixels of each color, and each set of pixels can be driven to grey scale levels, as described above. Each backlight segment can again simply be associated with a sub-array of pixels which are essentially all driven to grey scale levels, and this sub-array of pixels could include pixels of each color, or could include pixels of one color only. Thus, for color implementations where the pixel array is functioning simply as a light valve, the above considerations can be applied.

For color implementations which use multiple colored particles which are independently driven, the invention provides the opportunity of further modifications to the drive scheme. In particular, if the color of each backlight segment can be independently controlled, this provides a further opportunity to improve the drive scheme.

Figure 5 shows how multiple color particles are used to control a white light source. Yellow particles are used to absorb blue light, and this is represented by the step 50 in the transmission plot for the yellow particles at the wavelength corresponding to blue. Magenta particles are used to absorb green light, and this is represented by the step 52 in the transmission plot for the magenta particles at the wavelength corresponding to green. Cyan particles are used to absorb red light, and this is represented by the step 54 in the transmission plot for the cyan particles at the wavelength corresponding to red. The ability to change the backlight color can also be used to reduce the number of transitions of the different colored particles between frames of image data.

Color control can be achieved by using individual colored LEDs to form the backlight, for example red, green and blue. Additionally, a white output may be generated, and optionally other dedicated colors. It is therefore possible with existing technology to allow for setting of a segment to a desired color.

For a given backlight color, it is possible to have any amount of the colored particles within the visible portion of the pixel providing these particles do not absorb, reflect or scatter light of the wavelength range(s) which is being produced by their segment of the backlight. These particles do not then influence the image output. For example, with reference to Figure 5, if a yellow backlight color is provided, the quantity of yellow particles within the pixel aperture will not influence the output color of the pixel. In a similar manner to that explained above, this enables the yellow particles to be driven based on pixel data for the next frame, and this again provides a more efficient drive technique. A simplified example will be used to explain this principle with reference to

Figure 6.

Figure 6 shows three frames, in which an image is entirely red in the first frame, entirely magenta in the second frame (a combination of red and blue light), and entirely blue in the third frame. The magenta particles, which absorb only green light, may be present in the visible part of the pixel, during all images, if no green light source component is present. As shown, the transmission trough associated with the magenta particles does not align with the frequency of any of the light source components in this case. If instead a white backlight is used, the magenta particles are required to provide the desired color filtering. However, the use of a colored backlight, with the color controlled, can enable the presence of the magenta particles to be optional, so that the quantity of magenta particles in the pixel aperture may be chosen based on the next frame which requires them.

For example, for Frame 1, a red light source 60 can be used. This also enables the yellow particles to be prepared for Frame 2, as these have no influence on the color output.

For Frame 2, the backlight is switched from red to magenta. This could be a dedicated magenta light source, or alternatively both red and blue light sources operating together, as shown as 62. With a magenta light output from the backlight segment, any quantity of the magenta particles can be provided in the pixel aperture without influencing the color output.

A white light source could also be used for Frame 2, and this would require the magenta particles to be in the pixel aperture to remove the green component of the white light source. A dedicated white light source is preferably used, although white could be generated using red green and blue light sources operating together.

For Frame 3, the backlight is shown as switched to a blue light source 64. At this point, the cyan and magenta particles can be moved into the visible part of the pixel if they are to be required for a following image. The use of a selected backlight color instead of a white output enables the power to be reduced, and also gives the possibility of selecting the quantities of the particles which do not influence the color output in dependence on the next frame, or multiple future frames. Again, a maximum response time improvement of one frame time can be obtained if the switching of a particular color particle commences during the frame preceding the frame for which the color particle setting is required. If a larger response time improvement is necessary, the algorithm could again look at more than one future frame and start switching already when it is determined that the next two frames have no particular color component for this segment.

The example above is a simplified version of image colors that will be present in practice. The color point of the backlight segment can be selected using more intelligent algorithms taking into account the pixel outputs of the current and future frames. A lookup table can be used to implement the algorithm, and frame stores can be used to store the pixel data for future frames. These frame stores act as a buffer which delay the provision of the image data to the display by the required number of frames. When the quantity of a particular color particle can be selected based on the next frame, a reset phase may be used. For example, a pixel can be switched to fully magenta (with all magenta particles in the pixel aperture), then switched to 50% magenta, if this is the desired level in the next frame. In this way, there is more time for this transition and the transition artifacts are not visible. This has been described above in connection with the monochrome examples.

In the case of a color display with Cyan, Magenta, Yellow particles, maximum values of required light output for red, green and blue can be calculated independently taking into account the grey scale error and the clipping as discussed above, for example for the present frame only. The principles discussed above in connection with monochrome displays concerning clipping and grey scale error minimization can also be applied to color displays.

The color examples above describe systems using three color particles. Systems have also been proposed using six types of color particles (blue, green, red, yellow, magenta and cyan particles).

The invention enables a reduction in image transition artifacts, in particular less visible flashing and a reduction in image retention. The addressing can be made faster and simpler, with more transitions of pixels to fully on or off. A lower power consumption can be obtained, and an increased number of grey levels can be obtained at lower brightness images or image portions.

The number of backlight segments is lower than the number of pixels of the display, so that the pixels are divided into sub-arrays. Each sub-array has been shown as having 4x4 pixels in the example above.

A practical sub-array size will typically be a square or rectangular segment with a diagonal of 2.5cm for electronic price tags to 50cm for electronic billboards. The display itself may range from 7.5cm (electronic price tags) to 5m (electronic billboards).

The segmented backlight may be made using various known technologies. Light sources can range from organic or anorganic LEDs to CCFLs, or incandescent or halogen or other types of fluorescent lamps. There light could be coupled in to separate light guides, each of which couples out to a part of the display. Alternatively, a segmented optical element on top of a single light guide could selectively couple out a light output. The light sources could be positioned at the edges of a light guide or directly behind the panel. Mirrors, holographic structures, diffuse scattering layers, can be used to direct the light.

One example is a side-emitting LED which couples light into a respective light guide. In the light guide, the light is coupled out towards the viewer. A set of the light guides and LEDs can be positioned behind the panel and together form the backlight.

Figure 7 shows a display device of the invention, comprising a pixel array 70 and a segmented backlight 72. A processor system 74 is provided for implementing the drive scheme, and this has a field store 76 for storing one or more frames of data so that the drive of one frame can be dependent on the data for a future frame or frames. A look up table 78 implements the calculation of the desired backlight segment intensities and/or colors from the pixel data, and a processor 80 performs the required scaling or other conversion of the input data 84. The processor system controls a conventional row and column driver 82. The system 74 functions both as a pixel driver and as a backlight driver. Electrophoretic display systems 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. The display may also be used as a light source.

A number of different techniques have been described above taking advantage of the use of a segmented backlight. Various of these techniques may be combined, and the invention covers each of these possibilities. The physical design of the pixels has not been described in detail, as this will be known to those skilled in the art. Further details of example of pixel design can be found in the references cited above, and these are incorporated herein by way of reference material.

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

Claims

CLAIMS:

1. An electrophoretic display device, comprising:

- a plurality (70) of display pixels;

- a backlight (72); and - a backlight driver (74), wherein the backlight (72) comprises an array of independently drivable backlight portions, each backlight portion being associated with a sub-array of the plurality of display pixels, and wherein the backlight driver is operated to control each backlight portion in dependence on desired pixel outputs of the pixels of the associated sub-array.

2. A device as claimed in claim 1, wherein each pixel comprises particles suspended in a fluid, with a reservoir for housing the particles outside the pixel aperture.

3. A device as claimed in claim 2, wherein each pixel comprises three sets of colored particles suspended in a transparent fluid.

4. A device as claimed in any preceding claim, wherein the backlight portions each comprises a white light source (72).

5. A device as claimed in any one of claims 1 to 3, wherein the color of the output of each backlight portion is independently selectable.

6. A device as claimed in claim 5, wherein each backlight portion comprises a light emitting diode arrangement having a selectable output color.

7. A device as claimed in any preceding claim, wherein the output intensity of each backlight portion is independently selectable.

8. A device as claimed in any preceding claim, wherein each pixel comprises an in-plane switching arrangement.

9. A device as claimed in any preceding claim, wherein the plurality of display pixels comprises an array of multiple rows and multiple columns of pixels, and each sub- array comprises a sub-array of multiple rows and multiple columns of pixels.

10. A device as claimed in any preceding claim, wherein the backlight driver (74) is operated to control each backlight portion such that, with the pixel with the brightest desired output of the associated sub-array driven to full brightness, the intensity of the backlight output is selected to provide the desired pixel output.

11. A device as claimed in any preceding claim, wherein the backlight driver (74) is operated to control each backlight portion such that, with the pixel with the brightest desired output of the associated sub-array driven to full brightness, the intensity of the backlight output is selected to be below that required to provide the desired pixel output.

12. A device as claimed in any preceding claim, wherein the backlight driver (74) is operated to control a backlight portion to be turned off if all pixels of the associated sub- array of pixels are to be driven to black.

13. A device as claimed in claim 12, further comprising pixel drive circuitry, and wherein when a backlight portion is turned off, the pixel drive circuitry is controlled to drive the pixels to values based on the desired pixel output levels for the next image frame.

14. A device as claimed in claim 12 or 13, wherein when a backlight portion is turned off, the pixel drive circuitry is controlled to drive the pixels of the associated sub-array with one or more reset operations.

15. A device as claimed in any preceding claim, further comprising a multiple frame store, and wherein the backlight driver controls the backlight portions in dependence on the information in the multiple frame store.

16. A method of driving an electrophoretic display device, comprising a plurality (70) of display pixels and a backlight (72), the method comprising independently driving a plurality of backlight portions forming the backlight, each backlight portion being associated with a sub-array of the plurality of display pixels, the drive of each backlight portion being selected in dependence on desired pixel outputs of the pixels of the associated sub-array.

17. A method as claimed in claim 16, wherein the brightness level of each backlight portion is selected such that, with the pixel with the brightest desired output of the associated sub-array driven to full brightness, the intensity of the backlight output is selected to provide the desired pixel output.

18. A method as claimed in claim 16, wherein the brightness level of each backlight portion is selected such that, with the pixel with the brightest desired output of the associated sub-array driven to full brightness, the intensity of the backlight output is selected to be below that required to provide the desired pixel output.

19. A method as claimed in claim 18, wherein the number of pixels of the sub- array which are driven to a brightness below the desired pixel output is determined.

20. A method as claimed in claim 19, wherein the intensity of the backlight output is selected such that the counted number of pixels is below a threshold.

21. A method as claimed in any one of claims 16 to 20, wherein the difference between the desired pixel output and the produced pixel output is determined for each pixel, and the backlight intensity and pixel drive levels are selected to maintain a total difference value below a threshold.

22. A method as claimed in any one of claims 16 to 21, wherein if all pixels of the associated sub-array of pixels are to be driven to black, the backlight portion is turned off.

23. A method as claimed in claim 22, wherein when a backlight portion is turned off, the pixels of the associated sub-array are driven to values based on the desired pixel output levels for the next image frame.

24. A method as claimed in claim 22 or 23, wherein when a backlight portion is turned off, one or more reset operations are performed for the pixels of the associated sub- array.

25. A method as claimed in any one of claims 16 to 24, wherein the drive of each backlight portion is selected in dependence in addition on the desired pixel outputs of the pixels of one or more adjacent sub-arrays.

26. A method as claimed in any one of claims 16 to 25, comprising independently selecting the color of the output of each backlight portion.

27. A backlight driver (74) for controlling a backlight of an electrophoretic display device, in which the backlight comprises an array of backlight portions each associated with a sub-array of a plurality of display pixels, wherein the backlight driver is for independently controlling the operation of each backlight portion in dependence on desired pixel outputs of the pixels of the associated sub-array.

28. A backlight driver as claimed in claim 27, which is operated to control each backlight portion such that, with the pixel with the brightest desired output of the associated sub-array driven to full brightness, the intensity of the backlight output is selected to provide the desired pixel output.

29. A backlight driver as claimed claim 27 or 28, which is operated to control each backlight portion such that, with the pixel with the brightest desired output of the associated sub-array driven to full brightness, the intensity of the backlight output is selected to be below that required to provide the desired pixel output.

30. A backlight driver as claimed in claim 27, 28 or 29, which is operated to control a backlight portion to be turned off if all pixels of the associated sub-array of pixels are to be driven to black.