Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
2000-04-26
2003-02-18
Dudek, James (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C362S035000
Reexamination Certificate
active
06522373
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display device, a light guide plate and a method for producing a light guide plate. Particularly, it relates to a technique concerning a light guide plate in a back-lighting type or front-lighting type liquid crystal display device.
A portable computer called lap-top personal computer has been popularized with the advance of miniaturization of a personal computer in recent years. A liquid crystal display device is generally used as a display device in the lap-top personal computer. Color expression of the liquid crystal device has progressed in recent years. A so-called backlighting type liquid crystal display device in which an illuminating means is disposed on the back of a liquid crystal display panel so that a display surface is lit from the back holds the main current of such a liquid crystal display device. A back-lighting means in such a back-lighting type liquid crystal display device needs to light the whole flat surface of the liquid crystal display panel evenly with high and uniform luminance. It may be conceived that luminance of a light source is increased to improve back-lighting luminance. The increase of luminance of the light source is, however, self-limited because it causes increase of electric power consumption and temperature rise in the liquid crystal display device.
There are known various configurations for the back-lighting type liquid crystal display device. For example, the background art includes techniques disclosed in JP-A-4-162002, JP-A-6-67004, etc.
FIG. 2
is a view showing the configuration of the back-lighting means in the conventional back-lighting type liquid crystal display device using an edge-lighting system. In
FIG. 2
, the reference numeral
1
designates a light source;
2
, a light guide plate;
3
, a diffusing sheet;
5
, a first prism sheet;
5
′, a second prism sheet;
6
, a light scattering layer; and
7
, a reflection sheet.
In the configuration shown in
FIG. 2
, the light source
1
constituted by a lamp such as a cold cathode tube, a hot cathode tube, or the like, is disposed on an edge face of the light guide plate
2
made of a light-transmissible material so that illuminating light emitted from the light source
1
is led into the light guide plate
2
. The diffusing sheet
3
made of a translucent white synthetic resin and having a light scattering effect for making luminance of an illuminating surface uniform over the whole surface is provided on an upper surface (light emitting surface) of the light guide plate
2
. The first and second prism sheets
5
and
5
′ for converging scattered light to a certain degree to enhance the frontal luminance of the liquid crystal display device are further disposed on an upper surface of the diffusing sheet
3
.
On the other hand, the light scattering layer
6
is provided on a surface (rear surface) of the light guide plate
2
opposite to the light emitting surface of the light guide plate
2
so that light led into the light guide plate
2
is scattered in the direction toward the diffusing sheet
3
. The reflection sheet
7
is further disposed on a lower surface of the light scattering layer
6
.
The light scattering layer
6
is configured as follows.
FIG. 3
is a view showing the configuration of the light scattering Layer
6
depicted in FIG.
2
. As shown in
FIG. 3
, the light scattering layer
6
is formed by printing a plurality of light diffusing substances using titanium oxide, glass beads, or the like, as a predetermined pattern on the rear surface of the light guide plate
2
by a technique of screen printing, or the like. Generally, the intensity of light emitted from the light source
1
decreases as the position of the light becomes farther from the light source
1
. Therefore, the light scattering layer
6
is formed so that the pattern area of the light scattering layer
6
in the light guide plate
2
increases as the position becomes farther from the light source
1
.
JP-A-7-294745 has proposed also a light guide plate in which grating grooves as an alternative to the aforementioned light scattering layer
6
are formed in a surface (rear surface) of the light guide plate opposite to the light emitting surface of the light guide plate so that light incident on the light guide plate is reflected at the grating grooves.
On the other hand, a reflection liquid crystal display device as described in “Applied Physics; Vol. 67, No. 10, p.1159 (1998)” is known as a technique for achieving a low power-driven liquid crystal display device without use of back-lighting. In such a reflection liquid crystal display device, room light or sunlight taken in is reflected at a layer formed on the back of a liquid crystal and having a reflecting function to thereby achieve elimination of back-lighting. The visibility of the reflection liquid crystal display device is, however, lowered in the dark place. To apply the refection liquid crystal display device to a wider working environment, it is necessary that a measure counter to the lowering of visibility is taken while the characteristic of the reflection liquid crystal display device is kept the best.
A front-lighting type liquid crystal display device as shown in
FIG. 4
has been proposed to solve the aforementioned problem.
FIG. 4
is a view showing the configuration of a (front-lighting type) reflection liquid crystal display device having a front-lighting means (hereinafter merely referred to as front-lighting type liquid crystal display device).
In
FIG. 4
, the reference numeral
2
designates a light guide plate;
6
, a light scattering layer (formed by screen printing in the same manner as the light scattering layer in
FIG. 3
) formed on a surface (upper surface in
FIG. 4
) of the light guide plate
2
opposite to the light emitting surface of the light guide plate
2
; and
1
, a light source disposed on an edge face of the light guide plate
2
. The light source
1
and the light guide plate
2
including the light scattering layer
6
constitute a front-lighting means. In the front-lighting type liquid crystal display device shown in
FIG. 4
, the light source
1
is not turned on in the bright place and display is watched through the light guide plate
2
of high transparency. In the dark place, the light source
1
is switched on so that the front-lighting means operates in place of external light.
Little scattering, high transparency and smallness in the quantity of light exiting from the upper surface in
FIG. 4
are required as performance of the light guide plate in the aforementioned front-lighting type liquid crystal display device. Also in the light guide plate used in the front-lighting type liquid crystal display device, there is known a configuration in which grating grooves as an alternative to the light scattering layer
6
in
FIG. 4
are provided in a surface of the light guide plate
2
opposite to the light emitting surface of the light guide plate
2
.
In
FIG. 4
, the reference numeral
31
designates an absorption film;
32
, a reflection polarizer;
33
, a diffusing film (diffuser);
34
, a glass substrate;
35
, a thin-film transistor (TFT);
36
, a liquid crystal cell array;
37
, an LCD electrode;
38
, a color filter;
39
, a glass substrate;
40
, a diffusing film (diffuser);
41
, a phase-contrast film; and
42
, a polarizer. The detailed description of the respective parts will be omitted because such a front-lighting type liquid crystal display device (the reflection liquid crystal display device having the front-lighting means) configured as described above is commonly known.
The conventional back-lighting type liquid crystal display device as shown in
FIGS. 2 and 3
was configured so that light emitted from the light source
1
was led into the light guide plate
2
and scattered by the light scattering substances in the light scattering layer
6
. A considerable part of the quantity of light incident on the light guide plate
2
was, however, reflected at the reflection sheet
7
to thereby
Hira Yasuo
Miyawaki Toshitsugu
Taniguchi Hitoshi
Yano Shuji
Antonelli Terry Stout & Kraus LLP
Dudek James
Hitachi , Ltd.
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