Hologram synthesis

The invention relates to hologram synthesis using a two-dimensional lattice of basic holograms, each basic hologram being formed from a two-dimensional transparency. It provides a synthetic field hologram that has a diffraction efficiency unimpared by the number of basic holograms employed. The hologram may be reconstructed in white light. The method and hologram are useful in making and reconstructing holographic land maps.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to hologram synthesis from two-dimensional 
(2-D) transparencies and is particularly concerned with a new and improved 
method and hologram for reconstructing three-dimensional (3-D) objects 
from the multiple views of the objects recorded separately. 
Large outdoor scenes such as monuments, buildings, ships etc. cannot be 
directly recorded on usual holograms. However, there are methods of 3-D 
hologram synthesis from 2-D photographic transparencies that make 
multiplex holograms from which 3-D outdoor objects can be reconstructed. 
In priciple there are two basic techniques for constructing the multiplex 
holograms. Individual basic holograms are multiplexed either on a whole 
holographic plate overlapping each other or on adjacent small areas 
contiguous over a holographic plate. These areas are mostly vertical 
stripes, but theoretically small rectangles arranged in a 2-D lattice may 
be used instead of the stripes. Methods that are something between the two 
above-mentioned techniques have been also applied in practice. 
The method where the individual basic holograms overlap each other suffers 
from the serious fall-off of the diffraction efficiency with the 
increasing number of multiplexed holograms and is essentially useless from 
a practical standpoint. Attempts to alleviate this drawback, for instance 
by hologram copying or by using litium niobate for hologram recording, are 
not satisfactory enough. 
The stripe recording technique does not suffer from the above mentioned 
drawback. However, there are other shortcomings in this technique. First, 
the application of the stripe holograms brings the loss of one of two 
parallaxes. This means that a holographic stereogram constructed by this 
technique can reconstruct an image with only one parallax. It is the 
horizontal parallax which is mostly recorded. Second, the plane of such a 
hologram is the plane of adjacent pupils for the direction perpendicular 
to the stripes. For viewing such a multiplex hologram the eye pupil ought 
to be placed as close to the hologram as possible so as not to reduce the 
impression from the stereoscopic reconstruction. When the eye pupil is not 
close enough to this type of multiplex hologram, the different parts of a 
reconstructed object are viewed through different individual basic 
holograms (stripes) for a static position of the eye pupil and this fact 
causes the mentioned reduction of the impression from the stereoscopic 
reconstruction. The eye pupil at the plane of the hologram would have 
represented the optimum case for viewing the reconstructed synthetic 
image. Of course, this can never be achieved because of the impossibility 
of placing the eye onto a large mechanical plate. Moreover, it is obvious 
that it would not be suitable to view the hologram from some excessive 
distance. However, there is another reason which constrains one to view 
this type of hologram from a long distance. The other reason is connected 
with white-light reconstruction and the rainbow holographic technique. 
The white-light reconstruction of a hologram (which is a necessary 
condition for certain commercial purposes) can be realized by using a 
reflection hologram or by using some version of rainbow holography. The 
rainbow method unconditionally requires placing the pupil of an imagery 
for the direction perpendicular to the "holographic grating" far enough 
from the holographic plate. Consequently, we must also place the eye pupil 
viewing the reconstruction far enough from the hologram. This means that 
from the point of view of the successful rainbow reconstruction, the 
requirement of placing an eye far from the hologram is contrary to the 
requirement for a fully successful steroscopic reconstruction which 
requires placing the eye as close to the hologram as possible. In 
practice, the priority is given to the former requirement so that the 
steroscopic impression is partially lost. Of course it is impossible to 
view a reflection hologram from a vicinity without obstructing the 
reconstruction beam, so that the version replacing the rainbow method by a 
reflection hologram would not bring an improvement. 
To summarize, up to this time there has not been a method for making 
synthetic holograms from 2-D transparencies which would quite successfully 
reconstruct 3-D objects either in white-light or with help of only a 
laser. 
The present invention is directed to a new synthetic hologram and the 
method of its creation. This hologram is referred to as the synthetic 
field hologram and for shortness designated by the letters SFH. With help 
of the disclosed method and SFH it will be possible to record and 
reconstruct generally in white-light any 3-D object without loss of any 
parallax. This means, that with this invention it will be also possible to 
make 2-D holographic maps providing the 3-D images of recorded 
territories. The reconstructed synthetic image of an object made with a 
required magnification and with or without changed parallaxes can be 
one-color or full-color. The most suitable distance for viewing the SFH 
can be a priori selected. The diffraction efficiency of the SFH is not 
dependent on the number of basic holograms, i.e. on the number of the 
recorded views of the objects, and the reconstructed image can be viewing 
without vignetting and with the maximum possible impression from the 
volume reconstruction. It is also possible for a series of SFH's to be 
readily made. 
The aforementioned features, advantages and benefits of the invention, 
along with additional ones, will be seen in the ensuing description of one 
possible embodiment and claims which should be considered in conjunction 
with the accompanying drawing. 
The drawing promotes easier understanding of the embodiment of the 
invention showing only one of many possible arrangements that can be 
considered for the practicing the invention.

DESCRIPTION 
For purposes of explaining principles of the invention let it be assumed 
that it is desired to create a holographic map, i.e. a synthetic hologram 
of a territory serving as the large outdoor object. First multiple views 
of the selected territory are photographically recorded. This recording 
can be done by taking pictures of the same part of the territory from 
different positions of a camera placed for instance at the intersections 
of a fictitious square net at a plane high enough above the territory. The 
image views are taken from different directions on individual frames of a 
film. These directions are generally distributed in the 3-D space so that 
the information about the changes of the object profile in both the 
independent directions have been photographed. A sufficient number of 
views are used so that the desired information about the changes of the 
object profile in both the independent direction have been obtained. The 
suitable distance of the camera from the object and the spacing between 
individual points at which the photographs are taken will depend upon the 
object size and its profile. A black-white or color film is required for 
one-color or color reconstruction, respectively. 
Once these photographic images of the object have been taken they are used 
to construct a synthetic hologram referred to as a subsidiary synthetic 
hologram and for shortness designated by the letters SSH. All the frames 
are successively recorded on a holographic plate to yield, preferably, a 
transmission SSH. The SSH consists of the individual basic holograms where 
each basic hologram records the image of the object from one particular 
view. The individual basic holograms are arranged in a two-dimensional 
structure on the plate of the SSH in such a way that all reconstructed 
together yield the real synthetic image of the object, which would evoke 
the impression of the 3-D copy of the object if it had been possible to 
view this image from the direction of light propagation. The most suitable 
distance of the real synthetic image from the SSH equals the distance we 
want to use for viewing a final reconstruction from the synthetic field 
hologram, or SFH, which will be described in detail later on. This 
distance will be referred to as the distance d. Further, we will refer to 
the whole synthetic field of the synthetic image of the object as the 
synthetic signal field or briefly only as the signal field. Three SSH's in 
the three different wavelengths must be constructed if a final 
reconstruction from the SFH is required in color. 
One important aspect of the invention lies in the fact that, in principle, 
the basic holograms don't overlap each other on the surface of the SSH so 
that the diffraction efficiency of the SSH is not decreased due to mutual 
overlapping of the basic holograms. Moreover, the creation of the 
synthetic signal field is made in such a way that the pupils of the 
individual imagings of the synthetic signal field are directly the basic 
holograms on the SSH for both the independent directions perpendicular to 
the beams. The surface of these pupils, which is given by the surface of 
the SSH during construction and reconstruction of this hologram, will be 
referred to as the pupil surface, regardless of whether the SSH is present 
or not in the further steps of the invention. There are also two 
subsidiary conditions for obtaining the best possible final reconstruction 
from the SFH. First, each basic hologram should be equal to or larger than 
the eye pupil which will view the final reconstruction. This eliminates 
blurring of the final reconstruction. Secondly, all the basic holograms 
should be contiguous, i.e. they should be immediately adjacent each other 
without any gaps and with no overlapping. Gaps between the basic holograms 
would contribute to flickering of the final reconstruction during viewing 
and overlapping of the basic holograms would create dark moire fringes 
over the reconstructed synthetic image (due to double reconstruction). 
FIG. 1 illustrates a possible recording of one basic hologram on the SSH. A 
coherent wave I illuminates a transparency T containing the photographic 
image of one view of the object. The lens L yields the image T' of 
transparency T at the distance d from the plate of the SSH. The basic 
hologram of the SSH is created with help of the reference wave R. The mask 
M protects the recording medium of the SSH except the area of one basic 
hologram being recorded. The distance d and the magnitude of the image T 
can be selected by using suitable parameters of the lens L. The lens L 
should be as close as practically possible to the plate of the SSH in 
order that the lens exit pupil be not too far from the SSH where the pupil 
surface of the synthetic signal field is. 
For recording each new basic hologram from each new transparency the plate 
of the SSH together with the reference source R are shifted and tilted as 
one rigid body (with respect to the lens L and the transparency T) in such 
a way that a later simultaneous reconstruction of all the basic holograms 
of the SSH with help of one coherent reconstruction source results in the 
real synthetic image of the 3-D object without aberrations in the distance 
d from the SSH. The mask M is always tilted but not shifted together with 
the plate of the SSH. The construction of the SSH of the above prescribed 
properties can be also realized in any other suitable way than the 
specific one disclosed here. 
Once the SSH has been constructed the invention involves the subsequent use 
of the synthetic signal field reconstruction from the SSH as the optical 
coherent signal for construction of a further transmission or reflection 
hologram which is referred to as the subsidiary synthetic field hologram. 
For shortness we will designate this hologram by the letters SSFH. The 
distance of the SSFH from the SSH would not be smaller than the distance d 
in order that the real synthetic image reconstructed from the SSH is not 
behind the plate of the SSFH. The only function of the SSFH is to reverse 
the propagation of light of the synthetic signal field. This is realized 
by the technique of "reverse reconstruction" in which the reference wave 
propagating in the reverse direction (i.e. impinging on the SSFH from the 
opposite side) is used for reconstruction of the SSFH. Generally, only the 
signal beam propagating in the direction from the synthetic image to the 
pupil surface can evoke the successful impression of the required 
reconstruction. If the technique of "reverse reconstruction" can not be 
successfully applied on a color reconstruction of a final reflection SFH 
we construct the SSFH. However, for at least one color reconstruction the 
technique of "reverse reconstruction" can be applied directly on the final 
reconstruction of the SFH, regardless of whether it is transmission or 
reflection hologram, and in such a case the step of the construction of 
the SSFH can be skipped. The SSFH, or the SFH if made directly from the 
SSH, may be considered as a master synthetic field hologram. 
The SSFH has an important property that will also be important for the 
final SFH. The individual basic signals from the basic holograms of the 
SSH can be arbitrarily overlapped on the surface of the SSFH or final SFH 
without affecting the diffraction efficiency of the SSFH or SFH. This 
phenomenon of the invention is reached by the simultaneous reconstruction 
of all the basic holograms of the SSH. 
One possible variation of the construction of the SSFH is illustrated in 
FIG. 2 where only three basic holograms on the SSH are considered. R is 
the coherent reconstruction field reconstructing the synthetic signal 
field from the SSH. A coherent reference wave for creating the 
transmission SSFH is designated by the letter A. The broken line in front 
of the SSFH shows the position of the real synthetic image. 
Three SSFH from three SSH have to be constructed for three different 
wavelengths when the final reconstruction from the SFH is to be in full 
color. 
Having made SSFH we use the "reverse reconstruction" of this hologram for 
obtaining the reversely propagated synthetic signal field. This field is 
then used for the construction of the final SFH. The most suitable 
position of the plate for recording the SFH is the position in which the 
real synthetic image reconstructed from the SSFH, or directly from the SSH 
when the construction of the SSFH can be skipped, is focused on the plate 
of the SFH. Only in this case the synthetic image reconstructed later from 
the SFH will not be vignetted. Preferrably, the SFH is a reflection type 
of hologram which allows its reconstruction in white light. If the color 
reconstruction of the SFH is required three SSFH's each recording the 
synthetic signal field in a different wavelength have to be successively 
applied for making one relfection SFH. The special processing of the 
reflection SFH is necessary for keeping the emulsion from shrinking and 
ensuring a true color reconstruction when the recording medium of the SFH 
is a photographic emulsion. 
The step of the construction of the SFH from the SSFH is sketched in FIG. 
3. The coherent reconstruction wave A reconstructs the reversely 
propagated signal field which is recorded on the reflection SFH with help 
of the coherent reference beam B. 
The SFH has all the advantages which a "classical" synthetic hologram that 
has the individual basic holograms mutually overlapped over its surface 
would have been able to provide if its diffraction efficiency had been 
satisfactory. 
Be it noted, that from the SSFH or directly from the SSH can easily be 
prepared an arbitrary number of the final SFH so that, for instance, an 
arbitrary number of 2-D holographic maps providing 3-D images of a 
territory can easily in made. 
The reflection SFH can be reconstructed in white-light. The best 
recommended distance for the viewing of any SFH is the distance in which 
the eye pupil lies at the pupil surface of the synthetic field, i.e. in 
our case the distance d.