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Claims  |
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What is claimed is:
1. A planar illumination unit, comprising:
an optical guide layer having first and second, mutually opposing principal
surfaces, said optical guide layer being laterally defined by a side wall
and guiding a light incident thereto at said side wall, said optical guide
layer emitting said light from said first principal surface;
light source means, facing said side wall of said optical guide layer for
injecting said light into said optical guide layer through said side wall,
said light source means including first reflection means for reflecting
said light produced by said light source means toward said side wall of
said optical guide layer;
an optical scattering layer having first and second, mutually opposing
principal surfaces, said optical scattering layer being disposed such that
said second principal surface of said optical scattering layer faces said
first principal surface of said optical guide layer thereby for receiving
said light emitted from said first principal surface of said optical guide
layer, said optical scattering layer causing scattering in said light that
has been received from said optical guide layer and emitting said light
from said first principal surface;
second reflection means, provided on said second principal surface of said
optical guide layer, for reflecting a light incident to said second
principal surface of said optical guide layer; and
directivity means having first and second opposing principal surfaces and
provided on said first principal surface of said optical guide layer such
that said second principal surface of said directivity means faces said
first principal surface of said optical guide layer, said directivity
means being disposed such that said first principal surface of said
directivity means faces said second principal surface of said optical
scattering layer, said directivity means carrying a plurality of ridges on
said first principal surface so as to extend in parallel with each other,
each of said plurality of ridges having an apex angle such that said
plurality of ridges transmit an optical beam incident at any arbitrary
angle to said second principal surface of said directivity means.
2. A planar illumination unit as claimed in claim 1, wherein said
directivity means comprises a transparent film having first and second
principal surfaces coincident to said first and second principal surfaces
of said directivity means, respectively, said transparent film carrying,
on said first principal surface thereof, said plurality of ridges
extending parallel with each other.
3. A planar illumination unit as claimed in claim 2, wherein each of said
plurality of ridges has an isosceles-triangular cross section, with said
apex angle set to approximately 96.degree..
4. A planar illumination unit comprising:
an optical guide layer having first and second, mutually opposing principal
surfaces, said optical guide layer being laterally defined by a side wall
and guiding a light incident thereto at said side wall, said optical guide
layer emitting said light from said first principal surface;
light source means, facing said side wall of said optical guide layer for
injecting said light into said optical guide layer through said side wall,
said light source means including first reflection means for reflecting
said light produced by said light source means toward said side wall of
said optical guide layer;
an optical scattering layer having first and second, mutually opposing
principal surfaces, said optical scattering layer being disposed such that
said second principal surface of said optical scattering layer faces said
first principal surface of said optical guide layer thereby for receiving
said light emitted from said first principal surface of said optical guide
layer, said optical scattering layer causing scattering in said light that
has been received from said optical guide layer and emitting said light
from said first principal surface;
second reflection means, provided on said second principal surface of said
optical guide layer, for reflecting a light incident to said second
principal surface of said optical guide layer; and
directivity means having first and second opposing principal surfaces and
provided on said first principal surface of said optical guide layer such
that said second principal surface of said directivity means faces said
first principal surface of said optical guide layer, said directivity
means being disposed such that said first principal surface of said
directivity means faces said second principal surface of said optical
scattering layer, said directivity means carrying a plurality of ridges on
said first principal surface so as to extend in parallel with each other,
said directivity means including a transparent film having first and
second principal surfaces coincident to said first and second principal
surfaces of said directivity means, respectively, said transparent film
carrying, on said first principal surface thereof, said plurality of
ridges extending parallel with each other, each of said plurality of
ridges having a sawtooth cross section, with an apex angle set to
approximately 48.degree..
5. A liquid display panel, comprising:
a liquid crystal cell panel having first and second, mutually opposing
surfaces, said liquid crystal cell panel including liquid crystal
molecules therein; and
a planar illumination unit having first and second, mutually opposing
principal surfaces and being disposed such that said first principal
surface of said planar illumination unit faces said second principal
surface of said liquid crystal cell panel, said planar illumination unit
comprising:
an optical guide layer having first and second, mutually opposing principal
surfaces, said optical guide layer being laterally defined by a side wall
and guiding a light incident thereto at said side wall, said optical guide
layer emitting said light from said first principal surface;
light source means, facing said side wall of said optical guide layer, for
injecting said light into said optical guide layer through said side wall,
said light source means including first reflection means for reflecting
said light produced by said light source means toward said side wall of
said optical guide layer;
an optical scattering layer having first and second, mutually opposing
principal surfaces, said optical scattering layer being disposed such that
said second principal surface of said optical scattering layer faces said
first principal surface of said optical guide layer thereby for receiving
said light emitted from said first principal surface of said optical guide
layer, said optical scattering layer causing scattering in said light
received from said optical guide layer and emitting said light from said
first principal surface;
second reflection means, provided on said second principal surface of said
optical guide layer, for reflecting a light incident to said second
principal surface of said optical guide layer; and
directivity means having first and second opposing principal surfaces and
provided on said first principal surface of said optical guide layer such
that said second principal surface of said directivity means faces said
first principal surface of said optical guide layer, said directivity
means being disposed such that said first principal surface of said
directivity means faces said second principal surface of said optical
scattering means, said directivity means carrying a plurality of ridges on
said first principal surface so as to extend in parallel with each other,
each of said plurality of ridges having an apex angle such that said
plurality of ridges transmit an optical beam incident at any arbitrary
angle to said second principal surface of said directivity means. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to planar display devices and more
particularly to a liquid crystal display device having a planar light
source wherein the luminance is improved.
2. Description of the Related Art
In the planar display devices, particularly the liquid crystal display
panels, a backlight illumination unit is provided behind a liquid crystal
cell panel for improving the visibility of the displayed image. In
correspondence to the planar construction of the liquid crystal display
unit, the backlight illumination unit has also a planar construction.
FIG. 1 shows an example of the construction of a liquid display panel.
Referring to FIG. 1, the liquid crystal display panel includes a liquid
crystal cell panel 1 having a display area 2, wherein a display pattern is
formed on the display area 2 by inducing an orientation in the liquid
crystal molecules in the panel 1. The liquid crystal display device
further includes a planar illumination unit 3 behind the liquid crystal
cell panel 1, and the planar illumination unit 3 radiates light at a
luminous surface 4 that faces the liquid crystal cell panel 1. In such a
liquid crystal display panel, the planar illumination unit 3 is required
to have a high and uniform luminance throughout the luminous surface 4.
FIG. 2 shows the schematical construction of the planar illumination unit 3
used in the conventional liquid crystal display device.
Referring to FIG. 2, the unit 3 includes an optical guide plate 5 of a
transparent material such as an acrylic resin, and a cold cathode tube 6
is disposed so as to face a side wall 5a of the optical guide plate 5.
Thereby, the optical guide plate 5 guides the light that has been produced
by the cold cathode tube 6 and injected at the side wall 5a and emits the
same from a principal surface 11 that acts as a luminous surface.
The cold cathode tube 6 is held by a holder 7 in alignment with the optical
guide plate 5, wherein the holder 7 has a reflective surface 7a for
reflecting the light that has been produced by the cold cathode tube 6 to
the foregoing side wall 5a of the waveguide plate 5. In addition, there is
provided a planar reflector 8 at a side wall 5b opposite to the side wall
5a for reflecting back the light that has been emitted from the optical
guide plate 5. Further, a planar reflector 10 is provided adjacent to a
bottom principal surface 9 of the optical guide plate 5 such that the
reflector 10 faces the surface 9. Thereby, the light that has leaked from
the optical guide plate 5 through the bottom surface 9 is reflected back
to the optical guide plate 5.
Above the optical guide plate 5, there is provided an optical scattering
plate 12 such that the plate 12 faces the foregoing luminous surface 11.
The optical scattering plate 12 has a milky whitish appearance and causes
a scattering in the light that passes therethrough. As a result, the
backlight emitted from the optical scattering plate 12 has a uniform
luminance.
In such a liquid crystal display panel, it is necessary to increase the
luminance of the backlight that is emitted at the luminous surface 11 of
the optical guide plate 5. On the other hand, experience indicates that
mere increase of the optical power of the cold cathode tube 6 is not
effective for the desired increase of luminance of the planar illumination
unit 3. Contrary, such an increase of the optical power of the cold
cathode tube 6 invites an increased electrical power consumption and
increased size of the device. In view of the fact that the liquid display
panels are widely used in small, portable computers and word processors,
such an increase in the electrical power or size is not acceptable.
In designing the planar illumination unit 3, therefore, it is important to
reduce the loss of the light that has been emitted by the cold cathode
tube 6 and to maximize the proportion of the light that is emitted from
the luminous surface 11. The inventors of the present invention have
conducted a series of analysis with respect to the propagation of the
light that has been emitted from the cold cathode tube 6 and discovered
the following facts. The result of the investigation is reported in
MESAKI, Y., et al., TECHNICAL REPORT OF IEICE, EID92-126, ED92-159
(1993-02), pp.83-90, MESAKI, Y., et al., TECHNICAL REPORT OF IEICE,
EID93-33 (1993-07), pp. 45-50, MESAKI, Y., et al., TECHNICAL REPORT OF
IEICE, CPM93-33, OME93-19 (1993-07), all of which are published after the
priority date of the present application.
The light emitted from the cold cathode tube 6 propagates typically along
one of the paths P.sub.1 -P.sub.5 indicated in FIG.2, wherein P.sub.1
represents the path of the light that enters directly into the optical
guide plate 5 through the side wall 5a; P.sub.2 represents the path of the
light that enters the optical guide plate 5 after a reflection at the
holder 7; P.sub.3 represents the path of the light that is absorbed by the
holder 7; P.sub.4 represents the path of the light that is absorbed by the
cold cathode tube 6 after reflection at the holder 7; and P.sub.5
represents the light that has been emitted from the cold cathode tube 6
but failed to enter the optical guide plate 5. In correspondence to the
paths P.sub.1 -P.sub.5, there holds a relationship
LP.sub.1 +LP.sub.2 +LP.sub.3 +LP.sub.4 +LP.sub.5 =LP.sub.0,(1)
wherein LP.sub.1 -LP.sub.5 represents the amount or energy of the light for
each of the paths P.sub.1 -P.sub.5 while LP.sub.0 represents the total
energy of the light that has been emitted from the cold cathode tube 6.
It should be noted that the above analysis indicates that the energy of the
light that enters actually to the optical guide plate 5 is represented as
LP.sub.in =LP.sub.1 +LP.sub.2 ( 2)
where LP.sub.in represents the energy of the light entering into the
waveguide plate 5. The rest of the light has been lost. Thus, conventional
planar illumination unit 3 suffers from the problem that the efficiency of
injecting the light from the cold cathode tube 6 to the optical guide
plate 5 is poor when the contribution of the terms LP.sub.3 -LP.sub.5 is
substantial. With this regard, one should note that the cold cathode tube
6 carries a fluorescent material for emitting the light, while such a
fluorescent material also absorbs light that is incident thereto. Although
the light absorbed by the fluorescent material may be re-emitted,
substantial loss of light cannot be avoided during such a process.
Meanwhile, there is a proposal to align the direction of emission of the
light from the planar illumination unit 3 so as to increase the luminance
of the unit 3 with respect to the direction perpendicular to the unit 3.
For this purpose, a prismatic lens film carrying, on a transparent film, a
number of parallel ridges or stripes of triangular cross section, is
disposed on the scattering plate 12. However, such a prismatic lens film
tends to cause an excessively sharp directivity in that the luminance
drops sharply when the view angle has exceeded a certain threshold. When
such a light source unit 3 is used for a liquid crystal display device,
therefore, there occurs an inconvenience that the person who is not facing
the display panel directly cannot see the displayed image at all. It is
desirable to provide a more gentle change of the luminance as a function
of the view angle.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a
novel and useful planar illumination unit as well as a liquid crystal
display unit wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a
planar illumination unit wherein the loss of the light caused by a
fluorescent material on a cold cathode tube, after the light is emitted
from said cold cathode tube and is reflected back again to said cold
cathode tube, is successfully minimized.
Another object of the present invention is to provide a planar illumination
unit, comprising:
an optical guide layer having first and second, mutually opposing principal
surfaces, said optical guide layer being laterally defined by a side wall
and guiding a light incident thereto at said side wall, said optical guide
layer emitting said light from said first principal surface;
light source means provided so as to face said side wall of said optical
guide layer for injecting a light into said optical guide layer through
said side wall;
an optical scattering layer having first and second, mutually opposing
principal surfaces, said optical scattering layer being disposed such that
said second principal surface of said optical scattering layer faces said
first principal surface of said optical guide layer, for receiving said
light emitted from said first principal surface of said optical guide
layer, said optical scattering means causing a scattering in said light
that has been received from said optical guide layer and emitting said
light from said first principal surface; and
first reflection means provided on said second principal surface of said
optical guide layer for reflecting a light incident to said second
principal surface of said optical guide layer;
said optical source means including an optical source for emitting said
light and second reflection means for reflecting the light that has been
produced by said optical source toward said side wall, wherein said
reflection means reflects said light in a direction primarily offset from
said optical source.
According to the present invention, the problem of absorption of the light
by the fluorescent material provided on the optical source means after the
light being reflected back to the light source means by the reflection
means, is effectively eliminated.
Another object of the present invention is to provide a planar illumination
unit having an increased luminance in the direction perpendicular to the
plane of the light source unit while maintaining a smooth angular pattern
of radiation profile.
Another object of the present invention is to provide a planar illumination
unit, comprising:
an optical guide layer having first and second, mutually opposing principal
surfaces, said optical guide layer being laterally defined by a side wall
and guiding a light incident thereto at said side wall, said optical guide
layer emitting said light from said first principal surface;
a light source means provided so as to face said side wall of said optical
guide layer for injecting a light into said optical guide layer through
said side wall, said light source means including reflection means for
reflecting said light that has been produced by said light source means
toward said side wall of said optical guide layer;
an optical scattering layer having first and second, mutually opposing
principal surfaces, said optical scattering layer being disposed such that
said second principal surface of said optical scattering layer faces said
first principal surface of said optical guide layer, for receiving said
light emitted from said first principal surface of said optical guide
layer, said optical scattering means causing a scattering in said light
that has been received from said optical guide layer and emitting said
light from said first principal surface;
reflection means provided on said second principal surface of said optical
guide layer for reflecting a light incident to said second principal
surface of said optical guide layer; and
directivity means having first and second opposing principal surfaces and
provided on said first principal surface of said optical guide layer such
that said second principal surface of said directivity means faces said
first principal surface of said optical guide layer, said directivity
means being disposed such that said first principal surface of said
directivity means faces said second principal surface of said optical
scattering means, said directivity means carrying a plurality of ridges on
said first principal surface so as to extend parallel with each other.
According to the present invention, the optical beam is emitted from the
optical guide layer in the direction substantially perpendicular to the
plane of the optical guide layer with an improved directivity and the
luminance of the unit increases as long as viewed in the direction
perpendicular to the plane of the unit. Further, because of the fact that
the directivity means is disposed between the optical guide layer and the
optical scattering means, the optical beam emitted from the directivity
means experiences a scattering in the optical scattering means, and the
problem of undesirable sharp dropping of luminance when the view angle
exceeds a certain critical angle is substantially reduced. In a preferred
embodiment of the present invention, the peak angle of the ridges is
optimized such that the light having an arbitrary angle of incidence, with
respect to the first principal surface of the directivity means, is
emitted.
Another object of the present invention is to provide a planar illumination
unit having an optical guide layer for guiding a light injected at a side
edge thereof and for emitting the injected light from a principal surface,
wherein the principal surface is tilted with an optimized tilt angle such
that the light incident to the optical guide layer at a side edge part
thereof with an arbitrary angle of incidence, is emitted from the
foregoing tilted principal surface.
Other objects and further features of the present invention will become
apparent from the following detailed description when read in conjunction
with the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 (Prior Art) is a diagram showing the general construction of a
liquid crystal display panel;
FIG. 2 (Prior Art) is a diagram showing the construction of a conventional
planar illumination unit.
FIG. 3 is a diagram showing the construction of a planar illumination unit
according to an embodiment of the present invention;
FIG. 4 is a diagram for explaining a first feature of the present
invention;
FIG. 5 is a diagram showing the absorption of the light occurring at a cold
cathode tube from which the light has been emitted;
FIGS. 6(A) and 6(B) are diagrams showing an embodiment of a holder having
the first feature of the present invention and used in the planar
illumination unit of the present invention;
FIGS. 7-13 are diagrams showing embodiments of other holders having the
first feature of the present invention and used in the planar illumination
unit of the present invention;
FIGS. 14-16 are diagrams comparing various constructions of the holder used
in the planar illumination unit of the present invention;
FIG. 17 is a diagram showing the second feature of the present invention;
FIG. 18 is a diagram showing an embodiment of the planar illumination unit
having the second feature of the present invention;
FIG. 19 is a diagram showing another embodiment of the planar illumination
unit having the second feature of the present invention;
FIG. 20 is a diagram showing still other embodiment of the planar
illumination unit having the second feature of the present invention;
FIG. 21 is a diagram showing still other embodiment of the planar
illumination unit having the second feature of the present invention;
FIG. 22 is a diagram showing a third feature of the present invention;
FIG. 23 is a diagram showing an embodiment of the planar illumination unit
having the third feature of the present invention;
FIG. 24 is a diagram showing another embodiment of the planar illumination
unit having the third feature of the present invention;
FIG. 25 is a diagram showing still other embodiment of the planar
illumination unit having the third feature of the present invention;
FIG. 26 is a diagram showing still other embodiment of the planar
illumination unit having the third feature of the present invention;
FIG. 27 is a diagram showing an embodiment of the liquid crystal display
panel; and
FIGS. 28-31 are diagrams showing the experiment conducted on the planar
illumination unit including the one having the third feature.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 3 shows a planar illumination unit 20 according to a first embodiment
of the present invention. In FIG. 3 as well as other drawings, the
vertical axis is somewhat expanded for the sake of convenience of the
illustration. Thus, a cold cathode tube having a circular cross section is
represented to have an apparently elongated circular cross section.
Referring to FIG. 3, the planar illumination unit 20 includes an optical
guide plate 23 and a pair of cold cathode tubes 21a and 21b disposed at
both sides of the optical guide plate 23 so as to face side walls 23a and
23b of the optical guide plate 23. Adjacent to the cold cathode tubes 21a
and 21b, there are provided holders 22a and 22b respectively, wherein the
holders 22a and 22b carry respective reflectors 24a and 24b for reflecting
the light that has been produced by the cold cathode tubes 21a and 21b,
such that the light produced by the tube 21a is injected to the plate 23
at the side wall 23a after reflection at the reflector 24a while the light
produced by the tube 21b is injected to the plate 23 at the side wall 23b
after reflection at the reflector 24b. The reflectors 24a and 24b may be
formed of a metal coating and have a reflectivity of almost 100%.
In the illustrated example, there are provided materials 25a and 25b having
a high refractive index between the cold cathode tube and the reflector
such that the material 25a fills the space between the tube 21a and the
reflector 24a and the material 25b fills the space between the tube 21b
and the reflector 24b. The materials 25a and 25b have the foregoing high
refractive index set to be larger than the refractive index of the glass
that forms the tubes 21a and 21b, and the light that has been produced by
the tubes 21a and 21b is effectively guided to the optical guide plate 23
via the materials 25a and 25b.
The optical guide plate 23 may be formed by a transparent resin such as
acrylic resin (for example, a resin in which a copolymer of methyl
methacrylate and vinyl benzoate designated as poly(MMA-co-VB) or
poly(2,2,2-trifluoroethyl methacrylate) designated as PF3FMA is dispersed
in poly(methyl methacrylate) designated as PMMA), and guides the light
that has been emitted from the cold cathode tube 21a or 21b and injected
at the side walls 23a and 23b to a prismatic lens film 26 that is disposed
on an upper principal surface 28 of the plate 23. The prismatic lens film
26 will be described later. Further, a reflection layer 27 is disposed on
the lower principal surface 27a of the plate 23 for reflecting the light
in the optical guide plate 23 toward the prismatic lens film 26, wherein
there is formed a reflection diffusion structure or pattern at the
foregoing lower principal surface 27a of the plate 23 for causing a
diffusion in the light that is reflected by the reflection layer 27.
It should be noted that the upper principal surface 28 of the optical guide
plate 23 is formed to have a tilt angle to form a V-shape in the cross
section, for eliminating the total reflection of the light at the central
part of the surface 28. By providing such a tilt angle to the surface 28,
it is possible to reduce the incident angle of the light at the central
part of the optical guide plate 23, and the desired reduction of the total
reflection at the surface 28 is achieved. With such a construction, it is
possible to reduce the thickness of the optical guide plate 23, without
causing a total reflection of the light. In the present invention, as will
be described in detail later, the tilt angle .theta..sub.1, defined with
respect to a horizontal surface is optimized, and an efficient emission of
the light from the optical guide plate 23 can be achieved.
Hereinafter, the foregoing prismatic lens film 26 will be described
briefly. The prismatic lens film 26 has a number of ridges 29 extending
parallel with each other, wherein each ridge 29 has a triangular cross
section. Thereby, each ridge 29 acts as an optical prism, and such a
prismatic lens film 26 provides a directivity in the light that passes
through the film 26. In the illustrated example, the ridges 29 have a
shape of isosceles triangle with an apex angle .theta..sub.2 that is
optimized for an efficient radiation of the optical beam from the
prismatic lens film 26, as will be described later in detail.
Above the prismatic lens film 26, there is disposed an optical scattering
plate 30. The optical scattering plate 30 has a milky whitish appearance
and causes a scattering in the light that has been injected thereto. As a
result of the scattering, the light that is emitted from the scattering
plate 30 has a uniform luminance throughout the area of the plate 30.
Hereinafter, the features of the present invention will be described. The
first feature of the present invention resides in the point that the
planar illumination unit 20 has the holders 22a and 22b that are adapted
for optimum injection of the light, which is emitted from the cold cathode
tube 21a or 21b, into the optical guide plate 23.
It should be noted that one has to set the incidence angle of the light
with respect to the side walls 23a and 23b of the optical guide plate 23,
in order to achieve the injection of the light into the plate 23 at the
side walls 23a and 23b.
FIG. 4 shows the analysis for such an injection of the light.
Referring to FIG. 4, there holds the Fresnel's law represented at the side
wall 23a or 23b as
sin .theta.=n.multidot.sin .alpha. (3)
wherein .theta. stands for the incidence angle, .alpha. stands for the
refraction angle and n stands for the refractive index of the optical
guide plate 23.
On the other hand, from an elementary geometry, there holds a relationship
.alpha.=90.degree.-.theta..sub.c (4)
where .theta..sub.c stands for the incidence angle of the light that
impinges upon the lower principal surface 27a of the optical guide plate
23. In order that the light is guided along the plate 23, it is necessary
that the light impinging upon the lower principal surface 27a is
reflected. Thus, there must be a relationship
n.multidot.sin .theta..sub.c =1. (5)
Assuming that the optical guide plate 23 is formed of an acrylic resin of
which refractive index n is 1.49, the foregoing equation gives the value
of 42.degree. for the angle .theta..sub.c. Thereby, one obtains an angle
of about 90.degree. for the incidence angle .theta..
The foregoing analysis indicates that the light incident to the optical
guide plate 23 at the side walls 23a and 23b from the upper left direction
with any arbitrary angle can be guided through the plate 23, except for
the Fresnel reflection.
Here, the relationship of Eq. (1) mentioned previously is again noted.
LP.sub.1 +LP.sub.2 +LP.sub.3 +LP.sub.4 +LP.sub.5 =LP.sub.0 (1)
In Eq. (1), it will be noted that the optical energy corresponding to the
first and second terms LP.sub.1 and LP.sub.2 is injected into the optical
guide plate 23 without adjusting geometrical parameters, while the optical
energy corresponding to the rest of the terms LP.sub.3 -LP.sub.5 changes
depending on the adjustment of the optical construction. In other words,
it is possible to achieve the successful injection into the optical guide
plate 23 by adjusting the optical construction of the optical guide plate
23. Hereinafter, optimization of the individual terms will be examined
with respect to the third through fifth terms.
The third term LP.sub.3 represents the luminance or optical power that is
absorbed by the holders 22a and 22b. As described previously, the holders
22a and 22b carry reflective coatings 24a and 24b, and the improvement in
the quality of the reflective coatings reduces the optical loss. Today, a
reflective coating that provides a reflectivity of almost 100% is
available, and one can eliminate the loss caused by the term L.sub.3
substantially completely.
The fourth term LP.sub.4 represents the optical energy that is absorbed by
the cold cathode tubes 21a and 21b after reflection at the holders 22a and
22b. When the light, emitted from the cold cathode tubes 21a and 21b, has
returned again to the cold cathode tubes, the light may be absorbed by the
fluorescent materials applied upon a glass tube 31 of the cold cathode
tubes 21a or 21b as indicated by an arrow A.sub.1 in FIG. 5.
Alternatively, the light thus returned may be guided along the tube 31 as
indicated by an arrow A.sub.2 and absorbed as it travels along the tube.
It turned out that the effect of this term is substantial as reported by
Mesaki et al. (Mesaki, Y., Sotokawa, A., Tanaka, A., Tomatsu, M., Kaiwa,
K., TECHNICAL REPORT OF IEICE, EID92-126, ED92-159, 1993-02), wherein
about 38.8% of the optical energy is lost in this process.
The effect of the term LP.sub.5 can be minimized easily by providing a
reflective coating having an improved reflectance for the reflection layer
27.
In order to eliminate the light from returning to the cold cathode tubes
21a and 21b, one may provide an optical guide layer of high refractive
index such as a layer 25a or a layer 25b in correspondence to the space
between the holder 22a or 22b and the cold cathode tube 21a or 21b as
indicated in FIG. 6(A). Thereby, the difference in the refractive index
between the glass tube 31 forming the cold cathode tube 21a or 21b and the
optical guide layer 25a or 25b eliminates the light reflected at the
reflection coating 24a or 24b from entering into the glass tube 31 even
when the light has returned to the cold cathode tube, and the problem of
the absorption of the light by the fluorescent material is reduced. It
should be noted that the surface of the glass tube 31 reflects back the
light to the optical guide layer 25a or 25b, and the light thus reflected
are ultimately guided to the side wall 23a or 23b of the optical guide
plate 23 after repeating the reflection at the surface of the reflective
coating 24a or 24b.
FIG. 6(B) shows another embodiment for eliminating the absorption of the
light by the cold cathode tubes 21a and 21b, wherein the embodiment of
FIG. 6(B) employs fluorescent material introduced into the optical guide
layer 25a or 25b for absorbing and re-emitting the light that is guided
along the optical guide layer.
FIGS. 7 and 8 show another embodiment of the present invention wherein the
shape of the holders 22a and 22b is optimized such that the light
reflected at the reflective coating on the holders does not return to the
cold cathode tube 21a or 21b. In the embodiment of FIG. 7, there is formed
a flat surface 22f so as to face the side wall 23a or 23b of the optical
guide layer, and curved walls 22g and 22h connect the surface 22f to the
optical guide plate 23. Thereby, most of the light that has been emitted
by the cold cathode tube 21a or 21b and reflected at the flat surface 22f
are guided to the optical guide plate 23 after reflection at the curved
walls 22g or 22h. It should be noted that the flat surface 22f as well as
the curved walls 22g and 22h carry a reflective coating for this purpose.
In the embodiment of FIG. 8, the holder 22a or 22b has a polygonal cross
section defined by flat surfaces 22f.sub.1, 22f.sub.2, 22g.sub.1,
22g.sub.2, 22h.sub.1, and 22h.sub.2, and the light emitted from the cold
cathode tube 21a or 21b is guided to the optical guide plate 23 by causing
a reflection at these flat surfaces.
FIG. 9 shows an embodiment wherein a projection 22c is provided on the
holder 22a or 22b to change the direction of reflection of the light. By
providing such a modified surface, one can reduce the probability of the
light emitted from the cold cathode tube 21a or 21b and returning to the
tube again. Preferably, the projection 22c is provided so as to oppose the
side wall 23a or 23b of the optical guide plate 23 across the cold cathode
tube.
FIG. 10 shows a construction of the holders 22a and 22b, wherein the holder
has a conventional construction characterized by a curved wall having a
circular cross section except that the center line C.sub.1 for the holder
22a and the center line C.sub.2 of the tube are offset from each other by
a distance .DELTA.. Thereby, the majority of the light that has been
emitted from the cold cathode tube is reflected at the curved wall of the
holder 22a and is guided to the optical guide plate 23. FIG. 11 shows
another similar construction, wherein the position of the cold cathode
tube 21a is displaced upward with respect to the holder 22a or 22b in the
direction opposite to the case of FIG. 10, with a distance .DELTA..
FIG. 12 shows another construction of the holders 22a and 22b, wherein the
holder has a polygonal form defined by flat surfaces similarly to the
embodiment of FIG. 8, except that the flat surfaces form an apex
projecting toward the tube 21a or 21b contrary to the embodiment of FIG.
8. In the illustration, it will be noted that the flat surfaces carry
reflective coatings 24a or 24b thereon.
FIG. 13 shows a modification of the embodiment of FIG. 7, wherein a flat
surface 22f' is formed obliquely to the cold cathode tube with a reduced
area as compared with the embodiment of FIG. 7. In this embodiment, too,
it is possible to reduce the probability of the light emitted from the
cold cathode tube returning again to the cold cathode tube.
FIGS. 14-16 show the minimum angle 8 of the light that is emitted from the
cold cathode tube 21a or 21b and enters the optical guide plate 23 without
returning to the cold cathode tube, wherein FIG. 14 shows the case wherein
the holder 22a or 22b has a circular cross section, FIG. 15 shows the case
wherein the holder 22a or 22b has a hyperbolic cross section, and FIG. 16
shows the case wherein the holder 22a or 22b has a planar cross section in
correspondence to the embodiment of FIG. 7.
As will be seen, the minimum angle .theta. becomes largest in the case of
FIG. 14 that uses a mirror having the circular cross section, while the
angle .theta. becomes minimum in the case of FIG. 15 that uses a flat
mirror surface. The light within this angle .theta. is reflected to the
cold cathode tube. This indicates that the construction of FIG. 16
corresponding to the embodiment of FIG. 7 can emit the light with maximum
efficiency, by minimizing the light returning to the cold cathode tube.
The following TABLE I summarizes the result of the experiments conducted
in correspondence to the constructions of FIGS. 14-16.
TABLE I
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mirror shape
shape radius(mm) asph. coeff.*
.theta..sub.min
efficiency
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sphere 2.6 0 90.degree.
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