Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-04-29
2002-12-17
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S092000, C345S093000, C345S094000, C345S097000, C345S098000, C345S099000, C345S101000, C345S204000, C345S206000, C345S214000, C349S033000, C349S038000, C349S174000, C349S172000
Reexamination Certificate
active
06496170
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a liquid crystal apparatus for effecting an active matrix drive by using a liquid crystal having a spontaneous polarization.
At present, most of liquid crystal display apparatus for use in monitors of liquid crystal television sets, word processors and personal computers principally employ a TN (twisted nematic) mode or an STN (super twisted nematic) mode using a nematic liquid crystal as a display mode.
In the case of using such a TN or STN mode for multiplex driving scheme, however, an increase in the number of scanning signal lines is liable to lower a contrast. Even if a drive waveform is optimized in order to provide practical display qualities, the number of scanning signal line has been restricted to ca. 400-500 lines at best.
In order to sole such a problem that display qualities are lowered with an increased number of scanning signal lines in a liquid crystal display mode (TN or STN mode), there has been proposed an active matrix (display) mode using a plurality of switching devices or devices, such as MIM (metal-insulator-metal) devices or TFTs (thin-film-transistors), disposed in a matrix form on a substrate.
In this case, however, a nematic liquid crystal used as a liquid crystal material therefor shows a slow response speed of several hundred msec particularly for gradation display signals, thus failing to follow high-speed motion display. As a result, it is difficult to provide sufficient display qualities. Further, in the above-mentioned TN (or STN) mode, liquid crystal molecules cause switching between a state where they are twisted and in parallel with a substrate and a state where they are perpendicular to the substrate, thus resulting in a large viewing angle-dependence based on its principle.
On the other hand, there have been developed display devices using a liquid crystal having a spontaneous polarization, such as a ferroelectric or chiral smectic liquid crystal, in view of, e.g., a higher-speed responsiveness and a wider viewing-angle characteristic when compared with those of the TN or (STN) mode using the nematic liquid crystal.
For example, as a display device using a liquid crystal showing ferroelectricity, there has been proposed a surface-stabilized ferroelectric liquid crystal display device as described in Japanese Laid-Open Patent Application (JP-A) 56-107216, wherein multiplex driving scheme is practiced according to a simple matrix mode utilizing bistability of liquid crystal molecules. However, this driving scheme fails to continuously change a resultant transmittance since it performs two-value (binary) driving using bistable states of liquid crystal molecules, thus not facilitating gradational display. For this reason, there have been proposed various gradational display methods using, e.g., pixel division, time-division display, and image processing.
Further, there have been proposed active matrix driving schemes utilizing a high-speed responsiveness and a wide view-angle characteristic of a ferroelectric or antiferroelectric liquid crystal. For example JP-A 5-100208 discloses a method of effecting gradational display by performing active matrix driving of an antiferroelectric liquid crystal assuming three stable states and JP-A 9-68728 discloses a gradational display method using an active matrix driving scheme and a thresholdless-antiferroelectric liquid crystal (TL-AFLC).
However, in the case of the active matrix driving scheme using a chiral smectic liquid crystal (e.g., a ferroelectric or antiferroelectric liquid crystal), an effective voltage applied to the liquid crystal is substantially lowered to cause image-quality deterioration as described in, e.g., (1) A full-color thresholdless Antiferroelectric LCD exhibiting wide viewing angle with fast response time, T. Yoshida et al., SID (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 DIGEST, pp. 703-706. More specifically, in the case where an antiferroelectric (or ferroelectric) liquid crystal having a spontaneous polarization is driven (i.e., subjected to switching) by using an active element or device (e.g., TFT), an inversion of the spontaneous polarization of the liquid crystal causes a lowering in holding voltage to substantially decrease a voltage applied to the liquid crystal, thus resulting in a deterioration in image qualities, such as a low contrast.
The lowering in holding voltage leading to inferior image qualities will be simply described hereinbelow with reference to
FIGS. 3-5
.
FIG. 4
shows an equivalent circuit of one pixel portion of a liquid crystal device using a liquid crystal having a spontaneous polarization (in this instance, the TL-AFLC as described in the above document (1) is used), and
FIG. 3
shows a V-T (voltage-transmittance) curve as an optical response characteristic of the liquid crystal device using the TL-AFLC when a low-frequency triangular waveform is applied.
Referring to
FIG. 4
, the equivalent circuit includes a TFT
14
, a liquid crystal capacitance (C
lc
)
31
, a storage capacitance (Cs)
32
, a spontaneous polarization of the liquid crystal (Ps)
50
, a storage capacitance electrode
30
and a common electrode
42
. The TFT
14
includes a gate electrode, a source electrode and a drain electrode and supplies a voltage to the liquid crystal through the drain electrode. The storage capacitance (Cs)
32
for holding a voltage applied to the liquid crystal layer at the time of “OFF” state of the TFT
14
is disposed in parallel with the liquid crystal capacitance (Clc)
31
.
FIGS. 5A-5D
show drive waveforms applied to the pixel (shown in
FIG. 4
) and an optical response (transmittance) of the liquid crystal. More specifically,
FIG. 5A
shows a voltage waveform of a scanning selection signal applied to a scanning signal line (gate line) connected to the gate electrode of the TFT
14
.
FIG. 5B
shows a data signal voltage waveform applied to a data signal line (source line) connected to the source electrode of the TFT
14
.
FIG. 5C
shows a voltage waveform applied to the liquid crystal layer (portion) of the pixel concerned.
FIG. 5D
shows a transmittance of the pixel.
Referring to
FIG. 5A
, a gate voltage Vg as a signal for placing the gate of the TFT
14
in an “ON” state is periodically applied in every selection period Ton. In synchronism with the gate voltage Vg, a source voltage (data signal voltage) Vs is applied to the source electrode, thus being supplied to the liquid crystal layer via the drain electrode of the TFT
14
(FIG.
5
B). The source voltage Vs has a polarity which is inverted periodically in order to prevent a deterioration of the liquid crystal due to an asymmetrical voltage applied to the liquid crystal layer. Referring to
FIG. 5C
, a voltage Vpix applied to the liquid crystal layer is applied in a selection period Ton so that the Vpix has a polarity which is opposite to that in a period immediately before the selection period Ton. The liquid crystal starts to transfer its alignment state to that depending on the polarity of the voltage Vpix applied to the liquid crystal layer in the selection period Ton. If the response time of the liquid crystal is sufficiently shorter than the Ton, the transfer of the liquid crystal is continued to a (subsequent) non-selection period Toff by a voltage based on a storage capacitance. The liquid crystal used has a spontaneous polarization, so that a voltage decrease (lowering) (corresponding to &Dgr;V) is caused by the spontaneous polarization when the direction thereof is inverted (FIG.
5
C). As a result, the liquid crystal is finally placed in an alignment state corresponding to the voltage Vpix including the voltage drop &Dgr;V. Accordingly, as shown in
FIG. 5D
, an optical response of the liquid crystal is also continued to the non-selection period Toff.
As described above, in order to obtain a desired optical (display) state, it is necessary to effect a
Enomoto Takashi
Komiyama Katsumi
Yoshida Akio
Hjerpe Richard
Zamani Ali
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