Optics and Lasers in Engineering11 (1989)241—255

Electronic Speckle Pattern Interferometry (ESPI)

Brad Sharp

Newport Corporation,FountainValley, California, USA

(Received7 March 1989; accepted16 June1989)

ABSTRACT

Electronic SpecklePattern Interferometry (ESPI) is becominga verycommon test and measurementtool among industrial users. In itspresentform it rivals any current techniquesfor speedand easeofuse. With improvementsin imagequality it hasthe potential to becomethe definitivetool for non-destructivetesting.

INTRODUCTION

When it was first discoveredthat hologramscould be used to makeinterferometric measurements,there was a great deal of excitementwithin the test andmeasurementcommunity. The hopewas that thisnew techniquewould facilitate tests which were either difficult orimpossibleto do with conventionalmethods.Unfortunately,manyweredisappointedwhen this laboratory techniquewas applied to solvingindustrial problems.One of the difficulties was in using film plates asthe recording medium. Holographicinterferometry suffered a lag forseveral years while scientists searchedfor new, more user-friendlyrecording materials. This search went in several different directions,such as holographywith BSO crystals,thermoplastic,photoresist,andfinally with a video camera.Eachmethodhad certainadvantagesanddisadvantages,and many of these materials are currently used inindustrial test systems.

Electronic SpecklePatternInterferometry (ESPI) was developedinthe early 1970s as a methodof producinginterferometricdatawithout

241OpticsandLasersin Engineering0143-8166/89/$0350© 1989ElsevierSciencePublishersLtd, England. Printed in NorthernIreland

242 Brad Sharp

usingtraditionalholographicrecordingtechniques.’-”The premisewasto usea video camerain placeof film to recorda low spatialfrequencyhologram.The video imagecanbe storedin memoryfor later retrieval.The differences betweenESPI and holography are primarily in theoptical set-updataprocessing.

Since the early 1970s, ESPI has advancedto a point where severalcommercial systems are currently available. Due to this increasedexposure,the industrial community hasbecomeawarethat ESPI mayhavesignificant potentialas a test andmeasurementtool. At present,ESPI has capabilitieswhich are unmatchedby any technology. Asfurther refinementsare made, its scope of influence will continuetogrow.

This paperattempts to explain ESPI and its relationshipto holog-raphy, with an emphasison applications.In addition, we will discussthe important featuresof an ESPI system,including the use of diodetechnology.Finally, we will look at what the future may hold foradvancedcomputerprocessing.The speed and easeof use of ESPImakeit the kind of technologythat could find a very valuable role inindustrial testing.

ESPI VERSUS HOLOGRAPHIC INTERFEROMETRY

The data produced by an ESPI system are similar to that of aholographic system,in that each fringe representsa line of constantamplitude.The componentsof in-plane and out-of-planedisplacementand amplitude for both systemsare determinedin the samemanner.The main difference between a holographic interferogram and aspecklegram is the image quality. Because of the speckle noiseassociatedwith ESPI, detailsof the object’ssurfaceare oftenobscured.This samephenomenonputs a limit on the densityof resolvableESPIinterferometric fringes. By eye, one can resolve more than 40holographic fringes/cm, but with ESPI, the number is closer to 4fringes/cm.This affects the maximumdisplacementthat the systemcanmeasure.Sincespeckledatais createdandupdatedat video rate, ESPIis thousandsof times faster than traditional holographic techniques.This makes ESPI aperfect techniquefor on-line inspectionof produc-tion parts. The specklegramis producedin 1/30s, greatly reducingtheneedfor vibration isolation. As with any system which displays realtime fringes, the ESPI equipmentmust be stablewith respectto theobject.

Thereare methodsfor eliminating the out-of-planecomponentof an

Electronicspecklepattern interferometry(ESPI) 243

object’s vibration, but this does not affect the two componentsofin-plane motion. The one feature that ESPI and holographicfringesshareis the ability to measureboth a changein shapeandachangeinposition. This sensitivity to changesin position is what demandsareasonablelevel of object stability. Let’s exploresomeof the technicaldifferencesbetweenESPI andholography.

Off-axis versus on-axis

Severaltypes of optical configurationscan produceholographicinter-ferometric fringes. In general, the simple off-axis transmissioncon-figuration is the most common(seeFig. 1).

For this systemto work angle 0 must be chosenso that the spatialfrequency of the recording does not exceed the resolution of therecording medium. A simple example is silver halide film (resolutionapprox.1500 lines/mm)recordedwith a He—Nelaser.

(1500lines/mm)= (sin O)/(633x 10-6 mm)

0 = sin’ (1500x 633 x 10-6)= 71°

The referencebeam (0) in Fig. 1 can be off-axis by as much as 71°before the recordingexceedsthe resolutionof the film.

With ESPI, the recording is done with a video cameraratherthanfilm. Since most camerashave about 500 lines of resolution acrossa

Mirror

~~_splitter Mirror

Lens

— MirrorsinSSpatial frequency lines/mm

x,cosa 1Deformation fringe 2

Fig. 1. Off-axis transmission hologram.

244 Brad Sharp

CCD camera CCD chip ImagingIris lens

Referencebeam

Fig. 2. On-axisESPI optical set-up.

10 mm area, we can approximatethe resolution of a cameraat 50lines/mm. If we repeatthe abovecalculationsusing the resolution ofthecamera,we get:

(50 lines/mm)= (sin 0)/(633x 10~mm)

0 = sin’ (50 x 633 x 106) = 1•8°

Since it is logistically impossibleto design a traditional holographysystem with a reference angle of 1~8°,another method must beemployed. Figure 2 showsan alternativemethod to createa pseudo,on-axiscondition.

The recombinationbeamsplitter makes the camerabelievethat thepoint sourceof the referencebeamspatialfilter actuallycomesfrom thecenterof the object beam, limiting aperture. In this way, the on-axiscondition is simulated. By choosing the appropriatesetting for thediameterof the limiting aperture,the camerawill receiveonly thosespatialfrequencieswhich it can resolve(seeFig. 3).

Reduction lens

~b1ect?am ray

Reference beampoint source image

Fig. 3. Spatial frequencyversusobjectaperture.

Electronicspecklepattern interferometry(ESPI) 245

Image plane versus out-of-plane

With ESPI, it is necessaryto imagethe objectonto the recordingdeviceso that interferometricfringes are also in focus. Holographic inter-ferometry rarely images the object at the film plane. The basicdifferenceis that holographicdatacan be recordedby a video camerawhich hasno effect on the data itself, while ESPI usesthe sameopticsto createthe data as it does to record it. In order to zoom in on theESPIfringes, it is necessaryto createa new interferogram.This is dueto the fact that the cameralens is actually part of the recording.Changingthe lensposition hasthe sameeffect as distorting the objector adding an additional optical elementsubsequentto making therecording.

Correlation versuswave interference

The reasonwe are able to visualize depth with a hologram is thatinterferenceis createdbetweenlight from the object andan unmodu-lated referencebeam.The phasecomponentsfrom eachbeamcombineto give an intensity component,which can be recorded.This type ofinterferencealso occursin ESPI. A film plate may be placedinto anESPI systemto record a simple on-axis, image plane hologram.Themain differencecomesin thecreationof the interferometricfringes.

With holography,light from the distortedobject actually interfereswith the recreatedwave from the hologram,giving a new wave whichcarriesthe interferometricdata.ESPI, on the otherhand, takesall dataat discretevideo rate. When light from the distortedobjectreachestherecording plane, it interferes with the referencebeam and createsaslightly modified intensity distribution. The new distribution is thendigitally subtractedfrom the original pattern. The result is a videoframe containing the interferometricdata. The ESPI interferogramiscreatedthrough the processof correlation rather than through wavefront interference. This is why ESPI fringes are not consideredholographic.

Applications

Vibration analysisis by far the bestapplicationfor ESPIbecauseof themany techniquesavailableto reducethespecklenoise.Depictedbeloware both ESPI and holographymodal patternsof a pulley vibrating at5300 Hz. (Fig. 4)

The ESPI and holographicfringes have similar contrast.The ESPI

246 Brad Sharp

(

(.1)

_ ~i -_

~

________ ~,. ____

____ _____ ______ .~ ~ ~

(h)

Fig. 4. (a) holographic and (b) ESPI interterograrn~of a ~ihrating pulle~.

Electronicspecklepatterninterferornerrv (ESPI) 247

_ 4 _

ILi.~~ -

I ct

V...

~~ -

~ J~,~ ~

____ _____ ~

__:~ ~‘•:. ~ -

~ 4~.______ .‘. ‘~,j:

_____ _____ “V.. ~ ~ ~.

____ 1• ______ )~. _______.

________ _____ ‘~

________ ____ _______ .-‘-

(b)Fig. 5. (a) holographicand (h) ESPI interferogramsshowingthermaldistortions.

248 Brad Sharp

~:

(a)

p

- ___ __

_____ ~ ~ ~

(b)

Fig. 6. (a) holographicand(b) ESPI interferogram~at adehondedpanel.

Electronicspecklepattern interferometry(ESPI) 249

image appearsmuch more speckled. By using a technique calledspeckledecorrelation,the ESPI fringes can be processedto eliminatethis noise. This techniqueinvolves shifting the specklesspatially andcontinually averagingframes. The result is an interferogram whichapproachesholographic quality. The advantageto ESPI is that thisinterferogramwas producedin 1/30s, and could be updatedjust asquickly. Holographyat its bestis thousandsof times slower.

Mechanical and thermal distortions are both possible with ESPI.Becauseof thespecklenoise,ESPI is bestusedon objectswheredetailis not critical. Figure5 showsa holographicandESPI interferogramofaprintedcircuit board.Note that the boardhaslost mostof its detail inthe ESPIimageandresemblesa flat plate.

Although holographyhas a greatdeal more detail, the ESPI imagemay be improvedby using image processing.For example, the ESPIinterferogrammay be smoothedwith a low pass filter, anda secondwhite light image of the object recorded. The filtered (smoothed)interferogramis thensuperimposedontothe clean imageof the object,thus improving the imagedetail.

Flaw recognition is another techniquewhich is commonly appliedwith ESPI. Figure 6 depicts both ESPI and holographicimages of ahoneycombpanel with a debondedsurface.ESPI, in many cases,hasthe necessaryflaw resolution, and has the additional advantageofproviding dataat assemblyline rates.

ESPI SYSTEM CONSIDERATIONS

Delivery system

As with most interferometric devices, ESPI uses a single coherentoptical sourceto producethe desireddata.To create interference,thebeamis split into two parts,eachbeampassingthrough its path to therecordingplane. In the caseof ESPI, the objectbeamis expandedanddirectedonto the objectto be tested.

The resultantscatter is collected and imaged onto the recordingmedium. The referencebeam is applied directly onto the mediumwithout anymodulation.The two mostcommonschemesto accomplishthis task are (1) conventionaloptics (i.e. mirrors, beamsplitters,etc.),or (2) fiber optics.

Eachmethod (Figs 7 and 8) has its advantages.Systemscontainingconventional optics are generally more rugged and possibly lesssusceptibleto externalconditions. In addition,conventionaloptics tend

250 Brad Sharp

Laser

~era

Fig. 7. Conventionaloptics.

to be somewhateasier to align and maintain. Fiber optics offer theadvantageof flexibility andsize. An optical fiber canbe usedto directlight through pathsnot possiblewith conventionaloptics. Using opticalfibers, a laserdiode anda CCD camerahand-held,ESPI systemsarepossible.

Optical source and recording devices

Thereare a numberof different lasersourceswhich are appropriateforESPI, including He—Ne. While low in costand reliable,He—Ne lasersproduce a relatively large amount of heat and low output for theirphysical size. Argon lasers, although expensiveand requiring watercooling,are the bestchoicefor largeobjects.Unfortunately,air-cooledargonlaserslack the necessarycoherencelength andmodestability. Inaddition, vibrations and wind currentscreate difficulties in producinggood data.

The laserdiode is avery attractivealternative.The costper milliwattof a 30 mW diode system is roughly half that of He—Ne withapproximately 500 times the coherence length. The diode system

Laser

~~raoaJ

Fig. S. Fiber optics.

Electronicspecklepatterninterferometry(ESPI) 251

requiresan extremelystablecurrentdriver, andthermoelectriccoolingis highly advised. Shifts in ambient temperaturecan result in wildvariations in the drive current. These shifts can cause the diode tomulti-mode, resulting in a rapid decreaseof the coherencelength.Contraryto popularbelief, a 780nm diode is visible to the point that itcan be aligned with a standardwhite card. In order to view theexpandedbeam, it is necessaryto use an JR viewer. In addition tocontinuous wave (CW) lasers, pulsed lasers show a great deal ofpromise.The only requirementfor usingapulsedlaseris that the pulsebe synchronizedto the video camera.This will preventthe laserfromwriting on the camera during retrace. As pulsed lasers approachrepetition ratesof 60 Hz (e.g. YAG), it will be possible to record adouble pulsedhologram on every frame. Transientdata can then beviewed in real time.

Video cameras are the primary recording devicesused in ESPIsystems.The choiceof tubeor CCD cameradependson severalfactors.The tube provideshigh resolution and good sensitivity with very lownoise levels. A CCD has the advantageof providing high geometricalaccuracy,reasonablesensitivity, compact size and good sensitivity inthe nearJR. A CCD camerais required if a laserdiode is usedas theoptical source.

Imaging optics

Thereare two schoolsof thoughton how the object shouldbe imagedonto the recordingdevice.The first theory is to useaminimum numberof optical componentsbetweenthe object and the camerato conservelight. The secondis to obtain maximum intensity by usingthe imagingoptics of the system to obtain an image which matchesthe chip sizeexactly. Eachtheory is correct,basedupon the application.

An imaging system using a 35-mm cameralens at the front end(common with most systems),would produce a 44-mm image on an11-mmchip. This configurationresultsin an intensitylossof 16X due toover filling. If this set-upwere augmentedwith a field lensanda 25Xreductionlens, the resultantimageintensity would increaseby a factorof 16X. In order for this effort to be worthwhile, the loss factor duetoreflection should be minimized (seeFig. 9).

Accessories

Many commercialsystemsare equippedwith a mirror which can beshiftedby apiezoelectrictransducer(PZT). The advantagesof including

252 Brad Sharp

Field lens

D~oomi~ns

lrr~agingoptics

Fig. 9. ESPI imaging system.

this device are compatibility with phase shift fringe interpretationsoftware, and the ability to perform a variety of image processingtechniques. When an image is grabbed, and no deformation hasoccurred,the screenappearsblack.This is the natureof thesubtractionalgorithm. At this time, it is of valueto be ableto phaseshift theobjectbeamby 180°so the screendisplaysthe object at maximumbrightness.

In order to perform speckle decorrelation for vibration fringeenhancement,an additional piece of optics hardwareis required. Sincespeckleis a time independentnoise it is necessaryto find amethodforshifting the speckle’sposition without disturbingthe fringes. This leavesout phaseshifting, but the task can still be accomplishedby rotating awedgein theobjectpath,or tilting amirror aboutsomeaxiswhich doesnot increase/decreasepath length. Howeverit is done, the specklesareshifted enough betweenframes so the image can be averagedwithsubsequentimages.This is an integration technique,so the longer theaveragingprocessis run the cleanerthe imagebecomes.At somepoint,the increasein imagequality will be offset by inaccuraciesintroducedby externalvibrations.

COMPUTER PROCESSINGAND FUTURE ADVANCEMENTS

Speckledatafrom the camerasignal aresentto the processorin eitheranalog or digital form (dependingon the camera). The frequencybandwidthof thecamerarangesfrom DC to approximately5 MHz, withthe actualspeckledatacontainedin frequenciesabove 1~5MHz. Sincethe bulk of the data transmitted from the cameraare not useful, a1.5 MHz highpassfilter maybe usedto acceptonly thedataof interest.Filtering usually takesplace before the digitization processso that theresolution of the A—D converter is spread only over the data ofinterest. Once the data leave the filter, two basic methods can beemployedto createinterferometricfringes. Thesearereal time addition

Electronicspecklepatterninterferometry(ESPI) 253

and real time subtraction. Real time addition is implemented bysending the filtered data directly to the viewing monitor. Since noimage storagetakesplace, this method is only useful for looking attime-averagedevents which are significantly higher in frequencythanthe 30 Hz video rate. This narrows the field of applicationsdown totime-averagedinterferogramsof objects vibrating at resonance.Thistechniqueis similar to the old ‘non-electronic’speckleinterferometers,with the averagingtaking placeon thechip duringretraceinsteadof theviewer’seye. This methoddoesnot require computerprocessing,and itis immune to systemvibrations. The disadvantagein the imagequalityis usually inferior to a subtraction image, with its inherent noisecancelingqualities.

For real time subtraction,the signal is sent from the filter to thecomputerprocessorwhere it is digitized and storedas a video frame.With every video cycle, another frame enters the processorand isdigitally subtractedfrom the framecurrently residing in memory. If theresidentframe is not selectedas the reference,it is replacedwith thenew frame (seeFig. 10).

Theoretically, the video screenshowing the resultof the subtraction

Optics head

Frame Frame FrameCamera #10 #9 #8

Frame#7

Frame Filter#6

Computer_________________________ Frame#5

ALU

~ Framel Frame Frame#1 #2 #3 #4

Monitor

[Frarne~ Result

Discard

Fig. 10. Continuoussubtraction.

254 Brad Sharp

Optics head

Frame Frame FrameCamera #10 #9 #8

Frame#7

Frame Filter

Computer Frame

#5ALU

~1~m Frame Frame

Monitor

~h~suit __

Discard ________________

Fig. 11. Real time subtraction.

will be black, unlessthereare any transientvibrations. In order to dointerferometryon eventswhich happenoveraperiod longer than1/30sit is necessaryto retain a referenceframe in memorywhich becomesthe equivalentof aspecklehologram.From this point on the referenceframeis retainedin memory. Eachframe thatenterssubsequentto thiseventis subtractedfrom it, the result going to the monitor. After eachframe is subtracted from the referenceframe, it is discarded andreplacedby the new frame (seeFig. 11).

The video screendisplays the current subtractionresult which is atopographicalrepresentationof the deformation of the object. if asingle frame of the subtractedresult was stored,a numberof differentimageprocessingtechniquescould be applied to enhancethe quality ofthe fringe pattern. With phase shifting and the proper algorithms,several framescan be collected and processed.The purposeof thisexercise is to produce a 3-D graphic representationof the object’sdeformation from the 2-D fringes. Image processingcan be used tobring out flaws in a samplewhich are not visible to the naked eye.

Electronicspecklepattern interferometry(ESPI) 255

CONCLUSION

Holographyhasbeendescribedas ‘a solution searchingfor aproblem’,primarily becauseof the early failed attempts to introduce it to theindustrial community. Yearsof work have goneinto the advancementof this technology.However, the labor was always intensified by theneedfor specializedtechnologicaladvancements.ESPI is in a uniqueposition in that it relies on technologieswhich are driven by other,muchlarger, forces.Computers,CCD cameras,laserdiodesand imageprocessingboardsare all high volume deviceswhich are developedbymajor industrieswith large R + D budgets.In this way, ESPI hasthepotential to advancemuch more quickly than its predecessors.Withcontinuedadvancementsin image processingtechniques,it is probablethat ESPI will make big stridestoward producing holographicqualitydata.The improvedimage quality, coupledwith its inherentspeedandeaseof usecould eliminatethe needfor holographicinterferometry.

REFERENCES

1. Macovski,A., App!. Phys.Lett., 14 (1969) 166.2. Butters, J. N. & Leendertz, J. A., Proc. of the Technical Program

Electro-opticsConference.Kiver Communications,(1974) 43.3. Leendertz,J. A., Interferometricdisplacementmeasurementon scattering

surfacesutilizing speckleeffect. J. Phys. E., 3 (1970)214—18.4. Jones, R. & Wykes, C., Electronic speckle pattern interferometry. In

Holographic and Speckle Interferometry. Cambridge University Press,1983,chap.4.

5. Macovski, A., Ramsey, S. D. & Schaefer, L. F., Time-lapse inter-ferometry and contouring using television systems.App!. Opt., 10(12)(1971) 2722—7.

6. Burckhardt, C. B. & Enloe, L. H., Televisiontransmissionof hologramswith reducedresolutionrequirementson the cameratube. Bell Syst. Tech.J., 48(3) (1969) 1529—35.

7. Yamaguchi, I., Speckledisplacementand decorrelationin the diffractionandimagefields for small object deformation. Opt. Acta, 17 (1970) 761.

8. Stetson,K. A., Optik, 29 (1969) 386.9. Butters,J. N. & Leendertz,J. A., MeasControl, 4 (1971) 349.

10. Dainty, J. C. & Welford, W. T., Opt. Commun.,3 (1971) 289.11. Lokberg, 0. J. & Hogmoen,K., J. Phys. E., 9 (1976) 847.