Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-12-28
2001-07-10
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S139000, C349S129000
Reexamination Certificate
active
06259503
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to an active matrix liquid crystal display (LCD) device having a wide viewing angle.
(b) Description of the Related Art
Conventional LCD devices include a static drive LCD device wherein an electric field is applied to the liquid crystal (LC) by a constant voltage signal. The static drive LCD device has a drawback in that a large number of signal lines are required in the case of a large capacity display panel.
Thus, a large capacity LCD panel generally uses a multiplex drive scheme, wherein the signal voltages are supplied to the LC in a time sharing scheme. Among the LCD devices using the multiplex drive scheme, an active matrix LCD device, wherein electric charge supplied to the electrodes in a frame period is maintained to a next frame period, exhibits a high image quality. On the other hand, LCD devices are categorized also into two types including one wherein the electric field is applied to the LC perpendicular to the glass substrates sandwiching therebetween the LC, and the other wherein the electric field is applied parallel to the substrate. The latter is called “In-plane Switching Scheme” and especially suited to a large size monitor due to its wide viewing angle.
FIG. 1
shows the structure of electrodes in a pixel element of a conventional active matrix LCD device, such as described in JP-B-63-21907. The LCD device includes a plurality of pixel elements arranged in a matrix, a plurality of scanning lines
108
each connected to an external drive circuit and gates of a corresponding row of the pixel elements, a plurality of signal lines
102
each for supplying a corresponding column of the pixel elements, a common electrode
103
disposed for all of the pixel elements and having a comb-shape electrode portion in each pixel area. Each pixel element includes a thin film transistor (TFT)
109
and a pixel electrode
104
having a comb shape corresponding to the comb shape of the electrode portion of the common electrode.
Referring to
FIG. 2
taken along line II—II in
FIG. 1
, the conventional LCD device has a LC panel
300
including a TFT panel
100
and a counter panel
200
. The TFT panel
100
includes, from the front side thereof, a TFT glass substrate
101
having a first polarizing plate
110
on the front side thereof, the common electrode
103
, an insulator film
105
, and the pixel electrode
104
and the signal line
102
. The teeth of the comb-shape pixel electrode
104
and teeth of the comb-shape common electrode
103
are arranged alternatively with each other to oppose each other in the direction parallel to the LC panel
300
. These electrodes
103
and
104
are protected by a protective insulator film
106
, on which a first orientation film
107
is formed by coating. The first orientation film
107
is subjected to a rubbing operation in a first direction.
The counter panel
200
has, from the rear side thereof, a glass counter substrate
201
having a second polarizing plane
205
on the rear side thereof, a matrix shield film
203
for shielding light, a color film
204
for displaying multi-color image, a planarization film
202
, and a second orientation film
207
. The second orientation film
207
is subjected to rubbing operation in the second direction opposite to the first direction.
LC layer
301
is disposed between the TFT panel
100
and the counter panel
200
, wherein the LC molecules are oriented in the first direction adjacent to the TFT panel
100
by the first orientation film
107
and in the second direction adjacent to the counter panel
200
by the second orientation film
207
. The first polarizing plate
110
bonded onto the front side of the TFT substrate
101
has a light transmission axis perpendicular to the first direction, whereas the second polarizing plate
205
bonded onto the rear side of the counter substrate
201
has a light transmission axis perpendicular to the light transmission axis of the first polarizing plate
110
.
In operation, the TFT
109
is turned on/off by the corresponding scanning line
108
formed as the common layer with the pixel electrode
103
. When the TFT
109
is turned on, electric charge flows from the signal line
102
into the pixel electrode
104
, and after the TFT
109
is turned off in the subsequent period, the pixel electrode
104
stores the electric charge. The common electrode
103
is maintained at a constant potential, thereby generating a transverse electric filed due to the potential difference between the same and the pixel electrode
104
in the direction parallel to the LC panel
300
.
The transverse electric field rotates the crystal axis of the LC molecules due to the interaction between the electric field and the dielectric anisotropy of the LC molecules, as shown in
FIG. 3
, wherein the rotation is exemplarily shown in the case of a positive dielectric anisotropy of the LC molecules. If the dielectric anisotropy is negative, the rotation is opposite to the direction shown in FIG.
3
. The rotation of the LC molecules generates retardation change, wherein the transmission (or permeability) changes at the locations where the light shield layer
203
, pixel electrode
104
, common electrode
103
, scanning line
108
and TFT
109
are not disposed.
FIG. 4
shows the principle of the operation of the LCD panel, wherein the LC molecule exemplarily has a positive dielectric anisotropy. The direction of the initial orientation of the LC molecules
302
is determined depending on the rubbing direction of the first orientation film
107
on the TFT panel
100
, and thus aligned perpendicular to the polarizing axis of the first polarizing plate
110
of the TFT panel
100
. The incident light is polarized by the first polarizing plate
110
, and thus is substantially completely shielded by the second polarizing plate
205
because the polarized light is not subjected to the retardation of the liquid crystal layer
301
. In this state, the LCD panel exhibits black.
When the transverse electric field is applied to the LC molecule due to the potential difference between the common electrode
103
and the pixel electrode
104
, the LC molecule
302
rotates due to the interaction between the dielectric anisotropy of the LC molecule and the transverse electric field. The incident light is subjected to the retardation of the LC due to the dielectric anisotropy, and generally assumes an elliptical polarization just before passing the second polarizing plate
205
. The component of the elliptically polarized light aligned with the polarization axis passes the LCD panel
300
, and the time average of the light intensity is sensed by human eyes.
The shape of the elliptically polarized light is changed depending on the angle &psgr; defined by the mean orientation and the initial orientation of the LC molecules
302
, wherein the normalized transmission T/T
0
is determined by the following approximate expression:
T/T
0
=sin
2
(2&psgr;)sin
2
(&Dgr;
n×d
×&pgr;/&lgr;) (1)
wherein &Dgr;n,d and &lgr; are anisotropy in the refractive index, cell gap and wavelength of the transmitted light, respectively. In equation (1), the minimum transmission is obtained by &psgr;=0°, whereas the maximum transmission is obtained by &psgr;=45°.
In the active matrix LCD device as described above, a color tint phenomenon is generally observed due to the refractive index anisotropy of the LC modules, wherein the polarized light exhibits a blue or yellow tint when the LC panel is observed with a relatively large viewing angle. As schematically illustrated in
FIG. 5
, the view angle along the major axes of the LC molecules involves the blue tint, whereas the view angle along the minor axes of the LC molecules involves the yellow tint.
FIG. 6
shows x-y chromaticity change in the case of an intermediate gray scale level, wherein LCD device is observed with a viewing angle &thgr;=60°, with the azimuth &phgr; being between 0 and 360°. The definition for the viewing angle
Sukegawa Osamu
Watanabe Makoto
Chowdhury Tarifur R.
NEC Corporation
Sikes William L.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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