U.S. patent application number 09/984363 was filed with the patent office on 2002-05-16 for projection-type display apparatus.
This patent application is currently assigned to SEIKO EPSON CORPORATION. Invention is credited to Itoh, Yoshitaka, Nakayama, Tadaaki, Yajima, Akitaka.
Application Number | 20020057420 09/984363 |
Document ID | / |
Family ID | 13013415 |
Filed Date | 2002-05-16 |
United States Patent
Application |
20020057420 |
Kind Code |
A1 |
Nakayama, Tadaaki ; et
al. |
May 16, 2002 |
Projection-type display apparatus
Abstract
A projection-type display apparatus (1) is comprised of an
illumination optical system (2A), a color separating system (4) for
separating a white beam (W) from the illumination optical system
into color beams (R, G, B), three liquid crystal panels (5R, 5G,
5B) for modulating the separated color beams, a light guide system
(9) located on an optical path of the beam (G) having the longest
optical path among the separated color beams respectively incident
on the three light valves, a dichroic prism (6) for synthesizing
the beams modulated through the liquid crystal panels, and a
projection lens (7) for projecting the synthesized and modulated
beam onto a screen (8). The illumination optical system (2A) is
provided with a uniform illumination optical device (3) for
converting the white beam into a uniform rectangular beam. Since
the dichroic prism, which is an optical element rotationally
symmetrical about the center axis of a projection optical system,
is employed as a color synthesizing system and the uniform
illumination optical device for restricting unevenness in color and
luminous intensity is incorporated in the illumination optical
system, it is possible to realize a display apparatus which causes
little unevenness in color and luminous intensity and has a high
illumination efficiency.
Inventors: |
Nakayama, Tadaaki;
(Osaka-shi, JP) ; Itoh, Yoshitaka; (Suwa-shi,
JP) ; Yajima, Akitaka; (Suwa-shi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. Box 19928
Alexandria
VA
22320
US
|
Assignee: |
SEIKO EPSON CORPORATION
TOKYO
JP
|
Family ID: |
13013415 |
Appl. No.: |
09/984363 |
Filed: |
October 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09984363 |
Oct 30, 2001 |
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09526446 |
Mar 16, 2000 |
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09526446 |
Mar 16, 2000 |
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08783236 |
Jan 14, 1997 |
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08783236 |
Jan 14, 1997 |
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08335778 |
Jan 4, 1995 |
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Current U.S.
Class: |
353/53 ;
348/E5.141; 348/E9.027 |
Current CPC
Class: |
G03B 33/12 20130101;
G02B 3/0043 20130101; H04N 9/3105 20130101; G03B 21/208 20130101;
G02B 3/005 20130101; G02B 3/0062 20130101; G02B 3/0056 20130101;
H04N 9/3167 20130101; H04N 5/7441 20130101 |
Class at
Publication: |
353/53 |
International
Class: |
G03B 021/18; G03B
021/26 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 16, 1994 |
US |
PCT/JP94/00419 |
Mar 16, 1993 |
JP |
5-55952 |
Claims
What is claimed is:
1. A projector, comprising: a light source; color separating means
for separating a light beam emitted from said light source into a
plurality of color beams; a plurality of light valves for
modulating said separated color beams; light guide means located on
an optical path of a color beam having a longest optical path
length, the color beams being separated by said color separating
means and respectively incident on said light valves; color
synthesizing means for synthesizing a projection beam from said
color beams modulated through said light valves; a projection lens
for projecting said projection beam into a screen; uniform
illumination optical means interposed on an optical path between
said light source and said color separating means for converting
said light beam emitted from said light source into a uniform beam;
and a plurality of condenser lenses located respectively in
outputting portions of said color separating means for outputting
said color beams, the condenser lenses converting said color beams
separated by said color separating means into almost collimated
beams.
2. A projector according to claim 1, wherein said light valves are
liquid crystal panels, and a pixel pitch of each of said liquid
crystal panels is approximately 50 .mu.m or less.
3. A projector according to claim 1, wherein said uniform
illumination optical means has at least one lens plate comprising a
plurality of lenses.
4. A projector according to claim 3, wherein said uniform
illumination optical means comprises a first lens plate and a
second lens plate.
5. A projector, comprising: a light source; color separating means
for separating a light beam emitted from said light source into a
plurality of color beams; a plurality of condenser lenses located
respectively in outputting portions of said color separating means
for outputting said color beams, the condenser lenses converting
said color beams separated by said color separating means into
almost collimated beams; a plurality of light valves for modulating
said separated color beams; light guide means located on an optical
path of a color beam having a longest optical path length, the
color beams being separated by said color separating means and
respectively incident on said light valves; color synthesizing
means for synthesizing a projection beam from said color beams
modulated through said light valves; a projection lens for
projecting said projection beam onto a screen; and polarized beam
conversion means for converting the light beam emitted from said
light source into one kind of polarized light.
6. A projector according to claim 5, wherein uniform illumination
optical means is interposed on an optical path between said light
source and said color separating means for converting said light
beam emitted from said light source into a uniform beam.
7. A projector according to claim 5, wherein said polarized beam
conversion means comprises: polarized beam separating means for
separating a random polarized beam from said light source into two
kinds of linearly polarized beams; and polarization plane rotating
means for rotating a polarization plane of one of said two kinds of
linearly polarized beams through a 90 degree angle so that one kind
of polarized light is emitted from said polarized beam conversion
means.
8. A projector, comprising: a light source emitting a light beam; a
uniform illumination optical device that converts said light beam
emitted from said light source into a uniform beam; a color
separating optical system that separates the beam emitted from said
light source into a plurality of color beams; a plurality of
condenser lenses located respectively in outputting portions of
said color separating optical system, the condenser lenses
converting said color beams into almost collimated beams; a
plurality of light valves that modulate said separated color beams;
a light guide system located on an optical path of a color beam
having a longest optical path length; and a color synthesizing
system that synthesizes said color beams into a projection
beam.
9. The projector according to claim 8, further comprising a
projection lens that projects said projection beam.
10. The projector according to claim 8, wherein said light valves
are liquid crystal panels, and a pixel pitch of each of said liquid
crystal panels is approximately 50 .mu.m or less.
11. The projector according to claim 8, wherein said uniform
illumination optical device has at least one lens plate comprising
a plurality of lenses.
12. A projector according to claim 11, wherein said uniform
illumination optical device comprises a first lens plate and a
second lens plate.
13. A projector, comprising: a light source emitting a light beam;
a polarized beam conversion device that converts the light beam
emitted from said light source into a light beam having a single
polarization type; a color separating optical system that separates
the light beam into a plurality of color beams; a plurality of
condenser lenses located respectively in outputting portions of
said color separating optical system, the condenser lenses
converting said color beams into almost collimated beams; a
plurality of light valves that modulate said separated color beams;
a light guide system located on an optical path of a color beam
having a longest optical path length; and a color synthesizing
system that synthesizes said color beams into a projection
beam.
14. The projector of claim 13, further comprising a projection lens
that projects said projection beam.
15. The projector according to claim 13, further comprising a
uniform illumination optical device interposed on an optical path
between said light source and said color separating optical system
that converts said light beam having a single polarization type
into a uniform beam.
16. The projector according to claim 13, wherein said polarized
beam conversion device comprises: a polarized beam separating
device that separates a random polarized beam from said light
source into two kinds of linearly polarized light beams; and a
polarization plane rotating device that rotates a polarization
plane of one of said two kinds of linearly polarized light beams
through a 90 degree angle so that said light beam having a single
polarization type is emitted from said polarized beam conversion
device.
Description
TECHNICAL FIELD
[0001] The present invention relates to a projection-type display
apparatus which separates a white beam from a light source into
beams of three colors, red, blue and green, modulates these beams
through light valves according to image information, and
re-synthesizes and projects the modulated beams under magnification
onto a screen through a projection lens.
BACKGROUND ART
[0002] A projection-type display apparatus is comprised of a light
source lamp, a color separating means for separating a white beam
from the light source lamp into beams of three colors, three light
valves for modulating the separated color beams, a color
synthesizing means for synthesizing the modulated beams again, and
a projection lens for magnifying and displaying a light image
obtained by the synthesis onto a screen. As light valves, liquid
crystal panels are generally used.
[0003] A conventional projection-type display apparatus having such
structure in which a uniform illumination optical device referred
to as an optical integrator is incorporated in a light source
thereof is well known. For example, U.S. Pat. No. 5,098,184
discloses a projection-type display apparatus having such optical
integrator incorporated therein. This patent publication also
discloses a color synthesizing means consisting of dichroic mirrors
arranged in the shape of X. An ordinary color synthesizing means is
constituted by dichroic mirrors each of which has a dielectric
multilayer film on a glass plate.
[0004] Such projection-type display apparatus provided with a
mirror composite system in which a color synthesizing means is
constituted by dichroic mirrors has the following disadvantage.
Each dichroic mirror is an optical element which is rotationally
asymmetrical about the center axis of a projection lens. Therefore,
astigmatism arises in an image on the screen, and a Modulation
Transfer Function (MTF) representing the transfer characteristic of
a projection optical system is lowered. As a result, the image is
blurred and sharpness thereof is reduced. In a case in which the
size of a liquid crystal panel is large relative to the number of
pixels, in other words, when the pixel pitch is large, the lowering
of the MTF does not cause such a large problem. However, when the
pixel pitch is small, for example, as in a liquid crystal panel
using a polysilicon TFT as a switching device, such lowering cannot
be ignored.
[0005] Furthermore, a conventional projection-type display
apparatus having a prism composite system in which a color
synthesizing means consists of a dichroic prism is well known. The
dichroic prism is an optical element which is rotationally
symmetrical about the center axis of a projection lens. Therefore,
astigmatism caused by this prism can be easily removed by the
design of the projection lens. The MTF of the projection-type
display apparatus having such a prism composite system is generally
superior to that of the above-mentioned display apparatus having
the mirror composite system. Accordingly, such apparatus is
suitable in the case in which a liquid crystal panel having a small
pixel pitch is used as a light valve.
[0006] Another type of conventional projection-type display
apparatus is disclosed in, for example, U.S. Pat. No. 4,943,154. In
this apparatus, the decrease in the amount of light and the
unevenness in color are restricted by equalizing the optical path
lengths (the distances between a light source and liquid crystal
panels) of beams of three colors in a color separating means. In
other words, a light transmitting means constituted by a relay
lens, a field lens and so on is interposed on an optical path of
the beam having the longest optical path length in the apparatus
disclosed in the specification of this patent, thereby optically
equalizing the optical path lengths of the color beams.
[0007] However, in this apparatus, while the light amount of the
color beam having the longest optical path length is not reduced,
the brightness distribution thereof is made to do a complete
about-face by the relay lens. Therefore, if the initial brightness
distribution is not axially symmetrical, color unevenness arises in
the display on a screen, and the quality of the display is
degraded. Although such color unevenness does not arise if the
brightness distribution of the beam is axially symmetrical, in
fact, the brightness distribution is normally made axially
asymmetrical by the displacement of an attachment position of a
light source lamp and the slight asymmetrical properties of the
light source lamp and a reflecting mirror.
[0008] In a projection-type display apparatus, it is desirable to
increase the luminous intensity of an image to be projected, and to
obtain an image quality close to that of the image directly viewed
on a CRT, without unevenness in color and luminous intensity. For
such purpose, it is preferable to use a prism composite system
having a good transfer characteristic as a color synthesizing
system. It is also preferable to efficiently illuminate a liquid
crystal panel with uniform brightness by using an optical
integrator in a light source portion. However, if the optical
integrator is used in a case in which the optical path lengths of
the beams in the color separating system are different, the
decrease in the amount of light and change of brightness
distribution of the beam having the longest optical path are
remarkable. This results in color unevenness and a change in color
temperature of a projected image. Therefore, a sufficient effect of
the integrator cannot be shown. Furthermore, when the optical
integrator is used in the light source portion, the conventional
art cannot be utilized as it is. In other words, since a diverged
beam from a plane light source which exists in a finite position (a
beam outgoing plane of the integrator) away from the liquid crystal
panel illuminates the liquid crystal panel, the illumination with
the optical integrator is basically different from the illumination
from a point light source existing at an infinite distance from the
liquid crystal panel, as in the arrangement of the conventional
art.
[0009] An object of the present invention is to provide a
projection-type display apparatus which can generate a projection
image of higher quality, compared with the above-mentioned
conventional projection-type display apparatus, without any
unevenness in luminous intensity and color.
[0010] Another object of the present invention is to provide an
inexpensive projection-type display apparatus which can generate a
projection image of high quality.
[0011] Still another object of the present invention is to provide
a projection-type display apparatus which can generate a projection
image having higher luminous intensity than a conventional one.
[0012] A further object of the present invention is to provide a
compact projection-type display apparatus which can generate a
projection image of high quality.
[0013] A still further object of the present invention is to
provide a projection-type display apparatus suited to be used for
front projection.
SUMMARY OF THE INVENTION
[0014] In order to achieve the above objects, the present invention
provides a projection-type display apparatus comprising a light
source, a color separating means for separating a white light beam
emitted from the light source into beams of three primary colors,
three light valves for modulating the separated color beams, a
light guide means located on an optical path of the color beam
having the longest optical path length among the color beams
separated by the color separating means and respectively incident
on the three light valves, a color synthesizing means for
synthesizing the color beams modulated through the light valves,
and a projection lens for projecting the synthesized and modulated
beam onto a screen. A uniform illumination optical means is
interposed on an optical path between the light source and the
color separating means for converting the white beam from the light
source into a uniform rectangular beam, and for outputting the
beams toward the color separating means. Three condenser lenses are
located respectively in outputting portions of the color separating
means for emitting the color light beams to convert the diverged
beams outputted from the uniform illumination optical means into
almost collimated beams. The color synthesizing means consists of a
dichroic prism, and the light guide means is constituted by an
incident side reflecting mirror, an output side reflecting mirror,
and at least one lens.
[0015] In the projection-type display apparatus of the present
invention having such constitution, the light valves are
illuminated by the uniform illumination means, the diverged color
beams are collimated by the condenser lenses respectively located
on the optical paths of the color beams, and the optical path
lengths of the color beams are optically equalized by making one of
the color beams pass through the light guide system. Therefore,
according to the present invention, it is possible to form a
projection image having uniform illumination distribution, little
color unevenness, and more brightness and higher quality than
ever.
[0016] It is preferable that the light guide means have one
intermediate lens, and that the focal length of the intermediate
lens be set within a range of approximately 0.9 to 1.1 times the
optical path length of the light guide means.
[0017] The light guide means may also comprise an incident lens
located in the incident side of the incident side reflecting
mirror, an output lens located on the output side of the output
side reflecting mirror, and an intermediate lens located between
the incident and output side reflecting mirrors. In this case, it
is preferable that the focal lengths of the incident and output
side lenses each be set between approximately 0.5 and 0.7 times the
optical path length of the light guide means and that the focal
length of the intermediate lens be set between approximately 0.25
and 0.4 times the optical path length of the light guide means so
as to restrict any aberration.
[0018] Furthermore, in this case, it is preferable to make the
optical system compact by combining the abovementioned incident
lens and the above-mentioned condenser lens (for making the
collimated beam incident on the incident lens) into a single lens.
If the single lens is employed, it is preferable that the lens be
an aspherical lens in order to restrict aberration in the periphery
thereof.
[0019] Liquid crystal panels are available as the abovementioned
light valves. In this case, it is preferable that resolution of a
projection image be enhanced by setting a pixel pitch of each
liquid crystal panel at approximately 50 .mu.m or less.
[0020] On the other hand, the uniform illumination optical system
may be provided with at least one lens plate consisting of a
plurality of lenses arranged in a plane perpendicular to the chief
axis of the light emitted from the light source lamp. In this case,
it is preferable that the split number of the lens plate in one
direction be set between approximately 3 and 7.
[0021] A green light beam whose amount of light is normally larger
than that of other color beams, or a blue light beam in which the
influence on an image quality caused by change in the amount of
light is relatively difficult to detect is preferable as a color
beam to be passed through the above-described light guide
means.
[0022] The uniform illumination optical system may be constituted
by a first lens plate, a second lens plate and a reflecting mirror
interposed between the lens plates, and the optical path thereof
may be folded, for example, at a right angle.
[0023] Furthermore, it is preferable that a polarized beam
conversion means be located between the light source lamp and the
uniform illumination optical means. The polarized beam conversion
means is constituted by a polarized beam separating element for
separating a random polarized beam from the light source lamp into
two linearly polarized P and S beams, and a polarization plane
rotating means for rotating a polarization plane of one of the two
separated and polarized beams at an angle of 90.degree. so as to
coincide with that of the other linearly polarized beam. Since the
use of the polarized beam conversion means makes it possible to
enhance the use efficiency of light emitted from the light source
lamp, the luminous intensity of a projection image can be
increased.
[0024] The projection-type display apparatus of the present
invention is characterized in that the above light guide system is
provided with an incident side triangular prism located on the
incident side for folding an optical path at a right angle, an
output side triangular prism located on the output side for folding
the optical path at a right angle, and a light guide member located
between these triangular prisms. Even in the projection-type
display apparatus having such constitution, it is possible to form
a projection image with uniform illumination distribution, little
color unevenness and more brightness and higher quality than
ever.
[0025] As the light guide member, a quadratic prism may be used. It
is preferable that interfaces of the triangular prism and the
quadratic prism be covered with a anti-reflective coating.
Furthermore, it is preferable that a total reflection surface of
each triangular prism be coated with a metal film or a dielectric
multilayer film.
[0026] Another projection-type display apparatus of the present
invention is suited to be used as a front projector, and comprises
a light source, a color separating means for separating a white
light beam emitted from the light source into beams of three
primary colors, three light valves for modulating the separated
color beams, a light guide means located on the optical path of the
color beam having the longest optical path length among the color
beams separated by the color separating means and respectively
incident on the three light valves, a color synthesizing means for
synthesizing the color beams modulated through the light valves, a
projection lens means for projecting the synthesized and modulated
beam onto a screen, and a uniform illumination optical means
located on an optical path between the light source and the color
separating means for converting the white beam from the light
source into a uniform rectangular beam and for emitting the beam
into the color separating means. Three condenser lenses are located
respectively in emitting portions of the color separating means for
emitting the color light beams so as to convert the diverged beam
outputted from the uniform illumination optical means into almost
collimated beams. The color synthesizing means is constituted by a
dichroic prism. The light guide means comprises an incident side
reflecting mirror, an output side reflecting mirror, and at least
one lens. The optical path is formed so that the direction of the
beam from the projection lens is parallel and reverse to the onward
direction of the light beam emitted from the light source. A
cooling means for the light source is located on the output side of
the projection light in an apparatus case. A vent for the cooling
means is formed on a side surface of the case on the output side of
the projection light.
[0027] According to such constitution, since the cooling means is
located on the reverse side to a viewer of a projection image, it
is advantageous in preventing noises and exhausted air from the
cooling means from disturbing the viewer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a schematic view showing a general constitution of
a projection-type display apparatus according to a first embodiment
of the present invention;
[0029] FIG. 2 is a graph showing the relationship between the pixel
density and the transfer characteristic (MTF) of a liquid crystal
panel used as a light valve in the projection-type display
apparatus;
[0030] FIGS. 3(A), (B) and (C) are schematic perspective views
showing the structure of first and second lens plates constituting
a uniform illumination optical device as shown in FIG. 1;
[0031] FIG. 4 is a graph showing the relationship between the split
number of the lens plates of the uniform illumination optical
device and color unevenness;
[0032] FIGS. 5(A) and (B) each are diagrams explaining the
operation of the uniform illumination optical device;
[0033] FIG. 6 is a schematic structural view showing a variation of
a light guide system in the first embodiment of the present
invention;
[0034] FIGS. 7(A) and (B) are, respectively, a schematic structural
view showing another variation of the light guide system of the
first embodiment of the present invention, and an explanatory view
of the operation thereof;
[0035] FIGS. 8(A) and (B) are, respectively, a schematic structural
view showing a further variation of the light guide system of the
first embodiment of the present invention, and an explanatory view
of the operation thereof;
[0036] FIG. 9 is a schematic structural view showing a variation of
the light guide system shown in FIG. 8(A);
[0037] FIGS. 10(A) and (B) are, respectively, a schematic
structural view showing an optical system in a projection-type
display apparatus according to a second embodiment of the present
invention, and an explanatory view showing a light guide system
therein;
[0038] FIGS. 10(C) and (D) are explanatory views showing variations
of the light guide system shown in FIG. 10(B);
[0039] FIG. 11 is a schematic structural view showing an optical
system and a cooling fan of a projection-type display apparatus
according to a fourth embodiment of the present invention;
[0040] FIG. 12 is an explanatory view showing the structure of a
polarized beam conversion device incorporated in the illumination
optical system shown in FIG. 11;
[0041] FIG. 13 is a schematic structural view showing a variation
of the uniform illumination optical device shown in FIG. 1;
[0042] FIGS. 14(A) and (B) are schematic structural views showing a
projection-type display apparatus according to a third embodiment
of the present invention, and a variation thereof;
[0043] FIG. 15(A) is an explanatory view of a light guide system as
shown in FIG. 14(A); and
[0044] FIG. 15(B) is an explanatory view of a variation of the
light guide system shown in FIG. 15(A).
BEST MODE FOR CARRYING OUT THE INVENTION
[0045] Embodiments of the present invention will be described below
in reference to the drawings.
[0046] First Embodiment
[0047] FIG. 1 shows an optical system of a projection-type display
apparatus according to a first embodiment of the present invention.
A projection-type display apparatus 1 in this embodiment is
comprised of an illumination optical system 2A constituted by a
light source 2 and a uniform illumination optical device 3, a color
separating optical system 4 for separating a white beam W outputted
from the illumination optical system 2A through the uniform
illumination optical device 3 into color beams R, G and B of red,
green and blue, three liquid crystal panels 5R, 5G and 5B as light
valves for modulating the color beams, a color synthesizing optical
system 6 for synthesizing the modulated color beams again, and a
projection lens 7 for magnifying and projecting the synthesized
beam onto a screen 8. There is also provided a light guide system 9
for guiding the green beam G among the color beams, separated by
the color separating optical system 4, to the liquid crystal valve
5G.
[0048] The light source 2 in this embodiment is constituted by a
light source lamp 21 and a curved reflecting mirror 22. As the
light source lamp 21, a tungsten halogen lamp, a metal halide lamp,
a xenon lamp and so on can be used. Though the details of the
uniform illumination optical system 3 will be described below, the
optical system 3 is constituted by first and second lens plates 31
and 32 arranged on a plane perpendicular to a center optical axis
1a thereof.
[0049] The color separating optical system 4 is constituted by a
blue/green reflecting dichroic mirror 401, a blue reflecting
dichroic mirror 402 and a reflecting mirror 403. The blue and green
beams B and G contained in the white beam W are reflected at a
right angle by the blue/green reflecting dichroic mirror 401, and
directed toward the blue reflecting dichroic mirror 402. The red
beam R passes through this mirror 401, is reflected at a right
angle by the reflecting mirror 403, and is outputted from an
outputting portion 404 for the red beam toward the color
synthesizing optical system. As for the blue and green beams B and
G reflected by the mirror 401, only the blue beam B is reflected at
a right angle by the blue reflecting dichroic mirror 402, and
outputted from an outputting portion 405 for the blue beam toward
the color synthesizing optical system. The green beam G passes
through the mirror 402, and is outputted from an outputting portion
406 for the green beam toward the light guide system 9. In this
embodiment, the distances between the outputting portion of the
uniform illumination optical device 3 for the white beam and the
outputting portions 404, 405 and 406 for the color beams in the
color separating optical system 4 are set to be equal.
[0050] In this embodiment, condenser lenses 101, 102 and 103, each
of which consists of a planoconvex lens, are respectively located
on the outputting sides of the outputting portions 404, 405 and 406
of the color separating optical system 4 for the color beams.
Therefore, the color beams outputted from the outputting portions
come into the condenser lens 101-103 to be collimated.
[0051] The red and blue beams R and B, among the collimated color
beams R, G and B, come into the liquid crystal panels 5R and SB
located just behind the condenser lenses 101 and 102, and are
modulated and applied with image information corresponding to the
respective color beams. In other words, switching control
operations are performed according to the image information by
unillustrated drive means in these liquid crystal panels, thereby
modulating the color beams passing therethrough. As such drive
means, a well-known type of drive means can be used as is, and the
explanation thereof is omitted in this embodiment. On the other
hand, the green beam G is guided to the corresponding liquid
crystal panel 5G through the light guide system 9, and modulated
according to image information in the same manner as above. Each of
the liquid crystal panels employed in this embodiment has a pixel
pitch of 50 .mu.m or less, and uses a polysilicon TFT as a
switching device.
[0052] The light guide system 9 in this embodiment is constituted
by an incident side reflecting mirror 91, an output side reflecting
mirror 92 and an intermediate lens 93 located between the
reflecting mirrors 91 and 92. In this embodiment, the focal length
of the intermediate lens 93 is set to be equal to the total optical
path length of the light guide system 9. The focal length can be
set within a range of approximately 0.9 to 1.1 times of the total
optical path length of the light guide system 9. Among the optical
path lengths of the color beams, that is, the distances between the
light source lamp 21 and the liquid crystal panels, the distance of
the green beam G is the longest, and therefore, the green beam G
loses the most amount of light. However, the interposition of the
light guide system 9 in this embodiment can restrict the loss in
the amount of light. Therefore, the optical path lengths of the
color beams can be substantially equalized. A color beam passing
through the light guide system 9 may be red or blue. However, since
the amount of green light is more than those of other colors in an
ordinary projection-type display apparatus, it is generally
preferable to assign the green beam to the optical path passing
through the light guide system 9. If brightness or evenness in
image quality takes priority over color balance, it is allowable to
assign the blue beam, which has a low spectral luminous efficacy
and in which unevenness in luminous intensity is relatively
difficult to detect, to the light guide system 9.
[0053] The color beams modulated by the respective liquid crystal
panels 5R, 5G and 5B are made incident on the color synthesizing
optical system 6. The color synthesizing optical system 6 consists
of a dichroic prism in this embodiment. As the color synthesizing
optical system, a mirror composite system having dichroic mirrors
arranged in the shape of X may be employed. However, in a
projection-type display apparatus having a color synthesizing
system which has a mirror composite system comprised of dichroic
mirrors, each of the dichroic mirrors is an optical element which
is rotationally asymmetrical about the center axis of a projection
lens. Therefore, astigmatism arises in an image on a screen, and
the Modulation Transfer Function (MTF) of the projection optical
system is lowered. As a result, the image is blurred and sharpness
thereof is reduced. In a case in which the size of a liquid crystal
panel is large relative to the number of pixels, in other words,
when the pixel pitch is large, the lowering of the MTF does not
cause a large problem. However, when the pixel pitch is small, such
as in the case of a liquid crystal panel using a polysilicon TFT as
the switching device in this embodiment, such lowering cannot be
ignored. Since the dichroic prism is used as the color synthesizing
optical system 6 in this embodiment, such a bad effect can be
avoided.
[0054] That point will now be described with reference to FIG. 2.
This figure shows MTF characteristics for projection-type display
apparatus having a prism composite system in this embodiment, and
for a projection-type display apparatus having a mirror composite
system as a color synthesizing system. Referring to the figure, the
horizontal axis indicates the spatial frequency (line/mm)
representing the fineness of pixels of the display panel, and the
vertical axis indicates MTF characteristics (%). Solid lines each
indicate the characteristic of the projection optical system with
the prism composite system. A bold solid line indicates the
characteristic of the center portion of an image plane, and a thin
solid line indicates that of the peripheral portion of the image
plane. Similarly, broken lines each indicate the characteristic of
the projection optical system with the mirror composite system. A
bold broken line indicates the characteristic of the center portion
of the image plane, and a thin broken line indicates that of the
peripheral portion of the image plane.
[0055] In the case of the mirror composite system, since the mirror
is inserted at an angle of 45 degrees, astigmatism arises, thereby
lowering the MTF characteristic of the projection lens alone. In
the case of a liquid crystal panel using a polysilicon TFT as a
switching device and having a pixel pitch of less than 50 .mu.m, a
MTF characteristic of more than 30% is necessary relative to a
spatial frequency of 20 (line/mm). However, it is apparent that a
sufficient MTF characteristic cannot be obtained in the peripheral
portion of the image plane when the mirror composite system is
used. On the other hand, when the prism composite system is used in
this embodiment, the MTF characteristic is not lowered since
astigmatism caused by the prism can be removed by the design of the
projection lens.
[0056] In the apparatus of this embodiment, the color beams are
synthesized in the color synthesizing system consisting of a
dichroic prism, and an optical image can be obtained and projected
onto the screen 8 by the projection lens 7 under magnification. A
lens close to a telecentric system is preferable as a projection
lens. (Illumination Optical System)
[0057] An integrator lens generally used in an exposer is suitable
for the uniform illumination optical device 3 in the illumination
optical system of this embodiment. The basic structure of the
uniform illumination optical device 3 used in the projection-type
display device is illustrated in FIG. 3(A). As shown in this
figure, the uniform illumination optical device 3 consists of the
first and second lens plates 31 and 32. The first lens plate 31 is
formed from a matrix of a plurality of rectangular lenses 301, and
the second lens plate 32 is similarly formed from a plurality of
rectangular lenses 302. Each of the rectangular lenses 301 of the
first lens plate 31 is shaped similarly to the liquid crystal panel
to be illuminated. Images on these rectangular lenses 301 are
superimposed onto the liquid crystal panel by the corresponding
rectangular lenses 302 constituting the second lens plate 32.
Therefore, the liquid crystal panel is illuminated with uniform
illumination and little color unevenness.
[0058] In this embodiment, the rectangular lenses in the lens
plates 31 and 32 are respectively arranged in a 4 by 3 matrix. It
is preferable that the most split number of the lens plates in the
vertical or horizontal direction be within a range of approximately
3 to 7. Furthermore, it is not always necessary to separate the
first and second lens plates 31 and 32. These lens plates 31 and 32
can be brought closer together by making the size of each
rectangular lens smaller and increasing the split number of the
incident beam. Furthermore, the lens plates 31 and 32 may be
combined into a single lens plate.
[0059] Referring to FIG. 4, the relationship between the split
number of the rectangular lenses of the lens plates 31 and 32
constituting the uniform illumination optical device 3 and color
unevenness will be described. In a graph shown in FIG. 4, the
horizontal axis indicates the split number of the first and second
lens plates (integrator lenses), and the vertical axis indicates
color unevenness, differences in color among the center portion (1
portion) and the peripheral portions (4 portions) on the screen 8,
as differences on a U'V' chromaticity coordinate. The smaller the
value indicating color unevenness is, the smaller the degree of
color unevenness is. In the figure, a value indicated by a broken
line is the largest color unevenness which is regarded as
permissible as color unevenness.
[0060] As shown in this graph, it is preferable that the split
number be 3 or more. However, the increase of the split number
leads to an increase in cost from the viewpoint of production.
Therefore, a practical split number is within a range of
approximately 3 to 7.
[0061] FIG. 3(B) illustrates another arrangement example of the
first and second lens plates 31 and 32 constituting the uniform
illumination optical device 3. In the example shown in this figure,
each of the lens plates is also constituted by rectangular lens
plates of the same size. However, as for the arrangement of the
rectangular lenses, the split number in the vertical direction is
7. The split number in the horizontal direction is 3 in the top and
bottom lines, 5 in the center three lines and 4 in other lines.
[0062] The uniform illumination optical device 3 may be constituted
by a first lens plate 31 consisting of a plurality of cylindrical
lenses 301' and a second lens plate 32 consisting of a plurality of
cylindrical lenses 302' as shown in FIG. 3(C). In this case, the
luminous intensity is made uniform only in one direction, and the
luminous intensity of the center of an object to be illuminated is
higher than those of the cases shown in FIGS. 3(A) and (B).
Furthermore, the arrangement of the lenses is relatively simple,
thinning of the lenses can be easily performed.
[0063] The operation in illuminating the liquid crystal panels 5R,
5G and 5B by the uniform illumination optical device 3 having the
above-mentioned arrangement will now be described with reference to
FIG. 5(A). As mentioned above, a light emitting source close to a
point source, such as a tungsten halogen lamp, a metal halide lamp,
a xenon lamp and so on, is employed as the light source 2. The beam
emitted from the lamp is reflected by the reflecting mirror 22. The
shape of the reflection plane of the reflecting mirror 22 may be
elliptical, and in this case, a first focus is made coincident with
the emitting portion of the light source lamp 21 and the second
focus is made coincident with the center of the liquid crystal
panel 5 (5R, 5G and 5B). As a result, the beam reflected by the
reflecting mirror 22 advances toward the center of the liquid
crystal panel 5. In this case, the size of the second lens plate
32, that is, the size of each of the rectangular lenses 302
constituting the lens plate 32, is set smaller than that of the
first lens plate 31 so that the center of each rectangular lens 302
of the second lens plate 32 is positioned on a line between the
center of each corresponding rectangular lens 301 of the first lens
plate 31 and the center of the liquid crystal panel 5.
[0064] Each of the rectangular lenses 301 of the first lens plate
31 condenses the beam onto the center of the corresponding
rectangular lenses 302 of the second lens plate 32. The rectangular
lenses 302 of the second lens plate 32 superimpose images on the
corresponding rectangular lenses 301 of the first lens plate 31
onto a display area 5A (an area diagonally shaded in the figure) of
the liquid crystal panel 5. Since the image in the outputting
portion of the light source lamp 21 is thus formed on the center of
each rectangular lens 302 of the second lens plate 32, the whole
second lens plate 32 functions as a secondary light source.
Therefore, for example, a chief ray 303 of a beam incident on the
end of the display area 5A of the liquid crystal panel 5 coincides
with a line which links the center of the second lens plate 32 and
the end of the display area 5A. In other words, since the
illumination beam to the liquid crystal panel 5 is a diverged beam
from the second lens plate 32, it is necessary to collimate the
diverged beam in order to make a collimated beam incident on the
liquid crystal panel 5. The condenser lenses 101, 102 and 103 are
arranged in this embodiment to serve that purpose. The focal length
of each condenser lens is set equal to a distance b between the
second lens plate 32 and the condenser lens. In this embodiment, a
planoconvex lens which is located with a convex plane facing the
liquid crystal panel 5 is used as the condenser lens. The convex
plane may be set to face the second lens plate 32. A double-convex
lens or a Fresnel lens may be used instead of the planoconvex lens.
Thus, the chief ray of the beam outputted through the liquid
crystal panel 5 is made parallel to the center axis 1a of the whole
illumination optical system through the use of the condenser lenses
101, 102 and 103.
[0065] FIG. 5(B) illustrates a variation of the illumination
optical system. In this variation, a parabolical plane is used as a
reflection plane of the reflecting mirror 22 of the light source 2.
Since the focus of the parabolical plane is made coincident with
the emitting portion of the light source lamp 21 in this case, a
beam reflected by the reflecting mirror 22 is almost parallel to
the center axis 1a of the illumination system. Therefore, the
uniform illumination optical device 3 used in this case is
constituted by first and second lens plates 31' and 32' of the same
size, and rectangular lenses constituting the lens plates each have
the same focal length. Rectangular lenses 302' of the second lens
plate 32' form an image on corresponding rectangular lenses of the
first lens plate 31' at an infinite distance. Therefore, a lens 306
is added in this case in order to form the image to be formed at an
infinite distance onto the display area 5A of the liquid crystal
panel 5. The focal length of the lens 306 is set to be equal to the
distance between the lens 306 and the liquid crystal panel 5. The
lens 306 may be integrally formed with the second lens plate
32.
[0066] When the split number by the rectangular lenses of the lens
plates 31 and 32 is relatively small, the distance between the lens
plates 31 and 32 can be relatively long, and a reflecting mirror 33
can be interposed between the lens plates 31 and 32 as shown in
FIG. 13. In this case, it is advantageous that the volume of the
uniform illumination optical system is almost half of that in the
above embodiment. Furthermore, all the optical systems can be
arranged in an area close to a square as shown in the figure, and
it contributes to downsizing of the whole apparatus.
[0067] (Light Guide System)
[0068] As mentioned above, the light guide system 9 in this
embodiment is constituted by the two reflecting mirrors 91 and 92
and the intermediate lens 93 located therebetween. Another
arrangement of the light guide system applicable to this embodiment
will now be described below.
[0069] A light guide system 9A shown in FIG. 6 has the arrangement
obtained by omitting the intermediate lens 93 from the light guide
system 9 in this embodiment.
[0070] A light guide system 9B shown in FIG. 7(A) has the
arrangement in which an incident lens 94 is added on the incident
side thereof and an outputting lens 95 is added on the output side
thereof besides the arrangement of the light guide system 9 in this
embodiment.
[0071] Referring to FIG. 7(B), the operation of the light guide
system 9B having such arrangement will now be described. In the
figure, a linear system is employed without a pair of reflecting
mirrors 91 and 92 in order to make the description plain. As shown
in the figure, the intermediate lens 93 is located just at the
center of the whole optical path of the light guide system 9B, and
when it is assumed that the total optical path length is 2a, the
focal length of the intermediate lens 93 is set to be almost equal
to a/2. Therefore, the intermediate lens 93 forms an image of an
object 96 on the incident side of the light guide system 9B onto
the output side thereof as a reversed image 97. In other words, the
illumination distribution on the incident side is transferred with
an 180-degree turn on the emission side. However, since the
illumination optical system provided with the uniform illumination
optical device 3 is employed in this embodiment, the illumination
distribution is almost symmetrical about the 180-degree turn.
Therefore, even if the illumination distribution is turned or
reversed, no color unevenness arises on the display.
[0072] On the other hand, the incident lens 94 has a focal length
which equals a distance a to the intermediate lens 93, and directs
a main ray 9a of the beam G collimated through the condenser lens
103 toward the center of the intermediate lens 93. Therefore, an
image on the second lens plate 32 on the output side of the uniform
illumination optical device 3 is formed in the center of the
intermediate lens 93. Furthermore, the outputting lens 95 also has
a focal length set to be equal to a, and collimates and outputs the
chief ray of the diverged beam outputted from the center of the
intermediate lens 93. The incident lens 94 is, as shown in the
figure, a planoconvex lens, and is located with a convex side
thereof facing the incident side, thereby decreasing its spherical
aberration. The output lens 95 is also a planoconvex lens located
with a convex side thereof facing on the output side.
[0073] It is preferable that the focal lengths of the incident lens
94 and the output lens 95 be each set within a range of
approximately 0.5 to approximately 0.7 times of the total optical
path length (2a) of the light guide system 9B. In order to decrease
the spherical aberration, it is preferable that the focal length of
the intermediate lens 93 be a little longer than 1/4 of the total
optical path length (2a), and be set within a range of
approximately 0.25 to approximately 0.4 times of the total optical
path length.
[0074] FIG. 8(A) illustrates a variation of the abovementioned
light guide system 9B. In a light guide system 9C shown in this
figure, there is provided a lens 97 made by integrally forming the
incident lens 94 and the condenser lens 103 located on this side in
the direction of the optical path in the light guide system 9B. The
focal length of the lens 97 is set at a value obtained by adding
the refracting powers of the incident lens 94 and the condenser
lens 103, in short, ab/(a+b) as shown in FIG. 8(B). It is
preferable that the lens 97 be a double-convex lens in order to
reduce spherical aberration. In FIG. 8(B), the intermediate lens 93
is constituted by two planoconvex lenses 931 and 932. As shown in
the figure, the focal length of each of the planoconvex lenses 931
and 932 is set at a. By locating the lenses 931 and 932 so that
convex planes thereof face each other, spherical aberration can be
made extremely smaller than that in use of a single double-convex
lens. As a result, it is possible to transfer the illumination
distribution on the incident side of the light guide system to the
outputting side with extreme precision.
[0075] FIG. 9 illustrates a variation of the light guide system 9C.
In an illustrated light guide system 9D, an aspherical lens 98 is
employed instead of the integrated lens 97 in the above light guide
system 9C. The use of the aspherical lens makes spherical
aberration even smaller than that in use of the double-convex lens.
Therefore, the illumination distribution on the incident side of
the light guide system can be transferred to the output side with
extreme precision.
[0076] (Advantage of First Embodiment)
[0077] As described above, in the projection-type display apparatus
1 of this embodiment, the employed illumination optical system is
provided with the uniform illumination optical device 3, and a
dichroic prism, which is an axially symmetrical optical device, is
used as the color synthesizing optical system. Therefore, it is
possible to realize a projection-type display apparatus in which
unevenness in color and luminous intensity is small and the
illumination efficiency is high. Furthermore, since the color
synthesizing system including a dichroic prism is used, the focal
length of the projection lens can be shortened, and a large-scale
display at a short distance can be performed. Consequently, the
application of the constitution of this embodiment to a rear
projector makes it possible to shorten the depth of the projector,
and to make the projector compact.
[0078] Furthermore, since the focal lengths of the intermediate
lens, the incident lens and the outputting lens, which are optical
devices constituting the light guide system, are set at proper
values, it is possible to decrease the occurrence of color
unevenness and the loss in the amount of light of the color beams
passing through those optical devices, thereby restricting
unevenness in color and luminous intensity of a projection image,
and forming a bright image.
[0079] Still furthermore, when the incident lens and the outputting
lens are integrally formed in the light guide system, since the
number of components can be reduced, the optical system can be
compact and inexpensive. If the integrated lens is replaced with an
aspherical lens, it is possible to make the optical system compact
and reduce spherical aberration.
[0080] On the other hand, in this embodiment, since the split
number in the uniform illumination optical device is set within a
range of 3 to 7, and the pixel pitch of the liquid crystal panel is
set to less than 50 .mu.m, color unevenness, blurring and so on of
the projection image can be restricted. Therefore, it is possible
to realize a projection-type display apparatus capable of forming a
projection image of high quality.
[0081] Second Embodiment
[0082] FIG. 10 illustrates a projection-type display apparatus
according to a second embodiment of the present invention. A
projection-type display apparatus 100 in this embodiment is the
same as the above-mentioned projection-type display apparatus 1 in
the first embodiment except for the structure of a light guide
system. Therefore, like components are denoted by like numerals,
and the explanation thereof is omitted.
[0083] A light guide system 9E in the projection-type display
apparatus 100 of this embodiment is constituted by an incident side
triangular prism 901, an output side triangular prism 902 and a
quadratic prism 903 located between the triangular prisms 901 and
902.
[0084] The operation of the light guide system 9E in this
embodiment will be described with reference to FIG. 10(B). A light
beam collimated by the condenser lens 103 vertically enters an
incident plane 904 of the triangular prism 901, is reflected by a
total reflection plane 905, and outputted from an outputting plane
906. The total reflection plane 905 may be an optical flat surface
merely made of glass or plastic. However, if the incident beam
includes a light beam of an angle which is not totally internally
reflected, it is preferable that the total internal reflection
plane 905 be coated with a metal film, such as aluminum, silver and
so on. Instead, coating with a dielectric multilayer reflective
film may be conducted. Since the incident plane 904 and the
outputting plane 906 serve to guide light by total internal
reflection, as illustrated, each of them is should be an interface
between air and the glass material, and cannot be in contact with
adjacent optical elements. Therefore, it is necessary that the five
planes of the triangular prism 901 be optically flat planes and, in
some cases, the incident plane 904 and the outputting plane 906
thereof should have a reflection attenuation coating applied
thereto. In particular, it is preferable that a non-reflection
coating be applied to the interface between the triangular prism
901 and the adjacent quadratic prism 903.
[0085] The six planes of the quadratic prism 903 should all be
optically flat planes. The four planes 907 parallel to a main axis
of the beam passing therethrough guide the light beam by total
internal reflection. The triangular prism 902 on the output side of
the quadratic prism 903 has the same structure as that of the
triangular prism 901 on the incident side. The emitted beam enters
the display area 5A of the liquid crystal panel 5G.
[0086] In order to enhance the transfer rate of the beam, the shape
of the incident surface 904 of the triangular prism 901, and the
shape of the emitting plane of the triangular prism 902 are almost
the same as the rectangular shape of the display area 5A of the
liquid crystal panel 5G. A uniform illumination optical device 3 of
the illumination optical system is, as shown in FIG. 3, constituted
by first and second lens plates 31 and 32, each of which has
rectangular lenses arranged in a matrix. Therefore, the incident
surface 904 of the incident side triangular prism 901 is almost
uniformly illuminated in relation to the rectangular shape thereof.
The three prisms transfer the incident beam to the display area 5A
of the liquid crystal panel 5G while maintaining the amount of
light, the collimated state, and uniform brightness distribution of
the incident beam. Though it is necessary to locate the triangular
prism 902 on the output side and the liquid crystal panel 5G close
to each other, if there is a distance which between them cannot be
ignored, a prism or a lens for light guiding may be additionally
located.
[0087] The same advantage as that of the above-mentioned first
embodiment can be obtained by the projection-type display apparatus
having such constitution in this embodiment. Instead of the
quadratic prism 903 of the light guide system in this embodiment,
for example, a cylindrical light guide member formed from the
combination of four reflecting mirrors may be employed.
[0088] The quadratic prism 903 shown in FIG. 10(B) may be replaced
with a cylindrical light guide system constituted by four
reflectivity mirrors 903', as shown in FIG. 10(C). Though the
reflectance of the light guide surfaces is slightly lowered, the
operation is not changed. As shown in FIG. 10(D), the light guide
system may also be constituted by two upper and lower reflecting
plates 911 and 912, and two reflecting mirrors 913 and 914, for
folding the optical path. In this case, though the incident beam
cannot be transferred without a loss in the amount of light, the
loss amount can be reduced to some extent by shortening the focal
length of the lens 103. Since the illumination distribution also
cannot be maintained, this method is suited to the uniform
illumination optical device using the cylindrical lenses shown in
FIG. 3(C).
[0089] Third Embodiment
[0090] FIG. 14(A) illustrates a projection-type display apparatus
according to a third embodiment of the present invention. A
projection-type display apparatus 500 in this embodiment is the
same as the above-mentioned one of the first embodiment, except for
the structure of a light guide system thereof. Therefore, like
components are denoted by like numerals, and the explanation
thereof is omitted.
[0091] A light guide system 9F in the projection-type display
apparatus 500 of this embodiment is comprised of a field lens 921
on the incident side, a field lens 922 on the output side and a
concave mirror 923. A condenser lens 103 adjacent to the incident
portion of the light guide system 9F and the field lens 921 may be
combined into a single lens.
[0092] A light guide system 9G having such structure is illustrated
in FIG. 14(B). An integrally formed lens 924 consists of a
decentered double-convex lens as illustrated.
[0093] A concrete structure of the above-mentioned light guide
system 9F is shown in FIG. 15(A). If it is assumed that a distance
between the concave mirror 923 located in the center of the optical
path and the field lens 921 or 922 is a, the focal length of the
concave mirror 923 is almost equal to a/2. The curved surface of
the concave mirror 923 is spherical or elliptical. Therefore, the
concave mirror 923 forms an image of an object 802 in the incident
portion on the emitting portion as a reflected image 803, and in
fact, the illumination distribution in the incident portion is
reversed in the emitting portion. The field lenses 921 and 922 each
have a focal length equal to a, and optical axes 801 thereof
coincide with each other in the center therebetween. The incident
side field lens 921 focuses a collimated beam from the condenser
lens 103 onto the center of the concave mirror 923. The output side
field lens 922 refracts the reflected beam from the concave mirror
923 so as to be perpendicular to a liquid crystal panel 5G.
[0094] The light guide system 9F may be structured as shown in FIG.
15(B). In a light guide system 9H shown in the figure, two field
lenses 921 and 922 in the above light guide system 9E are replaced
by a single lens 806, the concave mirror 923 is replaced by a plane
mirror 804 which is located at a distance of a/2 from the lens 806.
Furthermore, a plane mirror 805 is arranged perpendicular to an
optical axis 807 of the lens 806. A collimated beam incident on the
light guide system 9H is reflected by the plane mirror 804 through
an end portion of the lens 806, and focused onto the center of the
plane mirror 805. The beam reflected by the plane mirror 805 is
reflected by the plane mirror 804, passes through an end portion of
the lens 806, and perpendicularly enters a display area 5A of a
liquid crystal panel 5G. An image of an object 802 on the incident
side is formed as a reflected image 803 by the center of the lens
806. Since the beam passes through the center of the lens 806
twice, it is the same as the case the beam passes through a lens
having a focal length of a/2. The constitution of this embodiment
has an advantage in making the size of the apparatus smaller than
that in use of the above-mentioned light guide system 9F. Fourth
Embodiment FIG. 11 illustrates a projection-type display apparatus
according to a fourth embodiment of the present invention. A
projection-type display apparatus 200 in this embodiment is
contrived so as to compactly house an optical system in a case 201.
The optical system in this embodiment is constituted by an
illumination optical system 2B, a color separating optical system
4, light valves 5R, 5G and 5B, a color synthesizing optical system
6, a projection lens 7 and a light guide system 9D. Among these
components, the color separating optical system 4, the light valves
5R, 5G and 5B, the color synthesizing optical system 6 and the
projection lens 7 are the same as those in the apparatus 100 of the
first embodiment. The light guide system 9D is the same as that
shown in FIG. 9(A). Therefore, components corresponding to the
abovementioned ones are denoted by like numerals, and the
explanation thereof is omitted.
[0095] In the apparatus 200 of this embodiment, the direction of an
emitted beam from a light source lamp 21 is folded at a right angle
in the illumination optical system 2B so that a center axis of a
beam emitted from the illumination optical system 2B is parallel to
an optical axis 7a of the projection lens 7. The illumination
optical system 2B is provided with a polarized beam conversion
system 11.
[0096] In other words, the illumination optical system 2B in this
embodiment is constituted by a light source 2 composed of the lamp
21 and a reflecting mirror 22, the polarized beam conversion device
11 located on the emission side of the light source 2, and a
uniform illumination optical device 3A on the emission side of the
polarized beam conversion device 11.
[0097] As shown in FIG. 12, the polarized beam conversion device 11
in this embodiment is constituted by a polarizing beam splitter
111, a reflecting mirror 112, and a .lambda./2 phase plate 113. A
random polarized beam 114 emitted from the light source 2 is
separated into two linearly polarized beams, a P polarized beam 115
and an S polarized beam 116, by the polarizing beam splitter 111
which is a polarized beam separating element. Since the polarized
beam separating function of the polarizing beam splitter 111 has a
dependence on an incident angle, a light source provided with a
lamp having a short arc length and capable of emitting a beam
excellent in parallelism is suitable. When the separated P
polarized beam 115 passes through the .lambda./2 phase plate 113
which is a polarizing plane rotating element, a polarization plane
thereof is turned at an angle of 90.degree. and the P polarized
beam 115 is converted into an S polarized beam. On the other hand,
the S polarized beam 116 is outputted as it is while an optical
path thereof is merely folded by the prismatic reflecting mirror
112. In this embodiment, the reflecting mirror 112 is made of, for
example, an aluminum evaporated film. Since the reflecting mirror
112 has a higher reflectivity rate for an S polarized beam than
that for a P polarized beam, the optical path of the S polarized
beam is folded by the reflecting mirror 112. As the reflecting
mirror 112, an ordinary plane reflecting mirror may be used instead
of such prismatic mirror. The random polarized beam 114 from the
light source is outputted as an S polarized beam by passing through
the polarized beam conversion device 11 having such structure.
Though the P polarized beam is converted into the S polarized beam
in this embodiment, it is also allowable for the S polarized beam
to be converted into a P polarized beam, and the P polarized beam
to be emitted from the polarized beam conversion device 11.
[0098] The uniform illumination optical device 3A located on the
output side of the polarized beam conversion device 11 is comprised
of a first lens plate 31 located on a plane perpendicular to the
chief axis of the outputted S polarized beam 116, a second lens
plate 32 orthogonal to the first lens plate 31, and a reflecting
mirror 33 located between the lens plates 31 and 32 for folding the
optical path at a right angle. The first and second lens plates
each have the same structure as that in the first embodiment. The
light beam incident on the uniform illumination optical device 3A
is thus folded at a right angle and outputted. The outputted white
S polarized beam is separated into beams of primary colors by the
color separating optical system 4. The separated color beams are
synthesized by the color synthesizing optical system 6 comprising a
dichroic prism, and magnified and projected onto a screen 8 through
the projection lens 7 under magnification.
[0099] As mentioned above, the optical path is formed in the
apparatus 200 of this embodiment so that the direction of the
projection beam is parallel and reverse to the emission direction
of the illumination optical system 2B, and a cooling fan 12 for
restricting heat generation of the light source lamp 21 is located
on the back side of the light source 2 in the case 201.
[0100] Therefore, air heated by cooling is exhausted in the same
direction as the projection beam in the apparatus 200 of this
embodiment. When an image is displayed on a reflection-type screen
to be viewed while using this projection-type display apparatus as
a front projector, a viewer is ordinarily present behind the
apparatus. Therefore, it is advantageous in preventing the seeing
and hearing of the viewer from being disturbed by the noise of the
cooling fan or the exhausted warm air. Furthermore, if the
apparatus is installed in a place whose space is relatively tight,
such as an audio rack, since air is exhausted from the front
thereof, the air is not exhausted into closed areas, which is
convenient.
[0101] In the apparatus 200 of this embodiment, the illumination
optical system 2B is provided with the polarized beam conversion
device 11. Therefore, the random polarized beam emitted from the
light source is converted into two specific linearly polarized
beams, and the converted beams are efficiently superposed and
output with little loss incident to the emission. The system can
realize a bright illumination and is capable of outputting
polarized beams at high efficiency. Furthermore, since the
outputted polarized beams pass through the uniform illumination
optical device 3A in this embodiment, unevenness in color and
luminous intensity caused in the light source is restricted, and
illumination light of high uniformity can be obtained.
[0102] INDUSTRIAL APPLICABILITY
[0103] As described above, the projection-type display apparatus of
the present invention comprises, an illumination optical system
having a uniform illumination optical device, a color synthesizing
system utilizing a dichroic prism, a light guide system located on
the optical path of a color beam having the longest optical path
length in a color separating system. Emitted color beams separated
through the color separating system are collimated by condenser
lenses and applied onto light valves. Therefore, according to the
present invention, unevenness in color and luminous intensity of
light from a light source is restricted by the uniform illumination
optical device. The color synthesizing system is a prism composite
system which causes less unevenness in color and luminous intensity
than a mirror composite system, and therefore, the unevenness in
color and luminous intensity hardly arises therein. Furthermore,
since light of the color beam having the longest optical path
length is transmitted with little loss in the amount of light
through the light guide system, and the collimated beams are
applied onto the light valves by the condenser lenses, the loss in
the amount of light is small, and the illumination efficiency is
enhanced. Therefore, according to the present invention, it is
possible to realize a projection-type display apparatus which
causes less unevenness in color and luminous intensity than ever,
and the apparatus has a high illumination efficiency.
[0104] In the present invention, the focal lengths of the lenses,
which are components of the light guide system, are set at
appropriate values, or prisms are used as the light guide system.
According to this constitution, since unevenness in color and loss
in the amount of light in the light guide system can be restricted,
it is possible to form a projection image having little unevenness
in color and a high illumination efficiency.
[0105] Furthermore, in the present invention, a dichroic prism,
which is an element rotationally symmetrical about the center axis
of the projection optical system, is used as the color synthesizing
system. A liquid crystal panel having a small pixel pitch of less
than approximately 50 .mu.m is used as a light valve. Therefore,
according to the present invention, it is possible to form a
projection image of high resolution, and to downsize the whole
apparatus by employing a liquid crystal panel using a polysilicon
TFT or the like, which is easy to make compact.
[0106] Since the split number of the lens plates constituting the
uniform illumination optical device is set to be within a range of
3 to 7 in the present invention, it is possible to form a
projection image whose unevenness in color is restricted.
[0107] Still furthermore, since the illumination optical system is
provided with a polarized beam conversion device in the present
invention, it is possible to restrict the emission loss of the
light emitted from the light source lamp, and therefore, to form a
bright projection image.
[0108] In addition, the optical path is formed in the
projection-type optical system of the present invention so that the
projection light can be output in the direction reverse and
parallel to the direction of the light beam emitted from the
illumination optical system. A cooling means for the light source
lamp is located on the output side of the projection light in the
apparatus case. According to such constitution, when the display
apparatus is used as a front projector, the cooling means is
located on the side opposite a viewer of the projection image, and
air from the cooling means is exhausted to the side opposite the
viewer. Therefore, the arrangement is advantageous in preventing
the noise and exhausted air from the cooling means from disturbing
the viewer.
[0109] In addition, according to the present invention, besides the
above advantages, since a back focus of the projection lens of the
optical system is short, a large-scale projection at a short
distance is easy to perform. Therefore, it is possible to realize a
projection-type display apparatus suitable for presentation use and
home theater use. Furthermore, since the back focus of the
projection lens is short, it is possible to realize a projection
lens having a small F number by a small number of lenses, and
therefore, to lower the production cost of the apparatus.
* * * * *