Liquid Crystal Displays

This invention relates to liquid crystal displays in which selected parts of a display are addressed in matrix form.

Liquid crystal displays are commonly formed by enclosing a thin layer e.g. 12 .mu.m thick, between glass slides provided on their inner faces with transparent electrodes. Application of a voltage to the electrodes causes a change in optical properties in the liquid crystal between the electrodes, removal of the voltage allows the liquid crystal to relax back to its original state. Thus for example a part of a display can be made to appear transparent in a coloured back ground, or vice versa, and this is usefully used in digital watch displays. For simple displays such as small numeric watch displays only a relatively few electrodes are required. However for a large display it becomes impractical to provide an individual pair of electrodes to each part of the display requiring addressing.

One solution to the addressing of large displays commonly used in electro-optic displays is to address in a matrix, i.e. use a series of long column electrodes on one of the glass slides and a series of long line electrodes on the other glass slide. Thus by applying a voltage to a particular column and a line electrode liquid crystal between the intersection of those two electrodes is addressed. A similar technique may also be used to multiplex the addressing of seven segment type digital displays.

Unfortunately the time required for a liquid crystal to change its optical property on application of an electrical voltage i.e. its `on` state, and the time required to relax back to its original `off` state after removal of the voltage is frequently incompatible with matrix addressing or multiplexing. This is because the whole display cannot be addressed before parts of the display decay back to their non-addressed state.

According to this invention a method of addressing a matrix liquid crystal display comprises the steps of applying a voltage across a plurality of active areas of liquid crystal to change them to their on state, applying a zero voltage between selected electrodes in sequence for sufficient time to cause a change in optical property in the liquid crystal at the intersection of those selected electrodes, and at other times applying a voltage to the electrodes to maintain a voltage across the liquid crystal layer at a sustaining value close to the threshold level, wherein the voltages are obtained by selective application of zero volts, and two sinusoidal waveforms 60.degree. out of phase.

Apparatus for carrying out the method of this invention includes a liquid crystal cell comprising a layer of liquid crystal material sandwiched between two slides at least one of which is transparent, each slide having on its inner surface spaced electrodes arranged to form a plurality of intersections across the liquid crystal layer, and further includes circuitry for applying a voltage to the electrodes to change the liquid crystal to its on state, circuitry for applying a voltage to the electrodes to maintain a sustaining voltage close to a threshold voltage across the liquid crystal layer, and circuitry for applying across selected intersections in sequence a zero voltage, the voltage being obtained by selective application of zero voltage, a first sinusoidal voltage waveform, and a second sinusoidal voltage waveform similar to the first voltage waveform but 60.degree. different in phase to obtain the required voltage across the intersections.

The two 60.degree. phased waveforms should be as near a sine waveform as possible since if the waveform becomes too distorted e.g. a square wave the device does not work. The liquid crystal responds to the r.m.s. value of the wave provided that its frequency has a period appreciably shorter than the response times of the liquid crystal.

To obtain a threshold voltage across an intersection a zero voltage is applied to one electrode and either the first or second voltage waveform applied to the other electrode at the intersection. To obtain a zero voltage between two electrodes at an intersection either the first or second voltage waveform is applied to both electrodes. The term active areas of liquid crystal is defined as the areas at the intersection's between opposing electrodes.

The liquid crystal may be a positive nematic with between 1% and 99% of a cholesteric material.

Such a material mixture, when formed as a thin layer in a liquid crystal cell, scatters light in its off state with no electric field applied across the layer. Application of a blanking voltage, typically 30 volts r.m.s. for a 12 .mu.m thick layer containing about 10% of cholesteric by weight, causes a realignment of liquid crystal molecules parallel to the applied field i.e. its on state. In this on state the layer is transparent. The time taken to turn from off to on depends on the amount of cholesteric material present, the layer thickness, and the applied voltage level. At a critical voltage, the threshold value, the molecules will start to change from their off state to their on state. If the voltage is suddenly removed from across the layer, the liquid crystal molecules return to their off state in typically 60 .mu.sec for a layer thickness 12 .mu.m containing 33% of cholesteric. However if a voltage having an r.m.s. value near this critical value is applied across the layer after the cell has been turned on it maintains its on state for up to about 40 seconds or longer. The liquid crystal mixture may contain a suitable quantity, say 0.25% to 10% w/w of a dichroic or pleochroic dye or a mixture of such dyes. In this case the liquid crystal is opaque or coloured in its off state and becomes clear and colourless or changes colour when turned fully on.

The invention will now be described by way of examply only in the accompanying drawings in which:

FIG. 1 is a front view of a three by three matrix display;

FIG. 2 is a sectional plan view of FIG. 1.

FIG. 3 is a waveform diagram showing how the display of FIG. 1 is addressed line by line.

As seen in FIGS. 1, 2 a cell 1 comprises two glass slides 2, 3. The slide 2 carries three transparent ribbon shaped electrodes 4, 5, 6 forming column or x electrodes whilst the slide 3 carries similar electrodes 7, 8, 9 forming line or y electrodes. Typically the electrodes are 500A thick tin oxide deposited as a continuous layer and etched to the desired shape. Between the slides 2, 3 is a 6 .mu. to 40 .mu.m thin layer 10 of liquid crystal material such as n-pentyl-cyanobiphenyl nematic material containing a cholesteric material such as cholesteryl nonanoate. An epoxy resin adhesive 11 is used to seal the edges of the cell 1. Electrical connections are made to each electrode.

A.C. voltages are applied to the cell 1 from a two phase (60.degree. apart) sinusoidal supply (not shown). If V is the r.m.s. magnitude of the supply voltage, the supply from each phase may be indicated by V.angle.0, V.angle.60. It may be shown that

in operation a blanking voltage of sufficiently high value is applied between each x and y electrode to cause an alignment of liquid crystal molecules parallel to the applied field. The blanking voltage may be obtained by applying + V.angle.60 to the x electrodes and - V.angle.60 to the y electrodes to give 2V across an intersection. At each intersection of x, y electrodes (an active area) the liquid crystal material 10 is transparent and light will pass through the cell 1. After the blanking voltage has been applied, a threshold voltage is applied as follows. A voltage V.angle.60 is applied to each x electrode and a zero voltage is applied to each y electrode. At each electrode intersection the voltage is therefore V.angle.60 and V.sub.RMS is arranged to approximately equal the threshold voltage V.sub.th. Each intersection continues to appear transparent for up to about 40 seconds or longer. Assume now it is required to make the intersection at x.sub.2, y.sub.2 appear opaque i.e. the liquid crystal scatter light, this is shown at 12 in FIG. 1. A voltage V.angle.0 is applied to both electrodes x.sub.2, y.sub.2 resulting in zero voltage across the liquid crystal 10 at their intersection. All other intersections continue to have V applied. For example at intersection x.sub.2, y.sub.1 the voltage on x.sub.2 is V.angle.0, and on y.sub.1 is zero resulting in V.angle.0. At intersection x.sub.1, y.sub.2 the voltage on x.sub.1 is V.angle.60, and on y.sub.2 is V.angle.0 resulting in V.angle.60 - V.angle.0 whose R.M.S. value is V, i.e. the threshold voltage. Once the intersection at x.sub.2, y.sub.2 becomes opaque return of V has insufficient effect to make the intersection appear transparent. Thus V is re-applied to x.sub.2 y.sub.2 and another intersection may be addressed with zero voltage. The whole of the cell 1 is addressed as required then the process is repeated i.e. a blanking voltage is applied to the whole cell, threshold voltage applied to the cell and selected intersections addressed.

For a 12 .mu.m thick layer nematic material with 10% cholesteric blanking voltage is typically 80 volts, threshold voltage i.e. V is about 20 volts. For such conditions the liquid crystal becomes clear in about 10.sup.3 .mu.s, and becomes scattering in about 100 .mu.s when zero voltage is applied to the electrodes. Thus a matrix of 10.sup.4 elements could be addressed intersection by intersection or 10.sup.4 lines could be addressed line by line in 1 second.

When addressing a large display the intersections x, y may be addressed in a raster manner first from say top left to bottom right of a display then from bottom right to top left. This assists in presenting a more uniform overall contrast. Alternatively the display may be divided into several blocks or individual lines which are blanked, and addressed in sequence.

The examples given illustrate the addressing of individual elements in the matrix. The addressing scheme may also be used to address the display line at a time using known techniques.

The technique need not be confined to x - y matrices. It may also be used with r-.theta. polar plot displays and with seven segments type numeric displays.

FIGS. 3a and 3b show the envelope of waveforms for applying to the three by three matrix of FIG. 1 when blanked and addressed in a line by line manner.

A blanking voltage Vb followed by V.angle.0 is applied to each y electrode in turn. For the remaining time periods each y electrode receives zero voltage. Meanwhile each x electrode receives V.angle.60 except for a time period when V.angle.0 is applied to the x.sub.2 electrode coincident with V.angle.0 being applied to y.sub.2 so that the x.sub.2 y.sub.2 intersection is supplied with V.angle.0 - V.angle.0 = 0 volts to change its associated liquid crystal state.