Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-09-03
2001-01-09
Malinowski, Walter J. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06172726
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a reflection type liquid crystal display device. More particularly, it relates to a reflection type liquid crystal display device of low power consumption employing a sole polarizer plate and which is capable of achieving high picture quality and high response characteristics.
2. Description of the Related Art
A liquid crystal display device (LCD) is lightweight, thin in thickness and low in power consumption, and is finding use in a wide field of application ranging from a small size device for computers or timepieces to a large size device such as a word processor or a personal computer.
From now on, use of the LCD as a display device for a portable information terminal or a personal data assist (PCD) is thought to be promising. In order for the LCD to be used as a display device for PCD, it is required to reduce the size and the weight of the device further and to reduce power consumption such as to enable driving for a prolonged time by e.g., an electric cell. As a display device capable of coping with these demands, a reflection type LCD is stirring up notice.
Among the reflective type LCDs, there is such a device employing two polarizer plates and a twist-nematic liquid crystal (TN liquid crystal) or a super-twist liquid crystal (STN liquid crystal). This reflection type LCD has a liquid crystal layer of the TN or STN liquid crystal sandwiched between two polarizer plates and a reflection plate on one side of the sandwiched structure.
Among the reflective type LCDs, there is also such a device employing a guest-host (GH) system. In this GH system, a two-color dyestuff (guest) is dissolved in a liquid crystal (host) and liquid crystal molecules are controlled in orientation by an electrical field to change the direction of orientation of dyestuff molecules simultaneously to utilize the change in light absorption due to dichroism in order to make display. This system, not in need of polarizer plates, may be exemplified by a phase change-GH system exploiting the phase transition, referred to hereinafter as a PC-GH system, a chiral nematic-GH system which has overcome the problem of hysteresis by exploiting the merit proper to the GH system, referred to hereinafter as the CN-GH system, an a-N*-GH system which randomly orients the liquid crystal to eliminate hysteresis to enable gradation representation, a &lgr;/4-GH system employing a quarter wave plate, and a three-layer GH system having a three-layer liquid crystal layer structure of the GH system.
Of the above described reflection type reflection type liquid crystal display devices, that employing two polarizer plates reportedly is insufficient in lightness and difficult to design as a color type device because the incident light has to traverse the polarizer plates four times before exiting the device so that 70% of the incident light is absorbed by the polarizer plates without being used for display.
Conversely, the above-mentioned reflection type liquid crystal display device employing a sole polarizer plate can be improved in lightness by diminishing absorption by the polarizer plate.
With the above-mentioned reflection type liquid crystal display device of the GH system not employing the polarizer plate, lightness can similarly be assured.
The following Table 1 summarizes the properties of the reflection type liquid crystal display device employing the sole polarizer plate and the reflection type liquid crystal display device of the GH system not employing the polarizer plate.
TABLE 1
angle
re-
of
light-
con-
grada-
sponse
visibi-
driving
mode
ness
trast
tion
speed
lity
voltage
sole
STN
&Dgr;
◯
⊙
&Dgr;
&Dgr;
up to
polar-
liquid
7V
izer
crystal
plate
TN
&Dgr;
⊙
⊙
⊙
&Dgr;
up to
liquid
3V
crystal
OCB
&Dgr;
⊙
⊙
⊙
◯
up to
liquid
3V
crystal
no
PC-GH
◯
◯
&Dgr;
◯
◯
up to
polar-
5V
izer
CN-GH
◯
◯
⊙
◯
◯
up to
plate
5V
a-N*
◯
◯
⊙
◯
◯
up to
GH
5V
&lgr;/4 GH
◯
◯
⊙
⊙
◯
up to
5V
three-
⊙
◯
⊙
&Dgr;
&Dgr;
up to
layer
5V
GH
times 3
In Table 1, lightness, contrast, gradation, response speed and the angle of visibility are evaluated in three stages. Specifically, ⊙, ◯ and &Dgr; denote extremely satisfactory, satisfactory and acceptable, respectively.
As may be seen from Table 1, the reflection type liquid crystal display device of the GH system is extremely satisfactory in lightness, herein white color display. However, the GH system cannot be said to be sufficient in reliability of the component material.
It cannot be said that all characteristics can be met sufficiently with the remaining types of the reflection type liquid crystal display device.
Recently, with progress in the polarizer plates, a material superior to the conventional material in light transmittance and polarization has been developed. Thus, attention is directed to the reflection type liquid crystal display device employing a sole polarizer plate. Moreover, since the use of the STN liquid crystal enables simple matrix driving, attention is directed to a reflection type liquid crystal display device employing a sole polarizer plate and the STN liquid crystal. However, this reflection type liquid crystal display device achieves high contrast by twisting the array of the liquid crystal molecules by 180° to 270° such that it is not optimum in picture quality or response speed.
SUMMARY OF THE INVENTION
It is an therefore object of the present invention to provide a reflection type liquid crystal display device assuring bright white color display and high contrast and which achieves low power consumption as well as a picture quality and a response speed capable of accommodating a moving picture.
The present invention provides a reflection type liquid crystal display device including a first substrate having a transparent electrode on its inner surface, a second substrate having a plurality of pixel electrodes provided facing the transparent electrode and a plurality of switching elements for driving the pixel electrodes, the pixel electrodes being formed of a light reflecting material, a nematic liquid crystal layer arranged between the first and second substrates and a polarizing plate arranged outside of the first substrate.
Preferably, a phase difference film is arranged between the first substrate and the polarizing plate.
Preferably, the nematic liquid crystal layer is constituted by a negative type nematic liquid crystal exhibiting negative anisotropy.
Preferably, the pixel electrodes have a light-scattering function.
Preferably, the light-scattering function is provided by irregularities on the surfaces of the pixel electrodes.
Preferably, a light-scattering layer is provided outside the polarizing plate.
Preferably, the first substrate has a color filter.
Preferably, the second substrate has a color filter.
Preferably, the switching element is a thin-film transistor coated with an organic resin layer.
Preferably, the pixel electrodes are formed by metal layers formed on irregularities formed on the surface of the organic resin layer.
In the reflection type liquid crystal display device of the present invention, the nematic liquid crystal is preferably a negative type one exhibiting negative anisotropy. However, a positive type nematic liquid crystal exhibiting positive anisotropy may also be used.
In the reflection type liquid crystal display device of the present invention, the surfaces of the plural pixel electrodes facing the liquid crystal layer present surface irregularities of the order of a few &mgr;m in size.
If, in the reflection type liquid crystal display device of the present invention, the surfaces of the plural pixel electrodes facing the liquid crystal layer are devoid of surface irregularities of the order of a few &mgr;m in size, a light-scattering layer is preferably provided on the transparent substrate.
In the reflection type liquid crystal display device of the present invention, a co
Arakawa Seiichi
Matsute Masataka
Malinowski Walter J.
Sonnneschein, Nath & Rosenthal
Sony Corporation
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