Optical: systems and elements – Diffusing of incident light
Reexamination Certificate
2002-05-15
2003-10-28
Sikder, Mohammad (Department: 2872)
Optical: systems and elements
Diffusing of incident light
C359S566000, C359S707000, C359S709000, C359S710000
Reexamination Certificate
active
06639725
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a protective, diffusive film and more particularly to a high-quality protective, diffusive film which can suppress curling and deflection, a process for producing the same, and a surface light source device and a liquid crystal display device using said protective, diffusive film.
2. Background Art
A large number of surface light source devices provided with a protective, diffusive film have hitherto been proposed. An example of a surface light source device using a protective, diffusive film
110
is shown in FIG.
5
.
FIG. 5
is a cross-sectional view of a liquid crystal display device
135
provided with a surface light source device
120
which is an edge-type flat light source. The surface light source device
120
comprises, for example, a light source
121
, a light guide plate
122
, a reflection film
124
, a light diffusive film
125
, a lens film
140
, and a protective, diffusive film
110
.
The light guide plate
122
is surface light projection means, and has a light source
121
on its side end and, on its non-light-outgoing surface remote from a light outgoing surface
122
a
, a dot pattern
123
for diffusing light from the light source
121
and directing the light to a light outgoing direction. The reflection film
124
is provided on the non-light-outgoing surface side of the light guide plate
122
and functions to block off unnecessary light beams and, at the same time, to reflect and return light beams to a predetermined direction.
A lens film
140
is disposed on the light outgoing surface
122
a
side of the light guide plate
122
through a light diffusive film (a diffusive plate)
125
for diffusing light to conceal the dot pattern
123
in such a manner that the lens film
140
on its prism surface
140
a
faces the light outgoing surface side.
In order to impart the function of diffusing light to the light diffusive film (diffusive plate)
125
, the light diffusive film (diffusive plate)
125
has hitherto been formed by incorporating, in a dispersed state, organic or inorganic beads as a light diffusing agent into a transparent resin substrate, or by coating an ink containing organic or inorganic beads as a diffusing agent onto a transparent resin substrate.
A protective, diffusive film
110
is provided on the light outgoing surface side of the lens film
140
, for preventing the prism
140
a
in the lens film
140
and a liquid crystal display element
133
from coming into direct contact and being damaged by each other, for example, due to vibration in transit. The protective, diffusive film
110
also has the function of slightly diffusing light for concealing stripes of the prism
140
a
in the lens film
140
, a spacer (not shown), and the like. To this end, the protective, diffusive film
110
has hitherto been formed by incorporating, in a dispersed state, organic or inorganic beads as a light diffusing agent into a transparent resin substrate, or by coating an ink containing organic or inorganic beads as a diffusing agent onto a transparent resin substrate. A transmission liquid crystal display element
133
comprising a liquid crystal layer
130
sandwiched between a lower substrate
132
and an upper substrate
131
is provided on the light outgoing side of the surface light source device
120
, and the liquid crystal display element
133
is illuminated from backside by the surface light source device
120
.
The conventional protective, diffusive film
110
is generally formed by coating an diffusing ink (an ink prepared by adding a diffusing agent in a resin) on both sides of a transparent substrate. As shown in Table 1, there is a difference in coverage between the coating on the top surface of the transparent substrate and the coating on the under surface of the transparent substrate.
TABLE 1
Difference in coverage between coating on top surface and
coating on under surface of protective, diffusive film
Coverage
Function
Viewer side (top
Large
Impartation of light diffusive
surface)
properties
Light source side
Small
Prevention of interference
(under surface)
fringe appearance defects
(prevention of sticking
In the conventional protective, diffusive film, when the difference in coverage between the coating on the top surface of the transparent resin substrate and the coating on the under surface of the transparent resin substrate is large, curling and/or deflection are likely to occur through the action of heat and/or humidity. This is considered attributable to the difference in stretching, derived from the difference in coverage, between the diffusing ink coating on the top surface of the transparent resin substrate and the diffusing ink coating on the under surface of the transparent resin substrate. In particular, under conditions within backlight for a liquid crystal display device which is brought to a closed space in which the temperature is raised to a high temperature, i.e., up to about 90° C., “deflection” of the film sometimes occurred. When the coverage of the coating on the light source side (under surface) was made substantially equal to the coverage of the coating on the viewer side (top surface), the following phenomena often occurred. (i) The concaves and convexes on the surface were reduced, and interference fringe appearance defects between the protective, diffusive film and the underlying light guide plate or lens film sometimes occurred. (ii) Increasing the amount of the diffusing agent (fine particles) enhanced diffusive properties (i.e., enhanced the haze value) and consequently lowered the utilization of light and sometimes deteriorated luminance.
In the conventional protective, diffusive film, the diffusive properties was imparted by adding a diffusing agent to the resin. Therefore, a variation in coverage caused a change in diffusive properties or a change in the shape of surface concaves and convexes. This sometimes affected the optical characteristics and the sticking preventive properties.
SUMMARY OF THE INVENTION
The present inventor has now found that, when resin layers are provided respectively on the top surface and the under surface of the substrate layer by not using any diffusing agent (for example, finely particulate material) and bringing the difference in coverage between the resin layer provided on the top surface of the substrate layer and the resin layer provided on the under surface of the substrate layer to a value falling within a specific range, curling and deflection can be effectively prevented and, consequently, a high-quality protective, diffusive film can be provided. The present invention has been made based on such finding.
Accordingly, it is an object of the present invention to provide a protective, diffusive film possessing excellent optical characteristics and sticking preventive properties.
According to one aspect of the present invention, there is provided a protective, diffusive film provided with a lens film, said protective, diffusive film comprising:
a transparent substrate layer; and
a resin layer provided on the top surface and the under surface of the transparent substrate layer,
said resin layers being free from fine particles and having fine concaves and convexes on the surface thereof,
the difference in coverage between the resin layer provided on the top surface of the transparent substrate layer and the resin layer provided on the under surface of the transparent substrate layer being in the range of −20% to +20% in terms of the percentage of the coverage of one of the resin layers to the coverage of the other resin layer.
According to another aspect of the present invention, there is provided a process for producing the protective, diffusive film according to the present invention, said process comprising the steps of:
introducing an ionizing radiation-curable resin into a cylinder having an inverted shape of concaves and convexes of the resin layer; and
applying an ionizing radiation to the ionizing radiation-curable resin to cure the ionizing radiation-curable r
Arakawa Fumihiro
Masaki Tadahiro
Dai Nippon Printing Co. Ltd.
Parkhurst & Wendel L.L.P.
Sikder Mohammad
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