Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-06-20
2003-03-18
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S177000
Reexamination Certificate
active
06535258
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquid crystal display device, especially, to a liquid crystal display device with the viewing angle dependency of the display screen abated by a combination of a liquid crystal display element and an optical retardation compensator plate.
BACKGROUND OF THE INVENTION
Conventionally, liquid crystal display devices incorporating nematic liquid crystal display elements have been in widespread use for numeral-segment-type display devices such as watches and calculators, and recently the applications are finding more places with word processors, notebook-type personal computers, liquid crystal televisions mounted in automobiles, etc.
Generally, a liquid crystal display element has a transparent substrate, electrode lines for turning on and off pixels, and other components formed on the substrate. For example, in an active-matrix type liquid crystal display device, active elements, such as thin-film transistors, are formed on the substrate together with the electrode lines as switching means for selectively driving pixel electrodes by which voltages are applied across the liquid crystal. Moreover, in liquid crystal display devices capable of color display, color filter layers having colors such as red, green and blue are provided on the substrate.
Liquid crystal display elements such as the one mentioned above adopt a liquid crystal display mode that is suitably selected depending on the twisted angle of the liquid crystal: some of well-known modes are active-driving-type twisted nematic liquid crystal display mode (hereinafter, referred to as the TN mode) and the multiplex-driving-type super-twisted nematic liquid crystal display mode (hereinafter, referred to as the STN mode).
The TN mode displays images by orientating the nematic liquid crystal molecules to a 90°-twisted state so as to direct rays along the twisted directions. The STN mode utilizes the fact that the transmittance is allowed to change abruptly in the vicinity of the threshold value of the applied voltage across the liquid crystal by expanding the twist angle of the nematic liquid crystal molecules to not less than 90°.
The problem with the STN mode is that the background of the display screen sustains a peculiar color due to interference between colors because of the use of the birefringence effect of liquid crystal. In order to solve this problem and to provide a proper black-and-white display in the STN mode, the application of an optical retardation compensator plate is considered to be effective. Display modes using the optical retardation compensator plate are mainly classified into two modes, that is, the double layered super-twisted nematic optical-retardation compensation mode (hereinafter, referred to as the DSTN mode) and the film-type optical-retardation compensation mode (hereinafter, referred to as the film-addition mode) wherein a film having optical anisotropy is provided.
The DSTN mode uses a two-layered construction that has a display-use liquid crystal cell and a liquid crystal cell which are orientated with a twist angle in a direction opposite to that of the display-use liquid crystal cell. The film-addition mode uses a construction wherein a film having optical anisotropy is disposed. Here, the film-addition mode is considered to be more prospective in the standpoint of light weight and low costs. Since the application of such an optical-retardation compensation mode makes it possible to improve black-and-white display characteristics, color STN liquid crystal display devices have been achieved that enable color display by installing color-filter layers in STN-mode display devices.
The TN modes are, on the other hand, classified into the Normally Black mode and the Normally White mode. In the Normally Black mode, a pair of polarizer plates are placed with their polarization directions in parallel with each other, and black display is provided in a state where no ON voltage is applied across the liquid crystal layer (OFF state). In the Normally White mode, a pair of polarizer plates are placed with their polarization directions orthogonal to each other, and white display is provided in the OFF state. Here, the Normally White mode is considered to be more prospective from the standpoints of display contrast, color reproducibility, viewing angle dependency, etc.
However, in the TN-mode liquid crystal display device, liquid crystal molecules have a refractive index anisotropy &Dgr;n, and are orientated so as to incline to the above and below substrates. For these reasons, the viewing angle dependency increases: i.e., the contrast of displayed images varies depending upon the direction and angle of the viewer.
FIG. 11
schematically shows the cross-sectional construction of a TN liquid crystal display element
31
. This state shows liquid crystal molecules
32
slanting upward slightly as a result of application of a voltage for halftone display. In such a liquid crystal display element
31
, a linearly polarized ray
35
passing through the surfaces of a pair of substrates
33
and
34
along the normals thereto, and linearly polarized rays
36
and
37
passing through those surfaces not along the normals thereto cross the liquid crystal molecules
32
at different angles. Besides, the liquid crystal molecules
32
have a refractive index anisotropy &Dgr;n. Therefore, the linearly polarized rays
35
,
36
and
37
, upon passing through the liquid crystal molecules
32
in different directions, produce ordinary and extraordinary rays. The linearly polarized rays
35
,
36
and
37
are converted to elliptically polarized rays according to the phase difference between the ordinary and extraordinary rays, which cause the viewing angle dependency.
In addition, in an actual liquid crystal layer, the liquid crystal molecules
32
show different tilt angles in the vicinity of the midpoint between the substrates
33
and
34
and in the vicinities of the substrates
33
and
34
. The liquid crystal molecules
32
are twisted by 90° around the normal.
For those reasons described so far, the linearly polarized rays
35
,
36
and
37
passing through the liquid crystal layer are affected by the birefringence effect in various ways depending upon, for example, the directions and the angles thereof, resulting in complex viewing angle dependency.
Such viewing angle dependency can be observed, as examples, in the following situations. If the viewing angle increases from the normal to the display screen in the standard viewing direction, i.e. downward, and exceeds a certain angle, the displayed image has a distinct color (hereinafter, referred to as the coloration phenomenon), or is reversed in black and white (hereinafter, referred to as the tone reversion phenomenon). If the viewing angle increases from the normal in the opposite viewing direction, i.e. upward, the contrast decreases abruptly.
The aforementioned liquid crystal display device has another problem that the effectual range of viewing angle narrows with a larger display screen. When a large liquid crystal display device is viewed from a short distance in the front thereof, the same color may appear different in the uppermost and lowermost parts of the large screen due to the effect of the viewing angle dependency. This is caused by a wider range of viewing angle required to encompass the whole screen surface, which is equivalent to a viewing direction which is increasingly far off center.
To restrain the viewing angle dependency, Japanese Laid-Open Patent Application NO. 55-600/1980 (Tokukaisho 55-600) and No. 56-97318/1981 (Tokukaisho 56-97318) suggest that an optical retardation compensator plate (retardation compensator film) be inserted as an optical element having optical anisotropy between the liquid crystal display element and one of polarize plates.
According to the method, the elliptically polarized ray converted from a linearly polarized ray by passing through liquid crystal molecules having refractive index anisotropy is directed through the optical retardation compensator plat
Conlin David G.
Dike Bronstein Roberts & Cushman IP Group
Parker Kenneth
Sharp Kabushiki Kaisha
Tucker David A.
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