Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-09-13
2002-07-23
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S122000, C349S178000
Reexamination Certificate
active
06424398
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquid-crystal display, and in particular, to a color liquid-crystal display in which molecules of liquid crystal are aligned vertically (homeotropically) between two substrates when an electric field is not applied to the liquid crystal molecules and in which each pixel is divided into a plurality of domains.
RELATED ART
FIGS. 10A
to
10
C are cross-sectional diagrams respectively showing black display, intermediate-tone display, and white display states of a prior-art liquid-crystal display of homeotropic alignment type in which liquid-crystal molecules are aligned homeotropically. A space or gap between a pair of substrates
100
and
101
is filled with liquid-crystal material
102
having a negative dielectric constant anisotropy. Polarizers are disposed respectively on outer surfaces of the substrates
100
and
101
such that polarization axes thereof are orthogonal to each other.
As shown in
FIG. 10A
, in a state in which a voltage is not applied to the liquid-crystal display, the liquid-crystal molecules
102
are aligned in a direction vertical or orthogonal to the substrates
100
and
101
and hence black display state is achieved. When a voltage is applied across the substrates
100
and
101
, the liquid-crystal molecules
102
are aligned in a direction horizontal to the substrates
100
and
101
as shown in FIG.
10
C. In this situation, a direction of polarization of light passing the liquid-crystal layer rotates to achieve white display state.
As shown in
FIG. 10B
, when a voltage lower than that applied for the white display state is applied across the substrates
100
and
101
, the liquid-crystal molecules
102
are aligned in an inclined direction with respect to the substrates
100
and
101
. A display state of an intermediate tone or color can be obtained by light L
1
propagating in a direction vertical to the substrates
100
and
101
. For light L
2
propagating from a lower-right corner to an upper-left corner in
FIG. 10B
, an effect of birefringence of the liquid-crystal layer rarely occurs. Therefore, the screen is black when viewed in a direction from the upper-left corner to the lower-right corner. Conversely, for light L
3
propagating from a lower-left corner to an upper-right corner in
FIG. 10B
, the effect of birefringence of the liquid-crystal layer considerably takes place. Therefore, the screen has a color like white when viewed in a direction from the upper-right corner to the lower-left corner. As above, in an ordinary liquid-crystal display of homeotropic type, the visual angle characteristic is lowered in the intermediate-tone display state.
To improve the visual angle characteristic, a liquid-crystal display of multidomain type has been proposed in which each pixel is divided into a plurality of domains. In the display of this type, liquid-crystal molecules are aligned in one direction in each domain in the intermediate-tone display state, and the alignment direction of liquid-crystal molecules in a domain is different from that in the adjacent domains. Referring to
FIGS. 11A and 11B
, description will be given of an example of a structure and operation of a liquid-crystal display of multidomain and homeotropic type (multidomain, vertically alignment (MVA) type).
FIG. 11A
shows a cross section of a liquid-crystal display in a no-voltage state in which a voltage is not applied thereto. A first protrusion pattern
16
is disposed on an inner surface of a glass substrate
1
. A second protrusion pattern
18
is disposed on an inner surface of an opposing substrate
36
facing the glass substrate
1
. The first and second protrusion patterns
16
and
18
are arranged alternately. On the opposing surfaces of the glass substrate
1
on which a thin-film transistor (TFT) is formed and the opposing substrate
36
, vertical alignment films
28
are respectively formed to cover the protrusion patterns
16
and
18
. A space between the glass substrate
1
and the opposing substrate
36
is filled with liquid-crystal material
29
including liquid-crystal molecules
30
. The molecules
30
have a negative dielectric constant anisotropy. On outer surfaces of the glass substrate
1
and the opposing substrate
36
, a polarizer
31
and a polarizer
32
are respectively disposed in a cross-Nicol layout.
In a voltage-applied state in which a voltage is applied to the display, the liquid-crystal molecules are aligned in a direction vertical to the surfaces of the substrates
1
and
36
. On inclined surfaces or planes of the first and second protrusion patterns
16
and
18
, the liquid-crystal molecules
30
a
tend to be aligned in a direction vertical to the inclined plane associated therewith. Consequently, these molecules
30
a
are aligned obliquely with respect to the substrate surfaces. However, since the molecules
30
are aligned in the vertical direction in a wide area of the pixel, black display state is achieved satisfactorily.
FIG. 11B
shows a cross-sectional view in an intermediate-tone display state in which a voltage is applied thereto to incline the direction of the liquid-crystal molecules. The liquid-crystal molecules
30
a
are inclined greater in inclined directions. Liquid-crystal molecules
30
around the inclined molecules
30
a
are also inclined in the same directions under the influence of the inclination thereof. Therefore, liquid-crystal molecules
30
between the first and second protrusion patterns
16
and
18
are aligned such that a longitudinal axis (a director) of each molecule
30
is aligned in a direction to an upper-right corner of FIG.
11
B. Liquid-crystal molecules
30
on the left side of the first protrusion pattern
16
and those on the right side of the left protrusion pattern
18
are aligned such that the director of each molecule
30
is aligned in a direction to a lower-right corner of FIG.
11
B.
As above, there are defined in one pixel a plurality of domains. The direction of inclination of liquid-crystal molecules in the domains varies from each other. The first and second protrusion patterns
16
and
18
define boundaries between the domains. By disposing the first and second protrusion patterns
16
and
18
in parallel to the substrate surfaces, two kinds of domains can be defined. By bending each protrusion pattern by 90°, four kinds of domains are defined. When a plurality of domains are defined in each pixel, the visual angle characteristic can be improved in the intermediate-tone display state.
The MVA-type liquid-crystal display obtains white display state using the effect of birefringence of the liquid-crystal material as described for FIG.
10
C. Since the effect of birefringence has wavelength dispersion, the transmittance varies among the red (R), green (G), and blue (B) pixels in the white display state. This leads to coloring of the display screen.
FIG. 12
shows a relationship between the transmittance and a cell gap for each of the red, green, and blue pixels. The abscissa represents the cell gap in micrometer (&mgr;m) and the ordinates represents the transmittance in percent (%). The transmittance is an overall transmittance of the entire liquid-crystal panel including the polarizers. The red, green, and blue pixels are equal in an opening ratio to each other. When the cell gap is set to a value from 4 &mgr;m to 4.5 &mgr;m for which the transmittance of the green pixel takes a maximum value, the transmittance of the blue pixel is lower than those of the red and green pixels. Therefore, the overall display screen becomes yellowish in the white display state.
DESCRIPTION OF THE INVENTION
It is therefore an object of the present invention to provide a liquid-crystal display capable of minimizing the coloring of the display screen in the white display state.
According to one aspect of the present invention, there is provided a liquid-crystal display, comprising a first substrate and a second substrate disposed in parallel to said first substrate with a gap therebetween; liquid-crystal material filled in a space be
Duong Tai V.
Fujitsu Limited
Greer Burns & Crain Ltd.
Sikes William L.
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