Liquid crystal display

Details Liquid crystal display Liquid crystal display

2001-04-05

2002-12-10

Dudek, James (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

Reexamination Certificate

active

06493054

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display. More particularly, the present invention provides an In-plane switching (IPS) type liquid crystal display, in which an electric field is formed laterally to its substrates and applied to a liquid crystal layer, having a wide viewing angle, a high contrast ratio and an excellent display quality.
In a liquid crystal display interposing a liquid crystal layer between a pair of substrates, at least one of them being transparent, some display mode have been used as a display method. Examples of the display mode which have widely been used include a twisted nematic (TN) mode. According to this mode, an electric field is applied to a liquid crystal layer to control its rotary polarization, thereby controlling a transparency of the liquid crystal layer and obtaining display of image.
On the other hand, there has been an in-plane switching (IPS) mode for applying, to a liquid crystal, an electric field which is almost parallel with a substrate by using interdigital shaped electrodes (hereinafter referred to as an interdigital electrode) or the like. According to this mode, an electric field is applied to the liquid crystal to control birefringence of the liquid crystal, thereby switching display of images. Japanese Patent No. 2743293 has disclosed that the IPS mode has a greater angle of view characteristic than that in a conventional TN mode.
FIG. 9
is an explanatory view showing a part of a conventional liquid crystal display using an IPS mode which is formed by a liquid crystal having a positive dielectric anisotropy.
FIG. 9
shows an electrode substrate
51
, two kinds of interdigital pixel electrodes
53
and
54
formed on the electrode substrate
51
, a counter substrate
52
, and liquid crystal molecules included in a liquid crystal layer
60
which constitute a liquid crystal panel in the liquid crystal display. As shown in
FIG. 9
, the electrode substrate
51
having the interdigital pixel electrodes
53
,
54
and the counter substrate
52
are provided in parallel with each other and the liquid crystal layer
60
including the liquid crystal molecules is present between the electrode substrate
51
and the counter substrate
52
. As shown in FIG.
10
(
a
), the electrode substrate
51
and the counter substrate
52
are subjected to an alignment treatment in a direction of angle
55
, thereby the liquid crystal molecules of the liquid crystal layer
60
are provided to have an angle of &thgr;
1c
with respect to a length-wise direction of pixel electrode
53
and
54
.
Next, a display principle of the liquid crystal display will be described with reference to FIG.
11
.
FIG. 11
is a partial view showing the liquid crystal display using the IPS mode.
FIG. 11
shows only the liquid crystal panel and two polarizing plates in the liquid crystal display. Referring to the liquid crystal panel, particularly, there are shown the electrode substrate
51
, interdigital pixel electrodes
53
and
54
(only one pixel electrode is shown respectively), the counter substrate
52
and liquid crystal molecules included in the liquid crystal layer
60
(only seven molecules are shown). In
FIG. 11
, the reference numeral
53
denotes a first pixel electrode, the reference numeral
54
denotes a second pixel electrode, the reference numeral
51
denotes an electrode substrate, the reference numeral
52
denotes an counter substrate, the reference numeral
59
denotes a first polarizing plate and the reference numeral
58
denotes a second polarizing plate. As shown in
FIG. 11
, the first polarizing plate
59
is provided such that a direction of a major axis of the liquid crystal molecule is parallel with a direction of a transmission axis of the first polarizing plate
59
(show in a double headed arrow) and a direction of a transmission axis of the second polarizing plate
58
(also shown in a double headed arrow) is orthogonal to that of the first polarizing plate
59
. Directions of aligning treatments of alignment films (not shown) formed on the electrode substrate
51
and the counter substrate
52
are both parallel with the transmission axis of the first polarizing plate
59
or the second polarizing plate
58
.
In a state in which no voltage is applied (that is, an electric field is not formed between the first pixel electrode
53
and the second pixel electrode
54
as shown in FIG.
11
A), a linearly polarized light incident on the liquid crystal layer
60
has an oscillation direction parallel with the liquid crystal molecule and does not receive a birefringence effect during passage through the liquid crystal layer. Therefore, a direction P of oscillation of the light passing through the counter substrate
52
is orthogonal to the transmission axis of the second polarizing plate
58
and the light transmitted through the counter substrate
52
cannot be transmitted through the second polarizing plate
58
and is set in a dark state.
In a state in which voltage is applied (that is, the electric field is formed between the first pixel electrode
53
and the second pixel electrode
54
) as shown in
FIG. 11B
, the liquid crystal molecule is rotated in the direction of the electric field (the degree of the rotation depends on a magnitude of the electric field) while maintaining a parallel orientation with respect to the surfaces of the electrode substrate
51
and the counter substrate
52
. For this reason, the linearly polarized light incident on the liquid crystal layer
60
receives the birefringence effect to be changed into an elliptically polarized light Q and a certain quantity of the light passes through the second polarizing plate. The quantity of the light transmitted through the second polarizing plate is changed depending on a rotating angle &thgr; of the liquid crystal molecule. The rotating angle &thgr; of the liquid crystal is a function of an applied voltage (V). Thus, display of image can be carried out by changing voltages to be applied to the first pixel electrodes
53
and the second pixel electrodes
54
.
At this time, an intensity of the transmitted light is expressed in the Equation 1:
I=I
o
·sin
2
(&pgr;
R
/&pgr;)·sin
2
(2&thgr;)  (Equation 1)
Wherein I
o
represents an intensity of the light incident on the first polarizing plate
59
, &lgr; represents a wavelength of the light, and R represents a retardation which is represented by an optical path difference (&Dgr;n)·d between an ordinary light and an extraordinary light, &Dgr;n representing an absolute value (|n
e
−n
o
|) of a difference between a refractive index n
o
of the ordinary light and a refractive index n
e
of the extraordinary light in the liquid crystal. As is apparent from the Equation 1, the transmitted light has a maximum intensity with &thgr;=&pgr;/4.
FIG. 12
shows a change in a quantity of the transmitted light with a variation in a voltage to be applied between the first pixel electrode
53
and the second pixel electrode
54
. When the applied voltage is increased, the rotating angle of the liquid crystal molecule becomes greater and the quantity of the transmitted light is increased. &thgr; in the Equation 1 corresponds to the average aligning direction of the liquid crystal layer realigned by an electric field formed laterally to the substrate.
Usually, the TN mode is used in a normally white mode in which white display is carried out without the application of an electric field and black display is carried out with the electric field applied. At this time, more black display can be obtained by the application of a higher electric field to the liquid crystal. As a result, a high contrast ratio can be achieved.
On the other hand, the IPS mode is used in a normally black mode in which the black display is carried out without the application of the electric field and the white display is carried out with the electric field applied.
FIGS. 9 and 10
show sectional and plan views showing a conventional IPS mode, respectively. In a liquid crystal displ

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