Showing papers on "Moiré pattern published in 1986"

Fractional moiré strain analysis using digital imaging techniques

Abstract: A new method for analyzing low-order moirefringe patterns of displacement fields is presented. This method adapts the techniques of half-fringe photoelasticity to moire and extracts continuous displacement information in the regions between integral fringes. The effectiveness of the technique is illustrated with three examples: a uniform uniaxial field, a tapered specimen in tension, and a disk in diametral compression.

Optical Sensing Techniques For 3-D Machine Vision

Abstract: Active optical vision systems rely on the analysis of images on an object illuminated by known structured optical patterns. Approaches differ by the choice of projected pattern (dots, stripes, grids, etc.) and the type of analysis used to extract shape information. A novel approach is reviewed, whereby a sinusoidal grating structure is projected onto an object, and the resulting deformed grating images are detected by a solid state array camera and analyzed by a microcomputer using digital interferometric phase-measuring algorithms. Large arrays of data points can be acquired and processed in parallel with very high accuracy and speed. Two systems are described, one that yields range measurement from a single point of view over a two-dimensional array of points, and a second that permits full 360 degree measurement. The case of parallel laser light illumination, as well as that of general white light diverging illumination are described. Experimental results and performance analysis are presented.

Semiconductor wafer and technical flat planeness testing interferometer

Abstract: An interferometer for planeness testing of technical surfaces is described and real-time interferometric evaluations of 10-cm (4-in.) diam silicon wafers were carried out. The interferometer, using the principles of phase sampling interferometry, is a combination of an oblique incidence Fizeau interferometer with a moire interferometer. The interference pattern is a pure two-beam pattern. To get low frequency moire fringes the live interference fringes must have about the same spatial frequency as the moving grating used for reference phase stepping. Assuming that an automatic alignment device is included in the design, up to three wafers per minute could be measured and classified.

Apparatus and method for measuring strain

Abstract: Two electro-moire systems provide strain values directly. By shifting a reference grating relative to a specimen grating along the direction perpendicular to the grating line the resulting moire fringes sweep across the field. In one method two photo-detectors are used to record the cyclic intensity signals. The phase difference between the two signals is used in a special formula for the calculation of strain. In the other method only one photo-detector is used, and the frequency information of the cyclic signal is used in another formula for the calculation of strain. The electrical signals from photo-detectors are converted into digital signals and processed using a digital computer through FFT (Fast Fourier Transformer) to eliminate noise. Strain-values as small as one microstrain can be measured.

Diffraction effects and special advantages in electronic heterodyne moire deflectometry.

Abstract: Effects of diffraction on the performance of electronic heterodyne readout of moire fringes are investigated. The sensitivity, accuracy, and resolution of the system are calculated, and it is shown that these features are significantly improved compared with the conventional intensity moire readout technique. The sensitivity of the system can be tripled without changing the distance between gratings. The system was evaluated experimentally by measuring the refractive-index derivatives of a weak phase object consisting of a large KD(asterisk)P crystal. Effects of nonlinear fringe modulation were studied both theoretically and experimentally. It is shown that in this case the electronic phase is not linearly related to the fringe shift, and calibration of the system is necessary.

Generation of the derivative of out-of-plane displacements using conjugate shear and moire interferometry

Abstract: A new method of obtaining the spatial derivative of out-of-plane displacements in a moire interferometry setup is presented. It is based on the recently developed concept of conjugate lateral shear interferometry applied to the diffracted beams generated by the specimen grating. The moire fringes produced by overlapping the two interferograms give the desired information using either the uniform or finite fringe detection method. Simulation experiments using specially prepared transmission-type distorted gratings are given to verify the principles.

Shadow speckle metrology.

Abstract: A computer-based method is developed to measure the out-of-plane motion of a surface which might arise, for example, from the deflection of a structural component or as a result of contouring a 3-D surface with respect to a reference plane. Artificial speckles are projected onto the test surface using an ordinary 35-mm projector equipped with a clear glass slide sprayed with black paint. Speckle patterns are digitally recorded as the surface changes its shape, and the apparent in-plane movements of the projected speckle are computed over the full field by numerically correlating small subsets extracted from each pattern. These shifts are related to the deflection of the surface with respect to its initial location.

Apparatus for detecting position of a rotating element using a two-grating moire pattern

Abstract: The position and motion of a rotating element is determined by using a two-grating moire pattern. Movement of the element causes a projected scale image representing radiation reflected from the element to shift relative to a fixed grating thereby generating moire fringes. The accumulated fringes serve to measure absolute position and motion of the element.

Automatic Alignment Technique For X-Ray Lithography Using Moire Signals In Reflection

Abstract: This paper deals with an automatic and precise alignment technique between a wafer and a mask in X-ray lithography, using two pairs of moire gratings, one on the wafer and another on the mask in each pair. The moire signals are detected in zeroth order beams in reflection. These two moire optical signals are converted to digital signals by A/D converters, and then the digital signals input to a microcomputer. The microcomputer controls the alignment of X stage through a stepping motor, using the difference between these two digitalized moire signals. The precision of the present system is resulted from the pitch of the gratings as an error signal, the relative displacement between the gratings per step of the stepping motor, the resolution of the A/D converters and S/N of the system. The response time of alignment is determined by the response time of the feed back loop. Experiments were carried out by using a grating of 25 μm pitch. The minimum displacement corresponding to one step of the stepping motor was 14 nm. The resolution of the A/D converters was 12 bits and the response time was 1.2 s. With these parameters a control reproducibility better than 50 nm was obtained. Improvement in the control precision by an order of magnitude is expected by using a pair of quadruple gratings, a combined unit of 4 matched photodiode and PZT controllers. The design considerations of a suitable X-ray lithographic mask alignment system are given.

Two-way gratings for strain analysis using moire and moire interferometry techniques

Abstract: A specimen grating design with two linear rulings with axes 60° apart is proposed for strain analysis using moire and moire interferometry techniques. The grating facilitates derivation of three displacement patterns using one analyzer grating or a single pair of illuminating beams. Advantages relating to the grating technology and experimental setup) simplification are discussed. The results of experiments are presented.

Diffraction effects in moiré deflectometry: reply to comment

Abstract: We show that placing the object in front of the gratings, as in moire deflectometry, rather than between them (as inTalbot interferometry) makes moire deflectometry capable of providing fully quantitative analyses of arbitrary objects. In addition, we review some differences in the philosophical aspects between the two methods (viz., ray approach versus wave interference).

Line Thinning Algorithm For Nearly Straight Moire Fringes

Abstract: A simple noniterative line-thinning algorithm, which is suitable for nearly straight interferometric and moire fringes, is proposed. The algorithm was found to be fast and accurate. An error analysis is presented.

The Phase Shift Method Applied to Reflection Moiré Pattern

Abstract: The moire methods are well-known for analyzing the contour and the deformation of an object ∣1∣ The moire fringes do not lead only to the wanted values but represent also a qualitative overview to the nature of the deformation However their automatic evaluation is relatively difficult by digital picture processing: First the fringes are generally of no regular structure and moreover their order cannot be determined without additional information of the related experiment Usually the input of the order must be done interactively

Optical measuring method for fine displacement

Abstract: PURPOSE:To measure displacement with high accuracy by enlarging a grating pattern installed on an object member as moire fringes and making it correspond to an image sensor with proper size. CONSTITUTION:The 1st and the 2nd equal-interval grating patterns 3 and 4 which generate a noise to moire fringes are suppressed by a spatial frequency filter. Then, when the 1st equal-interval grating pattern 3 moves together with the object member 1, the moire fringes move and shift in phase while holding an amplitude and a period constant for the object member 1. Then, the sensor 6 scans the moire fringes at every specific time and many photoelectric conversion parts arranged from one end side to the other end side convert the intensity of light at a corresponding part of the moire fringes into an electric signal successively at intervals of a clock pulse and outputs the intensity distribution as a serial signal to a data collecting and processing part C. Then, the processing part C performs arithmetic based upon a function showing the intensity distribution of the light and separates the phase signal of the moire fringes from an unnecessary signal. Thus, displacement is measured with extremely high accuracy.

Measuring device of dynamic displacement pattern by moire method

Abstract: PURPOSE:To enable indication of the displacement mode at each moment by setting up a standard grid movably and observing a momentary Moire interference fringes at a known slight displacement time continuously and recording in an image memory and thus by deciding the absolute order of interference based on a group of data of interference fringes consisting of each two values obtained by five segments CONSTITUTION:A three-dimensional surface condition at a standard time t1 is recorded as an image of a Moire interference fringe under a condition of the location of the first standard grid z=0 A surface condition at time (t1+DELTAt1) which can be considered as nearly the same as a surface condition at time t1 is recorded as an image of the secondary Moire interference fringe under a condition of the location of the standard grid z=+delta Bright Moire fringes are formed at the locations indicated by a set of blackround mark and white round mark approaching each other By selecting an amount of shift delta of the standard grid, a correspondence between a black round mark and a white round mark at the same fringe order before and after shifting the standard grid on an arbitrary cross-section, eg X-X axis is practiced by a simple calculation of comparison and also the shifting direction of the fringe of the same order cam be judged at the same time

On Randomization Techniques for the Suppression of Unwanted Moiré Patterns in Images Generated by a Scanning System with a Periodic Amplitude Defect

Abstract: Arrays of motifs, such as are found in half-tone images, may exhibit moire fringes when they are generated by a scanning system which introduces a periodic amplitude defect. It is shown that the fringes may be eliminated in the continued presence of the defect by a one-dimensional randomization of the positions of the motifs. The necessary Fourier theory is presented, and elimination of a particular moire pattern is demonstrated for four probability-density functions.

On the Elimination of Unwanted Moiré Patterns in Two-dimensional Images Generated by a Defective Optical Scanning Process

Abstract: Arrays of motifs, such as are found in half-tone images, may exhibit moire fringes when they are generated by a defective scanning system. These moire effects are analysed by Fourier methods, which suggest that alterations to the form of the motif can eliminate the fringes. Three such techniques are described and demonstrated.

Real-Time Moire Holography

Abstract: Interferometric techniques including hologrametry, both classical and electronic, present high sensitivity making difficult its practical use in real-time. The introduction of the differencial concept as moire evaluation techniques permits to use with advantage an arbitrary reference pattern within the correlation range. The carrier spatial spectrum can be directly the interferogram fringe pattern instead of the original interference pattern of wavelength dimensional scale. A moire techniques is in itself an optical processing method reducing evaluation time which is advantageous when real-time response is desired from hybrid metrological systems. The moire evaluation is performed via a dynamical digital memory that executes arithmetic operations on two frames temporally in sequence, at TV rate. These characteristics of the moire evaluation techniques can be implemented on a real-time holographic (or speckle based) hybrid system with great practical advantage for dynamical studies.

Scanning-Moire Method

Abstract: In the conventional moire method, a moire pattern appears as interference between a model grating and a master grating. However, a moire pattern also appears when the model grating lines are sampled by a TV camera. If the scanning lines of the TV camera are regarded as the master grating lines, the geometric relations among the moire fringe lines, the model grating lines and the scanning lines are obtained in the same way as the conventional moire method. When the TV camera scanning lines are thinned out, this method corresponds to a mismatch method in the conventional moire method. We call this the “Scanning-moire Method” (Morimoto 1984).

Real-time moire interferometry

Abstract: A high-frequency moire interferometric technique to continuously monitor in-plane surface displacement is presented. This technique employs fiber optics and a thermoplastic device to holographically record the initial carrier pattern produced on the surface of a test specimen. The carrier is modulated as the specimen deforms and moire fringes, indicative of in-plane displacement, are observed in real time. Unwanted holointerferometric patterns are eliminated by appropriately adjusting the polarization of the reference wavefront. This paper describes a demonstration of the real-time technique as applied to the study of deformations in a notched beam subjected to three-point loading.

The Influences Of Several Parameters Of Grating Sensor On The Modulation Depth Of Moire Signal Under Incoherent Illumination

Abstract: In this paper, the theoretical expressions of modulation depth of moire signal under the incoherent illumination are derived. It follows as a concequence that, the modulation depth of moire signal is in relation to the following factors: the geometric share and size of light source, the diffraction effect of grating,which relates to the line numbers of grating, the line and space ratio, the gap of grating pairs, the geometric shape and size of receiving window, etc. In addition, the influences of grating fair on the period and inclination of moire fringes under the noncollimated illumination are discussed, and the comparisons of the changes of modulation depth of moire signals and the noncollimated illumination with that under the collimated illumination are made. Finally, some exoeriment results are given to verify the theoretical expressions. This parer is useful to the actual design of grating sensors.© (1986) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Moire 간섭계에 의한 곡면 함수 결정

Abstract: A function of the curved surface is determined by Moire Interferometry which can be regarded as a form of holographic interferometry, especially for testing optics to be used at long wave length. A rouchi ruller, which has 10 line pairs/㎜, is placed on the curved surface and illuminated with collimated light beam, He-Ne laser or white light can be used as the light source. Moire fringe, which means the contour map of the surface, is photographed at angle 30° and 37°.

Moire Interferometry With Chromatic 1Ight

Abstract: Experimental observations and measurements are essential sources of information for the correct development of mathematical models of real materials. Moire interferometry offers high sensitivity in full-field measurements of the in-plane displacements on the surface of a specimen. Although it is a powerful method in experimental stress analysis, it has some shortcomings. One is that existing systems require highly coherent light. The only sufficient source of light for this application is a long cavity laser, which is relatively expensive and at best quite cumbersome. Another shortcoming is that measurements must be performed in a vibration-free environment, such as that found on a holographic table. These requirements limit the use of existing moire interferometers to a holographic laboratory. In this publication a modified concept of compensation is developed that permits the use of a chromatic source of light in a compact moire system. The compensator provides order in the angles of incident light for every separate wavelength, so that the virtual reference gratings created by each wavelength in a continuous spectrum are identical in frequency and spatial position. The result is a virtual reference grating that behaves exactly like one created in coherent light. With this development the use of a laser diode, which is a noncoherent light source of tiny dimensions, becomes practical. The special configuration of the optics that create the virtual grating allows its synchronization with the specimen grating and leads to the design of an interferometer that is relatively insensitive to the vibrations found in a mechanical testing laboratory. Sensitivity to relative motion is analyzed theoretically. This development provides the opportunity to apply moire interferometry to solid mechanics problems that cannot be studied in an optics laboratory. Experimental verification of the optical concepts is provided.

Strain measurement using moire by shifting model grating and its image processing.

Abstract: Strain measurement using moire by shifting a model grating is a useful method for full field measurement of strain distribution. We can obtain moire fringes which show a strain distribution by means of only superposing two copies of a deformed model grating shifted one with respect to the other. However, its disadvantage in applications has been the photographic process which involves time-consuming development of the photographic film. Also it has been difficult to shift a model grating exactly.This paper presents a method using a TV camera and a digital image processor which employs a personal computer whereby high speed and exact measurements of strain distribution are possible. Using this system, we present a method for strain analysis by means of superposing two copies of a deformed cross grating. In addition, the principle and limitation of this method are described and some applications are shown.

Digital Image Processing in Experimental Mechanics

Abstract: In several methods used in experimental mechanics an image is obtained as a result of an experiment. There are displacement measurement methods based on the phenomenon of light interference, like various moire techniques, holographic interferometry or speckle. In these methods a pattern of fringes appears on the specimen surface or in the camera. The fringes are contour lines of some specific deformation characteristics like in-plane displacements (e. g. intrinsic moire) or off-plane displacements (e. g. holographic interferometry). In the reflexion moire technique the contour map of the derivatives of the off-plane displacements is obtained. In all these methods usually specimen made of real structural materials are tested. Thus the fringe pattern frequently appears on the inhomogenously bright background and image filtration would be much-desired. There are also stress measurement methods based on the phenomenon of birefringence — various versions of photoelasticity. In these methods a pattern of fringes is also obtained. It is usually a contour map of the differences of principal stresses. Models made of special transparent materials are frequently tested instead of real structures or structural members. Thus, it is possible to assure homogenous background illumination and in many cases image filtration is not necessary. Further analysis is usually more complicated than in the previously mentioned methods, however, for fringe patterns in general the accurate determination of fringe axes is very important. The brittle lacquer technique can also be regarded as one giving visual information, although of a different type. The analysis of these images consists in the determination of densely cracked areas, thus being similar to texture analysis.

Optical error detector

Abstract: PURPOSE:To detect an error signal at a high speed with a low cost by shifting relatively slightly light with a wave surface dividing means arranged in the optical path of form interference fringes and providing a space modulating means in the space of these interference fringes to form a moire and detecting the moire with a photoelectric detecting means. CONSTITUTION:The light incident from a beam splitter is shifted slightly by a diffraction grating 31, and interference fringes in the direction of an arrow are formed in accordance with shift. Though intervals of interference fringes are as shown in a figure (b) when an objective lens is focused on a recording medium, said intervals are made wider as shown in a figure (a) if the objective lens is too close, and they are made narrower as shown in a figure (c) if the objective lens is too distant. Spatial intensity modulation is given to the wave surface by a space mounting means 32 to form the moire. As a result, the moire of interference fringes and an amplitude grating 32 is projected on a photoelectric detecting means 33 as shown in figures (g), (h), and (h) correspondingly to states (b), (a), and (c) of interference fringes. That is, the moire is horizontal in the in-focus state and is rotated counterclockwise if the distance between the objective lens and the recording medium is too short and is rotated clockwise if this distance is too long. Thus, the device is operated at a high speed with a low cost.