Three dimensional binocular correlator

A three dimensional binocular correlator is disclosed in which additive/differential discrimination can be provided by combining and comparing two optical inputs of different aspect angular views of a target of interest onto one matched filter. This technical approach records more information on a dual exposure matched filter which denotes two different aspect angular views of the same target, to allow better and more effective target discrimination, and also provides three dimensional information on the target. The binocular correlator can differentially monitor the signal strength of the signal outputs from the two transforms, which can provide a feedback signal for such control measures as a course correction or for robotic control. The signal strengths of the transforms can be compared, for the same output levels, to confirm that they recognize the same target, and the dual outputs thereof can increase the combined correlation signal strength by 3 db. A tri-state logic approach could also be utilized to provide discrimination under multitarget scenarios for improved performance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to the field of optical correlators 
which compare input image information with image information stored in one 
or more matched filters, and more particularly the subject invention 
pertains to a three dimensional binocular correlator in which inputs 
provide additive/differential discrimination by combining and comparing 
two optical inputs of different aspect angular views of a target of 
interest onto one matched filter. 
2. Discussion of the Prior Art 
A traditional optical correlation system utilizes matched filters to 
provide object identification, and possibly to provide aspect information, 
such as positional and size information, about an object, and utilizes 
primarily parallel optical processing therein. In a typical prior art 
optical correlator, an input image is incident upon a spatial light 
modulator, and the input image spatially modulates a coherent beam of 
radiation. A multiple holographic lens has the spatially modulated 
radiation beam incident thereon, and performs a multiple number of Fourier 
transformations thereon to obtain an array of a multiple set of Fourier 
transforms of the spatially modulated radiation beam. A corresponding 
array of matched filters has the array of Fourier transforms incident 
thereon, with each matched filter comprising a Fourier transform hologram 
of an aspect view of an object of interest, and each matched filter passes 
an optical correlation signal in dependence upon the degree of correlation 
of the Fourier transform of the spatially modulated radiation beam with 
the Fourier transform recorded by the matched filter. An inverse Fourier 
transform lens receives the optical correlation outputs of the array of 
matched filters, and performs an inverse Fourier transformation on each 
optical correlation output. A detector then detects the inverse Fourier 
transform of each optical correlation output, and produces a detector 
output signal representative of each optical correlation output. 
Traditional optical correlators process and provide only two dimensional 
information on a target of interest in one or more matched filters which 
comprise the optical memory of such correlators. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide a 
three dimensional binocular correlator in which additive/differential 
discrimination can be provided by combining and comparing two optical 
inputs of different aspect angular views of a target of interest onto one 
matched filter. This technical approach records more information on a dual 
exposure matched filter which denotes two different aspect angular views 
of the same target, to allow better and more effective target 
discrimination, and also provides three dimensional information on the 
target. 
Infra-red as well as visual inputs are sometimes susceptible to 
countermeasures, such as by the use of screening by smoke or flares. Under 
such hostile countermeasures conditions, signal to noise ratios frequently 
decrease. However, the additive inputs of the present invention may 
provide a sufficient signal in relation to background noise to see through 
these attempted countermeasures. The subject invention differentially 
monitors the signal strength of the signal outputs from two transforms, in 
a three dimensional binocular correlator which can provide a feedback 
signal for such control measures as a course correction or for robotic 
control. The signal strengths of the transforms can be compared, for the 
same output levels, to confirm that they recognize the same target, and 
the dual outputs thereof can increase the combined correlation signal 
strength by 3 db. A tri-state logic approach could also be utilized to 
provide discrimination under multitarget scenarios for improved 
performance. Moreover, in the case of an input failure, the dual signal 
inputs provide redundancy for increased reliability. Also, when both 
inputs are functioning, a wider search area for a target is provided. 
In accordance with the teachings herein, the present invention provides a 
three dimensional binocular optical correlator in which left and right 
angular aspect views of a target of interest are recorded simultaneously 
on a single matched filter to allow better and more effective target 
discrimination and also to provide three dimensional information on the 
target. The three dimensional binocular correlator includes left and right 
imaging optical systems for providing left and right images of left and 
right fields of view of a target area, with an overlapping target area 
therebetween. Left and right spatial light modulators are positioned 
substantially in the focal planes of the left and right optical systems 
for superimposing the left and right fields of view onto reference optical 
beams. Left and right optical transform systems provide left and right 
optical transforms of the left and right fields of view superimposed onto 
the reference beams in a common transform plane, and the dual exposure 
matched filter is positioned in the common transform plane. Left and right 
transform correlation detectors are provided for detecting the left and 
right correlation outputs of the dual exposure matched filter. The signal 
strengths of the two transform detectors are monitored differentially, as 
by a differential amplifier, to provide a control signal for control 
purposes such as a course correction or for robotic controls. Moreover, a 
correlation spot detector is also provided for detecting a common A+B 
correlation spot formed by the dual exposure matched filter. 
The present invention also provides an arrangement for fabricating a dual 
exposure matched filter by recording thereon left and right Fourier 
transform holograms of left and right angular views of a target. The 
arrangement includes left and right imaging optical systems for providing 
left and right images of left and right fields of view of a target with an 
overlapping target area therebetween. Left and right spatial light 
modulators are positioned substantially in the focal planes of the left 
and right optical systems for superimposing the left and right fields of 
view onto reference optical beams. Left and right optical transform 
systems provide left and right optical transforms of the left and right 
fields of view superimposed on the reference beams in a common transform 
plane. A photomedium for forming the dual exposure matched filter is 
positioned in the common transform plane of the left and right transform 
optical systems. The photomedium is also simultaneously illuminated with a 
reference optical beam to form and record thereon the left and right 
Fourier transform holograms of the left and right angular views of the 
target.

DETAILED DESCRIPTION OF THE DRAWINGS 
A number of elements and concepts relating to the present invention are 
used in this description and are essential to an understanding of the 
function and general principles of operation of the invention, and 
accordingly the nature and properties of several of those concepts are 
discussed initially herein below for convenience. 
When a lens is illuminated by a spatially modulated collimated beam (as 
when it is modified spatially by passing through a film of a scene, 
target, etc.), the lens creates at its focal plane the Fourier Transform 
of the object(s) on the film, which is a basic lens property. When the 
Fourier Transform is interfered with a collimated (or reference) beam from 
the same source, an interference pattern results. This is called a Fourier 
Transform hologram, or Matched Filter (MF). It is an optical spatial 
filter of the input object. When an arbitrary scene is played through the 
system, the matched filter will pick out and pass the object for which it 
was made. The signal passed by the filter is Fourier transformed again and 
a "correlation" plane results. If the matched filter target is present, a 
sharp correlation signal results, whereas non-target signals result in 
broad, low correlation signals in the correlation plane. 
A matched filter is a Fourier transform (FT) hologram with properties that 
are sensitive to an input object's size, angular aspect and input 
location. These parameters can be predetermined in order to prescribe a 
set of angle and range (size) lines covering the anticipated object's 
aspects. In the fabrication of a matched filter, the holographic fringe 
visibility is optimized at a particular spatial frequency that will 
satisfy the size and/or aspect sensitivity requirements. Because it is 
unlikely that both requirements can be satisfied simultaneously, a 
plurality of independent MFs are frequently utilized in such an optical 
correlator. The nature of different particular applications will generally 
require significantly different MF sensitivities. In summary, a matched 
filter is a complex holographic structure having size, wavelength, 
thickness of the storage medium, focal length of the Fourier transform 
lens, contrast ratio, overlap, placement, and spatial frequency 
dependence, all of which must be considered in the fabrication of a 
matched filter. 
A holographic lens (HL) is made by recording the interference pattern of an 
expanding point radiation source and a collimated radiation beam, which 
produces a hologram of a point source. When the holographic lens (after 
recording and processing, as on film) is illuminated, it recreates the 
point source, i.e., it behaves as a lens. If the recording process is 
initially repeated, a series of point source holograms, or a multiple 
holographic lens (MHL), can be recorded on the film. A multiple 
holographic lens array produces an array of Fourier Transforms of an input 
spatially modulated, laser radiation beam. In general, the particular 
requirements of the array are determined by the particular problem being 
addressed. In summary, a holographic lens takes a Fourier Transform (FT) 
of a laser beam illuminated scene or target, and a multiple holographic 
lens takes, simultaneously, a multiple set of Fourier Transforms. A 
multiple holographic lens array is usually used in conjunction with a 
corresponding multiple array of matched filters. 
Referring to the drawings in detail, FIG. 1 illustrates a relatively simple 
embodiment of an optical correlator employing a memory bank of matched 
filters. An object of interest 10 is moving past the input to the optical 
correlator, and is imaged by an input lens 12 onto a spatial light 
modulator (SLM) 14, which spatially modulates the image onto a laser beam 
from a laser 16, directed thereto by a mirror 18 and beam splitter 20. The 
spatially modulated laser beam is initially passed through a bandpass (BP) 
filter 21 to allow only the image carried by the laser beam to pass 
therethrough. The spatially modulated laser beam is then Fourier 
transformed by a multiple holographic lens 22 and directed onto a 
corresponding array of matched filters (MFs) 24. An inverse Fourier 
Transform lens array 26 inversely Fourier transforms the output of the MFs 
and directs the outputs thereof onto a detector array 28, the output 
signals of which are electronically processed at 30 to produce output 
control signals. 
FIG. 2 is a schematic arrangement of an exemplary embodiment pursuant to 
the teachings of the present invention for fabricating a dual exposure 
matched filter simultaneously and coherently for a three dimensional 
binocular correlator as disclosed and taught herein. Referring thereto, a 
laser beam 31 is directed through three consecutive beam splitters 42', 
43, and 42 to form respectively a laser beam input to a spatial light 
modulator 44', a reference laser beam illuminating a dual exposure matched 
filter photoplate 48, and a laser beam input to the spatial light 
modulator 44. The laser beam inputs to the spatial light modulator 44' and 
44 should be equal in intensity, and the beam splitters 42', 43, and 42 
can be so designed to apportion the laser beam into appropriate intensity 
beams. Of course, other optical arrangements could also be utilized to 
divide the laser beam into three laser beam inputs as shown. The target 
area is divided into a field of view A on the left and a field of view B 
on the right, with a common overlap target area 38 therebetween. 
The field of view A is imaged by a lens 40 through the cube beam splitter 
42 onto a Spatial Light Modulator 44 where it is superimposed on the 
reference laser beam, which is then filtered by a BP filter 45 to allow 
only the spatially modulated laser beam to be imaged by lens 46 onto a 
photomedium 48. The photomedium 48 is also illuminated with the reference 
beam provided by beamsplitter 43 to form a Fourier Transform hologram or 
matched filter of the field of view A on the photomedium. The field of 
view B is simultaneously processed in the same manner by a lens 40', a 
beam splitter 42', a Spatial Light Modulator 44', a BP filter 45', and a 
lens 46' to form a simultaneous matched filter of the field of view B on 
the photomedium 48. 
Accordingly, the arrangement of FIG. 2 fabricates a dual exposure matched 
filter which denotes two aspect angular views of the target, to allow 
better and more effective target discrimination, and also to provide three 
dimensional information on the target. Infra-red, as well as visual inputs 
are susceptible to countermeasures, such as by the use of screens provided 
by smoke or flares. Under such hostile countermeasures conditions, signal 
to noise ratios would frequently decrease. However, the additive outputs 
from the left and right views A and B may provide a sufficient signal in 
relation to background noise to boost the additive signal strength by 3 db 
to see through such countermeasures. 
FIG. 3 illustrates a three dimensional optical correlator for processing 
input images of interest through a dual exposure matched filter such as is 
produced by the arrangement of FIG. 2. Referring thereto, an arrangement 
similar to that of FIG. 2 is used as a three dimensional correlator in 
which reference laser beams are directed into the beam splitters 42, 42', 
to allow the fields of view A, B to be superimposed thereon in the Spatial 
Light Modulators 44, 44', which are then passed by bandpass filters 45, 
45' to be imaged by lenses 46, 46' onto the dual exposure matched filter 
which was developed after the processing of FIG. 2. The output thereof is 
then imaged by a Transform lens or lenses 50 in Fourier Transform planes 
where they are detected by respective detectors 52, 52', and then 
processed. An additional output is formed at a correlation spot A+B, which 
is also detected by a correlation spot A+B detector 54. This correlation 
spot signal A+B represents the combined zero order correlation spots of 
both channels A and B, and generally should be maximized during usage of 
the system, along with maximizing the separate output signals from 
detectors 52 and 52'. 
A differential amplifier circuit 56 can differentially monitor the signal 
strengths of the two transforms as detected by 52 and 52', to provide a 
signal for such measures as a course correction or robotic controls. When 
the correlator arrangement of FIG. 3 views a target head on, as recorded 
by the arrangement of FIG. 2, the outputs form the Transform plane 
detectors 52, 52' should be equal. A stronger amplitude signal from 
detector 52, for example, indicates that the optical correlator should be 
rotated clockwise to equalize the detector outputs. Accordingly, in a 
target guidance system, appropriate steering controls can steer the system 
towards the target. In a robotic control system, appropriate robotic 
control signals can be developed. The signal strengths of the transforms 
are compared for the same output levels, to confirm that they recognize 
the same target, and the combined dual outputs can increase the 
correlation signal by 3 db over that over that provided by a traditional 
single matched filter correlator. 
FIG. 4 illustrates a tri-state logic circuit for determining whether the 
A+B detector signal output from the correlation detector 54 indicates that 
a target is detected in one channel, both channels, or in neither channel. 
A reference signal 60 is representative of and monitors the incident or 
ambient light conditions in the search area, and provides a reference 
automatic gain control signal at 60. If the A+B detector signal is less 
than the reference signal 60, differential amplifier 62 produces a 
negative output, causing AND gate 64 to produce a negative output. The 
reference signal 60 is amplified by a factor of 2 in amplifier 66, and the 
A+B detector signal is also less than twice the output of amplifier 66, 
causing differential amplifier 68 to produce a negative output also. The 
two negative outputs on output lines A+B cause NOR gate 70 to produce a 
positive one output, representing that the very low amplitude of the A+B 
detector signal indicates that a target is being detected in neither 
channel. If the amplitude of the A and B detector signal is greater than 
the amplitude of the reference signal 60, but less than twice the 
amplitude of the reference signal 60 (output of amplifier 66), then the 
differential amplifier 62 produces a positive one output, and the 
differential amplifier 68 produces a negative output, which is inverted at 
72, such that the AND gate 64 now produces a positive signal, which 
indicates that the target is being detected in only one channel. If the 
amplitude of the A+B detector signal is greater than twice the amplitude 
of the reference signal 60 (produced by amplifier 66), then differential 
amplifier 68 produces a positive one output, which is inverted by 72, such 
that AND gate 64 produces a negative output. The positive output on 
channel C causes A to go low, and the positive output on channel C 
indicates that the relatively strong A+B detector signal is caused by 
detecting the target in both channels. 
Under a three target scenario in which a target occupies each field of view 
A, B, and a third target occupies the overlap area 38, additive targets 
would produce strong outputs from detectors 52, 52' and 54. As the 
distance between the optical correlator and the targets diminishes, the 
two targets that occupy the fields of view A and B would fall outside of 
the field of view of their prospective channels progressively, leaving 
only the target occupying the overlap area 38 as the singular target. A 
crossover condition occurs in which the signals of the targets that 
occupied field of views A and B diminishes as the signal from the target 
that occupies the overlap area increases. As the distance between the 
target and the correlator decreases even more, after this crossover 
condition, more of the target occupies more of the field of view, giving 
increasing output signals. Appropriate logic can be utilized to detect 
these conditions. 
A tri-state logic approach could provide discrimination under multi-target 
scenarios for improved performance. A tri-state logic approach would 
compare each of the signals from the detectors 52, 52', and 54. The 
signals from detectors 52 and 52' would be compared and used for control 
purposes as described hereinabove, while the signal from correlation spot 
A+B detector 54 should be maximized, while maintaining and maximizing 
equal strength signals from detectors 52 and 52', to provide for maximum 
signal strengths from the optical correlator. One advantage of the 
detector 54 is that its signal strength can be evaluated without any time 
delay, as might be caused by a comparator circuit for the signals from 
detectors 52 and 52'. 
Moreover, in the case of an output failure, the dual outputs from detectors 
52 and 52' provide redundancy for increased reliability. Additionally, 
when both inputs are functioning, a wider search area is provided for a 
target, as illustrated in FIG. 3. 
While several embodiments and variations of the present invention for a 
three dimensional binocular correlator are described in detail herein, it 
should be apparent that the disclosure add teachings of the present 
invention will suggest many alternative designs to those skilled in the 
art.