Showing papers on "Moiré pattern published in 1971"

Moiré Contour-Sum Contour-Difference, and Vibration Analysis of Arbitrary Objects

Abstract: A grid is projected onto an object and focused into a viewing camera which is offset from the angle of illumination. When the image is superimposed by double exposure or by filtering with the image of another object, contour-difference moire fringes appear. By the use of two offset projectors, measurements of contour sums or contour doubling may be accomplished. The properties, limitations, sensitivity, and procedures are analyzed and discussed. The method lends itself to image enhancement and is extended to the study of vibration analysis in real time. In the latter application, time-average moire fringes appear according to the zeroth-order Bessel function, which contains the modal amplitude function in its argument.

Moire Fringe Multiplication with a Nonsymmetrical Doubly Blazed Reference Grating

Abstract: A fine reference grating and coarse specimen grating in series are illuminated so that two dominant diffraction sequences take symmetrical paths in the intergrating space. In this optimum configuration, observation is on or near the axis of symmetry. Moire sensitivity is that of the reference grating and independent of the specimen grating. The contour map described by moire fringes depicts in-plane displacements uniquely, being insensitive to thickness and taper of gap between gratings. Moire fringes are nearly perfect two-beam interference fringes of superb contrast. Existing gratings blazed for single-order dominance can be used under conditions that produce the required double-order dominance. In experimental evaluations, coarse gratings of 200 rulings/cm were mated with various reference gratings up to 12,000 rulings/cm for fringe multiplications by factors up to 60.

Position measuring through the use of moire fringe multiplication

Abstract: A system for linear and angular measurements utilizing a form of moire fringe multiplication particularly in position measuring apparatus. The position of a table is determined by moire fringe multiplication with a coarse scale grating, an index member in juxtaposition with said scale grating, illumination, optical filtering, and observing means and means to indicate relative position of the table from changes of the said moire fringes.

Projected Interference Fringes in Holographic Interferometry

Abstract: Large surface movements can be conveniently measured with holographic interferometry by using a moire technique. A comparison grid is produced at the test surface by projecting onto it parallel interference fringes. Reconstruction of the surface shows a moire fringe pattern formed by the product of the holographically generated fringes and the projected grid. If the spatial frequency of the grid is within a certain range, the moire fringes are appreciably coarser than either the holographic fringes or the grid. A measurement of the moire fringe spacing and orientation allows determination of the form of the holographic fringes and, hence, surface deformations can be calculated. This method can be used to extend the range of holographic interferometry by at least an order of magnitude.

Apparatus for the relative positioning of two objects by means of a beam of radiation

Abstract: A displacement sensing device using superimposed dissimilar circular moire patterns. Groups of circles in one of the patterns alternately have a greater and a smaller pitch than those of the other pattern.

Shear and rotation moiré patterns obtained by spatial filtering of diffraction patterns

Abstract: Two superposed gratings can produce many other moire patterns in addition to the pattern commonly observed. They correspond to different forms of the indicial equation employed in parametric descriptions of moire phenomena. An analysis of the Fraunhofer diffraction pattern of two superposed gratings by the methods of Fourier optics shows that the different moire patterns can be separately observed by spatial-filtering techniques. Examples of additive and subtractive moire patterns obtained individually over the whole field of two superposed gratings are presented. This method is combined with a previously developed method for obtaining partial derivatives from deformed gratings as a direct moire pattern (rather than as moire of moire) to determine experimentally the whole field of cartesian shears and rigid rotations. The method is applied to a circular ring subjected to diametral compression.

Separation of Additive and Subtractive Moiré Patterns

Abstract: : A spatial filtering technique for observing several different moire patterns in the same image of superposed gratings and for separating them at will, is described. In addition to the commonly observed moire pattern, and the equivalent patterns obtained by fringe sharpening or multiplication, patterns can be observed that correspond to different forms of the indicial equation describing the parametric properties of moire phenomena. In particular both the additive and subtractive moires are obtained individually, over the field. This method, combined with a previously developed method for obtaining spatial partial derivatives from deformed gratings (as a direct moire rather than moire of moire), permits the experimental determination of the whole field of Cartesian shears and rigid rotations. The method is applied to a circular ring subjected to diametral compression. (Author)

Diffractometers for analysis of moire grid photographs

Abstract: Two diffractometers (linear spectrometers) are described, for spatial filtering of the moire fringes formed between a reference grid and a photographic copy of a strained specimen grid. Simple adjustments of the diffractometers enable initial apparent strains to be superimposed on the strained grid, and the location of the fringe pattern to be changed by known displacements of the reference grid. Both of these adjustments considerably increase the precision of the analysis.