Showing papers on "Moiré pattern published in 1974"

Measurement of slopes of structural deflections by speckle-shearing interferometry

Abstract: 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-moire-fringe pattern is generated. These fringes which depict derivatives of deflections of the specimen are made visible by spatial-filtering technique. Speckle-moire 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 moire techniques and holographic or speckle interferometry.

Moiré: Formation and interpretation

Abstract: The superposition of several periodic or quasiperiodic patterns produces moire effects. In particular, the combination of three grid structures adds great flexibility to the use of moire phenomena. Information about the fringe structure, given by vector addition in Fourier space, allows investigation of formations, interpretation, and expectation problems concerning moire-pattern parameters. Any frequency and orientation of the pattern is possible, using three-line gratings. On the other hand, three quasiperiodic gratings with variations of frequency and/or orientation result in a zone-plate shape of the moire pattern regardless of grating distortions. Both elliptical and hyperbolic zone plates occur. Several illustrations demonstrate these phenomena and indicate some potential applications.

Speckle-Shearing Interferometric Camera -- A Tool For Measurement Of Derivatives Of Surface-Displacement

Abstract: A camera having a lens with two apertures and equipped with a shearing mechanism is employed to image a specimen which is illuminated by coherent light. With this arrangement, the speckle from one point on the surface is allowed to interfere with the speckle from a neighboring point so that a speckle-grid pattern may be detected in the image plane. By double-exposure or real-time techniques, the speckle-grid pattern corresponding to the deformed state of the specimen is added to the speckle-grid pattern corresponding to the undeformed state of the specimen. The resulting mechanical interference produces a moire-fringe pattern which depicts derivatives of surface-displacements. The camera will be referred to as "Speckle-shearing Interferometric Camera". Three applications of the speckle-shearing interferometric camera are described: (1) mea-surement of slopes of structural deflections, (2) in-plane strain measurement, (3) vibration analy-sis. Experimental demonstrations for each case of the applications are presented. Although the speckle-shearing interferometric camera is based on interferometric principles, it overcomes several of the limitations associated with holographic and speckle interferometries, namely: (1) the setup is simple and does not re-quire laborous alignment of optical components, (2) it does not require stringent mechanical and ambient atmospheric stabilities, (3) coherent requirements of light are greatly relaxed, (4) the sensitivity is greatly reduced, and (5) it does not require differentiation to obtain strains.© (1974) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Focusing Errors in a Collimating Lens or Mirror: Use of a Moiré Technique

Abstract: The Fresnel diffraction pattern of a grating periodically gives an exact reproduction of the grating. By superposing a second grating on such an image, we form a moire pattern that depends upon the degree of parallelism of the incident beam. We present here the possibility of using this technique for studying the focusing errors in a collimating lens or a mirror.

An interference technique for the measurement of in-plane displacements of opaque surfaces

Abstract: A laser interferometer system is used to record two mutually perpendicular diffraction gratings on a photoresist coated specimen. When either grating is re-illuminated by the fringe pattern used to form it, real-time observations can be made of moire fringes corresponding to surface displacements which occur as a tensile load is applied to the specimen. The technique has been used to study the in-plane deformations of carbon-fibre composites, and typical results are given.

A new optical system for moiré methods

Abstract: A new optical system for moire methods is proposed in which the light path from the model grating and that from the master grating are diffent. They are combined by mirrors to form moire patterns. Its application to Ligtenberg's reflective moire method extends the method to dynamic problems and also facilitates the use of linearmismatch fringes. It can also be applied to in-plane moire method in cases where deformation is large and diect contact with model grating is to be avoided.

Two-axis moire fringe displacement transducer

Abstract: A two-axis Moire fringe displacement transducer is disclosed in which the scale grating is inscribed with a grid pattern consisting of two superimposed orthogonal optical line sets and the index grating is inscribed with at least two discrete optical grating patterns, each of the at least two grating patterns aligned with a respective grid grating set such as to generate Moire fringe patterns in a respective grating upon relative movement along each orthogonal direction.

Moire' Patterns and Two-Dimensional Aliasing in Line Scanner Data Acquisition Systems

Abstract: The basic mechanism underlying the generation of Moire' patterns in line scanner data acquisition systems is examined. A general expression is developed in terms of typical system parameters for the reproduced image of such systems and the interaction of the image spectrum; the raster frequency and digital sampling frequency of the A/D conversion process are discussed and examples given. System design requirements for avoiding Moire' pattern generation and two-dimensional aliasing are discussed.

Interference inspection procedure - compares specimen shape with master by superposition of interference patterns

Abstract: The method for checking the shape of a specimen against that of a master uses optical interference effects, and produces very sharply defined interference patterns. Light from a source, preferably a divergent beam laser is spread by a lens and passed through a diffraction grating. The interference pattern falls on the specimen and generates a pattern of contour lines which is imaged by a lens on to a photographic plate or other recording device. Superimposition of the resulting image with that obtained in similar fashion from the master shows deviations in the form of Moire fringe patterns.

Tilt moiré patterns

Abstract: Consideration is given to the image formed in the electron microscope of two crystals stacked one above the other when these show a relative rotation about an axis perpendicular to the electron beam. Dark‐field moire fringe patterns are predicted when certain conditions relating to the relative proximity to the Bragg condition of the two crystals are fulfilled. The limits of these conditions and the spacings of the fringes are formulated in terms of the angle of tilt and the direction of the tilt axis.

Flow visualization by an improved double exposure method in holography

Abstract: Some optical differential interferometric techniques were investigated in order to measure the density distribution in the air jet flow of a small scale flow cell while excluding the effects of the optical distortion of its glass windows. A technique using moire fringe formation by rotating the reference wave in one exposure of the double exposure method was found to be most suitable for this purpose. High contrast and multiplication of moire fringes can be obtained by a moire fringe sharpening and multiplication method.

Some Experience With The Use Of A Moire Test Pattern In The Measurement Of The Optical Transfer Function

Abstract: Incoherently illuminated sinewave objects are generally employed as tar-gets in transfer function studies. Since it is not easy to obtain good targets of this type, other targets have been proposed which, by the aid of appropriate filtering, can be transformed into effectively sinusoidal gratings. The rectangular-and triangu-larwave objects are mostly used in this connection (Ref.l) . The latter is especially advantageous since the higher harmonics of its Fourier-series representation converge-more rapidly than those in a rectangular wave. A practical method of generating test objects of triangular waveform is based on the utilization of moire fringes produced by two crossed gratings (Ron-chi rulings) -(]Ref.2 and 3). By ro-tating the two gratings in opposite directions the spatial frequency of the moire fringes can be varied continuously and linearly with time,observing simultaneously the condition that the scanning time-frequency remains the same for all spatial frequencies. In order to achieve the desired accuracy in OTF measurements with an equipment using the mentioned test ob-ject it is necessary to satisfy certain conditions. One of these is good quality of the basic gratings, another is correct positioning in the driving mechanism.© (1974) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A simplified holographic-interferometry technique for real-time flow visualization and analysis

Abstract: A holographic-interferometry technique for flow visualization and analysis that produces real-time moire fringes is described from both experimental and application considerations. It has three chief advantages: real-time data for continuous observation and photography, ease of optical adjustment, and capability of using ordinary-glass test-section windows without affecting the results. A theoretical discussion is presented describing the formation of the fringes in holographic terms and then comparing this result to that which is obtained from a conventional moire approach. A discussion on obtaining density information from the fringe pattern is also included.