Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within nematic phase
Reexamination Certificate
1999-03-12
2003-08-12
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within nematic phase
C349S117000
Reexamination Certificate
active
06606143
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device in which the angle of field of the display screen has been improved by combining a liquid crystal display element with a phase element having optical anisotropy.
2. Description of the Related Art
Conventionally, liquid crystal display devices (hereinafter, referred to as LCD devices) using a nematic liquid crystal material have been broadly used as numeral segmented type display devices for watches, clocks, portable calculators, and the like. In recent years, such LCD devices have found applications in broader fields, as display devices for wordprocessors, notebook type personal computers, and the like, as well as liquid crystal monitors for car navigation systems, and the like, for example.
An LCD device of this type includes a pair of light transparent substrates arranged to face each other with a liquid crystal layer interposed therebetween. Electrodes and interconnections for activating and inactivating pixels are formed on the substrates. For example, in an active matrix LCD device, pixel electrodes are arranged in a matrix on one of the substrates for applying a voltage to the liquid crystal layer. Active elements such as field effect transistors are provided on the same substrate together with interconnections as switching means for selectively applying the voltage to the respective pixel electrodes. In a color LCD device, a color filter layer composed of color filters of red, green, and blue, for example, is disposed on one of the substrates.
In such LCD devices, different display modes are appropriately selected depending on the twist angle of a nematic liquid crystal material used. Among such display modes, an active driving type twisted nematic liquid crystal display mode (hereinafter, referred to as a TN mode) and a multiplex driving type super-twisted nematic liquid crystal display mode (hereinafter, referred to as an STN mode) are well known.
In the TN mode, nematic liquid crystal molecules are oriented in a state twisted 90° between the pair of substrates, so that light is guided along the twisted direction. In the STN mode, the twist angle of nematic liquid crystal molecules is made greater than 90°, so that the light transmittance of the liquid crystal layer exhibits a sharp change when a voltage near a threshold voltage is applied.
Since the STN mode utilizes the birefringence effect of a liquid crystal material, the background of the display screen is likely to be uniquely colored due to color interference. In order to effect monochrome display in the STN mode without an occurrence of such coloring, using an optical compensation plate is conventionally considered effective. Two display modes using such an optical compensation plate are known; a double super-twisted nematic phase compensation mode (hereinafter, referred to as a DSTN mode) and a film type phase compensation mode (hereinafter, referred to as a film-added mode).
In the DSTN mode, the LCD device includes two liquid crystal cells, i.e., a liquid crystal cell for display and a liquid crystal cell in which liquid crystal molecules are twisted at an angle reverse to that in the liquid crystal cell for display. In the film-added mode, a film having optical anisotropy is provided. The film-added mode is considered advantageous since it is light in weight and low in cost.
By employing any of the above phase compensation modes, the LCD devices of the STN mode have been improved in the monochrome display characteristics. Accordingly, a color STN LCD device provided with a color filter layer has been realized as a color display.
The TN mode is roughly classified into a normally-black mode and a normally-white mode.
In the normally-black mode, a pair of polarizing plates are disposed, sandwiching a liquid crystal display element (hereinafter, referred to as an LCD element) therebetween, so that the polarizing directions thereof are parallel to each other. In this mode, black is displayed when no voltage is applied to the liquid crystal layer. In the normally-white mode, a pair of polarizing plates are disposed, sandwiching an LCD element therebetween, so that the polarizing directions thereof are orthogonal to each other. In this mode, white is displayed when no voltage is applied to the liquid crystal layer. The normally-white mode is advantageous when display contrast, color reproducibility, viewing angle dependence of display, and the like are taken into consideration.
The TN mode LCD device has viewing angle dependence in which the contrast of display images is changed or inverted depending on the direction in which and the angle at which an observer views the display screen. This occurs because liquid crystal molecules have refractive index anisotropy &Dgr;n and they are oriented in a tilted state with respect to the upper and lower substrates. As a result, a viewing angle characteristic of a wide angle of field is not obtained. This problem will be described below in detail.
FIG. 21
schematically illustrates a cross-sectional structure of an LCD element
31
of the TN mode. With an application of a voltage to a liquid crystal layer for gray scale display, liquid crystal molecules
32
are in a state of slight rise.
In the LCD element
31
, a linear polarized light beam
35
passing through the element in a direction normal to the surfaces of a pair of substrates
33
and
34
and linear polarized light beams
36
and
37
passing through the element in directions tilted from the normal are incident on the respective liquid crystal molecules
32
at different angles from each other. Since the liquid crystal molecules
32
have refractive index anisotropy &Dgr;n as described above, ordinary light and extraordinary light are generated when each of the linear polarized light beams
35
,
36
, and
37
passes through the liquid crystal molecules
32
. As a result, the linear polarized light beam is changed to an elliptic polarized light beam due to the phase difference between the ordinary light and the extraordinary light. This is a cause of the occurrence of viewing angle dependence.
Moreover, in an actual liquid crystal layer, the tilt angle of the liquid crystal molecules
32
located in the middle of the liquid crystal layer between the substrates
33
and
34
is different from that of the liquid crystal molecules
32
located in the vicinity of the substrates
33
and
34
. Also, the liquid crystal molecules
32
are twisted 90° around the normal of the substrate surface as an axis. These are also causes of the occurrence of the viewing angle dependence.
Thus, the linear polarized light beams
35
,
36
, and
37
passing through the liquid crystal layer are caused to have various birefringence effects depending on the directions and angles thereof. This results in the generation of complicated viewing angle dependence.
For example, when the viewing angle is gradually tilted from the direction normal to the screen in the positive viewing direction (toward the lower side of the screen), a phenomenon in which the display screen is colored (hereinafter, referred to as a coloring phenomenon) and a phenomenon where black and white are inverted (hereinafter referred to as an inversion phenomenon) occur at and after a certain viewing angle. On the other hand, when the viewing angle is gradually tilted from the direction normal to the screen in the negative viewing direction (toward the upper side of the screen), the contrast is abruptly reduced.
The above LCD device has another problem that as the display screen is larger the acceptable viewing angle becomes smaller. For example, when an observer views a large liquid crystal display screen from the front at a short distance, the observer may sometimes recognize that the color displayed on the upper portion of the screen is different from the color displayed on the lower portion thereof. This is because, as a display screen becomes larger, the apparent angle for viewing the entire screen becomes greater, causing the same phenomenon observe
Inoue Iichiro
Yamahara Motohiro
Conlin, Esq. David G.
Edwards & Angell LLP
Parker Kenneth
Roos, Esq. Richard J.
Sharp Kabushiki Kaisha
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