U.S. patent application number 10/827043 was filed with the patent office on 2005-03-17 for autostereoscopic optical apparatus.
This patent application is currently assigned to Eastman Kodak Company. Invention is credited to Cobb, Joshua M., Kessler, David, Roddy, James E..
Application Number | 20050057788 10/827043 |
Document ID | / |
Family ID | 34426363 |
Filed Date | 2005-03-17 |
United States Patent
Application |
20050057788 |
Kind Code |
A1 |
Cobb, Joshua M. ; et
al. |
March 17, 2005 |
AUTOSTEREOSCOPIC OPTICAL APPARATUS
Abstract
An autostereoscopic optical apparatus (10) for viewing a
stereoscopic virtual image comprises a left image to be viewed by
an observer (12) at a left viewing pupil (14l) and a right image to
be viewed by the observer at a right viewing pupil (14r). The
apparatus comprises a left pupil imaging system for forming the
left image. A right pupil imaging system forms the right image.
Inventors: |
Cobb, Joshua M.; (Victor,
NY) ; Kessler, David; (Rochester, NY) ; Roddy,
James E.; (Rochester, NY) |
Correspondence
Address: |
Mark G. Bocchetti
Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Assignee: |
Eastman Kodak Company
|
Family ID: |
34426363 |
Appl. No.: |
10/827043 |
Filed: |
April 19, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10827043 |
Apr 19, 2004 |
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10662208 |
Sep 12, 2003 |
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6834961 |
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Current U.S.
Class: |
359/7 ;
348/E13.027; 348/E13.058 |
Current CPC
Class: |
G02B 17/08 20130101;
H04N 13/363 20180501; H04N 13/302 20180501; G02B 30/26 20200101;
G02B 17/0804 20130101 |
Class at
Publication: |
359/007 |
International
Class: |
G02B 027/14 |
Claims
1. (cancelled)
2. (cancelled)
3. An autostereoscopic optical apparatus for viewing a stereoscopic
virtual image comprising a left image to be viewed by an observer
at a left viewing pupil and a right image to be viewed by the
observer at a right viewing pupil, the apparatus comprising: (a) a
left pupil imaging system for forming the left image, comprising:
(i) a left curved mirror having a left mirror center of curvature
and a left focal surface; (ii) a left image generation system for
projecting a left curved image toward the left focal surface;
wherein the exit pupil of the left image generation system and the
left curved image center of curvature are substantially coincident
and lie substantially at the left mirror center of curvature; (b) a
right pupil imagining system for forming the right image,
comprising: (i) a right curved mirror having a right mirror center
of curvature and a right focal surface; (ii) a right image
generation system for projecting a right curved image toward the
right focal surface; wherein the exit pupil of the right image
generation system and the right curved image center of curvature
are substantially coincident and lie substantially at the right
mirror center of curvature: and (c) a beamsplitter disposed to fold
both left and right image paths to form: (i) at the left viewing
pupil, a real image of the left image generation system pupil and a
virtual image of the left curved image; (ii) at the right viewing
pupil, a real image of the right image generation system pupil and
a virtual image of the right curved image; wherein said left image
generation system comprises an image source selected from a group
consisting of a CRT, an emissive array an LCD display an OLED; and
further comprising a focusing optical element adjacent to said
image source for directing light toward said left image generation
system entrance pupil.
4. An autostereoscopic optical apparatus according to claim 3
wherein said focusing optical element is taken from the group
consisting of a Fresnel lens, a holographic optical element, a
diffraction optical element, and a lens.
5. An autostereoscopic optical apparatus for viewing a stereoscopic
virtual image comprising a left image to be viewed by an observer
at a left viewing pupil and a right image to be viewed by the
observer at a right viewing pupil, the apparatus comprising: (a) a
left pupil imaging system for forming the left image, comprising:
(i) a left curved mirror having a left mirror center of curvature
and a left focal surface; (ii) a left image generation system for
projecting a left curved image toward the left focal surface;
wherein the exit pupil of the left image generation system and the
left curved image center of curvature are substantially coincident
and lie substantially at the left mirror center of curvature; (b) a
right pupil imaging system for forming the right image, comprising:
(i) a right curved mirror having a right mirror center of curvature
and a right focal surface; (ii) a right image generation system for
projecting a right curved image toward the right focal surface,
wherein the exit pupil of the right image generation system and the
right curved image center of curvature are substantially coincident
and lie substantially at the right mirror center of curvature; and
(c) a beamsplitter disposed to fold both left and right image paths
to form: (i) at the left viewing pupil, a real image of the left
image generation system pupil and a virtual image of the left
curved image; (ii) at the right viewing pupil, a real image of the
right image generation system pupil and a virtual image of the
right curved image; and wherein said viewing pupils range in size
between 5 mm and 60 mm.
6. An autostereoscopic optical apparatus for viewing a stereoscopic
virtual image comprising a left image to be viewed by an observer
at a left viewing pupil and a right image to be viewed by the
observer at a right viewing pupil, the apparatus comprising: (a) a
left pupil imaging system for forming the left image, comprising:
(i) a left curved mirror having a left mirror center of curvature
and a left focal surface; (ii) a left image generation system for
projecting a left curved image toward the left focal surface;
wherein the exit pupil of the left image generation system and the
left curved image center of curvature are substantially coincident
and lie substantially at the left mirror center of curvature; (b) a
right pupil imaging system for forming the right image, comprising:
(i) a right curved mirror having a right mirror center of curvature
and a right focal surface; (ii) a right image generation system for
projecting a right curved image toward the right focal surface;
wherein the exit pupil of the right image generation system and the
right curved image center of curvature are substantially coincident
and lie substantially at the right mirror center of curvature; and
(c) a beamsplitter disposed to fold both left and right image paths
to form: (i) at the left viewing pupil, a real image of the left
image generation system pupil and a virtual image of the left
curved image; (ii) at the right viewing pupil, a real image of the
right image generation system pupil and a virtual image of the
right curved image; and wherein said viewing pupils are separated
by an interocular distance ranging between 55 mm and 75 mm.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation-in-part of application Ser. No.
10/662,208, filed Sep. 12, 2003, entitled AUTOSTEREOSCOPIC OPTICAL
APPARATUS, by Cobb et al.
FIELD OF THE INVENTION
[0002] This invention generally relates to display apparatus and
more particularly relates to an autostereoscopic display apparatus
providing a wide field of view, large viewing pupils, and high
brightness.
BACKGROUND OF THE INVENTION
[0003] The potential value of autostereoscopic display systems is
well appreciated for a broad range of data visualization uses and
for a wide range of applications that include entertainment,
engineering, medical, government, security, and simulation fields.
Autostereoscopic display systems include "immersion" systems,
intended to provide a realistic viewing experience for an observer
by visually surrounding the observer with a three-dimensional (3-D)
image having a very wide field of view. As differentiated from the
larger group of stereoscopic displays that include it, the
autostereoscopic display is characterized by the absence of any
requirement for a wearable item of any type, such as goggles,
headgear, or special polarized or filter glasses, for example. That
is, an autostereoscopic display attempts to provide "natural"
viewing conditions for an observer.
[0004] An article entitled "3-D displays: A review of current
technologies" by Siegmund Pastoor and Mathias Wopking in Displays
17 (1997) surveys various approaches that have been applied for
obtaining autostereoscopic display images for one or more viewers.
Among the many techniques described in the Pastoor et al. article
are electro-holography, volumetric display, direction-multiplexed,
diffraction-based, refraction-based, and reflection-based methods
for autostereoscopic presentation. While each of these approaches
may have merit in one or more specific applications, these
approaches have a number of characteristic shortcomings that
constrain usability and overall performance. As a group, these
conventional approaches have been adapted for autostereoscopic
displays, but allow only a narrow field of view and provide limited
brightness and poor contrast. Imaging systems employing time-based
or spatial multiplexing require complex image processing algorithms
in order to provide left- and right-eye images in the proper
sequence or with the necessary spatial separation. Time-based
multiplexing introduces the inherent problem of image flicker.
Spatial multiplexing generally produces an image having reduced
resolution. Combining these multiplexing techniques, as is
disclosed in European Patent Application EP 0 764 869 A2 to Ezra et
al., may provide an increased number of views, but does not
compensate for these inherent drawbacks. A number of multiplexing
technologies also require tracking of view eye position and
compensation for changes in head position. As a further
disadvantage, each of the imaging technologies described in the
Pastoor et al. article present the viewer with a real image, rather
than with a virtual image.
[0005] In an article entitled "An Autostereoscopic Display
Providing Comfortable Viewing Conditions and a High Degree of
Telepresence" by Klaus Hopf in IEEE Transactions on Circuits and
Systems for Video Technology, Vol. 10, No. 3, April, 2000, a
teleconferencing system employing a spherical mirror is disclosed,
recommended particularly for its value in reducing chromatic
aberration. However, the optical system disclosed in this article
is subject to field curvature constraints, limiting its field of
view. Notably, the system described in the Hopf article provides a
virtual image; however, due to substantial field curvature, the
total field of view of such a system is limited to less than about
15 degrees. While such a narrow field of view may be acceptable for
videoconferencing applications, this level of performance would not
be useful for a desktop display system.
[0006] Virtual imaging provides an advantageous alternative to real
image projection, as is used in the apparatus described in the
Pastoor article and in EP 0 764 869 A2. In contrast to conventional
projection methods for forming a real image, a virtual image is not
formed on a display surface. That is, if a display surface were
positioned at the perceived location of a virtual image, no image
would be formed on that surface. Virtual image display has a number
of inherent advantages, as is outlined in U.S. Pat. No. 5,625,372
(Hildebrand et al.) As one significant advantage for stereoscopic
viewing, the size of a virtual image is not limited by the size or
location of a display surface. Additionally, the source object for
a virtual image may be small; a magnifying glass, as a simple
example, provides a virtual image of its object. Thus, it can be
seen that, in comparison with prior art systems that project a real
image, a more realistic viewing experience can be provided by
forming a virtual image that is disposed to appear some distance
away. Providing a virtual image also obviates any need to
compensate for screen artifacts, as may be necessary when
projecting a real image.
[0007] It is generally recognized that, in order to minimize
vergence/accommodation effects, a 3-D viewing system should display
its pair of stereoscopic images, whether real or virtual, at a
relatively large distance from the observer. For real images, this
means that a large display screen must be employed, preferably
placed a good distance from the observer. For virtual images,
however, a relatively small curved mirror can be used as is
disclosed in U.S. Pat. No. 5,908,300 (Walker et al.). The curved
mirror acts as a collimator, forming a virtual image at a
relatively large distance from the observer.
[0008] From an optical perspective, it can be seen that there would
be advantages to autostereoscopic design using pupil imaging. A
system designed for pupil imaging must meet a fairly demanding set
of requirements, including the following:
[0009] (a) form separate images at left and right pupils
correspondingly;
[0010] (b) provide the most natural viewing conditions, eliminating
any need for goggles or special headgear;
[0011] (c) present the largest possible pupils to the observer,
while limiting crosstalk between left and right views;
[0012] (d) allow reasonable freedom of movement;
[0013] (e) provide an ultra-wide field of view; and
[0014] (f) provide sufficient resolution for realistic imaging,
with high brightness and contrast.
[0015] It is recognized in the optical arts that each of these
requirements, by itself, can be difficult to achieve. An ideal
autostereoscopic imaging system must meet the challenge of each of
these requirements to provide a more fully satisfactory and
realistic viewing experience. Moreover, additional physical
constraints presented by the need for a system to have small
footprint, and dimensional constraints for interocular separation
must be considered, so that separate images directed to each eye
can be advantageously spaced and correctly separated for viewing.
It is instructive to note that interocular distance constraints
limit the ability to achieve larger pupil diameter at a given
ultrawide field by simply scaling the projection lens.
[0016] Clearly, the value and realistic quality of the viewing
experience provided by an autostereoscopic display system using
pupil imaging is enhanced by presenting the stereo 3-D image with a
wide field of view and large exit pupil. For fully satisfactory 3-D
viewing, such a system should provide separate, high-resolution
images to right and left eyes. To create a realistic illusion of
depth and width of field, the observer should be presented with a
virtual image that requires the viewer to focus at some
distance.
[0017] It is well known that conflict between depth cues associated
with vergence and accommodation can adversely impact the viewing
experience. Vergence refers to the degree at which the observer's
eyes must be crossed in order to fuse the separate images of an
object within the field of view. Vergence decreases, then vanishes
as viewed objects become more distant. Accommodation refers to the
requirement that the eye lens of the observer change shape to
maintain retinal focus for the object of interest. It is known that
there can be a temporary degradation of a viewer's depth perception
when the viewer is exposed for a period of time to mismatched depth
cues for vergence and accommodation. It is also known that this
negative effect on depth perception can be mitigated when the
accommodation cues correspond to distant image position.
[0018] There are also other basic optical limitations for immersion
systems that must be addressed with any type of optical projection
that provides a wide field of view. An important limitation is
imposed by the Lagrange invariant. A product of the size of the
emissive device and the numerical aperture, the Lagrange invariant
determines output brightness and is an important consideration for
matching the output of one optical system with the input of
another. Any imaging system conforms to the Lagrange invariant,
whereby the product of pupil size and semi-field angle is equal to
the product of the image size and the numerical aperture. An
invariant that applies throughout the optical system, the Lagrange
invariant can be a limitation when using, as an image generator, a
relatively small spatial light modulator or similar pixel array
which operate over a relatively small numerical aperture, since the
Lagrange value associated with the device is small. In practical
terms, the larger the size of the image source, the easier it is to
form an image having a wide field of view and large pupil.
[0019] In response to the need for more realistic autostereoscopic
display solutions offering a wide field of view, commonly assigned
U.S. Pat. No. 6,416,181 (Kessler et al.), incorporated herein by
reference and referred to as the '181 patent, discloses an
autostereoscopic imaging system using pupil imaging to display
collimated left and right virtual images to a viewer. In the '181
disclosure, a curved mirror is employed in combination with an
imaging source, a curved diffusive surface, a ball lens assembly,
and a beamsplitter, for providing the virtual image for left and
right viewing pupils. Overall, the monocentric optical apparatus of
the '181 disclosure provides autostereoscopic imaging with large
viewing pupils, a very wide field of view, and minimal
aberration.
[0020] While the autostereoscopic system of the '181 disclosure
provides a high-performance immersive display, there is still room
for improved embodiments. For example, while the '181 system
provides a large viewing pupil, there would be advantages in even
further increases in pupil size. At the same time, however, some
amount of correction may be needed, since eye movement within a
larger viewing pupil can cause some amount of "swim" effect, in
which pixels appear to shift position as the eye moves within the
viewing pupil. In addition, as is well known in the imaging arts,
some amount of spherical aberration is generally inherent in any
optical system that employs a curved mirror for image
collimation.
[0021] Generating its source image on a small spatial light
modulator device, the '181 system overcomes inherent Lagrange
invariant conditions by forming an intermediate curved image for
projection on a curved diffusive surface. Use of the diffuser with
the '181 apparatus is necessary because the image-forming device,
typically a reflective LCD or other spatial light modulator, is a
relatively small emissive device, measuring typically no more than
about 1 inch square. At the same time, however, the use of a
diffusive surface effectively reduces overall brightness,
introduces some level of graininess to the image, and limits the
achievable contrast.
[0022] There are other minor drawbacks to autostereoscopic displays
that use the design approach of the '181 disclosure. For example,
slight "keystoning" aberrations are detectable in a system using
the '181 design approach, due to the use of a single curved mirror;
moreover, this effect can be compounded by right and left images
exhibiting keystoning in opposite orientations with respect to the
final image. While spherical lenses such as the ball lenses of the
'181 disclosure have overall advantages for maximizing field of
view and for minimizing some types of imaging aberration, there are
some inherent disadvantages to the use of highly spherical optics,
requiring compensation for chromatic effects for example. Curved
images can be produced in a number of ways using more conventional
optics, which, while not providing some of the advantages of ball
lenses, might provide less expensive options for forming
intermediate images in an autostereoscopic system.
[0023] Thus, it can be seen that there is a need for an improved
autostereoscopic imaging apparatus that provides improved
brightness, enhanced viewing pupil dimensions, reduced image
aberrations, and higher resolution.
SUMMARY OF THE INVENTION
[0024] It is an object of the present invention to provide an
autostereoscopic display device having improved viewing pupil size,
brightness, and resolution, with reduced optical aberrations. With
this object in mind, the present invention provides an
autostereoscopic optical apparatus for viewing a stereoscopic
virtual image comprising a left image to be viewed by an observer
at a left viewing pupil and a right image to be viewed by the
observer at a right viewing pupil, the apparatus comprising:
[0025] It is a feature of the present invention that it provides a
completely specular autostereoscopic imaging display apparatus,
without the need for curved diffusive surfaces. This allows image
brightness to be optimized and allows improved contrast over
earlier design solutions.
[0026] It is an advantage of the present invention that it uses a
larger imaging display than previous solutions, relaxing Lagrange
invariant constraints on available luminance.
[0027] It is a further advantage of the present invention that it
provides an improved viewing pupil size when compared with earlier
solutions.
[0028] It is a further advantage of the present invention that it
provides a compact autostereoscopic display system providing a
virtual image.
[0029] These and other objects, features, and advantages of the
present invention will become apparent to those skilled in the art
upon a reading of the following detailed description when taken in
conjunction with the drawings wherein there is shown and described
an illustrative embodiment of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] While the specification concludes with claims particularly
pointing out and distinctly claiming the subject matter of the
present invention, it is believed that the invention will be better
understood from the following description when taken in conjunction
with the accompanying drawings, wherein:
[0031] FIG. 1 shows a prior art autostereoscopic display system
using pupil imaging to provide a virtual image;
[0032] FIG. 2 shows a detailed view of image generation system
components for the prior art system of FIG. 1;
[0033] FIG. 3 is a schematic diagram showing how an intermediate
image is formed in the apparatus of the present invention;
[0034] FIG. 4a is a schematic diagram showing optical behavior of a
ball lens;
[0035] FIG. 4b is a schematic diagram showing optical behavior of
an hemispheric lens with a reflective surface;
[0036] FIG. 5 is a schematic diagram showing an apparatus for
forming a curved intermediate image according to the present
invention;
[0037] FIG. 6 is a schematic diagram showing an alternate
embodiment of the present invention for forming a curved image,
using a field lens;
[0038] FIG. 7a is a schematic diagram showing an alternate
embodiment of the present invention for forming a curved
intermediate image using a focusing optical element, using a
conventional thin beamsplitter;
[0039] FIG. 7b is a schematic diagram showing an alternate
embodiment of the present invention for forming a curved
intermediate image using a focusing optical element and employing a
wedge beamsplitter;
[0040] FIG. 8 is a schematic diagram showing the overall function
of the focusing optical element of FIGS. 7a and 7b in the imaging
path;
[0041] FIG. 9 is a perspective view showing an arrangement of
components for pupil imaging in an autostereoscopic display system
of the present invention;
[0042] FIG. 10 shows corrected aberration curves for the ball lens
assembly of the apparatus of the present invention;
[0043] FIG. 11 shows uncorrected aberration curves for the curved
image generating optical system of the apparatus of the present
invention;
[0044] FIG. 12 shows corrected aberration curves for the curved
image generating optical system of the apparatus of the present
invention;
[0045] FIG. 13 shows a simplified image path using an aspheric
corrector element;
[0046] FIG. 14 shows corrected aberration curves using an aspheric
corrector element to obtain a larger viewing pupil;
[0047] FIG. 15 shows the sag profile for an aspheric surface
providing the improvement depicted in FIG. 14;
[0048] FIG. 16 is a perspective view showing an arrangement of
components for pupil imaging in an alternate embodiment of the
present invention, using separate mirrors for left and right
pupils; and
[0049] FIG. 17 is a perspective view showing another alternate 30
embodiment of the present invention using alternate imaging
components to provide intermediate images and using separate
mirrors for left and right pupils.
DETAILED DESCRIPTION OF THE INVENTION
[0050] The present description is directed in particular to
elements forming part of, or cooperating more directly with,
apparatus in accordance with the invention. It is to be understood
that elements not specifically shown or described may take various
forms well known to those skilled in the art.
[0051] For the purposes of the present application, a curved image
is an image for which best focus lies in a shape that is
substantially spherical. The optical path is simplest when curved
images are themselves spherically curved. By forming and using
curved intermediate images, for example, rather than flat, planar
images, the optics of the present invention take advantage of
various symmetrical arrangements and relationships that are
favorable for pupil imaging using virtual images, as is described
in this section. Curved intermediate images can be formed using
fully spherical lenses or using highly spherical lens segments as
well as using more conventional image projection optics.
[0052] Similarly, for reasons that become apparent upon reading
this detailed description, a curved mirror, as described in this
application, is preferably spherical, having a single center of
curvature.
[0053] In the prior art autostereoscopic projection apparatus 10
described in U.S. Pat. No. 6,416,181 and as shown in FIG. 1, a
curved mirror 24 is employed, in combination with a beamsplitter 16
to provide an autostereoscopic virtual image to a viewer 12 at left
and right viewing pupils 14l and 14r. For both left and right
viewing pupils 14l and 14r, a corresponding image generation system
70l and 70r provides an initial intermediate curved image that is
then projected through a corresponding left or right ball lens
assembly 30l, 30r in order to form an intermediate curved image 80
at a focal surface of curved mirror 24. As FIG. 1 shows, the left
and right optical paths cross between beamsplitter 16 and curved
mirror 24, due to the nature of imaging using curved mirror 24.
[0054] Referring to FIG. 2, there is shown, extracted from the more
detailed prior art description of the '181 disclosure noted in the
background section above, a portion of an image generation system
70 for providing intermediate curved image 80 for projection in a
stereoscopic projection system 82 for one viewing pupil 14. Here,
an image generator 74 provides a source image from a flat surface,
such as from a reflective LCD. A relay lens 54 directs light from
image generator 74 onto a diffusing element 32, so that a curved
intermediate image 76 is formed on a diffusive surface 40. Ball
lens assembly 30, cooperating with beamsplitter 16, projects curved
intermediate image 76 toward a front focal surface 22 of curved
mirror 24 to form intermediate curved image 80. Curved mirror 24
then provides a virtual image of intermediate curved image 80 for a
viewing pupil 14.
[0055] Forming a Curved Intermediate Image
[0056] To eliminate the need for diffusing element 32 as was
required using the approach of the '181 disclosure, the present
invention provides an alternate method for forming a curved
intermediate image using a specular optical system. Referring to
FIG. 3, an intermediate image 90 is formed by a curved mirror 92,
which is generally spherical according to the present invention.
Image-bearing light from image source 94 is directed toward curved
mirror 92 through an aperture stop location 96, which defines the
angle of light from image source 94 available for forming
intermediate image 90. Aperture stop location 96 is at the center
of curvature C.sub.s of curved mirror 92. With aperture stop
location 96 centered at mirror center of curvature C.sub.s, the
central rays that pass through C.sub.s are reflected back toward
this same point. As the traced light rays of FIG. 3 show, curved
intermediate image 90 is typically formed between mirror center of
curvature C.sub.s and the mirror focal point f. Curved intermediate
image 90 has an image center of curvature C.sub.i that is in a
different location from mirror center of curvature C.sub.s. As the
distance D between image source 94 and curved mirror 92 increases,
the respective centers of curvature C.sub.i and C.sub.s move toward
each other, coinciding as distance D approaches infinity. At this
idealized infinite distance D between image source 94 and curved
mirror 92, curved intermediate image 90 would lie on focal point f
and the center of curvature of the image C.sub.i would coincide
with the center of curvature C.sub.s of curved mirror 24 (FIG.
1).
[0057] Using the overall arrangement of FIG. 3, image source 94 can
be any of a number of image sources that emit light, such as a
display LCD, a CRT, or an OLED or PLED device, for example. Two
characteristics of image source 94 are particularly significant
with this arrangement:
[0058] (i) The image formed on image source 94 is substantially
flat. There may be some slight curvature to this image, such as
would be provided by a CRT; however, the arrangement of FIG. 3
works well when image source 94 is flat and shows how intermediate
image 90 is formed having a curvature using the methods of the
present invention. Since most image display devices form a flat
image, there is, then, no need for modification to off-the-shelf
display components with this arrangement.
[0059] (ii) Image source 94 can be several inches in diameter, and
may be well over one foot in diameter. In a preferred embodiment,
image source 94 is a large LCD display, such as a 17-inch display,
for example. This is unlike other apparatus for forming a curved
intermediate image, such as was shown with reference to FIG. 2, for
which a microdisplay, such as a liquid crystal on silicon (LCOS) or
DMD component, is used. Use of a larger display device for image
source 94 has particular advantages for increasing both image
resolution and brightness.
[0060] As described with reference to the Lagrange invariant in the
background section above, brightness in an optical system is a
product of the emissive area and the solid angle. By allowing image
source 94 to have a large emissive area, the method of the present
invention allows substantial brightness levels while, at the same
time, allowing light angles to be relatively small. Small light
angles are advantageous for maximizing image contrast and
minimizing color shifting and other related image aberrations.
[0061] Separate image sources 94 are used for left and right eyes,
respectively. Ideally, image source 94 for left image generation
system 70l and image source 94 for right image generation system
70r are well-matched for image size and color. CRT displays,
however, may be at a disadvantage if used as image sources 94.
Color differences between CRTs may degrade stereoscopic imaging
performance. Additionally, as a result of display ageing, CRT image
areas may vary dimensionally, effectively causing left/right pixel
misalignment. In contrast to CRT displays, LCD displays offer
dimensional stability with stable pixel locations, ease of
alignment, and simpler mounting.
[0062] Ideal Ball Lens Operation
[0063] Referring to FIG. 4a, there is shown the concentric
arrangement and optical characteristics of ball lens assembly 30
for directing light from a curved image 50. A central spherical
lens 46 is disposed between meniscus lenses 42 and 44. Spherical
lens 46 and meniscus lenses 42 and 44 have indices of refraction
and dispersion characteristics intended to minimize on-axis
spherical and chromatic aberration, as is well known in the optical
design arts. An aperture stop 48 defines a pupil 106 within ball
lens assembly 30. Aperture stop 48 need not be a physical stop, but
may alternately be implemented using optical effects such as total
internal reflection. In terms of the optics path, aperture stop 48
serves to define an entrance pupil and an exit pupil for ball lens
assembly 30.
[0064] In a preferred embodiment, meniscus lenses 42 and 44 are
selected to reduce image aberration and to optimize image quality
for the projected image projected. It must be noted that ball lens
assembly 30 could comprise any number of arrangements of support
lenses surrounding central spherical lens 46. Surfaces of these
support lenses, however many are employed, would share a common
center of curvature with C.sub.ball, the center of curvature of
central spherical lens 46. Moreover, the refractive materials used
for lens components of ball lens assembly 30 could be varied,
within the scope of the present invention. For example, in addition
to standard glass lenses, central spherical lens 46 could comprise
a plastic, an oil or other liquid substance, or any other
refractive material chosen for the requirements of the application.
Meniscus lenses 42 and 44, and any other additional support lenses
in ball lens assembly 30, could be made of glass, plastic, enclosed
liquids, or other suitable refractive materials, all within the
scope of the present invention. In its simplest embodiment, ball
lens assembly 30 could simply comprise a single spherical lens 46,
without additional supporting refractive components.
[0065] In ideal operation, curved image 50 shares the same center
of curvature C.sub.ball as ball lens assembly 30. When arranged in
this fashion, light from curved image 50 is imaged with low levels
of aberration, as is represented in the light rays of FIG. 4a.
[0066] The inherent advantages of a ball lens can be exploited
using a modified design, such as using a hemisphere combined with a
folding mirror, as is shown in the cross-sectional ray diagram of
FIG. 4b. In FIG. 4b, a hemispheric lens assembly 60 comprises at
least a hemispheric central lens 66 and a reflective surface 62
along the meridional plane of the hemisphere. Optionally, one or
more meniscus lenses 42 could also be part of hemispheric lens
assembly 60. Reflective surface 62 may be formed over the full
surface of the meridional plane or may be formed only along a
portion of this surface. As shown in FIG. 4b, hemispheric lens
assembly 60 forms, from curved image 50 as its object, a curved
image 64, folding the optical path at the same time. This
arrangement can have advantages, for example, where space for
optical components is at a premium.
[0067] For the purposes of this disclosure, the term "ball lens
segment" comprises both fully spherical ball lens assembly 30, as
shown in FIG. 4a and hemispheric lens assembly 60 as shown in FIG.
4b.
[0068] First Embodiment of Image Generation System
[0069] Referring to FIG. 5, there is shown a first embodiment of an
image generation system 100 for forming a curved image 110 for
projection according to the present invention, as disclosed in the
commonly-assigned copending U.S. patent application Ser. No.
10/393,236. Curved image 110 serves the function of intermediate
curved image 80 shown in FIGS. 1 and 2. As described with reference
to FIG. 3, image source 94 provides image-bearing light to curved
mirror 92 through aperture stop location 96. Referring now to FIG.
5, a beamsplitter 102 is used to direct an intermediate image 90'
so that it is concentric to ball lens assembly 30, which could
alternately be embodied as hemispheric lens assembly 60, as was
shown in FIG. 4b. Because the light is being directed by curved
mirror 92 toward its center of curvature C.sub.s, rather than
towards the center of curvature C.sub.ball of ball lens assembly
30, some portion of the light does not enter the pupil 106 of ball
lens assembly 30, thus causing vignetting. Overfilling pupil 106 of
ball lens assembly 30 compensates for vignetting. Ball lens
assembly 30 re-images intermediate curved image 90' to form curved
image 110. Beamsplitter 102 is disposed between the vertex V of
curved mirror 92 and its center of curvature C.sub.s as shown in
FIG. 5.
[0070] It must be emphasized that curved mirror 92 serves as an
image generation component that serves image generation system 100
for forming intermediate curved image 110, as shown in FIGS. 3 and
5. This is to be distinguished from ball lens imaging curved mirror
24 (used later in the optical path, as shown in FIG. 1 and in
subsequent figures) which, in conjunction with beamsplitter 16,
provides pupil imaging and forms the final virtual image observed
by viewer 12. Similarly, beamsplitter 102 serves image generation
system 100 for forming curved image 110 and is to be distinguished
from beamsplitter 16 (shown in FIG. I and in subsequent figures)
which cooperates with curved mirror 24 to provide pupil imaging and
form the virtual image.
[0071] Second Embodiment of Image Generation System
[0072] Referring to FIG. 6, there is shown an improved embodiment
of image generation system 100 in which a field lens 112 is
positioned along the output axis where intermediate image 90 is
formed. By positioning field lens 112 at this location,
intermediate image 90 is not substantially changed; however, light
from intermediate image 90 is directed toward center of curvature
C.sub.ball of ball lens assembly 30. Once again, it is significant
to observe that ball lens assembly 30 shares the same center of
curvature C.sub.ball as intermediate image 90, but that this is not
identical to the center of curvature C.sub.s of curved mirror 92 or
to the imaged center of curvature C.sub.s', towards which light
from curved mirror 92 is directed. The function of field lens 112
is, then, to image C.sub.s onto C.sub.ball without substantially
affecting the image quality of intermediate image 90. By doing
this, field lens 112 essentially redirects light in order to fill
pupil 106 of ball lens assembly 30 without vignetting.
[0073] Common to telescopic, microscopic, and similar "tube"
optical systems, field lenses are widely employed in the optical
arts, placed at the image location of a first lens, where the image
formed at that image location becomes the object of a second lens.
In this way, field lens 112 improves the overall brightness and
field of view of the optical system. Background information on
field lens use and theory can be found, for example, in Modern
Optical Engineering, the Design of Optical Systems, by Warren J.
Smith, McGraw-Hill, N.Y., pp. 212-213 and in other textbooks known
in the optics field.
[0074] In one embodiment, surface SI of field lens 112 is
concentric with mirror center of curvature C.sub.s and therefore
does not deviate chief rays towards C.sub.ball. In such an
embodiment, surface S2, not concentric with mirror center of
curvature C.sub.s, operates to bend chief rays toward C.sub.ball.
Alternately, surface S2 could be concentric with mirror center of
curvature C.sub.s, surface S1 performing the operation of bending
chief rays toward C.sub.ball. Embodiments with either surface S1 or
S2 concentric with C.sub.s or C.sub.ball represent the most
straightforward approaches to the design of field lens 112; other
designs could have neither surface S1 nor S2 concentric with mirror
center of curvature C.sub.s or C.sub.ball, however, these designs
could be more complex.
[0075] As was noted above with reference to FIG. 4b, the use of
hemispheric ball lens assembly 60 may have advantages for
simplifying the optical path. Referring to FIG. 7a, there is shown
an alternative arrangement to that of FIG. 6, using hemispheric
ball lens assembly 60 with field lens 112. In the arrangement of
FIG. 7a, field lens 112 again operates to image C.sub.s onto
C.sub.ball, where C.sub.ball is optically the center of curvature
of hemispheric ball lens assembly 60.
[0076] Providing Advantages of Telecentric Light
[0077] Still referring to FIG. 7a, an optional focusing optical
element 98 is employed for providing improved, uniform brightness
across the field. Disposed against the surface of image source 94,
or very near this surface, focusing optical element 98 acts as a
type of field lens for directing light emitted from image source
94. Referring to FIG. 8, there is shown, in schematic form, the
function of focusing optical element 98, focusing the emitted light
from image source 94 to its focal point foe, coincident with mirror
center of curvature C.sub.s, which is the entrance pupil of the
image generation system. By doing this, focusing optical element 98
forces telecentricity for light emitted from image source 94,
thereby optimizing the brightness and contrast of the image
provided to the optical system through aperture stop location 96.
As a result, curved image 110 has optimum brightness across the
field. In one embodiment, focusing optical element 98 is a Fresnel
lens. Among other devices that could be employed as focusing
optical element 98 are holographic optical elements, diffraction
optical elements, two-cylinder Fresnel lenses, or even a more
conventional curved surface lens, for example.
[0078] Considerations for Beamsplitter 102
[0079] As is shown in FIGS. 5 and 6, beamsplitter 102 must accept
incident light over a range of angles, so that where beamsplitter
102 is made of glass, rays at extreme sides of the field
effectively encounter different thicknesses of glass. For this
reason, it can be seen that there are advantages to providing
beamsplitter 102 having minimal thickness of glass or plastic.
Thus, beamsplitter 102 may be a thin glass or thin plastic type or
a pellicle type beamsplitting device.
[0080] Referring to FIG. 7b, a wedge beamsplitter 104 can be used
as an alternative. Wedge beamsplitter 104 has substrate thickness
varied so that the cross-sectional profile of wedge beamsplitter
104 is wedge-shaped. The difference in substrate thickness of wedge
beamsplitter 104 provides an alternative solution that helps to
compensate for optical path length and angle differences across its
surface. The use of wedge beamsplitter 104 within image generation
system 100 has advantages, since its thicker substrate is
mechanically less fragile than a pellicle or thin beamsplitter
102.
[0081] Embodiment for Stereoscopic Viewing
[0082] Referring to FIG. 9, there is shown a perspective view of a
left and a right eye image generation system 100l and 100r within
autostereoscopic display apparatus 10. The task of generating a
curved image to be displayed to each eye uses the basic components
of FIGS. 6, 7a and 7b. Left and right image sources 94l and 94r
provide, through respective left and right aperture stop locations
96l and 96r and reflected from left and right beamsplitters 102l
and 102r, light from images to their respective left and right
curved mirrors 92l and 92r. Respective left and right curved
intermediate images, through respective left and right
beamsplitters 102l and 102r, are formed near their respective left
and right field lenses 112l and 112r, which redirect light to their
respective left and right ball lens segments 130l and 130r. Left
and right ball lens segments 130l and 130r are basically
hemispheric in the configuration of FIG. 9, similar to hemispheric
lens assembly 60 as shown in FIG. 4b. Left and right ball lens
segments 130l and 130r are provided with left and right reflective
surfaces 132l and 132r and perform the dual function of folding the
light path and projecting the intermediate images towards curved
mirror 24. With the arrangement of FIG. 9, each image generation
system 100l and 100r can produce the appropriate image intended for
left and right viewing pupils 14l and 14r, the viewing pupils
typically range in size between 5 mm and 60 mm. The left and right
viewing pupils 14l and 14r are separated by the interocular
distance, which typically ranges between 55 mm and 75 mm. Curved
images generated by left and a right eye image generation systems
100l and 100r form left and right intermediate curved images 110l
and 110r for collimation using large curved mirror 24 to provide,
by cooperation with beamsplitter 16, virtual images at left and
right viewing pupils 14l and 14r, in the same manner as is
disclosed in U.S. Pat. No. 6,416,181. Referring back to FIG. 1,
curved image 110 of FIG. 7a or 7b corresponds to intermediate
curved image 80 in FIG. 1, for example.
[0083] Alternate Embodiment for Stereoscopic Viewing, Using Left
and Right Mirrors
[0084] The embodiment of FIG. 9 showed the use of a single curved
mirror 24 with beamsplitter 16 for forming both left and right
virtual images. In the alternate embodiment of FIG. 16, there is
shown how autostereoscopic display apparatus 10, using the same
left and a right eye image generation systems 100l and 100r
described with respect to FIG. 9, can employ separate left and a
right curved mirrors, 24l and 24r, one for each corresponding
viewing pupil 14l and 14r. The alternate arrangement of FIG. 16 is
advantaged over the arrangement of FIG. 9 that uses a single curved
mirror 24 because each left and right ball lens segment 130l and
130r can be positioned on-axis with respect to its corresponding
curved mirror 24l, 24r. In the arrangement of FIG. 9, on the other
hand, left and right ball lens segments 130l and 130r are offset
slightly to either side of the optical axis of single curved mirror
24. This causes a slight keystoning aberration in each optical
path. Disadvantageously, keystoning effects for left and right
images are in opposite directions, degrading the quality of the
displayed image at extreme edges of the field. To some extent, this
effect can be corrected or mitigated electronically, by
pre-distorting the image data.
[0085] Designs using left and right curved mirrors 24l and 24r,
such as shown in FIG. 16 allow on-axis imaging, minimizing or
eliminating keystoning effects. However, designs using multiple
mirrors can be disadvantaged due to mechanical placement
constraints; it is difficult to arrange both left and right curved
mirrors 24l and 24r and beamsplitter 16 without some obstruction
and consequent reduction of field width. Another design
consideration relates to the relative positioning of left and right
ball lens segments 130l and 130r. Using dual curved mirrors 24l and
24r, positioning constraints for both left and right ball lens
segments 130l and 130r can be relaxed somewhat, easing space
requirements for imaging support components and allowing the size
of ball lens segment 130 components to be relatively larger,
providing a larger viewing pupil 14. By contrast, the embodiment of
FIG. 9 requires that ball lens segments 130l and 130r be positioned
closely together, but allows a more compact design at the same
time.
[0086] The same general principles used for forming a virtual image
with the prior art configuration of FIG. 1 and the configuration of
FIG. 9 also apply to the dual mirror configuration of FIG. 16, with
necessary modifications for separate left and right image paths, as
would be appreciated by those skilled in the imaging arts. For
example, left and right intermediate curved images 110l and 110r
are formed at focal surfaces of left and right curved mirrors
respectively in order to provide virtual imaging.
[0087] Alternate Embodiment Using Broad Range of Image Generation
Systems 100
[0088] While ball lens segments 1301 and 130r shown in FIGS. 9 and
16 provide corresponding intermediate left and right curved images
110l and 110r having optimal curvature, a broader range of
image-forming optical systems could be used for this purpose. For
example, lower cost conventional projection optics could be
deployed to form intermediate left and right curved images 110l and
110r having reasonably good curvature, with some compromises in
imaging performance. Referring to FIG. 17, for example, there is
shown an arrangement of components using more conventional
projection optics for left and right image generation systems 100l
and 100r.
[0089] The ideal spatial relationships for pupil placement provided
by left and right image generation systems 100l and 100r, left and
right curved mirrors 24l and 24r, and beamsplitter 16, as
represented in FIG. 17, include the following:
[0090] (i) the center of curvature C.sub.24l of left curved mirror
24l is optically coincident with the exit pupil of left image
generation system 100l;
[0091] (ii) the center of curvature C.sub.24r of right curved
mirror 24r is optically coincident with the exit pupil of right
image generation system 100r;
[0092] (iii) the center of curvature C.sub.24l of left curved
mirror 24l and center of curvature C.sub.24r of right curved mirror
24r are separated by the ocular distance between left and right
viewer pupils 14l and 14r;
[0093] (iv) left viewer pupil 14l is optically at the center of
curvature C.sub.24l of left curved mirror 24l;
[0094] (v) right viewer pupil 14r is optically at the center of
curvature C.sub.24r of right curved mirror 24r;
[0095] (vi) left curved image 110l is optically centered at left
viewing pupil 14l and at the focal surface of left curved mirror
24l; and
[0096] (vii) right curved image 110r is optically centered at right
viewing pupil 14r and at the focal surface of right curved mirror
24r.
[0097] With this spatial arrangement of optical components, a real
image of the exit pupil of left image generation system 100l and a
virtual image of left curved image 110l are formed at left viewing
pupil 14l. Correspondingly, a real image of the exit pupil of right
image generation system 100r and a virtual image of right curved
image 110r are formed at right viewing pupil 14r.
[0098] It must be emphasized that the relationships listed in
(i)-(vii) above are ideal spatial relationships; optimal pupil
imaging is obtained when the requirements of (i)-(vii) are
satisfied. In practice, some amount of tolerance error is
acceptable, provided that viewing pupils 14l and 14r are formed at
suitable positions for the observer.
[0099] Correcting for Spherical Aberration
[0100] As was described with reference to FIG. 4a, ball lens
assembly 30 unavoidably exhibits some amount of aberration, which
is largely corrected using central lens 46 in combination with
meniscus lenses 42 and 44. Referring to FIG. 10, there is shown,
for ball lens assembly 30 of a preferred embodiment, tangential and
sagittal aberration curves 150 for a 20 mm pupil, showing a
significant amount of correction.
[0101] However, as a result of residual spherical aberration due to
higher order aberrations, the size of viewing pupils 14l, 14r is
still somewhat limited. Due to this residual aberration, movement
of the eyes of observer 12 within viewing pupils 14l, 14r can cause
some amount of image "swim".
[0102] Spherical aberration is a recognized problem in optical
systems that employ a concave mirror, such as astronomical
telescopes for example. To compensate for this type of aberration,
the Schmidt optical system, as described in Modern Optical
Engineering, the Design of Optical Systems, by Warren J. Smith
(cited above), pp. 393-394, employs an aspheric corrector plate. In
the Schmidt system, a thin, aspheric corrector plate is positioned
at the center of curvature of the curved mirror.
[0103] Comparing aberration curves 150 in FIG. 11 and FIG. 12, the
improvement using the Schmidt solution can be readily seen, for
similar imaging conditions and a 32 mm pupil. FIG. 11 corresponds
to the optical arrangement of FIG. 6, where there is no correction
for spherical aberration due to residual higher order aberrations.
FIG. 12 corresponds to the optical arrangement of FIG. 13.
[0104] FIG. 13 shows a side view of a simplified optical path for
image generation system 100 using an aspheric corrector element 140
disposed near the center of curvature of curved mirror 92. Light
from image source 94 is directed through aspheric corrector element
140, which is optically conjugate to the center of curvature of
curved mirror 92. The curved intermediate image formed by curved
mirror 92 lies near field lens 112, which directs the light to a
ball lens segment 130. Ball lens segment 130 creates curved image
110 for curved mirror 24 (not shown in FIG. 13).
[0105] Placing aspheric corrector element 140 near the center of
curvature of curved mirror 92 effectively images aspheric corrector
element 140 into the pupil of ball lens segment 130; that is,
corrector element 104 and the pupil of ball lens segment 130 are
optically conjugate. This allows aspheric corrector element 140 to
provide effective correction across the full field of view. As a
result, pupil size can be increased to 50 mm, with minimal
aberration, as shown in aberration curve 150 of FIG. 14. By way of
example, FIG. 15 shows the sag that is required of the aspheric
surface for a corrector plate as aspheric corrector element 140 in
a preferred embodiment.
[0106] In an alternate embodiment, aspheric corrector element 140
could be a compound lens that corrects chromatic as well as
spherical aberration. Such an arrangement would require more
complexity than the design of a single-component aspheric corrector
element 140, but would simplify the design requirements of ball
lens segment 130. For example, where a compound lens is used for
aspheric corrector element 140, it may be possible to use only a
single element as ball lens segment 130.
[0107] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the scope of the invention as described above, and as noted in the
appended claims, by a person of ordinary skill in the art without
departing from the scope of the invention. For example, field lens
112 can be more complex than is shown here, having different
curvature, composition, or coatings. Image source 94, a
transmissive LCD device in one embodiment, can be any of a number
of types of image source, including film, CRT, LCD, and digital
imaging devices. Image source 94 could be an emissive array, such
as an organic light emitting diode (OLED) array, for example. In
order to take advantage of the benefits of monocentric imaging,
curved mirror 92 will be substantially spherical in most
embodiments; however, some slight shape modifications might be
used, with the corresponding changes to supporting optics and to
optional aspheric corrector element 140. Either ball lens assembly
30 or hemispheric lens assembly 60 could serve as the ball lens
segment for either or both left and right image generation systems.
Separate left and right curved mirrors 24 could be used to improve
the image quality of each viewing pupil 14l, 14r, reducing
undesirable "keystoning" effects that can result from off off-axis
positioning of left and right ball lens segments 130l, 130r. Curved
mirror 24 could be fabricated as a highly reflective surface using
a number of different materials.
[0108] Thus, what is provided is an apparatus and method for
autostereoscopic image display having improved brightness, pupil
size, and resolution.
Parts list
[0109] 10 autostereoscopic display apparatus
[0110] 12 viewer
[0111] 14 viewing pupil
[0112] 14l left viewing pupil
[0113] 14r right viewing pupil
[0114] 16 beamsplitter
[0115] 22 front focal surface
[0116] 24 curved mirror
[0117] 24l curved mirror, left
[0118] 24r curved mirror, right
[0119] 30 ball lens assembly
[0120] 30l ball lens assembly, left
[0121] 30r ball lens assembly, right
[0122] 32 diffusing element
[0123] 40 diffusive surface
[0124] 42 meniscus lens
[0125] 44 meniscus lens
[0126] 46 central spherical lens
[0127] 48 aperture stop
[0128] 50 curved image
[0129] 54 relay lens
[0130] 60 hemispheric lens assembly
[0131] 62 reflective surface
[0132] 64 curved image
[0133] 66 hemispheric central lens
[0134] 70 image generation system
[0135] 70l left image generation system
[0136] 70r right image generation system
[0137] 74 image generator
[0138] 76 curved intermediate image
[0139] 80 intermediate curved image
[0140] 82 stereoscopic projection system
[0141] 90 intermediate image
[0142] 90' intermediate image
[0143] 92 curved mirror
[0144] 92l left curved mirror
[0145] 92r right curved mirror
[0146] 94 image source
[0147] 94l left image source
[0148] 94r right image source
[0149] 96 aperture stop location
[0150] 96l left aperture stop location
[0151] 96r right aperture stop location
[0152] 98 focusing optical element
[0153] 100 image generation system
[0154] 100l left image generation system
[0155] 100r right image generation system
[0156] 102 beamsplitter
[0157] 102l left beamsplitter
[0158] 102r right beamsplitter
[0159] 104 wedge beamsplitter
[0160] 106 ball lens pupil
[0161] 110 curved image
[0162] 110l left curved image
[0163] 110r right curved image
[0164] 112 field lens
[0165] 112l left field lens
[0166] 112r right field lens
[0167] 130 ball lens segment
[0168] 130l left ball lens segment
[0169] 130r right ball lens segment
[0170] 132l left reflective surface
[0171] 132r right reflective surface
[0172] 140 aspheric corrector element
[0173] 150 aberration curve
* * * * *