Illumination – Revolving
Reexamination Certificate
2001-04-04
2003-06-03
Cariaso, Alan (Department: 2875)
Illumination
Revolving
C362S330000, C362S561000, C349S065000
Reexamination Certificate
active
06572236
ABSTRACT:
PRIOR FOREIGN APPLICATION
This application claims priority from Japanese patent application number 2000-126706, filed Apr. 26, 2000, which is hereby incorporated herein by reference in its entirety.
TECHNICAL FIELD
The present invention relates to a back light unit for use in a liquid crystal monitor or the like, a liquid crystal display, and a method for manufacturing a light guide plate.
BACKGROUND ART
Liquid crystal displays are widely spread as image display devices for personal computers and various other monitors. Liquid crystal displays of this kind generally comprise a back light, planar light source for illumination, on a rear surface of a liquid crystal display panel to apply light to a liquid crystal surface with a predetermined spread in a fashion providing a uniform brightness, thereby visualizing images formed on the liquid crystal surface.
Such a back light uses a fluorescent lamp with a hot or cool cathode as a light source and must irradiate the overall surface of the liquid crystal display panel with light from what is called a linear light source comprising such a fluorescent lamp. Thus, the back light conventionally uses two methods, that is, a direct-light type and a side light type (an edge light type). The direct-light type has the fluorescent lamp placed immediately below the liquid crystal display panel and a uniformity plate and a diffusion plate installed thereon. On the other hand, the side light type has the fluorescent lamp installed on two sides or one side of a light guide plate made of a transparent resin so that light incident on the light guide plate is directed toward a liquid crystal display panel surface by means of a reflection section formed on a rear surface of the light guide plate; the light is then diffused into a uniform planar light.
The side light type back light may be thinner than the direct-light type back light and is thus suitable for display devices for portable equipment such as notebook computers.
FIG. 13
a
shows an example of the side light type back light. As shown in this figure, the back light
1
comprises a lamp
2
comprising a fluorescent lamp and acting as a light source, and a light guide plate
3
. The light guide plate
3
has an incoming surface
3
a
opposed to the lamp
2
and on which light from the lamp
2
is incident, an outgoing surface
3
b
that faces a liquid crystal display panel (not shown) and from which planar light is emitted, and an opposed surface
3
c
that is opposed to the outgoing surface
3
b.
The opposed surface
3
c
has, for example, dot-like printing (not shown) applied thereto as the reflection section for reflecting light. A reflection sheet
4
is disposed on the opposed surface
3
c
, and a diffusion sheet (whose functions are to diffuse the light and to suppress bright lines)
5
, one or more prism sheets
6
, a prism protective sheet
7
, and the like are laminated on the outgoing surface
3
b
. Furthermore, a metal reflector
8
is provided on the opposed surface
3
c
of the light guide plate
3
to reflect light.
In the light guide plate
3
, light from the lamp
2
which is incident from the incoming surface
3
a
essentially advances toward a side end surface on the other end side while being totally reflected in the interior of the light guide plate
3
. When, however, the light impinges on the dot-like printing or the like applied to the opposed surface
3
c
, the total reflection is compromised due to diffused reflection. Due to the reflection sheet
4
on the opposed surface
3
c
of the light guide plate
3
, the diffused-reflected light is emitted from the outgoing surface
3
b
of the light guide plate
3
. The dot-like printing or the like applied to the opposed surface
3
c
has coarser dots near the incoming surface
3
a
and denser dots on the other end side, in order to allow light to be uniformly emitted from the outgoing surface
3
b
(that is, to obtain a uniform luminance distribution).
Although attempts have been made to make the emitted light uniform by applying the dot-like printing to the light guide plate
3
as described above, bright lines K may occur on a screen, particularly near the lamp
2
, as shown in FIG.
14
. The bright lines K occur when a series of high luminance portions each extending linearly in parallel with the lamp
2
alternate with a series of low luminance portions each extending linearly.
Attempts have been made to find the causes of the occurrence of the bright lines K, and several occurrence mechanisms have been found which are based on a structure near the lamp
2
; that is, action has been taken to individual causes. In fact, however, the occurrence of the bright lines K has not completely been clarified yet and is still one of the problems to be solved by those skilled in the art.
SUMMARY OF THE INVENTION
It is thus an object of the present invention to provide a back light unit, a liquid display device, and a method for manufacturing a light guide plate in which the occurrence of the bright lines can be diminished.
Then, the inventors examined the bright line mechanisms to obtain the knowledge that an edge portion formed between the incoming surface
3
a
of the light guide plate
3
and the outgoing surface
3
b
or the opposed surface
3
c
is formed into shapes different from an originally designed one after molding, that is, manufacturing tolerance during molding contributes to the occurrence of the bright lines K.
That is, the light guide plate
3
is generally formed by injection-molding an acrylic resin or the like, and the inventors assume that after molding, an edge portion E is curled as if chamfered (the example in
FIG. 13
b
) or is thinned (the example in
FIG. 13
c
), compared to a predetermined designed angle. Such a phenomenon is assumed to occur if during the injection molding, a resin material cannot fill every corner of the die due to its insufficient filling capability or the like, resulting in “short mold.”
Under this assumption, the inventors manufactured the light guide plate
3
having an intentionally chamfered portion C on the incoming surface
3
a
, as shown in
FIGS. 15
a-c
, as a case corresponding to, for example, the example shown in
FIG. 13
b
(the example with the shape likened to have been chamfered), and used this light guide plate
3
to carry out an optical path simulation. That is, if the edge portion E of the light guide plate
3
is formed into a predetermined shape that is subjected to no chamfering or the like, no bright line K occurs theoretically; thus, in this simulation, the chamfered position C was formed on the edge portion to determine whether or not the bright lines K occur.
Only the prism sheet
6
was disposed on the outgoing surface
3
b
of the light guide plate
3
in order to eliminate other factors of the occurrence of the bright lines K. As shown in
FIG. 15
b
, the prism sheet
6
has recesses
6
a
and projections
6
b
alternately and continuously formed on a bottom surface thereof at pitches of 50 [&mgr;m] and having substantially triangular cross section a vertex of which has an angle of 68 [°]. In addition, as shown in
FIG. 15
c
, the chamfered portion C formed on the light guide plate
3
forms an angle of 45 [°] relative to the incoming surface
3
a
and the outgoing surface
3
b
and has a chamfer dimension C
1
, C
2
of 0.1 [mm]. As shown in
FIG. 15
a
, in respect of the lamps
2
, eight point light sources are arranged at equal intervals on a circumference of an imaginary circle of diameter 2 mm drawn in the center of the light guide body.
Then, the lamps
2
were lighted under the above described conditions for simulation to observe the intensity of light on a front surface of the prism sheet
6
. The surface of the prism sheet
6
was partitioned into 1000 portions between the incoming surface
3
a
of the light guide plate
3
and the opposite end, so that the intensity of light was measured for each partition.
FIG. 16
shows the results of observation of the first to 100th partitions relative to the incoming
Katsu Yoshihiro
Suzuki Masaru
Cariaso Alan
Heslin Rothenberg Farley & & Mesiti P.C.
Radigan, Esq. Kevin P.
Wojnicki, Esq. Andrew J.
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