EP1028347A1

EP1028347A1 - Liquid crystal display and method of driving the same

Info

Publication number: EP1028347A1
Application number: EP98940621A
Authority: EP
Inventors: Shinya Citizen Watch Co. Ltd. Techn.Lab KONDOH; Mie Citizen Watch Co. Ltd. Techn. Lab. OHARA
Original assignee: Citizen Watch Co Ltd
Current assignee: Citizen Watch Co Ltd
Priority date: 1998-08-28
Filing date: 1998-08-28
Publication date: 2000-08-16
Also published as: WO2000013057A1; EP1028347A4

Abstract

In a liquid crystal display comprising an antiferroelectric liquid crystal or a ferroelectric liquid crystal and a light source capable for successively emitting light of different colors, the light source has an arbitrary period during which the light source stops emission when switching from one color to another, and a reset period during which the antiferroelectric liquid crystal or ferroelectric liquid crystal in all pixels is reset to a predetermined state is included within the arbitrary period. The reset period may instead be included in the early part or the last part of a period during which the light source emits light.

Description

TECHNICAL FIELD

The present invention relates to a liquid crystal display constructed using a light source capable of emitting light of a plurality of different colors, in combination with a liquid crystal display panel, a liquid crystal optical shutter array, or a like component, having a matrix of pixels formed from a liquid crystal layer of antiferroelectric or ferroelectric liquid crystal. The invention also relates to a method of driving such a liquid crystal display.

BACKGROUND ART

In the conventional art, various methods have been proposed for accomplishing color display using a liquid crystal cell as a shutter and utilizing a successive additive color mixing phenomenon by placing a light emitting device (such as an LED or a CRT) behind the shutter. Prior art literature relating to such methods includes, for example, 7-9 "4 A Full-Color Field-Sequential Color Display" presented by Philip Bos, Thomas Buzak, Rolf Vatne et al. at Eurodisplay '84 (1984/9/18-20). This display method, unlike methods that use color filters with the respective color segments provided at each pixel position, produces color display by projecting differently colored lights in rapid succession. For the liquid crystal cell used with this method, the same structure as that of a cell used for monochrome display can be used. The light emitting device disposed behind the liquid crystal cell emits light of three primary colors, for example, R (red), G (green), and B (blue), which are successively projected onto the liquid crystal cell, each color for a predetermined duration of time (TS). That is, light of each color is projected onto the liquid crystal cell for the duration of time TS, in the order of R (red), G (green), and B (blue). These three primary colored lights are successively and repeatedly projected. The liquid crystal cell is controlled in synchronism with the time TS to vary the light transmittance of each display pixel. More specifically, the light transmittance for each of R, G, and B is determined by driving the liquid crystal cell in accordance with display color information. As an example, the light transmittance of the liquid crystal cell is set and held at 50% when R is being emitted for time TS, at 70% when G is being emitted for time TS, and at 90% when B is being emitted for time TS. Since the time TS is usually very short, the human eye does not perceive the respective colors as individually separate colors but as one color produced by mixing the respective colors.
Techniques utilizing such a method for ferroelectric liquid crystal display devices are disclosed in Japanese Patent unexamined Publication Nos. 63-85523, 63-85524, and 63-85525.
Unlike driving methods for conventional TN or STN liquid crystals, driving methods for ferroelectric or antiferroelectric liquid crystals employ unique techniques that make use of the properties of the liquid crystal to be driven.
Liquid crystal display devices using antiferroelectric liquid crystals have been researched vigorously since it was reported in Japanese Patent unexamined Publication No. 2-173724 by Nippondenso and Showa Shell Sekiyu that such liquid crystal displays provide wide viewing angles, are capable of fast response, and have good multiplexing characteristics.
In antiferroelectric or ferroelectric liquid crystal display devices, it is generally practiced to forcefully reset a pixel to a black display state or a white display state, irrespective of its display data, immediately before writing to the pixel.
In the case of antiferroelectric liquid crystal driving, for example, each selection period (Se) is immediately preceded by a reset period (Re) and, during this reset period, a voltage lower than a threshold voltage is applied to the pixel to reset it to an antiferroelectric state. In this way, by resetting the state of each pixel to a predetermined state immediately before writing necessary information to the pixel, a good display can be produced with each pixel being unaffected by its previously written state.
There are several factors that can cause the pixel state to be affected by the previously written state. In the case of an antiferroelectric liquid crystal display device, for example, this phenomenon is believed to be caused by the property that the antiferroelectric liquid crystal molecules form a layer structure which varies depending on the pixel state, that is, whether the pixel is in a first ferroelectric state, a second ferroelectric state, or an antiferroelectric state. That is, it is presumed that if a voltage waveform necessary for the next writing is applied while the previously written state is retained, it is difficult to change the layer structure since it is affected by the layer structure in the previously written state. Accordingly, if the reset period is not provided, a degradation in the contrast or another display characteristic occurs. Another factor that can be considered to contribute to the above phenomenon is the property that the antiferroelectric liquid crystal has a plurality of stable states and, therefore, the state of the liquid crystal molecules differs depending on the immediately preceding display state. Since the threshold at the time of voltage application differs depending on the stable state of the molecules, if all molecules are not set in the same stable state immediately before the selection period, it will become difficult to control all the pixels in the same display state by the voltage applied during the selection period.
On the other hand, when driving a ferroelectric liquid crystal display, a reset period is provided or is not provided, depending on the method employed. In the case of the driving method in which the reset period is not provided, when producing a white display or a black display depending on the polarity of the applied voltage, driving is performed by using two frames or constructing each selection period with four phases to write a screen. With such a driving method, the time required to write a screen becomes long.
When ferroelectric or antiferroelectric liquid crystals are employed for the liquid crystal display device, it is desirable that the reset period be provided. However, when performing driving for color display using the successive additive color mixing phenomenon, the reset period must be provided for each scanning electrode within the period that one light color is being emitted. In particular, in the case of conventional time division driving, this means that the reset period is provided each time one light color is emitted and each time a voltage is applied to each scanning electrode. During the reset period, information other than that necessary for writing is displayed. This gives rise to the problem that as the number of reset periods increases, the number of necessary display states increases, causing a significant degradation of display quality.
It is, accordingly, an object of the present invention to provide an antiferroelectric liquid crystal display and a ferroelectric liquid crystal display utilizing the successive additive color mixing phenomenon, and a method of driving such displays. More specifically, it is an object of the present invention to provide a liquid crystal display that produces a good display uniformly over a liquid crystal display panel utilizing the successive additive color mixing phenomenon, and a method of driving the same.

DISCLOSURE OF THE INVENTION

To achieve the above objects, the antiferroelectric liquid crystal display according to the present invention comprises: an antiferroelectric liquid crystal display element which includes an antiferroelectric liquid crystal sandwiched between a pair of substrates having a matrix of pixels; and a light source which successively emits light of different colors, wherein
the light source has an arbitrary period during which the light source stops emission when switching from one color to another,
the arbitrary period includes a reset period during which the antiferroelectric liquid crystal in all the pixels is simultaneously reset to one of three states consisting of an antiferroelectric state, a first ferroelectric state, or a second ferroelectric state, and
a selection period during which a voltage for determining the display state of a pixel is applied and a non-selection period during which the display state is maintained are included within a period during which the light source emits light of any one of the colors.
The antiferroelectric liquid crystal display according to the present invention, the reset period during which the antiferroelectric liquid crystal in all the pixels is simultaneously reset to one of the three states, i.e., the antiferroelectric state, the first ferroelectric state, or the second ferroelectric state, is included in the early part or the last part of the period during which the light source emits light of any one of the colors.
The ferroelectric liquid crystal display according to the present invention comprises: a ferroelectric liquid crystal display element which includes a ferroelectric liquid crystal sandwiched between a pair of substrates having a matrix of pixels; and a light source which successively emits light of different colors, wherein
the light source has an arbitrary period during which the light source stops emission when switching from one color to another,
the arbitrary period includes a reset period during which the ferroelectric liquid crystal in all the pixels is simultaneously reset to either one of two states consisting of a first stable state or a second stable state, and
a selection period during which a voltage for determining the display state of a pixel is applied and a non-selection period during which the display state is maintained are included within a period during which the light source emits light of any one of the colors.
The ferroelectric liquid crystal display according to the present invention, the reset period during which the ferroelectric liquid crystal in all the pixels is simultaneously reset to either one of the two states, i.e., the first stable state or the second stable state, is included in the early part or the last part of the period during which the light source emits light of any one of the colors.
In a method of driving the antiferroelectric liquid crystal display according to the present invention,
the light source, when switching from one color to another, stops emission for the arbitrary period,
in the arbitrary period, the antiferroelectric liquid crystal in all the pixels is simultaneously reset to one of the three states, i.e., the antiferroelectric state, the first ferroelectric state, or the second ferroelectric state, and
in the period during which the light source emits light of any one of the colors, the voltage for determining the display state of a pixel is applied during the selection period, and the display state is maintained during the subsequent non-selection period.
In another method of driving the antiferroelectric liquid crystal display according to the present invention, the antiferroelectric liquid crystal in all the pixels is simultaneously reset to one of the three states, i.e., the antiferroelectric state, the first ferroelectric state, or the second ferroelectric state, in the early part or the last part of the period during which the light source emits light of any one of the colors.
In a method of driving the ferroelectric liquid crystal display according to the present invention,
the light source, when switching from one color to another, stops emission for the arbitrary period,
in the arbitrary period, the ferroelectric liquid crystal in all the pixels is simultaneously reset to one of the two states, i.e., the first stable state or the second stable state, and
in the period during which the light source emits light of any one of the colors, the voltage for determining the display state of a pixel is applied during the selection period, and the display state is maintained during the subsequent non-selection period.
In another method of driving the ferroelectric liquid crystal display according to the present invention, the ferroelectric liquid crystal in all the pixels is simultaneously reset to one of the two states, i.e., the first stable state or the second stable state, in the early part or the last part of the period during which the light source emits light of any one of the colors.

ADVANTAGEOUS EFFECT OF THE INVENTION

In the antiferroelectric liquid crystal display or ferroelectric liquid crystal display according to the present invention utilizing the successive additive color mixing phenomenon, since all the pixels are simultaneously reset to a predetermined state, a good display can be presented by improving contrast and other display characteristics and minimizing the number of times that the resetting is performed.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a diagram showing the arrangement of an antiferroelectric liquid crystal cell and polarizers according to the present invention.
Figure 2 is a diagram showing a hysteresis curve for an antiferroelectric liquid crystal display element according to the present invention.
Figure 3 is a diagram showing a matrix of electrodes.
Figure 4 is a diagram showing driving waveforms for a conventional art antiferroelectric liquid crystal display and their corresponding light transmittance.
Figure 5 is a diagram showing the arrangement of a ferroelectric liquid crystal cell and polarizers according to the present invention.
Figure 6 is a diagram showing a hysteresis curve for a ferroelectric liquid crystal display element according to the present invention.
Figure 7 is a diagram showing driving waveforms for a conventional art ferroelectric liquid crystal display and their corresponding light transmittance.
Figure 8 is a diagram showing the structure of a liquid crystal panel according to the present invention.
Figure 9 is a block diagram showing a driving circuit configuration for the liquid crystal display of the present invention.
Figure 10 is a diagram showing driving waveforms for driving the antiferroelectric liquid crystal according to one embodiment of the present invention.
Figure 11 is a diagram showing driving waveforms for driving the antiferroelectric liquid crystal according to another embodiment of the present invention.
Figure 12 is a diagram showing driving waveforms for driving the antiferroelectric liquid crystal according to a further embodiment of the present invention.
Figure 13 is a diagram showing driving waveforms for driving the ferroelectric liquid crystal according to one embodiment of the present invention.
Figure 14 is a diagram showing driving waveforms for driving the ferroelectric liquid crystal according to another embodiment of the present invention.
Figure 15 is a diagram showing driving waveforms for driving the ferroelectric liquid crystal according to a further embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Driving methods for antiferroelectric and ferroelectric liquid crystals will be described in detail below.
Figure 1 is a diagram showing the arrangement of a liquid crystal cell and polarizers when the antiferroelectric liquid crystal is used as a liquid crystal display element. Between the polarizers 1a and 1b, arranged in a crossed Nicol configuration, is placed the liquid crystal cell 2 in such a manner that the average long axis direction X of molecules in the absence of an applied voltage is oriented substantially parallel to either the polarization axis, a, of the polarizer 1a or the polarization axis, b, of the polarizer 1b. Then, the liquid crystal cell is set up so that black can be displayed when no voltage is applied and white when a voltage is applied.
When a voltage is applied across the thus arranged liquid crystal cell, its light transmittance varies with the applied voltage, describing a loop as plotted in the graph of Figure 2. The voltage value at which the light transmittance begins to change when the applied voltage is increased is denoted by V1, and the voltage value at which the light transmittance reaches saturation is denoted by V2, while the voltage value at which the light transmittance begins to drop when the applied voltage is decreased is denoted by V5; further, when a voltage of opposite polarity is applied, the voltage value at which the light transmittance begins to change when the absolute value of the applied voltage is increased is denoted by V3, and the voltage value at which the light transmittance reaches saturation is denoted by V4, while the voltage value at which the light transmittance begins to change when the absolute value of the applied voltage is decreased is denoted by V6. As shown in Figure 2, a first ferroelectric state is selected when the value of the applied voltage is greater than the threshold of the antiferroelectric liquid crystal molecules. When the voltage of opposite polarity greater than the threshold of the antiferroelectric liquid crystal molecules is applied, a second ferroelectric state is selected. In either of these ferroelectric states, when the voltage value drops below a certain threshold, an antiferroelectric state is selected.
The antiferroelectric liquid crystal display can be constructed to produce a black display in the antiferroelectric state or a white display in the antiferroelectric state. The present invention is applicable to both modes of operation. The description hereinafter given assumes that the display is set up to produce a black display in the antiferroelectric state.
Next, a conventional liquid crystal driving method for driving the antiferroelectric liquid crystal will be described. Figure 3 is a diagram showing an example of an electrode arrangement in a liquid crystal panel having scanning electrodes and signal electrodes arranged in a matrix form on substrates. This electrode arrangement comprises the scanning electrodes (X1, X2, X3, ..., Xn, ... X100) and signal electrodes (Y1, Y2, Y3, ..., Ym, ..., Y220), and shaded portions where the scanning electrodes and signal electrodes intersect are pixels (A11, Anm). For these pixels, a voltage is applied to the scanning electrodes, in sequence and one scanning line at a time, in synchronism with the applied voltage, voltage waveforms corresponding to the display states of the associated pixels are applied from the signal electrodes. The display state of each pixel is written in accordance with a composite waveform produced by combining the voltage waveforms applied to the associated signal electrode and the selected scanning electrode.
As shown in Figure 4, writing to the pixel is accomplished by applying a scanning voltage (a) to the scanning electrode (Xn) and a signal voltage (b) to the signal electrode (Ym) and thereby applying the resulting composite voltage (c) to the pixel (Anm). In Figure 4, the first or second ferroelectric state or the antiferroelectric state is selected in a selection period (Se), and the selected state is held throughout the following non-selection period (NSe). That is, a selection pulse is applied in the selection period (Se), and the transmittance (d) obtained as the result of the selection is maintained throughout the following non-selection period (NSe) to produce the display. In an antiferroelectric liquid crystal display device, it is generally practiced to reset the antiferroelectric liquid crystal to the first or second ferroelectric state or the antiferroelectric state immediately before writing to the pixel. In Figure 4, each selection period (Se) is immediately preceded by a reset period (Re). During this reset period, a voltage lower than the threshold voltage is applied to the pixel to reset the antiferroelectric liquid crystal to the antiferroelectric state. By resetting the state of each pixel to a predetermined state immediately before writing necessary information to the pixel, as just described, a good display can be produced with each pixel being unaffected by its previously written state. In the first half of Figure 4, a white display is produced, and in the second half, a black display is produced. Further, the polarity of the drive voltage waveform is reversed alternately.
Figure 5 is a diagram showing the arrangement of polarizers when the ferroelectric liquid crystal is used as the liquid crystal display element. Between the polarizers 1a and 1b arranged in a crossed Nicol configuration is placed the liquid crystal cell 2 in such a manner that the long axis direction of liquid crystal molecules when the cell is in a first stable state or in a second stable state is oriented substantially parallel to either the polarization axis, a, of the polarizer 1a or the polarization axis, b, of the polarizer 1b.
When a voltage is applied across the thus arranged liquid crystal cell, its light transmittance varies with the applied voltage, describing a loop as plotted in the graph of Figure 6. The voltage value at which the light transmittance begins to change when the applied voltage is decreased is denoted by V1, and the voltage value at which the light transmittance reaches saturation is denoted by V2; on the other hand, the voltage value at which the light transmittance begins to drop when the applied voltage is increased into the region of the opposite polarity is denoted by V3, and the voltage value at and beyond which the light transmittance does not drop further is denoted by V4. As shown in Figure 6, the first stable state is selected when the value of the applied voltage is greater than the threshold of the ferroelectric liquid crystal molecules. When the voltage of the opposite polarity greater than the threshold of the ferroelectric liquid crystal molecules is applied, the second stable state is selected.
When the polarizers are arranged as shown in Figure 5, a black display (non-transmission state) can be produced in the first stable state and a white display (transmission state) in the second stable state. The arrangement of the polarizers can be changed so that a white display (transmission state) is produced in the first stable state and a black display (non-transmission state) in the second stable state. The description hereinafter given, however, assumes that the polarizers are arranged so as to produce a black display (non-transmission state) in the first stable state and a white display (transmission state) in the second stable state.
Next, a conventional liquid crystal driving method for driving the ferroelectric liquid crystal will be described. Figure 3, as previously explained, is a diagram showing an example of the electrode arrangement in a liquid crystal panel having scanning electrodes and signal electrodes arranged in a matrix form on substrates. For the pixels shown in Figure 3, a voltage is applied to the scanning electrodes, in sequence, one scanning line at a time, in synchronism with which voltage waveforms corresponding to the display states of the associated pixels are applied from the signal electrodes. The display state of each pixel is written in accordance with the composite waveform produced by compositing the voltage waveforms applied to the associated signal electrode and the selected scanning electrode.
Figure 7 shows examples of conventional drive voltage waveforms for the ferroelectric liquid crystal display. As shown in Figure 7, writing to the pixel is accomplished by applying a scanning voltage (a) to the scanning electrode (Xn) and a signal voltage (b) to the signal electrode (Ym) and thereby applying the resulting composite voltage (c) to the pixel (Anm). Waveform (d) represents the light transmittance.
When attention is paid to the composite voltage (c), it will be noted that, during a selection period (Se1), a pulse P1 having a pulse width T and a peak value +Vp greater than a threshold and a pulse P2 having a pulse width T and a peak value -Vp greater than a threshold are applied to the liquid crystal pixel. If it is assumed that the first pulse P1 is in the direction that switches the liquid crystal molecules from the second stable state (white display state) to the first stable state (black display state), then the second pulse P2, which is opposite in polarity, accomplishes switching in the reverse direction, i.e., from the first stable state to the second stable state. Accordingly, the state achieved by the pulse P1 is not retained, but the second stable state achieved by the pulse P2 is retained. In the second stable state achieved by the pulse P2, the light transmittance rapidly rises to create a white display. In the non-selection period (Nse1) that follows, since the pulses applied to the pixel are below the threshold, the previously obtained second stable state is retained and the light transmittance is maintained at the previously achieved level.
The drive waveform shown in Figure 7 is provided with a reset period for resetting the liquid crystal to the first stable state. The non-selection period (Nse1) is followed by the reset period (Re1) during which a pulse P3 having the peak value -Vp greater than the threshold and a pulse P4 having the peak value +vp greater than the threshold are applied to the liquid crystal pixel. Since the first pulse P3 is in the direction that switches the liquid crystal to the second stable state (white display state), the light transmittance remains unchanged. On the other hand, the second pulse P4 is opposite in polarity and works to switch the liquid crystal from the second stable state (white display state) to the first stable state (black display state), so that the light transmittance drops nearly to 0.
In the selection period (Se2) that follows, since the peak values of the pulses P5 and P6 applied to the pixel are below the threshold, the first stable state (black display state) is retained. In the next non-selection period (NSe2) also, since the pulses applied to the pixel are below the threshold, the previously obtained first stable state (black display state) is retained.
The non-selection period (NSe2) is followed by a reset period (Re2) during which a pulse P7 having the peak value -Vp greater than the threshold and a pulse P8 having the peak value +Vp greater than the threshold are applied to the liquid crystal pixel. Since the first pulse P7 is in the direction that switches the liquid crystal from the first stable state (black display state) to the second stable state (white display state), the light transmittance rises. However, since the second pulse P8 is opposite in polarity and works to switch the liquid crystal from the second stable state (white display state) to the first stable state (black display state), the light transmittance drops nearly to 0. During this process, the light transmittance rises momentarily but drops instantly, so that the change is not perceivable by the human eye.
The present invention will now be described in detail. The present invention allows the use of a conventionally-used liquid crystal panel in which the antiferroelectric or ferroelectric liquid crystal is injected into the liquid crystal display element. Further, a light source capable of successively emitting light of a plurality of colors is provided as a backlight behind the liquid crystal display element. For example, LEDs emitting red (R), green (G), and blue (B) light, respectively, are arranged for use as the light source. A driving method using such an RGB light source will be described below. To display a pixel in a desired state, the R, G, and B lights are sequentially turned on, each for a desired length of time; the time during which the R, G, and B lights are turned on in sequence (one cycle) constitutes the period required to write necessary information to the pixel. Further, a reset period during which the antiferroelectric or ferroelectric liquid crystal is reset, a selection period during which a voltage for determining the display state of the pixel is applied, a non-selection period during which the display state is maintained are provided within the ON period of each color light source. That is, when a light source of three RGB colors is used, the reset period, selection period, and non-selection period are provided in the ON period of each of the R, G, and B lights.
In the present invention, an arbitrary period is provided within the period that the light source switches from one color to the next; during this arbitrary period, the light emission is stopped and the antiferroelectric liquid crystal in all pixels is simultaneously reset to either the first or second ferroelectric state or the antiferroelectric state. Likewise, the ferroelectric liquid crystal is reset to either the first or second stable state. More specifically, the reset period is provided in such a manner as to be superimposed on the arbitrary period during which the light emission is stopped. By resetting all the pixels simultaneously in this way, the number of times that the resetting is performed can be minimized. Furthermore, by stopping the backlighting from the light source, the unwanted display produced during the reset period becomes unnoticeable, and a good quality display can thus be presented.
The length of time required for the resetting depends on the antiferroelectric or ferroelectric liquid crystal material used. That is, depending on the antiferroelectric or ferroelectric liquid crystal material employed, sufficient resetting may not be achieved unless the reset period is made long enough. When such a liquid crystal material is used, if the light emission is stopped during the reset period, the emission stop time becomes long, causing display flicker.
To address this, in an alternative embodiment of the present invention, the reset period for simultaneously resetting the antiferroelectric or ferroelectric liquid crystal in all pixels is included in the first part of the period that the light source starts emitting light of one color. By resetting all the pixels simultaneously in this way, the number of times that the resetting is performed can be minimized. Furthermore, by including the reset period in the early part of the period that the light source starts emitting light of one color, the selection period and non-selection period can be set in a contiguous manner within the period during which the light source emits light of one color. As a result, the emission time of the light source for writing the necessary display can be made long, which serves to enhance the display quality.
The same effect can be obtained if the reset period for simultaneously resetting the antiferroelectric or ferroelectric liquid crystal in all the pixels is included in the last part of the period at the end of which the light source stops emitting light of one color. If the reset period is included in the last part of the period when the light source stops emitting, since all the pixels are reset simultaneously, the number of times that the resetting is performed can be minimized. Furthermore, the selection period and non-selection period can be set in a contiguous manner within the period during which the light source emits light of one color. As a result, the emission time of the light source for writing the necessary display can be made long, which serves to enhance the display quality.

EMBODIMENTS

Embodiments of the present invention will be described in detail below with reference to drawings. Figure 8 is a diagram showing the structure of a liquid crystal panel used in the embodiments of the present invention. The liquid crystal panel used in the embodiments comprises: a pair of glass substrates 11a and 11b holding therebetween an antiferroelectric or ferroelectric liquid crystal layer 10 having a thickness of about 2 µm; and sealing members 12a and 12b for bonding the two glass substrates together. On the opposing surfaces of the glass substrates 11a and 11b are formed electrodes (ITO) 13a and 13b coated with alignment films 14a and 14b, respectively, and treated for alignment. On the outside surface of one glass substrate is disposed a first polarizer 15a with its polarization axis oriented parallel to the axis of alignment, while on the outside surface of the other glass substrate, a second polarizer 15b is arranged with its polarization axis oriented at 90° to the polarization axis of the first polarizer 15a. An LED, as a backlight 16, that emits three colored lights (R, G, and B) is mounted behind the thus structured liquid crystal device. The backlight is operated to emit light of R, G, and B in this order, each color for a duration of about 5.6 ms.
The arrangement of the liquid crystal panel is the same as that shown in Figure 3, and the scanning electrodes and signal electrodes are arranged as shown in Figure 3. In the electrode arrangement shown in Figure 3, there are 100 scanning electrodes and 220 signal electrodes, but their numbers can be changed arbitrarily.
Figure 9 is a block diagram showing a driving circuit configuration for the antiferroelectric or ferroelectric liquid crystal display. In the liquid crystal display 21 shown in the figure, the scanning electrodes to which scanning signals are applied are connected to a scanning electrode driving circuit 22, and the signal electrodes to which display signals are applied are connected to a signal electrode driving circuit 23. A power supply circuit 24 supplies the scanning electrode driving circuit 22 with a voltage Vx necessary for driving the scanning electrodes of the liquid crystal display, and the signal electrode driving circuit 23 with a voltage Vy necessary for driving the signal electrodes of the liquid crystal display. A control circuit 25, based on a signal from a display data generating source 26, supplies signals to the scanning electrode driving circuit 22 and signal electrode driving circuit 23 which then supply signals, respectively consisting of the voltages Vx and Vy, to the liquid crystal display 21 in accordance with the respectively supplied signals.

EMBODIMENT 1

Figure 10 shows drive voltage waveforms for driving the antiferroelectric liquid crystal according to the present invention. (BL) indicates the color of the light being emitted from the backlight (light source) as well as the period of the emission. (C1) indicates the composite voltage waveform applied to the pixels on the scanning electrode in the first row, (C2) the composite voltage waveform applied to the pixels on the scanning electrode in the second row, and (C3) the composite voltage waveform applied to the pixels on the scanning electrode in the third row. Further, (d1), (d2), and (d3) show the light transmittance of the pixels on the scanning electrodes in the first to third rows, respectively. In the waveforms (C1), (C2), and (C3), a reset period (Rs) is provided within a non-emission period during which the light source switches from one color to the next. During this reset period, the voltage applied to each pixel is set substantially equal to 0 to reset all antiferroelectric liquid crystal pixels simultaneously to the antiferroelectric state. Here, the voltage applied during the reset period (Rs) need not necessarily be set to 0 V, but need only be set to a value lower than the threshold. The reset period is followed by a selection period (Se) during which a voltage for determining the display state of each pixel is applied and a non-selection period (Nse) during which a voltage for maintaining the display state is applied. By resetting all the pixels simultaneously in this way, the number of times that the resetting is performed can be minimized. Furthermore, by stopping the backlighting from the light source, the unwanted display produced during the reset period becomes unnoticeable, and flickering can thus be suppressed.
In the voltage waveforms (C1), (C2), and (C3) shown in Figure 10, the selection voltage waveform applied during the selection period (Se) is the same in polarity; alternatively, a plurality of selection periods and a plurality of non-selection periods may be provided within the period during which one color light is emitted, and the polarity may be reversed alternately. Instead, the polarity may be reversed each time the light source switches from one color to the next, or the next time the same color of light is emitted. The same applies to the voltage waveforms hereinafter described.

EMBODIMENT 2

Figure 11 shows drive voltage waveforms for driving the antiferroelectric liquid crystal according to another embodiment. (BL) indicates the color of the light being emitted from the backlight (light source) as well as the period of the emission. (C1) indicates the composite voltage waveform applied to the pixels on the scanning electrode in the first row, (C2) the composite voltage waveform applied to the pixels on the scanning electrode in the second row, and (C3) the composite voltage waveform applied to the pixels on the scanning electrode in the third row. Further, (d1), (d2), and (d3) show the light transmittance of the pixels on the scanning electrodes in the first to third rows, respectively. In the waveforms (C1), (C2), and (C3), the reset period (Rs) is provided in the early part of the emission period of one color (in Figure 11, it is shown that Rs is provided in the early part of the emission period of each of R and G). During this reset period, the voltage applied to each pixel is set substantially equal to 0 to reset all antiferroelectric liquid crystal pixels simultaneously to the antiferroelectric state. The reset period is followed by a selection period (Se) and a non-selection period (Nse) in a contiguous manner. Since all the pixels are reset simultaneously in this way, the number of times that the resetting is performed can be minimized, and flickering can thus be suppressed. Furthermore, since the reset period is provided in the early part of the emission period of one color, the selection period and the non-selection period can be set contiguously within the period during which the backlight is emitting light. As a result, the emission time of the backlight for writing the necessary display can be made long, which serves to provide a bright screen and good display characteristics.

EMBODIMENT 3

Figure 12 shows drive voltage waveforms for driving the antiferroelectric liquid crystal according to a further embodiment. (BL) indicates the color of the light being emitted from the backlight (light source) as well as the period of the emission. (C1) indicates the composite voltage waveform applied to the pixels on the scanning electrode in the first row, (C2) the composite voltage waveform applied to the pixels on the scanning electrode in the second row, and (C3) the composite voltage waveform applied to the pixels on the scanning electrode in the third row. Further, (d1), (d2), and (d3) show the light transmittance of the pixels on the scanning electrodes in the first to third rows, respectively. In the waveforms (C1), (C2), and (C3), the reset period (Rs) is provided in the last part of the emission period of the light source of one color (in Figure 12, it is shown that Rs is provided in the last part of the emission period of each of B and R). During this reset period, the voltage applied to each pixel is set substantially equal to 0 to reset all antiferroelectric liquid crystal pixels simultaneously to the antiferroelectric state. The reset period is preceded by a selection period (Se) and a non-selection period (Nse) in a contiguous manner. Since all the pixels are reset simultaneously in this way, the number of times that the resetting is performed can be minimized, and flickering can thus be suppressed. Furthermore, since the reset period is provided in the last part of the emission period of one color, the selection period and non-selection period can be set contiguously within the period during which the backlight is emitting light. As a result, the emission time of the backlight for writing the necessary display can be made long, which serves to provide a bright screen and good display characteristics.

EMBODIMENT 4

Figure 13 shows drive voltage waveforms for driving the ferroelectric liquid crystal according to the present invention. (BL) indicates the color of the light being emitted from the backlight (light source) as well as the period of the emission. (C1) indicates the composite voltage waveform applied to the pixels on the scanning electrode in the first row, (C2) the composite voltage waveform applied to the pixels on the scanning electrode in the second row, and (C3) the composite voltage waveform applied to the pixels on the scanning electrode in the third row. Further, (d1), (d2), and (d3) show the light transmittance of the pixels on the scanning electrodes in the first to third rows, respectively. In the waveforms (C1), (C2), and (C3), the reset period (Rs) is provided within a non-emission period during which the light source switches from one color to the next. During this reset period, a pulse having peak values of ±20 V greater than the threshold is applied to each pixel to reset the ferroelectric liquid crystal in each pixel to the first stable state (black display state). Here, the ferroelectric liquid crystal is reset to the first stable state in the same way as described with reference to Figure 7. The reset period is followed by a selection period (Se) during which a voltage for determining the display state of each pixel is applied and a non-selection period (Nse) during which a voltage for maintaining the display state is applied. By resetting all the pixels simultaneously in this way, the number of times that the resetting is performed can be minimized. Furthermore, by stopping the backlighting from the light source, the unwanted display produced during the reset period becomes unnoticeable, and flickering can thus be suppressed.

EMBODIMENT 5

Figure 14 shows drive voltage waveforms for driving the ferroelectric liquid crystal according to another embodiment. (BL) indicates the color of the light being emitted from the backlight (light source) as well as the period of the emission. (C1) indicates the composite voltage waveform applied to the pixels on the scanning electrode in the first row, (C2) the composite voltage waveform applied to the pixels on the scanning electrode in the second row, and (C3) the composite voltage waveform applied to the pixels on the scanning electrode in the third row. Further, (d1), (d2), and (d3) show the light transmittance of the pixels on the scanning electrodes in the first to third rows, respectively. In the waveforms (C1), (C2), and (C3), the reset period (Rs) is provided in the early part of the emission period of light source of one color. During this reset period, a pulse having peak values of ±20 V greater than the threshold is applied to each pixel to reset the ferroelectric liquid crystal in each pixel to the first stable state (black display state). The reset period is followed by a selection period (Se) and a non-selection period (Nse) in a contiguous manner. Since all the pixels are reset simultaneously in this way, the number of times that the resetting is performed can be minimized, and flickering can thus be suppressed. Furthermore, since the reset period is provided in the early part of the emission period of one color, the selection period and non-selection period can be set contiguously within the period during which the backlight is emitting light. As a result, the emission time of the backlight for writing the necessary display can be made long, which serves to provide a bright screen and good display characteristics.

EMBODIMENT 6

Figure 15 shows drive voltage waveforms for driving the ferroelectric liquid crystal according to a further embodiment. (BL) indicates the color of the light being emitted from the backlight (light source) as well as the period of the emission. (C1) indicates the composite voltage waveform applied to the pixels on the scanning electrode in the first row, (C2) the composite voltage waveform applied to the pixels on the scanning electrode in the second row, and (C3) the composite voltage waveform applied to the pixels on the scanning electrode in the third row. Further, (d1), (d2), and (d3) show the light transmittance of the pixels on the scanning electrodes in the first to third rows, respectively. In the waveforms (C1), (C2), and (C3), the reset period (Rs) is provided in the last part of the emission period of the light source of one color. During this reset period, a pulse having peak values of ±20 V greater than the threshold is applied to each pixel to reset the ferroelectric liquid crystal in each pixel to the first stable state (black display state). The reset period is preceded by a selection period (Se) and a non-selection period (Nse) in a contiguous manner. Since all the pixels are reset simultaneously in this way, the number of times that the resetting is performed can be minimized, and flickering can thus be suppressed. Furthermore, since the reset period is provided in the last part of the emission period of one color, the selection period and non-selection period can be set contiguously within the period during which the backlight is emitting light. As a result, the emission time of the backlight for writing the necessary display can be made long, which serves to provide a bright screen and good display characteristics.
In any of the first, second, and third embodiments, the antiferroelectric liquid crystal was reset to the antiferroelectric state during the reset period. However, good display characteristics were also obtained when it was reset to the first or second ferroelectric state. Similarly, in the fourth, fifth, and sixth embodiments, the ferroelectric liquid crystal was reset to the first stable state. However, good display characteristics were also obtained when it was reset to the second stable state.

Claims

An antiferroelectric liquid crystal display comprising: an antiferroelectric liquid crystal display element which includes an antiferroelectric liquid crystal sandwiched between a pair of substrates having a matrix of pixels; and a light source which successively emits light of different colors, wherein

said antiferroelectric liquid crystal has an antiferroelectric state, a first ferroelectric state, and a second ferroelectric state that is entered when a voltage opposite in polarity to that for said first ferroelectric state is applied,

said light source has an arbitrary period during which said light source stops emission when switching from one color to another,

said arbitrary period includes a reset period during which said antiferroelectric liquid crystal in all the pixels is simultaneously reset to one of said three states, i.e., said antiferroelectric state, said first ferroelectric state, or said second ferroelectric state, and

a selection period during which a voltage for determining the display state of a pixel is applied and a non-selection period during which said display state is maintained are included within a period during which said light source emits light of any one of said colors.
An antiferroelectric liquid crystal display comprising: an antiferroelectric liquid crystal display element which includes an antiferroelectric liquid crystal sandwiched between a pair of substrates having a matrix of pixels; and a light source which successively emits light of different colors, wherein

said antiferroelectric liquid crystal has an antiferroelectric state, a first ferroelectric state, and a second ferroelectric state that is entered when a voltage opposite in polarity to that for said first ferroelectric state is applied,

a reset period, during which said antiferroelectric liquid crystal in all the pixels is simultaneously reset to one of said three states, i.e., said antiferroelectric state, said first ferroelectric state, or said second ferroelectric state, is included in the early part or the last part of a period during which said light source emits light of any one of said colors, and

a selection period during which a Voltage for determining the display state of a pixel is applied and a non-selection period during which said display state is maintained are included within the period during which said light source emits light of any one of said colors.
A ferroelectric liquid crystal display comprising: a ferroelectric liquid crystal display element which includes a ferroelectric liquid crystal sandwiched between a pair of substrates having a matrix of pixels; and a light source which successively emits light of different colors, wherein

said ferroelectric liquid crystal has a first stable state and a second stable state that is entered when a voltage opposite in polarity to that for said first stable state is applied,

said light source has an arbitrary period during which said light source stops emission when switching from one color to another,

said arbitrary period includes a reset period during which said ferroelectric liquid crystal in all the pixels is simultaneously reset to either one of said two states, i.e., said first stable state or said second stable state, and

a selection period during which a voltage for determining the display state of a pixel is applied and a non-selection period during which said display state is maintained are included within a period during which said light source emits light of any one of said colors.
A ferroelectric liquid crystal display comprising: a ferroelectric liquid crystal display element which includes a ferroelectric liquid crystal sandwiched between a pair of substrates having a matrix of pixels; and a light source which successively emits light of different colors, wherein

said ferroelectric liquid crystal has a first stable state and a second stable state that is entered when a voltage opposite in polarity to that for said first stable state is applied,

a reset period during which said ferroelectric liquid crystal in all the pixels is simultaneously reset to either one of said two states, i.e., said first stable state or said second stable state, is included in the early part or the last part of a period during which said light source emits light of any one of said colors, and

a selection period during which a voltage for determining the display state of a pixel is applied and a non-selection period during which said display state is maintained are included within the period during which said light source emits light of any one of said colors.
A method of driving an antiferroelectric liquid crystal display that comprises: an antiferroelectric liquid crystal display element which includes an antiferroelectric liquid crystal sandwiched between a pair of substrates having a matrix of pixels; and a light source which successively emits light of different colors, wherein

said antiferroelectric liquid crystal has an antiferroelectric state, a first ferroelectric state, and a second ferroelectric state that is entered when a voltage opposite in polarity to that for said first ferroelectric state is applied,

said light source, when switching from one color to another, stops emission for an arbitrary period,

in said arbitrary period, said antiferroelectric liquid crystal in all the pixels is simultaneously reset to one of said three states, i.e., said antiferroelectric state, said first ferroelectric state, or said second ferroelectric state, and

in a period during which said light source emits light of any one of said colors, a voltage for determining the display state of a pixel is applied during a selection period, and said display state is maintained during a subsequent non-selection period.
A method of driving an antiferroelectric liquid crystal display that comprises: an antiferroelectric liquid crystal display element which includes an antiferroelectric liquid crystal sandwiched between a pair of substrates having a matrix of pixels; and a light source which successively emits light of different colors, wherein

said antiferroelectric liquid crystal has an antiferroelectric state, a first ferroelectric state, and a second ferroelectric state that is entered when a voltage opposite in polarity to that for said first ferroelectric state is applied,

in the early part or the last part of a period during which said light source emits light of any one of said colors, said antiferroelectric liquid crystal in all the pixels is simultaneously reset to one of said three states, i.e., said antiferroelectric state, said first ferroelectric state, or said second ferroelectric state, and

in the period during which said light source emits light of any one of said colors, a voltage for determining the display state of a pixel is applied during a selection period, and said display state is maintained during a subsequent non-selection period.
A method of driving a ferroelectric liquid crystal display that comprises: a ferroelectric liquid crystal display element which includes a ferroelectric liquid crystal sandwiched between a pair of substrates having a matrix of pixels; and a light source which successively emits light of different colors, wherein

said ferroelectric liquid crystal has a first stable state and a second stable state that is entered when a voltage opposite in polarity to that for said first stable state is applied,

said light source, when switching from one color to another, stops emission for an arbitrary period,

in said arbitrary period, said ferroelectric liquid crystal in all the pixels is simultaneously reset to one of said two states, i.e., said first stable state or said second stable state, and

in a period during which said light source emits light of any one of said colors, a voltage for determining the display state of a pixel is applied during a selection period, and said display state is maintained during a subsequent non-selection period.
A method of driving a ferroelectric liquid crystal display that comprises: a ferroelectric liquid crystal display element which includes a ferroelectric liquid crystal sandwiched between a pair of substrates having a matrix of pixels; and a light source which successively emits light of different colors, wherein

said ferroelectric liquid crystal has a first stable state and a second stable state that is entered when a voltage opposite in polarity to that for said first stable state is applied,

in the early part or the last part of a period during which said light source emits light of any one of said colors, said ferroelectric liquid crystal in all the pixels is simultaneously reset to one of said two states, i.e., said first stable state or said second stable state, and

in the period during which said light source emits light of any one of said colors, a voltage for determining the display state of a pixel is applied during a selection period, and said display state is maintained during a subsequent non-selection period.