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Dual in-plane electronic speckle patterninterferometry system with electro-opticalswitching and phase shifting

Brian Bowe, Suzanne Martin, Vincent Toal, Andreas Langhoff, and Maurice Whelan

A dual in-plane electronic speckle pattern interferometry ~ESPI! system has been developed for in situmeasurements. The optical setup is described here. The system uses an electro-optical switch tochange between the illumination directions for x and y sensitivity. The ability of the electro-optic deviceto change the polarization of the laser light forms the basis of this switch. The electro-optic device is aliquid-crystal layer cemented between two optically flat glass plates. An electric field can be set upacross the layer by application of a voltage to electrodes. The speckle interferometry system incorpo-rates two additional liquid-crystal devices to facilitate phase shifting, and the overall system is controlledby advanced software, which allows switching between the two perpendicular planes in quasi real time.The fact that there are no moving parts is an advantage in any ESPI system for which mechanicalstability is vital. © 1999 Optical Society of America

OCIS codes: 120.5050, 120.6160, 230.3720.

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1. Introduction
Over the past few years there has been a significantincrease in both the scope and the volume of appli-cations of optical interferometric methods to surfacemetrology.1 Among the methods used are holo-graphic interferometry,2 white-light techniques,3 andspeckle techniques.4 These methods are applied tomany fields, such as optical testing, shape measure-ment, displacement and strain inspection, vibrationevaluation, and flow analysis. The advantages ofoptical interferometric techniques are that they arenondestructive, have high resolution, and can allowfull-field inspection. In both structural and mechan-ical engineering fields the measurement of object de-formations under loading conditions can provideuseful information about the mechanical propertiesof the material.5 This information is usually ob-ained in mechanical tests by means of strain gaugespplied directly to the object or by displacement

B. Bowe, S. Martin, and V. Toal are with the Centre for Indus-trial and Engineering Optics, School of Physics, Dublin Institute ofTechnology, Kevin Street, Dublin 8, Ireland. A. Langhoff and M.Whelan are with the Institute for Systems, Informatics and Safety,European Commission Joint Research Centre, Ispra, Italy.

Received 10 August 1998; revised manuscript 19 October 1998.0003-6935y99y040666-08$15.00y0© 1999 Optical Society of America

666 APPLIED OPTICS y Vol. 38, No. 4 y 1 February 1999

transducers connected at points of interest. How-ever, both these methods provide values averagedover the evaluated area only, and it is not usuallypossible to assume with certainty that their presencedoes not affect the measurements. For this reasonoptical nondestructive testing techniques are increas-ingly used for material characterization and struc-tural evaluation.7 One of the more common opticalmethods is electronic speckle pattern interferometry~ESPI!.8 In ESPI a speckle pattern is formed byilluminating the surface of the object to be tested withlaser light. This speckle pattern is imaged onto aCCD array and allowed to interfere with a referencewave, which may or may not be speckled. The re-sultant speckle pattern is then transferred to a framegrabber on board a computer, saved in memory, anddisplayed on a monitor. When the object has beendeformed or displaced, the resultant speckle patternchanges owing to the change in path difference be-tween the wave front from the surface and the refer-ence wave. This second resultant speckle pattern istransferred to the computer and either subtractedfrom or added to the previous stored pattern andrectified. The resulting interferogram is then dis-played on the monitor as a pattern of dark and brightfringes called correlation fringes. In real time ~stan-dard video rates! it is possible to grab frames contin-uously while a deformation is occurring and thensubtract them in succession from the first speckle

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pattern. This process makes it possible to observethe real-time formation and the progressive changesof the fringe pattern related to the deformation of thesurface.

Since the first study on ESPI was published, therehave been many different configurations and a vari-ety of different applications.9 Depending on the op-tical configuration of the ESPI system, it can be madesensitive to out-of-plane10 displacements, i.e., paral-el to the observation direction; to in-plane displace-

ents,11 i.e., perpendicular to the line of sight; or toboth. In this paper we describe an in-plane ESPIsystem.

2. In-Plane Electronic Speckle Pattern Interferometry

Figure 1 shows the basic optical setup of an in-planeESPI system. The surface of the object is illumi-nated by two beams that generate their own specklepatterns. This technique measures only the dis-placement along a particular vector direction that liesnormal to the viewing direction and is in the plane ofthe two illumination beams. In Fig. 1, only the dis-placement along the x axis can be measured. Tomeasure the displacement along the y axis, the illu-

ination beams would have to be in the y–z plane.To make a system sensitive to displacement in boththe x and the y axes, four illumination beams are

eeded. This makes complete in-plane displace-ent measurement possible by sequentially record-

ng the two separate interferograms and combininghem vectorially. The two pairs of beams should notlluminate the surface at the same time. This re-triction has lead to many different optical configu-ations. There are numerous methods available forwitching a beam between two paths, but most in-olve moving mirrors, prisms, or shutters. Anyoving part is disadvantageous in an ESPI system as

t may cause mechanical instability.The effects of vibration and instability become very

mportant as ESPI systems become more sensitiveecause of advances in fringe analysis. Using shut-ers has the additional disadvantage of being ineffi-ient with regard to laser power as typically the

Fig. 1. Basic setup of an in-plane ESPI system.

urface will only be illuminated with half the avail-ble power. Under static loading conditions, theectorial addition of the interferogram information isdequate for complete in-plane measurement.learly, this is not true for time-dependent loads be-

ause the surface changes between the sequentialecordings. There are systems that use two camerasnd different polarization states in the two planes12

to allow the measurement of the two interferogramsat the same time. The two speckle patterns pro-duced by the y–z and x–z plane illumination do notinterfere because they are linearly polarized with or-thogonal polarization states.

However, the efficiency of this system depends onthe surface not changing the polarization of the inci-dent beams, and this is not always the case. Also,unless two lasers are used, each plane uses only 50%of the available laser power. For switching, electro-optic devices are ideal because they lack moving partsand liquid crystals, and in particular, they do notrequire high driving voltages. In this paper we de-scribe a dual in-plane ESPI system that uses only onelaser and can switch between the x and the y planewith a liquid-crystal switch. The system obtainsand analyzes the fringe patterns in near real timebecause it is controlled by advanced ESPI software.This software allows fast and efficient switching aswell as fringe analysis. The software, ESPITEST Ver-sion 1.1, was designed and developed at and is li-censed by the Institute for Systems Informatics andSafety, European Commission Joint Research Cen-tre, Ispra, Italy.

As with all ESPI techniques, the interference thattakes place between the two speckle patterns is im-aged by a CCD camera and displayed. When theobject is being deformed, all subsequent interferencepatterns are subtracted from the initial pattern, pro-ducing a fringe pattern. The theory of fringe forma-tion is well documented.4 The fringes representcontours of equal displacement. The fringe spacingis inversely proportional to the gradient of the dis-placement, and the fringes are aligned perpendicu-larly to the direction of the displacements. The mostcommon technique for interferogram interpretationis the phase-shifting method.13 Phase shifting isused to obtain a fringe pattern that depicts the 2pand p range phase change that occurred between thetwo speckle patterns. A procedure called phase un-wrapping must be carried out to restore the unknownmultiple of 2p to each pixel. The displacement d ofthe fringes in an in-plane system is related to phasechange Df by the following expression

Df 5 ~4pd sin u!yl, (1)

where u is the angle between the illumination beamand the viewing direction and l is the wavelength ofthe laser. Use of this expression allows the phasemap to be converted into accurate displacement data.The ESPITEST software performs all this analysis.

Phase shifting is based on the introduction of aknown amount of lateral shift, called the phase step,

1 February 1999 y Vol. 38, No. 4 y APPLIED OPTICS 667

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into the interferometric pattern; the resulting effectis a movement of intensity peaks across the pattern.The phase step is introduced either as calibratedphase steps or as continuous periodic phase modula-tion. The most common phase-shifting techniquesare ones in which the phase in one arm of the inter-ferometer is stepped in N equal and controlled stepsso that the total phase shift is 2p. By combining theesulting N 1 1 frames, one can calculate the phase.

The most common way of implementing the phasechange is to displace a mirror with a piezoelectrictransducer. However, this can cause difficultieswhen large collimating mirrors are used. Attachingthese large mirrors to the piezoelectric transducersand maintaining mechanical stability can prove dif-ficult. Typical monolithic piezoelectric transducersalso require voltages of the order of 100’s of volts. Inthe system described here, two liquid-crystal deviceswere used to implement phase steps. These devicesuse relatively low voltages ~;10 V! and are easilyintegrated in the optical setup.

3. Liquid Crystals

Because of their anisotropic nature, liquid crystalsare birefringent.14–16 When light enters the liquid-crystal material its two perpendicular componentstravel at different velocities, resulting in a phasechange occurring between them. When they emergefrom the material and recombine, the resulting po-larization state has changed owing to this phasechange. If the incident light is linearly polarized,any polarization state can be produced with the rightcombination of birefringence and thickness. An ex-ternal electric field can cause significant changes inthe properties of a liquid-crystal material. The mol-ecules have the ability to align along an external fieldif a voltage is applied across the liquid crystal. Bychanging their alignment, the amount of retardationcan be controlled. Because of this alignmentchange, if linearly polarized light is incident on aliquid-crystal layer, the polarization state of theemerging light can be controlled by application of avoltage across the layer.

For the switch, a nematic liquid-crystal layer and apolymer coating are sandwiched between two glassplates. The polymer coating has the purpose ofaligning the molecules near the surface in a certaindirection. Two electrodes are attached to the glassplates by use of conductive paint, and the plates areplaced in a Perspex holder. The material used forthe phase-shifting devices are cholesteric liquid crys-tals. These two phase-shifting devices are con-structed in the same way as the switch. As theliquid-crystal molecules rotate under an applied elec-tric field, the refractive index also changes for a beamtravelling along the fast or the slow axes. Thereforethe phase shift induced in the beam can be controlledby the applied voltage. The liquid-crystal devicescan be optimized to have response times that arecompatible with the framing rate of the CCD camera~40 ms! and the frame grabber, thus making themdeal for ESPI.


The ESPITEST software controls the three liquid-crystal devices and has been designed to calibrate thephase shifters before each test automatically. Thisis to allow for small changes in the liquid-crystalsresponse that may occur over time. However, theneed for this has not yet been verified by quantitativeanalysis. The liquid-crystal devices used here weresupplied by the State Optical Institute, St. Peters-burg, Russia.

4. Optical Design

The optical setup of the ESPI system is essentiallytwo separate interferometers, one which gives sensi-tivity to in-plane displacement in the horizontal di-rection and one which gives sensitivity to in-planedisplacement in the vertical direction. The ESPITEST

software switches between the two in synchronismwith the frames grabbed, thus giving the requireddual in-plane sensitivity. Each interferometer con-sists of two beams emerging from a beam splitter,which are expanded, collimated, and directed towardthe object so that they overlap in the object plane andhave an angle of 60° between them. The illuminatedobject is imaged by a CCD camera, which is connectedto a frame grabber on board a computer. The beamsoriginate from a common point behind the CCD cam-era, and the two beams in each illumination geome-try have a common axis of symmetry. A laser diode,with a wavelength of 852 nm and a maximum outputpower of 150 mW is used as the light source. As wellas having the advantage of its wavelength being nearthe peak of a CCD array’s sensitivity, the diode laseris compact and portable. The beam produced by thelaser is elliptical and diverging, but customized col-limating and correcting optics produce a 4-mm-diameter collimated beam. The reflectivity ofdifferent objects varies greatly, so independent con-trol of the intensity of light striking the object isnecessary if the imaging system is to work efficiently.It is possible to control the laser power by varying thedrive current to the laser diode. There is also an852-nm filter on front of the imaging lens system sothat background light does not reach the CCD array.

The beam-splitting unit consists of a polarizingbeam splitter with a quarter-wave plate and a mirrorplaced in the transmitted beam path. This producestwo beams, angularly separated by 180°, with thesame plane of polarization. The ratio of intensities

Fig. 2. Liquid-crystal optical switch.

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of the two beams depends on the plane of polarizationof the incident light beam. For example, in Fig. 2,light polarized parallel to the z axis ~horizontal withrespect to PBS1! is transmitted, and light polarizedparallel to the x axis is reflected. The addition ofboth the quarter-wave plate ~QWP 1! with its axis at45° to the vertical and the mirror in the transmittedbeam rotate the plane of polarization of the transmit-ted light through 90° ~back to being parallel with thex axis! so that it is now reflected by the polarizingbeam splitter. Because the liquid-crystal device canbe used to rotate the plane of polarization of theincident light, it can be used in conjunction with oneof the beam-splitting units described above to form anoptical switch.

This liquid-crystal device, driven by an audio fre-quency square wave ~zero dc offset!, causes the planeof polarization to be rotated through 90°, on softwarecommand. Depending on the square-wave ampli-tude, the beam is either plane polarized parallel tothe x axis or parallel to the z axis and is thus directedinto one of the two possible beam paths ~for x and ysensitivity!. In each path, after the beam emerges

Fig. 3. Beam-directing optics.

Fig. 4. Beam expansi

rom the optical switch, it is split by an identicaleam-splitting unit. However, in this case a perma-ent 50y50 split of the beam power is required sohere is no need for the liquid-crystal polarizationotator. A 50y50 split is ensured if the plane of po-arization of the light incident on the beam splitter

akes a 45° angle with the vertical axis of the beamplitter. One could achieve this 50y50 split by usinghalf-wave plate to rotate the plane of polarization of

ach beam to the required angle, but physically ro-ating the optical switch ~all the optics shown in Fig.! through 45° in the x–y plane as shown in Fig. 3 is

equally convenient. Since the beam cross sectionsare circular, this has no effect other than the desiredrotation of the planes of polarization of the emergingbeams with respect to the beam splitters that follow.This option has the added advantage that the lasercan be mounted between two of the arms of the in-terferometer rather than in line with one of them,allowing for a more compact design. One of thebeam-splitting units is oriented so that the beams

d object illumination.

Fig. 5. Three-dimensional diagram of the beam-directing optics.

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produced are plane polarized parallel with the y axisand the other so that the beams produced are planepolarized parallel with the x axis.

Plano-concave lenses are used to expand eachbeam, which are then collimated by 75-mm-diameter,60° off-axis, diamond-turned, aspherical mirrors.These mirrors also direct the beams toward the ob-ject. This is shown in Fig. 4 for the vertical axisillumination system. The collimation allows a sur-face area of 5 cm 3 5 cm to be illuminated. If abigger area of illumination is needed, the aspherical

Fig. 6. Complete optical configuration of the ESPI system.

Fig. 7. Fringe patterns taken while the voltage across the liquid2.0 V, ~e! 3.4 V, ~f ! 4.2 V.


mirrors can be replaced with plane mirrors. In thisway the object surface is illuminated by divergingbeams. The components are mounted with a rigidstructural support.

The role of each component can be explained withthe schematic diagram shown in Fig. 5. The dia-gram is merely intended to show clearly how thesystem works; thus the laser axis is shown lying inthe y–z plane and not at 45° to it, and the half-waveplates on either side of pbs1 are not actually used.All optical faces are antireflection coated at 852 nm.The expanded, collimated beam is initially plane po-larized parallel to the x axis and has a diameter ofapproximately 4 mm. At point A, the beam entersthe liquid-crystal polarization rotator, which causesthe plane of polarization to be rotated through 90° onsoftware command.

The beam reflected by pbs1 is split into two orthog-onally plane-polarized beams by another polarizingbeam splitter ~pbs2!. A combination of quarter-wave plate and plane mirror returns the beam trans-mitted by pbs2 to pbs2, but it is now plane polarizedparallel to the x axis so that it is reflected at pbs2, isin alignment with the beam previously reflected bypbs2, and is in the same vertically plane-polarizedstate ~parallel to x axis!. These two beams are ex-panded by short-focal-length concave lenses and col-limated, as shown in Fig. 4, by the off-axisparaboloidal mirrors; they illuminate the surface forsensitivity in the y direction. One of them, however,first passes through the liquid-crystal phase modula-

al phase shifter was increased: ~a! 0.0 V, ~b! 1.2 V, ~c! 1.8 V, ~d!
-cryst

tor. This device provides as much as a 2p phaseshift, again under software control. For sensitivityin the x direction, an identical optical configuration isused.

Figure 6 shows that the system can provide sensi-tivity in x and y directions alternately as the switch-ing voltage applied to the liquid-crystal polarization

Fig. 8. Fringe pattern representing the displacement of a surfacealong the x axis.

Fig. 9. Wrapped phase fringes.

Fig. 10. Phase images: ~

rotator is changed. All of the laser power is directedalternately into each illumination geometry.

5. Results

Figure 7 shows six fringe patterns obtained as thevoltage across the liquid-crystal phase-shifting devicewas increased. The surface of an object was illumi-nated in the x–z plane, and after a controlled in-planerotation of the object, the first fringe pattern wassaved. The voltage across the liquid crystal thatwas in one arm of the interferometer was then in-creased in controlled steps. The phase change expe-rienced by the beam in that arm can be seen in the sixfringe patterns shown in Fig. 7. There is a nonlinearrelation between the voltage and the phase change.It is not necessary to calibrate the liquid-crystalphase shifters before each test. However, as the ef-fects of time on these devices is not certain, it isrecommended to calibrate as often as possible.

The interferogram, in Fig. 8, shows displacementfringes in the x direction. The object under investi-gation was a rotating disc. The software gives theuser the option to observe either the formation andprogression of the displacement fringes or thewrapped phase map. Updating of the wrappedphase maps is slower on account of the time needed tocalculate the phase. However, an update rate of 2Hz is typically achieved with an image size of 768 3512 with a 266 MHz computer with a peripheral con-nect interface frame grabber. Using the same rotat-ing disk with a different displacement, we obtainedthe wrapped phase map shown in Fig. 9. Figure 10shows the wrapped and the unwrapped phase imagesobtained from a three-point bend test of a section of ahigh-density polyethylene prismatic bar with holes.The user also has the option of performing ESPI in asingle plane or in both planes. In both planes the

apped and ~b! unwrapped.
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software continuously changes the voltage across theliquid-crystal switch. As the object is illuminated ineach plane, a frame is grabbed and subtracted fromthe reference frame recorded originally in that plane.The user can observe the two displacement fringepatterns form and change on the computer screen.Therefore we can measure the surface displacementsof both the x and the y components with a delay of arame between each frame grabbed in each plane.or example, if a frame was grabbed every 40 ms,hen the y data in Fig. 11 would have been obtained0 ms after the x data in the same figure. The nexthase of testing will examine switching responseimes and dual-axis testing of dynamic loads.

6. Conclusions

The system described here is a reliable, robust, andportable dual in-plane ESPI system. The stability re-quirements for this system are the same as they are forall typical ESPI systems that measure displacement ofa fraction of the wavelength. However, the sourcesof instability have been reduced in this system. Ithas no moving parts because it uses liquid-crystaldevices for both switching between illuminationplanes and phase shifting. Initial research showsthat the devices have proved to be both efficient andeffective, but extensive quantitative tests of theswitching speeds, reliability, and repeatability overtime will follow. The strong structural supportmeans the system is mechanically stable. The ESPIsystem is controlled by advanced software that per-forms all the analysis and calibration of the phase-shifting devices. The system, controlled by thissoftware, has proved to be very reliable in the pre-

Fig. 11. Displacement fringes show


liminary tests carried out to date. When measuringtime-dependent loads, the dual in-plane ESPI sys-tems, which measure both the x and the y planecomponents simultaneously, have a distinct advan-tage. However, these systems require either twocameras or two lasers. Also, unless parallel process-ing is used, the wrapped phase maps have to be cal-culated sequentially. There is a loss of data withthis system in each plane when data is being recordedin the other plane. However, with the fast switchingspeed, this loss is minimum. Also, in any ESPI sys-tems that uses phase-shifting algorithms, the speedof the data acquisition is limited by the time neededto calculate the wrapped phase maps. As stated pre-viously, we are presently testing the system’s perfor-mance under different laboratory conditions. Theliquid-crystal devices are also under continual devel-opment in the hope of further improving their perfor-mance.

This research was carried out under contractCSA96y112 for the European Commission Joint Re-search Centre, funded by CEC-DGXIII InnovationProgramme. We also acknowledge the support ofthe Dublin Institute of Technology’s Strategic Re-search and Development programme.

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~Chapman & Hall, London, 1993!.2. C. M. Vest, Holographic Interferometry ~Wiley, New York,

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7. E. Cadoni, B. Bowe, and D. Albrecht, “Application of ESPI tech-nique to evaluate the crack propagation zone of pre-notched clayelements,” presented at the International Conference on AppliedOptical Metrology, Balatonfured, Hungary, 8–11 June 1998.

8. J. N. Butters, R. C. Jones, and C. Wykes, “Electronic SpecklePattern Interferometry,” in Speckle Metrology, R. K. Erf, ed.~Academic, New York, 1978!, pp. 111–157.

9. R. S. Sirohi, Speckle Metrology ~Marcel Dekker, New York,1993!.

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~1992!.11. R. S. Sirohi, J. Burke, H. Helmers, and K. D. Hinsch, “Spatial

phase shifting for pure in-plane displacement and displacement-derivative measurements in electronic speckle pattern inter-ferometry ~ESPI!,” Appl. Opt. 36, 5787–5792 ~1997!.

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16. S. Chandrasekhar, Liquid Crystals ~Cambridge UniversityPress, Cambridge, UK, 1992!.


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