Reflective liquid crystal display device

Details Reflective liquid crystal display device Reflective liquid crystal display device

2001-05-30

2004-04-06

Nguyen, Ha Tran (Department: 2812)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S115000, C349S185000

Reexamination Certificate

active

06717639

ABSTRACT:

This application claims the benefit of Korean Application No. P00-29197, filed in the Republic of Korea on May 30, 2000, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a liquid crystal display having a reflective type display.
2. Description of the Related Art
A liquid crystal display (LCD) has many advantages. One advantage is that the LCD has a flat panel display. The flat display is advantageous because it is thin and not bulky. The LCD also has low power consumption. The LCD is used in many applications, for example, portable computers such as personal computer notebooks, office automation and audio-visual equipment. The LCD displays a picture or image when it manipulates an electric field that is applied to a liquid crystal material having a dielectric anisotropy to transmit or shut off a light. The LCD emits an external light rather than having a light generated from within, which is different from display devices such as electro-luminescence (EL) devices, cathode ray tubes (CRT), light emitting diodes (LED), and similar devices.
The LCD is largely classified into either a transmissive or reflective type display, depending on the manner in which the light is emitted. The transmissive LCD includes a liquid crystal panel having liquid crystal material injected between two glass substrates, and a back light for supplying a light to the liquid crystal panel. However, it is difficult to make a transmissive LCD, which is thin and light weight because of the added bulk and weight of the back light. Another drawback is that the back light causes excessive power consumption.
The reflective LCD has been widely used as a portable display device such as in an electronic passport or a personal data association (PDA) because it does not require a back light with low power consumption. The reflective LCD having less than sixteen scanning lines usually includes a twist nematic liquid crystal mode that has a 90° twist angle. The reflective LCD having more than sixteen scanning lines, however, usually includes a super twist nematic liquid crystal mode that engages a Diechroic polarizer or a phase compensating plate. The reflective LCDs that are available in the market have adopted a scheme where the device emits the difference between an electro-optical transmission curve of red, green, and blue by allowing &Dgr;n•d of the super twist nematics to be greater than 1.0 &mgr;m, or in the alternative, attaching color filters. A polarization-modulated type LCD that uses a polarizing plate and a reflecting plate that depends on a very large viewing angle, whereas a reflective LCD that uses an active matrix can realize various colors.
Referring to
FIG. 1
, the conventional reflective LCD includes a polarizer
2
for polarizing a natural light into a linear polarized light. The linear polarized light then transmits through a retardation film
4
, which converts the linear polarized light into a circular polarized light. A glass substrate
6
transmits the circular polarized light to a color filter
8
, which is arranged in red, green, and blue pixels. A liquid crystal layer
12
then converts the circular polarized light into a linear polarized light. A reflective plate
14
reflects light that passes through the liquid crystal layer
12
.
As shown in
FIG. 2A
, when a voltage is not applied to the conventional reflective LCD, only a first linear polarized light
31
(e.g., S wave) found in an incident light
30
that is mingled in a natural light and a peripheral light transmits through the polarizer
2
. The first linear polarized light
31
having transmitted through the polarizer
2
is then converted into a right-handed circularly polarized light
32
by means of a retardation film
4
having a phase difference value of &lgr;/4. The right-handed circularly polarized light
32
transmits through the glass substrate
6
as it is. After the right-handed circularly polarized light
32
transmits through the glass substrate
6
, it transmits through a red(R), green(G), or blue(B) color filter
8
of an absorptive color filter, thereby having a specific wavelength. The right-handed circularly polarized light
32
transmits through a liquid crystal layer
12
after having transmitted through the color filter
8
.
The liquid crystal layer
12
having a phase difference value of &lgr;/4 is injected into the liquid crystal display panel. It is changed into a second linear polarized light
33
(e.g., P wave) that is perpendicular to the first linear polarized light
31
. The light changed into the second linear polarized light
33
is again forward-reflected by the reflective plate to be irradiated onto the liquid crystal layer
12
. The irradiated second linear polarized light
33
a
is converted into a right-handed circularly polarized light
32
a
by transmitting through the liquid crystal layer
12
. The right-handed circularly polarized light
32
a
then transmits through the absorptive color filter
8
again. After the right-handed circularly polarized light
32
a
has transmitted through the absorptive color filter
8
, it is converted again into a first linear polarized light
31
a
by means of the retardation film
4
. This first linear polarized light
31
a
transmits through the polarizer
2
to display a specific color on a screen (not shown) of the reflective LCD.
As shown in
FIG. 2B
, when a voltage is applied to the conventional reflective LCD, only a first linear polarized light found in an incident light
30
that is mingled in a natural light and a peripheral light transmits through the polarizer
2
. The first linear polarized light
31
having transmitted through the polarizer
2
is then converted into a right-handed circularly polarized light
32
by means of a retardation film
4
having a phase difference value of &lgr;/4. The right-handed circularly polarized light
32
then transmits through the glass substrate
6
. After the right-handed circularly polarized light
32
transmits through the glass substrate
6
, it transmits through a red(R), green(G), or blue(B) color filter
8
of an absorptive color filter. The right-handed circularly polarized light
32
is then irradiated onto the liquid crystal layer
12
in the state of a right-handed circularly polarized light having a specific wavelength. Since the liquid crystal layer
12
is supplied with a voltage current, the right-handed circularly polarized light
33
irradiated onto the liquid crystal layer
12
is also irradiated onto a reflective plate
14
in the same state, without any change.
The right-handed circularly polarized light
33
irradiated onto the reflective plate
14
is then converted into a left-handed circularly polarized light
33
a
having a phase change of 180° and reflected. The reflected left-handed circularly polarized light
33
a
is then irradiated onto the liquid crystal, which is injected into the liquid crystal panel. When the liquid crystal layer
12
is supplied with a voltage current, the left-handed circularly polarized light
32
a
irradiated onto the liquid crystal layer
12
then transmits through the absorptive color filter
8
in the same state, without any change. The left-handed circularly polarized light
32
a
transmitted through the absorptive color filter
8
is then converted into a second linear polarized light
31
a
by means of the retardation film
4
. Light that is converted into the second linear-polarized light
31
a
fails to transmit through the polarizer
2
because the polarizer
2
is only capable of transmitting a first linear polarized light
31
, thus, the screen of the reflective LCD is only allowed to be in a blackened state.
Conventional reflective LCDs are disadvantageous in that the absorptive color filter is positioned on the upper substrate. With the absorptive color filter positioned on the upper substrate, a natural or peripheral light must transmit through the color filter twice between the time it enters and exits t

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