Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1997-11-06
2003-06-24
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06583839
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display unit, and more particularly to an in-plane-switching (IPS) active-matrix liquid crystal display unit.
2. Description of the Related Art
Liquid crystal display (LCD) units are generally characterized by low-profile shapes, lightweight structures, and low-power requirements. Particularly, active-matrix liquid crystal display (AM-LCD) units which comprise a two-dimensional matrix of pixels energizable by active devices are highly promising as high-image-quality flat panel displays. Among those active-matrix liquid crystal display units which are finding widespread use are thin-film-transistor liquid crystal display (TFT-LCD) units which employ thin-film transistors (TFTs) used as active devices for switching individual pixels.
Conventional AM-LCD units utilize a twisted-nematic (TN) electrooptical effect, and comprise a liquid crystal layer sandwiched between two substrates. The liquid crystal layer is activated when an electric field is applied substantially perpendicularly to the substrates.
U.S. Pat. No. 3,807,831 discloses an in-plane-switching liquid crystal display unit having a liquid crystal layer which is activated when an electric field is applied substantially parallel to two substrates which sandwich the liquid crystal layer therebetween, the liquid crystal display unit including interleaved arrays of alternate parallel electrodes.
Japanese patent publication No. 21907/88 reveals an AM-LCD unit based on a TN electrooptical effect and including interleaved or interdigitating arrays of alternate parallel electrodes for the purpose of reducing parasitic capacitance between a common electrode and a drain bus line or between a common electrode and a gate bus line.
FIG. 1
of the accompanying drawings shows a conventional in-plane-switching liquid crystal display unit. The illustrated conventional liquid crystal display unit comprises a liquid crystal layer sandwiched between two glass substrates
11
,
12
, and interdigitating arrays of alternate parallel electrodes
70
mounted on one of the glass substrates
11
. When a voltage is applied between the electrodes
70
, a liquid crystal activating electric field E
1
is generated parallel to the glass substrates
11
,
12
and perpendicularly to the interdigitating teeth of the electrodes
70
for thereby changing the orientation of liquid crystal molecules
21
. Therefore, the application of the voltage between the electrodes
70
is effective to control the transmittance of light through the liquid crystal layer. The term “the orientation of liquid crystal molecules” used in this specification means the direction of the longer axis of liquid crystal molecules.
With the in-plane-switching liquid crystal display unit shown in
FIG. 1
, it is necessary that when the voltage is applied, the liquid crystal molecules be rotated in a certain direction in order to achieve stable displays. To meet such a requirement, it is customary to initially orient the liquid crystal molecules in a direction that is slightly shifted from a direction perpendicular to the liquid crystal activating electric field. Specifically, the liquid crystal molecules are initially oriented at an angle of &phgr;
LC0
(<90°) with respect to a direction perpendicular to the parallel pairs of the interdigitating teeth of the electrodes. In the specification, the direction of the electric field and the orientation of the liquid crystal molecules will be described in the range of from −90° to 90° (the counterclockwise direction being positive) with respect to a reference direction (&phgr;=0) which is perpendicular to the parallel pairs of the interdigitating teeth of the electrodes. As described later on, in order to accomplish sufficient display contrast, it is necessary to rotate the liquid crystal molecules 45° from the initial orientation. Therefore, it is preferable to orient the liquid crystal molecules at an angle of &phgr;
LC0
in the range of 45°≦&phgr;
LC0
<90°. In the in-plane-switching liquid crystal display unit shown in
FIG. 1
, the initial orientation of the liquid crystal molecules is slightly shifted clockwise (as viewed from the upper substrate
12
) from the parallel pairs of the interdigitating teeth of the electrodes. When the voltage is applied, therefore, the liquid crystal molecules are rotated clockwise as indicated by the arrows.
The transmittance T of light passing through the liquid crystal cell shown in
FIG. 1
which is sandwiched between two confronting polarizers whose axes of polarization transmission (directions of polarization) are perpendicular to each other is expressed by the following equation (1):
T
=
1
2
sin
2
{
2
(
φ
P
-
φ
LC
)
}
sin
2
(
π
Δ
nd
λ
)
(
1
)
where &phgr;
LC
represents the orientation of the liquid crystal molecules when a voltage is applied thereto, &phgr;
P
the direction of the axis of transmission of the polarizer on which the light falls, &Dgr;n the refractive index anisotropy of the liquid crystal layer, d the thickness of the cell (the thickness of the liquid crystal layer, and &lgr; the wavelength of the light. The direction &phgr;
A
of the axis of transmission of the polarizer from which the light exits is expressed by &phgr;
A
=&phgr;
P
+90° or &phgr;
A
=&phgr;
P
−90°. It is possible to control the transmittance of the light by varying the orientation &phgr;
LC
of the liquid crystal molecules with a liquid crystal activating electric field parallel to the substrates based on the above equation (1). If the direction of the axis of transmission of one of the polarizers and the initial orientation of the liquid crystal molecules are in agreement with each other (&phgr;
LC0
=&phgr;
P
or &phgr;
LC0
=&phgr;
A
), then the liquid crystal display unit is brought into a dark display state when no voltage is applied. If the orientation of the liquid crystal molecules is rotated substantially 45° under a liquid crystal activating electric field, then the transmittance becomes highest, and the liquid crystal display unit is brought into a bright display state. Of course, the polarizers may be so arranged that the liquid crystal display unit will be brought into a dark display state when a voltage is applied.
It has been assumed for the sake of brevity that the liquid crystal molecules in the liquid crystal layer between the upper and lower substrates are uniformly rotated. Discussions based on such a simplified model do not essentially affect the principles of the present invention. Actually, however, those liquid crystal molecules which are held in contact with the surfaces of the upper and lower substrates are relatively firmly fixed in position, and do not basically change their orientation, whereas those liquid crystal molecules which are positioned nearly intermediate between the upper and lower substrates change their orientation to a greater extent. In view of these practical considerations, the in-plane angle &phgr;
LC
through which the liquid crystal molecules rotate under an applied electric field is represented as a function of coordinates in the transverse direction of the liquid crystal layer.
In order to accomplish sufficient display contrast, the orientation of the liquid crystal molecules may be rotated substantially 45° in the entire liquid crystal layer. However, for the reasons described above, the liquid crystal molecules which are positioned nearly intermediate between the upper and lower substrates are actually rotated more than 45°.
Published Japanese translation No. 505247/93 of a PCT international publication (International publication No. WO91/10936) describes improvements of angle of view characteristics, which have been poor in TN liquid crystal display devices, achieved by the in-plane-switching liquid crystal display unit. Because of their excellent angle of view characteristics, in-plane-switching active-matrix liquid cryst
Hirai Yoshihiko
Murai Hideya
Nishida Shin-ichi
Suzuki Masayoshi
Suzuki Teruaki
NEC Corporation
Parker Kenneth
Sughrue & Mion, PLLC
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