Illumination – Revolving
Reexamination Certificate
2002-06-19
2004-05-04
O'Shea, Sandra (Department: 2875)
Illumination
Revolving
C362S339000, C362S561000
Reexamination Certificate
active
06729737
ABSTRACT:
The present invention claims the benefit of Korean Patent Application No. 2001-67193, filed in Korea on Oct. 30, 2001, which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly to a backlight device for use in the LCD device.
2. Discussion of the Related Art
In general, since flat panel display devices are thin, low weight, and have low power consumption, they are increasingly being used for displays of portable devices. Among the various types of flat panel display devices, liquid crystal display (LCD) devices are widely used for laptop computers and desktop monitors because of their superiority in resolution, color image display, and display quality.
LCD devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce a predetermined image. Liquid crystal molecules have a definite orientation that results from their peculiar characteristics. The specific orientation can be modified by an electric field that is applied across the liquid crystal molecules. In other words, electric fields applied across the liquid crystal molecules can change the orientation of the liquid crystal molecules. Due to optical anisotropy, incident light is refracted according to the orientation of the liquid crystal molecules.
Specifically, the LCD devices have upper and lower substrates with electrodes that are spaced apart and face each other, and a liquid crystal material is interposed therebetween. Accordingly, when a voltage is applied to the liquid crystal material by the electrodes of each substrate, an alignment direction of the liquid crystal molecules is changed in accordance with the applied voltage to display images. By controlling the applied voltage, the LCD device provides various transmittances for rays of light to display image data.
The LCD device, however, does not emit the light by itself, and it only controls the light transmissivity. Therefore, the LCD device needs a light source additionally. As a light source, LCD devices often use backlight devices behind the LCD panel. The backlight devices are classified into direct back light type (or direct type) units and edge light type (or edge type) units, according to a position of a lamp. In the case where the direct type back light unit is used for a liquid crystal display device, incident rays irradiating from the lamp are directly incident to the LCD panel. In the case where the edge type back light unit is used, rays from the lamp are incident to the LCD panel via a light guide or a reflector. A detailed explanation of the edge type backlight will be provided subsequently.
FIG. 1
shows a conventional edge type backlight device for use in a LCD device. The conventional edge type backlight device includes a lamp
11
, a U-shaped lamp housing
12
surrounding the lamp
11
, a light guide plate
13
, and a reflector
14
. The light guide plate
13
coverts the light from the lamp into surface light and includes dot patterns (not shown) on the bottom thereof. The dot patterns (not shown) are formed, e.g., by printing of paint. The light guide plate
13
including the dot patterns diffuses light from the lamp in order to form the uniform surface light. The reflector
14
is disposed in the back side of the light guide plate
13
, and serves to reflect the rays from the light such that light leakage is prevented. A first diffusion sheet
15
, first and second prism sheets
16
and
17
, and a second diffusion sheet
18
are sequentially formed on the front surface of the light guide plate
13
.
FIG. 2
is a schematic perspective illustration of the first and second prism sheets
16
and
17
of FIG.
1
. Each of the prism sheets
16
and
17
includes a plurality of triangular prisms
16
a
and
17
a
on the front surface thereof. The first triangular prisms
16
a
of the first prism sheet
16
is arranged perpendicular to the second triangular prisms
17
a
of the second prism sheet
17
.
When the light guide plate
13
(in
FIG. 1
) includes the dot patterns printed by paint (i.e., a printing-type light guide plate), the backlight device preferably has more than two prism sheets which include perpendicular triangular prisms, respectively. However, as the number of prism sheets increases, total internal reflection frequently occurs in the angular prisms, thereby causing the optical transmission losses. Since the printing-type light guide plate utilizes light diffusion, it is very difficult to control the direction of the rays. Accordingly, prismatic patterns are widely used, on the bottom of the light guide plate instead of the dot patterns, as shown in FIG.
3
. The prismatic patterns are formed by a non-printing method, such as an etching method.
FIG. 3
shows a conventional backlight device having a plurality of prismatic patterns. As shown, the conventional backlight device includes a lamp
21
, a U-shaped lamp housing
22
surrounding the lamp
21
, a light guide plate
23
, and a reflector
24
. The light guide plate
23
is thinner, further from the lamp
21
. The light guide plate
23
includes a plurality of prismatic patterns
23
a
on the bottom thereof, which is formed by etching. The reflector
24
is disposed in the back side of the light guide plate
23
, and serves as reflecting the rays from the lamp
21
such that light leakage is prevented. A diffusion sheet
25
and a prism sheet
26
are sequentially formed on the front surface of the light guide plate
23
.
FIG. 4
is a simulation result showing viewing angle-dependence of luminance in cases of using the conventional backlight device of FIG.
3
. The dotted line of the graph in
FIG. 4
is a guide line that is expressed by cosine function. The guide line represents possible viewing angle distribution of the outgoing light from the backlight device. The line comprising an alternate sequence of long and short dashes indicates an up-and-down viewing angle distribution of the outgoing light from the backlight device of
FIG. 3
, while the full line indicates a right-and-left viewing angle distribution. As shown, the outgoing light from the backlight device distributes ranging from −30 to +30 degrees. When the viewing angle is zero, the intensity of the outgoing light is maximized.
Accordingly, since the backlight device having the prismatic patterns
23
a
of
FIG. 3
can control the direction of the light from the lamp
21
using the light refraction, only one prism sheet is applied. As a result, the costs of production are reduced.
Meanwhile, the LCD device generally includes an absorptive color filter layer to display color images. However, when the light passes through the absorptive color filter, portions of the light are absorbed by the absorptive color filter. Thus, the optical loses increases and the luminance of the LCD device decreases. To overcome this problem, cholesteric liquid crystal (CLC) has been researched and developed for use as a color filter. The CLC color filter utilizes the selective reflection characteristics of the cholesteric liquid crystal. The LCD devices having the CLC color filter have great color reproduction and contrast ratio compared to those LCD devices that use an absorptive color filter.
As widely known, the cholesteric liquid crystal (CLC) reflects the light having a certain wavelength in accordance with its helical pitch, i.e., selective reflection characteristics. However, the light travels different distances with respect to the CLC color filter depending on different angles of incidence. Thus, the light passing through the CLC color filter experiences different helical pitches whenever it strikes the CLC color filter at different angles of incidence. As a result, the LCD device produces color shift in accordance with the viewing angles.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a backlight device for a liquid crystal display (LCD) device, which substantially obviates
Jeon Seong-Man
Kim Je-Hong
LG.Philips LCD Co. , Ltd.
McKenna Long & Aldridge LLP
O'Shea Sandra
Ward John Anthony
LandOfFree
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