Illumination – Revolving
Reexamination Certificate
1998-03-03
2002-02-12
Cariaso, Alan S. (Department: 2875)
Illumination
Revolving
C362S331000, C349S061000
Reexamination Certificate
active
06345899
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to a surface light source device of side light type and more particularly to a surface light source device of side light type wherein uniformity in color characteristic of irradiation light is improved. This invention is applicable particularly to back lighting of a liquid crystal display.
2. Description of the Related Art
The surface light source device of side light type has been employed in, for example, a liquid crystal display apparatus. This device irradiates the liquid crystal display panel from its back surface. This disposition is suitable for thinning the entire shape of the apparatus.
In the surface light source device of side light type, ordinarily, a rod-like light source such as cold cathode tube is employed as a primary light source and disposed on a side of the light guide plate. Irradiation light emitted from the primary light source passes a side end face of the light guide plate so that it is introduced into the inside of the light guide plate. The introduced irradiation light is transmitted inside of the light guide plate. In that process, light emission occurs from a major surface of the light guide plate toward a liquid crystal display panel.
As the light guide plate employed in such a surface light source device of side light type, a type having substantially equal plate thickness and another type in which the plate thickness decreases as it goes far from the primary light source have been well known. Generally, the latter emits irradiation light more effectively than the former.
FIG. 12
is a disassembly perspective view showing the surface light source device of side light type using the light guide plate of the latter.
FIG. 13
shows a section along the line A—A of FIG.
12
. Referring to
FIGS. 12
,
13
, the surface light source device of side light type
1
comprise a light guide plate
2
, a primary light source
3
, a reflection sheet
4
, a light scattering sheet
5
, and prism sheets
6
,
7
acting as a light control member. The reflection sheet
4
, light guide plate
2
, light scattering sheet
5
and prism sheets
6
,
7
are laminatedly arranged and fixed by a frame member (not shown).
The light guide plate
2
has two major surfaces. One major surface provides an emission surface
2
C and another major surface provides a back surface
2
B. An end surface of the light guide plate
2
provides an incidence surface
2
A. The primary light source
3
is composed of a fluorescent lamp
8
with a reflector
9
on the back. As well known, the emission light of the fluorescent lamp
8
contains both short-wavelength component (blue component) and long-wavelength component (complementary color component to blue) of visible ray region.
To intensify brightness level and uniformity of the surface light source device, the back surface
2
B of the light guide plate
2
provides' a light scattering surface
2
D. The light scattering surface
2
D contains a distribution pattern composed of, for example, a great number of light scattering dot-like elements. This distribution pattern is called dot pattern.
FIG. 14
is a view for explaining a basic type of the distribution pattern on the light scattering surface
2
D, which depicts the light guide plate
2
viewed from the back surface
2
B. In
FIG. 14
, the light scattering elements are expressed by a great number of large and small squares. The light scattering elements are formed in the form of light scattering ink layer, fine uneven surface or the like. The light scattering ink contains pigment of, for example, magnesium carbonate, titanium oxide or the like. The light scattering property of the light scattering surface
2
D tends to increase corresponding to distance from the incidence surface
2
A along the lamp
8
to obtain an equal brightness level. This tendency is realized by design of the distribution of size, density or other factors of the light scattering element.
The primary light source
3
is provided with a reflector
9
having a semi-circular section, disposed on the back of a cold cathode tube (fluorescent lamp)
8
. The irradiation light is supplied to an end face of the light guide plate
2
through an opening of the reflector
9
. For the reflection sheet
4
, a sheet-like regular reflection member composed of metallic foil or the like or a sheet-like irregular reflection member composed of white PET film or the like is employed. The reflection sheet
4
brings back light leaking from the back surface
2
B to the light guide plate
2
so as to reduce loss of light.
The irradiation light L from the primary light source
3
is introduced into the light guide plate
2
through the incidence surface
2
A. The irradiation light L is transmitted toward an end (wedge distal end) while being repeatedly reflected between the back surface
2
B along which the reflection sheet
4
is laid and the emission surface
2
C. In this while, the irradiation light L is subjected to light scattering action by the light scattering surface
2
D formed on the back surface
2
B. If the reflection sheet
4
composed of the irregular reflection member is employed, the irradiation light is also subjected to the irregular reflection action.
Each time when the irradiation light L is reflected by the slope
2
B, the incidence angle relative to the emission surface
2
C gradually decreases. The decrease of the incidence angle increases component having a smaller angle than the optimum angle relative to the emission surface
2
C thereby inducing emission from the emission surface
2
C. Because this operation is intensified as it goes far toward the end of the light guide plate
2
, a shortage of the emission light in a region far from the incidence surface
2
A is compensated.
The irradiation light emitted from the emission surface
2
C passes the light scattering sheet
5
and is directed toward the prism sheets
6
,
7
. The light scattering sheet
5
scatters the irradiation light emitted from the light guide plate
2
, thereby preventing the light scattering surface of the back surface
2
B from being visually noticed from above the emission surface
2
C. The light scattering sheet
5
further suppresses local excessive brightness, shade and the like in the light guide plate
2
. Depending on the case, the light scattering sheet
5
is not employed.
The prism sheets
6
,
7
are formed of transparent sheet-like material of polycarbonate or the like. The prism surface is formed on an opposite side to a surface (outside surface) opposing the light guide plate
2
. The prism surface has a great number of projections which extend substantially parallel in a single direction and have a triangle section. In the inside prism sheet
6
, its projections are oriented so as to run parallel to the incidence surface
2
A. The outside prism sheet
7
is oriented so that the projections extend substantially perpendicular to the incidence surface
2
A.
The slopes of these projections correct main emission direction of the emission light to the frontal direction of the emission surface
2
C. A double-sided prism sheet having each prism surface on both sides may be used.
Generally, as compared with the surface light source device of side light type in which a substantially equal thickness light guide plate is employed, the surface light source device of side light type employing such a wedge-shaped light guide plate and a prism sheet is capable of emitting the emission light more effectively to the frontal direction.
However, the aforementioned conventional surface light source device of side light type 1 has a problem. That is, there is produced a phenomenon that the color of the emission light changes delicately depending on distance from the incidence surface
2
A. This tendency is more remarkable as size of the light guide plate
2
increases.
According to an experiment, as described in
FIG. 13
, the irradiation light becomes bluish near the incidence surface
2
A (blue light component tends to be excessive), and yellowish near the wedge distal end (end of t
Arai Takayuki
Ohkawa Shingo
Cariaso Alan S.
Enplas Corporation
Sawhney Hargobind
Staas & Halsey , LLP
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