Liquid crystal cells – elements and systems – Cell containing liquid crystal of specific composition – In nematic phase
Reexamination Certificate
2000-11-10
2003-07-29
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Cell containing liquid crystal of specific composition
In nematic phase
C349S081000, C349S084000, C349S177000, C349S076000, C345S038000
Reexamination Certificate
active
06600545
ABSTRACT:
This application is based on Japanese Patent Application Hei 11-331500 filed on Nov. 22, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a twist-nematic (TN) liquid crystal display, and in particular, to a twisted-nematic liquid crystal display of which display quality is improved as compared with the prior art.
In a nematic state of liquid crystal, liquid crystal molecules linearly elongated in a rod-like shape have optical axes aligned in one direction or orientation.
In a twist-nematic state of liquid crystal, all liquid crystal molecules sandwiched between two substrate surfaces are aligned in parallel with the substrate surfaces, but their orientation is 90° twisted between both substrate surfaces. Therefore, the orientation of alignment of liquid crystal molecules are continuously changed by a total of 90° between the substrate surfaces.
2. Description of the Related Art
FIG.
5
(A) and FIG.
5
(B) show schematic configurations of a twist-nematic liquid crystal display.
As can be seen in FIG.
5
(A), a twist-nematic liquid crystal display A includes two glass substrate
101
and
103
which are arranged to each other and apart from each other by predetermined distance d and which are opposing to each other, two transparent electrodes
105
and
107
formed on inner surfaces respective of the glass substrates
101
and
103
, nematic liquid crystal EM (to be represented as Np liquid crystal herebelow) which is sandwiched between two glass substrates and which is positive in dielectric anisotropy, and a pair of polarizing plates
111
and
113
disposed outside the glass substrates
101
and
103
, respectively.
Two glass substrates
101
and
103
, tow transparent electrodes
105
and
107
, and molecules of the nematic liquid crystal EM form pixels
115
. The transparent electrode
105
is a pixel electrode of segment type and the transparent electrode
107
is a shared or common electrode. The system further includes a voltage source
117
to apply a desired voltage between the transparent electrodes
105
and
107
. The system includes a plurality of transparent pixel electrodes
105
. FIG.
5
(
a
) shows a state in which no voltage is applied between the transparent electrodes
105
and
107
.
A large number of molecules of the nematic liquid crystal EM in the cell
115
has a twist pitch which is sufficiently larger than a wavelength of visible light. When linearly polarized light is vertically incident to the glass substrate
103
, the light is polarized 90° (90° polarization) along the twisted state of the molecules of the nematic liquid crystal EM while passing through the cell
115
. In the cell
11
, when two polarizing plates
111
and
113
have polarizing axes in a parallel Nicols state, namely, when the polarizing axes are parallel to each other, namely, the light is interrupted.
FIG.
5
(B) shows a state in which a voltage is applied between the transparent electrodes
105
and
107
.
When a predetermined voltage applied to the pixel
115
from the voltage source
117
and the voltage exceeds a threshold voltage Vth, long axes of liquid crystal molecules EM start changing their direction toward a direction of an electric field associated with the voltage.
When the voltage is about twice the threshold voltage Vth, the long axes of liquid crystal molecules in other than the neighborhood of surfaces of the transparent electrodes
105
and
107
are uniformly re-aligned to be parallel to the direction of the electric field and the polarization of 90° is lost. In this state, contrary to the state in which no voltage is applied between the transparent electrodes
105
and
107
, light is allowed to pass therethrough in the parallel Nicols state.
In the twist-nematic liquid crystal display A, when the polarizing plates
111
and
113
are arranged in the parallel Nicols state, the liquid crystal display can be operated in a normally black mode.
The liquid crystal molecules EM are sandwiched between two glass substrates
101
and
103
of the display A. To increase contrast between white and black in displayed images, thickness d of the cell of liquid-crystal molecules EM is determined as follows.
When 0 voltage is applied between the transparent electrodes
105
and
107
, the pixels of the liquid-crystal display has transimittivity expressed as
T
=sin
2
{&pgr;(1+
u
2
)
0.5
/2}/(1
+u
2
) (3)
where, u=2&Dgr;nd/&lgr; and &Dgr;n=(n
p
−n
1
).
In expression (3), &lgr; is a wavelength of the incident light and n
p
is a refractive index of liquid crystal molecules in an axial direction parallel to the long axis of the liquid crystal molecules, and N
l
is a refractive index of the liquid crystal molecules in an axial direction vertical to the long axis of the liquid crystal molecules. By determining the value of d for which the transmittivity T of the pixels takes a minimum (minimal) value in expression (3), images can be displayed with a high contrast ratio.
The function of expression (3) is a periodic function. Therefore, the transmittivity takes a plurality of minimal values. Namely, a plurality of values of d exist for the minimal values. Usually, a first or second minimal value of d at a lower order is used in ordinary cases.
In the liquid crystal displays using twist-nematic liquid crystal, images are displayed in primarily two ways, namely, display of segment type and display of matrix type.
The liquid crystal display of the segment type includes a relatively small number of segment-type pixel electrodes and is suitable when a limited number of symbols, numerals, and the like are repetitiously displayed. The liquid crystal displays of segment type are broadly used, for example, for desktop computers (calculators), watches, measuring instruments, computer game machines, and devices to display bar graphs.
In the liquid crystal display of matrix type, a large number of pixel electrodes are arranged, for example, in a contour of a simple matrix. Liquid crystal displays of matrix type are suitably used to display complex images and the like and are employed, for example, for personal computers and television sets.
The liquid-crystal displays are operated by static or multiplex driving.
The static driving is mainly employed when the display includes a small number of pixel electrodes. Each of the pixel electrode of the liquid crystal display receives a voltage individually from a driving circuit (driver). During a display period, the driver continuously applies a display voltage Vs between segment-type pixel electrodes of pixels to be displayed and the common electrode. All segment-type pixel electrodes can be simultaneously driven.
In the liquid-crystal display operated by static driving, 0 volt is applied to segment-type pixel electrodes for a non-display section (not selected for the display operation). Namely, this voltage is the same as that applied to a base section in which no segment-type electrode exists and only a common electrode exsist. The transmittivity and hue in the non-display section are almost the same as those in the base section. Namely, the non-display section and the base section can be readily discriminated from a display section.
The multiplex driving is also called time-division driving or dynamic driving and is used, for example, in a liquid crystal display of segment type including a relatively large number of segment-type pixel electrodes to be displayed. The multiplex driving is also applied to a liquid crystal display of matrix type having a large number of pixel electrodes.
FIGS,
6
and
7
A-
7
C show an example of the multiplex driving in a liquid crystal display.
FIG. 6
shows wiring of a liquid crystal display B in which numerals are displayed at 12 places or positions. Each numeral is displayed with seven segments by multiplex driving.
FIG. 6
shows an example in which only three places of numerals are displayed.
For each place, a common electrode
105
is subdivi
Chowdhury Tarifur R.
Frishauf Holtz Goodman & Chick P.C.
Stanley Electric Co. Ltd.
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