Liquid crystal cells – elements and systems – Particular structure – Interconnection of plural cells in series
Reexamination Certificate
2000-12-06
2003-09-09
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Interconnection of plural cells in series
C349S076000, C349S113000, C349S176000
Reexamination Certificate
active
06618102
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a display element capable of displaying multiple colors, a method of writing images to it, and an apparatus for writing images.
2. Description of the Prior Art
A reflective liquid crystal is suitable as a display element of small-size information equipment, portable information terminals and the like because it does not require a dedicated light source such as backlight, has low power consumption, and can be of a thin and lightweight construction.
There is known a reflective liquid crystal element capable of displaying multiple colors that, between a pair of substrates each having an electrode formed on an inner surface thereof, three liquid crystal cells forming display layers having cholesteric liquid crystals selectively reflecting blue, green, and red lights are stacked, and a light absorption layer is formed on the back of a liquid crystal cell opposite to a display side (a side through which outside light comes).
In the cholesteric liquid crystal display element of cell stacking type, by independently having the cholesteric liquid crystals of cells switch between a selective reflection state due to a planar state and a transmission state due to a focal conic state, eight colors—white, black, blue, green, red, cyan, magenta, and yellow—can be displayed within one pixel, and a display with low loss of light and high contrast can be obtained because no color filter is used.
However, the cholesteric liquid crystal display element of cell stacking type has the disadvantages that parallax becomes high because there are a substrate and an electrode between display layers of different colors and the interval between the display layers becomes large, and the display element and a display apparatus are expensive to fabricate because driving electrodes and driving circuits for three colors are required.
A cholesteric liquid crystal display element capable of displaying multiple colors is proposed in Japanese Published Unexamined Patent Application No. Hei 10-177191 (hereinafter referred to as a first conventional example) and Japanese Published Unexamined Patent Application No. Hei 11-149088 (U.S. patent application Ser. No. 09/192,402, hereinafter referred to as a second conventional example). According to the proposed cholesteric liquid crystal display element, three display layers having cholesteric liquid crystals selectively reflecting blue, green, and red lights are stacked between a pair of substrates each having an electrode formed on an inner surface thereof, and an image is written and displayed by applying a writing signal from the outside of the three display layers.
FIG. 28
shows a first conventional example. In a display element
31
of this example, between a substrate
32
having a writing electrode
34
formed on an inner surface thereof and a substrate
33
having a writing electrode
35
formed on an inner surface thereof, three display layers
38
A,
38
B, and
38
C of PDLC (Polymer Dispersed Liquid Crystal) in which cholesteric liquid crystals
41
A,
41
B, and
41
C selectively reflecting mutually different color lights are respectively droplet-dispersed in polymeric matrix
42
are stacked, and a light absorption layer
39
is formed on the back of a substrate
33
of a non-display side. Threshold voltages of orientation changes of the cholesteric liquid crystals
41
A,
41
B, and
41
C are set as described later. The writing electrodes
34
and
35
are connected to a writing apparatus (driving circuit)
50
.
FIG. 29
shows a second conventional example. In the display element
31
of this example, between the substrates
32
and
33
, three display layers
38
A,
38
B, and
38
C having the cholesteric liquid crystals
41
A,
41
B, and
41
C selectively reflecting mutually different color lights are stacked in a way that inserts spacers
37
A,
37
B, and
37
C into the display layers
38
A,
38
B, and
38
C, respectively, and puts a separating substrate
36
A between the display layers
38
A and
38
B and a separating substrate
36
B between the display layers
38
B and
38
C, and the light absorption layer
39
is formed on the back of the substrate
33
of the non-display side. Threshold voltages of orientation changes of the cholesteric liquid crystals
41
A,
41
B, and
41
C are set as described later. The writing apparatus
50
, which is formed separately from the display element
31
, includes the electrodes
54
and
55
sandwiching the display element
31
, and a driving circuit
51
for applying a writing signal between the electrodes
54
and
55
.
A cholesteric liquid crystal having positive dielectric anisotropy has three states: a planar state in which helical axes are vertical to cell surfaces and which causes a selective reflection phenomenon for incident light, as shown in
FIG. 26A
; a focal conic state in which helical axes are almost parallel to cell surfaces and which causes incident light to transmit while scattering a little forward, as shown in
FIG. 26B
; and a homeotropic state in which a helical structure collapses and liquid crystal directors face a field direction and which causes incident light to transmit almost perfectly, as shown in FIG.
26
C.
The planar state and the focal conic state of the three states can exist bistably when no electric field is applied. Therefore, the orientation states of cholesteric liquid crystals are not uniquely determined for electric fields; when an initial state is the planar state, as an applied voltage increases, the cholesteric liquid crystals change in the order of the planar, focal conic, and homeotropic states; and when an initial state is the focal state, as an applied voltage increases, the cholesteric liquid crystals change in the order of the focal conic and homeotropic states. On the other hand, if an electric field is suddenly set to zero, the planar and focal conic states remain unchanged, and the homeotropic state changes to the planar state.
Therefore, immediately after a pulse signal is applied, the cholesteric liquid crystal layers exhibit an electo-optical response as shown in
FIG. 27
; when an applied pulse voltage is Vfh
90
or more, it enters a selective reflection state representing a change from the homeotropic state to the planar state; and when an applied pulse voltage is between Vpf
10
and Vfh
10
, it enters a transmission state due to the focal conic state; and when an applied pulse voltage is Vfh
90
or less, it maintains the state in which it was before the pulse signal is applied, that is, enters the selective reflection state due to the planar state or the transmission state due to the focal conic state.
In the figure, the vertical axis represents normalized reflectivity and normalizes reflectivity by a maximum reflectivity of 100 and a minimum reflectivity of 0. Since change of reflectivity entails a transition area, a normalized reflectivity of 90 or more is defined as a selective reflection state; a normalized reflectivity of 10 or less, as a transmission state; threshold voltages of change between the planar state and the focal conic state, as Vpf
90
before a transition area and Vpf
10
after it; and threshold voltages of change between the focal conic state and the homeotropic state, as Vfh
10
before a transition area and Vfh
90
after it.
In the conventional display element
31
shown in
FIGS. 28 and 29
, these threshold voltages are mutually changed among the display layers
38
A,
38
B, and
38
C. Specifically, assuming that threshold voltages of the display layer
38
A are Vpf
90
(A), Vpf
10
(A), Vfh
10
(A), and Vfh
90
(A); threshold voltages of the display layer
38
B are Vpf
90
(B), Vpf
10
(B), Vfh
10
(B), and Vfh
90
(B); and threshold voltages of the display layer
38
C are Vpf
90
(C), Vpf
10
(C), Vfh
10
(C), and Vfh
90
(C), an expression (
6
) shown below is set.
Vpf
90
(C)<Vpf
10
(C)<Vpf
90
(B)<Vpf
10
(B)<Vpf
90
(A)<Vpf
10
(A)<Vfh
10
(C)<Vfh
90
(C)<Vfh
10
(B)<Vfh
90
(B)<Vfh
10
(A)<Vfh
90
(A) (1)
The order in which th
Arisawa Hiroshi
Harada Haruo
Akkapeddi P. R.
Chowdhury Tarifur R.
Fuji 'Xerox Co., Ltd.
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