Many technical advances have been made since the invention of holography in 1947, lasers circa 1960 and holographic interferometry in 1965, which have led to widespread use of holographic interferometry in the fields of nondestructive testing and structural development. This paper reviews some of these developments from the early days when the holographic tire tester was born to the current times of computer aided holometry. This is both a historic as well as a technical overview as seen from the authors' perspective and involvement since circa 1966. References are provided for the key technical publications, but only a small selection of the slide material is included in this paper.

This paper examines the application of Electronic Speckle Pattern Interferometry (ESPI), Electronic Shearography (ES), and Laser Doppler Vibrometry (LDV) to the detection of damage in fiber reinforced composite materials. Each of these techniques is sensitive to a different displacement parameter. ESPI detects changes in the out-of-plane displacement, ES indicates the change in the surface slope and LDV provides dynamic information on the structure (dynamic stiffness and loss factor). Experiments were done to evaluate the potential of each of these techniques for detecting impact damage in fiber reinforced composite materials. The results indicated that the three methods provide complimentary information. LDV is a fast screening procedure while ESPI and ES can provide information on the extent and location of the damage.

This paper presents a method for evaluating residual stresses. The approach is based on measuring the deformation due to the relief of stresses produced by a hole drilling technique. The deformation is measured by digital shearography al. In addition, the technique is extended to evaluating stresses in concrete structures. In this application, the stresses in concrete are treated as residual stresses. Since concrete materials are weak in tensile strength, one method of assessing the integrity of concrete structures is to look for the presence of tensile stresses in concrete. Unlike holography and ESPI, digital shearography does not require special vibration nor ambient light independence. These features make the method practical for detection of residual stresses in a production/field environment.

The present experiment demonstrates the application of high-sensitivity moire-interferometry to measurements of deformation around an artificial notch in a weld material on a specimen consisting of two 50 mm thick steel plates welded together. The tests were performed at the standard sensitivity of 417 mm per fringe order and the field of view was 20 mm by 20 mm. The 125 mm wide specimens were loaded in four-point bending under loads as high as 350 kN. The load was applied using a servo-controlled hydraulic testing machine. The compact moire interferometer was attached to one of the four loading points of the machine and was moving with it during load application. The experiment was performed in an industrial type facility without any means of reducing the vibrations of the experimental setup. The interferograms were analyzed using a digitizing tablet and hybrid technique of data processing.

In this paper the system for the automatic residual stress analysis in railway rails is presented. The modified grating interferometry and the software modifications are described. Finally the system calibration by the comparison between the theoretical and experimental results of the residual stress estimation in bending of a steel beam is shown.

Electronic Speckle Contouring (ESC) is a recently established technique for shape measurement. This technique is based in a modification of a in-plane and out-of-plane Electronic Speckle Pattern Interferometers (ESPI). It is capable of contour generation by translating the reference and object source illumination. The novel translating the reference and object source illumination. The novel contour fringe interpretation introduced by ESC, facilities their analysis. The ability of this system to determine shape has been demonstrated previously. Now the same system is employed for displacement and shape measuring without hardware modifications of an out-of-plane sensitive layout. Shape and displacement information obtained from a cylindrical surface under a load are combined for obtaining a full deformation field on the surface. The experimental result is shown as a 3D plot obtained from a phase map. Measure range and decorrelation effects are analyzed in order to give more information towards the design and construction of an ESC system.

This paper presents the results of a feasibility study to determine the suitability of using Hartmann screen techniques to measure the surface shape of reflection-coated automotive headlamp parabolic reflectors.

The holographic interferometry is a tool to detect internal defects of a specimen, due to the recording of the inhomogeneous surface deformation in the area of these defects. By means of a standard evaluation method, e.q. the phase-shifting, the Fourier-transformation or the heterodyning method, it is possible to determine the relative phase distribution of the interference pattern. However, for the quantitative evaluation it is necessary to evaluate the absolute fringe-order. A new method has been developed, which allows the determination of the absolute displacement, based on two interferograms of the specimen, recorded under different loads, provided that the dependence between the load and the displacement is linear.

Theory, experimental performance, and application of the noise-modulated interferometer are presented. This novel two-beam interferometer applies (1st) a laser diode source that is stochastically modulated in its frequency and (2nd) a common photodetector in the output arm followed by an averaging frequency counter. It is demonstrated that homodyne detecting of the two interferometer beams and counting the averaged beat frequency can be understood as a correlation process. By applying Horton's Anticorrelation' function to the signal processing channel of the noise-modulated interferometer, the range R <EQ 2 m of reflecting target is determined with high precision.

This paper presents a generalized approach to simultaneously compute displacements and its derivatives from a combined single and double illumination setup. If more than three independent illumination sets are used, the redundant data is reduced through a least squares approach. This paper also presents a generalized way to compute an equivalent measurement efficiency coefficient to express how much a specific combined setup is sensitive to each displacement component and also to estimate its random error level. By using this coefficient, one can easily optimize a holographic setup for any particular application just playing with the positioning and the number of illumination points. Both mathematical derivation and analysis and a practical example are presented in this paper.

Hologram interferometry is one of many testing techniques used to study and improve vehicle structures and subsystems. Other test methodologies serve to provide data on component loadings and input conditions for holometric testing or serve to correlate holometric results at discrete locations. This paper includes applications that show the benefits of holometric testing and its ability to predict the in-vehicle behavior of a wide variety of automotive components.

The solution of structural-acoustic problems in vehicle development needs improvement of calculation and experimental dynamic analysis methods. In this aim, new analysis methods based on 1-D and 3-D displacement measurements recorded by pulsed holographic interferometry have been developed. These essentially allow to perform 3-D finite-element model updating, improvement of physical understanding for particular vibration phenomena and analysis of dynamic responses in running conditions. We discuss, in particular, two different original methods which make possible to determine the modal participations in a dynamic response measured in non-steady state conditions. Several research and industrial measurement applications are presented.

The applicability of optical fiber modal domain sensors to impact monitoring in aircraft components is studied. This work pulls together a number of scattered ideas from previous research programs onto one working platform, and includes the incorporation of lead-in and lead-out fibers, integrated spatial filtering, a novel fiber termination technique, and an inertial microbender to enhance mode coupling effects. Impacts to both graphite/epoxy composite and metallic specimens were carried out using a unique test apparatus constructed to accommodate a wide variety of measurement conditions. Test results suggest that modal domain sensors can offer quantitative information about the energy associated with a particular impact, as well as its location. Various sensor configurations and signal processing options are discussed, and recommendations for optimized sensing given.

Pulsed Laser Electronic Speckle Pattern Interferometry for the measurement of in-plane strain on high speed rotating components uses a radially sensitive interferometer. In this form interference fringe patterns have successfully been demonstrated over a very wide range of component speeds up to 23,842 rpm and 364 ms^-1. The interferometer behavior and some of the errors due to simplification of the system for use in an industrial spin pit facility are discussed.

Use of CW holographic interferometry in order to experimentally analyze structural behavior of an aircraft turbine blade prototype is presented. Results have been used to verify expected turbine specifications worked out by finite element method.

The Zygo 5500 Optical Heterodyne Profiler was used to investigate the roughness of superpolished optical surfaces made of BK7, Fused Silica, and Zerodur. In opposite to other optical profiling instruments the Heterodyne Profiler makes no use of a reference surface, thus yielding more accurate results on supersmooth substrates. However, it has been found out that a systematic error with an amplitude of about 0.1 nm is generated by the system and interferes with the measuring scan. This error could be originated in a wobble of the air bearing turntable. It is demonstrated that a reference scan representing that error, which is stable in amplitude and phase, can be generated by averaging procedures. Thus, this reference can be removed from the scans of supersmooth substrates yielding to an rms roughness resolution down to 0.013 nm.

Sub-Nyquist interferometry and other extended range techniques have the potential to allow measurement of aspheric surfaces with large departures from a reference sphere. The optic is tested in a non-null configuration, and aberrations are introduced into the wavefront by the interferometer optics. Consequently, the wavefront measured at the sensor is different from the wavefront initially produced by the test surface, and the interferometer must be calibrated if useful measurements of aspheres are to be made. The aberrations produced by a Twyman- Green interferometer for this application were examined. To study the severity of these interferometer induced errors, a defocused spherical surface was used to generate a non-null configuration. With wavefront departures up to 400 waves, errors up to 12 waves rms were found to be introduced by the non-null test setup.

In this paper, a zero-mean, band-limited Fractal function is introduced to simulate rough surface profile. According to the Kirchoff Theory, the surface roughness measurement is simulated with the help of the fast Fourier Transformation technology. The relationship between the surface parameters and the reflected light intensity distribution is obtained. In order to quantify rough surfaces, some criteria are proposed. Numerical results show that these criteria have a strong correlation with the root mean square of the surface height variation and they can be used for surface roughness measurement and classification.

In the field of displacement measurement, the traditional method usually uses a linear receiver for 1D testing and a plane array receiver for 2D testing. In this paper we describe a new method of tiny displacement measurement by dimension reduction. According to this method, a testing system of a 2D displacements is established using a linear receiver, the accuracy of the system is under 3 micrometers , and the problem that 2N-dimensional variables can be measured by N linear receivers is discussed.

The difference between theoretical and experimental strength of solids, leads to the assumption that the presence of defects under the form of cracks is responsible for this difference. The classical work of Griffith on glass provided the foundations for the generalization ofthis model to explain the process of fracture of materials. The fracture of materials is then associated with the presence of very large stress and strain gradients in a small region, the crack tip. To experimentally verify the theoretical results of fracture mechanics, it is necessary to make observations at different levels of spatial resolutions depending of the actual physical size of the corresponding cracks. The actual level of resolution selected depends on the actual physical size of the crystal defect that one wants to observe. If one wants to observe individual dislocations, the level of the resolution is of the order of 10'°m to 105m. if one wants to observegrain domains, the level of resolution is iø8 to 104m, and if one wants to analyze composites, the level of resolution is 10 to 102m. Eight orders of magnitude are required to observe the different levels. To make observations at these different levels, different type of radiations are required depending on the resolution. Using conventional optics, one can make observations from 10m up. To proceed to make observations one must use a mathematical model, the most commonly used model is the continuum mechanics model. In this model, it is assumed that the deformations of a body can be represented by analytical functions with continuous derivatives up to the third order. This model implies to ignore the discrete nature of matter and replace it by an ideal medium, the continuum. In this model the deformations of the body are characterized by combinations of the derivatives of the displacement function. This model is used at all the levels of resolutions that we have referred to, from few atomic distances in the case of dislocations, to sizes of the order of 102m in concrete structures with large aggregates. If one looks at a given problem, for example concrete with large aggregates, one can look to the displacement field in a given region at the level of 102m, and go on observing the same field at the different levels of resolution that we have mentioned. One will observe different details of the same field with increasing resolutions. The situation is similar to that of fractal geometry, the deeper one looks the more detail appears. But unlike the simple rule of self-similitude existing in fractal geometry, the different levels are related by more complicated rules. If one considers the displacements, both in direction and magnitude, the displacements between two points are the average of the displacements of the points that appear between these two points at higher levels of resolution. In the case of fracture mechanics, the level of observation depends of how deep one wants to look in the chain leading to fracture. If one wants to look at the level used by the continuum mechanics approach that analyzes the elasto-plastic singular field, one should select a spatial resolution such that the statistical fluctuation caused by the presence of individual grains are smoothed out.

The Holographic Station is composed by a set of opto-electronic-mechanical devices, integrated by a powerful software, that has been designed to make it easier to apply TV holography for displacement, strain and stresses measurements. This paper presents a quick description about its main features, ranging from hardware aids, for fast and accurate setup, to advanced data acquisition and processing functions supported by the software.

Although a large number of nondestructive techniques based on laser interferometry have been developed over the last few decades, most of them are incapable of handling large structures. Shearography has shown potential as a full field nondestructive technique, capable of being used on large structures like bridges. The method however has been beset with the problem of poor fringe quality, and as any other interferometric technique, suffers from the problem of speckle de-correlation due to rigid body motions. This paper investigates the effect of rigid body motion on fringe visibility with an emphasis on applications in nondestructive testing of bridges. To this effect, a finite element and fracture mechanics analysis is carried out on a real bridge to identify the constraints on shearography in full scale structural testing.

This paper describes the development of tomographic techniques suited to the reconstruction of data obtained from holographic interferograms, and some initial results. The principle development discussed is the treatment of the data in three dimensions rather than the conventional approach of simplifying the problem to the reconstruction of a series of layered 2D slices. A discussion of different tomographic algorithms is also included.

The design of a low-noise holographic interferometer applied to the study of diffusely scattering objects, is presented. This system is a dual-beam arrangement used for the reconstruction process, the anisotropic self-diffraction effect in Bi₁₂TiO₂₀ photorefractive crystals (PRC). It is shown both theoretically and experimentally that holographic recording and reconstruction of complex wavefronts in PRCs, is different from the case of a plane wavefront. This fact is taken into account to optimize the setup. The main optimization criterion is the enhancement of the signal-to-noise ratio (SNR) of the interferogram. Our theoretical predictions of SNR dependence on the recording beam intensities ratio, are verified by the experimental results here shown. An experimental approach to optimize a holographic interferometer with PRC is also presented.

Automatic pattern recognition methods combined with a knowledge based approach are used for the detection and classification of fault indicating patterns. For this purpose the complex fringe pattern is reduced to a line pattern (fringe skeleton) and this line pattern is approximated by vectors. The new pattern representation is transformed in a data list preserving the topology and metrics of the original pattern. Based on characteristic symptoms within the pattern features for the description of different pattern classes are derived. The knowledge for the hierarchical classification procedure embedded in decision rules for the knowledge assisted system was derived from model based simulation of fringe patterns and preparation of test objects with different flaws.

A new 3D-Electronic-Speckle-Pattern-Interferometer, which was developed in the Institute for Experimental Mechanics of the Technical University of Braunschweig will be described. It is suitable for the whole field measurement of deformations of materials of elastic and non- elastic behavior. It is applicable in material testing, quality control and in dimensioning of structures. Because of its compact arrangement it can be attached to a testing machine or other loading constructions outside of an optical laboratory. The deformation components are determined nearly simultaneously for all three directions of a cartesian reference coordinate system. The presented arrangement gives a simple possibility to compare finite element calculations.

A study on stress corrosion cracking (SCC) of Ti₉₀-AL₆-V₄ wire in anhydrous methanol has been conducted. The study focused on developing a relationship between microscopic deformation and corrosion current density in the incubation period of the SCC of the wire. The study succeeded in non-destructively monitoring the initiation stage of SCC by holographic interferometry `Holometry'. The monitoring process was carried out in situ applying the real-time holographic interferometry. In the mean time, the corrosion current density was determined by typical methods of electrochemistry. Consequently, a relationship between the influence of microscopic deformations on the corrosion current density is established.

High-sensitivity grating interferometry is shown as the effective experimental tool for the composite material characterization, biaxial testing, stress and failure analysis. The hardware and software modifications enable complete determination u, v and w fields for the tests under static and dynamic conditions. The comparison with the results obtained from strain gauges measurements is given.

Measurement of Young's modulus of thin films is an important, yet challenging task. The purpose of this study, was to determine Young's modulus of an aluminum thin film using experimental data measured under static and dynamic loading conditions. The two test objects used in this study, a thin circular plate and a constant rectangular cross section cantilever beam, were constructed from an aluminum thin film. A nanoindenter was used to load the center of the thin circular plate, while simultaneously recording the applied force and displacement of the indenter tip. Then, using a basic equation relating the applied load and displacement to the geometry of the plate and its material properties, Young's modulus was determined. Using the experimental methods of electro-optic holography and laser vibrometry, the resonant frequency for the first bending mode of the cantilever beam was determined. Then, through an equation that relates the first resonant frequency to the material properties and the geometry of the cantilever beam. Young's modulus was determined. Uncertainty analyses of these basic equations, show that the uncertainty in Young's modulus is predominately affected by the measurement uncertainties of film thickness, and result, for the test samples used in this study, with uncertainties of 5 GPa for thickness errors on the order of 0.5 micrometers .

We have developed a novel technique for performing simple phase-sensitive optical measurements in a sub-micrometer area. We presently use this technique to measure film stacks commonly found in the semiconductor industry. This article explains the theory behind this technique and presents several examples demonstrating the capabilities of the system.

The design and application of scanning probe microscope primarily intended for topographic, magnetic force and magneto-optical investigations of magnetic thin films is described. A microfabricated silicon tip is scanned across the sample of interest, and the tip deflection detected using a modified form of the den Boef twin beam interferometric system. Initial studies of the recorded bit structure in magnetic and magneto-optic storage media, and the magnetic field from a thin film recording head are reported.

A new optical scheme of an absolute distance measuring interferometer with a variable optical frequency has been suggested. In this scheme the contrary beams from reference (stabilized) and measuring (tunable) lasers are used that allows in a simple optical design to ensure both the length measurement and the elimination of an optical path difference fluctuations influence. Usually these two functions are carried out by two channels of the variable optical frequency interferometer--measuring and control channels, each of which is, in essence, an independent separate interferometer with its own optical components. The usage of the contrary beams principle gives a possibility to construct both channels mentioned in the same optical components, providing essential simplification of the interferometer optical design.

The purpose of this paper is to describe a new precision displacement sensor--Compact Precision Holographic Optical Probe--with the use of holographic optical element (HOE) technology. It is a novel non-contact method for the measurement and inspection of displacement and surface quality control. The IR HOEs are made by using a computer generated hologram method. A mathematical model has been developed to analyze the performance of the probe. The initial experimental results are obtained. The probe is both compact and inexpensive.

With a wide spectrum light source, the auto-correlation function of the emitted beam is nil except for very short values of the optical time delay. This property is used in an interferometric set-up to identify the zero optical path difference between the two interfering arms and to analyze surface profile. The proposed technique is based on a simple intensity detection and does not compute any phase calculation. Therefore, we are not concerned with the phase ambiguity problem associated to classical phase shifting interferometers. We first present the acquisition principle and discuss the operating conditions which guarantee a correct detection. Secondly, the experimental results show the nanometric resolution of the proposed technique. We finally explain the interest in combining this method with actual confocal or phase shifting profilometers.

This paper reviews significant advances in fringe carrier techniques developed by the authors for acquiring derivative patterns of deformation and for automatic order determination of fringe patterns using optical and digital image processing. Various applications to many aspects of photomechanics are shown including, holographic-moire for the in-plane displacement derivative fringe patterns; speckle-shearing interferometry for the curvature patterns of bending plate and for the separated in-plane strain patterns; moire interferometry of real-time recordings for 3D displacement derivatives and phase-stepping technique based on the scanning of the fringe carrier for the phase demodulation; and the automatic order determination of the isochromatic fringes of dynamic photoelasticity and the transient fringes of geometrical moire.

In this paper, LSD wave ignition process is analyzed according to aerodynamics, and expressions of speed of LSD wave, pressure, internal energy is derived. Interaction of YAG laser and Al target is investigated synchronously with a Mach-Zehnder interferometer, and series of interferograms of ignition and propagation of LSD wave at the initial stage are obtained first. It presents reliable experimental datum for theoretical study of LSD wave.

A new method of determining the surface shape of various objects including diffusely reflecting ones is proposed. The method relies on the use of polarization measurements. As a result of the application of this method, the total (resulting) object wave at each point of the reconstructed image turns out to be rotated by an angle whose value is proportional to the relief height at a given point and the direction of the angle of rotation determines the direction of the relief change. This method permits determining not only the value, but also the sign of the relief height; it also makes it possible to increase the resolution and accuracy of measurements.

The question of informative character of special image points--wavefront dislocations, is discussed. They were observed in real phase images of microstructures, which had comparable and smaller linear dimensions than diffraction limit. According to the existing classification, phase patterns of the main types of wavefront dislocations were identified. Their influence on the phase imaging formation is investigated. The case of induced phase contrast due to considerable decrease, but not quite vanishing of the complex amplitude, is taken into consideration. The contour making and contrasting tendency in the presence of the combination of dislocations, is demonstrated. Observation of anomal small-size details in phase and polarization images and the possibility of their observation in general `functional' image, is discussed. The experimental pictures were obtained with the help of unique phase microscope `AiryScan' technique.