Back lighting device

In a back lighting device for a side light type panel, one end portion of a specular or light diffusing/reflecting film covering a linear light source is bonded to one end portion of a light conducting plate which is on the side of its light emerging surface and is in proximity of the linear light source, and a light diffusing/reflecting film is laid through an air layer on the part of the rear surface of the light conducting plate which corresponds to the junction. The back lighting device of the invention is small in size, large in effective light emitting area when compared with its external dimensions, and excellent in the distribution of luminance. Hence, it can be used as a back light device that is highly efficient in converting power to luminance.

BACKGROUND OF THE INVENTION 
This invention relates to a back lighting device for illuminating 
transmissive or semi-transmissive panels from the rear side. 
Recently, liquid crystal display devices with a back lighting mechanism 
which are thin and allow easy viewing of information, have been used with 
lap-top or book type word processors or computers. The back lighting 
mechanism in common use adopts an "edge lighting" method in which a linear 
light source, such as a fluorescent tube, is provided at one end portion 
of a transmissive light conducting plate as shown in FIG. 1. Further, as 
shown in FIG. 2, one surface of the light conducting plate is often 
partially covered with a light diffusing material having a higher 
refractive index than the material of the light conducting plate, and the 
one surface thus covered is almost entirely covered with a specular 
reflecting plate or light diffusing/reflecting plate. 
On the other hand, in order to improve the performance of word processors 
or personal computers, there have been strong demands for miniaturization, 
improvement in visibility, and increase in the efficiency of the 
conversion of power to luminance thereof. 
SUMMARY OF THE INVENTION 
In view of the foregoing, an object of this invention is to provide a back 
lighting device which, with respect to its external dimensions, is highly 
improved both in the effective light emitting area and in the efficiency 
of converting power to luminance. 
The inventors have conducted research on the structure of a back lighting 
device which is in the vicinity of a linear light source, and on the 
distribution of luminance of the light emerging surface of the light 
conducting plate, and found the following fact: when the junction of the 
end portion of the reflecting plate, or film, and the light conducting 
plate is in a certain state, the back lighting devices effective light 
emitting area is largest with respect to its external dimensions, and the 
conversion efficiency of power to luminance conversion is highest. 
On the above-described finding, the foregoing object of the invention has 
been achieved by the provision of a back lighting device for a panel which 
comprises a light conducting plate made of a light-transmissive material, 
the light conducting plate having a light diffusing function and covering 
the rear surface with a specular or light diffusing/reflecting plate or 
film, and a linear light source provided in proximity to the end portion 
of at least one side of the light conducting plate; in which, according to 
the invention, one of the end portions of a specular or light 
diffusing/reflecting plate or film covering the linear light source is 
bonded to the end portion of the light conducting plate, on the side of a 
light emerging surface of the light conducting plate, in proximity to 
which the linear light source is provided, and a light 
diffusing/reflecting plate or film is provided through an air layer on a 
part of the rear surface of the light conducting plate which corresponds 
to the end portion thus bonded.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention will be described with reference to the accompanying 
drawings. 
FIGS. 1 and 2 are a perspective view and a sectional view showing an 
embodiment of the invention. In those figures, reference numeral 1 
designates a light conducting plate, which is made of a material which 
transmits light with high efficiency, such as quartz, glass, transmissive 
natural or synthetic resin (for instance acrylic resin); and 2, a light 
diffusing plate which transmits light from the surface (front surface) of 
the light conducting plate 1 by diffusing it. In the invention, at least 
one light diffusing plate 2 is employed. 
One large surface of the light conducting plate 1 has a light diffusing 
function. This function is given, for instance, by printing dots on the 
surface of the light conducting plate with paint or printing ink 
containing light scattering material such as SiO.sub.2, BaSO.sub.4 or 
TiO.sub.2. Alternatively, the light diffusion function can be given by 
roughening the surface of the light conducting plate, or boring small 
holes, or forming small protrusions on it as indicated at 6 in FIG. 2, or 
the function is given by dispersing a high scattering material such as 
air, SiO.sub.2, BaSO.sub.4, TiO.sub.2, or a polymer in the light 
conducting plate. 
A specular or light diffusing/reflecting plate 3 is provided in such a 
manner as to cover substantially all of the rear surface of the light 
conducting plate. Linear light sources are designated by reference numeral 
4. Preferably, the linear light sources 4 are so arranged along the end 
faces of the light conducting plate so that the central axes thereof 
parallel with the end faces of the light conducting plate 1 to allow light 
therefrom to enter the latter. In addition, the surfaces of the linear 
light sources 4, except for the portions facing the end faces of the light 
conducting plate, are covered with specular or light diffusing/reflecting 
plates or films 5, respectively. 
In the invention, an example of a specular reflecting plate or film 5, 
which may be employed as the specular or light diffusing/reflecting plate 
or film 5, is formed as follows. That is, it is formed of a material of 
silver, aluminum, platinum, nickel or chromium which provides a 
specular-reflection of light; preferably a plastic film such as a 
polyester film is coated with silver or aluminum by vacuum deposition or 
sputtering. Employment of the specular reflecting plate or film 5 high in 
specular reflectivity (for instance, silver has a specular reflectivity of 
90 to 95%) and small in thickness (for instance about 75 .mu.m in the case 
where a layer of silver is formed on a commercially available polyester 
base by vacuum deposition) makes it possible to increase the efficiency of 
converting power to luminance, and to reduce the thickness of the linear 
light source sections which, heretofore, are larger than that of the light 
conducting plate section. The efficiency of converting power to luminance 
can be further increased when the method which has been filed by the 
inventors (Japanese Patent Application No. 67699/1991) is employed in 
combination with the above. 
In one example of the light diffusing/reflecting plate or film 5 covering 
the linear light source 4, a light diffusing material (such as SiO.sub.2, 
BaSO.sub.4 or TiO.sub.2) is mixed in a resin such as polyester resin 
having a light diffusing/reflecting characteristic. In another example, a 
light diffusing function is provided by foaming a resin such as polyester 
resin. In another example, an aluminum plate is coated with the 
above-described light diffusing material. That is, the light 
diffusing/reflecting plate or film 5 is not limited in its material; all 
that is necessary for the plate or film is to be able to sufficiently 
diffuse and reflect light. 
The junction of the end portion 5a of the specular or light 
diffusing/reflecting plate or film 5 covering the linear light source 4 
and the light conducting plate 1 is as shown in FIG. 3, in which a 
specific feature of the invention resides. More specifically, one end 
portion 5a, of the plate or film 5 covering the linear light source 4, on 
the light emerging surface side of the light conducting plate 1, is bonded 
to a part of the light emerging surface at the end of the light conducting 
plate 1 with double coated tape or an adhesive agent 8. However, the rear 
surface of the light conducting plate 1, which is at least opposite to the 
surface to which the one end portion of the plate or film 5 has been 
bonded, is covered through an air layer 11 with a light 
diffusing/reflecting plate or film 3. The above-mentioned structure is 
accommodated in a plastic case 10. 
The other end portion 5b of the specular or light diffusing/reflecting 
plate or film 5 covering the linear light source 4 (which is opposite to 
the one end portion 5a which has been bonded to the end portion of the 
light conducting plate 1), for example, is bonded to the outer surface 
(which does not face the light conducting plate) of the light 
diffusing/reflecting plate or film 3, as indicated at 9 in FIG. 3, is not 
bonded to the light conducting plate 1 or film 3, or is held with an 
external support means, so that the plate or film 5 surrounds the linear 
light source 4. 
The bottom surface of the light conducting plate 1 except the part covered 
through the air layer with the light diffusing/reflecting plate or film 5, 
may be covered with a light diffusing/reflecting plate or film 3 or a 
specular reflecting plate or film 3. 
The light diffusing/reflecting plate or film 3 may be any one which is able 
to diffuse and reflect light as was described above. That is, it may be 
formed by mixing a light diffusing material in a resin, such as polyester 
resin, or foaming a resin, such as polyester resin, to have a light 
diffusing function. Alternatively, it may be an aluminum plate coated with 
the above-described light diffusing material. That is, the plate or film 3 
is not limited in its material; all that is necessary for the plate or 
film 3 is to be able to sufficiently diffuse and reflect light applied 
thereto. 
The specular reflecting plate or film 3 is of a material such as sliver and 
aluminum. 
The air layer 11 formed between the rear surface of the light conducting 
plate 1 and the light diffusing/reflecting plate or film 3 is not 
particularly limited in thickness. However, in order to reduce the 
thickness of the back lighting device, the thickness of the air layer 
should be minimized, preferably 0.5 mm or less. That is, the minimum 
thickness may be such that air molecules are present in the form of a 
monomolecular layer. 
The junction (as indicated at 8 in FIG. 3) of the end portion of the 
specular or light diffusing/reflecting plate or film 5 and the light 
conducting plate 1 should be wide enough to be high in mechanical 
strength; however, it is preferable that the width be as small as 
possible. 
The linear light source 4 may be a fluorescent tube, a tungsten 
incandescent tube, an optical rod, or an LED array. Of those, the 
fluorescent tube is most suitable as the linear light source 4. In order 
to make the effective light emitting area uniform in the distribution of 
luminance, and to use electric power economically, it is preferable that 
the linear light source's 4 uniform light emission section, except the 
electrode section, be substantially equal in length to the end portion of 
the light conducting plate 1, near which the linear light source 4 is set. 
The back lighting device of the invention, with the essential components 
designed as described above, is to be used with display panels, 
particularly with liquid-crystal display panels. It is preferable that the 
back lighting device has the following constitutional features: 
The light diffusing material as indicated at 6 in FIG. 2 is formed in a dot 
pattern on the surface of the light conducting plate 1. The dots may be of 
any shape, circles or rectangles or cross lines. Those dots are formed in 
a grid pattern, with each dot being located at the point where any two 
imaginary lines are crossed at right angles. Adjacent crossed lines are 
preferably spaced apart by 0.5 to 3 mm, preferably 0.8 to 2 mm, with an 
appropriate distance being selected in accordance with the thickness of 
the light conducting plate 1. 
In addition, the surface of the light conducting plate 1 is covered with 
the light diffusing material in such a manner that the coverage percentage 
is preferably 1 to 50% in the areas which are near the linear light 
sources 4, and 20 to 100% in the area that is the farthest from the light 
sources 4. Preferably, the light conducting plate 1 is covered with the 
light diffusing material in such a manner that the coverage percentage 
increases gradually with the distance from the light source 4 starting at 
the point where the linear light source 4 is placed in proximity to the 
end portion of the one side of the light conducting plate 1. The term 
"coverage percentage" as used herein is intended to mean the rate in area 
of the light scattering material to the surface of the light conducting 
plate 1. 
The back lighting device of the invention is small in size, large in 
effective light emitting area with respect to its external dimensions, and 
excellent in the distribution of luminance. Hence, it can be used as a 
back lighting device that is highly efficient in converting power to 
luminance. 
CONCRETE EXAMPLES AND COMATIVE EXAMPLES 
As conducive to a full understanding of the invention, its concrete 
examples and comparative examples will be described below. The concrete 
examples relate to preferred embodiments, and the comparative examples 
relate to similar structures, but which do not provide results as 
desirable as the structures of the concrete examples. 
A rectangular acrylic plate (205 mm.times.160 mm) having a thickness of 4 
mm ("Delaglass (phonetic) A" manufactured by Asahi Kasei K.K.) as shown in 
FIG. 1 was provided. A cold-cathode fluorescent tube 4 (a normal tube of 
Harrison Denki K.K.) having a diameter of 3.8 mm and a length of 230 mm 
was set along each of the ends, in the longitudinal direction, of the 
rectangular acrylic plate 1. As shown in FIG. 2, each fluorescent tube 4 
was surrounded by a silver film 5 in such a manner that the silver film 5 
had a slit which was 4 mm in width and faced the end face of the light 
conducting plate 1, so that light emerging through the slit was allowed to 
enter the rectangular acrylic plate 1 (used as the light conducting plate) 
through the end face. 
An ink containing a light diffusing material (titania) was applied to the 
surface of the rectangular acrylic plate 1 (used as the light conducting 
plate) by screen-printing a pattern of circular dots with a pitch of 1 mm. 
A screen-image carrier was prepared by CAD in such a manner that the 
coverage with the light diffusing material would be 20% at the point of a 
minimum value (near the linear light source), and 95% at the point of a 
maximum value (the middle of the light conducting plate), with the 
coverage being gradually increased from the minimum value to the maximum 
value in the intermediate area. 
A white light diffusing/reflecting plate 3 of polyester 0.13 mm in 
thickness ("Merinex (phonetic) 329" of ICI Co.) was arranged to cover the 
entire surface of the rectangular acrylic plate 1 which was covered with 
the light diffusing material. A light diffusing plate 2 of polycarbonate 
0.18 mm in thickness ("8B36" of GE Co.) was arranged to cover 
substantially the whole of the light emerging surface of the rectangular 
acrylic plate 1 while its roughened surface set opposite to the 
rectangular acrylic plate 1. 
The cold-cathode tube 4 was driven by a constant current (a tube current of 
5 mA) with an AC voltage being applied from an invertor, and the surface 
luminance produced thereby was measured with a luminance meter (Topcon 
BM-8). 
Concrete Example 1 
As shown in FIG. 3, the end portion 5a of the specular reflecting film 5 
(the Ag film covering the light source 4), on the side of the light 
emerging surface of the light conducting plate 1, was bonded to a part of 
the light emerging surface of the light conducting plate 1 with a double 
coated tape having a width of 3.5 mm and a thickness of 0.16 mm (WPT-No 
750F of Teraoka Seisakusho K.K) as indicated at 8 in FIG. 3, and the part 
of the rear surface of the light conducting plate 1, which was opposite to 
the part bonded with the double coated tape 8, was covered with a light 
diffusing/reflecting film 3 through an air layer 11. In other words, the 
Ag film 5 was bonded (at (9)) to the surface of the light 
diffusing/reflecting plate 3 which was not facing the light conducting 
plate 1. In this case, the average luminance in the effective light 
emitting area was 1300 cd/m.sup.2. The abnormal light emission occurring 
in the vicinity of the end portion 5a of the light conducting plate 1, on 
the side of the light emerging surface, to which the Ag film 5 was bonded 
terminated within 2 mm from the linear light source 4, and its maximum 
luminance was 1700 cd/m.sup.2. 
In the invention, the effective light emitting area is 204 mm.times.152 mm, 
being considerably large when compared with the light conducting plate 1 
employed; that is, the region extending from a line 4 mm from the linear 
light source 4 may be employed as an effective light emitting region. In 
addition, with the fact taken into consideration that the double coated 
tape 8 is 3.5 mm in width, the region extending from a line 0.5 mm from 
the part where the Ag film 5 (used as the specular reflecting film) is 
laid on the light conducting plate 1 can be employed as an effective light 
emitting region. 
In the prior art, the distance is disclosed as 5 to 10 mm instead of 0.5 mm 
as mentioned above. Thus, the effective light emitting region is 
sufficiently large with respect to the external dimensions. 
Comparative Example 1 
As shown in FIG. 4, a back lighting device was formed which was equal in 
structure to the device of the above-described Concrete Example 1 except 
that the end portion 5b of the Ag film 5 is bonded to the part of the 
light diffusing/reflecting film 3 with the double coated tape 9 in such a 
manner that the part of the rear surface of the light conducting plate 1 
was covered through an air layer by the Ag film 5 (with the Ag film 5 on 
the rear surface of the light conducting plate being substantially equal 
in area to the Ag film 5 on the front surface). In the comparative 
example, the average luminance was 1270 cd/m.sub.2. The abnormal light 
emission occurring in the vicinity of the end portion 5a of the Ag film 5 
(used as the specular reflecting film 5), on the side of the light 
emerging surface of the light conducting plate 1, terminated at about 6 mm 
from the linear light source 4, and its maximum luminance was 2400 
cd/m.sup.2. 
With the light diffusing plate 2 peeled off, the abnormal light emitting 
region was observed obliquely, as a result of which the reason why the 
abnormal light emission occurred was found as follows. That is, light 
entering the light conducting plate 1 through its end face adjacent to the 
linear light source 4 is subjected to diffusion/reflection by the double 
coated tape as indicated at 8 in FIG. 3 through which the specular 
reflecting film 5 is bonded to the surface of the light conducting plate 
1. That is, the double coated tape acts as a light diffusing element. 
Thus, the double coated tape as indicated at 8 in FIG. 3 itself emits 
light abnormally. The light thus emitted is reflected by the Ag film 5 
(the specular reflecting film 5) on the side of the light conducting plate 
1 which is opposite to the side where the double coated tape 8 is 
provided. The light thus reflected is returned to the vicinity of the 
double coated tape 8, where it is diffused and reflected. The light thus 
diffused and reflected forms an angle with the surface of the light 
conducting plate 1 which is smaller than a critical refractive angle, thus 
directly emerging from near the double coated tape 8. This is the reason 
why the abnormal light emission occurs. 
In Concrete Example 1, the light from the double coated tape 8 is diffused 
and reflected by the light diffusing/reflecting plate 3 on the side of the 
light conducting plate 1 which is opposite to the side where the double 
coated tape 8 is positioned (in the diffusion/reflection, being different 
from the specular reflection, light is not regularly reflected; that is, 
light applied to the reflecting surface is reflected in many directions). 
Hence, the number of rays of light returned to the vicinity of the double 
coated tape 8 is small; that is, a larger part of the rays of light form 
angles with the surface of the light conducting surface which are larger 
than the critical refractive angle, thus contributing to improvement of 
the uniform planar light emission in the effective light emitting area. 
Comparative Example 2 
A back lighting device was provided which was equal in structure to the 
device of the above-described comparative example 1 except that the Ag 
film 5 on the rear surface of the light conducting plate 1 was painted 
black. The average luminance was 1265 cd/m.sub.2. 
In the vicinity of the end portion of the specular reflecting film on the 
side of the light emerging surface of the light conducting plate, no 
abnormal light emission occurred, and the luminance was low. The region 
low in luminance terminated at about 6 mm from the linear light source, 
and its minimum luminance was 850 cd/m.sub.2. 
Comparative Example 3 
A back lighting device was provided which was equal in structure to the 
device of the above-described comparative example 1 except that, before 
the Ag film 5 was bonded to a part of the light emerging surface of the 
light conducting plate 1, the part was painted black. In this case, the 
average luminance was 1000 cd/m.sup.2. In the vicinity of the end of the 
specular reflecting film 5 on the side of the light emerging surface of 
the light conducting plate 1, no abnormal light emission occurred, and the 
luminance was low. The region low in luminance terminated at about 6 mm 
from the linear light source, and its minimum luminance was 550 
cd/m.sub.2. 
Comparative Example 4 
A back lighting device provided was such that the device of the 
above-described comparative example 1 was so modified that the Ag film 5 
was bonded to both surfaces of the light conducting plate 1, and it was 
equal in the remaining arrangement to the device of the above-described 
concrete example 1. In this case, the average luminance was 1200 
cd/m.sup.2. The abnormal light emission in the vicinity of the end of the 
specular reflecting film 5 on the side of the light emerging surface of 
the light conducting plate 1 terminated at about 8 mm from the linear 
light source 4, and its maximum luminance was 2700 cd/m.sup.2. 
Comparative Example 5 
A back lighting device was provided which was equal in structure to the 
device of the above-described concrete example 1 except that the end 
portion 5a as indicated at 8 in FIG. 3 of the Ag film 5, on the side of 
the light emerging surface of the light conducting plate 1, was supported 
by an iron plate 1 mm in thickness, 3 mm in width and 240 mm in length 
instead of the use of the double coated tape 8 in such a manner that it 
was laid through an air layer on the light emerging surface of the light 
conducting plate. It was difficult to support it with the iron plate, and 
it was impossible to accurately measure the average luminance. The 
abnormal light emission in the vicinity of the end of the specular 
reflecting film on the side of the light emerging surface of the light 
conducting plate changed with the pressure applied through the iron plate. 
In order to miniaturize the back lighting device, the plate was pushed from 
both sides; however, the pressure was insufficient at the middle, and the 
abnormal light emission occurred in a range of from 5 mm to 10 mm from the 
linear light source, and its maximum luminance was 5000 cd/m.sup.2. The 
plate was not suitable for miniaturization of the back lighting device, 
particularly for reduction of the thickness. 
Concrete Example 2 
As shown in FIG. 3 a back lighting device was prepared which was equal in 
structure in the device of the above-described concrete example 1 except 
that each light source 4 was covered with a light diffusing/reflecting 
film 5 ("Merinex (phonetic) 329" of ICI Co.) instead of using the Ag film 
5. One end portion 5a of the light diffusing/reflecting film 5, on the 
side of the light emerging surface of the light conducting plate 1, was 
bonded to a part of the light emerging surface of the light conducting 
plate 1 with a double coated tape 3.5 mm in width and 0.16 mm in thickness 
(WPT-No 750F of Teraoka Seisakusho K.K.) as indicated at 8. The remaining 
part was used to cover the light source 4. The other end portion 5b of the 
film 5 was bonded to the end portion of the rear surface of the light 
diffusing/reflecting film 3 covering the surface of the light conducting 
plate 1 which was partially covered with the light diffusing material, 
with the double coated tape as indicated at 9, in such a manner that the 
front surface of the light diffusing/reflecting film 3 faced the light 
conducting plate 1, and the light diffusing/reflecting film 3 was laid 
through an air layer 11 on the light conducting plate. The luminance of 
the device was measured. The average luminance of 99 points (uniformly 
distributed) in the effective light emitting area was 1300 cd/m.sup.2. The 
abnormal light emission in the vicinity of the end portion of the light 
diffusing/reflecting film 5, on the side of the light emerging surface of 
the light conducting plate 1, terminated within 2 mm from the linear light 
source 4, and its maximum luminance was 1700 cd/m.sup.2. 
Comparative Example 6 
As shown in FIG. 5, a back lighting device was prepared which was equal in 
structure to the device of the above-described concrete example 2 except 
that the part of the rear surface of the light conducting plate 1 which 
faced the junction of the end portion 5b of the light diffusing/reflecting 
film 5 and the surface of the light conducting plate 1 was covered through 
an air layer with a specular reflecting film 20 (an Ag film manufactured 
by Nakai Kogyo K.K.). The average luminance was 1270 cd/m.sup.2. The 
abnormal light emission in the vicinity of the end portion of the light 
diffusing/reflecting film 3, on the side of the light emerging surface of 
the light conducting plate 1, terminated at about 6 mm from the linear 
light source 4, and its maximum luminance was 2400 cd/m.sup.2. 
With the light diffusing plate 2 peeled off, the abnormal light emitting 
region was observed obliquely, as a result of which the reason why the 
abnormal light emission occurred was found as follows: That is, the light 
entering the light conducting plate 1 through the end face adjacent to the 
linear light source 4 is subjected to diffusion/reflection by the double 
coated tape, through which the light diffusing/reflecting film 5 is bonded 
to the surface of the light conducting plate 1. That is, the double coated 
tape 8 acts as a light diffusing element. Thus, the double coated tape 
itself emits light abnormally. The light thus emitted is regularly 
reflected by the specular reflecting film 20 on the side of the light 
conducting plate 1 which is opposite to the side where the double coated 
tape 8 is provided. The light thus reflected is returned to the vicinity 
of the double coated tape, where it is diffused and reflected. Therefore, 
the light forms an angle with the surface of the light conducting plate 
which is smaller than a critical refractive angle, thus directly emerging 
from near the double coated tape. This is the reason why the abnormal 
light emission occurred. 
In Concrete Example 2, light from the double coated tape 8 is diffused and 
reflected by the diffusing/reflecting plate 3 on the side of the light 
conducting plate 1 which is opposite to the side where the double coated 
tape 8 is positioned (in the diffusion/reflection, being different from 
the specular reflection, light is not regularly reflected; that is, light 
applied to the reflecting surface is reflected in many directions). Hence, 
the number of rays of light returned to the vicinity of the double coated 
tape is small, which contributes to improvement of the uniform planar 
light emission in the effective light emitting area. 
Comparative Example 7 
As shown in FIG. 5, a back lighting device was provided which was equal in 
structure to the device of the above-described concrete example 2 except 
that the part of the rear surface of the light conducting plate 1 which 
faced the part of the front surface to which the double coated tape 8 was 
bonded was covered with a black film 21 through an air layer. The device 
was subjected to luminance measurement. The average luminance was 1265 
cd/m.sup.2. In the vicinity of the end portion of the light 
diffusing/reflecting film 5, on the side of the light emerging surface of 
the light conducting plate 1, no abnormal light emission occurred, and the 
luminance was low. The region low in luminance terminated at about 6 mm 
from the linear light source, and its minimum luminance was 850 
cd/m.sub.2. 
Comparative Example 8 
As shown in FIG. 5, a back lighting device was provided which was equal in 
structure to the device of the above-described comparative example 6 
except that the junction of the light diffusing/reflecting film 5 and the 
light conducting plate 1 was painted black. As a result of a luminance 
measurement given to the device, the average luminance was 1000 
cd/m.sup.2. In the vicinity of the end portion of the light 
diffusing/reflecting film, on the side of the light emerging surface of 
the light conducting plate, no abnormal light emission occurred, and the 
luminance was low. The region low in luminance terminated at about 6 mm 
from the linear light source, and its minimum luminance was 550 
cd/m.sub.2. 
Comparative Example 9 
A back lighting device provided was such that the device of the 
above-described comparative example 6 was so modified that the part of the 
specular reflecting film 20 was bonded to the light conducting plate 1, 
and it was equal in the remaining arrangement to the device of the 
above-described concrete example 2. The average luminance was 1200 
cd/m.sup.2. The abnormal light emission in the vicinity of the end portion 
5a of the light diffusing/reflecting film 5, on the side of the light 
emerging surface of the light conducting plate, terminated at about 8 mm 
from the linear light source, and its maximum luminance was 2700 
cd/m.sup.2. 
Comparative Example 10 
A back lighting device was provided which was equal in structure to the 
device of the above-described concrete example 2 except that the end 
portion of the light diffusing/reflecting film 5, on the side of the light 
emerging surface of the light conducting plate 1, was laid through an air 
layer on the light emerging surface of the light conducting plate without 
the double coated tape, and the junction of them was supported with an 
iron plate 1 mm in thickness, 3 mm in width and 240 mm in length. In 
practice, it was difficult to support the junction with the iron plate, 
and it was impossible to measure the average luminance accurately. The 
abnormal light emission in the vicinity of the end portion of the light 
diffusing/reflecting film 5 on the side of the light emerging surface of 
the light conducting plate 1 changed with the pressure applied through the 
iron plate. 
In order to miniaturize the back lighting device, the plate was pushed from 
both sides; however, the pressure was insufficient at the middle, and the 
abnormal light emission occurred in a range of 5 to 10 mm from the linear 
light source, and its maximum luminance was 5000 cd/m.sup.2. Thus, the 
plate was not suitable for miniaturization of the back lighting device, 
particularly for reduction of the thickness. 
The foregoing description of preferred embodiments of the invention has 
been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and modifications and variations are possible in light of the 
above teachings or may be acquired from practice of the invention. The 
embodiments were chosen and described in order to explain the principles 
of the invention and its practical application to enable one skilled in 
the art to utilize the invention in various embodiments and with various 
modifications as are suited to the particular use contemplated. It is 
intended that the scope of the invention be defined by the claims appended 
hereto, and their equivalents.