U.S. patent application number 09/796424 was filed with the patent office on 2001-09-13 for viewing optical system and image display apparatus using the same.
Invention is credited to Takeyama, Tetsuhide, Togino, Takayoshi.
Application Number | 20010021068 09/796424 |
Document ID | / |
Family ID | 18578985 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010021068 |
Kind Code |
A1 |
Togino, Takayoshi ; et
al. |
September 13, 2001 |
Viewing optical system and image display apparatus using the
same
Abstract
A compact, bright and high-performance viewing optical system
capable of color display has a reflection type image display
device, an ocular optical system and an illuminating device formed
from a plurality of juxtaposed illuminating light sources of
different colors. An illuminating light guide optical device is
placed between the entrance pupil and the reflection type image
display device. The ocular optical system includes a decentered
prism formed from a medium surrounded by three optical surfaces and
having a refractive index larger than 1. A chromatic aberration
producing device is placed between the entrance pupil and the
illuminating light guide optical device. The chromatic aberration
producing device is arranged to superimpose the images of the
plurality of illuminating light sources on one another in the exit
pupil.
Inventors: |
Togino, Takayoshi; (Tokyo,
JP) ; Takeyama, Tetsuhide; (Tokyo, JP) |
Correspondence
Address: |
Pillsbury Madison & Sutro LLP
Intellectual Property Group
Ninth Floor, East Tower
1100 New York Avenue, N.W.
Washington
DC
20005-3918
US
|
Family ID: |
18578985 |
Appl. No.: |
09/796424 |
Filed: |
March 2, 2001 |
Current U.S.
Class: |
359/630 ;
348/E5.096; 348/E5.138; 348/E5.145; 359/632 |
Current CPC
Class: |
G02B 27/0172 20130101;
H04N 5/44 20130101; G02B 2027/0132 20130101; G02B 2027/0178
20130101; H04N 5/7491 20130101; H04N 5/7408 20130101 |
Class at
Publication: |
359/630 ;
359/632 |
International
Class: |
G02B 027/14 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 2000 |
JP |
2000-058385 |
Claims
What we claim is:
1. A viewing optical system comprising: reflection type image
display means placed in an image plane; an ocular optical system
for leading an image displayed on said reflection type image
display means to a position of an exit pupil where an observer's
eyeball is to be placed; illuminating means placed at a position of
an entrance pupil conjugate to said exit pupil with respect to said
ocular optical system, said illuminating means being formed from a
plurality of juxtaposed illuminating light sources of different
colors; illuminating light guide optical means for guiding an
illuminating light beam from said illuminating means, said
illuminating light guide optical means being placed between said
entrance pupil and said reflection type image display means so that
the illuminating light beam can be applied to a display surface of
said reflection type image display means from a front side thereof;
and chromatic aberration producing means placed between said
entrance pupil and said illuminating light guide optical means,
said chromatic aberration producing means being arranged to
superimpose images of said plurality of illuminating light sources
on one another in said exit pupil; wherein said ocular optical
system includes a decentered prism formed from a medium surrounded
by three optical surfaces and having a refractive index larger than
1, said decentered prism having: an entrance surface through which
a display light beam reflected from said reflection type image
display means enters said decentered prism after passing through
said illuminating light guide optical means; a first reflecting
surface for reflecting the light beam entering through said
entrance surface; a second reflecting surface for reflecting the
light beam reflected from said first reflecting surface; and an
exit surface through which the light beam reflected from said
second reflecting surface exits said decentered prism; wherein said
first reflecting surface and said exit surface are formed from a
single surface serving as both a refracting surface and a
reflecting surface.
2. A viewing optical system according to claim 1, wherein said
reflection type image display means is a reflection type liquid
crystal display device.
3. A viewing optical system according to claim 1 or 2, wherein said
illuminating light guide optical means is a transparent member,
said transparent member having: a first surface through which the
light beam emitted from said illuminating means enters said
transparent member; a second surface that totally reflects the
light beam entering through said first surface; and a third surface
that reflects the light beam totally reflected from said second
surface; wherein said second surface transmits the light beam
reflected from said third surface to illuminate the display surface
of said reflection type image display means from the front side
thereof and also transmits the display light beam reflected from
the display surface of said reflection type image display means,
and said third surface forms a beam splitter surface that transmits
the display light beam passing through said second surface.
4. A viewing optical system according to claim 3, wherein a
diffractive optical element is provided on an entrance surface side
of said decentered prism.
5. A viewing optical system according to claim 4, wherein said
illuminating light guide optical means is a transparent member,
said transparent member having: a first surface through which the
light beam emitted from said illuminating means enters said
transparent member; a second surface that totally reflects the
light beam entering through said first surface; and a third surface
that reflects the light beam totally reflected from said second
surface; wherein said second surface transmits the light beam
reflected from said third surface to illuminate the display surface
of said reflection type image display means from the front side
thereof and also transmits the display light beam reflected from
the display surface of said reflection type image display means,
and said third surface forms a beam splitter surface that transmits
the display light beam passing through said second surface.
6. A viewing optical system according to claim 5, wherein a
deviation angle compensating member is placed on a third surface
side of said transparent member to compensate for an angle of
deviation caused by said transparent member.
7. A viewing optical system according to claim 5, wherein said
third surface of said transparent member is a polarization beam
splitter surface.
8. A viewing optical system according to claim 3, wherein at least
one reflecting surface of said decentered prism is decentered with
respect to an optical axis and has a rotationally asymmetric curved
surface configuration that corrects decentration aberrations due to
decentration of said at least one reflecting surface and gives a
power to the light beam.
9. An image display apparatus comprising said viewing optical
system according to claim 3, said viewing optical system being
provided for one of a right eye and a left eye.
10. An image display apparatus comprising a pair of said viewing
optical systems according to claim 3, said viewing optical systems
being provided for a right eye and a left eye, respectively.
11. An image display apparatus according to claim 9, further
comprising support means for supporting said image display
apparatus on a head of an observer so that said image display
apparatus is positioned in front of a face of the observer.
12. An image display apparatus according to claim 10, further
comprising support means for supporting said image display
apparatus on a head of an observer so that said image display
apparatus is positioned in front of a face of the observer.
Description
[0001] This application claims benefit of Japanese Application No.
2000-58385 filed in Japan on Mar. 3, 2000, the contents of which
are incorporated by this reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a viewing optical system
and an image display apparatus using the same. More particularly,
the present invention relates to a viewing optical system devised
to allow observation of a bright image of a display device of the
type in which an image is displayed by reflected light, e.g. a
reflection type liquid crystal display device, through an ocular
optical system arranged to be compact in size and to minimize the
loss of light quantity. The present invention also relates to an
image display apparatus, e.g. a head-up display, using the viewing
optical system.
[0004] 2. Discussion of Related Art
[0005] In recent years, with the development of head-up displays
and glasses-type displays, compact ocular optical systems have been
actively developed. As a result, ocular optical systems using a
thin and compact decentered prism have been proposed as disclosed,
for example, in Japanese Patent Application Unexamined Publication
Numbers (hereinafter referred to as "JP(A)") Hei 7-333551, 8-50256
and 8-234137. These are compact ocular optical systems in which
reflecting surfaces have a power and the optical path is folded,
and in which rotationally asymmetric decentration aberrations
produced by decentered reflecting surfaces with a power are
corrected by using an anamorphic reflecting surface or a
rotationally asymmetric reflecting surface having one plane of
symmetry.
[0006] Regarding liquid crystal display devices for displaying an
image for observation, reflection type liquid crystal display
devices have been developed to form images that are brighter and
easier to observe. As a reflection type liquid crystal display
device including an illumination structure therefor, JP(A) Hei
10-268306 has been laid open to public.
[0007] Under these circumstances, the present applicant proposed in
Japanese Patent Application Nos. Hei 11-96291, 11-176390 and
11-281031 an image display apparatus including a reflection type
liquid crystal display device and an ocular optical system formed
from a decentered prism, wherein an illuminating light guide prism
is placed between the reflection type liquid crystal display device
and the decentered prism.
[0008] Incidentally, the present applicant proposed in JP(A) Hei
11-194295 an image display apparatus for displaying color images,
wherein a chromatic aberration producing device, e.g. a spectral
prism, is provided to display a chromatic aberration-free color
image of a color image display device by using a light source
device formed from a plurality of juxtaposed illuminating light
sources, e.g. LEDs, of different colors, e.g. R (red), G (green)
and B (blue).
SUMMARY OF THE INVENTION
[0009] The present invention was made in view of the
above-described circumstances. Accordingly, an object of the
present invention is to provide a viewing optical system including
a reflection type liquid crystal display device, a decentered prism
and an illuminating light guide prism placed therebetween. More
specifically, the present invention provides a compact, bright and
high-performance viewing optical system capable of color display by
using a light source device formed from a plurality of juxtaposed
light sources of different colors as an illuminating light source,
and also provides an image display apparatus using the viewing
optical system.
[0010] To attain the above-described object, the present invention
provides a viewing optical system including a reflection type image
display device placed in an image plane and an ocular optical
system for leading an image displayed on the reflection type image
display device to the position of an exit pupil where an observer's
eyeball is to be placed. An illuminating device is placed at the
position of an entrance pupil conjugate to the exit pupil with
respect to the ocular optical system. The illuminating device is
formed from a plurality of juxtaposed illuminating light sources of
different colors. The viewing optical system further includes an
illuminating light guide optical device for guiding an illuminating
light beam from the illuminating device. The illuminating light
guide optical device is placed between the entrance pupil and the
reflection type image display device so that the illuminating light
beam can be applied to the display surface of the reflection type
image display device from the front side thereof. The ocular
optical system includes a decentered prism formed from a medium
surrounded by three optical surfaces and having a refractive index
larger than 1. The decentered prism has an entrance surface through
which a display light beam reflected from the reflection type image
display device enters the decentered prism after passing through
the illuminating light guide optical device, and a first reflecting
surface for reflecting the light beam entering through the entrance
surface. The decentered prism further has a second reflecting
surface for reflecting the light beam reflected from the first
reflecting surface, and an exit surface through which the light
beam reflected from the second reflecting surface exits the
decentered prism. The first reflecting surface and the exit surface
are formed from a single surface serving as both a refracting
surface and a reflecting surface. A chromatic aberration producing
device is placed between the entrance pupil and the illuminating
light guide optical device. The chromatic aberration producing
device is arranged to superimpose the images of the plurality of
illuminating light sources on one another in the exit pupil.
[0011] Thus, the present invention provides a compact, bright and
high-performance viewing optical system for head-up displays and
glasses-type displays and also provides an image display apparatus
using the viewing optical system by combining together a reflection
type image display device, an illuminating light guide optical
device, a decentered prism as an ocular optical system, and a
chromatic aberration producing device. Even in a case where a light
source device formed from a plurality of juxtaposed light sources
of different colors is used as an illuminating light source, the
viewing optical system allows observation of a clear color image
free from chromatic aberration even if the observer's pupil
position deviates from the exit pupil position.
[0012] Still other objects and advantages of the invention will in
part be obvious and will in part be apparent from the
specification.
[0013] The invention accordingly comprises the features of
construction, combinations of elements, and arrangement of parts
which will be exemplified in the construction hereinafter set
forth, and the scope of the invention will be indicated in the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a sectional view showing a basic arrangement of
the viewing optical system according to the present invention.
[0015] FIG. 2 is a diagram showing a head-mounted image display
apparatus for both eyes using the viewing optical system according
to the present invention in a state where it is fitted on an
observer's head.
[0016] FIG. 3 is a sectional view of the head-mounted image display
apparatus shown in FIG. 2.
[0017] FIG. 4 is a diagram showing a head-mounted image display
apparatus for a single eye using the viewing optical system
according to the present invention in a state where it is fitted on
an observer's head.
[0018] FIG. 5 is a ray path diagram showing a basic form of an
optical system based on a principle that allows color image display
free from chromatic aberration.
[0019] FIG. 6 is a diagram showing the relationship between the
images of exit and entrance pupils in FIG. 5.
[0020] FIG. 7 is a ray path diagram showing an optical system using
an example of a chromatic aberration producing device used in the
present invention.
[0021] FIG. 8 is a ray path diagram showing an optical system using
another example of the chromatic aberration producing device.
[0022] FIG. 9 is a ray path diagram showing an optical system using
still another example of the chromatic aberration producing
device.
[0023] FIG. 10 is a sectional view showing a viewing optical system
in which an illuminating light guide prism is placed between a
reflection type image display device and a decentered prism.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Examples of the viewing optical system according to the
present invention, together with examples of the image display
apparatus using the viewing optical system, will be described
below.
[0025] First, let us describe a principle that allows color image
display free from chromatic aberration by using an illuminating
light source formed from a plurality of light sources of different
colors placed in a side-by-side relation to each other as disclosed
in JP(A) Hei 11-194295. FIG. 5 shows a basic form of an optical
system based on this principle. The optical system has as a basic
structure an image-forming optical system 1 constituting an ocular
optical system or a projection optical system. With respect to the
image-forming optical system 1, an object point 2 and an image
plane 3 have been determined in a conjugate relation to each other.
When this optical system is actually used as an ocular optical
system or a projection optical system, the image plane 3 is the
display plane of an image display device or the film plane, and the
object point 2 is on a real image plane or a virtual image plane
(in the case of a virtual image, the position of the object point 2
is on the same side as the image plane 3 with respect to the
image-forming optical system 1).
[0026] Let us consider the above-described optical system by
backward ray tracing in which light rays are traced from the object
point 2 toward the image plane 3 (the reason for the backward ray
tracing is that the optical system is assumed to be an ocular
optical system). There is an exit pupil 4 in the image-forming
optical system 1. The pupil 4 is an exit pupil in the forward ray
tracing; it is an entrance pupil in the backward ray tracing. An
entrance pupil 5 (an exit pupil in the backward ray tracing) is
located at a position on the side of the image plane 3 remote from
the image-forming optical system 1. The entrance pupil 5 is in a
conjugate relation to the exit pupil 4. A chromatic aberration
producing device 6 as shown in examples (described later) is placed
between the image plane 3 and the entrance pupil 5. Consequently,
light from the object point 2 that passes through the center of the
exit pupil 4 passes through the image-forming optical system 1 and
the image plane 3 and enters the chromatic aberration producing
device 6 where chromatic aberration is introduced into the light.
As a result, the light separates for each wavelength on the
entrance pupil 5 as shown in FIG. 5. The chromatic aberrations on
the entrance pupil 5 are those in the image of the exit pupil 4
that is projected onto the entrance pupil 5. It should be noted
that FIG. 5 shows the positions of only three wavelengths R, G and
B on the entrance pupil 5.
[0027] FIG. 6 is a diagram showing the relationship between the
images of the exit and entrance pupils 4 and 5. An image in the
exit pupil 4 shown in part (a) of FIG. 6 is chromatically aberrated
by the chromatic aberration producing device 6 and projected onto
the entrance pupil 5 to form an image for each of the colors R, G
and B as shown in part (b) of FIG. 6. If monochromatic light
sources of R, G and B are placed individually on the entrance pupil
5, which is conjugate to the exit pupil 4, in alignment with the
positions of chromatic aberrations in the pupil image produced in
the backward ray tracing, it is possible to eliminate chromatic
aberrations at the exit pupil 4 in the forward ray tracing.
Accordingly, even if the observer's pupil position deviates from
the exit pupil 4 in the case of an ocular optical system, for
example, it is possible to observe a clear image, which is free
from chromatic aberration.
[0028] The above-described arrangement makes it possible to place a
plurality of illuminating light sources of different colors in a
side-by-side relation to each other and hence possible to reduce
the overall size and weight of a head-mounted image display
apparatus or color film projection apparatus that uses the
above-described optical system as an ocular optical system or a
projection optical system.
[0029] Furthermore, it is only necessary to provide at least one
chromatic aberration producing device 6. Therefore, the desired
performance can be obtained more easily and at lower cost than in
the case of the conventional optical system using a plurality of
color filters or dichroic mirrors. Moreover, because the structure
of the whole apparatus reduces in size, the overall weight of the
apparatus can be reduced.
[0030] The following is a description of examples of the chromatic
aberration producing device 6. All the following examples will be
described on the basis of the backward ray tracing. In actuality,
an image display device is placed in the image plane 3, and
illuminating light sources of three colors, i.e. R, G and B, which
may be LEDs or the like, are placed at the corresponding chromatic
aberration points on the entrance pupil 5. In addition, a virtual
image plane is located at the position of the object point 2. It
should be noted that the image display device is usually a
transmission or reflection type LCD (liquid crystal display), a
color film slide or the like. It should also be noted that the
following optical systems operate as stated in connection with FIG.
5.
[0031] FIG. 7 is a ray path diagram showing an optical system using
one example of the chromatic aberration producing device 6. With
respect to an image-forming optical system 1, an object point 2 and
an image plane 3 have been determined in a conjugate relation to
each other. In addition, an exit pupil 4 and an entrance pupil 5
have been determined in a conjugate relation to each other. The
entrance pupil 5 is located at a position on the side of the image
plane 3 remote from the image-forming optical system 1. The
image-forming optical system 1 is corrected for various
aberrations, including chromatic aberration, to such an extent that
the aberrations are ignorable. In this example, a dispersing prism
(spectral prism) 61 having chromatic dispersion is placed between
the image plane 3 and the entrance pupil 5 as a chromatic
aberration producing device 6. A plate glass having chromatic
dispersion may be placed at an angle to the optical axis (shown by
the chain line in the figure) in place of the dispersing prism 61.
The plate glass also functions effectively as a chromatic
aberration producing device 6.
[0032] FIG. 8 is a ray path diagram showing an optical system using
another example of the chromatic aberration producing device 6. An
image-forming optical system 1, an object point 2, an image plane
3, an exit pupil 4 and an entrance pupil 5 are placed as in the
case of FIG. 7. A diffractive optical element 62, e.g. a
diffraction grating or a DOE, is placed between the image plane 3
and the entrance pupil 5 as a chromatic aberration producing device
6 (see FIG. 5).
[0033] FIG. 9 is a ray path diagram showing an optical system using
still another example of the chromatic aberration producing device
6. An image-forming optical system 1, an object point 2, an image
plane 3, an exit pupil 4 and an entrance pupil 5 are placed as in
the case of FIG. 7. A gradient index optical element 63 having
chromatic dispersion is placed between the image plane 3 and the
entrance pupil 5 as a chromatic aberration producing device 6 (see
FIG. 5).
[0034] Incidentally, to observe an image displayed on a reflection
type image display device, e.g. a reflection type liquid crystal
display device, through an ocular optical system using a decentered
prism formed from three optical surfaces, a viewing optical system
is arranged as shown in FIG. 10 (Japanese Patent Application Nos.
Hei 11-96291, 11-176390 and 11-281031). In the viewing optical
system, an illuminating light guide prism is placed between the
reflection type image display device and the decentered prism to
illuminate the reflection type image display device.
[0035] The viewing optical system shown in FIG. 10 has, in order in
which light passes from the object side in backward ray tracing, an
exit pupil 4, a decentered prism 10, a diffractive optical element
28, a deviation angle compensating prism 27, an illuminating light
guide prism 26, and an image plane (i.e. the display surface of a
reflection type image display device) 30. A light source 21 and a
light collecting element 25, e.g. a Fresnel lens, are disposed on
the illuminating light entrance side of the illuminating light
guide prism 26. The light source 21 is disposed at the position of
the entrance pupil 5. The diffractive optical element 28, the
deviation angle compensating prism 27 and the light collecting
element 25 are not always necessary.
[0036] The decentered prism 10 has a first surface 11, a second
surface 12 and a third surface 13. The first surface 11 allows a
light beam from the object side to enter the prism 10 and reflects
the light beam reflected from the second surface 12 within the
prism 10. The second surface 12 reflects the light beam entering
through the first surface 11 within the prism 10. The third surface
13 allows the light beam reflected from the first surface 11 to
exit the prism 10. Thus, the first surface 11 is an optical
functional surface having both a transmitting optical action and a
reflecting optical action.
[0037] The illuminating light guide prism 26 is a triangular prism
having a first surface 26.sub.1, a second surface 26.sub.2 and a
third surface 26.sub.3. The first surface 26.sub.1 is a surface
through which a light beam from the light source 21 is introduced
into the prism 26 through the light collecting element 25. The
introduced illuminating light is totally reflected by the third
surface 26.sub.3 and further reflected by the second surface
26.sub.2, which is a cemented surface between the illuminating
light guide prism 26 and the deviation angle compensating prism 27
and forms a polarization beam splitter surface. Then, the reflected
light passes through the third surface 26.sub.3 to illuminate the
display surface of a reflection type image display device, e.g. a
reflection type liquid crystal display device, placed in the image
plane 3. The illuminating light is incident on the display surface
approximately at right angles thereto.
[0038] Display light from the display surface of the reflection
type liquid crystal display device placed in the image plane 3
enters the illuminating light guide prism 26 through the third
surface 26.sub.3 and passes through the polarization beam splitter
surface as the second surface 26.sub.2 to enter the deviation angle
compensating prism 27. Then, the light passes through the
diffractive optical element 28, which is placed on the exit-side
surface of the deviation angle compensating prism 27. The light is
diffracted by the diffraction surface of the diffractive optical
element 28 to enter the prism 10 through the third surface 13. The
light is totally reflected by the first surface 11 and further
reflected by the second surface 12 to exit the prism 10 while being
refracted through the first surface 11. Then, the light enters the
observer's eyeball placed at the position of the exit pupil 2 to
form an enlarged image of the displayed image of the reflection
type liquid crystal display device. It should be noted that the
object point 2, which is conjugate to the image plane 3, is far
away from the exit pupil 4.
[0039] In the present invention, a viewing optical system having a
basic arrangement as shown in FIG. 10 is allowed to display a color
image free from chromatic aberration of the light source by
providing the chromatic aberration producing device 6 as described
above in connection with FIGS. 5 to 9 between the entrance pupil 5
and the illuminating light guide prism 26. The arrangement of the
viewing optical system is shown in FIG. 1. The viewing optical
system will be described below.
[0040] The arrangement of the ocular optical system is the same as
that in FIG. 10. A decentered prism 10 constituting the ocular
optical system is formed from a transparent medium surrounded by a
first surface 11, a second surface 12 and a third surface 13 and
having a refractive index larger than 1. The first surface 11
allows a light beam from the object side to enter the prism 10 and
reflects the light beam reflected from the second surface 12 within
the prism 10. The second surface 12 reflects the light beam
entering through the first surface 11 within the prism 10. The
third surface 13 allows the light beam reflected from the first
surface 11 to exit the prism 10. Thus, the first surface 11 is an
optical functional surface having both a transmitting optical
action and a reflecting optical action.
[0041] The display surface of a reflection type liquid crystal
display device is placed in an image plane 30 at the third surface
13 side of the decentered prism 10. An illuminating light guide
prism 26, a deviation angle compensating prism 27 and a diffractive
optical element 28 are placed between the image plane 30 and the
third surface 13 of the decentered prism 10. The deviation angle
compensating prism 27 is cemented to a second surface 26.sub.2 of
the illuminating light guide prism 26 on the side thereof closer to
the decentered prism 10. The diffractive optical element 28 is
cemented to the side of the deviation angle compensating prism 27
closer to the decentered prism 10.
[0042] The illuminating light guide prism 26 is a triangular prism
having a first surface 26.sub.1, a second surface 26.sub.2 and a
third surface 26.sub.3. The first surface 26.sub.1 is a surface
facing the entrance pupil 5 of the ocular optical system, through
which light beams from light sources 21.sub.R, 21.sub.G and
21.sub.B juxtaposed in the entrance pupil 5 are introduced into the
prism 26. The third surface 26.sub.3 faces the image plane 30 and
totally reflects the illuminating light introduced through the
first surface 26.sub.1. The second surface 26.sub.2 is a
polarization beam splitter surface for reflecting a predetermined
polarized light component (e.g. p-polarized light component) of the
illuminating light totally reflected from the third surface
26.sub.3.
[0043] A light collecting element 25, e.g. a Fresnel lens, is
disposed to face the first surface 26.sub.1 of the illuminating
light guide prism 26. Light sources formed from LEDs or the like,
i.e. a light source 21.sub.R of R (red), a light source 21.sub.G of
G (green), and a light source 21.sub.B of B (blue), are placed in a
side-by-side relation to each other in the plane of the entrance
pupil 5, which is in a conjugate relation to the exit pupil 4 when
the image plane 30 is a reflecting surface. A chromatic aberration
producing device 6 having an arrangement and function such as those
described in connection with FIGS. 5 to 9 (e.g. the diffractive
optical element 62) is provided on the light source side of the
light collecting element 25.
[0044] With the above-described arrangement, R, G and B light from
the R light source 21.sub.R, the G light source 21.sub.G and the B
light source 21.sub.B, which are placed in a side-by-side relation
to each other in the plane of the entrance pupil 5, enter the
chromatic aberration producing device 6 at different incident
angles because the light emission positions are displaced with
respect to each other. However, after passing through the chromatic
aberration producing device 6, the R, G and B light exit at
approximately the same angle owing to the chromatic aberration
producing action of the device 6. Then, the R, G and B light become
approximately parallel light beams through the light collecting
element 25. The illuminating light is introduced into the
illuminating light guide prism 26 through the first surface
26.sub.1 and totally reflected by the third surface 26.sub.3. The
reflected illuminating light is reflected by the second surface
26.sub.2, which is a cemented surface between the illuminating
light guide prism 26 and the deviation angle compensating prism 27
and forms a polarization beam splitter surface. Then, the reflected
light passes through the third surface 26.sub.3 to illuminate the
display surface of a reflection type image display device, e.g. a
reflection type liquid crystal display device, placed in the image
plane 30. The illuminating light is incident on the display surface
approximately at right angles thereto.
[0045] Display light from the display surface of the reflection
type liquid crystal display device placed in the image plane 30
enters the illuminating light guide prism 26 through the third
surface 26.sub.3 and passes through the polarization beam splitter
surface as the second surface 26.sub.2 to enter the deviation angle
compensating prism 27. Then, the light is diffracted by the
diffraction surface of the diffractive optical element 28, which is
placed at the exit side of the deviation angle compensating prism
27. The light enters the prism 10 through the third surface 13 and
is totally reflected by the first surface 11 and further reflected
by the second surface 12 to exit the prism 10 while being refracted
through the first surface 11. Then, the light enters the observer's
eyeball placed at the position of the exit pupil 2 to form an
enlarged image of the displayed image of the reflection type liquid
crystal display device.
[0046] At this time, R, G and B light from the R, G and B light
sources juxtaposed in the plane of the entrance pupil 5 as shown in
part (b) of FIG. 6 are chromatically aberrated by the chromatic
aberration producing device 6, so that, as shown in part (a) of
FIG. 6, the images of the R, G and B light sources are superimposed
on one another in the exit pupil 4. Therefore, even if the
observer's pupil position deviates from the exit pupil 4 position,
it is possible to observe a clear image, which is free from
chromatic aberration.
[0047] The decentered prism 10 constituting the ocular optical
system is preferably arranged such that at least one surface having
a reflecting action, particularly the second surface 12, is
decentered with respect to the axial principal ray and has a
rotationally asymmetric surface configuration and further has a
power. By adopting such an arrangement, decentration aberrations
produced as the result of giving a power to the reflecting surface
can be corrected by the surface itself. In addition, the power of
the refracting surfaces of the prism is reduced, and thus chromatic
aberration produced in the prism can be minimized.
[0048] The above-described rotationally asymmetric surface should
preferably be a plane-symmetry free-form surface having only one
plane of symmetry. Free-form surfaces used in the present invention
are defined by the following equation (a). 1 Z = cr 2 / [ 1 + { 1 -
( 1 + k ) c 2 r 2 } ] + j = 2 66 C j X m Y n ( a )
[0049] In Eq. (a), the first term is a spherical surface term, and
the second term is a free-form surface term.
[0050] In the spherical surface term:
[0051] c: the curvature at the vertex
[0052] k: a conic constant
r={square root}(X.sup.2+Y.sup.2)
[0053] The free-form surface term is given by 2 j = 2 66 C j X m Y
n = C 2 X + C 3 Y + C 4 X 2 + C 5 XY + C 6 Y 2 + C 7 X 3 + C 8 X 2
Y + C 9 XY 2 + C 10 Y 3 + C 11 X 4 + C 12 X 3 Y + C 13 X 2 Y 2 + C
14 XY 3 + C 15 Y 4 + C 16 X 5 + C 17 X 4 Y + C 18 X 3 Y 2 + C 19 X
2 Y 3 + C 20 XY 4 + C 21 Y 5 + C 22 X 6 + C 23 X 5 Y + C 24 X 4 Y 2
+ C 25 X 3 Y 3 + C 26 X 2 Y 4 + C 27 XY 5 + C 28 Y 6 + C 29 X 7 + C
30 X 6 Y + C 31 X 5 Y 2 + C 32 X 4 Y 3 + C 33 X 3 Y 4 + C 34 X 2 Y
5 + C 35 XY 6 + C 36 Y
[0054] where c.sub.j (j is an integer of 2 or higher) are
coefficients.
[0055] In general, the above-described free-form surface does not
have planes of symmetry in both the XZ- and YZ-planes. However, a
free-form surface having only one plane of symmetry parallel to the
YZ-plane is obtained by making all terms of odd-numbered degrees
with respect to X zero. For example, in the above defining equation
(a), the coefficients of the terms C.sub.2, C.sub.5, C.sub.7,
C.sub.9, C.sub.12, C.sub.14, C.sub.16, C.sub.18, C.sub.20,
C.sub.23, C.sub.25, C.sub.27, C.sub.29, C.sub.31, C.sub.33,
C.sub.35, . . . are set equal to zero. By doing so, it is possible
to obtain a free-form surface having only one plane of symmetry
parallel to the YZ-plane.
[0056] A free-form surface having only one plane of symmetry
parallel to the XZ-plane is obtained by making all terms of
odd-numbered degrees with respect to Y zero. For example, in the
above defining equation (a), the coefficients of the terms C.sub.3,
C.sub.5, C.sub.8, C.sub.10, C.sub.12, C.sub.14, C.sub.17, C.sub.19,
C.sub.21, C.sub.23, C.sub.25, C.sub.27, C.sub.30, C.sub.32,
C.sub.34, C.sub.36, . . . are set equal to zero. By doing so, it is
possible to obtain a free-form surface having only one plane of
symmetry parallel to the XZ-plane.
[0057] Furthermore, the direction of decentration is determined in
correspondence to either of the directions of the above-described
planes of symmetry. For example, with respect to the plane of
symmetry parallel to the YZ-plane, the direction of decentration of
the optical system is determined to be the Y-axis direction. With
respect to the plane of symmetry parallel to the XZ-plane, the
direction of decentration of the optical system is determined to be
the X-axis direction. By doing so, rotationally asymmetric
aberrations due to decentration can be corrected effectively, and
at the same time, productivity can be improved.
[0058] It should be noted that the above defining equation (a) is
shown as merely an example, and that the feature of the present
invention resides in that rotationally asymmetric aberrations due
to decentration are corrected and, at the same time, productivity
is improved by using a rotationally asymmetric surface having only
one plane of symmetry. Therefore, the same advantageous effect can
be obtained for any other defining equation that expresses such a
rotationally asymmetric surface.
[0059] It should be noted that the first surface 11 of the
decentered prism 10 may be formed from a rotationally symmetric
surface, e.g. a spherical surface or an axially symmetric
aspherical surface, in place of a rotationally asymmetric
surface.
[0060] The above-described viewing optical system according to the
present invention can be used as an optical system of a
head-mounted image display apparatus, for example. Examples of such
image display apparatus will be described below.
[0061] FIG. 2 shows a head-mounted image display apparatus arranged
for two eyes in a state where the image display apparatus is fitted
on an observer's head. FIG. 3 is a sectional view of the image
display apparatus. As shown in FIG. 3, the viewing optical system
according to the present invention is used as an ocular optical
system 100 of the image display apparatus. A pair of combinations
of an ocular optical system 100 and a reflection type image display
device 101 are prepared for the left and right eyes and supported
apart from each other by the interpupillary distance, i.e. the
distance between the two eyes, thereby forming a stationary or
portable image display apparatus 102, such as a head-mounted image
display apparatus, which enables the observer to see with both
eyes.
[0062] More specifically, the display apparatus body unit 102 is
equipped with a pair of ocular optical systems 100 (left and
right). The above-described viewing optical system is used as each
ocular optical system 100. Reflection type image display devices
101, which are reflection type liquid crystal display devices, are
disposed in the respective image planes of the two ocular optical
systems 100. As shown in FIG. 2, the display apparatus body unit
102 is provided with temporal frames 103 that are contiguous with
the left and right ends thereof so that the display apparatus body
unit 102 can be held in front of the observer's eyes. As shown in
FIG. 3, to protect the first surface 11 (see FIG. 1) of the prism
10 in the ocular optical system 100 of each image display apparatus
102, a cover member 91 is placed between the exit pupil of the
ocular optical system 100 and the first surface 11. As the cover
member 91, any of a plane-parallel plate, a positive lens and a
negative lens can be used.
[0063] Further, a speaker 104 is provided on each temporal frame
103 to enable the user to enjoy listening to stereophonic sound in
addition to image observation. The display apparatus body unit 102
having the speakers 104 is connected with a replaying unit 106,
e.g. a portable video cassette unit, through an image and sound
transmitting cord 105. Therefore, the user can enjoy not only
observing an image but also listening to sound with the replaying
unit 106 retained on a desired position, e.g. a belt, as
illustrated in FIG. 2. Reference numeral 107 in FIG. 2 denotes a
switch and volume control part of the replaying unit 106. It should
be noted that the display apparatus body unit 102 contains
electronic parts such as image and sound processing circuits.
[0064] The cord 105 may have a jack and plug arrangement attached
to the distal end thereof so that the cord 105 can be detachably
connected to an existing video deck. The cord 105 may also be
connected to a TV signal receiving tuner so as to enable the user
to enjoy watching TV. Alternatively, the cord 105 may be connected
to a computer to receive computer graphic images or message images
or the like from the computer. To eliminate the bothersome cord,
the image display apparatus may be arranged to receive external
radio signals through an antenna connected thereto.
[0065] The viewing optical system according to the present
invention may also be used in a head-mounted image display
apparatus for a single eye by placing the ocular optical system in
front of either of the left and right eyes. FIG. 4 shows the
head-mounted image display apparatus for a single eye in a state
where it is fitted on an observer's head (in this case, the
apparatus is fitted for the left eye). In the illustrated
arrangement, a display apparatus body unit 102 includes a single
combination of an ocular optical system 100 and a reflection type
image display device 101. The display apparatus body unit 102 is
mounted on a front frame 108 so as to lie in front of the
associated eye of the observer. As shown in FIG. 4, the front frame
108 is provided with temporal frames 103 that are contiguous with
the left and right ends thereof so that the display apparatus body
unit 102 can be held in front of one eye of the observer. The
arrangement of the rest of the apparatus is the same as in the case
of FIG. 2. Therefore, a description thereof is omitted.
[0066] Incidentally, to allow the outside world image and the
displayed image to be viewed simultaneously or selectively in the
above-described head-mounted image display apparatus for both eyes
or a single eye using the viewing optical system according to the
present invention, it is desirable that, as shown in FIG. 3, a
semitransparent reflecting surface should be used as the reflecting
surface 12, which faces the exit pupil, of the decentered prism 10
constituting the ocular optical system 100, and another decentered
prism 41 for compensating for an angle of deviation or power
produced by the decentered prism 10 should be placed in contact
with or at a slight distance from the semitransparent reflecting
surface 12, thereby allowing the outside world to be viewed through
the two decentered prisms 10 and 41. In this case, it is desirable
that a shutter 42 such as a liquid crystal shutter that selectively
cuts off or passes outside world light shown by the dashed line
should be placed on the entrance side of the decentered prism 41
(i.e. on the side of the prism 41 remote from the observer's eye).
In this case, when the shutter 42 is opened, the outside world
image can be viewed (in a see-through manner), or a superimposed
image of the outside world image and the displayed image can be
viewed. When the shutter 42 is closed, the displayed image of the
display device 101 can be viewed.
[0067] As will be clear from the foregoing description, it is
possible according to the present invention to provide a compact,
bright and high-performance viewing optical system for head-up
displays and glasses-type displays and also provide an image
display apparatus using the viewing optical system by combining
together a reflection type image display device, an illuminating
light guide optical device, a decentered prism as an ocular optical
system, and a chromatic aberration producing device. Even in a case
where a light source device formed from a plurality of juxtaposed
light sources of different colors is used as an illuminating light
source, the viewing optical system allows observation of a clear
color image free from chromatic aberration even if the observer's
pupil position deviates from the exit pupil position.
* * * * *