Surface light source device of side light type

A surface light source device of side light type controls the intensity of light rays emitted therefrom. The surface light source device of side light type comprises a light scattering guide 20 having a light incidence surface 22, a light emission surface 23 and an inclined surface 25. The device further includes a primary light source L arranged around one side of the light incidence surface 22, a silver foil R surrounding the primary light source L, a prism sheet 4 arranged along the light emission surface 23, and a silver foil S arranged along the inclined surface 25. The inclined surface 25 of the light scattering guide 20 composed of three sections I, II and III. The section I is a convex cylindrical surface section having a radius of curvature of 330 mm. The tangential direction at the end of the guide on the side of the light incidence surface 22 is parallel to the light emission surface 23. The tangential direction at the boundary between the sections I and II is inclined at an angle 4.degree. with respect to the light emission surface 23. The sections II and III are straight sections whose inclinations are 4.degree. and 3.6.degree. with respect to the light emission surface 23, respectively. The distribution of emitted light intensity from the surface light source device has a higher luminance around its center. Various distributions of emitted light intensity are provided by varying the transition of inclinations of the inclined surface (particularly refer to FIG. 13).

FIELD OF THE INVENTION 
The present invention relates to a surface light source device of side 
light type having a plate-shaped light scattering guide and a primary 
light source arranged around one side of the light scattering guide, and 
more particularly to a surface light source device of side light type 
which provides illumination light rays having an intentional distribution 
of intensity and is advantageously applicable to back lighting for liquid 
crystal displays requiring a distributed brightness on its screen. 
RELATED ARTS 
Optical elements or devices for directing emitted light in a desired 
direction with the aid of scattering phenomenon have been known and have 
been applied to back lighting of liquid crystal displays. 
For example, according to the devices disclosed in Japanese Patent 
Application Laid-open Nos. 2-13925 and 2-245787, a primary light source is 
arranged around one side of a transparent light guide plate having a major 
surface along which a reflector element is arranged and light diffusion is 
caused on the other major surface (light emission surface) or in the 
proximity thereof to form a surface light source. 
With these technical means, light propagating directions are expanded by 
utilizing the diffuse reflection or specular reflection occurring around 
the light emission surface of the light guide plate or at reflector 
element and illumination rays are emitted out of the light guide plate. 
However, since no light scattering is volumetrically caused in the light 
guide plate, it is difficult to obtain a high efficiency in producing 
illumination rays. 
According to other known technical means as disclosed in Japanese Patent 
Application Laid-open Nos. 2-221924, 2-221925 and 2-221926, a light guide 
plate is used, whose transparent material includes particulate substances 
dispersed therein having refractive indexes different from that of the 
transparent material. Inside such a light guide, conversion of light 
propagation directions is caused in a volumetric region by scattering to 
increase the light rays directing toward a major surface (light emission 
surface). 
With a surface light source device of side light type having a primary 
light source arranged around one side of the light guide plate described 
above, there is generally a tendency of its luminance to be relatively 
lower in a zone relatively remoter from the light incidence surface 
(surface facing to the primary light source) of the light guide plate. 
Hitherto, most attempts and efforts have aimed to eliminate this tendency 
and to provide a light source having a characteristic in that the entire 
luminance of a light emission surface does not vary depending on the 
distance from the light incidence surface (flat distribution of emitted 
light intensity). 
In the Japanese Patent Application Laid-open Nos. 2-221924, 2-221925 and 
2-221926, for example, diameters and/or concentrations of the particles 
having different refractive indexes to be dispersed in the light guide 
plate are increased with an increase in distance from the primary light 
source. However, such a light guide plate having a gradient in diameter or 
concentration of particles is difficult to produce and unsuitable for 
mass-production and hence economically disadvantageous. 
In order to eliminate the disadvantages described above, a side light type 
surface light source device has been proposed, in which a light scattering 
guide in the form of a rectilinear wedge with a primary light source 
arranged around the thicker side face. FIGS. 1 and 1a are sections of the 
fundamental construction of such a proposed surface light source device. 
Briefly explaining this device, reference numeral 1 denotes the light 
scattering guide having a straight wedge shape in section, having an 
incidence surface 2 and a light emission surface 3. A primary light source 
(fluorescent light) L is arranged near to the incidence surface 2. Around 
the light emission surface 3 is arranged a prism sheet 4 with prism 
surfaces 4a and 4b in a row and a flat outer surface 40 which serves as a 
luminous portion to produce a flux of illumination rays having a 
directivity. A known liquid crystal display device is arranged above the 
light scattering guide 1 to provide a liquid crystal display. 
Reference letters R and S denote reflectors arranged around the rear face 
of the primary light source L and the rear face (inclined face) 5 of the 
light scattering guide 1. A silver foil of regular reflection may be 
usually used for the reflectors R and S. 
The light scattering guide 1 consists of a transparent matrix made of, for 
example, polymethyl methacrylate (PMMA, refractive index of 1.492) and 
particulates (different refractive index particulates) having a refractive 
index different from that of the above matrix. For example, silicone type 
resin particulates are added to the matrix at a rate of 0.08 weight % as 
different refractive index particulates. 
When the light scattering guide 1 constructed as above described is used, 
the light rays emitted from the light emission surface 3 have a 
preferentially propagating direction which directs forward but obliquely 
upward as viewed from the primary light source as described later. If a 
light ray proceeding in the preferentially propagating direction are 
referred to as "representative light ray" and designated by a letter "G", 
the angle .zeta. made by the propagating direction of the representative 
light ray with respect to the light emission surface 3 is 25.degree. to 
30.degree. roughly estimated. 
As shown in an enlarged figure of the encircled portion 50 in a broken line 
in FIG. 1a, if a prism sheet 4 having a vertical angle .phi. of around 
60.degree. to 65.degree. (63.degree. in the shown example) is used, the 
representative light ray G will be emitted in the direction substantially 
perpendicular to the outer surface 40 under the effect of the prism 
surfaces 4a and 4b. 
With the surface light source device using such a light scattering guide 1 
in the form of straight wedge, the level and uniformity in brightness as a 
surface light source will be improved by the repeated reflection effect 
occurring in the light scattering guide 1. The reason for achieving such 
an advantages will be summarily explained with reference to FIG. 2 
illustrating the behavior of the light in the straight wedge-shaped light 
scattering guide 1 used in the arrangement in FIG. 1. 
The light admitted through the incidence surface 2 into the light 
scattering guide 1 is representatively indicated by a light ray GO. It may 
be considered that the light ray GO forms a small angle with respect to 
the horizontal. 
Considering the behavior of the light ray GO, it is scattered and varies in 
direction at a rate, while it reflects repeatedly at the light emission 
surface 3 as one major surface and the inclined surface 5 as the other 
major surface, whereby the light ray GO approaches to the thinner end of 
the light scatting guide 1. Since the reflections of the light ray GO at 
the surfaces 3 and 5 are regular reflections, the incidence and reflection 
angles at each reflecting position are of course equal (.theta.1, 
.theta.2, .theta.3 . . . ). It should be noted that the reflection angles 
at the light emission surface 3 are in a relation 
.theta.2&gt;.theta.4&gt;.theta.6. 
Considering the interface transmittance at each reflection, a total 
reflection will occur under the condition of .theta.i&gt;.alpha.1 (critical 
angle; PMMA-air: 42.degree.), and the transmittance will become higher 
rapidly when .theta.i becomes smaller than .alpha.c. The transmittance 
becomes substantially constant if .theta.i is less than a certain small 
value (for instance, PMMA-air: around 35.degree.). FIG. 2 illustrates an 
example where light rays G4 and G6 are emitted under the relation of 
.theta.2&gt;.alpha.c&gt;.theta.4&gt;.theta.6. 
So long as the scattering caused in the light scattering guide 1 is forward 
scattering (the forward scattering characteristics of a light scattering 
guide will be explained later), such an effect will occur not only for the 
representative light ray GO (no scattering rays) entering the guide 1 
through the incidence surface 2 at the shown angle but also for the most 
of the light rays propagating in the light scattering guide 1 
substantially in the same manner. Therefore, the wedge-shaped sectional 
configuration of the light scattering guide 1 brings about a tendency in 
that the larger the distance from the light incidence surface 2, the 
higher is the rate of light emission from the light emission surface 3. 
Estimated the effect improving the rate of light emission by a function 
f(x) of the distance x from the light incidence surface 2, the f(x) is an 
increasing function of x. On the other hand, a proximity effect with 
respect to the light source L will occur in the part near to the light 
incidence surface 2. Estimated the proximity effect by a function g(x), it 
is a decreasing function of the distance x from the light incidence 
surface 2. 
Both functions considered, consequently, there is a tendency for the light 
emission surface 3 to emit light rays uniformly by canceling the decrease 
in the function g(x) and the increase in function f(x) each other. 
Moreover, opportunities for the light rays in the light scattering guide 1 
to enter the light emission surface 3 will probably increase as a whole 
owing to the wedge-shaped configuration of the guide 1. Accordingly, a 
surface light source with an improved illuminance level is provided. 
Use of a reflector of regular reflection (silver foil or the like) help the 
light rays, once transmitted through the rear surface of the guide 1, to 
enter the guide 1 again without diffusing. Therefore, the effect described 
above may become more prominent. 
By using such a light scattering guide 1 having a sectional shape 
rectilinearly reducing its thickness from the thick light incidence 
surface facing to the primary light source in the manner described above, 
it is possible to provide a surface light source device of side light type 
superior in uniformity of brightness. Examples of actually measured light 
intensity distributions will be described later for proving the uniformity 
in brightness. 
In recent years, however, there are increasing cases where a merely uniform 
brightness display hardly meets the user's requirements with increasing 
applications of liquid crystal displays and requirements for higher 
display performance. 
For example, for game machines displaying three dimensional scenes in 
moving pictures and for personal computers for multimedia, the displays 
superior in presence effect and three-dimensional effect are strongly 
required. However, the uniform brightness over the entire display gives a 
flat impression which is poor in presence effect and three-dimensional 
effect. For displays in such applications, therefore, they may be required 
to maintain the uniformity in brightness to a certain extent and at the 
same time to have an intentionally given gradient of brightness or 
specified brightness distribution. 
To meet such requirements, the surface light source device for backlighting 
has to provide an intentionally distributed light emitting intensity. An 
intentional distribution of emitting light intensity of a side light type 
surface light source device is accomplished by forming light scattering 
ink patterns on one major surface of a light guide or a light scattering 
guide, or by partially making the major surface rough. 
However, such measures will increase the proportion of light rays which 
dissipate finally as unused light rays for lightening thereby lowering the 
efficiency of the surface light source device. The formation of the ink 
patterns and rough surface will increase the manufacturing cost. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a surface light source device 
of side light type which is simple in construction and has a high 
efficiency in utilizing light rays and a controlled light emission 
intensity. 
It is another object of the invention to provide a surface light source 
device advantageously applicable to back lighting for a liquid crystal 
display requiring presence effect and three-dimensional effect. 
In a surface light source device of side light type including a 
plate-shaped light scattering guide having a volumetrically uniform 
scattering power and a primary light source arranged around one side of 
the light scattering guide, according to the invention, the light emitting 
surface (i.e. a major surface) is flat and the opposite surface (i.e. the 
other major surface) is provided with distributed inclinations depending 
on the distance from the incidence surface. At least a part of the latter 
major surface or inclined surface is inclined with respect to the light 
emission surface. The distribution of inclinations of the inclined surface 
is defined so as to permit the intensity of light rays emitted from the 
light emission surface to be changed depending on the distance from the 
light incidence surface. 
As constructional components of the surface light source device of side 
light type, may be employed any one or both of a prism sheet and a light 
reflector element, the former modifying the propagation direction of 
illumination rays and latter being arranged on the opposite side of the 
prism sheet with respect to the light scattering guide. Primary light 
sources may be provided on both sides of the light scattering guide. 
Various distributions may be adopted for inclinations of the inclined 
surface of the light scattering guide with respect to the light emission 
surface. According to one type of distribution, the inclination of the 
inclined surface with respect to the light emission surface continuously 
increases depending on the distance from the light incidence surface. For 
instance, the inclined surface is designed as a convex cylindrical 
surface. 
According to another type, the distribution of inclinations of the inclined 
surface with respect to the light emission surface is divided into a 
plurality of sections depending on the distance from the light incidence 
surface. Inclinations at boundaries between these sections are preferably 
smoothly changed. The sections of inclination may comprise a straight 
inclined section and a convex cylindrical section. 
In order to understand the fundamental light emission characteristic of the 
surface light source device of side light type using the light scattering 
guide, the scattering characteristic of the light scattering guide will be 
explained referring to Debye's theory. 
When the light having an intensity IO has been transmitted through a 
distance y(cm) in a medium (light scattering guide) and the intensity has 
decreased to I during the transmission, the effective scattering 
irradiation parameter E is defined by the following equation (1) or (2). 
EQU Ecm.sup.-1 !=-1n(I/IO)!/y (1) 
EQU Ecm.sup.-1 !=-(1/I).dI/dy (2) 
The above equations (1) and (2) are so-called "integral form" and 
"differential form", respectively and are equivalent in the physical 
meaning. This E may be called "turbidity". 
On the other hand, in the usual case that most of emitted light rays are 
longitudinally polarized lights for longitudinally polarized incidence 
lights, the scattered light intensity in the case causing light scattering 
owing to a nonuniform structure distributed in the medium (Vv scattering) 
is indicated in the following equation (3). 
##EQU1## 
In this case, 
EQU C=r.sup.2 sin(.upsilon.sr)!/.upsilon.sr (4) 
In case of natural incidence light, it is known that the following equation 
(5) can be considered as scattering light intensity. The equation (5) is 
obtained by multiplying the right side of the equation (3) by (1+cos.sup.2 
.phi.)/2 in consideration of Hh scattering. 
EQU Ivh=Vv(1+cos.sup.2 .phi.)/2 (5) 
where .lambda.O is wave length of incidence light, 
.upsilon.=(2.pi.n)/.lambda.O, and s=2sin(.phi./2). Moreover, n is 
refractive index of the medium, .phi. is scattering angle, and &lt;n.sup.2 
&gt;is permittivity fluctuation square mean value (&lt;n.sup.2 &gt;is represented 
by .tau. which will be suitably used hereinafter). The .gamma.(r) is a 
function called "correlation function". This correlation function 
.gamma.(r) is indicated by the following equation (6). 
EQU .gamma.(r)=exp(-r/a) (6) 
According to Debye's theory, the following equations (7) and (8) of 
relations among the correlation function .gamma.(r), correlation distance 
"a", and the permittivity fluctuation square mean value .tau. will be 
concluded, in the event that the nonuniform refractive index structure of 
the medium is dispersed divided in A and B phases with an interface 
therebetween. 
EQU acm!=(4V/S)..phi.A.phi.B (7) 
EQU .tau.=.phi.A.phi.B(nA.sup.2 -nB.sup.2).sup.2 ( 8) 
In case that the nonuniform refractive index structure can be regarded as 
being constructed by a spherical interface having a radius R, the 
correlation distance "a" is given by the following equation (9). 
EQU acm!=(4/3)R(1-.phi.A) (9) 
When natural light rays enter the medium based on the equation (5), the 
effective scattering irradiation parameter E is calculated using the 
equation (6) concerning the correlation function y(r). Results are as 
follows. 
EQU E=(32a.sup.3 .tau..pi..sup.4)/.lambda.0.sup.4 !.f(b) (10) 
where 
EQU f(b)={(b+2).sup.2 /b.sup.2 (b+1)}-{2(b+2)/b.sup.3 }.1n(b+1)!(11) 
EQU b=4.upsilon..sup.2 a.sup.2 ( 12). 
It will be understood from the above description that there are mutual 
dependence relations between the correlation distance "a", the 
permittivity fluctuation square mean value .tau. and the effective 
scattering irradiation parameter E. 
FIG. 3 illustrates two curves representing conditions rendering constant 
the effective scattering irradiation parameter E in cases of E=50 
cm.sup.-1 and E=100 cm.sup.-1, in a coordinate having an abscissa showing 
correlation distance "a" and an ordinate showing permittivity fluctuation 
square mean value .tau.. 
In general, if E is greater, the scattering power becomes larger, whereas 
if E is smaller, the scattering power becomes smaller. When E is 0, there 
is no scattering. From these facts, it is a general rule that when a light 
scattering guide is applied to a surface light source having a larger 
area, a smaller E should be selected, while when applied to a surface 
light source having a smaller area, a greater E should be selected. 
Showing one standard, for example, if the effective scattering irradiation 
parameter E is of the order of 0.001 cm.sup.-1, it is possible to give a 
uniform brightness to a very long light scattering guide having a length 
of a few tens meters. On the other hand, if E is of the order of 100 
cm.sup.-1 as shown in FIG. 3, it is suitable to illuminate a range from a 
few millimeters to a few centimeters uniformly. In the case of E=50 
cm.sup.-1 as shown in FIG. 3, it is probably suitable to give a uniform 
brightness to a light scattering guide having an intermediate size 
therebetween (for example, from a few centimeters to a few tens 
centimeters). 
Considering the standard described above, a preferable range of the 
effective scattering irradiation parameter E is 0.45 cm.sup.-1 to 100 
cm.sup.-1 for use in back lighting for liquid crystal displays having 
normal sizes. 
On the other hand, the correlation distance "a" is closely related to the 
direction characteristics of scattering light in individual scattering 
phenomena in a light scattering guide. In other words, as can be supposed 
from the above equations (3) to (5), the light scattering in a light 
scattering guide has generally a forward scattering property whose degree 
changes depending on correlation distances "a". 
FIG. 4 is a graph illustrating this fact with two values of the correlation 
distance "a" by way of example. 
In the graph of FIG. 4, the abscissa represents scattering angles .phi. 
(assumed that the traveling direction of incidence light is .phi.=0) and 
the ordinate represents scattering light intensity for natural light, 
namely, values of Vvh(.phi.)/Vvh(0) which are obtained by normalizing the 
equation (5) with respect to .phi.=0.degree.. 
As shown in FIG. 4, in case of a=0.13 .mu.m (particle diameter conversion 
value 2R=0.2 .mu.m), the graph of normalized scattering light intensity 
gives a slowly reducing function of .phi.. In contrast therewith, in case 
of "a"=1.3 .mu.m (particle diameter conversion value 2R=2.0 .mu.m), a 
graph of normalized scattering light intensity gives a rapidly reducing 
function in a range of small .phi.. 
Therefore, it can be concluded that the scattering caused by the nonuniform 
refractive index structure in a light scattering guide basically exhibits 
a forward scattering property, and the smaller the correlation distance 
"a" is, the weaker is the forward scattering property and the wider is the 
scattering angle range in scattering at one time. These facts have been 
experimentally ascertained. 
The above discussion is based on the scattering phenomenon itself in the 
nonuniform refractive index structure distributed in a light scattering 
guide. In order to estimate the direction characteristics of light 
actually emitted from the light emission surface of a light scattering 
guide, it is needed to consider the phenomenon of total reflection at the 
light emission surface as well as the transmittance (rate of leaving from 
the light scattering guide) at the light emission. 
As described in connection with FIG. 2, even if the light enters the light 
emission surface from the inside of the light scattering guide, any 
emission (escape) to the outside (the air layer) does not occur in the 
event that the incidence angle is in excess of the critical angle .alpha.c 
defined by refractive indexes of mediums inside and outside the light 
scattering guide (assuming that the direction of the normal at the light 
emission surface is 0.degree.). With PMMA (refractive index of 1.492) 
which is a typical material used in the present invention, 
.alpha.c=42.degree.. With other materials, the value of .alpha.ac is not 
greatly different from the above value (refer to Tables 1 and 2 latter 
described). 
As described above, as the scattering in a light scattering guide generally 
has a forward scattering property, it is understood that a primary 
scattering light generated from a straightly traveling light from 
incidence surface seldom meet the above critical angle condition in a 
surface light source device as shown in FIG. 1, wherein the primary 
scattering occurs when the straightly traveling light encounters a 
nonuniform refractive index structure (for example, different refractive 
index particles). 
In other words, it can be supposed that the formation of light rays meeting 
the above critical angle condition is in a close relation to the multiple 
scattering and reflection at a reflector disposed at the interface (or its 
near position) of the inclined surface of a light scattering guide. On the 
basis of this fact, the following matters are introduced with respect to 
the angle characteristics of emission light rays emitted from a light 
emission surface and emission light intensity characteristics depending on 
distances from a light incidence surface. 
1! Angle characteristics of light rays emitted from a light emission 
surface 
So long as only the light ray meeting the above critical angle condition 
concerned, the forward scattering property which is an attribute of the 
individual scattering phenomena becomes weaker to a considerable extent so 
that the distribution of light traveling directions expands to an extent 
correspondingly. As a result, the direction characteristics of light rays 
emitted from a light scattering guide is greatly influenced by the angle 
dependence of transmittance (escape rate) of light rays meeting the 
critical angle condition at the light emission surface. 
In general, the interface transmittance in case of scarcely meeting the 
critical angle condition is very low. For example, the interface 
transmittance at the interface between acrylic resin and the air is of the 
order of 40% for P-polarized light component, and 20% for S-polarized 
light component. When the incidence angle at the interface becomes less 
than the critical angle, the interface transmittance rapidly increases and 
becomes substantially constant when the incidence angle comes to a range 
less than the critical angle by more than 5.degree. to 10.degree.. For 
example, the substantially constant value for acrylic resin-air interface 
is more than 90% for P-polarized light component and 85% for S-polarized 
light component. 
Thus, the right rays entering the light emission surface of a light 
scattering guide at incidence angles around 35.degree., roughly estimated, 
contribute most intensively to the emission of light rays from the light 
emission surface. In consideration of the refraction at the light emission 
surface, the light rays entering the light emission surface at incidence 
angles around 35.degree. are emitted at angles around 60.degree. to 
65.degree. with respect to the normal on the light emission surface (light 
scattering guides usually having refractive indexes around 1.5). In other 
words, roughly estimated, the right rays emitted from the light emission 
surface of a light scattering guide have a directivity in a direction 
rising around 25.degree. to 30.degree. with respect to the light emission 
surface. The value .zeta. previously described corresponds to this rising 
angle. 
However, it should be noted that an excessively small correlation distance 
"a" give a weak directivity to the right rays emitted from the light 
emission surface because the forward scattering property becomes weaker so 
that the scattered light rays travel in a wide angle range after primary 
scattering. As a standard for preventing such a phenomenon from becoming 
prominent, the correlation distance "a" is preferably more than 0.01 
.mu.m, and more preferably "a"&gt;0.05 .mu.m. Such a property is referred to 
herein as "emitting directivity". A light scattering guide having such an 
emitting directivity may be preferably employed in the present invention. 
2! Emission light intensity characteristics depending on distances from 
the light incidence surface As clarified in the above explanation, the 
formation of light rays meeting the above critical angle condition is in 
close relation to the reflection from a reflector disposed at the 
interface, or its near position, of the inclined surface of a light 
scattering guide. 
If the sectional configuration of the light scattering guide in the form of 
a straight wedge as shown in FIG. 1 or 2 is modified to afford a 
distribution of inclinations of the inclined surface, the repeated 
reflections in the light scattering guide occur in different manners from 
that before being modified. As a result, the emission light intensity 
characteristics vary depending on distances from the light incidence 
surface. The present invention resides in the discovery that the emission 
light intensity depending on distances from the light incidence surface 
characteristics can be controlled by intentionally utilizing this 
phenomenon. 
In general, the configuration of the distribution of inclinations of the 
inclined surface of a light scattering guide with respect to the light 
emission surface is freely selected according to required distributions of 
emission light intensity. With a configuration of the distribution of 
inclinations, the inclination of the inclined surface with respect to the 
light emission surface continuously increases depending on the distance 
from the light incidence surface. According to an example, the sectional 
shape of a light scattering guide is defined as a wedge having a circular 
arc on one side. 
Such a sectional shape employed, it provides a tendency of emission light 
intensity from the light emission surface to increase depending on the 
distance from the light incidence surface. It is supposed that the 
repeated reflection effect previously described referring to FIG. 2 
appears more prominently with an increasing distance from the light 
incidence surface. 
In another pattern of distribution of inclinations, the inclined surface is 
divided into a plurality of sections and the transition of inclination is 
defined for each section. The transition of inclination at each of 
interfaces between the neighboring sections is preferably smooth. For 
example, the inclined surface may be formed by smoothly connecting a 
rectilinear inclined surface section and a convex cylindrical section. 
In general, the larger inclination of the inclined surface of a light 
scattering guide with respect to the light emission surface tends to 
stimulate the light emission. Therefore, if the inclination of the 
inclined surface at the portion near to the light incidence surface is 
larger, the amount of light rays emitted from the emission surface 
thereabout increases. On the other hand, if the increase in inclination of 
the part near to the light incidence surface is designed to be gentle, the 
amount of emission light rays from the part remote from the light 
incidence surface increases because a great amount of light rays are fed 
to the part remote from the light incidence surface. 
Various distributions of emission light intensity are obtained by varying 
the transition in inclination of the inclined surface depending on the 
distance from the light incidence surface on the basis of such a 
principle. 
The present invention is not limited by provision of a prism sheet or 
sheets, or construction, arrangement or configuration of the prism sheet. 
This is because even if the conditions in connection with the prism sheet 
are changed, the distribution of emitted light intensity on the light 
emission surface is maintained fundamentally while the preferentially 
propagating direction of the illuminating light rays is changed. 
The features of the invention described above and other features of the 
invention will be more clearly understood from the following description 
taken in connection with the accompanying drawings.

DESCRIPTION OF THE INVENTION 
In order to clarify the essential features of the invention, light emission 
characteristics were measured under a common condition for embodiments of 
the invention and the surface light source device, as a reference example, 
shown in FIG. 1. FIG. 5 is a diagram explaining the arrangement in these 
measurements. 
Referring to FIG. 5, reference numeral 40 illustrates the outer surface of 
a prism sheet employed in each of the surface light source devices in the 
measurements in the same manner as in FIG. 1. When the primary light 
source (L in FIG. 1) was switched on to form a luminous portion on the 
surface 40, its luminance was measured by means of a luminance meter M 
having a line of sight F (LS110, manufactured by Minolta Co., Ltd., field 
angle of view: 1/3.degree., provided with a close-up lens). 
The place on the left hand side of the surface 40 in FIG. 5 corresponds to 
the position of the incidence surface of the light scattering guide (not 
shown). For the sake of convenience, the direction shown by an arrow along 
the side of the left end of the surface 40 is referred to as "lamp 
parallel direction" and the direction shown by another arrow perpendicular 
to the lamp parallel direction is referred to as "lamp perpendicular 
direction" hereinafter. The center of the left end side of the surface 40 
is P0 while P2 is the point of intersection of a line extending in the 
lamp perpendicular direction from the center P0 with the right end side of 
the surface 40. The center of the two points P0 and P2 is P1 which is 
referred to as "central point". 
The luminance meter M is arranged so that its line of sight F always 
intersects the line P0P2. The point of intersection of the line of sight F 
with the line P0P2 is a luminance measurement point indicated by P. A 
distance between the points P0 and P is x. A vertical surface W extends 
perpendicularly to the surface 40 and in the lamp parallel direction 
through the luminance measurement point P. When the luminance meter M is 
so positioned and directed that its line of sight F is in the vertical 
surface W, the line of sight F forms an angle .PSI. with the normal N to 
the surface 40 at the luminance measurement point P. In the actual 
measurement, the distance from the measurement point P to the reference 
plane of the luminance meter M was 203 mm (the distance in FIG. 5 is 
showed shortened for the sake of convenience for drawing). 
In the measurements explained hereinafter, the luminance meter M is 
supported by a scanning mechanism (not shown) enabling the meter M to move 
along the overall length between the measured points P0 and P2, with the 
angle .PSI. being kept at a desired constant value for scanning. The 
surface 40 is of 68 mm.times.85 mm in all cases, which is equal to that of 
the light emission surface of the light scattering guide in each case. 
When the measurement point P is moved from the point P0 to the point P2, 
the distance x changes within the range from 0 mm to 68 mm. The distance x 
is referred to hereinafter as "scanning distance". 
FIGS. 6 to 8 are graphs showing the measured luminance distributions with 
the reference example employing the arrangement previously described (the 
surface light source device shown in FIG. 1). The angle .PSI. is 0 (FIG. 
6), 15.degree. (FIG. 7) and 30.degree. (FIG. 8). In each of the graphs, 
the abscissa indicates the scanning distance x(mm) and the ordinate 
indicates the luminance nt(=cd/m.sup.2). 
The light scattering guide (refer to numeral 1 in FIG. 1) used for the 
reference example has thicknesses of 4 mm at the thickest end (light 
incidence surface) and 0.2 mm at the thinnest end. Its light emission 
surface is of 68 mm (in the lamp perpendicular direction).times.85 mm 
(lamp parallel direction). As a primary light source (refer to reference 
letter L in FIG. 1), a fluorescent light having a length of 150 mm and a 
tube diameter of 3 mm (150C; manufactured by Harrison Electric Co., Ltd. 
HMBS3) is arranged, which was switched on at lamp current of 6.0 mA by the 
use of an inverter (CXA-M10L; manufactured by TDK Co., Ltd.). The 
fluorescent light was shielded except the portion facing to the light 
incidence surface of the light scattering guide. 
As can be seen from these graphs with the reference example, it has been 
found that the characteristics high in flatness can be obtained over the 
substantially overall length (over the scanning distance x from about 10 
mm to about 65 mm) in the lamp perpendicular direction, although there are 
slight differences in luminance depending upon the angles .PSI.. 
FIG. 9 illustrates the surface light source device according to the first 
embodiment of the present invention in a sectional view in a similar 
manner to that of the reference example (FIG. 1). The common components 
are designated by reference numerals similar to those in FIG. 1. 
The surface light source device according to the first embodiment of the 
present invention is different in construction from that of the reference 
example only in the sectional shape of the light scattering guide. The 
device of the reference example shown in FIG. 1 uses the straight 
wedge-shaped light scattering guide including the flat inclined surface 5 
inclined with respect to the light emission surface 3. In contrast 
herewith, the device shown in FIG. 9 according to the first embodiment of 
the invention uses the light scattering guide 10 including a flat light 
emission surface 13 and an inclined surface 15 which is cylindrical in 
section, thereby forming a one-side arc wedge-shaped guide 10. 
Referring to FIG. 9, the tangential direction of the inclined surface 15 
around the leftmost end (the light incidence surface 12) of the light 
scattering guide 10 is substantially parallel to the light emission 
surface 13. However, as the distance from the incidence surface 12 becomes 
larger, the inclination of the inclined surface 15 in the tangential 
direction progressively becomes larger. The degree of the change in the 
inclination can be indicated by the change in radius of curvature of the 
convex cylindrical surface forming the inclined surface 15. The material 
from which the light scattering guide 10 was made is the same as that of 
the scattering guide of the reference example. 
So long as the curvature of the inclined surface 15 does not become 
excessively large, it can be supposed that there is no essential 
difference in angular conditions between emitting from the light emission 
surface 13 and emission from the emission surface of the reference 
example. Then, the representative light ray G representing the light rays 
emitted from the light emission surface 13 is propagated in a direction 
which deviates obliquely upward with a rising angle around 25.degree. to 
30.degree. in the same manner as in the reference example. 
In the first embodiment, as the prism sheet 4 having a vertical angle .phi. 
of 63.degree. is used, the representative light ray G is emitted in the 
direction substantially perpendicular to the outer surface 40 under the 
effect of prism surfaces 4a and 4b. 
In the surface light source device using such a light scattering guide 10 
in the form of a wedge having the convex cylindrical inclined surface, 
repeated reflections occurring in the light scattering guide 10 are 
different from those in the reference example. As a result, the 
distribution of emitted light intensity aimed by the device according to 
the first embodiment will be different from that of the reference example. 
FIGS. 10 to 12 are graphs demonstrating the differences therebetween. The 
conditions for the measurements are the same as those in FIGS. 6 to 8 and, 
measured values of luminance on the outer surface 40 of the prism sheet 4 
are plotted in the graphs. How to define the scanning distance x with 
respect to the line of sight of the luminance meter M and the measurement 
point P is also shown in FIG. 9. 
The angle .PSI. for inclining the luminance meter M in the vertical surface 
W extending in the lamp parallel direction is 0.degree. (FIG. 10), 
15.degree. (FIG. 11) and 30.degree. (FIG. 12). In each of the graphs, the 
abscissa indicates the scanning distance x(mm) and the ordinate indicates 
the luminance nt(=cd/m.sup.2) as described previously. 
The light scattering guide 10 used in the measurements has thicknesses of 4 
mm at the thickest end (light incidence surface 12) and 0.2 mm at the 
thinnest end (these values are the same as those in the reference 
example). The radius of curvature of the cylindrical surface along the 
inclined surface 15 of the light scattering guide 10 is 1220 mm, such a 
cylindrical surface being formed for controlling the distribution of the 
emitted light intensity. The cylindrical arc in FIG. 9 is shown on an 
exaggerated scale. 
The light emission surface of the light scattering guide 10 is of 68 mm (in 
the lamp perpendicular direction) x 85 mm (lamp parallel direction) 
similar to that of the reference example. As the primary light source L, a 
fluorescent light is used, which is equivalent to that used in the 
reference example, under the same conditions as those in the reference 
example. 
As can be seen from these graphs in FIGS. 10 to 12, it has been found that 
there is a tendency of the luminance value to become higher progressively 
over the substantially overall length (over the scanning distance x from 
about 10 mm to about 65 mm) in the lamp perpendicular direction, although 
there are slight differences in luminance depending on angles .PSI.. Such 
a tendency could not find in the results of the reference example shown in 
FIGS. 6 to 8. 
It is supposed that this tendency results from the fact that the 
inclination of the inclined surface 15 becomes larger progressively with 
an increase in the scanning distance x and, accordingly, the repeated 
reflection effect explained with reference to FIG. 2 appears more 
prominently. 
If the two surface light source devices according to the first embodiment 
of the invention are juxtaposed in opposition to each other, a twin type 
surface light source device can be obtained, which has a characteristic 
such that the brightness is very high at its center and progressively 
decreases toward both ends. In this case, two surface light source devices 
may be integrally connected to form a unitary device. 
FIG. 13 illustrates the surface light source device according to the second 
embodiment of the invention in a similar sectional view to those in FIG. 1 
(reference example) or FIG. 9 (first embodiment). The common components 
are designated by the same reference numerals as those in FIGS. 1 and 9. 
The surface light source device of the second embodiment is different from 
that of the first embodiment only in the sectional shape. The light 
scattering guide 20 used in the second embodiment has the inclined surface 
25 whose sectional shape consists of following three sections. It should 
be noticed that the curvature of the circular arc and inclined angle of 
the inclined surface 25 are shown on an exaggerated scale. 
Section I: (section from 0 mm to 23 mm in scanning distance x) 
In the section I (convex cylindrical surface section), the inclined surface 
is a convex circular arc in section having a radius of curvature of 330 mm 
and the tangential direction at the end on the side of the light incidence 
surface 2 is parallel to the light emission surface 23. The tangential 
direction at the boundary between the sections I and II is at an angle of 
4.degree. with respect to the light emission surface 23. 
Section II: (section from 23 mm to 46 mm in scanning distance x) 
In the section II (inclined flat surface section), the inclined surface is 
straight and inclined at an angle 4.degree. with respect to the light 
emission surface 23. 
Section III: (section from 46 mm to 68 mm in scanning distance x) 
In the section III (inclined flat surface section), the inclined surface is 
straight and inclined at an angle 3.6.degree. with respect to the light 
emission surface 23. 
In other words, the tangential direction of the inclined surface 25 at the 
leftmost end of the light scattering guide 20 is substantially parallel to 
the light emission surface 13 while the tangential direction progressively 
increases in the range from the incidence surface 22 to the position about 
one third of its overall length referring to FIG. 13. The radius of 
curvature of the circular arc (cylindrical surface) of the inclined 
surface in the section I is less than the radius of circular arc of the 
inclined surface 15 of the first embodiment. The inclination is constant 
(4.degree.) in section I, although it changes into 3.6.degree. at the 
boundary between the sections II and III. The inclination in the section 
III is constant (3.6.degree.). 
With the surface light source device using such a light scattering guide 
20, repeated reflections in the guide 20 occur in a manner different from 
those in the first embodiment and the reference example. As a result, the 
distribution of emitted light intensity is also accordingly different from 
those in the first embodiment and the reference example. FIGS. 14 to 16 
illustrate graphs for demonstrating this fact. 
The conditions for measurements are similar to those in FIGS. 6 to 8 and 
FIGS. 10 to 12 while measured values of luminance on the outer surface 40 
of the prism sheet 4 are plotted in the graphs as functions of x. How to 
define the scanning distance x for the measurement point P and the line of 
sight of the luminance meter M is also shown in FIG. 13. 
The angle .PSI. for inclining the luminance meter M in the vertical surface 
W extending in the lamp parallel direction is 0.degree. (FIG. 14), 
15.degree. (FIG. 15) and 30.degree. (FIG. 16). In each of the graphs, the 
abscissa indicates the scanning distance x(mm) and the ordinate indicates 
the luminance nt(=cd/m.sup.2) as described previously. 
The light scattering guide 20 used for the measurements has thicknesses of 
4 mm at the thickest end (light incidence surface 22) and 0.2 mm at the 
thinnest end (these values are the same as those in the reference example 
and the first embodiment). 
The light emission surface of the light scattering guide 20 is of 68 mm (in 
the lamp perpendicular direction).times.85 mm (lamp parallel direction) 
while a fluorescent light is used as the primary light source L, which is 
equivalent to those used in the reference example and the first 
embodiment, under the same conditions as those in the reference example 
and the first embodiment. 
As can be seen from these graphs in FIGS. 14 to 16, it is understood that 
relatively higher luminance values are obtained on the range from the 
substantial center portion to somewhat on the right side thereof in the 
lamp perpendicular direction (over the scanning distance x from about 35 
mm to about 40 mm), although there are slight differences in luminance 
depending on angles .PSI.. Such a tendency could not find in any of the 
measured results of the reference example (FIGS. 6 to 8) and the first 
embodiment (FIG. 10 to 12). 
It is supposed that this tendency results from the fact that the 
inclination of the inclined surface 25 increases relatively rapidly from 
the section I toward the section II and is kept constant in the section II 
and the repeated reflection effect explained with reference to FIG. 2 
appears more prominently around the center portion. 
Considering the above results of two embodiments, the following facts are 
understood in a general. A large inclination of the inclined surface with 
respect to the light emission surface of a light scattering guide urges 
the light emission from the light emission surface. Therefore, if the 
inclination of the inclined surface at the part near to the incidence 
surface is increased rapidly, the amount of light emitted from the part of 
the light emission surface near to the incidence surface will increases. 
Moreover, if the inclination of the inclined surface at the part near to 
the incidence surface is slowly increased, a great amount of light rays 
will be fed to the remote part from the incidence surface so that the 
amount of light emitted from the part of the light emission surface remote 
from the incidence surface will increase. 
Therefore, by changing the configuration of the inclination of the inclined 
surface in various manners depending to the distance from the incidence 
surface, various distributions of emitted light intensity can be obtained 
accordingly. While the prism sheet is arranged so that its surface formed 
with prisms having the vertical angle of 63.degree. faces to the light 
emission surface of the light scattering guide in the above embodiments, 
it will be apparent that none of the existence, constitution and 
arrangement of the prism sheet limit the invention. 
This is because, even if the conditions in connection with the prism sheet 
are changed, the distribution of emitted light intensity on the light 
emission surface is not lost while but the preferentially propagating 
direction of the illuminating light rays is merely changed. For example, 
without using a prism sheet, a distribution of emitted light intensity 
having the similar tendency to those shown in FIGS. 10 to 12 and FIGS. 14 
to 16 can be obtained only by effecting the measurement under the 
condition that the surface W including the line of sight F of the 
luminance meter M is inclined forward at an angle around 55.degree. to 
60.degree. because the preferentially propagating direction of light rays 
emitted from the light emission surface directs obliquely upward with a 
rising angle around 25.degree. to 30.degree.. 
Various kinds of polymer based materials may be used for making the light 
scattering guide used in the present invention. Typical materials are PMMA 
(polymethyl methacrylate), PSt (polystyrene), PC (polycarbonate) and the 
like as shown in the following Tables 1 and 
TABLE 1 
______________________________________ 
Refractive 
Category 
Name of Polymer Index 
______________________________________ 
MA 1. PMMA polymethyl methacrylate! 
1.49 
2. PEMA polyethyl methacrylate! 
1.483 
3. Poly(nPMA) 1.484 
poly-n-propyl methacrylate! 
4. Poly(nBMA) 1.483 
poly-n-butyl methacrylate! 
5. Poly(nHMA) 1.481 
poly-n-hexyl methacrylate! 
6. Poly(iPMA) 1.473 
polyisopropyl methacrylate! 
7. Poly(iBMA) 1.477 
polyisobutyl methacrylate! 
8. Poly(tBMA) 1.463 
poly-t-butyl methacrylate! 
9. PCHMA polycyclohexyl methacrylate! 
1.507 
XMA 10. PBzMA polybenzyl methacrylate! 
1.568 
11. PPhMA polyphenyl methacrylate! 
1.57 
12. Poly(1-PhEMA) 1.543 
poly-1-phenylethyl methacrylate! 
13. Poly(2-PhEMA) 1.559 
poly-2-phenylethyl methacrylate! 
14. PFFMA polyfurfuryl methacrylate! 
1.538 
A 15. PMA polymethyl acrylate! 
1.4725 
16. PEA polyethyl acrylate! 
1.4685 
17. Poly(nBA) poly-n-butyl acrylate! 
1.4535 
XA 18. PBzMA polybenzyl acrylate! 
1.5584 
19. Poly(2-CIEA) 1.52 
poly-2-chloroethyl acrylate! 
______________________________________ 
TABLE 2 
______________________________________ 
Refractive 
Category 
Name of Polymer Index 
______________________________________ 
AC 20. PVAc polyvinyl acetate! 
1.47 
XA 21. PVB polyvinyl benzoate! 
1.578 
22. PVAc polyvinyl phenyl acetate! 
1.567 
23. PVClAc 1.512 
polyvinyl chloroacetate! 
N 24. PAN polyacrylonitrile! 
1.52 
25. Poly(.alpha.MAN) 1.52 
poly-.alpha.-methyl acrylonitrile! 
.alpha.-A 
26. PMA(2Cl) 1.5172 
polymethyl-.alpha.-chloroacrylate! 
St 27. Poly(o-C1St) 1.6098 
poly-o-chlorostyrene! 
28. Poly(p-FSt) 1.566 
poly-p-fluorostyrene! 
29. Poly(o, p-FSt) 1.475 
poly-o-, p-diflurostyrene! 
30. Poly(p-iPSt) 1.554 
poly-p-isopropyl styrene! 
31. PSt polystyrene! 1.59 
C 32. PC polycarbonate! 
1.59 
______________________________________ 
The light scattering guide made of such a polymer based material is 
produced by the following producing methods. 
First, one method of them utilizes a molding process including a step of 
kneading two or more polymers. 
In this method, two or more polymer materials having refractive indexes 
different from each other are mixed and heated to be kneaded (kneading 
step). The polymer materials before being kneaded may have any shapes. 
Pellet-shaped materials are preferable for industrial producing 
operations. The kneaded liquid material is injected under high pressure 
into a metal mold and cooled so as to be solidified. The molded material 
is removed from the metal mold to obtain a light scattering guide 
corresponding in shape to the inner surface of the metal mold. 
For example, using a metal mold having an inner shape corresponding to the 
sectional shape of the light scattering guide 10 shown in FIG. 9, a light 
scattering guide 10 to be used in the first embodiment is produced. 
The above two or more kneaded polymers having refractive indexes different 
from each other in the above kneading step are solidified before being 
completely mixed with each other, thereby causing a nonuniformity 
(fluctuations) in local concentrations in its solidified body. As a 
result, a uniform scattering power is given to the produced light 
scattering guide. 
Combinations and mixing rate of polymers in blending may be selected in a 
very wide range. They may be selected in consideration of difference in 
refractive index, degree and characteristics of nonuniformity in 
refractive index produced in the molding process (scattering irradiation 
parameter E, correlation distance "a", etc.). The typical polymer 
materials are shown in Tables 1 and 2. 
In another method for producing the light scattering guide, particulate 
materials having different refractive indexes are uniformly mixed into a 
polymer material or materials. The difference in refractive index between 
the polymer material and particulate materials is preferably more than 
0.001. 
A method utilizable for uniformly distributing the particulate materials is 
called "suspension polymerization". According to this method, the 
particulate materials are mixed into a monomer so that polymerization 
reaction proceeds under suspended condition of the particulate materials 
in the hot water to obtain a polymer material uniformly mixed with the 
particulate materials. Such a polymer is employed as a source material to 
produce a light scattering guide having a required configuration. 
The suspension polymerization is carried out with combinations of various 
particulate materials and a monomer (combinations of concentration of 
particles, particle diameters and refractive indexes) to prepare a 
plurality kinds of materials. These materials are selectively blended and 
the blended materials are then molded to produce light scattering guides 
having a variety of characteristics. Moreover, by further blending a 
polymer having no particulate material, the concentration of particles can 
be easily controlled. 
According to another method utilizable for uniformly mixing and 
distributing the particulate materials, a polymer material and particulate 
materials are kneaded. In this case, kneading and molding (into pellets) 
with combinations of various particulate materials and a polymer 
(combinations of concentration of particles, particle diameters and 
refractive indexes) are effected to obtain a plurality kinds of materials. 
These materials are selectively blended and the blended materials are then 
molded to produce light scattering guides having a variety of 
characteristics. 
The above polymer blending method may be combined with the particulate 
material distribution method. For example, when polymers having refractive 
indexes different from each other are blended and kneaded, particulate 
materials may be added into the polymers. 
It will be easily understood from the above detailed explanation that the 
surface light source device of side light type according to the invention 
is simple in construction and operates with high efficiency in utilizing 
light rays and with characteristics such that the emission light intensity 
provides an intentionally produced distribution. These unique features are 
very advantageous for applying the surface light source device to back 
lighting for liquid crystal displays which are required to afford the 
presence effect and three-dimensional effect to viewers.