Note: Descriptions are shown in the official language in which they were submitted.
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SELF-SUPPORTED OPTICAL CORRELATOR
FIELD OF THE INVENTION
The present invention relates to an optical correlator, and more specifically
to such
a correlator which is self-supported, and can be joined to other such optical
correlators in a laterally stacked fashion.
BACKGROUND OF THE INVENTION
Various screening tasks require massive computing capabilities. Although
computing devices have shown ever increasing processing power, there is still
a
need for high speed computing, especially when it comes to the screening of
images. Optical correlators could eventually fill the gap between the
applications
and the processing requirements.
An optical correlator takes advantage of the powerful capabilities of light to
perform real-time computation. As illustrated in Figure 7 (Prior Art), a light
beam
incoming from a laser source is directed through a first set of lenses to
expand its
diameter. The light passes through a first spatial light modulator on which an
image is displayed. Then, the modulated beam will undergo a first Fourier
transform by passing through another lens. The Fourier transform is performed
simply by the propagation of the light and as such is realised very rapidly.
It is an inherent property of a lens to perform a Fourier transform on an
input
image that will be observed at the front focal plane of the lens, provided
that this
image is displayed at the back focal plane of the lens. The optically-computed
2D
Fourier transform signal will cross the filter plane. It is on this second
spatial light
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modulator that the reference template corresponding to the searched object
(the
target) will be displayed. In fact, it is the Fourier transform of the
reference
template that is recorded. So after travelling trough this second spatial
light
modulator, a multiplication of two Fourier transforms is obtained. In the
spatial
domain this corresponds to a correlation. In order to achieve the conversion
between the frequency and the spatial domains, a second Fourier lens is used
and
the beam exits the optical system in a parallel way. The camera is the last
component of the correlator and detects the intensity all over the correlation
plane.
Basically, the system processing speed is limited only by the refresh rate of
the
io electro-optic components (spatial light modulator, camera), because the
computation itself is performed using the light.
The optical correlator principle has been known since the work of Vander Lugt.
Since then, a lot of work has been spent on generating filters to enhance
specific
recognition performances such as multiple target recognition with composite
filters,
enhanced discrimination with phase-only filters, or rotation invariant
recognition
with circular harmonic filters. Various optical correlator types have also
been
proposed such as a Vander Lugt correlator. In this correlator architecture,
similar
to the one illustrated in Figure 7, the image is displayed in the input plane
whereas
the filter is displayed in the frequency plane. The correlation is acquired at
the
output plane. The filter was at that time recorded on a spatial carrier. A
Joint
Transform correlator (JTC) was also proposed. In a JTC, both the image and the
reference template are recorded in the input plane. The interference pattern
is
recorded in the frequency plane and sent back to the input plane to obtain the
correlation in the frequency plane, after a second pass through the
correlator.
Despite extended work on optical correlator filters and architectures, it did
not
result in solutions which address the critical opto-mechanical structure
required to
obtain satisfactory optical correlation performances.
Various architecture implementations have been proposed for optical
correlators,
such as "Coherent Optical Correlator" (United States Patent 4,277,137), and
the
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optical correlator principle taught in "Holographic Information Storage and
Retrieval" (United States Patent 3,608,994). Architectures have also been
proposed to make the overall system more compact, such as "Compact 2F Optical
Correlator" (United States Patent 5,073,006).
These solutions usually result in optical set-ups where each individual
optical
element is inserted in a holder fixed on an optical table. This results in
excessive
production cost.
1o Furthermore, although optical correlator architectures were addressed in
these
patents, little or no consideration was devoted to the opto-mechanical
structure
that influences production cost and ease of alignment.
Nowadays, optical correlators are not widely spread either in terms of
commercial
applications or availability as commercial products. This is mainly due to the
high
production cost related to the aforementioned opto-mechanical structure and to
the difficulty of alignment of the optical correlator.
Lack of market penetration has also left unaddressed other considerations of
optical correlation implementation, such as heat dissipation and heat
stabilization.
The possibility to achieve multichannel optical correlators has been addressed
in
United States Patent 3,802,762. However, this possibility is limited by the
availability of powerful laser sources that can drive multiple correlators
simultaneously and by the interference that can be produced between the
various
channels.
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SUMMARY OF THE INVENTION
The present invention is directed to an optical correlator which solves the
above-mentioned deficiencies of the prior art.
In accordance with the invention, there is provided a self-supported optical
correlator, comprising:
a first holder having two opposite ends, one of said opposite ends being
io provided with anchor points, the other of said opposite ends being provided
with a
light source;
a second holder having two opposite ends, one of said opposite ends being
provided with anchor points, the other of said opposite ends being provided
with a
light receiving element,
a plurality of intermediary holders, each having two opposite ends, each of
said holders being provided with anchor points at each opposite end, at least
one
of said intermediary holders being provided with a spatial light modulator for
projecting an image and another of said intermediary holders being provided
with
another spatial light modulator for projecting a filter,
each of said intermediary holders being provided with optical components,
said optical components being secured within said holders,
said anchor points being adapted to secure said first, second and
intermediary holders together linearly end to end;
wherein, when said intermediary holders are assembled end to end, and
said first holder is assembled at one extremity and said other holder is
assembled
at another extremity, said
resulting assembly forms said optical correlator.
In accordance with another aspect of the invention, there is provided a self-
supported optical correlator, comprising:
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a first holder having two opposite ends, one of said opposite ends being
provided with anchor points, the other of said opposite ends being provided
with a
light source;
a second holder having two opposite ends, one of said opposite ends being
5 provided with anchor points, the other of said opposite ends being provided
with a
light receiving element,
at least one intermediary holder, each of said at least one intermediary
holder having two opposite ends, each of said at least one holder being
provided
with anchor points at each opposite end, at least one of said at least one
1o intermediary holder being provided with a spatial light modulator for
projecting an
image and another spatial light modulator for projecting a filter,
each of said at least one intermediary holder being provided with optical
components, said optical components being secured within said holders,
said anchor points being adapted to secure said first, second and
intermediary holders together linearly end to end;
wherein, when said at least one intermediary holder are assembled, and
said first holder is assembled at one extremity and said other holder is
assembled
at another extremity, said resulting assembly forms said optical correlator.
In accordance with yet another aspect of the invention, there is provided a
self-
supported optical correlator, comprising a housing for receiving a light
source at
one extremity and a light receiving element at another extremity, said housing
being further adapted to receive optical components therein, said optical
components being toleranced and forming an optical correlator, said housing
being further adapted to receive therein a display for projecting an image in
an
optical axis and a display for projecting a filter in said optical axis, said
housing
being tubular and having an opaque outer surface.
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BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood after having read a
description of a
preferred embodiment thereof, made in reference to the following drawings in
which:
Figure 1 is a cross-sectional view of an optical correlator according to a
preferred
embodiment thereof;
io Figure 2 is a perspective view of the correlator of Fig. 1;
Figure 3 is a partial cross-sectional view of the correlator of Fig. 1;
Figure 4 is a representation of a plurality of correlators stacked together;
Figure 5 is a schematic representation of tagging;
Figure 6 is a schematic representation of a polarity LUT application;
Figure 7 (Prior art) is a schematic representation of a typical correlator;
and
Figure 8 is a schematic representation of a system using a correlator.
DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
The tubular optical correlator optomechanical structure proposes a self-
supported
tubular architecture illustrated in a preferred embodiment in Figures 1 and 2.
However, although a combined structure is illustrated in the accompanying
Figures, the present invention also concerns an overall structure which may
differ.
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For example, as will be understood hereinafter, there may be more or less
individual holders.
One advantage of the structure of the present invention is that it combines
the
component holder and the optical structure into a single structure that
reduces the
overall number of components.
To that effect, the tubular optical correlator optomechanical structure
consists in a
single tubular assembly structure 10, where the holders of the optical
components
1 o are used at the same time as building blocks for the tubular optical
correlator
structure.
More specifically, the tubular optical correlator preferably consists in a
first and
second holders. The first holder 11 has two opposite ends 111, 113. A first
opposite end 111 is provided with anchor points 1 and the other opposite end
113
is provided with a light source 6, preferably a laser.
The second holder 16 has two opposite ends 115, 117. A first opposite end 115
is
provided with anchor points 1 and the other opposite end 117 is provided with
a
light receiving element 5, such as a camera.
The optical correlator 10 further preferably includes a plurality of
intermediary
holders 12, 13, 14, 15 which are longitudinally assembled together. Each
holder
12, 13, 14, 15 has anchor points 1 at each opposite end, and is further
provided
with optical components 2.
At least one intermediary holder is provided with a display 3 for projecting
an
image, and another intermediary holder is provided with a display 3 for
projecting a
filter. In a preferred embodiment of the invention, the displays are of course
adapted to the invention, and include spatial light modulators.
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Preferably, the holders 11, 12, 13, 14, 15 have an opaque outer surface, and
are
preferably tubular.
An example of an intermediary optical component holder is illustrated in
Figure 3
where the optical components are inserted in a monoblock tubular structural
element. The multiple structural elements are assembled together as
illustrated in
Figure 1. Each structural element is attached to the adjacent ones at anchor
points. Combined together, all the building blocks generate a single self-
supported
structure illustrated in Figures 1 and 2. No supplementary holding plate or
external
io structure is required to further position and support the component
holders.
Figure 3 illustrates a single structural element or tubular optical correlator
module.
The optical design of the correlator is toleranced. This means that the
optical
components may be slightly displaced either laterally or longitudinally,
within a
mechanical tolerance, without affecting significantly the correlation
obtained. The
maximum displacement is different for each element of the optical correlator.
The
optomechanical support must respect fabrication tolerances that are compatible
with the maximum displacement permitted for the various optical components.
Doing so the optical design prescriptions are respected when using the
optomechanical support. The optical components of Figure 3 are constrained by
the housing. Consequently alignment does not require translation or tilt
mechanisms reducing the number of components and the time required to align
the system.
The use of a tubular architecture provides a rigid self-supported structure
that can
be further mechanically isolated from the apparatus housing. This will prevent
the
environmental vibrations to affect the mechanical stability of the optical
correlator.
All building blocks are thermally connected, as illustrated in Figure 1,
yielding a
short stabilisation period for the tubular optical correlator structure.
Moreover, the
use a rugged tubular shape minimizes the thickness of the external structure
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illustrated in Figure 3 required for a given rigidity when compared to other
structures such as cubic or otherwise. With less material, the structure
exhibits a
smaller thermal inertia reducing consequently the period required to reach the
thermal equilibrium of the tubular optical correlator.
s
The tubular architecture illustrated in Figure 2 is preferably, as mentioned
above,
composed of holders exhibiting symmetry of revolution. These modules
necessitate mostly turning machining that is cheaper and faster to fabricate
than
more complex shapes.
The outer walls of the tubular optical correlator optomechanical structure are
opaque, as illustrated in Figure 3, and cover completely the optical path. The
light
emerging from the optical path is thus confined within the holding structure.
Doing
so, and taking advantage of the self-supported structure, multiple tubular
1s structures can be laterally stacked along each other (see Figure 4) without
mutually interfering. The tubular optical correlator architecture can thus be
easily
stackable.
The tubular optical correlator further contains an electronic control unit
making
use of a digital communication and addressing scheme that introduces onboard
image, filter and correlation tagging to uniquely identify source information
and
corresponding results
Based on this tubular optical correlator structure, real-life applications
require
some more specific items related to signal communication and driving
electronic
components. In a typical correlator the image and the filter are sent
together, then
after a certain lapse of time the correlation results is acquired. This
process is
based on a basic clock and is a continuous process. When the main control
system send images and filters to the correlator there is an uncertainty about
the
correlation retrieval identification. Due to potential delays in processing
time, in
copy time, in transfer time or simply in display time, the correlation
retrieved could
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come from the current image-filter pair sent, from the previous one, or from
the
ones sent some frames ago.
To obviate this uncertainty, according to a preferred embodiment of the
invention,
5 a tag is inserted in the image (Itag) and in the filter (Ftag) as
illustrated in Figure 5.
When the optical correlator electronic driver receives the image-filter pair,
the tag
is extracted and copied on the following correlation (Ctag). When the
correlation is
sent back to the control system there is no temporal uncertainty between the
filter
and the image that were correlated and the corresponding result.
Many optical correlators use spatial light modulators that are driven with
alternative polarity mode. Among others, this is done with liquid crystal
technologies. The image is displayed first in a positive polarity, then in the
following frame the image is displayed in inverted polarity. This prevents
electrolysis of the liquid crystal display. However, inverting the signal
polarity
usually implies using the driving electronic components at a slightly
different
operation point yielding different response curves. Driving the spatial light
modulator active medium with this signal can thus yield to a different
response
curve for the positive and the negative polarity.
To compensate for this effect, the present invention proposes the use of two
look-
up tables that can be used and applied alternatively to the positive polarity
and the
negative polarity frame.
Figure 6 illustrates the temporal sequence of the look-up table
implementation. At
Time 1, a first set of look-up tables is applied to the image and filter.
Then, at Time
2, the set of look-up tables corresponding to the reverse polarity is applied
to the
image and filter. Following this, at Time 3, the polarity is reversed back to
the initial
state and the first set of look-up tables is applied again. The look-up tables
are
A0 applied over time with the same sequence. Once the look-up tables applied,
the
image and the filter are displayed on their respective spatial light modulator
(SLM)
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destination. This makes the response more uniform and provides better temporal
stability to optical correlation.
The tubular optical correlator is further equipped with a digital
communication link
and addressing scheme. When interfacing with a control system, the use of
analog
video signal in the correlator requires video resampling that may induce
slight jitter
in the video signal can translate in slight modification of the image and
filter
positions or smoothing of the edges of the image and the filter. With a pixel-
to-
pixel addressing scheme each pixel of the memory is addressed to a single
pixel
1o on the spatial light modulator without spatial resampling. This provides
more stable
image and filter display as well as better conformity between the information
to be
displayed and the signal actually displayed.
A complete system is illustrated in Fig. 8. A camera 30, or any other external
sensor, captures an image such as scene 40. The output of the camera is sent
to
the input SLM. The driving electronics 21 apply a filter at filter SLM, and
are further
connected to the camera 5 for collecting the result of the correlation. The
driving
electronics further control the other aspects of correlation, such as tagging
and
using look-up tables. Alternately, the external sensor 30 could be connected
to the
driving electronics 21, which would in turn be connected to the input SLM.
As mentioned previously, the invention has been described made with reference
to
a preferred embodiment thereof. However, the invention does contemplate a
variety of different structures for the holder. For example, one could
envisage a
holder made of two pieces, each piece being in the shape of a half-pipe. The
pieces are machined to form receivers to receive the various components
therein,
so that when the half-pipes are joined together to form a tube, the components
fit
within the receivers and align in order to form the optical correlator.
Furthermore,
although a plurality of intermediate holders have been described, there may be
as
little as one, provided that the design allows for the insertion of the
various
components.
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Although the present invention has been explained hereinabove by way of a
preferred embodiment thereof, it should be pointed out that any modifications
to
this preferred embodiment within the scope of the appended claims is not
deemed
to alter or change the nature and scope of the present invention.
