Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-02-12
2004-01-13
Ngo, Julie (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S076000, C349S096000, C349S102000, C349S115000, C349S130000, C430S007000
Reexamination Certificate
active
06678024
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2000-6225, filed on Feb. 10, 2000, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reflective liquid crystal display device, and more particularly to a liquid crystal display device having a cholesteric liquid crystal color filter.
2. Discussion of the Related Art
LCD devices are usually classified into transmissive type and reflective type according to their difference in a light source.
The transmissive LCD device uses light incident from a back light that is attached to a rear surface of a liquid crystal panel. The light is incident to a liquid crystal layer of the liquid crystal panel, and is absorbed or passes through the liquid crystal layer according to different alignments of the liquid crystal layer. Therefore, dark or white mode is operated by the liquid crystal panel. Conventionally, the back light of the transmissive LCD device is an artificial light source. Therefore, high power consumption due to the back light is a greater disadvantage of the transmissive LCD device.
On the contrary to the above-mentioned transmissive LCD device, the reflective LCD device uses an ambient light incident from a natural light source or an exterior artificial light source. Because of its low power consumption, the reflective LCD device is focused on.
FIG. 1
is a cross-sectional view of a conventional reflective color LCD device.
As shown, upper substrate
13
includes a color filter
15
and a common electrode
17
, and a lower substrate
11
includes a reflective electrode
16
. Between the upper and lower substrates
13
and
11
, a liquid crystal layer
19
is interposed between. Since the liquid crystal layer
19
has an optical anisotropy, molecules of the liquid crystal layer
19
are aligned in a proper direction with an electric field applied there-across. Therefore, an incident light to the liquid crystal layer
19
is controlled by the electric field applied across the liquid crystal layer
19
via the common and reflective electrodes
17
and
16
. Instead of the liquid crystal layer
19
, a certain medium having the optical anisotropy may be used for an LCD device.
On each exterior surface of the upper and lower substrates
13
and
11
, a plurality of layers are formed to control a polarization of the incident light. For example, a retardation layer
23
, and a polarizer
25
are sequentially formed on the exterior surface of the upper substrate
13
. The retardation layer
23
, or a quarter wave plate (QWP), makes an incident light right-circularly polarized (RCP) or left-circularly polarized (LCP). The polarizer
25
serves to pass only a portion of an incident light that corresponds to a transmittance axis direction of the polarizer
25
. Other portions of the incident light that are different from the transmittance axis direction of the polarizer
25
are absorbed by the polarizer
25
. At this point, a RCP ray is a right-circularly polarized ray that goes from a viewer, while an LCP ray is a left-circularly polarized ray that goes from the viewer.
A twisted nematic (TN) liquid crystal is typically used for the liquid crystal layer
19
between the upper and lower substrates
13
and
11
. Long axes of the TN liquid crystal molecules are twisted to 90 degrees, and the TN liquid crystal layer is designed to have a phase difference (phase delay) of “&lgr;/4”.
Now, with reference to
FIGS. 2A and 2B
, “on and off” states of the conventional reflective LCD device shown in
FIG. 1
will be explained.
FIG. 2A
shows a passage of an incident light in the conventional reflective LCD device during its off state. At this point, a viewer (not shown) viewing the incident light is fixed at one position. A symbol “x” of ray denotes that the ray proceeds from the viewer, while another symbol “·” of ray denotes that the ray proceeds toward the viewer.
The incident light shown in
FIG. 1
first meets the polarizer
25
. As the light passes through the polarizer
25
, it is linearly polarized. A first linearly polarized ray “R
1
” meets the retardation layer
23
, passes through it, and becomes a first right-circularly polarized (RCP) ray “R
2
”. The RCP ray R
2
further meets the liquid crystal layer
19
, passes through it, and becomes a second linearly polarized light “R
3
”. Thereafter, the reflective electrode
16
reflects the second linearly polarized light R
3
such that the second linearly polarized ray R
3
change its direction and meets the liquid crystal layer
19
again. As a reflected ray R
4
that has the opposite direction to the second linearly polarized ray R
3
passes through the liquid crystal layer
19
, the reflected ray R
4
is right-circularly polarized with the phase difference of &lgr;/4. At this point, a second RCP ray “R
5
” is shown to rotate left and to proceed to the viewer. Then, the second RCP ray R
5
meets the retardation layer
23
, passes through it, and becomes a third linearly polarized ray “R
6
”. Since the third linearly polarized ray “R
6
” is parallel with the transmittance axis direction of the polarizer
25
, it passes through the polarizer
25
. Therefore, in the off state, the conventional reflective LCD device operates the white state.
Now, the TN liquid crystal layer
19
is on state such that molecules thereof arrange in one direction, for example perpendicular to the substrates
11
and
13
, corresponding to an electric field applied across the TN liquid crystal layer
19
. Therefore, the TN liquid crystal layer
19
is homeotropic-aligned.
In
FIG. 2B
, the incident light becomes the first RCP ray R
2
after it sequentially passes through the polarizer
25
and the retardation layer
23
. This is the same as shown in FIG.
2
A. Thereafter, since the TN liquid crystal layer
19
is on state, the first RCP ray R
2
passes through the TN liquid crystal layer
19
without rotation and phase difference. Then, the reflective electrode
16
reflects the first RCP ray R
2
such that it changes its direction to be a left-circular polarized (LCP) ray R
7
. The LCP ray R
7
passes through the TN liquid crystal
19
that is still on state without rotation and phase difference, and meets the retardation layer
23
. After the LCP ray R
7
passes through the retardation layer
23
, it is linearly polarized. At this point, a fourth linearly polarized ray R
8
is perpendicular to the transmittance axis direction of the polarizer
25
, and thus us absorbed by the polarizer
25
. Accordingly, in its on state, the conventional reflective LCD device operates a dark state.
Rays passing through the color filter
15
have colors corresponding to the color of the color filter
15
, for example red (R), green (G), and blue (B). When the conventional reflective LCD device is off state, a ray having a color passes through the polarizer
25
and displays the color. However, when the conventional reflective LCD device is on state, a ray having a color is absorbed by the polarizer
25
and does not display the color. Therefore, portions of the liquid crystal layer
19
respectively corresponding to colors of the color filter
15
are selectively on or off to display a color image.
However, in the above-mentioned conventional reflective LCD device, the incident light passes through too many layers. Whenever meeting new layers, or new mediums having different refractive indexes, each ray is somewhat reflected and absorbed by the new medium. Therefore, whenever passing through a new layer, the ray is weakened. For example, as the incident light passes through the polarizer
25
, it is weakened for the first time. Further, as a ray passes through the reflective electrode
15
, it is not only reflected but also absorbed and rapidly weakened.
In another aspect, color dispersion property and contrast ratios of the conventional reflective LCD device rapidly vary with respect to a viewing angle. That is to say, when a user looks at a color image displayed on the conventional reflective LCD device in a wide viewing an
LG.Philips LCD Co. , Ltd.
Morgan & Lewis & Bockius, LLP
Ngo Julie
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