Optics: image projectors – Distortion compensation
Reexamination Certificate
2003-06-19
2004-11-23
Gray, David (Department: 2851)
Optics: image projectors
Distortion compensation
C353S037000, C353S034000, C353S122000, C353S007000
Reexamination Certificate
active
06820982
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to projection apparatus and more particularly relates to an apparatus and method for forming a curved intermediate image from a substantially flat image source.
BACKGROUND OF THE INVENTION
In conventional projection apparatus, an image, provided from an essentially flat image-forming surface, is projected onto an essentially flat display surface. In film-based projection, for example, light is transmitted through a flat piece of film for projection onto a flat movie screen. Digital image-forming devices, such as transmissive and reflective LCDs and digital micromirror devices (DMDs) similarly produce an image on a flat surface. This allows projection apparatus employing these devices to use output optics that are similar to the projection optics employed in film-based projectors.
A number of optical systems, however, form images using curved surfaces, particularly optical systems of the immersive type that are designed to provide a wide field of view. One example system of this type is disclosed in commonly assigned U.S. Pat. No. 6,416,181 (Kessler et al.) and U.S. Pat. No. 6,522,474 (Cobb et al.), both incorporated herein by reference, herein referred to as either the '181 or '474 patent. In an autostereoscopic imaging apparatus 10 as described in the '181 disclosure and as shown in
FIG. 1
, a curved mirror
24
is employed, in combination with a beamsplitter
16
for providing an autostereoscopic virtual image to a viewer
12
at left and right viewing pupils
14
l
and
14
r
. For each viewing pupil
14
l
and
14
r
, an image generation system
70
provides an initial intermediate curved image that is then projected through a ball lens assembly
30
in order to form a left or right intermediate curved image at a focal plane of curved mirror
24
.
The monocentric optical apparatus of the '181 disclosure provides autostereoscopic imaging with large viewing pupils, a very wide field of view, and minimal aberration. In order to provide this type of imaging and take advantage of the inherent benefits of monocentric projection, the apparatus of the '181 disclosure, given its source image formed on a flat surface, must form an intermediate image having a suitable curvature. Referring to
FIG. 2
, there is shown, extracted from the more detailed description of the '181 disclosure, a portion of an image generation system
70
for providing an intermediate curved image
80
for projection, for either eye, in autostereoscopic imaging apparatus
10
. Here, an image generator
74
provides a source image from an image source
94
, where image source
94
has 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 a curved mirror
24
to form intermediate curved image
80
. Curved mirror
24
then provides a virtual image of intermediate curved image
80
at viewing pupil
14
.
Using the overall arrangement of
FIG. 2
, image source
94
can be any of a number of image sources that emit light, such as a transmissive or reflective LCD spatia light modulators, a digital micromirror device (DMD) spatial light modulator, a CRT, or an OLED or PLED device, for example. Significantly, 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. 2
works well when image source
94
is flat and shows how intermediate image
80
can be formed having the needed curvature. 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.
As the '181 disclosure points out, forming an intermediate image on a diffusive surface helps to overcome limitations 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. 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. Referring again to
FIGS. 1 and 2
and to the '181 disclosure, in order to maximize the light output from image generator
74
, it is necessary to provide a large angle of emitted light, using diffusing element
32
, in order to adequately fill left and right viewing pupil
14
l
and
14
r
. Diffusive surface
40
is shaped to provide curved intermediate image
76
having the desired curvature for the projection optical system.
While use of a diffusing element
32
provides a workable solution for forming a curved image, there are some drawbacks to projecting an image onto a diffusive component. In order to understand drawbacks with particular impact upon autostereoscopic imaging apparatus
10
, it is instructive to consider how ball lens assembly
30
operates. Referring to
FIG. 3
a
, there is shown the concentric arrangement and optical behavior of a 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
. Central 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 ball lens 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
.
In most embodiments, 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
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 central spherical lens
46
, without additional supporting refractive components.
In ideal operation, curved image
50
shares the same center of curvature C
ball
as ball lens assembly
30
. When arranged in this fashion, light from any point on curved image
50
is imaged with minimal aberration, as is represented in
FIG. 3
a.
The inherent advantages of a ball lens can be exploited using a modified design that employs a partial ball lens segment, such as using an hemisphere combined with a folding mirror, as is shown in the cross-sectional ray diagram of
FIG. 3
b
and described in the '474 patent. In
FIG. 3
b
, a hemispheric lens assembly
60
comprises a hemispheric central lens
66
, one or more optional meniscus lenses
42
, an
Kessler David
Liang Rongguang
Blish Nelson Adrian
Eastman Kodak Company
Gray David
Sever Andrew
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