Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-10-30
2002-01-29
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S144000
Reexamination Certificate
active
06342939
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display that can eliminate viewing angle dependence.
2. Description of the Related Art
A liquid crystal display (LCD) includes a pair of substrates and a liquid crystal layer (liquid crystal cell) sandwiched between them, and produces a display by altering the orientation of the liquid crystal molecules in the liquid crystal layer and thereby changing the optical refractive index within the liquid crystal cell. Accordingly, the liquid crystal molecules need to be aligned in an orderly manner within the liquid crystal cell.
One method commonly used to align the liquid crystal molecules in a given direction involves forming an alignment film on the liquid crystal layer side of each substrate and controlling the substrate surface liquid crystal alignment film material is first applied on the facing surfaces of the pair of substrates, and then dried and cured to form an alignment film on each surface, and preferential orientation is given by rubbing the surface of the alignment film with a nylon cloth or the like (rubbing method).
An inorganic alignment film or an organic alignment film may be used as the alignment film for the above purpose. Oxides, organic silanes, metals, and metallic complexes are examples of the inorganic alignment film materials. As the organic alignment film materials, polyimide resins are widely used; by rubbing the polyimide film surface formed on the substrate, the liquid crystal molecules can be aligned in a given direction.
Of such liquid crystal displays, thin-film transistor (TFT) liquid crystal displays (TFT-LCDs) are constructed using twisted nematic liquid crystals. In the TN liquid crystal display, the liquid crystal molecules are arranged with their long axes lying substantially parallel to the pair of substrates and gradually twisting through 90° between them; when a voltage is applied between electrode conductive lines formed on the respective substrates and an electric field is formed in a direction perpendicular to the substrates, the molecular alignment is altered with the liquid crystal molecules being caused to line up in the direction of the electric field by virtue of the dielectric anisotropy of the liquid crystal, thus producing a change on the optical refractive index within the liquid crystal layer.
In such a TN liquid crystal display, since the liquid crystal molecules have the property of refractive index anisotropy (birefringence), a phenomenon occurs in which the contrast varies depending on the angle at which the observer views the screen of the liquid crystal display. This phenomenon will be explained with reference to
FIGS. 1
,
2
, and
3
.
FIGS. 1 and 2
are a plan view and a perspective view, respectively, of a typical TN liquid crystal display, and
FIG. 3
shows a cross section taken along line F-F′ in FIG.
1
. The liquid crystal display is an active matrix display, and includes a pair of wiring substrates
131
and
132
and a liquid crystal layer
133
sandwiched between them. One wiring substrate
131
includes of a glass substrate
111
a
, a transparent pixel electrode
114
, and an alignment film
116
a
, while the other wiring substrate
132
includes of a glass substrate
111
b
, a transparent counter-electrode
115
, and an alignment film
116
b.
The edges of the two wiring substrates
131
and
132
are sealed with a resin or the like (not shown) in such a manner as to surround the liquid crystal layer
133
. Peripheral circuits for driving the liquid crystal layer
133
, etc. are mounted outward of the sealing resin. Around the pixel electrode
114
are arranged scanning lines
112
and signal lines
113
intersecting with each other. Electrical signals are applied to the scanning line
112
and signal line
113
connected to the pixel electrode
114
to drive the liquid crystal layer through a TFT
120
.
Liquid crystal molecules
133
a
in the liquid crystal layer
133
placed between the two wiring substrates
131
and
132
are oriented in such a manner that they twist through 90° between the two substrates
131
and
132
, the average orienting direction of the liquid crystal molecules projected on the substrate being substantially parallel to the direction of line F-F′. Also, the liquid crystal molecules
133
a
have a pretilt angle &dgr; with respect to the substrates
131
and
132
. This pretilt angle &dgr; is provided to prevent the occurrence of disclination lines due to multidomain; because of the pretilt angle &dgr;, when a voltage is applied between the pixel electrode
114
and the counter electrode
115
, the liquid crystal molecules
133
a
line up uniformly in the direction of the pretilt angle &dgr;. In
FIG. 2
, the arrow
134
indicates the rubbing direction of the substrate
131
and the arrow
135
the rubbing direction of the substrate
132
, while the arrow
136
indicates the positive viewing direction. Such an arrangement is also employed in liquid crystal displays of other types than the active matrix.
In conventional liquid crystal displays, however, since the direction in which the liquid crystal molecules line up when an electric field is applied is predetermined, a phenomenon occurs in which the contrast varies depending on the angle at which the observer views the liquid crystal display. The reason why this phenomenon occurs will be explained with reference to
FIG. 4
showing the voltage-transmittance (V-T) characteristics of a normally white mode liquid crystal display which produces a white display when no voltage is applied. Here, when the liquid crystal molecules
133
a
are viewed from the &thgr;1 side in
FIG. 3
, the viewing direction is said to be the positive viewing direction, and when viewed from the &thgr;2 side, it is said to be the negative viewing direction.
When the liquid crystal display is viewed from directly above (from a direction perpendicular to the substrate plane), a V-T characteristic such as shown by solid line L
1
in
FIG. 4
is obtained. As can be seen, as the applied voltage value increases, the light transmittance decreases until it becomes substantially zero at a certain applied voltage value, at voltages above which the transmittance remains substantially zero.
On the other hand, the viewing angle is shifted from the direct-above position toward the positive viewing direction (&thgr;1 side in FIG.
3
), a V-T characteristic such as shown by solid line L
2
in
FIG. 4
is obtained. As can be seen, the light transmittance decreases with increasing applied voltage until the voltage reaches a particular value, from which point the transmittance begins to increase and then gradually decreases. This means that at a particular angle of light incidence (viewing angle) the liquid crystal molecules are tilted in the same direction and the refractive index anisotropy of the liquid crystal molecules is lost, resulting in the loss of the optical rotatory power. That is, at a particular viewing angle an inversion phenomenon (contrast reversal) occurs in which the dark and light parts of an image appear as light and dark, respectively.
Conversely, when the viewing angle is shifted toward the negative viewing direction (&thgr;2 side in FIG.
3
), the refractive index of the liquid crystal molecules becomes difficult to change and the V-T characteristic shown by solid line L
3
in
FIG. 4
is obtained, which indicates that the light transmittance is hard to change. As a result, contrast between black and white drops markedly.
More specifically, when the applied voltage is zero or relatively low, the center molecule
133
a
appears as an ellipse to the observer
137
positioned in the positive viewing direction, as shown in FIG.
5
A. When the applied voltage is gradually increased, the center molecule
133
a
tilts toward the direction of the electric field and there is an instant in time at which the center molecule
133
a
appears as a true circle to the observer
137
, as shown in FIG.
5
B. At this time, the light transmittance is the highe
Hirata Mitsuaki
Mizushima Shigeaki
Watanabe Noriko
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Ton Toan
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