Optical: systems and elements – Prism – With refracting surface
Reexamination Certificate
1998-10-28
2001-04-24
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Prism
With refracting surface
C362S035000, C385S036000, C349S062000, C349S064000
Reexamination Certificate
active
06222689
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a surface light source device of side light type employing a directional emission guide plate and an(asymmetric prism sheet, and to an asymmetric prism sheet which can be applied to the device. In this specification, “asymmetric prism sheet” is an abbreviation for “sheet-like optical element having a prism surface which comprises a great number of asymmetric prism element rows”.
BACKGROUND
A surface light source device employing a directional emission guide plate, comprising a light-scattering guide body or a light-permeable guide body and a prism sheet, has been proposed, and is widely applied for uses such as backlighting of a liquid crystal display. The prism sheet comprises a plate-like member of optical material, having a prism surface comprising a great number of prism element rows. It is known that such a prism sheet is capable of correcting propagation direction properties of light fluxes.
FIG. 1
shows a partial cutaway perspective of a diagrammatic constitution of a liquid crystal display which uses as backlighting a surface light source device of side light type, employing a conventional prism sheet. Thickness of the prism sheet
4
and other elements, as well as formation pitch, and depth and others, are exaggerated for conveniences of illustration.
As shown in
FIG. 1
, a directional emission guide plate
1
comprises an optical member, which is wedge-shaped in cross-section, having a light-scattering guide body or a light-permeable guide body. The scattering guide body is a known optical member which performs functions of guiding and internal scattering, comprising a matrix of, for instance, polymethyl-methacrylate (PMMA) and a “substance of different refractive index” which is mixed and dispersed uniformly in the matrix. “Different refractive index” means a substance having a refractive index which is actually different from that of the matrix.
The end face of the thick side of the guide plate
1
constitutes a light incidence surface
2
, and a primary light source element (fluorescent lamp) L backed by a reflector R is provided near the incidence surface
2
. A reflector
3
is provided over one major surface (under face)
6
of the guide plate
1
. The reflector
3
comprises a frontal-reflecting silver-foil sheet or a diffused-reflecting white sheet. Illuminating light is extracted from the other major surface (light emission surface or light extraction surface)
5
, which is the front surface of the guide plate
1
. A prism sheet
4
is provided on the outer side of the front surface
5
with the prism surface facing inward.
In the explanatory cutaway portion, the outer face
4
c
of the prism sheet
4
is shown as an even surface. A liquid crystal panel LP is provided to the outer side of the even face
4
c
with a polarization separating sheet LS therebetween. The liquid crystal panel LP has a known constitution wherein a liquid crystal cell, light-permeable electrode and others sandwiched between two polarizing plates, arranged so that their axes of polarization intersect at a right angle.
The polarization separating sheet LS is an optical element in recent use, provided between the polarizing plate on the inner side of the liquid crystal panel and the prism sheet
4
. This polarization separating sheet LS has high permittivity with respect to polarization components in the same direction as the polarization axis of the polarizing plate on the inner side, and high reflectivity with respect to polarization components in the direction at a right angle to the same polarizing plate.
Proposals for this type of polarization separating sheet LS include a multi-refractive polarizer (see Japan Patent Laid-Open Publication No. 4-295804), comprising a laminated arrangement of a great number of alternate layers of a first polymer substance, having a positive stress optical factor, and a second polymer substance, having a negative stress optical factor, or a polarizer made by providing alternate layers having high refractive index and low refractive index, selected to satisfy McNiall conditions, on a prism surface of one of a pair of light-permeable substrates, having isosceles-triangular prisms arranged in a straight line on one face thereof, and then optically connecting the pair of substrates with their prism surfaces facing each other (see Japan Patent Laid-Open Publication No. 6-51399), and such like.
Normally, a space (vacant layer) is provided between the liquid crystal panel LP and the polarization separating sheet LS, or between the liquid crystal panel LP and the prism sheet
4
(when a polarization separating sheet cannot be used), to prevent these elements from sticking to each other, but this space is not shown in the diagram.
The prism surface, which forms the inner surface of the prism sheet
4
, has a great number of rows of prism elements. The great number of rows of prism elements are provided substantially parallel to the incidence surface
2
of the guide plate
1
. As shown in the partial enlarged cross-sectional view, each row of prism elements has a pair of slopes
4
a
and
4
b
which form a V-shaped groove.
Hereinafter, in the present specification, the slope angle of the slope
4
a,
which faces the light incidence surface
2
of the guide plate
1
, will be represented as &phgr;a, and the slope angle of the slope
4
b
facing the opposite side will be represented as &phgr;b. The slope angles &phgr;a and &phgr;b are defined with respect to the front surface direction (see reference numeral N). Many conventional devices use a symmetric prism sheet (i.e. &phgr;a=&phgr;b).
Light, sent from the light source element L into the guide plate
1
, is subject to the scattering and reflection action within the guide plate
1
while being guided toward the end surface
7
of the thin side of the guide plate
1
. Through this process, illuminating light is gradually emitted from the front surface
5
.
As is well known, light emitted from the front surface
5
of the guide plate
1
, which has received light supplied from the side, exhibits directivity of considerable clarity, and therefore such a guide plate
1
is known as a directional emission guide plate. When strong light diffusion properties are given to the front surface
5
or under face of the guide plate
1
, the guide plate
1
may lose its property of directional emission. FIG.
2
and
FIG. 3
are graphs illustrating emission characteristics of the front surface
5
of three example guide plates A (FIG.
2
), B (
FIG. 2
) and C (FIG.
3
).
In each of these graphs, the horizontal axis represents the direction of the brightness measurement. An angle of 0 degrees represents the frontal direction, minus represents the incidence surface
2
side and positive represents the end side (forward). The vertical axis represents relative brightness in unit (a.u.) to a peak value of 0.1. In
FIG. 2
, thick-line curve A represents characteristics of guide plate A, and thin-line curve B represents characteristics of guide plate B. And, in
FIG. 3
, curve C represents characteristics of guide plate C. The characteristics were measured using a brightness meter, to investigate brightness at different angles, in a plane perpendicular to the incidence surface
2
, near a center point in the front face of guide plate. The distance from the brightness meter to the center point was 203 mm.
Each data of guide plate A to C are following:
Guide Plate A
Material: light-scattering guide body=silicon-type resin particles were uniformly dispersed at 0.08 wt % within a matrix of PMMA (polymethyl-methacrylate). The refractive index was approximately 1.5;
Size: depth when viewed from the light incidence surface side was 51.5 mm, width 68.3 mm; thickness of light incidence surface side end portion was 3.0 mm, with thickness of end surface portion being 0.2 mm.
Guide Plate B
Material: light-scattering guide body=silicon-type resin particles were uniformly dispersed at 0.025 wt % within a matrix of PMMA (polymethyl-methacrylate). The refractive index
Higuchi Eizaburo
Koike Yasuhiro
Cherry Euncha
Enplas Corporation
Spyrou Cassandra
Staas & Halsey , LLP
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