Illumination – Revolving
Reexamination Certificate
2001-03-26
2002-11-26
O'Shea, Sandra (Department: 2673)
Illumination
Revolving
C362S333000, C362S346000, C362S026000, C349S065000, C359S599000, C385S901000
Reexamination Certificate
active
06485157
ABSTRACT:
BACKGROUND
1. Field of Invention
The present invention relates to a light guide plate which is supplied with light sideways and deflects the light to output from an emission face, further relating to a surface light source device employing the light guide plate as well as to a liquid crystal display employing the surface light source device for back-lighting or front-lighting.
2. Related Art
A surface light source device of a type comprises a light guide plate having an end face, through which light is introduced, and two major faces (i.e. faces larger than end faces) one of which provides an emission face, being employed for various uses such as back-lighting or front-lighting for a liquid crystal display. Basic performance of surface light source devices of such type greatly depends on light guide plates employed therein.
A basic function of a light guide plate is to change a propagation direction (roughly in parallel with an emission face of the light guide plate) of light introduced into the light guide plate through a side end face so that the light is emitted through the emission face. As known well, a simply transparent light guide plate without any modification is capable of deflecting light little, providing unsatisfactory brightness. Consequently, any means for promoting emission through the emission face is required.
In general, means for promoting emission from a light guide plate relies upon one of the followings or some of them as combined:
(1) Scattering power within the light guide plate (light scattering guide plate);
(2) Emission face (a major face) provided with light diffusibility;
(3) Back face provided with light diffusibility;
(4) Emission face provided with light-refractive unevenness;and
(5) Back face provided with light-refractive unevenness.
Ways based on (1) provide uniform and highly effective emission with ease. However, the emission is subject to have a preferential direction much inclined with respect to a frontal direction. Usually, the inclination is about 60 to 75 degrees to a normal with respect to the emission face.
Accordingly, an element for modifying the inclined direction to the frontal direction (prism sheet) must be arranged. A light diffusion sheet, if alternatively employed, can bring some increase in frontal emission. However, light diffusion involves a loss of light because of divergence toward useless directions.
Ways based on (2) or (3) hardly provide a highly effective emission. The emission is also preferentially directed to oblique directions as in the case of (1). An increase light diffusibility checks the efficiency because of factors such as wide range scattering or absorption by light scattering elements (for example, by a white ink).
Although ways based on (4) are capable of causing light to escape from the emission face with ease, positive direction conversions are less effected. Accordingly, emission with a high efficiency is less expected. In particular, it is not advantageous that they fail to generate light rays which travel from the back face to the emission face.
To the contrary, ways based on (5) positively generate light which travels from a back face to an emission face of a light guide plate, being free from wide range light scattering. Accordingly, the ways are latently capable of effectively generating emission directed to roughly frontal directions.
FIG. 1
a
to
FIG. 1
c
illustrate examples based on the above (5). Referring to the figures, reference number
1
indicates a light guide plate made of a transparent material such as acrylic resin, the plate having a side end face to provide an incidence face
2
. A primary light source L is disposed beside the incidence face
2
to be supplied with light from the primary light source L. One of two major faces
3
and
4
of the light guide plate
1
provides an emission face
3
. The other major face (called “back face”) is provided with a great number of recesses
5
having a cross section including slopes
5
a
and
5
b.
The primary light source L emits light which is introduced into the light guide plate
1
through the incidence face
2
. Upon encountering a recess, a propagation light within the light guide plate
1
(as represented by G
1
, G
2
) is inner-reflected by a slope
5
a
to be directed to the emission face
3
. Inner-incidence angle is denoted by &thgr; and emission beams derived from beams G
1
, G
2
are denoted by G
1
′, G
2
′. In other words, the slope
5
a,
which is relatively near to the incidence face
2
(or primary light source L) compared with the other slope
5
b,
provides an inner-reflection slope for direction conversion. This effect is sometimes called edge-lighting effect.
The recesses
5
are formed like dots or linear channels. As shown in
FIG. 1
a
to
FIG. 1
c,
formation pitch d, depth h or slope inclination &phgr; of the recesses
5
is varied depending on distance from the incidence face
2
. Such variations prevent brightness on the emission face
3
from varying depending on distance from the incidence face
2
.
However, prior arts as shown in
FIG. 1
a
to
FIG. 1
c
are subject to the following problems.
1. Light is hard to reach a region behind the slope
5
b
as viewed from the incidence face
2
. Therefore, a reduction of formation pitch d hardly rises direction conversion efficiency and the emission face
3
is apt to show an unevenness in brightness.
2. Sufficient direction control in a plane parallel to the incidence face
2
is not effected. For instance, if beams G
1
and G
2
are parallel to the emission face
3
but not perpendicular to the incidence face
2
, emission beams G
1
′ and G
2
′ will be diverged to the right or left as viewed from the incidence face
2
. Actually, there is considerable light components which propagate not perpendicularly with respect to the incidence face
2
within the light guide plate
1
. Accordingly, it is difficult to provide an emission to a desirable angle or within a desirable angle range spatially (i.e. in both planes parallel and vertical to the incidence face
2
).
3. Light leaking through the back face
4
occurs easily because direction conversion for generating light directed to the emission face
3
relies upon once-occurring-reflection (at slope
5
a
). That is, the condition for total reflection is broken with ease at the reflection for direction conversion. For instance, if beams G
1
′ and G
2
′ are required to be directed to approximately frontal directions, inner-incidence angle &thgr; is set at about 45 degrees. This is roughly the same as the critical angle for an interface between air and acrylic resin which is a typical material. Therefore, a considerable part of light propagating slightly downward leaks through the slope
5
a.
The present inventor proposed a light guide plate and surface light source device/LCD employing the light guide plate, which were disclosed Japanese Patent Application Tokugan-Hei 11-38977. A brief explanation of the proposed technique is as follows, being aided by FIG.
2
and
FIGS. 3
a,
3
b.
FIG. 2
is a plan view showing an arrangement of a surface light source device as viewed from a back side of a light guide plate arranged therein, the arrangement being disclosed in the above-mentioned patent application.
FIG. 3
a
is a partially enlarged perspective view of the light guide plate employed in the surface light source device shown in
FIG. 2
, and
FIG. 3
b
is a partially enlarged view of one of projection-like micro-reflectors formed on a back face of the light guide plate. Note that sizes of micro-reflectors are exaggerated for the sake of explanation.
Referring to
FIG. 2
, a light guide plate
10
made of a transparent material. The light guide plate
10
has an end face (minor face) to provide an incidence face
12
. A back face referenced with numeral
14
is a back face provided by one of major faces. The other major faces provides an emission face (See
FIG. 3
a
). The light guide plate
10
has right and left side end faces (minor faces)
15
and
16
.
A rod-like primary lig
Alavi Ali
Enplas Corporation
O'Shea Sandra
Staas & Halsey , LLP
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