nondestructive testing by using long-wave infrared interferometric … ndt... · 2013-11-08 ·...

Nondestructive testing by using long-wave infraredinterferometric techniques with CO2 lasers and

microbolometer arrays

Igor Alexeenko,1,4 Jean-François Vandenrijt,2 Giancarlo Pedrini,1 Cédric Thizy,2

Birgit Vollheim,3 Wolfgang Osten,1 and Marc P. Georges2,*1Institut für Technische Optik—Universität Stuttgart, Pfaffenwaldring 9, Stuttgart 70569, Germany

2Centre Spatial de Liège—Université de Liège, Avenue du Pré Aily, B-4031 Angleur (Liège), Belgium3InfraTec GmbH, Gostritzer Strasse 61-63, Dresden 01217, Germany

4Current address: Immanuel Kant Baltic Federal University, A. Nevskogo Street 14, Kaliningrad 236041, Russia

*Corresponding author: [email protected]

Received 6 July 2012; revised 20 September 2012; accepted 20 September 2012;posted 21 September 2012 (Doc. ID 171765); published 22 October 2012

We describe three different interferometric techniques (electronic speckle pattern interferometry, digitalholographic interferometry, and digital shearography), using a long-wave infrared radiation produced bya CO2 laser and recorded on a microbolometer array. Experimental results showing how these methodscan be used for nondestructive testing are presented. Advantages and disadvantages of these approachesare discussed. © 2012 Optical Society of AmericaOCIS codes: 090.2880, 120.6165, 120.4290.

1. Introduction

Methods based on holographic interferometry (HI)[1] and electronic speckle pattern interferometry(ESPI) [2] are commonly used for nondestructivetesting (NDT) of mechanical structures and dynamicanalysis. These techniques are based on the compar-ison of wavefronts recorded at different instants. Theresult of the wavefront comparison is the phase dif-ference containing the information about the objectdeformation during the recording.

There are many publications describing resultsobtained by HI and ESPI using visible or ultravioletlight; however, few investigations in the long-wave in-frared (LWIR) spectral range (wavelength 8–14 μm)were realized up to now.

The main reasons for the limited number of LWIRinterferometric developments are that working at

such wavelength requires (1) specific optical ele-ments usually made of germanium (Ge) or zincselenide (ZnSe), (2) an adequate hologram recordingmedium with good performance, (3) imagers sensi-tive in that range, and (4) a laser emitting in thatrange. The laser that we consider here is a singlelongitudinal mode CO2 laser, which can be foundwith power of several watts at low prices comparedto lasers with similar power in the visible range.With the growth of the thermal-imaging systemsmarket, optical components can be found in stockat affordable cost as well. At last, various thermalimager technologies exist with an increasing numberof pixels, and the uncooled microbolometer arrays [3]offer now advantages in terms of ease of use, afford-able costs, and state-of-the-art megapixel format [4].Therefore, it can be envisaged to build LWIR inter-ferometric NDT systems, which can offer severaladvantages over their visible equivalents. The ad-vantage of LWIR techniques is that the displacementmeasurement range is increased because it is

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directly proportional to the laser wavelength. Also,the sensitivity of the holographic setup to externalperturbations, which is driven by various factors re-lated to the object and the setupmechanical stiffness,are lower in LWIR than in the visible range. Indeed,the fringe pattern formed by interference of referenceand object beams at the holographic photosensitivemedium must not move by more than a tenth ofwavelength during capture.

Nevertheless the important question about theholographic recording medium remains, which wasalready addressed in the past for the case of LWIRphotosensitive plates. Let us cite the early works byChivian et al., who used a thermochromic materialfor recording at 10.6 μm and readout along with aHe–Ne laser [5]. These works were followed by othergroups using the same separated wavelengths for therecording and readout steps but applied to otherrecording media such as liquid crystals [6], wax andgelatin films [7], or other recording schemes withcuprous mercuric iodide [8]. In the same period, aCanadian groupmade a lot of investigations consider-ing severalmaterials andpresentedapplicationswithacrylic and thin films [9], wax [10,11], oil films depos-ited on glass plates [12], photoresists [13], and poly(acrylic acid) films [14].Until now, there is no evidenceof high-quality holographic interferograms based onsuch materials and arrangements. An alternative tochemical photosensitive materials is the electronicrecording of interference patterns formed by objectand reference beams in various configurations, whichrely either on ESPI or digital holography [15]. ESPIwith a CO2 laser was first introduced by Løkbergand Kwon in 1984 [16], who used pyroelectric vidiconcameras.Althoughofmajor interest for itsnovelty, thepaper reports the critical behavior of the vidicon cam-eras, which react to changes of the radiation energy.This was amajor drawback, and the authors were ob-liged to modulate the intensity of light to record thespeckle patterns. For that reason they only showedapplication of LWIR ESPI on vibrating objects.

More recently Allaria et al. [17] introduced LWIRdigital holographic reconstruction of a small maskobject in a Mach–Zehnder configuration. The holo-grams were recorded directly on a modern versionof a pyroelectric thermal imager equipped with aninternal chopper for coping with stationary imageintensities. Other works followed by the same groupwith the same sensor and with the aim of shapemeasurement [18].

In our work we consider microbolometer arrays forthe reasons discussed earlier and because they do notrequire artificial variations of light beams. RecentlyGeorge et al. showed digital holography based on amicrobolometer array [19] with an optical setupsimilar to that of [17,18].

Recently we have proposed several setups usingelectronic recording of holograms and specklegramswithmicrobolometer arrays and aCO2 laser for appli-cations in interferometry of scattering objects, morespecifically in-plane displacement measurements

by ESPI [20] and out-of-plane displacement by bothESPI and lensless digital HI (DHI) [21]. The resultspresented there were obtained with early microbol-ometer technologies with small formats (320 × 240pixels) and with the purpose of demonstrating thecapabilities of LWIR holography techniques in appli-cations of displacement measurement. The objectsconsidered in these papers were metallic plates mo-unted on rotation stages, with countermeasurementtechniques for cross-correlation of obtained values.

In[21]wediscussedtheproblemofthespecularityofthe surface in the LWIR range. Following Yamaguchi[22], the surface becomes completely scattering andspeckle appearswhen the laserwavelength is smallerthanthesurfaceroughness.Ontheotherside,specularreflection appears as soon as the wavelength is largerthantheroughnessandquicklydominates thespecklepattern intensity. Nevertheless, to some extent thespeckle pattern can still be observed, but its intensityis smaller than the specular peak. With metallic ob-jects, the surface often presents a roughness such thatit reflects in a specular way. We circumvented this byusing a white removable powder, which increases theroughnessinsuchawaythatobjectsscattersenoughtocreate the necessary speckle.

The purpose of the current paper is to show furtherdevelopments in the field of NDT of aeronauticalcomposite structures with various constructions,defects, and surface characteristics. We had a seriesof representative samples made of glass-fiber-reinforced plastic (GFRP), carbon-fiber-reinforcedplastic (CFRP), and Kevlar. Ideally these structuresneed to be investigated on the production site, aim-ing at flaw detection or assessment of mechanicalbehavior (displacement measurement) under loads.Our first step in this project was to assess the rough-ness of these samples, and we showed that most ofthe above cited materials have a roughness that ishigh enough to generate speckle with CO2 lasers[23]. Preliminary DHI experiments were alreadyreported with these samples [24].

In this paper, we present three techniques of inter-est for NDT that we extended to LWIR with a CO2laser. On one hand we have a first category that con-cerns measurement of displacement fields, namelyESPI and DHI, and on another hand we have digitalshearography (DS), which allows measuring the de-rivative of displacements along one particular direc-tion (strain).

The two former are studied on the basis of a similarsetup because of the high similarity of opticalschemes, with Section 2 being devoted to ESPI andSection 3 to DHI. Section 4 presents developmentsand results with DS.

2. LWIR Electronic Speckle Pattern Interferometry

A. Setup and Technique Description

Figure 1 shows the scheme of the ESPI setup. It in-cludes (1) a CO2 laser (from VM-TIM) with wave-length λ � 9.3 or 10.6 μm (selectable by the user),

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20 m coherence length, 1.6 mm beam diameter at thelaser output, beam divergence of 8.7 mrad, power upto 8W, and linear polarization; (2) an uncooledmicro-bolometer infrared camera (VarioCAM-hr, fromJenoptik) with 640 × 480 pixels of size 25 μm × 25 μm(giving a detector size of 16 mm × 12 mm), a dynamicrange of 16 bits, and a frame rate of 50 frames=s, run-ning with IRBIS software by Infratec GmbH. The ob-jective lens (OL) of the camera module has a 50 mmfocal length, is made of germanium, and is especiallydesigned for thermography in order to provide max-imal resolution and field of view.

As we will explain later, we have not detached theOL from the camera to incorporate the referencebeam. The camera is equipped with an internalaperture wheel, which is located between the objec-tive and the sensor and has two positions: a closedposition that is necessary for automated internal re-calibration of the camera from time to time (neces-sary in thermography application) and a fully openposition with diameter D � 21 mm, which constitu-tes the aperture stop (AS) of the OL. The distancebetween the aperture and the sensor is L � 21 mm.The lateral speckle size dsp can be found by the dif-fraction theory: it is given by separation of the firsttwo minima of the first-order Bessel function J1 andis expressed as dsp � 2.44λL=D � 22.7 or 26 μm as afunction of the laser wavelength used. Because thespeckle size is smaller than (or barely identical to)the pixel size of the LWIR detector, speckle grainsare not fully resolved. Due to the fact that the aper-ture diameter is fixed for LWIR systems, one cannotchange the L=D ratio for making speckle grains resol-vable. However, Maack et al. [25] have already shownthat ESPI can be performed under such circum-stances. Their works were carried out under visiblelight, and we extended this discussion in our earlierworks to LWIR ESPI [21].

The implementation of ESPI requires that thelaser beam is split in two parts. The first one (object

beam, represented by solid lines) illuminates theobject, which scatters light towards the camera.The second part is the reference beam (dotted lines)that is arranged to enter the camera through areference-beam injection setup that is discussed inthe next section.

The reference and speckle object beams can berepresented by their electric field vectors, respec-tively R�x; y� � R�x; y�eR exp�iφR�x; y�� and U�x; y� �U�x; y�eU exp�iφU�x; y��. Their interference at eachpoint �x; y� of the detector produces an intensitypattern (specklegram), which is given by

I�x; y� � jR�x; y�j2 � jU�x; y�j2 � R�x; y�U��x; y�� R��x; y�U�x; y�; (1a)

� IR�x;y��IU�x;y��eReU�2�IR�x;y�IU�x;y��1=2×cos�ϕ�x;y��; (1b)

where IR�x; y� � jR�x; y�j2 and IU�x; y� � jU�x; y�j2 arerespectively the intensities of reference and objectbeams at the sensor level, eR and eU are the unit vec-tors giving the orientation of their electric field vec-tor, and φR�x; y� and φU�x; y� are their intrinsic phaseand their relative phase difference, defined byϕ�x; y� � φR�x; y� − φU�x; y�. As is commonly known,in any interferometric setup the contrast of interfer-ence, given by m�x; y� � eReU2�IR�x; y�IU�x; y��1=2=�IR�x; y� � IU�x; y��, is maximized when the polari-zations are parallel (eReU � 1) and when IR�x; y� �IU�x; y�.

Therefore one has to ensure that both conditionscan be made possible in the setup. One unknownis the polarization behavior of the samples thatare observed. As mentioned earlier, the latter aremade of CFRP, GFRP, and Kevlar. In previous experi-ments [21] we also proposed the use of white remo-vable powder (developer powder for liquid penetranttechnique) in order to increase the roughness whenthe latter is too small to generate speckles. In thecase of the above mentioned materials, as well aswith the developer powder, we have observed thatthe scattered beam is globally linearly polarized,with a small depolarization (not exceeding 10%). Itis worth mention that, at least in the case of thewhite developer powder, the polarization behavioris totally different than when illuminated with a visi-ble laser, which completely depolarizes light. There-fore it was important to consider these aspects in thedesign of the LWIR ESPI setup because a highly de-polarized object beam would require a polarizer infront of the camera for interfering with the referencebeam in order to limit background noise. Indeed,such a noise can be represented by a supplementaryintensity term in Eq. (1), which further limits the in-terference contrast. In our case, a preserved object-beam polarization did require the use of such apolarizer. However, it is still necessary to try equal-izing the intensities of beams as much as possible by

Fig. 1. Scheme of the LWIR ESPI setup.


adequate components at the level of beam separa-tion. The separation is generally ensured by beamsplitters (BSs), which are made of ZnSe glass withcoating that allows either an equal ratio, that is, re-flection and transmission of 50% (R50/T50), or asym-metric ones. Various solutions were used in thesetup, and the results presented later make use ofone or another of those we will describe now. The firstone uses a R50/T50 BS from ULO Optics. Because alow level of light is scattered by the object towardsthe camera, a R50/T50 BS induces a reference beammuch more intense than the object when reachingthe camera sensor. Therefore attenuators (A) needto be placed in both arms to finely adjust the con-trast. These attenuators are made of two polarizersof which the transmission axes are rotated one toanother. Therefore their output polarizer also playsthe role of correctly orienting the polarization of bothbeams. The second possibility used for the beamsplitting is to consider an asymmetric-ratio BS:R90/T10 or R99/T1, manufactured by II-VI Company.In this case, the reference-beam intensity is intrin-sically small and better matches the scatteredobject-beam intensity. However, it was still too highand required a fine adjustment for beam ratio closerto unity. For that purpose we used a polarizer (P) inthe reference beam, which allows adjusting its inten-sity, the laser beam being polarized. Despite the factthat we changed the polarization direction of the re-ference beam (eReU < 1), the polarizer transmissiondirection was very close to the laser polarizationand this had no large influence on the final interfer-ence contrast.

After the BS, the object beam is rendered divergingby means of a ZnSe illumination lens (IL), of whichthe focal length is chosen to illuminate the wholeobject surface. The reference beam is travelling to-wards the reference injection setup. On its path, aset of folding mirrors are placed, one of them (MPZT)being mounted on a piezotranslator (Physical Instru-ments, P-611.1S, closed loop). The movement of thelatter allows changing the path length in the refer-ence arm in order to implement the phase-shiftingtechnique, which is necessary to determine the phaseof the wavefront.

During the acquisition of the specklegrams, thecamera is synchronized with the phase shifting ofthe piezo element: a set of four images is recordedfor positions of the MPZT mirror that induce a π=2phase shift between each acquisition. Therefore thephase-shifted specklegrams are expressed as follows(leaving aside the �x; y� dependency for a matter ofsimplicity):

In � IR � IU �m�IR � IU� cos�ϕ� n

π

2

�; (2)

with n � 1, 2, 3, 4. Eq. (2) is used to calculate thephase according to the well-known four-buckets algo-rithm: ϕ � tan−1��I4 − I2�=�I1 − I3��. If we record twosuch sets of four specklegrams In;a and In;b for twostates of the object, noted a and b, we can compute

their phases ϕa and ϕb. The phase difference Δϕ �ϕa − ϕb can be related to the displacement d of eachobject point, imaged in each point of the camera�x; y�. This relationship is given by

Δϕ � 2πλs · d; (3)

where the sensitivity vector defined by s � k1 − k2 isdetermined by the geometry of the setup, where k1and k2 are the unit vectors of illumination and obser-vation, respectively.

In Section 2.C we present a series of phase mapsΔϕ�x; y�, which were obtained when composite sam-ples undergo thermal loads. Before showing results,we have to explain an important part of the study,which is devoted to injection of the reference beamin the camera for producing the interference withthe object beam and recording the specklegrams.

B. Injection of Reference Beam

The injection of the reference beam into the camerais ensured by a beam combination device, which is akey element for any ESPI setup and is usually placedbetween the imaging lens and the sensor. In visiblelight, such a beam combiner (BC) is often a BS cube,which transmits the object beam and reflects thereference beam. The reference beam is a divergingbeam issued from a point source (e.g., from an opticalfiber or a pinhole), which is conjugate to the limitingdiaphragm of the imaging lens in the object arm.

In the design of a LWIR ESPI setup, we have toreach similar ideal conditions. Also we knew byexperience that CO2 laser beams can irreversibly de-stroy pixels of the microbolometer array. Thereforethe study of the reference injection appeared as animportant step of the development.

In our study we have analyzed the design of someOLs made for an uncooled LWIR camera in order toassess the feasibility of placing a BC between themand the focal plane array. The back focal distanceavailable between our 50 mm focal length OL andthe focal plane array is on the order of 20 mm. Ifwe take into account the fact that there is a germa-nium window at a few millimeters in front of thesensor and that optical elements of the OL can movetowards the sensor when adjusting the focus, thereremains little space for placing a BC. We have alsoto consider that such uncooled infrared cameras havebuilt-in aperture wheels out of the germanium win-dow. In such aperture wheels a blocking aperture ispresent because the cameras have to be recalibratedfrom time to time to ensure good thermal measure-ment when used for thermography purposes.

For our development, we could have removed thisdevice and built a new box for the sensor and electro-nics in order to have the sensor easily accessible forincorporating a BC. We already followed such an ap-proach in our preliminary developments presentedin [21]. Although those earlier results were encoura-ging, noisy images were obtained, which was partly


due to thermal noise coming from the environment ofthe sensor because the latter was not enclosed. In thenew developments presented here, we definitely seekbetter-quality results with a state-of-the-art cameraand OLs. Therefore we avoided dismantling ourequipment in any way for incorporating optics. Forthat reason we studied the injection of the referencebeam using different approaches that are describedin the next section.

We know that we ideally need a spherical referencebeam, which issues from the AS or diaphragm of theOL. The 50 mm OL considered here (and in generalall the OLs made for uncooled cameras) have theirAS after the last lens; moreover, it is attached tothe aperture wheel inside the camera. In order toaccurately study the injection of the reference beam,we need the OL design, but we were not allowed toobtain it from its manufacturer. Consequently, wemeasured some useful optical parameters, like thefront and back focal distances, on a separate opticalbench using a CO2 laser collimated beam. We foundthat the front focal distance (between the first lensand the object focal point) is 17 mm, whereas theback focal distance (between the last lens and the im-age focal point) is 24 mm. Figure 2 shows a represen-tation of the OL which has its AS a few millimetersaway from the last lens of OL. The figure shows theideal situation for the injection of the reference beam(dotted lines). With the OL parameters, we calcu-lated that, in order to cover the whole detector, theinjection lens L has to have a focal length of 252 mmand a diameter of 100 mm [26]. Such an idealelement is not in the catalog and has to be manufac-tured at high cost. Therefore we preferred alterna-tive ways: either keeping the ideal requirement ofcentering the reference point source on the centerof the AS (optimized configuration) or departing fromit with simpler working configurations, admittingthe possibility of lower quality of interference be-tween reference and object beams.

A drawback of the frontal injection of the referencebeam is the necessity of large BCs, which areessentially BSs similar to those already discussedin the previous section. Because the OL has a typical

diameter of 50 mm, the BC must be typically 80 mmwide to avoid vignetting. Such a large element can berealized in ZnSe, but it is quite expensive. In order tofind an alternative to such elements, we imagined tobring a point source in the middle of the AS throughan optical fiber embedded in a modified medical nee-dle. The needle biased aperture is sealed by a mirror,of which the reflecting face is turned inside (seeFig. 3). A 50 μm diameter hole was drilled on theupper part of the needle at an appropriate location.The beam launched by the LWIR fiber is then re-flected by the mirror and exits the needle throughthat hole. The ensemble constitutes a very compactpoint source. The LWIR fiber is a hollow silicawaveguide of 500 μm diameter and 1 m lengthmanufactured by Polymicro Company.

Figure 4 shows various possibilities that wereanalyzed. Any configuration for injecting the refer-ence beam must be such that the latter is large en-ough to cover the whole detector. Figure 4(a) consistsin focusing the reference beam on the object focalpoint F of the OL. This way the reference beam isnaturally collimated after having passed the OLand covers the entire detector (provide the injectionlens is sufficiently large). Figure 4(b) is an alterna-tive to the previous one and makes use of the mod-ified needle described above. It is sufficient to placethe needle point source at the object focal point F.Figure 4(c) is a further alternative that was also usedwith success. The LWIR fiber is used with a lens thatfocuses the beam well beyond the focal point F. A40 μm diameter pinhole at the output of the fiber per-forms filtering. This allows obtaining a homogeneousbeam, which is then passed through a ZnSe lens of50 mm focal length and 50 mm diameter. The refer-ence is not collimated anymore when reaching thedetector but is slightly converging. In this case,the interference with the object beam is limited ona central part of the detector.

Fig. 2. Ideal reference beam injection.Fig. 3. (Color online) Modified needle with embedded LWIRoptical fiber for generating miniature point source.


The BC in the schemes in Figs. 4(a) and 4(c) is awedged ZnSe plate with ratio R50/T50, which is in-sensitive to polarization. In order to avoid vignetting,the BC has a 80 mm diameter, which is not an advan-tage in compactness compared to the needle.

The different setups described above have theirpros and cons, and we selected one for further devel-opment of a mobile instrument. The needle solution[Fig. 4(b)] has the advantage of allowing verycompact setup and is cheaper than a BC. However,the alignment into the fiber is difficult because ofthe invisible wavelength. We experienced problemsof melting of the fiber input when slight misalign-ment occurs. Moreover, the fiber being multimode,the reference beam exhibits speckles at the fiber out-put. These speckles can move if the fiber is not wellstabilized mechanically and can prevent interferencewith the object beam. The first system with the BC[Fig. 4(a)] allows a stiffer setup without misalign-ment problem. The drawback of the R50/T50 BC isthat half of the light intensity is lost in the objectbeam, which requires more laser power for objectillumination. The third system [Fig. 4(c)] suffers fromthe same constraints related to the fiber as well as

losses due to the BC. Consequently, this is the leastinteresting scheme for reference injection. Finally,for building a mobile instrument for field applica-tions, the scheme depicted by Fig. 4(a) is our bestchoice.

Despite this choice, we have tried all these setupsin NDT experiments with actual samples made ofcomposites, and the results presented in the nextsection have to be understood rather for illustrationthan for deep comparison between reference injec-tion system performances.

C. Results

For the investigation, we used a series of compositesamples made of various skin materials, mainlyCFRP and Kevlar, with various layers and defectsof different natures and depths underneath thesurface. Figure 5 shows photographs and definitionof samples we used for the different results shownlater. Sample 1 [Fig. 5(a)] has a rectangular shapeof 36 cm × 30 cm. It is made of CFRP layers arrangedin four vertical zones with increasing thickness.A cross section along the sample thickness is shownunder the picture. The first zone on the left has threelayers with total thickness of typically 0.9 mm. Goingto the right, the second zone has six layers, the thirdone nine layers, and the last one on the right 12layers, with a final thickness of 3.5 mm. This samplehas no defect. Sample 2 [Fig. 5(b)] has a square shapeof 25 cm × 25 cm with a thickness of 4 mm and isalso made of CFRP layers. A delamination provokedby an impact is present (grey dots). Sample 3[Fig. 5(c)] has the same rectangular shape and lateralextent as sample 1 with four vertical zones. However,the sample has an 18 mm thick Rohacell core sand-wiched between two Kevlar skins. As shown in thecross section along the thickness, the upper skin hasthe same geometry as for sample 1 while the bottomskin has an overall thickness of three layers. TheRohacell core is glued on the two skins, and defectswere included between the upper skin and the core.Their location is superposed on the picture: each ver-tical zone has two debondings (black dots) of varioussizes, cracked core (grey dots) of various sizes, andtwo surface cracks (grey lines).

For all these samples, the surface roughness wassimilar to or larger than the wavelength; thereforespeckle was naturally present and no further surfacepainting was necessary. These samples were used ina typical NDTapplication where thermal stress is ap-plied and the sample mechanical response (deforma-tion) is observed. The thermal stresses are provokedby halogen or infrared lamps. In practice, a series ofphase-shifted specklegrams is recorded before heat-ing the sample. Then the lamp is switched on for afew tens of seconds. Afterwards it is switched offand one waits a few tens of seconds to record newphase-shifted specklegrams. The phase differencebetween the actual deformed state and the referenceone (before heating) is then computed in real timeand is related to the deformation, as explained in

Fig. 4. Various schemes for reference beam injection: (a) withfocusing reference beam at front focal point through BC, (b) withmodified needle at front focal point, (c) through BC and referencelaunch with fiber and lens beyond the focal point.


Section 2.A. Figure 6(a) shows the deformation ofsample 1. The entire object is imaged: it was placedat a distance of typically 1.3 m from the and wasilluminated by a lens with 25.4 mm focal length.The pattern shows shifts in fringes between the dif-ferent zones. The ESPI setup made use of the injec-tion scheme depicted in Fig. 4(a). Figure 6(b) isobtained using the injection setup with the needledepicted in Fig. 4(b) and with sample 2. In this case,the object was placed at around 70 cm from the OLand was illuminated by a negative lens with −20 mmfocal length. The total observed area is 20 cm ×15 cm and is represented by a rectangle in

Fig. 5(b). The delamination is well identified by astrong local deformation under the thermal stress.Figure 6(c) is obtained using the injection setup withthe fiber depicted in Fig. 4(c) and sample 3. In thiscase the object distance and illumination lens arethe same as for sample 1. The overall deformationis seen as large circular fringes, whereas defectscan be seen as local fringe patterns or tiny variationsof phase in some places. Figure 6(d) shows the post-processing of Fig. 6(c), where the global deformationhas been eliminated. The process consists of phaseunwrapping of the phase map in Fig. 4(c), fittingthe result with a two-dimensional (2D) second-degree polynomial and subtracting it from the phase.The remainder is the contribution of local deforma-tion induced by defects. It must be noted that sam-ples 1 and 3, which are observed at 1.3 m distancefrom the sensor, were placed out of the table wherethe remaining part of the optical setup was lying.Moreover, the samples were not firmly attached orclamped but rather were simply standing in a overallvertical position, with simply two pillars on the backside keeping the sample in place.

Figure 7 shows the effect of using a higher wave-length and the interest of using LWIR wavelengths.Figure 7(a) was obtained on sample 2 with the setupof Fig. 4(c), a CO2 laser with 9.3 μm wavelength andwith 60 s heating of sample. The change of wave-length is due to the fact that the previous laser at10.6 μm was no longer available. Nevertheless, asthe response of microbolometer arrays is almostidentical for both lines, results are comparable withthose obtained at 10.6 μm. Figures 7(c) and 7(d) show

Fig. 5. (Color online) Photographs and definition of the different samples used in our experiments.

Fig. 6. (Color online) (a)–(c) Phasemapsmodulo 2π obtained afterheating composite samples. (d) Defect map after postprocessingphase map (c).


how the result of the same deformation of Fig. 7(a)would have looked if shorter wavelengths were used.The figures were obtained by simple computation,noting that, for a given displacement and setup,the product s · d in Eq. (2) is conserved. Thereforechanging the wavelength λ allows computing newmodulo 2π phase maps. Figures 7(b), 7(c), and 7(d)are obtained respectively with 1.85, 0.93, and0.46 μm wavelength, and the latter is close thatwhich should be observed with a usual visible ESPIsystem. Equivalent large displacements could bemeasured by using interferometers based on visibleradiation, but in this case many more specklegramsshould be recorded during the object deformation(e.g., by using a high-speed camera) and consecutiveaddition of the phases is necessary. This approach istime consuming and increases the errors due to theaddition of phase maps.

3. LWIR Off-Axis DHI

A. Setup and Technique Description

The setup used for DHI is very similar to the one forESPI (see Fig. 1); the only difference is the off-axisconfiguration that allows obtaining the phase ofthe object wavefront without using a phase shifter.This configuration is obtained experimentally byadding a small angle between the reference and ob-ject beams. The DHI method consists in recordingthe interference of reference and object waves bythe matrix sensor, here the microbolometer arrayof the camera. The recorded intensity function candescribed by Eqs. (1a) and (1b) and is called herehologram, whereas we named it a specklegram inSection 2.A. The fourth term of Eq. (1a) containsthe information about the object wavefront ampli-tude. Digital holography consists in numerically re-constructing the object wavefront from the recordedpattern. We are restricted by the spatial resolution ofthe detector, and for recording the digital hologram,

the spatial frequency f should satisfy the samplingcondition <1=�2Δ�, where Δ is the camera pixel size,in order to ensure a correct spatial sampling of thehologram [27].

In the case of the off-axis configuration, a 2Dspatial Fourier transform of the recorded intensityallows separating the �1 order R��x; y�U�x; y� fromthe other terms described in Eq. (1a) [28]. Accordingto definition of phases given in Section 2.A, the phasedifference between two wavefronts Ua and Ubrecorded at different instants is expressed as

Δϕ � ϕa − ϕb � φR − φUa − �φR − φUb�

� tan−1

�Im�R�Ub�Re�R�Ub�

�− tan−1

�Im�R�Ua�Re�R�Ua�

�; (4)

where Re and Im denote respectively the real andimaginary part of the signal and where dependencyto coordinates �x; y� has been omitted for clarity. Thephase difference computed by Eq. (3) can be relatedto the object displacement d in every point throughEq. (2), similarly to ESPI. The advantage of thismethod is that only a single hologram is necessaryfor the wavefront phase calculation, as opposed toESPI or in-plane DHI, for which all the terms ofEq. (1a) are spatially superposed. This requiresthe phase-shifting method to be applied by capturingmultiple images, hence imposing stability of theobject during capture. The DHI technique presentedhere is thus well suited for the investigation ofdynamic processes.

This advantageous configuration has a limitationthat comes from the above mentioned samplingcondition. Indeed, the frequency f is given by f �1=df � �sin�θref=2� � sin�θobj=2��=λ, with df the dis-tance between adjacent maxima and minima of aninterference fringe on the detector and θref and θobjrespectively the angles that the reference and objectbeams make with the normal of the detector. There-fore these angles must be rendered as small as pos-sible by limiting the aperture of the optical system.

The camera AS located between the OL and thesensor was used to restrict the spatial frequency ofthe object beam. In order to estimate the aperturesize suitable for digital holography, the whole system(OL, AS, and microbolometer array sensor) was si-mulated by using ZEMAX ray tracing software; thisled us to choose an aperture size of 4 mm, which issmall compared with the maximal aperture, whichhas a diameter 21 mm. Because reducing this aper-ture is not possible with such a thermographicsystem (fixed diameter), we manufactured a thinaperture on a thick black paper sheet that was thenattached on the open position of the camera aperturewheel. By choosing the small aperture, we reduce theimage resolution and the intensity recorded by thedetector. The reference beam was focused throughthe objective in the plane of the aperture. Reducingthe aperture has the negative effect of reducingat the same time the intensity at the detector and

Fig. 7. (a) Phase map modulo 2π obtained by ESPI at micrometerwavelength. (b)–(d) Simulated equivalent phase map with shorterwavelengths, respectively 1.85, 0.93, and 0.46 μm.


the spatial resolution of the reconstructed objectwave. Nevertheless, we managed to obtain results,which are presented in the next section.

B. Results

Figure 8(a) shows the modulus of the Fourier trans-form of the recorded intensity. The central peak is thezero order containing information about referenceand object beam intensities according to Eq. (1).The two first orders (containing information on theobject wave field U and its conjugateU�) are also visi-ble. A similar digital hologram is reconstructed withthe deformed object. The phase variation due to thedeformation, described by Eq. (4), can be computed.Figure 8(b) shows the phase map due to heating ofsample 2, placed at typically 70 cm from the OLand illuminated by a negative lens with −20 mmfocal length, giving a total observed area of 20 cm ×15 cm. Because of the small aperture, the resolutionof the recorded images and the quality of the phasedifference is lower than the ones obtained with ESPI.Despite the lower quality, DHI requires only a singleacquisition per object state and not a set of phase-shifted ones. Therefore, DHI could be used for analy-sis of large dynamic displacement measurements asan alternative to ESPI, which is used for slower-speed phenomena.

4. LWIR DS

Shearography is usually used for investigation ofstrain [29], and mobile systems are nowadays usedincreasingly in industries for inspection in view offlaw detection [30]. Shearography does not makeuse of the interference of reference and object waves,contrary to both previous methods. Instead, a singlebeam coming from the laser is used to illuminate theobject, which scatters light towards the observationsystem, which is preceded by a shearing device. Inliterature one can find various configurations ofshearing devices. They can be based on (1) insertionof a wedged glass plate in half of the field of view [29],(2) birefringent elements introducing polarizationseparation in images followed by a polarizer [31], or(3) a modified Michelson interferometer [29]. For ourdemonstration, we used the Michelson configurationbecause this is the easiest to put in place with LWIRoptical components. Figure 9(a) shows the scheme ofthe setup. The BC is polarization insensitive and hasratio R50/T50. It splits the image into two similar,

equal-intensity images, the transmitted one (repre-sented by solid lines) and the reflected one (repre-sented by dotted lines), which are reflected bymirrors, respectively M1 and M2, to be further re-combined at the level of the camera. One of thetwo mirrors (M2) is tilted by an angle α, which allowsproducing a lateral shear between the two images.Figure 9(b) shows the situation where the shear isoriented along direction x. However, shear directioncan be adjusted in the image plane �x; y�. Interfer-ence occurs where the two images overlap [grey zonein Fig. 9(b)], and the intensity of the interference pat-tern is described by a function identical to Eqs. (1a)and (1b) except that R�x; y� and U�x; y� are replacedrespectively by the object field U�x; y� and itselflaterally displaced (or sheared), U�x�Δx; y�Δy�,where Δx and Δy are shear amounts respectivelyalong the x and y directions.

The strain of object (along the x direction) is re-lated to the phase difference by [29]

Δϕ � 2πλs∂d∂x

Δx; (5)

where s is the sensitivity vector and ∂d=∂x andΔx arerespectively the strain component and the shearamount in the x direction. A piezoelectric translatorwas set in one path of the interferometer to shift thepath length, and a four-step algorithm was used tocalculate the phase. One of the main applicationsof shearography in NDT is the detection of defectwhere the latter is seen as a local variation of strain.As for ESPI [Fig. 6(b)] and DHI shown previously, theinvestigated object is sample 2 with the same setupparameters (object distance and IL). Figure 10 showsthe phasemap, where the delamination defect under-neath the surface is clearly detected. The process ofthe measurement is identical to phase-shifting ESPIpresented in Section 2.A: a series of phase-shiftedshearograms (instead of specklegrams) are recorded

Fig. 8. DHImethod. (a) Reconstructed digital hologram. (b) Phasedifference due to the thermal deformation.

Fig. 9. (a) Shearography setup. (b) Overlap of images reflectedby mirrors M1 and M2 with lateral shear along x direction andamount Δx.


when the sample is at rest, and the phase ϕa is cal-culated by the four-bucket algorithm. Afterwards thesample is heated, for instance by a halogen lamp, fora few seconds, and then the heat source is switchedoff. During the temperature relaxation that follows,a series of phase-shifted specklegrams are againrecorded, giving new phase ϕb. The phase differencebetween ϕb and ϕa is then computed, which showsthe strain of the sample between the initial andstressed states following Eq. (5).

5. Discussion

Measuring displacement and deformation of compo-sites is a topic of interest for aeronautical industries,either for analyzing their behavior under varioustypes and magnitudes of stresses or for detecting lo-cal defects by abnormal local deformation induced bythe defect when the sample undergoes stress (usuallythermal). If we want to discuss the advantages anddrawbacks of the techniques presented in this paper,we have to put them in perspective with currentlyused NDT techniques.

Usual noncontact optical NDT techniques involveeither methods based on coherent light or noncoher-ent light. In the first category one finds methods likeESPI and shearography, which are used for full-fielddisplacement or strain measurements, respectively.Their measurement resolution is related to thewavelength of the laser. In the second category onefinds three-dimensional coordinate measurementssuch as fringe projection [32], stereocorrelation[32,33], or videogrammetry [32]. In this case, the dis-placements or the strains are obtained by capture ofcoordinates of the sample surface points at differentinstants of the stress and by computing numericalsubtraction. Their measurement resolution dependsmainly on the resolution of the camera and is a frac-tion of the volume observed. As a good rule of thumb,one can say that a megapixel camera offers a resolu-tion of 1=1000 of the object size, with equivalentperformance in term of displacement.

The noncoherent techniques are used in thermo-mechanical analyses or structural tests where the

mechanical displacements or strain of compositeparts are monitored as a function of stresses. Usuallythese parts have dimensions similar to the ones con-sidered in our study (a few tens of centimeters wide)or even higher (meters wide). Therefore resolutionsare on the order of a few hundred of micrometers.Better resolutions can be obtained but require stabi-lity and multiple image acquisitions [32], which isnot an advantage. Because of their noninterfero-metric character, these methods have the advantageof being quite immune to environmental perturba-tions. An important drawback is that surfaces haveto be prepared prior to measurements. For instance,stereocorrelation requires spraying the surface witha white color superposed by a random pattern ofblack dots whereas videogrammetry requires retro-reflecting targets glued on some surface points. Inthe last case, this strongly limits the lateral samplingof the object. Despite this, these techniques are usedin a wide variety of structural tests, with very highdisplacements or deformations and high temporaldynamics as in shocks or rupture (destructive)tests.

For displacement measurement with higher reso-lution, ESPI can be used, but as the wavelength isshort, maximum displacements remain limited to afew micrometers with resolutions of a few tens ofnanometers. For displacements that are too high forthe range of visible ESPI (because fringes are notresolved anymore), one needs to record a very largenumber of specklegrams and compute their phasedifferences Δϕ, which allow a resolvable number offringes, before adding these subsequent Δϕ for ob-taining the entire displacement. This is both timeconsuming and increases the measurement errors.Another drawback is that visible interferometrictechniques are generally used in laboratories wherequiet environmental conditions are present.

The LWIR ESPI techniques discussed in this paperclearly have the advantage of filling agapbetween thehigh resolution allowed by visible ESPI and the lowresolutions of noncoherent techniques. A few hun-dreds ofmicrometers can be reachedwith LWIRESPIin a single shot,whereas the equivalentmeasurementwould require 20 times more images in the visiblerange. Also the experiments were performed withsamples located outside of the optical table wherethe LWIR setup was placed, and moreover they werenot stiffly attached. This is an encouraging indicationfor the possibility of future development of a mobileinstrument for out-of-laboratory applications.

Concerning the dynamics of the phenomena ofinterest, the phase-shifting recording sequenceduration (typically 0.5 s) limits the velocities of dis-placements to a few micrometers per second (consid-ering that the phase-shifting objects do not move bya fraction of the wavelength during the recordingsequence). This is still 20 times better than for thevisible ESPI technique.

Typical applications addressed by LWIR ESPI arethe thermomechanical assessment of composites, for

Fig. 10. Phase map obtained with LWIR shearography afterheating of CFRP sample.


instance measuring the deformation of coupons orelements of larger sizes like the ones of the study.In these cases the samples may undergo a few tensof degrees of temperature variations and/or largemechanical stresses, like those experienced eitherby fuselage or wing parts in the different sequencesof flight, or in some parts of engines. Another appli-cation is the detection of defects, as was shown withsome samples, but this requires additional postpro-cessing for removing the large global deformation aswas shown in Figs. 6(c) and 6(d).

Concerning LWIR DHI, as already mentioned, fas-ter phenomena can be monitored compared to LWIRESPI. Indeed, single-shot measurements are possi-ble with recording time related to the frame rate ofmicrobolometer, say 1=50 of a second. Despite thelower lateral resolution of the off-axis DH configura-tion and the lower quality of interferograms reported(which in turn limits the maximum displacement re-solvable), millimeter-per-second-velocity phenomenaseem reasonable to achieve. This opens the way tovibration measurement or dynamic stresses like insome structural testing (such as fatigue).

Shearography in visible light is mostly used for in-ternal defect detection like delaminations in compo-site laminates and debondings between compositeskins and other structural elements. Such defectsinduce local strain under given stresses, like heatingwith a lamp, vibration by a piezoactuator, and soforth. Identifying the location of these locally higher-strain areas provides instant and easy position andextent of the defects underneath the surface. LWIRshearography showed the same typical results asthose obtained by visible shearography, but herestrains are 20 times higher. This means than largerstresses need to be applied for the same results,which is not necessarily an advantage. Due to thefact that the equipment of the LWIR shearographysetup is more cumbersome than in the visible range,LWIR shearography should have limited interest fordefect detection. However, it would keep the advan-tage of filling the gap between visible shearographyand noncoherent techniques for the measurement offull-field strain images.

6. Conclusion: Future Prospects

We have shown that it is possible to extend variousinterferometric NDT techniques (currently appliedwith visible lasers) to the LWIR spectral range, asso-ciating state-of-the-art thermographic cameras withmicrobolometer arrays and CO2 lasers. We have pre-sented different developments at the level of the op-tical setup. One critical point is the injection of thereference beam into the camera with its OL attached.Several schemes were studied and implemented forESPI and DHI. We also have demonstrated thefeasibility of LWIR shearography, which does notrequire an external reference beam. In all cases weperformed typical NDT experiments on the basis ofcomposite parts with size, materials, and defects thatare representative of aeronautical applications.

The sensitivity-to-displacements measurement ofLWIR interferometric NDT techniques is situatedbetween the one of their visible light counterpartsand the one of another category of NDT techniquesworking with noncoherent light. Therefore we nowfill a gap between two measurement ranges that donot overlap. Another advantage of using LWIR forinterferometric techniques is that the latter could beused in more perturbed environmental conditionsdue to the low sensitivity to phase variations be-tween reference and object beams. The disadvantageof longer wavelength is that the resolving power interm of imaging is 20 times lower, but this is not areal problem for object sizes larger than centimeters,as considered in our applications.

The results presented in this paper are initiallaboratory developments for a future mobile system.The most promising technique is LWIR ESPI, whichgives the best results and will be implemented forthermomechanical analyses and defect detection incomposites.

These works are funded by the FP7 Europeanproject FANTOM (ACP7-GA-2008-213457).

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