Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1995-11-03
2001-02-27
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C345S096000, C345S097000, C349S174000
Reexamination Certificate
active
06195137
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a liquid crystal apparatus for displaying various data by using a liquid crystal showing a ferroelectric state and an anti-ferroelectric state.
Hitherto, various liquid crystal apparatus for displaying various data by using a liquid crystal have been proposed, inclusive of one using a liquid crystal capable of assuming a ferroelectric state and an anti-ferroelectric state (hereinafter called a “ferroelectric liquid crystal”) as disclosed in Japanese Laid-Open Patent Application (JP-A) 6-20278.
The ferroelectric liquid crystal is disposed in a non-helical state within a small gap between a pair of substrates and assumes a smectic layer structure as shown in
FIG. 1A
under no electric field (E=0) wherein liquid crystal molecules are tilted in directions which are opposite to each other and alternate layer by layer of smectic layers, thereby providing an average optical axis parallel to the smectic layer normal (in a direction of arrow A in FIG.
1
A). If a liquid crystal device containing such a ferroelectric liquid crystal is sandwiched between a pair of polarizers arranged in cross nicols and having absorption axes A and B, respectively, the ferroelectric liquid crystal assumes a dark state (anti-ferroelectric state) under no electric field where the optical axis is parallel to the layer normal direction. If a voltage of positive polarity (E>0) is applied to the liquid crystal, a transition to a ferroelectric state is caused, wherein all the liquid crystal molecules
1
are tilted, e.g., rightwards as shown in
FIG. 1B
to provide a tilted optical axis in the direction of the tilted liquid crystal molecules. Accordingly, in the case where the polarizers are arranged in the above-described position the same as in
FIG. 1A
, the liquid crystal assumes a bright state (ferroelectric state) (hereinafter called a “first bright state” or a “first ferroelectric state”). Further, when the ferroelectric liquid is supplied with a voltage of negative polarity (E>0), all the liquid crystal molecules
1
are tilted, e.g., leftwards to provide a correspondingly tilted optical axis as shown in FIG.
1
C. Accordingly, in the case of the same polarizer arrangement, the liquid crystal also assumes a bright state (hereinafter called a “second bright state” or a second “ferroelectric state”).
The above-mentioned JP-A 6-202078 discloses a method of using the above-mentioned properties of a ferroelectric liquid crystal and displaying a bright state while inverting the applied voltage E for each of prescribed period (prescribed frame) (hereinafter, this method is called “polarity-inversion drive method”). The polarity-inversion drive method will now be described with reference to
FIGS. 2 and 3
.
FIG. 2
shows a voltage waveform applied to a pixel displaying bright states. In the polarity-inversion drive method, such a pixel is, for example, supplied with a positive-polarity voltage E
1
to form a first bright state in an odd-numbered frame and supplied with a negative-polarity voltage E
2
to form a second bright state in an even-numbered frame, whereby the pixel is designed to continuously display bright states while inverting the applied voltage polarity for each frame.
FIGS. 3A and 3B
are enlarged schematic views of respective pixels showing bright and dark states in a liquid crystal display device P using such a feroelectric liquid crystal, wherein pixels in a dark state are indicated with hatching and pixels in a bright state are indicated with no hatching.
FIG. 3A
shows a display state in an odd-numbered frame and
FIG. 3B
shows a display state in an even-numbered frame. Thus, the voltages E
1
and E
2
in
FIG. 2
are applied to the non-hatched pixels. In the display state of an odd-numbered frame shown in
FIG. 3A
, the non-hatched pixels are supplied with a positive-polarity voltage E
1
to provide a tilted liquid crystal optical axis as represented by a short line C. On the other hand, in the display state of an even-numbered frame shown in
FIG. 3B
, the non-hatched pixels are supplied with a negative-polarity voltage E
2
to provide a reversely tilted liquid crystal optical axis as represented by a short line C. The hatched pixels are supplied with no voltage in any frame, so that the optical axes are directed vertically to provide a dark state. Incidentally, the liquid crystal display device P is provided with large numbers of scanning lines
2
and data lines
3
running vertically and laterally disposed on a glass substrate (not shown), and a field-effect transistor
5
is disposed at each intersection of the scanning lines
2
and the data lines
3
. Each field-effect transistor
5
is connected to a pixel electrode
6
defining a pixel. (Details will be described later.) The arrows A and B in
FIGS. 3A and 3B
represent the absorption axes of the polarizers (not shown) arranged in cross nicols.
According to this method, the polarity of the voltage applied to pixels in a bright state is inverted for each frame as shown in
FIG. 2
, the voltage applied to the liquid crystal becomes averagely zero, thereby obviating the deterioration of the liquid crystal due to DC component.
In the above-described manner, the ferroelectric liquid crystal assumes one dark state and two bright states depending on the electric field applied thereto. The alignments of liquid crystal molecules in the respective states may be as shown in
FIGS. 4A-4C
when viewed in an oblique direction.
FIG. 4A
shows an alignment of liquid crystal molecules
1
in the dark state,
FIG. 4B
shows an alignment of liquid crystal molecules
1
in the first bright state (under application of a positive-polarity voltage, E>0), and
FIG. 4C
shown an alignment state in the second bright state (under application of a negative-polarity voltage, E>0). Now, when viewed in an oblique direction, the orientations of liquid crystal molecules are different between the first and second bright states (FIGS.
4
B and
4
C), so that the refractive index anisotropy in the second bright state in remarkably smaller than that in the first bright state, thus resulting in ai difference in transmittance. Further, when the liquid crystal display device P is viewed in an oblique direction in this manner, the optical path length is increased, the retardation is deviated from an optimum value in the first bright state to provide a yellowish tint when compared with that in the second bright state.
Thus, in the polarity-inversion drive method wherein the above-mentioned first and second bright state are alternately switched from each other for each frame, the liquid crystal display device P, when viewed in an oblique direction, provides transmittances and hues which periodically changes, thus causing “flickering” and inferior display image quality.
The above-mentioned problem becomes further serious when the ferroelectric liquid crystal is subjected to an active matrix drive. More specifically, when an ordinary case of using a transistor as an active element, a transistor shows a difference in its performance depending on whether it is used to charge a pixel to a positive voltage or to a negative voltage. Accordingly, different magnitudes of voltage are applied to the ferroelectric liquid crystal in the two ferroelectric states of
FIGS. 1B and 1C
, so that different transmittances result not only when viewed in an oblique direction as described but also when viewed normally or from a frontal position. As is well known, such an asymmetrical performance of a transistor is attributable to a phenomenon that a gate pulse exerts an induction voltage to a pixel voltage via a floating capacitance between the gate and drain. The same asymmetrical performance is caused also in active-matrix drive of a TN (twisted nematic) liquid crystal. However, in the case of a TN liquid crystal, a dark state is formed by a voltage application, so that the flickering is relatively alleviated. On the other hand, in the case of a ferroelectric liquid crystal, the flickering is caused
Inaba Yutaka
Nakamura Shin-ichi
Tsuboyama Akira
Canon Kabushiki Kaisha
Parker Kenneth
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