Driving method for optical modulation device

Abstract

A driving method for an optical modulation device comprising matrix picture elements each formed at intersecting points of scanning lines and data lines between which a bistable optical modulation material represented by a ferroelectric liquid crystal is interposed. The driving method comprises an erasure step of applying a voltage signal orienting the optical modulation material to the first stable state between the scanning and data lines, at all or a part of the matrix picture elements, and a writing step of sequentially applying a scanning selection signal to the scanning lines and applying an information orientation signal orienting the optical modulation material to the second stable state to the data lines in phase with the scanning selection signal.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a method of driving an optical modulation device, e.g., a liquid crystal device, and more particularly to a time-sharing driving method for an optical modulation device, e.g., a display device, an optical shutter array, etc.

Hitherto, liquid crystal display devices are well known, which comprise scanning lines (or electrodes) and data lines (or electrodes) arranged in a matrix manner, and a liquid crystal compound is filled between the lines to form a plurality of picture elements thereby to display images or information. These display devices employ a time-sharing driving method which comprises the steps of selectively applying scanning selection signals sequentially and cyclically to the scanning lines, and, in parallel therewith selectively applying predetermined information signals to the group of signal electrodes in synchronism with the scanning selection signals. However, these display devices and the driving method therefor have a serious drawback as will be described below.

Namely, the drawback is that it is difficult to obtain a high density of picture elements or a large image area. Because of relatively high response speed and low power dissipation, among prior art liquid crystals, most of liquid crystals which have been put into practice as display devices are TN (twisted nematic) type liquid crystals, as shown in "Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal" by M. Schadt and W. Helfrich, Applied Physics Letters Vol. 18, No. 4 (Feb. 15, 1971) pp. 127-128. In the liquid crystals of this type, molecules of nematic liquid crystal which show positive dielectric anisotropy under no application of an electric field form a structure twisted in the thickness direction of liquid crystal layers (helical structure), and molecules of these liquid crystals are aligned or oriented parallel to each other in the surfaces of both electrodes. On the other hand, nematic liquid crystals which show positive dielectric anisotropy under application of an electric field are oriented or aligned in the direction of the electric field. Thus, they can cause optical modulation. When display devices of a matrix electrode arrangement are designed using liquid crystals of this type, a voltage higher than a threshold level required for aligning liquid crystal molecules in the direction perpendicular to electrode surfaces is applied to areas (selected points) where scanning lines and data lines are selected at a time, whereas a voltage is not applied to areas (non-selected points) where scanning lines and data lines are not selected and, accordingly, the liquid crystal molecules are stably aligned parallel to the electrode surfaces. When linear polarizers arranged in a cross-nicol relationship, i.e., with their polarizing axes being substantially perpendicular to each other, are arranged on the upper and lower sides of a liquid crystal cell thus formed, a light does not transmit at selected points while it transmits at non-selected points. Thus, the liquid crystal cell can function as an image device.

However, when a matrix electrode structure is constituted, a certain electric field is applied to regions where scanning lines are selected and data lines are not selected or regions where scanning lines are not selected and data lines are selected (which regions are so called "half-selected points"). If the difference between a voltage applied to the selected points and a voltage applied to the half-selected points is sufficiently large, and a voltage threshold level required for allowing liquid crystal molecules to be aligned or oriented perpendicular to an electric field is set to a value therebetween, the display device normally operates. However, in fact, according as the number (N) of scanning lines increases, a time (duty ratio) during which an effective electric field is applied to one selected point when a whole image area (corresponding to one frame) is scanned decreases with a ratio of 1/N. For this reason, the larger the number of scanning lines are, the smaller is the voltage difference as an effective value applied to a selected point and non-selected points when scanning is repeatedly effected. As a result, this leads to unavoidable drawbacks of lowering of image contrast or occurrence of crosstalk. These phenomena result in problems that cannot be essentially avoided, which apepar when a liquid crystal not having bistability (which shows a stable state where liquid crystal molecules are oriented or aligned in a horizontal direction with respect to electrode surfaces, but are oriented in a vertical direction only when an electric field is effectively applied) is driven, i.e., repeatedly scanned, by making use of time storage effect. To overcome these drawbacks, the voltage averaging method, the two-frequency driving method, the multiple matrix method, etc., has already been proposed. However, any method is not sufficient to overcome the above-mentioned drawbacks. As a result, it is the present state that the development of large image area or high packaging density in respect to display elements is delayed because of the fact that it is difficult to sufficiently increase the number of scanning lines.

Meanwhile, turning to the field of a printer, as means for obtaining a hard copy in response to input electric signals, a Laser Beam Printer (LBP) providing electric image signals to electrophotographic charging member in the form of lights is the most excellent in view of density of a picture element and a printing speed.

However, the LBP has drawbacks as follows:

1) It becomes large in apparatus size. 2) It has high speed mechanically movable parts such as a polygon scanner, resulting in noise and requirement for strict mechanical precision, etc.

In order to eliminate drawbacks stated above, a liquid crystal shutter-array is proposed as a device for changing electric signals to optical signals. When picture element signals are provided with a liquid crystal shutter-array, however, 2000 signal generators are required, for instance, for writing picture element signals into a length of 200 mm in a ratio of 10 dots/mm. Accordingly, in order to independently feed signals to respective signal generators, lead lines for feeding electric signals are required to be provided to all the respective signal generators, and the production has become difficult.

In view of the above, another attempt is made to apply one line of image signals in a time-sharing manner with signal generators divided into a plurality of lines.

With this attempt, signal feeding electrodes can be common to the plurality of signal generators, thereby enabling to remarkably decrease the number of lead wires. However, if the number (N) of lines is increased while using a liquid crystal showing no bistability as usually practiced, a signal "ON" time is substantially reduced to 1/N. This results in difficulties that light quantity obtained on a photoconductive member is decreased, and a crosstalk occurs.

SUMMARY OF THE INVENTION

An object of the invention is to provide a novel method of driving an optical modulation device, particularly a liquid crystal device, which can solve the above-mentioned drawbacks encountered with prior art liquid crystal display devices or liquid crystal optical shutters as stated above.

Another object of the invention is to provide a liquid crystal device driving method which can realize a high response speed.

Another object of the invention is to provide a liquid crystal device driving method which can realize high packaging density of picture elements.

Another object of the invention is to provide a liquid crystal driving method which does not produce crosstalk.

To achieve these objects, there is provided a driving method for an optical modulation device having a plurality of picture elements arranged in the form of a matrix and comprising scanning lines, data lines spaced apart from and intersecting with the scanning lines, and a bistable optical modulation material assuming a first stable state or a second stable state depending on an electric field applied thereto interposed between the scanning lines and the data lines, each of the intersections between the scanning lines and the data lines forming one of the plurality of picture elements; the driving method comprising,

an erasure step wherein a voltage signal uniformly orienting the bistable optical modulation material to the first stable state is applied between the scanning lines and data lines constituting all or a part of the plurality of picture elements, and a writing step wherein a scanning selection signal is sequentially applied to the scanning lines, and an information selection signal orienting the bistable optical modulation material to the second stable state in combination with the scanning selection signal is applied to the data lines in phase with the scanning selection signal.

These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic perspective views illustrating the basic operation principle of a liquid crystal device used in the present invention,

FIG. 3A is a plan view of an electrode arrangement used in the present invention,

FIGS. 3B-1-3B-4 illustrate waveforms of electric signals applied to electrodes,

FIGS. 3C-1-3C-4 illustrate voltage waveforms applied to picture elements,

FIGS. 4A and 4E, in combination, illustrate voltage waveforms applied in time series,

FIGS. 5A-1-5A-4 illustrate waveforms of electric signals applied to electrodes in a different example,

FIGS. 5B-1-5B-4 illustrate voltage waveforms applied to picture elements in the different example,

FIGS. 6A to 10A in combination with FIGS. 6B to 10B, respectively, illustrate different examples of voltage waveforms applied in time series,

FIGS. 11A and 11D are plan views respectively showing an electrode arrangement used in a different embodiment of the driving method according to the present invention,

FIGS. 11B-1-11B-4 illustrate waveforms of electric signals applied to electrodes,

FIGS. 11C-1-11C-4 illustrate voltage waveforms applied to picture elements,

FIGS. 12A to 15A in combination with FIGS. 12B to 15B, respectively, illustrate still different examples of voltage waveforms applied in time series,

FIG. 16A is a plan view of an electrode arrangement in a different embodiment of the driving method according to the present invention,

FIGS. 16B-1-16B-4 illustrate waveforms of electric signals applied to electrodes in the different embodiment,

FIGS. 16C-1-16C-4 illustrate voltage waveforms in the different embodiment,

FIGS. 17A and 17B in combination show voltage waveforms applied in time series in the different embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As an optical modulation material used in a driving method according to the present invention, a material which shows either a first optically stable state or a second optically stable state depending upon an electric field applied thereto, i.e., has bistability with respect to the applied electric field, particularly a liquid crystal having the abovementioned property, may be used.

Preferable liquid crystals having bistability which can be used in the driving method according to the present invention are chiral smectic C (SmC*)- or H (SmH*)-phase liquid crystals having ferroelectricity. In addition, liquid crystals showing chiral smectic I phase (SmI*), J phase (SmJ*), G phase (SmG*), F phase (SmF*) or K phase (SmK*) may also be used. These ferroelectric liquid crystals are described in, e.g., "LE JOURNAL DE PHYSIQUE LETTERS" 36 (L-69), 1975 "Ferroelectric Liquid Crystals"; "Applied Physics Letters" 36 (11) 1980, "Submicro Second Bistable Electrooptic Switching in Liquid Crystals", "Solid State Physics" 16 (141), 1981 "Liquid Crystal", etc. Ferroelectric liquid crystals disclosed in these publications may be used in the present invention.

More particularly, examples of ferroelectric liquid crystal compound usable in the method according to the present invention include decyloxybenzylidene-p'-amino-2-methylbutyl cinnamate (DOBAMBC), hexyloxybenzylidene-p'-amino-2-chloropropyl cinnamate (HOBACPC), 4-o-(2-methyl)-butylresorcilidene-4'-octylaniline (MBRA8), etc.

When a device is constituted using these materials, the device may be supported with a block of copper, etc., in which a heater is embedded in order to realize a temperature condition where the liquid crystal compounds assume a smectic phase.

Referring to FIG. 1, there is schematically shown an example of a ferroelectric liquid crystal cell for explanation of the operation thereof. Reference numerals 11 and 11a denote base plates (glass plates) on which a transparent electrode of, e.g., In.sub.2 O.sub.3, SnO.sub.2, ITO (Indium-Tin oxide), etc., is disposed, respectively. A liquid crystal of an SmC*- or SmH*-phase in which liquid crystal molecular layers 12 are oriented perpendicular to surfaces of the glass plates is hermetically disposed therebetween. A full line 13 shows liquid crystal molecules. Each liquid crystal molecule 13 has a dipole moment (P.perp.) 14 in a direction perpendicular to the axis thereof. When a voltage higher than a certain threshold level is applied between electrodes formed on the base plates 11 and 11a, a helical structure of the liquid crystal molecule 13 is loosened a unwound to change the alignment direction of respective liquid crystal molecules 13 so that the dipole moments (P.perp.) 14 are all directed in the direction of the electric field. The liquid crystal molecules 13 have an elongated shape and show refractive anisotropy between the long axis and the short axis thereof. Accordingly, it is easily understood that when, for instance, polarizers arranged in a cross nicol relationship, i.e., with their polarizing directions crossing each other, are disposed on the upper and the lower surfaces of the glass plates, the liquid crystal cell thus arranged functions as a liquid crystal optical modulation device, of which optical characteristics vary depending upon the polarity of an applied voltage. Further, when the thickness of the liquid crystal cell is sufficiently thin (e.g., 1.mu.), the helical structure of the liquid crystal molecules is loosened even in the absence of an electric field whereby the dipole moment assumes either of the two states, i.e., P in an upper direction 24 or Pa in a lower direction 24a as shown in FIG. 2. When electric field E or Ea higher than a certain threshold level and different from each other in polarity as shown in FIG. 2 is applied to a cell having the above-mentioned characteristics, the dipole moment is directed either in the upper direction 24 or in the lower direction 24a depending on the vector of the electric field E or Ea. In correspondence with this, the liquid crystal molecules are oriented in either of a first stable state 23 and a second stable state 23a.

When the above-mentioned ferroelectric liquid crystal is used as an optical modulation element, it is possible to obtain two advantages. First is that the response speed is quite fast. Second is that the orientation of the liquid crystal shows bistability. The second advantage will be further explained, e.g., with reference to FIG. 2. When the electric field E is applied to the liquid crystal molecules, they are oriented in the first stable state 23. This state is kept stable even if the electric field is removed. On the other hand, when the electric field Ea of which direction is opposite to that of the electric field E is applied thereto, the liquid crystal molecules are oriented to the second stable state 23a, whereby the directions of molecules are changed. This state is also kept stable even if the electric field is removed. Further, as long as the magnitude of the electric field E being applied is not above a certain threshold value, the liquid crystal molecules are placed in the respective orientation states. In order to effectively realize high response speed and bistability, it is preferable that the thickness of the cell is as thin as possible and generally 0.5 to 20.mu., particularly 1 to 5.mu.. A liquid crystal-electrooptical device having a matrix electrode structure in which the ferroelectric liquid crystal of this kind is used is proposed, e.g., in the specification of U.S. Pat. No. 4,367,924 by Clark and Lagerwall.

A preferred embodiment of the driving method according to the present invention is explained with reference to FIG. 3.

FIG. 3A schematically shows a cell 31 having picture elements arranged in a matrix which comprise scanning lines (scanning electrodes), data lines (signal electrodes) and a bistable optical modulation material interposed therebetween. Reference numeral 32 denotes data lines. For the brevity of explanation, a case where two state signals of "white" and "black" are displayed is explained. It is assumed that hatched picture elements correspond to "black" and the other picture elements correspond to "white" in FIG. 3A. First, in order to make a picture uniformly "white" (this step is called an "erasure step"), the bistable optical modulation material may be uniformly oriented to the first stable state. This can be effected by applying a predetermined voltage pulse signal (e.g., voltage: +2V.sub.0, time width: .DELTA.t) to all the scanning lines and applying a predetermined pulse signal (e.g., -V.sub.0, .DELTA.t) to all the data lines. In the erasure step, an electric signal of polarity opposite to that of a scanning selection signal in the writing step described hereinbelow is applied to the scanning lines, and an electric signal of a polarity opposite to that of an information selection signal (writing signal) in the writing step is applied to the data line, in phase with each other.

FIGS. 3B-1 and 3B-2 show an electric signal (scanning selection signal) applied to a selected scanning line and an electric signal (scanning non-selection signal) applied to the other scanning lines (non-selected scanning lines), respectively. FIGS. 3B-3 and 3B-4 show an electric signal (information selection signal; V.sub.0 applied at phase T.sub.1) applied to a selected (referred to as "black") data line and an electric signal (information non-selection signal; -V.sub.0 at phase T.sub.1) applied to a non-selected (referred to as "white") data line, respectively. In the FIGS. 3B-1-3B-4, the abscissa represents time, and the ordinate a voltage, respectively. T.sub.1 and T.sub.2 in the figures represent a phase for applying an information signal (and a scanning signal) and a phase for applying an auxiliary signal. This example shows a case where T.sub.1 =T.sub.2 =.DELTA.t.

The scanning lines 32 are selected sequentially. It is assumed herein that a threshold voltage for providing the first stable state (white) of the bistable liquid crystal at an application time of .DELTA.t be -V.sub.th2, and a threshold voltage for providing the second stable state at an application time of .DELTA.t be V.sub.th1. Then, the electric signal applied to the selected scanning line comprises voltages of -2V.sub.0 at phase (time) T.sub.1 and 0 at phase (time) T.sub.2 as shown in FIG. 3B-1. The other scanning lines are placed in grounded condition as shown in FIG. 3B-2 and the electric signal is 0. On the other hand, the electric signal applied to the selected data line comprises V.sub.0 at phase T.sub.1 and -V.sub.0 at phase T.sub.2 as shown in FIG. 3B-3, and the electric signal applied to the non-selected data line comprises -V.sub.0 at phase T.sub.1 and +V.sub.0 at phase T.sub.2 as shown in FIG. 3B-4. In this instance, the voltage V.sub.0 is set to a desired value which satisfies V.sub.0 <V.sub.th1 <3V.sub.0 and -V.sub.0 >-V.sub.th2 >-3V.sub.0.

Voltage waveforms applied to respective picture elements when the above-mentioned electric signals are given are shown in FIGS. 3C-1 to 3C-4. FIGS. 3C-1 and 3C-2 show voltage waveforms applied to picture elements where "black" and "white" are displayed, respectively, on the selected scanning line. FIGS. 3C-3 and 3C-4 respectively show voltage waveforms applied to picture elements on the non-selected scanning lines.

At phase T.sub.1, on the scanning line to which a scanning selection signal -2V.sub.0 is applied, an information signal +V.sub.0 is applied to a picture element where "black" is to be displayed and, therefore, a voltage 3V.sub.0 exceeding the threshold voltage V.sub.th1 is applied to the picture element, where the bistable liquid crystal is oriented to the second optically stable state. Thus, the picture element is written in "black" (writing step). On the same scanning line, the voltage applied to picture elements where "white" is to be displayed is a voltage V.sub.0 which does not exceed the threshold voltage V.sub.th1, and accordingly the picture element remains in the first optically stable state, thus displaying "white".

On the other hand, on the non-selected scanning lines, the voltage applied to all the picture elements is .+-.V or 0, each not exceeding the threshold voltage. Accordingly, the liquid crystal at the respective picture elements retains its orientation which has been obtained when the picture elements have been last scanned. In other words, after the whole picture elements have been oriented to one optically stable state ("white"), when one scanning line is selected, signals are written in one line of picture elements at the first phase T.sub.1 and the written signal or display states are retained even after steps for writing one frame is finished.

FIG. 4 (combination of FIGS. 4A and 4B) shows an example of the above-mentioned driving signals in time series. S.sub.1 to S.sub.5 represent electric signals applied to scanning lines; I.sub.1 and I.sub.3 represent electric signals applied to data lines; and A.sub.1 and C.sub.1 represent voltage waveforms applied to picture elements A.sub.1 and C.sub.1, respectively, shown in FIG. 3A.

Microscopic mechanism of switching due to electric field of a ferroelectric liquid crystal having bistability has not been fully clarified. Generally speaking, however, the ferroelectric liquid crystal can retain its stable state semi-permanently, if it has been switched or oriented to the stable state by application of a strong electric field for a predetermined time and is left standing under absolutely no electric field. However, when a reverse polarity of an electric field is applied to the liquid crystal for a long period of time, even if the electric field is such a weak field (corresponding to a voltage below V.sub.th in the previous example) that the stable state of the liquid crystal is not switched in a predetermined time for writing, the liquid crystal can change its stable state to the other one, whereby correct display or modulation of information cannot be accomplished. We have recognized that the liability of such switching or reversal of oriented states under a long term application of a weak electric field is affected by a material and roughness of a base plate contacting the liquid crystal and the kind of the liquid crystal, but have not clarified the effects quantitatively. We have confirmed a tendency that a monoaxial treatment of the base plate such as rubbing or oblique or tilt vapor deposition of SiO, etc., increases the liability of the above-mentioned reversal of oriented states. The tendency is manifested at a higher temperature compared to a lower temperature.

Anyway, in order to accomplish correct display or modulation of information, it is advisable that one direction of electric field is prevented from being applied to the liquid crystal for a long time.

The phase T.sub.2 in the driving method according to the present invention is a phase for obviating a situation where a unidirectional weak electric field is continuously applied. As a preferred embodiment for this purpose, as shown in FIGS. 3B-3 and 3B-4, a signal with a polarity opposite to that of the information signal (FIG. 3B-3 corresponds to "black", FIG. 3B-4 to "white") applied at phase T.sub.1 is applied to the data line at phase T.sub.2. In a case where a pattern shown in FIG. 3A is intended to be displayed, for example, by a driving method not having such phase T.sub.2, picture element A is made "black" on scanning of the scanning electrode S.sub.1, but it is highly possible that the picture element A will be switched sometime to "white" because an electric signal or voltage of -V.sub.0 is continuously applied to the signal electrode I, during the steps for scanning of the scanning electrode S.sub.2 and so on and the voltage is continuously applied to the picture element A as it is.

The whole picture is once uniformly rendered "white", and then "black" is written into picture elements corresponding to information at the first phase T.sub.1. In this example, the voltage for writing "black" at phase T.sub.1 is 3V.sub.0 and the application time is .DELTA.t. The voltage applied to the respective picture elements except at the scanning time is .vertline..+-.V.sub.0 .vertline. to the maximum, and the longest time during which the maximum voltage is 2.DELTA.t as shown at part 40 in FIG. 4B. The severest condition is imposed when the information signals succeed in the order of white.fwdarw.white.fwdarw.black and the second "white" signal is applied at the scanning time. Even then, the application time is 4.DELTA.t which is rather short and does not cause crosstalk at all, whereby a displayed information is retained semipermanently after the scanning of the whole picture is once completed. For this reason, a refreshing step as required in a display device using a TN liquid crystal having no bistability is not required at all.

The optimum length of the second phase T.sub.2 depends on the magnitude of the voltage applied to the data line. When a voltage having a polarity opposite to that of the information signal is applied, it is preferred that the time length is shorter for a larger voltage and longer for a shorter voltage. When the time is longer, it follows that a longer time is required for scanning the whole picture. Therefore, T.sub.2 is preferably set to satisfy T.sub.2 .ltoreq.T.sub.1.

FIGS. 5 and 6 show another driving mode according to the present invention, FIGS. 5B-1 and 5B-2 show voltages applied to picture elements corresponding to "black" and "white", respectively, on a selected scanning line. FIGS. 5B-3 and 5B-4 show voltages applied to picture elements on a non-selected scanning line and on a data line to which "black" or "white" information signals are applied. FIG. 6 (combination of FIGS. 6A and 6B) illustrate these signals applied in time series.

FIG. 7 (combination of FIGS. 7A and 7B) illustrates another embodiment of the erasure step than the one explained with reference to FIG. 4. Thus, in this example, the polarities of electric signals applied to scanning lines and data lines in the erasure step are made opposite to those of the scanning selection signals and information selection signals in the writing step. The voltage V.sub.0 is also set to a value satisfying the relationships of V.sub.0 <V.sub.th1 <3V.sub.0 and -V.sub.0 >-V.sub.th2 >-3V.sub.0.

In the embodiment shown in FIG. 7, in the erasure step .DELTA.t, an electric signal of 2V.sub.0 is applied to the scanning lines at a time and, in phase with the electric signal, a signal of -V.sub.0 with a polarity oppoiste to that of the electric signal is applied to the data lines. In the next writing step, signals similar to writing signals explained with reference to FIGS. 3 and 4 are applied to the scanning lines and data lines.

FIG. 8 (combination of FIGS. 8A and 8B) and FIG. 9 (combination of FIGS. 9A and 9B) respectively show examples of driving modes according to the present invention in time series. In these driving modes, a voltage value V.sub.0 is so set that the threshold voltage for changing orientations for a pulse width .DELTA.t is placed between .vertline.V.sub.0 .vertline. and 2.vertline.V.sub.0 .vertline..

In FIG. 8 (FIGS. 8A and 8B), an electric signal of +V.sub.0 is applied to the scanning lines and, in phase therewith, an electric signal of -V.sub.0 is applied to the data lines for erasing a picture. Immediately thereafter and subsequently, in the writing step, scanning signals of S.sub.1, S.sub.2, . . . , each of -V.sub.0, are sequentially applied and, in phase with these scanning signals, information signals, each of +V.sub.0, are applied to data lines, whereby writing is carried out.

FIGS. 8 and 9 respectively show examples where no auxiliary signal is involved, whereas FIG. 10 (combination of FIGS. 10A and 10B) shows an example where an auxiliary signal is used. Voltage values in respective driving pulses are shown in the figure. In the example of FIG. 10, electric signals applied to scanning lines and data lines in the erasure step have polarities respectively opposite to those applied in the writing step, have magnitudes in terms of absolute values smaller (2/3 V.sub.0) than those of the latter and have larger pulse widths (2.DELTA.t) than those of the latter. This erasure mode is effective in a case where the threshold voltage depends on pulse widths and a threshold voltage V.sub.th.sup.2.DELTA.t for a width of 2.DELTA.t satisfies a relationship of V.sub.th.sup.2.DELTA.t .ltoreq.4/3 V.sub.0.

FIG. 11 (inclusive of FIGS. 11A, 11B and 11C) and FIG. 12 (combination of FIGS. 12A and 12B) illustrate a driving mode for an optical modulation device comprising:

a partial erasure step wherein electric signals are applied to selected scanning lines among the scanning lines and selected data lines; the selected scanning lines and selected data lines constituting a new image area where a new image is to be written, and the electric signals applied to the selected scanning lines and selected data lines having polarities opposite to those of a scanning selection signal and an information selection signal applied to the respective lines for writing images; whereby the optical modulation material constituting the new image area is oriented to the first stable state and an image written in a previous writing step is partially erased; and a partial writing step wherein a scanning selection signal is applied to the selected scanning lines and an information signal for orienting the optical modulation material to the second stable step is applied to the selected data lines corresponding to information giving the new image.

A preferred embodiment of the above mentioned driving mode will be explained with reference to FIG. 11.

FIG. 11A schematically shows a cell 111 having picture elements arranged in a matrix which comprise scanning lines (scanning electrodes), data lines (signal electrodes) and a bistable optical modulation material interposed therebetween. Reference numeral 112 denotes data lines. For the brevity of explanation, a case where two state signals of "white" and "black" are displayed is explained. It is assumed that hatched picture elements correspond to "black" and the other picture elements correspond to "white" in FIG. 3A. First, in order to make a picture uniformly "white" (this step is called an "erasure step"), the bistable optical modulation material may be uniformly oriented to the first stable state. This can be effected by applying a predetermined voltage pulse signal (e.g., voltage: +2V.sub.0, time width: .DELTA.t) to all the scanning lines and applying a predetermined pulse signal (e.g., -V.sub.0, .DELTA.t) to all the data lines. In the erasure step, an electric signal of a polarity opposite to that of a scanning selection signal in the writing step described hereinbelow is applied to the scanning lines, and an electric signal of a polarity opposite to that of an information selection signal (writing signal) in the writing step is applied to the data line, in phase with each other.

FIGS. 11B-1 and 11B-2 show an electric signal (scanning selection signal) applied to a selected scanning line and an electric signal (scanning non-selection signal) applied to the other scanning lines (nonselected scanning lines), respectively. FIGS. 11B-3 and 11B-4 show an electric signal (information selection signal; V.sub.0 applied at phase T.sub.1) applied to a selected (referred to as "black") data line and an electric signal (information non-selection signal; -V.sub.0 at phase T.sub.1) applied to a non-selected (referred to as "white") data line, respectively. In the FIGS. 11B-1-11B-4, the abscissa represents time, and the ordinate a voltage, respectively. T.sub.1 and T.sub.2 in the figures represent a phase for applying an information signal (and scanning signal) and a phase for applying an auxiliary signal. This example shows a case where T.sub.1 =T.sub.2 =.DELTA.t.

The scanning lines 112 are selected sequentially. It is assumed herein that a threshold voltage for providing the first stable state (white) of the bistable liquid crystal at an application time of .DELTA.t be -V.sub.th2, and a threshold voltage for providing the second stable state at an application time of .DELTA.t be V.sub.th1. Then, the electric signal applied to the selected scanning line comprises voltages of -2V.sub.0 at phase (time) T.sub.1 and 0 at phase (time) T.sub.2 as shown in FIG. 11B-1. The other scanning lines are placed in grownded condition as shown in FIG. 11B-2 and the electric signal is 0. On the other hand, the electric signal applied to the selected data line comprises V.sub.0 at phase T.sub.1 and -V.sub.0 at phase T.sub.2 as shown in FIG. 11B-3, and the electric signal applied to the nonselected data line comprises -V.sub.0 at phase T.sub.1 and +V.sub.0 at phase T.sub.2 as shown in FIG. 11B-4. In this instance, the voltage V.sub.0 is set to a desired value which satisfies V.sub.0 <V.sub.th1 <3V.sub.0 and -V.sub.0 >-V.sub.th2 >-3V.sub.0.

Voltage waveforms applied to respective picture elements when the above mentioned electric signals are given are shown in FIGS. 11C. FIGS. 11C-1 and 11C-2 show voltage waveforms applied to picture elements where "black" and "white" are displayed, respectively, on the selected scanning line. FIGS. 11C-3 and 11C-4 respectively show voltage waveforms applied to picture elements on the nonselected scanning lines.

At phase T.sub.1, on the scanning line to which a scanning selection signal -2V.sub.0 is applied, an information signal +V.sub.0 is applied to a picture element where "black" is to be displayed and, therefore, a voltage 3V.sub.0 exceeding the threshold voltage V.sub.th1 is applied to the picture element, where the bistable liquid crystal is oriented to the second optically stable state. Thus, the picture element is written in "black" (writing step). On the same scanning line, the voltage applied to picture elements where "white" is to be displayed is a voltage V.sub.0 which does not exceed the threshold voltage V.sub.th1, and accordingly the picture element remains in the first optically stable state, thus displaying "white".

On the other hand, on the nonselected scanning lines, the voltage applied to all the picture elements is .+-.V or 0, each not exceeding the threshold voltage. Accordingly, the liquid crystal at the respective picture elements retains its orientation which has been obtained when the picture elements have been last scanned. In other words, after the whole picture elements have been oriented to one optically stable state ("white"), when one scanning line is selected, signals are written in one line of picture elements at the first phase T.sub.1 and the written signal or display states are retained even after steps for writing one frame is finished.

FIG. 11A shows an example of a picture thus formed through the erasure step and the writing step. FIG. 11D shows an example of a picture obtained by partially rewriting the picture shown in FIG. 11A. This example shown in FIG. 11D illustrates a case where an X-Y region or area formed by scanning lines X and data lines Y is intended to be rewritten. For this purpose, an electric signal (e.g., 2V.sub.0 shown in FIG. 12) having a polarity opposite to that of a scanning selection signal (e.g., -2V.sub.0 in FIG. 12) applied in the previous writing step is applied at a time or sequentially to scanning lines S.sub.1, S.sub.2 and S.sub.3 corresponding to the new image region (X-Y region) to be rewritten. On the other hand, an electric signal (e.g., -V.sub.0 on line I.sub.1 in FIG. 12) having a polarity opposite to that of an information selection signal (e.g., V.sub.0 on I.sub.1 in FIG. 12) is applied to data lines I.sub.1 and I.sub.2 corresponding to the new image region. Thus, only a part (e.g., X-Y region) of one picture can be erased (Partial Erasure Step).

The writing in the partially erased region (X-Y region) is then effected by applying the same procedure as in the writing step, i.e., by applying an information selection signal (+V.sub.0) and an information non-selection signal (-V.sub.0) corresponding to predetermined rewriting image information to the data lines for the partially erased region in phase with a scanning selection signal (-2V.sub.0).

On the other hand, an electric signal below the threshold voltage of the ferroelectric liquid crystal is applied to the picture elements in the non-rewriting region (i.e., X.sub.a -Y, X.sub.a -Y.sub.a and X-Y.sub.a regions) so that the writing state of each picture element in the non-rewriting region is retained.

More specifically, in the partial erasure step, an electric signal (e.g., V.sub.0 on I.sub.3 in FIG. 12) having the same polarity as an electric signal (e.g., 2V.sub.0 FIG. 12) applied to the scanning signal in the erasure step is applied to the data lines not constituting the rewriting region (X-Y region). Further, in the partial writing step, an electric signal (e.g., -V.sub.0 on I.sub.3 in FIG. 12) having the same polarity as a scanning selection signal (e.g. -2V.sub.0 on S.sub.1, S.sub.2 and S.sub.3 in FIG. 12) is applied to the data lines not constituting the rewriting region (X-Y region) in phase with the selection scanning signal . On the other hand, the potential of the scanning lines not constituting the rewriting region is held at a base potential (e.g., 0 volt).

The above explained driving signals are shown in time series in FIG. 12 (combination of FIGS. 12A and 12B). S.sub.1 -S.sub.5 indicate electric signals applied to scanning signals; I.sub.1 and I.sub.3 indicate electric signals applied to data lines; and A.sub.2, C.sub.2 and D.sub.2 indicate waveforms applied to picture elements A.sub.2, C.sub.2 and D.sub.2 shown in FIGS. 11A and 11D.

A rewriting region can be appointed by a cursor in the present invention.

FIG. 13 (combination of FIGS. 13A and 13B) and FIG. 14 (combination of FIGS. 14A and 14B) show other examples of driving modes based on the present invention. In these driving modes, V.sub.0 is set to such a value that the threshold voltage for changing orientations for a pulse width of .DELTA.t is placed between .vertline.V.sub.0 .vertline. and .vertline.2V.sub.0 .vertline..

In the example shown in FIG. 13 (FIG. 13A and FIG. 13B), an electric signal of +V.sub.0 is applied to the scanning lines and, in parallel therewith, an electric signal of -V.sub.0 is applied to the data lines for erasing a picture. Immediately thereafter, in the writing step, scanning signals S.sub.1, S.sub.2 . . . , each of -V.sub.0, are sequentially applied and, in phase with these scanning signals, information signals, each of +V.sub.0, are applied to data lines, whereby a picture as shown in FIG. 11A is written in.

Next, in the partial erasure step, an electric signal of -2V.sub.0 is applied to the picture elements which have been written in the previous step in the X-Y region shown in FIG. 11D, whereby the picture elements are erased at a time. (This example of one time erasure is shown in FIG. 13. However, successive erasure is also possible by applying an electric signal of V.sub.0 successively to scanning lines as a scanning selection signal). Then, electric signals corresponding to new image information are applied to the X-Y region whereby the X-Y region is written as shown in FIG. 11D.

FIGS. 13 and 14 respectively show examples where no auxiliary signal is involved, whereas FIG. 15 (combination of FIGS. 15A and 15B) shows an example where an auxiliary signal is used. Voltage values in respective driving pulses are shown in the figure. In the example of FIG. 15, electric signals applied to scanning lines and data lines in the erasure step have polarities respectively opposite to those applied in the writing step, have magnitudes in terms of absolute values smaller (2/3 V.sub.0) than those of the latter and have larger pulse widths (2.DELTA.t) than those of the latter. This erasure mode is effective in a case where the threshold voltage depends on pulse widths and a threshold voltage V.sub.th.sup.2.DELTA.t for a width of 2.DELTA.t satisfies a relationship of V.sub.th.sup.2.DELTA.t .ltoreq.4/3 V.sub.0.

In the partial erasure step, an electric signal of -4/3 V.sub.0 is applied to effect partial erasure. In the next partial writing step, a new image is written in the X-Y region.

FIG. 16 (inclusive of FIGS. 16A, 16B and 16C) and FIG. 17 (combination of FIGS. 17A and 17B) illustrate another driving mode for an optical modulation device comprising: a writing step comprising a first phase wherein a voltage orienting the bistable optical modulation material to the first stable state is applied to picture elements on selected scanning lines among said plurality of picture elements, and a second phase wherein a voltage orienting the bistable optical modulation material to the second stable state is applied to a selected picture element among the picture elements on the selected scanning lines to write in the selected picture element, and a step of applying an alternating current to the written selected picture element.

A further preferred example of this driving mode is used for driving a liquid crystal device which comprises scanning lines sequentially and periodically selected based on scanning signals, data lines facing the scanning lines and selected based on predetermined information signals, and a bistable liquid crystal assuming a first stable state or a second stable state depending on an electric field applied thereto interposed between the scanning lines and data lines. The liquid crystal device is driven by applying to a selected scanning line an electric signal comprising a first phase t.sub.1 providing one direction of an electric field by which the liquid crystal is oriented to the first stable state regardless of an electric signal applied to signal electrodes and a second phase t.sub.1 having an auxiliary voltage assisting reorientation to the second stable state of the liquid crystal corresponding to electric signals applied to data lines, and a third step or phase t.sub.3 of applying to data lines an electric signal having a voltage polarity opposite to that of the electric signal applied at the phase t.sub.2 based on predetermined information.

A preferred embodiment according to this mode is explained with reference to FIG. 16.

FIG. 16A schematically shows a cell 16 having picture elements arranged in a matrix which comprise scanning lines (scanning electrodes), data lines (signal electrodes) and a ferroelectric liquid crystal interposed therebetween. Reference numeral 162 denotes data lines. For the brevity of explanation, a case where two state signals of "white" and "black" are displayed is explained. It is assumed that hatched picture elements correspond to "black" and the other picture elements correspond to "white" in FIG. 16A.

FIGS. 16B-1 and 16B-2 show an electric signal (scanning selection signal) applied to a selected scanning line and an electric signal (scanning non-selection signal) applied to the other scanning lines (nonselected scanning lines), respectively. FIGS. 16B-3 and 16B-4 show an electric signal (information selection signal) applied to a selected (referred to as "black") data line and an electric signal (information non-selection signal) applied to a non-selected (referred to as "white") data line, respectively. In the FIGS. 16B-1-16B-4, the abscissa represents time, and the ordinate a voltage, respectively. T.sub.1, T.sub.2 and T.sub.3 in the writing step represent first, second and third phases, respectively. This example shows a case where T.sub.1 =T.sub.2 =T.sub.3.

It is assumed herein that a threshold voltage for providing the first stable state (white) of the bistable liquid crystal for an application time of .DELTA.t be -V.sub.th2, and a threshold voltage for providing the second stable state for an application time of .DELTA.t be V.sub.th1. Then, the electric signal applied to the selected scanning line comprises voltages of 3V.sub.0 at phase (time) T.sub.1, -2V.sub.0 at phase (time) T.sub.2 and 0 at phase (time) T.sub.3 as shown in FIG. 16B-1. The other scanning lines are placed in grounded condition as shown in FIG. 16B-2 and the electric signal is 0. On the other hand, the electric signal applied to the selected data line comprises 0 at phase T.sub.1, V.sub.0 at phase T.sub.2 and -V.sub.0 at phase T.sub.2 as shown in FIG. 16B-3, and the electric signal applied to the nonselected data line comprises 0 at phase T.sub.1, -V.sub.0 at phase T.sub.2 and +V.sub.0 at phase T.sub.3 as shown in FIG. 16B-4. In this instance, the voltage V.sub.0 is set to a desired value which satisfies V.sub.0 <V.sub.th1 <3V.sub.0 and -V.sub.0 >-V.sub.th2 >-3V.

Voltage waveforms applied to respective picture elements when the above mentioned electric signals are given are shown in FIGS. 16C. FIGS. 16C-1 and 16C-2 show voltage waveforms applied to picture elements where "black" and "white" are displayed, respectively, on the selected scanning line. FIGS. 16C-3 and 16C-4 respectively show voltage waveforms applied to picture elements on the nonselected scanning lines.

As shown in FIG. 16C, a voltage -3V.sub.0 exceeding the threshold voltage -V.sub.th2 is applied to all the picture elements on the selected scanning line at phase T.sub.1, whereby these picture elements are once rendered white. In the second phase T.sub.2, a voltage 3V.sub.0 exceeding the threshold voltage V.sub.th1 is applied to the picture elements which are to be displayed as "black", whereby the other optically stable state ("black") is attained. Further, the voltage applied to the picture elements which are to be displayed as "white" is V.sub.0 not exceeding the threshold voltage, whereby the same optically stable state is maintained.

On the other hand, on the nonselected scanning lines, the voltage applied to all the picture elements is .+-.V or 0, each not exceeding the threshold voltage. Accordingly the liquid crystal at the respective picture elements retains its orientation which has been obtained when the picture elements have been last scanned. In other words, when a scanning line is selected, all the picture elements on the scanning line is uniformly oriented to one optically stable state ("white") at phase T.sub.1 and selected picture elements are transformed into the other optically stable state ("black"), whereby one line is written. The thus obtained signal or display state is retained even after writing steps for one frame is finished and until subsequent scanning.

FIG. 17 (combination of FIGS. 17A and 17B) shows an example of the above mentioned driving signals in time series. S.sub.1 to S.sub.5 represent electric signals applied to scanning lines; I.sub.1 amd I.sub.3 represent electric signals applied to data lines; and A.sub.3 and C.sub.3 represent voltage waveforms applied to picture elements A.sub.3 and C.sub.3, respectively, shown in FIG. 16A.

As has been described above, a reversal of orientation states (cross talk) can occur due to application of a weak electric field for a long period. In a preferred embodiment, however, the reversal of orientation states can be prevented by applying a signal capable of preventing continual application of a weak electric field in one direction.

FIGS. 16B-3 and 16B-4 illustrate a preferred embodiment for the above purpose wherein a signal having a polarity opposite to that of an information signal ("black" in FIG. 16B-3 and "white" in FIG. 16B-4 applied to a data line at phase T.sub.2 is applied to the data line at phase T.sub.3. In a case where a pattern shown in FIG. 16A is intended to be displayed, for example, by a driving method not having such phase T.sub.3, picture element A.sub.3 is made "black" on scanning of the scanning line S.sub.1, but it is highly possible that the picture element A.sub.3 will be switched sometime to "white" because an electric signal or voltage of -V.sub.0 is continuously applied to the signal electrode I.sub.1 during the steps for scanning of the scanning electrode S.sub.2 and so on and the voltage is continuously applied to the picture element A.sub.3 as it is.

The whole picture is once uniformly rendered "white" at the first phase T.sub.1, and then "black" is written into picture elements corresponding to information at the second phase T.sub.2 in the scanning. In this example, the voltage for providing "white" at phase T.sub.1 is -3V.sub.0 and the application time is .DELTA.t. Further, the voltage for writing "black" at phase T.sub.2 is 3V.sub.0 and the application time is also .DELTA.t. The voltage applied to the respective picture elements except at the scanning time is .vertline..+-.V.sub.0 .vertline. to the maximum, and the longest time during which the maximum voltage is 2.DELTA.t as shown at part 161 in FIG. 17. Thus cross talk does not occur at all, whereby a displayed information is retained semipermanently after the scanning of the whole picture is once completed. For this reason, a refreshing step as required in a display device using a TN liquid crystal having no bistability is not required at all.

The optimum length of the third phase T.sub.3 depends on the magnitude of the voltage applied to the data line at this phase. When a voltage having a polarity opposite to that of the information signal is applied, it is preferred that the time length is shorter for a larger voltage and longer for a shorter voltage. When the time is longer, it follows that a longer time is required for scanning the whole picture. Therefore, T.sub.3 is preferably set to satisfy T.sub.3 .ltoreq.T.sub.2.

The driving method according to the present invention can be widely applied in the field of optical shutters and display such as liquid crystal-optical shutters and liquid crystal TV sets.

Hereinbelow, the present invention will be explained with reference to working examples.

EXAMPLE 1

A pair of electrode plates each comprising a glass substrate and a transparent electrode pattern of ITO (Indium-Tin-Oxide) formed thereon were provided. These electrodes were capable of giving a 500.times.500 matrix electrode structure. On the electrode pattern of one of the electrode plates was formed a polyimide film of about 300 .ANG. in thickness by spin coating. The polyimide face of the electrode plate was rubbed with a roller about which a suede cloth was wound. The electrode plate was bonded to the other electrode plate which was not coated with a polyimide film, thereby to form a cell having a gap of about 1.6.mu.. Into the cell was injected a ferroelectric crystal of decyloxybenzylidene-p'-amino-2-methylbutyl cinnamate (DOBAMBC) under hot-melting state, which was then gradually cooled to form a uniform monodomain of SmC phase.

The thus formed cell was held at a controlled temperature of 70.degree. C. and driven by line-by-line scanning according to the driving mode explained with reference to FIGS. 3 and 4 under the conditions of V.sub.0 =10 volt, and T.sub.1 =T.sub.2 =.DELTA.t=80 .mu.sec, whereby extremely good image was obtained.

EXAMPLE 2

Writing of image was conducted in the same manner as in Example 1 except that the driving mode shown in FIG. 7 was used instead of the mode in Example 1, whereby good image was obtained.

EXAMPLE 3

Line-by-line scanning was carried out in the same manner as in Example 1 except that the driving waveforms shown in FIG. 12 was used, whereby extremely good image was formed. Then, a part of the image was rewritten according to driving waveforms shown in FIG. 12, whereby good partially-rewritten image was obtained.

EXAMPLE 4

Line-by-line scanning was carried out in the same manner as in Example 1 except that the waveforms shown in FIGS. 16 and 17 were used under the conditions of V.sub.0 =10 volt, and T.sub.1 =T.sub.2 =T.sub.3 =.DELTA.t=50 .mu.sec, whereby extremely good image was formed.

Claims

1. A display apparatus, comprising:

a liquid crystal device comprising a plurality of scanning electrodes, a plurality of signal electrodes disposed to intersect the scanning electrodes, and a chiral smectic liquid crystal disposed so as to form a matrix of picture elements defining a display area, each picture element being formed at an intersection of the scanning electrodes and the signal electrodes; and

driving means for:

(a) sequentially and periodically applying a scanning selection signal to the scanning electrodes to periodically select a particular scanning electrode, the scanning selection signal comprising a first voltage signal, a second voltage signal and a third voltage signal having mutually different waveforms;

(b) applying data signals to the signal electrodes, each data signal comprising an information signal for selecting a display state of a picture element on the particular scanning electrode and an auxiliary signal having a waveform different from that of the information signal,

wherein the picture elements on each periodically selected particular scanning electrode supplied with the first voltage signal are non-selectively erased into one display state,

wherein a selected picture element on the particular scanning electrode supplied with the second voltage signal is changed into the other display state depending on the selected information signal,

wherein the picture elements on the particular scanning electrode supplied with the third voltage signal are allowed to retain their previous display states, and

wherein a non-selected picture element on the particular scanning electrode supplied with the second voltage signal is held in said one display state, thereby providing a periodically refreshed display picture in the display area; and

(c) when a certain region of the display area is desired to be rewritten during a period of sequential and periodic application of the scanning selection signal to the scanning electrodes, designating a certain number of scanning electrodes corresponding to the certain region to be rewritten, applying the scanning selection signal to the designated scanning electrodes, and applying to the signal electrodes data signals carrying information for the partial rewriting in synchronism with the scanning selection signal, thereby providing a display picture including a partially rewritten region.

2. An apparatus according to claim 1, wherein the auxiliary signal has a voltage polarity opposite to that of the information signal with reference to a voltage level of a scanning electrode when not supplied with the scanning selection signal.

3. An apparatus according to claim 1, wherein said chiral smectic liquid crystal is a liquid crystal developing ferroelectricity.

4. A display apparatus, comprising:

a liquid crystal device comprising a plurality of scanning electrodes, a plurality of signal electrodes disposed to intersect the scanning electrodes, and a chiral smectic liquid crystal disposed so as to form a matrix of picture elements defining a display area, each picture element being formed at an intersection of the scanning electrodes and the signal electrodes; and

driving means for:

(a) sequentially and periodically applying a scanning selection signal to the scanning electrodes to periodically select a particular scanning electrode, said scanning selection signal comprising a first voltage signal, a second voltage signal and a third voltage signal having mutually different waveforms;

(b) applying data signals to the signal electrodes, each data signal comprising an information signal for selecting a display state of a picture element on the particular scanning electrode, each data signal having a waveform so as to provide an alternating voltage applied to the picture element on a scanning electrode not supplied with the scanning selection signal,

wherein the picture elements on each periodically selected particular scanning electrode supplied with the first voltage signal are non-selectively erased into one display state,

wherein a selected picture element on the particular scanning electrode supplied with the second voltage signal is changed into the other display state depending on the selected information signal,

wherein the picture elements on the particular scanning electrode supplied with the third voltage signal are allowed to retain their previous display states, and

wherein a non-selected picture element on the particular scanning electrode supplied with the second voltage signal is held in said one display state, thereby providing a periodically refreshed display in the display area; and

(c) when a certain region of the display area is desired to be rewritten during a period of sequential and periodic application of the scanning selection signal to the scanning electrodes, designating a certain number of scanning electrodes, designating a certain number of scanning electrodes corresponding to the certain region to be rewritten, applying the scanning selection signal to the designated scanning electrodes, and applying to the signal electrodes data signals carrying information for the partial rewriting in synchronism with the scanning selection signal, thereby providing a display picture including a partially rewritten region.

5. An apparatus according to claim 4, wherein the alternating voltage has an average voltage value of zero.

6. An apparatus according to claim 4, wherein said chiral smectic liquid crystal is a liquid crystal developing ferroelectricity.

7. A liquid crystal apparatus, comprising:

a liquid crystal device comprising a group of scanning electrodes and a group of signal electrodes intersecting each other to form an electrode matrix, and a liquid crystal having a memory function disposed so as to form a picture element at each intersection of the scanning electrodes and the signal electrodes; and

drive means for:

(a) sequentially and periodically applying a scanning selection signal, said scanning selection signal comprising a first voltage signal, a second voltage signal and a third voltage signal having mutually different waveforms, and being applied in a period for one scanning electrode,

(b) in a period of applying said first voltage signal, applying signals to the signal electrodes for uniformly causing an orientation state of said liquid crystal, thereby non-selectively erasing the picture elements on a scanning electrode to which the first voltage signal is applied,

(c) in a period of applying said second voltage signal, applying signals to the signal electrodes for selectively causing either one orientation state or another orientation state of said liquid crystal, thereby selectively writing in the picture elements on the scanning electrode to which the second voltage signal is applied,

(d) in a period of applying said third voltage signal, applying signal to the signal electrodes for allowing the picture elements to retain their display states on the scanning electrode to which the third voltage signal is applied,

(e) applying an alternating voltage to the pixels on the scanning electrodes not supplied with the scanning selection signal, and

(f) when a certain region of the display area is desired to be rewritten during a period of sequential and periodic application of the scanning selection signal to the scanning electrodes, designating a certain number of scanning electrodes corresponding to the certain region to be rewritten, applying the scanning selection signal to the designated scanning electrodes, and applying to the signal electrodes data signals carrying information for the partial rewriting in synchronism with the scanning selection signal, thereby providing a display picture including a partially rewritten region.

8. A liquid crystal apparatus according to claim 7, wherein said liquid crystal is a liquid crystal developing ferroelectricity.

9. A liquid crystal apparatus according to claim 7, wherein said liquid crystal is a chiral smectic liquid crystal.

10. A liquid crystal apparatus according to claim 7, wherein the voltage magnitude of said first voltage signal is larger than that of said second voltage signal.

11. A liquid crystal apparatus according to claim 7, wherein said alternating voltage has an average value of zero.

12. A driving method for a liquid crystal device comprising a group of scanning electrodes and a group of signal electrodes intersecting each other to form an electrode matrix, and a liquid crystal having a memory function disposed so as to form a picture element at each intersection of the scanning electrodes and the signal electrodes, said driving method comprising:

(a) sequentially and periodically applying a scanning selection signal, the scanning selection signal comprising a first voltage signal, a second voltage signal and a third voltage signal having mutually different waveforms, and being applied in a period for one scanning electrode;

(b) in a period of applying the first voltage signal, applying signals to the signal electrodes for uniformly causing an orientation state of the liquid crystal, thereby non-selectively erasing the picture elements on a scanning electrode to which the first voltage signal is applied;

(c) in a period of applying the second voltage signal, applying signals to the signal electrodes for selectively causing either one orientation state or another orientation state of the liquid crystal, thereby selectively writing in the picture elements on the scanning electrode to which the second voltage signal is applied;

(d) in a period of applying said third voltage signal, applying signal to the signal electrodes for allowing the picture elements to retain their display states on the scanning electrode to which the third voltage signal is applied;

(e) applying an alternating voltage to the pixels on the scanning electrodes not supplied with the scanning selection signal; and

(f) when a certain region of the display area is desired to be rewritten during a period of sequential and periodic application of the scanning selection signal to the scanning electrodes, designating a certain number of scanning electrodes corresponding to the certain region to be rewritten, applying the scanning selection signal to the designated scanning electrodes, and applying to the signal electrodes data signals carrying information for the partial rewriting in synchronism with the scanning selection signal, thereby providing a display picture including a partially rewritten region.

13. A driving method according to claim 12, wherein the liquid crystal is a liquid crystal developing ferroelectricity.

14. A driving method according to claim 12, wherein the liquid crystal is a chiral smectic liquid crystal.

15. A driving method according to claim 12, wherein the voltage magnitude of the first voltage signal is larger than that of the second voltage signal.

16. A driving method according to claim 12, wherein the alternating voltage has an average value of zero.