Liquid crystal display unit

Details Liquid crystal display unit Liquid crystal display unit

1999-01-11

2002-06-11

Sikes, William L. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Particular illumination

C349S062000

Reexamination Certificate

active

06404468

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to liquid crystal display units and, more specifically to a liquid crystal display unit capable of enhancing visibility at a large angle of view.
2. Description of the Background Art
A Liquid crystal display unit has conventionally been known as one type of display unit. One of major technological subjects of the liquid crystal display unit is to increase its angle of view. A liquid crystal display unit which has an IPS mode (In-Plane Switching mode) as one of modes for obtaining such large angle of view has been disclosed for example in Japanese Patent Laying-Open No. 8-254712. The liquid crystal display unit of an active matrix type having the IPS mode makes a direction of an electric field applied to a liquid crystal parallel to a substrate.
FIG. 18
is a schematic cross-sectional view showing a conventional liquid crystal display unit having an IPS mode, where a voltage is not applied to its electrode. Further,
FIG. 19
is a schematic plan view showing the liquid crystal display unit having the IPS mode shown in FIG.
18
. Referring to
FIGS. 18 and 19
, the liquid crystal display unit having the IPS mode will be described.
Referring to
FIG. 18
, the liquid crystal display unit having the IPS mode is provided with a back light member
101
and a liquid crystal display
103
. Liquid crystal display
103
is formed above back light member
101
. Liquid crystal display
103
includes polarizer
104
a
and
104
b
, glass substrates
113
a
and
113
b
, electrodes
106
a
and
106
b
, a source line
109
, orientation films
107
a
and
107
b
and liquid crystal molecules
108
a
and
108
b
. Glass substrate
113
a
is formed on polarizer
104
a
. Linear electrodes
106
a
,
106
b
and source line
109
are respectively formed on and above glass substrate
113
a
. Orientation film
107
a
for adjusting an orientation direction of liquid crystal molecules
108
a
and
108
b
(a major axis direction of liquid crystal molecules
108
a
and
108
b
) is formed above electrodes
106
a
and
106
b
. Glass substrate
113
b
is formed above orientation film
107
a
with a liquid crystal including liquid crystal molecules
108
a
and
108
b
interposed. The other orientation film
107
b
is transferred on glass substrate
113
b
. Polarizer
104
b
is formed on glass substrate
113
b.
Here, as shown in
FIG. 19
, when the voltage is not applied to electrodes
106
a
and
106
b
, the orientation direction of liquid crystal molecules
108
a
and
108
b
is adjusted to have some angle with respect to a direction in which electrodes
106
a
and
106
b
extend. It is noted that anisotropy of dielectric constant of the liquid crystal is assumed positive.
Referring further to
FIG. 18
, polarizer
104
a
and
104
b
are arranged with their transmission axes orthogonal to each other. It is noted that the transmission axes mentioned here correspond to directions of vibration components of light allowed to be transmitted through polarizer
104
a
and
104
b
. Then, the transmission axis of polarizer
104
a
positioned between back light member
101
and liquid crystal molecules
108
a
,
108
b
is aligned with the orientation direction of liquid crystal molecules
108
a
and
108
b
when the voltage is not applied to electrodes
106
a
and
106
b
. When the voltage is not applied to electrodes
106
a
and
106
b
, birefringence does not occur at liquid crystal molecules
108
a
and
108
b
. So the light directed from back light member
101
through polarizer
104
a
dose not transmit the polarizer
104
b
. As a result, a black screen (hereinafter represents a state in which a screen appears black) is achieved.
FIG. 20
is a schematic cross sectional view showing the liquid crystal display unit having the IPS mode shown in
FIG. 18
, where the voltage is applied to the electrode. Further,
FIG. 21
is a schematic plan view showing the liquid crystal display unit shown in FIG.
20
. Referring to
FIGS. 20 and 21
, an operation of the liquid crystal display unit having the IPS mode when the voltage is applied to the electrode is described.
Referring to
FIGS. 20 and 21
, when the voltage is applied to electrodes
106
a
and
106
b
, an electric field is formed in the direction indicated by an arrow
110
. When such electric field is formed, liquid crystal molecules
108
a
and
108
b
change their orientation directions on planes which are almost parallel to the surfaces of orientation films
107
a
and
107
b
. In this case, the birefringence occurs at liquid crystal molecules
108
a
and
108
b
. So the light directed from back light member
101
through polarizer
104
a
can transmit the polarizer
104
b
. As a result, a white screen (hereinafter represents a state in which the screen appears white) is achieved.
It is noted that although the liquid crystal having positive anisotropy of dielectric constant is used here, a liquid crystal having negative anisotropy of dielectric constant may be used. In this case, the orientation directions of liquid crystal molecules
108
a
and
108
b
in the case where the voltage is not applied to electrodes
106
a
and
106
b
are adjusted to have some angles with respect to electric field direction
110
.
As shown in
FIG. 18
, when the light is directed in the direction which is perpendicular to the surfaces of polarizer
104
a
and
104
b
which are arranged with their transmission axes orthogonal to each other, the light in the perpendicular direction is shielded. However, when the light is directed in a direction which is diagonal with respect to the surfaces of polarizer
104
a
and
104
b
, that is, when an angle of view &THgr; is large, there would be some light transmitting through polarizer
104
a
and
104
b
(leakage light). Here, angle of view &THgr; is defined as shown in FIG.
22
.
FIG. 22
is a diagram showing a relation between angle of view &THgr; and electric field direction
110
generated in the liquid crystal display unit having the IPS mode.
Referring to
FIG. 22
, assume that an x axis is almost parallel to electric field direction
110
. Then, a y axis is set on a plane which is almost parallel to the surfaces of polarizer
104
a
and
104
b
(with reference to FIG.
20
). Further, a z axis is set in the direction which is perpendicular to an xy plane and also in the direction from polarizer
104
a
toward
104
b
. Then, an azimuth &PHgr; is defined on the xy plane with respect to the electric field direction. Finally, on a plane defined by the direction determined by azimuth &PHgr; and the z axis, an angle of view &THgr; with respect to the z axis is defined.
FIG. 23
is a graph showing a relation between a relative luminance and an angle of view of the light directed from the back light member. Referring to
FIG. 23
, even when angle of view &THgr; attains 80°, the relative luminance is about 50%, and the light is directed from the back light member diagonally with respect to the liquid crystal display.
Then, a relation between the angle of view, relative luminance and contrast ratio is measured in the liquid crystal display unit having the IPS mode shown in
FIGS. 18
to
21
. The result is shown in
FIGS. 24
to
26
.
FIG. 24
is a graph showing the relation between the relative luminance and the angle of view in the case of the white screen in the liquid crystal display unit having the IPS mode.
FIG. 25
is a graph showing the relation between the relative luminance and the angle of view in the case of the black screen in the liquid crystal display unit having the IPS mode.
FIG. 26
is a graph showing the relation between the contrast ratio and the angle of view in the liquid crystal display unit having the IPS mode.
Referring to
FIG. 25
, in the conventional liquid crystal display unit having the IPS mode shown in
FIGS. 18
to
21
, the relative luminance increases around points at which &THgr;=±60° in the case of the black screen. This is because the light which is diagonally directed with respect to the liquid crystal display

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