Roger M. Groves

Shearography technology and applications: a review

Shearography is a full-field speckle interferometric technique used to determine surface displacement derivatives. For an interferometric technique, shearography is particularly resilient to environmental disturbances and has hence become an invaluable measurement tool outside of the optics laboratory. Furthermore, the inclusion of additional measurement channels has turned shearography from a qualitative inspection tool into a system suitable for quantitative surface strain measurement. In this review article we present a comprehensive overview of the technique, describing the principle of operation, optical configurations, image processing algorithms and applications, with a focus on more recent technological advances.

Kinematic and deformation parameter measurement by spatiotemporal analysis of an interferogram sequence

In recent years, optical interferometry has been applied to the whole-field, noncontact measurement of vibrating or continuously deforming objects. In many cases, a high resolution measurement of kinematic (displacement, velocity, and acceleration, etc.) and deformation parameters (strain, curvature, and twist, etc.) can give useful information on the dynamic response of the objects concerned. Different signal processing algorithms are applied to two types of interferogram sequences, which were captured by a high-speed camera using different interferometric setups: (1) a speckle or fringe pattern sequence with a temporal carrier and (2) a wrapped phase map sequence. These algorithms include Fourier transform, windowed Fourier transform, wavelet transform, and even a combination of two of these techniques. We will compare these algorithms using the example of a 1D temporal evaluation of interferogram sequences and extend these algorithms to 2D and 3D processing, so that accurate kinematic and deformation parameters of moving objects can be evaluated with different types of optical interferometry.

Shape and slope measurement by source displacement in shearography

Abstract Shearography is a full field non-contact optical technique usually used to measure the gradient of the displacement of a surface subjected to thermal or mechanical loading. This paper describes the use of shearography for surface slope and shape measurement. Interferometric speckle patterns obtained before and after displacement of the optical source are correlated to yield correlation fringes which are, in general, a mixture of slope fringes and carrier fringes. This paper contains a full treatment of the sensitivity of slope fringes to the parameters; the illumination and the imaging geometry and the magnitude and the direction of the source displacement. The slope fringes are corrected for distortion, which is due to the necessary off-axis illumination, and are scaled using parameters calculated using a mathematical model. Experimentally generated phase-stepped slope fringes are unwrapped and integrated to recover the object shape.

Surface strain measurement: a comparison of speckle shearing interferometry and optical fibre Bragg gratings with resistance foil strain gauges

The performance of two complementary optical strain measurement techniques, speckle shearing interferometry (shearography) and fibre Bragg grating (FBG) sensors, is compared with that of resistance foil strain gauges (RFSGs) and with theoretical predictions. The test object used for the surface strain measurements was a hydrostatically loaded ABS pipe. A multi-component shearography instrument, capable of full surface strain measurement, was used to determine the displacement gradient components, from which the surface strain components were calculated. Six surface mounted wavelength division multiplexed FBG sensors were used to measure the axial and the hoop strains. RFSGs located on the surface of the pipe, adjacent to the FBGs, were used for comparison. Reasonable agreement between theory and the axial and hoop strains determined by the different techniques was found. Issues associated with deploying and comparing the techniques are discussed.

Polarization-multiplexed and phase-stepped fibre optic shearography using laser wavelength modulation

A shearography system that measures the deformation gradient using two orthogonal shear directions and performs phase stepping, using no moving mechanical components and a single CCD camera, is described. The light exiting from a highly linearly birefringent optical fibre is switched between two orthogonal linearly polarized states by tuning the optical wavelength of a laser diode via injection current modulation. A polarization sensitive Michelson interferometer is used to shear the image in orthogonal directions for p- and s-polarized light. The change in the optical wavelength is also used to provide a phase step in the pathlength imbalanced interferometer. In this way wavelength modulation of a laser diode source is used to accomplish simultaneously polarization multiplexing and phase stepping in the shearing interferometer. By carefully matching the optical wavelength shift, the optical fibre length and the pathlength imbalance in the interferometer, the π/2 polarization shift can be matched to the required phase step.

Shearography as part of a multi-functional sensor for the detection of signature features in movable cultural heritage

This is one of a series of papers submitted by the Multi-Encode Project consortium (006427 (SSPI)). Shearography, a full-field speckle interferometry technique, is applied to the investigation of movable cultural heritage. For this project a portable shearography sensor was developed, with conventional sensor hardware supported by novel algorithms and instrument control software. The sensor has been used for an extensive measurement program, the purpose of which is to develop an Impact Assessment Procedure; this is to determine the capabilities of a shearography sensor in this measurement application. Data from the sensor, which shows the location of defects in the artwork, is stored in a database for future analysis and comparison. The particular advantages identified for incorporating shearography in a multi-functional sensor are the ability to measure unstable objects, such as gently vibrating canvas paintings, adjustable interferometric sensitivity and a different sensitivity parameter (displacement gradient) to the other sensors investigated. This work will assist in fulfilling the aims of the project, the detection of signature features for security purposes and the study of changes in artwork for conservation purposes.

Real-time extended dynamic range imaging in shearography

Extended dynamic range (EDR) imaging is a postprocessing technique commonly associated with photography. Multiple images of a scene are recorded by the camera using different shutter settings and are merged into a single higher dynamic range image. Speckle interferometry and holography techniques require a well-modulated intensity signal to extract the phase information, and of these techniques shearography is most sensitive to different object surface reflectivities as it uses self-referencing from a sheared image. In this paper the authors demonstrate real-time EDR imaging in shearography and present experimental results from a difficult surface reflectivity sample: a wooden panel painting containing gold and dark earth color paint.

Multifunctional Encoding System for Assessment of Movable Cultural Heritage

This is an introductory paper of a recent EC project dealing with research in cultural heritage and aiming to communicate new fields of application for optical metrology techniques. The project is in its initial state and more conclusive information is expected to be available at the time of the perspective conference. Nowadays safety, ethical, economical and security issues as well as the increase demand for loaning of art objects for exhibitions in transit, are forcing the Conservation Community to undertake strong initiatives and actions against various types of mistreatment, damage or fraud, during transportation of movable Cultural Heritage. Therefore the interest directs to the development of innovative methodologies and instrumentation to respond to critical aspects of increased importance in cultural heritage preservation, among which of prior consideration are: to secure proper treatment, assess probable damage, fight fraud actions in transportation.

Single-axis combined shearography and digital speckle photography instrument for full surface strain characterization

Full characterization of the surface strain requires the mea- surement of six displacement gradient components of the surface strain tensor. The out-of-plane displacement gradient component may be di- rectly measured using the full-field speckle interferometry technique of shearography, but to fully characterize the surface strain using shearog- raphy, a minimum of three illumination, or viewing, directions are re- quired. The image processing technique of digital speckle photography (DSP) is sensitive to in-plane displacement for normal collinear illumina- tion and viewing, with the displacement gradient components obtained by differentiation. A combination of shearography and digital speckle photography is used to perform full characterization of the surface strain using a single illumination and viewing direction. The increase in com- plexity compared with a standard single-channel shearography system lies predominantly in the additional image processing requirements. Digi- tal speckle photography image processing is performed using the optical flow field technique and the advantages of this technique compared with correlation are discussed. The design of the instrument is described and full surface strain measurements made with the system are presented.

Error analysis of 3D shearography using finite-element modelling

This paper describes the development of an opto-mechanical simulation of a complete shearography system, including the shearography instrument, the samples and the test environment. This simulation is applied to the measurement of 3D strains in engineering samples. The samples are a cylinder loaded by internal pressure and a flat plate under axial load. Finite elements models are used to obtain the displacements fields. A 3D shearography instrument consisting of a laser and four cameras has been simulated using the optical model. Combining the finite elements and optical simulations allows phase maps to be generated, which are the predictions for measurements using the complete test setup. Errors due to sample material properties, loading inaccuracy and dimensional tolerances are included in the model and this allows the calculation of phase maps at the minimum and maximum error limits. The simulation through path lengths and the simulation by inverted shearography processing provide similar results and the difference is associated with the approximation introduced by the sensitivity vector.