JPS6245535B2 - - Google Patents

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 an optical shutter array.

Conventionally, liquid crystal display elements have been well known, which display images or information by configuring a scanning electrode group and a signal electrode group in a matrix, filling a liquid crystal compound between the electrodes, and forming a large number of pixels. There is. The driving method for this display element is time-division driving, 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 have relatively high response speed and low power consumption, so most of them are in practical use as display elements. ”Vo.18, No.4
(1971, 2, 15), P.127-128 “Voltage―
DePendent Optical Activity of a Twisted
TN shown in “Nematic Liqvid Crystal”
(twisted nematic) type liquid crystal, which has a horizontal structure (helical structure) in which the molecules of the nematic liquid crystal, which has positive dielectric anisotropy, are twisted in the thickness direction of the liquid crystal layer in the absence of an electric field. The liquid crystal molecules form a structure in which they are arranged in parallel on both electrode surfaces. On the other hand, when an electric field is applied, nematic liquid crystals with positive dielectric anisotropy are aligned in the direction of the electric field, resulting in optical modulation. When a display element is constructed using this type of liquid crystal with a matrix electrode structure, in the region where both the scanning electrode and the signal electrode are selected (selected point), there is a A voltage higher than the threshold is applied, and no voltage is applied to areas where neither the scanning electrode nor the signal electrode is selected (non-selected points), so the liquid crystal molecules maintain a stable alignment parallel to the electrode surface. . By arranging linear polarizers above and below a liquid crystal cell in a cross-Nicol relationship with each other, light does not pass through selected points, but light passes through non-selected points, making it possible to use it as an image element. Become. However, when a matrix electrode structure is configured, there is a limited area in which scanning electrodes are selected and signal electrodes are not selected, or in areas where scanning electrodes are not selected and signal electrodes are selected (so-called "half-selected points"). The electric field becomes hot. 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. However, when the number of scanning lines (N) is increased, the time during which an effective electric field is applied to one selected point while scanning the entire screen (one frame) (duty ratio) increases. It decreases at a rate of 1/N. For this reason, when repeated scanning is performed, the effective voltage difference between selected points and non-selected points becomes smaller as the number of scanning lines increases, and as a result, reduction in image contrast and crosstalk can be avoided. This has become a serious drawback. 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 (ie, repeated scanning). In order to improve this point, voltage averaging methods, dual frequency driving methods, multiple matrix methods, 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.

On the other hand, looking at the field of printers, laser beam printers provide electrical image signals in the form of light to electrophotographic photoreceptors in terms of both pixel density and speed, as a means of obtaining hard copies using electrical signals as input. (LBP) is currently the best. However, LBP has the following disadvantages: 1. The printer is large; 2. It has a high-speed moving part like a polygon scanner, which generates noise, and requires strict mechanical precision. In order to overcome these drawbacks, a liquid crystal shutter array has been proposed as an element that converts electrical signals into optical signals. However, when providing pixel signals using a liquid crystal shutter array, for example, the pixel signals are transmitted at 20 dots/m within a length of 200 nm.
In order to write at a rate of This was difficult to manufacture.

For this reason, attempts have been made to time-divisionally provide pixel signals for one line using signal generators divided into several lines.

By doing so, it is possible to use a common electrode for applying a signal to a plurality of signal generating sections, and the amount of wiring can be substantially reduced. However, in this case, the number of lines (N) is
As the number of pixels is increased, the time during which the signal is ON becomes substantially 1/N, which leads to problems such as a decrease in the amount of light obtained on the photoreceptor and the problem of crosstalk.

SUMMARY OF THE INVENTION An object of the present invention is to provide a novel optical modulation element, particularly a method for driving a liquid crystal element, which solves all the problems in conventional liquid crystal display elements or liquid crystal light shutters 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.

Still 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 novel method for driving a liquid crystal device using a liquid crystal that is bistable with respect to an electric field, particularly a chiral smect 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.

That is, such an object of the present invention is to provide a method having a scanning electrode group and a signal electrode group, and disposing an optical modulating material (for example, liquid crystal) having bistability with respect to an electric field between the scanning electrode group and the signal electrode group. In a method for driving an optical modulation element such as a liquid crystal element having a structure, the liquid crystal having bistable property is arranged between a selected scanning electrode of the scanning electrode group and a selected signal electrode of the signal electrode group. A voltage is applied to align the liquid crystal in a stable state (one optically stable state), and the bistable liquid crystal is aligned between the scanning electrode and the unselected signal electrode of the signal electrode group in a second stable state (the other optically stable state). At the same time, the threshold voltage of the bistable liquid crystal -Vth ₂ (second stable state) is applied between the unselected scanning electrodes of the scanning electrode group and the signal electrode group. This is achieved by a driving method of the optical modulation element that applies a voltage set to a value between Vth 1 (the threshold voltage of the first stable state) and Vth ₁ (the threshold voltage of the first stable state).

In a preferred embodiment of the present invention, a scanning electrode group that is sequentially selected based on a scanning signal, a signal electrode group that faces the scanning electrode group and is selected based on a predetermined information signal, and a signal electrode group that is held between the two electrodes. Electric signals having phases t ₁ and t ₂ of different voltages 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 predetermined information is applied to a group of signal electrodes. By applying electrical signals with different voltages depending on the presence or absence of the information signal, in the part where the information signal is present on the selected scanning electrode line, the first signal is applied to the liquid crystal at phase t ₁ (t ₂ ).
_The liquid crystal element is driven by applying an electric field in _one direction that gives a stable state of can do. A specific example is the first
Although shown in the figure, details will be explained later.

The optical modulation material used in the driving method of the present invention has a bistable state consisting of a first optically stable state and a second optically stable state with respect to an electric field, and in particular has a bistable state with respect to an electric field as described above. A liquid crystal with bistability is 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.
Phase (SmH ^* ) liquid crystal is suitable. For more information about this ferroelectric liquid crystal, please refer to “LE JOURNAL DE
PHYSIQUE LETTERS” 36 (L-69) 1975,
“Ferroelectric Liquid Crystals”; “Applied
Physics Letters” 36 (11) 1980 “Submicro
Second Bistable Electrooptic Switching in
Liquid Crystals”; “Solid State Physics” 16 (141) 1981
The ferroelectric liquid crystals disclosed in these documents can be used in the present invention.

FIG. 2 schematically depicts an example of a ferroelectric liquid crystal cell. 21 and 21' are In ₂ O ₃ , SnO ₂
It is a substrate (glass plate) coated with a transparent electrode such as ITO (Indium-Tin Oxide), and the layer 22 is oriented perpendicular to the glass surface.
Enclosed is SmC ^* phase or SmH ^* phase liquid crystal. A thick line 23 represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment (P) 24 in a direction perpendicular to the molecule.
When a voltage equal to or higher than a certain threshold 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 aligned in such a way that all dipole moments (P) 24 are directed in the direction of the electric field. can be changed. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, if crossed Nicol polarizers are placed above and below the glass surface, the voltage application polarity can be changed. It is easily understood that the liquid crystal optical modulation element has optical characteristics that change depending on the temperature. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1μ),
As shown in Fig. 3, the helical structure of the liquid crystal molecules is unraveled even when no electric field is applied, and the dipole moment P or P' is upward 34 or downward 34'.
take either of the following states. Add a third cell to a cell like this
As shown in the figure, electric fields E with different polarities above a certain threshold value
or E′, the dipole moment is directed upward 34 corresponding to the electric field E or the electric field vector of E′.
or downward 34', and accordingly the liquid crystal molecules are oriented either in the first stable state 33 or in 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 has bistability. For example, change the second point to the third point.
To explain with the drawing, when an electric field E is applied, the liquid crystal molecules are oriented in a first stable state 33, and this state remains stable even when the electric field is turned off. When an electric field E' in the opposite direction is applied, the liquid crystal molecules are oriented to a second stable state 33' and the orientation of the molecules is changed, but they remain in this state even after the electric field is turned off. or,
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 that the cell be as thin as possible, and generally 0.5 to 20 μ, particularly 1 to 5 μ is suitable. A liquid crystal-electro-optical device with a matrix electrode structure using this type of ferroelectric liquid crystal is
For example, Clark and Ragabal, U.S. Patent No.
This is proposed in Publication No. 4367924.

A preferred example of the driving method of the present invention is shown in FIG.

FIG. 1A-a is a schematic diagram of a cell 11 having a matrix electrode structure in which a ferroelectric liquid crystal compound is sandwiched between. 12 is a scanning electrode group, and 13 is a signal electrode group. FIGS. 1A-b and 1A-c respectively show an electric signal applied to the selected scan electrode 12(S) and an electric signal applied to the other scan electrodes (unselected scan electrodes) 12(n), Figures 1A-d and 1E represent electrical signals applied to selected signal electrodes 13(s) and unselected signal electrodes 13(n), respectively. In each of FIGS. 1A-1B to A-E, the horizontal axis represents time and the vertical axis represents voltage. For example, when displaying a moving image, the scanning electrode group 12 sequentially
Selected periodically. Now, the threshold voltage to give the first stable state of the liquid crystal cell with bistability is
If Vth _{is 1} and the threshold voltage for providing the second stable state is -Vth ₂ , then the selected scan electrode 1
As shown in FIG. 1A-B, the electrical signal applied to 2(s) is an alternating voltage such that it is V at phase (time) _t1 and -V at phase (time) _t2 . Further, the other scanning electrodes 12(n) are in a grounded state as shown in FIG. 1A-c.
The electrical signal is 0. On the other hand, the selected signal electrode 1
The electrical signal given to the signal electrode 13(s) is V as shown in FIG. 1A-d, and the electrical signal given to the unselected signal electrode 13(n) is V as shown in FIG. 1A-e.
-V as shown in . In the above, the voltage value V is set to a desired value that satisfies V<V _th1 <2V and -V>V _th2 >-2V. FIG. 1B shows the voltage waveform applied to each pixel when such an electric signal is applied. B-e, B-b, B in Figure 1B
-c and B-d are pixel A in Figure 1A, respectively.
B, C and D correspond. That is, Figure 1B
As is clear from the above, in the pixel A on the selected scanning line, the voltage 2V exceeding the threshold _Vth1 at phase _t2
is applied. In addition, in pixel B existing on the same scanning line, the voltage exceeding the threshold value −V _th2 at phase t ₁ −
2V is applied. Therefore, depending on whether 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, on unselected scanning lines as shown in pixels C and D, the voltages applied to all pixels C and D are +V or -V, 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, a signal for that line is written, and that 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 and phase of the voltage value V (t ₁ +
The value of t ₂ )=T depends on the liquid crystal material used and the thickness of the cell, but is usually used in the range of 3 volts to 70 volts and 0.1 μsec to 2 msec. What is essentially different 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 applied to the selected scan electrode alternates from +V to -V. Further, the voltages applied to the signal electrodes are of opposite polarity to each other in order to designate a bright or dark state.

In order for the driving method of the present invention to be effectively achieved, the electric signal applied to the scanning electrode or the signal electrode must be
It is obvious that the signal does not necessarily have to be a simple rectangular wave signal as explained in FIGS. 1b to 1e. For example, it is also possible to drive with a sine wave or a triangular wave.

Further, FIG. 4 shows another specific example of the driving method of the present invention. Figures 4a, b, c and d respectively show the signals of the selected scanning electrodes, the signals of the unselected scanning electrodes, and the selected (information-containing) information signals.
and represents an unselected (no information) information signal. That is, as shown in FIG. 4, a voltage of +V is applied to the signal electrode with information only during phase (time) t ₂ , and a voltage of -V is applied to the signal electrode with no information only during phase (time) t ₁ . Even if a voltage of V is applied, the result is the same driving form as shown in FIG.

FIG. 5 shows a further modified example of the example shown in FIG. Figure 5 a, b, c and d are
Signals of each selected scanning electrode (Fig. 5a)
is the signal of the unselected scanning electrode (Fig. 5b)
, a selected (with information) information signal (FIG. 5c), and an unselected (without information) information signal (FIG. 5d). At this time, in order to drive correctly based on the present invention, the driving method shown in FIG. It is necessary to satisfy the following relationship.

FIG. 6 shows a schematic diagram of the matrix electrode structure when applied to a liquid crystal optical shutter. At this time, 41 is a pixel, and the electrodes on both sides of this portion are made of transparent material. 42 represents a scanning electrode group, and 43 represents a signal electrode group. Incidentally, examples of ferroelectric liquid crystal compounds include decyloxybenzylidene-P′-amino-2-
methylbutylcinnamate (DOBAMBC),
hexyloxybenzylidene―P′―amino―2―
chloropropyl cinnamate (HOBACPC) and 4
-O-(2-methyl)-butyl-resorcylidene-
Examples include 4′-octylaniline (MBRA8).

When constructing an element using these materials,
In order to maintain the temperature state such that the liquid crystal compound becomes the S _n ^* C phase or the S _n H ^* phase, the element can be supported by a copper block or the like in which a heater is embedded, if necessary.

The method of the present invention can be widely applied to the field of optical shutters or displays such as liquid crystal optical shutters and liquid crystal televisions.

[Brief explanation of the drawing]

FIG. 1Aa is a plan view schematically showing a liquid crystal element used in the driving method of the present invention. Figure 1 Ab is
FIG. 3 is an explanatory diagram showing signals of selected scanning electrodes.
FIG. 1 Ac is an explanatory diagram showing signals of unselected scanning electrodes. FIG. 1 Ad is an explanatory diagram showing information signals of selected signal electrodes. Figure 1 Ae
FIG. 2 is an explanatory diagram showing information signals of unselected signal electrodes. FIG. 1 Ba is a waveform diagram of the voltage applied to the liquid crystal of pixel A. Figure 1 Bb is pixel B
FIG. 3 is a waveform diagram of the voltage applied to the liquid crystal of FIG. Figure 1
Bc is a waveform diagram of a voltage applied to the liquid crystal of pixel C. FIG. 1Bd is a waveform diagram of the voltage applied to the liquid crystal of the pixel D. FIG. 2 is a perspective view schematically showing a liquid crystal element having a chiral smectoid phase liquid crystal. FIG. 3 is a perspective view schematically showing a liquid crystal element used in the present invention. Figure 4a is
FIG. 7 is an explanatory diagram showing signals of selected scanning electrodes in another specific example. FIG. 4b is an explanatory diagram showing signals of unselected scanning electrodes in another specific example. FIG. 4c is an explanatory diagram showing information signals of selected signal electrodes in another specific example. Fourth
FIG. d is an explanatory diagram showing information signals of unselected signal electrodes in another specific example. Figure 5a is
FIG. 7 is an explanatory diagram showing signals of selected scanning electrodes in another specific example. FIG. 5b is an explanatory diagram showing signals of unselected scanning electrodes in another specific example. FIG. 5c is an explanatory diagram showing information signals of selected signal electrodes in another specific example. Fifth
FIG. d is an explanatory diagram showing information signals of unselected signal electrodes in another specific example. FIG. 6 is a plan view of a liquid crystal optical shutter using the driving method of the present invention. 11...Liquid crystal element, 12...Scanning electrode group, 12
(s)...Selected scanning electrode, 12(n)...Unselected scanning electrode, 13...Signal electrode group, 13
(s) Selected signal electrode, 13(n)...Unselected signal electrode, 33...Liquid crystal oriented in first stable state, 33'...Liquid crystal oriented in second stable state, 34...Upward dipole moment P, 34'...
Downward dipole moment P′.

Claims (1)

[Claims] 1. In a liquid crystal device having a scanning electrode group and a signal electrode group, and in which a ferroelectric liquid crystal is arranged between the scanning electrode group and the signal electrode group, a scanning selection signal is applied to the scanning electrode, and the scanning selection signal is is the first phase and the second phase, with the voltage applied to the scan non-selected electrode as a reference,
each has one polarity voltage and the other polarity voltage,
In the first phase, a voltage exceeding the threshold voltage of one of the ferroelectric liquid crystals is applied to the selected signal electrode by combining with the scanning selection signal, and at the same time, a voltage exceeding the threshold voltage of one of the ferroelectric liquid crystals is applied to the selected signal electrode by combining with the voltage applied to the scanning non-selected electrode. A first information signal that provides a voltage between one threshold voltage of the ferroelectric liquid crystal and the other threshold voltage is applied, and in the second phase, the signal of the ferroelectric liquid crystal is applied to the other signal electrode by combining with the scanning selection signal. A second information signal is applied that exceeds the threshold voltage of the other and simultaneously provides a voltage between one threshold voltage and the other threshold voltage of the ferroelectric liquid crystal by combining with the voltage applied to the scanning non-selected electrode. A liquid crystal device having means for applying an voltage.