WO2006064459A2

WO2006064459A2 - Gamma correction in a bi-stable display

Info

Publication number: WO2006064459A2
Application number: PCT/IB2005/054200
Authority: WO
Inventors: Mark T. Johnson; Guofu Zhou; Rogier Cortie
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-12-17
Filing date: 2005-12-13
Publication date: 2006-06-22
Also published as: WO2006064459A3; TW200632832A

Abstract

A bi-stable electro-optic display (1) comprises pixels (5) which have an initial optical state to display an initial image. An input receives an input signal (IV) which defines a desired optical state of the pixels (5) to display a desired image succeeding the initial image. A temperature sensor (30) supplies a temperature signal (TE) which indicates a temperature of said display (1). An offset circuit (154) determines an adapted temperature signal (TA) which is the temperature signal (TE) adapted with an offset signal (TO) to indicate another temperature than the temperature of the display (1). A look-up table memory (152) comprises stored data (SD) that determines drive waveforms required for an optical state transition (OT) from the initial optical state to the desired optical state. A controller (153) determines actual drive waveforms (DA(J)) by looking up the stored data (SD) for the optical state transition (OT) using the adapted temperature signal (TA). A driver (3) supplies the actual drive waveforms (DA(J)) to the pixels (5).

Description

Gamma correction in a bi- stable display

The invention relates to a bi-stable electro-optic display, and a method of displaying an image on a bi-stable electro-optic display.

WO-A-03/044765 discloses a bi-stable electro-optic display with a plurality of pixels. Each pixel is capable of displaying at least three grey levels. The display is driven by a method which comprises the steps of: storing a look-up table containing data representing impulses necessary to convert an initial grey level to a final grey level; storing data representing at least an initial state of each pixel of the display; receiving an input signal representing a desired final state of at least one pixel of the display; and generating an output signal representing the impulse necessary to convert the initial state of the one pixel to the desired final state thereof, as determined from the look-up table.

The impulse required for a particular transition is affected by temperature. The look-up table may be expanded by an additional dimension for each temperature to be taken into account. The continuous variable temperature is quantized to maintain a look-up table which has a practical finite size. In order to find the waveform to be applied to the pixel, a calculation means may simply choose the look-up table entry for the table closest to the measured temperature. Alternatively, to provide more accurate temperature compensation, the calculation means may look up the two adjacent look-up table entries on either side of the measured temperature, and apply an appropriate interpolation algorithm to get the required entry at the measured intermediated value of the temperature. For example, if the look-up table comprises entries for temperature in increments of 1O⁰C, and the actual temperature is 25⁰C, the calculation would look up the entries for 2O⁰C and 3O⁰C, and use a value intermediate to the two. An alternative method for temperature compensation is to use look-up table entries in the form of coefficients of standard terms of functions. The calculation means retrieves these coefficients at the appropriate look-up table entry for the relevant initial and final states, and then uses the function to calculate the proper output signal having regard to the temperature to be taken into account. The spacing in the grey levels may be linear or may be selected to provide a specific gamma. A gamma of 2.2 is often adopted for monitors.

It is a drawback of the disclosed bi-stable electro-optic display that for different gammas, different waveforms have to be stored in the look-up table.

It is an object of the invention to provide a bi-stable electro-optic display in which the gamma can be controlled in a simple manner.

A first aspect of the invention provides a bi-stable electro-optic display as claimed in claim 1. A second aspect of the invention provides a method of displaying an image on a bi-stable electro-optic display as claimed in claim 11. Advantageous embodiments are defined in the dependent claims.

The bi-stable electro-optic display in accordance with the first aspect comprises a pixel which has an initial optical state. For example, if the bi-stable electro-optic display is a matrix display with a matrix of pixels, all the pixels have their initial optical state such that the matrix display displays an initial image. This initial image should be changed into a desired image represented by an input signal received via an input. A look-up table memory comprises a look-up table which stores data which determines drive waveforms required for converting the initial optical state of the pixels into the desired optical state. Usually, these desired optical state transitions of the pixels are found by storing the initial image in an image memory and by determining the difference between these stored initial optical states and the desired optical states indicated by the actual input signal.

A temperature sensor supplies a temperature signal which indicates a temperature of the display. An offset circuit determines an adapted temperature signal which is the temperature signal supplied by the temperature sensor offset by the offset signal. Thus, the adapted temperature signal represents another temperature than the temperature sensed by the temperature sensor.

A controller determines the actual drive waveform to be supplied by a driver to the pixel, or if a plurality of pixels is present: the actual drive waveforms to be supplied by the driver to the pixels. The actual drive waveform is determined by looking up the data in the look-up table memory for the optical state transition required using the adapted temperature signal.

The intentional offset when applied on the actual temperature sensed causes the controller to determine an actual drive waveform which is optimal for another temperature than the actual temperature of the display. Thus, in accordance with the invention, the dependence of the optical state on the temperature is used to shift the optical state with respect to the nominal optical state to vary the gamma. The nominal optical state is the optical state which is obtained if no temperature offset is applied. In contrast, in the prior art matrix display, the temperature sensed is used to obtain optical states which are independent of the temperature such that at each temperature the nominal optical states are obtained. If an electrophoretic display with white and black particles is used, the optical states are also referred to as grey levels.

The variation of the gamma by offsetting the temperature sensed is very simple. In the most simple embodiment in accordance with the invention no other changes with respect to the prior art are required than offsetting the temperature sensed. The actual effect of the offsetting is largely determined by the temperature behavior of the optical state of the pixels of the display as is elucidated in more detail with respect to Fig. 4.

In an embodiment in accordance with the invention, data is stored in the look- up table memory for a plurality of temperatures. For each temperature, data is stored which determines the drive waveforms required for the possible optical transitions. The controller determines the actual drive waveform from the stored data by looking up the stored data for the optical state transition, directly or by interpolation, at a temperature indicated by the adapted temperature signal. The stored data can be looked up directly if the data is stored for many temperatures, or if an error introduced by taking the nearest temperature is acceptable. If a more accurate drive waveform is required, an interpolation is possible between two stored data sets for the two temperatures which embrace the adapted temperature. In fact, this embodiment does operate exactly the same as known from the prior art, the only difference is that instead of the sensed temperature, the adapted temperature is used to look up the data for the drive waveform.

In an embodiment in accordance with the invention, the look-up table memory comprises stored data determining the drive waveforms for at least two different temperatures. The controller determines a scaling factor dependent on a difference between a value of the adapted temperature signal and the nearest one of the at least two different temperatures. The controller adapts a level or duration of at least one pulse of the drive waveform, which is determined by the stored data at the nearest one of the two different temperatures, in accordance with the scaling factor to obtain the actual drive waveform.

In an embodiment in accordance with the invention, data is stored in the lookup table memory which determines drive waveforms for a single temperature only. The controller determines the actual drive waveform from the stored data by looking up the stored data for the optical state transition required. The controller uses a function of the temperature to adapt the looked-up stored data in accordance with the adapted temperature signal to obtain the drive waveform at the adapted temperature. This embodiment has the advantage that it is not required to provide a plurality of stored data sets, one for each temperature.

In an embodiment in accordance with the invention, the bi-stable electro-optic display further comprises an input for receiving a user command which determines the offset value. This provides the display with a simple user controllable gamma.

In an embodiment in accordance with the invention, the offset circuit determines the offset value as a function of the temperature sensed. This allows keeping the gamma correction constant at different temperatures. Thus, once a desired gamma is set by offsetting the temperature sensed, this gamma is kept constant over the temperature range in a same manner as known from the prior art.

In an embodiment in accordance with the invention, the actual drive waveform comprises a reset pulse to reset the pixel to one of the two limit optical states. These reset pulse is also referred to as flash pulse or rail stabilizing pulse. The intermediate optical states (or grey scales) are then obtained by changing the optical state starting from the nearest limit optical state. Due to the reset pulse the limit optical states are always reached independent on the offset of the temperature. Thus, at least one of the limit optical states is not influenced by the offset value because the reset to this at least one of the limit optical states is sufficiently strong to always reach this one of the limit optical states independent on the offset value. This has the advantage that the brightness and/or contrast are independent on the actual gamma setting.

In an embodiment in accordance with the invention, the data stored in the look-up table memory determines waveforms of which the amplitude and/or duration of the drive pulse which succeeds the reset pulse determines the optical state transition starting from the limit optical state determined by the reset pulse. The amplitude and/or duration of the drive pulse may by varied to obtain the same grey levels at different temperatures, if the offset on the temperature sensed is set such that the desired gamma is obtained, and this desired gamma should be constant over the temperature range. The amplitude and/or duration of the drive pulse may be varied to obtain different grey levels at a particular temperature sensed if the temperature offset is applied. For example, the temperature offset causes a change of the duration and/or amplitude of the drive pulse with a scaling factor. It is of course also possible that due to the temperature offset another aspect of the drive waveform is changed such that the grey level changes.

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 diagrammatically a cross-section of a portion of a prior art electrophoretic display device, Fig. 2 shows diagrammatically a display apparatus in accordance with the invention and which comprises an equivalent circuit diagram of a portion of the electrophoretic display device,

Fig. 3 shows a drive waveform for an electrophoretic display, Fig. 4 shows an example of the dependency of the brightness levels of the optical states of an electrophoretic display on the temperature,

Fig. 5 shows an example of the effect of the offsetting of the sensed temperature, and

Fig. 6 shows an example of the effect of the offsetting of the sensed temperature.

The same references in different Figures indicate the same items. Fig. 1 diagrammatically shows a cross-section of a portion of an electrophoretic display device 1 which, for elucidation only, has the size of a few display elements 10 only. The electrophoretic display device 1 comprises a base substrate 11, an electrophoretic film with an electronic ink which is present between two transparent substrates 12 and 16 which, for example, are of polyethylene. One of the substrates 12 is provided with transparent picture electrodes 40, 40', and the other substrate 16 with a transparent counter electrode 50. The electronic ink comprises multiple micro capsules 14, of about 10 to 50 microns. The microcapsules 14 need not be ball-shaped, any other shape, such as for example, predominantly rectangular, is possible. Each micro capsule 14 comprises positively charged black particles 15 and negative charged white particles 13 suspended in a fluid 17. The dashed material 18 is a polymeric binder. The particles 13 and 15 may have other colours than black and white. It is only important that the two types of particles 13, 15 have different optical properties and different charges such that they act differently to an applied electric field. The layer 12 is not necessary, or could be a glue layer.

When a negative voltage is applied to the counter electrode 50 with respect to the picture electrodes 40, 40', an electric field is generated which moves the black particles 15 to the side of the micro capsule 14 directed to the counter electrode 50. Simultaneously, the white particles 13 move to the opposite side of the microcapsule 14 where they are hidden to the viewer and the display element will appear dark to a viewer. By applying a positive field between the counter electrodes 50 and the picture electrodes 40, 40', the white particles 13 move to the side of the micro capsule 14 directed to the counter electrode 50 and the display element will appear white to a viewer (not shown). When the electric field is removed the particles 13, 15 remain in the acquired state, the display exhibits a bi-stable character and consumes substantially no power.

Fig. 2 shows diagrammatically an equivalent circuit of a picture display apparatus which comprises the electrophoretic display device 1. The electrophoretic display device 1 comprises an electrophoretic film laminated on the base substrate 11 provided with active switching elements Sl, a row driver 2 and a column driver 3. Preferably, the counter electrode 50 is provided on the film comprising the encapsulated electrophoretic ink, but, the counter electrode 50 could be alternatively provided on a base substrate if a display operates based on using in-plane electric fields. The display device 1 comprises a matrix of display elements 10 at the area of intersecting row or select electrodes 20 and column or data electrodes 30. The row or data driver 2 supplies select voltages VS(I) to consecutively select the row electrodes 20. The column or data driver 3 provides data signals DA(J) to the column electrodes 30 for the display elements 10 of the selected row electrode 20. Preferably, a processor 4 firstly processes incoming data ID into the data signals DA(J) to be supplied by the column electrodes 30.

The select voltages VS(I) are referenced to by VS followed by an index (I) which indicates which one of the select voltages is meant. VS(I) is the select voltage for the first line (usually a row) of display elements 10. The reference VS(I) is used to indicate an arbitrary one of the select voltages. The data signals DA(J) are referenced to by DA followed by an index (J) which indicates which one of the data signals is meant. DA(I) is the data signals for the first column of display elements 10. The reference DA(J) is used to indicate an arbitrary one of the data signals. The display device may 1 may be constructed in that the select lines 20 extend in the column direction and the data lines 30 extend in the row direction. The pixel voltage VD across the pixels 5 are indicated by VD. The display elements 10 comprise the pixel 5, a counter electrode 50, a pixel electrode 40, a switch Sl and a capacitor 23. The voltage across the bi-stable material of the pixel 5 is applied with the counter electrode 50 and the pixel electrode 40.

The processor 4 supplies control signals CS and CS' to control the mutual synchronisation between the data driver 3 and the select driver 2. Select signals VS(I) from the select driver 2 which are electrically connected to the select electrodes 20 select the pixel electrodes 40 via the gates of the thin film transistors Sl. The sources of the thin film transistors Sl are electrically connected to the data electrodes 30. A data signal DA(J) present at the data electrode 30 is transferred to the pixel electrode 40 of the pixel 5 coupled to the drain of the TFT Sl. In the embodiment shown, the display elements 10 further comprise an optional capacitor 23 which is connected between the pixel electrodes 40 of the associated pixel 5 and one or more storage capacitor lines 24. Instead of a TFT other switching elements Sl can be applied such as diodes, MIM's, etc. The lines 24 may be connected to ground. The processor 4 may comprise an image memory 150, a comparator 151, a controller 153, a look-up table memory 152, and an offset circuit 154. The image memory

150 stores a previous image of the incoming data ID. The comparator 151 compares a present image of the incoming data ID with the stored previous image to determine desired optical transitions OT to be made by the pixels 5. The offset circuit 154 receives the sensed temperature signal TE and the offset signal TO to supply the adapted temperature signal TA. The sensed temperature signal TE, the offset signal TO and the adapted temperature signal TA may be, for example, digital values or analog levels.

In one embodiment, the look-up table memory 152 stores data SD which determines the drive waveforms required to obtain all the desired optical state transitions of the pixels 5 at a plurality of temperatures. It has to be noted that the drive waveforms which are actually present across the pixels 5 are referred to as the actual drive waveforms, while the drive waveforms which are determined by the stored data SD are referred to as drive waveforms. The controller 153 receives the desired optical transitions OT to be made and the adapted temperature signal TA and retrieves the stored data SD from the look-up table memory 152 for the temperature indicated by the adapted temperature signal TA. The data for this temperature may be directly available, or may be interpolated from data for other temperatures. It is possible to look up the drive waveform in the memory 152 for a temperature nearest to the adapted temperature and to accept the deviation caused by not using exactly the temperature indicated by the adapted temperature signal TA. Alternatively, the controller determines a scaling factor dependent on a difference between a value of the adapted temperature signal and the nearest one of the at least two different temperatures, and uses this scaling factor to adapt the drive waveform. For example, the drive waveform is adapted by modifying the frame period with the scaling factor, or by modifying an amplitude or duration of a pulse of the drive waveform with the scaling factor. For example, if the drive waveforms comprise a drive pulse, the amplitude or duration of the drive pulse found in the memory 152 is adapted in accordance with the scaling factor.

In another embodiment, the look-up table memory 152 stores data SD which determines the drive waveforms required to obtain all the desired optical state transitions of the pixels 5 at a single temperature. The controller 153 receives the desired optical transitions OT to be made and the adapted temperature signal TA and retrieves the stored data SD from the look-up table memory 152. The controller 153 uses a function of the temperature to determine the actual drive waveform from the stored data and the temperature indicated by the adapted temperature signal TA. The stored data is used as the coefficients of the function. The controller 153 supplies the control signal CS to the data driver 3 and the control signal CS' to the select driver 2. The control signal CS comprise the information required to supply the actual drive waveforms DA(J) to the pixels 5. The waveform data stored in the look-up table memory 152 defines the drive waveforms to be supplied to the pixels 5. The exact drive waveforms are not relevant to the invention; any drive waveform suitable for the actual bi-stable display used may be implemented. An example of a drive waveform suitable for an electrophoretic display as shown in Fig. 1 is shown in Fig. 3.

In the display in accordance with the invention, instead of the sensed temperature signal TE, the adapted temperature signal TA is used for looking up the drive waveform in the memory 152. Or the adapted temperature signal TA is used in the function to calculate the drive waveform from the stored drive waveform. The actual drive waveform across the pixel 5 is now a drive waveform intended for another temperature than the sensed temperature. The effect of the offset signal TO is that the gamma is changed. This will be elucidated in more detail with respect to Figs. 4 and 5.

In a display apparatus which comprises the display 1, an image processing circuit 25 is present which receives the input data signal IV to supply images as the incoming data ID to the processor 4. The incoming data ID determines the optical transitions to be made by the pixels 5.

Fig. 3 shows a drive waveform for an electrophoretic display. By way of example, Fig. 3 is based on an electrophoretic display with black and white particles and four optical states: black BL, dark grey DG, light grey LG and white WH. Fig. 3 shows an image update period IUP for a transition from light grey LG or white W to dark grey DG. The vertical dotted lines represent the frame periods TF (which usually last 20 milliseconds), the line periods TL occurring within the frame periods TF are not shown 3.

The pixel voltage VD across a pixel 5 comprises successively first shaking pulses SPl lasting from the instant t0 to the instant t3, a reset pulse RE lasting from the instant t3 to the instant t6, second shaking pulses SP2, and a drive pulse Vdr. The drive pulse Vdr occurs during the drive period Tdr which lasts from instant t7 to instant t8. The second shaking pulses SP2 immediately precede the driving pulses Vdr and occur during a second shaking period TS2 lasting from the instant t6 to t7. The reset pulse RE immediately precede the second shaking pulses SP2. The first shaking pulses SPl immediately precede the reset pulses RE. Each one of levels of the first and second shaking pulses SPl, SP2 is present during the standard frame period TF.

The first and second shaking pulses SPl, SP2 improve the reaction of the particles on the reset pulse RE and the driving pulse Vdr. The reset pulse RE moves all the particles to a limit position such that a limit optical state is reached. For example, at the end of the reset pulse RE all the black particles 15 are present near the substrate 16 and the white particles 13 are present near the substrate 12 and the pixel 5 appears black. Now, the dark grey appearance is obtained by the driving pulse Vdr which moves the black particles 15 towards the substrate 12 and the white particles 13 towards the substrate 16. The actual grey level reached depends on the amplitude and/or duration of the driving pulses Vdr. This rail stabilized driving has the advantage that the grey levels are more accurate because they are obtained starting from a well defined limit optical state.

Many different approaches are possible when using a reset pulse RE. The reset pulse may always have the same polarity such that always the same limit optical state is reached before the drive pulse Vdr is applied. Alternatively, the reset pulse RE may have a polarity such that the limit optical state is reached which is closest to the grey level which has to be obtained after the drive pulse Vdr. A duration of the reset pulse RE may be selected to be longer than required to move all the particles from one limit position to the other limit position. Such a reset pulse RE is commonly referred to as an over-reset pulse. A duration of the reset pulse RE may depend on the optical state of the pixel 5 such that this duration is not much longer than required to move the particles to the limit positions to prevent sticking of the particles. The first and/or second shaking pulses SPl, SP2 need not be present.

Together with the gamma control in accordance with the invention, the use of a reset pulse RE has the advantage that the limit state or states reached with the reset pulse RE are not or only marginally influenced by using the adapted temperature signal TA instead of the sensed temperature signal TE. Consequently, if the reset pulse always moves the particles to one of the limit optical states which is black, the black level is preserved when the gamma is varied. If the reset pulse is used to obtain the limit optical state closest to the desired optical state also the contrast is preserved when the gamma is varied.

Fig. 4 shows an example of the dependency of the brightness levels of the optical states of an electrophoretic display on the temperature. The electrophoretic display is a special embodiment of a bi-stable display. The gamma correction, which may be user definable, is based upon the realization that the response time of the bi-stable display is temperature dependent. Especially, the response time of an electrophoretic display is strongly temperature dependent. To account for this temperature dependency, prior art electrophoretic displays are provided with a temperature sensor and must be able to generate different drive waveforms for different temperatures. Fig. 4 shows an example of the change of the grey levels of an electrophoretic display when the temperature changes. The full vertical lines BLl, DGl, LGl, WHl indicate the brightness of the grey levels BL (black), DG (dark grey), LG (light grey), WH (white), respectively, at a first temperature. The dashed vertical lines BL2, DG2, LG2, WH2 indicate the brightness of the grey levels BL, DG, LG, WH, respectively, at a second temperature.

If other than black and white particles are used other colours are obtained. It is possible to generate more than four brightness levels. If different pixels 5 are provided which different coloured particles a full colour display can be provided.

Fig. 5 shows an example of the effect of the offsetting of the sensed temperature. This example is based on rail stabilized drive waveforms which comprise a reset pulse RE. The rail stabilized drive waveforms include, for example, over-reset schemes, or cyclic rail stabilized schemes. Rail stabilized drive waveforms enable to create intermediate optical states (intermediate grey levels if black and white particles are used in an electrophoretic display) with a low spread in the brightness levels. The intermediate optical state is obtained starting from one of the limit optical states. The limit optical states are also referred to as the extreme optical states which in an electrophoretic display occur when all particles are in a limit position. To achieve the most natural image update, it is preferred to reset the pixels 5 to the rail closest to the intermediate optical state the pixel 5 should change to. For example, in an electrophoretic display with black and white particles, a pixel 5 which should change its optical state to dark grey is first reset to black, while a pixel 5 which should change its optical state to light grey is first reset to white. Therefore, in rail stabilized schemes, a reset pulse RE precedes a drive pulse Vdr. The reset pulse RE has an amplitude and duration such that the pixel 5 reaches one of the two limit optical states, the drive pulse Vdr has an amplitude and duration such that the desired optical state is reached starting from the limit optical state. If a rail stabilized drive waveform is used, it is possible to adjust the intermediate optical states while the limit optical states are maintained. The adjustment of the intermediate optical states is obtained by intentionally providing an offset TO to the sensed temperature signal TE supplied by the temperature sensor 30 before the rail stabilized drive waveform is generated. The effect of providing the offset TO is illustrated in Fig. 5, by way of example, for an electrophoretic display with only four optical states and white and black particles. The four optical states are the grey levels: black BL, dark grey DG, light grey LG, and white WH. The vertical lines show the brightness BR at different temperatures. The full lines BLN, DGN, LGN, WHN indicate the brightness of the grey levels BL, DG, LG, WH at a nominal temperature which is the temperature sensed by the temperature sensor 30. These grey levels have equidistant brightness differences and thus a linear brightness curve is obtained. No gamma correction is applied.

The dashed lines BLL, DGL, LGL, WHL indicate the brightness of the grey levels BL, DG, LG, WH when a temperature offset TO is applied such that a temperature lower than the nominal temperature is used to determine the drive waveforms. Because at a lower temperature, the electrophoretic material reacts slower, the drive waveforms must have a higher amplitude and/or a longer duration. Thus, if due to the offset TO, the controller 153 receives a lower temperature than the actual temperature of the display 1, the drive waveforms generated will have a higher amplitude and/or a longer duration than required by the actual temperature and the intermediate grey levels move further away from the rails (which are the limit grey levels BL and WH) and thus closer towards each other. The display is now gamma corrected.

The dashed lines BLH, DGH, LGH, WHH indicate the brightness of the grey levels BL, DG, LG, WH when a temperature offset TO is applied such that a temperature higher than the nominal temperature is used to determined the drive waveforms. Thus, if due to the offset TO, the controller 153 receives a higher temperature than the actual temperature of the display 1, the drive waveforms generated will have a lower amplitude and/or a shorter duration than required by the actual temperature and the intermediate grey levels move less far from the rails and thus away from each other. Again, the display is gamma corrected. It has to be noted that the actual temperature of the display may be the instantaneous temperature of the display or the temperature of the display integrated over a period of time which is not longer than a few frame periods. The offsetting of the temperature signal TE supplied by the temperature sensor 30 may cause a change of the duration of the voltage pulses which form the driving waveform, for example by modifying the duration of the frame period TF with a scaling factor, or alternatively by altering the amplitude of at least one of the voltage pulses of the driving waveform with the scaling factor. Preferably, the reset pulse RE is an over-reset pulse which has a sufficient high amplitude and a sufficient long duration that the temperature offset TO has no influence on the limit optical states BL and WH.

Fig. 5 clearly shows that the limit optical states black BL and white WH do not vary with the temperature. This is due to the rail stabilized driving scheme which takes care that the pixels 5 are always reset to one of the two limit optical states BL and WH independent on the actual temperature. Therefore, the reset pulse should have an amplitude and duration such that the limit optical states BL and WH are always reached. The amplitude and duration of the reset pulse may be selected longer than required in the worst case. Such a reset pulse is often referred to as an over-reset pulse. The amplitude and/or duration may depend on the temperature and/or actual optical state transition to minimize over-reset effect which has the drawback that the particles may become sticky when in their limit position. Fig. 5 further clearly shows that the temperature offset TO causes the intermediate optical states and thus the intermediate brightness levels to vary with the offset TO and thus determine the gamma of the display in a very simple manner.

Fig. 6 shows an example of the effect of the offsetting of the sensed temperature. In this example all pixels 5 are reset to black (not the nearest rail). The nominal situation when the actual temperature is used, the grey levels black BL, dark grey DG, light grey LG, and white have the brightness levels BR indicated by the vertical full lines BLN, DGN, LGN, and WHN, respectively. If a positive temperature offset TO is applied, both the dark grey DG and light grey LG levels will become darker, see the dashed vertical lines DGH and LGH, respectively. The black level BL will be kept because of the resetting to black is not sensitive to the temperature. The white level WH can be held bright if we use an extra long reset to white. Using a negative offset TO, the DG and LG levels will both become lighter (not shown). This gives a different (more usual) type of gamma correction. Alternatively, it is possible to reset all the pixels 5 to white. 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. A bi-stable electro-optic display (1) comprising: a pixel (5) having an initial optical state, an input for receiving an input signal (IV) defining a desired optical state of the pixel (5) for succeeding the initial optical state, a temperature sensor (30) for supplying a temperature signal (TE) indicating a temperature of said display (1), an offset circuit (154) for determining an adapted temperature signal (TA) being the temperature signal (TE) adapted with an offset signal (TO) to indicate another temperature than the temperature of said display (1), a look-up table memory (152) comprising stored data (SD) determining drive waveforms required for an optical state transition (OT) from the initial optical state to the desired optical state, a controller (153) for determining an actual drive waveform (DA(J)) by looking up the stored data (SD) for the optical state transition (OT) using the adapted temperature signal (TA), and a driver (3) for supplying the actual drive waveform (DA(J)) to the pixel (5).

2. A bi-stable electro-optic display (1) as claimed in claim 1, wherein the look-up table memory (152) comprises the stored data (SD) determining the drive waveforms for a plurality of temperatures, and wherein the controller (153) is adapted for determining the actual drive waveform (DA(J)) from the stored data (SD) by looking up the stored data (SD) for the optical state transition (OT), directly or by interpolation, at the temperature indicated by the adapted temperature signal (TA).

3. A bi-stable electro-optic display (1) as claimed in claim 2, wherein the look-up table memory (152) comprises stored data (SD) determining the drive waveforms at at least two different temperatures, and the controller (153) is arranged for: determining a scaling factor dependent on a difference between a value of the adapted temperature signal (TA) and the nearest one of the at least two different temperatures, and adapting, in accordance with the scaling factor, a level and/or duration of at least one pulse of the drive waveform being determined by the stored data (SD) at the nearest one of the two different temperatures to obtain the actual drive waveform (DA(J)).

4. A bi-stable electro-optic display (1) as claimed in claim 1, wherein the look-up table memory (152) comprises stored data (SD) determining drive waveforms for a single temperature only, and wherein the controller (153) is adapted for determining the actual drive waveform (DA(J)) from the stored data (SD) by looking up the stored data (SD) for the optical state transition and using a function of the temperature to adapt the stored data (SD) for the optical state transition in accordance with the adapted temperature signal (TA).

5. A bi-stable electro-optic display (1) as claimed in claim 1, further comprising an input for receiving a user command determining the offset signal (TO).

6. A bi-stable electro-optic display (1) as claimed in claim 1, wherein the offset circuit (154) is adapted for determining the offset value (TO) being a function of the temperature sensed (TE) to keep a gamma correction constant at different temperatures.

7. A bi-stable electro-optic display (1) as claimed in claim 1, wherein the actual drive waveform (DA(J)) comprises a reset pulse (RE) for resetting the pixel (5) to one of two limit optical states.

8. A bi-stable electro-optic display (1) as claimed in claim 7, wherein, the stored data (SD) represents drive waveforms comprising a drive pulse (Vdr) succeeding the reset pulse (RE) and having a level or duration determining the optical state transition (OT).

9. A bi-stable electro-optic display (1) as claimed in claim 1, wherein the bistable electro-optic display is an electrophoretic display.

10. A bi-stable electro-optic display (1) as claimed in claim 1, wherein the bistable electro-optic display is a matrix display.

11. A method of displaying an image on a bi-stable electro-optic display (1) comprising a pixel (5) having an initial optical state, the method comprises: receiving an input signal (IV) defining a desired optical state of the pixel (5) succeeding the initial optical state, supplying (30) a temperature signal (TE) indicating a temperature of said display (1), determining (154) an adapted temperature signal (TA) being the temperature signal (TE) adapted with an offset signal (TO) and indicating another temperature than the temperature of said display (1), determining (153) an actual drive waveform (DA(J)) by looking up stored data

(SD) determining drive waveforms required for an optical state transition (OT) from the initial optical state to the desired optical state and using the adapted temperature signal (TA), and supplying (3) the actual drive waveform (DA(J)) to the pixel (5).