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Claims  |
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What is claimed is:
1. A liquid crystal apparatus comprising:
(a) a liquid crystal panel having
a matrix of electrodes composed of a group of scan electrodes and a group
of signal electrodes, said groups being arranged in a spaced and crossed
relationship to define pixels at crossed portions of the scan electrodes
and signal electrodes, and
a ferroelectric liquid crystal filled in a space between the groups, the
pixels each having a threshold voltage V.sub.th and a saturation voltage
V.sub.sat which vary among the pixels; and
(b) voltage applying means for achieving a uniform gradational display on
said liquid crystal panel regardless of variations in voltage response by
different ones of said pixels, said voltage applying means having
(1) first means for applying to said matrix of electrodes a voltage V.sub.4
of one polarity and a magnitude greater than V.sub.sat(max) so as to be
sufficient to reset all pixels of said matrix of electrodes to one optical
state, wherein maximum values of the threshold voltage V.sub.th and
saturation voltage V.sub.sat for all the pixels are defined as
V.sub.th(max) and V.sub.sat(max), respectively, and a minimum value of the
threshold voltage V.sub.th for all the pixels is defined as V.sub.th(min),
(2) second means for applying to said matrix of electrodes a voltage
V.sub.3 which causes a pixel having the highest threshold voltage to
assume a transmittance T.sub.1 % in a first intermediate optical state
between the one optical state and another optical state and which reverses
to the other optical state a pixel having the lowest threshold voltage,
wherein V.sub.3 is a voltage of another polarity opposite to the one
polarity and satisfies the relation:
V.sub.th(max) <V.sub.3 <V.sub.sat(max),
(3) third means for applying to said matrix of electrodes a voltage V.sub.2
which causes the pixel having the lowest threshold voltage to assume a
transmittance (100-T.sub.2) % in a second intermediate optical state
between the one optical state and the other optical state, wherein V.sub.2
is a voltage of the one polarity and satisfies the relation:
V.sub.th(min) .ltoreq.V.sub.2 <V.sub.th(max), and
(4) fourth means for applying to said matrix of electrodes a voltage
V.sub.1 having a magnitude sufficient to change the transmittance
(100-T.sub.2) % of the pixel having the lowest threshold voltage to the
transmittance T.sub.1 %, wherein V.sub.1 is a voltage of the other
polarity, whereby uniform gradation display can be performed regardless of
differences in threshold and saturation voltages.
2. A display apparatus comprising:
(a) a display panel having
a matrix of electrodes composed of a group of scan electrodes and a group
of signal electrodes, said groups being arranged in a spaced and crossed
relationship to define pixels at crossed portions of the scan electrodes
and signal electrodes, and
a ferroelectric liquid crystal filled in a space between the groups, the
pixels each having a threshold voltage V.sub.th and a saturation voltage
V.sub.sat which vary among the pixels; and
(b) voltage applying means for achieving a uniform gradational display on
said crystal panel regardless of variations in voltage response by
different ones of said pixels, said voltage applying means having
(1) first means for applying to said matrix of electrodes a voltage V.sub.4
of one polarity and a magnitude greater than V.sub.sat(max) so as to be
sufficient to reset all pixels of said matrix electrode to one optical
state, wherein maximum values of the threshold voltage V.sub.th and
saturation voltage V.sub.sat for all the pixels are defined as
V.sub.th(max) and V.sub.sat(max), respectively, and a minimum value of the
threshold voltage V.sub.th for all the pixels is defined as V.sub.th(min),
(2) second means for applying to said matrix of electrodes a voltage
V.sub.3 which causes a pixel having the highest threshold voltage to
assume a transmittance T.sub.1 % in a first intermediate optical state
between the one optical state and another optical state and reverses to
the other optical state a pixel having the lowest threshold voltage,
wherein V.sub.3 is a voltage of another polarity opposite to the one
polarity and satisfies the relation:
V.sub.th(max) <V.sub.3 <V.sub.sat(max),
(3) third means for applying to said matrix of electrodes a voltage V.sub.2
which causes the pixel having the lowest threshold voltage to assume a
transmittance (100-T.sub.2) % in a second intermediate optical state
between the one optical state and the other optical state, wherein V.sub.2
is a voltage of the one polarity and satisfies the following relation:
V.sub.th(min) .ltoreq.V.sub.2 <V.sub.th(max), and
(4) fourth means for applying to said matrix of electrodes a voltage
V.sub.1 having a magnitude sufficient to change the transmittance
(100-T.sub.2) % of the pixel having the lowest threshold voltage to the
transmittance T.sub.1 %, wherein V.sub.1 is a voltage of the other
polarity, whereby uniform gradation display can be performed regardless of
differences in the threshold and saturation voltages.
3. A display apparatus comprising:
(a) a display panel having
a matrix of electrodes composed of a group of scan electrodes and a group
of signal electrodes, said groups being arranged in a spaced and crossed
relationship to define pixels at crossed portions of the scan electrodes
and signal electrodes, and
a ferroelectric liquid crystal filled in a space between the groups, the
pixels each having a threshold voltage V.sub.th and a saturation voltage
V.sub.sat which vary among the pixels; and
(b) voltage applying means for achieving a uniform gradational display on
said crystal panel regardless of variations in voltage response by
different ones of said pixels, said voltage applying means having
(1) first means for applying to said matrix of electrodes a voltage V.sub.4
of one polarity and magnitude greater than V.sub.sat(max) so as to be
sufficient to reset all pixels of said matrix of electrodes to one optical
state, wherein maximum values of the threshold voltage V.sub.th and
saturation voltage V.sub.sat for all the pixels are defined as
V.sub.th(max) and V.sub.sat(max), respectively, and a minimum value of the
threshold voltage V.sub.th for all the pixels is defined as V.sub.th(min),
(2) second means for applying to said matrix of electrodes a voltage
V.sub.3 which causes a pixel having the highest threshold voltage to
assume a transmittance T.sub.1 % in a first intermediate optical state
between the one optical state and another optical state and reverses to
the other optical state the pixel having the lowest threshold voltage,
wherein V.sub.3 is a voltage of another polarity opposite to the one
polarity and satisfies the relation:
V.sub.th(max) <V.sub.3 <V.sub.sat(max),
(3) third means for applying to said matrix of electrodes a voltage V.sub.2
which causes a pixel having the lowest threshold voltage to assume a
transmittance (100-T.sub.2) % in a second intermediate optical state
between the one optical state and the other optical state, wherein V.sub.2
is a voltage of the one polarity and satisfies the relation:
V.sub.th(min) .ltoreq.V.sub.2 <V.sub.th(max), and
(4) fourth means for applying to said matrix of electrodes a voltage
V.sub.1 having a magnitude sufficient to change the transmittance
(100-T.sub.2) % of the pixel having the lowest threshold voltage to the
transmittance T.sub.1 %, wherein V.sub.1 is a voltage of the other
polarity; and
(c) control means for controlling said voltage applying means to determine
amplitudes and pulse widths of the voltages V.sub.1, V.sub.2 and V.sub.3
in accordance with image information, whereby uniform gradation display
can be performed regardless of differences in threshold and saturation
voltages.
4. A display apparatus according to claim 3, wherein said image information
comprises gradation information.
5. A method of driving a liquid crystal apparatus comprising a liquid
crystal panel having a matrix of electrodes composed of a group of scan
electrodes and a group of signal electrodes, the groups being arranged in
a spaced and crossed relationship to define pixels at crossed portions of
the scan electrodes and signal electrodes, and a ferroelectric liquid
crystal filled in a space between the groups, the pixels each having a
threshold voltage V.sub.th and a saturation voltage V.sub.sat which vary
among the pixels, for achieving a uniform gradational display on the
liquid crystal panel regardless of variations in voltage response by
different ones of the pixels, said method comprising the steps of:
applying to said matrix of electrodes a voltage V.sub.4 of one polarity and
magnitude greater than V.sub.sat(max) so as to be sufficient to reset all
pixels of said matrix of electrodes to one optical state, wherein maximum
values of the threshold voltage V.sub.th and saturation voltage V.sub.sat
for all the pixels are defined as V.sub.th(max) and V.sub.sat(max),
respectively, and minimum value of the threshold voltage V.sub.th for all
the pixels is defined as V.sub.th(min) ;
applying to said matrix of electrodes a voltage V.sub.3 which causes a
pixel having the highest threshold voltage to assume a transmittance
T.sub.1 % in a first intermediate optical state between the one optical
state and another optical state and reverses to the other optical state
the pixel having the lowest threshold voltage, wherein V.sub.3 is a
voltage of another polarity opposite to the one polarity and satisfies the
relation:
V.sub.th(max) <V.sub.3 <V.sub.sat(max),
applying to said matrix of electrodes a voltage V.sub.2 which causes a
pixel having the lowest threshold voltage to assume a transmittance
(100-T.sub.2) % in a second intermediate optical state between the one
optical state and the other optical state, wherein V.sub.2 is a voltage of
the one polarity and satisfies the relation:
V.sub.th(min) .ltoreq.V.sub.2 <V.sub.th(max), and
applying to said matrix of electrodes a voltage V.sub.1 having a magnitude
sufficient to change the transmittance (100-T.sub.2) % of the pixel having
the lowest threshold voltage to the transmittance T.sub.1 %, wherein
V.sub.1 is a voltage of the other polarity, whereby uniform gradation
display can be performed regardless of differences in threshold and
saturation voltages.
6. A method of driving a pixel in a liquid crystal device in which a
plurality of pixels are arranged, the pixel having a liquid crystal, which
can exhibit either one of two orientation states in accordance with a
polarity of an applied electric field, and a pair of electrodes for
applying the electric field to the liquid crystal, said method comprising
the steps of:
a) applying to the pixel a first voltage signal of one polarity having a
value of V.sub.4 not less than V.sub.sat, where V.sub.sat is an inversion
saturation value at which the pixel is fully inverted to one of the
orientation states under a predetermined environmental condition;
b) after said step a), applying to the pixel a second voltage signal of
another polarity opposite to the one polarity, having a value of V.sub.3
in accordance with gradation information and satisfying a relationship of
V.sub.th .ltoreq.V.sub.3 .ltoreq.V.sub.sat, where V.sub.th is an inversion
threshold value at which the pixel begins to invert to the one orientation
state under the predetermined environmental condition;
c) after said step b), applying to the pixel a third voltage signal of the
one polarity having a value of V.sub.2 equal to V.sub.th ; and
d) after said step c), applying to the pixel a fourth voltage signal of the
other polarity having a value of V.sub.3 .times.V.sub.th /V.sub.sat.
7. A method according to claim 6, wherein each of the voltage signals is a
pulse signal having a pulse amplitude and a pulse width, each of said
values being equal to a product of the respective pulse amplitude and
pulse width and set by pulse amplitude modulation.
8. A method according to claim 6, wherein each of the voltage signals is a
pulse signal having a pulse amplitude and a pulse width, each of said
values being equal to a product of the respective pulse amplitude and
pulse width and set by pulse width modulation.
9. A method according to claim 6, further comprising the steps of:
e) after said step d), applying to the pixel a fifth voltage signal of the
one polarity having a value V.sub.5 of V.sub.th.sup.2 /V.sub.sat, and
f) after said step e), applying to the pixel a sixth voltage signal of the
other polarity having a value V.sub.6 of
##EQU4##
10. A method of driving a pixel in a liquid crystal device in which a
plurality of pixels are arranged, the pixel having a liquid crystal, which
can exhibit either one of two orientation states in accordance with a
polarity of an applied electric field, and a pair of electrodes for
applying the electric field to the liquid crystal, said method comprising
the steps of:
a) applying to the pixel a first voltage signal of one polarity having a
value of V.sub.A not less than V.sub.sat, where V.sub.sat is an inversion
saturation value at which the pixel is fully inverted to one of the
orientation states under a predetermined environmental condition; and
b) after said step a), applying voltage signals having a value V.sub.i to
the pixel in an order i=1, 2, 3, . . .
where, when i is odd number,
##EQU5##
and when i is even number,
##EQU6##
wherein V.sub.th is an inversion threshold value at which the pixel beins
to invert under the predetermined environmental condition and n is a
constant determined on the basis of gradation information. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a matrix driving method of a liquid
crystal display apparatus using a ferroelectric liquid crystal and, more
particularly, to a liquid crystal display apparatus for executing a
gradation display and to a method of driving such an apparatus.
2. Related Background Art
With respect to a display device using a ferroelectric liquid crystal
(FLC), as shown in JP-A-61-94023 or the like, there is known a display
device in which a ferroelectric liquid crystal is injected into a liquid
crystal cell constructed by confronting glass substrates in which
transparent electrodes are formed on two inner surfaces so as to keep a
cell gap of about one to three microns and an orientation process has been
performed.
It is a feature of the display device using the ferroelectric liquid
crystal that since the ferroelectric liquid crystal has a spontaneous
polarization, a coupling force between an external electric field and the
spontaneous polarization can be used for switching and that since the
direction of the major axis of the ferroelectric liquid crystal molecule
corresponds to the polarizing direction of the spontaneous polarization in
a one-to-one corresponding manner, a switching operation can be performed
by the polarity of the external electric field.
Since the ferroelectric liquid crystal generally uses a chiral smectic
liquid crystal (SmC*, SmH*), the major axis of the liquid crystal molecule
in a bulk state exhibits a twisted orientation. However, the twisted state
of the major axis of the liquid crystal molecule can be eliminated by
inserting the liquid crystal into the cell having a cell gap of about 1 to
3 microns as mentioned above. (N. A. Clark et al., "MCLC", Vol. 94, pages
213 to 234, 1983)
A structure of an actual ferroelectric liquid crystal cell uses a simple
matrix substrate as shown in FIG. 2.
The ferroelectric liquid crystal is mainly used as a binary (white/black)
display device by setting two stable states into a light transmission
state and a light shielding state. However, a multivalue, that is, a
half-tone display can be also performed. As one of the half-tone display
methods, an intermediate light transmission state is produced by
controlling an area ratio of a bistable state in a pixel. The above method
(area modulating method) will now be described in detail hereinbelow.
FIG. 4 is a diagram schematically showing the relation between the
switching pulse amplitude of the ferroelectric liquid crystal device and
the transmittance. FIG. 4 is a graph in which a transmission light
quantity I after a single pulse of uni-polarity was applied to a cell
(element) which has originally been set in the complete light shielding
(black) state has been plotted as a function of an amplitude V of the
single pulse. When the pulse amplitude V is equal to or less than a
threshold value V.sub.th (V.ltoreq.V.sub.th), the transmission light
quantity doesn't change. As shown in FIG. 5(b), the transmission state
after the pulse was applied is equal to that of FIG. 5(a) showing the
state before the pulse is applied. When the pulse amplitude exceeds the
threshold value (V.sub.th <V<V.sub.sat), a state of a part in the pixel is
shifted to the other stable state, namely, the light transmission state
shown in FIG. 5(c) and exhibits an intermediate transmission light
quantity as a whole. Further, when the pulse amplitude V increases and is
equal to or higher than a saturation value V.sub.sat (V.sub.sat
.ltoreq.V), as shown in FIG. 5(d), the light transmission quantity reaches
a constant value because the whole pixel is set into the light
transmission state.
That is, the area modulating method intends to display a half-tone by
controlling the voltage so that the pulse amplitude V lies within a range
of V.sub.th <V<V.sub.sat.
However, the area modulating method has the following serious drawback as
will be explained hereinafter. Since the relation between the voltage and
the transmission light quantity shown in FIG. 4 depends on a thickness of
cell and a temperature, in other words, if there is a cell thickness
distribution or a temperature distribution in the display panel, a
different gradation level is displayed for the applied pulse of the same
voltage amplitude. FIG. 6 is a diagram for explaining such a drawback and
is a graph showing the relation between the voltage amplitude V and the
transmission light quantity I in a manner similar to FIG. 4. FIG. 6 shows
two curves showing the relations at different temperatures: that is, a
curve H indicative of the relation at a high temperature and a curve L
indicative of the relation at a low temperature. In the display (display
device) of a large display size, a temperature distribution often occurs
in the same panel (display section). Therefore, even if the operator tries
to display a half-tone at a certain voltage V.sub.ap, a variation of the
half-tone level occurs in a range from I.sub.1 to I.sub.2 as shown in FIG.
6, so that a uniform display state cannot be derived. Generally, since a
switching voltage of the ferroelectric liquid crystal is high at a low
temperature and is low at a high temperature and a difference between the
switching voltages depends on a temperature change of a viscosity of
liquid crystal, such a difference is ordinarily extremely larger than that
of the conventional TN type liquid crystal device. Therefore, a
fluctuation of the gradation level by the temperature distribution is
fairly larger than that of the TN liquid crystal. Such a point becomes the
maximum cause which makes it difficult to realize the gradation display of
the ferroelectric liquid crystal device.
The above influences increase as an area of the liquid crystal cell
increases (a variation of the cell thickness and a variation of the
temperature easily occur), so that the gradation, especially, the analog
gradation display in the cell of a large area using the FLC is impossible.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a liquid crystal display
apparatus and its driving method, in which the analog gradation display
can be realized by an FLC device and the stable gradation display can be
also performed for a threshold value fluctuation due to a variation of
temperature in a cell and a variation of thickness of the cell.
To accomplish the above object, according to the invention, there is
provided a liquid crystal display apparatus which has a display section in
which a group of scan electrodes and a group of signal electrodes are
arranged like a matrix and a ferroelectric liquid crystal having a
bistability in the direction of an electric field is filled between both
of the electrode groups and which displays an image or information,
wherein the apparatus has means for sequentially writing gradation
information from a pixel of a higher threshold value on a scan line by a
pulse to completely reset all of the pixels on the selected scan electrode
into a first stable state and one or a plurality of pulses subsequent to
the reset pulse. There is also provided a method of driving such a liquid
crystal display apparatus.
On the other hand, according to the invention, there is provided a liquid
crystal display apparatus which has a display section in which a group of
scan electrodes and a group of signal electrodes are arranged like a
matrix and a ferroelectric liquid crystal having a bistability in the
direction of an electric field is filled between both of the electrode
groups and which displays an image or information, wherein the apparatus
has drive control means for writing into a pixel of a high threshold value
on a scan line by applying a pulse to completely reset all of the pixels
on a scan line by the selected scan electrode into a first stable state
and to subsequently shift to a second stable state, and for further
changing a display state of the pixel of another lower threshold value
while keeping the writing state of the pixel of the high threshold value,
thereby enabling a half-tone display to be performed. There is also
provided a method of driving such a liquid crystal display apparatus.
Further, according to the invention, there is provided a liquid crystal
display apparatus having applying means which is expressed by the
relations
V.sub.sat(max) .ltoreq.V.sub.1
V.sub.th(max) <V.sub.2 <V.sub.sat(max)
V.sub.th(min) .ltoreq.V.sub.3 <V.sub.th(max)
where,
V.sub.th(max) : maximum value of the threshold voltage in the display
section of the liquid crystal display apparatus,
V.sub.sat(max) : maximum value of the saturation voltage,
V.sub.th(min) : minimum value of the threshold voltage,
V.sub.1 : amplitude of the pulse which is applied to reset in the first
step,
V.sub.2 : amplitude of the pulse which is applied to shift the level in the
second step,
V.sub.3 : amplitude of the pulse which is applied to change the display
state of the low threshold part in the third and subsequent steps.
There is also provided a method of driving such a liquid crystal display
apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are diagrams for explaining driving means in the invention;
FIGS. 2A and 2B are diagrams showing a structure of a liquid crystal
display device according to the first embodiment of the invention;
FIG. 3 is a diagram for explaining a display state in the case where the
driving means of the invention is used;
FIG. 4 is a schematic graph showing the relation between a switching pulse
amplitude and a transmittance with respect to a ferroelectric liquid
crystal;
FIG. 5 is a schematic diagram showing a light transmission state of a
ferroelectric liquid crystal display device according to an applied pulse;
FIG. 6 is a diagram showing the relations between a voltage amplitude V and
a transmission light quantity I at high and low temperatures;
FIGS. 7A to 7D are diagrams for explaining the first embodiment of the
invention;
FIG. 8 is a diagram showing an example of a scan signal, an information
signal, and an applied voltage to a pixel according to the first
embodiment of the invention;
FIG. 9 is a diagram showing a construction of scan signal electrodes and
information signal electrodes used in the embodiment;
FIG. 10 is a block diagram showing a circuit of a liquid crystal display
apparatus used in the embodiment;
FIG. 11 is a cross sectional view of a liquid crystal cell according to the
second embodiment of the invention;
FIG. 12 is a diagram showing an example of a scan signal, an information
signal, and an applied voltage to a pixel according to the second
embodiment of the invention; and
FIG. 13 is a diagram for explaining the second embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A writing method of the invention will be described by using graphs in
accordance with a practical procedure.
FIG. 1A shows a threshold curve of an FLC. An axis of ordinate shows the
transmittance I (0 to 100%) and an axis of abscissa indicates a pulse
voltage value by a logarithm scale.
A solid line (1) in FIG. 1A shows a threshold curve of a high threshold
part in the panel. A broken line (2) shows a threshold curve of a
fluctuation part of the threshold value. In both of the curves (1) and
(2), a pulse width is set to .DELTA.T.
A solid line (3) shows a threshold curve when a gradation signal voltage
V.sub.3 is set to a voltage (V.sub.sat) at which a transmittance is equal
to 100%. An inclination is the same as those of the curves (1) and (2).
An alternate long and two short dashes line (4) shows a threshold curve
having the same inclination as those of the curves (1) and (2) in the case
where V.sub.2 is set to V.sub.sat.
For explanation, pulses which are applied to the pixel are set as shown in
FIG. 1B and the time correlation among the pulses is omitted.
The writing operation comprises the following four steps.
1 The whole panel is written into a certain state Q.sub.0 by a pulse (A) of
a voltage V.sub.4 (whole surface reset).
2 The high threshold part in the panel is written by a pulse (B) of a
polarity opposite to that of the pulse (A) until the transmittance is
equal to T.sub.1. At this time, the portion in which the threshold value
has been fluctuated to a low value is completely inverted (which will be
explained by intersection points c' and a' of an alternate long and short
dash line and the solid lines (1) and (3) in FIG. 1A). Due to this, the
high threshold part is written into a gradation state Q.sub.1 of T.sub.1.
However, since the whole surface in the low threshold part has been
inverted, this means that it is again inverted to Q.sub.0 (Table 1).
Assuming that the transmittance of the state Q.sub.0 is equal to 0%, the
transmittance of Q.sub.0 is equal to 100% and the transmittance of
Q.sub.1 is equal to T.sub.1 %.
TABLE 1
______________________________________
Applied pulse (A) (B) (C) (D)
______________________________________
State of high threshold part
Q.sub.0 Q.sub.1 Q.sub.1
Q.sub.1
State of low threshold part
Q.sub.0 Q.sub.0 Q.sub.2
Q.sub.1
______________________________________
3 Subsequently, by applying a pulse (C) of a polarity opposite to that of
the pulse (B), the low threshold part is again inverted until the
transmittance is equal to (100-T.sub.2) %. At this time, no switching
occurs in the high threshold part shown by the threshold curve (1).
Therefore, the high threshold part keeps the state Q.sub.1 formed by the
pulse (B). On the other hand, the low threshold part is inverted from the
state of Q.sub.0 (transmittance of 100%) to the state Q.sub.2 of the
transmittance of (100-T.sub.2) %.
4 By applying a pulse (D) of a polarity opposite to that of the pulse (C),
the high threshold part keeps the state Q.sub.1 formed by the pulse (B).
However, the low threshold part is again inverted from the state Q.sub.2
until the transmittance is equal to (100-T.sub.2 +T.sub.3) % in FIG. 1A.
At this time, in the low threshold part, the state Q.sub.1 of the same
transmittance as that of the high threshold part is realized.
That is, as will be understood in FIG. 1A, the transmittance T.sub.1 % is
equal to (100-T.sub.2 +T.sub.3) %. This point will be also easily
understood because triangles abc and a'b'c' which are formed in the
diagram are congruent.
Table 1 shows a change in State by the supply of time-sequential pulses as
mentioned above and FIG. 3 shows an image diagram of such a state.
There are the following relations among the pulses (A), (B), (C), and (D)
which are applied in accordance with the above procedure.
1 First, in order to provide an enough resetting function, the pulse (A)
has a threshold value V.sub.4 of the whole surface inversion by the pulse
having a width of .DELTA.T. However, fundamentally, no problem occurs even
if the pulse width of the pulse (A) is set to .DELTA.T. For the pulse
having a pulse width of .DELTA.T (the same shall also apply hereinbelow),
the voltage of V.sub.4 is equal to the total inversion voltage V.sub.sat
between pixels in the high threshold part as shown in FIG. 1A.
2 V.sub.2 is equal to the partial inversion voltage V.sub.th in the pixel
in the high threshold part.
3 Therefore, under the condition such that the inclinations on the
threshold value curves are equal, there are the following relations
V.sub.2 =.xi.V.sub.4 =.xi.V.sub.sat
when it is assumed that
V.sub.th /V.sub.sat .ident..xi.
4 V.sub.3 is equal to the partial inversion information voltage in the
pixel in the high threshold part, that is, the voltage to write the
gradation state and there are the relations of V.sub.2 .ltoreq.V.sub.3
.ltoreq.V.sub.4. To obtain the transmittance of I %, V.sub.3 can be
written as follows.
##EQU1##
log V.sub.3 V.sub.2 =n.multidot.log V.sub.4 /V.sub.2
log (V.sub.3 /V.sub.2)=-n.multidot.log .xi.
V.sub.3 =V.sub.2 /.xi..sup.n
V.sub.3 =V.sub.4 .xi..sup.n-1
=V.sub.sat /.xi..sup.n-1
5 V.sub.1 can be regarded to be a voltage value corresponding to V.sub.th
of the threshold curve in which V.sub.3 is equal to V.sub.sat and can be
expressed as follows.
V.sub.1 =.xi.V.sub.3
V.sub.1 =V.sub.4 /.xi..sup.n-2
=V.sub.sat /.xi..sup.n-2
As mentioned above, the pulses (A), (B), (C), and (D) which are
sequentially applied are all determined by the following three factors.
(a) V.sub.sat of the high threshold part
(b) Constant .xi. (=V.sub.th /V.sub.sat) which is decided on the threshold
value characteristic
(c) Constant n (=I %/100%) which is determined by the gradation information
That is,
##EQU2##
The reason why an inequality sign has been added to V.sub.4 is because it
is sufficient to completely invert and the voltage V.sub.4 is not limited
by an equality sign.
The gradation expression can be realized by the pulses (A), (B), (C), and
(D). A fluctuation range of the threshold value which is now considered is
a region sandwiched by the threshold curves (4) and (1) in FIG. 1A.
This is because if an area such that V.sub.sat is set to a voltage of
V.sub.2 or less exists, even when the pulse (C) is applied, it is
impossible to distinguish from the area such that V.sub.sat is equal to
V.sub.2 and all of the parts are completely inverted, however, when the
pulse (D) is applied, the different inversions are executed, so that a
constant gradation level cannot be displayed.
On reflection, however, it will be understood that the relation between the
pulses (C) and (D) is the same as the relation between the pulses (A) and
(C) near the threshold curve (3).
This means that by adding a pulse (E) and, further, a pulse (F) and the
like after the pulse (D), a region of further low threshold curve can be
also incorporated. Assuming that peak values of the pulses (E) and (F) are
set to V.sub.5 and V.sub.6, they can be expressed as follows.
V.sub.5 =.xi..multidot.V.sub.2 =.xi..sup.2 .multidot.V.sub.sat
V.sub.6 =.xi..multidot.V.sub.1 =V.sub.sat /.xi..sup.n-3
The above relations will now be generalized and considered. When suffixes
are rewritten such as V.sub.1, V.sub.2, V.sub.3, . . . from the pulse
which is time-precedent, the pulses can be expressed as follows.
##EQU3##
Such a writing method is effective so long as a low threshold part such as
to exceed V.sub.th by only the applied information signal doesn't appear
upon matrix driving.
For instance, in the case of 1/4 bias (the information signal of a voltage
which is 1/4 of the selected voltage is used), since there is a relation
of
V.sub.th '>1/4 V.sub.sat
V.sub.sat ' corresponding to the minimum V.sub.th ' is
V.sub.sat '=V.sub.th '/.xi.>1/4 V.sub.sat.sup.2 /V.sub.th
Therefore, when the writing voltage is expressed by V.sub.sat ', it is
possible to drive in a range of
V.sub.sat >V.sub.sat '>V.sub.sat.sup.2 /4V.sub.th
According to the invention, it is desirable to construct the pixel such
that the inclination .alpha.T/.alpha.log(V.sub.sat /V.sub.th) on the
threshold curve is set to be constant.
It is also possible to correct the peak value in dependence on the relation
(in the case where a large pulse of the opposite polarity exists just
before the write pulse and the case where such a pulse doesn't exist; in
the case where a large voltage pulse of the opposite polarity exists just
after the write pulse and the case where such a pulse doesn't exist; and
the like) before and after the pulse which is applied to the FLC.
Generally, such a correction can be realized by adding a correction
coefficient.
V.sub.0i '=.alpha..xi..sup.i V.sub.sat, V.sub.1j '=.alpha..xi..sup.i
V.sub.1
(.alpha.=constant)
However, for instance, in the case where the write pulses continue, the
threshold value of the subsequent pulse fluctuates by the existence of the
preceding pulse. Therefore, in order to easily set the voltage, it is
preferable to provide an interval between the write pulses. It is
desirable to set such an interval to 100 .mu.sec or more.
Although the invention has been described above with respect to the case of
displaying the analog gradation, in the case of executing a discrete
gradation display (digital gradation display), an enough effect is derived
even if the inclinations of the threshold curves are slightly different.
In the case of executing such a digital gradation display, it is possible
to use a method whereby the value of .xi. is corrected and the number of
pulses is increased.
Further, although the invention has been described with respect to the case
where the pulse width is set to be constant and the voltage modulation is
used, even in the case where the voltage value is set to be constant and
the pulse width is made variable, the gradation display in which the
influences by the temperature change and the cell thickness change are
remarkably reduced can be executed by the similar means.
An embodiment will now be described hereinbelow.
[Embodiment 1]
FIGS. 2A, 2B, 7A to 7D, 8, 9, and 10 are diagrams for explaining an
embodiment.
FIGS. 2A and 2B show a liquid crystal cell according to the embodiment.
FIG. 7A shows an example of the gradation display of the embodiment. FIG.
7B shows driving waveforms of the embodiment. FIGS. 7C and 7D show
threshold curves of a pixel in a panel used in the embodiment.
FIG. 8 shows an example of a scan signal, an information signal, and an
applied voltage to the pixel which were used in the embodiment. In the
diagram, waveforms shown by S.sub.1 to I.sub.1 correspond to the driving
waveforms of FIG. 7B.
FIG. 9 is a diagram showing a construction of scan signal electrodes and
information signal electrodes used in the embodiment.
FIG. 10 is a block diagram showing a circuit of a liquid crystal display
apparatus used in the embodiment.
FIG. 11 shows a cross sectional view of a liquid crystal cell used in the
embodiment.
FIG. 9 shows a construction of the scan signal electrodes and information
signal electrodes used in the embodiment. Signal waveforms which are
applied to those electrodes are shown in FIG. 8.
In FIG. 8, S.sub.1, S.sub.2, and S.sub.3 show time charts of scan signal
waveforms which are successively applied to the selected scan signal
electrodes and are constructed by three pulses (pulses A, B, and C).
I.sub.1 and I.sub.2 show time charts of information signal waveforms which
are applied to the group of information signal electrodes. Only parts of
those signal waveforms are shown in the diagram for simplicity of
explanation.
FIG. 10 shows a block diagram of a circuit to supply those signals to the
liquid crystal cell. In FIG. 10, reference numeral 101 denotes a liquid
crystal cell (FLC cell); 102 a driving power source which can generate
voltages of various levels; 103 a segment side driving IC; 104 a latch
circuit; 105 a segment side shift register; 106 a common side driving IC;
107 a common side shift register; 108 an image information generator; and
109 a controller.
In the circuit construction of FIG. 10, as a method of executing a
gradation display (a plurality of voltage levels are supplied as signals),
there is used a method whereby a D/A converter is provided in the segment
side driving IC and a digital gradation signal which is supplied through
the latch circuit is converted into an analog signal and applied to the
information electrodes. In this case, the common side driving IC forms the
scan signal by a distributing method by an analog switch of the driving
power source.
In the embodiment, the liquid crystal cell shown in FIGS. 2A and 2B is used
as an FLC cell 101. FIG. 2A is a cross sectional view of the liquid
crystal cell. Reference numeral 21 denotes a glass substrate; 22 an ITO
stripe electrode; 23 an insulating film made of SiO.sub.2 ; 24 an
orientation film made of polyimide; 25 a sealing material; and 26 a liquid
crystal. FIG. 2B shows a pattern of the stripe electrodes on the substrate
of one side.
In the cell, the SiO.sub.2 layer 23 is formed as an insulating film onto
the ITO stripe electrodes 22, LQ-1802 (trade name) made by Hitachi
Chemical Co., Ltd. is coated onto the insulting film, the orientation film
24 is formed by executing a rubbing process to both of the upper and lower
substrates, a liquid crystal A having physical properties shown in Table 2
is used as a liquid crystal 26, and a substrate interval of about 1.49
.mu.m is held. The resultant cell is used.
TABLE 2
______________________________________
Liquid crystal A
##STR1##
Ps = 5.8 nC/cm.sup.2
30.degree. C.
Tilt angle = 14.3.degree.
30.degree. C.
.DELTA..epsilon. .about. 0
30.degree. C.
______________________________________
FIGS. 7C and 7D s | | |
