Cholesteric liquid crystal display

Details Cholesteric liquid crystal display Cholesteric liquid crystal display

2001-07-26

2004-06-01

Wu, Xiao (Department: 2673)

Computer graphics processing and selective visual display system

Plural physical display element control system

Display elements arranged in matrix

C345S100000

Reexamination Certificate

active

06744418

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cholesteric liquid crystal display used for a display panel in electronic equipment and a recording/display medium of images.
2. Discussion of the Related Art
The cholesteric liquid crystal display has attracted considerable attention in recent years as a display device for electronic paper such as electronic newspapers and electronic publications since it has the following advantages: being capable of utilizing reflection of surrounding lights to give a light display; having a storage property that holds display contents after the supply voltage is turned off; being capable of a large-capacity display by simple matrix drive utilizing the storage property; being capable of using a flexible substrate of a resin, etc., because an active matrix is not needed for the drive, and the like.
The cholesteric liquid crystal is made up of spirally oriented stick-like molecules, and exhibits a selective reflection phenomenon that reflects a light of a wavelength corresponding to a spiral pitch. The cholesteric liquid crystal display elements utilize this phenomenon. As an example of the sectional structure of this device is illustrated in
FIG. 18
, the device is made up of cells that sandwich a cholesteric liquid crystal
30
between two substrates
11
,
12
each having interventional transparent electrodes
21
,
22
, and a light absorptive layer
41
that absorbs a selective reflection wavelength is attached on the opposite face to the cell observation side. The light absorptive layer
41
is presumed to be a black color hereunder.
The orientation of the cholesteric liquid crystal takes on three types, namely, planer (P) orientation, focal conic (F) orientation, and homeotropic (H) orientation, as shown in FIG.
17
A through FIG.
17
C. The P orientation is a state in which the spiral axis is oriented almost vertically to the substrate plane, which assumes a color according to a selective reflection wavelength. The F orientation is a state in which the spiral axis is oriented almost in parallel to the substrate plane, which is colorless, and the black color of the light absorptive layer
41
is observed. The H orientation is a state in which the spiral structure is decomposed and the molecules are oriented vertically to the substrate plane, which is also colorless, and the black color of the light absorptive layer
41
is observed.
When a voltage is applied across the transparent electrodes
21
,
22
, both the P orientation and the F orientation stably exist in the applied voltage lower than V
T1
, showing a bi-stable state. When the voltage is increased, the F orientation does not change and the P orientation transitions gradually into the F orientation; and when the voltage is over V
T2
, the state completely transitions into the F orientation. When a still higher voltage than V
T3
is applied, the state is starting to transition into the H orientation, and when the voltage is over V
T4
, it completely transitions into the H orientation. Even though the applied voltage is sharply removed from the state of the F orientation, the F orientation is maintained; however, when the voltage is sharply removed from the state of the H orientation, it transitions into the P orientation.
As a result of the above transition characteristic, the measurement of the reflectance after a specific time from when the voltage is applied only for the time T as shown in
FIG. 20
gives the voltage vs. reflectance characteristic as shown in FIG.
21
. That is, when the initial orientation is the P orientation, the characteristic shows a high reflectance under V
T1
; in the range over V
T1
under V
T2
, the reflectance gradually lowers; in the range over V
T2
under V
T3
, the characteristic shows a low reflectance; in the range over V
T3
under V
T4
, the reflectance increases; and over V
T4
, it shows a high reflectance, which is the same as that in the initial orientation. On the other hand, when the initial orientation is the F orientation, the characteristic shows a low reflectance under V
T3
; in the range over V
T3
under V
T4
, the reflectance increases; and over V
T4
, it shows a high reflectance.
The above voltage vs. reflectance characteristic varies depending upon the time T during which the voltage is applied. When the initial orientation is the P orientation, as shown in
FIG. 23
, the whole voltage vs. reflectance characteristic shifts to a higher voltage side as the time T becomes shorter, and in the range over V
T2
under V
T3
, the reflectance increases. This is because the transition into the F orientation becomes incomplete by the time T becoming shorter to create a state in which the F orientation and the P orientation are microscopically mixed. On the other hand, when the initial orientation is the F orientation, as shown in
FIG. 22
, V
T4
shifts to a higher voltage side as the time T becomes shorter, and the range over V
T3
under V
T4
expands.
Utilizing the above voltage vs. reflectance characteristic, the cholesteric liquid crystal display is able to write image data by means of the simple matrix electrodes serving intersection portions of scan-electrodes and data-electrodes as pixels. As an example,
FIG. 19
illustrates a plan configuration of a simple matrix panel having 16×16 pixels. As shown in the drawing, the panel contains a scan-electrode group
23
made up of R
1
to R
16
and a data-electrode group
24
made up of C
1
to C
16
.
As a method of driving the cholesteric liquid crystal display elements, for example, the write method named as the FCR (ForcalConic Reset) method is disclosed in the Japanese Published Unexamined Patent Application No. Hei 11-326871. This method executes writing by a drive voltage made up of a reset time for making the pixels transition into the F orientation and a selection time for writing the P orientation, in which a drive voltage to make the pixels simultaneously transition into the F orientation is applied to all the scan-electrodes during the reset time, and next a selected voltage is applied to the scan-electrodes one by one sequentially.
FIG. 9
illustrates a timing chart of the drive voltage that is applied to the scan-electrode group
23
having 16 electrodes as an example. As in the drawing, during the reset time Tr, the method gives a voltage Vrh over V
T4
to make the pixels transition into the H orientation, thereafter brings the voltage once to zero, next gives a voltage Vrf being over V
T2
under V
T3
and again brings to zero to thereby attain the F orientation. During that time, the voltage given to the data-electrode group
24
is zero. During the selection time Ts, the method gives the voltage Vs of (V
T3
+V
T4
)/2 to the scan-electrodes, and simultaneously gives a data voltage of (V
T3
−V
T4
)/2 or (−V
T3
+V
T4
)/2 to the data-electrodes. Thereby, V
T4
or V
T3
being the difference of the scan-voltage and the data voltage is applied to the pixels, which makes the pixels selectively transition into the P orientation or the F orientation. The voltage applied to the scan-electrodes is zero except the reset time Tr and the selection time Ts.
While selecting a scan-electrode, the method applies the data voltage (V
T3
−V
T4
)/2 or (−V
T3
+V
T4
)/2 to a pixel on another scan-electrode. To condition the data voltage as |(V
T3
−V
T4
)/2|<V
T1
will permit writing the data in all the pixels without varying the reflectance of the already written pixels. Assuming that the number of the scanning lines is N, the full write time Tf is given by the following expression 1.
Tf=Tr+N×Ts
  [Expression 1]
Another method is disclosed in the specification of the U.S. Pat. No. 5,748,277, which is named as the DDS (Dynamic Drive Scheme) method. The DDS method takes on the drive voltage waveform, which is made up a series of reset time Tr, selection time Ts, and hold time Th, as shown in FIG.
24
. During the reset time Tr, a voltage Vrh is applied to make

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