JP2502292B2 - Driving method of optical modulator - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for driving an optical modulation element, and more particularly to a method for driving a ferroelectric liquid crystal element having at least two stable states.

[Description of Prior Art]

2. Description of the Related Art Conventionally, a liquid crystal display element in which a scanning electrode group and a signal electrode group are configured in a matrix, a liquid crystal compound is filled between the electrodes to form a large number of pixels, and an image or information is displayed is well known. . As a driving method of this display element, there is a time-division drive in which an address signal is sequentially and selectively applied to the scanning electrode group and a predetermined information signal is selectively applied to the signal electrode group in parallel in synchronization with the address signal. Has been adopted.

Most of these were put to practical use, for example, "Applied Physics Letters"("AppliedPhysics").
Letters ") 1971, 18 (4), pages 127-128," Voltage Day Pendant Optical Activity Of "written by M. Schadt and W. Helfrich.・ A twisted nematic / liquid crystal (“Voltage Dependent
Optical Activity of a Twisted Nematic Liquid Cryst
It was a TN (twisted nematic) type liquid crystal shown in al ").

In recent years, as an improved type of the conventional liquid crystal element, the use of a liquid crystal element having bistability has been disclosed in both Clark and Lagerwall by JP-A-56-107216.
Japanese Patent Publication No. 4367924 and the like. As the bistable liquid crystal, a ferroelectric liquid crystal having a chiral smectic C phase (SmC ^* ) or an H phase (SmH ^* ) is generally used, and in these states, a liquid crystal is generated in response to an applied electric field. It has either the first optical stable state or the second optical stable state, and has the property of maintaining that state when an electric field is not applied, that is, stability, and has a quick response to changes in the electric field. Therefore, it is expected to be widely used in the field of high-speed and memory type display devices and the like.

[Problems to be solved by the invention]

However, when the number of display pixels is extremely large and high speed driving is required, a problem occurs. That is,
A threshold voltage for giving a first stable state in a ferroelectric liquid crystal cell having bistability with respect to a predetermined voltage application time is −
V _th1 and the threshold voltage for giving the second stable state +
If V _th2 is set, the display state (for example, white state) written in the pixel is changed to another display state (for example, black state) when the voltage is continuously applied for a long time without _exceeding these threshold voltages. ) May be reversed. FIG. 5 shows the threshold characteristics of a bistable ferroelectric liquid crystal cell.

Figure 5 shows DOBAMBC (52 in the figure) as a ferroelectric liquid crystal.
And HOBACPC (51 in the figure) are used to plot the application time dependence of the threshold voltage (V _th ) required for switching.

As is clear from FIG. 5, it is understood that the threshold value V _th has an application time dependency, and the shorter the application time, the steeper the gradient. From this, when applied to an element that has a large number of scanning lines and is driven at high speed, for example, even if a certain pixel is switched to a bright state at the time of scanning, an information signal of V _th or less is always obtained after the next scanning. It can be understood that if the voltage is continuously applied, the pixel may be inverted to the dark state in the middle of the scanning of one screen. Alternatively, when driven for a long time, if the DC component becomes large as a whole, the same problem as described above may occur.

It is an object of the present invention to provide a driving method of a conventional optical modulation element as described above, particularly a novel optical modulation element which solves the problems in the liquid crystal display element or the liquid crystal optical shutter.

Another object of the present invention is to provide a driving method of an optical modulation element suitable for gradation expression.

[Means for Solving the Problems] and [Operation] That is, the present invention provides a method for driving an optical modulation element in which an optical modulation substance is arranged between a pair of substrates having a scanning electrode group and a signal electrode group, The selected scanning is performed by continuously selecting the scanning electrodes and supplying a signal voltage according to the information to be displayed to the signal electrode group during the continuous scanning selection period including the first to fourth periods. After the pixel on the electrode is once erased to a predetermined optical state, the optical state of the pixel is defined by either maintaining the erased state or changing the erased state. The erase voltage of the first polarity is applied to the pixel during the period, and the write voltage of the second polarity that changes according to the information to be displayed is applied to the pixel during the fourth period, and the erase voltage is applied during the first period. The write voltage has a polarity different from that of the write voltage. A first auxiliary voltage that is less than or equal to a threshold voltage that changes according to the voltage is applied to the pixel, and a second auxiliary voltage that has a polarity different from that of the erase voltage and has a constant voltage that is less than or equal to the threshold voltage is applied during the second period. Supplying a scan selection signal voltage and a signal voltage, each of which has a DC component of substantially zero, to the pixel, respectively, to the scan electrode and the signal electrode,
2 when each of the first to fourth periods is a unit period
A voltage waveform in which voltages of the same polarity do not continue for more than a unit period,
The feature is that it is applied to each pixel. Also, the present invention
In a method of driving an optical modulation element in which an optical modulation substance is arranged between a pair of substrates having a scanning electrode group and a signal electrode group, the scanning electrodes are continuously selected and a signal voltage corresponding to information to be displayed is set. By supplying to the signal electrode group, whether to maintain or change the erased state of the pixel on the selected scan electrode during the selected period after the erased period in which the pixel is once erased to a predetermined optical state. Any one of them includes a step of determining an optical state of the pixel, in which the erase voltage of the first polarity is applied to the pixel in the first half of the erase period, and the erase voltage is changed according to information to be displayed in the latter half of the selection period. And a first auxiliary voltage having a polarity different from that of the write voltage in the first half of the selection period, the write voltage having a second polarity that changes according to the write voltage and is equal to or less than a threshold voltage that changes according to the write voltage. Is applied to the pixel and the erase period In the latter half of the above, an erase selection signal voltage and a signal voltage having DC components of substantially zero for applying to the pixel a second auxiliary voltage having a polarity different from the erase voltage and having a constant voltage equal to or lower than a threshold voltage. A voltage waveform is supplied to each of the scan electrodes and the signal electrodes, and a voltage waveform in which voltages of the same polarity are not continuous for two or more unit periods when the first half or the second half is a unit period is applied to each pixel. According to the present invention, a voltage of zero DC component is applied to the scan electrode and the signal electrode, and the function of the voltage pulse in each period of the four unit periods (4 phases) is determined so that two or more unit periods (2 phases) are obtained. Voltages of the same polarity were not continuously applied to each pixel. In this way, so-called crosstalk in which the optical state of the pixel is inverted unintentionally can be prevented. Moreover, since the continuous application of the same polarity voltage can be prevented without adopting the idle period in which no scan electrode is scanned, the frame frequency for rewriting one screen does not decrease.
Another feature is that the screen does not flicker because it does not reverse to the opposite optical state immediately before erasing.

〔Example〕

The optical modulation substance used in the driving method of the present invention has at least two stable states, and particularly takes one of the first optical stable state and the second optical stable state depending on the applied electric field. That is, a substance having a bistable state with respect to an electric field, particularly a liquid crystal having such a property is used.

As a liquid crystal having bistability which can be used in the driving method of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable, and a chiral smectic C among them is most preferable.
Phase (SmC ^* ) and H-phase (SmH ^* ) liquid crystals are suitable. This ferroelectric liquid crystal is described in "Le Journal de
"Fujitik Lutheran"("Le Journal de Physioce l
etter ") Volume 36 (L-69), 1975" Ferroelectric Liquid Crystals "(" Ferroelectric Li
quid Crystals ");" Applied Physics Letters "Vol. 36 (No. 11)
1980's Submicron Second By Stable
Electro Optics Switching In Liquid Crystal "(" Submicro Second Bistable Elect
rooptic Switching in Liquid Crystals ”);“ Solid State Physics 16 (141) 1981 “Liquid Crystal” and the like, and the ferroelectric liquid crystals disclosed therein can be used in the present invention.

More specifically, examples of the ferroelectric liquid crystal compound used in the method of the present invention include desiloxybenzylidene-P'-
Amino-2-methylbutyl cinnamate (DOBAMBC),
Hexyloxybenzylidene-P'-amino-2-chloropropyl cinnamate (HOBACPC) and 4-o-
(2-methyl) -butyl resorcylidene-4'-octylaniline (MBRA8) and the like can be mentioned.

When a device is formed by using these materials, the liquid crystal compound is kept in a temperature state such that the liquid crystal compound is in the SmC ^* phase or the SmH ^* phase. Therefore, if necessary, the device is provided with a copper block in which a heater is embedded. Can be supported.

Further, in the present invention, it is also possible to use a ferroelectric liquid crystal which appears in the chiral smectic F phase, I phase, J phase, G phase or K phase in addition to the above-described SmC ^* and SmH ^* .

FIG. 6 schematically illustrates an example of a ferroelectric liquid crystal cell. 21a and 21b are substrates (glass plates) coated with transparent electrodes such as In ₂ O ₃ , SnO ₂ and ITO (Indium-Thein-Oxide), between which the liquid crystal molecular layer 22 is perpendicular to the glass surface. The liquid crystal of the SmC ^* phase oriented like this is enclosed. A thick line 23 represents a liquid crystal molecule, and the liquid crystal molecule 23 has a dipole moment (P⊥) 14 in a direction orthogonal to the molecule. When a voltage above a certain threshold is applied between the electrodes on the substrates 21a and 21b, the helical structure of the liquid crystal molecules 23 is unraveled, and all the dipole moments (P⊥) 24 are oriented in the electric field direction. Can be changed. The liquid crystal molecules 23 have an elongated shape, and exhibit refractive index anisotropy in the major axis direction and the minor axis direction thereof. Therefore, for example, if polarizers arranged in a crossed Nicols position above and below a glass surface are placed. It is easily understood that the liquid crystal optical modulation element has optical characteristics that change depending on the polarity of voltage application. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1 μ), the helical structure of the liquid crystal molecules is unraveled even when no electric field is applied as shown in FIG.
The dipole moment Pa or Pb is either in the upward (34a) or downward (34b) state. When electric fields Ea or Eb having different polarities equal to or higher than a certain threshold value are applied to such a cell for a predetermined time as shown in FIG. 7, the dipole moment is directed upward with respect to the electric field vector of the electric fields Ea or Eb.
a or downward 34b, and the liquid crystal molecules are aligned in either the first stable state 33a or the second stable state 33b accordingly.

There are two advantages of using such a ferroelectric liquid crystal as the optical modulation element. First, the response speed is extremely fast, and second, the orientation of the liquid crystal molecules has a bistable state. Explaining the second point with reference to FIG. 7, for example,
When the electric field Ea is applied, the liquid crystal molecules are aligned in the first stable state 33a, but this state is stable even when the electric field is cut off. or,
When the electric field Eb in the opposite direction is applied, the liquid crystal molecules are oriented in the second stable state 33b to change the orientation of the molecules, but they remain in this state even when the electric field is turned off. Electric field
As long as Ea does not exceed a certain threshold, each alignment state is also maintained. With such a fast response speed,
In order to effectively realize the bistability, it is preferable that the cell is as thin as possible, and generally 0.5 μ to 20 μ, particularly 1 μ to 5 μ is suitable.

A preferred specific example of the driving method of the present invention is shown by the following figures.

FIG. 4 is a schematic view of a typical ferroelectric liquid crystal cell 41 having a matrix pixel structure in which a bistable ferroelectric liquid crystal is sandwiched between a scan electrode group 42 and a signal electrode group 43. The present invention is applicable to multi-value or analog gradation display, but for simplification of description, a case of displaying three values of white, one intermediate level (gray), and black is taken as an example. Show. In FIG. 4, it is assumed that the pixel A indicated by cross-hatching corresponds to “black”, the pixel B indicated by half-hatching corresponds to the intermediate level, and the other pixels C correspond to “white”.

FIG. 1 shows an example of specific drive waveforms when an image erasing step and a writing step are used for each line,
The image after writing corresponds to FIG.

FIG. 5 (a) shows each scan electrode S _S , S _NS and each signal electrode I.
_S, shows the I _HS, the voltage applied to the pixel liquid crystal that is sandwiched voltage waveform and each of the scanning electrodes and the signal electrodes is applied to the I _NS. Here, the horizontal axis represents time and the vertical axis represents potential (voltage).

Here, S _S is a line in which image information is written, that is, a drive waveform applied to the selected scan electrode, and S _NS is a line in which image information is not written at that time, that is, a drive waveform applied to a non-selected scan electrode. is there. I _S is a drive waveform for writing “black” between the intersection with the selected line, I _HS is an intermediate level, and I _NS is a drive waveform for writing “white”. .

At this time, the voltages applied to the liquid crystals forming the pixels are I _S −S _S , I _HS −S _S , I _NS −S _S , I _S −S _NS , I _HS −S _NS, and I _HS −S _NS.
NS- S _NS is displayed.

Here, the inversion threshold of the bistable ferroelectric liquid crystal used is V _th
Then, the drive voltage V _o is selected so that | ± 2 V _o | <| V _th | <| ± 3 V _o | Here, normally, there may be a slight difference in V _th between the side and the side due to a difference in the alignment treatment applied to the liquid crystal cell, but in this case, the drive potentials on the side and the side of each drive waveform are slightly corrected. And so on,
Here, | + V _th | = | −V _th | for convenience of explanation.

In the above case, when the absolute value of the voltage applied to each pixel is 2V _o or less, liquid crystal molecule inversion does not occur, and when it is 3V _o or more, inversion occurs and the absolute value The larger the value, the stronger the inversion.

 Here, each waveform will be described.

The scan selection signal S _S applied to the selected scan electrode is
One line writing is divided into four phases, and the pixels on the scanning line are displayed in one display state (white) in the third phase.
By doing so, the previous writing display state is erased, and in order to perform pixel writing in accordance with the signal applied to the signal electrode in the fourth phase, the third phase is set in each.
2V _o, the voltage waveform of 2V _o the fourth phase is applied to the scanning electrodes. In the first phase and the second phase, they are maintained at almost zero (reference potential). This reference potential is at the same level as the voltage level applied to the scan electrodes during non-selective scanning.

On the other hand, the unselected scan electrodes S _NS are fixed to the reference potential (here, 0 _V ).

Next, the potential waveform applied to the signal electrode is synchronized with the scan selection signal and the erase signal is generated in the third phase.
2V _o is applied, and a voltage of 4V _o is applied to the pixel on the scan electrode selected in this phase, which exceeds the inversion threshold V _{th of the} liquid crystal, so all of this line is erased (white). Flip to. Then in the fourth phase,
The scanning selection signal S _S and the information signal I _HS , the selection information signal I _S, or the non-selection information signal corresponding to the gradation are applied. At this time, the potential for making the pixel “black” is 2V _o , the potential for making an intermediate level (gray) is V _o , and the potential for keeping it “white” is zero (reference potential). By doing this, in the fourth phase, the voltages applied to the pixels on this line are 4V _o , 3V _o , and 2V _o , respectively, and write "black", intermediate level, and "white" to the pixels, respectively. become.

Next, the first phase to be supplementarily applied, and the second phase
The second phase will be described. First, in the second phase, the voltage applied to all pixels was set to 2V _o , which is V _th or less, regardless of the write signal. A voltage signal corresponding to the pixel write signal in the fourth phase is applied as the first phase. It is preferable that this voltage has a polarity opposite to that of the voltage applied to the signal electrode in the fourth phase or is zero (reference potential), and that the absolute value of the potential is substantially equal to that of the potential of the fourth phase. That is, the voltage 2V _o applied to the pixel in the fourth phase, V _o, to zero, each zero, V _o, is applied to 2V _o. By doing so, the voltage applied to the pixel on the scan electrode selected in the first phase becomes 2V _o , V _o , and zero. That is, all the voltages applied here are below the threshold voltage V _th ,
Moreover, since the voltage is in the direction of turning the pixel white (the polarity opposite to the voltage applied in the fourth phase), the pixel does not invert in the “black” direction here, and the pixel is not reversed in the third phase. Thus, the image does not flicker before the voltage for whitening the pixels on the scan line is applied.

In the second phase, the voltage applied to the pixel on the selected scan electrode is below the threshold and is constant (-2V _o ).
Is.

Further, in FIG. 1 (a), unselected scan electrodes
The voltage applied between S _NS and each of the signal electrodes I _S , I _HS , and I _NS is as shown in the figure.

The drive waveforms shown in FIG.
FIG. 1 (b) shows a state in which the voltage is applied to 42 and the signal electrode group 43. As a typical example of the voltage applied to the pixel,
It cited _{I 1 -S 1, I 2 -S} 2 and S ₁ -S _3. The image of the fourth illustrated example is written in one frame by the waveform shown in FIG. 1 (b).

The mechanism of switching by the electric field of the ferroelectric liquid crystal in the bistable state is not necessarily clear microscopically, but generally after switching to a predetermined stable state with a strong electric field for a predetermined time, no electric field is generated. When left unapplied, it is possible to maintain that state almost semi-permanently, but a weak electric field that does not cause switching for a predetermined time (in the example described above, a voltage below V _th However, if an electric field of opposite polarity is applied for a long time, the orientation state will be reversed again to the opposite stable state, and as a result, correct information display and modulation cannot be achieved. Phenomena can occur. The inventors of the present invention have found that the ease of occurrence of the transition reversal phenomenon (a kind of crosstalk) of the alignment state due to the application of such a weak electric field for a long time is
We have recognized that it is affected by roughness and liquid crystal material, but we have not yet grasped it quantitatively. However, it was confirmed that uniaxial substrate treatment for aligning liquid crystal molecules such as rubbing or oblique vapor deposition of SiO 2 tends to increase the inversion phenomenon. In particular, it was confirmed that the tendency was stronger when the temperature was high than when the temperature was low.

In any case, it is preferable to avoid applying an electric field in a constant direction for a long time in order to achieve correct information display and modulation.

In the present invention, the above problems are solved by preventing voltages of the same polarity from being continuously applied in two or more phases.

That is, in FIG. 1, the range of voltage selection in each phase is within the range of the same polarity including almost zero, and in the continuous phase, the opposite polarity is always selected. Since there is always a voltage of almost zero or a voltage of opposite polarity between the voltages of, the voltage of the same polarity is not continuously applied to the pixel. Moreover, since the drive waveform can be configured so that the sum of the respective voltages becomes almost zero, the above-mentioned problem of crosstalk is solved. Further, in the present invention, since the auxiliary signal given in the first phase is the voltage of the reverse polarity whose peak value is almost the same as the voltage of the fourth phase which determines the state of the pixel, the auxiliary signal is applied to the fourth phase, for example. Since it is only necessary to invert the level signal for writing the pixel to be given by an analog or digital inverter or the like, the electric circuit for driving can be simply constructed, and a complicated arithmetic circuit etc. Not particularly required.

In the above description, a ternary image was explained, but in the drive waveform applied to the signal electrode in FIG. 1, the voltage at the fourth phase is, for example, from 2V _o to zero, and corresponds to this. by providing the first select a potential split to or analog manner value to multi-level to zero from 2V _o auxiliary signal in the phase Te, multitone image or analog gradation can be obtained.

FIG. 2 (a) shows an example in which an image is sequentially written line by line after erasing the entire surface of the display screen or the entire block.

In FIG. 2, S _CL is a signal applied to the scan electrode group in order to align all the pixels on the whole surface or the entire surface of the block (blocks corresponding to a predetermined number of scan electrodes) to “white” at the time of whole surface erasing, I _{CL CL} is a signal applied to the signal electrode group, and the voltage applied to the pixels of the scan electrode group and the signal electrode group at this time is I _CL −S _CL . The inversion threshold of the bistable ferroelectric liquid crystal used here is the same as that in the above-mentioned FIG. As a result, the first voltage applied to the pixel when erasing the entire surface
By setting the voltage of the second phase to 4V _o , all the pixels are uniformly arranged in “white”. After this, in the second phase
2V _o is applied, but this is below the threshold voltage V _th and there is no pixel change here.

Next, image information is sequentially provided for each line. Zero (reference potential) at the first phase and 2V _o drive waveform at the second phase are applied to the scanning electrodes of the selected line. give. Further, the scan electrodes that are not selected are maintained at zero (reference potential) in both the first phase and the second phase. Each signal electrode has a signal that determines the pixel state at the second phase, and a signal having the opposite polarity (second signal) at the first phase.
When the second phase is zero (reference potential), a potential having zero (reference potential) and substantially the same absolute value as that of the second phase is applied. The second phase -2 V _o and specifically as a signal I _S for writing "black", + 2V _o to the first phase, as an intermediate level I _HS, -V _o to the second phase, As a signal _{INS that} holds + V _{o as} the first phase and “white” as it is, a drive waveform of substantially zero (reference potential) is given to both the second phase and the first phase. In this way, I _S −S _S and I _S are respectively applied to the pixels by the signal applied to the signal electrode.
_{HS, -S S, I NS -S} S voltage illustrated is applied, the voltage -4 V _o Komu each write "black", an intermediate level -3
A voltage −2V _o holding V _o and “white” is applied in the second phase, and the state of each pixel is determined. On the other hand, the first
The voltage applied in the second phase is opposite or almost zero to that in the second phase and is not inverted to the "black" side. The pixels corresponding to the scanning electrodes which are not further selected, each I _S, I _HS, I _NS and polarity voltage waveform in which the reversed I _S -I _NS, I
_HS -S _NS, is applied as I _NS -S _NS, only holding the written state.

Also in this case, the range of voltage selection in each phase is within the range of the same polarity including almost zero, and in the continuous phase, it is always selected to the opposite polarity side. Since there is always a voltage of almost zero or a reverse polarity, the same polarity voltage is not continuously applied to the pixel.

Further, in the waveform shown in FIG. 2, the total voltage of the entire erase and the total voltage of the write are almost zero, and the total voltage of the non-selected state is almost zero. When the element is driven over, there is no DC component, and the harmful effects of this DC component are completely eliminated.

Also in this example, similarly to the above, by changing the absolute value of the signal applied to the signal electrode in an analog manner or in a multi-value manner, it is possible to perform good gradation expression.

FIG. 3 is a further modification, in which the voltage for erasing in FIG. 2 is applied to each line, and as in the previous case, there is no flicker and a good image without crosstalk is provided.

As described above, according to the present invention described above, it is possible to stably perform gradation expression even in a matrix optical modulation element having high-density pixels.

〔The invention's effect〕

According to the present invention, crosstalk can be prevented and a good gradation display image can be formed.

[Brief description of drawings]

FIGS. 1 (a) and (b), FIGS. 2 (a) and (b), and FIGS. 3 (a) and (b) are drive waveform diagrams showing a specific drive example of the present invention. FIG. 4 is a plan view of the matrix electrode structure used in the method of the present invention. FIG. 5 is a characteristic diagram showing the voltage application time dependency of the threshold voltage. 6 and 7 are schematic perspective views of the ferroelectric liquid crystal device used in the method of the present invention.

Claims (10)

(57) [Claims] 1. A method of driving an optical modulation element comprising an optical modulation substance disposed between a pair of substrates having a scanning electrode group and a signal electrode group, wherein the scanning electrode is continuously selected and is continuous.
Through supplying the signal voltage corresponding to the information to be displayed to the signal electrode group during the scan selection period including the fourth period,
After erasing the pixel on the selected scan electrode to a predetermined optical state once, the step of determining the optical state of the pixel by either maintaining or changing the erased state is included. Applying an erase voltage of the first polarity to the pixel during the third period,
A write voltage having a second polarity that changes according to information to be displayed in the fourth period is applied to the pixel, and a polarity that is different from the write voltage and that is different from the write voltage is applied in the first period. Applied to the pixel is a first auxiliary voltage equal to or lower than the threshold voltage, and a second auxiliary voltage having a polarity different from the erase voltage and a constant voltage equal to or lower than the threshold voltage is applied to the pixel during the second period. For supplying the scanning selection signal voltage and the signal voltage, each of which has a DC component of substantially zero, to the scanning electrode and the signal electrode, and each of the first to fourth periods is defined as a unit period. A method of driving an optical modulation element, characterized in that a voltage waveform in which voltages of the same polarity do not continue for two or more unit periods when applied is applied to each pixel. 2. The method for driving an optical modulation element according to claim 1, wherein the optical modulation substance is a chiral smectic liquid crystal. 3. The method for driving an optical modulation element according to claim 1, wherein the information to be displayed is multivalued information corresponding to gradation information. 4. The method for driving an optical modulation element according to claim 1, wherein the optical modulation substance is a ferroelectric liquid crystal. 5. In a method of driving an optical modulation element in which an optical modulation substance is arranged between a pair of substrates having a scanning electrode group and a signal electrode group, the scanning electrodes are continuously selected and information to be displayed is displayed. By supplying a corresponding signal voltage to the signal electrode group,
During the selection period after the erasing period in which the pixel is once erased to a predetermined optical state, the optical state of the pixel is changed by either maintaining or changing the erasing state of the pixel on the selected scan electrode. In the step, the first polarity erase voltage is applied to the pixel in the first half of the erase period, and the second polarity write voltage is changed in accordance with the information to be displayed in the latter half of the selection period. Is applied to the pixel, and in the first half of the selection period, a first auxiliary voltage having a polarity different from that of the writing voltage and being equal to or lower than a threshold voltage that changes according to the writing voltage is applied to the pixel, In the latter half of the above, an erase selection signal voltage and a signal voltage having DC components of substantially zero for applying to the pixel a second auxiliary voltage having a polarity different from the erase voltage and having a constant voltage equal to or lower than a threshold voltage. Provided to the scanning electrode and the signal electrode, respectively. A driving method of an optical modulation element, characterized in that a voltage waveform in which voltages of the same polarity are not continuous for two or more unit periods when the first half or the second half is set as a unit period is applied to each pixel. 6. The erasing selection signal voltage comprises an erasing signal voltage supplied to a plurality of scan electrodes all at once and a scanning selection signal voltage supplied to each scan electrode in sequence.
A method for driving the optical modulator according to the item 1. 7. The method for driving an optical modulation element according to claim 5, wherein the erase selection signal voltage comprises an erase signal voltage and a scan selection signal voltage which are sequentially supplied to each scan electrode. 8. The method for driving an optical modulator according to claim 5, wherein the optical modulator is a chiral smectic liquid crystal. 9. The method of driving an optical modulation element according to claim 5, wherein the information to be displayed is multivalued information corresponding to gradation information. 10. The method of driving an optical modulation element according to claim 5, wherein the optical modulation substance is a ferroelectric liquid crystal.