Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02243383 1998-07-20
METHOD FOR DRIVING A NEMATIC LIQUID CRYSTAL
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to a method for driving a liquid
crystal, especially, a nematic liquid crystal.
Description of the Related Art
When two transparent flat plates having transparent
electrodes and sandwiching a nematic liquid crystal are
placed between two polarizing plates, transmittance of light
passing through the polarizing plates changes with the
voltage applied across the transparent electrodes.
Since liquid crystal display devices based on the above
principle can be shaped flat and are operative with low
electric power, they have been widely used in wrist watches,
electronic calculating machines, and so forth.
In recent years, they are also used in combination with color
filters to form color display devices in note-type personal
computers and small liquid crystal TV sets, for example.
A problem with conventional liquid crystal display
devices is slow responses of liquid crystals. In this
respect, liquid crystal display devices have been inferior to
CRT displays especially when used as TV displays for
displaying moving images or as personal computer displays
required to quickly follow the movements of a mouse cursor.
In liquid crystal displays combined with color filters
to display color images, three dots of different colors,
namely, red, green and blue, are combined to display a
desired color. A problem with the use of color filters lies
in that color filters are very expensive and need a high
accuracy when bonded to panels. Moreover, they need a triple
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number of dots to ensure an equivalent resolution as compared
wit h black-and-white liquid crystal display panels.
Therefore, typical liquid crystal color panels require a
triple number of drive circuits in the horizontal direction.
This means an increase of the cost of drive circuits
themselves and the cost for an increased man-hour for
connecting drive circuits to the panel at a triple number of
points.
Another problem with the use of color filters is their
optical transmittance as low as 20% approximately. When
color filters are used in a liquid crystal panel, the
brightness decreases to approximately one fifth, and a large
electric power is consumed for back-lighting to compensate
the brightness.
Thus, the use of color filters with liquid crystal
panels to display color images involved many disadvantageous
factors from the economical viewpoint, and it was difficult
to manufacture an economical liquid crystal panel for color
images using this method.
Japanese Patent Laid-Open 1-179914 (1989) discloses a
color liquid crystal display device to display color images
by combining a black-and-white panel and tricolor back-
lighting instead of using color filters. This method
certainly appears more likely to realize high-fidelity color
images inexpensively. Practically, however, response speeds
of nematic liquid crystals by conventional liquid crystal
driving methods are as slow as several decades of
milliseconds through hundreds of milliseconds, and it has
been believed difficult to realize a response speed not
slower than 8 milliseconds required for color images by
tricolor back-lighting with a liquid crystal panel using a
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nematic liquid crystal.
There are also some proposals to use ferroelectric
liquid crystals or antiferroelectric liquid crystals to
provide liquid crystal panels operative at a high speed.
However, no such device has been brought into practice mainly
because the cell gaps of the liquid crystal must be as small
as 1 am and are difficult to make.
SUI IARY OF THE INVENTION
It is therefore an object of the invention to provide
a nematic liquid crystal driving method which increases the
response speed of any conventional nematic liquid crystal,
either of the TN type or of the STN type, to enable coloring
by tricolor back-lighting and to ensure the performance
equivalent to or higher than CRT displays in reproduction of
moving images.
The Inventor measured dynamic characteristics of
applied voltage waveforms and optical transmittance of
nematic liquid crystals to develop a liquid crystal panel
having a response speed enabling color images by tricolor
back-lighting, and has confirmed that, depending on the
waveform of the applied voltage, there occurs the phenomenon
that the optical transmittance changes very quickly in
response to changes in applied voltage. If this phenomenon
is repetitively produced, it must be possible to increase the
response speed of the liquid crystal. The present invention
is based on the above knowledge of the Inventor, and its
basic concept lies in increasing the response speed of a
liquid crystal by applying a voltage to the liquid crystal at
a unique timing different from those of conventional driving
circuits.
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That is, according to the invention, there is provided a
method for driving a nematic liquid crystal in a liquid
crystal display device which includes a nematic liquid
crystal, two electrodes confining the nematic liquid crystal
and a pair of polarizing plates sandwiching the electrodes
confining the nematic liquid crystal, comprising:
the voltage applied across two electrodes being returned to
and maintained in a predetermined value for a predetermined
duration of time in predetermined intervals.
In the duration of time other than the predetermined
duration of time in each interval, the voltage applied across
two electrodes may be inverted in polarity.
The nematic liquid crystal may be heated to a
predetermined temperature.
According to the invention, by returning or maintaining
the voltage across two electrodes to or in a predetermined
value for a predetermined time in predetermined intervals,
the liquid can be driven at a much higher response speed than
those of conventional driving methods. Therefore, a liquid
crystal panel suitable for color images by tri-color back-
lighting and for moving images with a high contrast ratio can
be realized. It is also possible to reduce the power
consumption.
Generally speaking, the present invention provides a
method for driving a nematic liquid crystal in a liquid
crystal display device which includes the nematic liquid
crystal, two electrodes confining the nematic liquid crystal
and a pair of polarizing plates sandwiching the electrodes
and the nematic liquid crystal, characterized in:
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applying a voltage corresponding to image data between
the two electrodes and thereby displaying an image on a
liquid crystal panel of the display device; and
applying a constant voltage between the two electrodes
in each frame period and thereby erasing the image displayed
on the panel within the same frame period.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a diagram showing the waveform of voltages
applied to a nematic liquid crystal by a nematic liquid
crystal driving method according to an embodiment of the
invention, together with absolute values of the voltages and
responsive changes with time in optical transmittance of the
nematic liquid crystal; and
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Fig. 2 is a diagram showing the waveform of voltages
applied to a nematic liquid crystal by a conventional driving
method, together with absolute values of the voltages and
responsive changes with time in optical transmittance of the
nematic liquid crystal.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Explained below is an embodiment of the invention with
reference to the drawings.
Fig. 1 shows an aspect where a voltage is applied to a
high-speed nematic liquid crystal panel using appropriate one
of conventional TN liquid crystals or STN liquid crystals and
optimizing the cell gap. Further, intervals, Ti through T6,
are equal in length, and the length is not longer than 8
milliseconds which is the slowest acceptable driving cycle
required for driving a liquid crystal for color images by
tricolor back-lighting.
As already known, optical transmittance of a liquid
crystal changes with absolute values of applied voltages
regardless of their polarities. However, the applied voltage
is usually changed in polarity in predetermined intervals
because continuous application of a d.c. voltage to a liquid
crystal will cause an electro-chemical reaction and will
deteriorate the liquid crystal. Therefore, also in the
embodiment of the invention, applied voltages are inverted in
polarity. However, inversion of polarities is substantially
immaterial to the subject matter of the invention, namely,
high-speed driving of a liquid crystal. Now explained below
is the operation of the embodiment of the invention with
reference to the drawings.
In Fig. 1 showing the driving method according to the
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embodiment of the invention, each of the intervals of time T1
through T6 includes two time zones. One of these time zones
(the former of each of Ti through T6 in Fig. 1) is the time
where a voltage responsive to image data is applied, and the
absolute value represents V1 or OV depending upon the image
data. The other time zone (the latter of each of Ti through
T6) is the time where the voltage of OV is applied
irrespectively of the image data. That is, in the present
embodiment, the applied voltage is forcibly changed to or
maintained in OV for a predetermined time in predetermined
intervals.
More specifically, in the interval T3 and the interval
T5 in Fig. 1, also the applied voltage responsive to image
data is OV, and the optical transmittance maintains the black
level throughout the intervals. In each of the intervals T1,
T2, T4 and T6, the applied voltage first becomes V1 in
response to image data, and is forcibly changed to OV later.
Responsively, the optical transmittance first changes from
the black level to the white level and then changes from the
white level to the black level. That is, the optical
transmittance changes from the black level to the white
level, and returns from the black level to the white level
within each interval, T1, T2, T4 or T6.
For a better understanding of the embodiment of the
invention, a conventional driving method is explained below
with reference to Fig. 2. Fig. 2 shows an aspect where a
voltage is applied by using the same nematic liquid crystal
panel as used in Fig. 1, and the same image data is supplied.
Also the intervals T1 through T6 are the same as those of
Fig. 1.
As shown in Fig. 2, in the conventional driving method,
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the applied voltage is determined exclusively by image data.
Therefore, the absolute value of the applied voltage becomes
V2 or OV, depending upon the image data to be displayed, but
the value is maintained throughout the interval, or beyond
the interval, until an image data is changed to the next
image data. In this case, the movement of the liquid crystal
is slow, and it takes time for the optical transmittance to
change. For example, even when the absolute of the applied
voltage changes from V2 to OV, like T2 to T3 in Fig. 2, the
optical transmittance does not change the full black level
within the interval T3. Further, when the absolute value of
the applied voltage changes from OV to V2 like T3 to T4 in
Fig. 2, the optical transmittance begins to change from an
incomplete black level toward the full black level, but fails
to return to the full white level within the interval Ti.
That is, the response speed of the liquid by the conventional
driving method is slow, and high-contrast images cannot be
displayed at a sufficient speed either on a TN liquid crystal
panel or on a STN liquid crystal panel.
It will be understood from comparison of Fig. 1 and
Fig. 2 that the embodiment can change the optical
transmittance from the black level to the white level or vice
versa more quickly by changing the applied voltage to OV for
a predetermined time in predetermined intervals.
Additionally, the embodiment can use a higher applied voltage
V1 than V2 of the conventional method to change the optical
transmittance to the white level. This is effective for more
quickly changing the optical transmittance from the black
level to the white level.
Consequently, the embodiment of the invention invert
the polarity within each interval (Ti through T6) so that the
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average voltage becomes substantially OV in each interval (Ti
to T6). Since the liquid crystal moves very quickly, if the
polarity is inverted between two adjacent intervals (for
example, if the polarity in the interval Ti is positive, the
polarity is changed to negative in the interval Ti), flickers
will occur due to a delicate difference between absolute
values of the positive applied voltage and the negative
applied voltage.
In order to ensure high-contrast images in the
embodiment of the invention, it is important to change and
return the optical transmittance of the liquid crystal panel
within each interval. Therefore, the frame cycle must be set
appropriately in accordance with characteristics of the
liquid crystal. If the frame period is short, the optical
transmittance of a certain liquid crystal fails to return to
the original level within the interval, and it results in a
decrease in contrast ratio. In contrast, if the frame
period is long, flickers are liable to occur.
The duration of time required for the optical
transmittance to return to the original level largely varies
with the property of the liquid crystal material, especially,
the viscosity of the liquid crystal material. Therefore, by
selecting an appropriate liquid crystal whose optical
transmittance quickly returns to the original level, high-
contrast images with substantially no flicker can be
realized. Even when a normal liquid crystal is used,. the
time for returning the optical transmittance to the original
level can be shortened by increasing the temperature to
adjust the viscosity, and high-contrast images can be
ensured.
Although the embodiment has been explained by way of a
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specific embodiment, it is not limited to these examples, but
involves various changes or modifications.
For example, the embodiment shown in Fig. 1 has been
explained as using a normally-black liquid crystal panel
which displays black under no applied voltage. However, the
same effects are promised even with a normally-white liquid
crystal panel configured to display white under no applied
voltage, by appropriately modifying the voltage to be applied
for a predetermined time in predetermined intervals. Also
with special liquid panels different from typical liquid
crystal panels in relation between the applied voltage and
the optical transmittance, substantially the same effects are
promised by appropriately modifying the voltage to be applied
for a predetermined time in predetermined intervals.
As described above, according to the invention, since
the applied voltage to the liquid crystal is returned to a
predetermined voltage value for a predetermined time in
predetermined intervals, the liquid can be driven very
quickly. Therefore, on a liquid crystal panel using the
invention, the operation for displaying and completely
erasing an image can be completed in a very short time, and
high-quality moving images are promised.
Additionally, since the waveform of the applied voltage
used in the invention is essentially the same as that used
for TFT systems, the invention is applicable also to TFT
liquid crystal panels. Also for other driving systems, the
operation speed of liquid crystals can be increased by
appropriately changing the applied voltage value for a
predetermined time in predetermined intervals.
Moreover, since the method according to the invention
is configured to complete the operation for displaying an
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image and erasing it completely within each frame interval,
it is optimum for color images by tricolor back-lighting, and
can realize high-performance, inexpensive color displays.
