Abstract:
An electro optic display ( 10 ) provides an electrode configuration adjacent each pixel which allows a non-uniform electrical field to be applied across the pixel ( 22 ) so that the optical output varies in the direction transverse to the pixel thickness. This allows only part of the pixel to be turned ON, or for shading within the pixel, depending on the characteristics of the electro-optic material. In a preferred embodiment the row and column electrodes ( 14, 17 ) are each made up of a group of conductive tracks ( 12, 18 ) connected into groups by impedance elements ( 13, 19 ). Voltage ramps are applied across the electrodes via input electrodes  15  and  20 . A multiphase drive scheme is described for ferro-electric and similar materials in which a number of different voltage ramps are applied to a row in succession, and at each phase the appropriate ramps are applied simultaneously to the column electrodes to build up the required pixel shape over a number of phases.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to electro-optic displays.  
         BACKGROUND OF THE INVENTION  
         [0002]    Many computer displays have a limited physical resolution—typically 70-100 dots per inch (2.76 to 3.94 dots per mm). Since the display is composed of an array of rectangular pixels, each of which is either ON or OFF, the edges of text, lines, etc that are displayed may often have a jagged “staircase” effect which is visually disturbing.  
           [0003]    There have been several attempts to solve this problem in a variety of ways e.g. by grey scale rendering or anti-aliasing. Some display technologies such as Twisted Nematic Liquid Crystal Displays (TN LCDs), cathode ray tubes, etc., allow the intensity of the whole area of a pixel to be varied. In these types of display, the intensity of each pixel may be selected in proportion to the area of the pixel that should be ON. Whilst this can reduce the visually disturbing staircase effect, it can make edges in the displayed image appear blurred, especially when viewed from close to the display.  
           [0004]    Other display technologies do not allow a range of intensities over the whole area of the pixel display, in which case a range of intensities may be simulated by rapidly turning the pixel ON and OFF, sufficiently fast so that the eye sees the average intensity. This may be used with Super Twisted Nematic (STN) LCDs. Alternatively, each pixel may be divided into sub pixels, and a varying number turned on according to the desired intensity. The display must then be viewed from such a distance that the eye cannot resolve the sub pixels, or some optical blurring introduced to average out the intensity over the whole pixel. An example of this type of technique is described in JP-A-3142260, where each pixel is effectively divided into four sub pixel slices. The image to be displayed is analysed and two-bit pixel data is added to each pixel to turn on selected sub pixel slices during a pixel sub-scanning period. This allows a range of intensities to be displayed by varying the area of the pixel that is ON, in four discrete steps. However, by its nature this system is only capable of modulating the pixel output slice-wise and in many instances this will not give good smoothing, particularly where the edge to be smoothed is nearly perpendicular to the slice direction of the pixels.  
           [0005]    A similar technique is disclosed in U.S. Pat. No. 4,824,218, which relates principally to Ferroelectric LCDs. Here a variable width portion of a pixel is turned on by driving a potential gradient across the width of the pixel by means of metal electrodes running along the edges of resistive transparent column electrodes. To allow the complete area of the pixel to be driven, whilst preventing crosstalk (i.e. unintentionally affecting other pixels in the same row), and to avoid a wasted area of the pixel nearest the “reference” metal electrode, the pixel is driven in two phases, swapping the role of the two metal electrodes between “reference” and “data”. This technique relies on the fact that a Ferroelectric Liquid Crystal (FLC) material stores its state and can be written to again, adding to the area that has already been turned ON, which is not true for all bistable materials. This scheme also requires a blanking pulse to clear the whole pixel before the two writing phases. U.S. Pat. No. 4,824,218 also refers to an extension of this technique in which the display has row and column transparent resistive electrodes each with metal electrodes on either side. A four field drive scheme is described in which the display is scanned four times, following a blanking pulse, to make up a frame. As before the metal electrodes swap roles between data and reference electrodes. Because alternate electrodes are set to a fixed reference this arrangement does not allow great flexibility in the creation of sub pixel shapes.  
         SUMMARY OF THE INVENTION  
         [0006]    Accordingly, in one aspect, this invention provides a display comprising:  
           [0007]    a first substrate having thereon a plurality of row electrode means,  
           [0008]    a second substrate having thereon a plurality of column electrode means,  
           [0009]    a layer of electro-optic material disposed between said first and said second substrate,  
           [0010]    row drive means associated with said first substrate for applying a respective selected voltage profile across each row electrode means in a direction transverse to the thickness of said layer,  
           [0011]    column drive means associated with said second substrate for applying a respective selected voltage profile across each column electrode means in a direction transverse to the thickness of said layer,  
           [0012]    whereby the electrical field in each pixel in the direction transverse to the thickness of the electro-optic layer may be selected to provide a non-uniform optical output.  
           [0013]    In a particularly preferred arrangement, each of said column electrode means and said row electrode means comprises a group of generally parallel conductive tracks.  
           [0014]    We have found that the multiple track architecture for each of the row and column electrodes provides important and unexpected advantages when used in conjunction with drive means which apply a selected voltage profile across each of the groups of tracks making up an electrode. In this way the magnitude of the electrical field across the pixel may be varied in a direction transverse to the thickness of the electro-optic layer to provide a non-uniform optical output across the pixel. By contrast to the arrangements of U.S. Pat. No. 4,824,218, which use resistive electrodes driven by metal electrodes to either side, the multiple conductive tracks of the present invention may be driven by electrical contacts well away from the image area, thus considerably improving the aperture ratio of the display. Also, the previous arrangement of U.S. Pat. No. 4,824,218 requires accurate alignment of fine metal electrodes with each of the transparent resistive electrodes, whereas in the present invention the conductive tracks in each group may be coupled together by a resistive element in contact with the end regions of the conductive tracks to one side of the display, and an input electrode provided at each end of the resistive element. Here the accuracy of alignment of the tracks and the resistive elements is not so important as the resolution of the conductive tracks may be effectively decoupled from that of the resistive elements.  
           [0015]    Thus, in one embodiment, the resistive elements may be formed on the first and second substrates, in electrical contact with the respective conductive tracks. Alternatively, the row and column resistive elements may be formed on separate substrates which are then placed in contact with the first and second substrates.  
           [0016]    For both the rows and the columns, the series of resistive elements driving the groups of conductive tracks may be replaced by a single resistive element in electrical contact with a substantial proportion of, or all the conductive tracks making up the complete set of row/column electrodes, with the drive means including an input electrode means between each group of conductive tracks.  
           [0017]    Preferably, said drive means includes means for applying an adjustable voltage across each of said resistive elements, so that the voltage profile across the group of conductive tracks is a ramp of positive, negative or zero slope. Instead of resistance coupling, the drives to the groups of tracks may be inductively or capacitively coupled.  
           [0018]    By the above arrangements, the electrical field may be configured to generate a wide range of different non-uniform outputs of selected shape for a pixel, to turn on an arbitrary portion of the pixel, so that the required edge of the line portion of the text character etc is maintained within the area of the pixel.  
           [0019]    In one preferred drive scheme, each of a set of predetermined voltage profiles is applied across a row electrode means in successive phases, and the columns driven in parallel with the required voltage profiles. In this way a broad range of pixel shapes may be provided in either a single write (i.e. just one of the phases) or a multiple write where the pixel output is incrementally rendered. It will be appreciated that this drive scheme could be modified so that the multiple successive phases are applied to the columns whilst the rows are driven in parallel.  
           [0020]    The electro-optic material may have a steep or “fast” electro-optic curve, i.e. where the electro-optic effect switches state abruptly at a particular threshold voltage, so that the optical output at a particular point within the pixel will vary between two generally discrete levels, dependent on whether the field strength at that point is above or below the threshold voltage. In general the boundary between ON and OFF regions in the pixel will be determined by an equi-potential line on the notional voltage surface within the pixel of magnitude corresponding to said threshold voltage. Alternatively, the electro-optic material may have a shallow or “slow” electro-optic curve, with the output varying continuously between ON and OFF through grey levels, so that the optical output may be shaded across the pixel, generally in accordance with the magnitude of the electrical field.  
           [0021]    We have found that the technique of providing modulation of the voltage field in a direction transverse to the thickness of the electro-optic layer may be extended to other forms of display, again to provide a non-uniform optical output, with remarkable results.  
           [0022]    Thus, in another aspect this invention provides a display comprising:  
           [0023]    first and second substrate means provided to either side of a layer of electro-optic material,  
           [0024]    the first substrate having thereon at least one electrically resistive surface,  
           [0025]    electrode means for applying selected voltages to respective points or regions across said resistive surface, and  
           [0026]    drive means for applying selected voltages to said electrodes to provide across a pixel an electrical field whose magnitude varies in at least one direction transverse to the thickness of the layer of electro-optic material, thereby to provide a non-uniform optical output across said pixel.  
           [0027]    The first substrate may have an array of discrete electrically resistive surface elements each defining a pixel, with each surface element having three or more electrodes attached thereto. Thus, three or more selected voltages may be applied to spaced points or regions of said discrete resistive layer element, for example a respective voltage at each corner of a rectangular pixel. In this arrangement, each pixel effectively interpolates the four corner voltage values around the pixel and, for an electro-optic material with a steep curve, at each point within the pixel where the voltage passes the threshold value, the optical output changes.  
           [0028]    We have also found that the ability to vary the optical output across a pixel instead of uniformly over the whole pixel means that we can provide displays which provide an effect similar to the half-tone type of process. In one arrangement the image is made up of pixels each containing a dot of controllably variable size from zero to fully filling the pixel, or even extending beyond its nominal boundary. Thus a display may comprise an electrode array of resistive electrodes with a plurality of connection points whereby an image may be created by a display of variable size dots.  
           [0029]    The electro-optic material may comprise a liquid crystal material such as twisted nematic, supertwist nematic, polymer dispersed liquid crystal (PDLC), or ferro-electric materials. Alternatively it may comprise other electro-optic materials or devices, such as field emissive devices.  
           [0030]    In operation of a typical example, the image to be displayed is analysed to determine those pixels where some form of intra-pixel variation is required, for example where the pixel is lying on an edge of the image or where grey-scaling or half toning is required. For each such pixel, the display driver determines the desired boundary lines between ON and OFF, and the electrical field intensity required, and then selects suitable voltages to be applied to the pixel to generate the required variation or level of electrical field across the pixel.  
           [0031]    Whilst the invention has been defined above, it extends to any inventive combination of the features set out above or in the following description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    The invention may be performed in various ways and, by way of example only, certain embodiments thereof will now be described with reference to the accompanying drawings, in which:  
         [0033]    [0033]FIG. 1 illustrates schematically a first embodiment of liquid crystal display device in accordance with the invention;  
         [0034]    [0034]FIG. 2 shows typical voltage profiles applied across a pixel and the non-uniform output obtainable in the embodiment of FIG. 1, with FIG. 2( a ) and ( b ) showing the voltage profiles applied across the row electrode and the column respectively, FIG. 2( c ) showing the resultant electrical field set up across the pixel, and FIG. 2( d ) showing the non-uniform optical output obtained;  
         [0035]    [0035]FIG. 3 illustrates an alternative driving electrode scheme for the embodiment of FIG. 1;  
         [0036]    [0036]FIG. 4 illustrates schematically a first type of drive scheme for driving the embodiment of FIG. 1;  
         [0037]    [0037]FIG. 5 illustrates a control system for controlling embodiment of FIG. 1;  
         [0038]    [0038]FIG. 6 illustrates schematically the pixel electrode scheme for a second embodiment of liquid crystal display device in accordance with the invention;  
         [0039]    [0039]FIG. 7 shows in FIG. 7( a ) a typical voltage surface effected in the embodiment of FIG. 6, and in FIG. 7( b ) the resultant non-uniform output obtained;  
         [0040]    [0040]FIG. 8 illustrates schematically the pixel electrode scheme for a third embodiment of liquid crystal display in accordance with the invention;  
         [0041]    [0041]FIG. 9 shows in FIG. 9( a ) a typical voltage surface effected in the embodiment of FIG. 8, and in FIG. 9( b ) the resultant non-uniform output obtained;  
         [0042]    [0042]FIG. 10 illustrates schematically a fourth embodiment of liquid crystal display in accordance with this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]    Referring to FIG. 1, the first embodiment of LCD display  10  of this invention is illustrated. For clarity, the upper glass or polymer cover layer has been omitted, together with the usual polarising, barrier and retardation layers, but these are of generally conventional design. The display  10  comprises a lower glass or polymer substrate  11  on which a series of electrically conductive tracks  12  is formed, for example from a high conductivity Indium Tin Oxide (ITO) layer.  
         [0044]    The conductive tracks  12  are connected together in groups of four by a resistive pad  13  to one side of the display, each group defining a composite row electrode  14 . A selected voltage ramp may be applied across the resistive pad  13  by means of input electrodes  15 . The voltage applied to each electrode is independently selectable to allow a range of voltage profiles to be applied. The effect of applying a voltage ramp across the resistive pad is to set each of the conductive tracks  12  making up the composite electrode to a predetermined adjustable voltage and effectively to set up a potential gradient across the lower surface of each of the pixels contained in the row, in the plane of the electrode. A polymer alignment layer (not shown) overlies the conductive tracks and serves to align the adjacent liquid crystal layer  16 . Adjacent the upper surface of the liquid crystal layer  16  there is provided a further polymer alignment layer and a set of composite column electrodes  17  each comprising a group of conductive tracks  18  interconnected by resistive pads  19 , and similar in construction and grouping to the row electrodes. A voltage ramp may be applied across the column electrode by means of input electrodes  20  at opposite ends of each of the resistive pads. In this arrangement a pixel  22  is defined at each crossing of a composite row electrode  14  by a composite column electrode  17 .  
         [0045]    As to be described the illustrated arrangement allows the orthogonal voltage profiles across the upper and lower surfaces bounding a pixel to be independently adjusted to apply a compound voltage profile to the pixel. The optical output at a particular point in the pixel will depend on whether the compound voltage at that point is or, for some materials, has been above the threshold voltage for that material. In this way a variety of complex geometrical shapes may be written within the pixel by suitable control of the voltage ramps applied to the relevant composite row and column electrodes.  
         [0046]    In the following, for convenience we refer to the electrodes applying continuously voltage surfaces across the upper and lower pixel faces but it will be appreciated that, due to the stepped nature of the composite electrodes, the voltage surfaces in practice may be stepped. It is assumed in this example, that the electro-optic material switches at 5 voltage units. FIG. 2( a ) shows the voltage profile applied across the lower surface of the pixel by means of the relevant composite row electrode  14 , by applying 10 voltage units to the upper row input electrode  15 ′ and 15 voltage units to the lower row input electrode  15 ″. Likewise FIG. 2( b ) shows the voltage profile applied to the upper surface by means of the composite column electrode  17 , by applying 10 voltage units to the left column electrode  20 ′ and 0 voltage units to the right column electrode  20 ″. FIG. 2( c ) shows the resultant field across the pixel corresponding to the difference between the two voltage surfaces, and FIG. 2( d ) gives the resultant optical output. In practice, due to the stepped nature of the voltage surfaces the dividing line may not be straight but this is unlikely to be perceptible. Naturally, the resolution may be improved by increasing the number of electrodes in a group.  
         [0047]    A wide range of sub pixel shapes may thus be generated by suitable selection of the adjustable voltages (four in all) applied to the top/bottom input electrodes of the resistive pad  13  on the relevant composite row electrode  14 ,and the left/right input electrodes of the resistive patch on the relevant composite column electrode  17 . It should be noted that selected sub portions of the pixel may be set or unset as required.  
         [0048]    A modified electrode structure is shown in FIG. 3. Here a single resistive pad  26  is in electrical contact with the resistive tracks  12  making up a number of composite row electrodes, and may interconnect with all the row tracks  12 . Between each group of tracks making up the electrode is an input electrode  28  so that the required voltage ramps may be set up across each of the composite row electrodes either singly, or in combination with a number of adjacent rows. Also, the whole, or a variable sized portion of the display may be driven, e.g. by driving the upper electrode of the first row and the lower electrode of the last row.  
         [0049]    Various drive schemes are possible and two will now be described with reference to FIGS. 4 and 5. The first scheme is for a LCD material such as Ferroelectric LCD and similar materials which can be selectively set and unset. Firstly a blanking or erasure pulse is applied to a composite row electrode  14  to clear the pixels in the row. The row is then written by a multi-phase write, in which a finite set of voltage ramps is applied across the upper and lower input electrodes  15 ′,  15 ″ on the resistive pad, for example “High” (both set high), “Low” (both set low), positive slope (upper set high, lower set low) and negative slope (upper set low, lower set high).  
         [0050]    At each phase, the composite column electrodes  17  that would require that particular “vertical” ramp on their lower surface have the appropriate respective horizontal ramps set on each of the input electrodes  20 ′,  20 ″ on each of the column resistive pads  19 . This allows complex shape pixels to be set up as the union or difference of the primitive shapes set up by any particular combination of row/column ramps. The maximum amplitude of the two ramps is set below half the threshold voltage of the material to prevent cross talk. FIG. 4 shows how the shapes in the pixels in a given row may be built up in successive phases. Thus in phase  1  a steep positive ramp is applied to the composite row electrode  14 . Simultaneously via the respective composite column electrodes  17 , a positive ramp is applied to the first pixel, a low ramp is applied to the second, a negative slope to the third and fourth, and a high is applied to the fifth, to give the first phase results as shown. In the second phase a gentle positive slope is applied to the composite row electrode  14 . Simultaneously, low ramps are applied to the first, third and fourth pixels (resulting in no change), a negative slope is applied to the second pixel and a positive slope applied to the fourth. This writes part of the second pixel and modifies the fifth pixel. In the next phase a gentle negative slope is applied to the composite row electrode. Simultaneously, low ramps are applied to the first second and fifth pixels, a positive slope is applied to the third pixel and a high slope applied to the fourth pixel.  
         [0051]    The display may be written with all the phases for a row completed before the next row is selected, as suggested by FIG. 4, or each phase may be written on a row by row basis, or a variety of interleaving schemes could be used, to maximise clarity and immediacy of the information displayed.  
         [0052]    Referring to FIG. 5, in this example the image data to be presented on the display  10  is processed using known processing techniques to determine the boundary pixels at the edge of text or images etc, where jagged edges would be visually unacceptable.  
         [0053]    The image processor  24  incorporates a look up table  26  which indicates the pixel shapes obtainable and the voltage ramps required to produce these shapes. The image processor  24  then selects the appropriate voltage ramps for the driving phases to match the shapes required and controls the row and column drives  27 ,  28  accordingly.  
         [0054]    In a preferred drive scheme for some materials such as Stabilised Cholesteric Liquid Crystals the previous state is wiped out on writing and thus no separate blanking pulse is applied. Here a modified approach is required. As before, a number of voltage ramps are set up on the row electrode in successive phases, but a drive ramp is applied to the column electrode of a particular pixel in only one row phase, that column being quiescent (i.e. at a level which will not induce switching), for the other phases.  
         [0055]    This scheme give straight edges (or approximating thereto) to the intra-pixel boundary. For situations where the pixels may appear quite large, e.g. for electronic signs, projection displays, or head mounted displays, it is desirable to have more control over the edge shape of the boundary of the ON region of the pixel e.g. to provide generally smooth curves.  
         [0056]    In the embodiment of FIG. 6, the output of each pixel may be varied non-uniformly by a multi-point electrode array provided here in the rear structure of the display, with a layer adjacent the upper surface of the liquid crystal layer providing a common ground plane. For clarity, the usual polymer alignment, polarising, barrier and retardation layers are not shown.  
         [0057]    The display  30  includes a multi-layer rear structure  32  which defines an array of resistive surface elements  34 , one adjacent each pixel, a liquid crystal layer  36 , and a transparent common conductive layer  38  serving as a voltage ground plane adjacent the upper surface of the liquid crystal layer  36 . Each resistive surface element  34  is energised by four control electrodes  40  or control points, one adjacent each corner of the surface element  34  respectively. The notional voltage surface generated over a pixel will depend on the levels of the voltages applied at the four corners tending to a smooth surface interpolating the four corner voltages, and can be adjusted by suitable selection of the four corner voltages, as illustrated in FIG. 7( a ). The resultant non-uniform output generated by the voltage surface of FIG. 7( a ) in an electro-optic material having a steep electro-optic effect is shown in FIG. 7( b ), and generally provides a curved boundary.  
         [0058]    Again a variety of drive schemes are possible depending on the number of pixels in the array. For a low number of pixels there may be sufficient space between or under the resistive surface elements to drive each corner voltage for all the pixels independently.  
         [0059]    Likewise, the image to be displayed may be analysed by an image processor as described in relation to FIG. 5, to determine those pixels requiring non-uniform outputs, the shapes required, and the appropriate voltage combinations that will achieve this in a single phase or in multiple phases as required. The appropriate signals are then supplied to the row and column drives.  
         [0060]    Referring now to FIG. 8, the third embodiment is similar in many respects to that of FIG. 6, as regards the general structure. In this embodiment, the display includes a rear multi-layer structure  46  on which is provided a continuous resistive layer  48 , which is energised by an array of control electrodes  50  spaced closely across the layer. The liquid crystal layer is bounded on its other side by a transparent common conductive layer  52  serving as a ground plane. In use, the voltages applied to the electrodes may be selected to give a voltage surface including at each electrode location a voltage peak of selected magnitude as shown in FIG. 9( a ). Where the liquid crystal material has a steep electro-optic curve, these peaks translate into dots of variable radius dependent on the radius of the peak at the threshold voltage. The drive voltages to the electrodes are therefore selected to give the required dot size, and a typical resultant image is shown in FIG. 9( b ). This embodiment allows images to be displayed using a half-tone type of process. Dots of variable size may be generated and merged if required to give the final image. In this embodiment, each control electrode effectively defines an unbounded pixel which, depending on the voltage applied, may encroach into or merge with adjacent pixels.  
         [0061]    The embodiments of FIGS. 6 and 7 may both use the multi-layer rear structure of the display to incorporate the electrode configurations required and this may be particularly suitable for displays using polymer dispersed liquid crystal material.  
         [0062]    [0062]FIG. 10 shows a further embodiment of this invention, in which respective voltage ramps may be set up across the upper and lower surfaces of a pixel by setting two adjustable voltages on the relevant row electrode and two further adjustable voltages on the relevant column electrode, thus providing a similar effect to the embodiment of FIG. 1, and capable of using similar drive schemes. In this embodiment, the upper glass or polymer cover layer has been omitted, together with the usual polarising, barrier and retardation layers, but these are of generally conventional design. The display  70  comprises a lower glass or polymer substrate  77  on which a series of electrically resistive tracks  74  is formed, for example from a high resistivity linearly conductive Indium Tin Oxide (ITO) layer. To either side of each resistive track  74  is deposited a low resistivity conductor  76  in good electrical contact therewith, for example of chromium or gold, so that a generally linearly varying electrical field may be set up across the surface of the resistive track  74 . In this example each resistive track  74  has its own pair of conductors  76 , although in other arrangements, adjacent resistive tracks  74  may share the same conductor. A polymer alignment layer  78  overlies the tracks  74  and conductors  76  and serves to align the adjacent liquid crystal layer  80 . Adjacent the upper surface of the liquid crystal layer  80  there is provided an orthogonal series of resistive tracks  82  and associated conductors  84  deposited on a cover layer (not shown). Only one upper resistive track  82  is shown, but it will be appreciated that in practice there will be many individually addressable parallel tracks  76  and  84  in both directions, defining rows and columns of pixels, e.g. at  86 .  
         [0063]    In use, the voltage profiles across the tracks  82  and  74  above and below a pixel  86  are selected to give a notional voltage surface across the pixel which generates the required electro-optic output. Thus, a typical voltage profile applied to the upper track  82  is an inclined ramp or plane bridging the voltage levels applied to the conductors  76 .  
         [0064]    Likewise, a typical profile applied to the lower track, is an inclined plane between the voltage levels applied to the conductors  76 .  
         [0065]    The ramps, and their combined effects are therefore similar to those described in connection with FIGS.  2 ( a ) to  2 ( d ). The various drive schemes mentioned above may be used to drive this embodiment.  
         [0066]    The above embodiments may be used with a wide range of combinations of electro-optic materials and addressing techniques. Thus the invention may also be embodied in Passive Matrix Twisted Nematic (TN) LCDS, Passive Matrix Supertwist Nematic (STN) LCDs, Active Matrix TN LCDs, Stabilised Cholesteric Liquid Crystal Devices, Passive Matrix Ferroelectric and Field Emissive Devices.