Backlighting device

A backlighting device for display panels operating on the edge lighting method that comprises a light conducting plate made of a light-transmissive material, a layer of light-diffusing material that partly covers one major surface of the light conducting plate, and a light diffusing/reflecting plate that covers the thus covered area of the light conducting plate. The light conducting plate is provided on its exit face with a film that is coarser grained on the exit face than on the entrance face (the side closer to the light conducting plate) and which, when provided on the exit face of the light conducting plate, will increase the luminance of light issuing from the exit face of the light conducting plate. This backlighting device produces satisfactory luminance and can be used as a high-performance model that achieves a high efficiency of power to luminance conversion at least in the direction of the line normal to the exit face.

BACKGROUND OF THE INVENTION 
1. Field of the Industrial Utility 
The present invention relates to a backlighting device for liquid-crystal 
panels that illuminates transmissive or semi-transmissive panels from the 
rear side. 
2. Prior Art 
Thin liquid-crystal displays provided with a backlighting mechanism that 
allows easy viewing of information on the screen are used with recent 
versions of lap-top or book type word processors and computers. The 
backlighting 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 
operating on the edge lighting method is often covered partially with a 
light diffusing material having a higher refractive index than the 
material of which said light conducting plate is made and the thus covered 
area is almost entirely covered with a specular reflecting or light 
diffusing/reflecting plate. 
In addition, as is often the case today, backlighting devices are driven 
with a battery and a further improvement in the efficiency of power to 
luminance conversion is desired. To meet this need, it has been proposed 
that a light reflector covering the linear light source be provided with a 
light reflecting plate having high light reflectance or that a reflecting 
plate having high reflectance be provided on the surface of the light 
conducting plate in the area covered with the layer of light-diffusing 
material. 
The methods described above achieve some improvement in the efficiency of 
power to luminance conversion but is still insufficient and an even better 
improvement is desired. 
SUMMARY OF THE INVENTION 
An object, therefore, of the present invention is to provide a backlighting 
device that has a high efficiency of power to luminance conversion and 
which hence is capable of achieving high luminance. 
The present inventors conducted various studies in order to attain this 
object and, as a result, they found the following: when the light 
conducting plate of a backlighting device operating on the edge lighting 
method is provided on the exit face with at least one film of a 
light-transmissive material that is coarser grained on the exit face than 
on the entrance face (the side closer to the light conducting plate), the 
luminous intensity distribution characteristics of the backlighting device 
are varied in such a way that greater directivity of light is attained in 
the direction of the line normal to the exit face, whereby the efficiency 
of power to luminance conversion is increased in the direction of a line 
substantially normal to the exit face. 
The present invention provides a backlighting device for display panels 
that comprises: 
a light conducting plate made of a light-transmissive material, said light 
conducting plate having a light-diffusing element; 
a specular reflecting or light diffusing/reflecting lo plate that covers 
the thus covered area of the light conducting plate; and 
a linear light source provided in proximity to the end portion of at least 
one side of said light conducting plate; 
characterized in that said light conducting plate is provided on its exit 
face with at least one film of a light-transmissive material that is 
coarser grained on the exit face than on the entrance face (the side 
closer to said light conducting plate).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is described below in detail with reference to the 
accompanying drawings. FIG. 1 is a perspective view of a backlighting 
device according to an embodiment of the present invention and FIG. 2 is a 
longitudinal section of a backlighting device operating on the edge 
lighting method. Shown by 1 in FIGS. 1 and 2 is a light conducting plate 1 
that may be made of any material that is capable of efficient light 
transmission, as exemplified by quartz, glass, light-transmissive natural 
or synthetic resins such as acrylic resins. Shown by 2 is a film of a 
light-transmissive material that is coarser grained on the exit face than 
on the entrance face (the side closer to the light conducting plate). This 
film changes the luminous intensity distribution characteristics of light 
issuing from the light conducting plate in such a way that the directivity 
of light in the direction of the line normal to the exit face is enhanced. 
In the present invention, at least one sheet of the film of the type 
described above need be used but, as will be apparent from Example 3 to be 
described hereinafter, a further improvement in luminance can be achieved 
by using more than one sheet, say, two sheets of the film. 
The most characterizing part of the present invention is that the 
above-described film which is made of a light-transmissive material and 
which is coarser grained on the exit face than on the entrance face (the 
side closer to the light conducting plate) is provided on the exit face of 
the light conducting plate. 
This condition is described below more specifically. The film indicated by 
2 in FIGS. 1 and 2 is made of a light-transmissive material and is coarser 
grained on the exit face than on the entrance face (the side closer to the 
light conducting plate). The film may be made of any light-transmissive 
material such as acrylic resins, polycarbonates or glass and there is no 
particular limitation on the materials that can be used. The roughening 
method for use in making the exit face of the film coarse grained is not 
limited in any particular way and various methods may be employed, 
including embossing, sand blasting, die molding with a hot press and a 
chemical treatment. There is no particular need to roughen the entrance 
face of the film (on the side closer to the light conducting plate) and 
the only requirement that has to be satisfied is that the exit face of the 
film should eventually be coarser grained than the entrance face. Whether 
the exit face of the film is "coarser grained" can be checked by measuring 
"ten-point average roughness", "center-line-average roughness" and other 
parameters in accordance with the methods specified in JIS B 0601. 
The grained state of the exit face of the film described above (and shown, 
for example, in FIG. 5) which is made of a light-transmissive material is 
not limited in any particular way and the grained exit face may have a 
regular pattern or an irregular pattern. To give a general guideline, the 
grained exit face of the film is preferably shown to have surface 
asperities as examined under a microscope at a magnification of 100, with 
the difference in height between a concave and an adjacent convex as taken 
in the direction of the line normal to the exit face or the distance 
between adjacent concaves or convexes as taken in the direction parallel 
to the exit face being in the range of 10 to 1,000 .mu.m. 
In another expression, the grained state of the exit face of the film under 
consideration may be such that said exit face in a section of the film 
taken at any point is composed of convex shapes (i.e., as shown in FIG. 6) 
such as prisms or cones similar to convex lenses, with the vertical angle 
being in the range of 40 to 170 degrees, preferably 80 to 150 degrees, 
further preferably 85 to 120 degrees, further more preferably 90 to 110 
degrees. (The vertical angle is directed to a vertical angle when the film 
is cut so as to obtain a minimum vertical angle.) 
Liquid-crystal displays provide a lower contrast as the angle at which the 
viewer looks at the display face increases as measured from the line 
normal to that display face. Hence, one of the performance indices that 
bear particular importance in practical applications of liquid-crystal 
displays is the luminance as measured in the neighborhood of the direction 
of the line normal to their display face. As already mentioned 
hereinabove, the film which is coarser grained on the exit face than on 
the entrance face is provided on the exit face of the light conducting 
plate and, by so doing, the luminance of light issuing from the light 
conducting plate is amplified and the directivity of the output light is 
further enhanced. This may be stated more specifically as follows: the 
luminance of light issuing from the exit face as measured in the direction 
of a line substantially normal to said exit face is amplified compared to 
the case where the film of the type described above is not provided or 
where a film that is not coarsely grained on the exit face than on the 
entrance face, and the luminance as measured at an angle, say, 60 
degrees, with respect to the line normal to the exit face decreases by a 
greater degree than does the luminance as measured in a direction that is 
substantially normal to the exit face (e.g., the decrease is ca. 50 to 
60%, sometime 20%, of the value as measured in the normal direction). 
These and other evidential facts show the more effectively enhanced 
directivity of the light issuing from the exit face. 
It should be noted here that luminance measurements may be performed with a 
customary commercial luminance meter. 
As will be described hereinafter, a light diffusing material (indicated by 
6 in FIG. 2) is printed in a dotted pattern on the surface of the light 
conducting plate; if necessary, a light diffusing plate may be provided 
between the film described above and the light conducting plate in order 
to make the dotted pattern indiscernible to the human eye. 
The light conducting plate of the present invention has a light-diffusing 
element on a surface thereof. The light-diffusing element is provided in 
such a manner that, for example, a light-diffusing material partly covers 
a surface thereof, or a concave/convex is formed on the surface thereof. 
The light-diffusing material which is to be applied to one major surface of 
the light conducting plate may be a paint or printing ink that contains a 
suitable pigment, such as titanium white, that has a higher refractive 
index and diffusion reflectance than the material of which the light 
conducting plate is made. Such light-diffusing materials are 
screen-printed or otherwise printed in dots on one major surface of the 
light conducting plate. 
The specular reflecting or light diffusing/reflecting plate (as indicated 
by 3 in FIGS. 1 and 2) is provided in such a way as to cover substantially 
the entire surface of the light conducting plate, opposite the surface 
covered by the film that is already covered with the light-diffusing 
material. Shown by 4 is a linear light source which, in a preferred 
embodiment, is covered with a light reflector 5 in such a way as to 
provide a certain clearance from the outer surface of said linear light 
source. The reflector 5 has a slit formed in the surface through which 
incident light from the linear light source is admitted into an end 
portion of the light conducting plate. The linear light source is provided 
in proximity to at least one end face portion of the light conducting 
plate in such a way that its central axis is substantially parallel to 
either end face of the light conducting plate. 
The linear light source 4 may be selected from among various types 
including a fluorescent tube, a tungsten incandescent tube, an optical rod 
and an array of LEDs, and a fluorescent tube is preferred. From the 
viewpoint of power saving, it is preferred that the length of the uniform 
light emitting portion except the electrode portion is substantially equal 
to the length of the end portion of the light conducting plate in 
proximity to that emitting portion. 
The backlighting device of the present invention which has its principal 
part composed in the manner described above is to be used with display 
panels, particularly with liquid-crystal display panels. In a particularly 
preferred case, the backlighting device of the present invention has the 
following constitutional features. 
1) The light diffusing material is formed in a dot pattern on the surface 
of the light conducting plate. Dots may be of any shape such as a circle 
or rectangle. They may also be formed of crosslines. Such 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, more preferably 0.8 to 2 mm, with 
an appropriate distance being selected in accordance with the thickness of 
the light conducting plate. 
The surface of the light conducting plate is covered with the light 
diffusing material in such a way that the percent coverage is preferably 1 
to 50% of the plate surface in areas near the linear light source and 20 
to 100% in the area that is the farthest from the light source. 
Preferably, the light conducting plate is covered with the light diffusing 
material in such a way that the percent coverage increases gradually with 
the distance from the light source starting at the point where the linear 
light source is placed in proximity to the end portion of one side of said 
light conducting plate. In the neighborhood of the end portion of the 
other side of the light conducting plate (remote from the linear light 
source), the percent coverage with the light diffusing material may be 
comparable to or smaller than the value for the preceding or most adjacent 
area. The term "percent coverage" as used herein means the proportion of a 
unit area of the light conducting plate that is occupied by the coating of 
the light diffusing material. 
2) More preferably, the percent coverage (Y%) with the light diffusing 
material increases in proportion to a power of the distance (X in mm) from 
the linear light source to the light diffusing material in a grid pattern, 
with the power ranging from 1.7 to 3.5. In other words, the percent 
coverage (Y) should increase at those values which lie between the lines 
represented by Y=aX.sup.1.7 and Y=aX.sup.3.5 (where a denotes the value 
that is determined from the percent coverage for the end portion of the 
surface of the light conducting plate and satisfies the relation 
0&lt;a.ltoreq.2), or Y should increase to satisfy the relation Y=a.sup.x 
(where a is the value determined by the same method as just described 
above and satisfies the relation 1&lt;a.ltoreq.2), with Y and X being taken 
on the vertical and horizontal axes, respectively. 
3) It is also preferred for the present invention that the percent coverage 
with the light diffusing material which is coated on the light emitting 
surface along grid forming lines that are parallel to the axis of the 
linear light source increases gradually with the distance by which the 
coating departs from a line on the surface of the light conducting plate 
that is dropped perpendicular to the linear light source from the center 
of each parallel line (i.e., the center of the length of the linear light 
source) towards both ends thereof. 
The backlighting device of the present invention is used in practice with 
an optical display panel such as a liquid-crystal panel being positioned 
on top of the exit face. 
The backlighting device of the present invention is comparatively small in 
size and yet produces satisfactory luminance; therefore, it can be used as 
a model having high efficiency of power to luminance conversion in the 
direction of the line normal to the exit face. 
EXAMPLES 1 TO 3 AND COMATIVE EXAMPLES 1 TO 4 
Comparative examples and working examples of the present invention are 
described below in order to further illustrate the invention. 
A rectangular light conducting plate (225 mm.times.127 mm) having a 
thickness of 2.0 mm (see FIG. 1) was provided. A cold-cathode fluorescent 
tube (a normal tube of Harrison Denki K.K.) with a diameter of 4.8 mm was 
positioned in contact with one of its shorter sides. The fluorescent tube 
was enclosed with a cylindrical aluminum reflector having a slit 2 mm wide 
in contact with the light conducting plate in such a way that light 
emerging through the slit would be admitted into the plate from one 
shorter side. 
A light-diffusing material (paint containing titanium white) was applied 
over the surface of the light conducting plate by screen-printing a 
pattern of circular dots in such a way that the coverage with the 
light-diffusing material would be 6% at minimum and 80% at maximum, with 
the coverage being proportional to a.sup.x in the intermediate area. 
A sheet of polycarbonate film having a thickness of ca. 200 .mu.m that was 
coarser grained on the exit face than on the entrance face (the side 
closer to the light conducting plate) by embossing was provided on the 
exit face of the light conducting plate. The surface asperities on the 
exit face of the film were examined under a microscope at a magnification 
of 100 and the results were as follows: the difference in height between a 
concave and an adjacent convex as measured in the direction of the line 
normal to the exit face of the film was 10 to 100 .mu.m; and the distance 
between adjacent concaves or convexes as measured in the direction 
parallel to the exit face of the film was 10 to 800 .mu.m. In addition, 
the vertical angle was in the range of 80 to 150 degrees when the film is 
cut so as to obtain a minimum vertical angle. 
The roughness of the coarse grained surface of the film was measured in 
accordance with JIS B 0601 under the following conditions: longitudinal 
recording magnification=500; transverse recording magnification=50; drive 
speed=0.3 mm/sec; probe=stylus equipped with a diamond tip of 5 .mu.m in 
radius. The results were as follows: maximum height of roughness (Rmax)=85 
.mu.m; ten-point average roughness (Rz)=60 .mu.m; and center-line-average 
roughness (Ra)=13 .mu.m. Similar measurements of the roughness of the none 
coarse grained surface of the film were conducted but under different 
conditions as follows: longitudinal recording magnification=2,000; 
transverse recording magnification=50; drive speed=0.3 mm/sec; and 
probe=stylus equipped with a diamond tip of 5 .mu.m in radius. The results 
were as follows: Rmax=12 .mu.m; Rz=7 .mu.m; and Ra=1 .mu.m. 
The areal luminance as produced when the cold-cathode tube was driven at a 
constant current (a constant power) with an alternating voltage (30 kHz) 
being applied from an invertor was measured with a luminance meter (Topcon 
BM-7) in the direction of the line normal to the exit face at a field 
coverage angle of 2 degrees and with the distance from the exit face to 
the luminance meter (as indicated by 8 in FIG. 3) being held at 40 cm. The 
result was 222 cd/m.sup.2 (Example 1). 
A backlighting device was constructed in the same way and operated under 
the same conditions as in Example 1 except that an ordinary unembossed 
light-diffusing film (D-204 of Tsujimoto Denki Seisakusho K.K.) was 
provided between the grained (i.e., embossed) film and the light 
conducting plate. A luminance measurement was conducted as in Example 1 
and the result was 221 cd/mm.sup.2 (Example 2). 
The surface roughness of the light-diffusing film (D-204) used in Example 2 
was measured under the following conditions: longitudinal recording 
magnification=2,000; transverse recording magnification=50; drive 
speed=0.3 mm/sec; probe=stylus equipped with a diamond tip of 5 .mu.m in 
radius. The results were as follows: Rmax=13 .mu.m; Rz=9 .mu.m; Ra=1 
.mu.m. 
A backlighting device was constructed in the same way and operated under 
the same conditions as in Example 1 except that two sheets of the same 
film as used in Example 1 which was coarser grained on the exit face than 
on the entrance face (the side closer to the light conducting plate) was 
provided in superposition on the exit face of the light conducting plate. 
A luminance measurement was conducted as in Example 1 and the result was 
239 cd/m.sup.2 (Example 3). 
A backlighting device was constructed in the same way and operated under 
the same conditions as in Example 1 except that a single sheet of 
unembossed light-diffusing film (D-204 of Tsujimoto Denki Seisakusho K.K.) 
was provided on the exit face of the light conducting plate in place of 
the film coarse grained by embossing. A luminance measurement was 
conducted as in Example 1 and the result was 192 cd/m.sup.2 (Comparative 
Example 1). 
A backlighting device was conducted in the same way and operated under the 
same conditions as in Example 1 except that the film coarse grained by 
embossing was provided in such a way that the coarse grained side would 
face the exit face side of the light conducting plate. A luminance 
measurement was conducted as in Example 1 and the result was 185 
cd/m.sup.2 (Comparative Example 2). 
A backlighting device was constructed in the same way and operated under 
the same conditions as in Comparative Example 2 except that the 
light-diffusing plate (D-204 of Tsujimoto Denki Seisakusho K.K.) used in 
Example 2 was provided between the coarse grained film and the light 
conducting plate. A luminance measurement was conducted as in Example 1 
and the result was 183 cd/m.sup.2 (Comparative Example 3). 
A backlighting device was constructed in the same way and operated under 
the same conditions as in Example 1 except that only two sheets of the 
light-diffusing film (D-204 of Tsujimoto Denki Seisakusho K.K.) used in 
Example 2 were provided in superposition on the exit face of the light 
conducting plate. A luminance measurement was conducted as in Example 1 
and the result was 188 cd/m.sup.2 (Comparative Example 4). 
The luminous intensity distribution characteristics of the backlighting 
devices constructed in Examples 2 and 3 and in Comparative Examples 1 and 
4 were investigated by the following procedure: with each device being 
driven with a constant current flowing under an alternating voltage 
applied at 30 kHz from an invertor to the cold-cathode tube, the areal 
luminance was measured with a luminance meter (Topcon BM-7) at a field 
coverage angle of 2 degrees and with the angle with respect to the line 
normal to the exit face (as indicated by 9 in FIG. 3) being varied from 0 
to 70 degrees, while the distance from the exit face to the luminance 
meter being held at 40 cm. The results are shown in FIG. 4, from which one 
can see that using the backlighting device of the present invention 
contributes a higher luminance and a marked improvement in the directivity 
of output light.