Note: Descriptions are shown in the official language in which they were submitted.
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LIGHT ADJUSTING SHEET HAVING A SINUSOIDAL
SURFACE AND A NON-OPTICALLY FLAT
SURFACE AND USABLE WITH AN LCD
This invention relates to a light adjusting sheet for a
planar lighting device used for a liquid crystal display of
personal computers or word processors or a liquid crystal
television set, and a planar lighting device and a liquid crystal
display using the sheet, and in particular to a light focusing
or adjusting sheet for a planar lighting device of the edge-light
type suitable for a back light of a thin display and a planar
lighting device and a liquid crystal display using the sheet.
Smaller, lighter and thinner word processors and personal
computers have been developed every year, and these having the
so-called laptop type or notebook type size have become today's
main trend. Since the liquid crystal display of the laptop type
or the notebook type is usually not luminescent by itself, it has
a back light on its back surface to improve visibility. The back
light is required to be thin and to illuminate the display
surface uniformly.
Usually, a planar lighting device is used as the back light,
and many kinds of planar lighting devices are used. For example,
as shown in Fig 1, one of them has a light source 11 on the back
side of a diffusing plate 10, wherein the light is emitted from
the back side of the diffusing plate 10 and emanates uniformly
from the surface of the diffusing plate 10 or from a regulator
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plate 12 attached to the surface of the diffusing plate 10, as
described in Japanese Patent Provisional Publication No.
257188/90.
However, the most widely used planar lighting device is that
of a so-called edge-light type. The edge-light type planar
lighting device has a light source on a side of a light guiding
plate, wherein the light emitted from the light source emanates
uniformly from the surface of the light guiding plate or from a
plate attached to the light guiding plate, and is widely used
because it is thin and light, etc.
Recently, along with the efforts to make thinner displays
and color displays, there is an increasing demand for improved
luminance of a planar lighting device as the back light for
displays. Since a user of the display looks at the display from
the front side in most of the time, it is necessary to regulate
the direction of the emanated light from the planar lighting
device so that the light emanates efficiently in the forward
direction to improve the luminance using the same light source.
However, if the light emanates within a too narrow range, a
slight change in the direction of the user's sight will make the
display invisible. Therefore, it is required to diffuse the
light across an appropriate range.
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The details are described here using this planar lighting
device of the edge-light type as an example. In general, if a
light source is provided on the edge side of a thin light guiding
plate, a light which enters the light guiding plate through the
edge surface repeatedly experiences total reflections and almost
no light leaves the light guiding plate because of the difference
in the refractive index between the air and the light guiding
plate. In the planar lighting device, an irregular reflection
layer is provided on the back surface of the light guiding plate
and a reflector plate is further provided on the back side of the
light guiding plate, so that the light which reaches the back
side of the light guiding plate is irregularly reflected by the
irregular reflection layer, and then goes out of the light
guiding plate directly or after being reflected by the reflector
plate. The irregular reflection layer is often formed by the
dot-printing using paint including beads of small diameters.
However, the light emitted from the light source on the edge
surface of the light guiding plate usually goes out from the
light guiding plate at a very small angle from the surface. That
is, the light has a strong directionality, as shown in Fig. 2.
A more detailed explanation of the above problem is given
hereby by referring to Fig. 2. A transparent light guiding plate
20 has an irregular reflection layer (not shown) on its back
surface. A reflector plate 21 is provided on the back side of
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the light guiding plate 20, and a linear light source 22 is
provided on the edge side of the light guiding plate 20. The
light emitted from the light source 22 travels through the light
guiding plate 20 and irregularly reflects at the irregular
reflection layer provided on the back surface of the light
guiding plate 20. Then, the light either directly goes out of
the light guiding plate 20 or reflects at the reflector plate 21
and then goes out of the light guiding plate 20. The light from
point A has a strong directionality making a very small angle
with the surface of the light guiding plate 20, indicated by B
in Fig. 2.
Since it is rare that the user looks at the screen of the
display at such a small angle with the surface of the light
guiding plate 20, it is necessary to change the angle of the
emanating light. In order to improve the directionality of the
light, a method in which a light diffusing plate 23 is provided
on the front surface of the light guiding plate 20 has been
proposed, as shown in Fig. 3. The light diffusing plate 23 is
a transparent plastic sheet applied with a paint containing a
white pigment on its surface, or a transparent plastic sheet
provided with fine unevenness on its surface by the mat
treatment, the crimp treatment, or such. By providing the light
diffusing plate 23, the light in the direction perpendicular to
the diffusing plate 23 increases, indicated by C in Fig. 3.
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Fig. 4 shows a detail of the light crystal display,
illustrating the basic structure of the planar lighting device
of the edge-light type. The planar lighting device 30 mainly
includes a light guiding plate 32 consisting of a transparent
plate having a dot pattern 31 on the back surface, at least one
linear light source 33 of a cathode ray tube (fluorescent lamp)
provided on at least one side of the light guiding plate 32, a
reflector plate 34 provided behind the light guiding plate 32,
and a light diffusing sheet 35 consisting of a resin plate
including a light diffusing material or a resin plate provided
with crimps on its surface, and a liquid crystal display element
36 is further provided in front of the planar lighting device 30
(refer to Japanese Patent Provisional Publication No. 244490/89,
U.S.P. No. 4775222 and U.S.P. No. 4729068). The dot pattern 31
is a light scattering printed dot pattern formed on the back
surface of the light guiding plate 32 so that the incident light
from the light source 33 on the side goes out uniformly from all
parts of the screen of the display, and it may be called a pseudo
light source. The light diffusing sheet 35 functions in such a
way that the back light source, i.e. the dot pattern 31 as the
pseudo light source, is not visible and the screen is seen as a
uniformly luminescent plane, when using the liquid crystal
display screen.
However, in the device using the light diffusing plate 35,
much of the light is emitted in directions not needed for the
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user and the light in the forward direction is scarce, so that
the luminance of the light in the forward direction from which
the user sees is low.
Conventionally, in order to make the dot pattern 31
invisible and increase the light diffusion efficiency, the
following measures have been taken: i) coating a light diffusing
material onto a plastic sheet surface or introducing it into the
inside of the sheet, and ii) creating a crimp-like unevenness or
a regular unevenness on the plastic sheet.
In the former measure i), the light diffusing sheet 35
consists of a sheet formed by an extruder from a material made
of a resin such as polyester, polycarbonate or
polymethylmethacrylate, mixed with a light diffusing material
such as fine-powder of calcium carbonate, titanium oxide, short
glass fibre or silicone resin particles containing polysiloxane
bonds (refer to Japanese Patent Provisional Publication No.
140343/78).
In the latter measure ii), the sheet having a crimp-like
unevenness or a regular unevenness on its surface is provided on
the front side of the light guiding plate 32 or on at least one
surface of the light diffusing sheet 35 (refer to Japanese Patent
Provisional Publication No. 257188/90). Such a sheet is called
"a light adjusting sheet"
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In one example of the latter measure ii), a light adjusting
sheet 37 consisting of a transparent prism sheet with convex
streaks and concave streaks each having a triangular cross
section, as shown in Fig. 5, is provided on the front side of the
light guiding plate 32 so that the opposite surface with no
convex streaks and concave streaks of the light adjusting sheet
37 is in contact with the light guiding plate 32 in a liquid
crystal display, as shown in Fig. 6. In such a planar lighting
device of the liquid crystal display, the strongly directional
light is redirected in the direction of the user's sight by the
slopes of the triangular cross section of the convex streaks and
concave streaks, and the emitting direction of the light is
confined to a predetermined range and converged in the direction
of the user of a wordprocessor or other such device, i.e. the
normal direction of the screen, and emanates from the front
surface of the light adjusting sheet 37, thus obtaining a higher
luminance in the screen.
However, in the former case i), of the planar lighting
device with the light diffusing sheet, if the amount of the light
diffusing material contained in the light diffusing sheet 35 is
increased to improve the light diffusion, the amount of the
emanated light is decreased because of the shielding
characteristic of the light diffusing material, resulting in
insufficient brightness in the screen, indicated by the dotted
line D in Fig. 7.
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On the other hand, in the latter case ii) , of the planar
lighting device with the light adjusting sheet, it utilizes the
prism effect of the convex streaks and concave streaks on the
surface of the light adjusting sheet, and is superior to the
former device in terms of the amount of the emanated light.
However, since the directionality of the light is too strong and
the incident direction and the outgoing direction of the light
are too strictly defined, so that the light emanating direction
is deflected in the forward direction within a narrow angle range
and the sight angle of the screen tends to be exceedingly narrow,
as indicated by E in Fig. 8.
Further, when the light adjusting sheet consisting of a
prism sheet of a serrate cross section is placed in such a way
that its flat back surface is brought into direct contact with
the light guiding plate, interference patterns of Newton ring
appear due to slight gaps generated between the back surface of
the light adjusting sheet and the light guiding plate. The
closer the convex streaks and the concave streaks are provided
to the final outgoing surface, the lattice-like partition lines
of the liquid crystal display surface and the ridge lines and
bottom lines of the prism sheet cause interference (the Moire
phenomenon), depending on the pitches of the convex streaks and
concave streaks.
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Further, the inventor experimented with this planar lighting
device and found that there is a problem in that the irregular
reflection layer provided on the back surface of the light
guiding plate is visible, although it can indeed alter the
direction of the light efficiently so that the light emanates in
the direction of the user's sight. When a display using the
liquid crystal is placed on the planar lighting device with the
visible irregular reflection layer, the screen of the display is
very hard to see.
Therefore, a light diffusing sheet 35 is required in the
planar lighting device when it uses the conventional light
adjusting sheet.
Accordingly, it is an object of this invention to provide
a light adjusting sheet for a planar lighting device in which the
emanated light distribution can be concentrated in the forward
direction of the screen, and no Moire fringes occur.
It is another object of this invention to provide a light
adjusting sheet for a planar lighting device which gives greater
luminance in the forward direction compared with the case when
no light adjusting sheet is used, gives a wider sight angle than
the conventional light adjusting sheet, and assures prevention
of an optically tight contact with the light guiding plate.
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It is a further object of this invention to provide a light
adjusting sheet for a planar lighting device which gives a
suitable span in the direction of the user's sight and an
improved brighter screen.
It is a still further object of this invention to provide
a light adjusting sheet for a planar lighting device with reduced
reflection at the front surface thereof, with no interference
fringes, and also with the irregular reflection layer made
invisible.
It is yet another object of this invention to provide a
planar lighting device of an edge-light type which can illuminate
a screen of a display brightly and uniformly.
It is yet a further object of this invention to provide a
liquid crystal display in which the screen is bright and easily
seen.
According to an aspect of the invention, a light adjusting
sheet has one surface provided with a plurality of convex streaks
each having a cross section of a convex arc and a plurality of
concave streaks each having a cross section of a concave arc
alternately arranged approximately parallel to each other, and
the other surface which is an optically non-smooth surface.
Throughout the specification, the term "sheet" includes not only
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a sheet in the strict sense limited by the thickness but also
thinner one which is usually called "film". Also, the term
"approximately parallel" means that a peak line of each of the
plurality of the convex streaks and a bottom line of each of the
plurality of the concave streaks are approximately parallel to
each other.
According to another broad aspect, the invention provides
a light adjusting sheet for a planar lighting device having a
plurality of convex streaks each having a lateral cross section
of a convex arc on one surface, and a plurality of concave
streaks each having a lateral cross section of a concave arc on
said one surface. Each convex streak and each concave streak are
arranged alternately and approximately parallel to each other.
According to another broad aspect, the invention provides
a light adjusting sheet for a planar lighting device which is
placed at the front of a light emitting surface of a light
guiding plate which composes said planar lighting device. The
light adjusting sheet has on one surface a plurality of convex
regions each having a lateral cross section of a convex arc, and
also on said one surface a plurality of concave regions each
having a lateral cross section of a concave arc. Each convex
region and each concave region are arranged alternately and
approximately parallel to each other. The angle between the two
slopes of said each convex regions and each concave regions is
in the range of 30° to 150°. The radius of curvature of said
convex arcs and concave arcs is 10 to 100 um.
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According to another broad aspect, the invention provides
a light adjusting sheet for a planar lighting device comprising
a light adjusting sheet, a light guiding plate, a light source
and a reflector plate. The light adjusting sheet has a plurality
of convex regions each having a lateral cross-section of a convex
arc on one surface, and a plurality of concave regions each
having a lateral cross-section of a concave arc on said one
surface. The convex regions and the concave regions are arranged
alternately and approximately parallel to each other. Another
surface opposite to said one surface has a non-optically flat
surface. The angle between the two slopes of each said convex
arc is in the range of 30° to 150° and the radius of curvature
of said convex arcs is 10 to 100~Cm.
As described above, the light adjusting sheet according to
the invention has one surface provided with a plurality of convex
streaks each having a cross section of a convex arc and a
plurality of concave streaks each having a cross section of a
concave arc alternately arranged approximately parallel to each
other and the other surface of an optically non-smooth surface,
so that the planar lighting device using this light adjusting
sheet has advantages in that the luminance in the forward
direction is improved compared with one without the light
adjusting sheet, the dot pattern on the back surface of the light
guiding plate is not visible in the least, the light diffusion
is adequate, the sight angle range is kept equivalent to that of
a liquid crystal display element itself, and the interference
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(Moire phenomenon) between the lattice-like partition lines of
the liquid crystal display surface and peak lines of the convex
streaks and bottom lines of the concave streaks of the sheet is
prevented.
Therefore, if this light adjusting sheet for a planar
lighting device is used in a planar lighting device of the edge-
light type or a planar lighting device which has a light source
on the back side of a light diffusing plate, then a bright and
very easy-to-see screen can be obtained by installing the planar
lighting device in a liquid crystal display. The light adjusting
sheet for a planar lighting device in the invention can be
effectively used not only in a liquid crystal display device but
also in stores, houses, offices, etc. as a thin planar light
source for thin facility lighting equipments.
In the drawings which illustrate the embodiments of the
invention,
Fig. 1 is an explanatory view showing the conventional
planar lighting device.
Fig. 2 is an explanatory view showing the light distribution
characteristics of the conventional planar lighting device of the
edge-light type.
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Fig. 3 is an explanatory view showing the light distribution
characteristics of the conventional planar lighting device of the
edge-light type.
Fig. 4 is a cross-sectional view showing the conventional
liquid crystal display device of the edge-light type.
Fig. 5 is a perspective view showing the conventional light
adjusting sheet for a planar lighting device.
Fig. 6 is a cross-sectional view showing the conventional
liquid crystal display device of the edge-light type.
Fig. 7 is a luminance distribution diagram for the planar
lighting device of the edge-light type.
Fig. 8 is a luminance distribution diagram for the planar
lighting device of the edge-light type.
Fig. 9 is a schematic diagram showing the luminance
measurement procedure for the planar lighting device.
Fig. 10 is a luminance distribution diagram for the planar
lighting device of the edge-light type.
Fig. 11 is a perspective view showing a light adjusting
sheet of an embodiment.
Fig. 12 is a cross-sectional view showing a planar lighting
device of the edge-light type of an embodiment.
Fig. 13 is a plan view showing a planar lighting device of
the edge-light of an embodiment.
Fig. 14 is a plan view showing a planar lighting device of
the edge-light type of an embodiment.
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Fig. 15 is a cross-sectional view showing a light adjusting
sheet of an embodiment.
Fig. 16 is a cross-sectional view showing a light adjusting
sheet of an embodiment.
Fig. 17 is a cross-sectional view showing a light adjusting
sheet of an embodiment.
Fig. 18 is a cross-sectional view showing a planar lighting
device of the edge-light type using the light adjusting sheet
shown in Fig. 17.
Fig. 19 is an explanatory view showing the light
distribution characteristics of the planar lighting device of the
edge-light type of an embodiment.
Fig. 20 is a cross-sectional view showing a light adjusting
sheet of an embodiment.
Fig. 21 is a cross-sectional view showing a light adjusting
sheet of an embodiment.
Fig. 22 is a cross-sectional view showing a light adjusting
sheet of an embodiment.
Fig. 23 is a perspective view showing a light adjusting
sheet of an embodiment.
Fig. 24 is a cross-sectional view showing a conventional
sheet which can be used with a light adjusting sheet of an
embodiment.
Fig. 25 is a cross-sectional view showing a sheet of an
embodiment.
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Fig. 26 is a plan view showing a liquid crystal display
device of the edge-light type of an embodiment.
Fig. 27 is a plan view showing a liquid crystal display
device of the edge-light type of an embodiment.
The measurement procedure of the outgoing light luminance
of the planar lighting device is described below. In Fig. 9, let
us assume that the normal angular coordinate of the light
emanating surface of the planar lighting device 30 is 0°, and
then the luminance of point P on the planar lighting device 30
is measured at several positions within the range from -90° to
+90° with the line which passes the point P and is parallel to
the axis of the light source 33 as the axis, using a luminance
meter 40. The maximum luminance value of these measurements is
represented as 1000, and values at other positions are
represented as percentages. The relationship between the
measurement positions and the luminance is plotted on a graph.
The solid line F and the dotted line G in Fig. 10 show the
luminance distributions of the planar lighting device of the
edge-light type with the basic structure shown in Fig. 4. The
solid line F in Fig. 10 shows the luminance distribution with a
wider sight angle, and the dotted line G in Fig. 10 shows the
luminance distribution which has a greater amount of the outgoing
light in the direction of the user (the normal direction of the
screen).
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In this lighting device, it is shown that the amount of
emanated light is small in the front direction while the user is
at the position just in front of the lighting device (0°). In
order to increase the luminance distribution in the forward
direction, there have been proposals in which the light adjusting
sheet 37 with the prisms shown in Fig. 5 in which convex streaks
and concave streaks with a serrate-shaped cross section are
formed on one or both sides of the sheet is used, or in which
fine unevenness is directly formed on the light emanating side
of the light guiding plate or on the reflector plate side. These
convex streaks and concave streaks provided on the light
adjusting sheet or on the light guiding plate are formed parallel
to the light source axis. However, in this case, the angle for
the maximum amount of emanated light differs depending on models
of the planar lighting device. Therefore, when a light adjusting
sheet which has premises with a specific configuration is used,
a certain model of the lighting device produces good emanated
light characteristics, while another model of the lighting device
produces not as good results.
The plurality of the convex streaks and the concave streaks
of the light adjusting sheet function to concentrate the outgoing
light from the sheet more in the normal direction of the screen.
As shown in Fig. 11, these plural convex and concave streaks are
provided on the light emanating surface of the light adjusting
sheet, and each peak 51a of the convex streaks 51 has a cross
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section of a convex arc and each bottom 52a of the concave
streaks 52 has a cross section of a concave arc. The angle a
between the two slopes of each convex streak (this angle is
defined as an angle made by two tangent lines in the middle of
the two slopes) is in the range of 30° to 150°, and preferably
of
about 60° to 120°. It is difficult to form the convex and
concave streaks if the angle is less than 30°, and the light
concentration capability of the sheet may decrease if the angle
is greater than 150°. The radius ~i of curvature of these convex
arcs and concave arcs in cross sections of the convex and concave
streaks is determined by the depth y of the convex and concave
streaks, the angle a made by the slopes, the pitch ~, etc., and
it is preferably 10 to 100 Vim. The convex and concave streaks
may have a lateral cross section of a sine curve represented by
the equation Y - a~sin bX (Y: Coordinate in the depth-wise
direction in mm units. X: Coordinate in the direction
perpendicular to the streaks in mm units, "a" and "b"' factors).
If the convex and concave streaks have a cross section of a sine
curve as described above, the light is converged by the streaks
into the forward direction, resulting a very easy-to-see light
when emanated.
As shown above, the light adjusting sheet of the embodiments
of this invention has the plurality of the convex streaks each
having a peak of a cross section of a convex arc and the concave
streaks each having a bottom of a cross section of a concave arc,
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so that the sheet has functions of adequate light concentration,
light diffusion and light directionality of the emanating light
to forward direction. Therefore, the surface provided with the
convex and concave streaks in the light adjusting sheet may
include flat surfaces which form less than 50~ of the total area
of the curved surface between each of the convex and concave
streaks. In this case, there is a tendency that the light
adjusting sheet has an increased light concentration function.
The thickness of the light adjusting sheet is 50 ~m or
greater, preferably 90 to 300 Vim.
The material of which the light adjusting sheet consists is
not limited, as long as it is a transparent organic or inorganic
material, that is a transparent material such as glass or
transparent synthetic resin formed into a sheet. A synthetic
resin sheet is particularly preferable. For the transparent
synthetic resin, polycarbonate, polymethyl methacrylate,
polyester, cellulose synthetic resins, polystyrene,
polyvinylchrolide and such are preferable. The light adjusting
sheet is preferably made of the same type of resin as the base
material of the light diffusing sheet which will be described
later.
Preferable methods for fabricating the light adjusting sheet
made of the synthetic resin include a method in which a
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transparent synthetic resin is extruded and goes through an
embossing roll to have convex and concave streaks formed on the
surface, a method in which a synthetic resin is pressed using a
die plate with convex streaks and concave streaks provided on the
surface, a method in which a synthetic resin is injected into a
die with convex streaks and concave streaks provided on the inner
surface, etc.
The back surface, which does not have convex streaks or
concave streaks, may be a non-smooth surface. For the non-smooth
surface, those on which fine and random unevenness, for example,
may be formed. The size and depth of the unevenness is not
limited as long as there is no optically tight contact of a size
recognizable by the naked eye when this surface is brought in
contact with a smooth surface. The non-smooth surface can also
be a moderate wave formed surface. For the method of forming the
non-smooth surface, the method in which a pattern on a roll or
a die is transferred to the surface at the same time of forming
the surface, and the method in which calendering, sand blasting,
chemical etching, the mat treatment, pressing, etc. are used to
give unevenness to an already formed sheet, are applicable. The
particularly preferable examples of the methods of forming the
non-smooth surface are the mat treatment, the sand blasting
method, the press method and such.
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For example, in the case of the planar lighting device
without a light guiding plate as shown in Fig. 1, the back
surface of the light adjusting sheet is not to be in contact with
anything, so that the light adjusting sheet with a flat back
surface can be suitably employed. In the case of the planar
lighting device with a light guiding plate as shown in Fig. 4,
the light adjusting sheet is used with the light guiding plate
in contact with its back surface, and interference fringes of
Newton ring may result because of the slight gap between this
back surface of the light adjusting sheet and the light guiding
plate if the back surface of the light adjusting sheet is a flat
surface. Therefore, in such cases, it is preferable to provide
unevenness on the back surface of the light adjusting sheet.
For example, it is preferable to form fine unevenness on the
back surface formed by sand blasting and such as shown in Fig.
11, because this will eliminate interference fringes of Newton
ring and also the light will be diffused. In addition, the
directionality of the light decreases if the back surface of the
sheet is provided with convex and concave streaks of a cross
section of a continuous curve shown in Fig. 20 or of a triangular
cross section shown in Fig. 21. According to the experiments by
the inventors, even better results can be obtained by making the
period of the uneven streaks on the back surface of the light
adjusting sheet the same as the period of the convex and concave
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streaks of the front surface and making the amplitude less than
a half of that of the front surface.
As described above, the back surface of the light adjusting
sheet opposite to the surface provided with the convex streaks
and concave streaks can be changed in various ways to produce
different results. Therefore, a suitable combination with the
convex and concave streaks of the front surface should be chosen,
depending on the refractive index and the thickness of the light
guiding plate, the refractive index of the light adjusting sheet,
etc.
Also, the light adjusting sheet may contain a light
diffusing material. The light diffusing material gives an
ability to diffuse light to the light adjusting sheet. For the
light diffusing material, white pigments such as calcium
carbonate powder, titanium oxide powder and zinc white, white
inorganic powder such as alumina powder, silica powder and white
clay, glass beads, glass fibre, synthetic resin powder with a
refractive index different from that of the light adjusting
sheet, etc., are preferable.
Also, regarding the diffusion material content, there is
little effect if the light diffusion material is equal to or less
than 0.01 weight units against 100 weight units of the
transparent material of glass, synthetic resin or such, of which
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the light adjusting sheet consists, and then the effect does not
increase any more. On the other hand, the light transmission
performance rather deteriorates if it is 10 weight units or more.
Therefore, the range of 0.01 to 10 weight units is preferable,
and 0.1 to 5 weight units is more preferable.
There are several methods for making the light adjusting
sheet contain the light diffusing material, that is, i) a method
in which the light adjusting sheet is formed by a transparent
material which is already mixed with the light diffusing
material, ii) a method in which the light adjusting sheet is
formed by stacking a plurality of layers of a transparent
material layer which mainly consists of the light adjusting sheet
and a transparent material which includes a light diffusing
material, and iii) a method in which a coating material
consisting of a transparent material which includes a light
diffusing material is coated on the surface of the light
adjusting sheet, etc.
The surface of the sheet on which the convex and concave
streaks are provided is used as the front surface through which
the light emanates. The light transmitted through the front
surface is refracted by the convex and concave streaks and
converged in the normal direction, i.e. the front direction, of
the light adjusting sheet. For example, when the sheet is used
in a planar lighting device of the edge-light type, the opposite
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surface which is not provided with the convex and concave streaks
is in contact with the front surface of the light guiding plate.
Thus, the light emanated from the front surface of the light
guiding plate at a low angle goes through the light adjusting
sheet and emanates as a converged light in the forward direction.
More detailed explanations are given below, by referring to
an example of the light adjusting sheet of an embodiment of this
invention installed in a planar lighting device of the edge-light
type. In the planar lighting device of the edge-light type, a
light emitted from a linear light source provided on the edge
side of the light guiding plate enters the light guiding plate
through the edge surface. The light which has entered the light
guiding plate is irregularly reflected by the irregular
reflection layer and either directly goes out from the front
surface or goes out of the front surface after being reflected
by the reflector plate provided on the back surface of the light
guiding plate. The light emanating from the light guiding plate
is very directional, as shown in Fig. 2. Since the light
adjusting sheet is provided on the front surface of the light
guiding plate, the light enters the light adjusting sheet.
Here, if the opposite surface of the light adjusting sheet
which is to be in contact with the light guiding plate has fine
unevenness, the opposite surface is not in tight contact with the
light guiding plate, and therefore interference fringes of the
24
CA 02244439 1999-O1-21
Newton ring which result from fine gaps at a tight contact do not
appear. Further, the light taking-in efficiency (a ratio of the
light entered the light adjusting sheet without reflection to the
light which goes out of the light guiding plate) is improved.
Then, the light travels through the light adjusting sheet
emanates from the front surface having the convex and concave
streaks. Here, the light is converged in the forward direction
by the convex and concave streaks, and the irregular reflection
layer such as dot printing provided on the back surface of the
light guiding plate becomes invisible, and thus the light
emanates as an easy-to-see light for the user. If a diffusing
material is mixed in the light adjusting sheet, the light is
suitably diffused.
When a liquid crystal display element is installed in front
of the light adjusting sheet, an optical contact of the light
adjusting sheet and the liquid crystal display element may not
occur by the uneven streaks provided on the surface of the light
adjusting sheet.
The light adjusting sheet configured as described thus far
is used for a planar lighting device. As shown in Fig. 12, the
planar lighting device 30 includes at least one light adjusting
sheet 50 with the configuration described above, a light guiding
plate 32 provided behind the light adjusting sheet 50 with a
CA 02244439 1999-O1-21
printed dot pattern 31 on the back surface, at least one linear
light source 33 provided on at least one side of the light
guiding plate 32, and a reflector plate 34 provided behind the
light guiding plate 32. As shown in Fig. 4, at least one light
diffusing sheet 35 may be provided as necessary between the light
adjusting sheet 50 and the light guiding plate 32 or in front of
the light adjusting sheet 50, i.e. the light emanating end.
Then, a liquid crystal display is obtained by installing a liquid
crystal display element 36 in front of the planar lighting device
30.
In the planar lighting device, each of the convex and
concave streaks of the light adjusting sheet 50 makes a
predetermined angle with an axis of the light source 33. In
order to provide the convex and concave streaks to make a
predetermined angle with the axis of the light source 33, a
square sheet provided with a plurality of convex streaks and
concave streaks which are parallel to each other and which make
a predetermined angle with an edge line of the sheet may be used
as a light adjusting sheet, wherein the sheet is arranged so that
the edge line of the sheet is parallel to the axis of the light
source 33.
As shown in Fig. 13, the plurality of the convex and concave
streaks are formed on one side of the sheet in such a way that
each peak line 54 of the streaks makes an angle B with the edge
26
CA 02244439 1999-O1-21
55 of the sheet. The angle 8 is preferably in the range of 5°
s B s 85°. If B exceeds 85°, it may not be possible to obtain
a sufficient light concentration effect. If 8 is smaller than
5° or 8 is 85° or greater, the peak lines of the streaks on the
light adjusting sheet and the pitch lines between dots of the
liquid crystal display may cause Moire fringes.
Additionally, Moire fringes may appear between specific
patterns displayed on the liquid crystal display element and the
streaks of the light adjusting sheet, though the angle B is in
the range of 5° s 8 <_ 85°. In order to effectively prevent this
problem, the angle 8 is preferably approximately 36°, determined
by frequency of occurrence of the patterns displayed on the
liquid crystal display element. If it is not required to
consider occurrence of Moire fringes with the patterns displayed
on the liquid crystal display element, the angle B is set by
determining the luminance distribution as a function of the
measurement direction with the procedure described above, and
determining the value at which the emanated light distribution
is concentrated towards the front of the screen.
The representative example of the light adjusting sheet
consists of two square sheets, each square sheet being provided
with a plurality of convex streaks and concave streaks
approximately parallel to each other, stacked together in such
a way that peak lines of the streaks of one square sheet and the
27
CA 02244439 1999-O1-21
other make an angle 5° or greater, the peak lines of the streaks
of one square sheet make an angle of 5° to 85° in the clockwise
direction with the sheet edge, and the peak lines of the streaks
of the other square sheet make an angle of 5° to 85° in the
counterclockwise direction with the sheet edges.
In the light adjusting sheet of the two sheet structure, as
shown in Fig. 14, supposing the angle between the peak lines of
the streaks of one square sheet and the sheet edge be B in the
counterclockwise direction, then the angle 8' in the clockwise
direction for the other square sheet is preferably in the range
of 5° s 8 <_ 85°. Also, the angle B for one square sheet and the
angle 8' for the other square sheet are preferably set in
opposite directions, clockwise and counterclockwise, against the
sheet edge, with the same absolute value. However, as discussed
above, the angle between the peak lines of the streaks of one
square sheet and the peak lines of the streaks of the other
square sheet must be 5° or greater, so that these square sheets
must be stacked in such a way that the absolute value of the
difference between the angles B and B' is not smaller than 5°.
If ~6 -B'~ < 5°, then Moire fringes may result from the convex
streaks and concave streaks of these square sheets.
When three or more square sheets are stacked together, the
sheet stacking is also conducted in such a way that the peak
lines of the streaks of one square sheet make an angle of 5° or
28
CA 02244439 1999-O1-21
greater with the peak lines of the streaks of any other square
sheet.
Next, each element which composes the planar lighting device
30 will be described.
First, the light adjusting sheet 37 (50, 60) has the
configuration described above. It is possible to use a plurality
of the light adjusting sheets stacking together, or to use it
jointly with a light diffusing sheet 35 of the conventional type.
For the linear light source 33, a cathode ray tube is
usually used. The cathode ray tube may be either a cold cathode
ray tube or a hot cathode ray tube. There is no limitation on
the size and such of the cathode ray tube. The light sources)
is provided on both left and right sides or on one side of the
light guiding plate 32. Also, the light sources may be provided
on three or all four sides of the light guiding plate 32.
The light guiding plate 32 is made of a material with good
transparency such as glass, polycarbonate, polyester and
polymethylmethacrylate, and a printed dot pattern 31 is applied
on its back surface, i.e. the surface on the reflector plate
side, to cause irregular reflection in such a way that the light
from the light source 33 emanates uniformly from each position
29
CA 02244439 1999-O1-21
of the light emanating surface. The thickness and such of the
light guiding plate 32 are not limited.
For the reflector plate 34, any plate can be used without
limitation as long as it functions to block and reflect a light,
such as a resin plate with a white pigment blended into it, a
foamed resin plate, a resin plate with a metal vapour coating or
a metal plate. The reflector plate 34 is placed on the non-
emanating side of the light guiding plate 32, and is in contact
with the printed dot pattern 31 of the light guiding plate 32.
The light diffusing sheet 35 diffuses the light so that the
user does not see the configuration of the printed dot pattern
31 on the light guiding plate 32. There are several types, that
are, a type in which a light diffusing material is mixed, a type
in which a light diffusing material is coated, a type in which
a random unevenness is formed, and a combination type of the
above types. The thickness of this sheet is not limited but is
usually 10 ,um or greater, and preferably 20 to 300 ,um. If the
thickness is less than 10~,cm, sufficient diffusion characteristics
may not be obtained.
The preferred embodiments according to the invention will
be described.
CA 02244439 1999-O1-21
(Embodiment 1)
As shown in Fig. 15, a plurality of convex streaks 51 and
concave streaks 52 are alternatively formed parallel to each
other on the light emanating surface of a square sheet made of
polycarbonate with a melt index of 4.0 (290°C, 1.9 kg). A peak
51a of each convex streak 51 has a lateral cross section of a
convex arc, and a bottom 52a of each concave streak 52 has a
lateral cross section of a concave arc. An angle between the two
slopes of each convex streak 52 (an angle between two tangent
lines at the middle of each slope) is roughly a right angle. The
total thickness of the sheet is 200 um, the height from the
bottom 52a of the concave streaks 52 to the peak 51a of the
convex streaks 51 is 120 Vim, the pitch of the convex streaks 51
and the pitch of the concave streaks 52 are both 350 um. The
radius of curvature for both the peak 51a of each convex streak
51 and the bottom 52a of each concave streak 52 is 67 um. The
convex streaks 51 and the concave streaks 52 are formed by the
thermal press method. The back surface 53 of the sheet is
provided with fine unevenness by the mat roll method. Such is
the configuration of the light adjusting sheet 50 which has a
plurality of the convex streaks 51 and the concave streaks 52 on
the front surface and the unevenness on the back surface 53 (a
light adjusting sheet (a)).
31
CA 02244439 1999-O1-21
(Performance test)
The forward luminance and the luminance ratio are measured
and the appearance is observed for the planar lighting device
equipped with the light adjusting sheet described above.
I) Luminance
The cathode ray tube (light source) 33, the light guiding
plate 32, the reflector plate 34, and the light diffusing sheet
35, described below, are used to compose the planar lighting
device for the measurements of the screen brightness.
Cathode ray tube: Cold cathode ray tubes of 3.0 mm diameter
and 130 mm length.
Light guiding plate: 130 mm long, 260 mm wide, 3.0 mm thick.
The material is an acrylic resin. Dot printing for irregular
reflection is applied on the back surface.
Reflector plate: 100 ~m thick. The material is
polycarbonate mixed with 20 wto white pigment (titanium oxide).
Light diffusing sheet: The material is polycarbonate mixed
with 10 wto calciumcarbonate.
In Fig. 12, which shows the basic structure of the planar
lighting device of the edge-light type, the cathode ray tube 33
32
CA 02244439 1999-O1-21
as a light source is placed on each side of the light guiding
plate 32 which has the printed dot pattern 31 on its back
surface. The reflector plate 34 is placed behind the light
guiding plate 32, and the light adjusting sheet 50 is placed in
front of the light guiding plate 32. The light adjusting sheet
50 is placed in such a way that the surface with the convex
streaks 51 and the concave streaks 52 is to be the light
emanating surface and that the convex streaks 51 and the concave
streaks 52 are parallel with the axis of the cathode ray tubes
33.
With the planar lighting device 30 described above, the
luminance is measured in the normal direction ( 0° ) and in the
direction of 30° from the normal direction. The procedure to
measure the luminance is as described before.
II) Appearance
For the planar lighting device 30 described above, the
screen is checked to see if there is any problem in appearance.
(Embodiments 2 to 4)
Light adjusting sheets (b), (c) and (d) are obtained by the
same procedure as for the sheet (a), except that the sheet
material, the size of the convex streaks and concave streaks and
the forming method of unevenness of the back surface are changed
to the items listed in Table 1. For the planar lighting devices
33
CA 02244439 1999-O1-21
equipped with either light adjusting sheet of (b), (c) or (d),
a performance test is conducted in the same manner as for
Embodiment 1. The light adjusting sheet (d) of Embodiment 4, as
shown in Fig. 16, has a back surface of non-smooth surface 53
formed into a moderate wave form surface by the press method.
(Control example 1)
A performance test is conducted in the same manner as for
Embodiment 1 for a planar lighting device which is not equipped
with the light adjusting sheet of this disclosure.
(Control examples 2 to 3)
The light adjusting sheets (e) and (f) are obtained by the
same procedure as for the sheet (a), except that the sheet
material, the size of the convex streaks and the concave streaks
and the forming method of the unevenness of the back surface are
changed to the items listed in Table 1. A performance test is
conducted in the same manner as for Embodiment 1 for the planar
lighting devices equipped with each of the light adjusting sheets
( a ) or ( f ) . The light adj usting sheet (e ) corresponds to the
conventional sheet shown in Fig. 5.
The sheet material, the size of the convex streaks, the
forming method of the back surface, and the results of the
performance tests for each light adjusting sheet of the
34
CA 02244439 1999-O1-21
Embodiments and the Control examples are summarized in Table 1.
In Table 1, the marks represent as following:
*: A light diffusing sheet which has a random uneven surface
on the front surface and a mat surface on the back surface.
0: No problem
X: There is some problem
As clearly shown in Table 1, compared with those of the
Control examples, the light adjusting sheets of the Embodiments
have superior luminance in the normal direction (0°) and in the
direction of 30° from the normal direction, and they are superior
in appearance as well.
CA 02244439 1999-O1-21
Table 1
Embodiments Control
examples
1 2 3 4 1 2 3
Light a b c d none a f*
adjusting
sheet
Material poly- poly- polymeth poly- - polymethpoly-
carbona carbonayl carbonate yl carbona
to to methacr methacr to
y-late
y-late
Total 200 170 400 250 - 500 250
thickness
( l~ )
Concave 120 70 180 120 - 180 -
depth _
( l-~ )
Peak angle 90 80 100 75 - 90 -
()
Pitch (um) 350 190 430 200 - 360 -
Radius of 67 50 30 30 - 0 -
curv-ature
of convex
and
concave
streaks
( 1-~ )
Forming Mat Sand Random sine curve- Mirror -
method of roll blast uneven with the surface
back die same phase
surface press as light
emanating
surface,
press with
15 ~.rm
concave
Luminance 405 400 415 420 150 530 330
(0 ,
cd/mZ )
Luminance 380 385 375 370 250 45 360
(30 ,
cd/mz )
Appearance o 0 0 o x x o
Dots Dots
are
are visible.
visibl Bright
e. points
by a
tight
contact.
36
CA 02244439 1999-O1-21
( Embodiment 5 )
Fig. 17 shows a part of a light adjusting sheet for a planar
lighting device in Embodiment 5, and Fig. 18 shows a planar
lighting device of the edge-light type in which the light
adjusting sheet shown in Fig. 17 is used.
In Fig. 17, the light adjusting sheet 50 has a front surface
56 provided with convex streaks and concave streaks forming a
sine curved surface and a back surface 53 of unevenness.
In Fig. 18, the numeral 32 represents a light guiding plate
consisting of a polymethylmethacrylic plate with a 3 mm
thickness, and a dot-printed irregular reflection layer (not
shown) is provided on the back surface of the light guiding plate
32. The particle diameter of the beads used in the dot printing
is in the range of 0.5 to 1.5 mm, the numeral 34 represents a
reflector plate. The reflector plate 34 is provided on the back
surface of the light guiding plate 32. The reflector plate 34
reflects the light emanating from the back surface of the light
guiding plate 32 back to the light guiding plate 32. The numeral
50 represents a light adjusting sheet consisting of a
polycarbonate transparent sheet. The light adjusting sheet 50
is made by the method in which polycarbonate is formed into a 200
~m thick sheet with convex streaks and concave streaks having an
amplitude of 120 ~m and a pitch of 350 um on the front surface
56 by the press process, and providing fine unevenness on the
37
CA 02244439 1999-O1-21
back surface 53 by the sand blasting method. The light adjusting
sheet 50 is used such that the back surface 53 which has fine
unevenness is in contact with the light guiding plate 32. The
numeral 33 represents a linear light source provided on the edge
side of the light guiding plate 32. The numeral 38 represents
a reflector cover. The reflector cover 38 is installed behind
the light source 33 and helps the light emitted from the light
source 33 efficiently enter the light guiding plate 32.
Next, operation of the light adjusting sheet 50 used in a
planar lighting device of the edge-light type shown in Figs. 17
and 18 are described below. When the light source 33 is turned
on, the light emitted from the light source 33 enters the light
guiding plate 32 directly or after being reflected by the
reflector cover 38. The light which has entered the light
guiding plate 32 emanates, directly or after being reflected by
the reflector plate 34, from the front surface as a strongly
directional light (the angle from the light guiding plate 32 is
approximately 20°), as shown in Fig. 2. The light emanating from
the front surface of the light guiding plate 32 enters the light
adjusting sheet 50. Here, since the back surface of the light
adjusting sheet 50 through which the light enters has the fine
unevenness, the light is diffused. Next, the light which has
entered the light adjusting sheet 50 travels through the light
adjusting sheet 50 and goes out of its front surface. When the
light goes out of the front surface, the direction of the light
38
CA 02244439 1999-O1-21
is altered to the forward direction by the convex streaks and the
concave streaks on the front surface.
The light distribution characteristics of the light which
has travelled through the light adjusting sheet 50 is analyzed
and the result is a very good distribution, indicated by H in
Fig. 19. Further, the irregular reflection layer provided on the
back surface of the light guiding plate 32 is invisible when
looking into the sheet 50. Additionally, when a liquid crystal
display element is placed in front of the planar lighting device
using the light adjusting sheet, there is no poor readability
caused by the irregular reflection layer and the screen is very
good-looking.
(Embodiment 6)
Next, Embodiment 6, as shown in Fig. 20 is described below.
A light adjusting sheet 70 shown in Fig. 20 has a front surface
71 provided with convex streaks and concave streaks forming a
sine curved surface and a back surface 72 provided with
unevenness of mat surface forming a sine curved surface by the
sand blasting method. The pitch of the streaks of the front
surface 71 is 400 ~,rm and the amplitude is 200 Vim. The unevenness
on the back surface 72 has the same pitch of 400 um as that of
the front surface 71, but the amplitude is 20 Vim. The thickness
of the light adjusting sheet 70 is 500 um. The light adjusting
sheet 70 is fabricated by extruding a methylmethacrylic resin
39
CA 02244439 1999-O1-21
into a sheet using an extruder and then forming the uneven
pattern on the sheet using an embossing roll.
The light adjusting sheet 70 shown in Fig. 20 is installed
on the same planar lighting device as the one in Embodiment 6,
and the light distribution characteristics are analyzed. The
light distribution characteristics are somewhat different from
the light distribution characteristics indicated by H in Fig. 19,
but the distribution is very good. When a liquid crystal display
element is placed in front of this planar lighting device, the
front surface of the liquid crystal display element is prevented
from being in tight contact with the front surface 71 of the
light adjusting sheet 70. Therefore, there is no interference
fringe of Newton ring caused by the tight contact. Further, the
light is converged in the forward direction by the unevenness on
the back surface 72 to produce an even better looking screen than
the planar lighting device in Embodiment 5.
(Embodiment 7)
Next, Embodiment 7 shown in Fig. 21 is described below. In
Embodiment 7 shown in Fig. 21, unevenness with a triangular cross
section is provided on the back surface 82 opposite to the front
surface 81 provided with convex streaks and concave streaks
forming a sine curved surface in the light adjusting sheet 80.
The unevenness of the back surface 82 has the same pitch of 350
~.rm as the pitch on the front surface 81, but the amplitude is 50
CA 02244439 1999-O1-21
um. Other than this, the structure is the same as that of
Embodiment 5.
The light adjusting sheet 80 of Embodiment 7 shown in Fig.
21 is installed on the planar lighting device used in Embodiment
5, and a liquid crystal display element is placed in front of
this planar lighting device. Since the front surface of the
light guiding plate 32 is prevented from being in tight contact
with the back surface of the unevenness with a triangular cross
section of the light adjusting sheet 80. Therefore, there is no
interference fringe of Newton ring caused by the tight contact,
and the light is better directed in the forward direction by the
unevenness on the back surface 82, the result is an even better
looking screen than the planar lighting device of Embodiment 5.
(Embodiment 8)
Next, Embodiment 8 shown in Fig. 22 is described below.
Embodiment 8 shown in Fig. 22 is different from Embodiment 5 in
that the light adjusting sheet 50 includes 3 weight units of
titanium oxide powder mixed in 100 weight units of polycarbonate
by which the sheet mainly consists of. Other than this, the
structure is the same as that of Embodiment 5.
The light adjusting sheet 50 of Embodiment 8 shown in Fig.
22 is installed on the planar lighting device used in Embodiment
5, and a liquid crystal display element is placed in front of
41
CA 02244439 1999-O1-21
this planar lighting device. Since the light is diffused by the
titanium oxide powder, the irregular reflection layer provided
on the back surface of the light guiding plate 32 is invisible,
and the screen is better looking than the planar lighting device
of Embodiment 5.
( Embodiment 9 )
Next, Embodiment 9 shown in Fig. 23 is described below. The
light adjusting sheet 50 of Embodiment 9 shown in Fig. 23 is
different from Embodiment 5 in that the back surface 57 opposite
to the front surface 56 provided with convex streaks and concave
streaks forming a sine curved surface is a flat surface. Other
than this, the structure is the same as that of Embodiment 5.
The light adjusting sheet 50 of Embodiment 9 shown in Fig.
23 is installed on the planar lighting device shown in Fig. 1,
and a liquid crystal display element is placed in front of this
planar lighting device. Thus a good-looking, brighter than
conventional, screen is obtained.
Next, the following testing is carried out in order to
compare the performance of conventional light adjusting sheets
and those of this disclosure.
42
CA 02244439 1999-O1-21
(Comparative testing)
The light adjusting sheet of Embodiment 5 shown in Fig. 17
and that of Embodiment 6 shown in Fig. 20 are used as samples.
For control samples, random unevenness of a 150 um depth is
provided on a sheet formed by extruding polycarbonate using an
extruder, and fine unevenness is provided, by the sand blasting
method, on the back surface (Control sample 4). The thickness
of this light adjusting sheet is 200 um.
The three samples described above are installed on the
planar lighting device shown in Fig. 18, and the forward
luminance (cd/m2), the angle range (deg) in which the luminance
brighter than a half of the forward luminance, and the light
emanation appearance quality are measured. The results are shown
in Table 2.
Table 2
Embodiment 5 Embodiment Control
6
sample 4
Forward luminance 420 440 350
( cd/mz )
Angle range (deg) 45 40 55
Light emanating o 0 0
appearance quality
As one can see in Table 2, the sheets of the preferred
embodiments of this invention, i.e. Embodiments 5 and 6, have a
high forward luminance, a suitably diffused angle range and a
good light emanation appearance quality. Therefore, they are
excellent sheets. On the other hand, Control Sample 4 has a
43
CA 02244439 1999-O1-21
large angle range and a small forward luminance. Therefore, one
can see that it does not emanate the light efficiently in the
direction of the user's sight, and that a liquid crystal display
element placed in front of it would produce a dim screen.
Next, embodiments of the light adjusting sheet in which a
plurality of sheets each having convex streaks and concave
streaks approximately parallel to each other on one surface are
stacked will be described.
First, two types of sheets (g) (corresponding to the
conventional light adjusting sheet) and (h) (corresponding to a
light adjusting sheet according to embodiments of the invention)
with different streak configurations are formed.
Sheet (g)
In Fig. 24, a plurality of parallel streaks 91 with a
lateral cross section of a right angled isosceles triangle are
formed on the light emanating surface of a square sheet (the peak
angle is a right angle). The total thickness of the sheet is 300
~.rm and the height of the streaks 91 is 150 Vim. The material of
the sheet is polycarbonate with a melt index of 4.0 (290°C, 1.9
kg). The streaks 91 are formed by the hot press method. Such
is the configuration of the sheet (g) with a plurality of streaks
91.
44
CA 02244439 1999-O1-21
Sheet (h)
In Fig. 25, a plurality of convex streaks 92 and concave
streaks 93 parallel to each other are formed on the light
emanating surface of a square sheet. Each of the convex streaks
92 has a lateral cross section of a convex arc with a radius of
curvature of 20 um and each of the concave streaks 93 has a
lateral cross section of a concave arc with a radius of curvature
of 20 um. The total thickness of the sheet is 250 um and the
height of the convex streaks 92 is 120 Vim. The material of the
sheet is polymethyl methacrylate. Such is the configuration of
the sheet (h) with a plurality of the convex streaks 92 and
concave streaks 93.
The following sets of a cathode ray tube 33, a light guiding
plate 32, a reflector plate 34 and a light diffusing sheet 35 are
used.
Set (A)
Cathode ray tube: One cold cathode ray tube of 3.5 mm
diameter and 150 mm length.
Light guiding plate: 150 mm long, 200 mm wide, 3.0 mm
thick. The material is polymethyl methacrylate. Dot printing
is applied on the bottom surface.
CA 02244439 1999-O1-21
Reflector plate: 100 ~m thick. The material is
polycarbonate mixed with 20 wto titanium oxide.
Light diffusing sheet: The material is polycarbonate mixed
with 10 wt~ calcium carbonate.
Set (B)
Cathode ray tubes: Two hot cathode ray tubes of 5.0 mm
diameter and 170 mm length.
Light guiding plate: 170 mm long, 230 mm wide, 5.0 mm
thick. The material is polycarbonate. Dot printing is applied
on the bottom surface.
Reflector plate: 100 um thick. The material is
polyethylene terephthalate foam.
Light diffusing sheet: The material is polycarbonate mixed
with 10 wto calcium carbonate.
One or a stacked unit of two or more sheets (h) (and the
sheet (g) additionaly) described above is used to compose light
adjusting sheets, and these sheets are combined with the two
types of sets (A) and (B). Each light adjusting sheet is placed
in such a way that the peak lines of the convex streaks and the
46
CA 02244439 1999-O1-21
concave streaks are at the required angle to the axis of the
cathode ray tube to compose a planar lighting device.
For example, two sheets 60 are placed on the light guiding
plate 32 as shown in Fig. 26, or placed between the light guiding
plate 32 and the light diffusing sheet 35 as shown in Fig. 27.
Control examples
Planar lighting devices in which one or two sheets (g)
described above is placed in such a way that the peak lines of
the convex streaks and the concave streaks are parallel to the
axis of the cathode ray tube are also constructed.
For each planar lighting device of Embodiments and the
control examples, the forward luminance and the luminance ratio
are measured and the appearance is observed. The results are
shown in Table 3. In Table 3:
* Angle of the streaks: The angle of the streaks is defined to
be 0° when the peak lines of the streaks of the light adjusting
sheets are parallel to the axis of the cathode ray tube.
* Luminance ratio (1): The ratio of the luminance to the
highest luminance of the luminance measurements in various
observation directions for one luminescent device.
* Luminance ratio (2): The ratio of the luminance to the
forward luminance of the planar lighting device when the peak
47
CA 02244439 1999-O1-21
lines of the streaks of one or multiple light adjusting sheets
are placed parallel to the axis of the cathode ray tube (with
other conditions unchanged).
The luminance measurements are conducted with the procedure
described earlier.
48
CA 02244439 1999-O1-21
Table 3
Embodiments Control
examples
10 11 12 5 6
Set of cathode A B B A A
ray tube, etc.
(A or B)
Sheet (number h + h (2) g + h (2) h+h+h (3) g (1) g + g
of (2)
sheets)
*Peak angle 15 +45 +30 0 0, 0
of
convex and -45 -20
concave streaks -50
()
Position of Between Between Light Light Light
light diffusinglight light emanating emanating emanating
sheet guiding guiding end end end
plate and plate and
light light
adjusting adjusting
sheet sheet
Forward 550 730 725 370 410
luminance
(Insuffici
(cd/m2) ent
luminance)
*Luminance ratio99~ 980 950 650 72$
(1)
*Luminance ratio135 140 125 - -
(2)
Appearance o 0 0 o x
Interfere
nce
pattern
(Moire)
Although the invention has been described with respect to specific
embodiment for complete and clear disclosure, the appended claims are
not to be thus limited but are to be construed as embodying all
modification and alternative constructions that may occur to one skilled
in the art which fairly fall within the basic teaching herein set forth.
20571723.1
49
