Showing papers on "Moiré pattern published in 1976"

Strain analysis by one-beam laser speckle interferometry. 2: Multiaperture method.

Abstract: A detailed analysis of using multiple apertures to record laser speckles for strain analysis is presented. It is shown that the basic fringe-forming mechanism is no different from that of single-aperture recording, except that at the Fourier filtering stage the diffracted light energy is concentrated at the spatial frequencies admitted by the apertures. As a result, better isothetic fringes at higher frequencies are obtainable. The concept of moire is not utilized in the analysis. Indeed, it is shown that some of the observed phenomena cannot be explained using the moire concept. Multiaperture arrangements studied include two, three, and four apertures.

Laser speckle interferometry for plate bending problems.

Abstract: A method is proposed whereby the spatial speckles created in front of a plate with an optically rough surface when illuminated by a coherent laser beam is used to generate slope contour fringes. This is done by photographing the speckles contained in a parallel plane in front of the plate before and after deformation via double exposure. The resulting speckle interferogram is then optically Fourier transformed to yield the fringe pattern of slope contours. It is shown that the method is analogous to the Ligtenberg reflection moire method with a grating of continuously variable pitch and orientation. The method can be applied to plates made of almost any material.

A holographic-moiré technique to obtain separate patterns for components of displacement

Abstract: A symmetrical double-beam illumination is used in conjunction with a fictitious fringe system to obtain a moire pattern which represents the projection of the displacement vector into a single plane. The fictitious system of fringes is generated by a rotation of the photographic plate. This additional degree of freedom makes it possible to optically superimpose holograms, to apply spatial filtering techniques, and to control fringe localization. The method is applicable for displacement determination throughout the entire holographic range. A disk subjected to diametral compression is used to demonstrate that displacements and strains on the order of magnitude of those found in real engineering problems can be determined very accurately.

Halftone method with edge enhancement and Moiré suppression

Abstract: A method has been found for separating control of high and low spatial frequencies in a continuous-tone image during conversion to a halftone (binary) form for printing. This method allows edge enhancement, while simultaneously limiting creation of spurious Moire patterns due to image frequencies near the halftone screen frequency. Macroscopic grey scale control is achieved by determination of average grey level by low-pass filtering of the image. This information is used to set halftone dot size. Microscopic or detail contrast is controlled as in the ordinary halftone process, that is, by screen shape and screen modulation, which therefore affects partial dot structure. In the absence of detail, the results of this process match ordinary halftones. In detailed areas, however, the low-frequency control limits generation of spurious low-frequency Moire patterns, while free choice of screen modulation allows enhancement of edges up to the common limit where image noise becomes dominant.

Sandwich hologram interferometry. 3:Contouring.

Abstract: This new holographic contouring method produces interference fringes formed by illumination shearing during recording in combination with observation shearing during reconstruction. One holographic plate is exposed after which the point source of light illuminating the object is displaced laterally to produce a shear of the illumination beam. A second exposure is made on another plate, whereupon the two plates are processed and bonded together with their emulsions outward. When this sandwich hologram is reconstructed, contouring fringes are formed that represent the intersection of the object by a set of interference surfaces. A tilt of the sandwich hologram produces a shear of the observation that results in a corresponding rotation of the intersecting surfaces of up to 180° in every direction of the three-dimensional reconstructed object space. Thus, sign, direction, and magnitude of object slant can be evaluated by measuring the analogous sandwich tilt. The possibility of studying cross sections in any random orientation from a single sandwich hologram could be very useful for measuring and demonstration purposes.

Geometric parameters of moiré fringes

Abstract: The incoherent illumination moire phenomenon between a periodic and a quasiperiodic structure is examined by the use of the spatial frequency analysis of the resultant transmittance. This approach has an advantage in that it completely explains all the fringe parameters without considering the averaging effect of the detecting system. We obtain the basic equation of the original moire pattern due to the beat phenomenon exclusively, without any external factors like, for example, the influence of the observing system. From the basic equation, we derive the fringe profile equation representing the moire fringe shape and the fringe equation expressing the spacing, the orientation, and the local displacement of the moire fringes. The fringe profile equation is experimentally verified. The sensitivity enhancement by the fringe multiplication method is readily interpreted by the use of the derived fringe equation.

Moiré Profile Prediction by Using Fourier Series Formalism

Abstract: One of the theories of moire pattern prediction uses the technique of partial sum extraction in the Fourier series expansion of the transmittance distribution which is obtained as the product of composite gratings transmittances. Based on this technique a simple method for the profile prediction of moire fringes produced by the interaction of two binary type gratings is developed. The theoretical model adopted easily clarifies the interdependence between the parameters of the interacting gratings and resulting specific fringe profiles. It also gives a simple and fast interpretation of the moire fringe shape encountered in a moire display using the fringe multiplication technique.

Moiré topography by means of a grating hologram.

Abstract: A new method of moire topography is described. This encompasses two processes: recording of the deformed grating projected onto a 3-D object, and posterior development of contour moire from the record. The development of contour moire is realized by projecting a linear grating onto a photographic print of the record of the deformed grating, using a photographic enlarger. By this new method, a technique for describing a 3-D object by means of the slope of facets composing the object is made possible.

Random quasiperiodic halftone process

Abstract: Images that have been halftoned with a periodic screen may suffer from several kinds of defects, including Moire patterns which result when the original image contains periodic structures with periods near that of the screen. A random quasiperiodic halftone process is proposed to eliminate these defects. The process can be implemented either as a contact screen or electronically. The statistical mean and covariance of the spectrum of the halftone image are derived. Experimental work confirms the results of the analysis and shows the effectiveness of the random quasiperiodic process in eliminating the defects found in periodically halftoned images.

Visual modeling of laser Doppler anemometer signals by moiré fringes

Abstract: This report describes the employment of moire patterns to model visually interference phenomena in general and laser Doppler anemometer signals in particular. The modeling includes signals created in dual beam and reference beam anemometers by both single particles and particle pairs. The considerations are extended to visual modeling of multiparticle signals and the decay of signal quality in the presence of many particles. The fringe model of the laser Doppler anemometer is also considered, and moire patterns are employed to demonstrate the interference fringes in the crossover region of two intersecting laser beams. Gaussian beam properties are taken into account to allow the effects of improperly designed optical systems to be studied. Instructions for using computer generated transparencies to produce the different moire patterns are provided to allow the reader to study in detail the various interference phenomena described.

Correlation and information processing using speckles

Abstract: In all methods of photographic optical processing presented here, the carrier frequency is derived from an intensity speckle pattern, the advantage of which is to spread out information in the Fourier plane. Furthermore, they are based on the fact that the Fourier spectrum of two laterally translated speckle patterns displays Young’s fringes, the visibility of which represents the correlation of the speckle patterns. The problems of comparing intensities of two transparencies, multiplexing optical signals, and detecting small translations of a diffuse object will be successively considered. The optical process of comparing two transparencies A and B is the following. The two signals are modulated by the same speckle pattern and recorded successively on a photographic plate which is laterally translated between the exposures. After processing, H displays Young’s fringes at infinity. If the intensity distributions of A and B are not identical, the two corresponding speckle images are not completely correlated and therefore the fringe visibility is not maximum. The difference A–B can easily be obtained by filtering the minima of the fringes. For multiplexing operations, each of the signals to be stored is modulated by a speckle pattern and recorded at least twice on a photographic plate which is laterally translated between successive exposures. The amount of translation given to the plate is different for each of the signals. The spectrum of the photographic record has as many fringe systems as the signals. By filtering the maxima of a particular fringe system, the corresponding signal is reconstructed and the others are removed. Small translations of a diffuse object can be detected by illuminating the object with a speckle pattern and recording its image before and after the motion in the same way as above. The decorrelation of the two recorded speckle patterns is only due to the motion of the object which can thus be revealed by the visibility of the corresponding Young’s fringes. With such a method it is not possible to detect the direction of the translation suffered by the object. We suggest therefore an interferometer consisting of a Michelson interferometer in which both mirrors are replaced by two scattering surfaces O1 and O2 shifted longitudinally. The interferometer is illuminated by a parallel beam of laser light, and a photographic plate twice records (before and after the translation of one of the surfaces) irradiances lying in the Fourier plane of O1 and O2. After processing, H exhibits Moire fringes. These fringes are rectilinear if the translation suffered by the object is lateral and they are circular if the translation is longitudinal. The sensitivity of the interferometer depends only on the geometrical characteristics of the recording setup.

System for measuring and recording three dimensional configuration of object

Abstract: An encoder for converting a distance to an electric signal is connected with a computer to put a distance signal into the computer. Th encoder is put on a moire pattern obtained by interference between a standard grating and a deformed image of the standard grating deformed by the surface of a three dimensional object. The distance between moire fringes is put into the computer. The computer is supplied with a signal indicting whether the surface is convex or concave. The depth or height of a point on the moire pattern is calculated by the computer, and a contour of the three dimensional object is obtained. An X-Y recorder is connected with the computer to draw the contour of the object.

Real time measurement of very small transverse displacements of diffuse objects by random moiré. 1: Theory.

Abstract: Illumination of a random diffuse object by a fringe pattern produced by two symmetrical laser beams leads to phenomena that are usually studied with speckle interferometric techniques. We first show that these phenomena are analogous to moire. We then propose to apply this method for measuring the transverse vibrations of a diffuse surface in real time with interferometric sensitivity. Further, we show that the discussed system can work as a linear displacement transducer; the theoretical aspects of this so-called random moire transducer are investigated.

Slotted Apertures for Multiplying Grating Frequencies and Sharpening Fringe Patterns in Moire Photography

Abstract: Slotted aperture filter masks can be used to "tune" a photographic system to an integral multiple of a fundamental grating frequency. Such a procedure can be used in moire photography for multiplying sensitivity and for obtaining improved rendering of fringe patterns. Depth of field is increased, and a camera lens of poorer quality than is normally required for high resolution photography of moire gratings is adequate. Also gained is flexibility in optical data processing of moire photographs and in the use of two-dimensional grid and dot specimen arrays.

Holographic interferometry: compensation for rigid body motion.

Abstract: A technique producing an optical effect that is unaffected by rigid body motion is described. It is shown that the resulting interference fringe pattern is proportional to the change in model thickness, which for the case of plane stress is proportional to the sum of the principal stresses, and thus provides a full field isopachic fringe pattern. This method combines holographic interferometry and the moire effect. Double exposure holograms are made from both sides of a model before and after deformation. The two sets of holographic interferometric fringes thus obtained are treated as random grids, and the superposition of these two grids produces a moire effect that is free from the effects due to rigid body motion. The theory is experimentally confirmed with several examples, and limitations of the techniques are also discussed.

On the Method of Measuring Strain by Use of Diffraction at Mis-matched Moire Fringes

Abstract: In the usual moire method and the multiplication method, the sensitivity to deformation is not sufficient at present. Then, how to increase the sensitivity is studied. In the measurement of a usual sheet metal specimen, moire fringes with large mis-match is photographed on a film at first. Such fringes are equivalent to grating lines on the specimen, as thy displace corresponding to strain, and they can be amplified by applying the multiplication method to that film. When strain is expressed by two beam interference fringes, one of the beams is tilted by a small amount, then fine fringes are obtained. They are equivalent to moire fringes with large mis-match and the multiplication method can be applied. Applications in various conditions have been shown. Consequently, concerning a sheet metal, measurement of elastic-plastic strain (eb 0.001) becomes possible and in the special condition that the surface of a specimen has been finished mirror-like, elastic strain (eb 0.001) can be measured.

Weighing system with a moire optoelectronic transducer

Abstract: A weighing scale includes an optoelectronic system having apparatus for generating a moire fringe pattern and for detecting the moving of the pattern in accordance with scale tare deflection. One of a pair of ruled parallel grating sets is fixed with respect to the scale frame while the other is mounted for movement with the tare and close to and in a plane parallel with the fixed set. When a beam of light is projected through both grating sets in a transmissive mode towards a photodetector array the grating interference modulates the light beam into parallel moire fringes. Upon placement of a load on the scale, deflection of the tare results in movement of successive fringes across the photodetectors resulting in the sequential generation of weight indicative signals. The photodetectors are arrayed on and secured to a movable carrier which is adjustably positionable so that the effective spacings between the photodetectors corresponds to the spacings and proper phase angle required between said fringes.

A Moire Technique for Rapid Alignment, Focus, and Scaling of Imagery

Abstract: A rapid procedure is outlined that permits accurate alignment, focus, and scaling of imagery at two separated locations in an optical system. The moire pattern formed by pairs of identical gratings permits the full image pupil to be observed and adjusted. Alignment and scaling can be adjusted to an accuracy of the order of the grating's fundamental spacing.

A new application of isodensitometry in the moiré technique: This paper deals with the production of moiré fringes of fractional order, in order to improve the determination of the displacements between the moiré fringes of full or half order. This can be accomplished by application of the Sabattier effect

Abstract: Some years ago, isodensitometry was introduced into the moire technique in order to improve the sensitivity of this method. The application of the Sabattier effect gives now a new possibility to produce more lines of equal displacement. The obtained experimental data are the additional lines and the moire fringes of full or half order. This method is applied on a beam under pure bending and a circular ring loaded diametrically. It will be shown in this paper that the calibration of the displacement curves can be accomplished by a calibration bar or by ‘self calibration’ which will be done on the same model to be studied. Further, the photographic process can be influenced in a way that different lines of equal displacement can be produced for a given system of moire fringes. The additional lines are shifted here by choosing different times of exposure.