JPS6031120A - Driving method of optical modulating element - Google Patents

Abstract

PURPOSE:To avoid a drop of a picture contrast and a crosstalk by applying a voltage which has set a threshold voltage of an optical mudulating substance having a bistability to a specified value. CONSTITUTION:When a threshold voltage for giving the first stable stage of a liquid crystal cell having a bistability, and a threshold voltage for giving the second stable state are denoted as Vth1 and -Vth2, respectively, a voltage 2V exceeding the threshold Vth1 is applied to a picture element A on a selected scanning line, and a voltage -2V exceeding the threshold -Vth2 is applied to a picture element B existing on the same scanning line. On the other hand, on a scanning line which is not selected, a voltage applied to all picture elements C and D is +V or -V, and both of them do not exceed the threshold voltage. Accordingly, a liquid crystal molecule in each picture element of C and D holds an orientation as it is corresponding to a signal state in case of the previous scanning without changing the oriented state. Accordingly, even if the number of scanning electrodes is increased, a substantial duty ratio is not changed, and a drop of a contrast, a crosstalk, etc. are not generated at all.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving an optical modulation element such as a liquid crystal element, and more particularly to a time division driving method for a liquid crystal element used in an optical modulation element such as a display element or a shutter array.

Conventionally, liquid crystal display elements display images or information by configuring a group of scanning electrodes and a group of signal electrodes in a matrix, filling a liquid crystal compound between the electrodes, and forming a large number of pixels.
well known. The driving method for this display element is a time-sharing method in which an address signal is selectively and periodically applied to a group of scanning electrodes, and a predetermined information signal is selectively applied in parallel to a group of signal electrodes in synchronization with the address signal. However, this display element and its driving method had major and fatal drawbacks as described below.

That is, it is difficult to increase the pixel density or enlarge the screen. Among conventional liquid crystals, most of them are practically used as display elements, for example, M, 5ch, because of their relatively high response speed and low power consumption.
adt and W, He1frich, IIApplied
Physics Letters”, Vol, 1B
, No. 4 (1971, 2, 15), P. 127
~128°' Voltage-Dependent
0Ptical Activity of a Tw
istedNematic Liquid Crystal
TN (twisted nemati) shown in “a+”
c) type liquid crystal, which has a structure (helical structure) in which nematic liquid crystal molecules are twisted in the thickness direction of the liquid crystal layer and has positive dielectric anisotropy in the absence of an electric field. The liquid crystal molecules form a structure in which they are arranged parallel to each other on both electrode surfaces. On the other hand, when an electric field is applied, nematic liquid crystals with positive dielectric anisotropy align in the direction of the electric field.
As a result, optical modulation can occur. When a display element is constructed with a matrix electrode structure using this type of liquid crystal, the area where both the scanning electrode and the signal electrode are selected (selected point) has a threshold value greater than or equal to the threshold required to align the liquid crystal molecules perpendicular to the electrode surface. A voltage is applied to the region where neither the scanning electrode nor the signal electrode is selected (unselected point), and therefore the liquid crystal molecules maintain a stable alignment parallel to the electrode plane. By arranging linear polarizers above and below such a liquid crystal cell in a cross Nicol relationship, light does not pass through selected points, but light passes through non-selected points, making it possible to use it as an image element. becomes. However, when a matrix electrode structure is configured, there are areas where scan electrodes are selected and signal electrodes are not selected, or areas where scan electrodes are selected and red and square electrodes are selected (so-called semi-selected points ′ ) is also subject to a finite electric field.

If the difference between the voltage applied to the selected point and the voltage applied to the half-selected point is sufficiently large, and the voltage threshold required to align liquid crystal molecules perpendicular to the electric field is set to a voltage value in between, the display element will function normally. This is why it works. However, in this method, when the number of scanning lines (N) is increased, the time during which an effective electric field is applied to one selected point (duty ratio) while scanning the entire screen (l frames) is /N. For this reason, the effective voltage difference between the selection point and the ratio selection point when repeated scanning is performed becomes smaller as the number of scanning lines increases, resulting in a decrease in image contrast and crosstalk. This is a drawback that is difficult to avoid. This phenomenon is caused by liquid crystals that do not have bistability (the stable state is when the liquid crystal molecules are aligned horizontally with respect to the electrode surface, and they are aligned vertically only while an electric field is effectively applied). ) is essentially an unavoidable problem that arises when driving using the temporal accumulation effect (that is, repeatedly scanning). In order to improve this point, voltage averaging method, dual frequency drive method, multiple matrix method, etc. have already been proposed, but all of these methods are insufficient, and it is difficult to increase the screen size and density of display elements. Currently, the number of scanning lines has reached a plateau due to the inability to increase the number of scanning lines sufficiently.

An object of the present invention is to provide a novel optical modulation element, particularly a liquid crystal element driving method, which solves all the problems of conventional liquid crystal display elements as described above.

Another object of the present invention is to provide a method for driving a liquid crystal element having high-speed response.

Another object of the present invention is to provide a method for driving a liquid crystal device having high density pixels.

Furthermore, another object of the present invention is to provide a method for driving a liquid crystal element that does not generate crosstalk.

Furthermore, another object of the present invention is to provide a method for driving a liquid crystal element suitable for a partially rewritable display device.

Furthermore, another object of the present invention is to provide a novel method for driving a liquid crystal device using a liquid crystal that is bistable with respect to an electric field, particularly a chiral smectic C-phase or H-phase liquid crystal that has ferroelectricity. There is a particular thing.

Furthermore, another object of the present invention is to provide a novel driving method suitable for a liquid crystal device having a high density of pixels and a large screen area.

The method for driving an optical modulation element of the present invention was developed to achieve the above-mentioned object, and more specifically includes a scanning electrode group and a signal electrode group for giving a predetermined information signal, In a method for driving an optical modulation element such as a liquid crystal element, which has a structure in which an optical modulation substance (for example, liquid crystal) having bistable properties with respect to an electric field is arranged between an electrode group and a signal electrode group, the scanning electrode group The bistable optical modulation material is placed between the selected scanning electrode of the signal electrode group and the selected signal electrode to provide new image information of the signal electrode group. applying a voltage that orients the scanning electrode to an optically stable state), and establishing the bistability between the selected scanning electrode and the unselected signal electrode that is to provide new image information of the signal electrode group; A voltage is applied to orient the optical modulation substance having the second stable state (the other optically stable state), and a voltage is applied between the unselected scan electrodes of the scan electrode group and the signal electrode group, and between all the scan electrodes. and a signal electrode that does not give new image information, the threshold voltage of the optical modulation material having bistability -
Vth2 (referring to the threshold voltage in the second stable state) and Vth
This is characterized by applying a voltage set to a value between x (referring to the threshold voltage in the first stable state).

In a preferred embodiment of the present invention, a group of scanning electrodes are sequentially selected based on a scanning signal, a group of signal electrodes facing the group of scanning electrodes are selected based on a predetermined information signal, and a group of signal electrodes is held between the two electrodes. Electric signals having different voltages at phases t□ and t2 are applied to selected scanning electrodes of a liquid crystal element having at least a liquid crystal that is bistable with respect to an electric field, and a predetermined voltage is applied to a group of signal electrodes. Depending on the presence or absence of information, or whether or not to retain the information from the previous scan,
Give electrical signals of different voltages. That is, in the portion where the information signal is present on the selected scanning electrode line, the phase t
1 (tj2), a unidirectional electric field that gives the first stable state is applied to the liquid crystal, and in the empty part, the phase t
2 (tt), apply an electric field in the opposite direction to give a second stable state, and in the part where you want to keep the information from the previous scan, change from one stable state at phases t1 and t2. The liquid crystal element can be driven by applying an electric field below the electric field threshold for transition to another stable state. Details of the specific example will be explained later with reference to the drawings.

The optical modulation material used in the driving method of the present invention is a material that takes either a first optically stable state or a second optically stable state depending on the applied electric field, that is, a material that has a bistable state with respect to the electric field. In particular, liquid crystals having such properties are used.

As the liquid crystal having bistability that can be used in the driving method of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable. ”) is suitable. Regarding this ferroelectric liquid crystal, ``LE J
OυRNAL DE PHYSIQUELETTERS
” 3B (L-8El) 1975. rFerro
electricLiquid Crystals J
; ” Applied Physics Let-
ters” 3B (11) 1880, rSubmi
cro 5econd B1-5table Elec
trooptic Switching in Liq
uidCrystalsJ; “Solid State Physics” 18
(141) 11181r Liquid Crystal", etc., and the ferroelectric liquid crystal disclosed in these can be used in the present invention.

More specifically, an example of a ferroelectric liquid crystal compound used in the method of the present invention is decyloxybenzylidene-E'-amino-2-methylbutylcinnamate (DOBAMBC).
), hexyloxybenzylidene-E'-amino-2-
Chloroprovir cinname-) (HOBACPC) and 4-o-(2-methyl)-butylresorsilitin-4
Examples include fuoctylaniline (MBRA8).

When constructing an element using these materials, a heater may be embedded in the element as necessary to maintain the temperature at which the liquid crystal compound becomes the S m C" phase or the S m H" phase. It can be supported by copper blocks, etc.

Figure 1 schematically depicts an example of a ferroelectric liquid crystal cell. 21 and 21' are In20B, 5n02 and I
It is a substrate (glass plate) coated with a transparent electrode such as TO (Indium-Tin Oxide), between which a liquid crystal molecular layer 22 is oriented perpendicular to the lath surface.
m C” phase liquid crystal is sealed. Line 2 shown by thick line
3 represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment (P) 24 in the direction perpendicular to the molecule.
have. When a certain threshold voltage is applied between the electrodes on the substrates 21 and 21', the helical structure of the liquid crystal molecules 23 is unraveled, and the liquid crystal molecules 23 are twisted so that all the dipole moments (P) 24 are oriented in the direction of the electric field. The orientation direction can be changed. The liquid crystal molecules 23 have an elongated shape,
It exhibits refractive index anisotropy in the long axis direction and the short axis direction, and therefore, if polarizers are placed above and below the glass surface in a crossed nicol positional relationship, the optical properties of the liquid crystal optical modulation element change depending on the polarity of voltage application. It is easy to understand that.

2 Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, i
As shown in Fig. 2, the helical structure of the liquid crystal molecules is released even when no electric field is applied, and its dipole moment) P or P' is directed upward (34) or downward (34') as shown in Figure 2.
). When an electric field E or E' with a different polarity above a certain threshold value is applied to such a cell as shown in FIG. 34', and accordingly the liquid crystal molecules enter the first stable state 33
or the second stable state 33'.

There are two advantages to using such a ferroelectric liquid crystal as an optical modulation element. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules is bistable. To explain the second point using, for example, Figure 2,
When the electric field E is applied, the liquid crystal molecules are aligned in a first stable state 33, and this state remains stable even when the electric field is turned off. or,
When an opposite electric field E' is applied, the liquid crystal molecules are oriented in a second stable state 33' and change their orientation, but they remain in this state even after the electric field is turned off. Further, as long as the applied electric field E does not exceed a certain threshold value, each orientation state is maintained. In order to effectively realize such fast response speed and bistability, it is preferable for the cell to be as thin as possible, and generally, the thickness is 0.5p to 20p.
4, especially IIl, to 5l are suitable. A liquid crystal-electro-optical device having a matrix electrode structure using this type of ferroelectric liquid crystal has been proposed, for example, by Clark and Ragabal in US Pat. No. 4,367,924.

A preferred specific example of the driving method of the present invention is shown in FIGS.
This will be explained using D).

Figure 3(A)-(a) shows a ferroelectric liquid crystal compound (
FIG. 2 is a schematic layout diagram of electrodes of a cell 11 having a matrix electrode structure in which electrodes (not shown) are sandwiched.

12 is a scanning electrode group, and 13 is a signal electrode group. FIGS. 3(A)-(b) and (A)-(c) show electrical signals applied to the selected scanning electrode 12(s) and other scanning electrodes (unselected scanning electrodes) 12(n), respectively. Figures 3 (A)-(b) and (A
)-(d) are the electrical signals given to the selected signal electrode 13(s) and the unselected signal electrode 13(n), respectively.
represents the electrical signal given to Figure 3 (A)-(b)
~(A)-(e) The horizontal axis represents time and the vertical axis represents voltage. For example, when displaying a moving image, the scanning electrode groups 12 are sequentially and periodically selected. Now, if the threshold voltage for providing the first stable state of a liquid crystal cell having bistable property is vth, and the threshold voltage for providing the second stable state is -Vth, then the selected scanning electrode 12
The electrode signals given to (s) are shown in Fig. 3 (A) - (b
), it is an alternating voltage such that it is V at phase (time) tr and 12 at phase (time) t2. It is said that by applying electrical signals having a plurality of phase intervals with different voltages to the scanning electrodes selected in this way, it is possible to easily and quickly convert between the two stable states of the liquid crystal that determine the display state of the element. Important effects can be obtained.

On the other hand, the other scanning electrodes 12(n) are shown in FIG. 3(A).
- As shown in (c), it is in a grounded state and the electrical signal is 0. Further, the electric signal given to the selected signal electrode 13(s) is V as shown in FIGS. 3(A)-(d), and the electric signal given to the unselected signal electrode 13(n) is V. is 1, as shown in FIGS. 3(A)-(e). In the above, the voltage value ■ is v<vth□
It is set to a desired value that satisfies <2V and -V>-Vt h2>-2V. The voltage waveforms applied to each pixel, that is, pixels A, B, C, and D shown in FIG. 3(A)-(a) when such an electric signal is applied, are shown in FIG. B) (a), (b), (C) and (d
). That is, as is clear from FIG. 3(B)-(a) to (d), a voltage of 2v exceeding the threshold value vth□ is applied to the pixel A on the selected scanning line at phase t2. Furthermore, for pixel B existing on the same scanning line, the phase is 11.
At , a voltage of -26 (2) exceeding the threshold value -Vth2 is applied. Therefore, depending on whether or not a signal electrode is selected on the selected scanning electrode line, if the signal electrode is selected, the liquid crystal molecules are aligned in the first stable state, and if not selected, the liquid crystal molecules are aligned in the first stable state. Align the orientation to the stable state of 2. In any case, it has nothing to do with the previous history of each pixel.

On the other hand, as shown in pixels C and D, on unselected scanning lines, the voltages applied to all pixels C and D are +V or -■, neither of which exceeds the threshold voltage. Therefore, the liquid crystal molecules in each pixel C and D maintain the orientation corresponding to the signal state when scanned last time without changing the orientation state. That is, when a scanning electrode is selected, signals for its l life are written,
The signal state can be maintained until the next selection after one frame ends. Therefore, even if the number of scanning electrodes increases, the actual duty ratio does not change, and contrast reduction and crosstalk do not occur at all. At this time, the value of the voltage value V and the value of the phase (t□+t2)=T depend on the liquid crystal material used and the thickness of the cell, but it is usually O, 1 pLsec ~ 2 m at 3 ports to 770 ports.
A range of 5ec is used. The essential difference from conventionally known driving methods is that in the method of the present invention, the electrical signal applied to the selected scanning electrode is in a first stable state (a "bright" state when converted into an optical signal). The point here is that it facilitates a change from a second stable state (which is assumed to be a "dark" state when converted to an optical signal) or vice versa.

For this purpose, the signal given to the selected scanning electrode is
For example, it alternates from +V to 1■. Further, the voltages applied to the signal electrodes are set to be different voltages to specify a bright or dark state.

Now, FIG. 3(C) shows an example of an image when scanning of one frame is completed in this manner. Next, for example, an example of partially rewriting this image is shown in Fig. 3(D)-
Shown in (a). As shown in the figure, when it is desired to rewrite only the area of the scanning electrode group X' and the signal electrode group Y', the scanning signal is sequentially applied only to the area of X'. In addition, as for the information signal, a signal corresponding to the presence or absence of information is added to the area Y', and in the area Y where it is desired to keep the information written in the previous scan as it is (that is, do not give new information), the signal shown in FIG. 3 (D) is added. - Add a signal as shown in (b) (in this case, a 0V signal). Therefore, the voltage applied to each pixel in area Y is as shown in FIGS. 3(D)-(C) during scanning, and as shown in FIGS.
As shown in Figure (D)-d, since the threshold voltage is not exceeded in any case, the image scanned last time is saved as is.

In order for the driving method of the present invention to be effectively achieved, the electrical signals applied to the scanning electrodes or the signal electrodes do not necessarily correspond to the values shown in FIGS. 3(A)-(b) to (e) and 3(D)-( b)～(
It is obvious that the signal does not have to be a simple rectangular wave signal as explained in d). For example, it is also possible to drive with a sine wave or a triangular wave, as long as an effective time span is provided.

Further, FIG. 4 shows another specific example of the driving method of the present invention. Figures 4 (a), (b), (C), 9 (d) and (e) respectively show the signal of the selected scanning electrode, the signal of the unselected scanning electrode, and the selected (with information) information. signals, information signals that are not selected (no information), and information signals that keep the signals from the previous scan as they are;
It represents. The value of Y' in FIG. 4(e) is IV'-Vl<1Vthl I, 1vth21, l v
' l < l V th t I, 1Vth21
is set to satisfy.

FIG. 5 shows yet another modification.

That is, FIGS. 5(EL), (b), (C), (d), and (e) represent the signals of the selected scanning electrode (FIG. 5(a)) and the signals of the unselected scanning electrodes (FIG. 5(a)), respectively. Figure 5 (
b) Selected (information-containing) information signal (Fig. 5(C)
), unselected (no information) information signal (Fig. 5(d)
)) and an information signal (FIG. 5(e)) that retains the signal of the previous scan as it is. At this time, in order to drive correctly based on the present invention, the following relationship must be satisfied in the driving method shown in FIG. That is, and (Va'Vo 2V)<-Vt h2

[Brief explanation of the drawing]

FIG. 1 is a perspective view schematically showing a liquid crystal element having chiral smectic phase liquid crystal. FIG. 2 is a perspective view schematically showing the bistability of the liquid crystal element used in the method of the present invention. FIG. 3(A)(a) is a plan view schematically showing the electrode arrangement state of a liquid crystal element used in the driving method of the present invention. FIGS. 3A and 3B are waveform diagrams of electrical signals applied to selected scanning electrodes. FIGS. 3A and 3C are signal waveform diagrams applied to unselected scanning electrodes. Figure 3 (A)
(d) is an information signal waveform diagram applied to a selected signal electrode. FIGS. 3(A) and 3(e) are information signal waveform diagrams applied to unselected signal electrodes. Figure 3 (B)
(a) is a waveform diagram of the voltage applied to the liquid crystal of pixel A. FIG. 3(B)(b) is a waveform diagram of the voltage applied to the liquid crystal of pixel B. FIGS. 3B and 3C are waveform diagrams of voltages applied to the liquid crystal of pixel C. Figure 3 (B) (
d) is a waveform diagram of the voltage applied to the liquid crystal of each pixel. FIG. 3(C) is an explanatory diagram of an example of an image given to the liquid crystal element after one frame scanning is completed. Figure 3 (D) (a)
is an example of an image after a part of the image in FIG. 3(C) has been rewritten. FIGS. 3(D) and 3(b) are information signal waveform diagrams applied to signal electrodes that do not provide new image information during rewriting. FIG. 3 (D) CG> and (d) show voltage waveforms applied to the liquid crystal between the signal electrode that does not give new image information during rewriting and the selected and unselected scanning electrodes, respectively. It is a diagram. FIG. 4(a) is a signal waveform diagram applied to selected scanning electrodes in another example. FIG. 4(b) is a signal waveform diagram applied to unselected scanning electrodes in this other specific example. FIGS. 4(C) and (d) are information signal waveform diagrams applied to selected and unselected signal electrodes, respectively, among the signal electrodes to which new image information is to be given in the other specific example. be. FIG. 4(e) is a signal waveform diagram applied to a signal electrode that does not provide new image information. FIG. 5(a) is a diagram of selected signal waveforms applied to the scanning electrodes in another specific example. Figure 5(b) shows
FIG. 7 is a signal waveform diagram applied to unselected scanning electrodes in the other specific example. FIGS. 5(C) and (d) are
FIG. 7 is a waveform diagram of information signals applied to selected and unselected signal electrodes among the signal electrodes to provide new image information in the other specific examples. FIG. 5(e) is a diagram of signal waveforms applied to the three signal electrodes that do not provide new image information. 11...Liquid crystal element 12...Scanning electrode group 12(a)...Selected scanning electrode 12(n)...Unselected scanning electrode 13...Φ signal electrode group 13(s)/Fist Selected signal electrode 13(n)...Unselected signal electrode 33--Liquid crystal 33' aligned in the first stable state.
... Liquid crystal 34 oriented in the second stable state - Fist - upward dipole moment P34' - Φ downward dipole moment P'X - -Φ - non-rewriting target area X' of scanning electrode group
...Rewriting target area Y of the scanning electrode group...Rewriting target area Y' of the signal electrode group...Rewriting target area 4 of the signal electrode group Electric l Kasumi rl! ;5 figure (A) Noodles No. 3 Soku (8) Yugeyuki figure 5 (D) (C) (ct) Roro uu mouth (b) 4 Good (d) 1e) 1gs <a) (b) UIL ( C) (d) Ce)

Claims (1)

[Claims] 1. An optical modulation material having a scanning electrode group and a signal electrode group that provides a predetermined information signal, and having bistability with respect to an electric field between the scanning electrode group and the signal electrode group. In the driving method of an optical modulation element having a structure in which: between a selected scanning electrode of the scanning electrode group and a selected signal electrode to give new image information of the signal electrode group, Applying a voltage to orient the bistable optical modulation material to a first stable state, and applying a voltage to the signal electrodes that are not selected among the selected scanning electrodes and the signal electrode group to provide new image information. A voltage is applied between the bistable optical modulating material to align it to the second stable state, and between unselected scan electrodes of the scan electrode group and the signal electrode group, and between all of the signal electrode groups. The threshold voltage -vth, (second
1. A method for driving an optical modulation element, comprising applying a voltage set to a value between Vth (referring to a threshold voltage in a stable state) and vth (referring to a threshold voltage in a first stable state). 2. Applying electrical signals having voltages with different phases to the selected scanning electrodes of the scanning electrode group, and providing new image information to the selected signal electrodes and unselected signal electrodes of the signal electrode group; 2. The method of driving an optical modulation element according to claim 1, wherein electrical signals of different voltages are applied to each signal electrode. 3. Applying electrical signals having different voltage polarities and phases to the selected scanning electrodes of the scanning electrode group, and applying electrical signals having different voltage polarities to the selected signal electrodes and unselected signal electrodes of the signal electrode group. A method for driving an optical modulation element according to claim 1, which provides a signal. 4. The method for driving an optical modulation element according to claim 1, wherein the optical modulation substance having bistability is a ferroelectric liquid crystal. 5. 5. The method of driving an optical modulation element according to claim 4, wherein the ferroelectric liquid crystal is a liquid crystal having a chiral smectic phase. 6. Claim 5, wherein the liquid crystal having a helical smetic phase is a liquid crystal phase that does not form a helical structure.
Driving method of the optical modulation element described in 2. 7. The method for driving an optical modulation element according to claim 5 or 6, wherein the liquid crystal having a radial smectic phase is a liquid crystal having a C phase or an H phase.