Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-09-30
2001-05-01
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06226064
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, in particular to a liquid crystal display apparatus which is used as a display apparatus of direct-view-type or projection-type.
2. Description of the Related Art
A liquid crystal display apparatus has such s structure that liquid crystal molecules are sandwiched between two substrates on which electrodes are formed, and a predetermined voltage is applied to the electrodes to control orientation directions of liquid crystal molecules for display.
As liquid crystals used for such liquid crystal display apparatus there exist ones having a positive dielectric constant anisotropy and ones having a negative dielectric constant anisotropy.
In the case where the liquid crystal having a positive dielectric constant anisotropy (hereinafter referred to as P-type liquid crystal) is used, horizontal orientation films are formed on substrate faces where the electrodes are formed, so that the liquid crystal molecules are oriented substantially parallel to the substrate faces at the time of non-select voltage application between the electrodes.
On the other hand, in the case where the liquid crystal having a negative dielectric constant anisotropy (hereinafter referred to as N-type liquid crystal) is used, perpendicular orientation films are formed on the substrate faces where the electrodes are formed, so that the liquid crystal molecules are oriented substantially perpendicular to the substrate faces at the time of non-select voltage application between the electrodes.
Although the N-type liquid crystal is generally useful for widening of viewing angle and increasing of response speed as compared with the P-type liquid crystal, and in practical application to display apparatuses it has not been so widely used for the reasons of difficulty in obtaining stable perpendicular orientation, unavailability of N-type liquid crystal of high reliability, etc.
In recent years, however, a perpendicular orientation film using a polyimide type material is available and thereby stable perpendicular orientation is achieved, and furthermore a liquid crystal material containing fluorine as a functional group has come on the market to make the N-type liquid crystal of high reliability obtainable, so that the development of the liquid crystal display apparatus using the N-type liquid crystal has been increasingly pursued.
In general, in the case where the N-type liquid crystal is used, as a voltage is applied to the liquid crystal, the liquid crystal molecules tilt to be substantially parallel to the substrate surface. However, if the tilt directions of the liquid crystal molecules are not uniform, disclination occurs due to irregular orientation of the liquid crystal molecules, with the result that display quality is largely lowered.
In order to avoid such a problem, in the case where the N-type liquid crystal is used, the major axial direction of the liquid crystal molecules is conventionally caused to be slightly inclined from the normal direction of the substrate surface to form a tilt angle &thgr;, at the time of non-select voltage application.
Here, the tilt angle &thgr; is, as shown in
FIG. 4
, an angle formed by the major axial direction
33
of the liquid crystal molecules
32
with respect to the normal direction
31
of the substrate
30
.
The prior art disclosed in Japanese Unexamined Patent Publication JP-A 6-337421 (1994) relates to a reflective-type liquid crystal display apparatus comprising a polarizer, a phase difference plate and a liquid crystal cell, which are arranged in this order. The liquid crystal cell is structured so that nematic liquid crystal molecules with a negative dielectric constant anisotropy are perpendicularly oriented and sandwiched between a pair of substrates. With respect to the liquid crystal molecules in vicinities of upper and lower substrates, the orientation directions of the liquid crystal molecules coincide with each other while the tilt angles thereof differ from each other. An optical axis of the phase difference plate is arranged in a direction intersecting at right angles with the orientation direction of the substrate disposed near the phase difference plate. A retardation value of the phase difference plate is selected in a range of 110 nm to 140 nm. The art disclosed in the Publication comprises a phase difference plate having a larger phase difference, whereby an improved viewing angle characteristic is attained.
Further, prior art disclosed in Japanese Unexamined Patent Publication JP-A 9-133913 (1997) also relates to a reflective-type liquid crystal display apparatus comprising a polarizer, a phase difference plate and a liquid crystal cell, which are arranged in this order, the apparatus further comprising at least one of light-diffusing means and light-gathering means, whereby an improved viewing angle characteristic is attained.
The projection-type liquid crystal display apparatus needs to have a panel of small size for miniaturizing the apparatus itself and reducing the cost for an optical system, for example. As the panel size becomes smaller, the definition of the liquid crystal display apparatus becomes higher, so that adjacent pixels becomes very proximate to each other. Therefore, there is a problem that a lateral electric field may be created among the adjacent pixel electrodes to newly cause orientation irregularity of the liquid crystal molecules, thereby causing a decrease in luminance.
With regard to the liquid crystal display apparatus using the N-type liquid crystal, a larger angle is often selected as the tilt angle &thgr; in order to prevent the liquid crystal molecules from being irregularly oriented. However, in the case where a larger angle is selected as the tilt angle &thgr;, there is a problem that a phase difference is generated due to the tilt of the liquid crystal molecules at the time of non-select voltage application, and light leakage is thereby caused and the liquid crystal display apparatus has significantly lowered contrast.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a liquid crystal display apparatus using N-type liquid crystal of which luminance and contrast is prevented from being lowered.
The present invention provides a liquid crystal display apparatus comprising at least a liquid crystal cell in which liquid crystal molecules having a negative dielectric constant anisotropy are sandwiched between two electrode substrates which confront each other and each have a perpendicular orientation film on a surface thereof,
a major axial direction of the liquid crystal molecules in a state of non-select voltage application between the electrodes being tilted toward a normal direction of the electrode substrates,
a phase difference plate for compensating a phase difference generated due to the tilt of the liquid crystal molecules being disposed adjacent to the liquid crystal cell.
Further it is preferable in the invention that a phase difference of the phase difference plate is set to be one to four times the phase difference generated due to the tilt of the liquid crystal molecules.
Further it is preferable in the invention that a tilt angle &thgr; formed by the major axial direction of the liquid crystal molecules in the state of non-select voltage application between the electrodes and the normal direction of the electrode substrates is set to be in a range of 1° to 15°.
Further it is preferable in the invention that the phase difference plate is made of norbornene resin.
According to the invention, since the liquid crystal display apparatus comprises the phase difference plate for compensating the phase difference due to the tilt angle &thgr; of the liquid crystal molecules in non-select voltage application, it is possible to prevent lowering of contrast and to obtain both high luminance and high contrast.
Since the phase difference plate is required to compensate at least the phase difference generated due to the tilt angle &thgr;, it is desirable that the phase differen
Shimada Shinji
Yoshikawa Hiroshi
Conlin David G.
Daley, Jr. William J.
Dike Bronstein, Roberts & Cushman LLP
Parker Kenneth
Sharp Kabushiki Kaisha
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