Illumination – Illuminated scale or dial – Edge illuminated modifier or light rod/pipe
Reexamination Certificate
1998-08-03
2002-07-02
O'Shea, Sandra (Department: 2875)
Illumination
Illuminated scale or dial
Edge illuminated modifier or light rod/pipe
C362S035000, C362S330000, C362S060000
Reexamination Certificate
active
06412968
ABSTRACT:
BACKGROUND
1. Field of Invention
The present invention relates to a surface light source device of side light type applied to backlighting, a liquid crystal display and a light guide plate suitable for use therein.
2. Related Art
Conventionally, a surface light source device of side light type is, for instance, applied to a backlighting arrangement of a liquid crystal display. This arrangement is suitable for making the overall shape of the device thinner.
Generally, the surface light source device of side light type uses a wedge-shaped light source, such as a cold cathode tube, as a primary light source which is provided at the side of a light guide plate (plate-like guide body). Illumination light emitted from the primary light source is fed into the guide plate through a side end face thereof. Having been fed inside, the illuminating light propagates through the guide plate, whereby light is output from a major surface of the guide plate toward a liquid crystal display.
Known types of guide plate include a guide plate having a generally uniform thickness and a guide plate which decreases in thickness as distance from the primary light source increases. Generally, the latter emits illuminating light output more effectively than the former.
FIG. 20
is an exploded perspective view of a surface light source device of side light type using the latter type of guide plate.
FIG. 21
is a cross-sectional view taken along the line A—A of FIG.
20
. As shown in FIG.
20
and
FIG. 21
, a surface light source device
1
comprises a guide plate
2
, a primary light source
3
, which comprises a wedge-shaped light source element
7
and a reflector
8
, a reflection sheet
4
and light control members consisting of prism sheets
5
and
6
.
The guide plate
2
comprises a scattering guide body, which is wedge-shaped in cross-section, known as a scattering guide plate. The scattering guide body is made of a material having a uniform scattering function, and comprises a matrix of, for instance, PMMA (polymethyl-methacrylate) and a great number of particles which are uniformly dispersed therein. The refractive index of these particles is different from that of the matrix.
The guide plate
2
has two major surfaces
2
B and
2
C. The major surface
2
C is called an emission surface and outputs illuminating light. The other major surface is called a back surface.
The light source element
7
comprises, for instance, a cold cathode tube (fluorescent lamp), and a reflector
8
, generally semicircular in cross-section, is provided to the rear of the light source element
7
. Illumination light is supplied through the opening of the reflector
8
toward the side end surface of the guide plate
2
. A sheet-like specular reflection member comprising metal foil or the like, or a sheet-like diffussive reflection member comprising white PET film or the like, is used as the reflection sheet
4
.
After light L from the primary light source
3
has been led through the incidence surface
2
A into the guide plate
2
, inside the guide plate
2
, the illuminating light L propagates toward the end while being repeatedly reflected between the emission surface
2
C and the back surface
2
B, along which the prism sheet
4
is arranged. During this time, the light receives the scattering action inside the guide plate
2
. When a reflection sheet
4
having light-diffusion properties is used, dispersing action of this reflection sheet
4
also has effect. As a general tendency, the incidence angle to the emission surface
2
C gradually decreases each time the light is reflected from the slope
2
B. This increases components which are below the critical angle to the emission surface, facilitating emission from the emission surface. Consequently, insufficient emission of light in regions which are far from the primary light source
3
is prevented.
As a result of the above scattering action, the illuminating light emitted from the emission surface
2
C has a great many propagation directions. However, the light is not completely scattered; the main direction of propagation inclines in the end direction (the opposite direction to the primary light source
3
) with respect to the front direction. In other words, light emitted from the guide plate
2
has directivity. This property of the guide plate
2
is known as directional emission.
The prism sheets
5
and
6
are provided in order to correct this directivity. The prism sheets
5
and
6
comprise light-permeable sheet-like material such as, for instance, polycarbonate. In many cases, the prism surface of the prism sheet
5
is provided facing the scattering guide plate
2
, while the prism surface of the prism sheet
6
is provided with its back facing the guide plate
2
.
Each of the prism surfaces comprises a great number of micro-projections, triangular in cross-section, which run generally parallel to one direction. The inner side prism sheet
5
is provided so that its projections run generally parallel to the incidence surface
2
A. The outer side prism sheet
6
is provided so that its projections run generally perpendicularly to the incidence surface
2
A.
The slopes of these projections correct the main propagation direction of emitted light to the frontal direction of the emission surface
2
C. Prism surfaces may provided on both faces; in other words, double-faced prism sheets may be used.
The above surface light source device of side light type generally emits light in the frontal direction more effectively than a device of the same type which uses a guide plate of practically uniform thickness.
However, the conventional device described above has a tendency whereby an undesirable region of low brightness is generated on the guide plate
2
. This reduces the uniformity of the light output.
As shown for instance by reference mark AR
1
in FIG.
20
and
FIG. 22
a
, this low brightness region occurs in the vicinity of the incidence surface
2
A.
Brightness distribution along the line B—B in
FIG. 22
a
varies depending on density of particles dispersed inside the scattering guide plate
2
.
FIG. 22
b
illustrates two examples of brightness distribution (high density/low density). As can be understood from this graph, it is difficult to make brightness distribution flat, and thereby eliminate the low brightness region AR
1
, merely by adjusting the particle density.
As shown in
FIG. 22
a
, the low brightness region AR
1
tends to spread near the corners. The positions of the corners correspond to electrodes
7
A and
7
B of a wedge-shaped lamp
7
. The low brightness region AR
1
is liable to spread here since the electrodes
7
A and
7
B have weaker light supply power than the center
7
C.
The following two solutions (1) and (2) to this problem have been proposed.
(1) According to a first proposed solution, as shown in the cross-section in
FIG. 22
c
, the back surface
2
B of the scattering guide plate
2
is shaped in a gentle curve, and particle density is adjusted so that brightness distribution becomes flat.
(2) According to another proposed solution, as shown in
FIG. 23
, groups of surface region elements
9
, which have scattering a function; are provided in patterns of pitch P on the back surface
2
B of the scattering guide plate
2
. These are known as scattering patterns. The scattering patterns
9
increase the amount of illuminating light components below the critical angle to the emission surface
2
C in the vicinity of the scattering patterns
9
, consequently facilitating light emission.
The covering rate (occupancy rate) of the scattering patterns
9
increases near the edges. The scattering patterns
9
can be formed by local deposition of scattering ink, or by locally making the back surface
2
B rough.
These proposed solutions are easily implemented when the back surface of the guide plate is basically a flat surface, but are difficult to implement when a great number of projections are provided on the back surface of the guide plate.
FIG. 24
is an exploded perspective view of a conventional surface light source device of
Enplas Corporation
O'Shea Sandra
Staas & Halsey , LLP
Ton Anabel
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