GB2178582A

GB2178582A - Liquid crystal apparatus and driving method therefor

Info

Publication number: GB2178582A
Application number: GB08616825A
Authority: GB
Inventors: Junichiro Kanabe; Masaki Kuribayashi; Toshihiko Nakazawa
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1985-07-16
Filing date: 1986-07-10
Publication date: 1987-02-11
Anticipated expiration: 2006-07-10
Also published as: GB8616825D0; DE3623895A1; FR2585163A1; DE3623895C2; GB2178582B; FR2585163B1

Abstract

A liquid crystal device comprises a plurality of picture elements A11-A55 comprising a ferroelectric liquid crystal showing a first stable orientation state and a second stable orientation state in response to an electric field thereto. The liquid crystal device is driven by applying a voltage signal capable of orienting the ferroelectric liquid crystal to the first orientation state to all or a part of the picture elements, applying an inversion voltage signal capable of inverting the ferroelectric liquid crystal oriented to the first orientation state to the second orientation state to selected picture elements, and variably controlling the waveform of the inversion voltage signal. Gradational display may be obtained by varying the pulse amplitude or the duty cycle. <IMAGE>

Description

SPECIFICATION Liquid crystal apparatus and driving method therefor FIELD OF THE INVENTION AND RELATED ART The present invention relates to a liquid crystal apparatus and a driving method therefor for effecting a gradational display in a display panel, and more particularly, to a liquid crystal apparatus and a driving method therefor for effecting a gradational display in a display panel such as a liquid crystal television panel using a liquid crystal (hereinafter sometimes abbreviated as "LC"), particularly a ferroelectric liquid crystal (hereinafter sometimes abbreviated as "FLC").

In a conventional LC-television panel using an active matrix driving scheme, a thin film transistor (TFT) is provided to each of picture elements arranged in a matrix, and a liquid crystal (e.g., a twisted nematic (TN) liquid crystal) at a picture element is driven by applying a gate-on-pulse to the TFT to establish continuity between the source and drain while at the same time applying a picture image signal through the source to be stored at a capacitor so as to drive the picture element based on the stored image signal. At the same time, a gradational display is provided by modulating the voltage of the picture image signal.

In such a television panel of an active matrix driving scheme using a TN liquid crystal, TFTs having a complicated structure are required, so that many production steps are required to result in a high production cost which has provided an obstacle. Moreover, there is a problem that is difficult to form a thin film semiconductor (e.g., of polysilicon or amorphous silicon) in a large area.

On the other hand, there is also known a display panel of a passive matrix driving scheme using a TN liquid crystal as one which can be produced at a low production cost. In this type of display panel, however, when the number (N) of scanning lines is increased, a time period (duty ratio) during which an effective electric field is applied to one selected point during the time in which one frame is scanned, is decreased to a ratio of 1/N. As a result, there arise problems that crosstalk occurs and a high contrast picture cannot be attained. Moreover, as the duty ratio is lowered, it becomes difficult to control the gradation of respective picture elements by voltage modulation. Thus, this type of display panel is not suited for a display panel comprising a high density of driving lines, particularly a LC-television panel.

SUMMARY OF THE INVENTION An object of the present invention is to provide a liquid crystal apparatus and a driving method therefor for effecting gradational display in a display panel comprising picture elements arranged at a high density over a wide area, particularly a LC-television panel.

According to a first aspect of the present invention, there is provided a liquid crystal apparatus, comprising: a liquid crystal device comprising a plurality of picture elements comprising a ferro-electric liquid crystal showing a first stable orientation state and a second stable orientation state in response to an electric field thereto, particularly one placed under bistability condition; means for applying a voltage signal capable of orienting the ferroelectric liquid crystal to the first orientation state to all or a part of the picture elements; means for applying an inversion voltage signal capable of inverting the ferroelectric liquid crystal oriented to the first orientation state to the second orientation state to selected picture elements; and means for variably controlling the waveform of the inversion voltage signal.

There is also provided a driving method for a liquid crystal device comprising a plurality of picture elements of the above described nature arranged in a plurality of rows and a plurality of columns to provide a picture; comprising: a first stage of applying a voltage signal capable of orienting the ferroelectric liquid crystal to the first orientation state to all or a part of the picture elements thereby to clear the picture elements, and a second stage of applying an inversion voltage signal for inverting the ferroelectric liquid crystal oriented to the first orientation state to selected picture elements to the second orientation state to selected picture elements to write in the picture elements, the inversion voltage signal having a voltage waveform controlled depending on given gradation data.

According to a second aspect of the present invention, there is provided a liquid crystal apparatus, comprising: a liquid crystal device comprising a plurality of picture elements of the above described nature arranged in a plurality of rows and a plurality of columns; means for applying a voltage signal capable of orienting the ferroelectric liquid crystal to the first orientation state row by row to the picture elements; means for applying an inversion voltage signal capable of inverting the ferroelectric liquid crystal oriented to the first orientation state to the second orientation state row by row to selected picture elements among the picture elements at which the ferroelectric liquid crystal has been oriented to the first orientation state; and means for variably controlling the waveform of the inversion voltage signal.

There is also provided a driving method for a liquid crystal device comprising a plurality of picture elements of the above described nature arranged in a plurality of rows and a plurality of columns to provide a picture, comprising: a first phase of applying a clear voltage signal for orienting the ferroelectric liquid crystal to the first orientation state to picture elements on a row to clear the picture elements, and a second phase of applying to selected picture elements among the cleared picture elements; said clear voltage signal and inversion voltage signal being applied row by row sequentially, the inversion voltage signal having a voltage waveform variably controlled depending on given gradation data.

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 DRA WINGS Figures I and 2 are schematic perspective views illustrating the basic operation principle of a liquid device used in the present invention; Figure 3 is a plan view showing a matrix electrode arrangement used in the present invention; Figures 4A-4D show driving waveforms used in the present invention; Figures 5A-5E show gradational waveforms applied to picture elements; Figures 7A-7E are sketches obtained through microscopic observation of picture elements; Figure 8 shows a curve indicating a relationship between transmittance and pulse height; Figure 9 shows time serial waveforms obtained by using driving waveforms shown in Figs. 4 and 5; Figure 10 shows a diagrammatic view showing an arrangement of a driving apparatus used in the present invention;; Figures 11A-11D show another set of driving waveforms used in the present invention; Figure 12 shows a time serial representation of the driving waveforms; Figures 13A-13F and Figures 14A-14E respectively show another set of gradation signals used in the present invention; Figures 15A-15D show another set of driving waveforms used in the present invention; Figures 16A-16D show voltage waveforms applied to picture elements; Figures 1 7A- 1 7E show gradational driving waveforms; Figures 18A-18E show gradation waveforms applied to picture elements; Figure 19 shows time serial waveforms obtained by using driving waveforms shown in Figs.

15, 16 and 17; Figures 20A-20D show another set of driving waveforms used in the present invention; Figures 2 lA-2 iD show voltage waveforms applied to picture elements; Figures 22A-22D show another set of driving waveforms used in the present invention; Figure 23 shows time serial waveforms obtained by using the driving waveforms shown in Figs. 22A-22D; Figures 24A-24F show another set of driving waveforms; Figures 25A-25F show still another set of driving waveforms used in the present invention; and Figures 26 shows time serial waveforms using the driving waveforms.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The optical modulation material used in the present invention may be a material which shows a first optical stable state (assumed to form, e.g., a "bright" state) and a second optically stable state (assumed to form, e.g., a "dark" state) depending on an electric field applied thereto, i.e., a material having bistability with respect to an electric field, particularly a liquid crystal having such a property.

As the ferroelectric liquid crystal having bistability used in the present invention, chiral smectic liquid crystals having ferroelectricity are most preferred. Among those liquid crystals, a liquid crystal in chiral smectic C phase (SmC-), H phase (SmH*), I phase (semi*, F phase (SmF*) or G phase (SmG-) is particularly suited. 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", "Kotai Butsuri (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 used in the present invention are decyloxybenzylidene-p'-amino-2-methylbutyl-cinnamate (DOBAMBC), hexyloxy-benzylidene-p'-amino-2-chloropropylcinnamate (HOBACPC), 4-o-(2-methyl)-butylresorcilidene-4'-octylaniline (MBRA 8), 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 an SmC"- or SmH*-phase.

Referring to Fig. 1, there is schematically shown an example of a ferroelectric liquid crystal cell. Reference numerals 11 a and 11 b denote base plates (glass plates) on which a transparent electrode of, e.g., In203, SnO2, ITO (Indium-Tin-Oxide), etc., is disposed, respectively. A liquid crystal of an SmC*-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 (P1) 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 a and 11 b, a helical or spiral structure of the liquid crystal molecule 13 is loosened or released to change the alignment direction of respective liquid crystal molecules 13 so that the dipole moment (Pl) 14 are all directed in the direction of the electric field. The liquid crystal molecules 13 have an elongated shape and show refractive anistropy 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,u), the helical structure of the liquid crystal molecules is loosened without application of an electric field whereby the dipole moment assumes either of the two states, i.e., Pa in an upper direction 24a or Pb in a lower direction 24b, thus providing a bistabilility condition, as shown in Fig. 2.When an electric field Ea or Eb 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 24a or in the lower direction 24b depending on the vector of the electric field Ea or Eb. In correspondence with this, the liquid crystal molecules are oriented to either a first orientation state 23a or a second orientation state 23b.

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 Ea is applied to the liquid crystal molecules, they are oriented in the first stable state 23a. This state is stably retained even if the electric field is removed. On the other hand, when the electric field Eb of which direction is opposite to that of the electric field Ea is applied thereto, the liquid crystal molecules are oriented to the second orientation state 23b, whereby the directions of molecules are changed. Likewise, the latter state is stably retained even if the electric field is removed.

Further, as long as the magnitude of the electric field Ea or Eb 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 , particularly 1 to 5 ij. 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. Patent No.

4367924 by Clark and Lagerwall.

An embodiment of the driving method according to the present invention is explained with reference to Figs. 3 to 9.

Fig. 3 schematically shows a cell 31 having picture elements arranged in a matrix which comprise scanning lines 32, data lines 33 and a ferroelectric liquid crystal under bistability condition interposed therebetween. 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. 3.

First, in order to make a picture uniformly "white", the ferroelectric liquid crystal under bistability condition is uniformly oriented to the first stable state. This can be effected by applying a predetermined voltage pulse signal (e.g., voltage: 3Vo, duration: At) to all the scanning lines.

Alternatively, it is also possible to apply a similar electric signal to all the data lines, or to apply to scanning lines or data lines constituting a prescribed block an electric signal capable of uniformly orienting the ferroelectric liquid crystal under bistability condition in the prescribed block to the first stable state. More specifically, it is possible to apply the above mentioned electric signal for clearng (3to) simultaneously to all or a prescribed part of the picture elements or alternatively to apply the above electric signal for clearing row by row. In any case, after a picture area is once uniformly written in "white", data are written therein corresponding to given data signal.

Figs. 4A-4D show driving signal waveforms used in an embodiment wherein writing is effected after the whole or a prescribed picture area is cleared.

Figs. 4A and 4B show an electric signal (-2V,) applied to selected scanning lines and an electric signal (0) applied to the other scanning lines (nonselected scanning lines), respectively.

On the other hand, Figs. 4C and 4D show an electric signal (VO) applied to a selected data line (assumed to present "black") and an electric signal (-V,) applied to a nonselected data line (assumed to present "white"), respectively.

Figs. 5A-5E show gradation signal waveforms applied to data lines, and Figs. 6A-6E show inversion signals for inverting "white" to "black") superposed with the above mentioned gradation signals. Fig. 5A shows a voltage waveform (0) of a first gradation signal, whereby a complete inversion voltage of 3Vo shown in Fig. 6A is applied to a picture element. At the picture element to which the complete inversion voltage 3Vo is applied, the whole picture element is inverted from the white state to the black state as shown in Fig. 7E. Fig. 5E shows a voltage waveform (V4) of a fifth voltage signal, whereby an inversion initiation voltage (3Vo-V4) as shown in Fig. 6E is applied to a picture element.At the picture element to which the inversion initiation voltage (3Vo-V4) is applied, a threshold state which is a state just before a domain 71 in the black state is formed in the white state as shown in Fig. 7A, is formed. Figs.

5B, 5C and 5D show a second gradation signal (V1), a third gradation signal (V2) and a fourth gradation signal (V3), respectively, set to satisfy the relationship of O < IV1l < lV2l < lV3l < lV4l. As the result, by applying the resultant voltages of 3Vo-V1, 3Vo-V2 and 3Vo-V3, which are set to above the inversion initiation voltage (3Vo-V4) and below the complete inversion voltage (3to), the ratio of the domain region 71 inverted to "black" to the white domain region 72 can be controlled depending on the magnitude of the voltages. Figs. 7B-7D respectively show the resultant states.More specifically, Fig. 7B shows the state of a picture element to which the voltage signal of 3V0-V3 has been applied; Fig. 7C shows the state of a picture element to which 3Vo-V2 has been applied; and Fig. 7D shows the state of a picture element to which 3Vo-V1 has been applied. As described before, in the white domain region 72, the ferroelectric liquid crystal is oriented to the first orientation state, and in the black domain region 71, the ferroelectric liquid crystal is oriented to the second orientation state. Both the orientation states are retained until the clearing signal (-3V,) is applied in the subsequent frame, so that a gradation display is effected in one frame period.Incidentally, Figs. 7A to 7E represent sketches obtained by microscopic observation through polarizers arranged in 90" cross nicols.

Fig. 8 shows a relationship between voltage and light transmittance at 38"C obtained with respect to a ferroelectric liquid crystal device which was prepared by providing a pair of glass plates each provided with an ITO (indium-tin-oxide) and a 1000 A-thick rubbing-treated polymide film disposed thereon, securing the glass plates to each other with a spacing of 3.8 ,am therebetween to form a cell and injecting the composition shown below: Liquid crystal composition

80 wt.% 20 wt.% The measurement was made by using pulses having a duration of 1 m.sec. and various pulse heights. According to Fig. 8, it is shown that the inversion initiation voltage 81 (3Vo-V4) was 5 V and the complete inversion voltage 82 (3to) was 15 V.When an intermediate voltage (3Vo-V3) of 9V was applied to a picture element, a domain distribution as shown in Fig. 7B was observed; at an intermediate voltage (3Vo-V2) of 10.2 V, a state as shown in Fig. 7C was observed; and at an intermediate voltage (3Vo-V1) of 11 V, a state as shown in Fig. 7D was observed. The marks shown in Fig. 8 represent measured values.

Fig. 9 shows time serial waveforms applied to picture elements A and B shown in Fig. 3. In this instance, the picture element A was brought to a light transmission state in the fourth gradation state shown in Fig. 7B, and the picture element B was brought to a light transmission state in the second gradation state shown in Fig. 7D.

Fig. 10 shows a driving circuit arrangement an liquid crystal display apparatus used in the present invention, wherein DSP denotes a liquid crystal display unit including picture elements A,1, A,2, ..., Ass. As denotes an input analog signal. The LC apparatus further includes a ^y- conversion circuit 101, an offset circuit 102, an analog shift register 103, a gate 104, a frequency divider 105, frequency demultipliers 106 and 107, a counter 108, and a monostable multivibrator 109. The y-conversion circuit 101 operates to control the input analog signal to the value of the voltage shown in Fig. 8. Further, the offset circuit 102 functions to add a V0 signal shown in Fig. 4C to the input analog signal subjected to y-conversion.

In a preferred embodiment of the present invention, an auxiliary signal having a polarity opposite to that of a writing signal is preferably applied as shown in Fig. 11 in order to prevent a picture element written in, e.g., "black" from being inverted to "white" when a signal for writing "white" as shown in Fig. 4 is continually applied to the picture element. In Figs.

11A-11D, a phase At corresponds to a writing period, and a phase t1 corresponds to an auxiliary signal application period. The waveform at phase At shown in Fig. 11A is a scanning selection signal which is the same as the one shown in Fig. 4A. The waveform shown in Fig.

11B is a scanning non-selection signal; the waveform in Fig. 11C is a signal for writing "black"; and Fig, 11D is a signal for retaining "white". Fig. 12 shows time serial waveforms applied to picture elements when unit driving signals shown in Figs. 11A-11D are used.

According to another preferred embodiment of the present invention, a liquid crystal device comprising a ferroelectric liquid crystal under bistability condition may be driven by applying an electric signal which comprises a first phase t1 wherein a voltage providing an electric field capable of orienting the liquid crystal to the first stable state is applied, and a second phase t2 wherein an inversion voltage for re-orienting the liquid crystal to the second stable state depending on an electric signal applied to a data line.

Figs. 15A and 15B show waveforms of a scanning selection signal and a scanning nonselection signal, respectively, applied to scanning lines, Figs. 15C and 15D show an inversion signal and a holding or retaining signal, respectively. In Figs. 15A-15D, the abscissas and the ordinates represent, time and voltage, respectively. For instance, when a motion picture is disposed, the scanning electrodes are sequentially and periodically selected.If a threshold voltage for giving a first stable state of the liquid crystal having bistability is referred to as Vth, and a threshold voltage for giving a second state thereof as VIb2, respectively, an electric signal applied to the selected scanning lines is an alternating voltage showingX2VO at a phase (time) t and -V, at a phase (time) t2, as shown in Fig. 15A. On the other hand, the other scanning lines to which a nonselection signal is applied are grounded as shown in Fig. 15B. Accordingly, the electric signals appearing thereon show zero volt.Further, the inversion voltage signal applied to the selected data lines is O at phase t and V0 at phase t2 shown in Fig. 15C, while the retaining voltage signal applied to the other data lines are 0 as shown in Fig. 15D. In this instance, the voltage V0 is set to a desired value which satisfies Vo < Vth1 < 2Vo and 2VO < V?hl < VO.

Voltage waveforms applied to respective picture elements are shown in Figs. 16A-16D. Namely, as seen from Fig. 16A, all the picture elements on a selected scanning line are once uniformly oriented to one optically stable state (first stable state), because a voltage -2V exceeding the threshold voltage V,2 is applied at a first phase t. Among these, picture elements to which the inversion signal with information is applied are transformed into the other optically stable state (second stable state), because a voltage 2Vo exceeding the threshold voltage Vthl is applied thereto at a second phase t2.Further, picture elements on the same scanning line to which the retaining signal with no information is applied remain in the above mentioned one optically stable state, because the application voltage at the second phase t2 is V0 not exceeding the threshold voltage Vthl.

On the other hand, a voltage applied to all the picture elements on the scanning lines to which a scanning nonselection signal is applied is V0 or 0, each not exceeding the threshold voltage.

Accordingly, the LC molecules at the picture elements on the scanning lines to which the scanning nonselection signal retain their orientation states corresponding to signal states produced when the picture elements have been last scanned. Thus, when a certain scanning line is selected, the picture elements on the scanning line are once uniformly oriented to one optically stable state at a first phase t, and at a second phase t2, one line of signals are written. The resultant signal states are retained until the scanning line is again selected after one frame operation is completed. Accordingly, even if the number of scanning lines increases, the duty ratio does not substantially change, thus avoiding possibility of lowering in contrast, occurrence of crosstalk, etc.

In this instance, the voltage V0 and the duration of phase T (=tt+t2) may ordinarily be selected in the ranges of 3 to 70 volts and 0.1 .sex. to 2 m.sec., respectively, while they also depend on the particular liquid crystal material and cell thickness used.

Figs. 17A-17E show gradation signals superposed with inversion voltage signals applied to data lines at phase t2, and Figs. 18A-18E show inversion signals (voltage signals for inverting "white" to "black") superposed with the above mentioned gradation signals at phase t2. Fig.

17A shows a voltage waveform (0) of a first gradation signal, whereby a complete inversion voltage of 2Vo shown in Fig. 18A is applied to a picture element. At the picture element to which the complete inversion voltage 2Vo is applied, the whole picture element is inverted from the white state to the black state as shown in Fig. 7E. Fig. 1 7E shows a voltage waveform (V4) of a fifth voltage signal, whereby an inversion initiation voltage (2Vo-V4) as shown in Fig. 18E is applied to a picture element. At the picture element to which the inversion initiation voltage (2Vo-V4) is applied, a threshold state which is a state just before a domain 71 in the black state is formed in the white state as shown in Fig. 7A; is formed. Figs. 17B, 17C and 17D show a second gradation signal (Vl), a third gradation signal (V2) and a fourth gradation signal (V3), respectively, set to a satisfy the relationship of O < lVll < lV2l < lV3l < lV4l. As a result, by applying the resultant voltages of 3Vo-V" 3Vo-V2 and 3Vo-V3, which are set to above the inversion initiation voltage (2Vo-V4) and below the complete inversion voltage (2to), the ratio of the domain region 71 inverted to "black" to the white domain region 72 can be controlled depending on the magnitude of the voltages.

Fig. 19 shows time serial waveforms applied to picture elements A and B shown in Fig. 3. In this instance, the picture element A is brought to a light transmission state in the fourth gradation state shown in Fig. 7B, and the picture element B is brought to a light transmission state in the second gradation state shown in Fig. 7D.

Figs. 20 and 21 show another embodiment of modification. The difference from the embodiment shown in Figs. 15 and 16 is that the voltage of the scanning selection signal at phase t1 is made half, i.e., VO, and corresponding thereto, -V, is added to all the information signals at phase t1. According to this embodiment, there can be attained an advantage that the maximum voltage value of the signals applied to the respective electrodes are made half compared with that required in the embodiment shown in Fig. 15.

More specifically, Fig. 20A shows the voltage waveform of a scanning selection signal applied to a selected scanning line, while a scanning nonselection signal in the grounded state as shown in Fig. 20B is applied to nonselected scanning lines. Fig. 20C shows the voltage waveform of an inversion signal applied to selected data lines, and Fig. 20D shows the voltage waveform of a retaining signal applied to nonselected data lines. Figs. 21A-21D show voltage waveforms applied to respective picture elements. More specifically, Fig. 21 A shows a voltage waveform applied to inverted picture elements; Fig. 21B a waveform to retaining picture elements; and Figs. 21C and 21D waveforms applied to picture elements on a line to which a scanning nonselection signal is applied.

According to another embodiment of the present invention, there may be used a driving method comprising a first phase wherein a ferroelectric liquid crystal under bistability condition at picture elements on an N-th scanning line is oriented to one stable state, a second phase wherein a writing signal is applied to a data line in synchronism with the scanning signal applied to the N-th scanning line; and a third phase wherein the ferroelectric liquid crystal under bistability condition at picture elements on an N-l-th scanning line is oriented to one stable state.

In a preferred embodiment, an optical modulation device comprising scanning lines selected sequentially and periodically depending on scanning signals, data lines disposed opposite to the scanning lines and selected based on prescribed information signals, and a ferroelectric liquid crystal showing bistability with respect to an electric field disposed between the scanning lines and data lines, may be driven by an electric signal comprising a first phase t, wherein a voltage (clearing signal) providing one direction of electric field capable of orienting the ferroelectric liquid crystal to the first stable state regardless of voltage signals applied to data lines is applied, and a second phase wherein an inversion voltage signal capable of orienting the ferroelectric liquid crystal to the second stable state depending on electric signals applied to data lines is applied.

Further preferably, there may be used an electric signal having a line-clear phase t,. a line-clear phase t2, and an auxiliary signal phase t3, wherein in the phase t3, a signal having a voltage polarity opposite to that of a signal applied to a data line in the phase t2 based on given data, is applied.

Figs. 22A and 22B show a scanning selection signal applied to a selected scanning line and a scanning nonselection signal applied to the other scanning lines (nonselected scanning lines), respectively. Figs. 22C and 22D respectively show inversion voltage signals applied to selected data lines (assumed to present "black"). Among these, Fig. 22C shows a signal to be applied in a case where the previous signal is one providing "black" (inversion voltage signal) and Fig. 22D shows a signal to be applied in case where the previous signal is one providing "white" (retaining signal).

Further, Figs. 22E and 22F respectively show retaining signals applied to nonselected scanning lines (assumed to present "white"). Among these, Fig. 22E shows a signal to be applied in a case where the previous signal is one providing "black" and Fig. 22F shows a signal to be applied in a case where the previous signal is one providing "white". In the figure, a phase t1 is for orienting all the picture elements on a scanning line uniformly to "white", and a phase t2 is for writing information signals. In this example, tl=t2=At.

In this driving method, a gradation may be displayed by superposing the above mentioned inversion voltage signal at the writing phase t2 with the gradation signals shown in Figs.

17A-17D.

Fig. 23 shows time serial waveforms applied to picture elements A and B shown in Fig. 3. In this instance, the picture element A is brought to a light transmission state in the fourth gradation state shown in Fig. 7B, and the picture element B is brought to a light transmission state in the second gradation state shown in Fig. 7D. In Fig. 23, S1-S5 denote signals applied to scanning lines; 11 and 13 denote signals applied to data lines 11 and i3, respectively; and A and B denote voltage waveforms applied to picture elements A and B, respectively, shown in Fig. 3.

When it is assumed for a liquid crystal cell showing bistability that a threshold voltage for providing a first stable state (assumed to present "white") at a duration At be Vth2, and a threshold voltage for providing a second stable state (assumed to present "black") at a duration At be Vthl, the value of V0 is set to satisfy: VO < Vhl < 2VO, and 2VO < Vlh2 < VO.

As seen from Fig. 23, all the picture elements on a scanning line are once uniformly cleared in "white", and then they are selectively written in "black" or "white" based on given data. At this time, at the picture elements to be written in "black", the inversion of "white""black" is caused to write in data. At this phase (time) for writing data on a scanning line, all the picture elements on the subsequent line are cleared in "white". As a result, the writing of the whole picture by one frame scanning can be effected at a high speed.

Figs. 24A-24F show driving signal waveforms used in another embodiment of the present invention.

Figs. 24A and 24B show a scanning selection signal applied to a selected scanning line and a scanning nonselection signal applied to a non-selected scanning line, respectively. Figs. 24C-24F show information signals applied to data lines. Figs. 24C and 24E correspond to cases where the previous signals have provided "black", and Figs. 24D and 24F correspond to cases where the previous signals have provided "white". At corresponding picture elements, information signals (inversion signals) showing V0 at phase t2 as shown in Figs. 24C and 24D are applied for providing "black", while information signals (retaining signals) showing -V, at phase t2 as shown in Figs. 24E and 24F are applied for providing "white".

Incidentally, 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 a prescribed (first) 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 the threshold value 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 a reverse (second) one, whereby correct display or modulation of information cannot be accomplished. We have recognized that the liability of such switching or reversal or 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 uniaxial 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.

Accordingly, in a preferred mode of the driving method according to the present invention, there is provided an auxiliary signal phase t3 for preventing a weak electric field in one direction from being continually applied to certain picture elements. A specific embodiment of this mode is explained with reference to Figs. 25 and 26.

Figs. 25A and 25B show a scanning selection signal applied to a selected scanning line and a scanning nonselection signal applied to a nonselected scanning line, respectively. To the data lines are applied signals as shown in Figs. 25C-25F which comprise a signal at phase t2 having a polarity opposite to that applied in a preceding phase t2. Figs. 25C and 25D correspond to "black", while Figs. 25E and 25F correspond to "white". Further, Figs. 25C and 25E correspond to cases wherein the previous signals have provided "black", and Figs. 25D and 25F correspond to cases where the previous signals have provied "white".For example, in a case where a pattern as shown in Fig. 3 is intended to be displayed by using a driving mode with no t2 phase, a picture element A is written in "black" on scanning of the scanning line S1 but is thereafter liable to be inverted into "white", as an electric signal of --V, is continually applied to the data line I, and the voltage is continually applied to the picture element A. If an auxiliary signal phase t3 is provided, however, such a liability of crosstalk is removed as will be understood from the time serial signals shown in Fig. 26.

Fig. 26 shows driving signals applied to respective lines and voltage waveforms applied to picture elements in order to obtain a display as shown in Fig. 3. More specifically, in Fig. 26, Sl-Ss denote signals applied to corresponding scanning lines; Ii and 13 denote signals applied to corresponding data lines; and A and B show voltage waveforms applied to the picture elements A and B, respectively, in time series.

In a preferred embodiment of the present invention, instead of the gradation signals having different pulse heights, it is possible to use gradation signals with different pulse durations as shown in Figs. 13A-13F or gradation signals with different numbers of pulses as shown in Figs.

14A-14E.

According to the present invention, picture images with gradation may be formed or displayed.

Further, in the present invention, by providing a liquid crystal device having bistability with color filters, e.g., in the form of stripes or mosaic, at respective picture elements, and driving the liquid crystal device in the manner as described above, color picture images with gradation may be displayed. As a result, the present invention may suitably be applied to a liquid crystal television system displaying a monochromatic or color picture image, particularly a LC portable color television set which is much smaller and lighter in weight compared with a conventional CRT color television set.

Claims

1. A liquid crystal apparatus, comprising: a liquid crystal device comprising a plurality of picture elements comprising a ferroelectric liquid crystal showing a first stable orientation state and a second stable orientation state in response to an electric field thereto; means for applying a voltage signal capable of orienting the ferroelectric liquid crystal to the first orientation state to all or a part of the picture elements; means for applying an inversion voltage signal capable of inverting the ferroelectric liquid crystal oriented to the first orientation state to the second orientation state to selected picture elements; and means for variably controlling the waveform of the inversion voltage signal.

2. A liquid crystal apparatus according to Claim 1, wherein the inversion voltage is set to a value above the absolute value of an inversion initiation voltage capable of initiating the inversion from the first orientation state to the second orientation state.

3. A liquid crystal apparatus according to Claim 1, wherein the inversion voltage is set to a value above the absolute value of an inversion initiation voltage capable of initiating the inversion from the first orientation state to the second orientation state and below the absolute value of a complete inversion voltage capable of inversion to the second orientation state.

4. A liquid crystal apparatus according to Claim 1, wherein said ferroelectric liquid crystal is a liquid crystal in a chiral smectic phase.

5. A liquid crystal apparatus according to Claim 4, wherein said chiral smectic phase is chiral smectic C phase, H phase, I phase, F phase or G phase.

6. A liquid crystal apparatus according to Claim 1, which comprises means for applying an auxiliary signal to the picture elements.

7. A driving method for a liquid crystal device comprising a plurality of picture elements comprising a ferroelectric liquid crystal showing a first stable orientation state and a second stable orientation state in response to an electric field applied thereto and arranged in a plurality of rows and a plurality of columns to provide a picture, comprising:: a first stage of applying a voltage signal capable of orienting the ferroelectric liquid crystal to the first orientation state to all or a part of the picture elements thereby to clear the picture elements, and a second stage of applying an inversion voltage signal for inverting the ferroelectric liquid crystal oriented to the first orientation state to selected picture elements to the second orientation state to selected picture elements-to write in the picture elements, said inversion voltage signal having a voltage waveform controlled depending on given gradation data.

8. A driving method according to Claim 7, wherein the voltage signal in the first signal is applied simultaneously to said all of a part of the picture elements, and the inversion voltage signal in the second stage is applied row by row to the picture elements.

9. A driving method according to Claim 7, wherein the voltage signal in the first stage is applied row by row to said all or a part of the picture elements, and the inversion voltage signal is applied row by row to the picture elements.

10. A driving method according to Claim 7, wherein the inversion voltage is set to a value above the absolute value of an inversion initiation voltage capable of initiating the inversion from the first orientation state to the second orientation state.

11. A driving method according to Claim 7, wherein the inversion voltage is set to a value above the absolute value of an inversion initiation voltage capable of initiating the inversion from the first orientation state to the second orientation state and below the absolute value of a complete inversion voltage capable of inversion to the second orientation state.

12. A driving method according to Claim 7, which comprises a third stage of applying an auxiliary signal to the picture elements.

13. A driving method according to Claim 12, wherein said auxiliary signal has a voltage below the absolute value of the inversion initiation voltage.

14. A driving method according to Claim 7, wherein the inversion voltage signal has a pulse, of which the height is controlled depending on the given gradation data.

15. A driving method according to Claim 7, wherein the inversion voltage signal comprises a pulse of which the duration is controlled depending on the given gradation data.

16. A driving method according to Claim 7, wherein the inversion voltage signal comprises a number of pulses which is controlled depending on the given gradation data.

17. A liquid crystal apparatus, comprising: a liquid crystal device comprising a plurality of picture elements comprising a ferroelectric liquid crystal showing a first stable orientation state and a second stable orientation state and arranged in a plurality of rows and a plurality of columns to provide a picture; means for applying a voltage signal capable of orienting the ferroelectric liquid crystal to the first orientation state row by row to the picture elements; means for applying an inversion voltage signal capable of inverting the ferroelectric liquid crystal oriented to the first orientation state to the second orientation state row by row to selected picture elements among the picture elements at which the ferroelectric liquid crystal has been oriented to the first orientation state; and means for variably controlling the waveform of the inversion voltage signal.

18. A liquid crystal apparatus according to Claim 17, wherein the inversion voltage is set to a value above the absolute value of an inversion intiation voltage capable of initiating the inversion from the first orientation state to the second orientation state.

19. A liquid crystal apparatus according to Claim 17, wherein the inversion voltage is set to a value above the absolute value of an inversion intiation voltage capable of initiating the inversion from the first orientation state to the second orientation state and below the absolute value of a complete inversion voltage capable of inversion to the second orientation state.

20. A liquid crystal apparatus according to Claim 17, wherein said ferroelectric liquid crystal is a liquid crystal in a chiral smectic phase.

21. A liquid crystal apparatus according to Claim 20, wherein said chiral smectic phase is chiral smectic C phase, H phase, I phase, F phase or G phase.

22. A liquid crystal apparatus according to Claim 17, which comprises means for applying an auxiliary signal to the picture element.

23. A driving method for a liquid crystal device comprising a plurality of picture elements comprising a ferroelectric liquid crystal showing a first stable orientation state and a second stable orientation state in response to an electric field applied thereto and arranged in a plurality of rows and a plurality of columns to provide a picture, comprising: a first phase of applying a clear voltage signal for orienting the ferroelectric liquid crystal to the first orientation state to picture elements on a row to clear the picture elements, and a second phase of applying to selected picture elements among the cleared picture elements; said clear voltage signal and inversion voltage signal being applied row by row and sequentially, said inversion voltage signal having a voltage waveform variably controlled depending on given gradation data.

24. A driving method according to Claim 23, wherein said clear voltage signal is applied to a row (N+ 1 -th row) subsequent to a writing row (N-th row), and said inversion voltage signal is applied to the writing row (N-th row) which has been cleared in advance.

25. A method of driving a liquid crystal display having a plurality of bistable pixels, in which at least some of the pixels are set or maintained in one of the states and then some of those pixels are set in the other state with a variable setting strength.

26. A liquid crystal apparatus adapted to perform the method of claim 25.

27. A liquid crystal apparatus substantially as described in the description with reference to the drawings.

28. A method of driving a liquid crystal display substantially as described in the description with reference to the drawings.