U.S. patent application number 12/490799 was filed with the patent office on 2010-02-04 for hologram optical element and surface light source device using the hologram optical element.
Invention is credited to Tetsuya HOSHINO, Yasushi Sugimoto.
Application Number | 20100027084 12/490799 |
Document ID | / |
Family ID | 33432060 |
Filed Date | 2010-02-04 |
United States Patent
Application |
20100027084 |
Kind Code |
A1 |
HOSHINO; Tetsuya ; et
al. |
February 4, 2010 |
HOLOGRAM OPTICAL ELEMENT AND SURFACE LIGHT SOURCE DEVICE USING THE
HOLOGRAM OPTICAL ELEMENT
Abstract
A hologram optical element having a thin form and a high degree
of light transmittance, moreover that provides superior handling
ease, as well as a surface light source device employing this
hologram optical element. The angle at which light can be bent in
this hologram optical element, has low wavelength dependency, and
the hologram optical element enables prevention of spectral
separation in white light incident from an oblique direction which
is bent to a vertical direction and emitted. A transmitting
diffraction grating, when light of wavelengths .lamda.1, .lamda.2
and .lamda.3 within the ranges 0.46.ltoreq..lamda.1.ltoreq.0.50
.mu.m (blue light), 0.53.ltoreq..lamda.2.ltoreq.0.57 .mu.m (green
light), 0.60.ltoreq..lamda.3.ltoreq.0.64 .mu.m (red light) is
incident at angle .theta.i, the maximum diffraction angle for
diffractive efficiency of each wavelength is within the range from
-5 degrees to +5 degrees.
Inventors: |
HOSHINO; Tetsuya;
(Tsukuba-shi, JP) ; Sugimoto; Yasushi;
(Tsukuba-shi, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET, SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
33432060 |
Appl. No.: |
12/490799 |
Filed: |
June 24, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10555591 |
Nov 7, 2005 |
|
|
|
PCT/JP2004/006486 |
May 7, 2004 |
|
|
|
12490799 |
|
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|
Current U.S.
Class: |
359/15 |
Current CPC
Class: |
G02B 5/0252 20130101;
G02B 6/0053 20130101; G03H 2001/264 20130101; G02B 5/32 20130101;
G02B 6/0038 20130101; G02B 5/1871 20130101; G02F 1/133607 20210101;
G02B 6/0065 20130101 |
Class at
Publication: |
359/15 |
International
Class: |
G02B 5/32 20060101
G02B005/32 |
Foreign Application Data
Date |
Code |
Application Number |
May 7, 2003 |
JP |
P2003-128929 |
Claims
1. A hologram optical element that is a transmitting diffraction
grating, wherein the angle at which light can be bent has a low
degree of wavelength dependency, spectral separation in white light
incident thereto from an oblique direction is prevented, and the
light is bent to a vertical direction and emitted, and wherein the
cross-sectional form of the grating is a sawtooth form in which the
lengths of the two sides (edges) intersecting at the teeth ends
differs by 10% or more, and the interior angle is equal to or below
60.degree..
2. The hologram optical element according to claim 1, wherein, when
light of wavelengths .lamda.1, .lamda.2 and .lamda.3 within the
range 0.46.ltoreq..lamda.1.ltoreq.0.50 .mu.m,
0.53.ltoreq..lamda.2.ltoreq.0.57 .mu.m,
0.60.ltoreq..lamda.3.ltoreq.0.64 .mu.m is incident at angle
.theta.i, the maximum diffraction angle for diffractive efficiency
of each wavelength is within the range from -5 degrees to +5
degrees.
3. The hologram optical element according to claim 1, in which,
when light of three wavelengths .lamda.1, .lamda.2 and .lamda.3
that are within the range 0.46.ltoreq..lamda.1.ltoreq.0.50 .mu.m,
0.53.ltoreq..lamda.2.ltoreq.0.57 .mu.m, and
0.60.ltoreq..lamda.3.ltoreq.0.64 .mu.m is incident at angle
.theta.i, the maximum order of diffraction for diffractive
efficiency of each wavelength is (m+m0), m, (m-m0) (provided that
m0=1, 2, . . . ), wherein m is within the range that fulfills
expression (1) and expression (2) following, and average period d
fulfills expression (3): m.times.{.lamda.2.times.(1-sin .delta./sin
.theta.i)-.lamda.1}.ltoreq.m0.times..lamda.1.ltoreq.m.times.{.lamda.2.tim-
es.(1+sin .delta./sin .theta.i)-.lamda.1} (1)
m.times.{.lamda.3-.lamda.2.times.(1+sin .delta./sin
.theta.i)}.ltoreq.m0.times..lamda.3.ltoreq.m.times.{.lamda.3-.lamda.2.tim-
es.(1-sin .delta./sin .theta.i)} (2) (Where .delta. is within the
range, 0.ltoreq..delta..ltoreq.5 (degrees)) and
d=m.times..lamda.2/sin .theta.i (3)
4. The hologram optical element according to claim 1, wherein the
cross-sectional form of the grating of the hologram optical element
approximates a stair like form of N levels (N=4, 5, 6, 7, 8, . . .
).
5. The hologram optical element according to claim 4, formed of
material having a refractive index n, where the average depth h of
the grating grooves is, h=.DELTA..times.d/(n-1)
(0.4.ltoreq..alpha..ltoreq.1.0, d being the average period of the
diffraction grating).
6. The hologram optical element according to claim 1, formed of
material having a refractive index n, where the average depth h of
the grating grooves is, h=.alpha..times.d/(n-1)
(0.4.ltoreq..alpha..ltoreq.1.0, d being the average period of the
diffraction grating).
Description
[0001] This application is a Divisional application of prior
application Ser. No. 10/555,591, filed Nov. 7, 2005, the contents
of which are incorporated herein by reference in their entirety.
No. 10/555,591 is a National Stage Application, field under 35 USC
.sctn.371, of International (PCT) Application No.
PCT/JP2004/006486, filed May 7, 2004.
TECHNICAL FIELD
[0002] The present invention relates to a hologram optical element
that bends incident white light entering from an oblique direction
to a vertical direction and a surface light source device that uses
the hologram optical element. More specifically, the present
invention relates to improving the degree of brightness in the
direction in front of an observer by using the invention in a
backlight of a liquid crystal display.
BACKGROUND ART
[0003] Liquid crystal displays are used as displays for computers,
displays for the control panels of consumer electronic products and
displays for mobile telephones. Desired improvements for liquid
crystal displays include achieving lower electricity consumption,
lighter weight and thinner form.
[0004] A liquid crystal display is not a self illuminating device
but must use an external light source or external light from its
surroundings. A typical example of an external light source is a
back light arrangement whereby a surface light source is disposed
at the rear surface of a liquid crystal panel. The back light
system requires that light emitted from the surface light source be
directed in the frontal direction to the observer.
[0005] FIG. 1 shows an example of a configuration of a backlight
system. The hologram optical grating 10 is, in the conventional
art, a prism sheet. Light obliquely emitted from the light guide
plate 12 is bent to a vertical direction at the prism sheet,
diffused, at a diffuser 32 to reduce color dispersion before being
irradiated to a liquid crystal panel 30 displaying an image. In
this backlight system the form of the light guide plate, and the
form of the prism sheet disposed between the light guide plate and
the liquid crystals are optimized such that the degree of
brightness at the front is high.
[0006] FIG. 2 shows the angle of incidence .theta.i to and the
angle of emission .theta.o from a diffraction grating, however a
prism sheet is used for the purposes of the description instead.
The angle of emission of light emitted from a light guide plate
depends on the design of that plate, but normally the angle of
incidence .theta.i is between 20.degree.-70.degree.. The role of
the prism sheet is to effectively bend this light such that
.theta.o is 0.degree., in other words, to bend the light in a
vertical direction. To do this, it is necessary to reduce Fresnel
reflection, that is, reflection at the interface of the atmospheric
layer and the prism, and to make the greater part of the light
proceed at 0.degree.. Further, when the emitted light is of diverse
angles, even if the angle of incidence .theta.i fluctuates to some
degree, by having light bending properties that ensure the degree
of brightness in the vertical direction does not decrease, the
degree of brightness in the frontal direction can be raised, rather
than having a constant light bending angle. Moreover, as the light
source is white light it is necessary to reduce bend angle
wavelength dependence and suppress spectral separation as much as
possible. Spectral separation causes a reduction in display
qualities such as in the deterioration of color reproduction in the
color display of liquid crystals.
[0007] A conventional prism sheet uses refraction and total
reflection to bend emitted light using geometric optics. In
contrast to this, an optical member (hologram optical element)
using refraction and interference phenomena based on wave optics,
realizes a plurality of functions in a single element, providing a
thinner form, and furnishing superior light focus and diffusion
characteristics in comparison to optical elements using geometric
optics. Such hologram optical elements however, have not been used
for bending white light for spectral separation or high order
diffraction, but thus far have been employed for diffusing white
light and broadening the viewing angle (Japanese Unexamined Patent
Application Publication No. 7-114015 (pages 1-2, elected drawing),
Japanese Unexamined Patent Application Publication No. 9-325218
(pages 1-2, elected drawing), Japanese Unexamined Patent
Application Publication No. 10-506500 (pages 1-4, FIGS. 1-5),
Japanese Unexamined Patent Application Publication No. 11-296054
(pages 1-2, FIGS. 2-5), Japanese Unexamined Patent Application
Publication No. 2000-39515 (pages 1-2, FIGS. 1-2)), or for spectral
separation of white light (Japanese Unexamined Patent Application
Publication No. 9-113730 (pages 1-5, elected drawing), and Japanese
Unexamined Patent Application Publication No. 10-301110 (pages 1-2,
FIG. 68)). Further, hologram optical elements are being used
employing the effects of white light diffusion to make dot matrix
display defects invisible (Japanese Unexamined Patent Application
Publication No. 5-307174 (pages 1-2, elected drawing), Japanese
Unexamined Patent Application Publication No. 6-59257 (pages 1-2,
elected drawing), Japanese Unexamined Patent Application
Publication No. 6-294955 (pages 1-2, elected drawing), Japanese
Unexamined Patent Application Publication No. 7-28047 (pages 1-2,
elected drawing) and Japanese Unexamined Patent Application
Publication No. 7-49490 (pages 1-2, elected drawing)). For design
methods for hologram optical elements please refer for example to
"Iterative Methods for Diffractive Optical Elements Computation" by
Victor Soifer, Victor Kotlyar and Leonid Doskolvich, US, Taylor
& Francis 1997 pages 1-10.
[0008] The method of geometric optics theory systems for bending of
emitted light presents a problem in that substantial height
irregularities mean that sheet film thickness increases, making it
difficult to achieve a thin form. Further, in the case of
conventional prism sheets, individual prisms perform the function
of bending light, and if there are defects on the prism or
impurities, light passing that prism may engender display
abnormalities such as a luminescent spot of abnormal light rays. A
display device of a conventional art being extremely sensitive to
defects and impurities, may give rise to display abnormalities that
degrade the quality of the product. Accordingly extreme care must
be taken in production and handling to ensure there are no such
defects and impurities affecting a prism.
[0009] Hologram optical elements have the problems that 1)
diffractive light other than that of an order of diffraction of
vertically refracted incident light arises, 2) the diffractive
efficiency of such an order of diffraction is low and 3) wavelength
dispersion is substantial. For example if the period is small,
there are orders that are not vertically diffracted and wavelength
dispersion becomes substantial. If the depth is not appropriate,
the diffractive efficiency of such an order of diffraction
deteriorates.
[0010] With the foregoing in view it is an object of the present
invention to use a hologram optical element that utilizes
diffraction and interference phenomena based on the wave movement
properties of light instead of a conventional prism sheet using
refraction, in providing a hologram optical element and a surface
light source device using the hologram element that realizes both a
high rate of light transmittance in a light bending film and a thin
form.
SUMMARY OF THE INVENTION
[0011] In order to achieve the above objective the present
invention provides a hologram optical element having a high
diffractive efficiency and low color and wavelength dispersion as
well as a surface light source device using that hologram optical
element, in order to bend white light emitted from the surface
light source in a vertical direction.
[0012] In the hologram optical element related to this invention
bend angle wavelength dependency is small, spectral separation of
white light incident from an oblique direction is prevented and the
white light is emitted, bent in a vertical direction.
[0013] This hologram optical element is a transmitting diffraction
grating wherein, when light collimated sufficiently close to
parallel light of wavelengths .lamda.1, .lamda.2 and .lamda.3
within the range 0.46.ltoreq..lamda.1.ltoreq.0.50 .mu.m,
0.53.ltoreq..lamda.2.ltoreq.0.57 .mu.m,
0.60.ltoreq..lamda.3.ltoreq.0.64 .mu.m is incident at angle
.theta.i, the maximum diffraction angle for diffractive efficiency
of each wavelength should be within the range from -5 degrees to +5
degrees. Here, it is preferable if the condition that .lamda.1=0.48
.mu.m, .lamda.2=0.55 .mu.m and .lamda.3=0.62 .mu.m is used to
determine the depth and period of grating.
[0014] This hologram optical element is a transmitting diffraction
grating in which, when light collimated sufficiently close to
parallel light of three wavelengths .lamda.1, .lamda.2 and .lamda.3
that are within the range 0.46.ltoreq..lamda.1.ltoreq.0.50 .mu.m,
0.53.ltoreq..lamda.2.ltoreq.0.57 .mu.m,
0.60.ltoreq..lamda.3.ltoreq.0.64 .mu.m is incident at angle
.theta.i, the maximum order of diffraction for diffractive
efficiency of each wavelength is (m+m0), m, (m-m0) (provided that
m0=1, 2, . . . ), and it is preferable that m is within the range
that fulfills expression (1) and expression (2) and that the
average period d fulfills expression (3). Here, it is preferable
that .lamda.1=0.48 .mu.m, and .lamda.2=0.55 .mu.m and .lamda.3=0.62
.mu.m.
m.times.{.lamda.2.times.(1-sin .delta./sin
.theta.i)-.lamda.1}.ltoreq.m0.times..lamda.1.ltoreq.m.times.{.lamda.2.tim-
es.(1+sin .delta./sin .theta.i)-.lamda.1} (1)
m.times.{.lamda.3-.lamda.2.times.(1+sin .delta./sin
.theta.i)}.ltoreq.m0.times..lamda.3.ltoreq.m.times.{.lamda.3-.lamda.2.tim-
es.(1-sin .delta./sin .theta.i)} (2)
[0015] (Where .delta. is within the range,
0.ltoreq..delta..ltoreq.5 (degrees))
d=m.times..lamda.2/sin .theta.i (3)
[0016] It is preferable that the hologram optical element is a
grating the cross-sectional form of which is a sawtooth form,
wherein the lengths of the two sides (edges) intersecting at the
teeth ends differs by 10% or more, and the interior angle is equal
to or below 60.degree..
[0017] This hologram optical element should preferably be a
transmitting diffraction grating, this diffraction grating being
preferably formed from material having a refractive index n, where
the average depth h of the grating groove is,
h=.alpha..times.d/(n-1) (0.4.ltoreq..alpha..ltoreq.1.0, d being the
average period of the diffraction grating).
[0018] This hologram optical element should preferably be a
transmitting diffraction grating wherein the grating groove is
formed as an arc shaped form.
[0019] This hologram optical element should preferably be a
transmitting diffraction grating used for bending white light for
which the angle of incidence .theta.i is in the visible regions
60.degree..+-.15.degree., in a vertical direction, having a
sawtooth form, such that when m1, m2=1, 2, 3 . . . , average period
d is m1.times.(6.0.+-.2.0) .mu.m, average depth h is
m2.times.(5.0.+-.1.0) micrometers, or this sawtooth form has a
surface form approximating N level (N=4, 5, 6, 7, 8, . . . ).
[0020] This hologram optical element should preferably be a film or
panel shape.
[0021] This hologram optical element should preferably have a
membrane having polarized light separating functionality, color
separating functionality or reflection preventing functionality
disposed in proximity thereto or disposed on the front and rear
thereof.
[0022] This hologram optical element should preferably have the
polarized light separation, color separation and reflection
prevention functionality provided in the form of a grating formed
in a relief form having a period equal to or below 0.6 .mu.m and a
depth equal to or below 0.5 .mu.m.
[0023] The surface light source device related to the present
invention has a hologram optical element arranged above the light
emitting surface of the surface light source.
[0024] The surface light source device should preferably operate
such that when the hologram optical element is not provided, the
light is emitted in a range of angles from 20.degree. to 70.degree.
in relation to the normal of the light emitting surface of the
light source, and when the hologram optical element is provided,
70% or preferably more than 70% of all light emitted from the light
source is emitted in a range of angles from -10.degree. to
+10.degree. in relation to the normal of the light emitting surface
of the light source.
[0025] This surface light source device should preferably employ a
diffuser in addition to the hologram optical element.
[0026] This surface light source device should preferably be a
hologram diffuser that diffuses incident light within a determined
range of angles in space.
[0027] This surface light source device should preferably have the
hologram diffuser formed as an integrated body with the light
emitting surface of a light guide plate.
[0028] This surface light source device should preferably have a
reflection preventing membrane arranged over the light emitting
surface of the hologram optical element.
[0029] This surface light source device should preferably also
simultaneously provide a film for polarized light or wavelength
selection.
[0030] This surface light source device should preferably be a
surface light source in which the light source is disposed in
contact with the end surface at one side of a light guide plate,
wherein the rear surface of the light guide plate has formed
thereon a plurality of grooves, almost vertical to the inclination
of light propagated in the plate.
[0031] This surface light source device should preferably have the
angle of light incident to the hologram optical element in the
vicinity of the Brewster angle, and polarized light of determined
directions in light emitted from the hologram optical element
should be strengthened.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 shows the structure of a liquid crystal display;
[0033] FIG. 2 illustrates the angle of incidence .theta.i and the
angle of emission .theta.o of hologram optical elements
(diffraction grating);
[0034] FIG. 3 shows the relationship between the order of
diffraction of light diffracted and the angle of diffraction;
[0035] FIG. 4 shows displacement of the sawtooth form of the
hologram optical element (diffraction grating);
[0036] FIG. 5 depicts the sawtooth form of the hologram optical
element (diffraction grating);
[0037] FIG. 6 shows a hologram optical element (diffraction
grating) having a fan shaped arrangement of grooves;
[0038] FIG. 7 illustrates how a hologram optical element
(diffraction grating) bends light emitted in an oblique direction
from a surface light source to a vertical direction;
[0039] FIG. 8 shows the structure of a liquid crystal display;
[0040] FIG. 9 illustrates a method for measuring and a method for
regulating diffusion properties of a transmitting hologram
diffuser;
[0041] FIG. 10 shows the structure of a liquid crystal display;
[0042] FIG. 11 is a cross-sectional view of a light guide
plate;
[0043] FIG. 12 is a cross-sectional view schematically depicting a
device for manufacturing a hologram optical element (diffraction
grating);
[0044] FIG. 13 is a graph illustrating the relationship between
diffractive efficiency and angle of diffraction of a hologram
optical element (diffraction grating);
[0045] FIG. 14 is a graph illustrating the relationship between
diffractive efficiency and angle of diffraction of a hologram
optical element (diffraction grating);
[0046] FIG. 15 provides a first example of a hologram optical
element (diffraction grating); and
[0047] FIG. 16 provides a second example of a hologram optical
element (diffraction grating).
BEST MODE FOR CARRYING OUT THE INVENTION
[0048] The embodiments of a hologram optical element according to
the present invention and a surface light source device using the
hologram optical element will now be described with reference to
the drawings. The described exemplary embodiments are intended to
assist the understanding of the invention, and are not intended to
limit the scope of the invention in any way.
[0049] The hologram optical element according to the first
embodiment of the present invention is one in which bend angle
wavelength dependency is small, and that operates to prevent
spectral separation of white light incident from an oblique
direction, and to bend that light in a vertical direction, emitting
that light.
[0050] This hologram optical element controls light emission by
multiple interference of diffracted light that has passed a
plurality of contoured forms, wherein emission of light is not
adversely affected even if one of the contours is damaged or
foreign matter is present. That is to say, this hologram optical
element provides superior redundancy. Accordingly, the handling and
processing of this hologram optical element is easier than that
required for a conventional prism sheet. Further, using this
hologram optical element enables not just the bending of light, but
provides additional functionalities for controlling light such as
light focusing functionality and the like. The method for design of
this hologram optical element can be found for example in the above
cited work by Victor Soifer et al.
[0051] In the case of for example a diffraction grating type
hologram optical element, generally, a grating the cross-sectional
form of which is a sawtooth form, has been found to provide
superior diffractive efficiency. If this form is further optimized
it enables bending to be achieved while preventing spectral
separation or diffusion of white light. If light in one color
passes a normal hologram optical element, a plurality of
diffractions known as first order light and second order light
arise and light is propagated at different diffraction angles
causing a decrease in light bending efficiency. Further, if white
light is to be bent by diffraction, generally the angles of
diffraction will be disparate due to different wavelengths causing
dispersion of colors. However, it is possible to avoid these
problems of reduced light bending efficiency and dispersion by
achieving an appropriate design for a hologram optical element.
Here, a hologram optical element refers to optical members in
general that use diffraction and interference phenomena based on
wave optics. Further, white light refers to light including the
three primary colors, blue, green and red and bending light in the
vertical direction means taking light incident to an optical member
surface having diffraction and interference effects from an oblique
direction, and changing that such that the direction is normal to
the surface before emitting that light.
[0052] A hologram including a plurality of pixels, like a CGH
(Computer Generated Hologram) is suitable for a hologram optical
element according to the first embodiment. The hologram optical
element can be of a surface relief type, or a volume phase type
hologram, and can have the film on one surface or both, or again,
the film may be layered on. Further, the hologram optical element
may be transmissive or reflective. Moreover, the hologram optical
element may be combined with a prism that operates based on the
principles of geometrical optics.
[0053] The hologram optical element according to the second
embodiment is the hologram optical element according to the first
embodiment, being a transmitting diffraction grating, in which,
when light collimated sufficiently close to parallel light of three
wavelengths .lamda.1, .lamda.2 and .lamda.3 within the range
0.46.ltoreq..lamda.1.ltoreq.0.50 .mu.m (blue light),
0.53.ltoreq..lamda.2.ltoreq.0.57 .mu.m (green light),
0.60.ltoreq..lamda.3.ltoreq.0.64 .mu.m (red light), for example,
.lamda.1=0.48 .mu.m, .lamda.2=0.55 .mu.m and .lamda.3=0.62 .mu.m,
is incident at angle .theta.i, the maximum diffraction angle for
diffractive efficiency of each wavelength is within the range from
-5 degrees to +5 degrees. Such a hologram optical element basically
regulates the permissible range close to the diffraction angles for
wavelengths in a transmitting diffracted grating. If, when light
collimated sufficiently close to parallel light of three
wavelengths .lamda.1=0.48 .mu.m, .lamda.2=0.55 .mu.m and
.lamda.3=0.62 .mu.m corresponding to blue, green and red light is
incident at angle .theta.i, the maximum angle of diffraction for
diffractive efficiency of each wavelength is included within the
range from -5 degrees to +5 degrees (0 degrees is normal to the
emission surface of the diffraction grating), then spectral
separation of white light including wavelength components other
than those three wavelengths can be prevented, and that light can
be bent to a vertical direction.
[0054] The hologram optical element according to the third
embodiment is the hologram optical element according to the first
embodiment or the second embodiment, being a transmitting
diffraction grating, in which, when light collimated sufficiently
close to parallel light of three wavelengths .lamda.1, .lamda.2 and
.lamda.3 within the range 0.46.ltoreq..lamda.1.ltoreq.0.50 .mu.m
(blue light), 0.53.ltoreq..lamda.2.ltoreq.0.57 .mu.m (green light),
0.60.ltoreq..lamda.3.ltoreq.0.64 .mu.m (red light), for example,
.lamda.1=0.48 .mu.m, .lamda.2=0.55 .mu.m and .lamda.3=0.62 .mu.m,
is incident at angle .theta.i, the maximum order of diffraction for
diffractive efficiency of each wavelength is (m+m0), m, (m-m0)
where m is within the range that fulfills expression (1) and
expression (2) and that the average period d fulfills expression
(3).
m.times.{.lamda.2.times.(1-sin .delta./sin
.theta.i)-.lamda.1}.ltoreq.m0.times..lamda.1.ltoreq.m.times.{.lamda.2.tim-
es.(1+sin .delta./sin .theta.i)-.lamda.1} (1)
m.times.{.lamda.3-.lamda.2.times.(1+sin .delta./sin
.theta.i)}.ltoreq.m0.times..lamda.3.ltoreq.m.times.{.lamda.3-.lamda.2.tim-
es.(1-sin .delta./sin .theta.i)} (2)
[0055] (Where .delta. is within the range,
0.ltoreq..delta..ltoreq.5 (degrees))
d=m.times..lamda.2/sin .theta.i (3)
[0056] A concrete example of the hologram optical element according
to the third embodiment that prevents spectral separation and bends
white light to a vertical direction can be illustrated by these
expressions. Consider a transmitting diffraction grating of average
period d wherein the maximum order of diffraction for diffractive
efficiency of each wavelength is (m+m0), m, (m-m0), (m0=1, 2, . . .
), when light of three wavelengths .lamda.1=0.48 .mu.m,
.lamda.2=0.55 .mu.m and .lamda.3=0.62 .mu.m is incident at angle
.theta.i. Here, if the angle of diffraction of m order in relation
to .lamda.2=0.55 .mu.m is .theta.2, expression (table 4)
results.
d.times.(sin .theta.i+sin .theta.2)=m.times..lamda.2 (4)
[0057] Thus, bending light of wavelength .lamda.2 to a vertical
direction, that is to .theta.2=0, requires
d=m.times..lamda.2/sin .lamda.i (5)
[0058] At this time, if, if the angle of diffraction of order
(m+m0) in relation to .lamda.1 is .theta.1, and angle of
diffraction of order (m-m0) in relation to .lamda.3 is .theta.3,
then
d.times.(sin .theta.i+sin .theta.1)=m.times..lamda.2.times.(1+sin
.theta.1/sin .theta.i)=(m+m0).times..lamda.1 (6)
d.times.(sin .theta.i+sin .theta.3)=m.times..lamda.2.times.(1+sin
.theta.3/sin .theta.i)=(m-m0).times..lamda.3 (7)
In order to prevent spectral separation, .delta. must be
-.delta..ltoreq..theta.1, .theta.3.ltoreq..delta. (8)
as a constant within the range 0.ltoreq..delta..ltoreq.5 (deg).
[0059] From expressions (6), (7) and (8), the extrapolated
expressions to be fulfilled by m are
m.times.{.lamda.2.times.(1-sin .delta./sin
.theta.i)-.lamda.1}.ltoreq.m0.times..lamda.1.ltoreq.m.times.{.lamda.2.tim-
es.(1+sin .delta./sin .theta.i)-.lamda.1} (9)
m.times.{.lamda.3-.lamda.2.times.(1+sin .delta./sin
.theta.i)}.ltoreq.m0.times..lamda.3.ltoreq.m.times.{.lamda.3-.lamda.2.tim-
es.(1-sin .delta. sin .theta.i)} (10)
[0060] If expressions (5), (9) and (10) are fulfilled, light of
wavelengths .lamda.1, .lamda.2 and .lamda.3 is diffracted within
the range .+-..delta. degrees. For example, where .theta.i=65
degrees, m0=1, .delta.=1 degrees, the appropriate transmitting
diffraction grating can be obtained. Here, from expressions (9) and
(10),
7.69.ltoreq.m.ltoreq.8.08 (11)
thus, m=8 is the only integer that fulfills the conditions.
Accordingly, from expression (5), average period d can be of
approximately 4.85 .mu.m. The appropriate cross-sectional form for
the grating can be selected such that in relation to .lamda.1=0.48
.mu.m maximum diffractive efficiency is 9th order, in relation to
.lamda.2=0.55 .mu.m, maximum diffractive efficiency is 8th order
and in relation to .lamda.3=0.62 .mu.m, maximum diffractive
efficiency is 7th order.
[0061] FIG. 3 shows the relationship between order of diffraction
and angle of diffraction. Light among light emitted from a hologram
optical element that propagates in the same direction as incident
light is 0 order light. Light that moves in a direction approaching
the normal to the exit surface is positive order diffracted light
and light moving to the opposite direction is negative order
diffracted light. Accordingly, light emitted in a direction normal
to the exit surface is definitely positive order diffracted
light.
[0062] The hologram optical element according to the fourth
embodiment is the hologram optical element according to the first,
second or third embodiment, being a grating the cross-sectional
form of which is a sawtooth form, wherein the lengths of the two
sides (edges) intersecting at the teeth ends differs by 10% or more
and the interior angle is equal to or below 60.degree..
[0063] The hologram optical element according to the fifth
embodiment is the hologram optical element according to the fourth
embodiment wherein the cross-sectional form of the grating is a
stair like form of N levels (N=4, 5, 6, 7, 8, . . . ).
[0064] The hologram optical elements according to the fourth and
fifth embodiments have a form suitable to a cross-sectional grating
form for a transmitting diffraction grating (hologram optical
element) used for bending white light to a vertical direction. A
form of sharp ended saw teeth or a form approaching a stair like
form of N levels is suitable for efficiently bending such light to
a vertical direction.
[0065] Displacement as shown in FIG. 4, from the ideal sawtooth
form, is still suitable for the cross-sectional form of the
grating. Here, the maximum value for the degree of displacement (28
in FIG. 4) from a straight line should be equal to or below 0.2
.mu.m. Depending on the conditions, diffractive efficiency may be
at the maximum at points slightly shifted from the sawtooth form.
The optimum grating form differs according to angle of incidence,
wavelength, period, depth and index of refraction. This optimum can
be worked out by calculating numerically the exact solution for
diffractive efficiency in a periodic grating, using varying values
in a process of trial and error.
[0066] The hologram optical element according to the sixth
embodiment is a hologram optical element according to the fourth
embodiment and the fifth embodiment that are transmitting
diffraction gratings, wherein the diffraction grating is formed
from material of refractive index n, the average depth h of the
grating grooves is h=.alpha..times.d/(n-1)
(0.4.ltoreq..alpha..ltoreq.1.0, d is average period of the
diffractive grating).
[0067] The above relational expression shows the desirable range
for the depth of grating grooves in the transmitting diffraction
grating (hologram optical element) used for bending white light in
a vertical direction according to the sixth embodiment.
[0068] FIG. 5 shows the relationship between period and depth of
grating grooves and sawtooth positional displacement. If the
average depth h of grating grooves of the diffraction grating is
deep or shallow the light reaches a vertical direction less
efficiently. In this way, when the refractive index of the
diffraction grating is n, the efficiency with which light reaches a
vertical direction is high when the average depth h of the grating
grooves is .alpha..times.d/(n-1) (0.4.ltoreq..alpha..ltoreq.1.0).
Here, the optimum depth h is dependent on period d and positional
displacement of the sawtooth contours. For example, where period is
5 .mu.m, u/d is 20%, one optimum depth is 5.5 .mu.m. When mass
producing a broad area diffraction grating having deep grooves as
used here, the resin material used can be transferred from a mold.
The resin thus transferred is hardened by a thermal process or UV
light. Methods for producing a mold having deep grooves as used for
the present invention involve excavating with RIE after applying an
electron beam resist over a substrate and performing electron beam
drawing, exposure and development using x-ray radiation, exposure
and development of gray scale mask patterns, or a machine
processing method using a cutting tool. Depending on the conditions
of usage, the transferred material may be a optically curable
resin, of an acrylic type with good optical transparency.
[0069] The hologram optical element according to the seventh
embodiment is the hologram optical element according to any of the
first to sixth embodiments, being a transmitting diffraction
grating, wherein the grating grooves are arc shaped.
[0070] This hologram optical element has a grating groove
arrangement for a diffraction grating suited to a back light of a
type employed when an LED is installed at the corner part of a
light guide plate. As the grating grooves are arc shaped, light
propagated from the LED of the corner part can be efficiently bent
to a vertical direction, enabling the degree of brightness in the
frontal direction to be raised. As shown in FIG. 6,
cross-sectionally the grating is made sawtooth shaped, it being
preferable to form the grating grooves in a concentric circle form
centered around a point. The grating grooves in a circular arc must
form a continuous groove.
[0071] The hologram optical element according to the eighth
embodiment is a hologram optical element according to any of the
first to seventh embodiments, being a transmitting diffraction
grating used for bending white light of a visible region in which
the angle of incidence .theta.i is 60.degree..+-.15.degree., to a
vertical direction, wherein when m 1, m2=1, 2, 3 . . . the grating
has a sawtooth form in which average period d is
m1.times.(6.0.+-.2.0) .mu.m and average depth h is
m2.times.(5.0.+-.1.0) .mu.m, or has a surface form in which this
sawtooth shape approximates N level (N=4, 5, 6, 7, 8, . . . ).
[0072] The above relational expression shows the desirable period
for a transmitting diffraction grating, grating groove depth and
cross-sectional form, particularly where the angle of incidence
.theta.i is within the range 60.degree..+-.15.degree..
[0073] In the case of any of the hologram optical elements
according to any of the first to eighth embodiments of the present
invention, the inclination of the grooves of the transmitting
diffraction grating may be vertical or parallel in relation to the
incident light. Further, this inclination may span lengthwise and
widthwise.
[0074] FIG. 2 shows the relationship between the angle of incidence
to and the exit angle from the diffraction grating. White light
including the three primary colors red, green and blue is emitted
from a surface light source that radiates light in a planar aspect
like a light guide plate used in a liquid crystal display. Here,
depending on the design of the surface light source device, the
angle resulting from the incident light and the direction normal to
the plane of incidence of the diffraction grating, that is to say,
the angle of incidence .theta.i, is normally within the range of
20.degree. to 70.degree.. At this time, it could be said that white
light passing the diffraction grating is bent to a vertical
direction within a range of .+-.10.degree., that is to say, from
the perspective of an observer, if more than 60% of the light is
focused in the frontal direction, it could be said to be bent to a
vertical direction. Further, at less than 10.degree., the range of
wavelength dependency of the diffraction angle is small. Besides
the above described wavelength dispersion it is also necessary to
consider polarization dispersion. When considering orders of
diffractive efficiency closest to vertical, if those having the
greatest diffractive efficiency are polarization A and those having
the smaller diffractive efficiency polarization B, then when
(A-B)/A is 20% or below, it could be said that polarization
dependency is small. When polarization dependency is 5% or above it
is preferable that polarization of the higher diffractive
efficiency is used at a liquid crystal display device. The
diffraction grating has not only light bending functions, but can
also function to focus or diffuse light, further, the surface of
the diffraction grating can be formed not only as a flat surface,
but in order to add additional optical functions, a curved surface
may also be formed. Moreover, the diffraction grating can also be
used together with a prism sheet. For example, when considering a
space of x, y z-coordinates, the light can be bent in the x
direction by the diffraction grating and in the y direction by the
prism sheet.
[0075] The hologram optical element according to the ninth
embodiment is a hologram optical element according to any of the
first to eighth embodiments wherein the hologram optical element is
a film or a plate.
[0076] In this way, the bulk of the hologram optical element of
film or planar form is less than that of a hologram optical element
of a cuboid or globe form.
[0077] The hologram optical element according to the 10th
embodiment is a hologram optical element according to any of the
first to ninth embodiments wherein a membrane having polarized
light separation, color separation and reflection prevention
functions is arranged in proximity to the hologram optical element
or on both sides thereof.
[0078] The hologram optical element according to the 11th
embodiment is a hologram optical element according to the 10th
embodiment, wherein polarized light separation, color separation
and reflection prevention functions are provided by a grating of
relief form having depth of equal to or below 0.5 .mu.m and a
period of equal to or below 0.6 .mu.m.
[0079] In this way, it is possible to achieve more efficient usage
of light by combining the hologram optical element used for bending
white light emitted from a surface light source in a vertical
direction with polarized light separation, color separation and
reflection prevention functions.
[0080] The polarized light separation, color separation and
reflection prevention functions can be realized by a fine period
construction.
[0081] The 12th embodiment of the present invention, is a surface
light source device having a hologram optical element according to
any of the first to 11th embodiments arranged over the exit surface
of the surface light source.
[0082] The hologram optical element of this 12th embodiment bends
light emitted from a surface light source in an oblique direction
to a vertical direction as shown in FIG. 7. Using a hologram
optical element as it is used by this 12th embodiment enables
realization of a surface light source device that efficiently bends
white light emitted from a surface light source, wherein the degree
of brightness in the frontal direction is high and there is only a
small occurrence of excess coloring due to spectral separation.
[0083] The 13th embodiment of the present invention is a surface
light source device according to the 12th embodiment wherein, when
the hologram optical element is not provided, light is emitted in a
range of angles from 20.degree. to 70.degree. in relation to the
normal of the exit surface of the light source and when the
hologram optical element is provided more than 60% of total light
emitted from the surface light source and preferably more than 70%,
is emitted in a range of angles from -10.degree. to +10.degree. in
relation to the normal of the exit surface of the light source.
[0084] Preferably, when the cross-sectional form of the grating of
the hologram optical element is a transmitting diffraction grating
having a sawtooth form, light emitted from the surface light source
should be largely parallel to the direction following the longer
side of the saw teeth shown in 18 of FIG. 5, and diffractive
efficiency should be high with regards to light incident to the
diffraction grating.
[0085] Further, normally, if light enters from or is emitted from a
direction oblique to a membrane there is a substantial Fresnel
loss. Accordingly, directing a grating surface having a sawtooth
form toward a surface light source results in less Fresnel loss
than the opposite arrangement. Further, if the grating is a planar
shape grating, Fresnel loss can be reduced as emitted light exits
at a direction vertical to the surface.
[0086] The 13th embodiment realizes a surface light source device
for a backlight, wherein as more than 60% and preferably 70% of
light is emitted within a range of angles of -10.degree. to
+10.degree., the degree of brightness in the frontal direction of
the liquid crystal display device is high, moreover, a high-quality
display with little spectral separation is achieved.
[0087] The 14th embodiment of the present invention is a surface
light source device according to the 12th embodiment or the 13th
embodiment which uses a diffuser in addition to the hologram
optical element.
[0088] As a slight degree of chromatic dispersion is apparent to
the human eye, the usage of a diffuser can be beneficial. A method
for combining a diffuser with a hologram optical element can be
found in Japanese Unpublished Patent Application No. 2002-23797 by
the same inventor as the present invention. The combination of the
hologram optical element and the diffuser may involve disposal of
the respective members on each side of a single film or may be
provided in an arrangement of two diffraction gratings and a single
diffuser. The arrangement shown in FIG. 1 having a light guide
plate 12, a hologram optical element 10 and diffuser 32 arranged in
succession, or the arrangement shown in FIG. 8 wherein the light
guide plate 12, diffuser 32 and hologram optical element 10 are
arranged in that order is also suitable. Further, the configuration
of light guide plate, diffuser, hologram optical element and then
diffuser is also suitable. The diffusion effect from the diffuser
may be due to the ruggedness on the surface or refractive index
distribution inside the film.
[0089] The 15th embodiment of the present invention is a surface
light source device according to the 14th embodiment, being a
device wherein a hologram diffuser diffuses incident light limited
within a range of determined angles in space.
[0090] Thus, a hologram diffuser that regulates the angle of
diffusion and achieves a high degree of diffusion efficiency
provides a suitable diffuser. When light is propagated in the z
direction, the inclination parallel to the grooves of the grating
is the x direction. The directions of dispersal of light from the
diffuser can be defined as unit vectors (Sx, Sy, Sz) as shown in
FIG. 9. Moreover, the maximum values for Sx and Sy are
respectively, sin(.theta.1) and sin(.theta.2). Here, chromatic
dispersion arises in the y direction, thus the range of .theta.1
should be appropriately small and the range of .theta.2 should be
set at the minimum angle necessary to eradicate chromatic
dispersion. A method for producing this kind of hologram diffuser
can be obtained by employing the method disclosed in the
embodiments of Japanese Unexamined Patent Application Publication
No. 2002-71959. The hologram diffuser may be a surface relief type
or a volume phase type. Further, the diffusion properties of the
hologram diffuser may vary in different locations.
[0091] The 16th embodiment of the present invention is a surface
light source device according to the 15th embodiment wherein the
hologram diffuser is integrally formed with the light exit surface
of a light guide plate.
[0092] When the order of arrangement used is, light guide plate,
hologram diffuser, hologram optical element, Fresnel loss can be
reduced by formation of the hologram optical element integrally
with the exit surface of the light guide plate.
[0093] The 17th embodiment of the present invention is a surface
light source device according to any of the 12th to 16th
embodiments, wherein a reflection preventing membrane is arranged
over the exit surface of the hologram optical element.
[0094] Light emitted from the light source is bent by a film having
a relief form and emitted in a vertical direction, from the
opposite side of the film. At that point when the light passes the
boundary between the atmosphere and the film approximately 4%
Fresnel reflection occurs. It is suitable to provide this
reflection preventing membrane (nonreflective membrane) in order to
prevent this. The reflection preventative function can be realized
by producing a reflection preventing membrane from multilayers of a
dielectric material membrane. A method for producing a reflection
preventing membrane from multilayers of a dielectric material
membrane is recorded for example in "Thin Optical Membranes, 2nd
edition", edited by Shirou FUJIMOTO, written by Kouzou ISHIGURO and
Hidetsugu YOKOTA, published by Kyoritsu Publishing Company, 1984,
pages 98-109. Further, this functionality can be realized by
providing a grating having a short period. Preferably, this period
should be 0.28.+-.0.08 .mu.m, while the depth of the grating
grooves should preferably be 0.22.+-.0.1 .mu.m. Again, in order to
reduce the boundary between the film and the atmosphere to minimize
Fresnel loss, it is preferable that the light bending relief form
and short period grating are disposed on the front and rear
surfaces of the same film respectively. Moreover, a plurality of
layers of this film can be provided. Also, the surface from which
light exits from the light guide plate can be provided with a
diffuser or reflection preventing membrane.
[0095] The 18th embodiment according to the present invention is a
surface light source device according to any of the 12th to 17th
embodiments wherein a polarized light or light length selection
film is also provided.
[0096] Providing a film for polarized light or light length
selection provides improved light usage efficiency. For example,
for light emitted from a surface light source of a light guide
plate incident to the film at approximately an angle of incidence
of 60.degree., where the relief form has a depth of equal to or
below 0.5 .mu.m and a period of equal to or below 0.6 .mu.m, only
light of specific wavelengths and polarization is reflected at an
efficiency of above 80% while the remainder is passed at efficiency
of 80%. Here, the optimum relief form is selected in accordance
with wavelength and angle of incidence. If the reflected light is
reused the light usage efficiency can be improved. For example, a
liquid crystal display device in which the usage rate of loss light
at the polarization film or color filter is improved, can be
achieved by combining a relief form with a depth of equal to or
below 0.5 .mu.m and a period of equal to or below 0.6 .mu.m with a
color filter red green and blue matrix, designing the period and
depth, combining this with a film that bends light in a vertical
direction, and positioning the matrix. The reason for this is that
one of the polarized lights at the light polarizing film, that is
to say, 50% of the quantity of light is lost, while two of the
three colors at the color filter, that is to say, 67% of the
quantity of light is lost, however if some colors only of some
polarizations are passed and the returning light is reused, it
becomes possible to substantially increase the usage efficiency of
the light. Further, a relief form that bends light and a small,
submicron period grating should preferably be disposed on the
respective sides of the same film in order to reduce Fresnel
reflection between the atmosphere and film boundary. Again, a
plurality of layers of the small, submicron period grating can be
provided. Also, it is preferable to provide a diffuser or
reflection preventing membrane on the light exit surface of the
light guide plate comprising the light generating layer of the
surface light source.
[0097] The 19th embodiment according to the present invention is a
surface light source device according to any of the 12th to 18th
embodiments, that is a surface light source having the light source
disposed in contact with the end surface at one side of a light
guide plate, and the rear surface of the light guide plate has
formed thereon a plurality of grooves, almost vertical to the
inclination of light propagated in the plate.
[0098] In FIG. 10, light incident from the end surface on the left
is reflected at the rear surface 50 of the light guide plate, is
next diffused at a diffuser 46 on the surface of the light guide
plate, and is bent by a hologram optical element (light bending
film) such as a diffraction grating or the like. With this kind of
arrangement, the degree of brightness in a vertical direction can
be improved by optimum adjustment of the angle of reflection from
the rear surface of the light guide plate and the angle of
diffusion at the surface of the light guide plate as well as the
bending angle of the hologram optical element (light bending
film).
[0099] The 20th embodiment according to the present invention is a
surface light source device according to any of the 12th to 19th
embodiments wherein the angle of light incident to the hologram
optical element is in the vicinity of the Brewster angle, and
polarized light of determined directions in light emitted from the
hologram optical element is strengthened.
[0100] Where n1 and n0 are respectively the indexes of refraction
of the film and the atmosphere, the Brewster angle when light is
incident to the film is defined by expression (12).
tan(.theta..sub.B)=n1/n0 (12)
[0101] If light is incident at the Brewster angle, light elements
the direction of oscillations of the electric field vectors of
which are vertical to the plane of incidence are passed, thus if
this polarized light (P polarized light) is selected, a 100% pass
efficiency at the boundary can be achieved. Further, a hologram
optical element also is polarized light dependent. Normally, the
inclination of polarized light having a high pass efficiency in
relation to a flat surface is the same as that of polarized light
having a high pass efficiency at a hologram optical element.
Accordingly, by making the angle of incidence of light incident to
the hologram optical element in the vicinity of the Brewster angle,
the strength of P polarized light is increased in light emitted
from the surface light source device. In this case, by combination
with a liquid display panel using P polarized light, the degree of
brightness in the frontal direction can be increased.
EMBODIMENTS
[0102] FIG. 10 shows the backlight structure using the light guide
plate 48 related to several embodiments of the present invention.
This backlight structure is like that used in a compact liquid
crystal display device such as in a mobile telephone or the like.
The backlight comprises, from the bottom of the drawing upward, a
reflective panel 56, light guide plate 48, hologram diffuser 46 and
hologram optical element (light bending diffraction grating) 10,
the light guide plate 48 being formed as an integrated body with
the hologram diffuser panel 46. An LED light source 54 is disposed
at that side of the light guide plate 48 at which the light
incidence end face 52 is provided. According to this construction,
light generated from the LED light source 54 enters the light
incidence end face 52 of the light guide plate 48 and after being
reflected a number of times at a reflective groove formed in the
rear surface 50 of the light guide plate, this light exits from the
hologram diffuser 46 formed on the exit surface.
[0103] The light guide plate 48 is produced by an injection molding
method using polycarbonate. This light guide plate 48 of a
thickness of 0.8 mm has the structure as depicted in FIG. 11, in
the rear surface reflective groove, where the period of the groove
is random within the range of 120 to 150 .mu.m in order to prevent
moire with pixels of the liquid crystal panel. Further, the
hologram diffuser 46 formed over the exit surface diffuses light at
60.degree. in a direction parallel to the light incidence end
surface 52 (the angle of diffraction at which optical power is half
is 60.degree.) and 1.degree. in a direction perpendicular to the
light incidence end face surface 52.
[0104] The optical hardened resin used for forming the hologram
optical element is an ultraviolet curable resin of an acrylic resin
base, such as urethane acrylate or epoxy acrylate for example. The
form of the diffraction grating of the hologram optical element in
FIG. 5 is h=6.2 .mu.m, d=5 .mu.m and u=1 .mu.m.
[0105] The production device 88 of the hologram optical element 10
and the production method will now be described. As shown in FIG.
12, in the production device 88 for the hologram optical element
10, resin supply head 68 that supplies optically curable resin 70
is disposed downwardly facing the mold roll 82, while a metering
roll 78, nip roll 80, ultraviolet irradiation device 86 and mold
release roll 84 are arranged in succession in the downward flow
direction of the rotation of the mold roll 82.
[0106] Diffraction grating grooves are formed around the surface of
the mold roll 82 so as to transfer the diffraction grating grooves
to the surface of the optically curable resin 70. The diffraction
grating grooves are formed by producing a diamond bit and then
forming the grooves on the surface of the mold roll 82 by precision
machining with the diamond bit. This mold roll 82 is produced of
brass material, and after the grooves are formed with the diamond
bit, chrome electroless plating is promptly performed, and the
surface is oxidized, glazed and strengthened for mechanical stress.
For the embodiments of this invention, a resin product called
Sanrad 201 (a product name of a product made by Sanyo Chemical
Industries, Ltd.) is used for the optically curable resin 70.
[0107] During production the optically curable resin 70 is supplied
to the mold roll 82 via a pressure control device 66 and resin
supply head 68. As this resin is supplied the pressure at which it
is being supplied is detected by a pressure sensor and is
controlled by the pressure control device 66, being adjusted to the
pressure required for application to the mold roll 82. The film
thickness of the optically curable resin applied to the mold roll
82 is adjusted by the metering roll 78. A doctor blade 72 is
provided on the metering roll 78 for cuffing of resin adhering to
the metering roll 78, thereby ensuring the resin is applied to the
mold roll 82 in a stable, uniform condition.
[0108] A transparent base film (light passing film) 74 is supplied
between the nip roll 80 and the mold roll 82 that are positioned
further down flow from the metering roll 78. The transparent base
film 74 enters between the nip roll 80 and the mold roll 82, and
fits close to the optically curable resin 70. When the film reaches
the ultraviolet irradiation device 86 in this condition, with the
transparent base film 74 fitted close to the optically curable
resin 70, the optically curable resin 70 is hardened by the
ultraviolet rays emitted from the ultraviolet irradiation device 86
and the transparent base film 74 sticks fast to the optically
curable resin 70 forming an integrated film body, whereafter the
integrated film sheet 76 is peeled away from the mold roll 82 by
the mold release 84. A long stream of film sheet 76 can be
continuously obtained in this way.
[0109] The hologram optical element 10 with the film sheet 76
produced in this way cut to the prescribed dimensions is thus
obtained. Hologram optical elements (diffraction grating) may be
produced by extrusion molding or thermal press processing.
[0110] Polyethylene terephthalate (PET) is used for this
transparent film base 74 for the embodiments, however, this is
offered as an example only and is not restrictive in its
application, thus, polycarbonate or acrylic resin, or thermoplastic
urethane or the like may also be used. Further, other material,
such as acrylic denatured epoxy or acrylic denatured urethane may
also be selected for the optically curable resin 70. A metal halide
lamp, with a maximum power of 8 kW, was used for the ultraviolet
irradiation device 86 light source, and the film sheet delivery
speed facilitates production at a rate of 3 m/minute. The delivery
speed may be altered according to the characteristics of the
hardening of the optically curable resin 70 and the light
absorption characteristics of the transparent base film 74, however
this can be increased by using a higher wattage metal halide
lamp.
[0111] The surface of light source device produced in this way has
sufficient brightness in the frontal direction and provides an
excellent backlight for a liquid crystal display device wherein
unevenness due to moire and coloring due to spectral separation are
not visible. FIGS. 13 and 14 show the optical characteristics of
this hologram optical element (diffraction grating). FIG. 13
relates to a total of six kinds of experiments performed with laser
light of a wavelength of 488 nm with angles of incidence set at
50.degree., 60.degree. and 70.degree., polarization angles of
0.degree. (P polarized light) and 90.degree. (S polarized light).
"50-0" in the legend of FIG. 13 means angle of incidence 50.degree.
and polarization angle .theta..degree.. On the other hand, FIG. 14
shows the results from the same experiments but with the wavelength
at 633 nm. When input at 60.degree. the light is diffracted to a
vertical direction of 0.degree. and emitted. Input at 70.degree.,
the light travels as it is without 10.degree. shift and distributes
largely in a vertical direction, that is to say, the light is
diffracted more in a frontal direction. The degree of brightness in
the frontal direction can be improved by using this effect.
[0112] FIG. 15 provides a first example of a hologram optical
element (diffraction grating).
[0113] This first example relates to a hologram optical element
according to the second, fourth and eighth embodiments. The
hologram optical element 10 is comprised of optically curable
acrylic resin having an index of refraction of 1.48, being a
sawtooth form grating having a period d=5 .mu.m. When light
incident to this hologram optical element at an angle of incidence
of 67.degree. is provided in the direction shown in the drawing
(light collimated sufficiently close to parallel light), the
results obtained are as displayed in the following Table 1.
TABLE-US-00001 TABLE 1 Wavelength Maximum order of (.mu.m)
diffractive efficiency Angle of diffraction 0.48 9 -2.3.degree.
0.55 8 -3.2.degree. 0.62 7 -3.0.degree.
[0114] FIG. 16 provides a cross-sectional view of a second example
of a hologram optical element that is a transmitting diffraction
grating.
[0115] This second example corresponds to the hologram optical
elements according to the second and the eighth embodiments. This
hologram optical element 10 comprises an optically curable acrylic
resin having an index of refraction of 1.48, and is a sawtooth form
grating having period d=5 .mu.m. When incident light (light
collimated sufficiently close to parallel light) enters the
hologram optical element in the direction shown in the drawing at
an angle of incidence of 67.degree., the results obtained are as
displayed in the following Table 2.
TABLE-US-00002 TABLE 2 Wavelength Maximum order of (.mu.m)
diffractive efficiency Angle of diffraction 0.48 9 -2.3.degree. 10
+2.3.degree. 0.55 8 -3.2.degree. 0.62 7 -3.0.degree. 8
+4.1.degree.
[0116] In this second example 9th order and 10th order light of
wavelength 0.48 .mu.m have approximately equivalent diffractive
efficiency, while 7th order and 8th order light of wavelength 0.62
.mu.m have approximately equivalent diffractive efficiency.
[0117] A backlight comprising the first example or the second
example of these hologram optical elements that are transmitting
diffraction gratings combined with a light guide plate according to
the above described embodiments realizes a sufficient degree of
brightness in the frontal direction and does not allow coloring due
to spectral separation to be seen.
[0118] As described, the hologram optical elements according to the
present invention control light emission by multiple interference
of diffracted light that has passed a plurality of contoured forms,
thus, in comparison to a conventional prism sheet, the handling and
processing of these hologram optical elements is easier, moreover
light emissions are not adversely affected even if one of the
contours is damaged or foreign matter is present. Further, the
hologram optical elements according to the present invention
realize simultaneously, a high rate of light transmittance and a
thin form. Using such a hologram optical element in a surface light
source device, enables prevention of spectral separation in white
light incident from an oblique direction and enables this light to
be efficiently bent to a vertical direction and emitted, thereby
enabling a higher degree of brightness in a frontal direction to be
realized.
* * * * *