Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-10-27
2004-02-24
Dudek, James (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S113000
Reexamination Certificate
active
06697135
ABSTRACT:
CROSS REFERENCE
This application claims the benefit of Korean Patent Application Nos. 1999-46946, 1999-56883, and 2000-19715, filed on Oct. 27, 1999, on Dec. 11, 1999, and on Apr. 14, 2000, respectively, under 35 U.S.C. §119, the entirety of each of which is hereby incorporated by reference.
BACKGROUND
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a transflective LCD device.
2. Description of Related Art
In general, liquid crystal displays are divided into transmissive LCD devices and reflective LCD devices according to whether the display uses an internal or external light source.
A typical transmissive LCD device includes a liquid crystal panel and a back light device. The liquid crystal panel includes upper and lower substrates with a liquid crystal layer interposed. The upper substrate includes a color filter, and the lower substrate includes thin film transistors (TFTs) as switching elements. An upper polarizer is arranged on the liquid crystal panel, and a lower polarizer is arranged between the liquid crystal panel and the backlight device.
The two polarizers have a transmittance of 45% and, the two substrates have a transmittance of 94%. The TFT array and the pixel electrode have a transmittance of 65%, and the color filter has a transmittance of 27%. Therefore, the typical transmissive LCD device has a transmittance of about 7.4% as seen in
FIG. 1
, which shows a transmittance (in brightness %) after light passes through each layer of the device. For this reason, the transmissive LCD device requires a high, initial brightness, and thus electric power consumption by the backlight device increases. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device. However, this has a problem that the battery can not be used for a lengthy period of time.
In order to overcome the problem described above, the reflective LCD has been developed. Since the reflective LCD device uses ambient light, it is light and easy to carry. Also, the reflective LCD device is superior in aperture ratio to the transmissive LCD device.
FIG. 2
shows a typical reflective LCD device in cross section. As shown in
FIG. 2
, the reflective LCD device includes upper and lower substrates
8
and
10
with a liquid crystal layer
12
interposed. The upper substrate
8
includes color filter layers
4
a
,
4
b
and
4
c
(e.g., red, green, and blue) and a common electrode
6
. The lower substrate
10
to includes a switching element (not shown) and a reflective electrode
2
.
Ambient light
100
passes through the upper substrate
8
and the liquid crystal layer
12
and is reflected on the reflective electrode
2
. When electrical signals are applied to the reflective electrode
2
by the switching element, phase of the liquid crystal layer
12
varies. Then, reflected light is colored by the color filter layers
4
a
,
4
b
and
4
c
and displayed in the form of images.
However, the reflective LCD device is affected by its surroundings. For example, the brightness of ambient light in an office differs largely from that outdoors. Even in the same location, the brightness of ambient light depends on the time of day (e.g., noon or dusk).
In order to overcome the problems described above, a transflective LCD device has been developed.
FIG. 3
shows a conventional transflective LCD device. As shown in
FIG. 3
, the conventional transflective LCD device includes upper and lower substrates
22
and
18
with a liquid crystal layer
20
interposed. The upper substrate
22
includes a color filter
104
, and the lower substrate
18
includes a switching element (not shown), a pixel electrode
14
and a reflective electrode
2
. The reflective electrode
2
is made of an opaque conductive material having a good reflectance and light transmitting holes “A” are formed therein. The transflective LCD device further includes a backlight device
16
. The light transmitting holes “A” serve to transmit light
112
from the backlight device
16
.
The transflective LCD device in
FIG. 3
is operable in transmissive and reflective modes. First, in reflective mode, the incident light
110
from the upper substrate
22
is reflected on the reflective electrode
2
and directed toward the upper substrate
22
. At this time, when electrical signals are applied to the reflective electrode
2
by the switching element (not shown), phase of the liquid crystal layer
20
varies and thus the reflected light is colored by the color filter
104
and displayed in the form of images.
Further, in transmissive mode, light
112
generated from the backlight device
16
passes through portions of the pixel electrode
14
corresponding to the transmitting holes “A”. When the electrical signals are applied to the pixel electrode
14
by the switching element (not shown), phase of the liquid crystal layer
20
varies. Thus, the light
112
passing through the liquid crystal layer
20
is colored by the color filter
104
and displayed in the form of images.
As described above, since the transflective LCD device has both transmissive and reflective modes, the transflective LCD device can be used without regard to the time of day (e.g., noon or dusk). It also has the advantage that it can be used for a long time by consuming low power. However, since the reflective electrode has the transmitting holes “A”, the conventional transflective LCD device has a very low light utilizing efficiency compared to either the reflective LCD device or the transmissive LCD device alone.
In the reflective mode of the transflective LCD device, incident light enters the color filter
104
and is reflected on the reflective electrode
2
and reenters the color filter
104
. That is, the light passes through the color filter twice. But, in the transmissive mode, light from the backlight
16
passes through the color filter once. Thus, the color purity that users perceive varies according to the mode of the LCD device.
FIGS. 4A and 4B
are graphs illustrating transmissivity with respect to the light wavelengths. The graphs are obtained by a spectrum analysis method.
As is well known, all objects have a wavelength-dependent reflectivity, and their color that an observer recognizes is determined by the wavelengths of the light reflected from or transmitted through the object. The wavelength range of visible light is about 380 nm to 780 nm. The visible light region can be broadly divided into red, green, and blue regions. The central wavelength of the red visible light region is about 660 nm, that of green is about 530 nm, and that of blue is about 470 nm.
Each pixel of the LCD device has three sub-pixels so that the reflected light is colored to red (R), green (G) and blue (B) colors and, therefore each color has a dominant wavelength band leading to a high color purity by transmitting the dominant wavelength band for a predetermined color and absorbing other wavelengths.
FIG. 4A
shows the relation of transmissivity and wavelength when light passes the color filter once, that is, in the transmissive mode. The blue color filter should transmit the blue color and absorb other colors. But as shown in the graph, since transmissivity of green color is relatively high in a band around 470 nm, the green color is also transmitted through the blue color filter with the blue color.
FIG. 4B
shows the relation of transmissivity and wavelength when light passes the color filter twice, that is, in the reflective mode. As known from the graph, since the lines representing each dominant wavelength are steep and distributed (spaced apart), light of the wavelengths other than the dominant wavelength band are well absorbed.
Thus, the color generated by combination of the color filters has different color purity depending on the selected mode. For example, in displaying green color, the color of the transmissive mode is lighter than that of the reflective mode.
Though the color purity varies according to the selected mode, the transflective LCD device only adopts color
Baek Heum Il
Ha Kyoung Su
Kim Yong Beom
Birch & Stewart Kolasch & Birch, LLP
Dudek James
LG. Philips LCD Co. Ltd.
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