Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-12-04
2003-06-24
Niebling, John F. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S123000
Reexamination Certificate
active
06583842
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reflective type liquid crystal display device, and more particularly, to a reflective type hybrid alignment fringe field switching mode LCD (hereinafter, referred to as FFS-LCD).
2. Description of the Prior Art
A conventional reflective type LCD has generally employed a twisted nematic (TN) mode LCD in which liquid crystal composition having positive dielectric anisotropy is twist-aligned. The reflective type TN-LCD has an advantage of low power consumption and is used in small-sized LCD such as electronic table clocks and digital clocks. However, the reflective type TN-LCD has disadvantages of poor viewing angle properties and low contrast ratio.
In order to realize better viewing angle properties, higher reflectance, and higher aperture ratio, recent efforts are focused on research and development of a reflective type FFS-LCD.
FIGS. 1 and 2
show outlines of the conventional FFS-LCD.
Referring to
FIGS. 1 and 2
, a lower substrate
11
and an upper substrate
12
are opposed with a predetermined distance. A liquid crystal layer
15
having a plurality of liquid crystal molecules
15
a
are interposed between the lower substrate
11
and the upper substrate
12
. A counter electrode
14
and a pixel electrode
13
are arranged on the inner sides of the lower substrate
11
, serving to form a fringe field for driving the liquid crystal molecules
15
a
. A color filter (not shown) is arranged on the inner side of the upper substrate
12
. A first horizontal alignment layer
20
is interposed between the lower substrate
11
and the liquid crystal layer
15
. And, a second horizontal alignment layer
19
is also interposed between the upper substrate
12
and the liquid crystal layer
15
.
The first and the second horizontal alignment layers
20
and
19
have rubbing axes, respectively. The rubbing axis of the first horizontal alignment layer
20
forms an angle of 180° (anti-parallel) with that of the second horizontal alignment layer
19
. Also, the rubbing axis of the first horizontal alignment layer
20
forms a predetermined angle with a line obtained on the surface of the substrate by projecting a fringe filed formed between the counter electrode
14
and the pixel electrode
13
. A polarizer
18
is attached on the outer side of the upper substrate
12
, the polarizing axis thereof being consistent with the rubbing axis of the first horizontal alignment layer
20
. A quarter wavelength plate
17
is arranged on the outer side of the lower substrate
11
so as to polarize the incident or reflective light. And, a reflector
16
is arranged on the outer side of the quarter wavelength plate
17
so as to reflect the light passed through the quarter wavelength plate
17
. A fast or slow axis of the quarter wavelength plate
17
forms an angle of 45° with the rubbing axis of the first horizontal alignment layer
20
.
The operation of the conventional reflective type FFS-LCD will be explained in the following.
Referring to
FIG. 1
, when no voltage difference is generated between the counter electrode
14
and the pixel electrode
13
, the liquid crystal molecules
15
a
are arranged, the major axes thereof being parallel with the rubbing axes of the horizontal alignment layers
20
and
19
. Therefore, natural light becomes incident light moving toward the direction of the polarizing axis after passing through the polarizer
18
. Afterwards, the incident light passes through the liquid crystal layer
15
wherein major axes of the liquid crystal molecules
15
a
are arranged to be parallel with rubbing axes of the horizontal alignment layers
20
and
19
, and therefore, the moving direction of the incident light is not changed. Since the incident light forms an angle of 45° with the fast or slow axis of the quarter wavelength plate
17
, the incident light is changed into right or left circular polarized light through the quarter wavelength plate
17
after passing through the liquid crystal layer
15
. The right circular polarized light is then reflected by the reflector
16
and changes left circularly polarized
A reflective light passes through the quarter wavelength plate
17
having the fast or slow axis forming an angle of 45° with the moving direction thereof. Therefore, the moving direction of the reflective light is shifted to a direction perpendicular to the polarizing axis. Since the shifted direction of the reflective light is perpendicular to the major axes of the liquid crystal molecules
15
a
, the reflective light passes through the liquid crystal layer
15
without a change of the moving direction. Then, the reflective light is at right angles with the polarizing axis after passing through the liquid crystal layer
15
, thereby not passing through the polarizer
18
. As a result, a screen shows a dark state.
Referring to
FIG. 2
, when a fringe field (E) is formed between the counter electrode
14
and the pixel electrode
13
, the liquid crystal molecules
15
a
are twisted in a shape of the fringe field. Therefore, optical axes of the liquid crystal molecules
15
a
form a predetermined angle with the polarizing axis. Passing through the polarizer
18
, natural light becomes incident light moving toward the direction of the polarizing axis. Then, the incident light forms an angle of 45° with the major axes of the liquid crystal molecules
15
a
arranged along the fringe field. Therefore, the incident light forms an angle of 45° with the polarizing axis after passing through the liquid crystal layer
15
. Since the incident light corresponds with the fast or slow axis of the quarter wavelength plate
17
after passing through the liquid crystal layer
15
, the moving direction of the incident light is not changed when passing through the quarter wavelength plate
17
. After passing through the quarter wavelength plate
17
, the incident light is reflected by the reflector
16
.
A reflective light passes through the quarter wavelength plate
17
without a change of the moving direction since the moving direction corresponds with the fast or slow axis of the quarter wavelength plate
17
. The moving direction of the reflective light passing through the quarter wavelength plate
17
forms an angle of 45° with the major axis of the liquid crystal molecules
15
a
of the liquid crystal layer
15
, so that the moving direction of reflective light through the liquid crystal layer
15
corresponds with the polarizing axis. As a result, the screen shows a white state.
The conventional reflective type FFS-LCD has, however, several problems.
The conventional reflective type FFS-LCD additionally uses optical members such as the quarter wavelength plate on the outer side of the substrate in order to improve contrast without employing back light as a light source. This may cause an increase of manufacturing cost. Moreover, the transmittance and the reflectance of the LCD are lowered since the quarter wavelength plate does not generally convert the linear polarized incident lights into circular polarized light across all the wavelength or visa versa.
In order to solve the above-mentioned problems, especially in cost point of view, other method has been proposed that the liquid crystal layer substitutes for the quarter wavelength plate by making &lgr;(2n+1)/4 of the phase retardation (d&Dgr;n) of the liquid crystal layer.
However, this method as well has several drawbacks, which will be explained in the following.
Although the phase retardation of the liquid crystal layer is controlled to &lgr;(2n+1)/4, this phase retardation may serve to realize a dark state in only a specific range of wavelengths considering that the phase retardation is a function of light wavelength (&lgr;). Therefore, complete contrast is not obtained in whole ranges of wavelengths.
Furthermore, in order to determine the initial arrangement direction of the liquid crystal molecules, it is required to perform a rubbing process for forming rubbing axes on alignment layers of the upper and lower substrates.
Hong Seung Ho
Jeong Youn Hak
Kim Jin Mahn
Lee Seung Hee
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