Carrier Fringes Research Articles

The magic of carrier fringes in moiré interferometry

Practical applications in which carrier fringes are used with moire interferometry for strain measurements are presented. Examples illustrate how moire 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 moire interferometry, the carrier fringes are produced easily by adjustments of optical elements that control the virtual reference grating.

Comparative Holographic Moire Interferometry: Separation Of Moire Fringes From The Carrier Interference Pattern

Comparative holographic moire interferometry (CHMI) permits defect detection by comparison of the mechanical responses of two macroscopically identical specimens at equal loading. In the present work a technique of moire fringe separation is suggested that is amenable to automatic readout. Two approaches are proposed and experimentally verified: phase shifting of the carrier interference pattern and auxiliary carrier fringe generation with subse-quent optical filtering. The phase shifting method is applicable to an additive moire scheme with two reference beams. Auxiliary carrier fringe generation can be used in both additive and multiplicative CHMI. The frequency constraints of the latter method due to the optical recording arrangement and mechanical responses of the tested specimen are analyzed.

Shearographic Fringe Carrier Method For Data Reduction Computerization

This paper presents some preliminary success in the computerizing of data reduction in shearography. Computerization is made possible by the introduction of a fringe carrier technique that removes the ambiguities in fringe interpretation. The technique is based on superposing a set of carrier fringes upon the deformation fringes. The resulting fringe pattern has monotonically increasing fringe orders, thereby allowing the fringe orders to be determined in a straightforward manner and without ambiguity. It is shown that the data deduction process can be computerized. The deformed carrier fringe pattern is digitized, and the displacement derivatives are then automatically deduced by means of digital image processing.

Automatic Analysis Of Holographic And Shearographic Fringes To Measure Flexural Strains In Plates

The technique of phase measuring by spatially modulated carrier fringes is applied to analyzing holographic and shearographic fringes. The algorithm employed is a simplified version of the Fourier transform method and does not require any special device such as the frequency modulator used in time domain phase detection techniques. Analyzed phase data are numerically differentiated to calculate full-field distributions of flexural curvature and twist, which correspond to flexural strains.

Cylindrical Fringe Carrier Technique In Holographic Interferometry

This paper presents a new holographic interferometry technique, called the "cylindrical fringe carrier technique," in which the carrier fringes are not equally spaced. This technique not only eliminates the ambiguity in interpretation of holographic fringes that occurs when the carrier fringes are equally spaced, but it also can detect larger deformations and yield the required fringe spacing without precise adjustment of the object beam.

Automated analysis of holointerferograms for the determination of surface displacement

A method for using an image digitizing system with a minicomputer to automatically determine surface-displacement fields by evaluating holographic fringe patterns is presented. The problem of determining the sign of the displacement vector as well as the need to produce a monotonic phase change across the surface under observation is solved by utilizing the method of carrier fringes. A set of programs has been written so that the camera/computer system can view the pattern of deformation-modulated carrier fringes and, with a knowledge of the initial carrier-fringe pattern, draw a profile plot of the deformed surface. The procedure is exemplified with a study of a centrally loaded disk.

A fringe carrier technique for unambiguous determination of fringe orders in shearography

Shearography is an interferometric method which measures displacement derivatives. This paper presents a technique which eliminates the ambiguity in shearographic fringe interpretation. The technique is based on generating a carrier fringe pattern. Introduction of the carrier fringes results in the formation of a fringe pattern which has monotonically increasing fringe orders, thus allowing fringe orders to be determined in a straightforward manner and without ambiguity. The phase change due to deformation alone is then obtained by simply subtracting the known undeformed carrier fringe orders from the deformed carrier fringe orders. Elimination of fringe ambiguity has paved a way for automatic data acquisition in shearography.

A windowing technique for the automated analysis of holo-interferograms

A systematic method for measuring surface deformations is described which includes the use of real-time holographic interferometry, the incorporation of a carrier fringe pattern to achieve fringe linearization, and the application of image digitization and automated computer analysis for rapid quantitative interpretation. In this method, regions of interest can be selected by the use of a windowing technique in which the operator specifies the desired subregion. The problem of correctly numbering fringes which originate or vanish at the boundary where the scanning begins is solved by an algorithm which ensures correct identification of the fringe orders. The output includes a perspective plot of the out-of-plane deformation field. This method is demonstrated by its application to a measurement of the out-of-plane deformation of a hermetically sealed plastic housing loaded by internal pressurization.

Circular fringe carrier technique in holographic interferometry

An improved fringe carrier technique can eliminate the ambiguity in holographic fringe interpretation. This paper presents a new fringe carrier technique-a circular fringe carrier technique that not only eliminates the ambiguity in holographic fringe interpretation but also determines the direction of displacements and obtains the required fringe spacings without precisely adjusting the object beam.

Shear strains in a graphite/PEEK beam by moiré interferometry with carrier fringes

A multispan quasi-isotropic graphite-PEEK beam exhibited dramatic shear strains in the interlaminar region between plies. Shear strains in the plies themselves varied in basic accord with fiber direction. The large anomalous shear strains were developed near the center of the beam height, where shear stresses were large. High-sensitivity moire interferometry with 2400l/mm (60,960l/in.) was used. A new technique of data extraction was developed, using carrier fringes to transform the pattern to one in which fringe slopes are proportional to derivatives of displacement. This technique enhanced detection and measurement of highly localized shear-strain gradients.

Conjugate lateral shear interferometry and its implementation

The concept of conjugate lateral shear interferometry is introduced. It involves producing two lateral shear interferograms with carrier fringes identically distorted in opposite directions. The moiré fringes formed by overlapping such interferograms display information proportional to the sum of the derivatives of the functions encoded in the interferograms. A simple method for generating conjugate interferograms by using a Ronchi test configuration and the self-imaging phenomenon is described. In the case of large phase gradients and shears, a spatial-filtering operation is required. The principles derived are verified by an experiment. The application of the technique is proposed.

Direct Measurement of Flexural Strains in Plates by Shearography

Strains in a flexed plate are directly related to the second derivatives of the plate deflections. A technique is developed which enables shearography to measure second derivatives of deflection directly. The technique is based on generating fictitious carrier fringes in the form of periodic linear fringes of uniform spacing. The carrier fringes are distorted when the plate is deformed. The change in the pitch of the carrier fringes due to deformation yields the flexural strain directly. The pitch of the carrier fringes is equivalent to gage length.

Moire interferometry for out-of-plane displacement measurement

A high sensitivity technique for out-of-plane displacement is demonstrated. A 600 lines mm phase grating is used in conjunction with a specularly reflecting (in some cases mirrored) object, to obtain contours of out-of-plane displacement. Initial patterns due to low quality optics are eliminated by a double exposure technique employing carrier (or mismatch) fringes. Beams with cracks are used to experimentally demonstrate this technique.

Sandwich hologram for displacement derivative

The suitability of sandwich holography for displacement derivative measurement is demonstrated. Holographic carrier fringes are produced by shearing one plate relative to the other in a sandwich pair. Some experimental results are presented.

Improved Fringe Carrier Technique For Unambiguous Determination Of Holographically Recorded Displacements

This paper presents an improved technique that eliminates the ambiguity in holographic fringe interpretation. The technique is based upon generating a high density linear phase carrier. Introduction of the carrier results in the formation of a fringe pattern that has monotonically increasing fringe orders. Any deformation of the object under study produces a corresponding perturbation of the carrier pattern. If the carrier dominates, the perturbed pattern continues to have monotonically increasing fringe orders and thus allows the fringe order to be determined at any point without ambiguity. The phase change due to deformation alone is obtained by subtracting the known linear carrier from the modulated fringe pattern. The process of data reduction is so straightforward that skillful human interpretation is no longer required. This simplification of the fringe interpretation process will pave the way for automatic data acquisition in holographic interferometry.

A new fringe carrier method for the determination of displacement derivatives in hologram interferometry

Holographic carrier fringes are produced by rotation of the object beam prior to the second exposure when the object is deformed. It is shown that the local pitch of the carrier is a linear function of displacement derivatives. The local pitch may be directly measured with a microscope or a digitized TV system. The theoretical foundation as well as an experimental demonstration are given in this paper.

Displacement fields (U,W) obtained simultaneously by moiré interferometry

A high-frequency phase grating on a specimen surface is illuminated symmetrically by two oblique beams. The diffracted beams emerge with wave front warpages that define both the in-plane U and out-of-plane W displacement fields. Contour maps of these wave fronts, with added carrier fringes, are obtained as a single photographic record. They are manipulated by moiré and optical filtering steps to yield whole-field fringe patterns of U and W. Sensitivities of 0.833 microm/fringe (32.8 microin./fringe) for in-plane displacements and 0.132 microm/fringe (5.2 microin./fringe) for out-of-plane displacements were demonstrated. Since data acquisition is experimentally simple, dynamic as well as static analyses are applicable.

Feasibility of high-resolution optical images from microwave far-field holograms

It is demonstrated, with the aid of simple facsimile far-field (Fourier-transform) holograms, that optical images can be obtained from limited far-field information and that the quality of these images is also dependent on the spatial frequency of the ‘carrier fringes’ which lace the hologram. The use of a synthetic reference wave in a practical case offers advantageously the possibility of high ‘carrier-fringe’ frequencies and hence high-quality optical images.