Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1996-12-17
2001-08-21
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S092000, C349S105000, C349S093000, C349S191000
Reexamination Certificate
active
06278506
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a liquid crystal display structure and, more particularly, to a holographic polymer dispersed liquid crystal structure, a color display unit using the holographic polymer dispersed liquid crystal structure and a process of fabrication thereof.
DESCRIPTION OF THE RELATED ART
A reflective liquid crystal display unit is attractive because of a low power consumption, and research and development efforts have been made on the reflective liquid crystal display unit with emphasis on a color display unit. The reflective color liquid crystal display unit is largely broken down into a display unit with a polarizing plate and a display unit without a polarizing plate.
A typical example of the color liquid crystal display unit with a polarizing plate is disclosed by Y. Miyama et. al. in “Direction of Field Induced Transition Between Different Twist States in STN”, Proceedings of Joint Lectures in 41th Applied Physics Meeting, page 833, 28a-A-1. However, the color variation is too narrow to satisfy user. Moreover, the polarizing plate does not allow the reflective color liquid crystal display unit to increase the reflectivity. Thus, the polarizing plate is causative of the undesirable results of the liquid crystal display unit.
On the other hand, a typical example of the reflective color liquid crystal display unit is disclosed by Keiji Tanaka et. al. in “Holographically formed liquid-crystal/polymer device for reflective color display”, Journal of the Society for Information Display, vol. 2, pages 37 to 40, 1994. Polymer dispersed liquid crystal is disclosed in the paper. The holographic polymer dispersed liquid crystal is produced as follows.
First, photo-curable polymer is mixed with liquid crystal, and the mixture
1
a
is sealed between transparent plates
1
b
and
1
c
so as to form a liquid crystal cell
1
(see FIG.
1
). The liquid crystal cell
1
is placed in a laser beam radiating system
2
. The laser beam radiating system
2
includes a laser beam generator
2
a,
a beam splitter
2
b
and reflecting mirrors
2
c,
2
d
and
2
e.
The laser beam generator
2
a
radiates a laser beam LB
1
to a beam splitter
2
b,
and the beam splitter
2
b
splits the laser beam LB
1
to laser sub-beams LB
2
and LB
3
. The reflecting mirror
2
c
reflects the laser sub-beam LB
2
toward the liquid crystal cell
1
, and the reflecting mirrors
2
d
and
2
e
reflect the laser sub-beam LB
3
toward the liquid crystal cell
1
. The laser sub-beam LB
2
is incident onto one of the transparent plates
1
b/
1
c,
and the other laser sub-beam LB
3
is incident onto the other of the transparent plate
1
b/
1
c.
A phase difference takes place between the incident laser sub-beams LB
2
and LB
3
.
The laser sub-beam LB
2
interferes with the laser sub-beam LB
3
, and, accordingly, the laser sub-beams LB
2
and LB
3
generate interference fringes in the mixture
1
a.
The interference fringes are repeated in a direction of the thickness of the liquid crystal cell
1
, and form strongly illuminated layers and weakly illuminated layers alternated with one another in the mixture
1
a.
The distance between the strongly illuminated layer and the weakly illuminated layer is dependent on the wavelength of the laser sub-beams LB
2
/LB
3
.
The photo-curable polymer in the strongly illuminated layers is polymerized, and the strongly illuminated layers are formed into polymer layers. On the other hand, liquid crystal droplets are formed in the weakly illuminated layers. As a result, the mixture
1
a
is formed into a multilayer structure consisting of the polymer layers
1
d
and the liquid crystal droplet layers
1
e
as shown in FIG.
2
. The prior art fabrication process is also disclosed in Japanese Patent Publication of Unexamined Application No. 4-355424.
Thus, the holographic polymer dispersed liquid crystal between the transparent plates
1
b/
1
c
has the multilayer structure. The polymer layers
1
d
are different in refractive index from the liquid crystal droplet layers
1
e,
and, for this reason, the holographic polymer dispersed liquid crystal structure is considered to be a diffraction grating in which the refractive index is spatially changed.
Assuming now that light LB
4
is obliquely incident into the holographic polymer dispersed liquid crystal, the multilayer structure of the holographic polymer dispersed liquid crystal reflects the incident light LB
4
, and the reflecting direction is determined from the angle A
1
of the multilayer structure to the transparent plates
1
b/
1
c
and the direction of the incident light LB
4
.
The incident light LB
4
is reflected on the liquid crystal droplet layers
1
e
as shown in
FIG. 2
, and reflected light LB
5
from one of the liquid crystal droplet layer
1
e
interferes with reflected light LB
6
from another liquid crystal droplet layer
1
e.
When reflected light is equal in phase to another light, the in-phase lights interfere with one another so as to increase the intensity. Thus, the multilayer structure strongly reflects the light having the wavelength determined from the periodicity of the multilayer structure and the spacious variation of refractive index. The multilayer structure provides virtual reflecting surfaces in the liquid crystal cell, and reflects the light of a specific wavelength.
When an appropriate electric potential is applied to the holographic polymer dispersed liquid crystal, the liquid crystal droplet layers
1
e
change the orientation of the liquid crystal, and break the interference conditions for the reflection. Thus, the reflection intensity of the holographic polymer dispersed liquid crystal is controlled by changing the electric potential applied thereto, and the holographic polymer dispersed liquid crystal is available for the liquid crystal display unit. No polarizing plate is incorporated in the holographic polymer dispersed liquid crystal display unit.
When a holographic polymer dispersed liquid crystal layer for reflecting red light, a holographic polymer dispersed liquid crystal layer for reflecting green light and a holographic polymer dispersed liquid crystal layer for reflecting blue light are laminated, the three-layer holographic polymer dispersed liquid crystal structure is available for a full-color liquid crystal display unit. When image signals independently control the holographic polymer dispersed liquid crystal layers, a color image is formed on the full-color liquid crystal display unit.
However, the prior art holographic polymer dispersed liquid crystal display unit encounters the following problems.
First, it is impossible for the prior art holographic polymer dispersed liquid crystal display unit to achieve a large refractive index. This is because of the fact that the multilayer structure merely reflects the light of a specific wavelength incident thereinto within a specific directional range.
Second, when the holographic polymer dispersed liquid crystal layers for primary three colors are laminated for a full-color liquid crystal display unit, each of the holographic polymer dispersed liquid crystal layers requires a pair of glass substrates, and six glass substrates are incorporated in the full-color liquid crystal display unit. This results in a heavy bulky liquid crystal display unit.
Third, the holographic polymer dispersed liquid crystal layers for the primary three color require three thin film transistor arrays formed on the respective substrates, because the three holographic polymer dispersed liquid crystal layers are independently controlled for a full-color image. The thin-film transistor array is so expensive that the manufacturer hardly decreases the production cost the full-color liquid crystal display unit.
Fourth, the prior art process for the full-color liquid crystal display unit requires a delicate fabrication process. It is necessary for the manufacturer to laminate the holographic polymer dispersed liquid crystal layers in such a manner that the pixels of one liquid crystal layer are precisely aligned with the pixels of
Hayama Hiroshi
Sumiyoshi Ken
NEC Corporation
Ngo Julie
Sikes William L.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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