Light reflector for use in a reflective-type liquid-crystal display

A light reflector for use in a reflective-type liquid-crystal display has a prism sheet, which is made from a transparent base material, the front surface of which has formed on it a light-diffusing layer that diffuses incident light and the reverse side of which has a plurality of unit prisms that are arranged in a striped arrangement and made of an optically transparent resin, these prisms extending in a vertical direction, and having a cross-section that is the shape of a scalene triangle, and also has a light-reflecting sheet that is in opposition to the group of prisms on the rear surface of the prism sheet, the opposing surface of this light-reflecting sheet reflecting transmitted light and being disposed in parallel to the prism sheet, so that incident light and exiting light are not mutually parallel, thereby achieving both a wide viewing angle and a bright display.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a light reflector for use in a reflective 
type of liquid-crystal display, which makes use of the internal reflection 
in a prism to reflect light. 
2. Description of the Related Art 
In above-noted type of light reflector for a reflective-type liquid-crystal 
panel, there is the need to have high intensity, meaning that the light 
reflectivity is high, the need for good diffusion characteristics, so that 
uniform light is diffused in a desired direction, and the need for high 
contrast. 
In the case of a reflective-type liquid crystal display 1, as shown in FIG. 
5, on the observation side with respect to the liquid-crystal display 
element 2, a surface material 3 is disposed, this surface material 3 
having the effects of preventing glare and reflection, so that light does 
not shine from the surface of the liquid-crystal display 1, and on the 
reverse side of the liquid crystal display 1, a matte reflective material 
4 is disposed, this matte reflective material 4 having a matte-type finish 
with minute vertical unevenness on its reflecting surface. 
A PET (polyethylene terephthalate) film that has a matte-finished film 
surface resin, or a PET film onto the surface of which is applied a matte 
paint layer having minute particles is used as the above-noted matte 
reflective material 4, a light-reflective layer is further formed thereon 
by vacuum deposition of a metal such as aluminum. 
In the above-noted reflective type of liquid-crystal display 1, as shown in 
FIG. 5, while the incident light that is reflected at the matte reflective 
material serves as the light (reflected light) for the display, part of 
the incident light is reflected at the surface material 3 or the 
liquid-crystal element 2, this representing a wasteful reflection of 
light. 
When the above-noted reflection occurs, because the reflective surfaces of 
the matte reflective material 4 and the surface material 3, for example, 
are mutually parallel, the angle of reflection .alpha. of the display 
light and the angle of reflection .beta. at the surface material 3 are 
equal, so that he display light and surface reflected light rays are 
parallel. In the above-noted, the term angle of reflection is not the 
angle of reflection that the light actually makes with the surface, but 
rather the apparent angle of reflection with respect to the display 
surface of the liquid-crystal display. 
As a result of the above, if observation is made from the direction of 
travel of the display light, the display surface appears the brightest. 
However, because that is also the direction of travel of the 
surface-reflected light, external light sources will appear on the display 
surface, making this the direction with the most display glare. For this 
reason, there is the problem that the direction from which the display 
appears the brightest is also the direction in which the reflected light 
is the strongest, thereby making the display difficult to view. 
Because of the above-noted problem, there is a reflective-type 
liquid-crystal display 5, such as shown in FIG. 6, in which the angle of 
reflection .alpha.1 of the display light and the angle of reflection 
.beta.1 of the surface-reflected light are made to be different. 
The reflective-type liquid-crystal display 5 of FIG. 6 has a 
sawtooth-shaped reflective material 6 in place of the matte reflective 
material 4 of the reflective-type liquid-crystal display 1 of FIG. 5. 
The cross-sectional shapes on the reflective surface of the sawtooth-shaped 
reflective material 6 are those of scalene triangles, the surface thereof 
that faces the liquid-crystal display element 2 being formed as a 
light-reflective surface by, for example, the vacuum deposition of a metal 
such as aluminum. 
In contrast to the arrangement shown in FIG. 6, because the surface of this 
type of sawtooth-shaped reflective material 6 is not parallel with respect 
to the display surface, the angle of reflection .alpha.1 of the display 
light and the angle of reflection .beta.1 of the surface-reflected light 
are mutually different, so that the display light and the 
surface-reflected light travel in different directions. 
In the above-noted reflective-type liquid-crystal display 5, however, 
because the sawtooth-shaped reflective material 6 has a mirror-like 
reflective surface, there is absolutely no diffusion of light at the 
reflective surface and, depending upon the direction of external light, 
the direction in which the display appears bright is limited to an 
extremely narrow angle range. 
Accordingly, it is an object of the present invention, in consideration of 
the above-described drawbacks, to provide a light reflector for use in a 
reflective-type liquid-crystal display that has a wide range of viewing 
angle, without interference from surface-reflected light, and with a 
bright appearance over a wide angle. 
SUMMARY OF THE INVENTION 
To achieve the above-noted object, the present invention as recited in 
claim 1 is a light reflector for use in a reflective-type liquid-crystal 
display that has a prism sheet, which is made from a transparent base 
material, the reverse side of which has a plurality of unit prisms that 
are arranged with a fixed pitch in either one or two dimensions, external 
light that is incident at the surface of this transparent sheet being 
internally reflected by the oblique surfaces of the above-noted unit 
prisms, so that it passes through the transparent base sheet and exits 
therefrom. In this light reflector, the unit prisms of the prism sheet 
have the shape of a triangle that has a base that is parallel to the 
reverse side of the sheet and a vertex that protrudes toward the side 
opposite the reverse side of the sheet and, if the angle that one of the 
oblique sides of this triangle makes with respect to the normal direction 
is .theta.1 and the angle that the other oblique side of this triangle 
makes with respect to the normal direction is .theta.2, the refractive 
index of the above-noted prism sheet material is n, the angle of incidence 
of external light with respect to the light-diffusing layer is .theta.0, 
under the condition that .theta.1&gt;.theta.2, a light reflector for a 
reflective-type liquid-crystal display that satisfies the conditions 
EQU .theta.2&lt;90.degree.-sin.sup.-1 (1/n) 
and 
EQU .theta.1=90.degree.+sin.sup.-1 (sin .theta.0/n)/2 
achieves the above-noted object of the present invention. 
In the above-noted light reflector for use in a reflective-type 
liquid-crystal display, it is also possible to have the angle .theta.2 
such that it satisfies the condition .theta.2&gt;sin.sup.-1 (sin 
.theta.0/n)/2. 
In the above-noted light reflector for use in a reflective-type 
liquid-crystal display, it is also possible to have .theta.1 and .theta.2 
so that the condition 
.theta.1+.theta.2.ltoreq.180.degree.-2{90.degree.-sin.sup.-1 (1/n)}. 
Additionally, in the above-noted light reflector for use in a 
reflective-type liquid-crystal display, it is possible to make the sum of 
.theta.1+.theta.2 be approximately 90.degree.. 
In the above-noted light reflector for use in a reflective-type 
liquid-crystal display, it is also possible to have on the surface of the 
transparent base material a prism sheet onto which is formed a 
light-diffusing layer that diffusing light. 
Additionally, in the above-noted light reflector for use in a 
reflective-type liquid-crystal display, it is possible to provide a 
light-reflecting sheet that is disposed in opposition to the unit prisms 
of the prism sheet, the surface of this light-reflecting sheet that 
opposes the unit prisms reflecting transmitted light. 
In the above-noted light reflector for use in a reflective-type 
liquid-crystal display, it is possible that the material that forms the 
prism sheet be either a UV-cured resin or an EB-cured resin. 
In the present invention, the cross-section shape of the unit prisms in the 
prism sheet is that of an scalene triangle, by virtue of which the 
directions of the incident light and the reflected light differ, thereby 
eliminating interference from the surface-reflected light, while providing 
a bright display over a wide angle and increasing the viewing angle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention are described below in detail, with 
reference being made to the relevant accompanying drawings. 
As shown in FIG. 1, an embodiment of a light reflector 30 for use in a 
reflective-type liquid-crystal display according to the present invention 
is disposed at the rear surface of a reflective-type liquid-crystal 
display panel(not shown in the drawing), and has a prism sheet 38, which 
is made up of a transparent base material 32, on the front surface 32A of 
which is formed a light-diffusing layer 34 that diffuses light, and on the 
rear surface 32B of which is formed a plurality of unit prisms 36 arranged 
at a uniform pitch in one direction, and a light-reflecting sheet 40, 
which is disposed in opposition and parallel to the unit prisms of the 
above-noted prism sheet 38, this opposing surface reflecting transmitted 
light in the direction of the prism sheet 38. External incident light from 
the front surface 32A that passes through the light-diffusing layer 34 is 
internally reflected by the oblique surfaces of the unit prisms 36 on the 
above-noted light-reflecting sheet 40 side, so that it exits from the 
light-diffusing layer 34, the unit prisms 36 of the prism sheet 38 having 
a cross-sectional shape, such as shown in enlarged form in FIG. 2, this 
being a triangle that is formed by a base 36A that is parallel with the 
rear surface 32B and a vertex that protrudes in the direction of the 
light-reflecting sheet 40, the oblique sides 36B and 36C of the triangle 
being such that the angles .theta.1 and .theta.2, respectively, that they 
make with a line normal to the prism sheet 38 surface are mutually 
different, so that light that is incident at an inclination exits 
perpendicularly from the surface. 
Stated in more detail, if the refractive index of the unit prisms 36 is n, 
the angle of incidence of external light with respect to the 
light-diffusing layer 34 is .theta.0, the angles made by the oblique sides 
36B and 36C with respect to a normal line at the rear surface 32B are 
.theta.1 and .theta.2, respectively, with .theta.1&gt;.theta.2, the angles 
.theta.1 and .theta.2 are established by the following equations (1) and 
(2). 
EQU .theta.2&lt;90.degree.-sin.sup.-1 (1/n) (1) 
EQU .theta.1=90.degree.-.theta.2'+sin.sup.-1 (sin .theta.0/n)/2(2) 
In addition, .theta.2&gt;sin.sup.-1 (sin .theta.0/n)/2. 
In actuality, the refractive index n of the unit prisms 36 is determined by 
the material thereof, and with respect to this value of n, the value of 
.theta.2 is determined from equation (1) and the specific value of 
.theta.2', which is the range of specific values of .theta.2 to be 
substituted in equation (2) are determined, and then, based on the angle 
of incidence .theta.0, the value of .theta.1 is determined from the 
above-noted values of n and .theta.2', using equation (2). 
If the refractive index of the resin of the unit prisms 36 is, for example, 
1.57, from equation (1), .theta.2 is less than 50.4.degree.. If .theta.2, 
which is the range of specific values of .theta.2 is .theta.2'=30.degree. 
to 40.degree., because the angle of incidence .theta.0 of light to a 
reflective-type liquid-crystal display panel is often in the range 
20.degree. to 30.degree., based on these values of n, .theta.2, and 
.theta.0, from equation (2), .theta.1=56.degree. to 70.degree.. 
Additionally, the condition under which incident light undergoes two total 
internal reflections is given by the equation (3) below. 
EQU .theta.1+.theta.2.gtoreq.180.degree.-2{90.degree.-sin.sup.-1 (1/n)}(3) 
The transparent base material sheet 32 can intrinsically be completely 
eliminated, and it is possible to form the unit prisms 36 directly onto 
the rear of the light-diffusing layer 34. The light-diffusing layer 34 
diffuses exiting light, and has is made of a light-diffusing material that 
is distributed within a transparent resin material. Exiting light is 
diffused by the light-diffusing layer 34, this having the effect of 
broadening the viewing angle. 
The unit prisms 36 are substantially of uniform shape in the form of 
stripes, and formed of an optically transparent resin, these being 
arranged so that they extend in a direction that is perpendicular with 
respect to the lines of view when the liquid-crystal panel is viewed with 
two eyes. The unit prisms 36 reflect transmitted light without loss, and 
have the effect of increasing the intensity of reflection at the screen. 
If the vertex angle of the unit prism 36 is made greater than 90.degree., 
even if the cross-section is that of a isosceles triangle, incident light 
rays, after being reflected at points P and Q on the prism oblique sides 
36B and 36C (with part of the light being transmitted in the direction of 
the light-reflecting sheet 40), are not parallel to the original incident 
light path, but are rather reflected in a diffused condition. In 
principle, in the case in which the vertex angle is 90.degree., an 
incident light ray is reflected in parallel and the gain becomes large. If 
the vertex angle is larger or smaller than 90.degree., however, the gain 
will be lowered in either case. 
The spacing part 39 between the prism sheet 38 and the light-reflecting 
sheet 40 need not absolutely be provided, and these sheets can be directly 
adhered to one another. At the points P or Q, part of the light that is 
not reflected, but rather is transmitted through the unit prisms 36 is 
reflected by the light-reflecting sheet 40, so that it is returned to the 
unit prisms 36. 
Next, a method for manufacturing the above-noted light reflector 30 will be 
described. 
First, a transparent sheet base material 32 is prepared. This transparent 
base material sheet 32 has transparency, is resistant to heat and 
solvents, has dimensional stability, and can be used repeatedly as a 
screen, enabling free selection of a material, as long as it has the 
required strength. Specifically, it is possible to use a sheet or plate of 
polyethylene terephthalate resin, triacetyl cellulose resin, polyethylene 
naphthalate resin, polyvinyl chloride resin, polypropylene resin, acrylic 
resin, polyimide resin, diacetate resin, triacetate resin, polystyrene 
resin or the like, having a thickness of 50 to 500 .mu.m, and preferably 
having a thickness in the range of 75 to 200 .mu.m. 
Next, a light-diffusing layer 34 is formed on the above-noted transparent 
base material sheet 32. To do this, a composition made of a transparent 
light-diffusing substance distributed in a resin binder is used. The 
binder resin can be polyester resin, polyvinyl chloride resin, acrylic 
resin, epoxy resin, or polyolefin, used singly or a mixture. It is 
desirable that the refractive index of these materials be in the range 
1.35 to 1.60.It is possible to use either an organic or an inorganic 
light-diffusing substance, and it is appropriate that the particle size 
thereof be in the approximately range from 1 .mu.m to 50 .mu.m. 
The application of a light-diffusing substance onto the transparent base 
material sheet 32 is can be done using roll coating, knife coating, 
gravure coating, reverse coating, bar coating or other such coating 
methods, as appropriate. The approximate applied thickness should be 10 
.mu.m to 50 .mu.m when dried. 
To form the prisms on the rear surface 32B of the transparent base material 
sheet 32, another transparent material is used, and this is preferably an 
epoxy, a polyester, an acrylic, or a urethane acrylate UV-cured or 
EC-cured resin. The required prism shape can be formed by supplying a 
continuously ejected resin composite between a die roll, on the peripheral 
surface of which is formed the prism shape, and the transparent base 
material sheet 32, the transparent base material sheet 32 being cured by 
exposure to ultraviolet light as it passes along the die roll. It is also 
possible to prepare a transparent base material sheet 32 onto which is 
formed a light-diffusing layer and a sheet onto which is formed a group of 
prisms separately, and then to laminate these sheets. 
The pitch of the unit prisms 36 will differ depending upon the size of the 
pixels that make up images, and this is usually a pitch of approximately 
0.02 mm to 2.00 mm. The lamination of a prism sheet 38 formed in this 
manner with a light-reflecting sheet 40 completes the fabrication of the 
light reflector 30 according to the present invention. 
The light-reflecting sheet 40 that opposes the unit prisms 36 is a 
mirror-finished reflecting sheet, a light-diffusing sheet, a sheet having 
a multilayer dielectric film, or a retroreflective sheet or the like, 
which reflects light that has been passed through the prism sheet 38 back 
in the direction of the prism sheet 38. 
The above-noted mirror-finished sheet can be a sheet such as one fabricated 
by vacuum depositing or plating a film of silver, aluminum, chromium, 
gold, or copper or the like onto a film or plate, and the light-diffusing 
reflective sheet can be, for example, a PET (polyethylene terephthalate) 
foam, or paper. 
The above-noted multilayer dielectric film can be multiple layers having a 
differing refractive index, produced by vacuum deposition or coating onto 
PET or polycarbonate and, in the case of a light-transmissive high polymer 
multilayer film, it is possible to use method that is disclosed in U.S. 
Pat. No. 4,310,584, or the method of manufacturing disclosed in the 
Japanese Unexamined Patent Application publication H4-295,804. 
In a light reflector 30 for use in a reflective-type liquid-crystal display 
as described above, because the unit prism 36 cross-sectional shape is 
that of a scalene triangle, as shown in FIG. 2, even if reflected light is 
emitted in a direction that is perpendicular to the rear surface of the 
transparent base material sheet 32, the incident light will not be 
parallel to this exiting light. 
Therefore, if the exiting light is viewed front-on, it is not parallel to 
surface-reflected light in the direction of the front surface 32A and rear 
surface 32B of the transparent base material sheet 32, the result being 
that even as viewed from a position at which the front of the 
reflective-type liquid-crystal display is best viewed, there is no 
disturbance of the appearance thereof by light reflected from a mirror 
surface. 
Furthermore, although in the above-described light reflector 30 for a 
reflective-type liquid-crystal display panel, there is a light-diffusing 
layer 34 formed on the transparent base material sheet 32, the present 
invention is not restricted in this manner, and it is possible, as shown 
in FIG. 3, to apply the present invention as well to a light reflector 42 
which is not provided with a light-diffusing layer. 
Next examples 1 and 2 of a light reflector according to the present 
invention will be described. The materials used in these examples are as 
follows. 
(1) Transparent base material sheet 
125-.mu.m-thick PET film (Type A4300, manufactured by Toyobo was used.) 
(2) Light-diffusing composition 
Binder: polyester resin 43 parts by weight (Vylon 200, manufactured by 
Toyobo, was used.) 
Light-diffusing substance: polymethyl metacrylate, average particle 
diameter 10 .mu.m 100 parts by weight 
(MBX-10, manufactured by Sekisui Kaseihin Kogyo, was used) 
Diluting solvent: methyl ethyl ketone 60 parts by weight 
Toluene 60 parts by weight (solids portion: 54%) 
(3) Prism formation material 
Resin material: UV-curable resin (epoxy acrylate) (Z9002A, manufactured by 
JSR) 
Refractive index after curing: 1.57 
(4) Prism formation material 
Resin material: UV-curable resin (urethane acrylate) 
(RC17-236, manufactured by Dai Nippon Ink Unideck) 
Refractive index after curing: 1.50 
(5) Light-reflecting sheet 
PET foam sheet (Toray E601 or ICI Melinex 329) 
Example 1 is described below. 
The above-noted (1) transparent base material sheet was used as the 
transparent base material sheet 32, and the above-noted (2) 
light-diffusing composition was applied to the surface of the transparent 
base material sheet, after which it was dried, thereby forming the 
light-diffusing layer 34. 
The above-noted application was done by the coating method, the amount of 
the application being established as 9 g/m.sup.2 after drying. 
On the rear surface of the transparent material sheet, which on the 
opposite surface from the surface on which the above-noted light-diffusing 
layer 34 was formed, the above-noted (3) prism formation material was used 
to distribute scalene triangle prisms. 
The above-noted scalene triangle prisms, as shown in FIG. 2, are in the 
form of a stripe, having a cross-sectional vertex angle of 97.degree. 
(.theta.1=58.8.degree., .theta.2=38.5.degree.), with the pitch .alpha. 
between prisms being 0.05 mm. 
By doing the above-noted operations, a light-diffusing layer is formed by 
light-diffusing ink composition that is applied to the front surface of 
the transparent base material sheet, and a prism sheet with scalene 
triangle shaped prisms is formed on the rear surface of the transparent 
base material sheet. The following type of light-reflecting sheet is 
superimposed on the side on which the scalene triangle prisms are formed 
on the prism sheet. 
The above-noted (5) light-reflecting sheet is laid over, thereby forming 
the light reflector as shown in FIG. 1. 
In this example 1, when used in a reflective-type liquid-crystal display, 
with respect to parallel incident light at an inclination, light exiting 
from the front, as indicated by the solid line in FIG. 4, is almost at an 
exit angle of 0.degree., meaning that light exits straight out toward the 
front, so that it does not overlap with surface reflections of incident 
light, thereby eliminating the perception of glare. In the case in which 
diffused light having an angle of incidence .theta.0 of 22.degree., 
similar to actual existing light, is caused to strike the display, as 
indicated by the double-dot broken line in FIG. 4, although the maximum 
intensity is reduced, reflection characteristics having a wide viewing 
angle are achieved. 
Next, example 2 will be described. In example 2, the above-noted (4) prism 
formation material is used to distribute scalene triangle prisms on the 
rear surface of the transparent base material sheet, which is on the 
opposite side from that on which is formed the above-noted light-diffusing 
layer 34. 
The above-noted scalene triangles have a cross-sectional vertex angle of 
100.degree. (.theta.1=60.0.degree., .theta.2=40.0.degree.), and all other 
conditions are the same as noted above with regard to example 1. 
In the above-noted example 2, when used in a reflective-type liquid-crystal 
display, with respect to parallel incident light at an inclination of 
30.degree., or diffused incident light with directionality, 
characteristics are obtained that are similar to those indicated by the 
solid line in FIG. 4. 
When used in a reflective-type liquid-crystal display, the light reflector 
according to the present invention causes the directions of incident light 
and reflected light to be different, so that there is no interference from 
surface-reflected light, and also achieves the effect of providing an 
easy-to-view display with a wide viewing angle.