Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
2001-08-15
2004-01-13
Eckert, George (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C349S062000, C349S064000, C349S095000
Reexamination Certificate
active
06678021
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, still further relating to a liquid crystal display employing the surface light source device for lighting of LCD panel, in particular, front-lighting of it.
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 a 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 from the emission face. As known well, a simply transparent light guide plate to which no modification is applied is capable of deflecting light slightly, providing an unsatisfactory brightness. Therefor any means for promoting emission from the emission face is required.
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 a 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;
(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.) Therefore, a member (prism sheet) for modifying the inclined direction to the frontal direction must be arranged. Although employment of a light diffusion sheet brings some increase in frontal emission, it involves a wide light diffusion which leads to reduction in light energy efficiency.
Ways based on (2) or (3) hardly provide uniform and effective emission. The emission is also preferentially directed to oblique directions as in the case of (1). An increased light diffusibility checks the efficiency because of factors such as wide range scattering or absorption by light scattering elements (e.g. white ink).
Ways based on (4) are capable of causing light to escape from the emission face with ease while positive direction conversions are less effected. Accordingly, it is hardly expected to realize a highly efficient emission. In particular, it is not advantageous that they fail to generate light which travels from the back face to the emission face.
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 efficiently generating an emission directed to approximately frontal directions. A further merit is that a good applicability to a front-lighting-type LCD, which has been used often recently, is realized.
However, in practice, prior arts fail to control propagating direction of emission sufficiently.
FIG. 1
a
to
FIG. 1
c
illustrate examples to which the above (5) is applied. Referring to the illustrations, reference number
1
indicates a light guide plate made of a transparent material such as acrylic resin, which has a side end face to provide an incidence end face
2
. A primary light source L is disposed beside the incidence end face
2
to be supplied with light from the primary light source L. One of two major faces
3
,
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
with slopes
5
a,
5
b
in profile.
The primary light source L emits light, which is introduced into the light guide plate
1
through the incidence end face
2
. Upon encountering a recess, the propagation light within the light guide plate
1
(as represented by G
1
, G
2
) is inner-reflected by one slopes
5
a
to be directed to the emission face
3
. Inner-incidence angle is denoted by &thgr; and an emission derived from beams G
1
, G
2
is denoted by G
1
′, G
2
′. In other words, the slope
5
a,
which is relatively near to the incidence end 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
FIGS. 1
a
to
1
c,
formation pitch d, depth h or slope inclination &phgr; of the recesses
5
is varied depending on distance from the incidence end face
2
. Such variations prevent brightness on the emission face
3
from varying depending on distance from the incidence end face
2
.
However, prior arts as shown in
FIGS. 1
a
to
1
c
are subject to the following problems.
1. There is a region which is located behind the slope
5
b
as viewed from the incidence end face
2
and is hardly supplied with light. Therefore, a reduced formation pitch d gives no increasing in direction conversion efficiency, with the result that the emission face
3
is apt to show an unevenness in brightness.
2. Direction control regarding in a plane parallel to the incidence end face is not sufficiently effected. For example, if travelling directions of G
1
, G
2
shown in
FIG. 1
a
are parallel to the emission face
3
but not perpendicular to the incidence end face
2
, emitted light G
1
′, G
2
′ will diverge to the right and left as viewed from the incidence end face
2
. There is a remarkable amount of light component which is not perpendicular to the incidence end face
2
in an actual light guide plate. Therefore, it is difficult to obtain an emission which is directed to a desirable spatial direction (regarding in both planes perpendicular and parallel to the incidence end face).
3. Leaking of light from the back face
4
occurs easily because direction conversion to produce light directed to the emission face
3
relies on a single reflection (slope
5
a
). In other words, a condition of total reflection is broken easily at the reflection for direction conversion. For example, if beams G
1
′, G
2
′ are to be directed toward a generally frontal direction, inner incidence angle is about 45 degrees. This value is roughly equal to the critical angle of an interface between acrylic resin, which is a typical material of a light guide plate, and air. Therefore, a remarkable part of light directed somewhat downward leaks through the slope
5
a.
The present inventor proposed a light guide plate having a back face provided with a great number of micro-reflectors shapes like projections as shown in
FIG. 2
b,
and surface light source device/LCD employing the light guide plate, which were disclosed (PCT/JP00/00871;WO00/49432).
FIG. 2
a
is a partially enlarged perspective view around a micro-reflector to illustrate light paths of an inner input light. Note that size of micro-reflector is exaggerated for the sake of explanation.
As shown in
FIG. 2
a,
a light guide plate
100
has a back face
114
provided with micro-reflectors
120
projecting from a general plane of the back face. The illustrated micro-reflector
20
has a shape like a block having six faces
121
,
122
,
123
,
124
,
127
and
128
.
The faces
121
and
122
provide a guiding portion to effect a smooth light input for direction-conversion. The faces
121
and
122
meet each other at a ridge portion
126
. On the other hand, the faces
123
and
124
effect reflections twice for
Eckert George
Enplas Corporation
Richards N. Drew
LandOfFree
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