U.S. patent application number 10/056388 was filed with the patent office on 2002-08-01 for liquid crystal display device, side backlight unit, lamp reflector and reflection member.
This patent application is currently assigned to INTERNATIONAL BUSINESS MACHINES CORPORATION. Invention is credited to Katsu, Yoshihiro, Nishikai, Akiko, Noguchi, Michikazu, Suzuki, Masaru.
Application Number | 20020101549 10/056388 |
Document ID | / |
Family ID | 18889851 |
Filed Date | 2002-08-01 |
United States Patent
Application |
20020101549 |
Kind Code |
A1 |
Katsu, Yoshihiro ; et
al. |
August 1, 2002 |
Liquid crystal display device, side backlight unit, lamp reflector
and reflection member
Abstract
A liquid crystal display (LCD) device having a side backlight
unit that minimizes or eliminates extraneous bright lines on the
LCD panel. The side backlight unit includes a lamp reflector and
lamp disposed on a light incident side surface of a light guide
plate. A light reflection layer is formed on the inner surface of
the lamp reflector, and a transparent protective layer is formed on
the reflection layer. The transparent protective layer has a
thickness less than 5 micrometers. The lamp reflector has arm
portions that sandwich the light guide plate on the front and back
surfaces at the lamp side of the light guide plate. A light
transmission region between the arm portions of the lamp reflector
and the front and back surfaces of the light guide plate has a
thickness less than 5 micrometers.
Inventors: |
Katsu, Yoshihiro;
(Sagamihara-shi, JP) ; Suzuki, Masaru;
(Yokohama-shi, JP) ; Noguchi, Michikazu;
(Sagamihara-shi, JP) ; Nishikai, Akiko;
(Yokohama-shi, JP) |
Correspondence
Address: |
Todd M. C. Li
IBM Corporation - Zip 482
2070 Route 52
Hopewell Junction
NY
12533
US
|
Assignee: |
INTERNATIONAL BUSINESS MACHINES
CORPORATION
ARMONK
NY
|
Family ID: |
18889851 |
Appl. No.: |
10/056388 |
Filed: |
January 24, 2002 |
Current U.S.
Class: |
349/65 |
Current CPC
Class: |
G02B 6/0086 20130101;
G02F 1/133615 20130101; G02B 6/0031 20130101 |
Class at
Publication: |
349/65 |
International
Class: |
G02F 001/1335 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2001 |
JP |
2001-024758 |
Claims
What is claimed is:
1. A liquid crystal display device comprising: a liquid crystal
display panel having a backside; a light guide plate including an
incident surface and an emitting surface, said light guide plate
being provided along said backside of said liquid crystal display
panel wherein said emitting surface of said light guide plate faces
toward said backside of said liquid crystal display panel; a lamp
disposed along said incident surface of said light guide plate; and
a lamp reflector having an inner circumference surface defining a
space for accommodating said lamp, a light reflection layer formed
on said inner circumference surface, and a transparent protective
layer formed on said light reflection layer, wherein said
transparent protective layer has a thickness less than about 5
micrometers.
2. The liquid crystal display device of claim 1 wherein said light
guide plate further includes a back surface opposing said emitting
surface, and wherein said lamp reflector further includes arm
portions disposed along said emitting surface and said back surface
at the incident surface side of said light guide plate, and wherein
light transmission regions defined by the space between said arm
portions and said emitting surface and said back surface have
thicknesses less than about 5 micrometers.
3. The liquid crystal display device of claim 1 wherein said
transparent protective layer has a thickness of 3.5 micrometers or
less.
4. The liquid crystal display device of claim 2 wherein said light
transmission regions have thicknesses of 3.5 micrometers or
less.
5. The liquid crystal display device of claim 1 wherein said lamp
reflector further comprises a sheet-shaped support body having a
specified rigidity.
6. The liquid crystal display device of claim 1 wherein said
transparent protective layer is formed on said light reflection
layer after said light reflection layer is formed on said inner
circumference surface.
7. A side backlight unit comprising: a light guide plate including
an incident surface, an emitting surface adjoining said incident
surface, and a back surface adjoining said incident surface and
opposing said emitting surface; a lamp disposed along said incident
surface of said light guide plate; and a lamp reflector for
reflecting light irradiated from said lamp toward said incident
surface, wherein said lamp reflector includes: an inner
circumference surface defining a space for accommodating said lamp;
arm portions each having an arm surface extending from said inner
circumference surface, said arm surfaces sandwiching said emitting
surface and said back surface of said light guide plate on said
incident surface side and defining light transmission regions
between said arm surfaces and said emitting surface and said back
surface; and a light reflection layer formed on said inner
circumference surface, wherein said light transmission regions have
thicknesses less than about 5 micrometers.
8. The side backlight unit of claim 7 wherein said light
transmission regions have thicknesses 3.5 micrometers or less.
9. The side backlight unit according to claim 7 wherein said lamp
reflector further comprises a transparent protective layer formed
on said light reflection layer.
10. The side backlight unit according to claim 9 wherein said
transparent protective layer has a thickness less than about 5
micrometers.
11. The side backlight unit according to claim 9 wherein said
transparent protective layer is deposited on said light reflection
layer after said light reflection layer is formed on said inner
circumference surface.
12. The side backlight unit according to claim 11 wherein said
transparent protective layer has a thickness less than about 5
micrometers.
13. A lamp reflector for use in a side backlight unit of a liquid
crystal display device, said lamp reflector comprising: a reflector
body having an inner circumference surface defining an
accommodation space for a lamp; a light reflection layer formed on
said inner circumference surface; and a transparent protective
layer formed on said light reflection layer wherein said
transparent protective layer has a thickness less than about 5
micrometers.
14. The lamp reflector according to claim 13 wherein said thickness
of said transparent protective layer is 3.5 micrometers or
less.
15. The lamp reflector according to claim 13 wherein said reflector
body comprises a sheet-shaped support body having specified
rigidity.
16. The lamp reflector according to claim 15 wherein said
sheet-shaped support body comprises a material selected from the
group consisting of brass and stainless steel.
17. The lamp reflector according to claim 13 wherein said
transparent protective layer comprises a film consisting of a
material selected from the group consisting of a metal-series
compound and a resin.
18. The lamp reflector according to claim 17 wherein said
metal-series compound is selected from the group consisting of
Sio2, TiO2, ZnO, MgO, ZnF, MgO and indium tin oxide.
19. The lamp reflector according to claim 17 wherein said resin is
selected from the group consisting of acryl-series resin, PET and
polycarbonate.
20. The lamp reflector according to claim 13 wherein said light
reflection layer comprises a material selected from the group
consisting of Ag, Al, Pt, and a white-colored material.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a reflection member, more
particularly to a lamp reflector for use in a side backlight unit
in a liquid crystal display (LCD) device.
BACKGROUND OF THE INVENTION
[0002] A liquid crystal display (LCD) device has become remarkably
widespread for use as an image display device for a personal
computer and other various monitors. Generally, a liquid crystal
display device is configured in such a manner that a backlight unit
is provided as a planar light source for illumination, disposed on
a backside of a liquid crystal display panel, and so that a liquid
crystal surface having a specified area is irradiated on the whole
with even brightness, thus enabling visualization of an image
formed on the liquid crystal surface.
[0003] The backlight unit employs a hot-cathode or cold-cathode
fluorescent lamp as a light source. Since light from the
fluorescent lamp is linear light, this linear light must be
converted into planar light for irradiating the entire surface of a
liquid crystal display panel. For this purpose, heretofore, two
systems of backlight units have been employed: a direct backlight
unit; and a side (edge-light type) backlight unit. A direct
backlight unit is configured in such a manner that a fluorescent
lamp is placed immediately under a liquid crystal display panel,
and a dimmer plate and a diffusion plate are disposed thereon. By
contrast, a side backlight unit is configured in such a manner that
one or two fluorescent lamps are disposed on one or two sides of a
light guide plate made of transparent resin. Light made incident
onto the light guide plate is then directed towards the liquid
crystal display panel by a reflection structure provided on a
front, back or both back and front of the light guide plate, thus
even planar light is given thereto by use of light diffusion. Since
a side backlight unit can be made thinner than the direct backlight
unit, it is suitable for use as a display device in a portable
device such as a notebook computer.
[0004] The following is a description of the principle of light
emission of a side backlight unit. Light incident onto the light
guide plate from the lamp travels through the light guide plate
while making total internal reflection resulting from a difference
in refractive indices (dielectric constants) between the material
constituting the light guide plate and air. On at least one surface
of the light guide plate, which is the back thereof in general, a
reflection structure is provided for promoting the emission of
light from the light guide plate. The total reflection with the air
interface is disordered when the light traveling through the light
guide plate strikes the reflection structure, and when the light is
emitted from the front of the light guide plate. Note that, with
reference to the light guide plate, the front signifies the surface
of the light guide plate facing towards the liquid crystal display
panel, and the back signifies the surface of the light guide plate
opposite the front. Moreover, the surface from which the light is
emitted is referred to as an emitting surface. Accordingly, the
front includes the emitting surface. Usually the reflection
structure is formed on the back of the light guide plate, and the
light incident onto the light guide plate from the light source is
emitted from the emitting surface of the light guide plate by the
reflection structure. In order to allow an even luminance
distribution at the emitting surface, a diffusion pattern is
provided on the reflection structure that is less dense on the lamp
side and grades to a higher density away from the lamp.
[0005] However, although the luminance distribution is made even,
in prior art LCD devices using side backlight units, a problem
occurs in that bright lines are generated at periodic positions
from the lamp. Moreover, as the size of screens has increased, the
distance from the screen to the lamp has decreased, and the bright
lines have become increasingly conspicuous.
[0006] FIG. 8 is a drawing illustrating the generation of the
bright lines. As shown in FIG. 8, particularly in the vicinity of a
lamp R, bright lines K are generated on the screen. The bright
lines K are portions having luminance higher than a periphery
thereof lying parallel to the lamp R in the shape of a line.
[0007] The cause of the generation of the bright lines K has been
pursued by those skilled in the art. As a result, a plurality of
generation mechanisms for the bright lines K, particularly due to
structural factors in the vicinity of the lamp R, have been
clarified, and countermeasures for individual causes have been
taken. However, the current situation is that the generation of the
bright lines K has not yet been completely eliminated, and this
problem remains to be solved.
SUMMARY OF THE INVENTION
[0008] In this connection, one objective of the present invention
is to provide a side backlight unit and a liquid crystal display
device that are capable of reducing or preventing the generation of
bright lines.
[0009] Light from the lamp is made incident onto the light guide
plate directly or after being reflected in the lamp reflector. Due
to the difference in refractive indices between the light guide
plate and air, regardless of the angle made by the light incident
from the incident surface of the light guide plate, the light is
converged within an angle of about 42 degrees inside the light
guide plate by refraction when the refractive index n is equal to
about 1.5. Thereafter, the light propagates through the inside of
the light guide plate while repeating the total internal reflection
with the air interface. However, the above is an ideal case which
assumes that the entirety of the light is made incident from the
incident surface, that the surface of the light guide plate as an
interface with air is a mirror surface, and that the incident
surface is set at an angle of 90 degrees with respect to the back
and front thereof. In fact, the surface of the light guide plate
constituting the interface is not a complete mirror surface, and
has some scattering properties. It has been found, by using an
atomic force microscope (AFM) to study the light guide plate of a
commercially available liquid crystal display device, that the
surface of the light guide plate has unevenness of about 100
nm.
[0010] The inventors of the present invention investigated the
possibility that light is incident from portions other than the
incident surface onto the light guide plate. In the side backlight
unit, the lamp is accommodated, and the lamp reflector for
reflecting the light to the incident surface of the light guide
plate is provided. The lamp reflector is disposed on an incident
surface side of the light guide plate, and is integrated with the
light guide plate by sandwiching the front and the back of the
light guide plate. What the inventors of the present invention paid
attention to is the place where the lamp reflector sandwiches the
light guide plate.
[0011] Here, the lamp reflector is configured in such a manner that
a sheet having specified rigidity, made of brass or stainless steel
(SUS) or the like, is bent in an approximately U-character shape,
and a light reflection layer is formed on an inner circumference
surface thereof. The lamp reflector serves as a protector for the
lamp and has a role in making the light emitted from the lamp
incident onto the light guide plate efficiently. In general, a
light reflection layer may be formed by using a sheet coated with a
reflective material, such as silver (Ag) or aluminum (Al), or a
resin sheet containing a white pigment. In many cases, a sheet
coated with Ag (an Ag sheet) is used as a reflection layer since Ag
has high reflectivity.
[0012] Ag has high reflectivity in its material property. However,
oxidation of Ag will significantly lower the reflectivity.
Therefore, in order to prevent the oxidation of Ag, a protective
film for protecting the Ag surface is formed on the Ag sheet. In
many cases, such a protective film is formed using transparent
polyethylene terephthalate (PET) for maintaining the reflective
function of Ag as well as for functioning to prevent the oxidation
of Ag. The Ag sheet is obtained by depositing an Ag film on the PET
sheet by a method such as sputtering. The Ag sheet is adhered to a
support sheet, which serves as a support body, made of brass,
stainless steel and the like, with the Ag film surface facing
thereto. For the adhesion, an adhesive or a tackiness agent is used
for example. Then, the support sheet with the Ag sheet adhered
thereto is bent into a specified shape, and thus the lamp reflector
is obtained. A very thin PET sheet cannot be used when productivity
is a concern since a specified strength is required therefor in the
step of depositing Ag. Hence, a PET sheet with a thickness of about
25 micrometers is conventionally used.
[0013] FIG. 3a is a sectional view schematically showing a
configuration of a backlight unit 10 and FIG. 3b is a partially
enlarged view of the same. As shown in FIG. 3, a lamp reflector 8
includes: a lamp accommodating portion 8a accommodating a lamp 2;
an upper arm 8b1; and a lower arm 8b2. The upper and lower arms 8b1
and 8b2 of the lamp reflector 8 sandwich a light guide plate 11 by
back and front thereof on the incident surface 11a side of the
light guide plate 11. As shown in the enlarged view FIG. 3b, the
lamp reflector 8 has a sectional structure, in which a light
reflection layer 82, composed of, for example, an Ag film, and a
transparent protective layer 83 for protecting the light reflection
layer 82 are sequentially laminated on a reflector body 81. As
described above, a conventional lamp reflector has used a PET sheet
with Ag deposited thereon as the light reflection layer 82 and also
the transparent protective layer 83. Note that the above-described
adhesive layer is not shown in FIG. 3b. The transparent protective
layer 83 can be regarded as a light transmission region existing
between the light guide plate 11 and the light reflection layer
82.
[0014] The inventors of the present invention paid attention to the
transparent protective layer 83. Specifically, the inventors
hypothesized that the light being incident onto the light guide
plate 11 via the transparent protective layer 83 could be one of
the factors contributing to the generation of the bright lines.
FIG. 4 is a drawing illustrating this hypothesis. As shown in FIG.
4, while light incident onto the transparent protective layer 83 is
reflected by the light reflection layer 82, light incident onto the
transparent protective layer 83 in the vicinity of the light guide
plate 11 is reflected by the light reflection layer 82 and then is
incident onto the light guide plate 11 from the back thereof. The
incident angle of the latter case does not coincide with the
incident angle of light incident from the incident surface 11a of
the light guide plate 11. Specifically, due to the existence of the
transparent protective layer 83, which acts as a light transmission
region, the condition that the entirety of light is incident from
the incident surface 11a is not satisfied, the condition being one
of the above-described assumptions for the light to travel through
the light guide plate 11 with total reflection.
[0015] Based on the above hypotheses, the inventors of the present
invention prepared a lamp reflector, using a PET sheet with an Ag
film deposited thereon similarly to the prior art was used, and
laminating the PET sheet on a reflector body with the Ag film
facing away from the reflector body. Then, the screen was observed
for the possible generation of the bright lines. According to the
lamp reflector thus prepared, since the Ag film is in direct
contact with the light guide plate, there will not exist a region
where the light is transmitted between the light guide plate and
the Ag film as seen in the prior art. As a result of the
observation, it was confirmed that the bright lines were hardly
generated. Some bright lines were generated due to the angle of the
light guide plate not being formed to have an ideal angle of 90
degrees. And when the relation between the thickness of the
transparent protective layer on the Ag film and the generation of
the bright lines was further observed, it was also determined that
the bright lines could be suppressed to the extent where, as a
practical matter, the problem does not occur. This result can be
achieved by setting the thickness of the protective layer to be
less than 5 micrometers. The present invention was made based on
the above observed results and the findings. According to the
present invention, provided is a liquid crystal display device,
comprising: a liquid crystal display panel; a light guide plate
including at least one incident surface and an emitting surface for
emitting light from the incident surface, the light guide plate
being provided on a backside of the liquid crystal display panel; a
lamp disposed along the incident surface of the light guide plate;
and a lamp reflector including a space for accommodating the lamp,
a light reflection layer, preferably composed of a metal film, and
a transparent protective layer formed on the light reflection
layer, the light reflection layer and the transparent protective
layer being formed on an inner circumference surface of the lamp
reflector, wherein the thickness of the transparent protective
layer is less than 5 micrometers.
[0016] According to the liquid crystal display device of the
present invention, since the thickness of the transparent
protective layer is set at less than 5 micrometers, the generation
of the bright lines can be reduced or prevented.
[0017] Moreover, according to the present invention, provided is a
side backlight unit, comprising: a light guide plate including at
least one incident surface and an emitting surface for emitting
light incident from the incident surface; a lamp disposed along the
incident surface of the light guide plate; and a lamp reflector for
reflecting light irradiated from the lamp and for guiding the light
to the incident surface, wherein the lamp reflector includes: an
arm unit for sandwiching back and front of the light guide plate on
the incident surface side; and a lamp accommodating portion for
accommodating the lamp, and a light reflection layer, preferably an
Ag film, formed on an inner circumference surface of the lamp
reflector, and a light transmission region between the Ag film on
the arm unit and any one of the back and front of the light guide
plate, wherein the thickness of the light transmission region is
less than 5 micrometers.
[0018] As described above, the transparent protective layer formed
on the lamp reflector becomes one of the factors contributing to
the generation of the bright lines. Accordingly, in the described
liquid crystal display device of the present invention, the
thickness of the transparent protective layer is limited to less
than 5 micrometers. However, if a light transmission region,
allowing the incidence of light from the back or front of the light
guide plate, exists in addition to presence of the transparent
protective layer of the lamp reflector, and if such a light
transmission region cannot be eliminated, it can be said that the
emission of the bright lines cannot be effectively controlled.
Therefore, a side backlight unit in accordance with the present
invention includes a light transmission region between the Ag film
on the arm unit and the back or front of the light guide plate that
has a thickness that is regulated to be less than 5
micrometers.
[0019] As apparent from the above description, the present
invention adopts a configuration that reduces the influence of
light incident onto the light guide plate through the transparent
protective layer formed on the lamp reflector. Hence, according to
the present invention, provided is a side backlight unit,
comprising: a light guide plate including at least one incident
surface and an emitting surface for emitting light incident from
the incident surface; a lamp disposed along the incident surface of
the light guide plate; and a lamp reflector including an
accommodation space for accommodating the lamp, a light reflection
layer for reflecting light irradiated from the lamp and guiding the
light to the incident surface of the light guide plate, and a
transparent protective layer for protecting the light reflection
layer, the light reflection layer and the transparent protective
layer being formed on an inner circumference surface of the lamp
reflector, wherein the transparent protective layer is deposited on
the light reflection layer after the light reflection layer is
deposited on a reflector body.
[0020] In the side backlight unit of the present invention, the
transparent protective layer is formed by deposition thereof on the
light reflection layer after the light reflection layer is
deposited. In conventional methods using a PET sheet, the thickness
of the PET sheet cannot be thinner than about 25 micrometers.
Specifically, in the case where a process is employed as in the
prior art, in which the Ag sheet is obtained by depositing the Ag
film on the PET sheet followed by adhesion of the Ag sheet on the
reflector body, at present, it would be difficult to realize the
results achieved by the present invention. Therefore, a process is
proposed in accordance with the present invention, in which the
transparent protective layer is deposited after the light
reflection layer is deposited. For example, the present invention
may be realized by first depositing on the inner circumference
surface of a lamp reflector body, an Ag film by a process such as
sputtering or vapor deposition, and then depositing an SiO2 film or
the like by a process such as sputtering or vapor deposition. As
well known, sputtering or vapor deposition are examples of
processes suitable for thin film formation.
[0021] Furthermore, according to the present invention, provided is
a lamp reflector for accommodating a lamp as a light source and
reflecting received light in a side backlight unit for use in a
liquid crystal display device, the lamp reflector comprising: a
reflector body defining an accommodation space for the lamp; a
light reflection layer formed on the surface facing toward the
accommodation space of the reflector body; and a transparent
protective layer formed on the light reflection layer, the
transparent protective layer having a thickness of less than 5
micrometers.
[0022] In the lamp reflector of the present invention, since the
thickness of the transparent protective layer for protecting the
light reflection layer is less than 5 micrometers, the generation
of the bright lines can be suppressed. Moreover, for further
suppression of the generation of the bright lines, the thickness of
the transparent protective layer is preferably 3.5 micrometers or
less, and more preferably 2 micrometers or less.
[0023] In the lamp reflector of the present invention, the
reflector body can be formed using a material similar to that of
the support body to be described below. Moreover, the light
reflection layer and the transparent protective layer can be formed
using materials similar to those of a reflection layer and a
protective layer to be described below.
[0024] According to the present invention, provided is a reflection
member suitable for the lamp reflector of the present
invention.
[0025] Specifically, the reflection member of the present invention
comprises: a sheet-shaped support body having specified rigidity; a
light reflection layer for reflecting received light, the light
reflection layer being formed on the support body; and a protective
layer composed of a transparent film having a thickness of less
than 5 micrometers, the protective layer being formed on the light
reflection layer.
[0026] In the reflection member of the present invention, since the
thickness of the transparent film formed on the reflection layer is
less than 5 micrometers, the generation of the bright lines can be
suppressed by using the reflection member as the lamp reflector of
the liquid crystal display device.
[0027] In the reflection member of the present invention, the
support body must be provided with specified rigidity required for
the lamp reflector when it is used as the lamp reflector. In the
case where the support body is used as the lamp reflector, a metal
sheet material made of brass, stainless steel or the like can be
used. Since stainless steel has higher rigidity than brass, it is
effective to use stainless steel when it is desired that the
thickness of the support body is to be thin.
[0028] In the reflection member in accordance with the present
invention, the reflection layer formed on the support body is not
particularly limited. However, in consideration that the reflection
layer is used as the lamp reflector, a metal material with high
reflectivity can be used. Examples of metal materials having high
reflectivity include Ag, Al and Pt. Among the above, Ag is the most
desirable for its high reflectivity.
[0029] As described above, the protective layer formed on the
reflection layer is composed of a transparent film having the
thickness of less than 5 micrometers. Although the protective layer
is a layer for protecting the reflection layer, which is an
underlying layer thereof, from a property change such as oxidation,
the protective layer is also a light transmitting film for securing
the light reflecting function of the reflection layer. Examples of
a substance that can be used as the protective layer include a
metal-series compound such as Sio2, TiO2, ZnO, MgO, ZnF, MgO and
indium tin oxide (ITO). Such a compound may be used alone, or two
or more of the compounds may be laminated. The protective layer may
be formed using a method such as vacuum deposition, sputtering or
chemical vapor plating. Moreover, a resin film, for example, made
of acryl-series resin or polycarbonate can be used as the
protective layer. When a resin film is used as the protective
layer, a method such as spin coating or dip coating can be
employed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] For a more complete understanding of the present invention
and the advantages thereof, reference is now made to the following
description taken in conjunction with the accompanying
drawings.
[0031] FIG. 1 is a perspective view for explaining an entire
configuration of a liquid crystal display device according to an
embodiment of the present invention.
[0032] FIG. 2 is a sectional view showing the principal
configuration of the liquid crystal display device according to an
embodiment of the present invention.
[0033] FIG. 3a is a sectional view of a backlight unit and
[0034] FIG. 3b is a partially enlarged view of the same according
to an embodiment of the present invention.
[0035] FIG. 4 is a view schematically illustrating the light
transmission behavior in the vicinity of an incident surface of a
light guide plate.
[0036] FIG. 5 is a view schematically illustrating the light
transmission behavior in the vicinity of the incident surface of a
light guide plate.
[0037] FIG. 6 is a table showing criteria for a sensory test
regarding bright lines.
[0038] FIG. 7 is a graph showing results of the sensory test
regarding the bright lines.
[0039] FIG. 8 is a view illustrating a generation state of the
bright lines.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] Hereinbelow, description will be made for the present
invention based on an embodiment with reference to the accompanying
drawings.
[0041] FIG. 1 is a perspective view for explaining an entire
configuration of the liquid crystal display device in this
embodiment. A reference numeral 41 denotes a metal shield case for
forming an upper frame, which forms a display window 42 defining an
effective screen of a liquid crystal display module. A numeral 43
denotes a liquid crystal display panel, in which thin film
transistors (TFTs), each having source/drain electrodes, a gate
electrode, an amorphous silicon layer and the like deposited
thereon, and color filters are laminated between two glass
substrates. On the liquid crystal display panel 43, a drain circuit
substrate 44, a gate circuit substrate 45 and an interface circuit
substrate 46 are formed, and furthermore, joiners 47, 48 and 49 are
provided for joining the circuit substrates. These circuit
substrates 44, 45 and 46 are fixed to the shield case 41 with an
insulating sheet 50 interposed therebetween.
[0042] Meanwhile, under, or the backside of, the liquid crystal
display panel 43, a light shielding spacer 52 is provided with a
rubber cushion 51 interposed therebetween, and furthermore, a
backlight unit 10 for irradiating planar light onto the liquid
crystal display panel 43 is provided.
[0043] FIG. 2 is a sectional view showing a configuration of the
backlight unit 10. As shown in FIGS. 1 and 2, the backlight unit
includes: a light guide plate 11; a prism sheet 12 provided on a
surface of the light guide plate 11; a diffusion sheet 5; and a
reflection sheet 4 provided under the light guide plate 11.
[0044] Moreover, on one side of the light guide plate 11, a lamp
unit 13 constituting the backlight unit 10 is provided.
Furthermore, as shown in FIG. 1, a lower case 54 having an opening
53 is provided under the reflection sheet 4.
[0045] For obtaining even planar light, the diffusion sheet 5 has a
function of diffusing light from the prism sheet 12 to be described
later. The diffusion sheet 5 may be formed, for example, using
acrylic beads, each having a diameter of, for example, about 30
micrometers to 50 micrometers, which are arranged as diffusing
agents on a surface of a sheet-shaped base material made of, for
example, PET. Here, a diffusion effect can be obtained due to the
uneven shape of the surface of the diffusion sheet 5, which is
formed by the acrylic beads. Alternatively, a diffusion sheet 5 may
be formed using acrylic beads, each having a diameter of, for
example 30 micrometers to 50 micrometers, which are kneaded as
diffusing agents into a sheet-shaped base material made of, for
example, polycarbonate (PC). Here, the diffusion effect is obtained
due to a difference in refractive indices between the base material
(polycarbonate) and the kneaded acrylic beads.
[0046] The prism sheet 12 is used for increasing the luminescence
in the front direction. Here, as shown in FIG. 2, a so-called
"downward" prism sheet 12 is used, in which concave grooves and
convex grooves acting as diffusing prism surfaces are alternately
formed on the lower surface of the sheet. However, the effect of
the present invention can also be obtained by using an "upward"
prism sheet.
[0047] The reflection sheet 4 is configured so as to be able to
reflect the light incident onto the light guide plate 11 from the
lamp unit 13 toward the direction of the liquid crystal display
panel 43. The reflection sheet 4 may be formed from one of a white
color series, one made of metal such as silver and aluminum, or one
coated with such metal or the like.
[0048] The lamp unit 13 is also constituted of a lamp 2, such as a
fluorescent lamp, and of a metal lamp reflector 8 made of a metal
plate such as a stainless steel plate or a brass plate, for
example, which covers a periphery of the lamp 2. The lamp reflector
8 forms a light reflection layer 82 by using a metal material
having a high reflectivity such as Ag, Al and Pt. This light
reflection layer 82 usually has a thickness of about several
hundred micrometers. This lamp reflector 8 has an opening on the
incident surface 11a side of the light guide plate 11. The light of
the lamp 2 is reflected by the light reflection layer 82 of the
lamp reflector 8, and the entirety of the light from the lamp 2 is
made incident onto the light guide plate 11 from the opening.
[0049] The light guide plate 11 is preferably formed of acrylic
resin having a thickness of about 1 mm to 4 mm. An example of an
acrylic resin having excellent light transmittance is represented
by polymethyl methacrylate (refractive index: 1.49, critical
reflection angle: 48 degrees), or an acryl-series
monomer/comonomer. Note that the light guide plate 11 may also
contain titanium oxide (Tio2) for scattering the light.
[0050] The light guide plate 11 includes: an incident surface 11a
for receiving the light from the lamp 2; an emitting surface
(front) 11b for emitting the light incident onto the incident
surface 11a to the outside; and an opposite surface (back) 11c
opposite the emitting surface 11b.
[0051] Preferably, printing of a pattern for reflecting light, for
example a dot-shaped pattern, is performed on the opposite surface
11c, the emitting surface 11b or both of the above of the light
guide plate 11 for causing diffuse reflection. Moreover, although
the thickness of the light guide plate 11 may be even from the
incident surface 11a to the other end, the light guide plate 11 is
preferably formed so as to become gradually thinner from the
incident surface 11a to the other end for improving its light
diffusion property.
[0052] FIG. 5 is a view illustrating the behavior of light
transmission in the vicinity of the incident surface 11a of the
light guide plate 11.
[0053] In FIG. 5, the lamp reflector 8 includes a light reflection
layer 82 on a reflector body 81, and further, a transparent
protective layer 83 is formed on the light reflection layer 82 for
protecting the light reflection layer 82 from a property change
such as oxidation.
[0054] As shown in FIG. 5, when the transparent protective layer 83
exists, light is incident onto the inside of the transparent
protective layer 83, and light is reflected by the light reflection
layer 82 to be made incident onto the light guide plate 11. The
incident angle of the light thus made incident does not coincide
with that of the light incident from the incident surface 11a. In
the case where the light is incident onto the incident surface 11a,
regardless of direction from which the light is made incident, the
light is converged due to the difference in the refractive indices
between air and the light guide plate 11. When the light guide
plate 11 is made of acrylic resin, since the refractive index of
the acrylic resin is about 1.49, the light incident onto the inside
of the light guide plate 11 is converged at an angle of about 42
degrees. By contrast, the light incident onto the inside of the
light guide plate 11 after being incident onto the transparent
protective layer 83 has a convergent axis shifted by 90 degrees
from that of the light incident from the incident surface 11a since
the refractive indices of the acrylic resin and the PET are
approximately equal to each other. The light incident at this angle
is emitted without satisfying the conditions for total reflection
at the interface between the light guide plate 11 and air.
Specifically, although the opportunity for emission of the light
from the light guide plate 11 should be triggered by the reflection
structure, such as the dot-shaped printing provided on the light
guide plate 11, the light is emitted regardless, thus causing the
generation of the bright lines.
[0055] Light converged toward the zenith direction by the downward
prism sheet 12 is the light emitted from the light guide plate 11
at an angle of about 75 degrees. In this case, the light incident
onto a length "L" portion at an angle of about 75 degrees goes
through an optical path as shown in FIG. 5, and is emitted from the
light guide plate 11. The light then becomes visible as a bright
line.
[0056] The following is a discussion of the thickness d of the
transparent protective layer 83.
[0057] The length L relating to the bright line is obtained by the
following equation:
L=2.times.d.times.tan(arcsin(sin(incident angle)/1.57)).
[0058] When the light is incident at an angle of 75 degrees, the
length L is: L=1.56.times.d. Specifically, it is understood that
the thickness d of the transparent protective layer 83 has a linear
relation with the intensity of the bright lines and that the
generation of the bright lines can be reduced or prevented if the
thickness d is made small.
[0059] Next, investigation was made to find out how thick the
transparent protective layer 83 must be in order to make the bright
lines inconspicuous.
[0060] Five types of lamp reflectors 8 were prepared, each having a
transparent protective layer 83 with thickness of: 0.1 micrometers;
1.0 micrometers; 2.0 micrometers; 12.5 micrometers; and 25
micrometers, respectively. Then, a sensory test was performed with
a subject. Concretely, perception of the intensity of the bright
lines was confirmed based on the intensity criteria shown in FIG.
6. FIG. 7 shows the test results.
[0061] When the thickness of the transparent protective layer 83
was 25 micrometers, conspicuous bright lines were observed. When
the thickness of the transparent protective layer 83 was 12.5
micrometers, although the bright lines were observed, the intensity
thereof was weaker. When the thickness of the transparent
protective layer 83 was 2.0 micrometers or less, no bright lines
were observed. Although it is desirable that no bright lines be
observed, meaning that the intensity level of the bright lines is
0, an intensity level of the bright lines is acceptable up to
1.
[0062] In order to set the intensity level of the bright lines at 1
or lower, it will be necessary to set the thickness of the
transparent protective layer 83 at less than 5 micrometers when a
tendency shown in FIG. 7 is concerned.
[0063] As described above, setting the thickness of the transparent
protective layer 83 at less than 5 micrometers, in accordance with
the present invention, preferably at less than 3.5 micrometers, and
more preferably at less than 2 micrometers, results in a
transparent protective layer 83 that is configured so as to reduce
the influence of the light from the lamp 2. Similarly, when a
transparent protective layer 83 is configured in such a manner in
accordance with the present invention, light not meeting the total
reflection conditions in the light guide plate 11 is controlled so
as not to be made incident from the front, including the emitting
surface 11b, and the back 11c of the light guide plate 11.
Consequently, the generation of the bright lines can be reduced or
prevented.
[0064] In the above embodiment, the transparent protective layer 83
has been the subject of the description. However, as described
above, the present invention is applicable also to the case where
other layers capable of light transmission therethrough exist
between the light reflection layer 82 and the light guide plate
11.
[0065] Moreover, in the above embodiment, description as been made
for the relation between the lower arm 8b2 of the lamp reflector 8
and the light guide plate 11. However, the present invention is
also applicable to the relation between the upper arm 8b1 and the
light guide plate 11.
[0066] As described above, according to the present invention, the
generation of bright lines can be reduced or prevented.
[0067] Although a preferred embodiment of the present invention has
been described in detail, it should be understood that various
changes, substitutions and alterations can be made therein without
departing from the spirit and the scope of the invention as defined
by the appended claims.
* * * * *