Illumination – Revolving
Reexamination Certificate
1997-10-31
2001-03-13
Tso, Laura K. (Department: 2875)
Illumination
Revolving
Reexamination Certificate
active
06199994
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to a surface light source device of side light type, more specifically, relates to a surface light source device of side light type in which uniformity of luminance is improved. The present invention is applied to back lighting of a liquid crystal display device, for example.
2. Related Art
It is know that a surface light source device of side light type provides a back lighting arrangement of thin type for lighting a liquid crystal display panel. In general, the surface light source device of side light type has a light guide plate which is a plate member composed of a light guide material, and a primary light source including a rod-type light source for supplying an illumination light to the light guide plate sideways.
The illumination light emitted from the primary light source is guided into the light guide plate through an end surface of the light guide plate (incidence surface). The guided illumination light is refracted to be emitted from one major surface of the light guide plate to a liquid crystal display panel. Since the primary light source is arranged beside the light guide plate, the whole device can be thinned easily.
As, the light guide plate employed in the surface light source device of side light type, a plate having substantially uniform thickness, and a plate whose thickness tends to become smaller as the distance from the primary light source increases, are known. In general, the latter plate emits an illumination light more efficiently than the former plate.
FIG. 4
is an assembly perspective view showing a general structure of the surface light source device of side light type using the latter light guide plate.
FIG. 5
is a cross section taken along line A—A in FIG.
4
. Referring to both the drawings, a surface light source device of side light type
1
has a light guide plate
2
, and a primary light source
3
is arranged besides the light guide plate
2
. The surface light source device
1
further has a reflection sheet
4
, a diffusible sheet
5
, and prism sheets
6
and
7
which serve as light control members, and they as well as the light guide plate
2
are laminatedly arranged as shown in FIG.
5
.
The primary light source
3
has a cold cathode tube (fluorescent lamp)
8
and a reflector
9
which surrounds the cold cathode tube
8
. The reflector
9
has an opening, and supplies an illumination light to the end surface
2
A of the light guide plate
2
through the opening. The reflector
9
is composed of a regular reflection or irregular reflection sheet material, etc. The light guide plate
2
with wedge-shaped cross section is formed by the injection molding method of acrylic (PMMA) resin, for example. The light guide plate
2
receives the illumination light of the primary light source
3
from the incidence surface
2
A which is one end surface.
The light guided into the light guide plate
2
is simultaneously reflected and propagated repeatedly between a major surface along the reflection sheet
4
(hereinafter, referred to as “slanted surface”)
2
B and another major surface along the diffusible sheet
5
(hereinafter, referred to as “emitting surface”)
2
C.
In every reflecting time, a component having an incidence angle not more than a critical angle is emitted from the slanted surface
2
B and emitting surface
2
C. Therefore, the main propagating direction of the illumination light emitted from the emitting surface
2
C is slanted to a direction of the point of the wedge shape. This is called directional emitting characteristic.
Further, a light diffusible surface
2
D is formed on the slanted surface
2
B. A diffusible characteristic which is increased from the incidence surface
2
A to the point of the wedge shape is given to the light diffusible surface
2
D. The diffusible characteristic is given by application of diffusible ink which includes pigment such as magnesium carbonate and titanic oxide. A degree of the diffusible characteristic can be adjusted according to an amount of application of the diffusible ink.
Instead of by the application of the diffusible ink, by forming the slanted surface
2
B on a satin finished surface (rough surface), the diffusible characteristic may be given. Also in this case, the diffusible characteristic is given so as to be increased gradually towards a direction from the incidence surface
2
A to the point of the wedge shape. For this reason, a region of the rectangular, for example, the satin finished surface is formed at constant or random pitch so as to be distributed with high density towards the direction from the incidence surface
2
A to the point of the wedge shape.
This light guide plate
2
corrects lowering of emission quantity in the vicinity of the point of the wedge shape, and makes a quantity distribution of the emitted light uniform. Here, a point to which special attention should be paid is that the diffusible characteristic is not enough strong to lose emission directivity of the light guide plate. Namely, even when the diffusible characteristic is given to the slanted surface
2
B, the main propagating direction of the illumination light emitted from the emitting surface
2
C is slanted to the direction of the point of the wedge shape.
The reflection sheet
4
is formed by a sheet-type regular reflection member composed of metallic foil, etc. or a sheet-type irregular reflection member composed of white PET film, etc. The illumination light leaking from the slanted surface
2
B is reflected by the reflection sheet
4
to enter the light guide plate
2
again, and as a result, the use efficiency of the illumination light is improved.
The prism sheets
6
and
7
are arranged in order to correct the emission directivity of the light guide plate
2
. The diffusible sheet
5
is arranged so as to prevent the light diffusible surface
2
D of the slanted surface
2
B from visually recognizing from an upper part of the emitting surface
2
c
, and to make brightness and shadow, etc. of each portion of the light guide plate
2
which is irradiated by the illumination light unnoticeable.
The diffusible sheet
5
diffuses the illumination light emitted from the light guide plate
2
. The prism sheets
6
and
7
are formed by transparent sheet materials such as polycarbonates. The prism surface is formed on a surface (outer surface) which is opposite with the light guide plate
2
. This prism surface is composed of a plurality of protrusions having rectangular cross section which are extended substantially parallel with one direction. In this example, the prism sheet
6
on the inner side is aligned that its protrusions are extended parallel with the incidence surface
2
A, and the prism sheet
7
is aligned so that its protrusions are extended to a direction which intersects perpendicularly to the incidence surface
2
A.
The prism sheets
6
and
7
correct the main emitting direction of the emitted light to a frontal direction of the emitting surface
2
C on the slanted surface of each protrusion. A so-called dual-side prism sheet in which the prism surfaces are formed on both the surfaces may be used.
In general, the surface light source device of side light type adopting the wedge-shaped light guide plate and prism sheets can emit an emitting light to the frontal direction more efficiently than the surface light source device of side light type adopting the light guide plate having substantially uniform thickness.
Such a surface light source device of side light type causes following one problem in connection with an electrode of the rod-type light source (fluorescent lamp
8
). In general, the fluorescent lamp
8
as the rod-type light source has electrodes
8
A and
8
B at its both ends. Regions of the inner wall of the tube to which a fluorescent material is not applied exist in the vicinities of the electrodes
8
A and
8
B. For this reason, regions which do not emit illumination lights L efficiently are formed in the vicinities of both the ends of the fluorescent lamp
8
. A portion other than these regio
Enplas Corporation
Staas & Halsey , LLP
Tso Laura K.
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