Acoustooptically addressed Fourier transformmatched filtering

Don A. Gregory and Laura L. Huckabee

Matched filters have been made using the standard VanderLugt technique and are addressed using an ac-oustooptic beam deflector to manipulate the Fourier transform of the coherent input scene with good corre-lation signals resulting. It may be possible to use this method to address large arrays of matched filters athigh scan rates; thus providing a large optical memory which is necessary for absolute object recognition anddiscrimination.

1. Introduction

A major problem in using Fourier transform matchedfilters for object recognition has always been the limitednumber of objects that could be recognized or thememory of the system.1 To date, many novel tech-niques have been proposed, including multiply exposingthe photographic plate and using a holographic elementto produce arrays of Fourier transformed images fromwhich matched filters could be made.2'3 The multipleexposure method is limited to about eight exposureswhile the holographic lens technique often contributesnoise to the correlation signals. In this paper anothertechnique is proposed and demonstrated to some degreeof satisfaction. In this experiment, an acoustoopticbeam deflector is used to manipulate a coherent inputscene and address matched filters made using the usualVanderLugt arrangement. 4 This technique offers somepromise of addressing large arrays of matched filters invery nearly real time owing to the high frequencyscanning abilities of modern acoustooptic beam de-flectors. This technique would not be limited by thenumber of times the photographic plate may be exposedbefore the diffraction efficiency decreases or by back-ground noise associated with a holographic element.

11. Experimental Arrangement and Procedure

A sketch of the basic experimental arrangement isgiven in Fig. 1. It is essentially the standard VanderLugt method for making matched filters, except for theaddition of the acoustooptic deflector between theFourier transform lens and the photographic plate or

The authors are with U.S. Army Missile Laboratory, U.S. ArmyMissile Command, Research Directorate, Redstone Arsenal, Alabama35898-5248.

Received 1 October 1984.

between the input scene and the Fourier transform lens.The input scene used in this investigation was a trans-parency of an aerial photograph of Huntsville, Ala.,containing both low and high spatial frequencies. Thetransparency was recorded on a high resolution Kodak649F plate.

The acoustooptical device used in this initial exper-iment was made by Isomet Corp. in September 1974,model LD-401-2Y, and used tellurium dioxide as thedeflecting medium. The device had an active apertureof 6.6 mm. An Andret Corp. frequency synthesizer,model 6100, in conjunction with an EIN rf power am-plifier, model 400 AP, provided the driving frequency.The operating frequency range was from 50 to 140 MHz.In acoustooptical devices, a piezoelectric transducer isused to create longitudinal waves in a crystalline de-flecting medium. These sound waves are produced ata frequency determined by an external frequency gen-erator. As these longitudinal wave fronts propagatethrough the deflecting medium, they alter the densityof the crystal and thus alter its index of refraction.When coherent light waves encounter the crystal, theyare deflected due to this modulation of the medium'srefractive index.5 For the first experiment, the fre-quency was varied until the negative first-order de-flection spot of highest intensity appeared. This oc-curred at a frequency of 70.2 MHz and at an angle of.40 from the zeroth (undeflected) order. The tilt of the

AO device was then adjusted until this order was max-imized; the positive first order was at a minimum at thisposition.

The coherent and collimated illumination of theinput scene was provided by a Hughes 1-mW He-Nelaser, model 32218-c. The beam was expanded andcollimated using a 25-pim pinhole and 20X microscopeobjective followed by a 10-cm focal length lens. Firstsurface mirrors were used to increase the object beampath length to match that of the reference beam. Po-

15 March 1985 / Vol. 24, No. 6 / APPLIED OPTICS 859

Fig. 1. Experimental arrangement for AO addressing of matchedfilters.

larizers were used as shown in Fig. 1 to insure the samepolarization state of the two beams for making thematched filters. The photographic plate was held inplace by a standard three point mount. Micrometerx-y-z translators allowed careful positioning of the platein the Fourier transform plane.

After exposing the 649F plate to the interference ofthe reference beam and the Fourier transformed zer-oth-order object beam, the plate was developed usingstandard techniques (D-19 developer, Kqdak stop bath,and Hunt's fixer).6 The plate was then replaced in thesystem, the reference beam blocked, and the correlationsignal detected using an RCA model TC-1160 TVcamera and displayed on a TV monitor or digitizedusing a Colorado Video, model 321, image digitizer andthe intensity displayed on a strip chart recorder or os-cilloscope.

111. Experimental Results

The acoustooptic deflector used in this initial inves-tigation was quite old but served to demonstrate thepotential of such a system. The efficiency of the devicein deflecting the input laser light into the negative firstorder was measured using an NRC 880 universal shuttersystem as a power meter. A well collimated beam wasincident on the cell in the first measurements made.The driving frequency was set at 70.2 MHz and the celltilted to maximize the -1 order and minimize the +1order. The intensities of the incident, zeroth, andpositive and negative first orders were present andmeasured. By dividing the intensity of the negativefirst order by the incident intensity, an efficiency of 20%was obtained. The positive first order was very weakand was therefore neglected.

The efficiency of the device was also determined fora tilt at which the positive and negative first ordersappeared nearly equal in intensity. The two deflectedintensities were added and divided by the incident in-tensity. Efficiency at this position was 10% with thesame (70.2-MHz) driving frequency as before. Theangle of the device during this measurement was -0°.The AO cell is most efficient when it is tilted at an angleadhering to the Bragg condition. The device was thenreturned to its original tilt, maximizing the negative first

Fig. 2. Image produced by the negative first-order deflection of theAO device with a Ronchi ruling as the input scene.

order. By inserting and removing various lenses, thebeam was then transmitted through the AO cell inseveral states of convergence and divergence. For allconditions, the efficiency was -20%. A strongly di-verging or converging beam would most likely be de-flected less efficiently, but for the usual f/No. (>5) usedin systems such as this, the effect is small.

Figure 2 is an indication of the image deflectingability of the AO device used in this investigation. Thecoherently illuminated input scene transparency of Fig.1 was a Ronchi ruling having 50 lines/in. The deflectionangle was 40. The photograph was made in the nextimage plane after the AO device. The AO deviceseemed to be quite effective in deflecting this low spatialfrequency scene. However, when higher spatial fre-quency scenes were used, some image degradation wasobserved. This was due, to some degree, to the colli-mation state of the laser beam carrying the image. Aminimum of distortion would occur if the beam werewell collimated as it entered the AO cell instead of beingconverging as in the present arrangement. Placing theAO device between the Fourier transform lens and thefilm plate alleviates this problem. This is discussedlater.

Initially, the experiment was arranged so that thedeflected negative first-order Fourier transformedimage was interfered with the reference beam to makea matched filter. The frequency shift induced in theobject beam by the AO deflector proved to be enoughto prevent the necessary interference at the film plate.Frequency shifting the reference beam with a secondAO device might have solved this problem, but it wasnot attempted. The alternative chosen was to make thematched filter with the undeflected zeroth order, whichpasses directly through the AO deflector, then attemptto address this filter with the deflected negative firstorder. The input scene used was the transparency ofHuntsville discussed earlier. The intensities of theobject and reference beams were fixed approximatelyequal and Kodak 649F plates were used to record thefilters. A typical filter is shown in Fig. 3. After ex-posing and developing the plate, it was replaced in theoriginal position. The correlation signal appeared asa very bright localized spot on the TV monitor shown

860 APPLIED OPTICS / Vol. 24, No. 6 / 15 March 1985

Ml MOR

PIMIZE

.. RRO

IRIS

COLL11tATISLENS MATTED

FILTERNeN. BASER

... RO

NANSH

LESS

Fig. 3. Photograph of a matched filter. The input scene was atransparency of an aerial photograph of Huntsville, Ala. The di-

ameter of the darkest area is -0.4 mm.

Fig. 6. Oscilloscope trace of a single TV line containing the corre-lation spot.

00b

Fig. 4. Detection of the correlation signal for a matched filter madewith the zeroth-order deflection and addressed with the negative firstorder. The dotted line represents the position of the photographic

plate when the matched filter was made.

70.0 70.1 70.2 70.3 70.4

FREQUENCY (MHZ)

Fig. 7. Correlation intensity vs driving frequency of the AOdevice.

The authors would like to thank J. G. Duthie of theArmy Missile Laboratory for suggesting this researchand giving valuable advice when it was needed.

Fig. 5. Correlation spot as it appeared on the TV monitor. Thediameter of the spot was -3 mm.

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11

, 114

PHOTORAPHI

in Fig. 1. The late was then carefully translated ver-tically until the deflected negative first-order Fouriertransformed image addressed the matched filter (seeFig. 4). The correlation signal was detected aftersearching in the area shown in Fig. 4. The correlationsignal was displaced by approximately the same angleas the deflected beam addressing the filter. The cor-relation signal itself was very well defined as shown inFig. 5, which is a photograph of the TV monitor used todisplay the correlation signal. An oscilloscope trace ofa single TV line containing the correlation spot is givenin Fig. 6. There is very little noise in the correlationsignal, which also had the usual translational invarianceexpected from filters made and addressed using theVanderLugt technique.

The final experiment involved measuring the corre-lation intensity as the driving frequency of the AO de-vice was changed. This provides information regardinghow carefully controlled the frequency generator mustbe. The results are given in Fig. 7. For a matched filtermade with the zeroth order then addressed with thenegative first order, the driving frequency may vary by100 kHz before the correlation intensity decreases by50%. This of course depends on the spatial frequencyof the input scene as well as other factors.

This information also lends some insight into theminimum physical spacing of matched filters addressedin this manner. The Rayleigh criteria provide aguideline for the minimum frequency separation-which corresponds to a spatial separation. Using theinformation in Fig. 7, the minimum resolvable separa-tion should be of the order of 130 kHz, assuming a fullwidth at half-maximum (FWHM) of 200 kHz. Thisalso depends on the spatial frequency content of theinput scene as well as other parameters.

The 50-140-MHz operating frequency of the AOdevice caused the Fourier transform of the input sceneto be swept through on an angle of 7.8°. If the Ray-leigh limit for separation is believed, approximately 700matched filters in a row or column could theoreticallybe addressed within the observed frequency range.Problems associated with making closely spacedmatched filters will probably limit the actual numbermade and addressed to about 50, assuming a 0.4-mmseparation which seems reasonable from the photographin Fig. 3. To date, 6 filters have been stored in a columnand addressed successfully using the techniques de-scribed here.

Acoustooptic deflectors are currently available whichproduce both x and y deflection of an incident beam.It is probably within reason to consider a 50 X 50 arrayof matched filters, all addressed within a few millisec-onds. This amount of optical memory would represent

a factor of 100 improvement over existing memories inoptical recognition systems. The number of matchedfilters possible will require experimental verificationand also depend heavily on the spatial frequency con-tent of the scenes used.

IV. Conclusions and Suggestions

In this brief paper a new method for addressingFourier transform matched filters has been demon-strated. It seems possible that this method may cir-cumvent the limited memory problem inherent withcoherent recognition systems, without creating sub-stantial new problems. It is not difficult to imagine alarge matrix of matched filters addressed with highlyefficient acoustooptic deflectors using a fast frequencysweeping x-y raster arrangement. The correlationsignals detected from this array of filters would bespatially separated at the detector, which provides an-other advantage, discrimination. The correlation signaldetected would correspond to one known particularfilter in the array. The addition of a real-time inco-herent-to-coherent image converter such as the Hughesliquid crystal light valve has been done which will alle-viate the need for transparencies as input scenes andmake the entire system operate in near real time,probably limited by the 25-30-msec reaction time ofmost light valves.; Locating the AO device between the input scene andthe Fourier transform lens has been investigated, andmay, in fact, be superior to locating the AO device be-tween the Fourier transform lens and the film plate asshown in Fig. 1. It was necessary to condense and re-collimate the image beam due to the small aperture ofthe AO device, but overall better image quality wasobtained in the deflected beam.

References1. H.-K. Liu and J. G. Duthie, "Real-Time Screen-Aided Multiple-

Image Optical Holographic Matched-Filter Correlator," Appl. Opt.21, 3278 (1982).

2. D. A. Gregory and H.-K. Liu, "Large-Memory, Real-Time Multi-channel Multiplexed Pattern Recognition," Appl. Opt. 23, 4560(1984).

3. A. Grumet, "Automatic Target Recognition System," U.S. Patent3,779,492 (1972).

4. J. Goodman, Introduction to Fourier Optics (McGraw-Hill, NewYork, 1968), p. 171.

5. R. Main, "Fundamentals of Acousto-Optic Devices," Lasers andApplications, 111 (June 1984).

6. A. Shulman, Optical Data Processing (Wiley, New York, 1970),p. 560.

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