Liquid crystal cells – elements and systems – Particular structure – Interconnection of plural cells in series
Reexamination Certificate
1999-11-01
2002-02-12
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Interconnection of plural cells in series
C349S121000, C349S165000, C349S175000
Reexamination Certificate
active
06346974
ABSTRACT:
BACKGROUND OF THE INVENTION
Today, every kind of electric device is being rapidly made multifunctional. For smoothly manipulating a multifunctional electric device, the significance of an interface is being increased as well. The liquid crystal display device (simply referred to as a liquid crystal display) is thin, lightweight, and energy-effective, so that it may be mounted on such a multi-functional electronic device without greatly changing the form of the electric device. Hence, the liquid crystal display is the most approximate to an interface provided in every kind of electric device.
A reflective liquid crystal display is arranged to reflect ambient light with a reflective plate for displaying an image without using a back light. Hence, the reflective liquid crystal display is far more energy-effective, thinner, and more lightweight among all kinds of liquid crystal displays. The field of utilization to which the present invention applies concerns with the reflective liquid crystal displays.
A guest host liquid crystal display does not employ a polarizer, so that it has a high efficiency of utilizing light. The application of this type liquid crystal display to the reflective liquid crystal display makes it possible to obtain a display with a high reflection factor. The guest host liquid crystal display may be divided into several types, one of which is a retardation film type guest host liquid crystal display.
For describing the display principle of the retardation film type guest host liquid crystal display, at first, the description will be oriented to the guest host liquid crystal display with no phase plate to be compared with the subject display with reference to FIG.
2
. The guest host liquid crystal display shown in
FIG. 2
is arranged so that a liquid crystal layer
15
has a twist angle of 0. The ray of light incident to the liquid crystal layer is decomposed into two optical eigen modes and then propagated through the liquid crystal layer. If the liquid crystal layer has a twist angle of 0, these two eigen modes are linear ones whose vibrating directions are perpendicular to each other. The ray whose vibrating direction is in parallel to the alignment direction of the liquid crystal is called extraordinary rays
43
and
45
, while the ray whose vibrating direction is perpendicular to the alignment direction thereof is called ordinary rays
44
and
46
.
A dichroic dye, on the average, has an alignment direction that is equal to that of the liquid crystal layer. Hence, if the liquid crystal layer has a twist angle of 0, for example, the extraordinary ray has an absorption coefficient &agr;
e
of 0.3 &mgr;m
−1
and the ordinary ray has an absorption coefficient &agr;
o
of 0.035 &mgr;m
−1
. The extraordinary ray
47
is sufficiently absorbed by the liquid crystal layer, while the ordinary ray
48
is hardly absorbed thereby. It means that lack of the polarizer results in lowering a contrast ratio.
The arrangement and the display scheme of the retardation film type guest host liquid crystal display are illustrated in FIG.
3
. The retardation film type guest host liquid crystal display includes a phase plate built inside of a liquid crystal cell. It means that a phase plate
33
is laid between a liquid crystal layer
15
and a reflective electrode
30
.
In the conventional retardation film type guest host liquid crystal display, the phase plate has a retardation of a quarter wave. That is, if the liquid crystal layer has a twist angle of 0, a phase difference between the extraordinary ray
44
in incidence and the ordinary ray
33
after reflection is a half wave. While the ray is passed through the phase plate twice, the ordinary ray in incidence is converted into the extraordinary ray after reflection. Hence, the retardation of the phase plate is made a half of a half wave, that is, a quarter wave.
The ordinary ray
50
is hardly absorbed in the liquid crystal layer and is incident to the phase plate. After the ordinary ray
50
is passed through the phase plate, it is converted into a circularly polarized light
48
. Then, the circularly polarized light
48
is reflected on a reflective layer. Next, the circularly polarized light
48
is converted into the extraordinary ray
33
while it is passed through the phase plate again. Since the ordinary ray that has been hardly absorbed in incidence is sufficiently absorbed after reflection (
50
), the reflection factor in dark representation is made sufficiently lower.
The retardation film type guest host liquid crystal display is described in JP-AP-8-286214, JP-A-9-90431, JP-A-9-152630, JP-A-10-62773, JP-A-10-82986, and JP-A-10-104565, for example.
The retardation film type guest host liquid crystal display has a problem that the dark representation is colored in purple. The ground therefor is shown in FIG.
4
. The wavelength dispersion of the retardation of an ideal phase plate is shown by a broken line of FIG.
4
. In case the liquid crystal layer has a twist angle of 0, the polarization shown in
FIG. 3
is executed over the overall visible wavelength if the retardation of the phase plate is a quarter wave over an overall visible wavelength.
However, the retardation of the phase plate is decreased with increase of the wavelength as shown by a solid line of FIG.
4
. Hence, the broken line is crossed with the solid line merely at one point of the visible wave area.
FIG. 4
shows the case both are arranged to coincide with each other at a wavelength of 550 nm where the relative luminous efficiency becomes maximum. Strictly, the polarization shown in
FIG. 3
is established only at a wavelength of 550 nm where the broken line coincides with the solid line.
On both ends of the visible wave area where the broken line is far away from the solid line, the polarization shown in
FIG. 3
is not established, so that the reflection factor of dark representation is not made sufficiently low. The reflective spectrum of the dark representation at that time indicates that the reflection factor is increased mainly in red and blue as shown by a solid line (thin line) of FIG.
5
. Hence, the dark representation is colored in purple.
Further, the retardation film type guest host liquid crystal display also has a problem that the driving voltage is increased. That is, in the retardation film type guest host liquid crystal display, as shown in
FIG. 3
, a phase plate is located between the liquid crystal layer and the electrode, so that the liquid crystal layer and the phase plate are aligned in series with respect to an electric field. Hence, the voltage to be applied to the liquid crystal layer is made lower by the application of a voltage to the phase plate. In order to obtain a sufficient reflection factor and contrast ratio, it is necessary to increase the driving voltage by the value applied to the phase plate.
SUMMARY OF THE INVENTION
An object of the present invention is to solve a problem of coloring dark representation in a retardation film type guest host liquid crystal display and make the display more energy-effective.
According to the present invention, a given amount of chiral dopant is added to a liquid crystal layer for making the structure of the liquid crystal layer twist alignment.
In a case that the alignment direction is turned clockwise from the second substrate located farther to the first substrate when the alignment direction of the liquid crystal layer is observed from the axial direction of the twist, the phase plate is formed so that the slow axis is located about 45 degrees counterclockwise against a closer alignment direction of the liquid crystal layer. In a case that the alignment direction is turned counterclockwise from the second substrate to the first substrate, the phase plate is formed so that the slow axis is located about 45 degrees clockwise against a closer alignment direction to the liquid crystal layer. The definition of about 45 degrees is ranged within 45 degrees ±5 degrees because a great difference takes place through the effect of the present application e
Itou Osamu
Komura Shinichi
Antonelli Terry Stout & Kraus LLP
Duong Tai V.
Sikes William L.
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