JPS63135920A - Driving method for liquid crystal optical device - Google Patents

Abstract

PURPOSE:To obtain a sufficient transmission liquid quantity in a shutter-ON state and to perform a fast shutter operation, by impressing an open operation voltage which holds an optical shutter in a state where a beam of light transmits for a time longer than a selective period allocated for the control of one optical shutter. CONSTITUTION:In the driving of a liquid crystal optical device which controls the light transmission states of plural optical shutters, by impressing the open operation voltage which holds one optical shutter in the state where the beam of light transmits for the time longer than the selective period allocated for the control of one optical shutter, out of one repeating period where the light transmission state of the optical shutter is controlled in order, the ON-state of the selective period can be expanded to the extent of a part of non-selective period. In such way, it is possible to obtain a sufficient transmission light quantity in the shutter-ON state, and to perform the fast shutter operation. Also, it is possible to control the transmission light quantity by combining the waveforms of liquid crystal driving signals and to improve a temperature characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method for driving a liquid crystal optical element using a dual-frequency driving liquid crystal.

[Prior art and its problems] In recent years, optical devices that use liquid crystals to control the transmission state of light have been widely used. Among these optical devices, liquid crystal shutters that switch between a light blocking state and a light transmitting state at high speed are used as light control elements in electrophotographic printers. In this kind of conventional liquid crystal shutter, since a high shutter speed is required, a liquid crystal element of electric field control birefringence mode is used and driven at two frequencies. That is, as shown in FIGS. 13(a) and 13(b), a high frequency electric field f and a low frequency electric field fL are combined to perform on/off control of the liquid crystal element.

When dynamically driving multiple shutters, - the write period consists of a selection period and a non-selection period, the selection period is specified by applying a high-frequency electric field fH and a low-frequency electric field fL every 1/2 period; The selection period is specified by applying the superimposed electric field fH/fL and the low frequency electric field fL every 1/2 period.

As mentioned above, in the conventional driving method for liquid crystal optical elements, - the shutter is turned on only during the selected period during the writing period (on period), so the amount of light that passes through the shutter is small, increasing the shutter speed. There is a problem in doing so.

[Object of the Invention] The present invention was made in view of the above circumstances, and an object of the present invention is to provide a method for driving a liquid crystal optical element that can obtain a sufficient amount of transmitted light in the shutter-on state and can perform high-speed shutter operation. .

[Summary of the Invention] The present invention provides a method for driving a liquid crystal optical element that controls the light transmission states of a plurality of light shutters, in which one light shutter is controlled in one repetition period in which the light transmission states of the light shutters are sequentially controlled. An opening voltage is applied to the electrodes of the liquid crystal element to maintain the one optical shutter in a state where light is transmitted for a longer time than the selection period allocated for the purpose of the present invention.

[Embodiments of the Invention] Hereinafter, a case where the present invention is implemented in a liquid crystal shutter will be described. First, the structure of the liquid crystal shutter will be explained with reference to FIGS. 1 and 2. In FIG. 2, reference numerals 11 and 12 denote a pair of substrates each made of a transparent glass plate, and these substrates 11 and 12 are adhered at a constant distance by a sealing material 13 in the shape of an oblong frame. And the substrate 11.1
An electrode 14a is provided between the two.

14b and the alignment films 15a and 15b, a two-frequency driving liquid crystal IB forming an optical shutter is enclosed. Further, polarizing plates 17 are provided on the outer surfaces of the substrates 11 and 12, respectively.
a, 17b are rejuvenated.

As shown in FIG. 1, on the inner surface of one of the substrates 11 (referred to as a signal electrode substrate), a large number of signal electrodes 18a..., 18b... are formed in two rows over the entire length thereof. , strip-shaped terminals I9 are led out in parallel from these signal electrodes 18a..., 18b... alternately on one side and the other side of the signal electrode substrate 11. These signal electrodes 18a..., 18b..., terminals 19... are integrally formed from a transparent conductive material such as indium oxide, and the signal electrodes 18a..., tab...
A metal film 20 of chromium or the like is deposited on the terminals 19 and on the terminals 19 except for the parts corresponding to the shutter parts S1, S2, and so on. One of the signal electrodes 18
a... is 1/1/1 with respect to the other signal electrode 18b...
The shutter parts S1..., S2... are arranged in two rows with a shift of 1/2 pitch.

Further, on the inner surface of the other substrate 12 (hereinafter referred to as a common electrode substrate), each signal electrode 18a of the signal electrode substrate 11...
, 18b... A pair of band-shaped common electrodes 21
a, 21b are formed of a transparent conductive material such as indium oxide at minute intervals in parallel along the longitudinal direction of the common electrode substrate 12, and shutter parts S1..., S2... are formed on these common electrodes 21a, 21b. A metal film 22 made of chromium or the like is deposited except for the portion corresponding to . The metal film 20 is used to reduce the electrical resistance of the signal electrodes 18a..., 18b... and the terminal 19, and the metal film 22 is used to reduce the electrical resistance of the common electrodes 21a, 21b. The shutter portions S1..., S2... are provided to regulate the light transmission area.

Furthermore, a light shielding member 23 is provided on the outer surface of the polarizing plate 17b laminated on the common electrode substrate 12 on the light output side by, for example, printing means. This light shielding member 23 is provided on the outer surface of the polarizing plate 17b so as to cover the sealing material 13 except for the portion facing the array area A of the shutter portions S1..., S2.... As described above, the shutter section S consists of the common electrode 21a and the signal electrodes 18aS18as...
t SS+ , . . . and shutter parts S2, S consisting of the common electrode 21b and the signal electrodes 18b, 18b, .
2,... are constructed.

FIG. 3 shows the liquid crystal molecule orientation direction (rubbing direction) of the liquid crystal shutter configured as described above and the polarizing plates 17a, 17.
This figure shows the relationship between the locations of b. In Figure 3, 3
1 is the liquid crystal molecule orientation direction, 32 is the transmission axis (or absorption axis) of the upper polarizing plate 17a, and 33 is the transmission axis (or absorption axis) of the lower polarizing plate L7b.

That is, the liquid crystal 1B for two-frequency driving is sealed in a horizontally aligned liquid crystal cell, and the polarization axis (transmission axis or absorption axis) of one polarizing plate 17a or 17b is set at 45 degrees with respect to the liquid crystal molecule alignment direction (rubbing direction). The polarizing axes of the upper and lower polarizing plates 17a and 17b are made perpendicular to each other while being tilted.

This arrangement method is similar to the electric field controlled birefringence mode.

Therefore, the transmitted light intensity I in the liquid crystal shutter is
It is determined by the following formula (1).

! -1o5in22θΦ 5in2(r・Δnd/λ) ・・・・・・(1) However, Io: Light intensity determined by the transmittance of the parallel polarizing plate θ: Angle Δn between the optical axis and the polarizing plate at initial orientation: Birefringence that depends on the angle φ between the normal direction of the substrate surface and the optical axis of the liquid crystal λ: Wavelength of incident light d: Cell thickness If the refractive index n11 and n are known, φ can be calculated using the following formula (
2) can be calculated.

In addition, Figure 4 shows the frequency dependence of dielectric anisotropy Δε when using a liquid crystal IB for dual-frequency driving that has the characteristics shown in Table 1 below. 23℃, 35℃, 40℃, 45℃, 50℃
The figures are shown for each temperature of 0155°C.

Table 1 The above two-frequency driving liquid crystal 1B has a dielectric anisotropy Δε of “0”.
It exhibits positive dielectric anisotropy with respect to an electric field fL with a frequency lower than the crossover frequency fe, and negative dielectric anisotropy with respect to an electric field fH with a frequency higher than the crossover frequency fc. In this case, the crossing frequency sequentially moves higher as the temperature increases.

The liquid crystal shack configured as described above is driven by a signal waveform such that the shutter is opened by a selected period of time when turned on, and is driven by, for example, the common signal CI and segment signals 81 to S4 shown in FIG. do. The common signal C1 is for the shutter section S1,
The common signal for the shutter section S2 is generated with a delay of 1/2 of one write period compared to the case of FIG. in this case,
Segment signals Sl and S2 are on signals for the shutter section S1, S3 . S4 is an off signal, and also a segment signal SL for the shutter section S2.

S3 is an on signal, S2. S4 is an off signal.

In FIG. 5 above, T1 is the selection period (1 msec)
2 is the non-selection period (1ms e c), T3 is the first f
, application period, T4 is the second fH application time, T5 is the no-field application period, T8 is the holding voltage application period set for the remaining period in the selection period T1, and T7 is the end of one writing period to turn off the shutter. f) an application period provided in
T8 is the remaining period in the non-selection period T2. In this case, the T6 period within the selection period Tl and the T8 period within the non-selection period T2 are periods in which a holding voltage is applied to maintain the current shutter-on or shutter-off state, and the T8 period is the non-selection period. The time width is set to be equal to the T7 period within T2.

Thus, the common signal CI gives the high frequency voltage fH in the selection period TI, but inverts its phase in the T5 period, and gives the superimposed wave voltage fH/fL in the T8 period in the non-selection period T2, and gives the high frequency voltage fH in the T7 period. A low frequency voltage fL is applied during the period. In this case, the superimposed wave voltage fo/fr is such that, for example, the T8 period is divided into eight equal parts and the high frequency voltage fH is alternately applied. The segment signal S1 for turning on the shutter sections sl and s2 provides a low frequency voltage fL during the periods T6 and T7, and provides a high frequency voltage fH during the other periods. The segment signal S2 for turning on the shutter section S1 and turning off the shutter section S2 is transmitted at T6 in the selection period T1.
The low frequency voltage f is applied throughout the period and the non-selection period T2.
L is applied, and a high frequency voltage f is applied during the other periods of the selection period TI. In addition, the shutter section S1 is turned off, and the shutter section S2 is turned off.
The segment signal S3 for turning on provides a low frequency voltage fL during the entire selection period Tl, a high frequency voltage fH during the T8 period in the non-selection period T2, and a low frequency voltage fL during the T7 period. Furthermore, the segment signal S4 for turning off the shutter sections St and S2 provides a low frequency voltage fL throughout the selection period T1 and the non-selection period T2.

In addition, in FIG. 5, C1-SL, C1-52, C
1-S3 and C1-54 indicate a composite waveform of the common signal CL and the segment signals 81 to S4. As shown in this composite waveform, T8 within the non-selection period T2
In the period, the holding electric field in which the superimposed electric field in which the high-frequency electric field f is superimposed on the low-frequency electric field fL and the no electric field are repeated at regular intervals is 2.
It is applied to the frequency driving liquid crystal cell 16.

FIG. 6 shows a liquid crystal shutter configured as shown in FIGS. 1 and 2 in which the two-frequency drive liquid crystal shown in Table 1 is sealed in a cell with a thickness of 4.36 μm. 55 degrees Celsius, 50 degrees Celsius, 45 degrees Celsius when the signal waveform shown is applied continuously,
The shutter characteristics at each temperature of 40°C are plotted with time (
msec), and the transmitted light intensity (arbitrary unit) is plotted on the vertical axis. Note that this shutter characteristic is obtained when a T4 period is provided consecutively to the T3 period, and a high frequency electric field f of 0.6 m5ec is continuously applied, and the light source is 543 m5.
In addition to using a fluorescent lamp with an emission peak in nm, the high frequency electric field f is 200KHz, the T7 period is ro, and 4m5e.
Measurements were taken with the setting at cJ. In the liquid crystal drive signal waveform shown in FIG. 5 above, the T6 period and the subsequent <78
Since a superimposed electric field or a holding electric field consisting of repetitions of a superimposed electric field and no electric field is applied during the period, the on state in the selection period TI extends to period T8 within the non-selection period T2. Then, during the final T7 period of the -write period, the low frequency electric field fL is applied to turn off the shutter, completing one shutter operation, but the shutter characteristics change depending on the temperature at that time.

That is, at a temperature of 55°C as shown in FIG. 6(a), the liquid crystal molecules to which the high-frequency electric field f is initially applied pass through the tilt angle corresponding to the first peak where the transmitted light is maximum. , where the first maximum value of the transmitted intensity appears. Next, when a superimposed electric field fH/fL is applied, at a temperature of 55°C, the force that tilts the liquid crystal molecules by 1ΔεH1 becomes IΔε
Since it is smaller than the force that causes the liquid crystal molecules to stand up due to L1, the influence of the low frequency electric field fL appears strongly, and the liquid crystal molecules slowly begin to stand up and pass through the above-mentioned tilt angle. Here, a second maximum value of transmitted light intensity appears. Thereafter, the liquid crystal molecules slowly rise, and finally, when the low frequency electric field fL is applied, the liquid crystal molecules suddenly rise, and the transmitted light intensity also drops rapidly.

In the case of a temperature of 50°C shown in Figure 6(b),
), but the force due to 1ΔεH1 and 1Δ
Since the difference with the force due to εL1 becomes smaller, the low frequency electric field f
The influence of L is reduced, and the movement of liquid crystal molecules due to the application of the superimposed electric field fH/fL becomes even more gradual. Therefore,
Until the low frequency electric field fL is applied, the transmitted light intensity decreases only slightly, and good shutter characteristics can be obtained.

In the case of a temperature of 45°C as shown in Fig. 6(c), the response speed becomes slow due to the decrease in temperature, and therefore the high frequency electric field fH
The inclination angle is reached just before the end of .

At this temperature, the force due to 1ΔεH1 and the force due to 1ΔεL1 are approximately equal, so the tilting speed of the liquid crystal molecules becomes extremely slow, so that the peak of the transmitted light intensity becomes flat. However, in this case, an unstable state may appear in which the transmitted light intensity fluctuates over time.

In the case of a temperature of 40°C shown in Figure 6(d), 1ΔεH
Since the force due to 1 becomes larger than the force due to 1ΔεL1, and the application time of the average high frequency electric field rH becomes longer, the high frequency hysteresis effect becomes larger. For this reason, the way the liquid crystal molecules stand when a low frequency is applied is different, and the behavior of the liquid crystal molecules itself does not behave as in (a) to (c) above. Furthermore, application of an electric field generates a portion that does not operate normally, that is, a domain, and the intensity of transmitted light decreases.

Therefore, when applying the high frequency electric field f continuously, it is desirable to set the operating temperature to about 50°C.

FIG. 7 shows the liquid crystal drive signal waveform shown in FIG.
T3 mT4 =0.25rns e C, T5-0.
1m5ec, TB heel T7-0.4m5ec, the shutter characteristics at each temperature of 55℃, 50℃, 45℃, 40℃ when continuously on signal (C1-st times signal is applied) are shown. It is something.

In this example, even at an fA degree of 40° C., there was little drop in the transmitted light intensity, and no unstable state that fluctuated over time was observed. At this time, the total application time of the high frequency electric field f is 0
．． It is desirable that it be 5m5ec, which is half the arrest time.

Therefore, by interposing the T5 period in which there is no electric field between the T3° and T4 periods, the alignment direction of the liquid crystal molecules becomes uniform,
Improved temperature characteristics and stable shirt opening/closing operation.

Next, let's examine the responsiveness of TB to the length of the 77 period. T3 mT4-0.25m5 e c fixed,
TB-T7 rO, IJ, rO, 2J.

ro, 3J, ro, 4Jmsec (At this time, T5
- rO,4J, rO,3J, ro, 2
J.

The shutter characteristics are measured as ro, IJmsec).

FIG. 7 above shows the shutter characteristics when T6-77-0.4m5ec is used.

When the TO-T7-0.2m5ec is continuously turned on, the characteristics shown in FIGS. 8(a) to 8(c) are obtained at temperatures of 55°C, 50°C, and 45°C. At 45° C. as shown in FIG. 8(C), the liquid crystal hardly responds and the brightness remains at a constant level. T
8-T7-0.1m5ec, although not shown, already reaches a state similar to that shown in FIG. 8(C) at 50°C. However, for one ON signal when OFF is continuous,
As shown in FIG. 8(d), it responds even at 45°C.

Moreover, at T8=T7m0.3m5ec, the same responsiveness as in FIG. 7 was shown.

As mentioned above, the length of the T8 and 77 periods contributes to the stability of the temperature characteristics. is necessary. However, if it is too long, the high frequency electric field f
Since the H application time becomes shorter and the amount of transmitted light becomes smaller, Tl.

It is desirable to set it to half of the T2 period.

A high frequency electric field fH is applied to perform the on operation, and the length of the T3 period at this time was studied and the following results were obtained. T3 +74 period f, total application time Q, 5m5ec, T5-0.1m5ec,
TO-77-0,4m5ec, T
The ratio between 3 and TAwI was varied.

Table 2 FIG. 9 shows the response characteristics in the case of "1" in Table 2 above. In this case, a shoulder appears before the maximum intensity of transmitted light is obtained, and the apparent responsiveness deteriorates. At this time, it is T5-0.1 m5ec, and since the high frequency electric field f is applied again after that, the transmitted light intensity becomes strong. However, T5 mm0.2m5ec, T3-0
．． 1m5ec.

T4=0.4m5ecS T1mT? -0.4m5ec
If the T5 period is long and the T3 period is short until the second high-frequency electric field fH is applied, the brightness will not be ensured as shown in Figure 10, so if the T3 period is shorter than the response time of the liquid crystal, Caution is required. Incidentally, the response time at 50° C. is approximately 0.15 m5ec. In other words, when the T3 period is shorter than the response time of the liquid crystal, the high frequency electric field fH
It is necessary to apply the voltage for the second time, and it is necessary to shorten the time until the second application. f, the total application time is related to the amount of transmitted light. In order to increase the amount of transmitted light, it is better to lengthen the fH application time, but as shown in FIG. 6, the temperature characteristics deteriorate.

In this case, the characteristics deteriorate particularly on the low temperature side.

In order to extend the on state in the selection period TI to the non-selection period T2, it is preferable to lengthen the fH application time in the T4 period. When a shutter with at least the amount of transmitted light and good temperature characteristics is desired, it is better to shorten the fH application time and make the Te,77 period longer. As mentioned above, T3 in the signal waveform shown in FIG. T4゜T5. Te, T7. The Tit period can be arbitrarily set within the above-mentioned range according to the usage conditions of the liquid crystal shutter.

The response when changing the total fH application time is the 11th
It becomes as shown in Fig. 12. Figure 11 shows T3-0.2
5m5ec, T4 =0.15m5ec.

T5=0.2m5ec, TO-77-0,4m5e
Figure 12 shows the case where T3-0.25 is set.
m5 ec, T4-0.05m5ec %75 mo
, 3ms ec, and T8-77-0.4m 5ec. In other words, the T3+T4 period is Q,4
This is the case where it is set to m5ec and 0.3m5ec. As is clear from FIGS. 11 and 12 above, the amount of transmitted light is larger than when the duty is 1/2. T3 +T4
As the period becomes smaller, the duty approaches 1/2, so by adjusting the T3+T4 period, it is possible to adjust the light amount. In any case, if the length of the T3+74 period is shorter than the response time of the liquid crystal, the transmitted light will not reach a sufficient intensity, so a longer period is required.

Note that the present invention is not limited to the above embodiments, and the opening operation voltage, holding voltage, and closing operation voltage can be created by arbitrarily combining high frequency voltage, low frequency voltage, and no voltage.

[Effects of the Invention] As detailed above, according to the present invention, in the method for driving a liquid crystal optical element for controlling the light transmission state of an imaginary light shutter, one repetition period for sequentially controlling the light transmission state of the light shutter is provided. The selection is made by applying to the electrodes of the liquid crystal element an opening voltage that maintains the one optical shutter in a state where light passes through it for a longer time than the selection period allotted for controlling one of the optical shutters. Since the ON state of the period is extended to a part of the non-selected period, a sufficient amount of transmitted light can be obtained in the shutter ON state, and high-speed shutter operation is possible. Furthermore, by combining the liquid crystal drive signal waveforms, the amount of transmitted light can be controlled and the temperature characteristics can be improved.

[Brief explanation of the drawing]

1 to 12 show an embodiment of the present invention, in which FIG. 1 is a plan view showing the main parts of a liquid crystal optical element, FIG. 2 is a sectional view, and FIG. 3 is a liquid crystal optical element shown in FIG. 2. FIG. 4 is a diagram showing the relationship between the orientation direction and the arrangement of polarizing plates. FIG. 4 is a diagram showing the frequency dependence of dielectric anisotropy in a liquid crystal for dual-frequency driving. FIG. 5 is a diagram showing an example of the drive signal waveform for driving a liquid crystal optical element. 6 to 12 are diagrams showing optical response characteristics to various drive signals, and FIGS. 13(a) and 13(b) are diagrams showing a conventional driving method of a liquid crystal optical element and its optical response characteristics. be. 11, l2-...Substrate, l2-...Sealing material, 14
a, 14b...electrode, 1t...liquid crystal, 17...
・Polarizing plate, 18a, 18b... No. electrode, l 9-
...Terminal, 20.22-...Metal film, 21a, 21
b-... Common electrode, 23... Light shielding member, 31...
Liquid crystal molecule alignment direction, 32... Transmission axis of upper substrate polarizing plate (
or absorption axis), 33...transmission axis of the upper substrate polarizing plate (
or absorption axis). Applicant's representative Patent attorney Takehiko Suzue No. 2 (!i Figure 3 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12

Claims (16)

[Claims] (1) A pair of opposing substrates, a plurality of electrodes formed in a plurality of arrays on the inner surface of one of the opposing substrates, and a plurality of electrodes on the other inner surface of the pair of substrates facing the plurality of electrodes. the other electrode formed in an array; a liquid crystal layer made of a liquid crystal material whose dielectric anisotropy is reversed in polarity depending on frequency; and a liquid crystal layer interposed between the pair of substrates and homogeneously aligned; and an alignment direction of the liquid crystal layer. a polarizing plate disposed with a polarization axis tilted at approximately 45 degrees with respect to the polarizing plate, and by applying a voltage between the one electrode and the other electrode, the one electrode and the other electrode are opposed to each other. A method for driving a liquid crystal optical device that controls light transmission states of a plurality of parts, comprising sequentially controlling light transmission states of a plurality of light shutters formed at a part where one of the one electrodes faces a plurality of the other electrodes. The above-mentioned 1 repeating period is longer than the selection period allocated for controlling one optical shutter.
1. A method for driving a liquid crystal optical element, comprising applying an opening voltage that maintains two optical shutters in a state in which light passes between the one electrode and the other electrode. (2) The open operation voltage includes the open operation voltage applied during the selection period and the holding voltage applied during the remaining selection period and a part of the non-selection period. Driving method for liquid crystal optical elements. (3) The method for driving a liquid crystal optical element according to claim 2, wherein the opening operation voltage further comprises a voltage for closing the shutter at the end of the repetition period. (4) The method of driving a liquid crystal optical element according to claim 2, wherein the opening operation voltage is applied for a period of 1/2 of the selection period or a period shorter than 1/2. (5) The liquid crystal optical element according to claim 2, wherein the opening operation voltage is applied for a period longer than the period until the liquid crystal molecules of the liquid crystal layer are oriented obliquely and the intensity of the transmitted light reaches its maximum value. driving method. (6) The liquid crystal according to claim 2, 4, or 5, wherein the opening operation voltage is a high frequency voltage higher than the crossing frequency at which the polarity of the dielectric anisotropy of the liquid crystal material is reversed. How to drive optical elements. (7) The method for driving a liquid crystal optical element according to claim 2, 4, or 5, wherein the opening operation voltage is a combination of a high frequency voltage and a non-voltage. (8) The open operating voltage is such that the total high frequency voltage is 1 in the selected period.
8. The method of driving a liquid crystal optical element according to claim 7, wherein the voltage is applied only for a period of /2 or less. (9) The method for driving a liquid crystal optical element according to claim 2, wherein the holding voltage is a superimposed voltage of a low frequency and a high frequency that are lower than the crossover frequency. (10) The method for driving a liquid crystal optical element according to claim 2, wherein the holding voltage is a repetition of a superimposed voltage and no voltage. (11) Holding voltage is superimposed voltage, no voltage, high frequency voltage,
and a low frequency voltage that does not close the optical shutter. (12) The closing operation voltage is applied for a period longer than 1/5 and less than 1/2 of the selection period.
A method for driving a liquid crystal optical element as described in Section 1. (13) The method for driving a liquid crystal optical element according to claim 3 or 12, wherein the closing operation voltage is a repetition of a low frequency voltage and no voltage. (14) The method for driving a liquid crystal optical element according to claim 3 or 12, wherein the closing operation voltage is a superimposed voltage of a high frequency voltage and a low frequency voltage. (15) The method for driving a liquid crystal optical element according to claim 3 or 12, wherein the closing operation voltage is a repetition of a superimposed voltage and no electric field. (16) The closing operating voltage includes a high frequency voltage or no voltage state, and the application time of each voltage is until the light intensity reaches 1/2 of the maximum light intensity when the optical shutter is in the open state. 4. A method for driving a liquid crystal optical element according to claim 3, wherein the voltage is applied for only a certain amount of time.