Okamoto, Msaaki

Shimizu, Eiji

Wei, Zhen

Osaka City University Repository

Stereoscopic Display with Recurrent Viewpoints Using Hologram of Multiple Focal Points
Zhen WEI* and Masaaki OKAMOTO** and Eiji SHIMIZU***
(Received October 15, 2001)
Synopsis
A pair of viewpoints is almost fixed in the case of stereoscopic vision without glasses, which causes inconve-
nience to an observer. This paper proposes a 3-D display system, by which the same stereoscopic image can be
observed from many horizontal directions. The holographic display is experimented by arranging many light sources
for the object beams. The several technical problems involved in the exposure process of the holographic optical
element are concretely examined.
KEYWORDS: Stereoscopic display, Multiple focal points, Holographic display, 3-D display
1. Introduction
A 3-D image is more naturally visible when compared to a plane image, and acts strongly on human sensitivity
and emotion. In recent-years, the progress of research on image-processing technology and stereoscopy, has en-
abled us to get the device that displays 3-D moving image. I) The holography that has a function of continuous
parallax in its 3-D image display has no inconsistency in convergence and regulation, and provides us with the
natural stereo effect. However, the holographic moving display involves a huge amount of information, and is
difficult to be realized.
In this paper, we proposed a 3-D display with which one can observe a stereogam from many horizontal direc-
tions. This is realized by applying the hologram that is made using many object light source to the stereogram.
The stereo-type stereoscopy without using glasses is inconvenient for an observer, since the observation point is
fixed. Moreover, the common stereogram has only two viewpoints, and then only one person can make use of it at
a time. The same is true for observation of the stereo image on the screen using the hologram. In this case, the
observation point is determined by the position of object light source at the time of photographing. Therefore, in
order for two or more persons to be able to observe simultaneously, it is necessary to set up large number of
observation points. In the case of hologram, it is achieved by carrying out a multiplex exposure by changing incli-
nation of a film plate gradually. However, the multiplex exposure causes a lowering of the diffraction efficiency of
the hologram and an observation under a general lighting turns out to be difficult. To overcome this, it has seen
proposed that a pixel unit is distributed along each direction. 2) A defect of this method is that, as the number of
parallax directions increases, the number of pixels per one direction decrease and the quality of image deteriorates.
In this paper, we propose a method of increasing a number ofviewpoints without making lows quality of image.
The key idea is a repetition ofgiving a same set ofstereogram at a time to the observers. We arrange a pixel by using
the slit-division method, and made a holographic optical element in the display by using plural object light sources.
When observation is made, as long as each eye is in the corresponding observation pupils, one can catch a ste-
reoscopyeven if the position of the eyes are not strictly fixed. Moreover, plural persons who sit in right and left can
observe the same stereoscopy simultaneously.
2. Composition of the System
As the reproduction with the holographic optical element, conjugate reproduction is occasionally made, in which
the illuminating light is of opposite direction to the reference light at the photography. In this case, the reproduced
diffraction light condenses into the position of the original object-light source. That is, the position of an image of
the object light source turns out to be the position of the observation point. Therefore, multi-viewpoint hologram
may be made by increase the number of the object light sources when taking the photograph.
* Student, Doctor Course of Dept. of Electrical Engineening
** Senior Researcher, Laboratories of Image Information Science and Technology
*** Professor, Department of Electrical Engineering
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Then, as shown in Fig. I , we consider the correspondence ofone pixel versus many viewpoints both for the left-
eye part and the right-eye part. An advantage of this system is to be sufficient to make only two original images,
the one for the right and the one for the left. Furthennore, despite the many viewpoints, the pixel size of the images
needs not to be very fine. In order to realize the system of, there must be plural object light sources. However, the
optical system constituting of many beam-splitters groups turns out to be complicated, and then we use a com-
bined parallel-plane mirrors and one object light source. 3)
The photographic optical system with a single stereo viewpoint is schematically shown in Fig.2. After the first
exposure, we move the slit mask in one unit and carry out the second exposure. When moving the mask, the
position of object light source for the left eye and the right eye are changed. At the reproduction, the slit images
of the position L condense in a left eye, and the slit images of the position R condense in a right eye. We then
project the mixed images which are made as the illuminating light of hologra from the back of a drawing holo-
gram. In this manner, a pair of stereoscopic images consisting of the image for the right eye and the one for the
light eye in provided.
film plate
reference beam
f f f f f f f f f f f f f f f
'\'JP
\......
L object light sources R
LRLR •••••••••••••• LRLR
Fig.2 Exposure for a pair of viewpoints
maskr right eye
holographic
optical element
r-----w -
eye fo
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i'
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, I; vi, J
,
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for left
ew pupil distance
"1" 62.5mm
view PUPI rows " /,,, • ' ~" ~
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"" / / ", , " I : , " \ ", ,IZl[z]Gd~[!JI:J[!]~~[JEJ
RL RLR LR LRLRL
Fig.l Display with viewpoints
mask
MI M2
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• II I
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• I
• I• • I I •• I I
_____.tIt.....tIl__.........·t__.etl C!~l.__ .
The system with many viewpoints is shown in Fig.3. The hologram is exposed by two plane mirrors M 1 and M2
which install in parallel. The object light sources consist of the multiplex virtual image (broken line) group of a
single object light source. The whole optical system is shown in Fig.4.
The distance between the two parallel plane mirrors (300mm X300mm), M1 and M2 are set to be about 62.5mm,
that is the distance between a pair of adult's eyes. For the purpose of the setting the size (IOOmm·X 125mm) ofa
hologram plate was also taken into consideration. The object light source (diffusion light source, 20mm X 60mm)
is set up close to the one of the two plane mirrors at the opposite position to the hologram plate. Setting the slit
mask on the leftmost position and using the plane mirror close to the object light source for the left (or right)
viewpoint, one takes a photo. Then one moves the slit in one unit and changes the position of object-light source
for the right (or left) viewpoint, and takes the second photo. At the time of reproduction, the viewpoint for left eyes
and the viewpoint for right eyes can arrange by turns in the unit of distance between eyes. One adjusts the image
of the contiguous object-light source for right eye and that for left eye, so that they completely are set each other at
the
L4 R3 L2 RI Lo Ro LJ R2 L3 R4 cylindricallens
Fig.3 Scheme for recurrent viewpoints Fig.4 Layout for exposure
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image for right eyeimage for left eye
/ " "
,," ,," ".> , ,
" \
If' ,," .," \
, " \vt ~;;~~~A' ,·,
M:.... LA
==::::1:=\ RB La RB
vi~ pupil from B ._slit for left eye
place near the boundary of the reflective side ofmirror. In order to prevent the appearance ofa space in the images,
a plane mirror with high reflectance should be used, and a reference beam should be of parallel and should be
projected in parallel to the mirror.
The width of the hologram plate is 125mm, which is twice the distance between eyes. Then, if the second
exposure is performed at moving the plate by 62.5mm after the first exposure, the whole plate can be used for the
purpose ofreproduction of the stereogram. A number of slits, light and darkness portions total, per one exposure are
25 and the slit width is 2.5mm, so that the width ofexposure side turns out to be 62.5mm. The space position in the
plate exposed before and after moving the plate is different. Then a device is required when one arranges by turns
the slits for right eye and the ones for left eye, such that no inconsistently arises at a boundary.
illumination beam
~ ~ ~ ~ ~ ~
IE-- A )IE B~
Fig.5 Generation of recurrent viewpoints Fig.6 Separated parallax images
As shown in Fig.5, the slits for the left eye and the slits for the right eye are arranged by turns both in the A
portion and in the B portion. If the slits for left eye and the one for right eye are arranged in the same way both in the
A portion and in the B portion, reversion occurs at the boundary of A and B. Therefore, it is necessary to take
photography ofA portion and B portion in right-left contrary mann. Then, the pupil LA for the left eye and the pupil
RA for right eye, which are reproduced from the A portion of the hologram plate, overlaps at the same position to
pupil La for the left eye and the pupil~ for right eye, in respective order, which are reproduced from B portion. The
arrangement of the slits is also in order at the boundary ofA and B.
The sheet that is used at the reproduction stage is made as follows. One slices each of the two 2-dimensional
images of different parallax by a slit mask. The area of the sliced picture is the half of the effective area of the
original picture. One compounds these two pictures to a sheet, which is used for reproduction. When we project the
synthetic image that is displayed on the LCD panel on the above-mentioned hologram screen, the stereograms
being divided into right and left, are condensed by turns into a horizontal direction. Then the stereoscopic observa-
tion may be make from any position or by two more persons at the same time.
In order to enlarge the image, one arranges in horizontal direction many of such screen successively, which is
photographed under the same condition as A and B.
3. Experiment Result and Examination
When stereoscopic observation is made of the image of slit-division method, the parallax images that observed
at the positions of a left eye and of a right eye are displayed in Fig.6. Some technical devices adopted in this
research are following.
a. Making a thin mask with high quality: We first make a master mask by arranging the wires with 2.5 mm
diameter at similar intervals of2.5mm. We then put this master mask on the emulsion side of the hologram plate for
masks and expose it by the incident parallel beam from the front. After development, we get a thin mask for
photography. If we use this thin mask, not only the object light from the front but the object light from transverse
direction can be exposed with sufficient accuracy on a hologram.
b. The reflective light from the rear surface of the plate interferes, and acts as a noise. As an object used for a
light source, we choose a diffusive object whose reflectance is high along the normal direction., Weakening the
brightness of the object light source, we take a photograph. In this way, the influence of back reflection can be
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reduced. In the experiment, the ratio of the brightness of reference beam to that of object beam is 3: 1.
c. Due to the position of the viewpoint at the time of reproduction, the inclination directions of the object beam
and of the reference beam are not in same plane to the hologram plate. Then, the photographic optical system is
three dimensions. Thus, due to the directivity of the object light, the degree of condensation of the reproduced light
to the transverse direction is debased. However, if the diffusive object light source is used the influence of directiv-
ity is reduced.
d. A faithful color reproduction is made through performing a triple-record in such a may that each of the RGB
three primary colors meets the corresponding Bragg condition. Due to the form of the object light source in this
experiment, reproduced light diffuses up and down. Then, we forecast that the light with fairly extensive zone is
reproduced by using a white light source, although it dose not satisfy the Bragg conditions.
e. Since the different object light source results in different optical path difference, as a number of images of
light sources increases, the spatial coherence of the beam is reduced and a noise arises. Then, it is necessary to
restrict the number of times of multiplex reflection used for reproduction.
4. Future Subject
As stated above, there are various factors that act to reduce the diffraction efficiency of a holographic optical
element in the fact. We forecast it is effective that by making use of the laser with sufficient spatial coherence length
or the photography system like to the edge-lit holography.
In order to carry out color reproduction, it is required to use the triple-exposures or 3-color laser reproduction,
etc.
These are future research subjects.
Reference
1) H.Harashima, Science of3-dimensionalpicture andhuman being, OHMSHA (2000)
2) J.Tsujiuchi, Holography, SHOKABO(1997)
3) K. Inoue, Holographic Display Study Group News, No.3 (1982)
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Stereoscopic Display with Recurrent Viewpoints Using Hologram of Multiple Focal Points

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Stereoscopic Display with Recurrent Viewpoints Using Hologram of Multiple Focal Points

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