U.S. patent application number 09/834899 was filed with the patent office on 2001-10-18 for surface light source device, and liquid crystal display device, sign display apparatus and traffic sign display apparatus using the surface light source device.
This patent application is currently assigned to Mitsubishi Rayon Co., Ltd.. Invention is credited to Chiba, Issei, Hayashi, Yasuko, Oda, Mosaharu.
Application Number | 20010030861 09/834899 |
Document ID | / |
Family ID | 27281622 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010030861 |
Kind Code |
A1 |
Oda, Mosaharu ; et
al. |
October 18, 2001 |
Surface light source device, and liquid crystal display device,
sign display apparatus and traffic sign display apparatus using the
surface light source device
Abstract
A surface light source device, comprising a light source (2); a
light conductor (1) which has a light incident face (11) on at
least one side end surface thereof which confronts the light source
(2), and a light emitting face (12) on one surface thereof which is
substantially perpendicular to the light incident face (11); and a
light angle varying sheet (3) which is disposed at a side of the
light emitting face (12) of the light conductor (1). wherein at
least one of the light emitting face (12) and a back surface (13)
of the light conductor (1) comprises a minute structure having an
average slant angle of 0.5 to 7.5 degrees. The light angle varying
sheet (3) may comprise a prism sheet having a plurality of prisms
(31) which are formed parallel to one another on at least one
surface thereof. The minute structure may comprise a roughened
surface which includes a plurality of fine convex members each
having a substantially spherical surface or a plurality of prism
arrays having slant surfaces which extend parallel to said light
incident face (11) and which have an average slant angle of 0.5 to
7.5 degrees.
Inventors: |
Oda, Mosaharu; (Kanagawa,
JP) ; Chiba, Issei; (Kanagawa, JP) ; Hayashi,
Yasuko; (Kanagawa, JP) |
Correspondence
Address: |
MORGAN, LEWIS & BOCKIUS LLP
1800 M Street, N.W.
Washington
DC
20036
US
|
Assignee: |
Mitsubishi Rayon Co., Ltd.
Tokyo
JP
|
Family ID: |
27281622 |
Appl. No.: |
09/834899 |
Filed: |
April 16, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09834899 |
Apr 16, 2001 |
|
|
|
09461342 |
Dec 15, 1999 |
|
|
|
6244719 |
|
|
|
|
09461342 |
Dec 15, 1999 |
|
|
|
09117505 |
Jul 30, 1998 |
|
|
|
6099135 |
|
|
|
|
09461342 |
Dec 15, 1999 |
|
|
|
PCT/JP97/00237 |
Jan 31, 1997 |
|
|
|
Current U.S.
Class: |
362/620 ;
359/599; 359/831 |
Current CPC
Class: |
G02B 6/0001 20130101;
G02B 6/0036 20130101; G02B 6/0038 20130101; G02B 6/0046 20130101;
G02B 6/0053 20130101 |
Class at
Publication: |
362/31 ; 359/599;
359/831 |
International
Class: |
F21V 007/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 1, 1996 |
JP |
8-16928 |
Feb 1, 1996 |
JP |
8-40719 |
Jul 4, 1996 |
JP |
8-175122 |
Claims
1. A surface light source device comprising: a light source; a
light conductor which has a light incident face on at least one
side end surface thereof which confronts said light source, and a
light emitting face on one surface thereof which is substantially
perpendicular to said light incident face; and a light angle
varying sheet which is disposed at a side of said light emitting
face of said light conductor, wherein at least one of said light
emitting face and a back surface of said light conductor comprises
a minute structure having an average slant angle of 0.5 to 7.5
degrees.
2. The surface light source device as claimed in claim 1, wherein
said light angle varying sheet comprises a lens sheet having a
plurality of lenses which are formed parallel to one another on at
least one surface thereof.
3. The surface light source device as claimed in claim 2, wherein
said lens sheet is a prism sheet having a plurality of prisms which
are formed parallel to one another on at least one surface
thereof.
4. The surface light source device as claimed in claim 1, wherein
said minute structure comprises a roughened surface which includes
a plurality of fine convex members each having a substantially
spherical surface.
5. The surface light source device as claimed in claim 4, wherein a
ratio of a minute average radius of curvature to an average period
of said convex members is set to 3 to 10, and a ratio of an average
deviation of a distribution of the minute average radius of
curvature to the minute average radius of curvature is set to 0.85
or less.
6. The surface light source device as claimed in claim 4, wherein
said roughened surface includes areas which have a minute average
slant angle of 20 degrees or more at an occupation rate of 2% or
less.
7. The surface light source device as claimed in claim 1, wherein
said minute structure comprises a plurality of lens arrays having
slant surfaces which extend parallel to said light incident face
and which have an average slant angle of 0.5 to 7.5 degrees.
8. The surface light source device as claimed in claim 7, wherein
said lens arrays comprise prism arrays.
9. The surface light source device as claimed in claim 7, wherein
said lens arrays comprise lenticular lens arrays each having an
arcuate shape in section.
10. The surface light source device as claimed in claim 1, wherein
the light emission rate from said light emitting face of said light
conductor is set to 1% to 4.5%.
11. The surface light source device as claimed in claim 1, wherein
a peak light having a maximum light intensity which is emitted from
said light emitting face of said light conductor is emitted at an
angle of 65 degrees or more with respect to a normal to said light
emitting face.
12. The surface light source device as claimed in claim 1, wherein
an intersecting angle between a direction of a peak light having a
maximum intensity emitted from said light emitting face of said
light conductor and a direction of light having 50% of the maximum
light intensity is equal to 20 degrees or less.
13. The surface light source device as claimed in claim 1, wherein
said light conductor is designed so that a ratio (L/t) of a length
(L) from the light incident face to an end face confronting said
light incident face and a thickness (t) of said light conductor is
set to 200 or less.
14. A liquid crystal display device, comprising the surface light
source device as claimed in claim 1 as a back light.
15. The liquid crystal display device as claimed in claim 14,
wherein a dispersion rate (R%) of the brightness of light emitted
from said light emitting face of said light conductor is set to 30%
or less.
16. A sign display apparatus, comprising the surface light source
device as claimed in claim 1 as a back light.
17. The sign display apparatus as claimed in claim 16, wherein a
dispersion rate (R%) of the brightness of light emitted from said
light emitting face of said light conductor is set to 250% or
less.
18. A traffic sign display apparatus, comprising the surface light
source device as claimed in claim 1 as a back light.
19. The traffic sign display apparatus as claimed in claim 18,
wherein a dispersion rate (R%) of the brightness of light emitted
from said light emitting face of said light conductor is set to
450% or less.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a surface light source
device for a display apparatus such as a liquid crystal display
device for use in a portable personal computer, a liquid crystal
television or the like; a sign display apparatus such as a guide
marking board, a large-size signboard or the like used in a
station, public facilities or the like; or for a traffic sign
display apparatus such as various types of guide signs, traffic
signs or the like on a highway road or a general road; and the
present invention relates more particularly to a surface light
source element for emitting light which has high brightness and a
uniform brightness distribution on a light emitting plane without
performing any uniformity-enhancing processing such as treatment
with a spot pattern or the like.
DESCRIPTION OF THE RELATED ART
[0002] Recently, a color liquid crystal display device has been
widely used in various applications such as in portable personal
computers, liquid crystal televisions, video built-in type liquid
crystal televisions, etc. This liquid crystal display device
comprises a back light portion and a liquid crystal display
portion. An under-lighting system in which a light source is
disposed just under the liquid crystal display portion and an
edge-lighting system in which a light source is disposed on the
side surface of a light conductor are used as a lighting system for
the back light portion.
[0003] Recently, the edge-lighting system has been more frequently
used because it is more suitable for reducing the size of the
liquid crystal display device. In the edge-lighting system, the
light source is disposed at a side surface portion of a planar
light conductor so as to emit light from the entire surface of the
light conductor, and thus the back light portion using this system
is called a "surface light source device".
[0004] According to such a surface light source device, a light
conductor is formed of a planar transparent member such as an
acrylic resin plate or the like, and light emitted from a light
source which is disposed at the side surface of the light conductor
is introduced through the side surface (light incident face) of the
light conductor into the light conductor. The incident light
totally (completely) reflects from the obverse surface of the light
conductor (light emitting face) and the back surface of the light
conductor and then passes through the light conductor. Further, a
light emitting function or light emitting portion such as a light
scattering portion is provided on the obverse surface or back
surface of the light conductor to emit the light from the whole
light emitting face. However, when the light emitting portion is
uniformly formed on the obverse surface or the back surface of the
light conductor, the brightness of the emitted light is more
reduced as the light is farther away from the light source, so that
the brightness distribution on the light emitting plane becomes
disuniform and thus a high-quality display image cannot be
obtained.
[0005] This result is more noticeable as the size of the liquid
crystal display device increases, and thus the surface light source
device cannot practically be used for a large-size liquid crystal
display device of 10-inches or more in size. Despite this,
large-size liquid crystal display devices have recently been in
demand and, following this recent demand, liquid crystal display
devices used for portable personal computers, liquid crystal
televisions or the like have additionally been required to have a
brightness distribution of very high uniformity on the screen
thereof.
[0006] Furthermore, marking apparatuses such as a guide signboard
or a large-size signboard, and traffic sign apparatuses such as a
guide sign, a traffic signboard or the like, have used two
illumination systems, i.e., an internal illumination system and an
external illumination system, to enhance visual recognition and
character recognition at night. According to the internal
illumination system, characters, figures, photographs, etc., are
formed on a semi-transparent plastic plate such as a methacrylate
plate or the like by cut-out, print or the like to form a display
plate. A light source is disposed at the inside of the display
plate, and the display plate is illuminated by the light source. A
rod (linear pipe shape) or annular type fluorescent lamp is
generally used as the light source. According to the external
illumination system, a light source is disposed at any of the upper
and lower sides or right and left sides of a display plate on which
an information display is formed, and the whole surface of the
display plate is illuminated by the light source. A rod type
fluorescent lamp is generally used as the light source.
[0007] In the conventional display devices as described above, the
brightness distribution on the entire surface of the display plate
is disuniform, that is, the ratio of the maximum value/minimum
value of the brightness is very large. Therefore, it is very
difficult to provide a display device having an uniform brightness
distribution by using these illumination systems. In particular,
this problem is more serious for the external illumination system.
Further, the internal illumination system has another problem in
that a fluorescent lamp or the like which is used as the light
source can be unintentionally seen through the display plate (i.e.,
a see-through phenomenon occurs).
[0008] Therefore, attempts have been made to apply the edge
lighting type back light system in which a light source is disposed
at the side surface portion of a planar light conductor to emit
light from the entire surface of the light conductor to the display
devices as described above. However, these display devices need a
large-size surface light source device, and thus they have the same
problem as the liquid crystal display device in that sufficient
uniformity in brightness cannot be obtained within the light
emitting face.
[0009] In order to solve this problem of "disuniformity of
brightness" of the surface light source device, various proposals
have been made. For example, Japanese Laid-open Patent Application
No. Hei-1-24522 proposes a surface light source device having a
light emitting portion which is obtained by coating or sticking
light scattering material to the back surface confronting the light
emitting face of the light conductor so that the density of the
light scattering material increases with increasing distance from
the light incident face. Further, Japanese Laid-open Patent
Application No. Hei-1-107406 proposes a light conductor comprising
plural laminated transparent plates on which fine spots formed of
light scattering material are formed in various patterns. In such a
surface light source device, since white pigment such as titanium
oxide, barium sulfate or the like is used as the light scattering
material, optical loss occurs due to light absorption or the like
when the light impinging against the light scattering material is
scattered. Therefore, although uniformity of the brightness
distribution can be achieved, the brightness of the emitted light
is reduced.
[0010] Further, Japanese Laid-open Patent Application No.
Hei-1-244490 and Japanese Laid-open Patent Application No.
Hei-1-252933 propose a surface light source device in which an
emitted light adjusting member or a light diffusion plate having a
light reflection pattern which is matched to the reciprocal of a
light emission distribution is disposed on the light emitting face
of the light conductor. However, in such a surface light source
device, since the light reflected from the emitted light adjusting
member or the light diffusion plate can not be reused, the same
optical loss also occurs. Therefore, the brightness of the emitted
light in a desirable direction is reduced.
[0011] Still further, Japanese Laid-open Patent Application No.
Hei-2-17 and Japanese Laid-open Patent Application No. Hei-2-84618
propose a surface light source device in which a satin-finished
face is or many lens units are formed on at least one of the light
emitting face or the back surface confronting the light emitting
face of the light conductor, and a prism sheet is mounted on the
light emitting face. In such a surface light source device, the
brightness of the emitted light is very high. however, the
uniformity of the brightness distribution on the light emitting
face is still unsatisfactory. Therefore, this type of surface light
source device is practically usable as only a small-size surface
light source element in a size of several inches.
[0012] In order to provide a surface light source device, which can
achieve uniformity in brightness of emitted light and reduce the
optical loss to enhance the brightness, Japanese Laid-open Patent
Application No. Hei-6-18879 proposes the following surface light
source device. In this surface light source device, a
satin-finished face is or many lens units are formed on the light
emitting face of the light conductor, a roughened surface portion
and a flat surface portion are formed on the back surface of the
light conductor so that the ratio of the roughened surface portion
to the flat surface portion increases with increasing distance from
the light source, and a prism sheet is mounted on the light
emitting face. In this surface light source device, the uniformity
of the brightness distribution of the emitted light can be achieved
and the optical loss can be reduced. However, when the surface
light source device is used for a display device such as a liquid
crystal display device, a marking apparatus or the like, a pattern
which is formed of the roughened surface portion and the flat
surface portion on the back surface of the light conductor can be
observed through the liquid crystal display panel or the display
plate, which prevents a viewer from seeing an image.
SUMMARY OF THE INVENTION
[0013] Therefore, an object of the present invention is to provide
a surface light source device for emitting light which has high
brightness and high uniformity in brightness distribution within a
light emitting face without performing a uniformity enhancing
treatment with a spot pattern or the like.
[0014] In view of the foregoing situation, the inventors of the
present application have made various earnest studies on the
structure of the light emitting face and the back surface of a
light conductor, and through these studies they have found out that
a surface light source device which can emit light having high
brightness and high uniformity of brightness distribution within a
light emitting face without performing a uniformity enhancing
treatment with a spot pattern or the like can be provided by
designing the light emitting face or the back surface thereof to
have a roughened surface having a fine uneven shape with a specific
average oblique angle, or to have an uneven surface comprising a
plurality of lens arrays with a specific average oblique angle.
[0015] That is, a surface light source device according to the
present invention comprises:
[0016] a light source;
[0017] a light conductor which has a light incident face on at
least one side end surface thereof which confronts the light
source, and a light emitting face on one surface thereof which is
substantially perpendicular to the light incident face; and
[0018] a light angle varying sheet which is disposed at a side of
the light emitting face of the light conductor,
[0019] wherein at least one of the light emitting face and a back
surface of the light conductor comprises a minute structure having
an average slant angle of 0.5 to 7.5 degrees.
[0020] A surface light source device according to a first aspect of
the present invention comprises: a light source; a light conductor
which has a light incident face on at least one side end surface
thereof confronting the light source and a light emitting face on
one surface thereof which is substantially perpendicular to the
light incident face; and a lens sheet which is disposed at the
light emitting face side of the light conductor and has a plurality
of parallel lens arrays on at least one surface thereof, wherein at
least one of the light emitting face and the back surface of the
light conductor comprises a roughened surface which includes a
plurality of fine convex members each having a substantially
spherical surface, and the average oblique angle of the roughened
surface is set to 0.5 to 7.5 degrees.
[0021] Furthermore, a surface light source device, according to a
second aspect of the present invention includes a light source, a
light conductor which has a light incident face on at least one
side end surface thereof confronting the light source and a light
emitting face on one surface thereof which is substantially
perpendicular to the light incident face, and a lens sheet which is
disposed at the light emitting face side of the light conductor and
has many parallel lens arrays on at least one surface thereof,
wherein at least one of the light emitting face and the back
surface of the light conductor comprises many lens arrays which
extend in parallel to the light incident face and each have slant
surfaces having an average oblique angle of 0.5 to 7.5 degrees.
[0022] Still furthermore, each of a liquid crystal display device,
a sign display apparatus and a traffic sign display apparatus
according to the present invention uses the surface light source
device as a back light.
[0023] According to the present invention, many fine convex members
which have the substantially spherical surface and the average
oblique angle of 0.5 to 7.5 degrees are formed on at least one of
the light emitting face and the back surface confronting the light
emitting face of the light conductor, or many lens arrays
comprising slant surfaces having the average oblique angle of 0.5
to 7.5 degrees are formed in parallel to the light incident face on
at least one of the light emitting face and the back surface of the
light conductor. With this construction, the light emission
efficiency of the light emitted from the light emitting face of the
light conductor can be reduced, thereby enabling greater amount of
light to propagate toward the tip end portion of the light
conductor. Therefore, the high uniformity of the brightness within
the light emitting face can be achieved without performing the
uniformity treatment using the spot pattern or the like.
BRIEF DESCRIPFION OF THE DRAWINGS
[0024] FIG. 1 is a perspective view showing a surface light source
device according to the present invention;
[0025] FIG. 2 is a schematic diagram showing an optical path of
light on a light emitting face of a light conductor according to
the present invention;
[0026] FIG. 3 shows a coordinate system in which the spherical
shape of a convex member according to the present invention is
simplified as a circle;
[0027] FIG. 4 is a partial cross-sectional view showing a prism
surface of the light conductor according to the present
invention;
[0028] FIG. 5 is a partial cross-sectional view showing a
lenticular lens surface of the light conductor according to the
present invention;
[0029] FIG. 6 is a side view showing a light conductor of the
surface light source device according to present invention;
[0030] FIG. 7 is a side view showing another light conductor of the
surface light source device according to present invention;
[0031] FIG. 8 is a partial perspective view showing a liquid
crystal display device according to present invention;
[0032] FIG. 9 is a graph showing a distribution of light emitted
from the light conductor;
[0033] FIG. 10 is a chart showing the surface roughness of a
roughened surface of a device of Example 1 according to the present
invention, and showing primary and secondary differentials of the
surface roughness;
[0034] FIG. 11 is a chart showing the surface roughness of a
roughened surface of a device of Comparative Example 1 of the
present invention, and showing primary and secondary differentials
of the surface roughness;
[0035] FIG. 12 is a partial cross-sectional view showing a prism
surface of a light conductor of a comparative example; and
[0036] FIG. 13 is a partial cross-sectional view showing a
lenticular lens surface of the light conductor of another
comparative example.
DETAILED DESCRIPTION OF THE INVENTION
[0037] FIG. 1 is a perspective view showing a surface light source
device of an embodiment according to the present invention. As
shown in FIG. 1, the surface light source device of this embodiment
includes an elongated light source 2, a light conductor 1 which has
at least one light incident face 11 confronting the light source 2
and a light emitting face 12 which is substantially perpendicular
to the light incident face 11, and a light angle varying sheet 3
comprising a lens sheet such as prism sheet mounted on the light
emitting face 12 of the light conductor 1. In the surface light
source device thus constructed, a part of the light which is
emitted by the light source 2 and incident onto the light conductor
1, and which has an incident angle distribution over a critical
angle, propagates in the light conductor 1 while being totally
reflected repetitively from the light emitting face 12 and a back
surface 13 of the light conductor 1. When the surface (the light
emitting face 12) of the light conductor 1 is designed to be
uneven, light which reaches an uneven portion at an angle below the
critical angle with respect to the uneven portion is refracted from
the uneven portion and emitted to the outside of the light
conductor 1. On the other hand, light which reaches an uneven
portion at an angle exceeding the critical angle is totally
reflected from the uneven portion and continues to propagate in the
light conductor 1. This phenomenon happens because the light
traveling direction (i.e., whether the light is reflected or
refracted) is determined according to Snell's law. In other words,
it is determined by the refractive index of a medium and the light
incident angle with respect to the normal of the surface to which
the light is incident.
[0038] FIG. 2 is a schematic diagram showing the light refraction
and reflection in the light conductor 1 having an uneven portion on
the surface thereof. Light A which is incident onto a slant surface
of the uneven portion at an incident angle i, which is below the
critical angle, is emitted from the light conductor 1 at an
refraction angle i' which satisfies the relationship of n
sin(i)=sin(i') (n represents the refractive index of the light
conductor 1) according to Snell's law. On the other hand, light B
which is incident onto the slant surface at an angle k, which
exceeds the critical angle, is totally reflected from the slant
surface at an angle k' (k'=k), and continues propagating in the
light conductor 1. The light which has been once incident onto the
uneven portion and then reflected therefrom is liable to change an
incident angle when it is incident again onto the uneven portion,
so that there is a probability that the light will be emitted to
the outside of the light conductor 1.
[0039] The inventors of this application have experimentally found
out that the relationship between the light emission intensity (I)
at a point and the light emission intensity (Io) at the light
incident face end of the surface light source device satisfies the
following equation (1):
I=Io(1-.alpha./100).sup.L'/t (1)
[0040] where .alpha. represents the light emission rate, L'
represents the distance from the light incident face end and t
represents the thickness of the light conductor 1.
[0041] It is apparent from equation (1) that if the length (L) and
the thickness (t) of the light conductor 1 are determined, the
uniformity of the brightness distribution of emitted light within
the light emitting face will be dependent on the emission rate
(.alpha.). The emission rate (.alpha.) of the light conductor 1
having a thickness of tmm, can be calculated from the following
equation (2):
.alpha.=(1-10.sup.K).times.100 (2)
[0042] K can be calculated by measuring the brightness at 20 mm
intervals from the light incident face end of the light conductor 1
and calculating the gradient (K(mm.sup.-1)) from the logarithmic
graph representing the relationship between the ratio (L'/t) of the
distance (L') from the light incident face end to the thickness (t)
of the light conductor 1 and the brightness thus measured.
[0043] In the present invention, a dispersion rate (R%) represented
by the following equation (3) is used as a criterion for the
uniformity of the brightness distribution to estimate and consider
the uniformity of the brightness distribution in the surface light
source device. The dispersion rate (R%) is measured as follows.
That is, the brightness on the light emitting face of the light
conductor 1 is measured on a substantially central area of the
light conductor 1, the area extending from a point of 5 mm interval
far away from the light incident face end, by 20 mm increments to
the end portion confronting the light incident face end of the
light conductor 1. The central area is positioned at substantially
central portion relative to lengthwise direction of the light
source 2. Then, the maximum value (Imax) of the measured
brightness, the minimum value (Imin) of the measured brightness,
and the average value (Iav) of the measured brightness are
calculated. Thereafter, the dispersion rate (R%) is calculated from
the following equation (3):
R%={(Imax-Imin)/Iav}.times.100 (3)
[0044] As a result, it has been found that the emission rate
(.alpha.) and the dispersion rate (R%) are dependent on the length
(L) and the thickness (t) of the light conductor 1, and satisfy the
specific relationship therebetween. That is, as the emission rate
(.alpha.) increases, the dispersion rate (R%) also increases. If
the emission rate (.alpha.) is constant, the dispersion rate (R%)
increases when the ratio (L/t) of the length (L) and the thickness
(t) of the light conductor 1 increases. That is, in the light
conductor 1 having a fixed size, the uniformity (dispersion rate)
of the brightness distribution within the light emitting face of
the light conductor 1 is dependent on the emission rate (.alpha.)
of the light conductor 1, and good uniformity of the brightness
distribution can be achieved by controlling the emission rate
(.alpha.).
[0045] In addition, the inventors of the present application have
also found that in the case where the surface (light emitting face
12, back surface 13) of the light conductor 1 is designed to have a
fine uneven roughened surface comprising many fine convex members
having substantially spherical surface, or to have many lens arrays
which extend parallel to the light incident face 11 of the light
conductor 1, the emission direction and emission rate of the light
emitted from the light conductor 1 and the emission rate vary in
accordance with the gradient of the uneven portion constituting the
roughened surface or the gradient of the slant surfaces
constituting the lens arrays.
[0046] Particularly in the case of the fine uneven roughened
surface, the fine uneven shape of the roughened surface can be
approximated by a slant surface having a gradient. Here, the
average slant angle (.theta.a) which is defined by ISO 4287/1-1987
may be used as the gradient. As the average slant angle (.theta.a)
increases, the light emitted from the light conductor 1 becomes
substantially parallel to the direction normal to the light
emitting face 12, i.e. the direction of the thickness t of the
light conductor 1. Further, as the average slant angle (.theta.a)
increases, the emission rate (.alpha.) of the light emitted from
the light conductor 1 also increases. Therefore, the uniformity of
the brightness distribution within the light emitting face 12 of
the surface light source device can be enhanced by lowering the
emission rate (.alpha.) of the light from the light conductor 1,
that is, the uniformity can be more enhanced by reducing the
average slant angle (.theta.a).
[0047] On the basis of the above new knowledge, according to the
present invention, at least one of the light emitting face of the
light conductor 1 or the back surface of the light conductor 1
which confronts the light emitting face is designed to have a
roughened surface or a plurality of lens arrays having an average
slant angle (.theta.a) of 0.5 to 7.5 degrees. With this design, the
emission rate (.alpha.) of the light emitted from the light
conductor 1 can be sufficiently reduced, and thus good uniformity
of brightness distribution within the light emitting face 12 of the
surface light source device can be achieved. If the average slant
angle (.theta.a) of the roughened surface is less than 0.5 degrees,
the total amount of the light emitted from the light emitting face
12 of the light conductor 1 decreases so that sufficiently high
brightness cannot be obtained, or, the light emission angle of the
light emitted from the light emitting face (the angle relative to
the normal to the light emitting face 12) increases, and thus the
emitted light cannot be directed toward the normal direction even
by using an angle varying member such as a prism sheet, lens sheet
or the like. On the other hand, if the average slant angle
(.theta.a) exceeds 7.5 degrees, the light emission rate (.alpha.)
of the light conductor 1 increases, and thus the uniformity of the
brightness distribution of the surface light source device is
lowered. Preferably, the average slant angle (.theta.a) is set
within a range from 1 to 6 degrees, and more preferably it is set
within a range from 2 to 5 degrees.
[0048] The average slant angle (.theta.a) on the roughend surface
having the fine uneven shape can be calculated as follows. First,
the surface roughness of the roughened surface which is formed on
the surface of the light conductor 1 is measured at a driving speed
of 0.03 mm/second by a probe type (tracing) surface roughness
tester, subtracting an average line from the measured chart to
correct the slant, and then calculating the average slant angle
(.theta.a) from the following equations (4) to (5). Here, L"
represents a distance which is scanned by the probe, x represents a
measurement position and f(x) represents a displacement of the
probe:
.DELTA.a=(1/L").intg.o.sup.L'.vertline.(d/dx)f(x).vertline.(dx)
(4)
.theta.a=tan.sup.-1.DELTA.a (5)
[0049] Furthermore, according to the present invention, in order to
sufficiently enhance the uniformity of the brightness distribution
of the surface light source device, it is preferable to set the
emission rate (.alpha.) of the light emitted from the light
emitting face 12 of the light conductor 1 to 1% to 4.5%. If the
light emission rate (.alpha.) from the light emitting face of the
light conductor is less than 1%, the light emission angle of the
light emitted from the light emitting face (the angle relative to
the normal of the light emitting face 12) increases, so that it is
increasingly difficult to sufficiently direct the emitted light
toward the normal direction even by using an angle varying member
such as a prism sheet or the like. Conversely, if the emission rate
(.alpha.) exceeds 4.5%, the uniformity of the brightness
distribution of the surface light source device of the liquid
crystal display device or the like trends to be lowered.
Preferably, the emission rate (.alpha.) is set to 1% to 4%, and
more preferably it is set to 1.2% to 3.5%.
[0050] In order to achieve good uniformity of the brightness
distribution in the surface light source device, it is preferable
that the following light emission characteristic be obtained. That
is, that the light emitted from the light emitting face of the
light conductor 1 be directed from the light emitting face so that
the peak light (i.e., having the maximum light intensity) is
emitted at an angle of 65 degrees or more with respect to the
normal direction to the light emitting face, or that the
intersecting angle between the direction of the peak light and the
direction of the light having half (50%) of the maximum light
intensity is equal to 20 degrees or less. If the peak light having
the maximum light intensity is emitted at an angle of less than 65
degrees to the normal of the light emitting face, or if the
intersecting angle between the direction of the peak light having
the maximum light intensity and the direction of the light having
the half (50%) maximum intensity exceeds 20 degrees, the emission
rate (.alpha.) of the light emitted from the light conductor 1
increases, and thus it is increasingly difficult to achieve good
uniformity of brightness distribution on the light emitting
face.
[0051] Still furthermore, in the present invention, a brightness
which is as high as possible is needed for the surface light source
device, and it is preferable that the light emitted from the
surface light source device is concentrated in an observation
(viewing) direction. Therefore, it is preferable that the
intersecting angle between the direction of the peak light having
the maximum intensity from the light conductor 1 and the direction
of the light having a light intensity of 10% of the maximum light
intensity be below 50 degrees. If the intersecting angle exceeds 50
degrees, the amount of light which is emitted in a direction other
than the observation direction increases even by using a light
angle varying sheet, so that a sufficiently high brightness cannot
be obtained.
[0052] In the present invention, when the surface of the light
conductor 1 comprises a fine uneven roughened surface comprising a
plurality of fine convex members each having a substantially
spherical surface, it is preferable to make the radius of curvature
of the convex members uniform, and also it is preferable to satisfy
a specific relationship among the average period (P), the minute
average radius of curvature (R) and the average deviation (S) of
the distribution of the minute average radius of curvature of the
fine convex members constituting the roughened surface of the light
conductor 1. That is, it is preferable that the ratio (R/P) of the
minute average radius of curvature (R) and the average period (P)
of the convex members be set to 3 to 10, and that the ratio (S/R)
of the average deviation (S) of the distribution of the minute
average radius of curvature and the minute average radius of
curvature (R) be set to 0.85 or less.
[0053] If the ratio (R/P) of the minute average radius of curvature
(R) of the convex members and the average period (P) of the convex
members is less than 3, the average slant angle (.theta.a) of
uneven surfaces of the convex members having the substantially
spherical surface increases, and thus the emission rate (.alpha.)
of the light conductor 1 also increases. Therefore, the uniformity
of the brightness distribution within the light emitting face of
the light conductor 1 tends to be lowered. On the other hand, if
the ratio (R/P) is more than 10, the average slant angle (.theta.a)
of the uneven surface of the convex members having substantially
spherical surface is reduced, and thus the emission rate (.alpha.)
of the light conductor 1 is excessively reduced. Therefore, the
total amount of light emitted from the light emitting face of the
light conductor 1 is reduced and thus sufficient brightness cannot
be obtained. Preferably, the ratio (R/P) is set to 5 to 7.
[0054] Furthermore, if the ratio (S/R) of the average deviation (S)
of the distribution of the minute average radius of curvature and
the minute average radius of curvature (R) exceeds 0.85, the
distribution of the convex members formed on the surface of the
light conductor 1 is made disuniform, and the uniformity of the
brightness distribution within the light emitting face of the light
conductor 1 tends to be lowered. Preferably, the ratio (S/R) is set
to 0.8 or less, and more preferably to 0.7 or less.
[0055] In the present invention, the average period (P) of the
convex members is defined as follows. That is, a linear area having
a fixed distance (for example, 1000 micrometers) in any direction
on the roughened surface of the light conductor 1 is measured by a
surface roughness tester to measure the number of crests of the
convex members, and the average period (P) is obtained as the
average value of the period which is calculated on the basis of the
measured number of crests (e.g., the average period P is equal to
the fixed distance (for example, the 1000 micrometers) divided by
the number of crests). Further, the minute average radius of
curvature (R) can be calculated from a chart which is obtained by
measuring the roughened surface comprising the convex members with
a surface roughness tester as follows. First, for simplicity, the
spherical shape of the convex members is simplified as or assumed
to be a circular arc. As shown in the coordinate system of FIG. 3,
a circle is represented by the following equation (6) where r
represents the radius of the circle:
y.sup.2-2ry+x.sup.2=0 (6)
[0056] By solving equation (6) for y, the following equation (7) is
obtained on the assumption that the projecting portion of each of
the convex members is directed in a negative direction of y:
y=x.sup.2/{r+(r.sup.2-x.sup.2).sup.1/2} (7)
[0057] As commonly used in the optical lens design field, when the
center portion of the spherical surface of the convex member is
used, r>x, and the following equation (8) is obtained as an
approximate version of equation (7):
y=x.sup.2/2r (8)
[0058] Further, the spherical surface (circular arc) can be
substituted by a quadratic curve as follows:
d.sup.2y/dx.sup.2=1/r (9)
[0059] Therefore, it is found that the secondary differential
coefficient is equal to the reciprocal of the radius. Accordingly,
the minute average radius of curvature (R) can be calculated by
calculating the secondary differential coefficient from the chart
curve measured by the surface roughness tester, and then by
calculating the average value of the reciprocal of the secondary
differential coefficient.
[0060] Further, if an area is divided into n equal sub-areas, and
if the radius of curvature of each sub-area is represented by
r.sub.i, then the minute average radius of curvature (R) can be
found by the following equation (10): 1 R = i = 1 n r i n ( 10
)
[0061] The average deviation (S) of the minute average radius of
curvature (R) shows a deviation from the average value, and thus it
is represented by the following equation (11): 2 S = i = 1 n r i -
R n ( 11 )
[0062] In the present invention, the minute average radius of
curvature (R) and the average deviation (S) are measured when the
size of the minute sub-area is set below 5 micrometers. The ratio
(S/R) of the average deviation (S) of the distribution of the
minute average radius of curvature and the minute average radius of
curvature (R) is represented by the following equation (12): 3 S /
R = i = 1 n r 1 - R nR ( 12 )
[0063] In order to enhance the brightness of the surface light
source device, it is preferable to concentrate the light emitted
from the surface light source device in the observation (viewing)
direction, and thus it is preferable to concentratedly emit the
light in one direction from the light conductor 1. According to the
present invention, in order to concentratedly emit the light in one
direction from the light conductor 1, the roughened surface
constituting at least one of the light emitting face of the light
conductor and the back surface thereof is preferably designed so
that an area having a minute average slant angle (.DELTA..theta.a)
of 20 degrees or more is located with an occupation rate (or
density) of 2% or less on the roughened surface. If the occupation
rate (or density) of such an area exceeds 2%, the degree of
concentration of the light emitted from the light conductor 1 is
lowered, and the rate of the light which is emitted in a direction
other than the observation (viewing) direction is increased even by
using a light angle varying member such as a prism sheet or the
like therewith in combination, with the result that the brightness
of the surface light source device cannot be sufficiently enhanced.
Preferably, the occupation rate (or density) of the area having the
minute average slant angle (.DELTA..theta.a) of 20 degrees or more
is set to 1% or less.
[0064] Particularly when the emission rate (.alpha.) of the light
emitted from the light conductor 1 is lowered, the rate of light
which propagates or goes and returns in the light conductor 1 while
being reflected is increased, so that the amount of the light
emitted from the light conductor 1 itself is reduced. Therefore, it
is preferable to enhance the light brightness of the surface light
source device by concentrating the light emission direction of the
emitted light to one direction. The occupation rate of the area
having the minute average slant angle (.DELTA..theta.a) of 20
degrees or more is calculated as follows. That is, the surface
roughness of the roughened surface of the light conductor 1 is
measured at a driving speed of 0.03 mm/second by the probe type
surface roughness tester to obtain a surface roughness chart. The
chart thus obtained is divided into minute areas of n portions
(n=L/xo) at a fixed minute interval (xo) to calculate the minute
average slant angle (.DELTA..theta.a) for each minute area (the
interval xo is taken as the interval between the measurement points
xa and xb) according to the following equation (13), and then a
rate of the number of the minute areas having the minute average
slant angle (.DELTA..theta.a) of 20 degrees or more with respect to
the total number of minute areas is found
.DELTA..theta.a=tan.sup.-1((f(xa)-f(xb))/xo) (13)
[0065] Further, when the surface of the light conductor 1 is
roughened by forming the fine uneven portions, the haze value
thereof is preferably set to 20% to 40%. The reason is as follows.
The surface light source device can provide a brightness having
high uniformity and a small dispersion rate (R%) by reducing the
emission rate (.alpha.) of the light emitted from the light
emitting face of the light conductor 1. However, when the emission
rate (.alpha.) is relatively small as described above, the rate of
light which goes and returns while being reflected in the light
conductor 1 is increased, and the amount of the light emitted from
the light conductor 1 is reduced. Therefore, it is preferable to
enhance the brightness of the surface light source device.
Therefore, a surface roughening treatment is performed so that the
haze value of the light conductor 1 is set to 20% to 40%, whereby
the brightness of the surface light source device can be enhanced.
If the haze value of the light conductor 1 is less than 20%, the
unevenness of the roughened surface is reduced, and the brightness
of the surface light source device cannot be sufficiently enhanced.
On the other hand, if the haze value exceeds 40%, the unevenness of
the roughened surface is disadvantageous, and spots are liable to
occur in the emitted light or the uniformity of the brightness
distribution tends to be reduced. Preferably, the haze value is set
to from 30% to 40%.
[0066] The processing method of uniformly forming a plurality of
fine convex members having substantially spherical surface on the
light conductor 1 is not limited to any specific one. For example,
there may be used a method of transferring a roughened surface by a
heat-press method, an injection molding method or the like by using
a mold or die made of metal or glass on which a roughened surface
is formed by a chemical etching method using hydrofluoric acid or
the like, a mold or die which is roughened by blasting with fine
particles such as glass beads or the like, or a mold or die on
which a roughed surface is formed by using the blasting and
chemically etching methods in combination, a method of unevenly
coating or sticking transparent materials onto the light conductor
1 by a printing method or the like, a method of directly processing
the light conductor 1 by a blasting method or an etching method, or
the like. Of these methods, the following method is preferable.
That is, fine particles such as glass beads or the like are blown
onto the surface of a glass plate to perform a blasting treatment
on the glass plate, and then the blast-treated surface of the glass
plate is subjected to chemical etching with hydrofluoric acid or
the like to form a roughened surface on the glass plate (i.e., a
mold or die having the roughened surface). By using the mold or die
thus formed, the roughened surface is transferred onto a
transparent plate by the heat-press method or the like, or
transparent resin is injected into the mold, whereby the light
conductor 1 having the roughened surface is formed.
[0067] The plural lens arrays to be formed on the surface of the
light conductor 1 are not limited to specific ones insofar as the
lens arrays are designed to have slant surfaces having an average
slant angle (.theta.a) of 0.5 to 7.5 degrees as shown in FIGS. 4
and 5. For example, a lenticular lens array having an arcuate shape
in section, a prism array having a saw-toothed shape in section, an
uneven array having a continuous wavelike shape in section or the
like are all possible. Of these arrays, the prism array (FIG. 4)
and the lenticular lens array (FIG. 5) which are symmetrical on the
right and left sides in section are particularly preferable. Such
lens arrays are formed so as to extend in parallel to the light
incident face 11 of the light conductor 1, and more preferably the
lens arrays are formed so as to be continuous to and parallel to
one another. The pitch of the lens arrays is suitably selected in
accordance with the application thereof, and normally it is
preferably set to 20 micrometers to 5 mm.
[0068] As the processing method of forming the plural lens arrays
comprising the slant surfaces having a specific average slant angle
(.theta.a) on the surface of the light conductor 1, there may be
used a method of performing a heat-press on a transparent substrate
or performing the injection molding of transparent resin by using a
mold or die made of metal or glass on which a lens pattern is
formed by a chemical etching method, a tool cutting method, a laser
processing method or the like, a method of coating a transparent
substrate with resin which can be cured by activation energy
irradiation, and then curing the resin by applying the activation
energy irradiation thereby transferring a lens pattern, a method of
directly processing the light conductor 1 by an etching method, a
tool cutting method, a laser processing method or the like.
[0069] The size of the light conductor 1 of the surface light
source device according to the present invention is not limited to
specific one. However, in order to enhance the advantageous effect
of the present invention even more, the ratio (L/t) of the length
(L) and the thickness (t) of the light conductor 1 is preferably
set to 200 or less. If L/t exceeds 200, the uniformity of the
brightness distribution within the light emitting face is not
sufficiently achieved even by reducing the average slant angle
(.theta.a) of the roughened surface or of the lens arrays of the
light conductor 1. Preferably, L/t is set to 150 or less.
Particularly when the surface light source device is used for a
liquid crystal display device, L/t is preferably set to 100 or
less, and more preferably set to 80 or less.
[0070] In the present invention, a transparent planar member of
glass or synthetic resin may be used as the light conductor 1. As
synthetic resin, there may be used various kinds of highly
transparent synthetic resins such as acrylic resin, polycarbonate
resin, vinyl chloride resin, etc. These resins may be molded into a
planar member by a normal molding method such as the extrusion
molding method, an injection molding method or the like to form a
light conductor. Particularly, methacrylate resin is excellent in
light transmission, heat resistance, dynamic characteristics and
molding and processing performance, etc., and thus it is more
suitable as the material for the light conductor. Particularly,
resin containing methyl methacrylate units of 80% by weight or more
is preferable as the methacrylate resin. Further, inorganic fine
particles such as glass beads, titanium oxide or the like, or fine
particles made of styrene resin, acrylic resin, silicone resin or
the like may be dispersed as light-diffusing material in the light
conductor 1.
[0071] In the surface light source device of the present invention,
the light source 2 such as a fluorescent lamp or the like is
disposed adjacent to one end portion (light incident face 11) of
the light conductor 1 described above, and a reflection layer 4 of
a reflection film or the like is formed on the back surface 13 of
the light conductor 1 confronting the light emitting face 12. In
order to effectively guide the light from the light source 2 to the
light conductor 1, the light source 2 and the light incident face
11 of the light conductor 1 are covered by a case or a film 5 which
is coated with a reflection agent on the inside thereof. Further,
various shapes such as a planar shape, a wedge shape as shown in
FIG. 6 (the thickness t gradually decreases along L' direction), a
shape as shown in FIG. 7 (the thickness t gradually decreases at
both end portions along L' direction toward the central portion),
etc., may be adopted for the light conductor 1.
[0072] In the surface light source device according to the present
invention, the light is normally emitted from the light conductor 1
with such a directivity that the emission direction thereof is at
an angle of 60 to 80 degrees to the normal of the light emitting
face 12. Therefore, in order to vary the emission direction of the
light to a specific direction such as in the normal direction or
the like, a light deflecting sheet or light angle varying sheet 3
is mounted on the light conductor 1. In this case, a diffusion
sheet, a lens sheet having a lens face on which a plurality of lens
units are formed in parallel on at least one surface thereof or the
like may be used as the light angle varying sheet 3. The shape of
the lenses formed on the lens sheet varies in accordance with the
particular application purpose. For example, a prism shape, a
lenticular lens shape, a wavelike shape or the like may be used.
The pitch of the lens units of the lens sheet is preferably set to
about 30 micrometers to 0.5 mm. When a prism sheet is used, the
apex angle of the prism is suitably determined on the basis of the
predetermined emission angle of the light emitted from the light
conductor 1. and generally it is preferably set to 50 to 120
degrees. Further, the direction of the lens sheet is suitably
determined on the basis of the predetermined emission angle of the
light emitted from the light conductor 1. The prism sheet may be
mounted so that the lens face is disposed at the light conductor
side or the opposite side.
[0073] In the light conductor 1 which has a roughened surface or
surface comprising the plural lens arrays having the specific
average slant angle (.theta.a), the prism sheet having the apex
angle of 50 to 75 degrees is usually mounted so that the prism face
is disposed at the light conductor side, whereby the light emitted
from the light emitting face can be directed substantially in the
normal direction with respect to the light emitting face 12.
[0074] In the surface light source device according to the present
invention, a plurality of light angle varying sheets 3 are usable
when overlaid upon each other. For example, when two lens sheets
are used, these lens sheets may be stacked so that the lens arrays
of these lens sheets intersect at an angle or are parallel to one
another. Each of the lens sheets may be disposed with the lens face
thereof laid face up or down. Further, the lens sheets may be
disposed so that the lens faces thereof are disposed at opposite
sides. In this case, it is preferable that the first lens sheet
adjacent to the light conductor 1 be disposed so that the lens face
thereof is located at the light conductor side and the lens arrays
31 are disposed parallel to the light source (see FIG. 1) while the
second lens sheet is disposed so that the lens face thereof is
located at the opposite side to the light conductor and the lens
arrays thereof are perpendicular to the lens arrays of the first
lens sheet. When the prism sheet is used as the lens sheet, it is
further preferable that the apex angle of the first prism sheet be
set to 50 to 75 degrees, and the apex angle of the second prism
sheet be set to 80 to 100 degrees.
[0075] Furthermore, according to the surface light source device of
the present invention, the lens sheet is preferably formed of
material having a high transmission to visible radiation and a
relatively high refractive index. For example, acrylic resin,
polycarbonate resin, vinyl chloride resin, activation energy
curable resin or the like may be used. Of these materials, the
activation energy curable resin is preferably used from the
viewpoint of abrasion resistance, ease of handling, productivity,
etc. Additive agents may be added to the lens sheet, such as
antioxidants, ultraviolet ray absorbent, yellowing preventing
agent, blueing agent, pigment, dispersing agent or the like. An
extrusion molding method, an injection molding method or any other
normal molding method may be used to manufacture the lens sheet.
When the lens sheet 3 is manufactured by using activation energy
curable resin, a lens portion made of the activation energy curable
resin is formed on a transparent substrate such as a transparent
film or sheet which is formed of transparent resin such as
polyester resin, acrylic resin, polycarbonate resin, vinyl chloride
resin, polymethacryl imide resin, polyolefine resin or the like.
First, activation energy curable resin liquid is injected into a
lens mold on which a predetermined lens pattern is formed, and then
it is overlaid on the transparent substrate. Subsequently,
activation energy such as ultraviolet rays, electron beams or the
like are irradiated through the transparent substrate to the
activation energy curable resin liquid to polymerize and cure the
resin liquid, and the cured resin is exfoliated from the lens mold
to form a lens sheet.
[0076] According to the surface light source device of the present
invention, together with the lens sheet as described above, there
may be used a diffusion sheet, a color filter, a polarizing
membrane, or various optical elements which can optically deflect,
converge or diffuse the light or vary the optical characteristics
thereof. Further, a general linear pipe type fluorescent lamp may
be used as the light source 2. When it is difficult to exchange or
replace the light source 2, a light line comprising plural optical
fibers may be used to guide light from another light source which
is separately disposed.
[0077] If a liquid crystal display element 7 is mounted at the
light emitting face side of the surface light source device thus
constructed as shown in FIG. 8, it can be used as a liquid crystal
display device for a portable personal computer, a liquid crystal
television or the like. In such a liquid crystal display device,
very high uniformity is required for the brightness distribution,
and it is required to reduce the dispersion rate (R%) to 30% or
less, preferably to 25% or less, and more preferably to 20% or
less.
[0078] Further, instead of mounting the liquid crystal display
element 7, by mounting a signboard on which characters, figures,
photographs or the like are formed on a semi-transparent plastic
plate comprised of methacryl plate or the like by cutting, printing
or the like, it may be used as a sign display apparatus such as a
guide signboard, a large-scale signboard or the like in a station,
public facilities or the like. In such a sign display apparatus, it
is required to reduce the dispersion rate (R%) to 250% or less,
preferably to 200% or less.
[0079] Further, instead of mounting the liquid crystal display
element 7, by mounting a signboard on which a traffic guide, a
traffic sign or the like is formed on a plastic plate comprising a
methacryl plate or the like by cutting, printing or the like, it
may be used as a traffic sign display apparatus for various guide
signs, traffic signs, etc., in a highway road or a general road. In
such a traffic sign display apparatus, it is required to reduce the
dispersion rate (R%) to 450% or less, preferably to 300% or
less.
[0080] Next, the present invention will be described in more detail
with the following Examples and Comparative Examples, wherein each
physical property and characteristic were measured as follows.
[0081] Emission Rate (.alpha.)
[0082] The brightness was measured at every interval of 20 mm
increments from the light incident face end of the light conductor,
and the gradient (K(mm.sup.-1)) of the logarithmic graph which
shows the relationship between the ratio (L'/t) of the distance
(L') from the light incident face end to the thickness (t) of the
light conductor 1 and the brightness was calculated to calculate
the emission rate (.alpha.) from equation (2).
[0083] Dispersion Rate (R%)
[0084] The light brightness on the light emitting face of the light
conductor 1 was measured on a substantially central area of the
light conductor 1, the area extending from a point of 5 mm interval
far away from the light incident face end by 20 mm increments to
the end portion confronting the light incident face end of the
light conductor 1. The central area is positioned at substantially
central portion relative to a direction parallel to the light
incident face. The maximum value (Imax) of the measured brightness,
the minimum value (Imin) of the measured brightness, and the
average value (Iav) of the measured brightness were calculated.
Thereafter, the dispersion rate (R%) was calculated from equation
(3).
[0085] Average Slant Angle (.theta.a)
[0086] The average slant angle was measured according to
ISO4287/1-1987. The surface roughness of the roughened surface was
measured at a driving speed of 0.03 mm/second by a probe type
surface roughness tester (SURFCOM 570A produced by Tokyo Seiki Co.,
Ltd.) using an E-DT-SO4A (1 micrometer R, 55.degree. circular cone,
diamond) as a probe. A chart was obtained, and a slant correction
was performed by subtracting the average line. The average slant
angle was calculated from equations (4) and (5).
[0087] Measurement of Angular Distribution of Light Emitted from
Light Conductor
[0088] A cold cathode tube was connected to a DC power source
through an inverter (CXA-MIOL produced by TDK), and a potential of
DC 12V was applied to the cathode tube to turn on the cathode tube.
The light conductor was mounted on a measuring table so as to be
rotatable at the center portion thereof around the rotational shaft
parallel to the axis of the cathode tube. Subsequently, a black
sheet having a pinhole of 3 mm in diameter was fixed onto the light
conductor so that the pinhole was disposed at the center of the
light conductor, and a luminance meter (nt-1.degree. produced by
Minolta) was suitably disposed while adjusting the distance between
the luminance meter and the light conductor so that the measurement
circle was set to 8 to 9 mm in diameter. After waiting for aging of
the cold cathode tube over a 30 minute period, the rotational shaft
was rotated from +85 degrees to -85 degrees every 1 degree to
measure the brightness of the emitted light by the luminance
meter.
[0089] On the basis of the measurement results, the angle (a) of
the direction of the peak light having a maximum light intensity
with respect to the normal, the intersection angle (b) between the
direction of the peak light having the maximum light intensity and
the direction of the light having half (50%) of the maximum light
intensity, and the intersection angle (c) between the direction of
the peak light having the maximum light intensity and the direction
of the light having 10% of the maximum light intensity were
measured as shown in FIG. 9.
[0090] Measurement of Brightness in Normal Direction (Normal
Brightness) of Surface Light Source Device (Compact-Size Surface
Light Source Device)
[0091] A cold cathode tube was connected to a DC power source
through an inverter (CXA-M10L produced by TDK), and a potential of
DC 12V was applied to the cathode tube to turn on the cathode tube.
The surface light source device was mounted on a measuring table so
as to be rotatable at the center portion thereof around the
rotational shaft parallel to the axis of the cathode tube.
Subsequently, a black sheet having a pinhole of 3 mm in diameter
was fixed onto the light conductor so that the pinhole was disposed
at the center of the light conductor, and a luminance meter
(nt-1.degree. produced by Minolta) was suitably disposed while
adjusting the distance between the luminance meter and the surface
light source device so that the measurement circle was set to 8 to
9 mm in diameter. After waiting for aging of the cold cathode tube
over a 30 minute period, the rotational shaft was set to 0 degrees,
and the brightness of the emitted light was measured by the
luminance meter. The measurement was conducted on the surface light
source device, except for an area within 5 mm from the edge of the
light conductor confronting the cold cathode tube. The area to be
measured was sectioned into square sub-areas of 20 mm.times.20 mm
in area, and the brightness was measured at the center of each
square sub-area. Thereafter, the respective measurement values were
averaged to obtain the normal brightness.
[0092] Measurement of Normal Brightness of Surface Light Source
Device (Large-Size Surface Light Source Device)
[0093] Except that a fluorescent lamp of 30 W was used as the light
source, the same measurement method as for the compact-size surface
light source device was used.
[0094] Occupation Rate of Area Having a Minute Average Slant Angle
(.DELTA..theta.a) above 20 Degrees
[0095] The surface roughness of the roughened surface was measured
in the same manner as the average slant angle (.theta.a). The chart
thus obtained was divided into n minute areas at 1 mm intervals,
and the minute average slant angle (.DELTA..theta.a) in each minute
area was calculated from equation (13). On the basis of this
calculation result, the occupation rate (or density) of the minute
areas in which the minute average slant angle (.DELTA..theta.a) was
above 20 degrees was found as the ratio of the number of the minute
areas of .DELTA..theta.a of above 20 degrees to the total number of
the minutes areas.
[0096] Measurement of Surface Roughness
[0097] The measurement of the surface roughness was performed at a
driving speed of 0.03 mm/second by a probe type surface roughness
tester (SURFCOM 570A produced by Tokyo Seiki Co., Ltd.) using a
1-micrometer R, 55.degree. circular cone diamond needle (010-2528)
as a probe. The measurement values (surface roughness) were
recorded at an interval of 5 micrometers. Furthermore, the primary
differential coefficient (Ki) and the secondary differential
coefficient (Li) were calculated on the basis of the measurement
values (Di) according to equations (14) and (15):
Ki=(D.sub.i+1-Di)/5 (14)
Li=(K.sub.i+1-Ki)/5 (15)
[0098] Average Period (S)
[0099] The primary differential coefficient (Ki) on a linear area
of 1000 micrometers in any direction of the light conductor was
measured at an interval of 5 micrometers. The primary differential
coefficients (Ki) thus obtained were successively linked to one
another, and from the following equation (16), the average period
(S) was calculated on the basis of the frequency m at which the
link thus obtained traversed the "0" level.
S=(1000.times.2)/m (16)
[0100] Minute Average Radius of Curvature (R)
[0101] The absolute value of the reciprocal of the secondary
differential coefficient (Li) which was obtained by the probe type
surface roughness tester was calculated, and values thus obtained,
except for values which are less than 10.sup.-6, were averaged. The
average value thus calculated was set as the minute average radius
of curvature (R).
[0102] Average Deviation (S) of Distribution of Minute Average
Radius of Curvature
[0103] From equation (11), the average deviation (S) was calculated
from the radius of curvature (ri) and the minute average radius of
curvature (R) which were obtained at an interval of 5 micrometers
on the linear area of 1000 micrometers in any direction of the
light conductor 1.
EXAMPLE 1
[0104] The surface of a glass plate was subjected to a blast
treatment using glass beads of 125 to 149 micrometers in particle
size (FGB-120 produced by Fuji Manufacturing Works Co., Ltd.) under
the condition that the distance between the glass plate and a blast
nozzle was set to 10 cm and the blast pressure was set to 4
Kg/cm.sup.2. Thereafter, a hydrofluoric acid treatment was
conducted to chemically etch the blast surface of the glass plate,
and a replica mold or die was obtained by conducting an
electroforming method on the glass plate. By performing a
heat-press using the mold, the roughened surface thereof was
transferred onto one surface of a transparent acrylic resin plate
of 4 mm in thickness and 165 mm.times.210 mm in area to form a
light conductor.
[0105] The roughened surface of the light conductor thus
constructed had a structure such that fine convex members having
substantially spherical surface were uniformly distributed. The
average slant angle (.theta.a) and the occupation rate of the areas
having the minute average slant angle (.DELTA..theta.a) of 20
degrees or more of the roughened surface were measured. The results
are shown in Table 1. The roughened surface of the light conductor
thus obtained was measured by the probe type surface roughness
tester to obtain the roughened surface chart shown in FIG. 10. The
primary differential coefficient and the secondary differential
coefficient were calculated from the chart, and the calculation
result is also shown in FIG. 10. Table 1 shows the structure
parameters for the light conductor surface. Further, the angular
distribution of the light emitted from the light conductor was
measured to obtain the angle (peak angle) of the peak light having
a maximum light intensity to the normal, the intersecting angle
(peak 50% angle) between the direction of the peak light having the
maximum light intensity and the direction of the light having 50%
of the maximum light intensity, and the intersecting angle (peak
10% angle) between the direction of the light having the maximum
light intensity and the direction of the light having 10% of the
maximum light intensity. The results are shown in Table 1.
[0106] A PET film on which silver was deposited was adhesively
attached to each of one end surface of 210 mm and two end surfaces
of 165 mm of the light conductor thus obtained, and further a PET
film on which silver was deposited was fixed to the back surface
confronting the light emitting face by an adhesive tape to form
reflection surface. A linear pipe type fluorescent lamp (KC230T4E
(4 mm in diameter.times.230 mm in length) produced by Matsushita
Electric Co., Ltd.) was disposed at the remaining end surface of
210 mm of the light conductor. Subsequently, a prism sheet
including a plurality of parallel prism arrays each having an apex
angle of 63 degrees and a pitch of 50 micrometers (which was formed
by ultraviolet ray curable acrylic resin having a refractive index
of 1.53 on the PET film) was disposed on the light emitting face of
the light conductor so that the prism face confronted the light
emitting face side of the light conductor, thereby fabricating a
surface light source device. The normal brightness of the surface
light source device thus fabricated was measured, and the
measurement results are shown in Table 1.
[0107] A light conductor was formed with a transparent acrylic
resin plate of 3 mm in thickness and 90 mm.times.300 mm in area in
the same manner as described above. A PET film on which silver was
deposited was adhesively attached to each of the two 300 mm end
surfaces of the light conductor thus obtained, and a PET film on
which silver was deposited was fixed to the back surface
confronting the light emitting face by an adhesive tape to form
reflection surface. A linear pipe type fluorescent lamp
(KC130T4E--4 mm in diameter.times.130 mm in length--produced by
Matsushita Electric Co., Ltd.) was disposed at one end surface of
90 mm of the light conductor. The emission rate and the dispersion
rate (R%) of the light conductor thus obtained were measured, and
the results are shown in Table 1.
EXAMPLE 2
[0108] The surface of a mirror-polished stainless steel plate was
subjected to the blast treatment using glass beads of 125 to 149
micrometers in particle size (FGB-120 produced by Fuji
Manufacturing Works Go., Ltd.) under the condition that the
distance between the stainless steel plate and a blast nozzle was
set to 10 cm and the blast pressure was set to 4 Kg/cm.sup.2. By
performing a heat-press using this stainless steel plate mold or
die, the roughened surface was transferred onto one surface of a
transparent acrylic resin plate of 3 mm in thickness and 165
mm.times.210 mm in area to form a light conductor.
[0109] The roughened surface of the light conductor thus
constructed had a structure such that the fine convex members
having substantially spherical surface were uniformly distributed.
The average slant angle (.theta.a) and the occupation rate of the
areas having the minute average slant angle (.DELTA..theta.a) of 20
degrees or more of the roughened surface were measured. The results
are shown in Table 1. The roughened surface of the light conductor
thus obtained was measured by the probe type surface roughness
tester, and the structure parameters of the surface of the light
conductor are shown in Table 1. Further, the angular distribution
of the light emitted from the light conductor was measured to
obtain the angle (peak angle) of the peak light having a maximum
light intensity to the normal, the intersecting angle (peak 50%
angle) between the direction of the peak light having the maximum
light intensity and the direction of the light having 50% of the
maximum light intensity, and the intersecting angle (peak 10%
angle) between the direction of the light having the maximum light
intensity and the direction of the light having 10% of the maximum
light intensity. The results are shown in Table 1.
[0110] A surface light source device was fabricated with the light
conductor thus obtained in the same manner as in Example 1. The
normal brightness of the surface light source device thus obtained
was measured, and the measurement results are shown in Table 1.
Further, by using the surface light source device which was
constructed with the above light conductor in the same manner as
Example 1, the emission rate and the dispersion rate (R%) of the
light conductor were measured, and the results are shown in Table
1.
EXAMPLE 3
[0111] By using the stainless steel plate mold used in Example 2,
the roughened surface was transferred onto one surface of a
transparent acrylic resin plate of 4 mm in thickness and 165
mm.times.210 mm in area by a thermal transfer method to obtain a
light conductor. The light conductor thus obtained had the same
structure, physical properties and characteristics as that of
Example 2. A surface light source device was fabricated with the
light conductor thus obtained in the same manner as the Example 1.
The normal brightness of the surface light source device thus
obtained was measured, and the measurement results are shown in
Table 1. By using the surface light source device which was
constructed with the above light conductor in the same manner as
Example 1, the emission rate and the dispersion rate (R%) of the
light conductor were measured, and the results are shown in Table
1.
EXAMPLE 4
[0112] A light conductor was obtained in the same manner as in
Example 1, except that a wedge-shaped plate having a thickness of 3
mm at one 210 mm end and a thickness of 1 mm at another 210 mm end
was used as the transparent acrylic resin plate. The light
conductor thus obtained has the same structure, physical properties
and characteristics as Example 1. Further, a surface light source
device was fabricated with the light conductor thus obtained in the
same manner as Example I except that the linear pipe type
fluorescent lamp was disposed at the end surface side having the
thickness of 3 mm of the light conductor. The the normal brightness
of the surface light source device thus fabricated and the emission
rate (.alpha.) and dispersion rate (R%) of the light conductor
thereof were measured, and the results are shown in Table 1.
COMPARATIVE EXAMPLE 1
[0113] A light conductor was obtained in the same manner as Example
2 except that glass beads of 74 to 88 micrometers in particle size
(FGB-200 produced by Fuji Manufacturing Works, Co., Ltd.) were used
for the blast treatment. The average slant angle (.theta.a) and the
occupation rate of areas having the minute average slant angle
(.DELTA..theta.) of 20 degrees or more of the light conductor thus
obtained were measured, and the results are shown in Table 1. The
roughened surface of the light conductor thus obtained was measured
by the probe type surface roughness tester to obtain the roughened
surface chart shown in FIG. 11. The primary differential
coefficient and the secondary differential coefficient were
calculated from the chart, and the calculation result is also shown
in FIG. 11. Table 1 shows the structure parameters of the light
conductor surface. Further, the angular distribution of the light
emitted from the light conductor was measured to obtain the angle
(peak angle) of the peak light having a maximum light intensity
with respect to the normal, the intersecting angle (peak 50% angle)
between the direction of the peak light having the maximum light
intensity and the direction of the light having 50% of the maximum
light intensity, and the intersecting angle (peak 10% angle)
between the direction of the light having the maximum light
intensity and the direction of the light having 10% of the maximum
light intensity. The results are shown in Table 1.
[0114] Like Example 1, a surface light source device was fabricated
with the light conductor thus obtained. The normal brightness of
the surface light source device thus obtained was measured, and the
results are shown in Table 1. Further, by using the surface light
source device which was constructed with the above light conductor
in the manner as Example 1, the emission rate and the dispersion
rate (R%) of the light conductor were measured, and the measurement
results are shown in Table 1.
COMPARATIVE EXAMPLE 2
[0115] By using the stainless steel plate mold used in Comparative
Example 1, the roughened surface was transferred onto one surface
of a transparent acrylic resin plate of 4 mm in thickness and 165
mm.times.210 mm in area by the thermal transfer method so as to
form a light conductor. The light conductor thus obtained had the
same structure, physical properties and characteristics as Example
2. A surface light source device was fabricated with the light
conductor in the same manner as Example 1. The normal brightness of
the surface light source device thus obtained was measured, and the
measurement results are shown in Table 1. Further, by using the
surface light source device which was constructed with the above
light conductor in the same manner as Example 1, the emission rate
and the dispersion rate (R%) of the light conductor were measured,
and the results are shown in Table 1.
COMPARATIVE EXAMPLE 3
[0116] A light conductor was obtained in the same manner as Example
2 except that glass beads of 53 to 62 micrometers in particle size
(FGB-300 produced by Fuji Manufacturing Works, Co., Ltd.) were used
for the blast treatment and the blast pressure was set to 5
Kg/cm.sup.2. The average slant angle (.theta.a) and the occupation
rate of areas having a minute average slant angle (.DELTA..theta.a)
of 20 degrees or more of the light conductor thus obtained were
measured, and the results are shown in Table 1. Further, the
roughened surface of the light conductor thus obtained was measured
by the probe type surface roughness tester, and the structure
parameters of the light conductor surface were as shown in Table 1.
Further, the angular distribution of the light emitted from the
light conductor was measured to obtain the angle (peak angle) of
the peak light having the maximum light intensity to the normal,
the intersecting angle (peak 50% angle) between the direction of
the peak light having the maximum light intensity and the direction
of the light having 50% of the maximum light intensity, and the
intersecting angle (peak 10% angle) between the direction of the
light having the maximum light intensity and the direction of the
light having 10% of the maximum light intensity. The results are
shown in Table 1.
[0117] The normal brightness of the surface light source device
thus obtained was measured, and the measurement results are shown
in Table 1. Further, by using the surface light source device which
was constructed with the above light conductor in the same manner
as Example 1, the emission rate and the dispersion rate (R%) of the
light conductor were measured, and the results are shown in Table
1.
EXAMPLE 5
[0118] A prism pattern comprising a plurality of prism arrays of
172 degrees in apex angle and 50 micrometers in pitch which were
formed continuously in parallel to one another corresponding to the
shape shown in FIG. 4, was formed on a brass plate by using a
diamond cutter to form a die or mold. By using the die thus
obtained, a prism face was transferred onto one surface of a
transparent acrylic resin plate of 4 mm.times.210 mm.times.165 mm
in size by the thermal transfer method to form a light conductor.
The average slant angle (.theta.a) of the light conductor thus
obtained was equal to 4.2 degrees. A silver-deposited PET film was
adhesively attached to each of the two 165 mm end surfaces of the
light conductor and one of the other two end surfaces of the light
conductor, and a silver-deposited PET film was fixed to the back
surface confronting the light emitting face having the prism face
by an adhesive tape to form reflection surface. Further, a cold
cathode tube (KC230T4E--4 mm in diameter.times.230 mm in length,
produced by Matsushita Electric Co., Ltd.) used as a light source
was disposed on the remaining end surface of the light conductor by
covering with a silver-deposited PET film. A plurality of parallel
prism arrays were formed of ultraviolet ray curable acrylic resin
having a refractive index of 1.53 on a PET film so as to have an
apex angle of 63 degrees and a pitch of 50 micrometers to form a
prism sheet. The prism sheet thus formed was disposed on the light
emitting face of the light conductor so that the prism face
confronted the light emitting face side of the light conductor,
thereby fabricating a surface light source device. The normal
brightness of the surface light source device thus fabricated was
measured, and the measurement results are shown in Table 2.
[0119] Further, a light conductor was formed in the same manner as
described above by using a transparent acrylic resin plate of 3
mm.times.90 mm.times.300 mm in size. The surface light source
device was fabricated in the same manner as described above, except
that a silver-deposited PET film was adhesively attached to each of
two 300 mm end surfaces of the light conductor thus constructed.
The emission rate and the dispersion rate (R%) of the light
conductor of the surface light source device were measured, and the
measurement results are shown in Table 2.
EXAMPLE 6
[0120] A lens pattern comprising a plurality of lenticular lens
arrays of 50 micrometers in pitch which were formed continuously in
parallel to one another corresponding to the shape shown in FIG. 5,
was formed on a brass plate by using a diamond cutter to form a
die. By using the die thus obtained, a lenticular lens face was
transferred onto one surface of a transparent acrylic resin plate
of 4 mm.times.210 mm.times.165 mm in size by the thermal transfer
method to form a light conductor. The average slant angle
(.theta.a) of the light conductor thus obtained was equal to 4.3
degrees. A surface light source device was fabricated with the
light conductor thus obtained in the same manner as in Example 5.
The normal brightness of the surface light source device thus
obtained was measured, and the measurement results are shown in
Table 2. Further, by using the surface light source device which
was constructed with the above light conductor in the same manner
as Example 5, the emission rate and the dispersion rate (R%) of the
light conductor of the surface light source device were measured,
and the measurement results are shown in Table 2.
COMPARATIVE EXAMPLE 4
[0121] A prism pattern comprising a plurality of prism arrays of
164 degrees in apex angle and 50 micrometers in pitch which were
formed continuously in parallel to one another corresponding to the
shape shown in FIG. 12, was formed on a brass plate by using a
diamond cutter to form a die. By using the die thus obtained, a
prism face was transferred onto one surface of a transparent
acrylic resin plate of 4 mm.times.210 mm.times.165 mm in size by
the thermal transfer method to form a light conductor. The average
slant angle (.theta.a) of the light conductor thus obtained was
equal to 8.2 degrees. A surface light source device was fabricated
with the light conductor thus obtained in the same manner as in
Example 5. The normal brightness of the surface light source device
thus obtained was measured, and the measurement results are shown
in Table 2. Further, by using the surface light source device which
was constructed with the above light conductor in the same manner
as Example 5, the emission rate and the dispersion rate (R%) of the
light conductor the surface light source device were measured, and
the measurement results are shown in Table 2.
COMPARATIVE EXAMPLE 5
[0122] A lens pattern comprising a plurality of lenticular lens
arrays of 50 micrometers in pitch which were formed continuously in
parallel to one another corresponding to the shape shown in FIG.
13, was formed on a brass plate by using a diamond cutter to form a
die. By using the die thus obtained, a lenticular lens face was
transferred onto one surface of a transparent acrylic resin plate
of 4 mm.times.210 mm.times.165 mm in size by the thermal transfer
method to form a light conductor. The average slant angle
(.theta.a) of the light conductor thus obtained was equal to 8.3
degrees. A surface light source device was fabricated with the
light conductor thus obtained in the same manner as in Example 5.
The normal brightness of the surface light source device thus
obtained was measured, and the measurement results are shown in
Table 2. Further, by using the surface light source device which
was constructed with the above light conductor in the same manner
as Example 5, the emission rate and the dispersion rate (R%) of the
light conductor of the surface light source device were measured,
and the measurement results are shown in Table 2.
[0123] As a comparative test, the surface light source devices
obtained in Examples 1 to 6 and Comparative Examples 1 to 5 were
used as back lights for a liquid crystal display device. In the
case of the surface light source devices of Examples 1 to 6, a very
light and uniform liquid crystal display image frame was obtained.
On the other hand, in case of the surface light source devices of
Comparative Examples 1 to 5, a relatively light image frame was
observed in the neighborhood of the light source. However, the
reduction in lightness became disadvantageously more noticeable
farther away from the light source.
EXAMPLE 7
[0124] In the same manner as Example 1, the roughened surface was
transferred onto one surface of a transparent acrylic resin plate
of 10 mm thick and 600 mm.times.1250 mm in area by the thermal
transfer method to form a light conductor. The average slant angle
(.theta.a) and the occupation rate of the areas having a minute
average slant angle (.DELTA..theta.a) of 20 degrees or more of the
light conductor thus formed were measured, and the measurement
results are shown in Table 3. The structure and the characteristic
of the light emitted from the light conductor were the same as in
Example 1. A silver-deposited PET film was adhesively attached to
one 600 mm end surface of the light conductor and to two 1250 mm
end surfaces of the light conductor, and a silver-deposited PET
film was fixed to the back surface confronting the roughened light
emitting face by an adhesive tape to form reflection surface.
Further, a 30 W fluorescent lamp (FSL30T6W, produced by Matsushita
Electric Co., Ltd.) was disposed on the remaining 600 mm end
surface of the light conductor. A plurality of parallel prism
arrays were formed of ultraviolet ray curable acrylic resin having
a refractive index of 1.53 on a PET film so as to have an apex
angle of 63 degrees and a pitch of 50 micrometers, thereby forming
a prism sheet. The prism sheet thus formed was disposed on the
light emitting face of the light conductor so that the prism face
confronted the light emitting face side of the light conductor,
thereby fabricating a surface light source device. The normal
brightness of the surface light source device thus fabricated was
measured, and the measurement results are shown in Table 3.
[0125] Further, a light conductor was formed in the same manner as
described above by using a transparent acrylic resin plate of 10 mm
thick and 600 mm.times.1250 mm in area. A silver-deposited PET film
was adhesively attached to each of two 1250 mm end surfaces of the
light conductor, and a silver-deposited PET film was fixed to the
back surface confronting the roughened light emitting face by an
adhesive tape to form reflection surface. Further, a 30 W
fluorescent lamp (FSL30T6W, produced by Matsushita Electric Co.,
Ltd.) was disposed on one 600 mm end surface of the light
conductor. By using the surface light source device thus
fabricated, the emission rate and the dispersion rate (R%) of the
light conductor were measured, and the measurement results are
shown in Table 3.
COMPARATIVE EXAMPLE 6
[0126] In the same manner as Comparative Example 1, the roughened
surface was transferred onto one surface of a transparent acrylic
resin plate of 10 mm in thickness and 600 mm .times.1250 mm in area
by the thermal transfer method to form a light conductor. The
average slant angle (.theta.a) and the occupation rate of the areas
having a minute average slant angle (.DELTA..theta.a) of 20 degrees
or more of the light conductor were measured, and the measurement
results are shown in Table 3. The structure of the roughened
surface of the light conductor and the characteristic of the light
emitted therefrom were the same as in Comparative Example 1. A
surface light source device was fabricated with this light
conductor in the same manner as in Example 7. The normal brightness
of the surface light source device thus fabricated was measured,
and the measurement results are shown in Table 3. By using the
surface light source device which was constructed with the above
light conductor in the same manner as Example 7, the emission rate
and the dispersion rate (R%)of the light conductor were measured,
and the measurement results are shown in Table 3.
EXAMPLE 8
[0127] In the same manner as Example 5, the prism face was
transferred onto one surface of a transparent acrylic resin plate
of 10 mm.times.600 mm.times.1250 mm in size by the thermal transfer
method to form a light conductor. The average slant angle
(.theta.a) of the light conductor thus obtained was equal to 4.2
degrees. A silver-deposited PET film was adhesively attached to
each of two 1250 mm end surfaces of the light conductor and to one
of the other 600 mm end surfaces of the light conductor, and a
silver-deposited PET film was fixed to the back surface confronting
the light emitting face having the prism face by an adhesive tape
to form reflection surface. Further, a 30 W fluorescent lamp
(FSL30T6 produced by Matsushita Electric Co., Ltd.) was disposed on
the remaining end surface of the light conductor and was wrapped by
a silver-deposited PET film.
[0128] A plurality of parallel prism arrays were formed of
ultraviolet ray curable acrylic resin having a refractive index of
1.53 on a PET film so as to have an apex angle of 63 degrees and a
pitch of 50 micrometers, thereby forming a prism sheet. The prism
sheet thus formed was disposed on the light emitting face of the
light conductor so that the prism face confronted the light
emitting face side of the light conductor, thereby fabricating a
surface light source device. The normal brightness of the surface
light source device thus fabricated was measured, and the
measurement results are shown in Table 4.
[0129] Further, a light conductor was formed in the same manner as
described above by using a transparent acrylic resin plate of 10
mm.times.600 mm.times.1250 mm in size. The surface light source
device was fabricated in the same manner as described above, except
that a silver-deposited PET film was adhesively attached to each of
two 1250 mm end surfaces of the light conductor. By using the
surface light source device thus fabricated, the emission rate and
the dispersion rate (R%) of the light conductor were measured, and
the measurement results are shown in Table 4.
EXAMPLE 9
[0130] In the same manner as Example 6, the lenticular lens face
was transferred onto one surface of a transparent acrylic resin
plate of 10 mm.times.600 mm.times.1250 mm in size by the thermal
transfer method to form a light conductor. The average slant angle
(.theta.a) of the light conductor thus obtained was equal to 4.3
degrees. A surface light source device was fabricated with the
light conductor thus obtained in the same manner as in Example 8.
The normal brightness of the surface light source device thus
fabricated was measured, and the measurement results are shown in
Table 4. Further, the surface light source device was formed with
the above light conductor in the same manner as in Example 8, and
the emission rate and the dispersion rate (R%) of the light
conductor of the surface light source device thus obtained were
measured, and the measurement results are shown in Table 4.
COMPARATIVE EXAMPLE 7
[0131] In the same manner as Comparative Example 4, the prism face
was transferred onto one surface of a transparent acrylic resin
plate of 10 mm.times.600 mm.times.1250 mm in size by the thermal
transfer method to form a light conductor. The average slant angle
(.theta.a) of the light conductor thus obtained was equal to 8.2
degrees. A surface light source device was fabricated with the
light conductor thus obtained in the same manner as Example 8. The
normal brightness of the surface light source device thus
fabricated was measured, and the measurement results are shown in
Table 4. Further, the surface light source device was formed with
the above light conductor in the same manner as Example 8, and the
emission rate and the dispersion rate (R%) of the light conductor
of the surface light source device thus obtained were measured, and
the measurement results are shown in Table 4.
COMPARATIVE EXAMPLE 8
[0132] In the same manner as Comparative Example 5, the prism face
was transferred onto one surface of a transparent acrylic resin
plate of 10 mm.times.600 mm.times.1250 mm in size by the thermal
transfer method to form a light conductor. The average slant angle
(.theta.a) of the light conductor thus obtained was equal to 8.3
degrees. A surface light source device was fabricated with the
light conductor thus obtained in the same manner as in Example 8.
The normal brightness of the surface light source device thus
fabricated was measured, and the measurement results are shown in
Table 4. Further, the surface light source device was formed with
the above light conductor in the same manner as in Example 8, and
the emission rate and the dispersion rate (R%) of the light
conductor of the surface light source device thus obtained were
measured, and the measurement results are shown in Table 4.
[0133] A semi-transparent acrylic plate on which a photograph was
printed and alternately a semi-transparent acrylic plate on which a
traffic sign was printed were disposed on the surface light source
device obtained in Examples 7 to 9 and Comparative Examples 6 to 8
to fabricate a large-size signboard or traffic sign apparatus. In
the signboards and the traffic sign apparatus which were formed by
using the surface light source devices of Examples 7 to 9 of the
present invention, the image was very light and uniform over the
whole frame. On the other hand, in the signboards and the traffic
sign apparatus which were formed by using the surface light source
devices of Comparative Examples 6 to 8, the image was relatively
light in the neighborhood of the light source. However, the
lightness of the image was quite reduced farther from the light
source, and the image was very dark in the neighborhood of the tip
end portion of the surface light source device.
INDUSTRIAL APPLICABILITY
[0134] According to the present invention, the roughened surface
which comprises a plurality of substantially spherical fine convex
members having an average slant angle (.theta.a) of 0.5 to 7.5
degrees, or a plurality of lens arrays having slant surfaces whose
average slant angle (.theta.a) is equal to 0.5 to 7.5 degrees, is
formed on at least one of the light emitting face of the light
conductor or the back surface of the light conductor which
confronts the light emitting face. Therefore, the surface light
source device of the present invention can provide light having
high brightness and a uniform brightness distribution within the
light emitting face without performing any uniformity treatment
using a spot pattern or the like. Therefore, the surface light
source device according to the present invention can be suitably
applied to a liquid crystal display device for a portable personal
computer, a liquid crystal television or the like, or to a display
apparatus such as a guide marking board or a large-size signboard
in a station or other public facilities, and to a guidepost or
traffic sign on a highway road or a general road.
1 TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Com. Ex. 1 Com. Ex. 2 Com. Ex. 3
L/t 41.3 55 41.3 -- 55 41.3 55 AVERAGE SLANT ANGLE 2.7 2.9 2.9 2.7
8.4 8.4 21.8 (.theta.a) (Degree) RATE OF AREA HAVING 0 0.5 0.5 0 3
3 4 .DELTA..theta.a ABOVE 20.degree. (%) DISPERSION RATE 14 19 18
18 163 121 850 (R %) (%) EMISSION RATE (.alpha.) 1.27 1.73 1.73
2.50 4.67 4.67 8.46 (%) NORMAL BRIGHTNESS 2424 2074 1991 2450 2324
2291 2060 (cd/cm.sup.2) MINUTE AVERAGE 206.6 255.8 255.8 206.6 69.0
69.0 49.8 RADIUS OF CURVATURE (R) (.mu.m) AVERAGE DEVIATION 135.0
196.7 196.7 135.0 61.1 61.1 44.9 (S) (.mu.m) S/R 0.657 0.769 0.769
0.657 0.886 0.886 0.902 AVERAGE PERIOD (P) 35.1 48.8 48.8 35.1 28.6
28.6 37.0 (.mu.m) R/P 5.86 5.20 5.20 5.86 2.41 2.41 1.35 PEAK ANGLE
(Degree) 71 70 70 71 63 63 67 ANGULAR DIFFERENCE 15 16 16 15 26 26
23 OF 50% LIGHT INTENSITY (Degree) ANGULAR DIFFERENCE 32 47 47 32
51 51 62 OF 10% LIGHT INTENSITY (Degree)
[0135]
2 TABLE 2 Ex.5 Ex.6 Com.Ex.4 Com.Ex.5 L/t 41.3 41.3 41.3 41.3
AVERAGE SLANT ANGLE 4.2 4.3 8.2 8.3 (.theta.a) (Degree) DISPERSION
RATE 17 18 110 115 (R%) (%) EMISSION RATE (.alpha.) 1.61 1.61 4.15
4.15 (%) NORMAL BRIGHTNESS 2303 2327 2176 2240 (cd/cm.sup.2)
[0136]
3 TABLE 3 Ex. 7 Com. Ex. 6 L/t 125 125 AVERAGE SLANT ANGLE 2.7 8.4
(.theta.a) (Degree) RATE OF AREA HAVING 0 3 .DELTA..theta.a ABOVE
20.degree. (%) DISPERSION RATE 180 650 (R%) (%) EMISSION RATE
(.alpha.) 3.40 9.10 (%) NORMAL BRIGHTNESS 397 345 (cd/cm.sup.2)
MINUTE AVERAGE 206.6 69.0 RADIUS OF CURVATURE (R) (.mu.m) AVERAGE
DEVIATION 135.0 61.1 (S) (.mu.m) S/R 0.657 0.886 AVERAGE PERIOD (P)
35.1 28.6 (.mu.m) R/P 5.86 2.41 PEAK ANGLE (Degree) 71 63 ANGULAR
DIFFERENCE 15 26 OF 50% LIGHT INTENSITY (Degree) ANGULAR DIFFERENCE
32 51 OF 10% LIGHT INTENSITY (Degree)
[0137]
4 TABLE 4 Ex.8 Ex.9 Com.Ex.7 Com.Ex.8 L/t 125 125 125 125 AVERAGE
SLANT ANGLE 4.2 4.3 8.2 8.3 (.theta.a) (Degree) DISPERSION RATE 170
180 630 670 (R%) (%) EMISSION RATE (.alpha.) 3.20 3.20 8.10 8.30
(%) NORMAL BRIGHTNESS 352 360 308 315 (cd/cm.sup.2)
* * * * *