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Claims  |
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What is claimed is:
1. An electro-optical modulating apparatus, comprising:
a liquid crystal device comprising a plurality of pixels forming a display
area having a low-threshold region including a pixel having a saturation
voltage of V.sub.sat(min), and a high threshold region, including a pixel
having a saturation voltage of V.sub.sat(max), each of said plurality of
pixels comprising a pair of opposite electrodes and an optical modulation
substance, capable of assuming a first molecular orientation state and a
second molecular orientation state, between the electrodes; and
voltage application means for sequentially applying, to each pixel, a first
voltage V.sub.l of one polarity of at least V.sub.sat(max), a second
voltage V.sub.2 of the opposite polarity of at most V.sub.sat(min) and a
third voltage V.sub.3 of the one polarity set to a value within the range
of V.sub.th(max) to the second voltage V.sub.2, wherein
V.sub.sat(max) denotes a maximum saturation voltage value among saturation
voltages occurring in the plurality of pixels,
V.sub.sat(min) denotes a minimum saturation voltage value among the
saturation voltages occurring in the plurality of pixels, and
V.sub.th(max) denotes a maximum threshold voltage value among threshold
voltages occurring in the plurality of pixels.
2. An apparatus according to claim 1, wherein said optical modulating
substance comprises a ferroelectric liquid crystal.
3. An apparatus according to claim 1, wherein said optical modulating
substance comprises a ferroelectric liquid crystal showing bistability.
4. An apparatus according to claim 1, wherein said third voltage comprises
a voltage signal depending on given gradation data.
5. An apparatus according to claim 1, wherein said plurality of pixels are
arranged in a plurality of rows and a plurality of columns so as to form a
matrix.
6. An apparatus according to claim 1, wherein the first voltage V.sub.1 is
applied immediately before the second voltage V.sub.2.
7. An apparatus according to claim 1, wherein said voltage application
means includes means for applying an alternating voltage between the
period of application of the second voltage V.sub.2 and the period of
application of the third voltage V.sub.3.
8. An apparatus according to claim 1, wherein said voltage application
means includes means for applying an alternating voltage after the
application of the third voltage V.sub.3.
9. An electro-optical modulating apparatus, comprising:
a liquid crystal device comprising an electrode matrix comprising scanning
electrodes and data electrodes intersecting the scanning electrodes, and
an optical modulating substance showing a first molecular orientation
state and a second molecular orientation state disposed between the
scanning electrodes and data electrodes so as to form a plurality of
pixels each at an intersection of the scanning electrodes and data
electrodes, said plurality of pixels forming a display area having a
low-threshold region, including a pixel having a saturation voltage of
V.sub.sat(min), and a high threshold region, including a pixel having a
saturation voltage of V.sub.sat(max), and
voltage application means for applying a scanning selection signal to a
selected particular scanning electrode among the scanning electrodes, and
for sequentially applying, to all or a prescribed number of the pixels on
the selected particular scanning electrode, a first voltage V.sub.l of one
polarity of at least V.sub.sat(max) a second voltage V.sub.2 of the
opposite polarity of at most V.sub.sat(min) and a third voltage V.sub.3 of
the one polarity set to a value within the range of V.sub.th(max) to the
second voltage V.sub.2, wherein
V.sub.sat(max) denotes a maximum saturation voltage value among saturation
voltages occurring in the plurality of pixels,
V.sub.sat(min) denotes a minimum saturation voltage value among the
saturation voltages occurring in the plurality of pixels, and
V.sub.th(max) denotes a maximum threshold voltage value among threshold
voltages occurring in the plurality of pixels.
10. An apparatus according to claim 9, wherein said optical modulating
substance comprises a ferroelectric liquid crystal.
11. An apparatus according to claim 9, wherein said optical modulating
substance comprises a ferroelectric liquid crystal showing bistability.
12. An apparatus according to claim 9, wherein said third voltage comprises
a voltage signal depending on given gradation data.
13. An electro-optical modulating apparatus, comprising:
(A) a liquid crystal device comprising an electrode matrix comprising
scanning electrodes and data electrodes intersecting the scanning
electrodes, and an optical modulating substance showing a first molecular
orientation state and a second molecular orientation state disposed
between the scanning electrodes and data electrodes so as to form a
plurality of pixels each at an intersection of the scanning electrodes and
data electrodes, said plurality of pixels forming a display area having a
low-threshold region, including a pixel having a saturation voltage of
V.sub.sat(min), and a high threshold region, including a pixel having a
saturation voltage of V.sub.sat(max) ; and
(B) voltage application means for:
in a first step, applying a first voltage of one polarity of at least
V.sub.sat(max) to all the pixels on all or a prescribed number of the
scanning electrodes, and
in a second step, (a) applying a scanning selection signal to a selected
particular scanning electrode among the scanning electrodes, and
(b) sequentially applying, to all or a prescribed number of the pixels on
the selected particular scanning electrode, a second voltage V.sub.2 of
the opposite polarity of at most V.sub.sat(min) and a third voltage
V.sub.3 of the one polarity set to a value within the range of
V.sub.th(max) to the second voltage V.sub.2, wherein
V.sub.sat(max) denotes a maximum saturation voltage value among saturation
voltages occurring in the plurality of pixels;
V.sub.sat(min) denotes a minimum saturation voltage value among the
saturation voltages occurring in the plurality of pixels, and
V.sub.th(max) denotes a maximum threshold voltage value among threshold
voltages occurring in the plurality of pixels.
14. An apparatus according to claim 13, wherein said optical modulating
substance comprises a ferroelectric liquid crystal.
15. An apparatus according to claim 13, wherein said optical modulating
substance comprises a ferroelectric liquid crystal showing bistability.
16. An apparatus according to claim 13, wherein said third voltage
comprises a voltage signal depending on given gradation data.
17. A driving method for a liquid crystal device comprising a plurality of
pixels each comprising a pair of opposite electrodes, and an optical
modulating substance assuming a first molecular orientation state and a
second molecular orientation state between the electrodes, said plurality
of pixels forming a display area having a low-threshold region, including
a pixel having a saturation voltage of V.sub.sat(min), and a high
threshold region, including a pixel having a saturation voltage of
V.sub.sat(max), said driving method comprising:
sequentially applying, to the plurality of pixels, a first voltage V.sub.1
of one polarity of at least V.sub.sat(max), a second voltage V.sub.2 of
the opposite polarity of at most V.sub.sat(min) and a third voltage
V.sub.3 of said one polarity set to a value within the range of
V.sub.th(max) to the second voltage V.sub.2 ; wherein
V.sub.sat(max) denotes a maximum saturation voltage value among saturation
voltages occurring in the plurality of pixels,
V.sub.sat(min) denotes a minimum saturation voltage value among the
saturation voltages occurring in the plurality of pixels, and
V.sub.th(max) denotes a-maximum threshold voltage value among threshold
voltages occurring in the plurality of pixels.
18. A method according to claim 17, wherein the optical modulating
substance comprises a ferroelectric liquid crystal.
19. A method according to claim 17, wherein the optical modulating
substance comprises a ferroelectric liquid crystal showing bistability.
20. A method according to claim 17, wherein the third voltage comprises a
voltage signal depending on given gradation data.
21. A method according to claim 17, wherein the plurality of pixels are
arranged in a plurality of rows and a plurality of columns so as to form a
matrix.
22. A method according to claim 17, wherein the first voltage V.sub.1 is
applied immediately before the second voltage V.sub.2.
23. A method according to claim 17, wherein an alternating voltage is
applied between the period of application of the second voltage v.sub.2
and the period of application of the third voltage V.sub.3.
24. A method according to claim 17, wherein an alternating voltage is
applied after the application of the third voltage V.sub.3.
25. A driving method for a liquid crystal device comprising an electrode
matrix comprising scanning electrodes and data electrodes intersecting the
scanning electrodes, and an optical modulating substance showing a first
molecular orientation state and a second molecular orientation state
disposed between the scanning electrodes and data electrodes so as to form
a plurality of pixels each at an intersection of the scanning electrodes
and data electrodes, said plurality of pixels forming a display area
having a low-threshold region, including a pixel having a saturation
voltage of V.sub.sat(min), and a high threshold region, including a pixel
having a saturation voltage of V.sub.sat(max), said driving method
comprising:
applying a scanning selection signal to a selected particular scanning
electrode among the scanning electrodes, and
sequentially applying, to all or a prescribed number of the pixels on the
selected particular scanning electrode, a first voltage V.sub.1 of one
polarity of at least V.sub.sat(max), a second voltage V.sub.2 of the
opposite polarity of at most V.sub.sat(min) and a third voltage V.sub.3 of
the one polarity set to a value within the range of V.sub.th(max) to the
second voltage V.sub.2, wherein
V.sub.sat(max) denotes a maximum saturation voltage value among saturation
voltages occurring in the plurality of pixels,
V.sub.sat(min) denotes a minimum saturation voltage value among the
saturation voltages occurring in the plurality of pixels, and
V.sub.th(max) denotes a maximum threshold voltage value among threshold
voltages occurring in the plurality of pixels.
26. A method according to claim 25, wherein the optical modulating
substance comprises a ferroelectric liquid crystal.
27. A method according to claim 25, wherein the optical modulating
substance comprises a ferroelectric liquid crystal showing bistability.
28. A method according to claim 25, wherein the third voltage comprises a
voltage signal depending on given gradation data.
29. A driving method for a liquid crystal device comprising an electrode
matrix comprising scanning electrodes and data electrodes intersecting the
scanning electrodes, and an optical modulating substance showing a first
molecular orientation state and a second molecular orientation state
disposed between the scanning electrodes and data electrodes so as to form
a plurality of pixels each at an intersection of the scanning electrodes
and data electrodes, said plurality of pixels forming a display area
having a low-threshold region, including a pixel having a saturation
voltage of V.sub.sat(min), and a high threshold region, including a pixel
having a saturation voltage of V.sub.sat(max), said driving method
comprising:
a first step of applying a first voltage V.sub.1 of one polarity of at
least V.sub.sat(max) to all the pixels on all or a prescribed number of
scanning electrodes,
a second step of (a) applying a scanning selection signal to a selected
particular scanning electrode among the scanning electrodes, and
(b) sequentially applying, to all or a prescribed number of the pixels on
the selected particular scanning electrode, a second voltage V.sub.2 of
the opposite polarity of at most V.sub.sat(min) and a third voltage
V.sub.3 of the one polarity set to a value within the range of
V.sub.th(max) to the second voltage V.sub.2, wherein
V.sub.sat(max) denotes a maximum saturation voltage value among saturation
voltages occurring in the plurality of pixels,
V.sub.sat(min) denotes a minimum saturation voltage value among the
saturation voltages occurring in the plurality of pixels, and
V.sub.th(max) denotes a maximum threshold voltage value among threshold
voltages occurring in the plurality of pixels.
30. A method according to claim 29, wherein the optical modulating
substance comprises a ferroelectric liquid crystal.
31. A method according to claim 29, wherein the optical modulating
substance comprises a ferroelectric liquid crystal showing bistability.
32. A method according to claim 29, wherein the third voltage comprises a
voltage signal depending on given gradation data. |
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Claims |
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Description |
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FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an optical modulating apparatus using a
ferroelectric liquid crystal and a driving method therefor, particularly
an optical modulation apparatus suitable for halftone display or
full-color display and a driving method therefor.
Hitherto, liquid crystal display devices are well known, which comprise a
group of scanning electrodes and a group of data electrodes arranged to
form a matrix, and a liquid crystal is filled between the electrode groups
to form a large number of pixels, thereby to display images or
information. These display devices are driven by a multiplexing drive
scheme wherein an address signal is applied sequentially and cyclically to
the scanning electrodes, and prescribed data signals are selectively
applied in parallel to the data electrodes in synchronism with the address
signal.
Most liquid crystals which have been put into practice for the above
purpose are TN (twisted nematic)-type liquid crystals, as shown in
"Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal"
by M. Schadt and W. Helfrich, Applied Physics Letters, vol. 18, No. 4
(1971), pp. 127-128.
In recent years, as an improvement to the conventional liquid crystal
devices, Clark and Lagerwall have proposed the use of a liquid crystal
device showing bistability, e.g., in U.S. Pat. No. 4,367,924. As the
bistable liquid crystal, a ferroelectric liquid crystal in chiral smectic
C phase (SmC*) or H phase (SmH*) is generally used. In these phases, such
a ferroelectric liquid crystal shows bistability, i.e., a property of
assuming either a first molecular orientation state or a second molecular
orientation state depending on an electric field applied thereto and
retaining the resultant state in the absence of an electric field.
Further, the ferroelectric liquid crystal quickly responds to a change in
electric field and is therefore expected to be widely used in the field of
high-speed and memory-type display apparatus.
A ferroelectric liquid crystal device has been generally used as a binary
(white and black) display device wherein the above-mentioned two stable
molecular orientation states and used for providing light-transmitting and
light-interrupting states but is able to also provide multiple display
states, i.e., a halftone or gray-scale display. According to a type of
halftone display method, an areal ratio of bistable states in a pixel is
controlled to provide an intermediate light-transmission. This method
(areal modulation method) is explained in more detail hereinbelow.
FIG. 2 is a view schematically illustrating a relationship between the
switching pulse amplitude and the resultant transmittance, more
specifically a graph showing a variation of a transmittance (or
transmitted light quantity) I of a cell (device) which has been originally
in a completely light-interrupting state as a function of the amplitude V
of a single pulse applied thereto. If the pulse amplitude V is below a
threshold V.sub.th (V<V.sub.th), the transmittance does not change and the
resultant transmission state is as shown in FIG. 3B which is not different
from the state before the pulse-application shown in FIG. 3A. If the pulse
amplitude exceeds the threshold (V.sub.th <V.sub.sat), the pixel is
partially transformed into the other optical state, to provide an
intermediate transmission state as shown in FIG. 3C, giving an
intermediate transmittance as a whole. If the pulse amplitude is further
increased to exceed a saturation value V.sub.sat (V.sub.sat <V), the whole
pixel is transformed into a light-transmitting state as shown in FIG. 3D
to reach a constant transmittance.
Thus, in the areal modulation method, the applied voltage is controlled to
provide an amplitude V satisfying V.sub.th <V <V.sub.sat to display a
halftone.
However, the areal modulation method has been found to involve a serious
defect as will be described below. This arises from a fact that the
relationship between the voltage and the transmittance shown in FIG. 2
depends on the cell thickness and temperature and if there is a
distribution in cell thickness or temperature over a display panel area,
different levels of gradation are displayed at the same amplitude of
applied voltage pulse. FIG. 4 is a view for describing this fact. FIG. 4
is a graph showing a relationship between voltage amplitude V and
transmittance I, similarly as FIG. 2, but shows two curves at different
temperatures including a curve H representing a relationship at a higher
temperature and a curve L representing a relationship at a lower
temperature. In a display panel of a large display size, it is common that
a temperature distribution occurs in the same panel, and as a result, even
if a prescribed halftone is intended to be displayed at a certain voltage
V.sub.ap, the resultant gradation levels can fluctuate ranging from
I.sub.1 to I.sub.2 as shown in FIG. 4, thus failing to provide a uniform
display of the prescribed halftone. The switching voltage of a
ferroelectric liquid crystal is generally high at a low temperature and
low at a high temperature, and the difference depends on the
temperature-dependent viscosity change of the liquid crystal and is
therefore much larger than in the conventional TN-type liquid crystal
devices. Accordingly, the change in gradation level due to temperature
distribution is much more noticeable than in the TN-liquid crystal and has
provided the most serious obstacle to realization of a halftone or
gradational display by a ferroelectric liquid crystal device.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an electro-optical
modulating apparatus, particularly a ferroelectric liquid crystal
apparatus, having solved the above-mentioned problems to provide a uniform
halftone level, and a driving method therefor.
According to an aspect of the present invention, there is provided an
electro-optical modulation system (apparatus and method) including a
liquid crystal display apparatus having a display unit for display of
images or data. Such display comprises scanning electrodes and data
electrodes arranged to form an electrode matrix, and a ferroelectric
liquid crystal showing bistability with respect to an electric field
applied thereto disposed between the scanning electrodes and data
electrodes. Therein, all the pixels on a selected scanning electrode are
completely reset into a first molecular orientation state of the liquid
crystal in a first step and incompletely reset into a second molecular
orientation state of the liquid crystal in a second step, and the
respective pixels on the selected scanning electrode are restored toward
the first molecular orientation state in a third step, so as to display a
halftone.
According to another aspect of the present invention, there is provided an
electro-optical modulating apparatus, comprising:
a liquid crystal device processing a plurality of pixels each comprising a
pair of opposite electrodes, and an optical modulating substance assuming
a first molecular orientation state and a second molecular orientation
state between the electrodes, and
voltage application means for applying to a pixel among said plurality of
pixels a first voltage for resetting the pixel to be occupied with the
first molecular orientation state, a second voltage for resetting the
pixel into a mixture state including a minor proportion of the first
molecular orientation state and a major proportion of the second molecular
orientation state, and then a third voltage for causing a prescribed ratio
of the first to second molecular orientation state at the pixel not
smaller than the ratio in said mixture state.
These and other objects, features and advantages of the present invention
will become more apparent upon a consideration of the following
description of the preferred embodiments of the present invention taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph illustrating examples of the amplitudes of the first and
second reset pulses and the gradation pulse used in the present invention
together with the resultant transmittances.
FIG. 2 is a schematic view illustrating a relationship between the
switching pulse amplitude and the transmittance of a ferroelectric liquid
crystal device.
FIGS. 3A-3D are illustrations of various transmission states of a
ferroelectric liquid crystal cell depending on applied pulses.
FIG. 4 is a graph showing a difference in relationship between voltage
amplitude V-transmittance I at higher and lower temperatures.
FIGS. 5A and 5B are time charts showing time serial waveforms of applied
pulses used for gradational display in multiplexing drive according to
prior art and the present invention, respectively.
FIGS. 6A-1 to 6A-3 and FIGS. 6B-1 to 6B-3 are schematic views illustrating
transmission states of a low-threshold pixel and a high-threshold pixel,
respectively, after application of reset pulses and a gradation pulse
according to the present invention.
FIG. 7 is a sectional view of a liquid crystal display device according to
an embodiment of the present invention wherein a minute unevenness is
provided to electrode surfaces on one substrate so as to form a threshold
distribution in a pixel.
FIG. 8 is a graph showing threshold characteristics at a highest
temperature point (26.5.degree. C.) and a lowest temperature point
(24.5.degree. C.) in the display region in the apparatus shown in FIG. 1.
FIG. 9 is a time chart showing a set of driving waveforms applied to
scanning lines and a data line of a device as shown in FIG. 3. According
to the present invention.
FIG. 10 is a time chart showing another set of driving waveforms according
"to another embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to a first embodiment of the present invention, a uniform
halftone is displayed by:
applying a first voltage for resetting all the pixels on a selected
scanning electrode to be occupied with the first molecular orientation
state,
applying a second voltage for resetting all or a prescribed number of the
pixels on the selected scanning electrode into a mixture state including a
minor proportion of the first molecular orientation state and a major
proportion of the second molecular orientation state, and
applying a third voltage for causing a prescribed ratio of the first to
second molecular orientation state not smaller than the ratio in said
mixture state at a pixel on the selected scanning electrode.
According to a second embodiment of the present invention, a halftone is
displayed by:
a first step of applying voltage signals to the scanning electrodes and
data electrodes of an electrode matrix so as to apply a first voltage for
resetting all the pixels on all or a prescribed number of the scanning
electrodes to be occupied with the first molecular orientation state,
a second step of applying voltage signals to the scanning electrodes and
data electrodes so as to apply a second voltage for resetting all or a
prescribed number of the pixels on said all or a prescribed number of the
scanning electrodes into a mixture state including a minor proportion of
the first molecular orientation state and a major proportion of the second
molecular orientation state, and
a third step of applying a scanning selection signal to a scanning
electrode and applying data signals to the data electrodes in synchronism
with the scanning selection so as to apply a third voltage for causing a
prescribed ratio of the first to second molecular orientation state at the
respective pixels on the scanning electrode not smaller than the ratio in
said mixture state.
A ferroelectric liquid crystal device has a memory characteristic, so that
it is generally necessary to apply a pulse for clearing a display state in
order to rewrite the display state. For this reason, in the conventional
driving method, a halftone display pulse is applied to a pixel after the
pixel is completely reset into one molecular orientation state. This
sequence of operation is directly affected by the above-mentioned
influences of the cell thickness distribution and the temperature
distribution, thus failing to provide a uniform halftone level. More
specifically, FIG. 5A shows an example of applied pulse waveform for
conventional gradational display by multiplexing drive. As shown in the
figure, it is conventional that prescribed pixels are reset into a first
molecular orientation state by application of a simultaneous clearing
pulse having an amplitude V.sub.1 in a period T.sub.1. Then a scanning
line is sequentially selected so that a pixel is supplied with a pulse
having an amplitude V.sub.3 corresponding to given gradation data to be
partially transformed into a second molecular orientation state in a
selection period T.sub.3 after an arbitrary non-selection period T.sub.4.
In contrast thereto, in the present invention, as shown in FIG. 5B, a pixel
is first completely cleared into one molecular orientation state by
applying a pulse with a voltage amplitude of -V.sub.1, then reset into the
other molecular orientation state by applying a pulse having an amplitude
V.sub.2 of the other polarity and thereafter supplied with a halftone
display pulse having an amplitude -V.sub.3. In this instance, the
amplitude V.sub.1 of the first clearing or reset pulse is set to be not
lower than the maximum value in the panel of the saturation voltage value
V.sub.sat(max), and the amplitude V.sub.2 of the second clearing or reset
pulse is set to be not higher than the minimum value in the panel of the
saturation voltage value V.sub.sat(min). Thus, V.sub.1
.gtoreq.V.sub.sat(max) and V.sub.2 .ltoreq.V.sub.sat(min). As a result,
after the application of the first clearing pulse V.sub.1, all the pixels
are completely placed in the first molecular orientation state, and after
the application of the second clearing pulse, the pixels are incompletely
placed in the second molecular orientation state with the first molecular
orientation state partially left in some pixels.
From actual points of view, it is preferred to set V.sub.1 =V.sub.sat(max)
so as not to excessively increase the drive voltage, and it is preferred
to set v.sub.2 =V.sub.sat(min) so as to provide as large a gradation
display range as possible. When V.sub.2 is set to satisfy V.sub.2
=V.sub.sat(min), pixels placed in an incompletely reset state by
application of the second clearing pulse V.sub.2 are those having a
saturation voltage close to V.sub.sat(max), i.e., pixels in a high
threshold region (at a low temperature or having a large cell gap), and
pixels having a saturation voltage close to V.sub.sat(min), i.e., pixels
in a low threshold region (at a high temperature or having a small cell
gap) are almost completely reset into the second molecular orientation
state. Hereinbelow, a case of V.sub.2 =V.sub.sat(min) is taken as an
example for explanation.
FIGS. 6A-1 to 6A-3 and FIGS. 6B-1 to 6B-3 illustrate the states of a pixel
in a low-threshold region (FIGS. 6A-1 to 6A-3) and a pixel in a
high-threshold region (FIGS. 6B-1 to 6B-3) after application of a first
reset pulse (-V.sub.1), a second reset pulse (V.sub.2) and a gradation
display data pulse (-V.sub.3) in this order. The numerals "0", "1", "1-x"
and "1-y" indicated near the pixels represent a ratio of the area in a
pixel occupied by the second molecular orientation state. After the
application of the second reset pulse, a low-threshold pixel assumes the
second molecular orientation state at a rate of nearly 100%, while a
high-threshold pixel partially remains in the first molecular orientation
state. The incompletely reset or cleared rate of a pixel is represented by
x:(1-x), i.e., an areal ratio between the first molecular orientation
state and the second molecular orientation state in the pixel.
Then, each pixel is supplied with a display data pulse having a pulse
amplitude V.sub.3 corresponding to given gradation data. The display data
pulse has a polarity in a direction of causing the first molecular
orientation state and its amplitude V.sub.3 to be set within the range of
V.sub.th(max) .ltoreq.V.sub.3 .ltoreq.V.sub.2. As a result, the pixel is
partially restored to the first molecular orientation state. The degree of
the restoration is represented by y:(1-y), i.e., an areal ratio between
the first molecular orientation state and the second molecular orientation
state after the application, which corresponds to an inversion ratio at a
low-threshold pixel which has been placed in the second molecular
orientation state at a rate of 100%.
A high-threshold pixel shows a lower inversion ratio than Z in response to
a data pulse having the same amplitude V.sub.3. More specifically, a
high-threshold pixel shows an inversion ratio lower by x than a
low-threshold pixel in response to the reset pulse having an amplitude
V.sub.2 and shows a lower inversion ratio than the low-threshold pixel by
nearly the same degree also in response to the gradation pulse having an
amplitude V.sub.3. Thus, the high-threshold pixel shows an inversion ratio
of y-x, and the resultant areal ratio between the first molecular
orientation state and the second molecular orientation state after the
gradation pulse application is (y-x):[1-(y-x)] if the pixel is assumed to
be placed in the second molecular orientation state at a rate of 100%.
However, a high-threshold pixel after the second reset pulse application
is not actually placed in the second molecular orientation state at a rate
of 100% but is placed in a mixture state having an areal ratio of x:(1-x)
of the first and second molecular orientation states. In the mixture
state, the portion x in the first molecular orientation state is a portion
which has not been inverted by the application of the second reset pulse
V.sub.2 and is relatively difficult to invert, i.e., having a higher
threshold, in the pixel. Accordingly, the portion x is not affected by
application of the gradation pulse V.sub.3 but retains the first molecular
orientation state. On the other hand, the portion (1-x) in the second
molecular orientation state is a portion which is relatively easy to
invert, so that it is transformed into the first molecular orientation
state according to the above-mentioned inversion rate. As a result, the
high-threshold pixel after the gradation pulse application assumes the
following mixture state:
Area of 1st. molecular orientation state =.times.+(y-x)=y;
Area of 2nd. molecular orientation state=(1-x)-(y-x)=1-y;
The areal ratio in a high-threshold pixel is identical to that in a
low-threshold pixel. This is shown in FIGS. 6A-3 and 6B-3.
The above results are summarized in Table 1 appearing hereinafter.
In contrast thereto, according to the conventional driving method, a second
reset pulse is not used, but both a low-threshold pixel and a
high-threshold pixel are reset into the first molecular orientation state
at a rate of 100% over the entire area by application of a first reset
pulse (V.sub.1 of a polarity opposite to that shown in Table 1),
immediately followed by application of a gradation pulse. As a result, the
areal ratio between the first molecular orientation state and the second
molecular orientation state is y:(1-y) for a low-threshold pixel which is
the same as in the present invention but is (y-x):[1-(y-x)]. The results
in the conventional case are summarized in Table 2 below.
TABLE 1
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After application of
1st reset
2nd reset
gradation
pulse (-V.sub.1)
pulse (V.sub.2)
pulse (-V.sub.3)
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Low- 1st molecular
1 0 y
threshold
orientation state
pixel 2nd molecular
0 1 1-y
orientation state
High- 1st molecular
1 x y
threshold
orientation state
pixel 2nd molecular
0 1-x 1-y
orientation state
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TABLE 2
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After application of
reset pulse
gradation
(V.sub.1)
pulse (-V.sub.3)
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Low- 1st molecular 0 y
threshold
orientation
pixel 2nd molecular 1 1-y
orientation state
High- 1st molecular 0 y-x
threshold
orientation state
pixel 2nd molecular 1 1-(y-x)
orientation state
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As described above, according to the present invention, reset pulses are
applied in two steps, of which the second one is made an incomplete reset
pulse, whereby an irregularity of gradation over a panel due to a
fluctuation in temperature or cell thickness is eliminated.
Accordingly, as is apparent in view of Tables 1 and 2 in comparison, a
uniform gradation level can be attained according to the present
invention, while a difference in gradation level occurs between a
low-threshold pixel (or region) and a high-threshold pixel (or region)
according to the conventional system. Further, according to the
conventional system, it has been difficult to display a fine gradation
over a certain number of levels due to the above-mentioned fluctuation in
gradation level, whereas a finer gradation display has become possible due
to an improved uniformity according to the present invention.
EXAMPLE
Hereinbelow, the present invention will be explained by way of an example.
FIG. 7 shows a partial schematic sectional view of a liquid crystal cell
(device) which comprises a pair of glass substrates 1a and 1b, of which
the substrate 1b had a roughened surface as a result of etching with a
hydrofluoric acid. The substrates were provided with 1500 .ANG.-thick and
200 .mu.m-wide transparent electrodes 2a and 2b forming scanning
electrodes and data electrodes. The transparent electrodes 2b retained a
minute unevenness so as to provide a threshold distribution in a pixel
because of the roughened substrate lb. The electrodes 2a and 2b were
covered with a pair of alignment films 3a and 3b of 300 .ANG.-thick
rubbled polyimide film, between which a ferroelectric liquid crystal
"CS-1014" (trade name, available from Chisso K.K.) was hermetically sealed
in a thickness of 1.4 .mu.m. A liquid crystal device thus prepared having
a JIS A4 size showed a temperature distribution over a display area
including a maximum temperature of 26.5.degree. C. and a minimum
temperature of 24.5.degree. C., and these maximum temperature point and
minimum temperature point showed threshold characteristics as represented
by curves H and L, respectively, shown in FIG. 8.
From the figure, the amplitudes of a first reset pulse V.sub.1, a second
reset pulse V.sub.2 and a gradiation pulse V.sub.3 were set as follows:
V.sub.1 =22 V, V.sub.2 =20 V, and 12 V.ltoreq.V.sub.3 .ltoreq.20 V.
Further, a scanning selection signal having an amplitude V.sub.s of 16 V
was sequentially applied to scanning lines S.sub.1, S.sub.2, S.sub.3 . . .
and data signals having an amplitude V.sub.I changing within the range of
-4 V.ltoreq.V.sub.I .ltoreq.4 V depending on given gradation data as shown
in FIG. 9 corresponding to the above-mentioned second embodiment were
applied. As a result, during the operation for display of halftones in the
above described manner, a luminance irregularity between the
high-temperature point and the low-temperature point was not substantially
observed.
In the above example, a driving mode for gradational display through pulse
amplitude modulation was adopted, but the present invention is also
applicable to other known driving modes wherein the pulse duration or
pulse number is varied depending on given gradation data.
Further, in the above example, the voltages V.sub.1 and V.sub.2 were set so
as to satisfy the conditions of V.sub.1 =V.sub.sat(max) and V.sub.2
=V.sub.sat(min), but it is possible to adopt a setting of V.sub.2
<V.sub.sat(min) if a coarser degree of gradation is tolerable. Even in
such a case, the effect of suppressing gradation irregularity is not
impaired.
On the other hand, FIG. 10 shows another set of driving waveforms for
gradational display corresponding to the above-mentioned first embodiment
of the present invention.
As described above, according to the present invention, reset pulses are
applied in two steps, of which the second reset pulse is applied as an
incomplete reset pulse, whereby an irregularity of gradation over a panel
due to a fluctuation in temperature or cell thickness is eliminated to
afford a display at a uniform gradation level. Further, according to the
conventional system, it has been difficult to display a fine gradation
over a certain number of levels due to the above-mentioned fluctuation in
gradation level, whereas a finer gradation display has become possible due
to an improved uniformity according to the present invention.
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