Title: Image display apparatus
Abstract: An image display apparatus forms illumination light from a light source into image light by using a reflective display device, and directs the image light through an eyepiece optical system to an eye to present a virtual image of the image carried by the image light. To realize an image display apparatus of this type that is compact, affords a sufficiently long eye relief, and offers wide-field, high-quality images, a combiner for making the optical path of the illumination light from the light source overlap with that of the image light is disposed inside the eyepiece optical system, a positive lens is provided in the rear potion of the eyepiece optical system between the display device and the combiner, a concave reflective surface is provided in the front portion of the eyepiece optical system between the combiner and the observation point, and the eyepiece optical system is designed to be overtelecentric as a whole.
Patent Number: 7,002,751 Issued on 02/21/2006 to Kobayashi, et al.
| Inventors:
|
Kobayashi; Yasushi (Itami, JP);
Ohzawa; Soh (Toyonaka, JP)
|
| Assignee:
|
Minolta Co., Ltd. (Osaka, JP)
|
| Appl. No.:
|
963308 |
| Filed:
|
September 26, 2001 |
Foreign Application Priority Data
| Sep 27, 2000[JP] | 2000-293975 |
| Current U.S. Class: |
359/631; 359/630; 359/633 |
| Current Intern'l Class: |
G02B 27/14 (20060101) |
| Field of Search: |
359/629-631,633,636,638
345/7-9
|
References Cited [Referenced By]
U.S. Patent Documents
| 6023253 | Feb., 2000 | Taniguchi et al.
| |
| 6313950 | Nov., 2001 | Hayakawa et al.
| |
| 6333820 | Dec., 2001 | Hayakawa et al.
| |
| 6342871 | Jan., 2002 | Takeyama.
| |
| 6417970 | Jul., 2002 | Travers et al.
| |
| 6445507 | Sep., 2002 | Togino et al.
| |
| 6493146 | Dec., 2002 | Inoguchi et al.
| |
| 6512635 | Jan., 2003 | Takeyama.
| |
| 6552854 | Apr., 2003 | Kuramochi et al.
| |
| 6655820 | Dec., 2003 | Jung et al.
| |
| 6814442 | Nov., 2004 | Okuyama et al.
| |
| 2001/0022689 | Sep., 2001 | Takeyama.
| |
| 2002/0060851 | May., 2002 | Yamazaki.
| |
| Foreign Patent Documents |
| 08-320451 | Dec., 1996 | JP.
| |
| 2000/-081591 | Mar., 2000 | JP.
| |
| 2000/-147422 | May., 2000 | JP.
| |
Primary Examiner: Mack; Ricky
Attorney, Agent or Firm: Sidley Austin Brown & Wood LLP
Claims
What is claimed is:
1. An image display apparatus comprising:
a display device of a reflective type for displaying an image and reflecting
illumination light fed thereto from ahead so as to produce image light representing
the image;
an eyepiece optical system, disposed in front of the display device and composed
of a rear portion nearer to the display device and a front portion farther from
the display device, for directing the image light from the display device through
the rear and front portions to a predetermined observation point so as to permit
a virtual image of the image displayed by the display device to be observed at
the observation point;
a light source, disposed in a position substantially conjugate with the observation
point, for emitting the illumination light fed to the display device; and
a combiner, disposed between the rear and front portions of the eyepiece optical
system, for introducing the illumination light from the light source into the rear
portion of the eyepiece optical system in such a way that a path of the illumination
light overlaps with a path of the image light,
wherein
the rear portion of the eyepiece optical system includes a refractive optical
element having a positive power,
the front portion of the eyepiece optical system includes a concave reflective
surface, and
the eyepiece optical system has an exit pupil behind and at a finite distance
from the display device.
2. An image display apparatus as claimed in claim 1,
wherein the following relations are fulfilled
1<
fa/ft≦1.4
0.3≦
Epd/fb≦0.9
where
ft represents a focal length of the eyepiece optical system as a whole;
fa represents a focal length of the front portion of the eyepiece optical system;
fb represents a focal length of the rear portion of the eyepiece optical system; and
Epd represents a distance from a rear end of the eyepiece optical system to the
exit pupil of the eyepiece optical system.
3. An image display apparatus as claimed in claim 1,
wherein the concave reflective surface included in the front portion of the eyepiece
optical system is a semitransparent reflective surface that partially reflects
and partially transmits light, and is so disposed as to point to the observation
point, and
the front portion of the eyepiece optical system includes a selective reflective
surface that is so disposed as to face the concave reflective surface and that
reflects or transmits light selectively according to a polarization direction of
the light.
4. An image display apparatus as claimed in claim 3,
wherein the concave reflective surface included in the front portion of the eyepiece
optical system is formed as a concave surface of a meniscus lens, and
the selective reflective surface included in the front portion of the eyepiece
optical system is formed on a flat surface of a member that has a flat surface
and that transmits light.
5. An image display apparatus as claimed in claim 4,
wherein a convex surface of the meniscus lens included in the front portion of
the eyepiece optical system is formed as an aspherical surface.
6. An image display apparatus as claimed in claim 5,
wherein the meniscus lens included in the front portion of the eyepiece optical
system is composed of a concave surface side portion made of glass and a convex
surface side portion made of resin.
7. An image display apparatus as claimed in claim 1,
wherein the refractive optical element having a positive power included in the
rear portion of the eyepiece optical system is a planoconvex lens.
8. An image display apparatus as claimed in claim 1,
wherein the refractive optical element having a positive power included in the
rear portion of the eyepiece optical system has an aspherical convex surface.
9. An image display apparatus as claimed in claim 1,
wherein the display device is a reflective liquid crystal panel, and
the combiner is a reflective polarizing plate.
10. An image display apparatus as claimed in claim 1,
wherein an optical axis of the rear portion of the eyepiece optical system coincides
with an optical axis of the front portion thereof,
the combiner is planar, and
an angle between an optical axis of the eyepiece optical system and a normal
to the combiner is in a range from 30° to 40°.
11. An image display apparatus as claimed in claim 1,
wherein the rear and front portions of the eyepiece optical system each include
a prism, and
the combiner is disposed between the prism included in the rear portion of the
eyepiece optical system and the prism included in the front portion thereof.
12. An image display apparatus as claimed in claim 11,
wherein the prism included in the rear portion of the eyepiece optical system
has a convex surface so as to be shared as the refractive optical element having
a positive power.
Description
This application is based on Japanese Patent Application No. 2000-293975 filed
on Sep. 27, 2000, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image display apparatus that is used in front
of an eye, such as those used as viewfinders in various cameras and as head-mounted displays.
2. Description of the Prior Art
Many video cameras and digital cameras are equipped with an image display apparatus
as a viewfinder. On the other hand, image display apparatus of the type called
head-mounted displays (HMDs) have been becoming increasingly popular in recent
years. These image display apparatus are used in front of an eye, and are typically
composed of a display device for displaying an image, a light source for feeding
illumination light to the display device, and an eyepiece optical system for directing
image light (the light carrying the image) from the display device to the eye so
as to present a virtual image of the image displayed on the display device.
Among many types of display devices, reflective liquid crystal panels are in
wide use because they are compact but nevertheless offer high resolution and in
addition high light use efficiency. However, a reflective liquid crystal panel
needs to be illuminated from in front of its display surface, and therefore, unless
a complicated arrangement is adopted in which the path of image light deviates
greatly from that of illumination light, it is inevitable to dispose in front of
the panel a combiner for combining the paths of image and illumination light.
Moreover, a reflective liquid crystal panel reflects illumination light
substantially regularly, and therefore, to make the panel uniformly bright with
respect to the observer's eye, it is necessary to dispose on the path of illumination
light a condenser lens so that the light source, or the secondary light source
such as a diffuser panel that is illuminated by the light source, is substantially
conjugate with the observation point at which the observer's eye is located.
U.S. Pat. No. 6,023,253 discloses an arrangement in which, between a reflective
liquid crystal panel and an eyepiece lens, a polarization separation (PBS) block
is disposed as a combiner and, between the combiner and a light source, a condenser
lens is disposed. However, in this arrangement, the beam diameter of the illumination
light incident on the PBS block is so large that it is impossible to make the block
satisfactorily slim. Moreover, the condenser lens is located to the side of the
eyepiece optical system, and this makes the apparatus as a whole unduly large.
Japanese Patent Applications Laid-Open Nos. 2000-81519 and 2000-147422 disclose
arrangements that permit the omission of the condenser lens between the combiner
and the light source. FIG. 18 schematically shows those arrangements. In FIG. 18,
reference numeral 51 represents a reflective liquid crystal panel, reference
numeral 52 represents an eyepiece optical system, reference numeral 53
represents a light source, and reference numeral 54 represents a combiner.
The eyepiece optical system 52 includes positive lenses 52
a and
52
b, and the combiner 54 is disposed between these two positive
lenses 52
a and 52
b. That is, the positive lens 52
b
is located between the panel 51 and the combiner 54, and this
positive lens 52
b functions as a condenser lens as well. This makes
it possible to place the light source 53 nearer to the combiner 54
to some degree and thereby make the apparatus as a whole more compact.
However, in the arrangements disclosed in Japanese Patent Applications Laid-Open
Nos. 2000-81519 and 2000-147422 mentioned above, the eyepiece optical system has
a positive refractive optical element in its portion located on the observation
point side of the combiner, and this positive refractive optical element, together
with the panel-side positive lens, contributes to a large Petzval sum. As is well
known, a large Petzval sum causes large, negative curvature of field. This makes
it impossible to present the observer with a high-quality image (virtual image).
Moreover, in the arrangements disclosed in the aforementioned laid-open
applications, the observation point is made conjugate with the light source by
designing the eyepiece optical system to be substantially telecentric with respect
to the liquid crystal panel and by making the illumination light substantially
parallel with respect to the liquid crystal panel. Thus, the observation point
is located at the front focal point of the eyepiece optical system as a whole,
and the light source is located at a point equivalent to the front focal point
of that portion of the eyepiece optical system which is located between the combiner
and the panel.
As a result, it is extremely difficult to shorten the focal length of the eyepiece
optical system and thereby widen the field of view of the image presented while
securing a sufficiently long eye relief (the distance from the front end of the
eyepiece optical system to the observation point). Moreover, attempting to make
the apparatus as a whole still more compact by placing the light source nearer
to the combiner leads to an increase in the power of the positive lens disposed
between the panel and the combiner and thus to a further increase in the Petzval
sum. This makes further miniaturization also difficult.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an image display apparatus that
is compact, affords a sufficiently long eye relief, and offers wide-field, high-quality images.
To achieve the above object according to one aspect of the present invention,
an image display apparatus is provided with: a display device of a reflective type
for displaying an image and reflecting illumination light fed thereto from ahead
so as to produce image light representing the image; an eyepiece optical system,
disposed in front of the display device and composed of a rear portion nearer to
the display device and a front portion farther from the display device, for directing
the image light from the display device through the rear and front portions to
a predetermined observation point so as to permit a virtual image of the image
displayed by the display device to be observed at the observation point; a light
source, disposed in a position substantially conjugate with the observation point,
for emitting the illumination light fed to the display device; and a combiner,
disposed between the rear and front portions of the eyepiece optical system, for
introducing the illumination light from the light source into the rear portion
of the eyepiece optical system in such a way that the path of the illumination
light overlaps with the path of the image light. Here, the rear portion of the
eyepiece optical system includes a refractive optical element having a positive
power, the front portion of the eyepiece optical system includes a concave reflective
surface, and the eyepiece optical system has an exit pupil behind and at a finite
distance from the display device.
In this image display apparatus, the rear portion of the eyepiece optical system
includes a refractive optical element having a positive power, and this eliminates
the need to dispose a condenser lens between the combiner and the light source.
Moreover, the front portion of the eyepiece optical system includes a concave reflective
surface, and this helps reduce the Petzval sum of the eyepiece optical system as
a whole and thereby suppress curvature of field. In addition, the exit pupil of
the eyepiece optical system is located behind and at a finite distance from the
display device, i.e. the eyepiece optical system is designed to be overtelecentric,
and this makes it possible to secure a sufficiently long eye relief while making
the field of view of the presented image satisfactorily wide. Furthermore, the
position substantially conjugate with the observation point is nearer to the combiner
than is the position equivalent to the front focal point of the rear portion of
the eyepiece optical system, and this permits the light source to be disposed near
the combiner.
In this image display apparatus, it is advisable that the following relations
be fulfilled:
1<
fa/ft≦1.4 (1)
0.3≦
Epd/fb≦0.9 (2)
where
- ft represents the focal length of the eyepiece optical system as a whole;
- fa represents the focal length of the front portion of the eyepiece
optical system;
- fb represents the focal length of the rear portion of the eyepiece optical
system; and
- Epd represents the distance from the rear end of the eyepiece optical
system to the exit pupil of the eyepiece optical system.
The greater the value of fa/ft, i.e. the ratio of the focal length of the front
portion of the eyepiece optical system to that of the eyepiece optical system as
a whole, the stronger the positive power of the rear portion needs to be. However,
making the positive power of the rear portion too strong causes large aberrations,
such as coma aberration, that degrade off-axial performance, and also causes large
chromatic aberration. Within the range defined by expression (1) above, such aberrations
are satisfactorily suppressed.
- The greater the value of Epd/fb, i.e. the ratio of the distance from
the eyepiece optical system to the exit pupil to the focal length of the rear portion
of the eyepiece optical system, the longer the distance from the combiner to the
light source located in a position conjugate with the observation point. Within
the range defined by expression (2) above, it is easy to avoid the interference
of the light source with the eyepiece optical system while placing the light source
near the combiner.
In the image display apparatus described above, it is advisable that the concave
reflective surface included in the front portion of the eyepiece optical system
be a semitransparent reflective surface that partially reflects and partially transmits
light, and be so disposed as to point to the observation point, and that the front
portion of the eyepiece optical system include a selective reflective surface that
is so disposed as to face the concave reflective surface and that reflects or transmits
light selectively according to the polarization direction of the light. In this
arrangement, the image light reciprocates between the semitransparent reflective
surface, acting as the concave reflective surface, and the selective reflective
surface. This makes it possible to widen the field of view of the presented image
while making the front portion of the eyepiece optical system smaller.
Here, it is advisable that the concave reflective surface included in the front
portion of the eyepiece optical system be formed as a concave surface of a meniscus
lens, and that the selective reflective surface included in the front portion of
the eyepiece optical system be formed on the flat surface of a member that has
a flat surface and that transmits light. This helps make the construction of the
front portion very simple.
It is advisable that the convex surface of the meniscus lens included in the
front
portion of the eyepiece optical system be formed as an aspherical surface. Forming
a refractive surface as an aspherical surface makes it easy to prevent degradation
of off-axial performance.
It is advisable that the meniscus lens included in the front portion of the eyepiece
optical system be composed of a concave surface side portion made of glass and
a convex surface side portion made of resin. Since the selective reflective surface
reflects light by exploiting the difference in the polarization direction, if the
concave surface side portion of the meniscus lens that directs light to the selective
reflective surface is made of resin, which exhibits a high degree of birefringence,
ghosts are likely to result. This inconvenience can be avoided by forming the concave
surface side portion of the meniscus lens out of glass, and forming the convex
surface side portion thereof out of resin makes it easy to form the convex surface
as an aspherical surface.
It is advisable that the refractive optical element having a positive power included
in the rear portion of the eyepiece optical system be a planoconvex lens. This
helps make the construction of the rear portion very simple.
It is advisable that the refractive optical element having a positive power included
in the rear portion of the eyepiece optical system have an aspherical convex surface.
Forming a refractive surface as an aspherical surface makes it easy to suppress
curvature of field.
It is advisable that the display device be a reflective liquid crystal panel,
and that the combiner be a reflective polarizing plate. Using a reflective liquid
crystal panel makes it possible to benefit from the advantages described earlier
that it offers, and using a reflective polarizing plate instead of a semitransparent
mirror as the combiner makes it possible to use light efficiently for image presentation.
In this case, the illumination light from the light source is formed into linearly
polarized light that suits the reflective polarizing plate, and the liquid crystal
panel is so controlled that the image light has its polarization plane rotated
by 90°.
It is advisable that the optical axis of the rear portion of the eyepiece optical
system coincide with the optical axis of the front portion thereof, that the combiner
be planar, and that the angle between the optical axis of the eyepiece optical
system and a normal to the combiner be in the range from 30° to 40°.
When the angle θ between the optical axis of the eyepiece optical system
and a normal to the combiner equals 45°, the combiner occupies equal dimensions
in the directions perpendicular and parallel to the optical axis of the eyepiece
optical system. The smaller the angle θ, the nearer the light source is located
to the rear portion of the eyepiece optical system. This makes it necessary to
increase the distance from the light source to the combiner to avoid the interference
between the light source and the rear portion of the eyepiece optical system. By
setting the angle θ within the aforementioned range, it is possible to reduce
the dimension that the combiner occupies in the direction parallel to the optical
axis of the eyepiece optical system, and thus to place the rear and front portions
of the eyepiece optical system nearer to each other. In addition, it is then easy
to place the light source near the combiner while avoiding the interference between
the light source and the rear portion of the eyepiece optical system. Furthermore,
in a case where a reflective polarizing plate is used as the combiner, it is possible
to benefit from the characteristic of the reflective polarizing plate that exhibits
increasingly high transmittances as the angle of incidence decreases, and thereby
suppress the loss of the image light.
It is advisable that the rear and front portions of the eyepiece optical system
each include a prism, and that the combiner be disposed between the prism included
in the rear portion of the eyepiece optical system and the prism included in the
front portion thereof. This makes it possible to shorten the optical length, as
converted into the equivalent length in air, according to the thicknesses and refractive
indices of the prisms, and thereby shorten the focal length of the eyepiece optical
system, i.e. widen the field of view of the presented image.
Here, it is advisable that the prism included in the rear portion of the eyepiece
optical system have a convex surface so as to be shared as the refractive optical
element having a positive power. This eliminates the need to provide a refractive
optical element separately, and thus helps make the construction of the rear portion
of the eyepiece optical system very simple.
BRIEF DESCRIPTION OF THE DRAWINGS
This and other objects and features of the present invention will become clear
from the following description, taken in conjunction with the preferred embodiments
with reference to the accompanying drawings in which:
FIG. 1 is a diagram schematically showing the overall construction of the image
display apparatus of a first embodiment of the invention;
FIG. 2 is a diagram schematically showing the overall construction of the image
display apparatus of a second embodiment of the invention;
FIG. 3 is a diagram showing the construction and optical path of a first practical example;
FIG. 4 is a diagram showing the spherical aberration, astigmatism, and distortion
observed in the first practical example;
FIG. 5 is a diagram showing the curvature of field observed in the first practical example;
FIG. 6 is a diagram showing the construction and optical path of a second practical example;
FIG. 7 is a diagram showing the spherical aberration, astigmatism, and distortion
observed in the second practical example;
FIG. 8 is a diagram showing the curvature of field observed in the second practical example;
FIG. 9 is a diagram showing the construction and optical path of a third practical example;
FIG. 10 is a diagram showing the spherical aberration, astigmatism, and distortion
observed in the third practical example;
FIG. 11 is a diagram showing the curvature of field observed in the third practical example;
FIG. 12 is a diagram showing the construction and optical path of a fourth practical example;
FIG. 13 is a diagram showing the spherical aberration, astigmatism, and distortion
observed in the fourth practical example;
FIG. 14 is a diagram showing the curvature of field observed in the fourth practical example;
FIG. 15 is a diagram showing the construction and optical path of a fifth practical example;
FIG. 16 is a diagram showing the spherical aberration, astigmatism, and distortion
observed in the fifth practical example;
FIG. 17 is a diagram showing the curvature of field observed in the fifth practical
example; and
FIG. 18 is a diagram schematically showing the overall construction of a conventional
image display apparatus.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, image display apparatus embodying the present invention
will be described with reference to the drawings. FIG. 1 schematically shows the
overall construction of the image display apparatus
1 of a first embodiment
of the invention. The image display apparatus
1 is provided with a reflective
liquid crystal panel
11, an eyepiece optical system
12, a light source
13, and a combiner
14.
The liquid crystal panel
11 displays an image, modulates illumination
light fed from ahead with the displayed image to produce image light representing
the image, and emits the image light ahead. The liquid crystal panel
11
is driven by a driver circuit (not shown) according to an image signal.
The eyepiece optical system
12 is disposed in front of the liquid crystal
panel
11 with the optical axis Ax of the former perpendicular to the latter.
The eyepiece optical system
12 directs the image light from the liquid crystal
panel
11 to a predetermined observation point P. The observer, by viewing
the liquid crystal panel
11 through the eyepiece optical system
12
with his or her eye E positioned at the observation point P, can observe an enlarged
virtual image of the image displayed by the liquid crystal panel
11. The
pupil of the observer corresponds to the aperture stop of the eyepiece optical
system
12, and the observation point P is located at the aperture stop of
the eyepiece optical system
12.
The eyepiece optical system
12 is composed of two portions, namely a rear
portion
12b nearer to the liquid crystal panel
11 and a front
portion
12a farther from the liquid crystal panel
11, and
the optical axes of the rear and front portions
12b and
12a
coincide with each other. The rear portion
12b of the eyepiece
optical system
12 is composed of a positive lens
21, and the front
portion
12a thereof is composed of a meniscus lens
25 and
a planoconvex lens
26. In the front portion
12a, the meniscus
lens
25 is disposed with its concave surface pointing to the observation
point P, and the planoconvex lens
26 is disposed on the observation point
P side of the meniscus lens
25 with the flat surface of the former pointing
to the latter.
The combiner
14 is disposed between the rear and front portions
12b
and
12a of the eyepiece optical system
12 so as to obliquely
cross the optical axis Ax of the eyepiece optical system
12. In the image
display apparatus
1, a flat-surfaced half mirror is used as the combiner
14. The angle θ between a normal to the combiner
14 and the
optical axis Ax of the eyepiece optical system
12 is in the range form 30°
to 40°.
The light source
13 emits illumination light that is fed to the liquid
crystal panel
11. The illumination light emitted by the light source
13
is unpolarized light. The light source
13 is disposed in a position conjugate
with the observation point P with respect to the eyepiece optical system
12.
The image display apparatus
1 is provided with, in addition to the components
mentioned already, a polarizing plate
31, a polarizing plate
32,
and a quarter-wave plate
33. The polarizing plate
31 is disposed
in the vicinity of the light source
13. The polarizing plate
32 and
the quarter-wave plate
33 are disposed between the half mirror
14
and the front portion
12a of the eyepiece optical system
12,
with the polarizing plate
32 placed nearer to the liquid crystal panel
11.
On the concave surface of the meniscus lens
25 is provided a semitransparent
reflective film
35, and on the flat surface of the planoconvex lens
26
is provided a cholesteric liquid crystal layer
36. The semitransparent reflective
film
35, like a common half mirror, partially reflects light and partially
transmits light. The cholesteric liquid crystal layer
36 reflects one and
transmits the other of two types of circularly polarized light that are polarized
in opposite rotation directions.
The illumination light emitted by the light source
13 is formed into linearly
polarized light by the polarizing plate
31, and then strikes the half mirror
14, by which a half of the illumination light is reflected. The illumination
light reflected from the combiner
14 passes through the positive lens
21
constituting the rear portion
12b of the eyepiece optical system
12, and then strikes the display surface of the liquid crystal panel
11.
Here, the positive lens
21 functions as a condenser lens so that the illumination
light illuminates the entire display surface of the liquid crystal panel
11 uniformly.
The liquid crystal panel
11 modulates the illumination light, which is
now linearly polarized, by rotating the polarization plane of part of the illumination
light by 90°. The liquid crystal panel
11 is controlled either in such
a way that the part of the linearly polarized light of which the polarization plane
has been rotated by modulation is used as image light representing an image or
in such a way that the part of the linearly polarized light of which the polarization
plane has not been rotated by modulation is used as image light representing an image.
The image light from the liquid crystal panel
11 travels along the same
optical path as the illumination light in the opposite direction, then passes through
the positive lens
21 again, and then strikes the half mirror
14,
by which a half of the image light is transmitted. The image light transmitted
through the half mirror
14 then strikes the polarizing plate
32.
The polarizing plate
32 is so configured, according to how the liquid crystal
panel
11 is controlled, as to transmit light that is linearly polarized
in the way that the image light is. Thus, the part of the light coming from the
liquid crystal panel
11 of which the polarization plane is perpendicular
to that of the image light is eliminated by the polarizing plate
32.
The image light transmitted through the polarizing plate
32 then passes
through the quarter-wave plate
33 and is thereby formed into right-hand
or left-hand circularly polarized light. This image light then enters the meniscus
lens
25 through its convex surface, and a half of the image light is transmitted
through the semitransparent reflective film
35 provided on the concave surface
of the meniscus lens
25. The image light transmitted through the semitransparent
reflective film
35 then strikes the cholesteric liquid crystal layer
36.
The cholesteric liquid crystal layer
36 has its chirality so set as to reflect
light that is circularly polarized in that rotation direction in which the light
that has been transmitted through the quarter-wave plate
33 is, and thus
reflects the image light that strikes it. The image light reflected from the cholesteric
liquid crystal layer
36, while keeping its rotation direction, strikes the
semitransparent reflective film
35, by which a half of the image light is reflected.
The image light, by being reflected by the semitransparent reflective film
35,
is formed into light circularly polarized in the opposite rotation direction, and
then strikes the cholesteric liquid crystal layer
36 again, by which the
image light is transmitted this time. The image light transmitted through the cholesteric
liquid crystal layer
36 then passes through the planoconvex lens
26,
and then reaches the observation point P.
While traveling along the optical path described above, the image light from
the liquid crystal panel
11 is subjected to the positive powers resulting
from the refraction at the positive lens
21, the convex surface of the meniscus
lens
25, and the convex surface of the planoconvex lens
26 and from
the reflection at the concave surface of the meniscus lens
25. As a result,
an enlarged virtual image of the image displayed on the liquid crystal panel
11
is presented to the eye E positioned at the observation point P. By adding a concave
reflective surface to the front portion
12a of the eyepiece optical
system
12 so that it is not composed solely of refractive optical elements,
it is possible to avoid increasing the Petzval sum of the eyepiece optical system
12 as a whole and thereby obtain images with almost no curvature of field.
The eyepiece optical system
12 is so configured as to have its exit pupil
(the pupil with respect to the observation point P located at the aperture stop)
behind the liquid crystal panel
11 (on the opposite side of the liquid crystal
panel
11 to the eyepiece optical system
12). That is, the eyepiece
optical system
12 is designed to be overtelecentric.
Let the focal length of the eyepiece optical system
12 as a whole, i.e.
its rear and front portions
12b and
12a together, be
ft, and let the focal length of the front portion
12a of the eyepiece
optical system
12 be fa. Then, these are so determined as to fulfill the
relation given by expression (1) below (the same as the identically numbered one
presented earlier).
1<
fa/ft≦1.4 (1)
Setting an upper limit to the ratio fa/ft of the focal length of the front
portion
12a to the focal length of the eyepiece optical system
12
as a whole in this way eliminates the need to give the rear portion
12b
a very strong power and thereby helps suppress aberrations occurring in the
rear portion
12b composed of the positive lens
21, which is
a refractive optical element. Within the range defined by expression (1), almost
no coma or chromatic aberration, which degrades off-axial performance, occurs.
Moreover, let the focal length of the rear portion
12b of
the eyepiece optical system
12 be fb, and let the distance from the rear
end of the eyepiece optical system
12 (i.e. the liquid crystal panel
11
side surface of the positive lens
21) to the exit pupil of the eyepiece
optical system
12 be Epd. Then, these are so determined as to fulfill the
relation given by expression (2) below (the same as the identically numbered one
presented earlier).
0.3≦
Epd/fb≦0.9 (2)
The greater the ratio Epd/fb of the distance from the eyepiece optical system
12 to the exit pupil to the focal length of the rear portion
12b
of the eyepiece optical system
12, the longer the distance from the
half mirror
14, i.e. the combiner, to the light source
13 located
in a position conjugate with the observation point P, and thus the larger the beam
diameter of the light that strikes the half mirror
14. This requires the
half mirror
14 itself to be made larger. Setting an upper limit to the ratio
Epd/fb as defined by expression (2) makes it possible to place the light source
13 near the half mirror
14, and thus helps avoid making the image
display apparatus as a whole larger. Moreover, setting a lower limit to the ratio
Epd/fb as defined by expression (2) helps avoid a situation in which the light
source
13 is so near the half mirror
14 that it is difficult to place
the eyepiece optical system
12 in such a way that it does not interfere
with the rear portion
12b of the eyepiece optical system
12.
The smaller the half mirror
14, which is disposed so as to obliquely cross
the optical axis Ax of the eyepiece optical system
12, the nearer the front
and rear portions
12a and
12b can be placed to each
other. This is desirable for the miniaturization of the image display apparatus
as a whole. In addition, as described earlier, the inclination of the half mirror
14 relative to the optical axis Ax is in the range from 30° to 40°,
i.e. not 45°, and therefore the dimension of the space required to dispose
the half mirror
14 as measured in the direction along the optical axis Ax
is smaller than the dimension of the same space as measured in the direction perpendicular
to the optical axis Ax. This makes it easier to place the front and rear portions
12a and
12b nearer to each other.
To suppress aberrations, it is desirable that the convex surfaces of the positive
lens
21 and of the meniscus lens
25, which act as refractive surfaces,
be formed as aspherical surfaces. A lens with an aspherical surface can be formed
out of glass or resin, and forming it out of resin is easier. However, considering
that a cholesteric liquid crystal layer reflects or transmits light selectively
by exploiting the difference of the polarization direction of the light, it is
essential to prevent the image light arriving at the cholesteric liquid crystal
layer
36 from containing a light component that is polarized otherwise than
desired, and therefore it is not desirable to form the concave surface side portion
of the meniscus lens
25, which ultimately directs the image light to the
cholesteric liquid crystal layer
36, out of resin, which exhibits birefringence.
Thus, when the convex surface of the meniscus lens
25 is formed as an aspherical
surface, it is preferable to form the entire meniscus lens
25 out of glass,
or form the concave surface side portion thereof out of glass and the convex surface
side portion thereof out of resin.
A meniscus lens of which the concave surface side portion is made of glass and
of which the convex surface side portion is made of resin can be produced easily
by first forming a meniscus lens having a spherical convex surface out of glass,
then forming a resin layer on its convex surface side, and then forming the surface
of the resin layer into an aspherical shape. Alternatively, one can be produced
also by cementing together a concave lens made of glass and an aspherical convex
lens made of resin.
FIG. 2 schematically shows the overall construction of the image display apparatus
2 of a second embodiment of the invention. The image display apparatus
2
is obtained by modifying part of the constituent components of the image display
apparatus
1 and adding some components thereto. In the following descriptions,
such components as are identical with or similar to those used in the image display
apparatus
1 are identified with the same reference numerals, and overlapping
explanations will not be repeated.
The rear and front portions
12b and
12a of the eyepiece
optical system
12 include prisms
22 and
27, respectively.
The prisms
22 and
27 hold a combiner
15 by sandwiching it
from both sides. If the liquid crystal panel
11 side surface of the prism
22 and the observation point P side surface of the prism
27 are formed
as flat surfaces that are parallel to each other, the prisms
22 and
27
have no power; if these surfaces are formed as curved surfaces, the prisms
22
and
27 have a power of their own. In particular, if the liquid crystal panel
11 side surface of the prism
22 is formed as a convex surface so
as to function as the positive lens
21, there is no need any longer to provide
a lens separately in the rear portion
12b.
In the image display apparatus
2, instead of a half mirror
14, a
reflective polarizing plate is used as the combiner
15. Moreover, on the
flat surface of the planoconvex lens
26 included in the front portion
12a
of the eyepiece optical system
12, instead of a cholesteric liquid crystal
layer
36, a reflective polarizing plate
37 is provided. Moreover,
between the meniscus lens
25 and the planoconvex lens
26, a quarter-wave
plate
38 is provided.
The illumination light, which is unpolarized, from the light source
13
is formed into linearly polarized light by the polarizing plate
31, then
passes through the prism
22, and then strikes the reflective polarizing
plate
15, i.e. the combiner. The polarizing plate
15 is so configured
as to reflect light that is linearly polarized in the way that the light that has
been transmitted through the polarizing plate
31 is, and thus reflects all
the illumination light that strikes it. The illumination light reflected by the
polarizing plate
15 passes through the prism
22, then passes through
the positive lens
21, and then strikes the entire display surface of the
liquid crystal panel
11 uniformly.
The liquid crystal panel
11 is controlled in such a way that the part
of the linearly polarized light of which the polarization plane has been rotated
by modulation is used as image light. The image light from the liquid crystal panel
11 travels along the same optical path as the illumination light in the
opposite direction, then passes through the positive lens
21 and the prism
22 again, and then strikes the polarizing plate
15. The image light
has its polarization plane rotated by 90°, and is therefore transmitted through
the polarizing plate
15. On the other hand, the part of the linearly polarized
light of which the polarization plane has not been rotated by modulation is reflected
by the polarizing plate
15 and is thereby eliminated.
The image light transmitted through the polarizing plate
15 then passes
through the prism
27, and then strikes the polarizing plate
32. The
polarizing plate
32 is so configured as to transmit light that is linearly
polarized in the way that the image light is, and thus transmits the image light.
The polarizing plate
32 may be omitted; however, in a case where the elimination
of unnecessary polarized light components by the polarizing plate
15 is
incomplete, it serves to eliminate the remnants of such light components and thereby
sharpen the presented image.
The image light transmitted through the polarizing plate
32 then passes
through the quarter-wave plate
33 and is thereby formed into either right-hand
or left-hand circularly polarized light. The image light thus formed into circularly
polarized light enters the meniscus lens
25 through its convex surface,
and a half of the image light is transmitted through the semitransparent reflective
film
35 provided on the concave surface of the meniscus lens
25.
The image light transmitted through the semitransparent reflective film
35
is then transmitted through the quarter-wave plate
38 and is thereby formed
into a linearly polarized light, and then strikes the reflective polarizing plate
37. The reflective polarizing plate
37 is so configured as to reflect
light that is linearly polarized in the way that this linearly polarized light
is, and thus reflects the image light.
The image light reflected from the polarizing plate
37 is then transmitted
through the quarter-wave plate
38 again and is thereby formed into circularly
polarized light, and then strikes the semitransparent reflective film
35,
by which a half of the image light is reflected. The image light, by being reflected
from the semitransparent reflective film
35, is formed into light circularly
polarized in the opposite rotation direction, is then transmitted through the quarter-wave
plate
38 again, by which the image light is formed into a linearly polarized
light. This linearly polarized light has its polarization plane rotated by 90°,
and is therefore transmitted through the reflective polarizing plate
37.
The image light transmitted through the reflective polarizing plate
37 then
passes through the planoconvex lens
26, and then reaches the observation
point P.
In the image display apparatus
2, as in the image display apparatus
1,
the exit pupil of the eyepiece optical system
12 is located behind the liquid
crystal panel
11, the relations defined by expressions (1) and (2) are fulfilled,
and the angle between a normal to the reflective polarizing plate
15, i.e.
the combiner, and the optical axis Ax of the eyepiece optical system
12
is in the range from 30° to 40°.
The image display apparatus
2, which uses a reflective polarizing plate
15 as the combiner, offers higher light use efficiency and thus presents
brighter images than the image display apparatus
1, which uses a half mirror
14. In general, with a reflective polarizing plate, the larger the angle
of incidence, the lower its transmittance toward light. However, in the image display
apparatus
2, where the inclination of the polarizing plate
15 relative
to the optical axis Ax of the eyepiece optical system
12 is as small as
in the range from 30° to 40°, the image light can be transmitted without
a loss.
In the image display apparatus
2, the prisms
22 and
27 are
provided in the vacant portion of the space that a combiner
15 would occupy.
Thus, the substantial optical path length through this space equals the thicknesses
of the prisms
22 and
27 (i.e. their lengths along the optical axis
Ax) divided by their respective refractive indices, i.e. shorter than when a combiner
15 is used. This makes it possible to place the rear and front portions
12b and
12a of the eyepiece optical system
12
still nearer to each other than in the image display apparatus
1. As a result,
it is possible to make the eyepiece optical system
12 smaller and its focal
length shorter, and thereby widen the field of view of the presented image.
In the image display apparatus
2 also, the polarizing plate
37
reflects
or transmits light by exploiting the difference in the polarization direction of
the light, and therefore, when the convex surface of the meniscus lens
25
is formed as an aspherical surface, it is preferable to form the entire meniscus
lens
25 out of glass, or form the concave surface side portion thereof out
of glass and the convex surface side portion thereof out of resin.
In the image display apparatus
2, the front portion
12a of
the eyepiece optical system
12 is described as having a different construction
than in the image display apparatus
1; however, it may have the same construction
as in the image display apparatus
1. The prisms
22 and
27
may be omitted.
Hereinafter, practical examples of image display apparatus embodying
the present invention will be presented on the basis of the construction of the
second embodiment. In all of the practical examples described below, to eliminate
unnecessary polarized light components and thereby make the presented image still
sharper, a polarizing plate
39 is provided between the planoconvex lens
26 and the reflective polarizing plate
37.
PRACTICAL EXAMPLE 1
FIG. 3 shows the construction and optical path of a first practical example,
and Table 1 shows the construction data thereof FIGS. 4 and 5 show the aberrations
observed in this practical example. In the construction data, the surfaces are
numbered in the order reverse to the direction in which the image light travels
along its optical path. The refractive indices given are for light having a wavelength
of 587.6 nm. These apply also to Tables 2 to 5 described later.
In this practical example, the focal length of the eyepiece optical system 12
as a whole is ft=18.000 mm, the focal length of the front portion 12
a
of the eyepiece optical system is fa=19.752 mm, the focal length of the rear
portion 12
b of the eyepiece optical system is fb=27.948 mm, and the
distance from the rear end of the eyepiece optical system to the exit pupil is
Epd=19.296 mm. Hence, fa/ft=1.097, and Epd/fb=0.690. The angle between the optical
axis of the eyepiece optical system and a normal to the reflective polarizing plate
15 functioning as a combiner is θ=35°. The positive lens 21
constituting the rear portion 12
b of the eyepiece optical system
12 is a planoconvex lens.
PRACTICAL EXAMPLE 2
FIG. 6 shows the construction and optical path of a second practical example,
and Table 2 shows the construction data thereof FIGS. 7 and 8 show the aberrations
observed in this practical example.
In this practical example, ft=18.000 mm, fa=19.234 mm, fb=38.912 mm, Epd=26.809
mm, fa/ft=1.069, Epd/fb=0.689, and θ=35°. The prisms 22 and 27
are omitted.
The meniscus lens 25 has its convex surface (surface 11) formed
as an aspherical surface, which is defined by expression (3) below.
Z=C·h2/{1+[1-(1+
K)·
C2·h2]
1/2
}+A4·
h4+A6·
h6+A8·
h8+A10·
h
where Z represents the displacement along the optical axis Ax, C represents
the curvature (the reciprocal of the radius of curvature), h represents the distance
from the optical axis, K represents a conic constant, and A4 to A10
represent the coefficients for the terms of orders 4 to 10.
The aspherical surface of the meniscus lens 25 has the following coefficients:
K=0, A4=-0.962037×10-4, A6=0.275900×10-5,
A8=-0.324928×10-7, A10=0.142936×10-9.
PRACTICAL EXAMPLE 3
FIG. 9 shows the construction and optical path of a third practical example,
and Table 3 shows the construction data thereof FIGS. 10 and 11 show the aberrations
observed in this practical example.
In this practical example, ft=18.000 mm, fa=18.804 mm, fb=38.123 mm, Epd=14.818
mm, fa/ft=1.045, Epd/fb=0.389, and θ=35°. The prisms 22 and 27
are omitted.
The meniscus lens 25 is composed of a concave surface side portion 25a
made of glass and a convex surface side portion 25b made of resin,
and has its convex surface (surface 12) formed as an aspherical surface.
This aspherical surface has the following coefficients: K=0, A4=0.147835×10-3,
A6=0.811462×10-6, A8=-0.127597×10-7, A10=0.767565×10-10.
Moreover, the planoconvex lens 26 has its observation point P side
surface (surface 1) formed as an aspherical surface. This aspherical surface
has the following coefficients: K=0, A4=0.331917×10-4, A6=-0.159489×10-5,
A8=0.188617×10-7, A10=-0.770228×10-10.
PRACTICAL EXAMPLE 4
FIG. 12 shows the construction and optical path of a fourth practical example,
and Table 4 shows the construction data thereof FIGS. 13 and 14 show the aberrations
observed in this practical example.
In this practical example, ft=18.000 mm, fa=18.926 mm, fb=51.060 mm, Epd=31.461
mm, fa/ft=1.051, Epd/fb=0.616, and θ=35°. The prisms 22 and 27
are omitted.
The positive lens 21 has its polarizing plate 15 side surface (surface
17) formed as an aspherical surface. This aspherical surface has the following
coefficients: K=0, A4=0.301331×10-3, A6=-0.487364×10-5,
A8=0, A10=0.
PRACTICAL EXAMPLE 5
FIG. 15 shows the construction and optical path of a fifth practical example,
and Table 5 shows the construction data thereof FIGS. 16 and 17 show the aberrations
observed in this practical example.
In this practical example, ft=18.000 mm, fa=18.341 mm, fb=53.696 mm, Epd=26.371
mm, fa/ft=1.019, Epd/fb=0.491, and θ=35°. The prism 22 has its
liquid crystal panel 11 side surface formed as a convex surface, which is
shared as the positive lens 21.
The planoconvex lens 26 has its observation point P side surface (surface
1) formed as an aspherical surface. This aspherical surface has the following
coefficients: K=0, A4=0.605233×10-4, A6=-0.215823×10-5,
A8=0.283409×10-7, A10=-0.138355×10-9.
Obviously, many modifications and variations of the present invention
are possible in light of the above teachings. It is therefore to be understood
that within the scope of the appended claims, the invention may be practiced other
than as specifically described.
| TABLE 1 |
|
| |
|
Radius of |
Axial |
|
| |
Reference |
Curvature |
Distance |
Refractive |
| Surface |
Numeral |
(mm) |
(mm) |
Index |
|
| |
| 0 |
P |
∞ |
18.000 |
1.0 |
| |
air |
| 1 |
26 |
∞ |
0.800 |
1.4914 |
| 2 |
39 |
∞ |
0.100 |
1.5834 |
| 3 |
37 |
∞ |
0.100 |
1.5834 |
| 4 |
38 |
∞ |
0.200 |
1.5834 |
| 5 |
air |
∞ |
5.008 |
1.0 |
| 6 |
35 |
-39.862 |
-5.008 |
(reflective) |
| 7 |
38 |
∞ |
-0.200 |
1.5834 |
| 8 |
38 |
∞ |
0.200 |
1.5834 |
| 9 |
air |
∞ |
5.008 |
1.0 |
| 10 |
25 |
-39.862 |
1.800 |
1.5168 |
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