Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-04-12
2002-08-13
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S113000, C349S121000, C349S096000
Reexamination Certificate
active
06433845
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reflection type liquid crystal display which can increase the contrast ratio.
Further, the present invention relates to a reflection type liquid crystal display in which angles of a polarization plate, a half wavelength phase difference film, and a quarter wavelength phase difference film are adjusted so that change in retardation of a liquid crystal layer depending on the viewing angle is cancelled by the change in retardation of the phase difference films depending on the viewing angle.
This application is based on Japanese Patent Applications Nos. 10-099144 and 10-346241, the contents of which are incorporated herein by reference.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In recent years, reflection type liquid crystal displays which do not use a back light, which can reduce consumption of power, and which are used in various mobile devices driven by batteries have become widely used. The most popular system currently in use is a twisted nematic system (hereinafter referred to as TN). In the following, the prior liquid crystal display will be explained with reference to figures.
FIG. 20
is a cross-sectional side view showing the first reflection type liquid crystal display using the TN system of the background art. The first conventional reflection type liquid crystal display
400
comprises a pair of transparent substrates which are an upper substrate
401
and an lower substrate
402
arranged in parallel and separated from each other by approximately 5 &mgr;m, and a liquid crystal layer
403
whose orientation is twisted by 90 degrees and which is provided between the substrates
401
and
402
.
Polarization plates
406
and
407
are laminated on the surfaces of the transparent substrates which are not in contact with the liquid crystal layer. The lower substrate
402
has a reflector plate
108
on the outer surface of the polarization plate
407
. Transparent electrodes
404
made of a transparent conductive material such as indium tin oxide (ITO) are laminated on the surfaces of the upper and lower substrates
401
and
402
in contact with the liquid crystal layer. On these transparent electrodes
404
, an orientated film
405
is formed. By applying an electric field via the transparent electrodes
404
to the liquid crystal layer
403
, the orientation of the liquid crystal is changed. Because of this change, incident light is reflected by the reflector plate
408
, changing the strength of the incident light and producing images.
However, there is a problem in the first reflection type liquid crystal display
400
using the conventional TN system, which is described below. The incident light passes through the two polarization plates
406
and
407
twice one way, that is, four times return trip. Because the two polarization plates
406
and
407
absorb not only polarized light in a direction of the absorption axis but also slightly polarized light in the direction of the transmission axis, the light incurs an optical loss every time the light passes the polarization plates
406
and
407
. The optical loss decreases brightness of the display so that the first reflection type liquid crystal display using the two polarization plates
106
and
107
cannot produce bright images.
Between the liquid crystal layer
403
and the reflector plate
408
, the transparent lower substrate
402
with a thickness of approximately 1 mm is provided. When a viewer looks at the device obliquely, a doubled image may appear because of parallax. Although one of the reflection type liquid crystal displays uses a super twisted nematic (hereinafter referred to as STN) in which the twist angle of the liquid crystal layer
203
is increased to approximately 200 degrees, the STN device has the disadvantages of decreasing brightness and the problem of parallax, like the TN system, because the STN device uses the two polarization plates.
To solve the problem, a report by A. R. Kmetz, which appears in “Proceedings of the SID” Vol. 21, No. 2, pp. 63-65, describes a reflection type liquid crystal display using a single polarization plate. Because the incident light passes through the polarization plate only twice, this reflection type liquid crystal display reduces the optical loss as compared with the first conventional liquid crystal display. Since the polarization plate is eliminated from the rear side of the liquid crystal layer, the electrodes of the substrate facing the light inputting side may be formed of metal, etc., as a reflector electrode, which can solve the problem caused by the parallax.
Japanese Patent Application, First Publication No. 4-116515 discloses a reflection type liquid crystal display using a single polarization plate, and the optimization of three parameters of the twist angle of a liquid crystal layer, the product (hereinafter referred to as &Dgr;nd) of anisotropy of the refractive index of the liquid crystal layer and its thickness (hereinafter referred to as a cell gap), and the angle of the polarization plate. The angle of the polarization plate is between either the transmission axis or the absorption axis and the direction of the orientation of the liquid crystal in the light inputting side.
FIG. 21
is a cross-sectional view showing the structure of the second reflection type liquid crystal display disclosed in Japanese Patent application, First Publication No. 4-116515. A transparent electrode
404
and a stretched film
405
are formed on an upper substrate
401
. A reflector electrode
409
and a stretched film
405
are formed on a lower substrate
402
. The liquid crystal layer
403
is provided between the upper and lower substrates
401
and
402
, which are arranged in parallel. The light inputting side of the upper substrate
401
, that is, the surface opposite to the transparent electrode
404
, is coated with a polarization plate
406
.
According to the optimized design parameters taught by Japanese Patent Application, First Publication No. 4-116515, the twist angle of the liquid crystal layer
403
is 63 degrees, the &Dgr;nd of the liquid crystal layer
103
is 0.2 &mgr;m, and the angle of the polarization plate is zero.
In the second conventional liquid crystal display
400
designed according to the above parameters, linearly polarized light output from the liquid crystal layer
403
is converted into circularly polarized light. Further, the light, which is reflected and is transmitted through the liquid crystal layer
403
, is converted into a linearly polarized light whose polarization plane is rotated from that of the incident light by approximately 90 degrees. Therefore, the polarization conversion by the liquid crystal layer
403
is remarkably efficient.
Further, a report by S. -T. Wu in “1997 SID International Symposium Digest of Technical Papers” Vol. 28, pp. 643-646, 1997 teaches optimized design parameters in the second conventional reflection type liquid crystal display
400
using a single polarization plate. The design parameters described in the report are shown in the following: the twist angle of the liquid crystal layer is 90 degrees, the value &Dgr;nd of the liquid crystal layer is 0.25 &mgr;m, and the angle of the polarization plate is 20 degrees. The report teaches other design parameters for the reflection type liquid crystal display: the twist angle of the liquid crystal layer is 70 degrees, the value &Dgr;nd of the liquid crystal layer is 0.28 &mgr;m, and the angle of the polarization plate is 20 degrees.
The element designed according to these parameters is a normally black element. That is, the element is in a dark state when no voltage is applied, while the element is in a light state when a voltage is applied.
FIG. 22
is a cross-sectional side view showing a third conventional reflection type liquid crystal display. Except that a quarter wavelength plate
410
is inserted between the polarization plate
406
and the upper substrate
401
, the third conventional reflection type
Suzuki Teruaki
Yanai Koichi
Qi Mike
Sikes William L.
Young & Thompson
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