espi - project report

EML 5111L Experimental Stress Analysis (Spring, 2016) Date of Submission: 03/23/2016

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ElectronicSpecklePatternInterferometry

AkashMarakani,NiveditaMadanalaAbstractInterferometryis ameasurementmethodusing thephenomenonof interferenceofwaves (usually light, radioorsoundwaves). Themeasurementsmay include those of certain characteristics of thewaves themselves and thematerials that thewaves interactwith. Interferometry is an investigative technique that uses lightwaves for thestudyofchanges indisplacement.Electronicspecklepattern interferometry (ESPI) isa techniquewhichuses laserlight, together with video detection, recording and processing to visualize static and dynamic displacements ofcomponentswithopticallyroughsurfacesintheformoffringes.Thispaperreviewstheworkingprinciple,benefitsandlimitationsofthistechnique.Thepaperalsotalksaboutthevariousapplicationsandprovidesabriefsummaryofthelatestadvancementsinthisfield.

1.IntroductionElectronicSpecklePatternInterferometry(ESPI),whichisalsoknownasTVHolographyisanon-contactopticalmethod which is generally used for studying surfacedeformations.Surfacedeformationareacharacteristicproperty of three dimensional displacements, whichcan be further translated into 3D strains and stresses,andareakeyparameterfordesign,manufacturingandqualitycontrol.Duetotheadvancementsintechnologyvarious industrial sectors like automotive,manufacturing has already adapted rapid optimizationdesignconcepts.Theseconceptsallrequirethesupportofhighsensitivemeasurementof3Ddisplacements.The traditional technique for measurement ofdisplacement and strain is the resistance strain gagemethod,howeverdueto its lowspatial resolutionandtime consuming methodology, advanced opticalmethods due to their non-contact, full fieldcharacteristics,andhighmeasurement sensitivityhavebeen widely accepted as displacement and strainmeasurement tool in industry. Of these methods,electronic speckle pattern interferometry is the mostsensitive and accurate method for full field 3Ddisplacementmeasurement.Electronic Speckle Pattern Interferometry (ESPI) relieson the interference between the reflected light fromtheobjecttobetestedandareferencebeamfromthesame laser source. It is an optical full-fieldmeasurement method used to determine thedeformationsonobjectthatmustbeanopticallyrough.The reflected beamand the reference beam from the

same laser light source are superimposed on a videocameraandinterferetoformaspecklepattern.Specklepattern recorded before and after deformation of theobjectyieldanon-uniquefringepattern.Usingaphaseshiftingmethod,thisnon-uniquenesscanbesolvedandthefringepatternisfurtherevaluatedusingacomputeralgorithm.Thistechniqueisveryaccurateandalsohasthe capability to detect fractures, micro-cracks andsurfaceflaws.In this report, we provide a literature review onElectronic Speckle Pattern Interferometry (ESPI), theirexperimental setup, procedure and their applications.This report also summarizes the areas of currentadvancements in ESPI and how they have beenaccepted as a displacement and strain measurementtoolintheindustrialenvironment.2.ExperimentalSetupTheElectronic Speckle Pattern Interferometry (ESPI) isusedforcollectingdataforsurfacestrainanalysisandisbased on a holographic interferometric (HI) techniqueinvolving photographic recording of a light beam andvideorecording.Thevideorecordingfromthesourceisfiltered and displayed on the TV monitor. The mainadvantageofESPIisitsrealtimecapability,andthefactthattherecordingscanbeeasilystoredandprocessedfor later use. Further it uses an automatic fringeanalysistechnique.The setup consists of a laser diode (LD), microscopeobjective (MO), directional coupler (DC), charge-coupled diode camera (CCD), digital to analog


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convertor (D/A), piezoelectric phase shifter (PZT) asshowninthefigurebelow.

Fig.1ExperimentalsetupofESPI

Anillustrativespecklepatternsubtractionwhereintwopatternsarerecordedatthesurfaceofthespecimenbeforeandaftertheapplicationofloadhasbeendemonstratedinthefigurebelow.

Fig2.Before(A)andafter(B)afluidpressuregradient

wasappliedtothesurface

Fig3.(A)issubtractedfrom(B)toobtainthespecklepatterntoobtainthedisplacementofthefringes

2.1Components2.1.1LaserDiode(LD)A laser diode (LD) is an electrical diode in which thelaser beam medium is formed by a p-n junction of asemiconductordiodewhichissimilartothatfoundina

lightemittingdiode.Themodulationoflightintensityisdone to obtain different sensitivity and phase shiftingcanbeachievedbytuningthelaserbeamwavelength.Ideally,Helium-Neon(HeNe)lasersareusedforlookingat relatively small objects, while Argon (Ar) lasers areused for largeobjects and for two-wavelength surfacegeometrymeasurements.Inrarecases,Co2canalsobeused.2.1.2MicroscopeObjective(MO)The laser beamwhich is coherent in nature is passedthroughthemicroscopicobjectivetogetasinglemodeoptical light to avoid feedback. The beam is furtherdividedintwobeamsofequalintensitybyadirectionalcoupler.2.1.3PhaseShifterA piezoelectric phase shifter is generally glued to amirror and it also has a voltage input from thecomputerinterface.Thevoltagethendistortsthephaseshifter and themirrorsmoveswhichproduces a smallvariation of the optical phase. For example, adisplacementofλ/8producesaphaseshitofπ/2.2.1.4Charge-CoupledDevice(CCD)CameraThe reference beam and the reflected beam beforeincidentontheCCDcameraaremadetopassthroughamirror with a pin hole. The pin hole is necessary toensure the two beams are in line and to cleanse thereferencewave.TheCCDcameraisusedtoresolvethebeamsbeforeprojectinginontotheTVcamera.2.1.5Computer–TVInterfaceThevideosignalfromthecameraisdigitizedbythePC.The computer is providedwith a video digitizer boardgiving it a wide spectrum of image processingcapabilities. It is also used to read out fringecoordinates. The video signal from the computer ispassed througha filter rectifierbeforedisplayingontotheTVmonitor.Anillustrativefigurecomprisingofthecomputerinterfacehasbeenshownbelow.

Fig4.ESPIcomputerinterface


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3.WorkingPrincipleofESPIThe coherent laserbeam from the laser source is firstsplitintoareferencebeamandanobjectbeamusingabeam splitter. The object beam illuminates the objectandisthenscattered,producinganobjectimageontherecording device. The object image and the referencebeamare superimposedon thechargecoupleddevice(CCD) camera, and the interference patterns obtainedon thedevice are called specklesor an interferogram.The intensity, phase and amplitude of the scatteredbeamaredependentonthestructureoftheareaoftheobjectfromwhich itreflects.Thespecklepattern isaninherentattributeoftheobject.The reference beam and the object beam arerecombined using either a pinholemirror aperture oran optical component like a glass wedge. However,using a mirror gives the best results as the beam issubjectedtolessintensityfluctuationswhencomparedtopassingthebeamthroughanopticalcomponentlikealens.When the object under consideration is subject toloading, the surface of the object undergoesdeformation. Since the interference pattern is aninherentpropertyoftheobjectsurface, it toochangesaccording to the deformation, resulting in a newspeckle pattern. Comparing this to the originalundeformed interferencepattern,we canqualitativelyobtain the displacement the surface undergoes in theform of contour lines or by calculating the order offringes. However, the presence of speckles gives us alowcontrast,noisyimage.Amorequantitativeanalysisof the patterns can be carried out by a phase shiftingprocedure to remove the non-uniqueness in thepattern. A deformation in the object will change thedistance between the object and the image, andtherefore the interference pattern of the deformedsurfacewillundergoaphasechange.Theinterferogramfor the deformed object is subtracted pixel by pixelfromtheoriginal interferogram.The resultobtained issentthrougharectifiertogiveacontourmapmadeofbright and dark fringes called correlation fringes thatgiveusthedisplacementoftheobject.When there is a phase difference between thereferenceandreflectedbeamweobtaingreyorwhitefringes and dark fringes when there isn’t a phasedifference.

TheESPIhastwobasicconfigurations:

1. In-planemeasurementsystem2. Outofplanemeasurementsystem

3.1DifferenttypesofESPIConfigurationDepending on the direction of deformation of thespecimen we have two different types of ESPIexperimentalsetupconfiguration.3.1.1Out-of-planeconfigurationIn this setup the laser beam is initially split using a

beamsplitterintotwobeams,namelyobjectbeamandreferencebeamasseeninthefig(5).Theobjectbeamis used to illuminate the surface of the object andscatteredbacktothecamera.Thereferencebeamandthe reflected beam are combined together using abeamsplitteranddirectedbacktowardsthecamera.Asthe object displaces in the direction parallel to thedirection of viewing, the distance travelled by theobject beam changes, due to which there is a phasechange.Thefinal imagerecordedbythecamera isthespeckle pattern or interference pattern formed bythese two beams. If φ is considered as the phasedifference between reference beam and object beambefore any displacement and (φ+ Δ) is the phasechangeoftheobjectbeamafterdisplacementwhere,Δis the change due to deformation. The two specklepatternsaresubtractedtogetthefringedpatternthatgivestheinformationaboutthedisplacement.

Fig5.Schematicrepresentationofthesetupandspecklepatternofoutofplaneconfiguration


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3.1.2In-planeconfigurationFor the in-plane measurement, two symmetric laserbeamsfromthesamesourcearedirectedtowardstheobject fromopposite directions. This is producedwiththe help of beam splitter and mirror setup. Theinterference pattern is formed by subtracting the twobeamsreflectedbacktothecamera.Theinterferenceisobtainedonlyifthereisadeformationordisplacementin the direction perpendicular to the direction ofviewing. There is phase change as one beam phaseincreasesand theotherbeamphasedecreasesdue todisplacementandisrecordedasΔ.

Fig6.Schematicrepresentationofthesetupand

specklepatternofinplaneconfiguration3.2TheoreticalCalculationConsider the in-plane ESPI system. The two coherentlaser beams of wavelength λ, and equal and oppositeangles θ fall on the object and get scattered. Theintensityofthescatteredlightalongthenormaltothesurfaceisgivenby

𝐼 = 𝐼# + 𝐼% + 2 𝐼#𝐼%𝑐𝑜𝑠φ (1)

where𝐼#and𝐼%are the intensities of the two beamsandφisthephasedifferencebetweenthem.Ondeformation,ifapointonthesurfaceoftheobjectmoves by a displacement d, the phase differencesbetweenthelightbeamchangesbyΔφ,

∆𝜑 =4𝜋𝜆𝑑𝑠𝑖𝑛𝜃 (2)

Therefore,usingEq.(1)wecanwritethenewintensityas,

𝐼5 = 𝐼# + 𝐼% + 2 𝐼#𝐼%cos(φ + ∆𝜑) (3)Thedifferenceintheintensitiesgives

𝐼 − 𝐼5 = 4 𝐼#𝐼% sin φ +∆𝜑2

sin∆𝜑2

, (4)

which denotes a fringe patternwith intensitymaximaat∆𝜑 = (2𝑝 + 1)𝜋, andminima at∆𝜑 = 2𝑝𝜋, wherepisaninteger. The bright and dark fringes are obtainedaccording to Eq. (2) and Eq. (4), if d is spatiallydependent. Then the in plane displacement of theobjectisobtainedas

𝑑 =𝑛𝜆

2 sin 𝜃

(5)

wherenisthenumberoffringesatdisplacementd.

Similarly, in case of out-of-plane configuration bycarrying out a similar analysis we find the spatialdependent,

where, λ=wavelengthoflaserlight d=outofplanedisplacementoftheobjectdue totheappliedstress

α=anglebetweenthedirectionofobjectnormalandcameraviewingangleβ=anglebetweenthedirectionofobjectnormalandtheobjectbeam

4.ApplicationsElectronic speckle pattern interferometry (ESPI) haswiderangeofapplicationindiversefieldsbecauseofitsability to measure deformation or displacement withvariable sensitivity for in-plane and out-of-planedirections, 3D object shape, surface roughness andvibrationsetc.BelowaretheareaswhereESPIiswidelyusedinindustries.

𝑑 =𝑛𝜆

𝑐𝑜𝑠𝛼 + 𝑐𝑜𝑠𝛽

(6)


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4.1Deformationmeasurements of solder joints onelectronicdeviceboardsElectronic speckle pattern interferometry is preferredfor the investigationof solder jointdeformation, sinceitisanon-contactmeasurementmethodwithaccuracyin the range of the order of the wavelength of thesource light, and has a full field image of thedeformation. It uses digital information, which suitsindustrialapplication.4.2 3D shape measurement with light-in-flightelectronicspecklepatterninterferometryThere are various techniques that can be used forthree-dimensional shape measurements of smallcomponents. The common problems with traditionalmethodsarethattheyarenotefficientenough,andaretime consuming. ESPI is much more effective andprovidesmeasurementsmuchfaster.4.3ContinuousdeformationmeasurementMeasurement of continuous deformation is difficult,since collection of useful information is hard as thestateoftheobjectchangesdynamically.However,withthehelpofhighspeedcameras,multi-camerasystems,and piezoelectric translators, ESPI is a very favorablemethodfordynamicsystemmeasurements.4.4CuttingtoolmonitoringTheefficiencyofthecuttingprocessdirectlydefinesthecost and productivity of machining operations. This isbecausethecuttingtimeincreasesasmaterialstrength,complexity of work pieces’ increases and morestringent machining tolerances are desired. The idealconditionsvary significantly for the tool-machine-workpiece combination. Because of these reasons,traditionalmethods like stylus probes cannot be usedto check the cutting tool condition. In such cases ESPIperformsmuchbetterandisafavorablemeasurementtool.4.5HoledrillingmethodThe ‘locked in’ stresses which are also known asresidualstressesofamaterialwhichareformedduringcommon manufacturing processes can also becalculatedbyusingaESPIopticalphenomenonastheyavoid the lengthy procedure of attaching strain gagesasshownFig9.

Fig7.Specklepatternobtainedforthermaldistortions

inPrintedcircuitboard(PCB)

Fig8.Specklepatternforflawrecognitionforasurface

withcracks

Fig9.Specklepatternforaprismformedbyexamining

adrilledhole


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5.AdvantagesandDisadvantagesTheESPIsystemhasseveralbenefitswhencomparedtoother traditional methods for determining surfacedeformation,• It is a non-contact measurement method with

wavelengthorderaccuracy.• Thismethodprovidesafullfieldmeasurement.• It iswell suited forcomputeraidedmeasurements

asinformationisacquiredandevaluateddigitally.• The sensitivity is much higher than that of

holographic plates and thus allows one to useshorter exposure times than those in classicalholography.

• Almost a real-time operation. The correlationfringes canbedisplayedonamonitorwithout therecourse to any form of photographic processing,orplaterelocation.

• Theresolutionoftherecordingmediumused,neednot to be high compared with that required fortraditionalholography.

• With phase modulation, the sensitivity can beincreasedby20times.

LimitationsofanESPIsystem,• ThemeasurementrangeofESPIissmallandlimited

bythespecklecorrelation.• Forlargeobjects,highpowerlasersarerequiredto

increasetheaveragespecklepatternsize.• Equipmentandinstallationcostofthesetupishigh.

6.AdvancementsESPIreliesontechnologieswhichhaveahugescopeofimprovement and thoseofwhich are drivenby other,much larger, forces. Components such as computers,CCDcameras,laserdiodesandimageprocessingboardsare all high volume devices which are developed bymajor industrieswhichhavehighR&Dbudgets. In thisway, ESPI has the potential to advance much morequickly than others. With continued advancements inimage processing techniques, it is probable that ESPIwill take a big leap towards producing holographicquality data which could eliminate the need forholographicinterferometry.The future scope of ESPI appears to be even brighterthanthealreadydocumentedsuccess.Thelaserdiodes,phaseshifters,PC’s,CCDcamerathatwillbedevelopedintheforthcomingyearsseemstobeatleastanorder

of magnitude more powerful than those availabletoday. Future advancements in optical and imageprocessing techniques will help to develop a betterfringe contrast, reduce the noise and increase theresolution of the speckle pattern. Finally, a completedisplacementmapofthespecimenunderdeformationcan be obtained by implements phase shift methods.The phase shift can be achieved by modulating thewavelengthofthelaserdiode.7.ConclusionThis paper explains how surface deformations can beevaluated using electronic speckle patterninterferometric (ESPI) optical method. Two differentconfiguration of ESPI have been explained in detailfollowedbythetheoreticalcalculationfordeterminingspatialdependent(d).The speckle pattern is obtained by the contouringmethod which is based on the holographic two-beamilluminationtechnique.Therecording,digitizing,storingand processing of video signals, as well as the datacollection and all calculation are done by using apersonal computer (PC) which follows an algorithmderivedfromthevideodigitizedboard.We can also conclude that ESPI has a short exposuretime(<1/25sec),highrepeatabilityrate(1/25sec)andless sensitive to noise when compared to otherholographictechniques.ESPIhaswiderangeofapplicationsbecauseofwhichitisthemeasurementtoolusedinmostindustries.

Further, this paper also summarizes the advantages,disadvantagesandapplicationsofESPIandoutlinestheadvancements and some potential futureimprovementstoESPI.


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8.References1. Jones R & Wykes C, Holographic and Speckle

Interferometry, 1989, Cambridge UniversityPress

2. Winther Svein, 3D StrainMeasurements usingESPI,1987,NorwegianInstituteofTechnology

3. Moore J. Andrew & Tyrer R. John, 2D Strainmeasurement with ESPI, 1995, LoughboroughUniversityofTechnology

4. Moore,A.J.&Tyrer,J.R.,Anelectronicspecklepattern interferometer for complete in-planedisplacement measurement. Meas. Sci.Technol.,1(1990),1024-1030.

5. Sharp Brad, Electronic Speckle PatternInterferometry, 1989, Newport Corporation –CA,USA

6. Angel F Doval, “A systematic approach to TVholography,”Meas.Sci.Technology11,R1-R36,(2000)

7. Zoltan Füzessy, Jüptner Werner, Osten

Wolfgang, Simulation and Experiment in lasermetrology,pp.120-155

8. Amalia Martínez, J.A. Rayas, R. Cordero, KatiaGeenovese,AnalysisofopticalconfigurationforESPI,Vol.46[1],2008

9. Xin XIE, Lianquin ZHU, Sijin WU, YonghongWANG, Review of ESPI for 3D displacementmeasurement, Chinese journal of ME, Vol. 27[1],2014

10. Erf K. Robert, Speckle Metrology, AcademicPress,1978

11. Moore R. Thomas, A simple design for anelectronic speckle pattern interferometer,RollinsCollege,2004

12. Raghavendra Jallapuram, Con Healy, EmiliaMihaylova,andVincentToal, In-Planesensitiveelectronicspecklepatterninterferometerusinga diffractive holographic optical element,DublinInstituteofTechnology,2010

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