Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-06-28
2001-10-23
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S117000, C349S118000
Reexamination Certificate
active
06307608
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display having a wide viewing angle range.
Conventional liquid crystal displays generally use TN (Twisted Nematic) or STN (Super Twisted Nematic) type apparatuses.
Each of these liquid crystal displays comprises a liquid crystal cell having a liquid crystal layer in which liquid crystal molecules are twisted at a predetermined twist angle between a pair of transparent substrates on the front and rear sides, each of which has a transparent electrode on its inner surface, and front and rear polarizers sandwiching the liquid crystal cell.
However, the TN- or STN-type liquid crystal display has a narrow observation angle range, i.e., viewing angle in which display with satisfactory contrast can be observed.
More specifically, in the conventional TN-type liquid crystal display, light obliquely incident on the liquid crystal display with respect to light incident from the normal direction has an optical path length through the liquid crystal layer, which changes depending on the incident angle. For this reason, retardation corresponding to the incident angle occurs, and the transmittance changes depending on the direction of light incidence, i.e., the observation direction. Hence, the observation angle range, i.e., viewing angle in which display with satisfactory contrast can be observed becomes narrow, and the display suffers color tinge.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a liquid crystal display capable of obtaining a wide viewing angle.
In order to achieve the above object, according to the first aspect of the present invention, there is provided a liquid crystal display comprising:
a liquid crystal cell having alignment films formed on opposing surfaces of a pair of front and rear substrates having electrodes formed on inner surfaces, respectively, so as to be aligned in predetermined directions, and a liquid crystal layer provided between the pair of substrates;
a pair of polarizers arranged on the front side as an observation side of the liquid crystal cell and the rear side as an opposing side, respectively, so as to have absorption axes making predetermined angles with aligning treatment directions of the alignment films formed on the adjacent substrates;
a first retardation film inserted between the liquid crystal cell and the polarizer on the front side of the liquid crystal cell so as to have an axial direction, in which a refractive index is minimized, in a direction tilted in a predetermined direction with respect to a normal line to a film plane, and a direction component along the film plane in the axial direction, which is set along the direction of the aligning treatment of the alignment film formed on the substrate on the front side of the liquid crystal cell;
a second retardation film inserted between the liquid crystal cell and the polarizer on the rear side of the liquid crystal cell so as to have an axial direction, in which a refractive index is minimized, in a direction tilted in a predetermined direction with respect to a normal line to a film plane, and a direction component along the film plane in the axial direction, which is set along the direction of the aligning treatment of the alignment film formed on the substrate on the rear side of the liquid crystal cell; and
a third retardation film having refractive index anisotropy in a film plane and inserted to at least one of a position between the first retardation film and the front polarizer and a position between the second retardation film and the rear polarizer so as to have a direction in which a refractive index in the film plane is maximized is substantially parallel or perpendicular to the absorption axis of the adjacent polarizer.
According to the liquid crystal display of the first aspect of the present invention,
when a voltage is applied to the electrodes sandwiching the liquid crystal layer, residual retardation generated due to the influence of liquid crystal molecules which are not sufficiently aligned in the application direction near the front and rear substrates of the liquid crystal layer is compensated by the first and second retardation films, and
the retardation difference that changes depending on the incident angle when light that has passed through one of the polarizers passes through the liquid crystal layer is compensated by the third retardation film.
Hence, the liquid crystal display of the present invention can realize bright display and dark display within a wide viewing angle range and can also realize display with high contrast as the ratio of the maximum transmittance to the minimum transmittance.
In the liquid crystal display of the present invention, each of the first and second retardation films comprises a discotic liquid crystal layer in which liquid crystal molecules are aligned to gradually rise from an aligned state along one surface, from one surface toward the other surface of the first or second retardation film, and has the axial direction, in which the refractive index is minimized, in a direction along an average molecular axis direction of discotic liquid crystal molecules in an intermediate direction in a direction of thickness of the discotic liquid crystal layer, and the direction component along the film plane in the axial direction of each of the first and second retardation films is set parallel to the direction of the aligning treatment of the corresponding adjacent substrate.
According to this liquid crystal display, the discotic liquid crystal layers of the first and second retardation films can compensate residual retardation due to the liquid crystal molecules near the front and rear substrates.
In the liquid crystal display of the present invention,
the liquid crystal cell in which the direction of aligning treatment of the alignment film formed on the front substrate is perpendicular to that of the alignment film formed on the rear substrate has the twisted liquid crystal layer sandwiched by the substrate,
the pair of polarizers have the absorption axes parallel to the direction of aligning treatment of the alignment films formed on the adjacent substrates of the liquid crystal cell; and
the third retardation film has the direction in which the refractive index in the film plane is maximized in substantially parallel to the absorption axis of the adjacent polarizer.
According to this liquid crystal display, when light that has passed through one of the polarizers passes through the liquid crystal layer, the retardation difference due to the difference in incident angle is compensated by the third retardation film, so the transmittance does not change depending on the incident angle. In addition, when a voltage is applied to the electrodes sandwiching the liquid crystal layer, the first retardation film compensates residual retardation due to the alignment of liquid crystal molecules near the front substrate, and the second retardation film compensates residual retardation due to the alignment of liquid crystal molecules near the rear substrate. Hence, a decrease in contrast due to the residual retardation can be prevented.
More specifically, retardation of light that becomes incident on the liquid crystal layer obliquely with respect to the normal direction to the liquid crystal cell is different from that of light incident in the normal direction. When this retardation difference is almost canceled by refractive index anisotropy in the film plane of the third retardation film when the light passes through the film, the retardation difference generated when the light passes through the liquid crystal layer can be compensated.
Hence, irrespective of the incident angle, when a voltage is applied to the electrodes, the light that has passed through the rear polarizer is polarized to oscillate in a direction almost parallel to the absorption axis of the front polarizer, so the light is absorbed. When no voltage is applied to the electrodes, the light is polarized to oscillate in a direction perpendicular to the absorption
Casio Computer Co. Ltd.
Chowdhury Tarifur R.
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Sikes William L.
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