Showing papers on "Moiré pattern published in 1991"

Phase-shifting and logical moiré

Abstract: Logical moire uses the logical operators (and, or, xor) to generate moire fringes from two 1-bit binary gratings within the computer. Phase shifting of these moire fringes is easily achieved by translating the computer-generated reference gratings in steps of one picture element (pixel). We analyze the phase-shift expressions based on three, four, and five shifted patterns by using logical moire patterns. Both simulated and real moire demonstrations are shown.

Method and apparatus for generating digital, angled halftone screens using pixel candidate lists and screen angle correction to prevent moire patterns

Abstract: In digital halftone color printing, low spatial frequency patterning can occur due to the interaction of the overlapping angled halftone screens. The moire patterning can be minimized by automatically constructing digital haltone angled screens from screen tiles which are designed to minimize the low spatial frequency patterning. The screen tile boundaries are generated in response to selected integers which, together with the number of screen dots along a side of the tile in the direction of screen pitch measurement, determine the a limit on the magnitude of screen angle error. Based upon these values, each pixel within the tile boundary is associated with one of the screen dots of the image. Typically, the pixels are added to potential candidate lists from which they can be selected for inclusion within a particular screen dot.

Digital phase-step interferometry: a simplified approach

Abstract: In a two-beam optical interferometer, introduction of a tilt between the beams produces a set of nominally straight and equispaced fringes. The phase information represented by deformations in this fringe pattern can then be calculated at each point in a video image by using intensity samples from a group of neighbouring positions. Deformations of a linear grid pattern as encountered in moire methods can be analysed in the same manner. This procedure of spatial phase stepping has been implemented on a fast microcomputer, enabling quantitative results to be obtained in a particularly simple qay.

Logical moiré and its application

Abstract: Moire fringes are generated using logical operations on one-bit binary gratings. A traditional moire effect is observed using the AND operator while an enhanced moire is observed using the XOR operation. Since the reference grating in this type of moire is computer generated, fringe shifting to increase sensitivity can be easily accomplished. An alternative scheme, called the average step method, resulting from the logical operations with values averaged over the pitch of the reference grating, can also be used for fractional fringe-order determination. These principles with some applications are demonstrated in this paper.

3-D Contouring of Diffuse Objects by Talbot-projected Fringes

Abstract: Talbot fringe projection, a moire technique, is applied to three-dimensional contouring of diffuse targets for absolute shape measurement. The basic system relies on depth coding the test target surface by projecting the Talbot image of a linear grating. A second grating, similar to that used for the Talbot image, is employed to obtain the moire fringes. These fringes represent surface contours of equal depth. Using a phase measurement technique and digital image processing algorithms, the surface shape information is obtained from the contour maps. Experimental results, merits and limitations of the system are discussed.

Three-dimensional shape analysis by use of a projected grating image

Abstract: A grating pattern projected onto an object deforms in accordance with the three-dimensional shape of the surface. This deformed image can be analyzed by detecting the shifted phase of the original grating pattern with the aid of Fourier transformation. The profile of the object is expressed in moire contours as well as in a wire-frame model for the intuitive understanding of the shape. In this procedure, multiplication and visibility enhancement of moiré fringes are performed with an improved result.

Diffraction theory of optical interference Moiré and a device for production of variable virtual reference gratings : a Moiré microscope

Abstract: Optical interference moire methods are analyzed using Fraunhoffer diffraction theory to relate general large surface deformations to the fringes observed. This analysis determines the Almansl strain in the current configuration from the gradients of the fringe number function. The analysis shows the advantages of an experimental scheme that allows the virtual reference grating to be varied. The ability to vary the virtual reference grating results in a larger dynamic range and the ability to maintain a fringe spacing for maximum accuracy. A moire microscope has been constructed which has this ability. Digital image processing coupled with optical filtering and phase control is used to enhance the accuracy of the fringe measurements. The variable virtual-reference-grating capability is demonstrated by using it to highlight several aspects of the deformation field near a crack tip in a single crystal of iron-silicon.

Three-channel phase stepped system for moire interferometry

Abstract: A use of the multichannel phase stepped interferometry approach to automated fringe pattern analysis in moire interferometry is described. Polarization optics is used to implement the appropriate phase shifts in each channel. The channels are created by inserting a diffraction grating at the output of the interferometer. It is possible to work at one frame rate. This system enables studies in unstable environmental conditions and studies of time dependent events.

Sampled-grating and crossed-grating models of moire patterns from digital imaging

Abstract: Traditional "crossed-grating" moire as well as the newer "sampled-grating" (scanning) moire have proved to be effective methods of shape measurement. There is speculation that the moire patterns of a sampled grating, which are due to aliasing, can be modeled with crossed gratings. We compare the two by writing notationally consistent models of each and show that while crossed gratings can correctly predict the frequencies of a sampled grating, they cannot correctly predict the amplitudes. Our sampled-grating model is a new formulation that accounts for multiple stages of sampling and transmission. We show how neglecting multiple stages can lead to mistakes in moire analysis. We demonstrate our models with an experiment using a digital imaging system.

Contouring by Moire Interferometry

Abstract: A double-exposure moire-interferometry technique for topographic contour measurement of an arbitrarily curved object is presented. A curved surface coated with light-sensitive material is exposed twice in a volume of virtual gratings formed by the interference of two coherent light beams split from a laser. An adequate rotation of the curved surface relative to the virtual grating between the two exposures produces moire fringes which reveal topographic contour, or contour under some conditions, of the surface. The advantage of the present method in comparison with others is that it offers both reasonably good fringe quality and easily adjustable high sensitivity. The sensitivity of the technique is shown to be from the order of micrometer to that of millimeter depending on the frequency of the virtual grating and the amount of the relative rotation. This technique was successfully applied to the topographic contour measurement of a cylindrical shell with and without a diametrical point loading. The principle of this paper and some early results were presented at the SPIE conference held at Dearborn, MI on June 27–30, 1988 and appeared in its proceeding.1

Interferometer employing reference images

Abstract: An interferometer for measuring the degree of deformation of an object, wherein alignment detection is facilitated by producing a moire pattern. An image signal data for reference fringes is compounded with interference fringe data that has been used for alignment detection. In generating a moire pattern, the interference fringe image signal data is process using the reference fringe data as a filter, resulting in improvement in visibility of the moire pattern.

Method and apparatus for cancellation of Moire interference in color cathode ray tube displays

Abstract: Visible Moire interference is eliminated by alternately shifting the phase of the horizontal sync signal or video signals such that the phase of each video line, and hence the phase of the resulting Moire interference associated with that video line, is also alternately shifted. The phase of the Moire interferences are shifted such that persistence of vision in the human eye averages oppositely phased phosphor intensity variations occurring on alternating scan lines and/or vertical fields. When viewed by a user of the CRT, optical cancellation of the Moire interference patterns results.

Phase-stepped fractional moiré

Abstract: Two computer-aided methods have been developed for analysis of in-plane and out-of-plane surface displacements of structures under load. Both methods are whole-field techniques which combine phase-stepped geometric moire with video and computer technologies. With these methods, a displacement field of interest is determined by computer-processing phase stepped, geometric moire image data with fringe ordering done automatically within the software. The theory of the techniques is described and results of accuracy tests and application problems are given. It is shown that very good agreement is obtained between theory and experiment for in-plane strain determinations. For out-of-plane displacement determinations errors are only a few percent over the entire field of view. The application problems discussed are: (1) the measurement of composite column buckling, and (2) the determination of the shape of a slightly distorted, thin aluminum plate.

Determination of basic grids for subtractive moire patterns.

Abstract: The simple formula derived in the paper establishes a direct relationship between moire beat patterns and basic grids for minute displacements. The possibilities of finding a basic grid for a desired moire pattern are pointed out. The analysis is illustrated by several examples of Fresnel moire zone plate patterns and concentric equidistant circular moire patterns obtained by changes of scale and rotation. Possible advantages of the practical use of this element are outlined.

Color halftone screen utilizing preselected halftone dots placed at preselected distance and screen angles from center halftone dots

Abstract: A plurality of halftone screen density data structures utilizing halftone dot patterns which arrays the halftone dots at preselected screen angles and at a preselected distance to create a screened halftone separation. By arraying the halftone dots at preselected screen angles, the human eye is confused into perceiving continuous patterns and is less likely to perceive any Moire pattern than when using conventional methods. The data structures are created by arranging for every halftone dot in at least one halftone dot pattern of the data structures to be surrounded by a plurality of dots having their dot centers at preselected screen angles and at a preselected distance from the dot center of the center halftone dot. The halftone dots are superimposed over a plurality of bits arranged as a bit map which is representative of a pixel format utilized to produce screens wherein each bit in the bit map is assigned a value in a predetermined fashion. The values are utilized when binary representations of the density of an image for a sampled area are transformed to pixel format. Through this process, the halftone screen density data structures are utilized to transform binary representations of images to a pixel format which is utilized to reproduce a desired artwork.

Acquisition of 3-D data by focus sensing utilizing the moire effect of CCD cameras.

Abstract: A new technique was recently presented for finding the 3-D shape of diffusely reflecting object surfaces [ Appl. Opt.27, 4684– 4689 ( 1988)]. The technique is based on grid projection with small depth of focus, confocal observation, and focus sensing by evaluation of the local grating contrast. In this paper a modification of the measuring system is demonstrated that allows the use of a 2-D CCD array instead of a vidicon as the detector. In this modification the moire effect between the projected grid and the CCD array is utilized. Depth resolution is increased, and almost no unwanted moire terms arise in the detector output as long as the grating frequency is chosen above the Nyquist frequency of the array. The technique can be useful in robot vision.

Experimental Deformation Mechanics of Materials from their Near-Atomic-Resolution Defect Images

Abstract: A novel approach to quantitative interpretation of high-resolution electron microscopy images of defects in materials has been developed. The emphasis of this paper is on the methodology, which has been named Computational Fourier Transform Moire Analysis. The essential principle of this technique is to extract an accurate displacement field about a defect from its near-atomic-resolution picture using digital Fourier transformation procedures. From this data, the displacement gradient can be calculated which yields much information on the experimental deformation mechanics of the material under investigation. As a by-product, we produce the computational Moire pattern without the need of an external perfect reference lattice image normally associated with the interference phenomena. This method is illustrated using a bounding Frank partial dislocation for a Frank loop of the vacancy type. Results are presented on its strain field, Burgers vector and dislocation core shape and dimensions. Further mention will be made on the types of J-integral calculations that can result from this experimental study.

Off-axis mirror alignment

Abstract: An apparatus and method are disclosed for increasing the alignment accuracy for off-axis mirrors in which a circular phase grating (14) is illuminated from behind onto the mirror (12) to be aligned creating a reflected image of the grating. The circular grating is aligned with the reflected image produced by the mirror so as to create at interference pattern when observed along an observation axis (16) which is an approximate mirror optical axis. The generated or observed interference pattern is observed for the presence of moire lines and the mirror is translated or rotated about the observation axis until moire lines are minimized in, or disappear from, the observed interference pattern. The observation axis is then aligned with the optical axis of the mirror which can be recorded or marked. In further embodiments of the invention, the fringe pattern is transferred, using one or more lenses (28), to an image processing element such as a camera (18). The image can be digitized and transferred to a process controller which is coupled to position translators connected to the mirror under test. The process controller analyzes the observed fringes and moves the mirror until a desired minimum number of moire lines are present.

Holographic Moiré for strain fringe patterns

Abstract: New holographic-moire methods are presented to obtain strain fringe patterns. To introduce two carriers of different directions, two kinds of recording procedures are proposed: (1) triple-exposure recording by two object beams and one reference beam (all of these beams are identical in polarization state) and (2) double exposure or real-time recording by two orthogonally polarized object beams and two corresponding reference beams. In these recordings, the carriers are introduced by changing the directions of the reference beams and are modulated by deformation. Separated in-plane displacement derivative patterns are then obtained by optical processings. Both the principles and the experimental results are shown in the paper.

Computer moire deflectometry using the Talbot effect

Abstract: Computer moire deflectometry (CMD) using one grating, a TV camera, and a computer image processor (CIP) is developed. The TV camera is located in a Talbot plane, allowing the recording of a (disturbed) image of the grating with maximum contrast and without any other optical component. CIP is used for obtaining, processing, and automated interpretation of the fringes. Double-exposure, real-time, time-average, and phase-measuring methods of CMD for transparent and reflective objects are demonstrated.

Use of the moire effect to improve diameter measurements with charge coupled imagers

Abstract: An optoelectronic setup has been developed for in-situ measurements of thin glass cylinder diameters. The principle of the method is based on the in-line far field diffraction pattern analysis. The photometric signal is recorded with a 2-D CCD array sensor. The running time is reduced (0.25 s) using the Newton method to solve the inverse problem of the diffraction. It is worth noting that the 2-D analysis of an 1-D diffraction pattern provides several spatial sampling lines at a given time. Thus, the signal to noise ratio (SNR) is improved with an average along the cylinder axis. Other filtering techniques and interpolation algorithms can be applied. After this signal processing, a moire pattern is observed. This pattern is strongly dependent on the object angular position and the distance between the object and the detector. Then, it provides additional information which can be used to validate the recording cycle.

Moire patterns in three-dimensional Fourier space

Abstract: Mathematics are developed to transform a moire pattern into a 3-D Fourier space. Grating shapes, tilt, heterodyning, phase shifting, and Fourier filtering are all easily visualized in the 3-D Fourier space. It is shown why, in general, a moire pattern cannot be interpreted in the 2-D spatial domain by a simple algorithm operating on a point. A moire pattern is defined in terms of the 3-D Fourier space. A simple algorithm is developed to generate the value of the moire pattern at a point. The filter characteristics of a solid state camera are developed and exploited. Phase modulation and synchronous detection are shown to filter out only components due to the fundamental harmonic of the reference grating. The specimen frequency vector is recovered from the moire phase field.

High-resolution computer-aided moire

Abstract: This paper presents a high resolution computer assisted moire technique for the measurement of displacements and strains at the microscopic level. The detection of micro-displacements using a moire grid and the problem associated with the recovery of displacement field from the sampled values of the grid intensity are discussed. A two dimensional Fourier transform method for the extraction of displacements from the image of the moire grid is outlined. An example of application of the technique to the measurement of strains and stresses in the vicinity of the crack tip in a compact tension specimen is given.

Nondestructive evaluation of turbine blades vibrating in resonant modes

Abstract: The paper presents the analysis of the strain distribution of turbines blades. The holographic moire technique is used in conjunction with computer analysis of the fringes. The application of computer fringe analysis techniques reduces the number of holograms to be recorded to two. Stroboscopic illumination is used to record the patterns. Strains and stresses are computed.

Observation of nonprojective moiré fringe patterns produced with an X-ray interferometer

Abstract: Moire fringes produced with an X-ray interferometer have been found to give an extraordinary nonprojectiveness, an oscillatory change of the fringe position along the beam paths of the transmitted and the Bragg reflected waves behind the interferometer. Densitometric measurements of the moire fringes showed that, along with the fringe position, the fringe profile changes in an unusual manner along the beam paths. This nonprojectiveness is presumably of the same nature as that observed for moire fringes produced with a bicrystal [Yoshimura (1989). J. Phys. Soc. Jpn, 58, 1283–1295].

Analysis of combined moire and laser speckle grating methods used in 3-D crack tip deformation measurements

Abstract: The paper provides analyses for two optical schemes developed for mapping 3-D crack tip fields in ductile materials. The first scheme combines the in-plane moire method with the projection moire method to determine all three displacement components simultaneously. In the second method, we combined one-beam laser speckle photography with the projection moire method to measure 3-D deformation fields. The experimental results obtained using these methods are presented.

White-light moire phase-measuring interferometry

Abstract: Moire is a common technique for contouring large scale features on diffuse surfaces. Typically, this is accomplished by projecting a high-frequency line grating onto the object and viewing its image through another high-frequency line grating. The resultant pattern, produced by aliasing between the two gratings, shows lines of surface contour. The surface height between these contour lines is determined by triangulation between the illumination and observation beams. Although moire is really a geometric ray technique, it is typically referred to as interferometry because its contour patterns resemble interferograms; Moire contouring can be classified as either shadow or projection moire. Shadow moire uses the same grating for both illumination and observation, while projection moire uses separated gratings. Another surface contouring technique is projected fringe. This approach is similar to moire in that it projects a line grating and uses triangulation to measure surface height, however, there is no second grating or aliasing. Rather, the projected pattern is.viewed directly. Although it is not really moire (because it does not use aliasing with a second grating), projected fringe is typically considered a moire technique. This paper has four parts. First, it reviews the theory behind moire contouring. Second, it outlines how to phase modulate a moire 'interferogram'. Third, it discusses practical considerations associated with performing moire contouring such as projection and observation geometry, imaging, and illumination. And fourth, it presents two examples of objects that were contoured using a white-light moire phase-measuring interferometer.

Carrier pattern analysis of moire interferometry using Fourier transform

Abstract: The Fourier transform moire and grid method (FTMGM) which we have previously presented is extended to analyze strain distribution from the image of a fringe pattern with carrier fringes obtained by moire interferometry. The characteristics of the carrier fringes are explained in the frequency domain. The fringe pattern without carrier fringes is obtained from the fringe pattern with carrier fringes by extracting the first harmonic of the spectrum of the fringe pattern with carrier fringes and shifting it. The strain distribution is obtained by analyzing the phase of the image obtained from the inverse Fourier transform of the shifted first harmonic. Strain analysis of composites under compressive loading are shown.

Study of plate vibrations by moire holography

Abstract: Advantages of moire holography with single illumination beam as a real-time method to detect vibrational modes of diffusing objects have been already experimentally demonstrated by several authors. The main obstacle to automatizing the analysis of the moire pattern is the difficulty in filtering the carrier pattern. At the present, it seems affordable to realize filtering in quasi-real time by means of a digital image-processing system. However, several problems arise in this operation, for example, the spread of the spatial frequency spectrum of the carrier fringes. Different filtering algorithms, with and without FFT, are compared. Different ways to combine both carriers are examined.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Phase-corrected Moire pattern generation with electronic grating - applying e.g. iterative least-squares fit to achieve phase constancy of fringes in real=time processing

Abstract: A conventional image of a scene including the generative grating is produced by a high-resolution video camera having no analytic grating. Special periodic assessment of regions in the memory of an image-processing computer is used to perform the function of the grating. By interpolation, the values of brightness of the Moire fringes are converted into virtual intermediate positions of the grating. USE/ADVANTAGE - With objects having pronounced extension in depth, precise geometry is based on exact alignment of grating w.r.t. pixels of e.g. CCD video camera and rapid, inertialess virtual movement.