Liquid crystal display apparatus

A display panel includes a matrix of pixels each constituted by a pair of oppositely disposed electrodes and a liquid crystal disposed between the electrodes. A current signal, particularly one associated with inversion of spontaneous polarization of the liquid crystal is detected at plural pixels. The display panel is driven by applying drive signals thereto while correcting the drive signals based on the detected current signal. As a result, a threshold distribution typically attributable to a temperature distribution on the display panel is accurately compensated for. The display system thus constituted is particularly useful for gradational display.

FIELD OF THE INVENTION AND RELATED ART 
The present invention relates to a liquid crystal display apparatus for 
computer terminals, television receivers, word processors, typewriters, 
etc., inclusive of a light valve for projectors, a view finder for video 
camera recorders, etc. 
There have been known liquid crystal display devices including those using 
twisted-nematic (TN) liquid crystals, guest-host(G-H)-type liquid 
crystals, cholesteric (Ch) liquid crystals, smectic (Sm) liquid crystals, 
etc. 
There have also been a well-known type of liquid crystal display devices 
wherein a liquid crystal compound is disposed between a group of scanning 
electrodes and a group of data electrodes constituting an electrode matrix 
so as to form a large number of pixels. 
As a driving method for such a liquid crystal display device, there has 
been generally adopted a multiplexing drive scheme wherein an address 
signal is sequentially and selectively applied to the scanning electrodes 
and prescribed data signals are selectively applied to the data electrodes 
in parallel and in synchronism with the address signal. 
The practical application of such a multiplexing drive scheme has been made 
by using a TN (twisted nematic) liquid crystal as disclosed in "Voltage 
Dependent Optical Activity of a Twisted Nematic Liquid Crystal" written by 
M. Schadt and W. Helfrich in Applied Physics Letters, 1971 18(4), p.p. 
127-128. 
In recent years, as an improvement for such a conventional liquid crystal 
device, Clark and Lagerwall have disclosed a bistable ferroelectric liquid 
crystal device using a surface-stabilized ferroelectric liquid crystal in, 
e.g., Applied Physics Letters, Vol. 36, No. 11 (Jun. 1, 1980), p.p. 
899-901; Japanese Laid-Open Patent Application (JP-A) 56-107216, U.S. Pat. 
Nos. 4,367,924 and 4,563,059. Such a bistable ferroelectric liquid crystal 
device has been realized by disposing a liquid crystal between a pair of 
substrates disposed with a spacing small enough to suppress the formation 
of a helical structure inherent to liquid crystal molecules in chiral 
smectic C phase (SmC*) or H phase (SmH*) of bulk state and align vertical 
(smectic) molecular layers each comprising a plurality of liquid crystal 
molecules in one direction. 
Further, as a display device using such a ferroelectric liquid crystal 
(FLC), one is known wherein a pair of transparent substrates respectively 
having thereon a transparent electrode and subjected to an aligning 
treatment are disposed to be opposite to each other with a cell gap of 
about 1-3 .mu.m therebetween so that their transparent electrodes are 
disposed on the inner sides to form a blank cell, which is then filled 
with a ferroelectric liquid crystal, as disclosed in U.S. Pat. Nos. 
4,639,089; 4,655,561; and 4,681,404. In such a device, the ferroelectric 
liquid crystal in its chiral smectic phase shows bistability, i.e., a 
property of assuming either one of a first and a second optically stable 
state depending on the polarity of an applied voltage and maintaining the 
resultant state in the absence of an electric field. Further, the 
ferroelectric liquid crystal shows a quick response to a change in applied 
electric field. Accordingly, the device is expected to be widely used in 
the field of e.g., a high-speed and memory-type display apparatus. 
A liquid crystal display apparatus having a display panel constituted by 
such a ferroelectric liquid crystal device may be driven by a multiplexing 
drive scheme as described in U.S. Pat. Nos. 4,655,561, 4,709,995, 
4,800,382, 4,836,656, 4,932,759, 4,938,574, and 5,058,994. 
A ferroelectric liquid crystal (FLC) has been principally used in a binary 
(bright-dark) display device in which two stable states of the liquid 
crystal are used as a light-transmitting state and a light-interrupting 
state but can be used to effect a multi-value display, i.e., a halftone 
display. In a halftone display method, the areal ratio between bistable 
states (light transmitting state and light-interrupting state) within a 
pixel is controlled to realize an intermediate light-transmitting state. 
The gradational display method of this type (hereinafter referred to as an 
"areal modulation" method) will now be described in detail. 
FIG. 1A-1 is a graph schematically representing a relationship between a 
transmitted light quantity I through a ferroelectric liquid crystal cell 
and a switching pulse voltage V. More specifically, FIG. 1A-1 shows plots 
of transmitted light quantities I given by a pixel versus voltages V when 
the pixel initially placed in a complete light-interrupting (dark) state 
is supplied with single pulses of various voltages V and one polarity as 
shown in FIG. 1A-2. When a pulse voltage V is below threshold Vth (V&lt;Vth), 
the transmitted light quantity does not change and the pixel state is as 
shown in FIG. 1B-2 which is not different from the state shown in FIG. 
1B-1 before the application of the pulse voltage. If the pulse voltage V 
exceeds the threshold Vth (Vth&lt;V&lt;Vsat), a portion of the pixel is switched 
to the other stable state, thus being transitioned to a pixel state as 
shown in FIG. 1B-3 showing an intermediate transmitted light quantity as a 
whole. If the pulse voltage V is further increased to exceed a saturation 
value Vsat (Vsat&lt;V), the entire pixel is switched to a light-transmitting 
state as shown in FIG. 1B-4 so that the transmitted light quantity reaches 
a constant value (i.e., is saturated). That is, according to the areal 
modulation method, the pulse voltage V applied to a pixel is controlled 
within a range of Vth&lt;V&lt;Vsat to display a halftone corresponding to the 
pulse voltage. 
However, in actually, the voltage (V)-transmitted light quantity (I) 
relationship shown in FIG. 1A-1 depends on the cell thickness and 
temperature. Accordingly, if a display panel is accompanied with an 
unintended cell thickness distribution or a temperature distribution, the 
display panel can display different gradation levels in response to a 
pulse voltage having a constant voltage. FIG. 2 is a graph for 
illustrating the above phenomenon which is a graph showing a relationship 
between pulse voltage (V) and transmitted light quantity (I) similar to 
that shown in FIG. 1A-1 but showing two curves including a curve H 
representing a relationship at a high temperature and a curve L at a low 
temperature. In a display panel having a large display size, it is rather 
common that the panel is accompanied with a temperature distribution. In 
such a case, however, even if a certain halftone level is intended to be 
displayed by application of a certain drive voltage Vap, the resultant 
halftone levels can be fluctuated within the range of I.sub.1 to I.sub.2 
as shown in FIG. 2 within the same panel, thus failing to provide a 
uniform gradational display state. As shown in FIG. 2, FLC shows a higher 
switching voltage at a lower temperature and a lower switching voltage at 
a higher temperature, and the difference in switching voltage is generally 
much larger than that of a conventional TN-liquid crystal since the 
difference depends on a change in viscosity of the liquid crystal caused 
by a temperature change. Accordingly, the difference in gradation level 
due to a temperature distribution is much larger than that encountered in 
a TN-type liquid crystal, and this has been a main factor which makes 
difficult the realization of gradational display by FLC. 
Further, in a conventional FLC device, a temperature change causes a 
remarkable change in drive margin, i.e., the range of voltage value or 
pulse width of a drive pulse allowing a practical display. As a result, 
there is no set of drive conditions, including application of a constant 
voltage and a constant pulse widths, capable of retaining a good display 
state over a temperature range of, e.g., 10.degree. C. to 40.degree. C. 
In view of the above problems, it has been proposed to dispose a planar 
heater in the vicinity of a display section so as to keep the temperature 
at constant or to detect a temperature in the vicinity of a display panel 
so as to control the drive conditions, However, the resultant drive margin 
is still small so that the provision of a large-area panel remains 
difficult because it has been impossible to absorb threshold 
irregularities caused by cell thickness irregularity, waveform 
irregularity caused by delay in transmission of signal waveform, 
irregularity in liquid crystal alignment state, etc., besides temperature 
irregularity. 
Further, in the case of gradational display using a conventional FLC 
device, the voltage value and pulse width of a drive pulse for displaying 
a desired gradation level vary remarkably so that, even if the 
above-mentioned method of providing a planar heater for keeping the 
temperature at a constant level, or the method of detecting a temperature 
in the vicinity of the display panel to control the drive conditions is 
adopted, it would still be impossible to absorb the threshold change due 
to a temperature irregularity over the display panel. 
The above-mentioned problems are not restricted to the areal modulation 
method but are common to the binary display scheme of displaying two 
states of bright and dark. 
SUMMARY OF THE INVENTION 
A generic object of the present invention is to solve the above-mentioned 
problems. 
A more specific object of the present invention is to provide a liquid 
crystal display apparatus capable of effecting a good display even if an 
nonuniformity in threshold occurs in a display area. 
According to the present invention, there is provided a liquid crystal 
display apparatus, including: 
a display panel comprising a matrix of pixels each comprising a pair of 
oppositely disposed electrodes and a liquid crystal disposed between the 
electrodes, 
detection means for detecting a current signal flowing across the liquid 
crystal at plural pixels on the display panel, 
drive means for applying drive signals to the display panel, and 
correction means for correcting the drive signals based on the current 
signal detected by the detection means. 
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, wherein like parts are denoted 
by like reference numerals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First of all, a pixel current signal detection means used in the present 
invention will be described. 
FIG. 3 is a schematic view for illustrating a current signal detection 
system. Referring to FIG. 3, the system includes a detection waveform 
application circuit 106 for applying a current signal detection input 
signals and a current detection circuit 107 for taking out a current 
signal detection output signal. The application circuit 106 is connected 
to a scanning electrode 201 and the detection circuit 107 is connected to 
a data electrode 202 which constitutes, together with the scanning 
electrode 201, a pair of electrodes sandwiching a liquid crystal 305 to 
form a pixel as a detection object. 
FIG. 4A shows a detection input signal and FIG. 4B shows a detection output 
signal. 
Referring to FIG. 3, a voltage in a rectangular or ramp waveform is applied 
from the detection waveform application circuit 106 to the liquid crystal 
305 through the scanning electrode 201 so as to detect a current flowing 
to the data electrode 202 including an internal current accompanying 
inversion of the spontaneous polarization of liquid crystal molecules. 
When a voltage waveform shown in FIG. 4A is applied, a current response as 
shown in FIG. 4B is obtained. When the temperature changes, the internal 
current due to the inversion of the spontaneous polarization changes its 
peak and total quantity. Similar changes occur corresponding to a change 
in applied voltage. Further, if the input waveform is delayed, the rising 
form of an external current accompanying the switching of an external 
electric field changes. Accordingly, if the parameters, such as a peak 
time .tau., a total charge Q, a peak half-value width .tau..sub.w, etc., 
of a current response as shown in FIG. 4B are measured, the current 
threshold characteristic of a pixel concerned can be detected. 
In the present invention, the display operation is corrected based on the 
detected threshold data. 
FIG. 5 is a block diagram of a liquid crystal display apparatus including 
such a detection system. Prescribed pixels (hatched pixels) among a large 
number of pixels constituted by an electrode matrix in a liquid crystal 
display device 101 are supplied with a detection input signal from a 
detection signal application circuit 106 (also functioning as a data 
signal application circuit). Output signals from the prescribed pixels are 
outputted through electrodes concerned (scanning electrodes in this case) 
and a changeover circuit 102 to a control circuit 116. 
The changeover circuit 102 may be constituted as shown in FIG. 5 so that it 
includes changeover switches on only electrodes leading to previously 
determined pixels to be detected or may be constituted so that all 
electrodes are provided with a changeover switch so as to allow selection 
of an arbitrary pixel. 
In a case where a correction is required, corrected display drive signals 
are supplied from a scanning electrode driver 103 and the data electrode 
driver 106 (also functioning as a detection signal application circuit as 
described above) based on a correction instruction issued from the control 
circuit. At this time, the changeover switches in the circuit 102 are 
switched to connect the electrodes concerned to the scanning electrode 
driver 103. 
The detection signal may be applied to data electrodes as in the above 
embodiment or alternately to scanning electrodes, as desired, so as to fit 
an entire system. 
FIG. 6 is a partial top plan view of a liquid crystal display panel 101 
(display section), and FIG. 7 is a partial sectional view taken along line 
A--A' as viewed in the direction of arrows. 
Referring to FIG. 6, the panel 101 includes scanning electrodes 201 and 
data electrodes 202 intersecting the scanning electrodes 201. The scanning 
electrodes and data electrodes form an electrode matrix (pixel matrix) 
constituting a pixel 222 as a display unit at each intersection of the 
scanning electrodes and the data electrodes. Referring to FIG. 7, the 
panel further includes glass substrates 302 and 302a carrying the data 
electrodes 202 and the scanning electrodes 201, respectively, insulating 
films 303 and 303a, alignment films 304 and 304a, a liquid crystal 305, 
and a sealing member 310, which form a cell (panel) structure in 
combination, and further an analyzer 301 and a polarizer 309 disposed in 
cross nicols sandwiching the cell structure. The liquid crystal 305 usable 
in the present invention may include a nematic liquid crystal, a 
cholesteric liquid crystal and a smectic liquid crystal. It is 
particularly suitable to use a smectic liquid crystal showing 
ferroelectricity. 
A representative example of such a liquid crystal may be a ferroelectric 
liquid crystal mixture containing a pyrimidine component and showing a 
phase transition series as shown in the following Table 1 and spontaneous 
polarization Ps and an optical response time .tau. (causing a 
transmittance change of 0.fwdarw.90%) under application of rectangular 
pulses of .+-.4 volts and 5 Hz as shown in Table 2 below. 
TABLE 1 
__________________________________________________________________________ 
##STR1## 
__________________________________________________________________________ 
TABLE 2 
______________________________________ 
10.degree. C. 
20.degree. C. 
30.degree. C. 
40.degree. C. 
50.degree. C. 
______________________________________ 
Ps (nC/cm.sup.2) 
7.0 6.2 5.0 3.9 2.6 
.tau. (.mu.sec) 
230 170 125 95 85 
______________________________________ 
The present invention aims at providing a good display on a large-area 
panel regardless of temperature and further provides a stable gradational 
display. For this purpose, it is necessary to accurately compensate for 
threshold irregularity over the display panel. Accordingly, the apparatus 
of the present invention includes means for defining a current flowing 
through the liquid crystal layer including a polarization inversion 
current and correcting data signals and scanning signals for display. In 
order to more accurately detect the current without impairing the image 
quality thereby, it is desirable to satisfy at least one of the following 
features (1)-(7). 
(1) Measurement at plural points. 
At plural points on a pixel matrix, the current is detected to evaluate a 
threshold distribution over the display panel, particularly over the 
display area constituted by a pixel matrix (electrode matrix) and, based 
on the threshold distribution, corresponding correction of display signals 
is effected. Herein, the term "pixel" is intended to also mean a cell unit 
having a structure identical to a pixel, i.e., a pair of electrodes and a 
liquid crystal disposed therebetween, but actually not contributing to the 
display as by masking or disposition at a marginal part of the display 
panel, in addition to a pixel in an ordinary sense, i.e., a display 
element. 
The increase in number of detected pixels tends to result in difficulties 
such that a longer time is required for the measurement and a complicated 
current detection circuit is required, thus leading to necessity of an IC 
of a large capacity and an increase in production cost. On the other hand, 
mutually adjacent pixels are at a substantially equal temperature, so that 
the measurement at all the pixels is unnecessary. 
In the case of a display panel having about 1000.times.1000 pixels, 
measurement at about 100.times.100 pixels may be sufficient. In this 
instance, such measured pixels (hereinafter referred to as "detection 
pixel(s)" may preferably be distributed not uniformly but in different 
densities such that detection pixels are disposed at a higher density at a 
part having more severe temperature irregularity such as a part close to 
electrode drivers, or a part having a more severe alignment irregularity, 
such as a part close to the liquid crystal injection port of the panel. In 
a preferred embodiment, in connection with the IC structure designed to 
output 128 bits as a unit, one detection pixel is selected per 16.times.16 
pixels and locally per 8.times.8 pixels. A better result may be obtained 
if correction data for non-detection pixels are obtained by interpolation 
based on correction data at the detection pixels. 
(2) Applying the current detection signal to scanning electrodes. 
If the current detection is performed, pixels in a region or along a 
detection current input electrode cannot retain the display state but are 
brought into a first or second stable state or a mixture of such two 
stable states. In order to maintain a good image quality, the pixels 
having disordered images should be rewritten quickly. In the case of 
applying a detection waveform to a scanning electrode and detecting 
through a data electrode, only the scanning electrode supplied with the 
detection waveform is required to be scanned for image display. For 
example, in the case of using three scanning electrodes for current 
detection at a time, the image disorder caused thereby over the entire 
display area can be removed by scanning and writing only on the three 
scanning electrodes. This is faster by 300-400 times than one frame period 
which is required to remove an image disorder caused in the case where a 
current detection signal is applied to a data electrode, thus causing an 
image disorder along the data electrode. As a result, the required time 
for removing image disorder is limited to a very short time which does not 
leave a noticeable image disorder. As will be described hereinafter, a 
larger S/N ratio is attained if the current detection is performed at a 
larger number of pixels at one time but it is important that the rewriting 
for removing image disorder as a post treatment of the current detection 
is completed within a period unnoticeable to the human eye. 
(3) Detecting the current from plural pixels at one time. 
If a current from a pixel at a central part of the display panel is 
detected at an end of the data electrode concerned, the current may be 
very small, so small that it can be hidden by a noise. The S/N ratio is 
liable to be decreased in a larger-size display panel. In order to 
increase the signal, it is desired that the currents from plural pixels 
are collectively detected. In a case where, the output currents from 
plural pixels along a data electrode are detected collectively, the 
rewriting after the detection may require increased time as described 
above so that the number of detection pixels at one time should be limited 
so as to complete the rewriting within an unnoticeable time. Further, in 
the case of detecting the currents from pixels along a data electrode, it 
is necessary to change over between the state wherein plural data 
electrodes are connected to a current detection element for detection and 
the state wherein such plural data electrodes are connected to respective 
data signal application elements separately, so that the number of the 
detection pixels along a data electrode should be limited within an extent 
of not making the IC complicated or excessively enlarged thereby. In any 
case, the detection pixels should be disposed collectively in a narrow 
region so that the respective pixels are at a substantially identical 
temperature. 
(4) Controlling the current detection condition based on temperature data. 
The peak and integrated value of the current vary remarkably depending on 
temperatures, e.g., by 2-4 times between 10.degree. C. and 40.degree. C. 
Accordingly, if a single detection condition is fixedly used for the 
entire temperature region, a higher temperature region causing a quicker 
response of the liquid crystal requires a shorter period for detection 
than at a lower temperature region but is liable to result in a relatively 
coarse measurement accuracy. Accordingly, it is sometimes appropriate to 
change the resetting pulse, detection pulse, detection period, sampling 
(detection) frequency, detection timing, location, number and area of 
detection pixels, etc., depending on whether it is located in a 
high-temperature region or in a low-temperature region, so as to retain a 
certain level of measurement accuracy regardless of the temperature. 
(5) Common use of a pixel. 
A pixel used for display is also used as a current detection element 
(detection pixel) for direct measurement the current therefrom. This may 
be advantageous for removing measurement error due to an indirect 
measurement. In this instance, the pixel concerned is subjected to the 
displaying operation and the current detection operation alternately by 
time-sharing. 
(6) Comparison of both the peak and integrated value. 
The influence of temperature can be understood by the peak time alone or by 
the integrated value alone of a detected current response; but, if the 
peak time and integrated value are measured in combination, it is possible 
to estimate the temperature, cell thickness and delay of a waveform 
applied to the electrode simultaneously. It is also possible to apply 
different waveforms depending on causes of threshold irregularities in 
such a manner that an amplitude-modulated signal is applied to a pixel at 
a high temperature and a pulse width-modulated signal is applied to a 
pixel accompanied with a severe delay in waveform. 
(7) Correction based on a current differential. 
The response current includes a charging current, an ionic current, etc., 
in addition to a Ps inversion current. The threshold characteristic of a 
detection pixel well corresponds to the Ps inversion current. Accordingly, 
by taking a differential current so as to minimize the contribution by 
factors other than the Ps inversion current, it is possible to obtain a 
higher sensitivity to the Ps inversion current. For this purpose, a 
current response in the case of no pixel state inversion in response to a 
detection input signal is detected and compared with a current response in 
the case of pixel state inversion to obtain a differential, based on which 
display drive signals are corrected. 
The following Examples are presented for describing embodiments wherein one 
or more of the above features are adopted alone or in combination. The 
present invention is however not restricted to such embodiments but should 
be understood to cover modifications by substituting alternative or 
equivalent features for some features characterized in the embodiments. 
(EXAMPLE 1) 
FIG. 8 is a block diagram of a liquid crystal display apparatus according 
to an embodiment of the present invention. The display apparatus includes 
a liquid crystal display panel 101, signal changeover switches 102, a 
scanning signal application circuit 103, a data signal application circuit 
104, a display signal control circuit 105, a detection waveform 
Application circuit 107, 106, a current detection circuit 107, a current 
detection control circuit 108, a temperature detection element 
(temperature sensor) 109, a temperature detection circuit 110, a general 
control circuit 111, and a graphic controller 112. 
A temperature in the vicinity of the display panel 101 is detected by the 
temperature sensor 109, and the resultant temperature data is inputted 
through the temperature detection circuit 110 to the display signal 
control circuit 105 and the current detection control circuit 108. The 
current detection control circuit 108 instructs the detection waveform 
application circuit 106 to apply an appropriate current detection waveform 
based on the temperature data. The waveform applied via the signal 
changeover switches 102 to the display panel 101 and a response signal 
from the panel 101 is received by the current detection circuit 107 to be 
converted into current data, which is inputted to the current detection 
control circuit 108. 
The display signal control circuit 105 receives display data from the 
graphic controller 112, converts and corrects the display data based on 
the above obtained temperature data and current data and supplies address 
data and display data based thereon to the scanning signal application 
circuit 103 and the data signal application circuit 104, respectively. The 
scanning signal application circuit 103 and the data signal application 
circuit 104 apply scanning signals and data signals, respectively, to the 
liquid crystal display panel 101 to effect image display thereon. 
Whether the image display or current detection is determined by the general 
control circuit 111 with reference to the temperature data and the current 
data and controlled by the signal changeover switches 102. 
A set of display signal waveforms used in this embodiment are shown in 
FIGS. 9A-9F. At FIGS. 9A-9E are shown data signals applied to the data 
electrodes, and at FIG. 9F is shown a scanning selection signal applied to 
the scanning electrodes. By appropriate selection of waveforms of FIGS. 
9A-9E, good display may be effected regardless of a threshold irregularity 
distributed on a scanning electrode. For example, in the case of 
gradational display, the waveform of FIG. 9A is used to display a 0% 
transmission state, the waveform of FIG. 9B is used to display a 
50%-transmission state and the waveform of FIG. 9C is used to display a 
100%-transmission state in a high temperature region on a scanning 
electrode. On the other hand, in a low temperature region on the scanning 
electrode, the waveform of FIG. 9C is used for a 0% state, the waveform of 
FIG. 9D is used for a 50% state, and the waveform of FIG. 9E is used for a 
100% state. 
The amplitude and pulse width of each waveform may preferably be controlled 
based on a threshold distribution on a scanning electrode, as a matter of 
principle. However, in a case where the threshold distribution on the 
panel and the threshold distribution on the scanning electrode are not 
substantially different, the waveforms can be controlled solely based on 
the temperature data while disregarding the current data in order to 
simplify the control circuit. 
FIGS. 10A and 10B respectively show another example of waveform applied to 
a scanning electrode for current detection. In each waveform, a first 
pulse is applied so as to completely reset a detection pixel into a first 
stable or a second stable state, and a second pulse is applied so as to 
detect a current from the detection pixel after the application thereof. 
The amplitude and pulse width of each of the first and second pulses are 
both controlled by the temperature data. The pixel after application of 
the second pulse and before writing thereafter does not display image 
data. In this instance, in the case where neighboring pixels 
preferentially display the first stable state, the second pulse for 
current detection may preferably be set to a polarity providing the second 
stable state so as to make the non-displaying pixels less noticeable. 
(EXAMPLE 2) 
FIG. 11 is a block diagram of a liquid crystal display apparatus according 
to another embodiment of the present invention, having a control system 
somewhat different from the one in the embodiment of FIG. 8. 
Different from the embodiment of FIG. 8, the control circuit 111 in this 
embodiment of FIG. 11 supplies correction data calculated from the 
temperature data and current data to the graphic controller 112, from 
which already corrected display data are supplied to the display signal 
control circuit 105. 
FIGS. 12A-12F show another example set of display signal waveforms 
different from that shown in FIGS. 9A-9F. Also in FIGS. 12A-12F, at FIGS. 
12A-12E are shown data signals and at FIG. 12F is shown a scanning 
selection signal. 
(EXAMPLE 3) 
FIG. 13 is a schematic view of a current signal detection system applicable 
to the display apparatus according to the present invention and usable in 
association with a control system as shown in FIG. 5. 
Different from the one shown in FIG. 3, the system shown in FIG. 13 is used 
for detection for plural objects (pixels a and b), from which two 
independent detection output signals are derived. 
Referring to FIG. 13, the current signal detection system includes 
detection waveform application elements 152, current detection elements 
153, scanning electrodes 201, data electrodes 202 and a liquid crystal 
305. Regions a and b each encircled with a dotted line represent a first 
and a second detection region each comprising at least one pixel. The 
current detection system further includes a differential circuit 151 for 
taking a difference between outputs from the first and second detection 
regions. 
A rectangular or ramp waveform is applied from the detection waveform 
application circuit 152 to cause a switching of the liquid crystal 
molecules, thereby detecting an internal current due to inversion of the 
spontaneous polarization (hereinafter referred to as a "Ps inversion 
current") by the current detection element 153. For example, when a 
waveform shown in FIG. 14A is applied, a current response as shown in FIG. 
14B may result. As it is known that the shape of a Ps inversion current 
changes depending on the temperature of the liquid crystal and the 
electric field intensity, it is possible to know the temperature, cell 
thickness and threshold characteristic of the detection region (or pixel) 
by measuring the quantity of charge Q, peak time .tau. and half-value 
width .tau..sub.w of the waveform shown in FIG. 14B. 
However, the responsive current can further include a charging current 
accompanying a potential change in the liquid crystal layer, a current 
accompanying localization of ions in the liquid crystal layer, etc., in 
addition to the Ps inversion current. Accordingly, in the case of a small 
Ps inversion current or a quick Ps inversion as shown in FIG. 14C or 14D, 
respectively, the measured values of the charge quantity Q, peak time 
.tau., half-value width .tau..sub.w, etc., are liable to contain increased 
errors. 
Accordingly, after the first detection region (or pixel) a is reset into a 
white state and the second detection region (pixel) b in a drive condition 
substantially equal to the first detection region is reset into a black 
state, then both the first and second detection regions are supplied with 
a waveform for switching the liquid crystal molecules into a black state. 
As a result, the Ps inversion current is contained only in the output 
current from the first detection region and not in the output current from 
the second detection region. Accordingly, if two outputs are inputted into 
the differential circuit to take a differential, a Ps inversion current 
can be obtained. 
From the data regarding the Ps inversion current thus obtained, it is 
possible to know the threshold characteristics, based on which signals 
applied to the respective pixels may be corrected to effect a stable 
display. 
(EXAMPLE 4) 
FIG. 15 is a schematic view of another current signal detection system 
applicable to the present invention. The system is basically characterized 
by adding a thermocouple 171 and a temperature detection device 172 to the 
system shown in FIG. 13. 
First, a first detection region and a second detection region in a 
different condition from the first detection region are both placed in a 
first state and then supplied with a detection waveform for switching the 
liquid crystal molecules into a black state. Then, the outputs from the 
first and second detection regions are inputted into a differential 
circuit 151 to take a differential. If the first and second detection 
regions have an equal area and an equal cell thickness, the output of the 
differential circuit 151 is attributable to a temperature difference 
between the two detection regions. Now, the temperature of the second 
(left) detection region is known by the thermocouple 171 and, therefore, 
can teach the temperature of the first detection region in combination 
with the output of the differential circuit 151. As in the embodiment of 
FIG. 13, it is possible to measure the difference in Ps inversion current 
at a good accuracy by subtracting the contribution of the charging 
current, the ion current, etc. 
As the temperature sensor 171 for the second detection region, it is 
desirable to dispose a thermocouple of alumel-chromel, chromel-constantan, 
copper-constantan, etc., within the liquid crystal layer, but it is also 
possible to dispose a thermister on a glass substrate. The latter is 
simple in disposition while the accuracy is somewhat inferior. 
(EXAMPLE 5) 
In some further embodiments, the current signal detection is effected by 
performing the measurement plural times under different measurement 
conditions so as to provide different liquid crystal inversion rates, and 
the correction of an error is effected based thereon. These embodiments 
may be divided into several types. One embodiment is presented herein as 
Example 5. 
The relationship between the factors such as the Ps inversion current, 
charging current, ionic current, etc., and the error in measurement values 
of the charge quantity Q, peak time .tau., half-value width .tau..sub.w, 
etc., is the same as in the embodiment of FIG. 13. 
In view of a possibility that different liquid crystal molecular states can 
be present before the measurement, the initial state of a detection region 
is set in a white state for a first measurement and in a black state in a 
second measurement, and the detection region is supplied with a detection 
waveform for switching into a black state in both the first and second 
measurements. As a result, the output current in the first measurement 
contains a Ps inversion current whereas the output current in the second 
measurement does not contain a Ps inversion current, whereby a 
differential between the two outputs provides a Ps inversion current 
accompanying the switching from the white state to the black state. 
The differential between outputs may be taken by a method of storing the 
output waveforms in a memory, followed by comparison of the waveforms in 
memory, or a method of integrating the output waveforms, followed by 
comparison of the integrated values. The former method provides more 
detailed information regarding the threshold characteristics but requires 
a higher cost. On the other hand, the latter method requires only a low 
cost but can require a long integration period, thus resulting in a slower 
measurement speed, in some cases. 
With reference to FIG. 5 already mentioned for description, an objective 
pixel for detection (detection pixel) is first reset into a white state by 
the circuits 103 and 106. Then, the circuit 102 is changed over, and an 
input signal for switching the pixel into a black state is applied, 
whereby an output signal thereby is read by the circuit 116. Then, by 
using the same circuits, the same detection pixel is reset into a black 
state and then supplied with the same input signal for switching into the 
black state as in the previous measurement. Then, a differential between 
the signal thus measured by time-sharing is taken, and display drive 
signals are corrected based on the differential. 
(EXAMPLE 6) 
In this embodiment, a number (N) of input signals are applied each after 
resetting. More specifically, a detection region (or pixel) is initially 
reset into a white state and then supplied with a detection waveform. This 
cycle is repeated N times while gradually increasing the pulse width of 
the detection waveform. As a result, the liquid crystal which may not be 
switched into black in the first cycle is gradually switched to increase 
the black state area and make the entire detection region black after the 
N times of application cycles. 
FIG. 16 is a graph showing a relationship between pulse width AT and Ps 
inversion current Ps' based on measured data through such N times of 
signal application. Generally, the inverted area and the Ps inversion 
current Ps' correspond to each other. Accordingly, referring to FIG. 16, 
points giving constant Ps' represented by (.DELTA.T.sub.0, Ps'.sub.0) and 
(.DELTA.T.sub.100, Ps'.sub.100) are used to define inversion rates of 0% 
and 100%, respectively, and a pulse width-inversion rate characteristic 
(threshold characteristic) is obtained based thereon. 
Display drive may be performed by applying writing waveforms based on such 
a .DELTA.T-Ps' characteristic. 
In the above detection method, the .DELTA.T-Ps' characteristic is obtained 
by N times of signal application while gradually increasing the pulse 
width. This is effected for making easy the data processing. For a similar 
purpose, it is also possible to gradually shorten the pulse width. 
Reversely, in case of obtaining the .DELTA.T-Ps' characteristic at a time 
(in a short time) without including a substantial display period during 
the first to N-th measurements, the pulse widths may preferably be given 
at random so as to obviate a threshold change due to hysteresis of the 
liquid crystal inversion state. 
A V-Ps' characteristic similar to the .DELTA.T-Ps' characteristic may be 
obtained by applying pulses having a fixed pulse width and varying 
voltages. For realization of this, a scanning-side waveform application 
device capable of providing analog outputs or multi-level outputs, thus 
requiring a higher cost. However, in the case of gradational display by 
modulating amplitudes of display data signals, the V-Ps' characteristic 
provides an advantage of a simple correlation between the detection 
waveform and the display waveform. 
(EXAMPLE 7) 
As described above, the response current can contain a charging current in 
a substantial proportion. In this embodiment, therefore, two measurement 
periods are provided for a single detection waveform for subtracting the 
charging current. 
A detection waveform is set as shown in FIG. 17 so as to invert the liquid 
crystal by a single polarity pulse. A first measurement is performed in a 
period .DELTA.T.sub.1 for applying the pulse and, in a subsequent period 
.DELTA.T.sub.2 of an equal length to .DELTA.T.sub.1 immediately after the 
pulse termination, a second measurement is performed. As a result, the 
first period .DELTA.T.sub.1 and the second period .DELTA.T.sub.2 include 
responses to the rising and the falling, respectively, of the same pulse, 
so that the charging current can be canceled by adding the outputs of the 
first and second measurements. 
In this scheme, it is desired that the inversion of the liquid crystal is 
completed within a period of .DELTA.T.sub.1 for catching a current 
response waveform and within a period of .DELTA.T.sub.1 -.DELTA.T.sub.2 
for catching an integral value of current response. 
This embodiment may be effected by applying a control system identical to 
the one shown in FIG. 8 or 11. A temperature in the vicinity of the 
display panel is inputted as temperature data to the display signal 
control circuit 105 and the current detection control circuit 108 via the 
temperature detection element 109 and the temperature detection circuit 
110. The current detection control circuit 108 instructs the detection 
waveform application circuit 106 to apply appropriate current detection 
waveforms based on the temperature data. The detection waveforms applied 
to the liquid crystal display panel 101, and response signals therefrom 
are received via the signal changeover switches 102 by the current 
detection circuit 107, through which current data are inputted to the 
current detection control circuit 108, wherein a differential is taken in 
this embodiment. 
Then, in the display signal control circuit 105, display data received from 
the graphic controller 112 are converted and corrected based on the 
above-mentioned temperature data and current data into address data and 
display data, which are then inputted to the scanning signal application 
circuit 103 and the data signal application circuit 104, respectively. The 
scanning signal application circuit 103 and the data signal application 
circuit 104 respectively apply scanning signals and display signals 
synchronously to the liquid crystal display panel 101 to effect image 
display thereon. 
On the other hand, as shown in FIG. 11, it is also possible to supply 
correction data calculated based on the temperature data and current data 
or the temperature data and the differential of current data to the 
graphic controller 112, from which already corrected display data is 
supplied to the display signal control circuit. 
(EXAMPLE 8) 
FIG. 18 shows another embodiment of the current detection waveform used in 
Example 3 or 4. The waveform includes a period T.sub.1 for resetting a 
pixel and a period T.sub.2 for current detection. Referring to FIG. 18, at 
A is shown a (voltage) waveform applied to a scanning electrode, at B is 
shown a waveform applied to a data electrode for a first detection region, 
and at C is shown a waveform applied to a data electrode for a second 
detection region. In the period T.sub.1 the first detection region is 
reset to a white state and the second detection region is reset to a black 
state. Then, in the period T.sub.2 after a pause period of, e.g., 100 
.mu.s so as to avoid the influence of the pulse applied in the period 
T.sub.1, an input signal is applied to an associated scanning electrode so 
as to apply a voltage for switching into a black state to both the first 
and second detection regions. At this time, current signals outputted from 
the two data electrodes are read to provide a differential therebetween, 
based on which display drive signals are corrected. 
(EXAMPLE 9) 
FIG. 19, similarly as FIG. 18, shows another embodiment of the current 
detection waveform used in Example 3 or 4. Similarly as in FIG. 18, at A 
is shown a waveform applied to a scanning electrode, at B is shown a 
waveform applied to a data electrode for a first detection region, and at 
C is shown a waveform applied to a data electrode for a second detection 
region. The second detection region is used as a reference for the first 
data electrode and therefore should desirably be identical to the first 
detection region with respect to the area as well as the other factors, 
such as the temperature, cell thickness, and degree of delay in wave 
transmission, so that the second detection region is disposed in the 
neighborhood of the first detection region. The first and second detection 
regions may be set without being fixed but while being changed at 
locations at prescribed timing for current detection so as not to be 
localized or biased. 
(EXAMPLE 10) 
In this embodiment, a current detection system identical to the one shown 
in FIG. 3 is used by applying waveforms shown in FIG. 20. The waveforms 
include a period T.sub.1 for resetting a pixel and a period T.sub.2 for 
current detection. At A is shown a waveform applied to a scanning 
electrode for a first measurement, and at B is shown a waveform applied to 
a scanning electrode for a second measurement. A pixel is reset to a white 
or black state in the period T.sub.1 and set to a black state in the 
period T.sub.2 
More specifically, in the first measurement using the waveform at A, a 
pixel is reset to a white state in the period T.sub.1 and, after a 
prescribed period, inverted to a black state by applying an input signal 
in the period T.sub.2, so that a current signal is detected through a data 
electrode. 
Then, in the second measurement using the waveform at B, the pixel is reset 
to a black state in the period T.sub.1, and, after the same prescribed 
period, supplied with the same input signal as in the waveform at A in the 
period T.sub.2, so that a current signal is read through the data 
electrode. 
A differential is taken between the current signals obtained in the first 
and second measurements, and display drive signals are corrected based 
thereon. 
(EXAMPLE 11) 
This embodiment is a modification of Example 10 described above and uses a 
current detection waveform shown in FIG. 21. The waveforms, similarly as 
those shown in FIG. 20, include a reset period T.sub.1 and a detection 
period T.sub.2. Referring to FIG. 21, at A.sub.1 is shown a waveform 
applied to a scanning electrode for a first measurement, at A.sub.2 is 
shown a waveform applied to the scanning electrode for a second 
measurement, at A.sub.3 is shown a waveform applied to the scanning 
electrode for a third measurement, and . . . at A.sub.N is shown a 
waveform applied to the scanning electrode for an N-th measurement. For 
the pulse width .DELTA.T, an initial value and an increment are set based 
on temperature data, and the pulse width .DELTA.T is gradually increased 
as the measurement is repeated from the first, 2nd, 3rd, . . . to the N-th 
measurement. 
(EXAMPLE 12) 
FIG. 22 is a block diagram of a liquid crystal display apparatus according 
to this embodiment including the control system. 
This embodiment is different from the one shown in FIG. 8 in that a large 
number of temperature sensors 109 are disposed at discrete points on a 
display panel 101. FIG. 23 shows a detection input signal used in this 
embodiment including a reset pulse (T.sub.1) and an inversion signal 
(T.sub.2) serially applied to scanning electrodes with a prescribed 
spacing therebetween, so that current signals are taken through associated 
data electrodes. 
(EXAMPLE 13) 
FIG. 24 shows a modification including a modified control system of the 
liquid crystal display apparatus shown in FIG. 8 or FIG. 22. 
In this embodiment, temperature sensors 109 comprising a thermistor are 
disposed in adhesion on a non-display part 113 (not observable from the 
outside) of the liquid crystal display panel. 
However, as the response current includes not only the Ps inversion current 
but also a charging current accompanying a potential change within the 
liquid crystal layer and an ionic current due to localization of ions 
within the liquid crystal layer, the measured values of the charge 
quantity Q, peak time .tau., half-value width .tau..sub.w, etc., can 
include substantial errors in the case of a small Ps inversion current or 
a quick Ps inversion as shown in FIG. 14C or 14D. 
Accordingly, in this embodiment, a relaxation period is disposed so as to 
improve the measurement accuracy. 
FIG. 25 illustrates how directors of liquid crystal molecules in a uniform 
alignment state in a chevron structure showing a black display state are 
changed in response to an applied voltage. 
At (a) is shown a state when a minute pulse in a direction of setting a 
white state is applied, at (b) is shown a state of no voltage application, 
at (c) is shown a state when a minute pulse in a direction of setting a 
black state is applied, and 
at (d) is shown a state when a pulse sufficient to set a back state is 
applied. 
In FIG. 25, each radius 121 represents a director, an arrow 122 represents 
a spontaneous polarization of a liquid crystal molecule, numerals 123 
denote a pair of substrates, and an arrow 124 represents a spontaneous 
polarization as a total of liquid crystal molecules between the 
substrates. As shown in the figure, the director directions can be 
different in the same black state depending on the voltage application 
states. The spontaneous polarization of each liquid crystal molecule is 
oriented in a direction perpendicular to the director and is represented 
by an arrow 122. However, the total spontaneous polarization between the 
substrates is caused to have a different magnitude which depends on the 
uniformity of director directions. 
In other words, a pixel having an identical inverted domain area can have 
different quantity of spontaneous polarization depending on the magnitude 
of a pulse applied or the time since application or termination of a 
pulse. 
FIG. 26 is a graph showing a relationship between inverted domain area and 
charge quantity in case where a pixel comprising a liquid crystal used in 
this embodiment is changed from its initial black state to a halftone 
state by application of a pulse so that the charge quantity is determined 
as a difference in charge quantity between immediately before and after 
application of the pulse. FIG. 26 shows the results obtained by applying 
drive voltages of +10 volts, .+-.15 volts and .+-.20 volts. As shown, the 
characteristics are clearly different depending on the drive voltages 
applied. 
For the above reason, in order to obviate the error in measurement of a 
spontaneous polarization, it is appropriate that the current detection is 
performed with reference to a constant director state, i.e., the no 
voltage application state shown at FIG. 25(b) or the largest spontaneous 
polarization state shown at FIG. 25(d). 
Accordingly, it is appropriate to dispose a relaxation period after a pulse 
application so as to effect a measurement when the influence of the pulse 
is removed, or to effect a measurement during or immediately after 
application of a sufficiently large pulse (reset pulse). Further, in order 
to obtain a varying domain area, it is necessary to applying a pulse for 
placing a pixel in a halftone state. Accordingly, measurement may 
appropriately be effected by using a combination of "a halftone pulse+a 
relaxation period" and "immediately after application of a reset pulse". 
For the above reason, the current response is measured by using a group of 
waveforms as shown in FIG. 26. In FIG. 26, T.sub.1 denotes a period for 
applying a first waveform for setting a pixel in a halftone state. T.sub.2 
denotes a relaxation period wherein the director moved by application of 
the first waveform is set in the state shown at FIG. 25(b). T.sub.3 
denotes a period for applying a second waveform by which the pixel is 
reset to a black state. The directors immediately after the application of 
the second waveform are in the state shown at FIG. 25(d). Accordingly, a 
charge quantity difference between the points immediately before and 
immediately after application of the second waveform. FIG. 28 shows a 
relationship between the domain area inverted into the black state by 
application of the second waveform and the charge quantity (difference) 
thus measured, under different drive voltages of .+-.10 volts, .+-.15 
volts and .+-.20 volts for the first and second waveforms while changing 
the pulse widths (FIG. 27(a) to FIG. 27(d)) so as to provide various 
inverted domain areas. As shown in FIG. 28, a good agreement is obtained 
among the drive voltages of .+-.10 volts, .+-.15 volts and .+-.20 volts, 
thus showing a constant relationship between the inverted domain area and 
the Ps inversion current (i.e., charge quantity as an integrated value). 
FIG. 29 shows another group of waveforms for such measurement. T.sub.3 is a 
period for applying a second waveform for resetting a pixel to a black 
state. T.sub.1 is a period for applying a first waveform for setting the 
pixel in a halftone state, and T.sub.2 is a relaxation period. A relation 
similar to the one shown in FIG. 28 is obtained by taking a charge 
quantity (difference) between the points immediately after the application 
of the second waveform and after the relaxation period. However, compared 
with the scheme using the waveforms shown in FIG. 27, a longer period is 
required for the current detection, so that the measurement result is 
liable to be accompanied with a noise by that much. 
In the above, in order to obtain the state shown at FIG. 25(b), it is 
desirable to design the first waveform and the second waveform to be free 
from DC components as shown in FIG. 27 or 29. 
The periods required of T.sub.1, T.sub.2 and T.sub.3 vary depending on the 
temperature and drive voltages, and the period T.sub.1 can also vary 
depending on the halftone level to be displayed. At 30.degree. C. and 
under application of .+-.20 volts, for example, a uniform display could be 
obtained by roughly T.sub.1 =200 .mu.s, T.sub.2 =300 .mu.s, and T.sub.3 
=200 .mu.s. At higher temperatures, the respective periods could be 
shortened but T.sub.2 required 100 .mu.s at the minimum for a uniform 
display. 
As described above, it is possible to provide a liquid crystal display 
apparatus capable of stably retaining a good display state regardless of a 
temperature change and a threshold distribution along a liquid crystal 
display panel by providing current detection means and means for applying 
two waveforms with a relaxation period for current detection. 
(EXAMPLE 14) 
As the shape of Ps inversion current varies depending on the temperature, 
the shape of a detection waveform, etc., it is possible to know the 
temperature, cell thickness and threshold characteristic at a detection 
region from the charge quantity, peak time, etc. A threshold change may be 
obtained by comparing the threshold characteristic with a reference 
threshold characteristic, and a correction factor may be obtained 
therefrom within a pause period during or in parallel with image display 
drive. During the display drive, given display data are corrected by 
adding correction factors for respective pixels concerned, thereby 
controlling the drive signals applied to the respective electrodes. 
FIG. 30 is partial plan view of a display panel used in this embodiment, 
wherein a detection region is denoted by hatching and a black spot 
represents a center of a related detection region. 
Display compensation may for example be performed in such a manner that a 
display panel is divided into an appropriate number of sections as shown 
and a common correction factor is used for each section. For example, a 
display at point E is corrected by using a correction factor at point A 
and a display at point F is corrected by using a correction factor at 
point B. According to this scheme, however, the correction factors in the 
vicinity of section boundaries are discontinuous, so that there arises a 
difficulty of providing two different display states for identical display 
data. 
In order to obviate such an irregularity at such section boundaries, it is 
preferred that correction factors obtained from current data at respective 
detection regions are used for deriving correction factors over the entire 
display area. 
For example, a correction factor Mx for a point E surrounded by four points 
A, B, C and D may be calculated by interpolation based on the following 
formula 1 or 2: 
##EQU1## 
wherein M.sub.1 -M.sub.4 denote correction factors for points A-D, 
respectively, and L.sub.1 -L.sub.4 denote distances between the point E 
and the points A-D, respectively. 
Generally, the correction factor My for an arbitrary point may be 
calculated by interpolation by using corrections factors M.sub.1 . . . . 
Mn of an appropriate number (n) of points having distances L.sub.1. . . 
Ln, respectively, from the arbitrary point based on the following formula 
3 or 4: 
##EQU2## 
The number n is at most the number S of detection regions set on the 
display panel and should be an appropriate number of detection points in 
the neighborhood of the objective arbitrary point. 
In some cases, it is desirable to effect interpolation with respect to 
time. For example, if a correction factor for a point G changes rapidly or 
periodically, the display state of the corresponding pixel can also cause 
a rapid contrast change or flicker. In such a case, the change in 
correction factor may be moderated by interpolation. For example, if the 
correction factor for the point G is M at time T.sub.1 and then 10M at a 
subsequent current detection, the correction factor for the point G is 
gradually changed to 2M, 3M, . . . 10M at time T.sub.2, T.sub.3, . . 
T.sub.10. 
As described above, a good display can be ensured by interpolation with 
respect to position and time, so that the current detection need not 
performed at every pixel or frequently and thereby the cost for the 
current detection can be saved by minimizing the time and space for the 
current detection. 
In a specific example, a display panel having 1280.times.1024 pixels was 
provided with 2500 detection regions each comprising 10 pixels (5 pixels 
along a scanning electrode and 2 pixels along a data electrode). The 
correction factors for respective pixels were calculated by interpolation 
based on the formula 3 using correction factors from the surrounding 
detection regions within a display control circuit in parallel with 
control of the drive signals while changing a correction factor once per 
0.5 sec (interpolation at a 0.5 sec cycle) based on current detection data 
obtained once per 5 sec at the respective detection regions. 
As described above, according to this embodiment, it is possible to retain 
a good display state over an entire display panel regardless of a 
threshold change while suppressing a rapid or discontinuous change or 
flicker accompanying the compensation. 
(EXAMPLE 15) 
In order to effect a good display and further a stable halftone display on 
a large display panel regardless of a temperature distribution thereover, 
it is necessary to effect an accurate compensation for a threshold 
irregularity over the display panel. Therefore, an apparatus according to 
this embodiment is provided with means for detecting a threshold 
characteristic of a certain specific region (data electrode) on the matrix 
display panel, i.e., means for detecting charge migration accompanying an 
inversion from a first stable state to a second stable state or vice versa 
of liquid crystal molecules in the detection region, and means for 
correcting data signals and scanning signals based thereon. 
In order to accurately detect the charge migration, the following factors 
are important: 
1) Molecules in a detection region are inverted. 
2) The migrated charge or a part thereof accompanying the detection 
("responsive current") can be taken out to a current detection circuit 
outside the electrode matrix. 
3) Responsive current other than from the detection region does not enter 
the current detection circuit. 
Based on the above, in order to accurately measure the detection current 
while avoiding image quality degradation, this embodiment is characterized 
by the following features. 
FIG. 31 is a schematic illustration of a detection system according to this 
embodiment. Referring to FIG. 31, the system includes a detection waveform 
application circuit 801, a scanning signal application circuit 802 for 
display drive, a current detection circuit 803 including an amplifier and 
a terminal resistor, a data signal application circuit 804 for display 
drive, switches 805 for changeover between detection operation and display 
drive, and a differential circuit 805. These members are connected to a 
liquid crystal display panel including scanning electrodes 201a, 201b, . . 
. , data electrodes 202a, 202b, . . . and a ferroelectric liquid crystal 
305 disposed between the scanning electrodes and data electrodes. A 
detection region may be formed as a region x encircled by a dotted line. 
In the detection operation, the switches 805 are set to a position for 
detection, and a detection waveform as shown in FIG. 4A is applied from 
the detection waveform application circuit 801 to the scanning electrode 
201a to switch the liquid crystal molecules in the detection region X, 
whereby a response current (FIG. 4B) including a Ps inversion current is 
inputted to the current detection circuit 803 via the data electrode 202a. 
As it is known that the shape of a Ps inversion current changes depending 
on the temperature of the liquid crystal and the electric field intensity, 
it is possible to know the temperature, cell thickness and threshold 
characteristic of the detection region (or pixel) by measuring the 
quantity of charge Q, peak time .tau. and half-value width .tau..sub.w of 
the waveform shown in FIG. 4B. 
In order to prevent the response current from outside the detection region 
from entering into the current detection circuit, the image display 
operation is switched to the current detection operation in a step within 
a sequence shown in FIG. 32A so as to cause the inversion of liquid 
crystal molecules only at the detection region. More specifically, the 
sequence includes the following steps. 
1) The scanning for image display drive is interrupted. As a result, a 
static picture is displayed because of the memory characteristic of the 
ferroelectric liquid crystal. 
2) Pixels including the detection region (at least one pixel) on a scanning 
electrode concerned are written. At this time, the pixel in the detection 
region is in a first stable state or a mixture of the first stable state 
and a second stable state, and pixels outside the detection region are 
reset to the second stable state. 
3) The pixels are allowed to stand until the molecular perturbation or 
perturbation due to the writing at 2) is substantially removed (a 
relaxation period is disposed). 
4) Associated data electrodes are connected to the current detection 
circuit to start the current detection. 
5) the scanning electrode (detection-selection scanning electrode) 
including the detection region is supplied with a detection waveform to 
reset all the molecules in the detection region to the second stable 
state. 
6) The response current is detected. 
7) After the current detection, the display drive is resumed by first 
scanning the detection-selection scanning electrode to form an image. 
By performing the steps 1)-7) above sequentially, the liquid crystal 
molecules in the detection region can be selectively switched into the 
second stable state. 
Data electrodes not related with the detection region or a region for a 
differential purpose as described below may be provided with a ground 
level potential from the data signal application circuit 804 or grounded 
via the terminal resistor so as to suppress the noise, thereby providing 
an increased S/N ratio. 
The response current occurring in the detection region enters the current 
detection circuit 803 via the data electrode. However, a part thereof can 
flow to the scanning electrode side during the period it flows through the 
data electrode. 
Accordingly, at the time of the current detection, scanning electrodes not 
associated with the detection region (detection-nonselection scanning 
electrodes) may be placed in a high impedance state so as to remove a 
potential difference from the opposite data electrodes, thereby preventing 
the response current from flowing toward the scanning electrode side. As a 
result, the detection current entering the current detection circuit may 
be increased. Examples of the current detection sequence including such a 
high impedance placement step are shown in FIGS. 32B and 32C. 
Incidentally, in case where data electrodes not associated with the 
detection region are grounded via a resistor, a response current is 
inputted to the current detection circuit; in case where such 
non-associated pixels are grounded, substantially identical to the 
integral value of the response current is inputted to the current 
detection circuit and, in case where such non-associated data electrodes 
are placed in a high impedance state, a potential almost identical to that 
of the scanning electrode side is inputted to the current detection 
circuit. The former two cases are more effective. 
FIG. 33 is a time-serial waveform diagram showing a set of waveforms for 
current detection. The waveforms include a period T.sub.1 for resetting 
the liquid crystal molecules into a state suitable for current detection, 
a period T.sub.2 for current detection and a relaxation period 
therebetween. Referring to FIG. 33, at A and B are shown waveforms applied 
to detection-selection scanning electrodes, at C is shown a waveform 
applied to detection-nonselection scanning electrodes, at D is shown a 
waveform applied to data electrodes associated with (i.e., constituting) 
the detection region, at E is shown waveform applied to data electrodes 
associated with a detection region for a differential purpose, and at F is 
shown a waveform applied to data electrodes not associated with (i.e., not 
constituting) the detection region. 
In the period T.sub.1, the detection region is set to a first stable state 
or a mixture of the first stable state and a second stable state, and the 
pixels outside the detection region on the detection-selection scanning 
electrode(s) are reset to the second stable state. 
In the period T.sub.2 following the relaxation period, all the pixels on 
the detection selection scanning electrode(s) are reset to the second 
stable state for current detection at the pixels constituting the 
detection region. After the current detection, the scanning for image 
display is resumed from the detection-selection scanning electrode(s) to 
resume an image display state within 2 ms. 
In this instance, in order that the image disorder due to the current 
detection is not recognizable by eyes, the second stable state may 
preferably be set to an optical state close to a display state immediately 
before the detection. For example, in case where a current detection is 
performed during display of a picture having a bright state as the 
background, it is preferred that the second stable state is set to a 
bright state. 
Further, a region for taking a differential with the detection region may 
preferably have factors, such as area, temperature, cell thickness, and a 
delay in waveform transmission, affecting the current response identical 
to those of the detection region and is therefore preferably set at a 
position close to the detection region. 
In a specific example, a detection region was set to include 10 pixels (5 
pixels along each scanning electrode and 2 pixels along each detection 
region), and the current detection was performed while setting T.sub.1 at 
150 .mu.s, the relaxation period at 1.5 ms, T.sub.2 at 100 .mu.s and a 
display restoring period at 200 .mu.s (corresponding to two lines), so as 
to suppress the image display interruption period within 2 ms, whereby no 
image disorder was visually recognized. 
(EXAMPLE 16) 
FIG. 34 is a time-serial waveform diagram showing a set of waveforms used 
for current detection in another embodiment. This embodiment is different 
from the embodiment shown in FIG. 33 in that the detection non-selection 
scanning electrodes and data electrodes not associated with the detection 
region are all placed in a high-impedance state during current detection, 
and the period of connecting the data electrodes for detection to the 
detection circuit is restricted to within the detection pulse-application 
period. The connection may be effected at any time after application of 
the detection pulse and before commencement of the polarity inversion of 
the liquid crystal. The disconnection from the detection circuit and 
connection to the display drive circuit may be at any time after 
completion of the polarity inversion. 
In a specific example, the connection to the detection circuit was 
performed at a point of 10 .mu.s after application of the detection pulse, 
and the disconnection was performed simultaneously with the termination of 
the detection pulse. The detection-nonselection scanning electrodes and 
data electrodes not associated with the detection region were placed in a 
high-impedance state simultaneously with the connection of the associated 
data electrodes to the detection circuit. 
In the embodiment of FIG. 13, the associated data electrodes are connected 
to the detection circuit prior to the application of the detection pulse 
and are thus placed in a high-impedance state, so that the potential of 
the data electrodes is also affected by the application of the detection 
pulse and is restored to zero potential through the terminal resistor 
within the detection circuit, thus applying a voltage to the liquid 
crystal. For this reason, the voltage application can be delayed 
substantially depending on the magnitude of the terminal resistor, thus 
taking a longer time for the detection. 
In contrast thereto, if the connection to the detection circuit is effected 
immediately after the application of the detection pulse as in this 
embodiment of FIG. 34, the liquid crystal is supplied with the voltage 
simultaneously with the pulse application, so that the detection time can 
be shortened and the terminal resistor can be omitted. 
(EXAMPLE 17) 
FIG. 35 is a block diagram showing a liquid crystal display apparatus 
including a current detection system according to this embodiment. 
Referring to FIG. 35, the system includes scanning electrodes 1701 
including a scanning electrode 1701a associated with a detection region 
1707 and scanning electrodes 1701b not associated with the detection 
region, data electrodes 1702 including a data electrode 1702a associated 
with the detection region and data electrodes 1702b not associated with 
the detection region, a scanning electrode drive circuit 1703, a data 
electrode drive circuit 1704, a current detection circuit 1705, and 
changeover switches 1706 for switching the connection of the scanning 
electrodes to the drive circuit 1703 or to the detection circuit 1705. 
FIG. 36 is a time-serial waveform diagram showing a set of waveforms 
applied to the system shown in FIG. 35 for the current detection. 
Referring to FIG. 36, at 1801 is shown a voltage waveform applied to a 
scanning electrode associated with the detection region, at 1802 is shown 
a voltage waveform applied to the other scanning electrodes, at 1803 is 
shown a voltage waveform applied to a data electrode associated with the 
detection region, at 1804 is a voltage waveform applied to the other data 
electrodes, at 1805 is shown a voltage waveform applied to pixels in the 
detection region, and at 1806 is shown a voltage waveform applied to 
pixels outside the detection region on the scanning electrode associated 
with the detection region. Further, the waveforms shown in FIG. 36 include 
a period T.sub.1 for ordinary image display, a period T.sub.2 for 
resetting all the pixels on the scanning electrode associated with the 
detection region into a black state prior to the detection, a period 
T.sub.3 for the detection, a period T.sub.4 for connecting the detection 
scanning electrode to the detection circuit, and a period T.sub.5 for 
restoring the pixels associated with the current detection to the original 
display state. 
FIG. 37 is a block diagram of an embodiment of the detection circuit. 
Referring to FIG. 37, a detection signal is inputted through a line 901 to 
an input terminal 903 of an operational amplifier 902 to be amplified 
therein. To another terminal 904 is inputted a difference (1801-1803 in 
FIG. 36) between outputs from the scanning side drive circuit and the 
data-side drive circuit, so that only a potential change is amplified. The 
amplified signal is converted by an analog/digital converter 905 into a 
digital signal, which is time-divided with the aid of high frequency clock 
pulses inputted through a line 906 to the D/A converter 905, so that the 
time-divided signals are stored in a memory 907 for respective time. 
The detection is performed at a prescribed time between ordinary display 
drives. More specifically, ordinary scanning in period T.sub.1 is 
interrupted, and all the pixels on a scanning electrode 1801a associated 
with the detection region 1707 are reset into a black state in period 
T.sub.2. In this embodiment, the black resetting is performed so as to 
place the pixels outside the detection region 1707 in a black state and 
make the pixels not readily recognizable. 
Then, in period T.sub.3, a detection voltage pulse is applied as a 
combination of pulses applied to the associated scanning electrode and 
data electrode so that the voltage applied to the detection region exceeds 
a threshold for inversion to a white state and the voltage applied to the 
non-detection region is below the threshold. 
Slightly after the commencement of the detection pulse, a period T.sub.4 
for disconnecting the scanning electrode 1701a from the drive circuit 1703 
and connecting the scanning electrode 1701a to the detection circuit 1705. 
The period of shift (T.sub.3 -T.sub.4) is a period required for the 
respective electrodes to reach the potentials for detection, and is 
disposed in view of a possibility that an electrode portion remote from 
the drive circuit does not immediately reach a saturation potential due to 
a delay in pulse transmission. If the scanning electrode is disconnected 
from the drive circuit before the detection pulse voltage reaches the 
remote end thereof, a correct detection voltage is not applied to the 
detection pixel so that the detection becomes inaccurate. 
The scanning electrode 1701a is connected to the detection circuit 1705 
within the period T.sub.4. The detection circuit is principally 
constituted by an operational amplifier 902 which can be designed to have 
a sufficiently large impedance, so that the scanning electrode is placed 
in a high-impedance state. At the detection pixel, the spontaneous 
polarization of the liquid crystal is inverted and, as a result, the 
scanning electrode potential is changed by 
EQU .delta.V=2PSA/C.sub.line, 
wherein Ps denotes the spontaneous polarization of the liquid crystal, A 
denotes the area of the inverted region, and C.sub.line denotes a static 
capacitance for one scanning electrode with the opposite data electrodes. 
The pixels outside the detection region are not inverted, thus not 
contributing to the potential change. 
The detection is terminated when the liquid crystal inversion is completed, 
and the scanning electrode 1701a is disconnected from the detection 
circuit 1705 and connected to the drive circuit 1703. Simultaneously 
therewith, the pulses in period T.sub.5 are applied to restore the pixels 
on the detection-selection scanning electrode 1701a to the original 
display state, and then the ordinary scanning is resumed in period 
T.sub.1. 
FIG. 38 shows a potential change with time of the detection-selection 
scanning electrode. The potential change occurs within a time on the order 
of the inversion response time .tau., and the magnitude .delta.V thereof 
is proportional to Ps. Accordingly, by detecting the potential, it is 
possible to know .tau. or Ps. The temperature-dependence of .tau. and Ps 
has been known as a function of temperature, so that it is also possible 
to know the temperature of the detection region. 
In some cases, the cell gap of the detection region is unknown in addition 
to the temperature. In such cases, both Ps and .tau. are measured, and the 
temperature is obtained from Ps and further the viscosity .eta. of the 
liquid crystal is obtained based on the temperature. The viscosity is a 
property intrinsic to the liquid crystal material and the 
temperature-dependence thereof has been known similarly as Ps. 
Accordingly, from these values and the applied voltage V, the cell gap d 
can be calculated based on a well known formula: 
EQU .tau.=.eta.d/(PsV). 
According to this embodiment, the following advantages may be attained. 
(1) The detection may be performed by using only one scanning electrode, so 
that the image disorder is suppressed to a slight degree compared with the 
case wherein plural scanning electrodes are used at a time for detection, 
thus requiring a longer time for restoring the original display. 
(2) The detection-nonselection scanning electrodes are placed on a 
non-selection potential so that the circuit is simple compared with the 
case wherein the detection-nonselection scanning electrodes and the other 
data electrodes are all placed in a high-impedance state, thus requiring 
changeover switches on both sides. 
FIGS. 39 and 40 are respectively a block diagram of a liquid crystal 
display apparatus including a current detection system according to this 
embodiment. 
The example ferroelectric liquid crystal having properties shown in Tables 
1 and 2 appearing hereinbelow was found to show .gamma. (i.e., V-T) 
characteristics shown in the following table. 
TABLE 3 
______________________________________ 
30.degree. C. 
35.degree. C. 
40.degree. C. 
______________________________________ 
.gamma..sub.10-90 
1.43 1.55 1.63 
.gamma..sub.0-100 
1.71 1.88 1.80 
______________________________________ 
.gamma..sub.10-90 in Table 3 is a value defined by .gamma..sub.10-90 
.ident.V.sub.T=90 /V.sub.T=10 wherein, when a liquid crystal initially 
placed in a wholly black state is supplied with voltage pulses having a 
fixed pulse width and varying voltages (amplitudes), V.sub.T=10 denotes a 
voltage providing a transmittance of 10 % and V.sub.T=90 denotes a voltage 
providing a transmittance of 90%. Similarly, .gamma..sub.0-100 is defined 
by .gamma..sub.0-100 =V.sub.T=100 /V.sub.T=0 and is identical to a ratio 
Vsat/Vth shown in FIG. 1A-1. Hereinafter .gamma..sub.0-100 is simply 
denoted by .gamma.. In other words, .gamma. represents an inclination of a 
V-T curve and may preferably be in a certain suitable range when the drive 
scheme according to the invention is applied to a halftone display. 
Hereinbelow, this point will be described in more detail. 
The compensation range is first considered. Referring to FIG. 41 showing a 
threshold curve H at a high temperature pixel and a threshold curve L at a 
low temperature pixel, V.sub.I denotes a data signal amplitude, Tb denotes 
a maximum crosstalk quantity, Va denotes a threshold voltage at the high 
temperature pixel, and Vb denotes a threshold voltage at the low 
temperature pixel. As the voltage for providing T=100% at the low 
temperature pixel is Vb.gamma., a condition of 
EQU 2V.sub.I .gtoreq.Vb.cndot..gamma.-Va (1) 
is required. On the other hand, a condition of 
EQU Tb.gtoreq.Va (2) 
is required in order to avoid crosstalk. 
Accordingly, in case of V.sub.I =Tb, a condition of 
EQU V.sub.I .ltoreq.Va (3) 
is required. From (1) and (3), the following condition is derived: 
EQU .gamma..ltoreq.3Va/Vb (4). 
On the other hand, in case of V.sub.I =2Tb, the following condition is 
derived from the formula (2): 
EQU (1/2)V.sub.I .ltoreq.Va (3a). 
From (1) and (3a), the following condition is derived: 
EQU .gamma..ltoreq.5Va/Vb (4a). 
Accordingly, in case where the high temperature pixel and low temperature 
pixel have a large difference in threshold characteristic or, in other 
words, in order to compensate for a broad temperature range, it is 
preferred that .tau. is close to 1 (.tau.(=Vsat/Vth) cannot be 1 or 
below). 
Next, a display accuracy is considered. FIG. 42 shows two threshold 
characteristic curves M.sub.1 and M.sub.2 which are slightly different 
from each other, wherein .delta.T denotes a change in transmittance, and 
.delta.V denotes a change in voltage. Now, in case of effecting a 
gradational display of n levels, an allowable transmittance change is 
given by 
EQU .delta.T.ltoreq.100/n (%) (5). 
As a relationship of .delta.T.ltoreq..delta.V exists, the following is 
derived: 
EQU .delta.V/.gamma..ltoreq.100/n, i.e., .gamma..gtoreq.(n/100).delta.V(6). 
If a voltage output accuracy .delta.V is assumed to be a constant 
determiend by a circuit structure, .gamma. is required to be large in 
order to increase the number of gradation levels. As a result of 
combination of the constraint (4) or (4a ) regarding the compensation 
range and the constraint (6) regarding the display accuracy, the following 
range for .gamma. is given for the driving scheme according to the 
invention: 
EQU (n/100).delta.V.ltoreq..gamma..ltoreq.3Va/Vb, or 
EQU (n/100).delta.V.ltoreq..gamma..ltoreq.5Va/Vb. 
In this embodiment, it has been formed that .gamma. is preferably in the 
range of 1.3.ltoreq..gamma..ltoreq.2.0, particularly around 1.5. 
On the other hand, if the drive scheme according to the invention is 
applied to a binary state display, the constraint on .gamma. is given by: 
EQU .gamma..gtoreq.3Va/Vb (4), or 
EQU .gamma..ltoreq.5Va/Vb (4a). 
In this embodiment, .gamma..ltoreq.2.0 is preferred for such binary display 
and particularly as close as possible to 1.