Computer graphics processing and selective visual display system – Display driving control circuitry
Reexamination Certificate
1999-04-28
2001-11-27
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Display driving control circuitry
C345S087000, C345S095000, C345S097000, C345S206000, C345S210000, C349S037000, C349S172000, C349S174000
Reexamination Certificate
active
06323850
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a driving method for a liquid crystal device allowing a high-speed drive for gradational display according to the active matrix scheme.
Various types of liquid crystal materials are used in liquid crystal display apparatus, inclusive of nematic liquid crystals, smectic liquid crystals and polymer dispersion-type liquid crystals.
Particularly, a liquid crystal device exhibiting a spontaneous polarization and bistability has been proposed by Clark and Lagerwall in U.S. Pat. No. 4,367,924, etc. As the bistable liquid crystal, a ferroelectric liquid crystal in chiral smectic C phase (SmC*) or H phase (SmH*) is generally used. In this phase, the liquid crystal exhibits bistable states, i.e., a first optically stable state and a second optically stable state, in response to an electric field applied thereto, and also a memory characteristic, i.e., a property that the resultant first or second optically stable state is retained as it is in the absence of an electric field. The liquid crystal device also quickly responds to a change in electric field and accordingly is expected to be widely utilized in the field of high-speed memory-type display apparatus.
As liquid crystal devices using a liquid crystal having a spontaneous polarization, there are also known in recent years an anti-ferroelectric liquid crystal device using a liquid crystal exhibiting two ferroelectric states and one anti-ferroelectric state (J.J.A.P., 28, L1265, 1989), and a so-called thresholdless anti-ferroelectric liquid crystal device wherein the optical axis of liquid crystal molecules is continuously changed in a plane parallel to the substrates in response to the strength and polarity of an applied electric field (Asia Display '95 Digest, P. 61, 1995).
The former anti-ferroelectric liquid crystal device effects a picture display by utilizing the stability of an alignment state possessed by the anti-ferroelectric liquid crystal. More specifically, the anti-ferroelectric liquid crystal assumes three stable states in alignment of liquid crystal molecules. In response to a voltage exceeding a first threshold, the liquid crystal is oriented to a first ferroelectric phase wherein liquid crystal molecules are aligned in a first direction or a second ferroelectric phase wherein liquid crystal molecules are aligned in a second direction depending on the polarity of the applied voltage, and in response a voltage below a second threshold which is lower than the first threshold, the liquid crystal is oriented to an anti-ferroelectric phase which is an intermediate alignment state between the first and second ferroelectric phases. If the transmission axes of a pair of polarizers disposed on both sides of the liquid crystal device are set with reference to the optical axis in the anti-ferroelectric phase, the optical transmittance through the device can be controlled to effect a picture display.
A driving method for a display device comprising the above-mentioned anti-ferroelectric liquid crystal device equipped with active drive devices or elements is disclosed in Japanese Laid-Open Patent Application (JP-A) 7-64056 which discloses a scheme wherein a writing voltage is applied to a liquid crystal placed in a ferroelectric phase or an anti-ferroelectric phase.
On the other hand, several studies have been made on active matrix drive of the above-mentioned thresholdless anti-ferroelectric liquid crystal device exhibiting high-speed responsiveness and wide viewing angle characteristic, e.g., as disclosed in the following references:
(1) “A full-color thresholdless Antiferroelectric LCD exhibiting wide viewing angle with fast response time”, T. Yoshida et al., SID 97 (Society for Information Display 97) DIGEST, pp. 841-844, and
(2) “Voltage-holding properties of thresholdless Antiferroelectric liquid crystals driven by active matrices”, T. Saishu, et al., SID 96 (Society for Information Display 96) DIGEST, pp. 703-706.
The above-mentioned ferroelectric liquid crystal and antiferroelectric liquid crystal both have a spontaneous polarization and therefore cause a current (i.e., an inversion current) accompanying the inversion of the spontaneous polarization at the time of the switching of liquid crystal molecules. The inversion current flows in a direction of obstructing the external electric field, i.e., in a direction of consuming an electric charge stored in a liquid crystal capacitance via a switching device. Accordingly, there occurs no problem if all liquid crystal molecules are switched and charges consumed by an inversion current accompanying the switch are supplemented during a period of switching element being ON, i.e., during a scanning selection period, but if the switching is not completed within the scanning selection period and some liquid crystal molecules are switched in a subsequent non-selection period, the voltage applied to the liquid crystal layer is lowered by the inversion current accompanying the switching of the liquid crystal molecules. This phenomenon is explained with reference to FIG.
6
.
FIG. 6
is an example of time chart for driving a thresholdless antiferroelectric liquid crystal device as described above according to a known active matrix scheme. Referring to
FIG. 6
, at (a) is shown a scanning signal voltage waveform applied to switching devices on an arbitrarily selected scanning signal line wherein T
G
represents a scanning selection period. At (b) is shown a data signal voltage waveform applied to a pixel electrode via a switching device at a certain pixel on the selected scanning signal line. At (c) is shown a voltage waveform applied to the liquid crystal layer at the pixel. At (d) is shown a transmittance change at the pixel wherein the darkest state is represented as 0% and the brightest state is represented as 100%.
At (d) of
FIG. 6
is illustrated a pixel intended to display a 100% display state in a frame period T
F1
and a 0% display state in a frame period T
F2
. However, as shown for a selection period T
G
in the frame period T
F2
at (d), if the switching to a 0% state is not completed within the selection period T
G
, the voltage applied across the liquid crystal layer at the pixel is raised by an inversion current due to liquid crystal molecules switched in a subsequent non-selection period as shown at (c), whereby the intended 0% display is failed as shown at (d). On the other hand, if the selection period Tg is extended so as to ensure the liquid crystal switching to the 0% state as shown in
FIG. 7
, the frame frequency is lowered (i.e., the frame period T
F1
, T
F2
. . . is increased).
SUMMARY OF THE INVENTION
A principal object of the present invention is to provide a driving method for a liquid crystal device using a liquid crystal having a spontaneous polarization capable of a high-speed drive for desired gradational display.
According to the present invention, there is provided a driving method for a liquid crystal device of the active matrix-type comprising a pair of substrates, a layer of liquid crystal having a spontaneous polarization disposed between the substrates so as to form two-dimensionally arranged pixels disposed along a plurality of rows and a plurality of columns, and a switching device disposed at each pixel so as to control a voltage applied to the liquid crystal at the pixel; the driving method comprising a frame operation including: dividing a scanning selection period for each selected row into a first period and a second period in a current frame, in the first period, applying a reset pulse to the liquid crystal at each pixel on the selected row, the reset pulse having a polarity opposite to that of a writing pulse voltage applied to the liquid crystal at the pixel in a previous frame, thereby resetting the pixels on the selected row to a first transmittance, and in the second period, applying a writing pulse of a prescribed voltage to the liquid crystal at each pixel to establish a prescribed transmittance for current frame display at the pixel.
As a result, accordin
Enomoto Takashi
Iba Jun
Katakura Kazunori
Miura Seishi
Takiguchi Takao
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Kovalick Vincent E.
Shalwala Bipin
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