Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within cholesteric phase
Reexamination Certificate
1998-12-29
2001-11-13
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within cholesteric phase
C345S111000, C345S111000, C349S078000, C349S080000
Reexamination Certificate
active
06317189
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to the field of liquid crystal displays. More particularly this invention is directed to multi-color reflective liquid crystal displays that reflect multiple colors from individual cells.
2. Description of Related Art
Current reflective liquid crystal displays (LCDs) can provide reduced power consumption, lighter weight, thinner packages, and better adaptability to a wider range of ambient conditions (e.g., in-doors and out-doors) than transmissive or emissive flat panel displays (FPDs) can. These features make reflective LCDs suitable for many applications where portability and/or viewability under high ambient illumination are desired. Personal document readers (PDR), personal information tools (PIT), and hand-held maps and manuals are exemplary applications for these reflective LCDs.
The reflective nature of reflective LCDs can reduce power consumption by one-half by eliminating backlight requirements. Further reduction in power consumption in reflective LCDs can be achieved in polymer-stabilized cholesteric texture (PSCT) liquid crystal displays. These polymer-stabilized cholesteric texture liquid crystal displays comprise a cholesteric liquid crystal medium which contains a stabilizing polymer network.
FIGS.
1
(
a
)-
1
(
c
) illustrate the operation of such a polymer-stabilized cholesteric texture liquid crystal display
10
, as described in “Reflective Color Displays for Imaging Applications,” by G. P. Crawford et al., Proceedings of the IS & T/SID 1995 Color Imaging Conference: Color Science, Systems and Applications, pp. 52-58, incorporated herein by reference in its entirety. The display
10
comprises a pair of transparent substrates
12
, a pair of transparent electrodes
14
, and a polymer stabilized cholesteric liquid crystal medium
16
located between the electrodes
14
. A polymer network
18
stabilizes the liquid crystal material
20
.
As shown in FIG.
1
(
a
), in the voltage off-state, the planar texture is stable and the helical axes of the cholesteric liquid crystal material
20
are substantially perpendicular to the surface
22
of the substrate
12
on which natural or artificial light impinges. The planar texture selectively reflects incident light centered at the Bragg wavelength, &lgr;
B
=nP, where n is the average index of refraction of the liquid crystal material
20
and P is the pitch length of the helical structure of the liquid crystal material
20
. The pitch P can be selectively varied by adding chiral agents to the liquid crystal material
20
. The chiral agents affect the pitch of the cholesteric liquid crystal material
20
, and thus also the Bragg wavelength &lgr;
B
.
As shown in FIG.
1
(
b
), when a low voltage is applied between the electrodes
14
by a voltage source
24
, the planar texture of the cholesteric liquid crystal material
20
is transformed into a focal conic texture, in which the helical axes of the liquid crystal are randomly aligned. In this state, the cell is substantially light transparent. If the applied voltage is then removed, the focal conic texture remains fixed due to the stabilizing effect of the polymer network
18
. As shown in FIG.
1
(
c
), if a greater field is applied to the display, the cholesteric liquid crystal material
20
becomes completely aligned and completely transparent.
The response of the polymer cholesteric stabilized liquid crystal material
16
to the release of the applied field is dependent on the rate of release of this field. Quickly releasing this field causes the cholesteric liquid crystal material
20
to relax back to the planar texture shown in FIG.
1
(
a
). If the field is released more slowly, then the cholesteric liquid crystal material
20
will relax back to the focal conic texture shown in FIG.
1
(
b
). This bistable memory capability of polymer-stabilized cholesteric texture LCDs can significantly reduce the power consumption of the displays in many applications, because the displays consume no power when viewed and only need to be powered for short periods of time to change the displayed image.
FIGS.
2
(
a
)-
2
(
d
) show another type of known reflective color display device, a holographically structured polymer dispersed liquid crystal (HPDLC) display
30
. These displays are also described in the incorporated Crawford reference. Holographically structured polymer dispersed liquid crystal displays are formed using optical interference techniques applied to a mixture of liquid crystal material and photocurable polymer material. This technique forms fringe planes
33
of liquid-crystal-filled droplets at predetermined positions within a polymer matrix separated by a plurality of polymer-rich planes,
35
. As a result, the liquid crystal droplet densities are spatially modulated in the direction perpendicular to the planar structure. As shown in FIG.
2
(
a
), a holographically structured polymer dispersed liquid crystal material
32
is sandwiched between a pair of transparent electrodes
34
and a pair of transparent substrates
36
. In particular, in each liquid-crystal-rich plane
33
, a plurality of liquid crystal droplets
38
are dispersed in a network of the polymer
39
. In contrast, in the polymer-rich planes
35
, there is essentially only the network of the polymer
39
. In the field-off state shown in FIGS.
2
(
a
) and
2
(
b
), the liquid crystal molecules in the liquid crystal droplets
38
dispersed in the polymer
39
are randomly oriented, or misaligned, within the liquid crystal droplets
38
. The effective refractive index of the droplets
38
is therefore significantly higher than the index of refraction, n
p
, of the polymer matrix
39
. The directions of the extraordinary index of refraction, n
e
, and the ordinary index of refraction, n
o
, of the liquid crystal droplets
38
are shown. Consequently, the holographically structured polymer dispersed liquid crystal display
30
reflects light at the Bragg wavelength &lgr;
B
. The reflectance under ambient illumination conditions for these holographically structured polymer dispersed liquid crystal displays
30
is larger than that for the polymer-stabilized cholesteric texture display
10
shown in FIGS.
1
(
a
)-
1
(
c
).
When voltage is applied across the liquid crystal display
30
, as shown in FIGS.
2
(
c
) and
2
(
d
), the liquid crystal molecules align within the liquid crystal droplets
38
, as shown in FIG.
2
(
d
). As a result, the index of refraction of the liquid crystal droplets
38
approximately equals the index of refraction of the polymer matrix
39
along the direction of light propagation. As shown, n
e
is parallel to the incident and reflected light direction, and n
o
is perpendicular to the incident and reflected light direction. The periodic refractive index modulation vanishes if n
o
of the liquid crystal material in the liquid crystal droplets
38
approximately equals the index of reflaction n
p
of the polymer matrix
39
. In this state, the holographically structured polymer dispersed liquid crystal display
30
is essentially light transparent and does not reflect or diffract the light incident on the holographically structured polymer dispersed liquid crystal display
30
.
The effective refractive index of the liquid crystal droplets
38
can be varied with applied voltage, enabling the reflected light intensity to be controlled electrically.
In addition, the spectral reflectance of the holographically structured polymer dispersed liquid crystal display
30
can be selectively controlled by fabrication. To date, such holographically structured polymer dispersed liquid crystal displays
30
represent an especially promising reflective technology because of their high peak reflectance capability.
Holographically structured polymer dispersed liquid crystal reflective displays are described in detail in U.S. Pat. No. 6,133,971, incorporated herein by reference in its entirety.
SUMMARY OF THE INVENTION
One of the major challenges for high-performance, reflective color LCDs is to achieve h
Fiske Thomas G.
Silverstein Louis D.
Yuan Haiji
Oliff & Berridg,e PLC
Parker Kenneth
Qi Mike
Xerox Corporation
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