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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical correlator utilized for
photometry, optical information processing and the like. More
particularly, the present invention relates to an optical correlator which
identifies a target object automatically from among two-dimensional images
through a coherent optical correlation process.
2. Description of the Prior Art
Various types of optical correlators are known.
One type of optical correlator utilizes a method for making a correlation
filter by means of holography for detecting correlation. However, the
method requires holograms which make use of Fourier transform patterns for
comparison of specifically prepared images which is time consuming and
since a pertinent space modulator is not provided for the holograms of the
prior art, the holography utilizes a method for recording images lacking
in real time efficiency.
Therefore, K. Kasahara, Japanese Patent Laid-Open Nos. 138616/1982,
210316/1982, 21716/1982, discloses an optical correlator utilizing a
method for transforming two coherent images into first Fourier transform
images through a Fourier transform lens, transforming first Fourier
transform images into second Fourier transform images through a Fourier
transform lens again, and generating a self-correlation peak and a
cross-correlation. The optical correlator is realized with a quasi-real
time operation by using a liquid crystal display device for forming two
pictorial information sets for comparison with one another however, the
two compared images or sets must be spaced apart substantially, thus the
operation requires a large optical system or resolution decreases.
Further, in case one of the two compared images moves relative to the
other, the prior art optical correlation has an extremely narrow field of
view and is not operable for minute positioning.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical correlator
which erases a self-correlation peak of two images to be compared and
detects only a cross-correlation peak of the two images to be compared at
a high S/N ratio.
Another object of the present invention is to provide an optical correlator
which precisely indicates a positional relationship of the two images
without depending on a positional relationship of input images.
A further object of the present invention is to provide an optical
correlator which is stable against disturbance such as noise so that
errors are prevented.
To realize the above objects, the optical correlator of the present
invention has first transforming means for transforming two sets or
patterns of pictorial information to be compared into coherent images,
first generating means for generating a phase conjugate waveform, second
generating means for generating pictorial patterns of a sum of the two
patterns of pictorial information and a difference between the two
patterns of pictorial information, second transforming means for
transforming the pictorial patterns into Fourier transform images, and
shifting means for shifting pictorial patterns of Fourier transform images
to the first transforming means.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings,
FIG. 1 is an illustration representing one embodiment of an optical
correlator according to the present invention; and
FIG. 2 is an illustration representing another embodiment of an optical
correlator according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention will now be described in detail with reference to its
embodiments.
FIG. 1 is an illustration representing one embodiment of an optical
correlator according to the present invention.
A coherent light 1a generated by a laser 1 such as argon ion laser or the
like is transformed into a parallel light expanded in beam width by a beam
expander 2, passes a beam splitter 3, and is incident on a beam splitter
4. In this case, the transmissivity and reflectivity of the beam splitters
3, 4 are 50% each.
The light reflected on the beam splitter 4 passes a space modulator 6 such
as a liquid crystal display device or the like for displaying a first
input image 6a (not shown) thereon. The light is then reflected by a
mirror 8, passes a lens 10, is reflected by a mirror 11, and is incident
on a non-linear optical crystal 12 such as BaTiO.sub.3 or the like. The
first input image 6a is focused on a surface of the non-linear optical
crystal 12.
Furthermore, the light which was passed through the beam splitter 4 passes
a space modulator 5 such as a liquid crystal display device or the like
for displaying a second input image 5a (not shown) thereon, which is
placed at a spot equivalent optically to the input image 6a, is reflected
by a mirror 7, passes a lens 9, and is incident on the non-linear optical
crystal 12. The second input image 5a is focused on a surface of the
non-linear optical crystal 12.
In the case when BaTiO.sub.3 is used as the non-linear optical crystal 12,
it is desirable that the first input image 6a is incident on a face
vertical to the C-axis of the BaTiO.sub.3 at about 15.degree. and the
second input image 5a is incident on a face vertical to the C-axis at
about 19.degree..
A phase conjugate waveform generated by the non-linear optical crystal 12
is incident on the beam splitter 4 and the beam splitter 3 through the
same route as that for incidence of the coherent light input from opposite
sides of the beam splitters 3 and 4. In this case, as disclosed in
"Optical Engineering" May 88, Vol. 27 No. 5 385, the light perpendicularly
reflected in a direction perpendicular to the incident axis on which it is
incident through the space modulator 5 and the light passed axially to the
incident axis on which it is incident through the space modulator 6 are
focused at a point A which is symmetrical to the point on the space
modulator 5 about the normal to the beam splitter 4. And its intensity is
as follows;
I.sub.A =I.sub.1 .vertline.E.vertline..sup.2
.vertline..sigma..vertline..sup.2 RT.vertline.T.sub.1 (X, Y)-T.sub.2 (X,
Y).vertline..sup.2 (1)
T.sub.2 (X,Y) includes the images which are located at a predetermined
distance away from the optical axis and which do not overlap each other on
formation of the sum of the images and the difference between the images.
Furthermore, a light which is incident on the beam splitter 3 through the
space modulator 5 and the beam splitter 4, and a light which is incident
on the beam splitter 3 through the space modulator 6 and the beam splitter
4, are reflected at the beam splitter 3, and are focused at a point B
which is symmetrical to the point on the space modulator 5 about the
normal to the beam splitter 3. The intensity of this focused light is as
follows:
I.sub.B =I.sub.1 R.sub.1 .vertline.E.vertline..sup.2
.vertline..sigma..vertline..sup.2 .vertline.TT.sub.1 (X, Y)+RT.sub.2 (X,
Y).vertline..sup.2 (2)
In Eqs. (1) and (2), I.sub.1, R.sub.1 represent transmissivity and
reflectivity of the beam splitter 3 respectively, and T, R represent
transmissivity and reflectivity of the beam splitter 4 respectively. Then,
.sigma. represents a reflection coefficient of a phase conjugate mirror,
when the non-linear optical crystal 12 operates as the phase conjugate
mirror. E represents an amplitude of the incident light. Further, T.sub.1
and T.sub.2 represent a transmission distribution of the first and second
input images 6a, 5a each.
Now, if transmissivity and reflectivity of the beam splitters 3 and 4 are
specified at 50% each, then:
I.sub.A =1/8.vertline.E.vertline..sup.2 .vertline..sigma..vertline..sup.2
.vertline.T.sub.1 (X, Y)-T.sub.2 (X, Y).vertline..sup.2 (3)
I.sub.B =1/16.vertline.E.vertline..sup.2 .vertline..sigma..vertline..sup.2
.vertline.T.sub.1 (X, Y)+T.sub.2 (X, Y).vertline..sup.2 (4)
Thus, the image focused at the point A represents a difference between the
first and second input images 6a, 5a, and on the other hand, the image
focused at the point B represents a sum of the first and second input
images 6a, 5a.
Next, when Fourier transform lenses 13, 14 are disposed at positions where
the points A and B become front focal points of the Fourier transform
lenses 13, 14, the rear focal planes of the Fourier transform lenses 13,
14 are Fourier transform planes of both the input images. Light receiving
elements 15, 16 such as CCD and the like are placed at the positions which
are the rear focal planes of the Fourier transform lenses 13, 14, and
sensitivities of the light receiving elements are adjusted so to equalize
outputs of both light receiving elements 15, 16 when the input is not
operative through Fourier transform lenses 13, 14. As a result,
intensities on the Fourier transform planes will be:
I.sub.A '=.alpha..vertline.F(T.sub.1 (X, Y)-T.sub.2 (X,
Y)).vertline..sup.2(5)
I.sub.B '=.alpha..vertline.F(T.sub.1 (X, Y)+T.sub.2 (X,
Y)).vertline..sup.2(6)
In Eqs. (5) and (6), .alpha. represents a proportionality constant, which
is decided according to a reflection coefficient of the input light
intensity phase conjugate mirror, sensitivity of the light receiving
element and so forth.
Next, Fourier transform images received by the light receiving elements 15,
16 are sent to a frame memory 17 of a computer for storage. Then, images
formed by intensity patterns of each of the Fourier transform images are
again written in the space modulators 5, 6 such as a liquid crystal
display device or the like. The subsequent process is as described above
and hence is omitted here. Because of the shift invariance of Fourier
transformation, the images written in the space modulator 5 and 6 overlap
each other centering around the optical axis on formation of the sum of
the images and the difference between the images. However, according to
the phase conjugate waveform generated by the non-linear optical crystal
12, the difference between Fourier transform images is outputted to the
point A with the following intensity:
I.sub.A "=.beta.(F(T.sub.1 (X, Y)T.sub.2 *(X, Y)+T.sub.1 *(X, Y)T.sub.2 (X,
Y)) (7)
and the sum of Fourier transform images is outputted likewise to the point
B with the following intensity:
I.sub.B "=.beta.(F (T.sub.1 (X, Y).sup.2 +T.sub.2 (X, Y).sup.2)(8)
and then these image are transformed again to Fourier transform images
through the Fourier transform lenses 13, 14, therefore outputs of the
light receiving elements 15, 16 will have the following intensities:
I.sub.A '".varies.T.sub.1 (X, Y) T.sub.2 (X, Y) (9)
I.sub.B '".varies.T.sub.1 (X, Y) T.sub.1 (X, Y)+T.sub.2 (X, Y) T.sub.2 (X,
Y) (10)
Here, a correlation operation.
Thus, only a cross-correlation peak output is obtainable from the light
receiving element 15, and only a self-correlation peak output is
obtainable from the light receiving element 16.
Accordingly, the luminous intensity of self-correlation peaks for the first
and second input images does not appear at all on the light receiving
element 15, therefore, even in case one of the two comparison images moves
relative to the other, a cross-correlation peak will never be buried in a
self-correlation peak. Thus, a target object can be continually tracked,
and absolute position coordinates can be derived for utilization on minute
positioning. Then, since noise and other disturbances which are included
in Eqs. (5) and (6) concurrently and which are generated by speckle, dust
on each element and other contaminants will be erased, an identification
error due to generation of a false correlation peak or the like will be
prevented, and detection at a high S/N ratio will be realizable.
FIG. 2 is an illustration representing another embodiment of an optical
correlator according to the present invention.
The space modulators 5, 6 such as a liquid crystal display device or the
like used in the above-described embodiment are substituted by
photosensitive films 18, 19 for reproducing input images in the form of
transmissivity distributions, and the light receiving elements 15, 16 are
substituted by photosensitive films 20, 21 which are capable of
reproducing output images in the form of transmissivity distributions. A
procedure for obtaining output images is the same as the foregoing
embodiment and hence is omitted here. In this case, the photosensitive
films 20, 21 upon which output images are reproduced are shifted to
substitute light receiving elements 15,16 to accommodate the
photosensitive films 18, 19 such that output images are again generated
through a procedure similar to that of the foregoing embodiment, thus a
self-correlation peak and a cross-correlation peak are generated
separately from each other as in the case of the foregoing embodiment. In
this case, for example, although a real time efficiency may be lost,
information travelling in a special wave envelope will be obtainable by
using a plate used in X-ray photography for recording an internal defect
of an object or an internal defect of the human body as an input image.
Since resolution and contrast ratio of the plate are high normally as
compared with the space modulator such as a liquid crystal display device
or the like, a correlation of details detected using the latter embodiment
can be compared instantly.
As described above, since the optical correlator of the present invention
erases self-correlation peaks of input images and detects only
cross-correlation peaks of input images without using means such as
holography or the like, the optical correlator can track a target object
moving arbitrarily at all times, makes use of absolute position
coordinates for targeting, and is utilized in minute positioning.
Additionally, the optical correlator eliminates noise which is generated
by dust and marring of each element, or speckle, and it detects a
cross-correlation peak at a high S/N ratio.
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