Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 95/14960 2 1 7 7 3 ~ ~ PCT/U594/13639
.
Apparatus for Reconstructing 1 I~,luu~phic Imaûes
CRQSS-REFERENCE TO RELATED APPLICATIONS
5 This is a continuation-in-part (CIP) of United States Patent Application Serial
No. 07/982,3f 6 of the same title filed November 27, 1992 by inventor Stephen J. Hart;
and of International Patent Application No. PCT/US93/11501 of the same title andinventor filed November 26, 1993.
TECHNICAL FIFI n
0 The present invention relates, generally, to methods and apparatus for making
holograms, and more particularly to a technique for sequentially exposing a film substrate
to a plurality of two-dimensional images ~ tel)ldlive of a three-dimensional physical
system to thereby produce a hologram of the physical system.
BAÇKGRQU~ID ART AND TECHNIÇ~I PRQBI F1~5
A hologram is a three-dimensional record, for example a film record, of a physical
system which, when replayed, produces a true three-dimensional image of the system.
Holography differs from ~ os-u,uic ,~ ulo~ld,ully in that the holographic image exhibits
full parallax by affording an observer a full range of viewpoints of the image from every
angle, both horizontal and vertical, and full perspective; i.e., it affords the viewer a full
20 range of perspectives of the image from every distance from near to far. A holographic
~pl~st~ dlion of an image thus provides significant advantages over a stereoscopic
l~p~ dlion of the same image. This is particularly true in medical diagnosis, where
the examination and understanding of volumetric data is critical to proper medical
treatment.
While the examination of data which fills a three-dimensional space occurs in all
branches of art, science, and engineering, perhaps the most familiar examples involve
medical imaging where, for example, Computerized Axial Tomography (CT or CAT),
Magnetic Resonance (MR), and other scanning modalities are used to obtain a plurality
of cross-sectional images of a human body part. Radiologists, physicians, and patients
30 observe these two-dimensional data "slices" to discern what the two-dimensional data
implies about the three-dimensional organs and tissue I~ d by the data. The
integration of a large number of two-dimensional data slices places great strain on the
human visual system, even for relatively simple volumetric images. As the organ or tissue
WO 9~/14960 ~ 1 7 ~ 3 5 8 PCT/US94/13639
underinvestigationbecomesmorecomplex,theabilitytoproperlyintegratelargeamounts
of two-dimensional data to produce meaningful and understandable three-dimensional
mental images may become overwhelming.
In prior art holograms employing a small number of superimposed holographic
s images on a single film substrate, the existenoe of a relatively small ~e,~:"~d~ of spurious
exposed and/or developed photosensitive elements (fog) does not d,l J,I~ idbly degrade the
quality of the resulting hologram. In contrast, holograms made in accordance with the
subject invention, discussed below, typically employ up to 100 or more hologramssuperimposed on a single film substrate; hence, the presence of a small amount of fog on
0 each hologram would have a serious cumulative effect on the quality of the final product.
A method and apparatus for producing holograms is therefore needed which
permits a large number, for example up to several hundred or more different holograms
to be recorded on a single film substrate, thereby facilitating the true, three-dimensional
holographic reproduction of human body parts and other physical systems which are
currently viewed in the form of discrete data slioes.
SUMMARY OF THE INVENTION
The present invention provides methods and apparatus for making holograms
which overcome the limitations of the prior art.
In accordance with one aspect of the present invention, a hologram camera
20 assembly comprises a single laser source and a beam splitter configured to split the laser
beam into a reference beam and an obied beam and to direct both beams at a film
substrate. The assembly further comprises a spatial light modulator configured to
sequential ly project a plural ity of two-di mensional i mages, for example a p l ural ity of sl ices
of data ~-,""~ i"g a CT scan data set, into the object beam and onto the film. In this
2s man ner, a three-d i mensional holograph ic record of each two-di mensional sl ice of the data
set is produoed on the film.
In accordance with another aspect of the invention, the entire data set, consisting
of one to two hundred or more individual two-dimensional slices, is su~e, i",~.osed onto
the film, resulting in the superposition of one hundred Qr more individual, interrelated
30 holograms on the single substrate (the master hologram~. In contrast to prior art
techniques wherein a small number (e.g., one to four) of holograms are superimposed
onto a single film substrate, the present invention contemplates methods and apparatus
for recording a large number of relatively weak holograms, each consuming an
-2-
WO 95/14960 ~ ~ ~7 7 ~ 5 ~ PCT/US94113639
d,U,UI U~il, Idlely equal, but in any event proportionate, share of the photosensitive elements
within the film.
In accordance with a further aspect of the invention, a reference-to-object copy(transfer) assembly is provided whereby the d~u,~",~lltioned master hologram may be
5 quickly and efficiently reproduced in a single exposure as a single hologram.
In a.-u,-ld"~ with yet a further aspect of the invention, a ho~ogram viewing
device is provided for viewing the hologram produced in accordance with the invention.
In particular, an exemplary viewing box in accordance with the present inventioncomprises a suitably enclosed, rectangular apparatus LU~ g a broad spectrum light
0 source, e.g, a white light source mounted therein, a collimating (e.g., Fresnel) lens, a
broad spectrum light source, e.&, diffraction grating, and a Venetian blind (louver). The
collimating lens is configured to direct a collimated source of white light through the
diffraction grating. In the context of the present invention, a collimated light refers to
light in which all LU~ JO~ thereof have the same direction of propagation such that
5 the beam has a substantially constant cross-sectional area over a reasonable propagation
length.
The diffraction grating is configured to pass light therethrough at an angle which
is a function of the wavelength of each light component. The hologram also passes light
therethrough at respective angles which are a function of the corresponding wavelengths.
20 By inverting the hologram prior to viewing, all wavelengths of light emerge from the
hologram substantially orthogonally thereto
BRIEF DES(:RIPTION OF THE DRAWING FIGVRES
The subject invention will hereinafter be described in conjunction with the
appended drawing figures, wherein like numerals denote like elements, and
Figure 1 shows a typical computerized axial IUIllU~;ld~JIly (CT) device;
Figure 2 shows a plurality of two-dimensional data slices each containing
data such as may be obtained by x-ray devices typically employed in the CT
device of Figure 1, the slices cooperating to form a volumetric data set;
Figure 3 shows a schematic diagram of a camera system in d.Lolddl~cl: with
a preferred ~ budil~ of the present invention;
Figure 4 shows a schematic diagram of a beam splitter assembly in
accordance with a preferred embodiment of the present invention;
-3 -
WO95/14960 2 ~ 7~ 8 PCT/US94113639
Figure 5A to 5D are graphic illustrations showing the effect of Fourier
Llal~ru~ g the laser beam utilized in the camera system of Figure 3;
Figure 6A shows an enlarged schematic diagram of a portion of the camera
system of Figure 3;
Figure 6B shows a schematic diagram of an alternative embodiment of the
spatial light modulator shown in Figure 3;
Figure 7 shows an enlarged schematic diagram of another portion of the
camera system of Figure 3;
Figure 8 shows an enlarged schematic diagram of a portion of the
0 projection assembly utilized in the camera assembly of Figure 3;
Figure 9 shows a schematic layout of an exemplary copy rig in accordance
with the present invention;
Figures 1 ûA and 1 ûB set forth orthoscopic and pseudoscopic views,
respectively, of a master hologram being replayed in accondance with one aspect
of the present invention;
Figure 11 shows a schematic diagram of a hologram viewing apparatus;
Figure 12 is a schematic diagram of an alternative embodiment of a "single
step" camera system in accordance with the present invention;
Figure 13 is a schematic diagram of an alternative embodiment of the
viewing apparatus shown in Figure 11 in accordance with the present invention;
Figure 14 is a schematic cross-section view of a first alternative embodiment
of a laminated, composite light control film (LCF) useful in the context of the
viewing apparatus shown in Figure 11;
Figures 15 shows a front view of an exemplary one of the film sheets
2s shown in Figure 14;
Figures 16 and 17 are schematic cross-section views illustrating the effect
of manipulating the film sheets of Figure 14 on the passage of first order lightthrough the LCF; and
Figure 18 is a schematic cross-section view of a second alternative
embodiment of the LCF of Figure 14.
r~ETAlLED DESCRIPTION OF PREFERRED EXE~PLARY EMBODIMENTS
In the context of the present invention, a volumetric data set corresponding to a
three-dimensional physical system (e.g., a human body part) is encoded onto a single
1-
WO 95/14960 2 1 7 7 3 ~ 8 PCT/US94/13639
recording material, e.g., a pllvlols~d,uhic substrate, to thereby produce a master hologram
of the object. The master hologram may be used to produce one or more copies which,
when replayed by directing an appropriate light source therethrough, recreates a three-
dimensional image of the object exhibiting full parallax and full perspedive. Thus, for
5 a particular data set, the present invention contemplates a plurality of separate,
ldled optical systems: a camera system for producing a master hologram; a copy
system for generating copies of the master hologram; and a viewing system for replaying
either the master hologram or copies thereof, depending on the particular configuration
of the camera system.
0 THE DATA SET: -
Presently known modalities for generating volumetric data ~ ding to aphysical system include, inter alia, computerized axial lmllo~ld~Jlly (CAT or CT) scans,
magnetic resonance scans (MR~, three-dimensional ultra sound (US), positron emission
~nllo~5~dplly (PET), and the like. Although a preferred embodiment of the present
5 invention is described herein in the context of medical imaging systems which are
typically used to investigate internal body parts (e.g., the brain, spinal cord, and various
other bones and organs), those skilled in the art will appreciate that the present invention
may be used in conjunction with any suitable data set defining any three-dimensional
distribution of data, regardless of whether the data set represents a physical system, e.g,
20 numerical, graphical, and the like.
Referring now to Figures 1 and 2, a typical CT device comprises a gantry 10 and
a table 12, as is known in the art. Table 12 is advantageously configured to move axially
(along arrow A in Figure 1) at ,~"~d~l~"~,i"~d increments. A patient (not shown) is placed
on table 12 such that the body part to be i,,~..,v~dl~d is generally disposed within the
2s perimeter of gantry 10.
Gantry 10 suitably comprises a pluQlity of x-ray sources and recording devices
(both not shown) disposed about its circumference. As the patient is moved axially
relative to gantry 10, the x-ray devices record a succession of two-dimensional data slices
14A, 14B, . . . 14X ~,""-,;,i"g the three-dimensional space (volume) 16 containing data
30 obtained with respect to the i~ dl~d body part (see Figure 2). That is, the individual
dataslicesl4combinetoformavolumetricdatasetl6which,intotal,comprisesathree-
dimensional image of the ill~ dl~d body part. As used herein, the terms "volume" or
"volumetric space" refers to volumetric data set 16, including a plurality of two-
_5_
WO 95114960 2 1 7 7 ~ 5 ~ PCTI[JS94/13639
dimensional data slices 14, each slice containing particular data regarding the body partu~;d~d by the given modality.
Typical data sets comprise on the order of 10 to 70 (for CT systems) or 12 to 128
(for MR) two- dimensional data slices 14. Those skilled in the art will appreciate that the
s thickness and spacing between data slices 14 are configurable and may be adjusted by
the CT technician. Typical slice thicknesses range from 1.5 to 10 millimeters and most
typically d~ ly 5 millimeters. The thickness of the slices is desirably selected so
that only a small degree of overlap exists between each successive data slice.
Presently known CT scan systems generate data slices having a resolution defined0 by, for example, a 256 or a 512 square pixel matrix. Furthermore, each address within
the matrix is typically defined by a twelve bit grey level value. Cr scanners are
conventionally calibrated in Houndsfield Units whereby air has a density of minus 1,000
and water a density of zero. Thus, each pixel within a data slice may have a grey level
value between minus 1,000 and 3,095 (inclusive) in the context of a conventional CT
1s system. Because the human eye is capable of simultaneously perceiving a maximum of
approximately one hundred (100) grey levels between pure white and pure black, it is
desirable to manipulate the data set such that each data point within a slice exhibits one
(1) of d~J,ul~)~dllld~ly fifty (50) to one hundred ~100) grey level values (as opposed to the
4,096 available grey level values). The process of redefining these grey level values is
20 variously referred to as "windov.~ing" (in radiology); "stretching" (in remote sensing/satellite
imaging); and "photometric correction" (in astronomy).
The present inventor has d~l~llllillt:d that optimum contrast may be obtained bywindowing each data slice in accordance with its content. For example, in a CT data
slice which depicts a cross-section of a bone, the bone being the subject of examination,
2s the relevant data will typically exhibit grey level values in the range of minus 600 to
1,400. Since the regions of the data slice exhibiting grey level values less than minus 600
or greater than 1,400 are not relevant to the examination, it may be desirable to clamp
all grey level values above 1,400 to a high value corresponding to pure white, and those
data points having grey level values lower than minus 600 to a low value corresponding
30 to pure black.
As a further example, normal grey level values for brain matter are typically in the
range of about 40 while grey level values ~"t:~,uonding to tumorous tissue may be in the
120 range. If these values were expressed within a range of 4,096 grey level values, it
WO 95114960 2 1 ~ 7 ~ 5 8 PCI/US94/13639
.
would be extremely difficult for the human eye to distinguish between normal brain and
tumorous tissue. Therefore, it may be desirable to clamp all data points having grey level
values greater than, e.g., 140, to a very high level ~nlt:a,uOI)d;llg to pure white, and to
clamp those data points having grey scale values of less than, e.g., minus 30, to a very
s low value corresponding to pure black. Windowing the data set in this manner
contributes to the production of sharp, unambiguous holograms.
In addition to windowing a data set on a slice-to-slice basis, it may also be
advantageous, under certain circumstances, to perform differential windowing within a
particular slice, i.e., from pixel to pixel. For example, a certain slice or series of slices
0 may depict a deep tumor in a brain, which tumor is to be treated with radiation therapy,
for example by irradiating the tumor with one or more radiation beams. In regions which
are not to be irradiated, the slice may be windowed in a relatively dark manner. In
regions which will have low to mid levels of radiation, a slice may be windowed
somewhat more brightly. In regions of a high ~u~ dlion of radiation, the slioe may
5 be windowed even brighter. Finally, in regions actually containing the tumor, the slice
may be windowed the brightest. In the context of the present invention, the resulting
hologram produces a ghostly image of the entire head, a brighter brain region, with the
brightest regions being those regions which are either being irradiated (if the data set was
taken during treatment) or which are to be irradiated.
A further preprocessing technique useful in the context of the present inventionsurrounds manipulating the aggregate brightness level for some or all of the slices within
a particular data set to thereby reduce the differences in aggregate brightness level from
slice to slice and to reduce the need for long exposure times for same or all of the slices.
This technique is sometimes referred to herein as adding "asteroids" to certain data slices
25 to enhance their brightness level.
More particularly and as discussed below in greater detail, each slice ~olllp~ gthe finished hologram desirably consumes its proportionate share of available photo-
sensitive elements within the holographic substrate during processing of the hologram.
In accordance with one aspect of the invention, this is achieved by coordinating various
30 processing ~dldlll~la for each data slice including, for example, the beam ratio, the
aggregate brightness level for the particular data slice and the exposure time during which
the particular data slice is projected on to the film substrate. As a general principle,
brigh~r d~ta slices requi ~ Iess exposure tim~ and relatively faint data slices require a
WO 95/14960 2 1 7 7 3 5 8 PCI/13S94/13639
higher exposure time. in order to reduce the exposure time for faint slices, the aggregate
brightness level for a particular faint slice may be artificially boosted by adding a random
or otherwise irregular pattern of bright spots to the data slice, preferably in the wings of
the data slice remote from the image under examination. Alternatively, the portion of the
5 object beam laser light may be diverted prior to passing through the data slice, for
example through the use of an additional beam splitter, and controllably projected onto
the film service. If desired, the diverted beam may be passed through as variable intensity
polarizer, which polarizer comprises a random pattern of white spots, the intensity of
which may be modulated to achieve a desired "astroid" beam intensity. In this regard,
0 the asteroid may ~u~p~u~;se a small pattern of bright spots, a large pattern of relatively
diffused spots, or a ~ur~ dlioll of both. In accordance with a further aspect of the
invention, the aforementioned polarizer may comprise a polaroid disk configured with
asteroids, which disk may be rotated to modulate the asteroid intensity. Further, the
asteroid disk may be equipped with a shutter to effectively shunt the asteroid beam for
5 those slices which do not require an artificially elevated aggregate brightness level. This
pattern of random white spots, or asteroids, artificially enhances the aggregate gray scale
value of the slice, thereby reducing the exposure time for the slice. If desired, the
asteroids may be subsequently masked from view in the final, finished composite
hologram.
Another step in preparing the data set involves cropping, whereby regions of each
data slice or even an entire data slice not germane to the examination are simply
eliminated. Cropping of unnecessary data also contributes to the formation of sharp,
unambiguous holograms.
~lore particularly, each point within the volume of the emulsion exhibits a
25 microscopic fringe pattern ~u~ JUll~;lillg to the entire holographic image from a unique
viewpoint. Stated another way, an arbitrary point at the lower left hand corner of a
holographic film comprises an interference fringe pattern which encodes the entire
holographic image as the image is seen from that particular point. Another arbitrary point
on the holographic film near the center of the film will comprise an interference fringe
30 pattern ,~,u,~st:"~dlive of the entire holographic image when the image is viewed from the
oenter of the film. These same phenomenon holds true for every point on the hologram.
As briefly discussed above, a suitable ,ullù~o~;,dul~ic substrate preferably comprises a
volume of photographic emulsion which adheres to the surface of a plastic substrate, for
WO95/14960 i~ ~ 7j'~5~ PCI/US94/13639
example triacetate. The emulsion typically comprises a very large number of silver halide
crystals (grains) suspended in a gelatinous emulsion. Inasmuch as the emulsion contains
a finite quantity of crystals, the elimination of u"l1e~ , y data (cropping) within a data
slice ensures that substantially all of the silver halide grains which are converted (exposed)
5 for each data slice ~u~uol~d the relevant data from each slice. By conserving the
number of silver halide grains which are converted for each data slice, a greater number
of slices may be recorded onto a particular piece of film.
THE CAMERA SYSTEM
Once a data set is properly prepared (e.g., windowed and cropped), an individual10 hologram of each respective data slice is su,ue,i",,uos~d onto a single film substrate to
generate a master hologram. In accordance with a preferred embodiment, each individual
hologram corresponding to a particular data slice is produced while the data
~u"~uoi,di"g to a particular slice is disposed at a different distance from the film
substrate, as discussed in greater detail below.
Referring now to Figures 3-4, a camera system 3û0 in accordance with the presentinvention suitably comprises a laser light source 302, a shutter 306, a first mirror 308, a
beam splitting assembly 310, a second mirror 312, a reference beam expander 314, a
collimatinglens316,afilmholder318,athirdmirror320,anobjectbeamexpander322,
an imaging assembly 328, a projection optics assembly 324, a rear projection screen
20 comprising a diffusing surface 472 having a polarizer 327 mounted thereto, and a track
assembly 334. Imaging assembly 328, projection optics assembly 324, and rear
projection screen 326 are suitably rigidly mounted to track assembly 334 so that they
move in unison as track assembly 334 is moved axially along the line indicated by arrow
F. As discussed in greater detail below, track assembly 334 is advantageously configured
2s to replicate the relative positions of data slices ~u"".ii~i"g the subject of the hologram.
In a preferred embodiment, total travel of track assembly 334 is suitably sufficient to
accommodatetheactualtraveloftheparticularscanningmodalityemployedingenerating
the data set, for example on the order of 6 inches.
Camera assembly 300 is illustratively mounted on a rigid table 304 which is
30 suitably insulated from environmental vibrations. Laser source 302 suitably comprises a
conventional laser beam generator, for example an Argon ion laser including an etalon
to reduce the bandwidth of the emitted light, preferably an innova 306-SF manufactured
by Coherent, Inc. of Palo Alto, California. Those skilled in the art will appreciate that
_9,
WO95/14960 2 1 773~ PCT/US94/13639
laser 302 suitably generates a monochromatic beam having a wavelength in the range of
400 to 750 I~dllOIll~l~ (nm), and preferably about 514.5 or 532 nm. Those skilled in the
art will appreciate, however, that any suitable wavelength may be used for which the
selected,ul,u~uE;,d,ul~icmaterialis~u",,~,dliL,le,includingwavelengthsintheultravioletand
5 infrared ranges.
Alternatively, laser 302 may comprise a solid state, diode-pumped frequency-
doubled YAG laser, which suitably emits laser light at a wavelength of 532 nm. These
lasers are capable of emitting in the range of 300 to 600 million watts of pure light, are
extremely efficient and air-cooled, and exhibit high stability.
Laser 302 should also exhibit a coherence length which is at least as great as the
difference between the total path traveled by the reference and object beams, and
preferably a coherence length of at least twice this difference. In the illustrated
embodiment, the nominal design path length traveled by the reference beam is equal to
that of the object brain (d~plu~illldl~ly 292 centimeters); however, due to, inter alia, the
15 geometry of the setup, the particular reference angle employed, and the size of the film,
some components of the reference and object beams may travel a slightly greater or lessor
path length. Hence, laser 302 suitably exhibits a coherence length in excess of this
difference, namely, c~ uxi~dlely two (2) meters.
Shutter 306 suitably comprises a conventional r~ u",e~l,d"ical shutter, for
20 example a Uniblit~ 35 model no. Lr~54Z manufactured by Vincent Associates of
Rochester, New York. In a preferred embodiment, shutter 306 may be remotely actuated
so that a reference beam and an object beam are produced only during exposure of the
film substrate, effectively shunting the laser light from the system (e.g., via shutter 306~
at all other times. Those skilled in the art will appreciate that the use of a shutter is
25 u~",e~ d~y if a pulse laser source is employed. Moreover, it may be desirable to
incorporate a plurality of shutters, for example a shutter to selectively control the
reference beam and a different shutter to separately control the object beam, to permit
independent control of each beam, for example to permit independent measurement
and/or calibration of the respective intensities of the reference and object beams at the
30 film surface.
The various mirrors (e.g., first mirror 3û8, second mirror 312, third mirror 320,
etc.) employed in camera assembly 300 suitably comprise conventional front surface
mirrors, for example a dielectric mirror coated on a pyrex substrate, for example stock
--10-
WO 95/14960 2 1 7 7 3 ~ 8 PCT/US94113639
mirror 10D20BD.1, manufactured by Newport. For a typical laser having a beam
diameter on the order of 1.5 millimeters, mirror 308 suitably has a surface of
d~Jplo~ ly 1 inch in diameter.
First mirror 308 is suitably configured to direct a source beam 402 to beam
splitting assembly 310. In the illustrated t:,llL,o.li",~"~, first mirror 308 changes the
direction of beam 402 by 90 degrees. Those skilled in the art, however, will appreciate
that the relative disposition of the various optical components ~U~pli~illg camera
assembly 300, and the particular path traveled by the various beams, are in large measure
a function of the physical size of the available components. As a working premise, it is
0 desirable that the reference beam and object beam emanate from the same laser source
to ensure proper ~u"~ld~iu" between the reference and the object beam at the surface of
film holder 318, and that the path traveled by the reference beam from beam splitter 310
to film 319 is dp!,lu,.il"dlt:ly equal to the path traveled by the object beam from beam
splitter 310 to film 319.
With momentary reference to Figure 4, beam splitter assembly 310 preferably
comprises a variable wave plate 404, respective fixed wave plates 408 and 412,
respective beam splitting cubes 406 and 414, and a mirror 416. On a gross level, beam
splitting assembly 310 functions to separate source beam 402 into an object beam 410
and a reference beam 418. Moreover, again with reference to Figure 3, beam splitter
assembly 310 also cooperates with imaging assembly 328 and polarizer 327 to ensure
that the reference beam and the object beam are both purely polarized in the same
polarization state, ;.e., either substantially S or P polarized as discussed in greater detail
below, when they contact an exemplary film substrate 319 mounted in film holder 318.
By ensuring that the reference and object beams are pure polarized in the same
25 polarization state, sharp, low noise i"l~,r~ ".~ fringe patterns may be formed.
With continued reference to Figure 4, beam 402 generated by laser source 302
enters beam splitting assembly 310 in a relatively pure polarization state, for example as
S polarized light. In the context of the present invention, S polarized light refers to light
which is polarized with its electric field oscillating in a vertical plane; P polarized light
30 refers to light having its electric field oriented in a horizontal plane. Beam 4û2 then
passes through variable wave plate 404 whereupon the beam is converted into a beam
403, convenientlydefinedas-~ risillgamixtureofSand Ppolarized lightcomponents.
Beam 403 then enters beam splitting cube 406, which is suitably configured to split beam
1 1
WO gS/14960 2 1 7 ~ 3 5 8 I'CTIUS94113639
403 into a first beam 405 ~u~ n i~ g the P polarized light ~ulll,uol~l ll of beam 403 and
a second beam 407 ~u"~u~ illg the 5 polarized light .u"IpUIl~lll of the beam 403. Beam
splitting cube 406 suitably comprises a broad band beam splitter, for example a broad
band polarization beam splitter, part no. 05FC1 6PB.3, manufactured by Newport.
5 Although beam splitting cube 406 is ideally configured to pass all of (and only) the P
polarized component of beam 403 and to divert all of (and only) the S polarized
~ur~Ju~ ll of 403, it has been observed that such cubes are generally imperfect beam
splitters, ignoring small losses due to reflection off of beam splitter surfaces. More
precisely, such cubes typically exhibit an extinction ratio on the order of a thousand to
0 one such that approximately 99.9 percent of the S polarized component of beam 403 is
diverted into beam 407, and such that dp~lu7~illldl~ly 9û percent of the P polarized
component of beam 403 passes through cube 406. Thus, beam 407 comprises 99.9
percent of the S polarized ~ulll,uo~ of beam 403, and approximately 10 percent of the
P polarized component of beam 403; similarly, beam 405 comprises d,uplu~dllldlt:ly 90
s percent of the P polarized ~u~ elll of beam 403 and dpplu~dllldl~ly 0.1 percent of the
S polarized cûmpûnent of beam 403.
Wave plates 404, 408, and 412 suitably comprise half wave plates for the laser
wavelength in use, e.g, part no. 05RP02 manufactured by Newport. Wave plate 404 is
configured to convert the S polarized beam 402 into a pl~d~ ed ratio of S and P
20 polarized u",~ "ls. In a preferred embodiment, variable wave plate 404 comprises
an LCD layer, which layer changes the polarization of the incoming beam in accordance
with the voltage level at the LCD layer. A suitable wave plate 404 may comprise a Liquid
Crystal Light Control System, 932-VIS available from Newport. Accordingly, wave plate
404 divides S polarized beam 402 into a mixture of S and P polarized light as a function
25 of applied voltage. By manipulating the voltage on wave plate 404, the operator thereby
controls the ratio of the intensity of the reference beam to the intensity of the object beam
(the beam ratio). In a preferred embodiment, this ratio as measured at the plane of film
319 is approximately equal to unity.
In any event, beam 405 is almost completely pure P polarized, regardless of the
30 voltage applied to wave plate 4û4; beam 407 is ideally pure S polarized, but may
nonetheless contain a substantial P polarized component, depending on the voltage
applied to wave plate 404.
-12-
WO95114960 2 1 7 7 3 ~ ~ PCT/US94/13639
With continued reference to Figure 4, beam 4û5 then travels through wave plate
408 to convert the pure P polarized beam 4û5 to a pure S polarized object beam 41û.
Beam 407 is passed through wave plate 412 to convert the substantially S polarized beam
toasubstantiallyPpolarizedbeam4û9whichthereafterpassesthroughsplittingcube414
5 to eliminate any extraneous S ~u~po~ . In particular, 99.9 percent of the residual S
un~uol~llL of beam 409 is diverted from cube 414 as beam 415 and shunted from the
system. In the context of the present invention, any beam which is shunted from, or
otherwise removed from the system may be conveniently employed to monitor the
intensity and quality of the beam.
0 The p,~dD",in.l"tly P ~ur~,uul~ of beam 409 is passed through cube 414 and
reflected by respective mirrors 416 and 312, resulting in a substantially pure P polarized
reference beam 418. As discussed in greater detail below, by dividing source beam 402
into object beam 410 and reference beam 418 in the foregoing manner, both the object
beam and reference beam exhibit extremely pure polarization, for example on the order
5 of one part impurity in several thousand. Moreover, a high degree of polarization purity
is obtained regardless of the beam ratio, which is conveniently and precisely controlled
by controlling the voltage applied to variable wave plate 404.
With continued reference to Figures 3 and 4, beam 418 is reflected off mirror 312
and enters beam expander 314. Beam expander 314 preferably comprises a conventional
20 positive lens 421 and a tiny aperture 420. The diameter of beam 418 at the time it enters
beam expander 314 is suitably on the order of approximately 1.5 millimeters (essentially
the same diameter as when it was ~ l,a,~5ed from laser 302). Positive lens 421 is
configured to bring beam 418 to as small a focus as practicable. A suitable positive lens
may comprise ",'~,us~upe objective M-20X manufactured by Newport. Aperture 420
2s suitably comprises a pin-hole aperture, for example a PH-15 aperture manufactured by
Newport. For good quality lasers which emit pure light in the fundamental transverse
rullld~ lic mode (TEMoo), a good quality lens, such as lens 421, can typically focus
beam418downtotheorderofdl.p,uxi,,,dlely10to15micronsindiameter. Atthepoint
of focus, the beam is then passed through aperture 420, which suitably comprises a small
30 pin hole on the order of 15 microns in diameter. Focusing the beam in this manner
effects a Fourier transform of the beam.
More particularly and with reference to Figures 5A-5D, the TEMoo mode of
propagation typically exhibited by a small diameter laser beam follows a Gaussian
-1 3-
WO95/14960 2 ~ 7~ 3 5 8 PCTNS94/13639
distribution transverse to the direction of propagation of the beam. With specific
reference to Figure 5A, this means that the intensity (I) of beam 418 exhibits a Gaussian
distribution over a cross-section of the beam. For a Gaussian beam having a nominal
diameter of one millimeter, a small amount of the beam at very low intensity extends
5 beyond the one millimeter range.
With reference to Figure 5B, a more accurate ~,ul~sellLdlion of the ideal condition
shown in Figure 5A illustrates a substantially Gaussian distribution, but also including the
random high frequency noise inevitably imparted to a beam as it is bounced off mirrors,
polarized, etc. Note that Figure 5B exhibits the same basic Gaussian profile of the
10 theoretical Gaussian distribution of Figure 5A, but further including random high
frequency noise in the beam form ripples.
It is known that the Fourier transform of a Gaussian with noise produces the same
basic Gaussian profile, but with the high frequency noise ~u",,uo"~"I~ shifted out onto the
wings, as shown in Figure 5C. When the Fourier transform of the beam is passed through
an aperture, such as aperture 420 of beam expander 314, the high frequency wings are
clipped, resulting in the extremely clean, noise free Gaussian distribution of Figure 5D.
Quite literally, focusing the beam to dp~lJIUXillldl~ a point source, and thereafter passing
it through an apenure has the effect of shifting the high frequency noise to the outer
bounds of the beam and clipping the noise.
Beam expander 314 thus produces a substantially noise free, Gaussian distributeddivergent reference beam 423.
In a preferred embodiment of the present invention, lens 421 and aperture 420
suitably comprise a single, integral optical ~UIllUUllt:llt, for example a Spatial Filter model
900 manufactured by Newport. Beam expander assembly 314 advantageously includes
a screw thread, such that the distance between lens 421 and aperture 420 may be
precisely controlled, for example on the order of about 5 millimeters, and two onhogonal
set screws to control the horizontal and vertical positions of the aperture relative to the
focus of lens 421.
With continued reference to Figure 3, mirror 312 is suitably configured to direct
beam 423 at film 319 at a ~u~L~",i"ed angle which closely dpplu~dllldI~ Brewster's
angle for the material ~UII~UIi~illg film 319. Those skilled in the an will appreciate that
Brewster's angle is often defined as the arc tangent of the refractive index of the material
upon which the beam is incident (here, film 319~. Typical refractive indices for such films
-14-
WO 95/1~96~ 2 1 7 ~ 3 5 8 PCT/IJS94113639
.
are in the range of au,u,u~i,,,dlely 1-5 plus or minus 0.1. Thus, in aordance with a
preferred ~",bodi",~"t of the invention, mirror 312 is configured such that beam 423
strikes film 319 at a Brewster's angle of a~uu,uxi,,,dl~ly 56 degrees (arc tan 1.5 - 56
degrees). Those skilled in the art will also appreciate that a P polarized beam incident
5 upon a surface at Brewster's angle will exhibit minimum reflection from that surface,
resulting in maximum refraction of reference beam 423 into film 319, thereby facilitating
maximum interference with the object beam and minimum back reflected light whichcould otherwise eventually find its way into the film from an incorrect diredion.
Referring now to Figures 4 and 6-7, object beam 410 is reflected by mirror 320
0 and directed into beam expander 322 which is similar in structure and function to beam
expander 314 described above in conjunction with Figure 4. A substantially noise free,
Gaussian distributed divergent object beam 411 emerges from beam expander 322 and
is collimated by a collimating lens 434, resulting in a collimated object beam 436 having
a diameter in the range of approximately 5 ~:"ti"~t:le~s. Collimating lens 434 suitably
5 comprises a bi-convex opticai glass lens KBX148 manufadured by Newpûrt. Collimated
object beam 436 is applied to imaging assembly 328.
With reference to Figures 7 and 8, imaging assembly 328 suitably comprises a
cathode ray tube (CRT) 444, a light valve 442, a wave plate 463, and a polarizing beam
splitting cube 438. In a preferred embodiment, beam spl itting cube 438 is dl~pl UXill IdLt~ly
20 a 5 centimeter square (2 inch square) cube. As discussed in greater detail below, a beam
460, ~u~ul i,il,g a P polarized beam which incorporates the data from a data slice through
the action of imaging assembly 328, emerges from imaging assembly 328 and is applied
to projection optics assembly 324.
As discussed above, a data set ~u~ll,u~ lg a plurality of two-dimensional images25 corresponding to the three~i",~"siu"al subject of the hologram is prepared for use in
producing the master hologram. The data set may also be maintained in an electronic
data file in a conventional multi-purpose computer (not shown). The computer interfaces
with CRT 444 such that the data slices are lldll~llliLI~d, one after the other, within imaging
assembly 328.
More particularly, a first data slice is projected by CRT 444 onto light valve 442.
As explained in greater detail belûw, the image corresponding to the data slice is applied
to film 319. The reference and object beams are applied to film 319 for a pr~d~Lt:"" ,e i
amount of time sufficient to permit film 319 to capture (record) a fringe pattern associated
-15-
WO 95/14960 2 ~ 7 7 3 5 8 PCTIU594113639
with that data slice and thereby create a hologram of the data slice within the emulsion
~ulllu~ lg film 319. Thereafter, track assembly 334 is moved axially and a subsequent
data slice is projected onto fil m 319 in accordance with the distances between data sl ices;
a subsequent hologram ~u~ ol~ding to the subsequent data slice is thus s~,uel illluosed
5 onto film 319. This process is sequentially repeated for each data slice until the number
of holograms superimposed onto film 319 ~ur~,uoll~ to the number of data slices 14
~u~,uli~illg the particular volumetric data set 16 which is the subjed matter of the master
hologram being produced.
More particularly and with continued reference to Figures 7 and 8, CRT 444
0 suitably comprises a conYentional fiber-optic face-plate CRT, for example, 41397T1
manufactured by the Hughes Aircraft Company of Carlsbad, California. CRT 444 is
configured to project an image corresponding to a particular data slice onto the left hand
side of light valve 442 (Figure 7).
In a preferred embodiment, light valve 442 is a Liquid Crystal Light Valve H416015 manufactured by Hughes Aircraft Company of Carlsbad, California. With specific
reference to Figure 8, light valve 442 preferably comprises a photocathode 454, a mirror
450, having its mirrored surface facing to the right in Figure 8, and a liquid crystal layer
452. Liquid crystal layer 452 comprises a thin, planar volume of liquid crystal which
alters the polarization of the light passing therethrough as a fundion of the localized
20 voltage level of the liquid crystal.
Photocathode 454 comprises a thin, planar volume of a photovoltaic material
which exhibits localized voltage levels as a function of light incident thereon. As the
image corresponding to a particular data slice 14 is applied by CRT 444 onto
photocathode 454, local photovoltaic potentials are formed on the surface of
2~ photocathode 454 in direct correspondence to the light distribution within the cross-
section of the applied image beam. In particular, the beam generated by CRT 444
~u~ ,uùl)di~g to the data slice typically comprises light regions corresponding to bone,
soft tissue, and the like, on a dark background. The dark background areas predictably
exhibit relatively low grey scale values, whereas the lighter regions of the data slice
30 exhibit correspondingly higher grey scale values. A charge distribution corresponding to
the projected image is produced on the surface of photocathode 454.
The static, non-uniform charge distribution on photocathode 454, corresponding
to local brightness variations in the data embodied in a particular data slice 14, passes
-16-
WO 95/14960 2 1 7 7 3 ~ ~ PCT/[JS94/13639
through mirror450 and produces ~u~ ùl~dil lg localized voltage levels across the surface
of liquid crystal layer 452. These localized voltage levels within liquid crystal layer 452
rotate the local liquid crystal in plùpulliu~- to the local voltage level, thereby altering the
pure S polarized light diverted from cube 438 onto mirrored surface 450, into localized
5 regions of polarized light having a P ~u~po~ associated therewith, as the light passes
through crystal layer 452 and is reflected by mirror 450. The emeging beam 460 exhibits
(in cross-section) a distribution of P polarized light in accordance with the voltage
distribution within crystal layer 452 and, hence, in accordance with the image
corresponding to the then current data slice 14.
0 Substantially all (e.g., 99.9%) of the S polarized light ~ulll~JIisillg beam 436 is
diverted by cube 438 onto liquid crystal layer 452. This S polarized light is converted
to P polarized light by liquid crystal layer 452 in accordance with the voltage distribution
on its surface, as described above. The P polarized light is reflected by the mirrored
surface of mirror 450 back into cube 438; the P polarized light passes readily through
cube 438 into projection optics assembly 324.
The S ~c""po"e,~l of the beam reflected off of the mirrored surface of mirror 450
will be diverted 90 degrees by beam splitting cube 438. To prevent this stray S polarized
light from re-entering the system, cube 438 may be tilted slightly so that this S polarized
light is effectively shunted from the system.
The resultant beam 460 exhibits a distribution of P polarized light across its cross-
section which directly corresponds to the data embodied in the data slice currently
projected by CRT 444 onto light valve 442. As a result of the high extinction ratio of
cube 438, beam 460 comprises essentially zero S polarization. Note also that the small
portionofSpolarizedlight~u",~,~i,i"gbeam436which isnotreflectedbycube438into
light valve 442 (namely, a beam 440) may be conveniently shunted from the system.
Beam splitting cube 438 is similar in structure and function to beam splitting cubes
406 and 414, described herein in connection with Figure 4, and preferably comprises a
large broad band polarization beam splitter, for example a PBS-514.5-200 manufactured
by CVI Laser Corporate of Albuquerque, New Mexico. In a preferred embodiment, beam
splitting cube 438 has a cross-section at least as large as the image projected by CRT 444
onto light valve 442, e.g., 2 inches. This is in contrast to beam splitting cubes 406 and
414whichcanadvantageouslybeofsmallercross-section,e.g.,one-halfinch,~u""ud~c,Lle
to the diameter of the unexpanded beam 402 from laser 302.
-17~
WO95/14960 2 1 77358 PCI/US94113639
In the context of the present invention, light which is variously described as
removed, eliminated, or shunted from the system may be disposed of in any number of
convenient ways. For example, the light may be directed into a black box or onto a
black, preferably textured surface. The precise manner in which the light is shunted, or
s the particular location to which the light is shunted is largely a matter of convenience;
what is important is that light which is to be removed from the system be prevented from
striking the film surface of a hologram (for reasons discussed herein), and further that the
light be prevented from reentering the laser source which could disturb or even damage
the laser.
0 Althoughprojectionoptics328illustrativelycompriseslightvalve442,anysuitablemechanism which effectively integrates the image ~u~,uo~ lg to a data slice into the
object beam will work equally well in the context of the present invention. Indeed, light
beam 460, after emerging from cube 438, merely comprises a nonuniform distribution of
P polarized light which varies in intensity according to the distribution of data on the then
5 current data slice 14. The cross-section of beam 46û is substantially identical to a
hypothetical beam of P polarized light passed through a ul,ulu~,d,ul~ic slide of the instant
data sl ice.
Moreover, any suitable ",~.I,d~ l" may be employed in addition to or in lieu of
CRT 444 to project data onto light valve 442. For example, a reflective, transmissive or
20 transflective LCD may be employed, which panel may be selectively energized on a pixel-
by-pixel basis to thereby replicate the data corresponding to each particular data slice.
Alternatively, an dupluplidLe beam, for example a laser beam, may be suitably
Id~ dlllled across the rear surfaoe of light valve 442 to thereby replicate the data
corresponding to each data slice.
2s In yet a further embodiment, although CRT 444 is shown in Figure 7 as abutting
light valve 442, it may be desirable to configure the projection assembly such that CRT
444 is separated from light valve 442. Such a separation may be desirable, for example,
if the diameter of CRT 444 is larger than the diameter of light valve 442 such that the
image projected by CRT 444 is desirably projected onto the rear surface of light valve
30 442, for example, through the use of an dpu~u,ulidl~ lens disposed therebetween.
Moreover, it may also be desirable to employ a fiber optic coupling between light valve
442 and CRT 444, regardless of whether an intervening lens is employed, and further
regardless of the magnitude of the separation therebetween.
-18-
WO95/14960 2 ~ 77~S~ PCT/US94/13639
Moreover, projection optics 328 may be wholly replaced by a suitable spatial light
modulator (SLM; not shown) conveniently mounted in the object beam path. In this way,
the laser light ~u~,u~ lg the object beam would pass through the SLM, with the SLM
imparting to the object beam information corresponding to a particular image. Depending
- 5 on the type of SLM used, such an dl I dl l~ may be employed either with or without
the use of a diffuser between the SLM and film holder 319, as appropriate.
With continued reference to Figures 7 and 8, wave plate 463 is suitably interposed
between light valve 442 and beam splitting cube 438. Wave plate 463 functions tocorrect certain undesirable polarization which light valve 442 inherently produces.
0 More particularly, light valve 442 polarizes the light which passes through liquid
crystal layer 452 in accordance with the local voltage distribution therewithin.Specifically, the applied voltage causes the liquid crystals to rotate, e.g, in an elliptical
manner, the amount of rotation being proportional to the localized voltage level. That
is, a very high voltage produces a large amount of liquid crystal rotation, resulting in a
5 high degree of altercation of the polarization of the light passing through the rotated
crystals. On the other hand, a very low voltage produces a correspondingly small degree
of liquid crystal rotation, resulting in a ~or,~uol~di"gly small amount of altercation in the
level of polarization. However, it has been observed that a very small degree of liquid
crystal rotation (pre-tilt) exists even in the absence of an applied voltage. Thus,
20 d~Jplu,~i",dI~lyonepercentoftheSpolarizedlightpassingthroughliquidcrystallayer452
is converted to P polarized light, even within local regions of liquid crystal layer 452
where no voltage is applied. While this very small degree of spurious polarization does
not generally degrade the p~,~u""a".~ of light valve 442 in most contexts, it can be
p~ublenldLic in the context of the present invention. For example, if one percent of pure
2~ S polarized light is inadvertently converted to P polarized light, the contrast ratio of the
resulting hologram may be substantially limited.
Wave plate 463 is configured to ~o" ,,u~, ISdl~ for the foregoing residual polarization
by, for example, imparting a ~,~d~ "" ,ed polarization to the light passing therethrough,
which is calculated to exactly cancel that amount of polarization induced by liquid crystal
30 layer 452 in the absence of an applied voltage. By eliminating this undesiredpolarization, the effective contrast ratio of the resulting hologram is limited only by the
degree of control achieved in the various process Udldlll~ , as well as the inherent
capabilities of the equipment ~UIIIpli~illg camera assembly 300.
_19_
W095114960 ~ 7 77~ PCTIUS94113639
of the SLM output beam will be subsequently disrupted and repolarized in any event.
Referring now to Figure 6B, an alternative embodiment of the camera assembly
shown in Figure 6A will now be described. In particular, incoming beam 410 is passed
through beam expander 322 and collimating lens 434. Collimated beam 436 is then
5 passed through a liquid crystal display (LCD) SLM 1302, whereupon the image
w"t~i onJ;"g to a data slice is interposed into the collimated beam.
In accordance with the alternate embodiment set forth in Figure 6B, LCD 1302
suitably comprises a transmissive, pixelated LCD, for example, a 640 by 480 pixel screen.
Inasmuch as transmissive LCD 1302 will typically impart a "pretilt" to the light passed
0 through it, it may also be desirable to pass incoming beam 410 through a wave plate
1308 to ~,",i e~ for the pretilt.
The output from LCD 13û2 comprises a collimated object beam having local
variations in the degree of polarization corresponding to the data embodied in the
particular data slice displayed by LCD 1302. As such, it is necessary to convert the
15 variations in polarization within the beam may be conveniently converted into variations
in intensity, for example, through the use of a suitable output polarizer (transducer) 1304.
Because high quality polarizers tend to be quite expensive, the polarization/intensity
conversion may be suitably effected through the use of a smaller transducer 1306interposed within the object beam downstream of projection lens 462 (where the beam
20 has a smaller cross-section).
Commercially available liquid crystal display (LCD) SLM's are typically designedfor use with unpolarized light. Hence, conventional SLM's generally include an input
polarizer such that unpolarized input light is converted to a desired polarization state
before being modulated within the SLM. In addition, ~,"""~,~ially available SLM's
2s typically rotate the polarization at each pixel and include an output polarizer (transducer)
configured to convert variations in polarization into corresponding variations in intensity.
Inasmuch as high-quality polarizers tend to absorb light and are typically quite expensive,
it may be advantageous to employ an SLM in the context of the present invention which
does not include one or both of an input polarizer and an output polarizer indeed, the
30 light entering the SLM in accordance with the preferred embodiments discussed herein
will be typically purely polarized in any event, thus rendering a separate input polarizer
for the SLM unneoessary. Moreover, the output of the SLM in the context of the preferred
embodiments discussed herein will often be manipulated, e.g., by applying it to a
-20-
WO 95/14960 2 1 7 7 3 5 8 PCI`IUS94/13639
.
diffusing screen or the like which may disrupt the polarization state of the output beam,
whereupon the disrupted beam may be subsequently repolarized, as desired. That being
the case, it is unnecessary to pass the beam as it ieaves the SLM through a polarizer
inasmuch as the polarization state of the SLM output beam will be subsequently disrupted
s and repolarized in any event.
By eliminating one or both of the input and output polarizer in the SLM, two levels
of efficiency may be achieved:
(1) laser light is conserved to the extent u~ e~Sdly absorption
of light in the input/output polarizer is eliminated; and
(2) eliminating ~ dly hardware components reduces the
cost of the assembly.
With reference to Figures 6 and 7, projection optics assembly 324 suitably
comprises a projection lens 462, a mirror 464, and an aperture 466. Lens 462 preferably
comprises a telocentric projection lens optimized for specific image sizes used on light
valve 442 and rear projection screen 326. Lens 462 converges collimated beam 46û until
the converging beam, after striking mirror 464, converges to a focal point, whereupon it
thereafter forms a divergent beam 470 which effectively images the data corresponding
to the then current data slice 14 onto projection screen 326 and onto film 319. Beam
470 passes through an aperture 466 at dp~ XirlldL~Iy the point where beam 470 reaches
a focal point. Aperture 466 preferably comprises an iris diaphragm ID-0.5 manufactured
by Newport. Note, however, that aperture 466 is substantially larger than the diameter
of beam 470 at the point where the beam passes through aperture 466. This is in contrast
to the pinhole apertures comprising beam expanders 314 and 322 which function toremove the high frequency ~""pu"~ from the beam. The high frequency components
2~ within beams 460 and 470 are important in the present invention inasmuch as they may
correspond to the data which is the subject of the hologram being produced. Aperture
466 simply traps and shunts scattered light and otherwise misdirected light carried by
beam 470 or otherwise visible to projection screen 326 and which is not related to the
information corresponding to the data on data slice 14.
With continued reference to Figure 6, beam 470 is projected to apply a focused
image onto rear projection screen 326. Screen 326 is suitably on the order of 14 inches
in width by 12 inches in height, and preferably comprises a thin, planar diffusing material
adhered to one surface of a rigid, ~rd~ dle~llL substrate, for example a 0.5 inch thick glass
_~1_
WO 95114960 2 t 7 7 ~ 5 8 PCTIUS94/13639
sheet 472. Diffuser 472 is fabricated from a diffusing material e.g., Lumiglas-130
manufactured by Stewart Filmscreen Corporation of Torrance, California. Diffuser 472
diffuses beam 470 such that each point within beam 470 is visible over the entire surface
area of film 319. For example, an exemplary point Y on beam 470 is diffused by diffuser
5 472 so that the object beam at point Y manifests a conical spread, indicated by cone Y,
onto film 319. Similarly, an arbitrary point X on diffuser 472 casts a diffuse conical
spread X onto film 319. This phenomenon holds true for every point within the projected
image as the image passes through diffuser 472. As a result, every point on film 319
embodies a fringe pattern which encodes the amplitude and phase information for every
point on diffuser 472.
Since light from every point on diffusing diffuser 472 is diffused onto the entire
surface of film 319, it follows that every point on film 319 "sees" each and every point
within the projected image as the projected image appears on diffuser 472. However,
eachpointonfilm319necessarilyseestheentireimage,astheimageappearsondiffuser
472, from a slightly different perspective. For example, an arbitrary point Z on film 319
"sees" every point on diffuser 472. Moreover, an arbitrary point W on film 319 aiso
"sees" every point on diffuser 472, yet from a very different perspective than point Z.
Thus, after emerging from diffuser 472 and polarizer 327, the diffuse image carried by
object beam 473 is applied onto film 319.
Presently known diffusing screens typically comprise a sheet of plastic, glass, or
the like which is either rough or which comprises particles which scatter light. Such
diffusers are w~ dilled by the simple physics of particle scattering, such that very little
control may be exerted over the extent and direction of the random scatterlng.
To increase the efficiency of the diffusing screen, diffusing screen 472 may
25 alternately comprise a holographic optical element (HOE).
More particularly, a hologram may be thought of as a controlled diffuser which
diffuses the incoming uniform reference beam into an output of any desired pattern,
which pattern may be of any desired degree of complexity.
A HOE diffuser may be conveniently constructed by applying diffused laser light
30 to a holographic film substrate. Indeed, a very high-quality conventional plastic diffusing
screen, which is itself extremely inefficient, may be employed to project a diffused pattern
onto the holographic film. That is, the object beam used to create the HOE simply
comprises the output of a high-quality conventional diffuser. By recording a hologram of
-22-
WO 95/14960 2 ~ 7 ~ 3 ~ ~ PCTIUS94113639
this diffused laser light, a very efficient holographic diffusing screen may be produced.
The HOE diffuser, being a hologram, comprises a highly efficient diffuser, which does not
suffer from the inherent limitations of a conventional plastic diffuser, and particularly the
undesirable scattering, absorption, and depolarizing characteristics of the conventional
5 diffuser.
Polarizer 327 is advantageously mounted on the surface of diffusing diffuser 472.
Although the light (beam 470) incident on diffusing diffuser 472 is substantially P-
pold,i,dliù,, diffuser 472, by its very nature, scatters the light passing therethrough,
typicdlly depolarizing some of the light. Polarizer 327, for example a thin, planer,
0 polarizing sheet, repolarizes the light so that it is in a substantially pure P-polarization
state when it reaches film 319. Note that polarizer 327 is disposed after diffuser 472, so
that the light improperly polarized by diffuser 472 is absorbed. This ensures that a high
p~ llLd~t: of the object beam, being substantially P polarized, will interfere with the
reference beam at film 319, further enhancing the contrast of each hologram.
With continued reference to Figure 6A, diffuser 472 may alternatively comprise aholographic optical element constructed in a known manner to i~l"ul~"l~"l the diffusing
function. In yet a further alternative embodiment, an additional lens (not shown) may be
placed adjacent to diffuser 472, for example between diffuser 472 and imaging assembly
328. Through the use of an d,l,)UlUUl idl~ lens, substantially all of the light emerging from
diffuser 472 may be cdused to emerge substantially orthogonally from diffuser 472.
Consequently, the object beam may be caused to strike film substrate 319 in a
substantially parallel manner, i.e., substantially all ~olll,uo~ s of the object beam strike
film substrate 319 substantially o,LIlugolldlly thereto.
Returning briefly to Figure 6B, those skilled in the art will appreciate that liquid
2s crystal devices typically exhibit good contrast on the axis, with increasingly poor contrast
the further off axis the liquid crystal display is viewed from. In the context of the present
invention, it is highly desirable that the composite hologram exhibit high contrast from
quite large angles, both up and down and from left to right. In this way, detailed analysis
of the medical data may be viewed with full parallax from substantially large off-axis
30 angles in all directions. Indeed, high contrast is desirable for viewing angles of up to 30 -
40 degrees off axis and greater from essentially all directions.
In accordance with an alternate embodiment of the present invention, high off-axis
contrast may be achieved at the film surface by using an LCD to sequentially project each
-23-
wog5/l4960 2 1 77~ ~ ~ PCTIUS94/13639
data slice on to the film. However, because of the poor off-axis contrast exhibited by
typical LCD's, it is also desirable to plaoe a diffuser i~""e.lid~ly after the LCD, i.e., at the
downstream surface of the LCD between the LCD and the film substrate. A lens may then
be placed at the downstream surface of the diffusing screen li.e., between the diffusing
5 screen and the film surface). In this way, the high contrast on axis output of the LCD is
scattered (diffused) by the diffuser, with the diffused image captured by the lens and
projected on to the film. Because the image captured by the lens corresponds to the on-
axis image scattered by the diffuser, the poor off-axis charaderistics of the LCD are
overcome, resulting in high off-axis contrast at the film surface.
0 The manner in which the complex object wave front travelling from diffuser 472
to film 319 is encoded within the film, namely in the form of a static interferenoe pattern,
is the essence of holographic reprodudion~ Those skilled in the art will appreciate that
the interferenoe (fringe) pattern encoded within the film is the result of constructive and
destructive interadion between the obied beam and the reference beam. That being the
5 case, it is important that the object beam and reference beam comprise light of the same
wavelength. Although two light beams of different wavelengths may interact, the
interadion will not be constant within a particular plane or thin volume (e.g, the "plane"
of the recording film). Rather, the interadion will be a time-varying function of the two
wavelengths.
The static (time invariant) interaction between the obied and reference beams inaccordance with the pre-sent invention results from the monochromatic nature of the
source of the reference and obied beams (i.e., monochromatic laser sources 302
exhibiting an adequate coherence length). Moreover, those skilled in the art will further
appreciate that maximum interadion occurs between light beams in the same polarization
25 state. Accordingly, maximum interaction between the object and reference beams may
be achieved by ensuring that each beam is purely polarized in the same polarization state
at the surface of film 319. For films mounted in the configuration shown in Figure 6A,
the present inventor has determined that P polarized light produces superior fringe
patterns. Thus, to enhance the interference between objed bedm 470 and referenoe30 beam 423, beam 47 passes through polarizing screen 327 adhered to the surface of
diffuser 472.
The pure P polarized reference beam 423 passes through a collimating iens 316
and is collimated before striking film 319. Inasmuch as the reference and object beams
-2~
WO 95114960 ~ 7~ Pi~/US94113639
both emanate from the same laser 302, and further in view of the relatively longcoherence length of laser 302 relative to the differential path traveled by the beams from
the laser to film 319, the reference and object beams incident on film 319 are mutually
coherent, monochromatic (e.g, 514.5 nm), highly purely P polarized and, henoe, highly
- s correlated. In addition, reference beam 423 is highly ordered, being essentially noise free
and collimated. Object beam 470, on the other hand, is a complicated wave front which
uiiJoldLrr~ the data from the current data slice. These two waves interact extensively
withinthevolumeoftheemulsionru",u,i~i"gfilm319,producingastatic,standingwave
pattern. The standing wave pattern exhibits a high degree of both constructive and
10 destructive i, llel ~el el ,ci . In particular, the energy level E at any particular point within the
volume of the emulsion may be described as follows:
E -- [Ao cos Bo + Ar cos B,]Z
where Ar~ and A, represent the peak amplitude of the ûbject and reference beams,respectively, at a particular point, and Bo and Br represent, the phase of the object and
5 reference beams at that same point. Note that since the cosine of the phase is just as
likely to be positive as negative at any given point, the energy value E at any given point
will range from 0 to 4A2 (ArJ _ A, for a unity beam ratio). This constructive and
destructive wave interference produces well defined fringe patterns.
For each data slice, film 319 will be exposed to the standing wave pattern for a20 iJle~ielellllilled exposure time sufficient to convert that data slice's pro rata share of silver
halide grains.
After film 319 is exposed to the interference pattern corresponding to a particular
data slice, track assembly 334 is moved forward (or, alternatively, backward) by a
uleil~ellll ledamountproportionaltothedistancebetweenthedataslices. Forexample,
25 if a life size hologram is being produoed from CT data, this distance suitably corresponds
exadly to the distance travelled by the subject (e.&, the patient) at the time the data slices
were generated. If a less than or greater than life size hologram is being produced, these
distances are varied dcrLu,dil~,,ly.
In a~uli~dl~e with a preferred ellli~odilllellt of the invention, film 319 suitably
30 comprises HOLOTEST~ holographic film, for example film No. 8E 56HD manufactured
by AGFA, Inc. The film suitably comprises a gelatinous emulsion prepared on the surface
of a plastic substrate. An exemplary film may have a thickness on the order of .015
inches, with an emulsion layer typically on the order of dUUlU~illldLely 6 microns.
-2s-
WO95/14960 2 1 77358 PCTIUS94113639
Incontrasttoconventional,ul~ulu~;,d,ul,y,whereinamplitudeinformationpertaining
to the incident light is recorded within the film emulsion, a hologram contains a record
of both amplitude and phase information. When the hologram is replayed using the same
wavelength of light used to create the hologram, the light emanating from the fi!m
5 continues to propagate just as it did when it was 'frozen' within the film, with its phase
and amplitude information substantially intact. The mechanism by which the amplitude
and phase information is recorded, however, is not widely understood.
As discussed above, the reference beam and object beam, in accordance with the
present invention, are of the same waveiength and polarization state at the surface of film
10 319. The interaction between these two wave fronts creates a standing (static) wave front,
which extends through the thickness of the emulsion. At points within the emulsion
where the object and reference beam constructively interact, a higher energy level is
present than would be present for either beam independently. At points within the
emulsion where the reference and object beam destructively interact, an energy level
5 exists which is less than the energy level exhibited by at least one of the beams.
Moreover, the instantaneou3 amplitude of each beam at the point of interaction is defined
by the product of the peak amplitude of the beam and the cosine of its phase at that
point. Thus, while holographers speak of recording the amplitude and phase information
of a wave, in pradical effect the phase information is 'recorded' by virtue of the fact that
20 the instantaneous amplitude of a wave at a particular point is a function of the phase at
that point. By recording the ill~ldl)tdl~UUs amplitude and phase of the static interference
pattern between the reference and object beams within the three-dimensional emulsion,
a "three~' "~"siu"al picture" of the object as viewed from the plane of film 319 is
recûrded. Since this record contains amplitude and phase information, a three-
25 dimensional image is recreated when the hologram is replayed.
Aher every data slice ~UIIIIJli~ill~ a data set is recorded onto film 319 in theforegoing manner, film 319 is removed from film holder 318 for processing.
Asdiscussedabove7theul,ul~ld,ullicemulsionemployedinthepresentinvention
comprises a large number of silver halide crystals suspended in a gelatinous emulsion.
30 While any suitably photosensitive element may be employed in this context silver
halide crystals are generally on the order of 1,000 times more sensitive to light than other
known phu~us~"~i~ive elements. The resulting short exposure time for silver halide
renders it extremely compatible with holographic applications, wherein spurious
-26-
WO 95114960 2 1 7 7 3 5 8 PCTIUS94/13639
vibrations can severely erode the quality of the holograms. By keeping exposure times
short in duration for a given luer power, the effects of vibration may be minimized.
As also discussed above, a hologram corresponding to each of a plurality of dataslices is sequentially encoded onto film 319. After every slice ~u~ g a particular
- 5 data set has been recorded onto the film, the film is removed from camera assembly 3ûû
for prooessing Before discussing the particular processing steps in detaiL it is helpful to
understand the photographic function of silver halide crystals.
In conventional l~hùlu~d~ly, just as in amplitude holography, a silver halide
crystal which is exposed to a threshold energy level for a threshold exposure time
0 becomes a latent silver halide grain. Upon subsequent immersion in a developer, the
latent silver halide grains are converted to silver crystals. In this regard, it is important
to note that a particular silver halide grain carries only binary data; that is, it is either
converted to a silver crystal or it remains a silver halide grain throughout the process.
Depending on the processing techniques employed, a silver halide grain may ultimately
5 ~u~ Jol~d to a dark region and a silver crystal to a light region, or vice versa. In any
event, a particular silver halide grain is either converted to silver or left intact and, hence,
it is either "on" (logic hi) or "off" (logic low) in the finished product.
In conventional photography as well as in amplitude holography, the exposed filmis immersed in a developing solution (the developer) which converts the latent silver
20 halide grains into silver crystals, but which has a negligible affect on the unexposed silver
halide grains. The developed film is then immersed in a fixer which removes the
unexposed silver halide grains, leaving dear emulsion in the unexposed regions of the
film, and silver crystals in the emulsion in the exposed areas of the film. Those skilled
in the art will appreciate that the converted silver crystals, however, have a black
2s appearance and, henoe, tend to absorb or scatter light, decreasing the efficiency of the
resulting hologram.
In phase holography, on the other hand, the exposed film is bleached to remove
the opaque converted silver, leaving the unexposed silver halide grains intact. Thus, after
bleaching, the film comprises regions of pure gelatinous emulsion comprising neither
30 silver nor silver halide (corresponding to the exposed regions), and a gelatinous emulsion
U~ lg silver halide (~oll~ u~ding to the unexposed regions). Phase holography is
predicated on, inter alia, the fact that the gelatin containing silver has a very different
-27-
WO 95/14960 2 1 7 7 ~ ~ 8 PCTlllS94/13639
refractive index than the pure gelatin and, hence, will diffract light passing therethrough
in a correspondingly different manner.
The resulting bleached film thus exhibits fringe patterns ~u~ g alternating
lines of high and low refractive indices. However, neither material comprises opaque
5 silver crystals, so that a substantially insignificant amount of the light used to replay the
hologram is absorbed by the hologram, as opposed to amplitude holographic techniques
wherein the opaque silver crystals absorb or scatter a substantial amount of the light.
More particularly, the present invention contemplates a six-stage processing
scheme, for example, performed on a Hope RA201 6V photoprocessor manufactured by10 Hope Industries of Willow Grove, Pennsylvania.
In stage 1, the film is developed in an aqueous developer to convert the latent
silver halide grains to silver crystals, which may be made by mixing, in an aqueous
solution (e.g., 1800 ml) of distilled water, ascorbic acid (e.g., 30.0 g), sodium carbonate
(e.g., 40.0 g), sodium hydroxide (e.g., 12.0 g), sodium bromide (e.g., 1.9 g), phenidone
1s (e.g., 0.6 g), and distilled water resulting in a 2-liter developing solution.
In stage 2, the film is washed to halt the development process of stage 1.
Stage 3 involves immersing the film in an 8 liter bleach solution ~ul~ illg
distilled water (e.g., 72û0.0 ml), sodium dichromate (e.&, 19.0 g), and sulfuric acid (e.g.,
24.0 ml). Stage 3 removes the developed silver crystals from the emulsion.
Stage 4 involves washing the film to remove the stage 3 bleach.
Stage 5 involves immersing the film in a 1 liter stabilizing solution ~ g
distilled water (50.0 ml), potassium iodide (2.5 g), and Kodak PHOTO-FLO (5.0 ml). The
stabilizing stage .i~",i~ the remaining silver halide grains to enhance long-term
stability against subsequent exposure.
2s In stage 6, the film is dried in a conventional hot-air drying stage. Stage 6 is
suitably performed at 100 degrees fahrenheit; stages 1 and 3 are performed at 86 degrees
rdl.,~"l":i~; and the remaining stages may be performed at ambient temperature.
Upon completion of the processing of film 319, the resulting master hologram maybe used to create one or more copies.
In accordance with one aspect of the invention, it may be desirable to produce acopy of the master hologram and to replay the copy when observing the hologram, rather
than to replay and observe the master hologram directly. With reference to Figure 10,
Figure 1 OA depicts a collimated replay beam PB replaying a master hologram, with beam
-28
WO95/14960 2 1 77~58 PCT/US94/13639
PB being directed at the film from the same direction as the collimated reference beam
used to create the hologram (H1). This is reflected to as orthoscopic reconstruction.
This is consistent with the layout in Figure 3, wherein the data slice corresponding to
respective images 1002 in Figure 10, were also illuminated onto the film from the same
5 side of the film as the reference beam. However, when observed by an observer 1004,
the reconstructed images appear to be on the opposite side of the film from the observer.
Although the reconstructed images 1002 are not literally behind hologram H1, they
appear to be so just in the same way an object viewed when facing a mirror appears to
be behind the mirror.
0 With momentary reference to Figure 10B, hologram H1 is inverted and again
replayed with the replay beam PB. In this configuration, known as r~P~ os~oric
construction, the images 1002 appear to the observer as being between the observer and
the film being replayed. When master hologram H1 is copied using copy assembly 900,
the rSPl Idr~SfpiC construction set forth in Figure I OB is essentially reconstructed, wherein
5 the master hologram is shown as H1, and a holographic film corresponding to the copy
hologram is positioned within the images 1002 in a plane P. The assembly shown in
Figure 10B illustrates the copy film (plane P~ as being centered within the images 1002,
thereby yielding a copy hologram which, when replayed, would appear to have half of
the three-dimensional image projecting forward from the film and half the three-20 dimensional image projected back behind the film. However, in (~ UlddllU~: with an
alternate embodiment of the present invention, the copy assembly may be configured
such that plane P assumes any desired position with respect to the data set, such that any
~u~ Jull~i~lg portion of the three~ siv"al image may extend out from or into theplane in which the film is mounted.
25 COPY A~SEMBLY
Referring now to Figure 9, copy assembly 900 is suitably mounted to a table 904
in much the same way camera assembly 3 is mounted to table 304 as described in
conjunction with Figure 3. Copy assembly 900 suitably comprises a laser source 824,
respective mirrors 810, 812, 820, and 850, a beam splitting cube 818, a wave plate 816,
30 respective beam expanders 813 and 821, respective collimating lenses 830 and 832, a
master film holder 834 having respective legs 836A and 836B, and a copy film holder
838 having a front surface 840 configured to securely hold copy film substrate H2 in
place.
-29-
WO 95/14960 2 1 7 7 3 ~ 8 PCT/US94113639
Film holder 838 and, if desired, respective film holders 834 and 318 are suitably
equipped with vacuum equipment, for example, vacuum line 842, for drawing a vacuum
between the film and the film holder to thereby securely hold the film in place. By
ensuring intimate contact between the film and the holder, the effects of vibration and
5 other spurious fi Im movements which can adversely impact the interferenoe fri nge patterns
recorded therein may be substantially reduced.
Film holders 838 and 318 desirably comprise an opaque, non-reflective (e.g.,
black) surface to minimize unwanted reflected light therefrom. Film holder 834, on the
other hand, necessarily comprises a Lldl~LJdl ~ surface inasmuch as the obiect beam must
0 pass therethrough on its way to film holder 838. Accordingly, the opaque film holders,
may, if desired, comprise a vacuum surface so that the film held thereby is securely
vacuum-secured across the entire vacuum surface. Film holder 834, on the other hand,
being ~d"~Lart:"l, suitably comprises a perimeter channel wherein the corresponding
perimeter of the film held thereby is retained in the holder by a perimeter vacuum
s channel. A glass or other Lld~ Jdl~ t surfaoe may be conveniently disposed within the
perimeter of the channel and a roller employed to remove any air which may be trapped
between the film and the glass surface.
Although a preferred embodiment of the present invention employs the foregoing
vacuum film holding techniques, any mechanism for securely holding the film may be
20 conveniently used in the context of the present invention, including the use of an
electrostatic film holder; a pair of opposing glass plates wherein the film is lightly
sandwiched therebetween; the use of a suitable mechanism for gripping the perimeter of
thefilmandmaintainingsurfaoetensionthereacross;ortheuseofanairtightcellwherein
~u~LJIt:~ed air may be IlldillLdill~l within all to securely hold the film against one surface
25 of the air tight chamber, the chamber further including a bleed hole, disposed on the
surface of the cell against which the film is held, from which the ~r"LJ,t ~ d air may
escape.
With continued reference to Figure 9, laser source 824 is sul-tably similar to laser
302, and suitably produces laser light of the same wavelength as that used to create the
30 master hologram (e.g., 514.5 nm). Alternatively, a laser source for producing the copy
may employ a different, yet ,~ lll;"ed, wavelength of light, provided the angle that
the referenoe beam illuminates film H1 is varied in accordance with such wavelength.
Those skilled in the art will appreciate that the wavelength (~1) of the reference beam
-30-
WO 95/14960 2 1 7 7 3 ~ 8 PCTIUS94/13639
illuminating hologram H1 is p~ nliol)dl to the sine of its incident angle, e&"l ~
K sin ~. Moreover, by manipulating the processing pdlcllll~ to either shrink or swell
the emulsion, the relationship between the wavelength and the incident angle can be
further adjusted in accordance with the relationship between the incident angle and a
- 5 reference beam wavelength.
Asourcebeam825fromlaser824isreflectedoffmirror812throughawaveplate
816 and into cube 818. Variabie wave plate 816 and cube 818 function analogously to
beam splitting assembly 310 discussed above in conjunction with Figure 3. Indeed, in
a preferred e",bodi~"e,~ of the present invention, a beam splitting assembly nearly
0 identical to beam splitter 310 is used in copy system 900 in lieu of wave plate 816 and
cube 818; however, for the sake of clarity, the beam splitting apparatus is schematically
:p~s~t~ as cube 18 and wave plate 816 in Figure 9.
Beam splitting cube 818 splits source beam 825 into an S polarized object beam
806 and a P polarized reference beam 852. Object beam 806 passes through a wave
plate 814 which converts beam 806 to a P polarized beam, which then passes through
a beam expanding assembly 813 including a pin-hole (not shown); reference beam 852
passes through a similar beam expander 821. Respective beam expanding assemblies 813
and 821 are similar in structure and function to beam expanding assembly 314 discussed
above in conjunction with Figure 3.
Object beam 806 emerges from beam expander 813 as a divergent beam which
is reflected off mirror 850 and collimated by lens 832. Reference beam 852 is reflected
off mirror 820 and collimated by lens 830. Note that virtual beams 802 and 856 do not
exist in reality, but are merely illustrated in Figure 9 to indicate the apparent source of the
object and reference beams, respectively. Note also that object beam 806 and reference
25 beam 852 are both pure P polarized.
The master hologram produced by camera assembly 300 and discussed above is
mounted in a Lldn~a~ film holder 834 and referred to in Figure 9 as H1. A secondfilm H2, suitably identical in structure to film substrate 319 prior to exposure, is placed
in film holder 838. Object beam 806 is cast onto master hologram H1 at the Brewster's
30 angle associated with film H1 ~d,u,l~lo~d~dl~ly 56).
Film substrate H2 records the standing wave pattern produced by object beam 806
and reference beam 852 is the same manner as described above in connection with film
319 in the context of Figures 3 and 4. More particularly, the plurality of images
-31-
WO 9S/14960 2 ~ 7 7 3 ~ 8 PCT/US94/13639
~ulle~,uO~ g to each data slice within a data set are simultaneously recorded onto film
H2. The amplitude and phase information ~ulle~,uu~ ; to each data slice is accurately
recorded on film H2 as that amplitude and phase information exists within the plane
defined by film H2. When copy hologram H2 is subsequently replayed, as discussed in
5 greater detail below, the image ~ulle~JOI~Jillg to each data slice, with its amplitude and
phase ill~lllldLiull intact, accurately recreates the three-dimensional physical system
defined by the data set.
It will be duple~idled that large collimating lenses such as refe~ence beam
collimating lens 316 (Figure 3) are quite expensive. Although it is desirable in aordance
0 with the illustrated ellluC ' "~"L to employ a collimated reference beam and a collimated
object beam, one or both of the reference and object beam collimating lenses may be
dispensed with in the context of an alternating embodiment.
More particularly, a divergent reference and/or object beam may be employed as
opposed to two collimated reference and object beams in accordance with an alternate
embodiment of the present invention. It is generally known, however, that the use of
such divergent beams may result in distorted images at the film plane. However, the
nature and extent of these distortions may be fairly precisely modeled and quantified.
See, for example, the discussion by Edwin Champagne in the January, 1967 issue of the
Journal of Eledrical Society of America.
Specifically, by calculating the manner in which the use of one or more divergent
beams will distort the image at the fi!m plane, the date embodied in a data slice may be
manipulated mathematically to ~ulllluellsdle for this distortion. In this way, a properly
recûnstructed image may be obtained at the film surface notwithstanding the use of non-
collimated reference and/or object beams.
With continued reference to Figure 9~ the present inventor has deLe~ l leu that the
emulsion .u",~,i,i"g the film within which holograms are made in accordance with the
present invention may undergo subtle volumetric changes during processing. In
particular, the emulsiûn may shrink or expand on the order of 1% or more, depending
upon the particular chemistry involved in processing the substrate.
Although such shrinkage or expansion has a relatively minimal effect on a masterhologram, this effect may be eXd~;el dLed in the conte~a of a copy hologram. Specifically,
a 1% shrinkage in a typical hologram on the order of, for example, 10 ~ellLillleLel~ may
be il"~Je,~e~Lible to the observer; however, when the master hologram (H1) is copied
-32-
WO95/14960 2 ~ 7735~ PCTIUS94113639
onto a copy hologram (H2), a 1% change in master hologram H1 may manifest itself as
a 1% change in the distance between master hologram holder 834 and copy hologramholder 838, which distance is generally far greater than the actual size of the hologram.
Indeed,fora141/2inchseparationbetweenmasterfilmholder834andcopyfilmholder
5 838, a 1% shrinkage in the substrate ~uil~,ulisillg hologram H1 may result in the copy
hologram being displaced from the film plane on the order of 5 millimeters.
To correct for such shrinkage/expansion and thereby ensure that copy hologram
holder 838 H2 closely corresponds to the film plane of the hologram, the distance
between master hologram holder 834 and copy hologram holder 838 may be suitably
0 manipulated. In particular, if the emulsion ~ù",,~,,isi"~ master hologram H1 shrinks by,
for example, 1%, the distance between master hologram holder 834 and copy hologram
holder 838 may be suitably decreased by ap,ulu~i"~d~ 1%. Similarly, to the extent the
emulsion ~u",,ur;~i"g the master hologram expands during processing, the foregoing
distance may be correspondingly increased.
Moreover, the distance between master hologram holder 834 and copy hologram
holder 838 may also be manipulated such that copy hologram holder 838 cuts through
any desired position in the hologram. In particular, while it is often desirable for the copy
hologram to straddle the film plane, i.e., for dpu,u~ill,d~ly one-half of the holographic
image to be projected in front of the viewing screen and one-half of the hûlogram to be
20 projected behind the film screen, by manipulating the distance between the master
hologram holder and the copy hologram holder any desired portion of the hologram may
be positioned in front of or behind the film plane, as desired.
In the preferred embod iment discussed herein, master holograms H 1 are producedon a camera assembly 300, and copy holograms H2 are produced on a copy assembly
25 900. In an alternate embodiment of the present invention, these two systems may be
conveniently combined as desired. For example, film holder 318 in Figure 3 may be
replaced with film holder 834 from Figure 9, with a subsequent H2 film holder disposed
such that the object beam is l~dn~",.ll~i through film holder 834 onto the new H2 film
holder. In this way, the relationship between film holders H1 and H2 (Figure g) would
30 be substantially replicated in the hybrid system. To complete the assembly, an additional
reference beam is confined to strike the new H2 film holder at Brewster's angle. As
altered in the foregoing manner, the system can effectively produce master holograms and
copies on the same rig. More particularly, the master hologram is produced in the
-33-
WO9S/14960 2177~58 PCT/US94/13639
manner described in conjunction with Figure 3 and, rather than utilizing a separate copy
rig, the master hologram may simply be removed from its film holder, inverted, and
utilized to create a copy hologram. Of course, the original object. beam would be
shunted, and replaced by a newly added reference beam configured to illuminate newly
5 added film holder H2.
In yet a further ~ "l,~ ' "~"L of the present invention, which master holograms may
be produced substantially in a.~u,dd,~ with the foregoing discussion copy holograms
may be suitably produced through a method known as contact copying. Specifically, a
master hologram (H1) may be placed in intimate contact with a suitable sheet of film and
0 a reference beam applied thereto, as is known in the context of producing copies of
conventional holograms.
As discussed above, a master hologram (H1) produced in accordance with the
illustrated embodiment results in a non-image plane hologram; the copy assembly
described herein may thus be employed to generate an image planed hologram (H2) from
the master hologram. Alternatively, various apparatus and techniques may be employed
to generate an image planed hologram in a single step.
More particularly and with momentary reference to Figure 3, in the preferred
embodiment discussed above, the images corresponding to the slices are projected from
screen 472 on to film holder 319 at a variable distance in the range of, e.g., 14 inches.
20 Alternatively, the projedion assembly may be brought very close to the film surface, such
that some of the data slices (e.g., half of them) are projected on to one side of the film
and, after turning the film over (and rotating it by 180 degrees) and projecting the
remaining slices on to the other side of the film. In this way, an image planed hologram
may be theoretically produced. However, it becomes difficult to apply a reference beam
2s at a desired reference angle (e.&, Brewster's angle) to the film surface, in view of the
close proximity of the projedion assembly to the film plane.
Referring now to Figure 12, a diffusing screen 1202 may be advantageously
disposed very close to Film 319, such that the object beam 1204 applied to the input
service of diffusing screen 1202, is diffused onto the film. In accordance with the
30 ~",L,odi",e"lshowninFigure 12~diffuserl2o2issuitablyd~iso~,upi~thatis~screen12o2
fundions as a diffuser when object beam 1204 is applied to it, whereas screen 1202
permits reference beam 1206 to pass through it in a substantially ~Idl~,Jdlt~ manner. In
this regard, such an angle defective diffuser may be conveniently fabricated as a
-34-
WO95/14960 2 ~ 7735~ PCIIUS94113639
holographic optical element, such that it functions as diffuser for the object beam, yet acts
like a lld~ dl~ window with respect to the reference beam.
By properly configuring and positioning such a lens, the image on the projectionscreen (i.e., diffuser 47~) may be focused on to film 319. By moving the projection
5 assembly along track assembly 334 to thereby vary the distanoe between the diffusing
screen/lens assembly and the film plane, the relative positions of the data slices within a
data set may be preserved. However, the image for a particular slice will not necessarily
be focused at the plane of the film substrate; rather, the image for each slice will be
focused at a point in front of or behind the film substrate in accordance with the relative
0 position of the particular data slice in the data set. However, sinoe the film substrate will
capture the phase as well as amplitude information of each slice, a properly positioned
and properly focused image will be produced for each data slice upon replay of the
finished hologram. Moreover, by properly configuring the d~u,~"l~"lioned lens, an image
planed hologram may be produced in a single step.
In accordance with a further alternative ~ Ludilllellt, to reduce the size, weight,
and cost of such a projection lens, a HOE lens may be employed.
More particularly, such a HOE lens may be made by creating a hologram of a
point source of light, for example a spherically irradiating point source. When the HOE
lens is replayed, the output of the HOE lens will converge to the point source, effectively
20 focusing light from a parallel beam to a point. As such, the HOE lens functions in a
manner optically equivalent to a conventional glass lens.
As also discussed above, the present invention contemplates, for a data set
comprising N slices, recording N individual, relatively weak holograms onto a single film
substrate. To a first d~uplukilllaLiul~ each of the N slices will consume (convert)
25 a,u~nukillldl~ly 1/N of the available silver halide grains consumed during exposure.
As a starting point, the total quantity of photosensitive elements within a filmsubstrate may be inferred by sequentially exposing the film, in a conventional
photographic manner, to a known intensity of light and graphing the extent to which
silver halide grains are converted to silver grains as a function of applied energy (intensity
30 multiplied by time). At various time intervals, the extent to which the film is fogged, i.e.,
the extent to which silver halide grains are converted to silver grains, is measured by
simply exposing the film to a beam of known intensity, developing the film, and
measuring the amount of light which passes through the film as a function of incident
-35-
WO 95/14960 2 1 7 7 ~ 5 8 PCT/I~S94113639
light. Although typical HD curves are nonlinear, they may nonetheless be used in the
context of the present invention to ascertain various levels of fog as a fundion of applied
energy.
In accordance with the present invention, the HD curve for a particular film
5 (generally supplied by the film manufadurer) may be used to determine the amount of
light, expressed in microjoules per square cm, necessary to prefog the film to apredetermined level, for example, to 10/0 of the film's total fog capacity as determined
by the HD curve. After prefogging the film to a known level, a very faint, plane grating
hologram is recorded onto the film, and the diffradion efficiency of the grating measured.
0 Thereafter, a different piece of film from the same lot of film is prefogged to a higher
level, for example to 20% of its total fog capacity based on its HD curve, and the same
faint hologram superimposed on the fogged film. The diffradion efficiency of the faint
hologram is again measured, and the process repeated for various fog levels. Thee
diffraction efficiency of the grating for each fog level should be essentially a function of
15 the pre-fog level, inasmuch as the prefogging is wholly random and does not produce
fringe patterns of any kind.
A particular film lot may be conveniently characteri~ed in terms of its multipleexposure holographic exposure capacity. For a data set ~u"",,i~i"g N slices, the film's
total exposure capacity may be conveniently divided into N equal amounts, such that
20 each data slice may consume 1/N of the film's total exposure capacity. Recalling that the
energy for a particular slice is equal to the product of the intensity of the incident light
and time of exposure, and further recalling that the intensity of the incident light (e.g.,
object beam~ is determined for eaçh slice in the manner described below in connedion
with the beam ratio determination, the time of exposure for every slice may be
25 conveniently d~L~" "i"ed.
In a.~u,dd".~ with a further aspect of the present invention, each lot of film may
be conveniently coded with data ~u~ Jondi~g to its total exposure capacity and/or
dl diffraction effficiency. Analogously, most conventional 35 mm film is
encoded with certain information regarding the film, for example, data relating to the
30 exposure ~l,a,d~L~ Lics of the film. In a similar way, the information pertaining to the
film's diffraction efficiency curve may be conveniently appended to each piece of
holographic film for use in the present invention, for example by applying it to the film
or to the packaging therefor. The computer (not shown) used to control camera assembly
-36-
WO 95J14960 2 1 773~ PCT/US94/13639
300 may be conveniently configured to read the data imprinted on the film, and may
thereafter use this data to compute the exposure time for each data slice in the manner
described herein.
As stated above, the relative intensities of the reference beam to the object beam
at the film plane is known as the beam ratio. Known holographic techniques tend to
define beam ratio without reference to a polarization state; however, an alternate
definition of the term, particularly in the context of some aspects of the present invention,
surround outside the relative intensities of the reference and object beams (at the film
plane) at a particular common polarization state, i.e., either a common P polarization state
0 ora common S portion state. Moreover, beam intensity, for purposes of determining
beam ratio, may alternatively be defined in terms of any other desired ~I,dld~ c or
quality of a beam, for example by monitoring the mode of a beam through the use of a
mode detector, or by monitoring beam uniformity, i.e., the amplitude of the beam a cross-
section of the beam.
The intensity of a beam may be suitably detected at the film surface through theuse of a photo-diode. In accordance with one aspect o:f the present invention, one or
more photo-diodes may be suitably embedded in a convenient location within the
hardware ~o,~ ,g camera system 300, for example, as part of film holder 319 In this
regard, such a photo-diode may be embedded on the perimeter of the film holder (to the
20 side of the film) or within the film holder itself, behind the transparent film Alternatively,
one or more photo-diodes may be suitably disposed on arms or similar lever mechanisms
which may selectively inserted into and removed from the beam path, as desired.
For purposes of understanding the role of beam ratio in the present invention, it
is helpful to point out that holography may be conveniently divided into display25 holography, in which the hologram is intended to show a three~ "siol~al image of a
selected object, and Holographic Optical Elements (HOE) in which a basic holographic
fringe pattern is recorded on a film which thereafter functions as an optical element
having well-defined properties, for example, as a lens, mirror, prism, or the like
HOEs are formed with simple directional beams leading to simple repetitive fringe
30 patterns which tend to dominate weak secondary fringes which are also formed by
scattered and reflected light within the emulsion Since the secondary fringe patterns are
typically ignored to the first approximation, conventional holographic theory states that
-37-
wo95/14960 2 1 77 ~58 PCT/US9~/13639
to achieve the strongest interference between the two beams, a beam ratio of one should
be employed.
In display holography, on the other hand, while the reference beam is still a simple
directional beam, the object beam can be extremely complex, having intensity and5 direction variations imposed by the object. In addition, objects typically exhibit any
number of bright spots which diffuse light at fairly high intensities.
The resulting fringe pattern is extremely complex, bearing no simple relationship
to the object being recorded. Moreover, the bright spots (highlights) on the object act
as secondary reference beams, producing unwanted fringe patterns as they interfere
0 with the reference beam and with each other, resulting in many sets of noise fringes,
effectively reducing the relative strength of the primary fringe pattern. The resulting
"intermodulation" noise (also referred to as self-r~r~ "~i"g noise) causes an unacceptable
loss of image quality unless it is suppressed.
Conventional holographic theory states that intermodulation noise may be
5 suppressed by increasing the relative strength of the reference beam, with respect to the
object beam, by selecting a beam ratio in the range of 3 to 30, and most typically
between 5 and 8. This results in strong primary fringes and greatly reduced secondary
fringes (intermodulation noise). Thus, existing holographic techniques suggest that, in the
context of display holography, a beam ratio higher than unity and preferably in the range
20 of 5-8:1 substantially reduces intermodulation noise.
The diffraction effici-ency oFa hologram, i.e., how bright the hologram appears to
an observer, also exhibits a maximum at a beam ratio of one. At beam ratios higher than
one, the diffraction efficiency falls off, resulting in less bright holograms when replayed.
The conventional wisdom in existing holographic theory, however, states that since
2s intermodulation noise falls offfaster than diffraction efficiency as the beam ratio increases,
a beam ratio of between 5-8:1 minimizes intermodulation noise (i.e., yields a high signal
to noise ratio) while at the same time producing holograms exhibiting reasonablediffradion efficiency.
In the context of the present invention, a very low reference-to-object beam ratio,
30 for example on the order of 3:1 and particularly on the order of unity, is desirably
employed, resulting in optimum (e.g., maximum) diffraction efficiency for each hologram
associated with every data slice in a particular data set. In the context of the present
invention, however, intermodulation noise (theoretically maximum at unity beam ratio)
-38-
W0 95/14960 2 ~ ~ ~ 3 5 8 PCTIUS94113639
does not pose a significant problem as compared to conventional display holography
More particularly, recall that intermodulation noise in conventional l1oluE;Idplly results
from, inter alia, bright spots associated with the objects. In the present invention, the
"objects" ~u.,~,uu"d to a two~lil.., ,.siu"al, windowed, gamma-corrected (discussed
5 below) data slice. Thus, the very nature of the data employed in the context of the
present invention results in inherently low intermodulation noise, thus permitting the use
of a unity beam ratio and permitting maximum diffraction efficiency and very high signal
to noise ratio images.
Moreover, the selection of a near-unity or unity beam ratio for each slice in a data
0 set may be accomplished quickly and efficiently in the context of a preferred embodiment
of the present invention.
More particularly, variable wave plate 404 may be calibrated by placing a photo-diode in the path of the reference beam near film 319 while shunting the object beam,
and vice versa. As the applied voltage to wave plate 4û4 is ramped up at p,~:d~le.".i,.ed
5 i.~ , from ~ero to a maximum value, the intensity of the reference beam may be""i,.ed as a function of input voltage. Since the intensity of the reference beam, plus
the intensity of the object beam (before a data slice is in~uruu. dl~d into the object beam),
is a,up,uAi,,,d~ly equal to the intensity of their common source beam and the intensity of
the common source beam is readily a~u:,LdillaL,lc, the pure object beam intensity as a
20 Function of voltage applied to wave plate 4û4 may also be conveniently derived. It
remains to determine the proper input voltage to wave plate 404 to arrive at a unity beam
ratio for a particular slice.
At a f~.,dd",el~tdl level, each data slice comprises a known number of "pixels"
(although not literally so after having passed through imaging assembly 328), each pixel
25 having a known grey level value. Thus, each data slice may be assigned a brightness
value, for example, as a percent of pure white. Thus, the particular voltage level required
to obtain a unity beam ratio for a particular data slice having a known brightness value
may be conveniently determined by selecting the unique voltage value corresponding to
a pure object beam intensity value which, when multiplied by the brightness value, is
30 equal to the reference beam intensity value for the same voltage level. This computation
may be quickly and efficiently carried out by a conventional computer programmed in
accordance with the relationships set forth herein.
-39-
WO95/14960 2 1 77358 PCT/US94/13639
Accordingly, each data slice has associated therewith a voltage value
~u~ olldillg to the input voltage to wave plate 404 required to achieve a unity beam
ratiû~
With momentary reference to Figure 6A, as the diffusing screen is placed further5 frûm the film substrate, the object beam intensity at the film surface becomes more
uniform. Conversely, as the diffusing screen gets closer and closer to the film surface, the
object beam at the film surface becomes less uniform, i.e., localized regions of high
intensity and low intensity may be observed as a function ûf the particular data
~urlluli~illgtheobjectbeam~notwi~ ldl~dil)gthepresenoeofevenlyrelativelyhigh-quality
10 diffusers.
In order to enhance control of the beam ratio at the film surface, it may be
desirable to modulate the reference beam intensity (amplitude) distribution to more
closely correspond to the obied beam intensity distribution at the film surface. Enhanced
control of the beam ratio at the film surface is particularly advantageous when producing
15 a copy (H2), but may also be helpful to a lesser extend in the context of the master (H1)
hologram.
At a first level of d,UUlU~illld~iOn, the intensity distribution of the referenoe beam
over a cross-section of the reference beam may be modeled as a Gaussian distribution
(see, e.g., Figure 5). Thus, in accordance with one ~",uuuilll~ of the present invention,
20 the reference beam at the film surface may exhibit an essentially Gaussian intensity
distribution, such that a different beam ratio will be observed near the center of the film
than may be observed at the outer edges of the film.
Thismaybecorrectedtoafirst-levelofapproximationbyin...,~uldli,,gafilter(not
shown) into the reference beam, which filter is configured to flatten out the Gaussian
25 intensity distribution within a cross-section of the reference beam. In particular, such a
filter may be configured to minimally suppress (e.g, absorb, scatter, or redirect) the beam
near the outer edges of the film, while more substantially suppressing the beam near the
center of the film. In this way, a substantially uniform reference beam intensity
distribution may be obtained at the film surface, thereby resulting in a more uniform beam
30 ratio at the film surface.
In d~Ul~d~ with an alternate embodiment, the reference beam intensity
distribution may be modulated through the use of a SLM or similar device interposed into
the reference beam. The intensity distribution within the object beam at the film surface
~o-
WO 95/14960 2 ~ 7 7 3 5-8 PCT/US94/13639
.
may also be measured, inferred, or calculated in any convenient manner, for example,
through the use of a video camera or otherwise photo-voltaically or photo-optically
measuring the object beam brightness level at various points on the film surface. Having
d,.e~ i ~ed the intensity distribution in the object beam at the film surface, this
5 illtUIIIIdtiOII may be fed back into the SLM in the reference beam, such as the SLM
modulates the reference beam in a-~,,Jd"-~ with the intensity distribution of the object
beam at the film plane. This permits substantially improved control over the localized
beam ratio across the film surface.
Alternatively, the reference beam projection optics may be configured to expand
10 the cross-section of the beam and to clip the relatively low intensity perimeter of the
beam, for example by electronically, optically, or mechanically masking the outer edges
of the beam, leaving the expanded higher intensity portion of the middle of the beam
intact.
In a further alternative embodiment, a SLM, LCD, or similar functional device may
5 be interposed in the reference beam, and configured to ~,"I~ dl~ for the Gaussian or
other reference beam intensity distribution, for example by making the LCD dark in the
middle and lighter at the edges, in radial fashion, to thereby flatten the intensity
distribution of the reference beam. In this way, the SLM may be configured to implement
the function of an apedizing filter. As a further alternative, a glass filter which is darker
20 in the middle than at the edges may be interposed in the reference beam, either alone or
in combination with a SLM for controlling the intensity distribution of the reference beam.
In a further alternativeembodiment, the intensity distribution of the reference beam
may be manipulated optically, for example, through the use of a lens or series of lenses
to redirect portions of the reference beam to achieve a substantially uniform cross-
25 sectional intensity distribution.
Inyetafurtheralternatee",L,o~i",t"l,theintensitydistributionoftheobjectbeam
atthefilmsurfacemaybecalculatedbasedonthevariousphysicalandopticall.d,~,",~associated with the hologram camera and/or copy assembly.
More particularly, for a given data slice applied to diffusing Screen 472, the
30 intensity distribution at the input of Screen 472 may be derived as a function of the data
on the slice and the optics employed to project the image onto Screen 472. In
conjunction with, inter alia, the known optical properties of diffusion Screen 472, the
distance between the diffuser and film plane, and the optical properties of any polarizers
Il -
WO 95/14960 2 1 7 7 3 5 8 PCT/lJS94113639
or other hardware employed in the projection optics, the intensity distribution at the film
plain may be conveniently computed, at least to a l~d~UlldLI~ dU,UlU~CillldliUII.
In accordance with another aspect of the present invention, each data slice
~u""u,i~i"g a data set may be ~urther prepared subsequent to the windowing procedures
s set forth above. In particular, imaging assembly 328 generates an image ~ur"uli,illg
various brightness levels (grey levels) in accordance with data values applied to CRT 444.
However, it is known that conventional CRTs and conventional light valves do notnecessarily project images having brightness levels which linearly correspond to the data
driving the image. Moreover, human perception of grey levels is not necessarily linear.
10 For example, while a image having an arbitrary brightness value of 100 may look twice
as bright as an image having a brightness value of 50, an image may require a brightness
level of 200 to appear twice as bright as the image having a brightness value of 100.
Because human visual systems generally perceive brightness as an exponential
function, and CRTs and hot valves produce images having brightness which are neither
5 linearly nor exponentially related to the levels of the data driving the images, it is
desirable to perform a gamma correction on the data slices after they have been
windowed, i.e., after they have been adjusted at a gross level for brightness and contrast
levels. e,y gamma correcting the windowed data, the grey levels actually observed are
evenly distributed in terms of their perceptual differences.
In accordance with a preferred embodiment of the present invention, a gamma
lookup table is created by displaying a series of ,u~d~L~ ;"ed grey level values with
imaging assembly 328. A photo-diode (not shown) is suitably placed in the path of the
output of imaging assembly 328 to measure the actual brightness level corresponding to
a known data value. A series of measurements are then taken for different brightness
2s levels corresponding to different grey level data values, and a gamma lookup table is
constructed for the range of grey values exhibited by a particular data set. Depending on
the degree of precision desired, any number of grey level values may be measured with
the photo-diode, allowing for computer interpolation of brightness ievels for grey values
which are not measured optically.
Using the gamma lookup table, the data corresponding to each data slice is
translated so that the brightness steps of equal value in the data correspond to visually
equivalent changes in the projected image, as measured by the photo-diode duringcreation of the lookup table.
~2-
WO 95/14960 2 ~ 7 7 3 ~ ~ PCT/U594/13639
Moreover, light valve 442, when used in conjunction with wave plate 463 as
discussed in the context of Figures 7-8, is typically capable of producing a blackest black
image on the order of about 2000 times as faint as the brightest white image. This level
of contrast range is simply ~""e.~d,y in view of the fact that the human visual system
5 can only distinguish within the range of 50 to 100 grey levels within a single data slice.
Thus, the maximum desired contrast ratio (i.e., the brightness level of the blackest region
on a slice divided by the brightness level of the brightest white region on a slice) is
desirably in the range of 10020:1, allowing for flexibility at either end of the brightness
scale. Since the contrast ratio of a particular slice is thus on the order of one-tenth the
0 available contrast ratio producible by the right valve, a higher aspect of the gamma
correction scheme employed in the context of the present invention surrounds defining
absolute black as having a brightness level equal to zero. Thereafter, a subjective
determination is made that the darkest regions of interest on any slide, i.e., the darkest
region that a radiologist would be interested in viewing on a slice, would be termed
5 "nearly black." These nearly black regions would be mapped to a value which is on the
order of 100-200 times fainter than pure white. Moreover, any values below the
nearly black values are desirably clamped to absolute black (zero grey value). These
absolute black regions or super black regions comprise all of the regions of a slice which
are darker than the darkest region of interest.
An additional gamma correction step employed in the present invention surrounds
clamping the brightest values. Those skilled in the art will appreciate that conventional
CRTs and light valves are often unstable at the top of the brightness range. More
particularly, increasing the brightness level of data driving an image in any particular
CRTAight valve combination above the 90% brightness level may yield images having
2s very unpredictable brightness levels. Thus, it may be desirous to define the upper limit
of brightness level for a data set to coincide with a p,~dt:~""",ed brightness level
exhibited by imaging assembly 328, for example, at 90% of the maximum brightnessproduced by imaging assembly 328. Thus, pure white as reflected in the various data
slices will actually correspond to 10% less white than imaging assembly 328 is
30 theoretically capable of producing thereby avoiding nonlinearities and other instabilities
associated with the optical apparatus.
Finally, if any slice is essentially black or contains only irrelevant data, the slice
may be omitted entirely from the final hologram, as desired.
~3-
WO 9S/14960 2 1 7 7 3 ~ 8 PCT/IJS94/13639
Thus, in a~u,.ld~ with one aspect of the present invention, the intensity of theobject beam may suitably be controlled as a function of one or more of a number of
factors, including, inter alia, the voltage level applied to wave plate 404, the data
distribution for a particular data slice, the axial position of a data slice with respect to the
5 film holder, and the effects of gamma correction performed on the data.
As discussed above, the exposure time for each data slice may be conveniently
dt:Le"" ,ed as a function of one or more pdldlllt~ , including the desired beam ratio,
the total number of slices in the data set, and the aggregate gray scaie value (brightness
level) for a particular data slice. In accordance with one aspect of the present invention,
0 relatively bright slices require a relatively short exposure time, whereas relatively faint
(dark) slices require a longer exposure time. In this way, each slice may thus consume
an app~uplid~ (e.g., proportional) share of pl~u~u~ ive elements within the filmemu Ision.
Relatively long exposure times may be disadvantageous in several respects. For
example, the longer the exposure, the more likely it is that spurious phenomena may
adversely effect the quality of the hologram. Such spurious effects include, among other
things, vibration, drift in beam intensity or in various projection optics Udldlll~ 15,
temperature, humidity, coherence length of the laser source, and the like. It may thus be
desirable to reduce the exposure time for relatively faint slices.
In accordanoe with one aspect of the present invention, the exposure time for
some or all of the slices ~.ulll,uli~il,g a particular data set may be reduoed by artificially
boosting the aggregate brightness level for one or more of the slices by a ,ul~d~ ed
amount. This is suitably accomplished by i~ ,uo~illg phantom bright pixels to the slice,
in a minimally intrusive manner.
For example, an asteroid may be placed in a dark region of the slice remote fromthe relevant data. In this way, the aggregate brightness level for a particular slioe may be
boosted without affecting the brightness levels of the pixels which comprise the relevant
data embodied in the slice.
In a..u,dd"-~ with a further aspect of the foregoing asteroid technique, the
30 phantom brightness regions may take any desirable form or shape, but are preferably
configured as clouds, asteroids, or other random (e.g., irregular) shapes. In this regard,
the use of regular shapes having sharp contrast edges (e.&, rectangular shapes) may result
in undesirable side effects. For example, to the extent similar geometric patterns appear
-44-
WO 9~/14960 2 1 7 7 ~ ~ ~ PCT/US94/13639
from slice-to-slice, erroneously strong or weak fringes for this pattern may be inadvertently
produced. This may result in aliasing, undesired intermodulation noise, or the like.
As discussed in greater detail below, once a composite hologram (master
hologram) is produced it may be desirable to make a copy of the master hologram, which
s copy is suitably an image plane hologram. In this context, it may be desirable to mask
the various asteroids such that they do not appear on the image plane (copy) hologram.
- This may be done by simply physically masking the holographic asteroids to optically
isolate them from the copy mechanism. In this regard, masking of the holographicasteroids is facilitated if all of the asteroids for the various slices within a data set fall
10 within a single plane, for example, in the plane of the filmholder for the copy hologram
(discussed in greater detail below in conjunction with Figures 9 and 10). In order to
facilitate the placement of all asteroids in a single plane, it may be desirable to project the
asteroids onto the master hologram from a fixed location for all slices; that is, as ~he
camera assembly moves relative to the master hologram film plane as the master hologram
5 is produced, it may be desirable to maintain the asteroid projection mechanism (e.g., the
variable intensity polarizer discussed above) at a fixed location corresponding to the H2
(copy hologram) film plane during production of the H1 (master) hologram.
By artificially boosting the brightness level for faint slices in accordance with the
foregoing asteroid technique, the dynamic brightness range among the various slices may
20 be desirably reduced, such that the range of exposure times for the various slices
~u"~ isi~g a particular data set may also be reduced.
In a typical data set, it may be desirable to artificially boost the brightness level of
only those data slices falling below a predetermined aggregate brightness threshold.
Alternatively, it may also be desirable to add an asteroid to even the brighter (i.e., high
2s gray scale value) slices, for example preserve the relative aggregate gray scale levels for
the various slices comprising the data set or if all data slices within a particular data set
are too faint. In this regard, it should be noted that the brightness level of each asteroid
may be selected to boost each slice to a desired gray scale value.
By adding an asteroid to a relatively faint data slice, it is believed that the fringe
30 patterns produced in the film substrate for a particular data slice will be sharper and,
hence, a higher contrast ratio for each data slice will be achieved, thereby producing a
sharper composite hologram. This is so even though the gray scale values for the various
pixels ~,",~ i"g the relevant data under examination for each data slice remains
~s- ~
WO 9~/14960 - ~ 1 7 ~ 8 PCT/US94/13639
unaltered. That is, by adding an asteroid to a faint data slice, the fringe pattern for that
data slice is enhanced even though the amount of light passing through the pixels which
comprise the relevant data remains unchanged.
Viewin~ Assemblv
Copy hologram H2 is suitably replayed on a viewing device such as the VOXBOX~
viewing apparatus manufactured by VOXEL, Inc. of Laguna Hills, California. Certain
featuresoftheVOXBOX~viewingapparatusaredescribed in U.S. Patent Nos. 4,623,214
and 4,623,215 issued November 18, 1~86.
Referring now to Figure 11, an exemplary viewing apparatus 1102 suitably
0 comprises a housing 1104 having an internal cavity 1106 disposed therein, housing 1104
being configured to prevent ambient or room light from entering the viewing device.
Viewing apparatus 1102 further comprises a light source 1108, for example a
spherically irradiating white light source, a baffle 1132, a mirror 1134, a Fresnel lens
1110, a diffraction grating 1112, and a Venetian blind 1114 upon which copy hologram
H2 is conveniently mounted. Venetian blind 11 t4 and hologram H2 are schematically
illustrated as being separated in space from diffraction grating 1112 for clarity, in a
preferred embodiment of the device, Fresnel lens 1110 suitably forms a portion of the
front surface of housing 1104, diffraction grating 1 1 t 2 forms a thin, planar sheet secured
to the surface of lens 1110, and Venetian blind 1 114 forms a thin planar sheet secured
to grating 1112. Hologram H2 is suitably removably adhered to Venetian blind 1114 by
any convenient ",~ dll~ for example by suitable clips, vacuum mechanisms, or anyconvenient manner which permits hologram H2 to be intimately yet removably bonded
to the surface of Venetian blind 1114.
Fresnel lens 1110 collimates the light produced by light source 1108 and directs2s the collimated through diffraction grating t 112. The desired focal length between source
1108 and lens 1110 will be determined by, inter alia, the physical dimensions of lens
1110. In order to conserve space and thereby produce a compact viewing box 1102, the
light from source 1108 is suitably folded along its path by mirror 1134. Since source
1108 may be placed near lens 1110 in order to maximize space utilization, baffle 1132
30 may be conveniently disposed intermediate source 1108 and lens 1110, such that only
light which is folded by mirror 1134 strikes 1110. As discussed above, the relationship
between this angle and wavelength are similarly governed by the equation ,1 ~s K sin e.
~6-
WO 95114960 2 ~ 7 7 3 5 B PCTIUS94/13639
In a preferred embodiment of the present invention, the focal length of lens 1110 is
approximately 12 inches.
Diffraction grating 1112 suitably comprises a holographic optical element (HOE),for example one produced by a holographic process similar to that described herein.
More particularly, diffraction grating 1112 is suitably manufactured using a reference and
an object beam having a wavelength and incident angle which ~o"t~ to that used
in producing hologram H2 (here 514.5 nm~. In a preferred embodiment, diffractiongrating 1112 is advantageously a phase hologram.
Diffraction hologram 1112 suitably diffracts the various components of the white0 light incident thereon from source 1 108 as a function of wavelength. More particularly,
each wavelength of light will be bent by a unique angle as it travels through diffraction
grating 1112. For example, the blue component of the white light will bend through an
angle P; the higher wavelength green light component is bent at a greater angle Q; and
the higher wavelength red light is bent at an angle R. Stated another way, diffraction
5 grating 1112 collimates each wavelength at a unique angle with respect to the surface of
the grating. Those skilled in the art will appreciate, however, that diffraction grating 1112
is an imperfect diffractor; thus, only a portion of the incident light is diffracted (e.g., 50%),
the remainder of the ulldirr d-~td light passes through as collimated white light.
Venetian blind (louvers) 111 4 comprises a series of very thin, angled optical slats
20 which effectively trap the undiffracted white light passing through grating 1112. Thus,
substantially all of the light passing through louvers 1114 passes through at an angle, for
example the angle at which the light was diffracted by grating 1112. Of course, a certain
amount of light will nonetheless be deflected by the louvers and pass through at various
random angles.
2s Moreover, the geometry of the slats comprising louvers 1114 may be selected to
produce a resulting hologram with optimum colorization. More particularly, the slat
geometry may be selected so that certain wavelengths pass through louvers 1 114
essentially intact (the nominal wave band), whereas wavelengths higher or lower than
the nominal wavelength will be clipped by the louvers. Moreover, the geometry of the
30 slats may be selected such that light which passes through grating 1112 undiffracted does
not pass directly through louvers 111 4. By ~ooldilldli"g slat geometry, undiffracted light
may be substantially attenuated, for example, by causing such undiffracted light to reflect
a number of times (e.g., four) between adjacent slats before reaching hologram H2.
~7~
wo 95~l4960 2 1 7 ~ 3 5 8 Pcr/uss4/l3639
Louvers 1114 suitably comprise a thin, pianar light control film manufactured bythe 3M Company. On one surface, louvers 1114 are slightiy convex; moreover, a greasy
or waxy substance is apparently applied to this surface by the manufacturer. To avoid
damage to the delicate slats, it may be desirabie to adhere the louvers to a protective
5 surface, for example, an acrylic sheet (not shown). Improper application of the "greasy"
side of louvers 1114 to an acrylic sheet may, however, produce a nonuniform contact
interface between the two surfaces, which could produce undesirable optical
characteristics.
The present inventor has determined that applying a thin coating to a high-lubricity
0 particulate substance (e.g., talc) at this interface tends to yield a contact surface between
the acrylic sheet and the louvers having improved optical ~l,a,d.~ cs.
Hologram H2 is illustratively placed onto the viewing screen, for example by
adhering it to the surface of louvers 1 114. In this regard, the viewing screen suitably
comprises one or more of the following components. Iens 1110; grating 1112; and
s Venetian blind 1114. Alternatively, the viewing screen may simpiy comprise a thin,
planar sheet of L~d"~a,~"l material for example glass, upon which one or more of the
foregoing components may be conveniently mounted. In accordance with one aspect of
the present invention, such a viewing screen is suitably on the order of 10 to 16 inches
in width, and on the order of 14 to 20 inches in height, and most preferably on the order
20 of 1~ by 17 inches. Consequently, it is also desirable that the various holograms made
in accordance with the present invention, namely master hologram H 1 and copy
hologram H2, be of suitable dimensions so that they are either smaller than or
a~ i",~.~.'y as large as the viewing screen. In a particularly preferred embodiment
master hologram H1 and copy hologram H2 each are suitably 14 by 17 inches.
Since hologram H2 is suitably produced using the same wavelength and reference
beam angle as was used to produce grating 1112, light passing through hologram H2 is
bent in accordance with its wavelength. Specifically, blue light is bent at an angle of
minus P, green light is bent at an angle of minus 0, and red light is bent at an angle of
minus R (recall that master hologram H1 was inverted during the production of copy
30 hologram H2). Consequently, all wavelengths pass through hologram H2 substantially
orthogonally to the plane of lens 1110. As a result, an observer 1116 may view the
reconstructed hologram from a viewpoint substantially along a line orthogonal to the
plane of hologram H2.
18-
WO 95/14960 2 1 7 7 :~ ~ 8 PCT/US94113639
By coordinating the wavelength-selective diffraction capacity of diffraction grating
1112 with the wavelength-selective diffraction properties of hologram H2, substantially
all of the light diffracted by diffraction grating 1112 may be used to illuminate the
hologram. Thus, even the use of a relatively inefficient diffraction grating 1 112 produces
5 a relatively bright holographic image. Moreover, the holographic image is not
unnecessarily cluttered by spurious white hot which is not diffracted by grating 1112,
inasmuch as a substantial amount of this spurious light will be blocked by louvers 1114.
Moreover, by mounting the thin, planar hologram, louvers, and diffraction grating
on the surface of a lens which forms a portion of the viewing apparatus, the replay beam
10 used to illuminate the hologram is substantially exclusively limited to the collimated light
from source 1108; that is, spurious noncollimated light is prevented from striking the rear
surface (right-hand side in Figure 11) of hologram H2.
Altern~tive Li~ht Control Film [~ o~ t~
Referring now to Figure 13, in accordance with an alternative embodiment of
15 ewing assembly 1102, a light control film 1310 may be suitably employed in lieu of
louvers 1114.
Moreparticularly, lightcontrolfilm 1310suitablycomprisesathin, lld~ Jdl~"tfilm
laminate made from a plurality of thin planar sheets sandwiched together, as described
in greater detail below. In the embodiments set forth in Figure 14, light control film (LCF)
20 1310 comprises three laminated sheets, namely a front sheet 1402, a core sheet, 1404,
and a back sheet 1406. Each of the foregoing sheets comprises a thin, I~d~ a,~"l film,
with a series of thin, parallel, opaque lines extending across the entire surface of the film.
To illustrate the optical properties of LCF 1310, these sheets are shown in cross-sections;
for clarity, a front view of an exemplary sheet 1402 is shown in Figure 15, with the
25 thickness of the opaque lines ~xd~ ldled for illustration purposes. Respective opaque
lines 1402A, 1402'3, 1402C, etc. shown in Figure 15 may be seen in cross-section in
Figure 14. Respective sheets 1404 and 1406 are suitably similar or identical to sheet
1 402.
With continued referenceto Figure 14, LCF 1310 isconvenientlyviewed asa light
30 filter, such that the duty cycle of a constituent sheet (e.g., sheet 1402) is a function of the
width W1 of an exemplary opaque line (e.g., line 1402A relative to the width W2 of the
distance between consecutive lines). I the embodiments shown in Figure 14, each of the
~9-
wo 95114960 2 1 ~ 7 ~ 5 ~ PCTIIsS94ll3639
respective sheets 1402-1406 suitably exhibit an opaque duty cycle on the order of 50%,
i.e., W1 is d5J5~1U~illldl~ly equal to W2.
The quality of grating 112 may be expressed in terms of its ability to selectively
diffract incoming white like 1408. As discussed above in connection with Figure 11,
s diffraction grating 1112 diffracts light at an angle as a function of wave length. For
example, red light rays 1410 are diffracted at a relatively steep angle from the horizontal,
green light 1412 is diffracted at less than red light, and blue light 1414 is diffracted at a
relatively small angle from the horizontal.
Diffraction gratings are typically not 100% efficient. Thus, a ~u~1sid~l dule amount
0 of undiffracted light inevitably passes through grating 112. In the context of the present
invention, undiffracted light which passes through grating 1112 is referred to herein as
zero order light 14-16, whereas the diffracted light (e.g., rays 1410-1416) are referred to
as first order diffracted light.
To facilitate the reconstruction of a sharp, high contract, hologram, LCF 1310 is
~s suitably configured to block zero order light 1416 such that it is nût viewed by viewed
1116, and at the same time to pass the diffracted first order light therethrough. As
discussed above in conjunction with Figure 11, the first order light which passes through
LCF 1310, will be inversely diffracted by the hologram and directed horizontally to be
viewed by the observer.
In accordance with a first embodiment of LCF 1310 shown in Figure 14, front
sheet 1402 îs suitably dispDsed with respect to back sheet 14û6 such that their respective
opaque and ~Idn~d,~"I lines are aligned. Core sheet 1404, on the other hand, is suitably
disposed such that its opaque lines 1404A, 1404B, etc. are in registration with the
~Sdl ,~.a,~l~t portions of front sheet 1402 and back sheet 1406, while the opaque portions
2s of core sheet 1404 are disposed in registration with the transparent portions of front sheet
1402 and back sheet 1406. Consequently, most of the zero order light which passes
through grating 1112 will be blocked by LCF 1310. However, the present inventor has
observed that a small amount of zero order light, as shown for example at ray 1416A,
inevitably passes through LCF 131 û. The zero order light 141 6A which passes through
30 LCF 1310 may be attributed to several factors, including: vertical misalignment of one or
more of sheets 1402-1406; flaws or imperfections in the opacity, width W1, parallelism,
or position of one or more of the opaque lines ~u",5,,ising one or more of sheets 1402-
_5~
WO 951149C0 2 1 7 7 ~ 5 8 PCT/US94/13639
1406; refraction of light through LCF 1310, and diffraction of zero order light around oneor more of the edges of the opaque lines ~u~luliSirlg LCF 1310.
Accordingly, while the ~ o~ , set forth in Figure 14 produces acceptable
results, alternate configurations of light control film may also be employed in the context
5 of the present invention.
Referring now to Figure 16, an alternate embodiment of light control film 1610
suitably comprises a front sheet 1602, a core sheet 1604, and a back sheet 1606, wherein
the relative dimensions of the various opaque lines ~u~,u~i~illg sheets 1602-1606 are
suitably manipulated such that opaque lines 1604A, 1604B, etc. "overlap" the edges of
0 the opaque lines ~u"~u,i~ir,g respective front and back sheets 1602 and 1606. The
configuration shown on Figure 16 substantially reduces the extent to which zero order
light may diffract around the opaque lines embodied in LCF 1610. Alternatively, the
plurality of core sheets having substantially thinner (dimension W1 ) opaque lines may be
employed, with the opaque lines of the core sheets being staggered in various
s configurations to preclude the passage of zero order light through the light control film.
However, the usefulness of such ~ "~odi~ are limited to the extent they also tend to
block the passage of first order light therethrough, for example, narrowing the band width
of first order light which can pass through the various layers comprising the light control
film.
More particularly and with Illolll~ ly reference to Figure 17, even a relativelysmall overlap 170~ in the width of opaque lines 1704A and 1704B can significantly
reduce the amount of first order light which passes through LCF 1710 for certain wave
lengths from a first amount defined by pathway 1712 to a second amount defined by
pathway 1714.
With continued reference to Figures 14-17, it can be seen that the vertical uplifting
of one or both of the front and back sheets tends to block wave lengths at the extreme
ends of the band width for which the light control film is designed to pass. It can also
be seen that vertical shifting of the core (intermediate) sheets tends to reduce the amount
of intermediate wave lengths which pass through the LCF. Ideally, an LCF should be
30 configured to pass all desired first order wave lengths equally well, while blocking
substantially all zero order light.
51
WO 95/14960 2 1 7 7 ~ 5 ~ PCT/US94/13639
Referring now to Figure 18, an alternate LCF embodiment is shown which
substantially decouples the zero order blocking cdpability of the LCF from the LCF's
cdpacity to pass first order light.
Referring now to Figure 18, an alternative embodiment of an LCF 1810 suitably
5 comprises a front layer 1802, a core layer 1804, and a back layer 1806 In accordance
with one aspect of LCF 1810, back layer 1806 may be thought of as a datum, whereupon
shifting of front layer 1802 results in color selectivity, and a w~,uol~dillg shift in core
layer 1804 provides good zero order blocking.
As shown in Figure 18, substantially all of the zero order light 1416 which passes
0 throughgrating1112willbeblockedbyLCF1810. Inaddition,LCF1810isconfigured
to facilitate passage of a desired band width of first order diffracted light therethrough.
The particular dlldl~ ",e:"l of the various sheets ~ul~ g composite LCF 1810 areconveniently described in the context of a preferred embodiment whereby LCF 1811~ is
constructed. Accordingly, a detailed methodology for manufacturing LCF 1810 will now
5 be described.
With continued reference to Figure 18, LCF 1810 is suitably manufactured using
a sturdy, flat, viewing apparatus of the type discussed above in conjunction with
Figure 11, rotated d~ ly 90 such that the viewing screen is substantially
horizontal and may thus be viewed by the operator during assembly of LCF 1810. In the
20 context of Figure 18, grating 1112 would thus be oriented horizontally, with respective
sheets 1802, 1804, and 1806 assembled with back sheet 1806 on the bottom and fron~
sheet 1802 on top, as described in greater detail below.
As an initial manufacturing step, a protective glass sheet 1816, for example a 3/8"
slab of glass, is suitably laid horizontally on top of the surface of viewing screen 1818 to
25 avoid damage to the viewing screen during assembly of LCF 1810. Thereafter, it may be
desirable to place a protective coating over the glass, for example a thin, lldll~j~dl~
polyester, sheet (polyester sheet 1820) to prevent any adhesives used during assembly
from contacting glass sheet 1816.
To facilitate handling and installation of laminated LCF 1810, it may be desirable
30 to construct the laminate as a composite, wherein LCF 1810 is sandwiched between
respectjve sheets of glass 1822 and 1824. Accordingly, rear sheet 1822 is suitably
thoroughly cleaned and plaoed on top of polyester protective sheet 1820. Respective
-s2-
WO 95114960 2, 7 7 3 5 8 PCIIUS94/13639
glass sheets 1822 and 1824 are suitably on the order of one to five millimeters in
thickness, and most preferably in the range of about 2.3 millimeters thick.
As best seen in Figure 18, it is desirable that glass sheet 1822 comprise rectangular
dimensions on the order 14 7/16" in height by 7 7/16" wide, and for respective fi!m
5 sheets 1806, 1804, and 1802 to exhibit successively smaller rectilinear dimensions, with
glass sheet 1824 having the smallest rectangular dimensions. The various sheets can be
stacked on top of one another and conveniently manipulated by the operator during
assembly.
With continued reference to Figure 18, the first active layer of the LCF is suitably
0 placed on top of glass sheet 1822. Specifically, back sheet 1806 is disposed on top of
glasssheet 1822,with respectofopaquestrips 1806A, 1806B,andthelikerunningfrom
left to right as viewed by the operator 1116. In a particularly preferred embodiment, the
film comprising respective sheets 1802-1806 is Kodak Accumax 2000 ALI7. In a preferred
~",I,o,li",e,~l, respective sheets 1802-1806 are on the order of 7 mils. thick, and suitably
15 comprised polyester, acetate, or any convenience lld"~.a,~"l material.
In accordance with the further aspect of the present invention, the respective
opaque lines ~u~ Jri~ g the various sheets within the laminate are suitably on the order
of 12 mils. in width (dimension W1; see Figure 14), with spaces on the order of 11 mils,
such that that duty cycle of the various light block films are in the range of ~0% to 60%,
20 and preferably in the range of 50% to 60%. In addition, the emulsin which comprises
the opaque stripes may be imbedded within the thickness of the film or, alternatively, may
be deposited on the surface of the film at a thickness of ap~,,.,xil"dlely 6 microns.
After positioning film sheet 1806 on top of glass sheet 1822, film sheet 1806 issuitably secured to the glass sheet, for example by wiping a hypodermic needle across the
2s tip of a bottle of Locktite~ Unlocktite 351, or any other suitable general purpose
ultraviolet (UV) adhesive.
The adhesive is then wiped onto the underside of two or more corners of film
1806, and a UV light applied to the adhesive region to cure the adhesive, thereby
securing film sheet 1806 to glass sheet 1822.
In accordance with the preferred embodiment, a suitably ultraviolet curing lamp
comprises a 100 wan UV flood lamp, for example a Spectronics SB 1 OOC hand held UV
lamp.
-s3-
WO 9sl14960 ~ ~ 7 ~ 3 5 ~ PCTNS941~3639
Core sheet 1804 is this placed on top of back sheet 1806, with the opaque lines
running from left to right. In order to properly position core sheet 1804 on top of back
sheet 1806, the operator suitably looks directly downwardly toward the film, such that
the operators line of vision is substantially orthogonal to the film plane. Sheet 1804 is
5 then manipulated until the opaque lines of sheet 1804 are in ~ ,dliu" with the opaque
lines of sheet 1806, such that sheet 1806 is essentially hidden behind sheet 1804. Once
the two sheets are exactly aligned, any air between the sheets is suitably wiped out to
provide for intimate sliding contact between the two film sheets. Through the use of
small portable",i.,u~up~,forexample,aTasco30XmicroscopeavailablefromtheH&R
0 catalogue, core film 1804 is suitably slid toward the operator slightly (downwardly in
Figure 18), so that the opaque lines of sheet 1804 overlap the opaque lines of sheet 1806
by app,u;~i",dl~ly 50%. Though the use of the drur~",~:"lioned microscopes, this may be
accomplished visually with relative ease. By using the ~ u~up~s at the four corners
of the composite, it is also relatively easy to ensure that the opaque lines of sheet 1804
15 are substantially parallel to the opaque lines of sheet 1806 throughout the entire surface
of the sheet. In this position, an exemplary edge 1826 of an arbitrary opaque line of
sheet 1804 is suitably disposed d~Jpluxillldl~:ly one half way between respective edges
1828 and 1830 of an adjacent opaque line 1806B on sheet 1806.
Core sheet 1804 is then temporarily taped in place, for example by placing two
20 pieces of tape at the bottom edge of sheet 1804, temporarily securing sheet 1804 to film
sheet 1806 and glass sheet 1822.
Front sheet 1802 is then placed on top of core sheet 1804, such that the variousopaque lines 1802A, 18û2B, etc. are aligned with the opaque regions defined by the
overlapping opaque lines of sheets 1804 and 1806. Sheet 1802 is then slowly urged
25 slightly toward the operator (downwardly in Figure 18) until all zero order light is
completely blocked. This will be apparent to the operator in that all zero order light
which passes through grating 112 and the various components set forth in Figure 18 is
totally blocked. To confirm that the zero order light is essentially totally blocked, the
operator may turn the brightness level in the bulb disposed within the viewing apparatus
30 to a maximum level.
Specifically, zero order light will be totally blocked when edge 1832 of opaque
stripe1802Aisslightlyaboveedge18340fanopaquestripe1826foreachofthevarious
opaque stripes ~u",u,isi~g respective sheets 1802 and 1804. The degree of overlap
-54-
WO 95/14960 2 1 7 7 3 ~ ~ PCTNS94/13639
between respective edges 1832 and respective edges 1834 may be conveniently defined
as dimension L. In accordance with the preferred embodiment of the present invention,
dimension L should be as small as possible while insuring complete blockage of zero
order light.
As an additional step in ~u,~ri,ll, ,g that sheet 1802 is properly positioned, the
operator may lean forward over the assembly, such that he looks downwardiy and
rearwardly at the assembly, for example from position 1814B. From position 1814B, the
operator can observe any "backlight" which may shine through LCF composite 1810.While the backlight will typically be substantially lower in intensity than the zero order
0 lighl, it is nonetheless desirable to block as much backlight as possible; this may be
accompl ished by minimizing dimension L wh ile insuring complete biockage of zero order
light.
Front layer 1802 is then secured to layer 1804, for example by applying a few
pieces of adhesive tape to fasten layer 1802 to layer 1804.
s Having confirmed that all zero order light is blocked, one or both of sheets 1802
and 1804 may be moved slightly to achieve optimum color balance. In this regard, the
operator may step away from the assembly, and/or bend down slightly, such that he
observes the assembly from position 1814A such that he is "looking up" the tunnel
defined by the series of opaque lines. In the configuration set forth in Figure 18, it is
20 possible to observe what appears to be a comet, which the present inventor believes to
be a scattered image of the filament of the source light within the viewing apparatus.
The operator then urges core layer 1804 toward him (downwardly in Figure 18),
while maintainin~ sheet 1802 essentially stationary. This manipulation effectively
increases dimension L, further insuring total zero order blockage. Alternatively, front
2s sheet 1802 may be urged upwardly in Figure 18, either in addition to or in lieu of urging
layer 1804 downwardly, to effect a slight increase in dimension L without increasing the
amount of backlight observed from position 1814B.
An exemplary hologram may then be placed on top of the assembly to insure
proper color and zero order blockage. To the extent that the operator desires to fine tune
30 the color spectrum passing through LCF 1810, he may manipulate core layer 1804
upwards or downwards slightly, while insuring complete zero order blockage, to obtain
desired variations in color.
-55-
WO 95/14960 ~ 1 7 7 ~ 5 8 PCT/US94/13639
Once the three layers ~ illg LCF 1810 are properly positioned, they are
secured to one another at their corners through the aforementioned UV adhesive. Glass
plate 1824 is then placed on the assembly, and the enlarge planar pressing tool is placed
on the entire assembly to expel air from between the various laminates in the assembly.
5 A bead of UV cement is then applied around the perimeter of the assembly, leaving a
small gap in the perimeter bead. A hypodermic needle is then inserted into the gap,
which hypodermic needle suitably comprises a 303 stainless steel, 25 gauge, thin wall
tube.
The peripheral, adhesive bead is then completed, essentially completing the
0 perimeter seal with the hypodermic needle in place. A vacuum lead, for example a teflon
hose, is then secured to the distal end of the hypodermic needle, and a vacuum, on the
order of 25 inches of mercury pressure, is applied to the hypodermic. This insures that
any residual air within the laminated assembly is withdrawn through the hypodermic.
When all air is withdrawn from the assembly, a hot lamp or blow torch may be
15 used to soften the hypodermic, such that the hypodermic collapses upon itself, creating
an airtight region inside the adhesive bead. During the heating process, the hypodermic
needle is suitably squeezed with needle-nosed pliers to flatten it out and may be
advantageously gripped ~vith channel grip pliers to insure a strong, light, mechanical
airtight seal. The end of the hypodermic needle is then folded back into the adhesive
20 bead, and the entire perimeter of the assembly is taped to insure a stable, air tight,
mechanically sound composite laminate structure.
Modifications and ~nhancements
When a hologram (H2), produced in accordance with the present invention, is
mounted on box 1102, a three~li",el-siol-al ~ "I~Lion of the object may be seen,2s affording the viewer full parallax and perspectives from all viewpoints. The present
inventor has further determined that the hologram may be removed from the viewbox,
inverted, and placed back on the viewbox. The inverted hologram contains all of the
same data as the noninverted view of the same hologram, except that the observer is
looking at the hologram from the opposite direction; that is, points on the hologram
3C which previously were furthest away from the observer are no~ closest to the observer,
and vice versa. This feature may be particularly useful to physicians when mapping out
a proposed surgical procedure, for example, by allowing the physician to assess the
various pros and cons of operating on a body part from one direction or the other.
-s6-
WO 95/14960 2 1 7 7 3 ~ 8 PCT/US94/13639
The present inventor has also determined that two or more holograms may be
simultaneously viewed on the same viewbox, simply by pldcing one hologram on top of
the other hologram. This may be particularly significant in circumstances where, for
example, the first hologram comprises a body part (e.g, hip) which is to be replaced, and
5 the second hologram comprises the prosthetic replacement device. The physician may
thus view the proposed device in proper context i.e., as the device would be implanted
in the three~;""~ iv"dl space within the patient.
Moreover, it may be advantageous to overlay a hologram of a coordinate grid, e.g,
a three-dimensional coordinate grid, with the hologram which is the subject of inspection.
0 In this context, a suitable coordinate grid may simply comprise a hologram of one or more
rulers or other measuring devices having spatial indicia encoded thereon. Alternatively,
the coordinate grid may simply comprise a series of intersecting lines or, alternatively, a
matrix of dots or other visual markings spaced apart in any convenient manner, for
example linearly, logarithmically, and the like. In this way, three-dimensional distances
5 may be easily computed by counting the coordinate markings, particularly if the
coordinate grid is of the same scale or of a convenient multiple of the dimensional scale
~vr~,u~ g the hologram.
The present inventor has also observed that very faint patterns of light and dark
rings are occasionally visible when viewing a hologram in accordance with the present
20 invention. More particularly, these rings appear to be a great distance behind the
hologram when viewed. The present inventor theorizes that these rings constitute an
inL~ lv~;la~ which results from taking a llhologramll of diffusing diffuser 472 along with
each data slice. To overcome this problem, diffuser 472 may be shifted slightiy (e.g., ten
millimeters) within its own plane after each data slice is recorded. In this way, the image
25 ~vll~Jol~d;l,g to each data slice is still projected onto film 319 as described herein, yet
a slightly different portion of diffuser 472 is projected for each data slice, thereby avoiding
projection the same pattern attributable to diffuser 472 for each data slice.
It is also possible to add textual or graphical materiaL for example to one or more
data slices, thus permitting the resulting hologram of the data set to reflect this textual or
30 graphic material. Such material may comprise identification data (e.g., patient name,
model or serial number of the object being recorded), or may comprise pure graphical
information (arrows, symbol and the like).
-57-
WO 95/149~0 2 t 7 ~ ~ ~ 8 PCT/US94/13639
In this regard, it is interesting to note that text which is viewed in the orthoscopic
view will be inverted in the pseudoscopic view of the same hologram; that is, if text
appears right-side up in the orthoscopic view, it will appear upside down in theps~dc\scopic view. Thus, to the extent it is desirable to utilize text within a hologram,
it may be advantageous to insert the same text right-side up at the top of the hologram
and upside down at the bottom of the hologram, so that text may be properly observed
regardless of whether the hologram is viewed in the orthoscopic or pseudoscopic
construction .
Moreover,textwhichisinthefilmplanewillgenerallyappearsharpduringreplay,
0 whereas text disposed out of the film plane, I.e., along axis A in Figure 1, generally
appears less sharp. This may be advantageous in accordance with one aspect of the
invention, inasmuch as "out of film plane" text would be legible when viewed on a
Voxbox, but illegible without a Voxbox. In the context of holograms used for medical
diagnosis, it may thus be desirable to place confidential patient information, for example
a patient's name, condition, and the like, out of the film plane so that such information
may be most easily viewed by proper personnel with the aid of a Voxbox, thereby
ensuring patient confidentiality.
In addition to textual and graphical material, it may be desirable to include
additional images, for example a portion of the image ~o",p, i,i"g a particular hologram,
20 or image data from other holograms, onto a master hologram. For example, consider a
master hologram of a fractured bone .u" ,~., i,i"~5 one hundred or more sl ices. For the few
slices which comprise the key information, it may be desirable to separately display this
data spaced apart from the overall hologram, yet adjacent to the hologram and at the
proper depth with respect to the hologram.
2s As briefly discussed above, wherein, a hologram produced in accordance with the
present invention is viewed on a Voxbox or other suitable viewing device, the
orthoscopic view of the hologram may be observed when the hologram is in a firstposition, and the p5F~Ilrlr)crr)pjc view may be observed when the hologram is rotated
about its horizontal axis. Since it may be difficult to determine whether a particular
30 orientation of the holographic film corresponds to the orthoscopic or rs~ lr~s~rjc view
with the naked eye, it may be desirable to place convenient indicia on the holographic
film to inform the viewer as to which view of the hologram may be observed when the
holographic film is placed on a viewing apparatus. For example, it may be desirable to
-~8-
WO95/14960 2 ~ 77~ PCT/US94113639
place a notch or other physical indicium on the film, for example in the upper right hand
corner of the orthoscopic view. Alternatively, a small textual graphical or color coded
scheme may be employed by placing appropriate indicia at a corner, along an edge, or
at any convenient position on a holographic fiim or on any border, frame, or packaging
5 therefor.
In accordance with another aspect of the present invention, it may be efficient to
window onIy a portion of the data slices and nonetheless achieve satisfactory contrast and
shading. For example, for a 1ûO slice data set, it may be possible to manually window
every tenth data slice, for example, and through the use of computerized interpolation
10 techniques, automatically window the interstitial data slices.
In accordance with a further aspect of the present invention, it is possible to select
the film plane among the various data slice planes ~ur~uliSillg the data set. More
particularly, each data slice within a data set occupies its own unique plane. In
accordance with the preferred ~ bodilll~ of the present invention, track assembly 334
15 is moved forward or backward such that the data slice which is centered within the
volume of the data set corresponds to the data slice centered within the length of travel
of track assembly 334. The relative position of imaging assembly 328 and film 319 may
be varied, however, so that the plane of film 319 is located nearer to one end of the data
set or the other, as desired. The resulting hologram H2 will thus appear to have a greater
20 or lesser portion of the holographic image projected into or out of the screen upon which
the hologram is observed, depending on the position that the film plane has been selected
to cut through the data set.
In accordance with a further aspect of the invention, a plurality of different
holograms may be displayed on a single sheet. For example, a hologram of a body part
25 before surgery may be displayed on the upper portion of a film, with the lower portion
of the film being divided into two quadrants, one containing a hologram of the same body
part after surgery from a first perspective, and the other portion containing a view of the
same body part after surgery from another perspective. These and other holographic
compositions may be suitably employed to facilitate efficient diagnostic analysis.
In accordance with a further aspect of the present invention, the entire beam path
is advantageously enclosed within black tubing or black boxes, as appropriate. This
minimizes the presence of undesirable reflections. Moreover, the entire process of maker
master and copy holograms is advantageously carried out in a room or other enclosure
-59-
WO 95/14960 2 1 7 7 3 5 8 PCTIUS94/13639
which is devoid of spurious light which could contact any film surface. Alternatively, the
path travelled by any of the beams in the context of the present invention may be
replaced with fiber optic cable. By proper seledion of the fiber optic cable, the
polarization and Transverse Cle.~lu",dE;"etic Mode (TEM) of the light travelling through
s the cable is preserved. Use of fiber optic cable permits the system to be highly
~UI 1 Ipl ~d, and further permits the e~ imination of many of the components of the system
entirely (e.&, mirrors). Finally, fiber optic cables may be used to compensate for a
differential path length between the reference beam and the object beam. Specifically,
to the extent the path travelled by one of the beams differs from the other, a
0 p,ed~ "" ,t:.l Iength of fiber optic cable may be employed in the path of the beam
travelling the shorter length to ~U~ dl~ for this difference in length and, hence, render
the two paths equal.
Returning briefly to the rc~ cr~pic construction shown in Figure 10B, it may
be desirable under certain circumstances to replay the master hologram and view the
5 three-dimensional image in free space. For example, it may be beneficial to a surgeon
to rehearse a surgical technique on a particular body part prior to performing the surgery.
In this regard, a 6 space digitizer, for example a BirdlM part no. 60û102-A manufactured
by the Ascension Technology Corporation of Burlington, Vermont, may be advantageously
employed in the context of a rS~r~os~lric construction.
More particularly, a 6 space digitizer is capable of being manipulated in free
space, and reporting its position to a computer, much like a conventional computer
mouse reports two-dimensional position data to its computer. By moving through the
holographic space, size and other dimensional data may be unambiguously obtained with
respect to the hologram.
With continued reference to Figure 10B, it may also be desirable to replay a
hologram partially or wholly out of its film plane, for example in free space, in order to
perform various diagnostic and experimental tasks. For example, it may be advantageous
to project a holographic display of a portion of human anatomical structure, for example
an injured hip, and to physically place into the holographic space a prosthetic device
intended to replace the hip or other anatomical element. In this way, the "fit" of the
prosthetic device may be ascertained and any dp~Jruplid~ corrections made to theprosthetic device prior to implanting the device.
~o-
2 ~ 77358
WO 951~4960 PCT/US94/13639
.
In addition, it may be desirable to replay a hologram in free space and place a
diffusing screen or oth~r l,a"~-a,~:,lI or opaque structure into the holographic space to
permit interaction with the subject matter of the hologram for various ~x!J~,i",~ dl and
diagnostic purposes.
5 Although the invention has been described herein on con junction with the
appended drawings, those skilled in the art will appreciate that the scope of the invention
is not so limited. For example, while the view box has been described as being
rectangular, those skilled in the art will appreciate that any suitably mechanical
configuration which conveniently houses the various components of the viewing
0 apparatus will suffice. Moreover, although the camera and copy assemblies are illustrated
as separate systems, they may suitably be combined into a single system.
These and other modifications in the selection, design, and al,dl,;Se",~"I of the
various ~ ,uol~ s and steps discussed herein may be made without departing from the
spirit of the invention as set forth in the appended claims.
-61 -
