Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-03-28
2004-02-17
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S106000, C349S115000, C349S141000
Reexamination Certificate
active
06693689
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Korean Patent Application No. 1999-11108, filed on Mar. 31, 1999, and the benefit of Korean Patent Application No. 1999-48411, filed on Nov. 3, 1999, each of which are hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE RELATED ART
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly to a reflective LCD device including a cholesteric liquid crystal (CLC) color filter.
2. Description of Related Art
In general, LCD devices are divided into reflective LCD devices and transmissive LCD devices. The transmissive LCD device uses an internal light source such as a back light, while the reflective LCD device uses ambient light.
Particularly, since the reflective LCD device uses ambient light, the brightness of the display depends on circumstances. In an office, the reflective LCD device is lower in brightness than the transmissive LCD device and, accordingly the color purity of an absorption-type color filter used in the LCD should be sacrificed to increase the brightness.
FIG. 1
is a cross-sectional view of a conventional reflective liquid crystal display.
As shown in
FIG. 1
, the liquid crystal panel includes a linear polarizer
26
, a retardation film
24
, a diffuser film
22
, a first substrate
10
, a color filter
20
, a common electrode
18
, a liquid crystal layer
16
, a reflective electrode
14
and a second substrate, each are stacked in the above-described order.
The reflective electrode
14
reflects light transmitted from outside the display and also functions as a pixel electrode. The reflective electrode
14
and the common electrode
18
apply a voltage to the liquid crystal layer
16
and change the orientation of liquid crystal molecules. The diffuser film
22
reduces a surface reflection of light and increases a viewing angle. The retardation film
24
such as a &lgr;4 plate converts linearly polarized light into circularly polarized light. Further, the linear polarizer changes the natural light into linearly polarized light.
The reflective LCD device described above functions and acts as follows.
When natural light is incident into the LCD device, the natural light is converted into linearly polarized light by the linear polarizer
26
, then converted into circularly polarized light by the retardation film
24
. The circularly polarized light is converted into linearly polarized light while passing through the liquid crystal layer
16
and is reflected on the reflective electrode
14
. The reflected polarized light is converted into circularly polarized light while passing through the liquid crystal layer again, then passes through the color filter to produce colored light.
The circularly polarized light is diffused to increase the viewing angle while passing through the diffuser film
22
, then is converted again into linearly polarized light while passing through the retardation film
24
. The linearly polarized light is displayed to the user after passing through the linear polarizer
26
in the form of images.
FIG. 2
shows the state of light while it passes through each of the components described above when an electric field is not applied to the liquid crystal layer.
The natural light is first converted into linearly polarized light through the linear polarizer
26
. The linearly polarized light is changed into circularly polarized light through the retardation film
24
. The circularly polarized light is converted again into linearly polarized light through the liquid crystal
16
, then reflected by the reflective electrode
14
. The reflected linearly polarized light is changed into circularly polarized light through the liquid crystal layer
16
. The circularly polarized light is finally converted into linearly polarized light through the retardation film
24
.
FIG. 3
shows the state of light while it passes through each of the components described above when an electric field is applied to the liquid crystal layer.
The natural light is first converted into linearly polarized light through the linear polarizer
26
. The linearly polarized light is changed into circularly polarized light through the retardation film
24
. The circularly polarized light is not changed when passing through the liquid crystal
16
as an electric field is applied to the liquid crystal
16
, then reflected by the reflective electrode
14
. The reflected circularly polarized light is not varied even when passing through the liquid crystal
16
. The circularly polarized light is finally converted into linearly polarized light through the retardation film
24
, then absorbed by the linear polarizer
26
.
FIG. 4
is a graph illustrating the reflectivity of light with respect to the incident light of the LCD device described above. In
FIG. 4
, the X-axis indicates a wavelength &lgr;, and the Y-axis indicates a reflectivity. Note that a dominant wavelength region is referred to as region A and other wavelengths are referred to as region B. As shown in the graph, though light's reflective index is relatively high in the region A, because light reflection is also carried out in the region B, the color purity of the LCD is reduced. It is required that the color purity is reduced in order to increase the transmissivity of the color filter, but just lowering the color purity to increase the brightness has a limitation.
Further, since the LCD having the configuration described above has a multi-layered structure in which each layer, i.e., each component differs from one another in reflective index, the intensity of the light is reduced while the light passes through each component. For example, the intensity of the light first is reduced while passing through the linear polarizer
26
, and then also prominently is reduced after passing through the color filter
20
, because part of the light is absorbed or reflected while passing through the color filter
20
.
Further, though the observer can clearly see the image displayed due to a good contrast ratio in the center of the screen, the contrast ratio becomes lower as it gets far from the center of the screen, thereby deteriorating the display characteristic.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a reflective liquid crystal display device having increased brightness without sacrificing color purity.
Another object of the present invention is to provide a reflective liquid crystal display device having high color purity and an improved contrast ratio.
In order to achieve the objects, in a first embodiment, a reflective liquid crystal display including a linear polarizer for converting natural light into linearly polarized light; a retardation film for converting the linearly polarized light into a circularly polarized light; a liquid crystal layer for varying the phase of the light differently depending on the presence of an electric field; a cholesteric liquid crystal color filter for selectively reflecting the light from the liquid crystal layer; and a black background for absorbing the light passing through the color filter.
The present invention also provides, in the first embodiment, a reflective liquid crystal display including first and second substrates opposite to and spaced apart from each other; a liquid crystal layer interposed between the first and the second substrates, the liquid crystal layer having a first switching mode in which the phase of light is changed while passing through it and a second switching mode in which the phase of light is not changed while passing through it; first and second electrodes for applying an electric field to the liquid crystal layer; a semiconductor element located on the second substrate, for switching an electric signal applied to the liquid crystal layer; a retardation film located on the first substrate, for converting a linearly polarized light a circularly polarized light; a linear polarizer located on the retardation film, for converting natural light
Baek Heume Il
Kim Yong Beom
Kwon Soon Bum
Di Grazio Jeanne
Kim Robert H.
LG Philips LCD Co., Ltd.
LG. Philips LCD Co., Ltd.
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