A neoteric interferometer for use in holographic photoelasticity

A shortcoming of the conventional holographic interferometer used in photoelasticity is that, for the double-exposure cases, the resulting fringe patterns are a complex combination of conventional isochromatic and isopachic fringes. This paper describes a holographic interferometer that may be used to obtain separate but simultaneous isochromatic- and isopachic-fringe patterns for photoelastic models in states of plane stress. The method requires a model with a partially reflecting front surface. Isopachics, which are proportional to the thickness change, are recorded using holographic interferometry from the transmitted light. The isochromatics are obtained from the transmitted light by conventional means. General equations relating the surface displacement of the specimen to the observed fringe patterns are developed, and examples of static and dynamic loadings are shown.     

Dual hologram shearing interference technique for wind tunnel flow fields testing

 A novel optical diagnostic technique, dual hologram shearing interferometry, for measuring density gradients of different phase objects is proposed and demonstrated. The lateral shearing is achieved by using a phase grating. A holographic interferometer has been developed and designed on the base of a single pass Z type conventional schlieren device. The interferometer’s scheme is insensitive to acoustical disturbances, similarly to the conventional schlieren layout, and is capable of recording holograms with a continuous wave laser during the wind tunnel run. The features of the technique make it tolerant to both the temporal coherence of the laser light source and to the relatively low, schlieren quality optical windows of the wind tunnel’s test section. The obtained reconstructed lateral shearing interferograms with a large region of overlap have high contrast and may have an arbitrary orientation and/or spacing of the background interference fringes. It is believed that the proposed approach will become a useful tool for visualization and accurate mapping of the density gradients of gas dynamic flow fields, in wind and shock tunnels, where acoustic noise problems may dramatically affect reference beam holographic schemes. Received: 9 January 1997 / Accepted: 12 April 1997     

Comparison between Digital Fresnel Holography and Digital Image-Plane Holography: The Role of the Imaging Aperture

Optical techniques are now broadly used in the field of experimental mechanics. The main advantages are they are non intrusive and no contact. Moreover optical techniques lead to full spatial resolution displacement maps enabling the computing of mechanical value also in high spatial resolution. For mesoscopic measurements, digital image correlation can be used. Digital holographic interferometry is well suited for quantitative measurement of very small displacement maps on the microscopic scale. This paper presents a detailed analysis so as to compare digital Fresnel holography and digital image-plane holography. The analysis is based on both theoretical and experimental analysis. Particularly, a theoretical analysis of the influence of the aperture and lens in the case of image-plane holography is proposed. Optimal filtering and image recovering conditions are thus established. Experimental results show the appropriateness of the theoretical analysis.     

The magic of carrier fringes in moiré interferometry

Practical applications in which carrier fringes are used with moiré interferometry for strain measurements are presented. Examples illustrate how moiré carrier fringes are applied to obtain the desired data in complex laminated composite specimens. In many cases, carrier fringes permit extraction of much more detailed information, with procedures that are easier and more accurate than those using loadinduced fringes alone. The fringe vector for carrier fringes is introduced and its application to the interpretation of fringe patterns is explained. In moiré interferometry, the carrier fringes are produced easily by adjustments of optical elements that control the virtual reference grating.     

Techniques for the determination of absolute retardation in photoelasticity

The purpose of this paper is to give an interpretation to the fringes observed in holographic interferometry when plane-polarized light or circularly polarized light is utilized. It is shown that, when plane-polarized light is utilized and both the loaded and the unloaded states are considered, the obtained patterns are formed by the superposition of three families of fringes: the two families of absolute optical retardation and the family of relative retardation. The intensity distribution is also a function of the orientation of the plane of polarization, and along the points where the plane of polarization is parallel to one of the principal directions, only one of the families of absolute retardation is observed. By utilizing circularly polarized light, the dependence on the orientation of the principal axis is eliminated and patterns consisting of the superposition of the three above-mentioned families are obtained. If only the loaded state is considered, the holographic interferometer behaves as an ordinary polariscope with the reference beam playing the role of the analyzer. The relationships between the observed families are discussed. Examples of application to the disk and ring under diametral compression are also given.     

Application of digital phase-shift holographic interferometry to weak shock waves propagating at Mach 1.007

Digital phase-shift holographic interferometry was applied to visualize weak shock waves and related phenomena quantitatively. This method of interferometry is an improved version of double-exposure holographic interferometry using digital image processing and a phase shift method. The obtained interferograms were analyzed using the Carré method. To evaluate the applicability of the interferometry to quantitatively visualize the phenomena, density profiles behind weak spherical shock waves generated with 500 μg of silver azide were examined. The results of the numerical analysis performed with the hydrocode AUTODYN were compared with those of the experiment. The Mach number of visualized shock waves was estimated to be 1.007 ± 0.001 at the pressure transducer near the test section. At the shock fronts, the density difference between the experimental and numerical results was within 0.3%.     

二维变线距光栅密度的检测

变线距全息光栅平面单色仪由于具有自动聚焦、消像差、高分辨率以及减少杂散光等能力，因而在同步辐射装置、激光核聚变装置以及太空望远镜等设备中有着重要的应用。由于变线距全息光栅的制作及检测存在着相当的难度，制约了变线距全息光栅的推广应用。因此，变线距全息光栅的制作及检测研究，具有十分重要的理论及应用意义。变线距全息光栅的制作必须有高精度的密度检测技术相配合。比如，在变线距全息光栅的制作过程中，需要对用于形成变线距光栅的全息干涉条纹的线密度进行检测；对于已经制成的变线距全息光栅的线密度也需要进行检测，以检查光栅的制作质量。因此，变线距全息光栅线密度的检测，对提高变线距全息光栅的制作质量起着十分关键的作用。文中给出了二维变线距光栅的密度描述公式，讨论了干涉条纹的物理意义；应用干涉云纹法和条纹图像处理技术，对二维变线距光栅的密度分布进行了检测研究，给出了光栅密度的全场分布，并讨论了检测精度。     

A simple holographic interferometer for static and dynamic photomechanics

A particular variation of holographic imaging system is described which, when used as a multiple-exposure holographic interferometer, possesses advantages for applications in static and dynamic photomechanics. Large fields of view can be obtained. Rigid-body motions produced by loading are automatically eliminated. The holograms can be recorded on medium-resolution films which have high sensitivity. Specimens manufactured with readily available materials can be used for the determination of isochromatics and isopachics. An attempt is made to describe this contribution in the background of previous developments in interferometry. Operation of the system is interpreted by showing the equivalence of the holographic interferometer to a combination of two systems presently in use in experimental stress analysis: a Fizeau interferometer and an optical spatial filter. The interpretation of isochromatics and isopachis as moiré phenomena is emphasized. Isochromatics and isopachics are presented as illustrations of the applications of the method to the solution of static-stress problems, and they are used in the solution of some not yet solved dynamic-stress problems. Whole-field static isochromatics obtained as absolute-retardation interference are shown. Also shown are whole-field dynamic isopachics.     

Measurement of slopes of structural deflections by speckle-shearing interferometry

A specimen illuminated by coherent light is imaged by a camera through a shearing mechanism so that the speckle from one point on the surface can be made to interfere with the speckle from a neighboring point. The resultant speckle pattern is recorded. By mechanically interfering the recorded speckle pattern corresponding to deformed and undeformed states of the specimen, respectively, using double-exposure technique, a speckle-moiré-fringe pattern is generated. These fringes which depict derivatives of deflections of the specimen are made visible by spatial-filtering technique. Speckle-moiré fringes can also be obtained in real time. This method is a new interferometry and will be referred to as “speckle-shearing interferometry”. Speckle-shearing interferometry has the same function as Ligtenberg's technique. However, it does not have the sometimes inconvenient requirement of Ligtenberg's technique that the surface of the specimen must be of mirror quality. The new technique will be particularly useful in studies of flexural deformation such as flexed beams and plates. Although speckle-shearing interferometry is an interferometric method, it overcomes several of the limitations associated with holographic and speckle interferometries, namely: (1) the setup is simple and does not need laborious alignments of optical components, (2) it does not require stringent mechanical and ambient stabilities, (3) coherent requirement of light is greatly relaxed, and (4) the sensitivity is reduced that somehow fills the gap in sensitivity between moiré techniques and holographic or speckle interferometry.     

Impact-induced transient deformation analysis by means of digital speckle pattern interferometry

Transient in-plane displacements generated in a metal plate subjected to impact loading are measured using a pulsed digital speckle pattern interferometry system. Two separated speckle patterns produced by a ruby laser that freezes the object motion are recorded using a CCD camera and stored in a frame grabber. Transient displacements are evaluated by digital analysis of the fringes generated by the subtraction of these speckle patterns. The computer used for image processing is also utilized to control image acquisition and to synchronize the laser pulses to the CCD camera and the object position. Experimental results are compared with numerical calculations obtained using the finite-element method.     

Comparative holographic interferometry: A nondestructive inspection system for detection of flaws

Comparative holographic interferometry addresses a key problem of nondestructive evaluation: comparison of the resistance to strength in real time of two nominally identical specimens. After outlining the basis of comparative holography in flaw detection, the present paper reports on some new and complementary developments in comparative holographic interferometry. The proposed systems stand out by their ability to store the master displacement field in the interferometric setup. The integration of the storage capacity in the instrument considerably augments its potential in nondestructive routine inspection tasks. The visual display of the fringes contouring the difference in mechanical response is shown to improve considerably with the addition of auxiliary phase difference satisfying certain conditions. Methods for the generation of corresponding fringes are considered and their localization investigated in brief. Particular attention is devoted to the formation of the holographic moiré fringes. The influence of system misalignment on the moiré fringe interpretation is examined. A potential application of comparative holography to the quantitative evaluation of fatigue is described. Experimental evidence supporting the operational feasibility of the technique along with the results obtained in application to flaw detection are finally presented.     

测量温度场分布的激光散斑照相计量术

本文讨论了激光散斑照相术在测量密度(或温度)扬中的应用。提出了激光散斑——纹影干涉仪的光学系统,可在同一瞬时得到一张散斑图和一张差分干涉图。对沿竖直加热平板自然对流的温度边界层进行了定量地测量。从散斑图和从干涉图中获得的实验结果符合极好。为了说明本方法的应用范围,对一个非稳定的波动甚大的本申灯火焰进行了测量。实验结果表示,激光散斑照相术更适于研究湍流场,因为干涉术在这种情况下已失去其定量计量的作用。     

Contact holographic interferometer for studying the deformation of small-curvature shells of revolution

The strains of a curvilinear surface are determined using holographic interferometry. Equations are derived to interpret the interference fringes for a shell of revolution. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 3, pp. 176–182, May–June, 2006.     

A new application of differential interferometry for stress analysis

In the past, differential interferometry has found interesting applications in gas dynamics. The gradients of density could be measured in gas flows. Now, a first trial is made to extend this method to the experimental treatment of stress problems. A Wollaston prism with polarizing elements is used in the optical arrangement. This prism combines two beams of light which have penetrated the model at locally separated points. A field of interference fringes can be produced behind the Wollaston prism. The deflections of the different conjugated light beams, which are caused by the deformed elements of the model, lead to a shifting of the interference fringes. A Stress Differential-interferometer Law is derived theoretically in order to interpret the optical data According to this theory, the optical effect caused by the deflection in this arrangement is proportional to the gradient of the sum of principal stresses. A calibration test is performed by using a circular disk, this method is applied to a circular ring for measuring the stress gradients. Under special conditions, interference fringes could be produced which represent the loci of equal stress gradient. Plexiglas plane models are loaded diametrically by single loads. The experimental results verify the statements of the developed theory.     

Expanding the applications of holographic interferometry to the quantitative visualization of oscillatory thermofluid processes using temperature as tracer

 In this paper we discuss a novel application of holographic interferometry in the simultaneous quantitative visualization of high-speed, oscillatory flow and temperature fields in complex flow geometries. We consider cases of (i) self-sustained oscillatory flows with main flow imposed in grooved and communicating channels as well as (ii) oscillating thermofluid processes with zero mean velocity in a thermoacoustic refrigerator model. Examples showing unsteady temperature distributions obtained by real-time holographic interferometry combined with high-speed cinematography illustrate the possibilities of the approach introduced in the paper. Our study shows that temperature distributions accurately mirror flow structures in certain types of complex, unsteady flows, thus allowing, apart from the measurement of temperature profiles and heat transfer, also the measurement of oscillatory amplitudes, frequencies, wavelengths as well as the speed of propagation of traveling waves by applying digital image processing techniques. In the grooved and communicating channels it is possible to visualize the structure of the Tollmien–Schlichting waves through isotherms by using the infinite fringe field alignment of holographic interferometry. In the thermoacoustic refrigerator model, small amplitude temperature oscillations generated by the acoustic standing wave are visualized and measured. Image processing as well as data reduction procedures used in the analysis of these flow fields are discussed in the paper. Experimental data obtained by applying the techniques introduced in the paper show good agreement with theory and results of numerical simulations. Our study suggests that using temperature as tracer offers numerous advantages in the study of certain types of complex, unsteady flows. Received: 7 January 1997/Accepted: 18 September 1997     

1999 William M. Murray lecture

The history of holographic interferometry is one of problems. It has progressed from a novel discovery to a recognized technology only by the systematic pursuit of explanations for the puzzling phenomena it presents: unusual fringe patterns, fringe localization phenomena and bizarre fringe parallax. How to use the fringes to find three-dimensional deformation patterns is also a problem. Furthermore, the problem of how to handle high-volume testing spurred the evolution from photographic plates to fiexible roll film for all electronic media. Finally, the use of CCD television cameras and digital video processing has made phase-step interferometry the dominant method of fringe analysis but presents a major problem in the form of the phase-unwrapping quandary. This paper will attempt to trace the history of this technology in terms of the problems it has presented and their solutions. This paper was presented as an invited lecture at the 1999 SEM Annual Conference and Exposition, June 9, 1999, in Cincinnati, Ohio.     

A new holographic interferometer for stress analysis

In holographic interferometry with a photoelastic model, two families of fringes are generated simultaneously when the model is stressed. One family represents the isochromatic-fringe pattern normally associated with photoelasticity which yields the difference between the principal stresses. The other family represents the isopachic-fringe pattern associated with interferometry which yields the sum of the principal stresses. From these complementary patterns, the magnitudes of the principal stresses can readily be determined throughout the field of observation. Unfortunately, these fringe patterns are not completely independent but interact in such a way as to make interpretation difficult in critical regions of the model. A new system has been developed which readily permits simultaneous acquisition of these fringe patterns without their undesirable mutual interaction, as well as providing increased sensitivity. This new interferometer uses a double-pass object beam and an optical rotator to eliminate the isochromatic-fringe pattern and its effect from the isopachic interferogram. Such a system has considerable value in experimental mechanics for applications to both static and dynamic model studies and to materials investigation.     

Holographic vibration study of a rotating propeller blade

Holographic interferometry has often been used to determine the natural frequencies and the associated mode patterns of vibrating objects with rigidly fixed or stationary boundaries. This paper describes a new pulse-laser holographic technique whereby the vibration-mode patterns of a rotating propeller blade were determined. This technique consists of rotating the holographic plate on the same axis as the rotating object and using spherically symmetric wavefronts for object and reference beams.     

Full-field surface-strain and displacement analysis of three-dimensional objects by speckle interferometry

A method of full-field measurement of displacements as well as strain on arbitrarily curved surfaces is introduced. The speckle effect of coherent light is utilized to produce fringes due to displacements. Unlike the fringes produced by holographic interferometry, these fringes have a unique interpretation in relating to displacements and they localize on the surface. Three measurements are required to determine the three components of displacement; and, knowing the geometry of the object, its surface strains can be deduced. Three ways of recording displacement fringes, namely, real time, double exposure and superposition, are described.     

利用光学干涉条纹图相位分析测量纳米位移

Optical methods such as Twyman-Green interferometry, moiré interferometry, holographic interferometry and speckle interferometry are useful to measure displacement and strain in the full-field of structures. Recently phase analysis method of fringe patterns obtained by these optical methods becomes popular, and it provides accurate quantitative results in the full-field. In this paper, in order to measure displacement and strain, real-time or high-speed nano-meter displacement measurement methods developed by the authors are introduced. That is, (1) out-of-plane displacement analysis by Twyman-Green interferometry using integrated phase-shifting method using Fourier transform phase-shifting method, (2) simultaneous two-dimensional in-plane displacement analysis by moiré interferometry and (3) out-of-plane displacement analysis by phase-shifting digital holographic interferometry. The theories and applications are shown.