Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1995-04-13
2001-12-04
Lao, Lun-Yi (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S097000, C349S072000
Reexamination Certificate
active
06326943
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a display device and, more particularly, to a display device having a memory function, e.g., a display device using a ferroelectric liquid crystal element.
2. Related Background Art
A known liquid crystal element using a liquid crystal compound comprises scanning and signal electrodes arranged in a matrix form, and a liquid crystal compound filled between the electrodes to constitute a large number of pixels, thereby displaying image information.
According to a conventional time-divisional method of driving such a display element, voltage signals are sequentially and periodically applied to the scanning electrodes, and predetermined information signals are in parallel applied to the signal electrodes in synchronism with the scanning electrode operations. According to the above-mentioned display element and its driving method, it is difficult to increase both the pixel density and the screen size.
The most popular liquid crystal element is a TN (twisted nematic) element since it has a relatively short response time among the liquid crystal materials and low power consumption. In a state of no electric field applied, twisted nematic liquid crystal molecules having positive dielectric anisotropy have a twisted structure (helical structure) in a direction of thickness of a liquid crystal layer, as shown in FIG.
41
A. The liquid crystal molecules of the respective molecular layers are twisted and parallel to each electrode surface between the upper and lower electrodes. However, as shown in
FIG. 41B
, in an electric field, the nematic liquid crystal molecules having positive dielectric anisotropy are oriented in the direction of the electric field, thereby causing optical modulation. When a display element is arranged in a matrix electrode structure by using such a liquid crystal material, a signal voltage higher than a threshold value required for orienting the liquid crystal molecules in a direction perpendicular to each electrode surface is applied to a selected area (i.e., a selected point) as an intersection between the corresponding scanning and signal electrodes. The signal voltage is not applied to non-selected intersections (non-selected points) between the non-selected scanning and signal electrodes. Therefore, in these points, the liquid crystal molecules are twisted and parallel to each electrode surface. When linear polarizers in a relationship of crossed nicols are arranged on the upper and lower surface of this liquid crystal cell, light is not transmitted at the selected point(s), but light is transmitted at the non-selected point(s) due to the twist structure of the liquid crystal and an optical rotary power, thereby providing an imaging element.
With a matrix electrode structure, a limited electric field is applied to an area (so-called “semi-selected point”) where the scanning electrode is selected and the signal electrode crossing this scanning electrode is not selected, and vice versa. If the difference between the voltage applied to the selected point and the voltage applied to the semi-selected point is sufficiently large, and a voltage threshold required for vertically aligning the liquid crystal molecules with respect to the electrode surface can be set to an intermediate value between the above voltages, the display element can be normally operated.
When the number (N) of scanning lines is increased in the above system, a duration (i.e., a duty ratio) for which an effective electric field is applied to one selected point during scanning of one frame is decreased at a rate of 1/N. For this reson, the difference between voltages, i.e., effective values, applied to the selected and non-selected points upon repetition of the scanning cycle is decreased when the number of scanning lines is increased. As a result, a decrease in image contrast and a crosstalk phenomenon cannot be avoided inevitably.
The above phenomena inevitably occur when a liquid crystal without a bistable state (i.e., liquid crystal molecules are stably oriented in a direction parallel to the electrode surface and their orientation is changed in a direction perpendicular to the electrode surface during an effective application of the electric field) is driven by utilizing an accumulation effect as a function of time (i.e., scanning is repeated). In order to solve this problem, various driving schemes such as a voltage averaging scheme, a 2-frequency driving scheme, and a multiple matrix scheme are proposed. However, none of these conventional schemes are satisfactory. Therefore, a large screen and a high packing density of a display element cannot be obtained since the number of scanning lines cannot be sufficiently increased.
In order to solve the above problem, the present applicant filed a U.S. Ser. No. 598,800 (Apr. 10, 1984) entitled as a “Method of Driving Optical Modulation Device”. In this prior art, the present applicant proposed a method of driving a liquid crystal having a bistable state with respect to an electric field. An example of the liquid crystal which can be used in the above driving method is preferably a chiral smectic liquid crystal, and more preferably a chiral smectic C-phase (SmC*) or H-phase (SmH*).
The SmC* has a structure in which liquid crystal molecular layers are parallel to each other, as shown in FIG.
42
. The direction of a major axis of each molecule is inclined with respect to the layer. These liquid crystal molecule layers have different inclination directions and therefore constitute a helical structure.
The SmH* has a structure in which the molecular layers are parallel to each other, as shown in FIG.
43
. The direction of a major axis of each molecule is inclined with respect to the layer, and the molecules constitute a six-direction filled structure on a plane perpendicular to the major axis of the molecule.
The SmC* and SmH* have helical structures produced by the liquid crystal molecules, as illustrated in FIG.
44
.
Referring to
FIG. 44
, each liquid crystal molecule e
3
has electrical bipolar moments e
4
in a direction perpendicular to the direction of the major axis of the molecule e
3
. The molecules e
3
move while maintaining a predetermined angle &thgr; with respect to the Z-axis perpendicular to a layer boundary surface e
5
, thereby constituting a helical structure.
FIG. 44
shows a state when a voltage is not applied to the liquid crystal molecules. If a voltage exceeding a predetermined threshold voltage is applied to the X direction, the liquid crystal molecules e
3
are orientated such that the electrical bipolar moments e
4
are parallel to the X-axis.
The SmC* or SmH* phase is realized as one of the phase transition cycles caused by changes in temperatures. When these liquid crystal compounds are used, a proper element must be selected in accordance with the operating temperature range of the display device.
FIG. 45
shows a cell when a ferroelectric liquid crystal (to be referred to as an FLC hereinafter) is used. Substrates (glass plates) e
1
and e
1
′ are coated with transpatent electrodes comprising In
2
O
2
, SnO
2
or ITO (indium-tin oxide). An SmC*-phase liquid crystal is sealed between the substrates e
1
and e
1
′ such that liquid crystal molecular layers e
2
are oriented in a direction perpendicular to the substrates e
1
and e
1
′. The liquid crystal molecules e
3
represented by thick lines have bipolar moments e
4
in directions perpendicular to the corresponding molecules e
3
. When a voltage exceeding a predetermined threshold is applied between the substrates e
1
and e
1
′, the helical structure of the liquid crystal molecules e
3
is changed such that the directions of orientation of the liquid crystal molecules e
3
are aligned with the direction of the electric field. Each liquid crystal molecule e
3
has an elongated shape and exhibits refractive anisotropy in the major and minor axes. For example, when polarizers having a positional relationship of crossed nicols with the orientation direction
Inoue Hiroshi
Kanno Hideo
Mizutome Atsushi
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Lao Lun-Yi
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