thermal luminescence sensor for ground-path contamination detection

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Thermal luminescence sensorfor ground-path contamination detection

Arthur H. Carrieri, Irving F. Barditch, David J. Owens, Erik S. Roese, Pascal I. Lim, andMichael V. Talbard

A standoff method of detecting liquids on terrestrial and synthetic landscapes is presented. The inter-stitial liquid layers are identified through their unique molecular vibration modes in the 7.14–14.29-mmmiddle infrared ~fingerprint! region of liberated thermal luminescence. Several seconds of 2.45-GHzbeam exposure at 1.5 W cm21 is sufficient for detecting polydimethyl siloxane lightly wetting the soilthrough its fundamental Si–CH3 and Si–O–Si stretching modes in the fingerprint region. A detectionwindow of thermal opportunity opens as the surface attains maximum thermal gradient followingirradiation by the microwave beam. The contaminant is revealed inside this window by means of asimple difference-spectrum measurement. Our goal is to reduce the time needed for optimum detectionof the contaminant’s thermal spectrum to a subsecond exposure from a limited intensity beam. © 1999Optical Society of America

OCIS codes: 300.6300, 120.0280, 070.5010, 070.4790, 040.3060.

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1. Introduction

In military maneuvering and industrial cleanup op-erations it is often vital to ascertain, in situ, the pres-ence and extent of land area chemical contaminationcaused by a deliberate or accidental spill and dealwith the threat rapidly. The hazardous liquid masscan be neutralized and life and health safeguardedonce maps are obtained outlining its surface and vol-ume coverage. Several noninvasive methods of re-motely interrogating terrestrial and syntheticlandscapes have or can be applied to this problem.They include reflectance,1–6 stimulated emissionspectroscopy or pulsed photoluminescence,6–13 and

polarized light scattering,14–16 among others. Inthis paper we examine and apply thermal lumines-cence ~TL! as a possible solution. Detection is basedon revealing absorption or emission moieties by the

A. H. Carrieri and I. F. Barditch are with the U.S. Army Soldierand Biological Chemical Command, Edgewood Chemical BiologicalCenter, 5183 Blackhawk Road, Aberdeen Proving Ground, Mary-land 21010-5424. The e-mail address for A. H. Carrieri [email protected]. D. J. Owens, E. S.Roese, P. I. Lim, and M. V. Talbard are with Quetron Systems,Incorporated, 4 Newport Drive, Suite F, Forest Hill, Maryland21050.

Received 5 February 1999; revised manuscript received 16 June1999.

0003-6935y99y275880-07$15.00y0© 1999 Optical Society of America

5880 APPLIED OPTICS y Vol. 38, No. 27 y 20 September 1999

contaminant in the TL light liberated over an irradi-ated zone. We show how a TL sensor is built andused to detect liquid nerve and blister chemicalagents wetting the soil through their simulant ~sim-lar physical properties but nontoxic! counterpart or-

ganic compounds. A description of sensor optics anddata collection, reduction, and pattern recognitionmodules that implement the TL technique follows.

2. Technique and Instrument

In TL the energy of an irradiating beam is absorbedinto a material following a thermal flux over its sur-face boundary, yet all manifestations of the incidentbeam ~namely, scattering! are absent in the detectedadiance. TL is the broad portion of infrared radi-nce that is liberated from the beam-irradiated zone.irchhoff ’s law states that the ratio of absorptivity tomissivity of a heated radiator is constant, indepen-ent of the type of material and dependent only onemperature of the medium ~good absorbers are goodmitters!. We exploit this in a thermal lumines-ence sensor ~TLS! through irradiation of a suspecturface with an energetic beam coinciding with anntense absorption line of water ~terrain that con-ains water-bearing materials!. A magnetron cavityuned to 2.45 GHz—the W-band microwave regionoinciding with a strong rotation resonance in theater molecule—is the chosen beam source for TLroduction. By scanning the irradiated surface, aichelson interferometer produces digital wave-

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forms of summed constructive and destructive inten-sities ~interferograms! of the TL light. Pluralities ofthese interferograms are coadded and Fourier trans-formed into graybody spectra. Typical of most inho-mogeneous dielectric materials, the graybody is aGaussian-like distribution of spectral amplitudesspanning 7.14–14.29-mm wavelengths of the middleinfrared region. For example, at 23 °C soil exhibitsa peak emission amplitude in the neighborhood of10.31 mm. This extremum will grow as temperatureof the soil volume elevates in time of beam irradia-tion, whereas its corresponding frequency will shifttoward higher energy.13 A contrast of emissivity be-tween analyte ~contaminant! and substrate ~soil! willuild and then collapse from incident beam exposurep until thermal equilibrium is attained at an ele-ated temperature. During this heating period ahermal gradient attains maximum strength; i.e.,2Tsy]t2 5 0, where Ts is the surface temperature and

t is the irradiation time. It is most prudent to mea-sure a difference spectrum along the maximum gra-dient event:13 DS 5 S~n, B! 2 S~n, A!, where n spansthe interferometer’s middle infrared detection band-width, and A and B are adjacent heating periods;T2 . tA $ T1 and T3 . tB $ T2 of interferogramcquisitions. Inside the entire heating period ~T1,3! a detection window of opportunity develops be-

cause the absorption bands of the contaminant re-vealed in DS are quite strong, provided that thegraybody envelope S~n, t! shifts at a maximum rate~i.e., the maximum gradient condition! and that theshift in amplitude and frequency between contiguousspectra S~A! and S~B! is slight ~which is related to theinterferogram acquisition rate and quantity of coad-ditions!. Elaborate ~and awkward! background fil-tration is not required for a clear identification of theanalyte if these conditions are met.

In summary, the benefit of TL remote sensing isresonant absorption of energy by an incident beam,outside the middle infrared band, into the groundwith a concomitant release of broadband middle in-frared emissions. TL excites the contaminant intoits fundamental molecular vibration modes and istherefore a carrier of thermal radiance containinginformation on chemical identity. Absorption by theincident beam causes a spectral contrast ~separation!to form between the contaminant and its substrate.This contrast builds as an induced thermal gradientgrows and then collapses as irradiation brings themedium to equilibrium at an elevated temperature.A search is conducted for emissions by suspect or-ganic compounds from difference spectra capturedwhen the maximum gradient thermodynamic state isattained.

A TLS was built around the technique just de-scribed, which is a result of our previous applied re-search13 now in early development. The TLSomputer model and prototype are shown in Fig. 1.t was built for mobile or stationary use, it is self-ontained ~all subsystems are integrated and power

is generated and distributed onboard!, and it pro-vides tactical mission support ~automated computer

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intercommunication and radio extracommunicationsystems are also onboard!. Our design objectivesare to use a low-cost, continuous-wave, shielded beamsource~s! to liberate heat from suspect surfaces with-out posing a radiation health hazard, to provide asystem for detecting contaminants on terrestrial sur-faces in situ by analyzing stimulated infrared emis-sions for fingerprint spectra, and to do this in nearreal time. To accomplish these objectives we builtand bench tested an optical head assembly ~Fig. 2!.The receiver section of this optical head consists of atrimirror scanner system for directing TL, over a con-stant ground field of view ~FOV!, to a Fourier-ransform infrared ~FTIR! spectrometer. Thepectrometer instrument, a fast-scan Michelson in-erferometer with a vibration-isolation linear mirrorrack, resolves spectral composition of the collectedL in 2 cm21 ~inverse wavelength or wave-number!

resolution. The head transmitter section is config-ured for microwave beam generation and groundstimulation of TL.

A. Transmitter

The transmitter section of the TLS optical headshown in Fig. 2 contains a magnetron ~top box com-ponent! emitting W-band, 2.45-GHz microwave radi-ation that is piped through sections of a waveguideflange. ~This is the same energy that is commonly

Fig. 1. Computer model ~top! and actual system ~bottom! of theTLS. The sensor’s optical head is housed on a modified M116A2military trailer with pedestal. The refurbished S-250 militaryshelter is equipped with data acquisition and processing and au-tomation electronics, environment control hardware, global posi-tioning system, and radio. Electrical power is supplied by a dcgenerator mounted in the HMMWV engine compartment, a dcbattery bank is located in a side compartment of the vehicle, and adc-to-ac converter is housed inside the shelter.

September 1999 y Vol. 38, No. 27 y APPLIED OPTICS 5881

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used for heating in commercial microwave ovens.!Duty cycle and switching of the beam from full power~1500 W! to nil power is accomplished by a modula-or circuit integrated in magnetron and power sup-ly. The T-component flange shown in Fig. 2 is ahree-port waveguide circulator used to protect theagnetron by redirecting backreflected energy to a

iquid-cooled blackbody absorber. A small part ofhe incident beam traversing the circulator impingesn a directional coupler, of the cross-guide type, andasses to a coaxial Bird wattmeter for monitoringeam performance. A three-stub straight wave-uide tuner regulates the forward beam power that isocused onto the ground through horn and antennand Fresnel lens optics. Side lobe intensities of thencident beam are truncated by the horn and lensombination, delivering an elliptical ground imprintf a 0.0324-m2 cross-sectional area at a 0.95-m range.

The magnetron source with load, tuner, and recircu-lator waveguide components are products of Astex,Inc. of Woburn, Mass., Seavey Engineering Associ-ates, Inc., Cohasset, Mass., fabricated the antennaand lens assembly.

B. Receiver

The TLS receiver’s ground-tracking scanner, beamdirector and condenser, and FTIR spectrometer op-tics are all seated on an inner-cradle frame. Fourstages of mechanical vibration damping meet theoperation specifications of the FTIR spectrometerunit when the TLS is mobile. The specifications

Fig. 2. Principal components of the TLS transmitter and receiveroptics.


are stiff-spring trailer suspension; FTIR and con-denser optics shock mounted to the inner cradle;inner cradle shock mounted to a rigid outer frame;and, as mentioned above, a separate damping sys-tem inside the interferometer cube isolating themotion of its transduced mirror. The triangularground scanner ~a belt to pulley that is driven abouta axle and camshaft! comprises three flat 30.5-cm-diameter mirrors that are coaxial centered and ori-ented 120° apart. A drive mechanism rotates themirrors at angular velocity v that is proportional tothe linear velocity v of a high mobility multipurpose

heeled vehicle ~HMMWV!, towing the sensoread, so that downward-looking FOV of a fixedround object is stationary. When v 5 0 ~TLS istationary! the scanner FOV is centered on the ir-adiated area ~home position!. While the sensoread is in tow, a microprocessor controller contin-ously probes the HMMWV’s speedometer encoderhile relaying feedback to a driver circuit control-

ing the scanner motor. The drive controller com-ares two sets of pulse rates—one pulse rate isroportional to v and the other to v. Regulation of, so that the FOV stays fixed, is accomplished byne continually updating the condition d~sec2 u!uvuyvu 5 1, where d 5 1.70 m is the perpendicularistance from the scanner rotation axis to theround object, and the range of the scan angle is° # u # 40°. Pulse-ratio computing is performedvery 200 ms to assure a smooth and accurateracking of the ground. Moreover, a parallel com-unication protocol system is used to monitor the

canner’s angular orientation precisely during TLeasurement cycles, including the three positions

f start of scan, crossover, and end of scan. Thisction synchronizes all three mirror orientations toata collection and processing functions ~see Sec-ion 3!. TL radiance from the ground, emitted overconstant FOV, is sent through a beam-condensingewtonian telescope ~of narrow-FOV optical de-

ign! where it is collimated, reduced 10 times to a.05-cm-diameter beam, and directed along the op-ical axis of a MIDAC M2401-C Michelson inter-erometer cube. We enhanced the moving flatirror component of the interferometer to oscillate

t 33 Hz ~interferograms per second!. Interfero-ram sets are stored at the 33-Hz rate, grouped,oadded, and transformed into spectral amplitudesy a fast-Fourier-transform ~FFT! algorithm in-cm21 resolution. Strings of contiguous spectra,

in the 700–1400-cm21 band, are dynamically storedinto a succession of computer RAM memory bins.Adjacent spectra between bins are subtracted everyone-third rotation of the ground scanner and sub-mitted in sequence to a fully trained neural networkpattern recognition system. Training of the neuralnetwork is performed against several known ana-lyte signature spectra.

Optical alignment of the TLS receiver is conductedin three phases as shown in Fig. 3. We managed analignment tolerance of 60.25 mm in centering pin-hole irises 1, 2, and 3. With the mirrors correctly

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positioned, annular rings were inserted on the scan-ner’s timing gear ~with magnetic sensors! that definecritical start-of-scan, crossover, and end-of-scan posi-tions. With these reference settings known, and bypulse-ratio tracking of the scanner as describedabove, a microcomputer flawlessly regulates v of the35-lb scanner in millisecond intervals as the HM-MWV changes velocity during its course of travel.

3. Data Collection, Processing, Reduction, and Results

Data collection by the moving TLS is graphically de-picted in Fig. 4, where the bottom photograph wastaken of the actual receiver component. An elec-tronic monitoring of the scanner synchronizes inter-ferogram acquisitions to these stages of datahandling: coaddition of a interferogram data set,Fourier transformation of the coadded interferogram,subtraction of contiguous spectra, parity check of thedifference-spectrum and baseline correction, noisefiltration, and spectral pattern recognition. To man-age these operations, we built a serial communica-tions protocol system with interrupt-driven input andoutput to provide for command and control status

Fig. 3. Optical alignment of the TLS receiver. ~a! The condens-er’s primary mirror is coaligned with the horizontal axis ~opticaltable!. Incident from the left, a He–Ne alignment beam is retro-reflected and superimposed by a flat mirror mounted to the tele-scope’s spider vertical frame ~the spider is a radial mount for a 90°central flat reflector in the Newtonian telescope!. Pinhole irises 1and 2 are positioned in the superimposed beam. ~b! A perpendic-ular axis is established by inserting a 90° reflector in the horizontalbeam, just before the condenser, and retroreflecting it from a flathorizontal mirror on the optics table while iris 3 is inserted in thesuperimposed beam. ~c! The triangular scanner structure isplaced on its mount and an upward-vertical He–Ne beam is madeto pass through iris 3, reflect 90° twice by reflector ~b! and scannermirrors, respectively, and then retroreflect. The scanner is ro-tated in exact increments of 120°, and angle adjustments ~shim-ming! to its three mirror disks superimpose the retroreflectedalignment beam.

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transfers routed between the scanner controller, cen-tral computer, and the interferometer’s direct mem-ory access ~DMA! board. Integrity of data transferss kept to a high level through software modules thatontrol handshaking, packetization, and check sumsperations. When the digitized interferograms areoved into separate computer memory buffer regions

t 33 Hz, the DMA card creates a ping-pong effect.hen buffer region 1 is full, an interrupt routine

ignals the DMA controller to start transferringewly collected interferograms into region 2, while aimultaneous coaddition operation is performed inegion 1. Fast Fourier transformation ~a 1024-pointomplex spectrum is computed in 0.0013 s by an ar-ay digital signal processing board!, spectrum sub-raction, parity adjustment, baseline correction, andoise filtration operations follow. Numerical files ofrocessed difference-spectrum amplitudes—outputy the end noise filtration operator—are formattedhen serially submitted to the input layer of a neuraletwork. Pattern recognition by the weight matrix

Fig. 4. Data collection by the TLS while traveling an open landarea. The scanner tracks a fixed ground area ~507 cm2! in theperiod T1 f T4 while projecting infrared radiance to a 103 beamcondenser. The condensed radiance is directed to a spectrometerwhere interferograms are produced, grouped and coadded, Fouriertransformed, and spectrally processed. A regulated magnetronbeam source heats the ground, generating a thermal gradient anddetection window for conducting a difference-spectrum ~DS! mea-surement ~during maximum gradient!. DS contains enhancedthermal emission and absorption bands that identify the surfacecontaminant. The bottom photograph was taken of the receiver’sinner cradle that houses scanner, beam condenser, and FTIR spec-trometer optics.


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of this network will yield a succession of yes and nodetection decisions.

Figure 5 shows a consolidated flow chart of opera-tions by the TLS data-acquisition and processinghardware and software modules. Amplified inter-ferogram waveforms, synchronized to movement bythe ground-tracking scanner, are digitized by twoparallel circuits, converted into graybody spectraS~A! and S~B!, and stored in computer memory.S~A! is derived from the Fourier transformation of acoadded interferogram set measured during the~cold! scanning period T1 # tA , T2, while S~B! iscomputed in the adjacent ~hot! period T2 # tB , T3,where the dwell time of tA and tB are identical. Themodules on the right-hand side of Fig. 5 show how theTLS regulates magnetron beam power so as to pro-duce ideal ground-heating conditions ~maximumthermal gradient!. Two pieces of information areextracted from S~A! and S~B!: their graybody max-imum amplitude Smax~n! and the corresponding en-rgy at this peak emission nmax. The absolute

difference values d 5 uSmax ~B! 2 Smax ~A!u and ε 5unmax ~B! 2 nmax ~A!u are tested with tolerances Dd andDε, respectively. If they are low ~high! the micro-wave beam power is increased ~decreased!. Modules

Fig. 5. Operation flow chart of the TLS data-acquisition and procemissions collected from the surface ~Fig. 4! are directed to andwaveforms that are amplified and digitally recorded. The right-hto produce maximum TL flux from the irradiated ground. Thetransformed into thermal spectra, and prepared for submission to apostprocessing events including localization of the contaminant arethe threat. AyD, analog-to-digital; MCT, mercury cadmium tellu


on the left-hand side of Fig. 5 perform the followingsuccessive operations: ~1! apodization of raw inter-ferograms and coaddition, ~2! fast Fourier transfor-mation, ~3! subtraction of spectra DS 5 S~B! 2 S~A!,~4! polarity check and positive selection ~the differ-ence spectrum can have negative or positive parity!,~5! baseline correction, and ~6! noise filtration.

Searching DS for the fingerprint spectral pattern ofone or several analytes is the final TLS data process-ing function. This pattern recognition is done by aneural network system trained against feature spec-tra ~chemical targets! and is implemented throughhardware @electronically trainable analog neural net-work ~ETANN! or Ni1000 chips# or software ~C11

code! computer models. The network architecturethat we built and tested for the TLS consists of aninput layer of 350 nodes ~accepting DS in a format ofone node per spectral amplitude spanning 700–1400cm21 in 2-cm21 resolution!; two hidden layers of 256nd 129 nodes, respectively; and a output layer ofine nodes. The quantity of nodes per hidden layeras built up from many training epochs and is rele-ant to network convergence and prediction perfor-ance. The end product of network training is ane-tuned weight matrix and pattern recognition

g systems with in-loop beam power regulation. Scanned radiantrated on by a Michelson interferometer, producing interferencesection of the chart shows how the TLS beam source is regulated-hand section depicts how raw interferogram sets are coadded,al network pattern recognition system. The bottom section showsthe global positioning system and instructions on how to deal with

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filter—an array of signal magnitudes interconnectingall nodes between layers of the network architecture~strength of conduction between nodes!. As the TLScompletes one scan cycle of the ground ~one-thirdscanner rotation!, DS is feed forwarded through thisweight matrix by the network’s input layer, produc-ing nine real numbers at the output layer. Theseoutputs are treated as components of a nine-dimensional vector T. A normalized inner productis computed as s 5 ~TzRi!yN, where Ri are vectorepresentations defining the analytes used to trainhe network, and N is the inner product norm:t1r1 1 t2r2 1 t3r3 1 . . . 1 t9r9!1y2. If the quantity

s lies within some set interval, say 1 # s # 0.98, thana detection event is established and an alarm will trip~absorption and emission moiety detected, Fig. 5, left-hand side!. The bottom modules of Fig. 5 show thatthe alarm activates several sensor-specific postpro-cessing tasks such as the generation of reports onphysical properties of the contaminant~s!, global po-sitioning system mapping of the contaminated area,and avoidance and decontamination procedures.These reports are transmitted electronically throughan internal router and radio located inside the TLSshelter. Our results of exhaustively training theabove four-layer neural network architecture ~300In-put 2 256Hidden1 2 129Hidden2 2 9Output! yieldetter than 99% true positive and nearly zero falseositives. The weight matrix of this particular net-ork was derived from numerous training and vali-ation trials performed against nine standardbsorption spectra of chemical nerve and blistergents and their simulant liquids.17

Figure 6 illustrates TLS stages of data collectionand processing leading to a DS measurement fromsoil that is wetted by polydimethyl siloxane ~codename SF96!, a simulant of nerve agent O-ethylS-2diisopropylaminoethylmethylphosphonothiolate~code name VX!. Sensitivity of detection is a fewdroplets ~0.3 mlydrop! dispersed over 127-cm2 surfacerea. This level of sensitivity is one order of magni-ude improvement, or better, over a laser differentialolume-reflectance method that we previously inves-igated in our laboratory. Accuracy of detectionith the TLS is also improved ~namely, fewer false

alarms! because a full data spectrum is measuredand analyzed, compared with lidar detection per-formed by scattering at two or a few discrete wave-lengths of a CO2 laser. There is, however, a concernwith the TL sensing method ~and all similar remote-sensing schemes! on how to deal with spectral inter-ferences from compounds overlapping some or allabsorption bands of a targeted analyte @e.g., see Fig.6~f !# and absorption band overlap between analytesin the 7.14–14.29-mm spectrum. Retraining theneural network with additional mixture-spectrumfeatures, and thereby expanding its weight matrix toaccommodate such analyte mixtures, will solve thelatter case. In the former case of overlapping bandsbetween analytes, additional preprocessing is re-quired that will narrow the spectral features in DS

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over the interferometer’s optical bandwidth. For ex-ample, an operation called Fourier self-convolutioncould be added after the final filtration stage of pre-processing to determine all overlapping principalbands in DS.

4. Conclusion

The signature infrared bands of contaminant inter-stitial layers on natural and synthetic surfaces wereinterferometrically detected in TL by heating from anabsorbed beam. Interferograms of the TL light arecollected as an induced thermal gradient rises tomaximum amplitude. From these data, differencespectra over the middle infrared region are producedand analyzed for the signature pattern of anyN-targeted contaminants. The TLS system is de-signed to generate, detect, and process TL radiance inthis optimum detection time frame. For example, asufficiently large thermal gradient produced by micro-

Fig. 6. Processing of TLS data from a soil sample that is wettedwith SF96 before submission to the neural network: ~a! Coaddedinterferogram sets represent ambient and heated ground regions;~b! Fourier transformation of coadded interferogram sets; ~c! slightgraybody shift is an effect of microwave heating; ~d! subtraction ofsimilar spectra ~DS! during the peak thermal gradient event iden-tifies an emissions contrast by the SF96 layers; ~e! specializedalgorithms operate on DS clarifying the contaminant’s spectralpresence; and ~f ! comparison of SF96 standard spectrum and TLSmeasurement. The neural network causes an alarm to tripagainst SF96 in this case. Note the presence of a second contam-inant in ~f !.


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wave beam exposures ranging from 10 to 20 s at1.5 W cm21 separate the emissions by liquid SF96from emissions by its soil substrate. We are nowinvestigating ways to reduce the minimum heatinginterval for making detection viable from 10 s to asubsecond. One approach is to increase the scanrate of the interferometer and thus increase thequantity of interferograms per spectrum ~the signal-to-noise ratio of the TL spectrum is proportional tothe square root of the quantity of interferograms com-prising it!. We modified a commercial Michelson in-erferometer to scan at 33 Hz for this purpose. Aoubling of the scan rate to 66 Hz or beyond may beossible in that instrument as DMA technology ad-ances. However, the limitation of applying a mov-ng mirror interferometer to the TL detectionroblem is realized. Photoelastic-modulation-basedTIR spectroscopy, in which interferogram acquisi-ion rates can exceed 10 KHz, is ideally suited here tolleviate or eliminate the signal-to-noise ratio prob-em. Tudor Buican of Semiotic Engineering Associ-tes Ltd., Albuquerque, New Mexico, is now pursuingesearch on developing an infrared photoelastic mod-lation spectrometer for this and other applications.n addition to increasing the interferometer scan rateor better signal-to-noise ratio, we are applying alter-ate source beams for generating an optically en-anced TL signature radiance. In this study weredict a greater heating rate, and thus increased TLux in the 7.14–14.29-mm middle infrared region,

rom illumination by several ~15 or more! focused5-W halogen lamps emitting in the 2–5-mm near-nfrared region.

This research was sponsored by the U.S. Army Sol-ier and Biological Chemical Command, Edgewoodhemical Biological Center, Research and Technol-gy Directorate, Aberdeen Proving Ground, Mary-and.

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thermal luminescence sensor for ground-path contamination detection

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