Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
2003-02-04
2004-05-11
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C362S035000
Reexamination Certificate
active
06734929
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an illumination device and a liquid crystal display device. In particular, the invention relates to an illumination device which can illuminate a wide area brightly and uniformly though having only a single light source as well as to a liquid crystal display device using it.
2. Description of the Related Art
Conventionally, front lights of reflection-type liquid crystal display devices employ a unit that is composed of a light source, an intermediate light guide, a light guide plate, a case body that holds those components in an integral manner and has an inner reflective surface, and other components.
FIG. 23A
is a perspective view showing the configuration of a liquid crystal display device having such a configuration.
FIG. 23B
is a plan view of the liquid crystal display device of FIG.
23
A. The liquid crystal display device shown in
FIGS. 23A and 23B
is composed of a liquid crystal display unit
120
and a front light
110
that is disposed on the front side of the liquid crystal display unit
120
. Although not shown in detail, the liquid crystal display unit
120
is a reflection-type liquid crystal display unit that performs display by reflecting light coming from the front side. In the liquid crystal display unit
120
, a liquid crystal layer is held between a top substrate
121
and a bottom substrate
122
that are opposed to each other. Display is performed by varying the light transmission state by controlling the liquid crystal orientation state of the liquid crystal layer.
The front light
110
is composed of a flat light guide plate
112
, a rod-shaped intermediate light guide
113
that is disposed to a side end face
112
a
of the flat light guide plate, and a light-emitting element
115
that is disposed adjacent to one end face of the intermediate light guide
113
. The top surface of the light guide plate
112
is formed with a prism structure in which a plurality of wedge-shaped (in cross-section) projections
114
are arranged parallel with each other. To prevent a moiré pattern, the projections
114
are formed so as to be somewhat inclined with respect to a light-guide-plate-side end face
112
a.
In the front light
110
, light that is emitted from the light-emitting element
115
is applied to the side end face
112
a
of the light guide plate
112
via the intermediate light guide
113
and is thereby introduced into the light guide plate
112
. The light is reflected by the prism-shaped top surface of the light guide plate
112
and is thereby changed in traveling direction. As a result, the light is emitted from the bottom surface (see
FIG. 23A
) of the light guide plate
112
toward the liquid crystal display unit
120
.
In portable electronic equipment such as PDAs and portable game machines, the battery drive time greatly influences the ease of use. Therefore, liquid crystal display devices that are used as display sections of such equipment have come to employ, to lower the power consumption of a front light, a single-light front light that is equipped with only a single light-emitting element like the front light
110
shown in
FIG. 23A
is. That is, it is intended to reduce the power consumption by decreasing the number of light-emitting elements. Further, miniaturization of portable electronic equipment requires that the thickness of the front light
110
be decreased to about 1 mm.
However, in such single-light front lights, it is almost impossible to uniformly illuminate a wide display screen of several inches or more with the combination of the thin light guide plate and the single light-emitting element. More specifically, in the front light
110
shown in
FIG. 23A
in which the light-emitting element
115
is provided on one side, to uniformly introduce light coming from the light-emitting element
115
into the light guide plate
115
, first it is necessary to make the incident light uniform in the longitudinal direction of the side end face
112
a
of the light guide plate
112
by means of the intermediate light guide
113
. However, since making the incident light on the light guide plate
112
uniform by means of the intermediate light guide
113
is difficult in itself, it is very difficult to obtain output light that is uniform over the entire light exit surface of the light guide plate
112
. In a serious case, this results in a problem that, as shown in
FIG. 23B
, a triangular (in a plan view) dark portion
118
may occur close to the light-emitting-element-
115
-side sideline (left-hand-sideline in
FIG. 23B
) of the light guide plate
112
, which lowers the legibility of the liquid crystal display device.
Where the light guide plate
112
is made thinner to reduce the thickness and size of portable electronic equipment, light traveling inside the light guide plate
112
is prone to leak out of the light guide plate
112
in being reflected by the surfaces of the light guide plate
112
. This results in a problem that the light quantity decreases as the position goes away from the light-emitting element
115
.
As described above, whereas the demand for the front light using a single light-emitting element as a light source is increasing, no front light has been realized which can illuminate a large area brightly and uniformly though it is thin.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problems, and one object of the invention is therefore to provide an illumination device which can illuminate a large area brightly and uniformly though its power consumption is low.
Another object of the invention is to provide a liquid crystal display device that is equipped with such an illumination device and is high in luminance and superior in display quality.
To attain the above objects, the invention employ the following configurations.
The invention provides an illumination device comprising a light source, and a light guide plate for receiving light emitted from the light source through one side end face thereof, and for outputting the light traveling inside the light guide plate through one major surface thereof as a light exit surface, the other major surface, being a light reflecting surface, of the light guide plate being formed, in stripe form in a plan view, with a plurality of prism grooves each of which is formed by a slight slope and a steep slope having an inclination angle larger than that of the slight slope, the inclination angle &thgr;1 of the slight slope and the inclination angle &thgr;2 of the steep slope being in ranges of 1° to 10° and 41° to 45°, respectively, the width of the steep slope in the reflecting surface increasing as the steep slope goes away from the light incident surface in such a manner that the width of the steep slope amounts to 1.1 to 1.5 at a position farthest from the light incident surface if it is assumed that the width of the steep slope is equal to 1.0 at a position closest to the light incident surface.
According to the above configuration, since the shapes and the dimensions of the prism grooves which are formed in the reflecting surface of the light guide plate are set within the above ranges, a high-luminance illumination device can be obtained in which the exit light quantity is uniform in the light exit surface of the light guide plate and the efficiency of utilization of the light source is high. If the inclination angle &thgr;1 of the slight slope is smaller than 1°, the desired amount of luminance for the illumination device is not obtained. If it is larger than 10°, the uniformity of the quantity of exit light that is output from the light exit surface of the light guide plate lowers. Therefore, such ranges of the inclination angle &thgr;1 are not preferable. If the inclination angle &thgr;2 of the steep slope is smaller than 41° or larger than 45°, the luminance of the illumination device lowers. Therefore, such ranges of the inclination angle &thgr;2 are not preferable.
The width of the steep slope is measured in the direction perpendicular to the
Mitsuo Ohizumi
Sugiura Takuro
Yamashita Tatsumaro
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Chowdhury Tarifur R.
Duong Tai
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