surface mine signature modeling for passive polarimetric ir

Preprint Proc.SPIEVol. 4742, Det. andRem.Techn.for MinesandMinelikeTargetsVII , OrlandoFL, USA, Apr. 2002 1

Surfacemine signaturemodeling for passivepolarimetric IR

FrankCremerabc, Wim deJonga, KlamerSchuttea, JoelT. Johnsond andBrianA. Baertleind

aTNO PhysicsandElectronicsLaboratory, P.O. Box 96864,2509JG,TheHague,TheNetherlandsbPatternRecognitionGroup,Delft University of Technology, Delft, TheNetherlands

cSectionof AppliedGeophysics,Delft University of Technology, Delft, TheNetherlandsd OhioStateUniversity, ElectroScienceLaboratory, Columbus, OH, USA

ABSTRACT

A specularmodel hasbeenusedto predict the passive polarimetric infrared (IR) signatureof surface-laid landmines. Thesignature dependson the temperatureof the landmine andthe sky radiance. The temperatureof the landmine is measuredusinga thermocouple. The signature itself is measuredusinga polarimetric IR camera setup. The predictions arefit to themeasurementsusingtherefractive index asanoptimisationparameter. Theobtainedrefractive indicesof eachlandmine typeareconsistent,but for thePMN landmine muchlower thandeterminedin a previousindoor experiment.

Throughout the measurement day, the average landminepolarimetric signature washigher thanthe average backgroundsignature. Moreover thepolarimetricsignatureappears to beamorerobustindicator of theshapeof thelandmine’s topsurfacethanthenormal IR signature.

A simulatorof passive polarimetric imageryis alsobeingdeveloped.Thatwork is basedon a physicalmodelfor boththethermalandradiometricprocesses,andit includesafinite-element solutionfor theheattransferproblem,raytracingto describetheincident sunlight andtheeffectsof shadowing, andanalyticalmodelsfor theMuellermatricesof rough dielectricsurfaces.Preliminaryresultsfrom thatmodel show substantialqualitative agreementwith measured images.

Keywords: infrared polarisationmodel, model validation, outdoor measurements, landmine detection

1. INTRODUCTION

Thethermalinfraredcamerais onesensorusedfor thedetectionof surfaceandflushburiedlandmines.Clutteris alimitation forthedetectionof landmineswith IR cameras.It is known that,within thevisualandIR bands,polarisationgivesextrainformationabout objectsandtheirsurfaces.With a(rotating) polarisationfilter in frontof amidwaveinfrared(MWIR) camera,polarimetricMWIR canbemeasured.Thepolarimetric IR signatureof landmines differsfrom theclutteredbackground, becauseit hasadifferentsurfaceroughness.To predict thepolarimetric IR signaturefor diverseconditionsa validatedmodel is needed.

In previous work a specularmodel of the polarisationsignaturehasbeendevelopedandvalidated in a controlled indoorexperiment.

�� This modelhasprovento beableto predict thepolarisationsignature sufficiently in this situation.For further

validation,anoutdoorexperimentwasperformed.In thisoutdoor experiment,severaltypesof landmineshavebeenusedamongthemthe PMN landmine thatwasusedin the previous experiment. Themodelpredictions arecomparedto the polarimetricsignature asmeasuredby thepolarimetriccamerasetup.

As shown in this paper, the landmines show a pronouncedpolarimetric signatureand this agreesreasonable well withthe predictions. Moreover the polarimetric imagesseemto be betterfor automaticdetection of surface-laid landminesthanthenormalMWIR images.To reachhigh detectionratesfor surfaceandburied landmines,this polarimetric cameramustbeintegratedinto a multi-sensorsystem.

�This paper will concentrateon themeasuredandpredictedpolarimetric signatureof

surfaceAP andAT landminesin abaresandbackground.

To betterunderstandthephenomenathatarisein polarimetric signatures,we arealsodeveloping an imagesimulatorthatbuilds on prior work in thermal modeling of buried mines.

�This effort permitsus to explore in a unified, self-consistent

manner, the effects of spatialvariations in material types,surfaceproperties,and time-dependent changes in temperature,incident radiation, andshadowing.

Furtherauthorinformation:(Sendcorrespondenceto FrankCremer)F.C.: E-mail: [email protected],Telephone:+3170 3740795,Fax: +3170 3740654B.A.B.: E-mail: [email protected], Telephone:+1 6142920076


In thenext two sections,Sections2 and3, thespecularpolarimetric model andthe imagesimulatorarebriefly discussed.Thenin Section4 themeasurementsaredescribed.Thesemeasurements areanalysedandcomparedto themodelpredictionsin Section5. In this sectiontherefractive index is determinedfor thedifferentlandmines. Finally in Section6 theconclusionsaregiven.

2. POLARIME TRIC IR MODELING

Previously, asimplepolarimetric IR modelwasderived.��

In thatmodel it is assumedthatasinglesource’illumin ates’onthelandmine. This model hasbeenvalidatedindoorsandhasprovento besufficient to describetheindoor measurements.

�� The

assumptions onwhich thismodelis basedare:

1. Thematerialof thelandmine canbedescribedby a singlerefractive index for thewavelength band used(MWIR).

2. Thematerialof thelandmine is opaque,meaning thatthereis no transmissionof radiation through thelandmine.

3. Thesurfaceof thelandmine is specularfor reflectionandobeys theKirchhoff law for radiation.

4. The temperature of the landmine is constant, sincethe landmine is assumedto be in thermalequilibrium with its sur-roundings.

5. Thesinglesource,thatis ’illuminating’ thelandmine is unpolarised.

6. Thespectralsensitivity of theMWIR camerais constantthroughout thewavelength band, ranging from 3 to 5 � m.

7. Thepolarisationfilter is idealfor thewavelength band.

8. Thetransmissionthroughair is 100%and,thus,thereis nopathradiance.

For ouroutdoorexperiment,it is assumedagainthatthereis only onesource:thesky. Only linearpolarisationis consideredfor this model; circular polarisationis not modelled andnot measured. Thenaccording to this model, theStokesparameters�, and � for aflat horizontal surfacearegivenby

�� :

��! #" $%'& ( ��)��* ,+ (.- �)��/ /0 & �121!�)��3�4 657�121!�)�8�9 /0�+:�121;��; �� (1)

��! #" $%'& ( ��)��* �5 (.- �)��/ /0 & �121!�)��3�4 657�121!�)�8�9 /0<� (2)

� �� #" =>�(3)

with�therefractive index,��theobservation angle(0 degreeis downwardlooking),

( � thereflectioncoefficient for theperpendicularorientation,

( - thereflectioncoefficient for theparallelorientation,� ��3�thetemperatureof thesky,��

thetemperature of thelandmine,and 121 �)�! theblackbody radiancefor MWIR for abodyof temperature

�.

This polarimetric IR modelrequiresa numberof input parameters.Firstly, therearetwo parametersthat remainconstantthroughout themeasurement: the refractive index

�andtheobservationangle

��. Furthermore therearetwo parametersthat

will vary during the measurement: the temperatureof the sky� ��

andthe temperatureof the landmine� �

. Undernormalcircumstances(i.e., in a minefield) it is not possibleto measurethetemperatureof thelandminedirectly usingthermocouplesashasbeenperformedin this paper, seeSection4. An estimateof this temperaturemaybederived from theimagesimulatorthatis discussedin Section3.


3. IMA GE SIMULA TOR COMBINING EMITTED AND SCATTERED SOURCES

Themodel presentedin Section2 describesthe behavior of theStokesparameters at a givenpoint in thescene.In addition,thespatialdependenceof theseparameters(i.e., theshapeof theminesignature) is animportantcuein targetdetection.In thissectionwe briefly describea model that includesboth thepolarimetric characteristicsandthespatialfeaturesof polarimetricmineimagery. Sucha model servesthedualpurposesof facilitatingphysicalunderstandingof thesignatures andproviding asourcefor simulateddatausedin testsof detectionalgorithms.Becauseof spaceconstraints,only asummaryis presentedhere.Additional informationonthesimulatorcanbefound in a separatepaper.?

Simulating a polarimetricsignatureinvolves severalsteps.First,aCAD modelis createdto describethemine’ssurface,itsinternalstructureandtheunderlying soil (roughly a squaremeterin areaand40 cm deep).A tetrahedral representationof themineandsoil is generated from this representation.

A description of the thermalenvironment is alsocompiled, which mustencompassthe thermalpropertiesof the mine’sconstituentsandthesoil, thereflectivity of themineandsoil atsolarwavelengths,thetimehistoryof theincidentsolarradiation,andthe time historyof theair temperature.Thesedatadefineboundaryconditions on theexposedsoil andminesurfaces.Afinite elementsolver is usedto determine thetemperaturedistribution throughout thediscretizedvolume over adiurnalcycle.

Theresultingsurfacetemperaturedistributionis thencombinedwith a radiometricdescription of theenvironment atsensorwavelengths. This descriptionincludesMueller matricesfor themineandsoil in thesensorband,a descriptionof thenaturalradiation, therelativegeometry of themineandsensor, andthetime-varying solargeometry.

Sceneradiance, whichis proportionalto imageintensity, is calculatedin parallelfor eachpixel in thesceneandrepeatedforeachobservation time. First, thesurfacefacetviewedby eachpixel is determinedby ray tracing.Second, another ray tracingcalculationis doneto determine if the projectedpixel is illuminatedby direct solar irradiance. Unlike the determination ofvisible facets,theeffectsof shadowing aretimedependent,andthiscalculationmustbedonefor eachobservationtime. Third,thetemperature of thevisible facetsaredeterminedby interpolating theFEM nodaltemperaturedataproducedby thethermalmodel. Finally, theemittedandreflectedradiancearecalculatedusingthetemperature of themineandreflectionsof thesolarandsky radiation(if present).

Theemissivity andreflectivity of soil andminesurfaceshave primary rolesin theobservedsignature. Somecomponentsof theMueller matrix have beenmeasuredfor a few mines,but therequired dataarenot availablefor mostminesnor for thesurrogatesof interesthere. To meetthis need, we explored analyticalmethods of estimatingtheMueller matrices.Both thedielectricconstantof thesurfaceandits roughnesshave importanteffects on polarimetric emissivity andreflectivity, andtheywere the primary parametersexplored in this work. By varying a parameter of surfaceroughness(the slopevariance) onecanaffect the relationof specularanddiffusereflectionby thesurface. Theemissivity andreflectivity models usedherearebasedon thosedescribedby Tsanget al., @ which usea Kirchhoff approximation andresultin expressionssimilar to thoseofBeckmannandSpizzichino. A Another important phenomena,multiplescatteringby particlesembeddedin apenetrablesurface(e.g.,paints),hasnotyetbeenexploredtheoretically, but adiffusecomponentis addedempirically.

4. MEASUREMENT S

4.1. Measurement instruments

The main measurementinstrumentis, of course, the polarimetric IR camerasetup. This device consistsof a RadianceHSmidwave infrared camerawith a polarisationfilter in front of the lens.

�The camerais configured to run at a framerateof

100Hz. Thepolarisation filter rotatessynchronously with this framerateataspeedof 6 B perframe.In afull rotation60imagescanbeacquired for estimationof theStokesparameters.

Thepolarimetric camerais placedata height of 2.88m andis looking downwardsat anadirangleof 70 B . With the25mmlens,thecamerahasa field of view of 18 B by 18B .

To measurethe temperaturesof the landmines,thermocouples have beenused. They areattachedto the top andbottomof the landmines. Specialglue hasbeenusedto ensurethat the thermocouplesstay in placeandmaintaina good contactwith thelandmines.Thetemperatureasmeasuredby thethermocouplewasrecordedcontinuouslyfor severaldays around themeasurementday.

A weatherstationwasusedto acquire themeteorological data.This weatherstationregisterstheair temperature,thewindspeedanddirection,theprecipitation, theshortwave (or solar)radianceandthelong wave radiance.Themeteorologicaldatawasrecordedcontinuouslyfor severaldaysaround themeasurement day.


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viewing

PMN

thermocouplepole

TM62

DM31

tc17

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(a) (b)

Figure 1. The layout of the testfield: schematicallyin (a) anda photoin (b). Thethermocouplesindicatedby ’ tc#’ thataredrawn in themiddle of thelandmine areattachedto thetopof thelandmine,while thethermocouplesdrawn on thelower rightsideof thelandmine areattachedto thebottom. Thethermocouplesindicatedby dotsin a boxmeasurethetemperatureof thesandat differentdepths.

Finally, a handradiometerwasusedto measurethe apparentsky temperature. This handradiometer (sensitive between8 � m and12 � m) wasorientedtowardsthesky atanelevationangle of 20 B . Theapparent sky temperaturewasconvertedto anequivalentblackbodyradiancein theMWIR. This is basedontheassumptionthatthesky radiatesasablackbody. Theapparentsky temperaturewasrecordedmanuallyevery timea polarimetric IR sequencewastaken.

4.2. Testfield layout

Tocomparetheoutdoor resultswith theindoor resultsobtainedpreviously, thesamePMNlandminewasincludedin theoutdoorexperiment. Themajorconsideration for the layout wasthat it canbeobservedfrom thefour principal compassorientationsandstill have a similar placementof landmines. Furthermore theremustbeenough freespaceto evaluatethepolarisationofthebackground. Finally, differentlandmine materials(metal,plasticandrubber) areusedto determinetheir influence. Theseconsiderationshave led to thetestfield layout asshown in Figure1. This layoutconsistsof a mix of Anti-Personnel(AP) andAnti-Tank(AT) landmines.

TheAP landmine usedhereis thePMN, seeFigure2(a). With a diameterof around 12 cm, this is oneof the larger APs.This landmine hasa flat rubber top. In the indoor experiment, the refractive index of the rubber for MWIR wasfound to be1.45.

�TheAT landmineDM31, seeFigure2(b),hasametalcasing,whichis notcompletelyflat. Finally, theTM62 is aplastic

AT landmine,seeFigure2(c). This landminehasacenterthatis elevatedfrom therestof thelandmine.

Theselandmines,which areall replicasof real landmines,areplacedon top of sand.This testfield is locatedwithin theareaof theTNO testfacility for humanitariandemining. r4.3. Measurements

Themeasurementsweretakenon 27 November2001, between07:30and19:00. This daystartedwith clearsky andtempera-turesbelow thefreezing point. Laterin theafternoon,after14:00,cloudsdrifted in andafter18:00therewassomeprecipitation.Therelevant meteorologicaldatafor this dayareshown in Figure 3.

Threesetsof imageswererecordedwith thepolarimetricIR cameraevery 15minuteswith 1 minuteinterval. Also anumberof additional measurementsweremadeduring these15minutesintervals.For everyimageset,thesky temperatureasindicatedby thehandradiometerwasrecorded.

Every 30minutesacameraandfilter calibrationwasperformed,asdescribedin Section4.4.


(a) (b) (c)

Figure2. Thelandminesusedfor theexperiments:(a) thedummyPMN, (b) thedummy DM31, and(c) thedummyTM62.

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Figure 3. Therelevant meteorological dataduringtheexperimentof 27 November2001: (a) theair temperature,(b) theshortwave irradiaton, (c) thelongwave irradiationand(d) thesky radiance.


4.4. Calibratio n procedure

Thesamecameraandpolarisationfilter calibration procedurehasbeenfollowedfor this measurement asfor previous indoorexperiments.

�Thatis, 30setsof images(onefor every 6 B of rotation)wereacquired from botha coldanda warmblackbody.

The temperaturesof the cold andwarm blackbodiesweremeasured using the hand radiometerandaredenoted�ts

and�vu

respectively. The30 setsareaveraged to reduce thenoisein thecalibration. Thecalibrated radiance for orientation w of thepolarisationfilter is givenfor eachpixel

�)xv��y> by:

{z{� w ��xv��y> |"} 1,1 �� s v+�~ � w ��xv��y> 65 ~ s � w ��x,��y> ~ u�� w ��xv��y> 65 ~ s�� w ��x,��y> & 121 �� u 657 1,1 �� s �0�� (4)

with ~ � w ��xv��y� themeasuredvalueof thescene(in bits), ~ s�� w ��xv��y> and ~ u�� w ��xv��y� themeasuredaveragecalibration valueoftheblackbody with low andhightemperature respectively.

First, all images werecalibratedusingthecalibration closest(in time) to themeasurements. Although this producesgoodresultsfor a singlemeasurement set, it suffers from continuity problems. Due to switchingfrom onecalibrationset to theother, discontinuitiesoccuredin themeasuredradiance.To compensatefor this,thecalibrationvalueswereinterpolatedsotheychange smoothly from onecalibration to thenext. Usingthis interpolation,acontinuousradianceestimatewasobtained.

5. MEASUREMENT ANALYSESAND MODEL PREDICTIONS

5.1. Stokesparameters

Usingthecalibratedradiance in Equation 4, anestimateof theStokespolarisationparameters � , � , �� andthemodelerror erris givenby:

� " %�� z � w � (5)

� " �� z�� w � �� % w � (6)

�� " �� z � w � ��v� % w � (7)

err� " $��5 $

�� ;+ � �� % w � v+ �� v� % w � 65�{z�� w � �� (8)

with��"��.=

thenumberof frames,� theframenumberand w � " �� theangle of thelinearpolariserfor frame� .The threeStokesparametersandthe error for oneparticularmeasurementareshown in Figure 4. At the moment of the

measurements,acloudwasjustblocking thedirectsunlight. However, in theintensityimagein Figure4(a),shadestill appearsto bepresent.This socalled’shade’appears becausethis surfacehasnot beenheatedby thesun. The landminesareclearlyvisible in theintensityimage.However, thetopsof thelandminesaremuchcoolerthanthesidesthathavebeenwarmedupbythesun.

Whenemissiondominatesreflection,the Stokesparameter is negative for horizontal surfaces andpositive for verticalsurfaces.Emissiondominatesreflectionhere,sincethe sky appears colder thanthe landmines. The Stokesparameter inFigure4(b) showsclearlyall thetopsof thelandmineswith negativevaluesfor . Moreover, variationsin thebackgroundhavebeensurpressed.Thisclearlyshowsthatfor automaticdetection, which is outsidethescopeof thispaper, theStokesparameter is very usefull. Thesidesof thelandmines have a positive Stokesparameter . Theleft sideis morepositive thantherightside,sincethatsideis warmerascanbeseenin theintensityimage.

TheStokesparameter� in Figure4(c) giveslessinformationthantheprevious two images.Notice that thescaleof thisimageis a factorof 10 lower thanthe Stokesparameter . Mainly the heatedleft sidesof the TM62 have a high negativeradiance. This radianceis muchlesspronouncedon theDM31, probably dueto lower temperatureand/orlower reflectivity.Thispolarisedradianceis causedby a changingsurfacenormal at thesidesof thelandmines.


[W m

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Stokes model error (133102)

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Figure 4. TheestimatedStokesimagesandtheerror in thecalculation. Theseimagesarejust onescenetakenat 13:31 on 27November2001. All landmines areclearlyvisible in the intensityimage. Thescenecontains plastic(TM62 andPMN) andmetallandmines,with different surfacecharacteristics.Noticethatdifferent scaleshavebeenusedfor eachimage.

ThemismatchbetweentheStokesmodelprediction andtheactualmeasurementis shown in Figure4(d). This imageshowsall theedgesof theobjectsin thescene,notonly thelandmines,but alsothepolesandwiresin thetopright cornerof theimage.Theseedges aremostprobablycausedby therotationof thepolarisationfilter causinga small translationof theimage,whichis dependenton theorientationof thefilter. A correctionfor this effect canbemadeby usingmotion correctionalgorithms.

��For thescopeof thispaperthatis notnecessary, sincetheedgesaresmallandonly thetopof thelandmine is considered.

5.2. Time analyses of the Stokesparameters

For eachmeasurement,theStokesparameters, and � areestimated.For further analysesonly thetopof thelandminesthat

areinstrumentedwith a thermocouple areselected.Two areasof sandthatalsohave a thermocouplein placeareselectedforreferenceaswell.


tc11

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Figure5. Thepixelsof thelandmines(in black)andthebackground(in gray)selectedfor timeanalyses.

Thecriteria for selectingthe top of eachlandmine is to selectall pixelson that landmine that have a lower � parameterthan-0.01Wm � � sr� � in thesequencetakenat13:38:46. Thissequenceis selected,becauseit hasthehighestpolarisation.Theselectedpixels areshown in Figure5. For this time analysestheaverage is takenover all theselectedsurfacepixelsof eachlandmine. Theresultsof this timeanalysesarediscussedin Section5.4.

5.3. Model predictions

Themodelpredictionfor theStokesparameters, and � is givenin Section2. Thesource in this model is thesky andthis

radiancewasmeasuredwith thehandradiometer. Thetemperaturesof thelandmine havebeenmeasuredwith thermocouples.Therefractive indicesof thematerialof thetwo AT landminestypes(DM31 andTM62) arenot known. This is theonly freeparameterin themodel. Therefractive index of theAP landmine wasestimatedbefore,

�but usingtheoutdoor measurements

this is verifiedagain.

Thefollowing errorfunction is minimisedto find therefractive index�

:

� �)�v �" �, �� - �� ¡ � 3�� )¡ � � 65 �¢�)¡ � � � +¤£ �, ��

� - �� 3� ��¡ � 3�� )¡ � � 65 � ��¡ � � � � (9)

with�¥�

thenumber of measurementsand �¢�)¡ � and � ��¡ � theaverageestimatedStokesparametersof theobject(eitherthetop of thelandmine or thesand). Thefactor

£is includedto balance thefit betweentheStokesparameters

and . Sincethe

magnitudeof theStokesparameter is roughly a factorof 10 lower thantheStokesparameter, thefactorwasfixedat 100.

Now by minimisingthis function� �)�v

over therefractive index, thebestfit betweentheprediction andthemeasurementwasmade.In thepreviousindoorexperiment,theimaginarypartof therefractive index wasfixedat0.05,sinceit couldnotbedeterminedaccurately. Sofor this experiment,thesameassumption wasmade.

5.4. Results

The plots of the measured Stokesparameter

and of eachlandminetype and the sandbackground as function of timeareshown in Figure6. The predicted Stokesparameters arealsoshown. The only parameter that wastunedfor the modelprediction is therefractive index

�, seeSection5.3. Only themeasurementstakenafter11:00 aretakeninto account, because

it wasnoticed that in theearlymorning therewasfrozendew on top of the landmines. TheagreementbetweenthepredictedandmeasuredStokesparameter

is reasonable. However, themeasuredintensitychangesfasterthanexpected. This maybe

explainedby theway the thermocoupleis attachedto thesurface.For thePMN it wasactuallymounted in contactwith therubber, but on the inside. Becauseof this it may take sometime to heatup the thermocouple andthusthe responsewill beslowerandof lowermagnitude.

ThemeasuredStokesparameter is in goodagreementwith thepredictedStokesparameterafter14:00. Between11:00and14:00themagnitudeis aboutcorrect,but againit does not reactasfast.Thismayalsocausedby thetemperaturemeasurementwith thethermocouple. Themagnitude of thepolarisationdependson thedifferencebetweenthetemperature of thelandmineandthetemperatureof thebackground.


Theestimatedrefractive index for thePMN wasaround1.16with a variation of 0.01over thethreePMNs. This refractiveindex is muchlower thantherefractive index foundin theindoor modelverification

�(1.45). If this refractive index wasto be

used,thentheStokesparameter

will be too low (up until about 14:00) andtheStokesparameter a factorthreetoo highin magnitude. Oneof theexplanationsmaybethatassumption3 (seeSection2) of themodel is invalid. Thatmeansthat thesurfaceof the landmine is not strictly specularfor reflection.This causesothersources to bereflectedby the landmine, eachwith a differentangle andthuscanceling outsomeof thepolarisedradiance.

Not fullfilling assumption number 5 (i.e., the sky radiance is polarised)may alsohave the effect of either increasingordecreasing the measuredStokesparameter . However, this is not likely sincethe sky doesnot appearto be polarisedinMWIR for elevation angles above a few degrees.

��For theotherassumptions,it cannot beseenhow they mayexplain this

lower thanexpectedpolarisation.

Theassumptionin Section4.1 that the sky is a blackbody is a likely sourcefor the differencebetweenthe expected andmeasuredrefractiveindex. Simulationwith Modtranwith asubarticatmosphereshowsthattheestimatedsky radiancein MWIRis muchtoo low. A highersky radiance would leadto a higher estimateof therefractive index.

The measuredandpredicted Stokesparameters

and for the AT landmines of type TM62 andDM31 areshown inFigures6(c) through (f). The measurementsand predictions agree moreor lessequallyas well as the measurementsandpredictions for thePMN landmine. However, thethermocouple measurementof theDM31 seemsto bereactingslower. Thismay be causedby the big metalmassof this landmine andmay be an indication that the paint reactsfasterto temperaturechangesthanthemetalunderneath.

The refractive index for the TM62 is 1.19andthe refractive index for the DM31 is 1.15. Sincerefractive indicesof thesurfacesof thesetwo landmineswerenot measured in the indoor experiment, it is impossibleto statewhetheror not theyareconsistent.However, basedon the differentresults(indoors andoutdoors)of the refractive index of thePMN, it is to beexpectedthatthetruerefractive indicesarein actualfacthigher. Againothersourcesof radiation combinedwith anotspecularsurfacemayresultinto a lowerpolarisationresponseandthusa lower refractive index.

For thetwo selectedsandbackgroundsasimilarmodel prediction andfit hasbeencarriedout. Thematchbetweenthemea-suredandpredicted Stokesparameter

and is very poor. This meansthatthemodel cannot beapplied to sandbackground.

Themainexplanation for this is thatsandis a diffusereflectorandthatradiancefrom thespecularorientation hasonly a minorinfluence on themeasuredradiance.

If onecomparesthe -plots of the sandbackgroundwith the -plots of the landmines in Figure6, thenit is clearthatthe polarisationof the landmine is alwayshigherthanthe polarisationof the sandbackground. This meansthat on averagethe polarisationof the landmines is higherthanthe polarisationof the background. However, becauseof local variation intemperatureandreflection,this is not valid for individual pixels: somelandmine pixels canhave a lower polarisationthantheaverage background andsomebackgroundpixelscanhavea higherpolarisation thantheaveragelandmine.

For the-plotsthesituationis verydifferent. Thesandbackground hasa verysimilar Stokesparameter

comparedto the

topsof thelandmines.It is sometimeshigherandsometimeslowerandquiteoftenvery closeor equal to theStokesparameterof thelandmines.This is anindicationthattheStokesparameter

is notagoodbasisfor reliablydetectinglandminesin this

situation.

5.5. Image Simulations

Thesimulatordescribedin Section3 wasusedto producesyntheticimagescomparableto thoseof Figure4. Thecomponentsofasimulatedminesignatureareshown in Figure7. Theresultsreportedherearefor thesurrogateplasticTM62 minewith plasticcaseshown in Figure2(c). Theshapeof thatmineis well approximatedby two stackedconcentric cylindersof diameter30and13cmrespectively. A smallprotuberanceof diameter1.6cmextendsroughly 2cmabovetheuppercylinder. Theenvironmentalconditions simulatedincluded direct sunlight anda cloud-free sky. (Somecloudswerepresentduring the measurement, butthey did not obscurethe sun.) Measured

, and � componentsappearin Figures7(a) through 7(c), while the respective

model calculationsappearin Figures7(d) through7(f). In generating theseresultswe have specifieda relative permittivity of1.16for theminecasing(asnotedabove) and1.6+j0.002for thesoil (asnotedin a prior work

��). For thesurfaceroughness

we usedslopevariancesof zero(a smoothsurface)and0.3for theminecasingandsandrespectively. Theminesimulantusedin thesetestshasa similar painted finish on all exterior surfaces.Thatsurfaceis smoothto thetouch,but a diffusecomponentwasincluded becauseof thepotentialfor multiplescatteringwithin thepaintlayer.

A studyof themeasuredandmodeledresultssuggeststhatsolarirradiancehasastrongrole in the

componentvia heatingandalsovia reflection. Becauseof thesunangle,thesecontributionsarestrongestonthevertical facesof themine,whichwere


8 9 10 11 12 13 14 15 16 17 18 190

0.2

0.4

0.6

0.8

1

Time [h]

Rad

ianc

e [W

m−

2 sr−

1 ]

I−parameter of PMN (tc21)

MeasuredPredicted (n=1.171)

(a)

8 9 10 11 12 13 14 15 16 17 18 19−0.06

−0.05

−0.04

−0.03

−0.02

−0.01

0

Time [h]

Rad

ianc

e [W

m−

2 sr−

1 ]

Q−parameter of PMN (tc21)


(b)

8 9 10 11 12 13 14 15 16 17 18 190

0.2

0.4

0.6

0.8

1

Time [h]

Rad

ianc

e [W

m−

2 sr−

1 ]

I−parameter of TM62 (tc19)


(c)

8 9 10 11 12 13 14 15 16 17 18 19−0.06

−0.05

−0.04

−0.03

−0.02

−0.01

0

Time [h]

Rad

ianc

e [W

m−

2 sr−

1 ]

Q−parameter of TM62 (tc19)


(d)

8 9 10 11 12 13 14 15 16 17 18 190

0.2

0.4

0.6

0.8

1

Time [h]

Rad

ianc

e [W

m−

2 sr−

1 ]

I−parameter of DM31 (tc16)


(e)

8 9 10 11 12 13 14 15 16 17 18 19−0.06

−0.05

−0.04

−0.03

−0.02

−0.01

0

Time [h]

Rad

ianc

e [W

m−

2 sr−

1 ]

Q−parameter of DM31 (tc16)


(f)

8 9 10 11 12 13 14 15 16 17 18 190

0.2

0.4

0.6

0.8

1

Time [h]

Rad

ianc

e [W

m−

2 sr−

1 ]

I−parameter of sand (tc09)

Measured

(g)

8 9 10 11 12 13 14 15 16 17 18 19−0.06

−0.05

−0.04

−0.03

−0.02

−0.01

0

Time [h]

Rad

ianc

e [W

m−

2 sr−

1 ]

Q−parameter of sand (tc09)

Measured

(h)

Figure 6. ThemeasuredandpredictedStokesparameter: (a) PMN, (c) TM62, (e) DM31 and(g) sand.And themeasured

andpredictedStokesparameter : (b) PMN, (d) TM62, (f) DM31 and(h) sand.


0.75 0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2 1.25

5 10 15 20 25 30 35 40 45 50 55

5

10

15

20

25

30

(a) MeasuredStokesparameter¦ .−0.03 −0.02 −0.01 0 0.01 0.02

5 10 15 20 25 30 35 40 45 50 55

5

10

15

20

25

30

(b) MeasuredStokesparameter§ .

−20 −15 −10 −5 0 5

x 10−3

5 10 15 20 25 30 35 40 45 50 55

5

10

15

20

25

30

(c) MeasuredStokesparameter .

0.1 0.2 0.3 0.4 0.5 0.6 0.7

5 10 15 20 25 30 35 40

5

10

15

20

25

30

35

40

(d) CalculatedStokesparameter¦ .−0.015 −0.01 −0.005 0 0.005 0.01 0.015

5 10 15 20 25 30 35 40

5

10

15

20

25

30

35

40

(e) CalculatedStokesparameter§ .

−10 −8 −6 −4 −2 0 2

x 10−3

5 10 15 20 25 30 35 40

5

10

15

20

25

30

35

40

(f) CalculatedStokesparameter .

Figure7. Comparisonof measuredandsimulatedpolarimetric data.

not examinedin thespecularmodeldescribedpreviously. Conversely, thermalandsky radiationarethedominantcontributorsto the and � components.Shadowing of thesolarcomponentis clearlyevidentin the

image,but notin the and � results.

Themeasured and � componentsshow evidence for strongdepolarizationat singlepixelsnearthespecularpoints on theupper cylinder, which maybetheresultof edgediffractionby thesolarcomponent(a contribution not included in themodel).While thesemodelingresultsreproduceanumberof thefeaturesfound in themeasurements,theneedfor morework is evident.Amongthedifferences,we noteanapparent reflectionof theuppercylinder in thesurfaceof thelower cylinder anda possiblereflectionof thesoil in thesidewall of thelowercylinder. Thosedoubly-reflectedcomponentsarenotcurrently includedin thesimulation.

6. CONCLUSIONSAND DISCUSSION

In this paperour specularIR polarisation modelwasverified in an outdoor test,usinga polarimetric cameraandadditionalmeasurements(meteorologicaldata,thermocouples). In Section5.4,it is shown thatthemodel performsareasonable predictionof both the (normal IR) intensityandthe polarisation(specificly the Stokesparameter ). The refractive index is the onlyparameterusedto fit the model to the measureddata. The refractive index for the PMN landmine was1.16, for the TM62landmine 1.19andfor theDM31 landmine 1.16. Therefractive index found for thePMN landmine differs from thepreviousvalueof 1.45 obtainedin the indoor experiment. This differencemay be explainedby surfaceroughnessof the landminescombinedwith thepresenceof othersources(sunandsky) or adifferencebetweentherealandestimatedsky radiance.

In this paper it hasalsobeenshown that passive IR polarisationis of value for detection of landmines under a varietyof circumstances(freezing temperatures, sun,cloudsandeven a small amount of precipitation). Furthermore, under thesecircumstancesthe polarimetric componentgives a morerobust imageof the topsof the landmines thannormal IR images.Automatic detectionof landminesin IR imagesmaybeimprovedwith this additional polarimetriccomponent.

Imagesformedfrom measuredStokesparameters werealsocomparedwith theresultsof a combinedthermal-radiometricimagesimulator. Although themeasurementsshow evidence for a numberof phenomenanot currently included in themodel,thesimulationis largely in qualitative agreementwith themeasurements.


ACKNOWLE DGEMENTS

Thework presentedin thispaperis sponsoredby theNetherlandsMinistry of Defense.

Theeffortsof BAB andJTJweresupportedin partby funds from DukeUniversityunder anawardfrom theARO (theOSDMURI program).Thefindings,opinionsandrecommendationsexpressedthereinarethoseof theauthorandarenotnecessarilythoseof DukeUniversity or theARO.

REFERENCES

1. F. Cremer, W. de Jong, andK. Schutte. Infraredpolarisation measurementsandmodelling appliedto surfacelaid anti-personnel landmines. Optical Engineering, May 2002. To bepublished.

2. F. Cremer, W. deJong,andK. Schutte.Infraredpolarisationmeasurementsof surfaceandburiedanti-personnellandmines.In A. C. Dubey, J. F. Harvey, J. T. Broach, andV. George, editors,Proc. SPIE Vol. 4394, Detection and RemediationTechnologies for Mines and Minelike Targets VI, pages164–175,Orlando(FL), USA, Apr. 2001.

3. F. Cremer, K. Schutte,J.G. M. Schavemaker, andE. denBreejen.A comparisionof decision-level sensor-fusionmethodsfor anti-personnellandmine detection. Information Fusion, 2(3):187–208,Sept.2001.

4. I. K. Sendur andB. A. Baertlein. Numericalsimulationof thermalsignatures of buriedminesover a diurnal cycle. InA. C. Dubey, J. F. Harvey, J. T. Broach,andR. E. Dugan, editors,Proc. SPIE Vol. 4038, Detection and RemediationTechnologies for Mines and Minelike Targets V, pages156–167, Orlando (FL), USA, Apr. 2000.

5. E. Hecht.Optics. Addison-Wesley publishing company, Reading(MA), USA, secondedition, 1987.6. B. A. Baertlein,J. T. Johnson, andW. J. Liao. Model-basedsurfaceminedetectionin polarimetric IR imagery. In J. T.

Broach,R. S. Harmon, andG. J. Dobeck,editors,Proc. SPIE Vol. 4742, Detection and Remediation Technologies forMines and Minelike Targets VII, Orlando (FL), USA, Apr. 2002. This volume.

7. L. Tsang,J.A. Kong, andR. T. Shin. Theory of Microwave Remote Sensing. Wiley-Interscience,New York, NY, 1985.8. P. Beckmann andA. Spizzichino. The Scattering of Electromagnetic Waves From Rough Surfaces. ArtechHouse,Nor-

wood, MA, 1987.9. W. deJong,H. A. Lensen,andY. H. L. Janssen.Sophisticatedtestfacility to detectlandmines.In A. C. Dubey andJ.F.

Harvey, editors,Proc. SPIE Vol. 3710, Detection and Remediation Technologies for Mines and Minelike Targets IV, pages1409–1418,Orlando (FL), USA, Apr. 1999.

10. W. de Jong,F. Cremer, K. Schutte,andJ. Storm. Usageof polarisationfeatures of landminesfor improved automaticdetection. In A. C. Dubey, J. F. Harvey, J. T. Broach,andR. E. Dugan, editors,Proc. SPIE Vol. 4038, Detection andRemediation Technologies for Mines and Minelike Targets V, pages241–252,Orlando(FL), USA, Apr. 2000.

11. F. Cremer, P. B. W. Schwering, W. deJong, andK. Schutte.Infraredpolarisationmeasurementsof targetsandbackgroundsin a marineenvironment. In W. R. Watkins,D. Clement,andW. R. Reynolds,editors,Proc. SPIE Vol. 4370, Targets andBackgrounds VII: Characterization and Representation, pages169–180, Orlando (FL), USA, Apr. 2001.

12. I. K. Sendur, J. T. Johnson, andB. A. Baertlein. Analysisof polarimetric IR phenoma for detectionof surfacemines.In A. C. Dubey, J. F. Harvey, J. T. Broach, andV. George, editors,Proc. SPIE Vol. 4394, Detection and RemediationTechnologies for Mines and Minelike Targets VI, pages153–163,Orlando(FL), USA, Apr. 2001.

surface mine signature modeling for passive polarimetric ir

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