Note: Descriptions are shown in the official language in which they were submitted.
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THE APPLICATION OF THE LIGHT VALVE TO HOLOGRAPHIC
STEREOGRAMS
FIELD OF THE INVENTION
The present invention relates to a method and device for producing
holographic stereograms.
BACKGROUND OF THE INVENTION
Liquid crystal display (LCD) panels are often used in the production of
holographic stereograms, however the size, resolution, contrast ratio, and
insertion losses of this technique create serious limitations for holographers
and
stereograms produced this way contain the familiar "fish-scale" patina. Motion
picture film is used to produce larger images in greater detail, however,
registration is often a problem and film-recording costs have largely limited
production. Full color work requires the additional expense of producing color
separated film footage. Few holographers are able to afford the production
costs
associated with medium and large format stereograms. This direct digital link
between a variety of 3D digital imaging processes and holography represents a
significant improvement to the holographic process.
United States Patent No. 5,560,17 discloses direct holographic recording
which eliminates many troublesome aspects of conventional holography. The
process relies on known ray-tracing algorithms to predict the pattern of light
that
would be present on the holographic recording material when making a
conventional hologram of the object and prints the calculated pattern directly
on
the recording material. Images are then tiled to form large mural-sized
holographic scenes. The technique is enormously computationally intense.
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Holographic pixels that make up the scene are often too large to be understood
well at close viewing distances. The cost of each custom made panel is
prohibitive for most users. Mural-sized holograms also have extremely limited
application because of strict illumination requirements. The current capital
costs
to implement this technology are over 10 million dollars.
It would be very advantageous to provide an economic method and
apparatus for producing holographic stereograms that avoids the expense of
present commercial systems.
SUMMARY OF THE INVENTION
The present invention provides a method of producing holographic
stereograms, comprising:
a) focusing an image of an object onto an image light amplifier means
which encodes said image in a birefringent material in said image light
amplifier
means;
b) directing a beam of coherent polarized light first through said
birefringement material such that the polarization of said beam of polarized
light
is spatially varied in a manner that reflects the image encoded in the
birefringement material and then through a polarizer to produce a coherent
polarized beam of light that is spatially imprinted with said image;
c) focusing said coherent polarized beam of light that is spatially imprinted
with said image onto a light diffusing means; and
d) capturing coherent polarized light scattered from the light diffusing
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medium on a holographic recording medium and focusing a reference beam of
coherent polarized light on said holographic film which interferes with said
coherent polarized light scattered from the light diffusing means light to
holographically record said image.
In another aspect of the present invention there is provided an apparatus
for producing holographic stereograms, comprising:
a) image light amplifier means having a back surface onto which an image
is projected and containing a birefringement material, said image light
amplifier
means including encoding means for encoding an image in said birefringent
material that has been projected onto said surface;
b) a light source and means for polarizing and directing a light beam into
said birefringent material through a transparent front surface of said image
light
amplifier, reflection means for reflecting said light beam back through said
birefringent material and out through the transparent front surface to produce
a
coherent polarized beam that is spatially imprinted with said image;
c) means for polarizing and focusing said coherent polarized beam that is
spatially imprinted with said image onto a means for diffusing light which
produces scattered coherent polarized light;
d) a holographic recording medium positioned to capture said scattered
coherent polarized light; and
e) means for producing and focusing a reference beam of coherent
polarized light onto said holographic recording medium wherein said reference
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beam interferes with said scattered coherent polarized light to
holographically
record said image on said holographic recording medium.
In another aspect of the invention there is provided an apparatus for
producing a
coherent polarized beam that is spatially imprinted with an image, comprising:
a) image light amplifier means containing a birefringement material;
b) image projection means for projecting an image onto a back surface of
said image light amplifier means and encoding means for encoding said image in
said birefringent material; and
c) a light source and means for polarizing and directing a light beam into
said birefringent material through a transparent front surface of said image
light
amplifier, reflection means for reflecting said light beam back through said
birefringent material and out through the transparent front surface to produce
a
coherent polarized beam that is spatially imprinted with said image.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described, by way of
example only, with reference being had to the drawings, in which:
Figure 1 shows a light valve stereogram printer apparatus constructed in
accordance with the present invention for producing holographic stereograms;
Figure 2 shows a block diagram of a light valve projector forming part of
the apparatus of Figure 1;
Figure 3 is cross section of an image light amplifier forming part of the
light
valve projector of Figure 2;
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Figure 4 shows a block diagram of an alternative embodiment of a light
valve projector forming part of the apparatus of Figure 1;
Figure 5 is a cross section of a single pixel of a digital light amplifier
forming part of the digital light valve projector of Figure 4;
Figure 6 illustrates an array of holographic elements for a full-color master
transmission hologram; and
Figure 7 illustrates an array of holographic elements for full-parallax, full-
color reflection master holograms.
DETAILED DESCRIPTION OF THE INVENTION
Referring to Figure 1, a light valve stereogram printer apparatus
constructed in accordance with the present invention is shown generally at 10.
Apparatus 10 includes a light valve 12 for producing and projecting a
polarized
light beam 14 that is spatially imprinted with an image to be converted to a
holographic stereogram. Referring to Figure 2, the light valve projector 12
includes an Image Light Amplifier (ILA) 16 onto which an image from a high
resolution cathode ray tube (CRT) 18 is focused. The image may be taken from a
video source, or output from a computer. The image from the CRT 18 is
projected onto the rear of the ILA 16. Referring to Figure 3, the ILA 16 is a
multi-
layer device with a central layer 22 being comprised of liquid crystal
sandwiched
between liquid crystal alignment films 24 and 26. A dielectric mirror 28 is
sandwiched between film 24 and a light blocking layer 30 and an amorphous
photoconducting silicon layer 32 is located on light blocking layer 30. A
s
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transparent conducting electrode 36 is located on the back surface of silicon
layer 32 and a bias is applied between electrode 36 and a transparent
conducting counter electrode 34 located on the front face of alignment film
26.
The entire assembly is sandwiched between glass plates 38.
The image on the rear face of the ILA 16 is picked up by amorphous
silicon layer 32 after passing through conductive glass layer 36. The light
striking
the silicon layer 32 will induce photo-conductivity in that layer in
proportion to the
brightness of the light at that point. The result is that the image displayed
on the
CRT 18 is transferred, or encoded, into a matching resistance pattern in the
amorphous silicon layer 32. The bias across the device then results in the
same
pattern of voltage across the liquid crystal 22 and the liquid crystal is
aligned to
varying degrees corresponding to the image intensity. The overall functioning
of
the device is then to convert the image into a pattern of liquid crystal
alignment.
Referring again to Figures 2 and 3, the projection step occurs using
polarized light and birefringence of the liquid crystal. The liquid crystal 22
(Figure
3) is a birefringent material when it is aligned so that the index of
refraction is
different for light polarized in different directions. Polarized light passing
through
a birefringent material can undergo a change in polarization. Referring
particularly to Figure 2, the ILA 16 is then read-out by bringing in a
polarized light
beam 40 from for example a laser or arc lamp (not shown). The polarized beam
40 is directed to a polarizing beam splitter 42 which splits the beam so that
one
beam is directed towards the front surface of the ILA 16. Referring to Figure
3,
this beam passes through the liquid crystal layer 22, reflects from the
dielectric
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mirror 28, and passes back through the liquid crystal 22 again. The
polarization is
then altered to varying degrees spatially as dictated by the alignment of the
liquid
crystal 22, which is in turn determined by the original image on the CRT 18
encoded in the photoconducting layer 32. Passing the returning beam through
polarizer 42 (Figure 2) then produces a polarized beam 14 that is spatially
imprinted with the image which is then focused by a projection lens 46. The
split
light beam directed back through collimating lens 44 by the polarizer 42
contains
the negative image. The light blocking layer 30 in ILA 16 (Figure 3) prevents
any
leakage from the high intensity read-out light causing photo-conductivity in
the
silicon layer, which would reduce contrast.
The polarizing beam splitter 42 is the preferred device for separating the
image from the returning beam due to the high efficiency and large aperture
achievable. Although less desirable, other embodiments that would perform the
same function are possible. One such configuration is a simple 50% beam
splitter (for example a partially silvered mirror) with a polarizing film
inserted in
the projected beam. In this case, the efficiency would be very poor as half
the
incident light is rejected on the way in and half of the light of the correct
polarization that is imprinted with the image is also unnecessarily rejected.
It is
also possible to configure the ILA such that the reflected light is not
directly
counter-propagating to the incident light. Then no beam splitter is required
and
only a polarizer is needed. In this case, the contrast is reduced, possibly
significantly, since the off-normal reflection through the birefringent liquid
crystal
will result in less pure polarization encoding of the image information.
Another
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possible device would involve another appropriately oriented birefringent
material
such as quartz or calcite. In these materials differently polarized light is
refracted
through different angles and the beams of each polarization are physically
separated as they pass through the material. The big disadvantage to this
technique would be the massive size of pure crystal that would be required to
separate the beams, which would be prohibitively expensive and difficult to
manufacture.
In the case of other types of ILA that encode information in intensity rather
than polarization (for example the micromirror array described hereinafter)
either
a simple 50% beam splitter alone is required (again leading to high
inefficiencies)
or preferentially the off-axis reflection method could be employed without
loss of
contrast.
Referring again to Figure 1, the coherent wavefront 14 projected from the
ILA 16 is then projected onto to a diffusing medium, or diffuser 92, such as a
ground glass plate or, preferably, a light shaping diffuser (LSD). The light
scattered from the diffuser 92 is then holographically recorded. The light
shaping
diffusers are holographically recorded randomized surface structures that
provide
nearly 90% transmission. These devices can be considered as a superposition of
numerous random gratings, having the effect of efficiently scattering light
through
a multitude of angles. By design, light-shaping diffusers can be made to have
viewing angles ranging from a few to over a hundred degrees and can have
different spreads in different directions. This provides holographers with
dramatically increased control over the uniformity of illumination while still
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concentrating the light within an effective area. This results in a
substantial
saving of laser energy, which permits shorter exposure times. Furthermore, the
light shaping diffusers do not rely on multiple scattering events, so there is
no
loss in coherence or polarization information. The overall result is
considerably
superior to results achieved using ground glass or similar diffusers.
Following projection from light valve 12 and scattering from the diffuser 92,
the still coherent and still polarized scattered light (preferentially
scattered in a
cone 17 containing the holographic recording medium (e.g holographic film) for
maximum efficiency) is interfered or mixed with a reference beam 94 that is
coherent with the scattered beam 21 and of the same polarization, see Figure
1.
The reference beam 94 is from the same source as the beam 40 (Figures 1 and
2). A light source (not shown) produces a beam 96 which is focused by a lens
98
and upon hitting beam splitter 100, one of the beams 102 is reflected by
mirror
104 and expanded through lens 106 to produce beam 40 which enters the light
valve 12. The other beam 110 is directed by a mirror 112 to expanding lens 114
and a collimating mirror 116 directs the reference beam 94 to the holographic
film
98. The holographic film 98 is placed in the plane where the beams 94 and 21
interfere and the interference pattern thus captured on film 98 is a
holographic
recording.
The holographic stereogram is a plurality of holograms of two-dimensional
images. These images may come from 3D-computer animation software, video,
motion picture film or multiple photographs taken from different views. They
may
also be obtained with image capture technology. Each two-dimensional scene is
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presented on the LSD light shaping diffuser and recorded as described below.
To
make a stereogram, a single image is projected onto the diffuser 92 and
holographically recorded in a specific position on the film 98 by masking the
film
to expose only a vertical slit (or checkerboard pattern of exposures) using
the
translating aperture apparatus 122. A fresh image of the same object taken
from
a slightly different viewpoint is subsequently projected along with a
simultaneous
repositioning of the slit to capture the hologram of the new image in an
appropriate place on the film. By repeating this procedure numerous times in
this
way, the stereogram is built up with a number of frames of the object as
viewed
from different positions. To record master holograms for monochromatic and
achromatic (colorless) holograms, long, narrow slits such as seen in
translating
aperture apparatus 122 in Figure 1 are used.
For master holograms, variations on this process include: the production
of full-color transmisson holograms that form overlapping spectra from red,
green
and blue (RGB) strips of holographic pixels (hogels), see Figure 6; production
of
full color, full parallax holograms (having both vertical and horizontal "look-
around" views) reflection holograms from RGB hogels in a "corn row" array, see
Figure 7.
For transfer holograms in the full-color transmission model, rows of hogels
form the look around view for each of the red, green and blue elements which
are
later combined to form the full color transfer hologram, see Figure 6. In the
full
color, full parallax reflection master model, the hogels form an array much
like
to
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conventional television. This master is then transferred using known color
control
techniques to form a full color transfer, see Figure 7.
The light valve stereogram printer apparatus 10 disclosed herein
eliminates the time, effort and expense of production from motion picture
film.
The near photographic resolution and extremely high contrast ratio produce
detailed holograms. The smoothing effect introduced by the finite bandwidth of
the CRT 18 (Figure 2) reduces the evidence of pixels adding to the film-like
appearance. Full color work is also made easy with almost immediate color
separations and absolute registration from frame to frame.
Figures 4 and 5 illustrate an alternative embodiment of an apparatus for
producing holographic stereograms in which the analog image light amplifier
has
been replaced by a digital image light amplifier (D-ILA) as recently developed
by
Hughes/JVC Technologies that incorporates an ILA with newly developed liquid
crystal technology, as disclosed in "D-ILA PROJECTOR TECHNOLOGY: The
Path to High Resolution Projection Displays", W.P. Bleha, JVC Digital Image
Technology Center, 2310 Camino Vida Roble, Carlsbad, CA 92009. This has
radically reduced the size of the high resolution digital projection systems.
Referring to Figure 4, the digital light valve projector 15 includes a Digital
Image Light Amplifier (DILA) 19 which directly receives image information in
digital form from a computer or digital video source. Referring to Figure 5,
the
DILA 19 is a multi-layer device with a central layer 22 being comprised of
liquid
crystal sandwiched between liquid crystal alignment films 24 and 26, as in the
ILA device. The transparent electrode 34 and supporting glass layer 38 remain
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the same as in the ILA device 15 shown in Figure 3. The layers on the opposite
side of the central layer 22 have been replaced by an array of specialized
transistors, each making up a single pixel as shown in Figure 5. The
transistor is
built on a silicon substrate 62 and consists of a source electrode 65, a gate
electrode 67, a drain electrode 69, and a capacitor 72. Above the electrodes
is a
silicon dioxide layer 50 in which are embedded three metal layers consisting
of a
wiring layer 59, a light blocking layer 56, and a reflective electrode layer
52. The
digital signal for the pixel is first converted to an analog voltage using an
analog-
to-digital converter. The resulting voltage is then applied to a sample-and-
hold
amplifier where the analog voltage stored in this way is applied to the gate
electrode 67 of the pixel. This voltage bias in turn controls the voltage of
the drain
electrode 69 which is connected through the wiring layer 59 to the reflective
electrode 52. The voltage on the reflective electrode 52 acts to align the
liquid
crystal layer 22 to a degree proportional to the voltage of the reflective
electrode
52. As well as aligning the liquid crystal, the reflective electrode 52 is
also made
highly reflective to act as a mirror. A small amount of the projection light
incident
on the reflective electrode layer 52 through the central layer 22 passes
through
or around the reflective electrode 52. In between the wiring layer 59 and the
reflective electrode 52 is a light blocking layer 56 that prevents this light
from
reaching the wiring or substrate layers, an event which would induce an
undesirable photocurrent and voltage change in the device, thus reducing
contrast. While only a single pixel is illustrated in Figure 5, it is clear
that an array
of pixels, each with an appropriate voltage applied to the gate electrode 67,
will
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reproduce an image as a pattern of voltages on the reflective electrode layer
52.
This voltage pattern will in turn produce a matching pattern in the degree of
liquid
crystal alignment in the central layer 22 corresponding to the original
digital
pattern applied to the array.
Referring to Figures 4 and 5, the projection step occurs as before using
polarized light and birefringence of the liquid crystal. The liquid crystal 22
(Figure
5) is a birefringent material when it is aligned so that the index of
refraction is
different for light polarized in different directions. Polarized light passing
through
a birefringent material can undergo a change in polarization. Referring
particularly to Figure 4, the DILA 19 is then read-out by bringing in a
polarized
light beam 40 from for example a laser or arc lamp (not shown). The polarized
beam 40 is directed to a polarizing beam splitter 42 which splits the beam so
that
one beam is directed towards the front surface of the DILA 19. Referring to
Figure 5, this beam passes through the liquid crystal layer 22, reflects from
the
reflective electrode 52, and passes back through the liquid crystal 22 again.
The
polarization is then altered to varying degrees spatially as dictated by the
alignment of the liquid crystal 22, which is in turn determined by the digital
information encoded as a voltage on the gate electrode 67. Passing the
returning
beam through polarizer 42 (Figure 2) then produces a polarized beam that is
spatially imprinted with the image which is then focused by a projection lens
46.
The split light beam directed back through collimating lens 44 by the
polarizer 42
contains the negative image. The light blocking layer 56 in DILA 19 (Figure 5)
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prevents any leakage from the high intensity read-out light causing photo-
conductivity in the silicon or metal layers, which would reduce contrast.
The coherent wavefront projected from the DILA 19 (Figure 5) is then
holographically recorded as described above with respect to the embodiment
using the ILA in Figures 1 to 3. The use of D-ILAs in the holographic
apparatus
disclosed herein is very advantageous as it eliminates many of the heat
generating components, such as the CRT 18 (Figure 2) and associated
electronics, as well as the potentially dangerous high voltages required by
the
CRT 18.
It is noted that an alternate method of high brightness projection that is in
principle available and suitable for holographic application is the Digital
Light
Processing projector. This device is based on an array of micromirrors where
each micromirror represents a single pixel. In this case, the mirror has 2
positions: the "off' position in which the incident light is reflected away
from the
projection system and the "on" position in which the incident light is
reflected into
the projection system to be imaged as a pixel on the diffusing medium. Because
the mirror is a binary device, gray scale is more difficult to implement and
is
achieved by rapidly moving the mirror between the on and off positions with
the
relative amount of time spent in each position determining the light level.
The
motion inherent to this method will necessarily introduce some degradation of
the
hologram, which may be anywhere from not significant to unusable.
Researchers in the field of biomedicine routinely utilize volumetric displays
of data from CAT scans, MRI and con-focal microscopy for various applications
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including treatment planning of radiation therapy. Users can send images from
hospital PAC (Picture Archive and Communications) systems via the Internet
using utilities such as E-film to be printed as holograms.
Holographic stereograms have made possible the creation of holograms
of both real objects in natural and studio lighting and virtual objects
created using
three-dimensional graphics. Holography represents a powerful visual medium for
the presentation of volumetric imagery. By eliminating the extra step to
motion
picture film the light valve printer disclosed herein provides an inexpensive
means of production of 3D hard copy for dimensional imaging. The use of D-ILAs
mean ever increasing resolution SLMs for use in holographic applications.
Medical researchers and physicians using volumetric displays advantageously
benefit from an inexpensive, high resolution, direct recording system for auto-
stereoscopic displays.
The method and apparatus disclosed herein for producing holographic
stereograms based on the image light amplifier provides a direct link from
digital
graphics to holography. Its high resolution and contrast ratio makes it an
excellent choice for small and medium format (50cm X 60cm) holograms.
The foregoing description of the preferred embodiments of the invention
has been presented to illustrate the principles of the invention and not to
limit the
invention to the particular embodiment illustrated. It is intended that the
scope of
the invention be defined by all of the embodiments encompassed within the
following claims and their equivalents.
is
