Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Patent
1993-07-02
1995-10-17
Hjerpe, Richard
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
345210, G09G 336
Patent
active
054594812
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a driving method for display elements, light valves, etc., and more particularly it relates to a driving method for display elements that use a liquid crystal substance.
2. Related Art
The tristable switching of antiferroelectric liquid crystal is expected to solve some of the problems inherent in prior art surface stabilized ferroelectric liquid crystal (SSFLC), and its research is actively going forward. (Refer to A. D. L. Chandani, et al.: Jpn. J. Appl. Pys., 27, L729 (1988) and A. D. L. Chandani, et al.: Jpn. J. Appl. Phys., 28, L1265 (1988).)
The main features of tristable switching are: application has a steep threshold characteristic with respect to DC voltage (FIG. 33). wide optical hysteresis, and the selected state can be maintained as long as a bias voltage is applied after an antiferroelectric phase or a ferroelectric phase is selected. phase can be made optically equivalent. substance can be prevented, there is no deterioration over time of the electro-optical characteristic such as is seen in SSFLC. possible in a simple matrix with no restriction on the duty ratio. Examples of previously known driving methods are noted in M. Yamawaki, et al., Digest of Japan Display '89, p. 26 (1989) (FIG. 30). In FIG. 30, V.sub.t and V.sub.d are the voltage waveforms supplied to the scanning electrodes and signal electrodes, respectively, and V.sub.LC is a composite waveform applied to the liquid crystal layer V.sub.LC =V.sub.t -V.sub.d. In this driving method, frame F(+) on which a positive polarity voltage is applied and the subsequent negative polarity frame F(-) are a pair.
The principle of display by means of this driving method is explained using FIGS. 32A and 32B. Referring if FIG. 32A, optical axis OA in the antiferroelectric phase is perpendicular to the smectic layer. As shown in FIG. 32B, when a cell comprising liquid crystal 6 sandwiched between two glass substrates 1, 2 on which transparent electrodes 4, 5 and alignment films 9, 10 are formed is disposed between two polarizers 11, 12 whose polarization axes are perpendicular to each other such that optical axis OA is parallel to one of the polarization axes, the element goes to a light-blocking condition (tentatively OFF). Even if the voltage waveform in frame F'(+) or F'(-) in FIG. 30 is applied on this condition, as long as .vertline.V.sub.W2 .vertline.<.vertline.V(A-F)t.vertline. (see FIG. 33), the light transmittance changes very little and the OFF condition can be maintained. In case of which the voltage waveform of F(+) or F(-) in FIG. 30 is applied, the liquid crystal will respond if .vertline.V.sub.W1 .vertline.>.vertline.V(A-F)s.vertline., and change to ferroelectric phase(+) or ferroelectric phase (-). Ferroelectric phase(+ ) and ferroelectric phase (-), have the respective optical axes OF(+) and OF(-) and spontaneous polarizations Ps(+) and Ps(-). Since the optical axes form angle .theta.(+) or .theta.(-) with the polarization axis, a light transmission condition (tentatively ON) is set. Since angles .theta.(+) and .theta.(-) are equal, they can both be treated as being optically equivalent.
However, the prior art driving method has the two problems explained below.
One problem concerns the stability of the antiferroelectric phase. The antiferroelectric phase generally has a steep threshold characteristic with respect to DC voltage. Even if a single-polarity bias voltage is applied during the non-selection period (T.sub.22 in the figure) after the antiferroelectric phase has been selected in the selection period (T.sub.12 in the figure) as shown in FIGS. 31A and 31B, the state of the antiferroelectric phase can be maintained regardless of the duration in which the bias voltage is applied. However, in further research by the inventors, a phenomenon was observed in several liquid crystal materials in which the state gradually changed from the antiferroelectric phase to the ferroelectric phase as time elapsed from when the bias voltage was fi
REFERENCES:
patent: 5033822 (1991-07-01), Ooki et al.
Electro-Optical Properties of Fluorine-Containing Ferroelectric Liquid Crystal Cells, M. Yamawaki et al., Japan Display '89, pp. 26-29.
Ferroelectric Liquid Crystal Display Using Tristable Switching, Yuichiro Yamada et al., Japanese Journal of Applied Physics, vol. 29, No. 9, Sep. 1990, pp. 1757-1764.
Sato Yuzuru
Tanaka Takaaki
Hjerpe Richard
Janofsky Eric B.
Lao Lun-Yi
Seiko Epson Corporation
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