demonstration of differential backscatter absorption gas imaging

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Demonstration of differential backscatter absorptiongas imaging

Peter E. Powers, Thomas J. Kulp, and Randall Kennedy

Backscatter absorption gas imaging ~BAGI! is a technique that uses infrared active imaging to generatereal-time video imagery of gas plumes. We describe a method that employs imaging at two wavelengths~absorbed and not absorbed by the gas to be detected! to allow wavelength-differential BAGI. From theframes collected at each wavelength, an absorbance image is created that displays the differentialabsorbance of the atmosphere between the imager and the backscatter surface. This is analogous to atwo-dimensional topographic differential absorption lidar or differential optical absorption spectroscopymeasurement. Gas plumes are displayed, but the topographic scene image is removed. This allows amore effective display of the plume image, thus ensuring detection under a wide variety of conditions.The instrument used to generate differential BAGI is described. Data generated by the instrument arepresented and analyzed to estimate sensitivity. © 2000 Optical Society of America

OCIS codes: 280.1120, 280.1910, 280.3420, 280.3640, 010.3640.

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1. Introduction

Backscatter absorption gas imaging ~BAGI! is a laseremote-sensing technique1 that allows the real-time

visualization of gas plumes. The basic principle ofBAGI is straightforward—a scene is imaged in theinfrared ~IR! as it is illuminated by narrow-bandwidth IR laser radiation. The image is createdfrom laser radiation backscattered from solid sur-faces in the illuminated area. Gas visualization oc-curs if a plume capable of absorbing the laserradiation is located in the area being viewed. Itspresence causes a dark plume image to form in thevideo picture. BAGI is useful for many gas detectionoperations ~such as leak surveying at pipelines orrefineries! because the video picture conveys leakpresence and location in a simple format. Severalinstrumental implementations of BAGI have beendeveloped. The original version1 employed acontinuous-wave ~cw! laser illuminator that was in-tegrated with a raster-scanned IR camera. Imagers

At the time of this research, the authors were with the Chemicaland Radiation Detection Laboratory, Sandia National Laborato-ries, Livermore, California 94551-0969. P. E. Powers is now withthe Department of Physics, University of Dayton, Dayton, Ohio45469. The e-mail address for T. J. Kulp is [email protected].

Received 3 May 1999; revised manuscript received 15 November1999.

0003-6935y00y091440-09$15.00y0© 2000 Optical Society of America

1440 APPLIED OPTICS y Vol. 39, No. 9 y 20 March 2000

based on this design were developed for operation indifferent spectral regions using line-tunable ~;50lines between 9- and 11-mm! CO2 lasers,1–3 an IRhelium–neon laser4 ~3.39 mm!, and a tunable optical

arametric oscillator based on periodically poled lith-um niobate5 ~continuously tunable from 3.1 to 3.6

mm; extendable to operation between 1.3 and 4.5mm!. Imagers capable of stand-off ranges from 6 to60 m were demonstrated.3,4 Recently,6 a system

was developed that employs a flood-illuminatingpulsed laser ~tunable from 3 to 3.5 mm! used in con-unction with a staring IR focal-plane array ~FPA!amera. Extension to operation with pulsed lasersllows efficient use of nonlinear conversion in gener-ting wavelengths that are inaccessible with existingw laser technology. Use of a FPA camera has someistinct advantages over a single-element, raster-canned system, including a larger collection aper-ure and a lower noise floor. Each enhancementontributes to a longer imaging range at a given av-rage laser power.In this paper we present a new methodology for

AGI that uses illumination at two laser wave-engths ~one absorbed by the gas to be detected andne not absorbed! to operate in a differential absorp-ion mode. Past systems imaged only at the absorb-ng wavelength and relied on visual detection of thepatial contrast caused by the gas plume. Althoughhis is effective when viewing scenes of uniform re-ectivity, it can present problems when scenes ofighly variable reflectivity are observed. In those

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cases the intensity contrasts caused by the gas plumecan be masked by those inherent in the scene itself.This arises because the image information is mappedonto a gray scale, and sensitivity is determined by theminimum gray-scale change that the eye can per-ceive. In a high-contrast scene the gray-scale rangemust be spanned across a wide range of return sig-nals to adequately represent the scene image,whereas in a low-contrast scene the gray scale spansa range of return signals that is fairly narrow. Thisimplies that the resolution with which changes inreturn signal ~and therefore gas attenuation! can beperceived is high when viewing a low-contrast scenebut can be rather coarse when viewing a high-contrast scene.

The new approach uses dual-wavelength imagingto eliminate this problem. Images are collected un-der illumination at each wavelength and displayed asthe natural logarithm of the ratio of the on wave-length ~that absorbed by the gas! to the off wave-length ~that not absorbed by the gas!. The resultantimage, hereafter referred to as a ln-ratio image, isequivalent to a display of the path-integrated differ-ential optical depth of the atmosphere between thesensor and the target. Processing in this mannerremoves all scene information ~assuming that it isspectrally flat over the wavelength difference—an as-sumption that is obviously better for small than forlarge differences7! and leaves only the gas image.

his allows the gas image to be optimally displayed.ecause of its two-wavelength nature, the technique

Fig. 1. Diagram of the apparatus used to demonstrate the

can be considered to be an extension of topographicdifferential absorption lidar8,9 ~DIAL! or differentialoptical absorption spectroscopy10 ~DOAS! to an imag-ng format. As such, it is referred to as differentialAGI.In the remainder of this paper the differential

AGI instrumentation that was used for this demon-tration is described, as is the signal processing pro-edure and some sample results. Measurements areresented that indicate the performance and sensi-ivity limitations of the system.

2. Imager Description

The instrumentation used to demonstrate differen-tial imaging is a variation of the pulsed imager de-scribed in a previous paper.6 It consists of threecomponents: a tunable infrared laser source, abeam formatter projector, and a synchronously gatedFPA receiver. The overall system is illustrated inFig. 1 and is described in the following paragraphs.It should be noted that this system was used for thisdemonstration because of the availability of the hard-ware and that it does not represent an optimal dif-ferential BAGI instrument. A more appropriatefieldable device would use a more compact lightsource than the laboratory-grade laser that was used.For example, the pulse energies used are attainableby optical parametric oscillators pumped by compactdiode-pumped or flash-lamp-pumped Nd:YAG lasers.A simpler method of attaining the frequency dither-ing described below would be to use injection seeding

rential BAGI measurement. HSVB, high-speed video bus.
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20 March 2000 y Vol. 39, No. 9 y APPLIED OPTICS 1441

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at the signal wavelength by one ~tunable! or two~fixed-frequency! diode lasers. Other limitations as-sociated with the described configuration are indi-cated below.

A. Frequency-Dithered Tunable Infrared Laser Source

The laser source, shown schematically in Fig. 2, isbased on the design of Milton et al.11 Figure 2 isncluded to assist in explaining the present system;owever, readers are referred to Ref. 11 for specificetails of operation. The laser consists of aifference-frequency generation ~DFG! stage followedy an optical parametric amplifier ~OPA!. A pulsed,

injection-seeded Nd:YAG, operating at a repetitionrate of 30 Hz and a pulse energy of 600 mJypulse wasthe starting point for both stages. A portion ~400mJ! of the 1.064-mm output beam was frequency dou-bled and was used to pump a PDL-1 dye laser. AnLDS-821 laser dye was used to allow tuning ~by use ofan intracavity grating! near 800 nm. With 160mJ of 532-nm frequency-doubled light, the dye laserproduced 17 mJ at 805 nm. The tunable output ofthe dye laser was mixed with 50 mJ of the YAGfundamental in the 5-cm-long uncoated LiNbO3 DFGcrystal ~cut at 47°! to produce a difference-frequencybeam that is tunable from ;3.1 to 3.5 mm. Its out-put energy was typically ;1 mJ. It is subsequentlyamplified by mixing it with the remaining 150 mJ ofpump in the OPA crystal ~identical to the DFG crys-tal!. The OPA output energy at 3.3 mm was 4.5 mJ.The short-wavelength output of the OPA ~tunablefrom ;1.5 to 1.6 mm! was not used. Energies as highas 17 mJ have been reported in Ref. 11 when ahigher-power Nd:YAG pump laser is used.

The laser used in the differential imager differsfrom that described in Ref. 6 in its ability to ditherbetween two wavelengths on a frame-to-frame timescale ~switching time of ,33 ms!. This is accom-plished with a piezoelectric translator ~PZT! attachedto the dye laser grating arm. The PZT deflects the

Fig. 2. Diagram of the frequency-dithered pulsed la


grating arm up to a maximum displacement of 100mm, which tunes the dye laser over a frequency rangeof up to ;25 cm21. In addition, both the DFG andthe OPA crystals are mounted on galvanometers thatallow their phase-matching angles to be tuned as thegrating is dithered. When operated in thewavelength-dither mode, the Nd:YAG pulses weresynchronized with the motion of all tuning elements.Prior to running, the grating is positioned to set thewavelength of the DFG beam near to that of the gasabsorption. Then the amplitude of the tuning ele-ment motion is adjusted to set the frequency-ditherspacing to some value between 0 and 25 cm21 and toposition the end points of the dither at the appropri-ate on and off wavelengths. For the results pre-sented in this paper the end points were set towavelengths on and off the Q branch of methane,located at 3.313 mm. Typically, the frequency spac-ing between the two positions was chosen to be ;3m21, but was changed systematically for some mea-

surements.

B. Beam Homogenizer Projector

Shot-to-shot fluctuations in the spatial profile of theprojected illumination give rise to unwanted baselinenoise in a ln-ratio differential measurement. Staticnonuniformity in the illumination is also a problembecause it leads to spatial variations in the returnsignal and, hence, the dynamic range. Reduction ofthese effects is achieved by use of beam homogeniza-tion optics. These consist of an f 5 21.55-cm lensserving as a beam expander! followed by a facetedens ~Laser Power Optics, San Diego, Calif.!. ThenSe faceted lens is formed to contain the equivalentf sixteen 0.25-in. ~0.64-cm! facets and sixteen partialacets around the edge of the lens on a 1.5-in. ~3.81m! diameter with an effective f-number of 1.7. Itperates as a prism array—the expanded beam isegmented into 32 different square beamlets that areubsequently overlapped at a distance of 2 in. ~5.08

ource used in the differential BAGI imaging system.
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cm! from the surface of the lens. The square-shapedoverlap region is then imaged onto the target by useof an fy1.7, 3.3-in. ~8.38-cm! focal-length ZnSe lens.As a unit, the system converts the highly asymmetricbeam profile of the DFG laser into a uniform squareillumination that fills a 5° field of view ~FOV! at thetarget. Figure 3 shows a comparison of ln-ratiomeasurements made with the beam homogenizer andwith an ordinary beam-expanding telescope. Theimages illustrate the difference in uniformity pro-duced by these two approaches. Additional consid-erations regarding the design and performance ofsimilar beam homogenizers can be found in Refs. 12and 13.

C. Synchronously Gated Focal-Plane Array and ControlElectronics

Differential imagery is collected by use of a snapshot-mode 256 3 256 pixel InSb FPA camera ~customizedModel AE-173, Amber Engineering! that is the sames that used in the single-wavelength pulsed imager.s described in Ref. 6, some aspects of the cameraerformance were modified from those of the off-the-helf AE-173 to optimize its response to nanosecond-uration pulsed light. During the performance ofhe research described in this paper, the camera wasperated at 77 K with a reverse bias on the FPA of

Fig. 3. Comparison of ln-ratio images generated with a ~a! con-entional telescope beam expander to format the laser illuminationnto the target ~b! to that generated with the beam homogenizer

projector to format the illumination.

3.34 V. Imaging was performed through a 88-mmfocal-length fy1 lens. Collected rays were transmit-ted through a cold filter ~bandpass of 440 nm, cen-tered at 3520 nm! to remove unwanted passivebackground radiation.

In the differential mode of operation, the camerawas synchronized to the laser, so that the laser firedwhile the FPA was gated to integrate the signal.The FPA data collection electronics ~Amber ProView!served as the master clock for the system. The Pro-View generated clock pulses that triggered the flashlamps and Q-switch of the Nd:YAG pump laser. TheQ-switch clock pulses also synchronized a waveformgenerator whose sinusoidal output was used to drivethe dye laser grating PZT. The amplitude of thesinusoid was adjusted for the desired wavelengthseparation. The same signal was also used to gen-erate the drive signals for the galvanometers thatpositioned the DFG and OPA crystals. Prior to in-put into the galvanometer drivers, the amplitude andphase of these signals were adjusted to set the angu-lar amplitude of the crystal motion and to account forelectronic and mechanical timing delays.

The rate of collection of frames ~total of on- andoff-wavelength frames! is 30 Hz; thus, the collectionate of on–off frame pairs is 15 Hz. However, theroView electronics were clocked at a frame rate of 90z to allow operation in bias–sum–subtract mode.

n that mode, each ~on- or off-wavelength! frame isollected by use of a three-step sequence. In the firsttep ~bias!, an internal ~to ProView! frame register isnitialized with a midrange ~32,768! integer value.n the next step ~sum!, a laser-illuminated frame isollected and added to the frame register. In the lastsubtract! step, a passive frame is collected and sub-racted from the register. The resultant frame inhe register is then displayed. Laser firing is syn-hronized with the second step. The bias–sum–ubtract sequence is carried out each time an on or offrame is collected. The frame integration time was00 ms, which was found6 to be the minimum time

required to read out the direct-injection backplaneFPA when operated in a pulsed-illumination mode.

Using this apparatus, we could collect and processshort ~100-frame! differential BAGI movies. Speedimitations of the software package that we usedAmberView! prohibited real-time processing andisplay of the differential data. Thus, a contiguousequence of on- and off-wavelength frame pairs wasollected, stored in memory, and subsequently post-rocessed. Postprocessing consisted of normalizingndividual frames to the laser energy and then cal-ulating ln-ratio images. We accomplished normal-zation by dividing each frame by the average pixelalue of that frame. This is not an optimum methodecause the gas plume can affect the average inten-ity in a manner that is independent of laser-pulsenergy. In the future, normalization could be ac-omplished with a detector that would directly mon-tor the illumination beam energy. Also, theifferential algorithm could be automated. For theata presented in this paper, the processing was car-


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ried out by the operator who sequentially applied theimage processing operations to the images. Thistypically took several minutes to accomplish andwould not be suitable for ultimate application. Theprocesses used, however, are consistent with thespeed of available real-time processing hardware andsoftware and could be implemented in this way in thefuture.

As alluded to above, some aspects of this configu-ration limit its use as a practical gas sensing system.For example, the 33-ms time period between on- andoff-wavelength frames and that ~11 ms! between ac-tive and passive frames in the bias–sum-subtract op-eration are sufficiently long to make the systemsensitive to platform movement. Motion causespixel registration errors that result in incompletesubtraction of passive background or improper ratio-ing of on- and off-wavelength values. The passiveradiation collected could be reduced to a negligiblelevel with a narrower-bandpass cold filter or by onereducing the integration time. The filter used now,however, is the narrowest bandpass that is compati-ble with the fy1 collection optics; reduction in band-width would lead to loss of the extreme rays of the fy1bundle because of shifting of the center bandpass forthat incidence angle. The FPA integration time can-not be made shorter than the present 200 ms becauseof peculiarities associated with the response of thedirect-injection backplane to short- ~nanosecond-! du-ration pulsed light. Use of a different backplane~such as a charge transimpedence amplifier! couldpotentially lead to shorter integration. As discussedin Section 4, other hardware configurations ~such asa cw raster scanner using two separate laser beams!may be more suitable for a fieldable differential BAGIinstrument.

3. Results and Discussion

The system described above was used to make differ-ential images of scenes containing methane gasleaks. Figures 4~a!–4~c! contain image examplesthat allow a visual comparison of single-wavelengthand differential imagery. The imaged scene con-tains a 0.4 standard cubic foot per hour methanerelease that is occurring near a set of gas cylindersarranged in a six-pack configuration. The first twoimages are single-wavelength frames collected at theon wavelength ~the methane Q-branch absorption at;3018.5 cm21, having an absorption cross section of2.04 3 1023 ppm21 ~inverse parts per million! m21 forhe 0.3-cm21 laser bandwidth used! and the off wave-

length ~;3021.1 cm21!. The third image shows theresultant ln-ratio image that we obtained by process-ing the first two frames. It is clear that the ln-ratioimage effectively displays a plume that is difficult todiscern in the single-wavelength frame. The en-hancement occurs because differential processing re-moves scene contrasts generated by objects otherthan the gas plume ~such as the gas bottles!, therebyllowing the plume contrast to be optimally spannedcross the image gray scale. Differential imaginglso relaxes the reliance on plume motion that exists


in single-wavelength BAGI.1 The existence ofplume motion can, however, still be helpful in distin-guishing against the residual noise floor in differen-tial BAGI.

The differential imaging photon signal at pixeli, j~Di, j! can be related to that specified for the single-wavelength imager, namely,

Di, j 5 lnFna:i, j~lon!

na:i, j~loff!G , (1)

where na:i, j~lon! and na:i, j~loff! are the number ofactive-signal photons collected in the FPA unit cell i, jduring on- and off-wavelength illumination. Insert-

Fig. 4. Representative single-wavelength and differential BAGIimages: ~a! collected at the wavelength absorbed by methane3018.5 cm21!; ~b! collected at a wavelength off the methane ab-

sorption ~3021.1 cm21!; and ~c! differential BAGI image derivedfrom the other two images.

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ing the expression for na:i, j ~derived from a standardlidar equation! from Ref. 6, we obtain

here lL is the laser wavelength, EL:i, j is the pulseenergy of the laser incident upon pixel i, j, Isp:i, j is afunction describing intensity variations that are dueto speckle ~see below!, bij is the ~assumed Lamber-ian! reflectivity of the target at pixel i, j and lL,

ka~lL! is the atmospheric attenuation coefficient atlL, sg is the absorptivity of the target gas, Cg:i, j is the

ath-integrated concentration of the target gas inront of pixel i, j, lij is the effective plume thickness inront of pixel i, j, R is the range to the target surface,nd on and off indicate that the laser wavelength isn or off the target gas absorption.The term Isp was added to represent the random

intensity modulation caused by laser speckle. It is arandom function that varies according to a Rayleighstatistical distribution with a mean value of 1 and astandard deviation given by

ssp 54lL

pdupx, (3)

where d is the diameter of the receiver aperture andupx is the full-angle divergence of a single-pixel FOV.

quation ~3! is the noise-to-signal ratio for a speckle-ontaining signal and is derived by taking the squareoot of the quotient of the speckle correlation areand the receiver collection area ~see Ref. 14!. Thus,hen ssp is multiplied by the average signal ~in unitsf energy! it yields the standard deviation of the sig-al in units of energy.Equation ~2! can be recast to separate the gas im-

ge from the scene image. Using Dka 5 ka~loff! 2

a~lon! and Dsg 5 sg~loff! 2 sg~lon!, we obtain Di, j by

Di, j 5 lnFEL:i, jbi, j~lon!Isp:i, j~lon!

EL:i, jbi, j~loff!Isp:i, j~loff!G

1 lnSlon

loffD 1 2Dka R 1 2Dsg Cg:i, jli, j. (4)

Equation ~4! is the basis of differential imaging.he first and second terms are logarithms of the ratiof quantities that should change little between the onnd off wavelengths ~it is assumed that the framesre normalized for transmitted laser-pulse energyrior to processing!. Thus, these terms are nearlyero, and differential gas image information is con-ained entirely in the last two terms, which add toive the differential absorbance of the interveningtmosphere ~including the gas plume! between the

two wavelengths. The hard target image drops outbecause it is represented entirely in the first term.

Di, j 5 lnEL:i, jbi, j~lon!Isp:i, jlon ex

EL:i, jbi, j~loff!Isp:i, jloff ex

If the attenuation caused by the atmospheric back-ground varies little from pixel to pixel, its differential

absorbance adds only a subtractable dc offset to theimage. Once the atmospheric offset is removed, onlythe target gas plume contributes to spatial contrastin the image. Its intensity can then be spannedacross the gray scale for optimal display.

Noise in differential imagery results from a combi-nation of electro-optical noise ~shot noise, FPA readand dark noise, digitization noise! in the on- andff-wavelength images and from wavelength depen-ences in the ratioed terms in Eq. ~2!. The magni-ude of the latter can vary from pixel to pixel and canause a residual fixed-pattern noise in the differentialmage that must be distinguished from the gas plumemage. Wavelength dependence in surface reflectiv-ty ~b! is determined by the nature of the surface

aterial and its orientation.7 Changes in the spa-tial distribution of the illumination ~EL! with wave-length can arise from laser beam-profile fluctuationsthat are not removed by the homogenizer or fromwavelength-dependent interference fringe patternsthat occur when the segmented beamlets overlap.Laser speckle ~Isp! can cause noise that is a functionof wavelength, time, and space. Temporal and spa-tial noise that is due to speckle is present in single-wavelength imaging as well and can be considered tobe a component of the electro-optical noise; its effectis summarized here to assist in discussion of specklenoise in differential imaging. For close-rangesingle-wavelength imaging, under conditions of notarget motion, speckle appears as a static patternthat is referred to here as correlated speckle. Whenthere is motion of the target or imager, or when thereis significant atmospheric turbulence, the pattern ex-hibits frame-to-frame temporal variation15,16 ~i.e., itecomes decorrelated!. The degree of correlation inhe pattern can vary from total to partial to zero. Inhe limit of totally decorrelated speckle, Isp is a func-ion of pixel and time, and the standard deviation ofhe frame-to-frame temporal noise amplitude for aingle-wavelength frame is equal to ssp times the

mean signal amplitude.Speckle decorrelation can occur as the wavelength

is tuned from the on to the off wavelength. It isexpected that the magnitude of the change ~for agiven wavelength difference! will increase as the tar-get depth viewed by a given pixel increases. Theintensity on a single pixel is a result of the coherentsuperposition of light reflected from the various tar-get facets in the pixel FOV. If these facets have alarge variation in depth, the resultant interferencepattern becomes more sensitive to a wavelengthchange. This can be better understood if we con-

2@ka~lon!R 1 sg~lon!Cg:i, jli, j#%

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sider a pixel with a FOV that contains only two sur-faces separated in depth by a distance z. For a givenwavelength, constructive or destructive interferencedepends on the number of wavelengths in the sepa-ration z. For normal incidence this number of wavesis simply 2zyl. As the wavelength changes, thechange in the number of waves between the two sur-faces is given by

DN 52zl2 Dl, (5)

where Dl is the wavelength difference between the onand off wavelengths. Hence for a rougher surface~i.e., larger z! the interference pattern varies moreapidly because the number of waves changes moreapidly. A tilted surface will give the same effect asrough surface because the FOV of a pixel samples

ifferent target distances.The implications of speckle for differential imaging

iffer in the correlated and uncorrelated limits.hen the on- and off-wavelength frames are tempo-

ally decorrelated, the temporal noise in each is equalo ssp times the mean signal amplitude and the noise

in the resultant differential image is equal to =2times this. Under these conditions, wavelength-dependent speckle changes add no extra noise. Inthe limit of perfect temporal correlation of the on andoff frames, Isp is a function of space but not time, andthe variation of the fixed-pattern intensities along aline of pixels ~assuming that one is viewing a surfacef uniform reflectivity! is described by the same stan-

dard deviation. In differential images derived fromthese frames, speckle would be removed by ratioing ifthere was no wavelength dependence of Isp. If

resent, however, wavelength-dependent changesan determine the speckle noise contributions to dif-erential imagery.

The noise characteristics of the imager described inhis paper are illustrated in Fig. 5. Figure 5~a! wasade when the formatted laser illuminates a portion

f a uniform-reflectivity wall. Figure 5~b! showsorizontal slices of data taken from ten similar con-ecutive images. The data show significant spatialuctuations that vary little in relative intensity fromrame to frame. The signal falls to zero at the edgesf the target where there is no return signal. Theagnitude of the spatial intensity fluctuations are

ot consistent with those expected from the smallarget reflectivity variations and are attributed to aorrelated speckle pattern. The speckle noise stan-ard deviation predicted with Eq. ~3! is 14%. Thisgrees reasonably well with the 12.7% standard de-iation observed in Fig. 5~b! and with previous mea-

surements made by use of the pulsed imager, thusconfirming that the noise in the image is speckledominated.

Having established partial speckle decorrelation asthe dominant noise source, it is of interest to deter-mine how its magnitude changes under differentmeasurement conditions. The nature of the noise indifferential BAGI imagery can be investigated by


subjecting these frames to ln-ratio processing. Fig-ure 5~c! shows a horizontal slice of a ln-ratio framederived from two consecutive frames of the ten-framesequence. At the edges, where there is no backscat-tered light, the baseline fluctuation is large becauseof division by near-zero values. At the center, thedifferential signal has a zero average baseline with astandard deviation corresponding to 0.038 units ofpath-integrated gas absorbance. This noise is largerthan that attributable to the sum of passive thermaland FPA noise, which should be equal to the loga-rithm of =2 times the fluctuations in the baseline atthe edges of the line scans in Fig. 5~b!. Its mostlikely sources include ~1! partial speckle decorrela-tion ~as a result of changes in the speckle patternwith time or over the wavelength change between theon and off frames!, ~2! changes in the spatial intensityf the transmitted illumination that are due to laseream-profile fluctuations that are incompletely re-oved by the homogenizer, ~3! changes in the loca-

Fig. 5. ~a! Single-wavelength image. ~b! Superposition of tenprofiles along the indicated line in ~a! for ten consecutive images.~c! A ln-ratio profile derived by dividing one of the lines by thatfrom the next frame.

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tion of interference fringes produced by thehomogenizer, and ~4! slight motion of the target orcamera. We made an attempt to determine themagnitude of the contribution of fringing by compar-ing ln-ratio data collected using the homogenizerwith that collected using an illumination beamformed by broadly expanding the laser output with asimple telescope. The differential baseline fluctua-tions were similar in both cases, indicating that fring-ing and beam-profile changes are probably not thecause of the noise. Thus, it is assumed that the mostlikely cause of the noise is partial decorrelation of thespeckle pattern, possibly caused by the wavelengthchange or by slight motion of the target or imager.

To determine the magnitude of decorrelationcaused by wavelength change and target depth, wemade ln-ratio measurements using successive framescollected as a function of wavelength difference be-tween frames and as a function of target angle ~whichsimulates a change in target depth or surface rough-ness!. We measured the baseline noise by determin-ng the standard deviation of the single-pixel ln-ratioignal about the zero background within a 60 3 60

pixel region in the target image. This standard de-viation corresponds to a minimum detectable absorp-tion for a representative pixel in that area. Theresults are shown in Fig. 6. At zero wavelength sep-aration, the baseline noise gives a minimum detect-able absorbance of ;0.03. This gradually increasesto ;0.13–0.14 as the wavelength separation is in-creased to a value of up to 25 cm21. The increase ismore rapid as the incidence angle on the target isincreased. In attempting to draw conclusions fromthis data, care must be taken that the decrease in thesignal-to-noise ratio at larger incidence angles is notsimply the result of a lower return signal ~relative tothe camera noise floor!. The zero wavelength differ-ence results indicate that this is not the case becausethe noise level for the different incidence angles is

Fig. 6. Plot of the baseline standard deviation of ln-ratio imagesmeasured as a function of wavelength separation between the onand off wavelengths and the target angle.

similar, as expected when the speckle patterns re-main correlated. It can be concluded that the in-creased noise observed at nonzero wavelengthdifferences originates primarily from speckle decor-relation. The plateau at an absorbance of 0.13–0.14is somewhat less than the expected ~0.14!~2!1y2 5 0.20alue predicted by use of speckle statistics, indicatinghat decorrelation is still not complete between the onnd off wavelengths for those wavelength separationnd target angle combinations.At this point, some comments regarding the prac-

ical implications of these observations can be made.he magnitude of the speckle noise resulting fromavelength-induced decorrelation will depend on then–off wavelength separation required to detect theas of interest. Molecules exhibiting sharp features,uch as isolated rovibrational lines or Q branches areetectable at separations of ;0.1–1 cm21, whereavelength-induced speckle decorrelation shouldave little effect. Molecules having broad, fullyverlapped rotational bands requiring separations ofeveral to tens of inverse centimeters will be moretrongly impacted. At those separations, however,ther systematic effects such as differential albedoay also have a measurable effect.Another issue is pixel averaging. In the discus-

ion above, the minimum detectable absorption wasetermined for individual pixels. This does not ac-ount for visual averaging that can occur as the eyeimultaneously views patterns of pixels. It is con-eivable that this effect would improve detectivityeyond that inferred from Fig. 6. As alluded tobove, temporal ~or spatial! averaging could be usedo improve detectivity. Care must be taken how-ver, that the temporal or spatial scale of the aver-ging is compatible with the time scale of motion ofhe plume or its spatial dimensions. If the averag-ng is too coarse in either dimension, it will do morearm than good by fading the image into the back-round.

4. Conclusion

The results of this study show the feasibility of dif-ferential BAGI. To the best of our knowledge, this isthe first description of an imaging-mode topographicDIAL or DOAS system for gas plume detection. Theimagery presented demonstrates that operation in adifferential mode can significantly improve the visi-bility of low-contrast gas plumes, especially whenviewed simultaneously with a high-contrast back-ground. Quantitative analysis of the image datashows that the primary source of noise ~under thecondition of the measurement! is changes in specklethat occur between the on- and off-wavelength frame.Measurements of the variation of this noise with on–off wavelength difference and with target roughnessshow that relatively low ~2–6%! absorbance noise ispossible for small ~up to 3-cm21! wavelength changes~regardless of target roughness!, such as might becompatible with the detection of molecules havingisolated rovibrational features or sharp Q branches.

or molecules requiring larger on–off separations,


4. T. G. McRae and L. L. Altpeter, “Application of backscatter

1

the noise floor can be expected to be significantlyhigher ~up to 18%!.

Improved performance may be achievable with amore optimal instrument. Certainly, some of thesystematic effects mentioned above that make themeasurement susceptible to platform motion could bealleviated by one reducing the time between collec-tion of on- and off-wavelength data. An example ofsuch a system would be a raster scanner that usedsimultaneous illumination of the instantaneous FOVusing two ~on and off ! laser beams. Greater immu-nity to speckle may be attained by one adjusting thegeometry of the averaging scheme. Unfortunately,the degree of correlation in the speckle makes it un-likely that simple time averaging will allow its re-moval. Perhaps averaging could be employed inconjunction with some process that ensures that ran-dom speckle patterns are averaged over time. Sucha process might include averaging as the angle to thetarget is changing or as the viewing area is ditheredover a small region ~that is large enough to ensurethat different speckle realizations are sampled overthe averaging times!. It is also possible that passiveimaging ~using a spectrally filtered imager collectingpassive images at the on and off wavelengths! wouldbe a speckle-free alternative to active imaging. Anadditional issue that would have to be considered inthat case, however, is its reliance on thermal differ-ences between the gas and its surroundings.

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demonstration of differential backscatter absorption gas imaging

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