U.S. patent number RE38,305 [Application Number 09/512,735] was granted by the patent office on 2003-11-11 for lcd device including an illumination device having a polarized light separating sheet between a light guide and the display.
This patent grant is currently assigned to Asahi Glass Company Ltd.. Invention is credited to Tomoki Gunjima, Hiroshi Hasebe, Hiroaki Ito, Tetsuro Matsumoto, Yutaka Nakagawa, Yoshiharu Ooi, Masao Ozeki.
United States Patent |
RE38,305 |
Gunjima , et al. |
November 11, 2003 |
LCD device including an illumination device having a polarized
light separating sheet between a light guide and the display
Abstract
An illumination device for a direct viewing type display element
comprising a flat light guide; a light source set such that light
is incident on a side portion of said flat light guide; a polarized
light separating flat set on a first side of a light emitting side
of the flat light guide for transmitting a p polarized light
component and reflecting at least a portion of an a polarized light
component with respect to a light ray substantially having a
predetermined direction of incidence; and a light reflecting sheet
disposed on a second side opposite to said light emitting side of
the flat light guide in parallel with the light emitting site.
Inventors: |
Gunjima; Tomoki (Chikushinoshi,
JP), Ooi; Yoshiharu (Koriyama, JP), Ozeki;
Masao (Yokohamashi, JP), Ito; Hiroaki
(Yokohamashi, JP), Hasebe; Hiroshi (Kowloon,
JP), Matsumoto; Tetsuro (Chibashi, JP),
Nakagawa; Yutaka (Iseharashi, JP) |
Assignee: |
Asahi Glass Company Ltd.
(Tokyo, JP)
|
Family
ID: |
27564784 |
Appl.
No.: |
09/512,735 |
Filed: |
February 24, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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016409 |
Jan 30, 1998 |
Re37377 |
Sep 18, 2001 |
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132864 |
Oct 7, 1993 |
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Reissue of: |
530012 |
Oct 19, 1995 |
05587816 |
Dec 24, 1996 |
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Foreign Application Priority Data
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Oct 9, 1992 [JP] |
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4-298021 |
Dec 16, 1992 [JP] |
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4-354651 |
Feb 17, 1993 [JP] |
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5-051594 |
May 28, 1993 [JP] |
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5-151260 |
Jun 2, 1993 [JP] |
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5-156142 |
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Current U.S.
Class: |
349/9; 349/113;
349/62; 349/96 |
Current CPC
Class: |
G02B
6/0056 (20130101); F21V 9/14 (20130101); G02F
1/13362 (20130101); G02B 6/0053 (20130101); G02F
1/13355 (20210101); G02F 1/133615 (20130101) |
Current International
Class: |
F21V
8/00 (20060101); F21V 9/14 (20060101); F21V
9/00 (20060101); G02F 1/1335 (20060101); G02F
1/13 (20060101); G02F 001/133 () |
Field of
Search: |
;349/9,62,64,96,115
;359/487,490 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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JP |
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2-17 |
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Jan 1990 |
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3-15002 |
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04-110990 |
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JP |
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5-184524 |
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JP |
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WO 92/04648 |
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WO |
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PCT/US94/14323 |
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Mar 1995 |
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WO |
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Other References
"Polarized Backlight for Liquid Crystal Display", vol. 33, No. 1B
(Jun. 1990) IBM Technical Disclosure Bulletin, pp. 143-144.* .
Patent Abstracts of Japan, vol. 016, No. 506(P-1440), Oct. 20, 1992
JP-A04 184 429, Jul. 1, 1992.* .
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|
Primary Examiner: Spikes; William L.
Assistant Examiner: Ton; Toan
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Parent Case Text
.Iadd.More than one reissue application has been filed for the
reissue of U.S. Pat. No. 5,587,816. The reissue applications are
application Ser. Nos. 09/512,735, and 09/016,09, filed on Jan. 30,
1998, now U.S. Patent RE37,377, issued on Sept. 18, 1002, all of
which are divisional reissues of U.S. Pat. No.
5,587,816..Iaddend..[.This application.]. .Iadd.U.S. Pat. No.
5,587,816 .Iaddend.is a Continuation of application Ser. No.
08/132,864, filed on Oct. 7, 1993, now abandoned.
Claims
We claim: .[.
1. An illumination device for a direct viewing type display
element, comprising: a flat light guide; a light source set such
that light is incident on a side portion of said flat light guide;
a polarized light separating sheet set on a light emitting side of
the flat light guide for transmitting a p polarized light component
and reflecting at least a portion of an s polarized light component
with respect to a light ray substantially having a predetermined
direction of incidence; and a light reflecting sheet disposed on
another side opposite to and facing said light emitting side of the
flat light guide in parallel with the emitting side, said light
reflecting sheet converting said portion of s polarized light
component reflected by said polarized light separating sheet into p
polarized light and reflecting the converted p polarized light to
said polarized light separating sheet for transmission of the
converted p polarized light through said polarized light separating
sheet in overlapping relationship with the p polarized light
component..]. .[.
2. The illumination device for a direct viewing type display
element according to claim 1, wherein the polarized light
separating sheet is comprising a multi-layered structure wherein
light transmitting media having a relatively large refractive index
and light transmitting media having a relatively small refractive
index are laminated..]..[.
3. The illumination device for a direct viewing type display
element according to claim 1, wherein the polarized light
separating sheet comprises a transparent supporter and at least one
dielectric thin film laminated on said transparent supporter having
a thickness which is equal to or smaller than a wavelength of
visible light..]..[.
4. The illumination device for a direct viewing type display
element according to claim 1, wherein the polarized light
separating sheet comprises a plurality of laminated transparent
polymer layers having different refractive indices..]..[.
5. A liquid crystal display device, wherein the illumination device
according to claim 1 is disposed on a rear side of a direct viewing
type liquid crystal display element such that a principle
polarization direction of emitted light from the illumination
device substantially agrees with a direction of an optical axis of
polarization of a polarizing sheet on a light-incident side of a
liquid crystal display element..]..[.
6. The liquid crystal display device according to claim 5 further
comprising: a light deflecting means disposed between the polarized
light separating sheet and the liquid crystal display element for
deflecting a direction of a light ray maximizing a light intensity
among light distributing directions to a direction substantially
perpendicular to a display face of the liquid crystal display
element..]. .[.
7. The liquid crystal display device according to claim 5, further
comprising: a means for rotating polarization direction disposed
between the illumination device and the liquid crystal display
element for rotating the principle direction of emitted light..].
.[.
8. An illumination device for a direct viewing type display
element, comprising: a flat light emitting means for emitting a
diffused light including a first polarized light component having a
first direction of polarization and a second polarized light
component having a second direction of polarization perpendicular
to the first direction of polarization; and a polarized light
converting and emitting means disposed in an optical path of a
light emitted from said flat light emitting means for emitting said
first polarized light component and for emitting, in overlapping
relationship with the emitted first polarized light component, at
least a portion of said second polarized light component after
selectively converting said portion of said second polarized light
component into the first polarized light component..]. .[.
9. An illumination device for a direct viewing type display
element, comprising: a flat light guide; a light source set such
that light is incident on a side portion of said flat light guide;
a polarized light separating sheet set on a light emitting side of
the flat light guide for transmitting a p polarized light component
and reflecting at least a portion of an s polarized light component
with respect to light rays substantially having a predetermined
direction of incidence; and a light reflecting sheet disposed on
another side opposite to and facing said light emitting side of the
flat light guide in parallel with the emitting side, said light
reflecting sheet converting said portion of s polarized light
component reflected by said polarized light separating sheet into p
polarized light and reflecting the converted p polarized light to
said polarized light separating sheet for transmission of the
converted p polarized light trough said polarized light separating
sheet in overlapping relationship with the p polarized light
component..]. .[.
10. The illumination device for a direct viewing type display
element according to claim 9, wherein the polarized light
separating sheet comprises a transparent supporter and at least one
dielectric thin film laminated on said transparent supporter having
a thickness which is equal to or smaller than a wavelength of
visible light..]..[.
11. The illumination device for a direct viewing type display
element according to claim 9, wherein the polarized light
separating sheet is composed of a multi-layer structure including
alternately laminated light transmitting media, having a refractive
index, n.sub.0, and other transmitting media, having a refractive
index, n.sub.1, smaller than the refractive index
n.sub.0..]..[.
12. The illumination device for a direct viewing type display
element according to claim 9, wherein a .lambda./4 phase
interference plate is provided on the back face of the light guide
and a side face of the light guide opposing the light
source..]..[.
13. The illumination device for a direct viewing type display
element according to claim 10, wherein a .lambda./4 phase
interference plate is provided on the back face of the light guide
and a side face of the light guide opposing the light
source..]..[.
14. The illumination device for a direct viewing type display
element according to claim 9, wherein a prism shape is formed on a
surface of the light guide..]..[.
15. The illumination device for a direct viewing type display
element according to claim 14, wherein the prism shape is a
lenticular lens array..]..[.
16. The illumination device for a direct viewing type display
element according to claim 9, further comprising a light
deflector..]..[.
17. The illumination device for a direct viewing type display
element according to claim 16, wherein a micro lens array or prism
array having a lenticular shape or Fresnel shape is employed as the
light deflector..]..[.
18. An illumination device for a direct viewing type display
element, comprising: a flat light guide; a light source set such
that light is incident on a side portion of said flat light guide;
a polarized light separating sheet set on a light emitting side of
the flat light guide for transmitting a p polarized light component
and reflecting at least a portion of an s polarized light component
with respect to a light ray substantially having a predetermined
direction of incidence; and a light reflecting sheet disposed on
another side opposite to and facing said light emitting side of the
flat light guide in parallel with the emitting side, wherein said
polarized light separating sheet is planar and parallel to the
light emitting side of the flat light guide and is set in proximity
to the flat light guide in the light emitting direction..].
.Iadd.
19. In an illumination device for a direct viewing type display
element including a display face, the improvement comprising: a
planar polarized light separating sheet comprising at least two
opposed planar layers with different refractive properties, said
planar polarized light separating sheet separating incident light
into a first polarized light component which is transmitted through
the display face and a second polarized light component which is at
least partially reflected; and a reflecting element disposed
opposite said planar polarized light separating sheet to produce
phase-interference of the second polarized light component and
reflect light resulting from said phase-interference to said planar
polarized light separating sheet..Iaddend..Iadd.
20. The illumination device according to claim 19, wherein said
polarized light separating sheet comprises at least one planar
plastic layer..Iaddend..Iadd.
21. The illumination device according to claim 20, wherein said at
least one planar plastic layer is obtained by
extrusion..Iaddend..Iadd.
22. The illumination device of claim 21, wherein said polarized
light separating sheet comprises plural planar layers including
said at least one plastic layer obtained by multi-layer
extrusion..Iaddend..Iadd.
23. The illumination device according to claim 22, further
comprising: a microlens array or a prism array which converts a
direction of light distribution of the light for a display
element..Iaddend..Iadd.
24. The illumination device according to claim 23, wherein the
direction of light distribution of light for said display element
is substantially perpendicular to said display
element..Iaddend..Iadd.
25. The illumination device according to claim 21, further
comprising: a microlens array or a prism array which converts a
direction of light distribution of the light for a display
element..Iaddend..Iadd.
26. The illumination device according to claim 20, further
comprising: a microlens array or a prism array which converts a
direction of light distribution of the light for a display
element..Iaddend..Iadd.
27. The illumination device according to claim 26, wherein said
polarized light separating sheet comprises at least one planar
polymer layer..Iaddend..Iadd.
28. The illumination device according to claim 27, wherein said at
least one planar polymer layer is obtained by
extrusion..Iaddend..Iadd.
29. The illumination device of claim 28, wherein said polarized
light separating sheet comprises plural planar layers including
said at least one polymer obtained by multi-layer
extrusion..Iaddend..Iadd.
30. The illumination device according to claim 29, wherein said
polarized light separating sheet comprises plural planar polymer
layers having a thickness not less than 0.05 .mu.m and not more
than 0.45 .mu.m..Iaddend..Iadd.
31. The illumination device according to claim 30, further
comprising: a microlens array or a prism array which converts a
direction of light distribution of the light for a display
element..Iaddend..Iadd.
32. The illumination device according to claim 31, wherein the
direction of light distribution of light for said display element
is substantially perpendicular to said display
element..Iaddend..Iadd.
33. The illumination device according to claim 29, further
comprising: a microlens array or a prism array which converts a
direction of light distribution of the light for a display
element..Iaddend..Iadd.
34. The illumination device according to claim 33, wherein the
direction of light distribution of light for said display element
is substantially perpendicular to said display
element..Iaddend..Iadd.
35. The illumination device according to claim 28, further
comprising: a microlens array or a prism array which converts a
direction of light distribution of the light for a display
element..Iaddend..Iadd.
36. The illumination device according to claim 27, wherein said
polarized light separating sheet comprises plural planar polymer
layers having a thickness not less than 0.05 .mu.m and not more
than 0.45 .mu.m..Iaddend..Iadd.
37. The illumination device according to claim 27, further
comprising: a microlens array or a prism array which converts a
direction of light distribution of the light for a display
element..Iaddend..Iadd.
38. The illumination device according to claim 36, further
comprising: a microlens array or a prism array which converts a
direction of light distribution of the light for a display
element..Iaddend..Iadd.
39. The illumination device according to claim 36, wherein the
direction of light distribution of light for said display element
is substantially perpendicular to said display
element..Iaddend..Iadd.
40. The illumination device according to claim 19, further
comprising: a microlens array or a prism array which converts a
direction of light distribution of the light for a display
element..Iaddend.
Description
The present invention relates to a direct viewing type liquid
crystal display device which is employed in a liquid crystal
television set, a liquid crystal display for a computer and the
like.
In recent years, the technical progress of a direct viewing type
liquid crystal display device is significant especially in a device
employing a color display element. There are many displays having a
display quality which is comparable to that of a CRT. In the
black-white display, until several years before, the main stream of
technology had been a reflecting type liquid crystal display
element which did not employ a backlight. However, currently,
almost all of them are replaced by a transmitting type liquid
crystal display element in use of a backlight even in the
black-white display. In the color liquid crystal display, the
display can not work without a backlight, and the backlight is an
indispensable device in the direct viewing type liquid crystal
display device.
In a so-called "notebook type personal computer" which has come in
use in recent years, the portability is important, and therefore,
driving thereof by a battery is indispensable. However, currently,
the time capable of driving the device without charging the battery
is only several hours, and does not reach a level wherein a day's
operation can continuously be performed. From this viewpoint, the
prolongation of the time of continuous use is extremely important.
Especially, the illumination device is a device consuming much
power in that system, and reducing the power consumption of the
illumination device is of great significance.
In the mean time, there is a specific contrast ratio distribution
in accordance with the viewing angle in a liquid crystal display
element employed in the notebook type personal computer. A
representative example is shown in FIG. 5 in case of a super
twisted nematic liquid crystal display element. FIG. 5 shows the
representative viewing cone of the super twisted nematic liquid
crystal display element and a diagram designating a change in the
contrast ratio of a section on a horizontal line passing through
the center of the contrast ratio of the viewing cone. According to
FIG. 5, the viewing angle of the liquid crystal display element is
widened from a direction perpendicular to the image plane
substantially by 40.degree. through 50.degree., and it is revealed
that there is a region having an especially high contrast ratio in
the vicinity of the center.
In an actual use, the device is often set such that the contrast
ratio is maximized on the image plane viewed from an operator in
case of the notebook type personal computer. Accordingly, the
illuminating efficiency can substantially be promoted, when the
maximum brightness of illumination is formed in the viewing
direction (mostly a direction perpendicular to an image plane or a
direction a little deviated therefrom) which causes the maximum
contrast ratio.
The viewing angle of the liquid crystal display element employed in
the notebook type personal computer is widened substantially to not
less than 40.degree. through 50.degree., and investigations are
being performed for widening the viewing angle. Therefore, it is
important to adjust the light direction distribution of the
illumination device so that the display can be viewed from a more
or less oblique direction.
The adapting of the brightness distribution of the illumination
device to this contrast ratio distribution in such a manner is
significant as a means for promoting substantial brightness.
The color liquid crystal display device is grossly classified into
two systems, namely, a twisted nematic (TN) liquid crystal display
device by the active matrix driving using TFTs and a super twisted
nematic (STN) liquid crystal display device by the multiplex
driving. Both are provided with a construction wherein polarizing
sheets are disposed on the light-incident side and the
light-emitting side of an element wherein the liquid crystal layer
is hold by glass substrates, and the liquid crystal display system
is operated by modulating the polarization state of an incident
linearly polarized light.
However, the direction of polarization of the incident light of the
conventional liquid crystal display element, is not uniform due to
the randomly polarized light. Therefore, more than half of the
incident light is absorbed by the polarizing sheet disposed on the
light-incident side of the display element, and does not
substantially contribute to the illuminating light. It is proposed
as a structure for reusing the light to be absorbed by the
polarizing sheet, wherein a polarized light separator is interposed
between a light source lamp and a liquid crystal display device in
a projecting type liquid crystal display element, for separating a
non-polarized light to mutually orthogonal polarized lights, one
polarized light is directly emitted from the polarized light
separator, and the other polarized light is converged to the light
source lamp and is reused as a light source light (Japanese
Unexamined Patent Publication No. 184429/1992).
However, this method is performed on the premise of a projector
(projecting type), and it is necessary to provide a sufficient
distance between the light source and the polarized light
separator. Further, the device effectively functions as an
illumination for the projecting type liquid crystal display
element, only when the light is a considerably collimated parallel
ray. Accordingly, the device is not suitable to adopt as the
illumination for a direct viewing type display element wherein
thinning thereof is an indispensable condition, and the brightness
distribution of the illumination device should adapt to the
contrast ratio distribution of the liquid crystal display.
Further, it is proposed that a prism array is interposed between a
light source for illumination and a display element, as a means for
converging light in a direction perpendicular to a display face.
However, according to this means, the brightness in the direction
perpendicular to the display face is promoted by narrowing the
direction of an illuminating light in a specified range, and
therefore, the light direction distribution of the illuminating
light is narrowed. Further, the brightness in the direction
perpendicular to the display face is not sufficient even by this
means. Accordingly, an illuminance distribution suitable for the
direct viewing type liquid crystal display element can not be
provided.
In this invention, attention has been paid to the fact wherein only
a polarized light having a specific direction of polarization
contributes to the promotion of the illuminance of the directly
viewing type liquid crystal display element, and a light in a
specific direction is selectively converted its polarization
direction, among polarized lights which do not contribute to the
promotion of the illuminance of the liquid crystal display element
by themselves. In this way, the intensity of light can be enhanced
in the specific direction while maintaining a wide light direction
distribution, with respect to a polarized light having a direction
of polarization which can contribute to the promotion of the
illuminance, and the light distribution is suitable for the
directly viewing type liquid crystal display.
According to a first aspect of the present invention, there is
provided an illumination device for a direct viewing type display
element comprising: a flat light guide; a light source set such
that light is incident on a side portion of said flat light guide;
a polarized light separating sheet set on a first side of a light
emitting side of the flat light guide for transmitting a p
polarized light component and reflecting at least a portion of an s
polarized light component with respect to a light ray substantially
having a predetermined direction of incidence; and a light
reflecting sheet disposed on a second side opposite to said light
emitting side of the flat light guide in parallel with the light
emitting site.
According to a second aspect of the present invention, there is
provided the illumination device for a direct viewing type display
element according to the first aspect, wherein the polarized light
separating sheet is comprising a multi-layered structure wherein
light transmitting media having a relatively large refractive index
and light transmitting media having a relatively small refractive
index are laminated.
According to a third aspect of the present invention, there is
provided the illumination device for a direct viewing type display
element according to the first aspect, wherein the polarized light
separating sheet comprises a transparent supporter and at least one
dielectric thin film laminated on said transparent supporter having
a thickness which is equal to or smaller than a wavelength of
visible light.
According to a fourth aspect of the present invention, there is
provided the illumination device for a direct viewing type display
element according to the first aspect, wherein the polarized light
separating sheet comprises a plurality of laminated transparent
polymer layers having different refractive indices.
According to a fifth aspect of the present invention, there is
provided a liquid crystal display device, wherein the illumination
device according to Claim 1 is disposed on a rear side of a direct
viewing type liquid crystal display element such that a principle
polarization direction of emitted light from the illumination
device substantially agrees with a direction of an optical axis of
polarization of a polarizing sheet on a light-incident side of a
liquid crystal display element.
According to a sixth aspect of the present invention, there is
provided the liquid crystal display device according to the fifth
aspect further comprising: a light reflecting means disposed
between the polarized light separating sheet and the liquid crystal
display element for deflecting a direction of a light ray
maximizing a light intensity among light distributing directions to
a direction substantially perpendicular to a display face of the
liquid crystal display element.
According to a seventh aspect of the present invention, there is
provided the liquid crystal display device according to the fifth
aspect, further comprising: a means for rotating polarization
direction disposed between the illumination device and the liquid
crystal display element for rotating the principle direction of
emitted light.
According to an eighth aspect of the present invention, there is
provided an illumination device for a direct viewing type display
device comprising: a flat light emitting means for emitting a
diffused light including a first polarized light component having a
first direction of polarization and a second polarized light
component having a second direction of polarization perpendicular
to the first direction of polarization; component and a polarized
light converting means disposed in an optical path of a light
emitted from said flat light emitting means for emitting said first
polarized light component and for emitting at least a portion of
said second polarized light component after converting said portion
of the second polarized light component into the first polarized
light component selectively with respect to a light ray
substantially having a light direction maximizing a brightness
thereof.
In the drawings:
FIG. 1 is a sectional digram showing an example of the present
invention;
FIG. 2 is a sectional diagram showing another example of the
present invention;
FIG. 3 is a sectional diagram showing another example of the
present invention;
FIG. 4 is a sectional diagram showing a construction of a polarized
light separator employed in this invention;
FIG. 5 is a graph showing a contrast ratio curve of an STN liquid
crystal display element;
FIG. 6 is a sectional diagram showing another example of the
present invention;
FIG. 7 is a plane view showing an arrangement of optical axes of
FIG. 6;
FIGS. 8(a) through 8(d) are sectional diagram respectively showing
examples of uniform light forming means of this invention; and
FIGS. 9(a) and 9(b) are sectional diagram respectively showing
other examples of uniform light forming means of this
invention.
Although, there are various systems in manufacturing a flat
illumination device, they are grossly classified into two kinds.
The most general one is a system which is called the "internal
illuminating system" of the "direct method", wherein a light source
is disposed just under the display. On the other hand, in the other
system which is called the "edge light" type system, the light
source or sources are disposed outside the display, for instance,
an approximately linear light emitting body or bodies such as
fluorescent lamps (mostly cold cathode fluorescent lamp) and the
like are attached to one side or two sides of a light guide made of
a transparent acrylic resin plate or the like, which is a light
illuminating plane, and light is introduced to the light guide by
providing a lamp cover composed of a reflector.
It is preferable that the light emitting means of this invention is
the edge light type system, which is composed of a flat light
emitter and a light source or sources attached such that light is
incident on the side portion or portions of the flat light guide,
since the edge light type illumination device is compact and is
mostly preferable in view of promoting the portability of the
liquid crystal display device.
Further, it is preferable to provide a polarized light separating
sheet which is installed on the light-emitting face side of the
flat light guide, and which transmits the p polarized light
component (first polarized light component) and reflects at least a
portion of the s polarized light component (second polarized light
component) with respect to a light ray having substantially a
specific direction of incidence, and a polarized light converting
means composed of a guide and a light-reflecting sheet provided to
a side of the flat light guide opposite to the light-emitting plane
approximately in parallel with the light-emitting plane. When the
light-reflecting plane is employed in such a construction, the
separated polarized light can be reused and the direction of
polarization is changed in the reflection. Accordingly, the
light-reflecting plane operates as the polarized light converting
means in cooperation with the polarized light separating sheet. In
the following, an element having the polarized light separating
sheet is called a polarized light separator. However, this does not
mean that the polarized light separator is necessary as an element
separated from the flat light guide. The flat light guide may be
provided with a polarized light separating function.
In this construction, with respect to light having substantially a
specified angle as the angle of incidence to the polarized light
separator, the p polarized light component which has transmitted
through the polarized light separator, is incident on the liquid
crystal display element after passing through a polarizing plate,
and the s polarized light component is reflected to the inside of
the flat light guide. When the reflected and returned s polarized
light component is reflected on the surface of the flat light
guide, a phase change is caused, and a "p" polarized light
component is formed which can transmit through the polarized light
separator. Accordingly, a component which is converted into the p
polarized light component is formed by reflecting the s polarized
light component which has been reflected by the polarized light
separator, on the surface of the flat light guide, which
contributes to the component transmitting towards the liquid
crystal display element. As a result, the flat illumination device
is provided with a high illuminance, with respect to a specified
viewing direction.
It is preferable that the polarizing sheet provided on the
light-incident side of the liquid crystal display element, is
disposed such that the transmittance thereof is maximized with
respect to the p polarized light component which is emitted from
the polarized light separator, for employing the illumination
device as the backlight of the liquid crystal display element. That
is, an average direction of an optical axis of polarization of a
light ray emitted from the flat light guide in the flat
illumination device approximately agrees with the optical axis of
polarization of the polarizing sheet on the light-incident side of
the liquid crystal display element.
The polarized light separator of this invention can employ a
multi-layered structure wherein light transmitting media having a
relatively large refractive index and other light transmitting
media having a relatively small refractive index are alternately
laminated, a structure wherein at least one layer of a dielectric
film having a thickness of preferably not larger than 1,000 nm is
formed at least on one face of a flat light transmitting supporter,
or a structure wherein a plurality of kinds of transparent polymer
layers having different refractive indices are laminated.
An explanation will be given of a polarized light separator
composed of a multi-layer structure wherein light transmitting
media having a relatively large refractive index and other light
transmitting media having a relatively small refractive index are
alternately laminated, as follows.
This multi-layer structure is provided with a property wherein,
with respect to an oblique incident light, the transmittance and
the reflectance are dependent on the polarization of the oblique
incident light. Therefore, the structure can be employed as a light
non-absorbing type polarization element. Generally, when, on an
interface between an optical material having the refractive index
of n.sub.0 and another optical material having the refractive index
of n.sub.1, the angle of incidence .theta..sub.01 of light which is
incident from the optical material having the refractive index of
n.sub.0 to that of n.sub.1 is expressed by the following
equation,
there is no p polarized light component in the reflected light, the
reflected light is composed of the s polarized light, and the
transmitted light is composed of the residual s polarized light
component and the p polarized light, which is well known. The angle
of incidence .theta..sub.01 is called Brewster's angle
.theta..sub.B. Accordingly, when the optical material layers having
the refractive indices of n.sub.0 and n.sub.1 are alternately
laminated, with respect to light having the angle of incidence
which is substantially equal to Brewster's angle, the p polarized
light component transmits through the structure, but the s
polarized light component is reflected by the plurality of
interfaces and its transmitted light component is almost nullified.
Accordingly, the emitted light from the above multi-layer structure
has a polarization.
The multi-layer structure may be of any components, so far as it is
composed of at least two layers of light-transparent materials
having different refractive indices. However, since the polarized
light dependency of the reflectance at the interface is very
effective, when a difference between the interface is very
effective, when a difference between the refractive indices is
large, a combination wherein the difference of refractive indices
is large, is preferable. For instance, there is a combination of
air (n.apprxeq.1.0) and a transparent resin, such as a plastics
(n.apprxeq.1.5); acrylic resin, polycarbonate, polyurethane,
polystyrene and the like. The above combination is preferable since
a multi-layer structure having a large area can be provided
cheeply.
There is basically no restriction on the thicknesses of the
respective layers of the multi-layer structure. Further, there may
be a structure which is inhomogeneous depending on locations, or a
structure wherein flat air bubble layers are dispersed in a
homogeneous plastics in a lamellar form, so far as the respective
layers in the multi-layer structure are disposed substantially in
parallel with each other.
It is also possible to employ a transparent dielectric multi-layer
film as a material of the structure. When a dielectric multi-layer
film is employed for the multi-layer structure, the thickness of
layer is to be not less than approximately ten times as much as an
order of a wavelength of light, such that lights reflected from
interfaces of the respective layers of the multi-layer structure do
not interfere with each other, thereby enabling to provide a
polarization characteristic having a small wavelength dependency
with respect to a white light source. On the other hand, when the
thicknesses of the respective layers are too large, the thickness
of a total of the multi-layer structure is large, which is not
suitable for the light-weight and the thin-shape thereof.
Accordingly, the thickness of layer of approximately 3 .mu.m
through 100 .mu.m is suitable for the purpose. Further, when the
film thicknesses are non-uniform, a "coloring" due to the
interference of light can be suppressed. Accordingly, there is a
case wherein it is preferable to make the thicknesses of the
respective layers non-uniform.
In case of a construction wherein flat air bubble layers are
dispersed in a homogeneous plastics in a lamellar form, the
thickness of the flat air bubble layer is to be approximately 3
.mu.m through 100 .mu.m. As another structure, there is a
multi-layer structure wherein a transparent thin plate having a
thickness of approximately 3 .mu.m through 100 .mu.m, is laminated
on a scattered gap controlling material such as beads, glass fibers
or the like. In this case, compared with a structure wherein flat
air bubble layer are dispersed in a homogeneous plastics in a
lamellar form, a polarization operation having a high light
distinguishing ratio can be provided, since the angle of incidence
of an interface between materials having different refractive
indices does not differ with locations as in the flat air bubble
layers.
The reflectance R.sub.S of an s polarized light on an interface
between an optical material having a refractive index of n.sub.0
and another optical material having a refractive index of n.sub.1
under the condition of Brewster's angle, is shown by the following
equation (1). ##EQU1##
The larger the difference between the refractive indices, the
larger the reflectance R.sub.S. Therefore, in an approximation
(which is sufficiently applicable to this case) not considering
multiple reflections between layers, wherein 100% of the p
polarized light component in an incident light to the multi-layer
structure, is transmitted and x % of the s polarized light
component is transmitted, the necessary number of layers N is
provided by the following equation (2).
Accordingly, when the multi-layer structure is constructed by a
combination of air (n.sub.1.apprxeq.1.0) and a plastics
(n.sub.0.apprxeq.1.5), Brewster's angle .theta..sub.B
=56.3.degree., R.sub.S =14.8%, the number of layers which is
necessary for making the transmittance of the s polarized light
component not larger than 2%, is 12.
Therefore, in case of a structure wherein flat air bubble layers
are scattered in a homogeneous plastics in a lamellar form, when
six or more flat air bubble layers are formed in the depth
direction, with respect to a light having substantially an angle of
incidence of Brewster's angle, the transmittance of the s polarized
light component is not larger than 2% and more than 98% of the
component is reflected. On the other hand, the p polarized light
component is provided with the transmittance of approximately 100%
without the loss of light quantity.
As explained above, the polarization function of the multi-layer
structure operates most effectively when the angle of incidence is
at Brewster's angle. Accordingly, when the multi-layer structure is
provided in the flat illumination device composed of the light
source and the flat light guide, it is preferable for the
substantial promotion of brightness to provide a construction
wherein the angle of incidence of incident light to the multi-layer
structure is substantially Brewster's angle, by the means of the
light source, the flat light guide and an optical element further
added with a light reflecting means and the like.
The reason that the polarized light separator in use of a
dielectric film employing light interference works, is as follows.
This polarized light separator is provided with a property wherein
the transmittance and the reflectance thereof are dependent on the
polarization of the angle of oblique incident light, and therefore,
it can be employed as a light non-absorbing type polarization
element.
When an optical element having a film thickness in the order of a
wavelength of visible light and the refractive index of n.sub.1 is
interposed between an optical material having the refractive index
of n.sub.0 and another optical material having the refractive index
of n.sub.2, light will interfere. The angle of incidence and the
angle of emittance of light which is incident from the optical
material having the refractive index of n.sub.0 to the optical
material having the refractive index of n.sub.1, are determined to
be .theta..sub.0 and .theta..sub.1, respectively. The angle of
incidence and the angle of emittance of light which is incident
from the optical material having the refractive index of n.sub.1 to
the optical material having the refractive index of n.sub.2, are
determined to be .theta..sub.1 and .theta..sub.2, respectively. The
reflection complex amplitude in consideration of the effect of
interference when the optical material has the refractive index of
n.sub.1 and the film thickness is d.sub.1, is provided by the
following equation (3).
where .rho..sub.a and .rho..sub.b designate amplitude reflectances
of Fresnel reflection caused on the interfaces between the optical
material having the refractive index of n.sub.0 and the optical
material having the refractive index of n.sub.1, and between the
optical material having the refractive index of n.sub.1 and the
material having the refractive index of n.sub.2.
The reflectance is provided by the following equation (4), where
.rho.* is the complex conjugation of .rho.. ##EQU2##
The p polarized light component is provided by the following
equation (5).
The s polarized light component is provided by the following
equation (6).
From these equation, it is revealed that, when the refractive
indices n.sub.0 and n.sub.2, the refractive index n.sub.1 and the
film thickness d.sub.1 satisfy a certain condition, the ratio of
the transmitted light intensity of the p polarized component as
compared to the transmitted light intensity of the s polarized
light component, increases in comparison with a case wherein there
is no interference, at a certain angle of incidence of
.theta..sub.0. The above equations are concerned with a case
wherein the interference film is a single layer. However, the same
way of thinking is applicable similarly when the interference film
is of a multi-layer.
As stated above, the light emitted from the polarized light
separator wherein at least one layer of dielectric films having a
thickness of not larger than 1,000 nm, is formed on at least one
face of a flat light-transmitting supporter, is provided with high
degree of polarization. The film thickness of not larger than 1,000
nm signifies that the thickness is mainly not larger than the order
of a visible light wavelength, and it is preferably not larger than
800 nm.
Since the dielectric film of the polarized separator having the
thickness of the order of a visible light wavelength, utilizes the
light interference, it is generally possible to promote the degree
of polarization of a specified wavelength when the number of layers
increases, and conversely, the wavelength dependency also
increases. When the spectrum of the employed backlight light source
is of a narrow band wavelength, it is possible to construct a
multi-layer film wherein the degree of polarization increases with
respect to the backlight wavelength range of light. However, a film
having too many layers causes a poor productivity. Therefore, the
number of layers is preferably 5 through 15. On the other hand,
when a white backlight is employed for color display, it is
preferable to employ an interference film having not larger than 5
layers, especially a single layer, to restrain the wavelength
dependency of the degree of polarization to a low value. It is
preferable to form a single layer film of TiO.sub.2 or ZrO.sub.2
since a flat degree of polarization is provided over the whole
region of visible light and it is easy to control the film
thickness, although a high degree of polarization can not be
provided.
The material of the flat light-transmitting supporter employed in
the polarized light separator is glass or plastics such as acrylic
resin, polycarbonate, polyurethane, polystyrene and the like. It is
preferable that the material is light and surface thereof is
smooth.
As a material of a dielectric film, TiO.sub.2, ZrO.sub.2, ZnS,
Y.sub.2 O.sub.3, SiO.sub.2, MgF.sub.2, Na.sub.3 AlF.sub.6, Ta.sub.2
O.sub.5 and the like, are pointed out. The refractive indices of
these dielectric films are normally in a range of 1.4 through 2.5,
and the film formation may be performed by selecting dielectric
films having pertinent refractive indices. Further, the film
forming can be performed by normally employed methods such as vapor
deposition, sputtering, and the like.
Further, the polarized light separator of the invention can be
formed by laminating plural kinds of transparent polymer layers
having different refractive indices. Also in this case, as in the
above-mentioned multi-layer structure or the dielectric thin film,
it operates as a light non-absorbing polarized light separator.
The polarized separator can be formed by a multi-layer film
construction whereby the degree of polarization is promoted with
respect to the wavelength range of the backlight. Since the film
having too many layers causes the poor productivity, the number of
layers is preferably not smaller than 30, more preferably 100
through 400. It is preferable that the materials of the
light-transmitting polymers having different refractive indices are
suitable for forming a multi-layer laminated body. Further, the
larger the difference in the refractive indices, the more
preferable.
It is preferable that the materials of the transparent polymers are
selected from at least two of plastics such acrylic resin,
polycarbonate resin, polyurethane, polystyrene, triacetyl
cellulose, polymethylpentene, polyether sulfone and the like.
Further, it is preferable that the difference in the refractive
indices is not less than 0.03, to improve the efficiency.
A consideration should be given to the method of making the
polarized light separator in the above selection. Although there
are the casting method, the multi-layer extrusion method and the
like as the method of manufacturing the multi-layer laminated
product, it is preferable to adopt the multi-layer extrusion method
whereby a multi-layer film having not less than 30 layers can
economically formed.
The method of manufacturing is disclosed in U.S. Pat. No. 3,773,882
and U.S. Pat. No. .[.3,883,606.]. .Iadd.3,884,606.Iaddend.. In case
of a polarized separator having a thickness enough to decrease the
interference operation of light, the total thickness increases.
Accordingly, when the interference of light is used, the optical
thickness of at least one of polymers having different refractive
indices, is not less than 0.05 .mu.m and not more than 0.45 .mu.m.
Further, the thicknesses of both polymers are considerably
different rather than almost equal, since a so-called nacreous
color due to the variation in the thicknesses of the respective
layers, is not manifested. Further, a hard coat layer of silicone
and the like may be provided on the surface.
FIG. 4 is an outline sectional diagram of a polarized separator
which employs the interference of light. First polymer layers 21
and second polymer layers 22 are alternately laminated, and at
least the second polymer layers 22 are provided with a thickness
which is enough to cause the interference of light.
The examples of preferable combinations of preferable polymers in
consideration of the method of manufacturing, are acrylic resin and
polycarbonate, acrylic resin and polystyrene, polymethylpentene and
polycarbonate and the like.
A detailed explanation will be given of a liquid crystal display
device of this invention using FIG. 1 which is a representative
construction diagram, as follows.
A fluorescent lamp 1 (cold cathode fluorescent lamp) having a
length corresponding to a length of a side face of a light guide 3,
is attached to the side of the light guide 3 (an acrylic resin
plate) which is transparent and which is an illuminating sheet, and
a lamp cover having a reflecting body on the innerface thereof, is
provided thereby introducing a light emitted from the lamp into the
light guide 3.
As stated above, in case of using an illumination device in a
direct viewing type liquid crystal display element, the light
distribution property of the illumination light is extremely
important. The directivity (angular distribution) of the light
transmitting through the light guide 3, is determined by the light
direction distributing property of the fluorescent lamp, the light
gathering property of the reflecting body, the light transferring
property of the light guiding plate and the like. Further, in the
light transferring property of the light guide, the functions of
sending the light incident on an edge portion of the light guide to
the inside of the light guide, and the function of emitting the
transmitted light to a predetermined direction, are important.
The function of sending the light which is incident on the edge
portion of the light guide to the inside of the light guide, is
determined in accordance with the material employed in the light
guide and the interface reflecting property. That is to say, on the
side of the liquid crystal display element 12 of the light guide 3,
the light having the angle of incidence which is not smaller than
the total reflection angle .theta..sub.c which is determined by the
refractive index of the light guide 3, is totally reflected and
transmitted through the light guide 3, and light having the angle
of incidence which is below the total reflection angle
.theta..sub.c, is refracted on the surface of the light guide 3 and
emitted on the side of the liquid crystal display element 12. For
instance, the total reflection angle .theta..sub.c on the interface
of air (n.apprxeq.1.0) and a transparent plastics (for example,
n.apprxeq.1.5), is determined by the following equation (7), and
the incident light having the angle of incidence of not more than
42.2.degree. can emit from the illumination plane of the light
guide 3.
As a preferable transparent resin employed in the light guide, for
instance, acrylic resin, polycarbonate, polyurethane, polystyrene,
silicone and the like are pointed out.
When a reflecting plane 5 such as an aluminum reflecting plane is
formed on a face opposite to the liquid crystal display element of
the light guide, the reflected light is guided to the inside of the
light guide. Further, the reflecting plane 5 may be a diffused
reflection plane to increase light emitted from the face on the
side of the liquid crystal display element 12 of the light guide
3.
On the other hand, when most of light which is incident on the
light guide 3 is provided with the angle of incidence which is not
smaller than the total reflection angle .theta..sub.c, the light
emitted from the light guide is very little. Therefore, it is
necessary to provide a function wherein the light is emitted on the
side of the liquid crystal display element 12 of the light guide 3
by avoiding the total reflection condition. As the means of
avoiding the total reflection condition, there is a method such as
a method of forming a white light diffusing material on the surface
of the light guide 3, a method of forming a Fresnel shape
(microlens array, prism array and the like) having a lenticular or
a prism shape on the surface of the light guide.
The Fresnel shape in this case, may be formed on a face of the
light guide on the side of the liquid crystal display element 12,
or may be formed on the opposite side. Further, a film-like plate
having a Fresnel shape may be placed on the face of the light
guide. When it is placed on the light guide, it is necessary that
an air layer is not present between the film and the light guide.
Therefore, it is preferable that a deaeration is performed after
pasting the film, or the film is pasted by employing an adhesive
agent having the refractive index which is comparable to that of
the film. Further, it is preferable that the refractive index of
the film is approximately the same with that of the light
guide.
The polarized light separator employed in this invention manifests
a strong polarized light separating function with respect to light
having the angle of incidence (Brewster's angle) in a specified
range. Therefore, it is preferable that the angle of light incident
on the polarized light separator is provided with a maximum value
at Brewster's angle of the polarized light separator, and the light
quantity is substantially concentrated on Brewster's angle, to
promote the illuminance. Accordingly, it is important to control
the angle of light emitted from the light guide.
To control the angle of emittance, the distribution of the white
light diffusing material, the Fresnel shape, (microlens array,
prism array and the like) with a lenticular or a prism, and the
like are adjusted. For instance, by placing a film of a prism array
13 (see FIG. 2), made by a component which is the same with that of
the light guide, on the surface of the light guide, the maximum of
intensity of light emitted from the light guide is concentrated in
a deviated range of +40.degree. through +80.degree. and -40.degree.
through -80.degree..
Specifically, when a multi-layer structure wherein flat air bubble
layers are dispersed in a lamellar form, as above, is formed,
Brewster's angle .theta..sub.b on the interface between the acrylic
resin and the air bubble layer is 33.7.degree. according to the
equation (1). Therefore, only the p polarized light component is
emitted on the side of the liquid crystal display element 12, by
optimizing the light direction distributing property of a
fluorescent lamp, the light gathering property of the light
reflector, the light transferring property of the light guiding
plane and the like, such that the incident light from the acrylic
resin side to the multi-layer structure is substantially
33.7.degree.. On the other hand, the s polarized light component is
transmitted through the light guide 3 as in the case of total
reflection. Further, the polarization characteristic of this
polarized light separator 6 is an effect which is sufficiently
manifested even when the incident light is a little deviated from
the Brewster's angle condition. In this case, the polarized light
separating operation is significant even with respect to a light
ray having the angle of incidence of 20.degree. through 40.degree.
which is proximate to the total reflection angle, although the
light extinguishing ratios of the s and p polarized light
components are a little deteriorated.
The directivity of light emitted from the polarized light
separator, is not necessarily distributed in the viewing angle of
an observer of the liquid crystal display element, that is, in a
direction orthogonal to a face of the liquid crystal display
element. Rather, in an ordinary case, the direction of emittance of
light which is incident on the polarized light separator by the
angle of incidence proximate to Brewster's angle, concentrates in a
range out of viewing angle. For instance, the direction of
emittance concentrates in the ranges of -40.degree. through
-70.degree. and 40.degree. through 70.degree. with respect to the
vertical direction of the face of the liquid crystal display
element, in a plane including an optical axis of light transferring
through the light guide, and the light which reaches the viewing
angle range of an observer is very little. Therefore, a clear
display is not provided.
For instance, in FIG. 1, in case wherein the incident light from
the side of the acrylic resin to the multi-layer structure is
provided with the angle of incidence of 33.7.degree., the angle of
emittance of the multi-layer structure is sin.sup.-1, ((sin
33.7.degree.)/n)=56.3.degree.. To convert the light direction
distribution of the flat illumination device having the light
direction distribution in the deviated viewing angle plane
(.+-.40.degree. through .+-.70.degree.), to the direction
perpendicular to the illuminating plane, it is effective to further
provide a light deflector on the side of the light emittance of the
polarized light separator. As the light deflector, a microlens
array or prism array or the like having a lenticular shape or
Fresnel shape can be employed.
FIG. 1 shows a case wherein a prism array having Fresnel shape on
the surface is disposed between the light guide 3 laminated with
the polarized light separator 6 and a polarizing sheet 9 on the
light-incident side of the liquid crystal display element 12 with
the prism face in parallel with the progressing direction of light
transferring in the light guide 3. The prism array 7 is of a
columnar prism, the intersection including an average optical axis
of light ray emitted from the flat light guide 3 is triangular.
With respect to the prism array 7, in accordance with the shape and
arrangement (whether the apex of prism is on the light-incident
side or light-emitting side), there is a case wherein the
refraction is caused on the light-incident face and the
light-emitting face of the prism, and a case wherein the total
reflection is caused, and the orientation of the light direction
distribution of the emitting light can be controlled. The optimum
shape and arrangement may be determined by a finally necessary
orientation of the light direction distribution and the orientation
of the light direction distribution of the light emitted from the
polarized light separator.
For instance, a prism array 7 having a sectional shape of an
isosceless triangle having the apex angle of approximately
60.degree. is employed and is arranged such that the apex faces the
face of the polarized light separator. In this case, the light
emitted from the polarized light separator by the angle of the
emittance which is substantially equal to 60.degree., is incident
on a side face of the prism, and totally reflected by the other
side face, and thereafter, emitted from the bottom face of the
prism corresponding to the perpendicular incident direction on the
side of the liquid crystal display element. Therefore, it is
possible to convert the light direction of the light emitted from
the polarized light separator with the angle of emittance of
substantially 60.degree., to the light direction having the
direction perpendicular to the face of the liquid crystal display
element.
In this way, a linearly polarized light flat illumination device
which illuminates the liquid crystal display element in the
perpendicular light direction distribution, is provided. When the
directivity of the light transferring through the light guide, is
large, as a result, the light direction distribution of the light
emitted from the flat illumination device concentrates on the
perpendicular direction, and the range of viewing angle which
corresponds to a clear display, is too narrow. In this case, it is
possible to dispose an optical element such as a light diffusing
sheet 8 which deteriorates the directivity, between the liquid
crystal display element and the light deflecting means such as the
above prism array.
Further, the reflecting face 5 formed on the face opposite to the
liquid crystal display element of the light guide, may be converted
into a light diffusing plane, to deteriorate the directivity of
light transferring in the light guide. Further, the polarized light
separator per se may be provided with a fine rugged structure such
that the light scattering is caused on the interface of the
structure. In case of the polarized light separator having a
construction wherein flat air bubble layers are scattered in a
homogeneous plastics in a lamellar form, as mentioned above, the
shape of the interface of the air bubble layer is random, and the
fine rugged structure is easy to cause. Therefore, the light
diffusing effect is easy to manifest simultaneously.
In FIG. 1, a case is shown wherein the multi-layered structure is
formed on the surface of the light guide. However, it is not
necessary to form the multi-layered structure on the surface of the
light guide, and the multi-layered structure may be disposed inside
the light guide.
It is important in this invention to convert the s polarized light
which is reflected and returned to the inside of the light guide in
the multi-layer structure, to a light including the p polarized
light component in efficiently transferring it in the light guide,
and reuse it, to efficiently provide the linearly polarized light
from the flat illumination device. There are various methods to
convert the s polarized light to a light including the p polarized
light component. The representative examples are described as
follows.
Generally, it is known that, in case wherein a linearly polarized
light is incident on a metal face in a oblique direction and is
reflected, the linearly polarized light is converted into an
elliptically polarized light in accordance with the optical
physical constants (refractive index n, absorption coefficient k)
of a metal. That is, even when the s polarized light is incident, a
p polarized light component is formed in the reflecting light.
Accordingly, when the reflecting plane 5 formed on a face of the
light guide 3 opposite to the liquid crystal display element 12 is
of a metal such as aluminum in this invention, a portion of the s
polarized light is converted into the p polarized light, every time
the light is reflected by this reflecting plane. As another method,
there is a method wherein a phase difference plate composed of a
transparent .[.high.]. polymer material, is employed. For instance,
by disposing the phase shift plate having a pertinent film
thickness, is disposed between the reflecting plane 5 of the light
guide 3 and the polarized light separator 6, the s polarized light
reflected by the polarized light separator becomes an elliptically
polarized light and a portion thereof can be converted into the p
polarized light. FIG. 1 shows an Example of a construction wherein
the polarized light conversion is efficiently performed by
attaching the phase shift sheet 4 on the reflecting plane 5
provided on the light guide 3. Further, the phase shift sheet 4 may
be disposed between the polarized light separator 6 and the light
guide 3.
FIG. 2 shows an example wherein a polarized light separator in
which one layer of a dielectric film is formed on one face of a
flat transparent supporter, is employed, and a prism array 13 for
emitting light to the liquid crystal display element avoiding the
total reflection condition of the light guide. A notation 6a
designates a flat light-transmitting supporter, and 6b, a
dielectric film. Since the other construction is almost the same as
in FIG. 1, parts of FIG. 2 which are the same with those in FIG. 1
are attached with the same notations, and the explanation will be
omitted.
As stated above, the polarized light separator may be of a
component which is different from the light guide. However, it may
be of a single component. For instance, a polarized light
separating layer such as a dielectric body interference film may be
formed on the light guide. The respective interfaces among the
light guide, the outside of the light guide and the dielectric body
interference film achieve an effect which is similar to that of the
polarized light separator. In case wherein the structure of the
light guide is formed into a prism array or the like, to enhance or
to make uniform the light quantity of light emitted from the light
guide, the dielectric interference film may be formed on the
surface having a prism array shape.
Such an example is shown i n FIG. 3. A prism array shape is formed
on the surface of the light guide 3, and a dielectric film 6c is
further formed on the surface of the prism array. A polarized light
separation function is manifested on the interface between the
light guide and the dielectric film. Since the other construction
is approximately the same as in FIG. 1, parts of FIG. 3 which are
the same with those in FIG. 1 are attached with the same notation,
and the explanation will be omitted.
To maximize the quantity of light which transmits through a liquid
crystal panel, the direction of the optical axis of polarization of
the polarizing plate of the liquid crystal panel on the side of the
polarized light separator, should coincide with the direction of an
optical axis of polarization of light emitted from the polarized
light separator. However, the optical axis of polarization of light
emitted from the polarized light separator, is dependent on the
position of the light source disposed on the side portion of the
flat light guide. For instance, in case of FIG. 1, light is emitted
which is polarized in the direction perpendicular to the linear
light source. On the other hand, thee is a direction wherein the
contrast ratio is high, and a direction wherein the contrast ratio
is low depending on the viewing angle, in the liquid crystal panel.
Normally, the liquid crystal panel is designed such that the
contrast ratio is maximized in the direction of viewing the liquid
crystal panel. This viewing angle is influenced by the angle of the
optical axis of the polarization of the polarizing plate.
Accordingly, when the angle of the optical axis of polarization of
the polarizing plate of the liquid crystal panel on the side of the
polarized light separator, suffers a restriction, the direction of
the viewing angle can not freely be determined.
It is possible to provide an optical axis of polarization rotating
means between the illumination device and the display device in
this invention, to cope with such a case.
Generally, the rotation of the optical axis of polarization of
light, is caused in case wherein light transmits through a medium
having the birefringence, or wherein light transmits through a
medium having the optical rotating power. The optical axis of
polarization is rotated when media having the birefringence are
laminated in multi-layers which rotates the optical axis.
Especially, when a linearly polarized light is incident on a medium
having the birefringence, an elliptically polarized light is
emitted therefrom. The ellipticity and the direction of the long
axis of ellipse are determined by the amount of birefringence and
the direction of optical axis of the birefringence medium. However,
when a linearly polarized light is incident on a substrate having
the birefringence the amount of which is a half of the wavelength
.lambda. of the incident light, the emitted light is always a
linearly polarized light. Further, when a medium having the
birefringence of .lambda./2 is provided with an advanced phase axis
direction which is inclined by .theta. with respect to the
direction of the optical axis of polarization of the incident
linearly polarized light, the linearly polarized light is emitted
inclined by 2.theta. with respect to the direction of the optical
axis of the polarization of the incident linearly polarized
light.
It is possible to convert the optical axis of polarization of a
linearly polarized light which is polarized in an arbitrarily
direction, to a specified direction while the linearly polarized
light remains as it is, by utilizing the above property.
The region of wavelength required for a liquid crystal display
device is that of all the visible light, and the property of the
illumination light is considerably different by a selection of
wavelength whereby the amount of the birefringence of plate is
determined. As the .lambda./2 plate, it is preferable to employ a
flat film, judging from the aspects of the light-weightedness, the
thinness, the cost and the like. Since there is no film which
satisfies the condition of .lambda./2 with respect to all the
visible light, it is preferable to generally employ a film
satisfying the condition of .lambda./2 at a wavelength
approximately equals to 550 nm wherein the viewing sensitivity is
maximized. That is, the film is provided with the birefringence in
the vicinity of the wavelength of 275 nm.
The quantity of birefringence .lambda./2 of the optical axis of
polarization rotator signifies a quantity along a locus of a light
ray. The direction wherein a light ray maximizing the light
quantity transmits through the optical axis of a polarization
rotating means, is not always in the direction perpendicular to a
flat sheet of the optical axis of polarization rotating means. It
is preferable to design the setting of the size of birefringence of
the film, optimally in consideration of the locus of light rays
maximizing the light quantity.
As the material of the film, polyvinyl alcohol, polycarbonate,
polystyrene, polymethyl methacrylate and the like, are
employed.
The birefringence of the film is generally provided by an uniaxial
elongation. That is, a difference of refractive indices between a
direction of an orientated axis and a direction perpendicular to
the orientated axis is caused by the uniaxial elongation. The
birefringence is caused in direction of thickness, and the
ellipsoid of the refractive index is uniaxial.
Further, it is effective in the construction of this invention in a
viewpoint of making uniform the illumination to further provide a
uniform light forming means. There is a case with respect to light
emitted to the edge light type backlight, wherein the larger the
distance from the light source, the smaller the light quantity.
This is not preferable in a case wherein the edge light type
backlight is employed as an illumination device for display
elements of a large image area. Accordingly, a means is provided
which makes uniform an in-plane intensity distribution of light
emitted from a flat light guide. In other words, the further the
emitting light is disposed from a light source, the better a light
emitting efficiency of light emitted from the flat light guide.
The uniform light forming means may be provided on both surfaces of
the flat light guide, or on one side thereof. The design of the
uniform light forming means for making uniform the emitted light,
is considerably dependent on whether the light source is disposed
only on one side of the flat light guide or on both sides.
A light diffusing means can be employed as one of the uniform light
forming means. When the light source is disposed on one side of the
flat light guide, it is preferable that the light diffusing effect
is small on the side of the light source, and the further from the
light source, the more improved is the light diffusing effect.
However, a light reflecting means is normally provided on a side
face of the light guide opposite to the light source, it is
preferable that the diffusing effect is a little improved on the
side of the light source, in the vicinity of the side face of the
light guide opposite to the light source. When light sources are
disposed on the both sides of the flat light guide, it is
preferable that the diffusing effect at the central portion is
large.
It is simple and effective for controlling the diffusing effect to
perform a mesh printing of a white ink on the light guide and
control the size or the density of the mesh. However, the use of
the light diffusing material has a possibility of deteriorating the
directivity of light emitted from the uniform light forming means,
and may reduce an incident light having Brewster's angle which is
suitable for the polarized light separator to separate the s
polarized light component and the p polarized light component.
Therefore, it is more preferable if the uniform formation of light
is achieved by a means other than the light diffusing means.
As a uniform light forming means other than the light diffusing
means, a means can be employed wherein a lenticular shape is formed
on the surface of the flat light guide. When the light source is
disposed only on one side of the flat light guide, it is preferable
that the light emitting efficiency on the side of the light source
is low, and the further the emitted light is disposed from the
light source, the more improved the light emitting efficiency.
Further, in case wherein a reflecting means is disposed on a side
face of the light guide opposing the light source, it is preferable
that the light emitting efficiency is more improved on the side of
the light source, in the vicinity of the side face of the light
guide opposing the light source. In case wherein the light sources
are provided on the both sides of the flat light guide, it is
preferable that the light emitting efficiency is large at the
central portion.
There are means for controlling the light emitting efficiency of
the lenticular lens which are schematically shown in FIGS. 8(a)
through 8(d). In these Figures a numeral 61 designates a light
source, 62, a reflecting plate, and 63, a flat light guide.
FIG. 8(a) shows an example wherein the distribution of arcs is
changed. FIG. 8(b) shows an example wherein the heights h of arcs
are changed. FIG. 8(c) shows an example wherein the heights and the
widths of arcs are changed. FIG. 8(d) shows an example wherein
aspect ratios of portions of ellipses are changed. Naturally, these
curves can be employed in combinations.
Further, as a uniform light forming means, a means can be employed
wherein a prism shape is formed on the surface of the flat light
guide. In case wherein the light source is disposed only on one
side of the flat light guide, it is preferable that the light
emitting efficiency is low on the side of the light source, and the
further the emitted light is disposed from the light source, the
more improved is the light emitting efficiency. In case wherein a
reflecting means is provided on the side face of the light guide
opposing the light source, it is preferable that the light emitting
efficiency is more improved on the side of the light source, in the
vicinity of the side face of the light guide opposing the light
source. In case wherein the light sources are provided on the both
sides of the flat light guide, it is preferable that the light
emitting efficiency is large at the central portion.
There are means for controlling the light emitting efficiency of a
prism array, which are extremely schematically shown in FIGS. 9(a)
and 9(b). In these Figures, a numeral 61 designates the light
source, 62, the reflecting plate, and 63, a flat light guide.
FIG. 9(a) shows an example wherein the distribution of prisms is
changed. FIG. 9(b) shows an example wherein the heights of prisms
are changed. Naturally, these can be employed in combinations.
Further, prisms and lenticular lenses may be employed.
The pitch of a prism or lenticular lens is preferably selected from
a range of 0.1 through 1 mm, since it is conspicuous when it is
large and it is hard to manufacture when it is fine.
Although such a uniform light forming means may be provided
separately from the light guide, it is generally preferable to
integrate it with the light guide, in view of reducing the number
of parts.
EXAMPLES 1 to 3
An explanation will be given of Examples of this invention in
reference to FIG. 1.
In an edge light type backlight wherein a fluorescent lamp 1 (cold
cathode fluorescence lamp) attached to one side of a transparent
acrylic resin flat light guide 3 which is an illumination plane,
light is introduced into the light guide by providing a lamp cover
2 composed of a light reflecting body, and a polarized light
separator 6 composed of a multi-layered structure is
integrated.
As the fluorescent lamp 1, a cold cathode fluorescence lamp of 10 W
or 16 W having a length corresponding to a side face length (152
mm) of a 10 inch liquid crystal display plane and a small tube
diameter are used. Further, as the lamp cover 2, a reflecting
mirror having a cylindrical shape or an elliptic column shape
surrounding the cold cathode fluorescence lamp is employed and as
the light guide 3, a light-transmitting light guiding plate
(n=1.49) made of an acrylic resin and having a size of 160
mm.times.220 mm.times.5 mm, is employed. Further, a retardation
plate 4 is provided on the backface of the light guide 3 and a side
face of the light guide opposing the face wherein the fluorescent
lamp is disposed, and a reflecting plane composed of an aluminum
metal reflecting film is formed thereon.
As a multi-layered structure of a polarized light separator 6, a
structure is adopted wherein approximately five layers of flat air
bubble layers having a height in the thickness direction of about
10 .mu.m and a radius of about several mm, are dispersed in a
homogeneous transparent plastic plate (n.apprxeq.1.5), in a
lamellar form, and the structure is attached on the side of the
light emitting plane of the light guide 3.
Further, as a prism array 7, a prism array each prism having a
sectional shape of an isosceless triangle having the apex angle of
58.degree., is employed, and is disposed such that the apex faces
the polarized light separator 6. The thickness of the prism array
plate is 2 mm and the pitch of the prism array is about 1 mm.
Further, a light diffusing plate 8 is employed on the side of the
light emitting face of the prism array 7, to widen the viewing
angle.
As a liquid crystal cell 11, an RGB color TFT driving TN liquid
crystal display cell having the pixel number corresponding to VGA,
is employed.
As a light-incident side polarizing sheet 9, a normal light
absorbing type organic polarized plate is employed. When the
required contrast ratio is approximately 10:1, there is a case
wherein the above polarizing sheet is not employed and only the
above-mentioned multi-layered structure is employed. However, in
this case, since the light distinguishing ratio of a polarized
light is low (about 10:1; about 1,000:1 in case of the light
absorbing type organic polarizing plate), it is necessary to
provide the incident-side polarizing sheet, in a TFT driving liquid
crystal cell television set wherein a contrast ratio of not smaller
than 100:1 is required. With respect to the optical axis of
polarization of the light absorbing type organic polarizing sheet,
the optical axis of polarization of light emitted from the
polarized light separator 6 and the optical axis of polarization of
the polarizing sheet 9 agree with each other such that the p
polarized light emitted from the multi-layer structure is provided
with the maximum transmittance.
A light-emitting side polarizing sheet 10 similarly employs a
light-absorbing type organic polarizing sheet. The direction of the
optical axis of polarization is suitably selected in accordance
with the display mode (normally white, normally black). However, in
this Example, as the normally white display, the optical axis of
polarization of the light-emitting side polarizing sheet 10 is
provided in the direction wherein the optical axis of polarization
is rotated by 90.degree. with respect to the optical axis of
polarization of the light-incident polarizing sheet 9.
Examples 1 through 3 were carried out by adjusting the power
consumption of the lamp and the brightness of a viewing field in
the perpendicular direction, by variously changing the properties
of the light source and the light guide. Table 1 shows these
Examples and a Conventional Example.
TABLE 1 Brightness Power of perpen- Range of consump- dicular
viewing angle tion of viewing at 1/2 maximum lamp field brightness
(W) (cd/m.sup.2) Horizontal Vertical Conventional 16 100
.+-.50.degree. .+-.40.degree. Example 1 16 150 .+-.50.degree.
.+-.40.degree. Example 2 10 100 .+-.50.degree. .+-.40.degree.
Example 3 16 100 .+-.60.degree. .+-.40.degree.
In Example 1, the brightness was improved by 1.5 times as much as
that of the conventional Example, and the viewing angle range was
not narrowed. In Example 2, both the brightness and the viewing
angle were approximately the same as in the conventional Example,
however, the power consumption of lamp was reduced to 2/3 of that
in the conventional case, and the time for driving a battery was
prolonged. In Example 3, the power consumption of lamp and the
brightness of the perpendicular viewing field were the same as
those in the Conventional Example, but the viewing angle was
widened.
In this way, various light direction distribution can be provided
in accordance with the contrast ratio of an employed liquid crystal
display element. Especially, it is possible to selectively enhance
the brightness of the perpendicular viewing field.
EXAMPLES 4 to 6
An explanation will be given of other examples of this invention in
reference to FIG. 2.
An edge light type backlight is employed wherein a fluorescent lamp
1 (CCFL) is attached to a side of a transparent acrylic resin plate
light guide 3 which is an illumination plane, and light is
introduced to the light guide by providing a lamp cover 2 which is
composed of a light reflecting body.
As the fluorescent lamp 1, CCFL of 2 W and 4 W are employed which
are provided with a length corresponding to a side face length (120
mm) of a general notebook type personal computer. Further, as a
lamp cover 2, a reflecting mirror having a cylindrical shape or an
elliptic column shape surrounding the cold cathode discharge tube,
is employed, and as the light guide 3, a transparent light guiding
plate (n=1.49)is employed which is made of an acrylic resin and is
provided with a size of 128 mm.times.225 mm.times.2.8 mm.
Further, a retardation plate 4 is provided on the backface of the
light guide 3 and a side face of the light guide opposing the
fluorescent lamp, on which a reflecting face composed of an
aluminum metal reflecting film is formed. The retardation plate is
a 1/4 wavelength plate.
As a prism array 13, a prism array each prism having a sectional
shape of an isosceless triangle having the apex angle of
160.degree., is employed, and is disposed such that the apex faces
the polarized light separator 6. The thickness of the prism array
plate is 2 mm and the pitch of the prism array is approximately 1
mm. The prism array 13 and the light guide 3 employ an acrylic
resin of the same material. Further, an optical adhesive agent
having the refractive index of 1.49 which is the same with that of
the acrylic resin, is employed between the prism array 13 and the
light guide 3.
As a polarized light separator 6, one layer of a titanium oxide
(TiO.sub.2 :n=2.35) film 6a is formed on the surface of a
homogeneous glass substrate (n=1.52) 6b by the film thickness of
approximately 640 .ANG., and is provided on the light-emitting
plane side of the light guide 3. The light separating angle of the
polarized light separator is approximately 72.degree..
Further, as a prism array 7, a prism array each prism having the
sectional shape of an isosceless triangle having the apex angel of
60.degree., is employed, and is disposed such that the apex faces
the polarized light separator 6. The thickness of the prism array
is 2 mm and the pitch of the prism array is approximately 1 mm.
Further, a light diffusing plate 8 is employed on the
light-emitting face side of the prism array 7 to widen the viewing
angle.
As a liquid crystal display element 12, an STN liquid crystal
display cell of a monochromatic display wherein films having the
birefringence are laminated, is employed. The twist angle is
240.degree..
As alight-incident side polarizing sheet 9, a normal light
absorbing type organic polarizing sheet is employed. With respect
to the optical axis of polarization of the light-absorbing type
organic polarizing sheet, the optical axis of polarization of light
emitted from the polarized light separator 6 and the optical axis
of polarization of the polarizing sheet 9 agree with each other,
such that the p polarized light emitted from the polarized light
separator is provided with the maximum transmittance.
A light-emitting side polarizing sheet 10 similarly employs the
light absorbing type organic polarizing sheet. Although the
direction of the optical axis of polarization is pertinently
selected, in this Example, the optical axis of polarization of the
light-emitting side polarizing sheet 10 is in the direction wherein
the optical axis of polarization is rotated by 85.degree. with
respect to the optical axis of the polarization of the
light-incident side polarizing sheet 9.
Examples 4 to 6 were carried out by adjusting the power consumption
of the lamp and the brightness of the perpendicular viewing field,
by variously changing the properties of the light source and the
light guide. Table 2 shows a comparison between the Examples and a
Conventional Example having no polarized light separator.
TABLE 2 Brightness Power of perpen- Range of consump- dicular
viewing angle tion of viewing at 1/2 maximum lamp field brightness
(W) (cd/m.sup.2) Horizontal Vertical Conventional 2 60
.+-.50.degree. .+-.20.degree. Example 4 2 90 .+-.50.degree.
.+-.20.degree. Example 5 1.3 60 .+-.50.degree. .+-.20.degree.
Example 6 2 60 .+-.60.degree. .+-.30.degree.
In Example 4, the brightness was improved by 1.5 times as large as
that in the Conventional Example, and the viewing angle range was
not narrowed down. In Example 5, both of brightness and the viewing
angle were approximately the same as in the Conventional Example,
but the power consumption of the lamp was reduced by 2/3 of that in
he conventional Example, and the time for driving a battery was
prolonged. In Example 6, both the power consumption of the lamp and
the perpendicular brightness were the same as those in the
conventional Example, but the viewing angle was widened.
In this way, various light direction distributions can be provided
in accordance with the contrast ratio curve of the employed liquid
crystal display element. Especially it is possible to selectively
enhance the brightness of the perpendicular viewing field.
EXAMPLE 7
An explanation will be given of an Example in reference to FIG. 3.
The shape of the surface of a light guide 3 is in a prism array
shape. Three layers of thin films of ZrO.sub.2 and SIO.sub.2 are
alternately and uniformly formed on the surface. Specifically, the
three layers are formed on the light guide in the order of
ZrO.sub.2, SiO.sub.2 and ZrO.sub.2, and the degree of polarization
at Brewster's angle is maximized in the vicinity of the wavelength
of 530 nm. The light guide and the dielectric interference film are
integrated. The apex angle of the prism array of the light guide 3
is 160.degree.. Since the direction of light emitted from the
dielectric body interference film is provided with a distribution
with respect to the direction perpendicular to the face of the
light guide, a prism array is further provided on the side opposite
to the light guide of which apex of each prism faces the light
guide. The construction other than the above is the same as in
Example 4. The brightness a little increases compared with that in
Example 1. This is because the interface reflection is reduced
since the interface is reduced in Example 7. Further, the thickness
can be reduced compared with that in Example 4 by the integral
forming. Further, there is a merit wherein the cost is reduced in
the mass production.
EXAMPLE 8
As a polarized light separator, a laminated product having the same
shape as in FIG. 2 is employed, which is provided with 400 layers
of acrylic resin and polycarbonate. Compared with a case wherein a
polarized light separator is not employed, the brightness in the
perpendicular direction is enhanced about 1.5 times as much as that
in the case.
EXAMPLE 9
An explanation will be given of another Example of this invention
in reference to FIGS. 6 and 7.
An edge light type backlight is employed wherein a fluorescent lamp
31 (CCFL) is attached to a side of a transparent acrylic resin
plate light guide 34 which is an illumination plane, and light is
introduced into the light guide by providing a lamp cover 32
including a reflecting body 33.
As the fluorescent lamp 31, a CCFL having a length corresponding to
a side face length (125 mm) of a general notebook type personal
computer and a tube diameter of 3 mm. Further, as the lamp cover
32, a reflecting mirror having a cylindrical shape or an elliptical
column shape surrounding the CCFL, is employed, and as the light
guide 34, a light-transmitting light guiding plate (n=1.49) is
employed which is made of an acrylic resin and is provided with a
size of 128 mm.times.225 mm.times.2.8 mm.
Further, a .lambda./4 phase interference plate 35 is provided on
the backface of the light guide 34 and a side face of the light
guide opposing the fluorescent lamp, on which a reflecting plane 36
made of an aluminum metal reflecting film is formed.
A lenticular lens array 37 is employed and is disposed such that
the protruded portions thereof face a polarized light separator 38.
The thickness of the lenticular lens array is 2 mm and the pitch
thereof is approximately 30 .mu.m. The lenticular lens array 37 and
the light guide 34 employ an acrylic resin of the same material.
Further, an optical adhesive agent having the refractive index of
1.49 which is the same with that of the acrylic resin, is employed
between the lenticular lens array 37 and the light guide 34.
As the polarized light separator 38, one layer of titanium oxide
(TiO.sub.2 :n=2.35) is formed on the surface of a homogeneous glass
substrate (n=1.52) by the film thickness of approximately 640
.ANG., and is provided on the light emitting plane side of the
light guide 33. The separating angle of the polarized light
separator is 72.degree.. That is, approximately 100% of the p
polarized light component having the angle of incidence of
72.degree. transmits through the polarized light separator, and
there is almost no reflection, but only approximately 15% of the s
polarized light component transmits therethrough and 85% thereof is
reflected.
The polarized light emitted from the polarized light separator, is
polarized in the perpendicular direction with respect to the linear
light source.
Further, as a prism array 39, a prism array each prism having a
sectional shape of an isosceless triangle having the apex angle of
65.degree., is employed, and is disposed such that the apex faces
the polarized light separator 38. The thickness of the prism array
plate is 2 mm and the pitch thereof is approximately 30 .mu.m. In
this way, the light quantity of a light ray of which transmitting
direction is approximately perpendicular to the light guiding plate
can be enhanced.
Further, a .lambda./2 retardation plate 40 is provided on the
external side of the prism array 39. The fast axis of the
.lambda./2 retardation plate is inclined by .theta.=45.degree. with
respect to the direction perpendicular to the light source as in
FIG. 7. The material employs a PC (polycarbonate), which is
provided with the birefringence of .lambda./2 when measured by the
wavelength of 550 nm. In FIG. 7, a numeral 51 designates a
fluorescent lamp, 52, a light guide, 53, the direction of fast
axis, 54, an optical axis of polarization for a light-incidence
side polarizing sheet, 55, an optical axis of polarization for a
light-emitting side polarizing sheet, 56, a rubbing direction on
the light-incident side, and 57, a rubbing direction on the
light-emitting side.
A TFT liquid crystal display cell of color display is employed for
a liquid crystal panel 41. A normal light absorbing type organic
polarized sheet is employed for a light-incident side polarizing
sheet 42. The optical axis of polarization is .theta.=90.degree.. A
normal light-absorbing type organic polarizing sheet is also
employed for a light-emitting side polarizing sheet 43. The optical
axis of polarization is .theta.=0.degree.. The rubbing direction on
the light-incident side is .theta.=90.degree., and the rubbing
direction on the light-emitting side is .theta.=0.degree..
Since the light emitted from the polarized light separator is
provided with much linearly polarized light having a direction
perpendicular to the light source, when the optical axis of
polarization of the light-incident side polarizing sheet which is
in use in this Example, is 90.degree., the increase of brightness
of approximately three times as much as the brightness of a liquid
crystal display device, is provided by employing the .lambda./2
retardation plate which is inclined by 45.degree., and the
utilization efficiency of light is promoted.
EXAMPLE 10
Further, one layer of titania is formed on the retardation plate
PC, in place of the polarized light separator and the .lambda./2
phase difference plate in Example 9, which is provided with the
both functions, and which is disposed between the lenticular lens
and the prism array, with the side of the interference film on the
side of the light source. The retardation plate of PC is provided
with the birefringence of .lambda./2, when measured by a wavelength
of 550 nm with respect to the incident light having the angle of
incidence of approximately 60.degree.. A result approximately
similar to those in the above Examples were provided thereby.
According to the present invention, an illumination device for a
direct viewing type display element having especially high
substantial brightness with respect to a specified region having a
high contrast ratio, can be provided. Especially, in this
invention, the brightness is promoted with respect to a desired
viewing direction, by converting light which does not substantially
contributes to the illumination light of the display device, among
lights of a desired viewing direction, into a polarized light,
which is different from a case wherein a single prism array and the
like is employed. Accordingly, an illumination device having a high
brightness in a specific direction, is provided while maintaining a
wide illuminance distribution. This is most pertinent to an
illumination device for a direct viewing type display element
having a wide viewing angle.
Further, a defused light which is pertinent as an illumination
light for a direct viewing type display element can easily be
provided by employing a so-called edge light type light source as a
light emitting means. Further, when especially a polarized light
converting means is constructed by cooperating a polarized light
separating plane provided on the light-emitting phase side of a
light guide for an edge light, with a light reflecting plane
provided on a side of a flat light guide opposite to the
light-emitting face, the light guide for an edge light can be
employed as a space for separating a polarized light. Accordingly,
this is more preferable since a very compact construction can be
provided.
Further, a direct viewing liquid crystal display device having a
high illuminance in a practical viewing angle and a small power
consumption can be provided, by disposing the illumination device
of this invention on the backface of the liquid crystal display
element, such that the direction of an optical axis of a light ray
emitted from the illumination device approximately agrees with the
direction of an optical axis of polarization sheet on the
light-incident side of the liquid crystal display element.
* * * * *