WO1999066487A1

WO1999066487A1 - Method and apparatus for driving a field-sequential color antiferroelectric liquid crystal display device with polarity inversion

Info

Publication number: WO1999066487A1
Application number: PCT/GB1999/001618
Authority: WO
Inventors: Paul William Herbert Surguy
Original assignee: Central Research Laboratories Limited
Priority date: 1998-06-18
Filing date: 1999-05-21
Publication date: 1999-12-23
Also published as: GB9813226D0; TW455830B

Abstract

The invention relates to a method of, an apparatus for, driving a liquid crystal display device incorporating anti-ferroelectric liquid crystal (AFLCD). A switching sequence is provided whereby all colour fields are addressed according to a sequence of the type red, green, blue, green, red, green, blue, etc., with polarity inversion. Therefore, charge balancing is achieved. Moreover, flicker coming from the asymmetry of the transmission characteristics curve of AFLC is avoided, without doubling the switching speed, since each field is always addressed with a voltage having the same polarity (e.g. red always positif, green always negatif, etc.).

Description

METHOD AND APPARATUS FOR DRIVING A FIELD-SEQUENTIAL COLOR ANTIFERROELECTRIC LIQUID CRYSTAL DISPLAY DEVICE WITH

POLARITY INVERSION

The present invention relates to a method of, and apparatus for, driving a liquid crystal

display (LCD) device. One type of liquid crystal display (LCD) device is a liquid crystal,

silicon back-plane device. Liquid crystal, silicon back-plane devices are electro-optic

modulators where a liquid crystal layer is fabricated directly on a silicon memory chip.

An example of the aforementioned device is described in the Applicant's granted

European Patent EP-Bl-548179. The liquid crystal material is sandwiched between a

front plate and a back plate. The back plate is usually a mirror. The liquid crystal

material is sub-divided into a grid of pixellated regions. The display device described

includes a driver which is a type of control device and operates a plurality of switches.

The switches are arranged to switch region of the liquid crystal material from one state to

another. When it is desired to display an image, the driver selectively switches

predetermined regions of the liquid crystal material so that some of the pixels become

transparent whilst others become opaque. The result is that a viewer sees an image

which is formed by pixels which either reflect or absorb light.

Several liquid crystal types have been used in silicon back-plane devices. Two of these

are: ferroelectric liquid crystals and twisted nematic liquid crystals. Both types of liquid

crystals have been used in a variety of alignment configurations. By applying a voltage

across the faces of the front and back plates, the orientation of liquid crystals changes.

The consequence is that more or less light is transmitted through the device. This results

in a display becoming brighter or darker. Another class of liquid crystals which have been used in silicon back-plane devices are those known as anti-ferroelectric liquid crystals (AFLCs). Anti-ferroelectric liquid crystals have an electro-optical response which is shown by the graph in Figure 1. Unlike ordinary ferroelectric liquid crystal materials, anti-ferroelectric liquid crystal materials (AFLCs) are not bistable. That is when an applied voltage is removed anti- ferroelectric liquid crystal materials return to their original orientation; whereas ferroelectric liquid crystals remain in a switched state. This is sometimes referred to as "memory". Moreover, the electro-optic response of AFLCs is only approximately symmetric about a zero applied voltage and therefore is not truly polarity insensitive. That is when a voltage +V volts is applied the transmission of an AFLC device is Ti. However, if an opposite polarity voltage, (i.e. -V volts is applied), then the transmission of the device is different namely T₂.

UK Patent Application No 2173 629 (STC) discloses a driving sequence which employs a strobe type unipolar pulse sequence which combines with a sequence of bipolar pulses. The polarity of strobe pulses is reversed in order to maintain change of balance. This switching sequence would not be suitable for anti-ferroelectric liquid crystal (AFLC) material because they exhibit different properties according to the applied voltage polarity. As there is no reference to colour switching it is assumed that the sequence is only suitable for monochrome devices.

It is possible to use AFLCDs in a colour display. Such a device may be a silicon backplane device and has three separate colour fields, namely: red (R), green (G) and blue (B). Information about the colour of an image is written to each of the appropriate fields sequentially. An advantage of colour sequential LCDs is that the number of pixels required in a display is reduced by a factor of 3 compared to a colour filter display.

In colour sequential liquid crystal display devices, colour is introduced into an image by field sequential reflection or transmission from three different light sources namely red, green or blue. A colour sequential device is preferred over using three separate colour filters because in smaller devices it is extremely difficult (and therefore expensive) to produce different colour pixels on this scale. This makes colour sequential displays cheaper and easier to manufacture, because there are no colour filters.

United States Patent US 5113274 (Seiki) employs a spatial sharing of common pixels. This implies that a low resolution is available. However, as stated above the use of colour filters imposes a limit to the size of the device.

Typically information for a liquid crystal display device is written using alternate polarity electric fields, as this maintains an approximate direct current balance for the whole liquid crystal device. In the case of AFLCDs this is not easily possible because the device is not entirely polarisation insensitive.

In the case of an AFLCD device a flicker problem arises when simple dc balancing is implemented due to the aforementioned polarity sensitivity of the liquid crystal material. This can be overcome by using complex electronic processing or faster writing of the colour fields. However, drivers operating at very high frequencies, are expensive and consume more energy. The present invention arose in an attempt to solve the forgoing problem.

According to a first aspect of the present invention there is provided a method of driving a liquid crystal display device comprising the steps of: applying a first voltage pulse, of a first polarity, to a first group of pixels; subsequently applying a second voltage pulse, of opposite polarity to the first voltage pulse, to a second group of pixels; subsequently applying a third voltage pulse of identical polarity to the first voltage pulse, to a third group of pixels; subsequently applying a fourth voltage pulse, of opposite polarity to the first voltage pulse, to the second group of pixels thereby achieving a desired grey level; and repeating the sequence of voltage pulse application so that voltage pulses of the same polarity are always applied to the same group of pixels and thereby obtaining direct current (dc) charge balancing for the device.

According to a second aspect of the present invention there is provided a method of driving a liquid crystal display device comprising the steps of: in a first time interval applying a first voltage pulse, of a first polarity to a first colour display driver; in a subsequent time interval applying a voltage pulse of opposite polarity to the first voltage pulse, to a second colour display driver; in a subsequent time interval applying a voltage pulse of identical polarity to the first pulse, to a third colour display driver; and in a subsequent time interval applying a voltage pulse of identical polarity to the second voltage pulse to said second display driver, then repeating the pulse application sequence so that regions of pixels are illuminated during adjacent time intervals, with reversing polarities, thereby achieving direct current (dc) charge balancing. In a preferred embodiment of each aspect, the liquid crystal display device includes an anti-ferro-electric liquid crystal material (AFLCD).

Preferably red (R) and blue (B) fields are addressed with the first polarity voltage pulses and the green (G) field is addressed with the second polarity voltage pulses. The overall result is that, in any given time period (that is during a plurality of adjacent time intervals), a green field is always addressed before and after a blue field is addressed and a green field is always addressed before and after a red field is addressed. Alternatively, a red field is never addressed immediately before or immediately after a blue field is addressed. Instead there is always an intervening green field which is addressed. The effect of this address sequence is to achieve direct currently balancing and each colour is always addressed with the same polarity voltage pulse, thus avoiding flicker which would otherwise arise from the asymmetric transmission / voltage characteristic material, as shown by the graph of properties of AFLC material in Figure 1.

Corresponding apparatus is provided in accordance with the aforementioned statements of invention. Preferably the apparatus is supported on a silicon back plane device.

Preferably drive c means is integrated into the device.

Because the human eye is most sensitive to green light, it is possible to reduce the intensity of the driving voltage of the green field. Preferably therefore, the drive? means (or driver) for the green light source is always of a slightly lower amplitude than that for the red and blue drivers means. As the eye is more sensitive to green light it would appear to be intuitive to use one of the other colours as the colour which is addressed most often. However, in the preferred sequence a brighter display results from using green as the most frequently addressed colour with less energy consumption. It will be appreciated that alternative colour sequences may be used. Thus, for example an alternative sequence may be: red, green, red, blue, red, green, red, blue.

Preferably the sequence repeats itself after 6N adjacent time intervalswhere N is any positive whole number, and preferably 1. The colour sequence may then change so that alternate interlacing patterns may be used.

The overall frame time is 4/3 faster than before and consequently does not place such a great demand upon electronic processing. Previously drivers had to operate at twice the speed in order to achieve direct current (dc) balancing and avoid flicker.

An example of a preferred embodiment of the invention will now be described with reference to the Figures generally, and particularly to Figures 5 to 8, in which:

Figure 1 shows a graph of AFLCD transmission characteristics between positive and negative pulses;

Figure 2 shows an example of a sequence of illumination fields used in the prior art;

Figure 3 shows diagrammatically how alternate voltage polarities are applied to the sequence of illumination of fields in Figure 2; Figure 4 shows diagramatically how flicker arises in a prior art display as a result of asymmetry in optical response characteristics of certain liquid crystals;

Figure 5 shows an example of a colour field sequence according to the present invention;

Figure 6 shows diagrammatically how alternate voltage polarities are applied to the sequence of colour fields in Figure 5;

Figure 7 shows overall optical output obtained when combining the sequences in Figures 5 and 6; and

Figure 8 is a sketch of a block diagram of one example of an apparatus according to the present invention.

Referring to the Figures generally there is shown in Figure 1 a graph of the response of a liquid crystal material having asymmetry in transmission between positive and negative voltage pulses. This asymmetry occurs in certain anisotropic materials such as anti- ferroelectric liquid crystals (AFLC). The result is that when a negative voltage -V_a volts is applied, a transmission T₂ is achieved. When a reverse voltage polarity, +V_a volts is applied, a different transmissivity Tj is achieved. The overall result, when using this type of liquid crystal material in rapid switching devices, where dc balancing is achieved, is that a flicker effect is observed. This is undesirable from a viewer's point of view. One way that the so called "flicker" has been removed is by doubling the rate at which the display has been addressed. This is expensive and requires high data rates to the display from a controller or driver (not shown). Also certain types of AFLC materials do not readily lend themselves to rapid switching.

"Flicker" arises as a result of the addressing sequence of alternate colour fields. An example of such a sequence is shown diagrammatically with reference to Figures 2, 3 and 4. As mentioned above, "flicker" occurs as a result of combining dc balancing with alternating colour fields. Colour fields red, green and blue are addressed, always in that order, but with alternate polarity voltage pulses, (shown diagrammatically in Figure 3). Figure 4 shows diagrammatically how "flicker" arises when alternating polarity pulses of (Figure 3) are used to drive a display illuminated by a colour field sequence, for example of the type shown in Figure 2.

The invention overcomes the problem of d.c. charge balancing by adopting a different sequence of addressing colour drivers. This is achieved in a preferred embodiment by using a different sequence of address pulses. The resultant pulse train is described briefly with reference to Figures 5 to 7. The following notation is used: a "plus" (+) indicates a positive applied voltage and a "minus" (-) indicates a negative applied voltage. Thus by combining the pulse sequences shown in Figures 5 and 6 there is a obtained the following address sequence:

R+, G-, B+, G-, R+, G-, B+.

the overall brightness level is depicted, diagrammatically and it is apparent, from Figure 7, that the brightness of the green field is less than either the red or blue fields. (Alternatively the green field may be brighter than red or blue fields). Referring briefly to Figure 8, there is shown a display controller 10, which combines a polarity signal from a polarity selector 12, with picture data from picture data source 14. This forms a train of pulses similar to that depicted in Figure 6. Controller 10 then sequentially switches respective pulses to one of three different colour light sources 16, 18 or 20. Light source 16 is for a red field, light source 18 is for a green field and light source 20 is for a blue field. Illumination optics 22 couple light from sources 16, 18 and 20 to a display 24. The combination of light source and display controller may also be referred to as drive means or driver. These light sources illuminate selectively groups of pixels, as is known in the art, and thereby define an image (not shown). The device 10 may be supported on a silicon back plane device.

The invention has been described by way of one embodiment only, to which variation may be made, without departing from the scope of the invention.

Claims

1. A method of driving a liquid crystal display device comprising the steps of:

applying a first voltage pulse, of a first polarity, to a first group of pixels; subsequently

applying a second voltage pulse, of opposite polarity to the first voltage pulse, to a

second group of pixels; subsequently applying a third voltage pulse of identical polarity

to the first voltage pulse, to a third group of pixels; subsequently applying a fourth

voltage pulse, of opposite polarity to the first voltage pulse, to the second group of pixels

thereby achieving a desired grey level; and repeating the sequence of voltage pulse

application so that voltage pulses of the same polarity are always applied to the same

group of pixels and thereby obtaining direct current (dc) charge balancing for the device.

2. A method of driving liquid crystal display (AFLCD) device comprising the steps

of: in a first time interval applying a first voltage pulse of a first polarity to a first colour

display driver; in a subsequent time interval applying a voltage pulse of opposite polarity

to the first voltage pulse to a second colour display driver; and in a subsequent time

interval applying a voltage pulse of identical polarity to the first pulse, to a third display

driver; and in a subsequent time interval applying a voltage pulse of identical polarity to

the second voltage pulse to said second display driver then repeating the pulse

application sequence so that regions of pixels are illuminated during adjacent time

intervals, with reversing polarities, thereby achieving direct current (dc) charge

balancing.

3. A method according to either claim 1 or claim 2 wherein the first polarity pulses address pixels in the red and blue fields and pulses of opposite polarity address pixels in the green field.

4. A method according to claim 3 wherein the green field is addressed using a voltage pulse of lower amplitude than that used for the red or blue fields.

5. A method of addressing an AFLCD substantially as herein described with reference to Figures 5 to 8.

6. Apparatus for driving a liquid crystal display device (AFLCD) comprising: means

for applying a first voltage pulse, of a first polarity, to a first group of pixels; means for

second group of pixels; means for applying a third voltage pulse of identical polarity to

the first votlage pulse, to a third group of pixels, means for applying a fourth voltage

pulse, of opposite polarity to the first voltage pulse, to the second group of pixels, control

means for controlling the sequence of pulse application so as to achieve a desired grey

level; and direct current (dc) charge balancing for the device.

7. Apparatus for driving a liquid crystal display (AFLCD) device comprising: means

for applying a first voltage pulse of a first polarity to a first colour display driver in a first

time interval; means for applying a voltage pulse of opposite polarity to the first voltage

pulse to a second colour display driver in a subsequent time interval; means for applying

a voltage pulse of identical polarity to the first pulse, to a third display driver in a third

time interval; means for applying a voltage pulse of identical polarity to the second

voltage pulse to said second driver in a subsequent time interval, and control means for

repeating this sequence so that regions of pixels are illuminated during adjacent time

intervals with reversing polarities thereby achieving direct current (dc) charge balancing.

8. Apparatus according to claim 6 or 7 wherein means is provided for addressing the green field using a voltage pulse of lower amplitude than that used for the red and blue fields.

9. Apparatus according to claim 6 to 8 which is supported on a silicon back plane device.

10. Apparatus substantially as herein described with reference to Figure 8.