Surface light source device and a liquid crystal display

Provided are a surface light source device whose bright surface is excellent in whiteness and softness and a liquid crystal display that ensures a high visual quality. A relatively thick side end surface of a light scattering guide 1, which has a wedge-shaped profile, forms an incidence surface 2. A fluorescent lamp L is located in the vicinity of the incidence surface 2, and a silver foil R is arranged around it. A prism sheet 4 is located outside an exit surface 5. The orientation of the prism sheet 4 is selected so that the running direction of prismatic rows is in line with the transverse direction of the light scattering guide 1. The prismatic surface may be directed inward or outward. A prism sheet 8 is interposed between a back surface 6 of the light scattering guide 1 and a reflector (silver foil) 3. The prism sheet 8 is arranged so that its prismatic surface is directed inward. The orientation of prismatic rows formed on the prismatic surface is selected so that the running direction of the prismatic rows is in line with the vertical direction of the light scattering guide 1.

SPECIFICATION 
1. Technical Field 
The present invention relates to a surface light source device and a liquid 
crystal display, and more specifically, to a surface light source device 
utilizing an optical element that shows a light scattering effect and a 
light guiding effect, i.e., "light scattering guide," and a planar element 
that has a prismatic effect, i.e., "prism sheet," and a liquid crystal 
display using the same for backlighting. 
2. Background Art 
A surface light source device utilizing a light scattering guide and a 
prism sheet is proposed and used for backlighting of a liquid crystal 
display and the like. The prism sheet is formed of a platelike optical 
material having a surface (i.e., "prismatic surface") that is formed with 
V-shaped repetitive corrugated rows. 
It is known that the prism sheet functions to modify the propagation 
direction properties of light fluxes. In the conventional surface light 
source device, therefore, the prism sheet is located solely on the 
exit-surface side of the light scattering guide. 
FIG. 1 shows an arrangement of the principal part of a conventional surface 
light source device that employs a prism sheet. Referring to FIG. 1, 
numeral 1 denotes a light scattering guide having a wedge-shaped profile. 
The light scattering guide 1 is composed of a matrix of, for example, 
polymethyl methacrylate (PMMA) and particles uniformly dispersed therein, 
the particles having a refractive index different from that of the matrix. 
These particles will be referred to as "particles of different refractive 
index" hereinafter. A relatively thick end surface of the light scattering 
guide 1 provides an incidence surface 2. A light source element (e.g., 
fluorescent lamp) L is located in the vicinity of the incidence surface 2. 
A reflector 3 is arranged along one surface (back surface) 6 of the light 
scattering guide 1. A specular silver foil or diffusive white sheet is 
used as the reflector 3. An illumination light is emitted from the other 
surface, i.e., an exit surface 5, of the light scattering guide 1. The 
prism sheet 4 is located outside the exit surface 5. 
For ease of illustration, the distance between the light scattering guide 1 
and the prism sheet 4 and the pitch of prismatic rows are exaggerated. One 
surface of the prism sheet 4 is composed of V-shaped prismatic surfaces 4a 
and 4b, while the other surface is a flat surface (bright surface) 4e from 
which an illumination flux 4f is emitted. The liquid crystal display is 
constructed by locating a conventional liquid crystal display device 
(i.e., "liquid crystal display panel") outside the prism sheet 4. 
Since the thickness of the light scattering guide 1 is reduced with 
distance from the incidence surface 2, reflection repeatedly occurs 
between the exit surface 5 and the slanting back surface 6 in the light 
scattering guide 1. In consequence, a uniform high luminance can be 
obtained. 
The light introduced from the light source element L into the light 
scattering guide 1 is subjected to scattering and reflecting actions as it 
is guided toward a relatively thin end surface 7. In this process, light 
emission from the exit surface 5 occurs gradually. 
The light emitted from the exit surface 5 is provided with directivity and 
parallelized, depending on the diameter (correlation distance of a 
nonuniform-refraction structure, in general) of the particles of different 
refractive index dispersed in the light scattering guide 1. In other 
words, the illumination light taken out of the exit surface 5 has a 
tendency to propagate preferentially in a specific direction. 
The greater the diameter of the particles of different refractive index 
(the greater the correlation distance of the nonuniform-refraction 
structure, in general), the more positively the light emitted from the 
exit surface 5 is parallelized. Usually, the preferential propagation 
direction (main propagation direction of the illumination flux) is raised 
at an angle of about 25.degree. to 30.degree. to the exit surface as 
viewed from the side of the incidence surface 2. 
Thus, the function of the prism sheet 4 employed in the surface light 
source device shown in FIG. 1 can be described in the following manner. 
FIG. 2 is a diagram for illustrating the behavior of the light in a 
vertical section, in the arrangement shown in FIG. 1. Here "vertical" 
means "perpendicular to the incidence surface 2." On the other hand, 
"parallel to the incidence surface 2" is expressed as "transverse." 
As shown in FIG. 2, the prism sheet 4 is located along the exit surface 5 
of the light scattering guide 1, with its prismatic surface inward. A 
preferred value of the vertical angle of each prism formed on the 
prismatic surface is .phi.3=about 60.degree.. 
The direction of incidence is indicated by arrow L'. The preferential 
propagation direction of the light flux emitted from the exit surface 5 is 
at an angle of .phi.2=about 60.degree. to a line normal to the exit 
surface 5. If the refractive index of the light scattering guide 1 is 
1.492 (PMMA matrix), the angle of incidence upon the exit surface 5 that 
gives .phi.2=about 60.degree. is .phi.1=about 35.degree.. A light beam 
corresponding to the preferential propagation direction is called a 
"representative beam" and designated by symbol B1. 
After the representative beam B1 emitted from the exit surface 5 advances 
straight through an air layer AR (refractive index n0=1.0), it impinges on 
the prismatic surface 4a of the prism sheet 4 substantially at right 
angles thereto (.phi.3=about 60.degree.). It is to be noted that the 
percentage for the incidence upon the prismatic surface 4b on the opposite 
side is relatively small. 
The representative beam B1 advances substantially straight to the prismatic 
surface 4b on the opposite side through the prism sheet 4, and is 
reflected specularly. The reflected beam is projected on the flat surface 
4e of the prism sheet 4 substantially at right angles thereto, and is 
emitted from the prism sheet 4. Through this process, the preferential 
propagation direction, a modified direction substantially perpendicular to 
the exit surface 5, is established. 
However, the modified preferential propagation direction is not always 
perpendicular to the exit surface 5. In other words, a considerable angle 
adjustment is allowed depending on the material (refractive index) of the 
prism sheet 4, the prism vertical angle 3, and the material (refractive 
index) of the light scattering guide 1. 
FIG. 3 shows another conventional prism sheet arrangement. In this 
arrangement, the prismatic surfaces of the prism sheet 4 face outward. A 
preferred prism vertical angle is .phi.4=about 70.degree.. A preferred 
range of the vertical angle for this arrangement is wider than that for 
the arrangement of FIG. 2. 
The direction of incidence is indicated by arrow L'. As in the case of FIG. 
2, a representative beam B2, which represents the preferential propagation 
direction, is projected on the exit surface 5 at the angle .phi.1=about 
35.degree., and is mostly emitted into the air layer AR (refractive index 
n0=1.0). At this time, the emission angle .phi.2 is about 60.degree.. 
After advancing straight through the air layer AR, the representative beam 
B2 is projected aslant on the flat surface 4e of the prism sheet 4, traces 
a refractive path, such as the one shown in FIG. 3, and is emitted from a 
surface 4c substantially at right angles to the exit surface 5. Here it is 
to be noted that the percentage for the emission from the surface 4d is 
relatively small. 
Since the path for the light after the incidence upon the flat surface 4e 
changes depending on a refractive index n2 and the prism vertical angle 
.phi.4 of the prism sheet 4, an adjustment in the preferential propagation 
direction is allowed in accordance with these parameters. 
The conventional surface light source device described above is 
advantageous in that its depth is relatively small and that it can 
generate uniform bright illumination fluxes that propagate in a desired 
direction. 
However, the liquid crystal display that employs the above-described 
conventional surface light source device for backlighting is 
unsatisfactory in the macroscopic visual feeling of the bright surface 
(upper surface of the prism sheet). More specifically, it has not been 
realized to obtain a bright surface that is fine in texture, soft, 
glareless, and also white enough. 
The following is an explanation of possible causes of this problem. The 
visual diffusing power given to the light scattering guide 1 of the 
surface light source device shown in FIG. 1 is not very strong. The wider 
the bright surface, the weaker the diffusing power that is given to secure 
uniform brightness is. Thus, a considerable quantity of reflected light 
from the reflector 3, which is arranged along the back surface of the 
light scattering guide 1, impinges on the eyes of an observer without 
being subjected to plenty of diffusive action. 
As a result, a specular sheet, such as a silver or aluminum foil, which is 
used as the reflector 3, gives the observer a visual feeling peculiar to a 
specular surface. This visual feeling involves, so to speak, "lack of 
whiteness" and "lack of softness" (i.e., "glare"). This problem on the 
visual feeling is supposed to be associated compositely with the color 
temperature and the propagation direction characteristics of the 
illumination flux, as well as with the light quantity level. 
If a diffusive white sheet is used as the reflector 3, the lack of 
whiteness can be improved in some measure. However, the uniformity of the 
brightness of the bright surface and the light quantity level lower. 
Whether the reflector is specular or diffusive, moreover, unevenness 
(e.g., local wrinkles or irregularity), if any, of the surface of the 
reflector 3 results in visual unevenness. 
SUMMARY OF THE INVENTION 
Accordingly, the object of the present invention is to overcome the 
aforementioned drawbacks of the prior art. The present invention provides 
a surface light source device, which ensures a high brightness level and 
whose bright surface gives improved visual sensation (whiteness and 
softness). Also, the present invention provides a liquid crystal display 
in which the improved surface light source device is used for backlighting 
so that its visual quality standard, as well as power saving property, is 
high. 
A surface light source device according to the present invention comprises 
a light scattering guide, primary light source means for supplying light 
from at least one side end surface of the light scattering guide, a prism 
sheet located on the exit surface side of the light scattering guide, a 
reflector located in the vicinity of the back surface side of the light 
scattering guide, and at least one prism sheet interposed between the 
light scattering guide and the reflector. The at least one prism sheet has 
a prismatic surface that are formed with prismatic rows. The orientation 
of the prism sheet is selected so that the prismatic surface is directed 
toward the light scattering guide, and that the running direction of the 
prismatic rows is in line with the direction of light supply from the 
primary light source means. 
Preferably, the reflector, which is located further outside the prism 
sheet, is specular. 
A liquid crystal display that ensures a high visual quality standard, as 
well as an excellent power saving property, may be provided if the above 
surface light source device is applied to backlighting for the liquid 
crystal display. 
The basic and most important feature of the present invention lies in that 
at least one prism sheet is interposed between the light scattering guide 
and the reflector. 
This feature makes the behavior of light in the vicinity of the back 
surface of the light scattering guide different from that in the 
conventional arrangement. That is, in the vicinity of the back surface of 
the light scattering guide, a considerable quantity of light is guided 
away from the primary light source means while being confined within the 
prism sheet. This process of light guidance involves internal reflection 
and light divergence. In consequence, the diversity of optical paths 
leading to the eyes of an observer increases, and the visual brightness 
(i.e., "whiteness") and "softness" are improved remarkably.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 4 shows an arrangement of the principal part of one embodiment of the 
present invention. Like numerals are used to designate common elements 
that are shared with the surface light source device shown in FIG. 1. For 
ease of illustration, moreover, FIGS. 1 and 4 are reverse to each other in 
lateral positional relation, and the thicknesses of prism sheets and a 
reflector are neglected. 
The present embodiment is equivalent to an arrangement that is obtained by 
adding another prism sheet to the conventional arrangement shown in FIG. 
1. More specifically, a relatively thick side end surface of a light 
scattering guide 1 having a wedge-shaped profile provides an incidence 
surface 2, and a light source element (e.g., fluorescent lamp) L is 
located in the vicinity of the incidence surface 2. Symbol R designates a 
reflector sheet (e.g., silver foil; not shown in FIG. 1), which is 
provided surrounding the rear side of the light source element (e.g., 
fluorescent lamp) L. 
A prism sheet 4 is located outside an exit surface 5. The running direction 
of prismatic rows of the prism sheet 4 is in line with the transverse 
direction of the light scattering guide 1. The prismatic surface may be 
directed inward (so that the prismatic surface faces the light scattering 
guide 1) or outward, as in the prior art case. 
The light scattering guide 1, like the conventional one, is composed of a 
matrix of, for example, polymethyl methacrylate (PMMA) and a material of 
different refractive index (e.g., silicone-based fine particles) are 
uniformly dispersed therein. The content (wt. %) of the material of 
different refractive index is adjusted so that the light scattering guide 
1 has a suitable diffusing power. 
In general, the larger the vertical size of the light scattering guide 1, 
the lower the content of the material of different refractive index to be 
selected is. If the light scattering guide 1 is provided with an excessive 
diffusing power, propagation of light to a portion distant from the 
incidence surface 2 is hindered, so that the bright surface may possibly 
be subject to a brightness gradient. 
On the other hand, the particle size of the particles of different 
refractive index is a factor that affects the intensity of forward 
diffusiveness for individual scattering processes in the light scattering 
guide 1. In general, the larger the particle size, the higher the forward 
diffusiveness is. If the particle size is relatively large, the light flux 
emitted from the exit surface 5 has a distinct directivity, thereby a 
relatively well parallelized light flux being obtained. 
If the particle size is relatively small, in contrast with this, the 
directivity of the light flux emitted from the exit surface 5 lowers. 
It is preferable, therefore, to adjust the particle size in accordance with 
the level of the directivity required for the illumination flux. According 
to the present invention, no special restrictions are imposed on such 
conditions for the composition of the light scattering guide 1 arranged as 
described above. 
The following is a description of an arrangement in the vicinity of a back 
surface 6 of the light scattering guide 1, which provides an essential 
feature of the present invention. As in the conventional arrangement, a 
reflector 3 is provided parallel to the back surface 6 of the light 
scattering guide 1. However, the arrangement of the present invention 
basically differs from the conventional arrangement in that a prism sheet 
8 is interposed between the back surface 6 and the reflector 3. 
The construction of the prism sheet 8 itself may be similar to that of the 
prism sheet 4 outside the exit surface 5. The prism sheet 8 is arranged so 
that its prismatic surface is directed inward and faces the light 
scattering guide 1. The orientation of the prismatic rows formed on the 
prismatic surface is selected so that the prismatic rows face in the 
vertical direction of the light scattering guide 1. 
Either a specular or a diffusive reflector may be used as the reflector 3. 
It is advisable, however, to use the former (e.g., silver foil) in order 
to secure a higher brightness level. 
In the prior art, as mentioned before, the visual feeling is subject to a 
problem if the specular reflector 3 is employed. According to the present 
invention, however, the specular reflector can be used without creating 
such a problem. 
The surface light source device according to the present embodiment or 
another embodiment, which will be mentioned later, can be used for 
backlighting of a liquid crystal display. In this case, a conventional 
liquid crystal display panel LP (partially indicated by broken line) is 
located outside the prism sheet 4. 
According to the present embodiment, a commercially available fluorescent 
lamp for backlighting was used as the light source element L (i.e., 
primary light source means). When it was lit and observed macroscopically 
from outside the prism sheet 4, a bright surface with satisfactory 
"softness" and "whiteness" was identified. Visual properties such as 
"softness" and "whiteness" cannot be easily demonstrated by only the 
conventional photometry using a luminance meter. 
Accordingly, two measurement results will first be described, which 
indicate that the surface light source device according to the present 
invention has a capacity to provide a high luminance level. Then, reasons 
for the remarkable improvement in visual feeling will be described. 
FIGS. 5 and 6 are graphs showing the difference in luminance depending on 
the presence or absence of the prism sheet 8 according to the embodiment 
shown in FIG. 4. 
In both these graphs, the axis of the ordinate represents the luminance of 
the bright surface (i.e., outer surface of the prism sheet 4), while the 
axis of the abscissa represents the direction of luminance measurement 
(i.e., direction of vision of the luminance meter). The luminance is given 
in cd (candela)/m.sup.2. 
Referring to FIG. 5, the direction of luminance measurement was subjected 
to angular scanning within the vertical section of the light scattering 
guide 1. 
Referring to FIG. 6, on the other hand, the direction of luminance 
measurement was subjected to angular scanning within the transverse 
section of the light scattering guide 1. The way of settling scanning 
angles .theta. and .theta.' is shown in FIG. 4. 
Specifically, the angle .theta. represents a forward tilt angle 
(.theta.&gt;0.degree.) or backward tilt angle (.theta.&lt;0.degree.) with 
respect to a perpendicular g to the bright surface. Also, the angle 
.theta.' represents a leftward tilt angle (.theta.'&gt;0.degree.) or 
rightward tilt angle (.theta.&lt;0.degree.) as viewed from the side of the 
incidence surface 2, with respect to the perpendicular g to the bright 
surface. The direction of the perpendicular g corresponds to a direction 
given by .theta.=.theta.'=0. 
In either graph, a thick line represents a result for the present 
embodiment, while a thin line represents a result for an arrangement 
obtained by removing the prism sheet 8 from the present embodiment. The 
latter is equivalent to the arrangement shown in FIG. 1. These results 
involve the following facts. 
(1) The presence of the prism sheet 8 causes neither of the entire graphs 
of FIGS. 5 and 6 to get out of shape. 
(2) As seen from the graph of FIG. 5, the presence of the prism sheet 8 
causes the frontal luminance in the vertical section of the light 
scattering guide 1 to be improved by about 10%, and restrains light from 
being emitted in directions at angles of .+-.30.degree. or more. 
(3) As seen from the graph of FIG. 6, the presence of the prism sheet 8 
causes the luminance in the transverse section of the light scattering 
guide 1 to increase as a whole, especially with respect to the frontal 
direction. The rate of increase of the luminance in the frontal direction 
is about 10%. 
Thus, in the case where the prism sheet is located on the back side of the 
light scattering guide, the illumination flux can be efficiently radiated 
in a desired direction (frontal direction in this case) without ruining 
the general directional property of the conventional surface light source 
device. 
Referring also to FIG. 7, the reason why the visual sensation is 
considerably improved by the use of the prism sheet 8 will now be 
described. 
FIG. 7 is an enlarged view of part of the prism sheet located on the back 
side of the light scattering guide shown in FIG. 4, combining a 
perspective view and a sectional view taken from the side of the light 
source element and arranged vertically. 
As mentioned before, the prism sheet 8 is arranged so that the prismatic 
surface faces the back surface 6 (not shown in FIG. 7; see FIG. 4) of the 
light scattering guide 1. A very thin air layer (not shown) exists between 
the prism sheet 8 and the reflector (silver foil) 3. 
The prismatic surface of the prism sheet 8 is formed with prismatic rows 
that are composed of slanting surfaces 8a and 8b. The running direction of 
the prismatic rows is in line with the vertical direction of the light 
scattering guide 1. The value of the prism vertical angle is not specially 
restricted, and may be in a normal angular range (e.g., about 60.degree. 
to 110.degree.). 
Now let us consider the propagation of light from the back surface 6 of the 
light scattering guide 1 to the prism sheet 8. Most of the light, which is 
representatively designated by symbol C1, is projected on the one slanting 
surface 8a of each prismatic row, and gets into the space inside the prism 
sheet 8. 
Part of the light is reflected (totally reflected, in some cases) by the 
flat surface of the prism sheet 8, and the remainder by the surface of the 
silver foil 3. These light components trace substantially the same path, 
and considerable portions of them get out from the other slanting surfaces 
8b of the prismatic surface into the air layer. 
Part of the light, having gotten out into the air layer, is projected into 
the light scattering guide 1, while the remainder advances again toward 
the prism sheet 8. The path C1 for this light is not substantially 
different from the one obtained without the presence of the prism sheet 8. 
On the other hand, light represented by a path C2 is projected on the one 
slanting surface 8a of each prismatic row, enters the prism sheet 8, and 
advances toward the other slanting surface 8b. Most of the light advances 
toward the flat surface of the prism sheet 8 after being reflected 
(totally reflected, in many cases). Much of the light temporarily gets out 
of the prism sheet 8, and is projected again on the prism sheet 8 after 
being reflected by the surface of the silver foil 3. Then, the light gets 
out from the other slanting surfaces 8b of the prismatic surface into the 
air layer. 
Part of the light, having gotten out into the air layer, is projected into 
the light scattering guide 1, while the remainder advances again toward 
the prism sheet 8. The path C2 for this light is considerably different 
from the one obtained without the presence of the prism sheet 8. 
Here the presence of the prism sheet 8 implies that regions in which the 
refractive index is higher than in the ambient air layer are distributed 
in the vertical direction of the light scattering guide 1. Thus, the prism 
sheet 8 has a function to guide the light away from the light source 
element L while confining therein the light having got out of the back 
surface 6 of the light scattering guide 1. 
Since the light confined within the prism sheet 8 is repeatedly reflected 
therein, there is a general effect to make the advancing direction of the 
light diverge. As may be also inferred from the consideration for the 
paths C1 and C2, however, this effect, unlike a general disorderly light 
diffusion effect, maintains a definite directivity. 
Accordingly, the capacity to guide the light to a region distant from the 
light source element L is rather enhanced. Moreover, a loss attributable 
to "return light" that is bound for the light source element L cannot be 
easily caused. 
In this manner, the prism sheet 8 causes the light from the back surface 6 
to diverge variously while providing a smooth light guiding effect (see 
divergence of C1 and C2). Thus, the surface of the silver foil 3 can be 
prevented from direct visual reflection while maintaining the brightness 
of the illumination flux, so that an observer can perceive "whiteness." 
This effect should be understood from the following two results of 
observation. First, when the silver foil 3 is observed through the prism 
sheet 4 and the light scattering guide 1 with the light source element L 
of the surface light source device of FIG. 4 switched off, the silver foil 
3 looks white. This indicates that the surface of the silver foil 3 cannot 
be directly observed in this condition. 
When the silver foil 3 is observed through the prism sheet 4 and the light 
scattering guide 1 with the prism sheet 8 removed from the arrangement of 
FIG. 4 and with the light source element L switched off, on the other 
hand, the appearance of the silver foil 3 is reflected as it is, and a 
color (dark silver) quite different from white color is observed directly. 
Although the embodiment shown in FIG. 4 has been described in detail 
herein, the present invention is not limited to this embodiment. 
Specifically, two or more prism sheets 8 may be arranged between the 
reflector 8 and the back surface 6 of the light scattering guide 1. 
In this case, the aforementioned orderly light divergence effect is 
enhanced. Also, the flat surface of the prism sheet 8 may be satinized to 
have light diffusing power. 
The shape of the light scattering guide 1, number and form of the light 
source elements L, etc. may be also modified variously. FIGS. 8 to 12 are 
brief partial sketches enumeratively illustrating examples of the 
modifications. In connection with these modifications, individual 
descriptions of the general construction of the surface light source 
device, arrangements and functions of the individual elements, and the 
arrangement and orientation of the prism sheets are omitted. 
Referring first to FIG. 8, a planar light scattering guide 1 is employed, 
and a light source element L is disposed on one side end surface of the 
guide. This modification differs from the embodiment shown in FIG. 4 only 
in the profile of the light scattering guide 1. 
Referring to FIG. 9, a planar light scattering guide 1 is employed, and a 
light source element L is disposed on each of two opposite side end 
surfaces of the guide. This modification differs from the embodiment shown 
in FIG. 4 in the profile of the light scattering guide 1 and in the 
arrangement and number of the light source elements L used. 
Referring to FIG. 10, a planar light scattering guide 1 and an L-shaped 
light source element L are employed. In this case, the main direction of 
light supply is in line with the direction of a diagonal line of the light 
scattering guide 1. Accordingly, the orientation of the prismatic rows of 
the prism sheet 8 interposed between the light scattering guide 1 and the 
reflector 3 is selected so that the rows extend in the direction of a 
diagonal line that crosses the aforesaid diagonal line. 
Referring to FIG. 11, an employed light scattering guide 1 has a profile 
composed of two butting straight wedges. A light source element L is 
disposed on each end of the guide. 
Referring to FIG. 12, the back surface of an employed light scattering 
guide 1 has the shape of an arch. A light source element L is disposed on 
each end of the guide. 
Finally, a supplementary explanation will be given for the material of the 
prism sheets and the light scattering guide used according to the present 
invention and a manufacturing method therefor. 
Various materials based on polymers can be employed as the materials of the 
prism sheets and the light scattering guide. Tables 1 and 2 show typical 
examples of the materials. 
Since a prism sheet is transparent, normally, those materials may be used 
directly. If the flat surface of the prism sheet is satinized, the 
conventional blasting method or the like is applicable. Also, the 
generally-known plastic film forming technique is applicable to the 
formation of V-grooves that provide a predetermined prism vertical angle. 
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-phenylmethyl 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-b-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-ClSt) 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 
______________________________________ 
A light scattering guide based on a polymeric material can be manufactured 
by the following methods. 
According to one of the methods, a molding process including a stage of 
kneading two or more polymers. More specifically, two or more polymeric 
materials having different refractive indexes are mixed, heated, and 
kneaded together (kneading stage). Although the materials in the kneading 
stage may be in any desired form, pellet-type materials are available, for 
example. 
The kneaded liquid-like material is injected under high pressure into a 
mold of an injection molding machine. After being cooled and solidified, 
it is taken out of the mold, whereupon a light scattering guide having a 
shape corresponding to that of the mold is obtained. 
The two or more polymers having different refractive indexes cannot be 
fully mixed together by the kneading. In the process of solidification, 
therefore, unevenness (fluctuation) for the local concentration is fixed, 
and uniform diffusing power is given. 
Moreover, a molded piece of the light scattering guide can be obtained by 
injecting the kneaded material into a cylinder of an extruding machine and 
extruding it in the conventional manner. 
The combination of and the mixture ratio for these polymer blends can be 
selected from very wide choices, and must only be settled in consideration 
of the difference in refractive index and the strength and properties of a 
nonuniform-refraction structure produced in the molding process. In 
general, the strength and properties of the nonuniform-refraction 
structure of a light scattering guide can be described in terms of a 
scattering irradiation parameter E, correlative distance a, etc. 
Typical examples of available polymeric materials are shown in Tables 1 and 
2 above. 
According to another manufacturing method for the material of the light 
scattering guide, a particulate material with a refractive index different 
from that of the polymeric material is uniformly dispersed therein. In 
general, in this case, a refractive index difference of 0.001 or more is 
required at the least. 
The suspension polymerization method is one of available methods for 
uniformly dispersing the particulate material in the polymer. 
According to the suspension polymerization method, the particulate material 
is mixed with a monomer, and a polymerization reaction is accomplished in 
a manner such that it is suspended in hot water, whereupon a polymeric 
material having the particulate material uniformly dispersed therein is 
obtained. The light scattering guide with a desired configuration can be 
manufactured by molding the polymeric material as a raw material. 
A plurality of kinds of materials may be prepared by executing the 
suspension polymerization for various combinations (combinations of 
particle concentration, particle diameter, refractive index, etc.) of 
particulate materials and monomers. A light scattering guide with diverse 
properties can be manufactured by selectively blending and molding these 
materials. Also, the particle concentration can be easily controlled by 
blending polymers that contain no particulate materials. 
According to still another method available for uniform dispersion of 
particulate material, a polymeric material and a particulate material are 
kneaded. Also in this case, a plurality of kinds of materials may be 
prepared by kneading and molding (pelletizing) various combinations 
(combinations of particle concentration, particle diameter, refractive 
index, etc.) of particulate materials and polymers. A light scattering 
guide with diverse properties can be manufactured selectively blending and 
molding these materials. 
The aforementioned polymer blending method and the particulate material 
dispersion method may be combined together. For example, polymers with 
different refractive indexes may be further loaded with a particulate 
material as they are blended and kneaded. 
Since these manufacturing methods are generally known, a detailed 
description of them is omitted. 
As described herein, the present invention provides a surface light source 
device, which ensures a high brightness level and whose bright surface 
gives improved visual sensation ("whiteness" and "softness"). 
Also, there is provided a liquid crystal display in which the improved 
surface light source device is applied to backlighting for the liquid 
crystal display so that its visual quality standard, as well as power 
saving property, is high.