Showing papers on "Moiré pattern published in 2017"

Laboratory-based X-ray phase-imaging scanner using Talbot-Lau interferometer for non-destructive testing.

Abstract: An X-ray Talbot-Lau interferometer scanning setup consisting of three transmission gratings, a laboratory-based X-ray source that emits X-rays vertically, and an image detector on the top has been developed for the application of X-ray phase imaging to moving objects that cannot be tested clearly with conventional absorption contrast. The grating-based X-ray phase imaging method usually employs a phase-stepping (or fringe-scanning) technique by displacing one of the gratings step-by-step while the object stays still. Since this approach is not compatible with a scanner-type application for moving objects, we have developed a new algorithm for achieving the function of phase-stepping without grating displacement. By analyzing the movie of the moire pattern as the object moves across the field of view, we obtain the absorption, differential phase, and visibility images. The feasibility of the X-ray phase imaging scanner has been successfully demonstrated for a long sample moving at 5 mm/s. This achievement is a breakthrough for the practical industrial application of X-ray phase imaging for screening objects carried on belt-conveyers such as those in factories.

STEM moiré analysis for 2D strain measurements.

Abstract: A moire pattern is created in a scanning transmission electron microscope (STEM) when the scan step is close to a crystalline periodicity. Usually, fringes are visible in only one direction, corresponding to a single set of lattice planes, but fringes can be formed in two directions or more. Using an accurate independent calibration, the strains in silicon devices have been determined from the spacing and orientation of one-directional STEM moire fringes. In this report, we first discuss the origin of the STEM moire, and then we show how an accurate calibration of the scan step can be obtained from the STEM moire pattern itself, providing that we know initially only an approximate scan step and the planar spacing. The new calibration scheme also makes the STEM moire experiments easier, since it can be applied for the moire where the scan direction is not precisely aligned with the crystalline lattice. Finally, we show how the two-dimensional strain information will be readily extracted from two one-directional moire patterns using the concept of geometric phase.

Computer-generated Moiré profilometry.

Abstract: A new 3D measuring method based on computer-generated moire fringes is proposed. The two AC components of the 0-degree and 90-degree phase-shifted fringe patterns on reference plane are prepared in advance. While the AC component of the single-shot deformed pattern is multiplied by the two prepared AC components, respectively, two computer-generated moire fringes can be retrieved. The ratio of the two computer-generated moire fringes is just the tangent of the phase modulated by the object. It is of great potential in real-time or even dynamical 3D measurement due to its single-shot deformed pattern feature, and it avoids the influences of the object's reflectivity simultaneously. Compared to the Fourier transform profilometry, its error is smaller due to its higher first-order spectrum. Experimental results show the feasibility and validity of the proposed method.

Two-dimensional Moiré phase analysis for accurate strain distribution measurement and application in crack prediction.

Abstract: Aimed at the low accuracy problem of shear strain measurement in Moire methods, a two-dimensional (2D) Moire phase analysis method is proposed for full-field deformation measurement with high accuracy. A grid image is first processed by the spatial phase-shifting sampling Moire technique to get the Moire phases in two directions, which are then conjointly analyzed for measuring 2D displacement and strain distributions. The strain especially the shear strain measurement accuracy is remarkably improved, and dynamic deformation is measurable from automatic batch processing of single-shot grid images. As an application, the 2D microscale strain distributions of a titanium alloy were measured, and the crack occurrence location was successfully predicted from strain concentration.

Demoiréing for screen-shot images with multi-channel layer decomposition

Abstract: Moire patterns on screen-shot images are mainly due to the aliasing of the grid of the display and the camera sensor, which heavily degenerated the image quality. This paper proposes an demoireing method for screen-shot images via layer decomposition on polyphase components (LDPC). The layer decomposition model separates the image into a background layer and a moire layer, which are both regularized by a patch-based Gaussian Mixture Model (GMM) prior. To enhance the distinguishability between the image patches and moire patches, the input image is first subsampled into four polyphase components, each of which is decomposed with the GMM-based layer decomposition model. The proposed model is applied on luminance (Y) channel to weaken the intensity of moire patterns, and on red, green, blue channels respectively to further remove moire patterns. Experimental results demonstrate that the proposed method is able to efficiently remove moire artifacts for screen-shot images and outperform several other methods.

Calibrated phase-shifting digital holography based on a dual-camera system.

Abstract: A calibrated phase-shifting digital holography system based on the sampling Moire technique is proposed. Two synchronized cameras are used in this system. One is to record the conventional holograms that include the object information, and the other is to record the interference fringes to analyze phase-shifting errors. An algorithm for improving the quality of the reconstructed images is proposed. In this Letter, the effectiveness of the proposed system and algorithm is demonstrated in four-step phase-shifting digital holography. The quality of the reconstructed images is greatly improved from both the numerical simulation and experiment.

Design methodology for moiré magnifier based on micro-focusing elements

Abstract: Moire effect is a well-known interference phenomenon occurred between repetitive structures. Conventional moire fringes are produced by superposing gratings or dots arrays. However, when the micro-image array as base layer and the corresponding micro-focusing elements as revealing layer overlap each other, a special kind of moire effect, so-called moire magnifier, can be observed. Micro-image units in the base layer are enlarged and projected to moire space. To our knowledge, there has no complete design methodology for the realization of the moire magnifier. With the combination of the Fourier transform and spectral approach, a new algorithm based on transfer matrix is investigated, which is capable of predicting the location of any arbitrary point in the base layer mapping to the moire space, thus it provides a simple way to explore the physical insight into the field of moire imaging. The magnification factor and the orientation of the synthetically enlarged image are determined not only by the scaling ratio but also by the interrelation between the primitive vectors in the base and revealing layers. Experimental results are in good agreement with theoretical predictions. By using the proposed method, the moire magnifier can extend appealing applications in esthetic security devices, highly accurate measurements and precise color printing.

Micro/Nano-scale Strain Distribution Measurement from Sampling Moiré Fringes.

Abstract: This work describes the measurement procedure and principles of a sampling moire technique for full-field micro/nano-scale deformation measurements The developed technique can be performed in two ways: using the reconstructed multiplication moire method or the spatial phase-shifting sampling moire method When the specimen grid pitch is around 2 pixels, 2-pixel sampling moire fringes are generated to reconstruct a multiplication moire pattern for a deformation measurement Both the displacement and strain sensitivities are twice as high as in the traditional scanning moire method in the same wide field of view When the specimen grid pitch is around or greater than 3 pixels, multi-pixel sampling moire fringes are generated, and a spatial phase-shifting technique is combined for a full-field deformation measurement The strain measurement accuracy is significantly improved, and automatic batch measurement is easily achievable Both methods can measure the two-dimensional (2D) strain distributions from a single-shot grid image without rotating the specimen or scanning lines, as in traditional moire techniques As examples, the 2D displacement and strain distributions, including the shear strains of two carbon fiber-reinforced plastic specimens, were measured in three-point bending tests The proposed technique is expected to play an important role in the non-destructive quantitative evaluations of mechanical properties, crack occurrences, and residual stresses of a variety of materials

Principle and observation of fluorescence moiré fringes for alignment in print and imprint methods

Abstract: The authors report the principle and experimental observation of fluorescence moire interference fringes for alignment in the print and imprint methods Concave bar and cross alignment marks on a silica mold and substrate without the deposition of any light-shielding or high-refractive-index layers could be visualized by fluorescence microscopy with a fluorescent liquid sandwiched between the patterned surfaces Fluorescence moire interference fringes with a pitch of 44 μm were generated by the superimposition of two sets of mold bar arrays with respective pitches of p1 = 40 μm and p2 = 44 μm on substrate bar arrays with different periodicity of p2 and p1 through a thin fluorescent liquid layer The fluorescence moire fringes were attributed to an additive-type generation by the interference of two luminous gratings with different periodicities, which was different from a multiplicative-type generation by the superimposition of two light-shielding metal gratings with different periodicities under illumi

Color moiré reduction and resolution enhancement technique for integral three-dimensional display

Abstract: In the case of using a flat-panel display for displaying elemental images in an integral three-dimensional (3D) display, the color moire on the 3D image caused by the sub-pixel structure for colorization becomes a visual obstacle. Generally, a method of displaying an elemental image through a diffusion screen, or a defocusing method of the elemental image from the focal length position of the elemental lens have been used for color moire reduction. However, on the other hand, this lowers the resolution of the elemental image, which is one of the causes of lowering the depth reproducibility of the 3D image. We developed a technology to reduce color moire, while improving the resolution of reconstructed 3D images by multiple 3D display systems.

Measurement and simulative proof concerning the visibility loss in x-ray Talbot-Lau Moiré imaging

Abstract: X-ray grating-based phase-contrast imaging is a very promising imaging technique for medical imaging and non-destructive testing. Simultaneously to the attenuation properties additional information about the phase-shift and the scattering properties of an object can be gained. Nevertheless, to obtain this information multiple shots per image have to be taken in the so-called phase-stepping process. Thus, it often is a time consuming imaging procedure. In contrast, Moire imaging with a Talbot-Lau interferometer is a very fast single-shot imaging method. To obtain the Moire pattern which encodes the object information the interferometer has to be slightly detuned. This leads to a loss in visibility and therefore to a loss in image quality compared to the more time consuming phase-stepping method. Thus, it is important for the Moire imaging method to evaluate the mechanisms of visibility loss. In this publication we present measurements and simulations investigating the visibility loss due to detuning the Talbot-Lau interferometer. Thus, a deeper understanding of Moire imaging can be gained.

Accurate Strain Distribution Measurement Based on the Sampling Moiré Method

Abstract: To obtain accurate measurement of non-uniform strains over the entire surface of materials or structures, in this study, the sampling moire method is applied to two-dimensional measurement of in-plane displacement and strain distributions. The two-dimensional displacement distribution is obtained by detecting the phase difference of the moire fringes, and the strain distribution is determined by measuring the change of grating pitches on the specimen in vertical and horizontal directions before and after deformations. A compensation technique of fringe order is proposed to eliminate the pitch error of fabricated grating. In addition, a spatial filtering procedure in phase distribution is suggested to reduce the random measurement error. The deformations of an aluminum specimen with a through hole under different tensile loads were measured by the proposed method. Experimental results indicated that the measurement accuracy of strain distribution can be dramatically improved by combination the above-mentioned two techniques. This method is useful to evaluate mechanical properties of various materials under tension/compression or heating/cooling testing.

Digital moire fringe phase extraction method combining wavelet analysis and low pass filtering

Abstract: The invention discloses a digital moire fringe phase extraction method combining wavelet analysis and low pass filtering and belongs to the optical measurement and image processing technology field. The method is characterized by using actual interference graph and phase-shift virtual interference graph moire synthesis to generate a phase shift moire composite graph, and using low pass filtering and combining a phase shift interferometry to extract a preliminary result of a moire fringe phase; according to a virtual interference wavefront acquired by a virtual interferometer, combining a filtering radius of a filter to determine a distribution center and a range of a low pass filtering error in the preliminary result of the moire fringe phase; after a low pass filtering error range and the center are positioned, carrying out wavelet decomposition on the preliminary result containing the low pass filtering error; and according to the positioned error center and the range, processing a wavelet coefficient so that the filtering error can be removed, according to an approximation coefficient and a processed wavelet coefficient, carrying out reconstruction and acquiring the moire fringe phase where the low pass filtering error is removed, which means that a measurement range of digital moire interference measurement is expanded.

High speed displacement/strain distribution measurement method by moire method and measurement device

Abstract: PROBLEM TO BE SOLVED: To solve a problem that, though a scan moire method and a sampling moire method have many advantages in a displacement strain distribution measurement in a static state, it takes a time to obtain a high resolution image when a scanning type laser microscope is used in the case of performing a dynamic displacement strain distribution measurement, and alternately, in the measurement of plane deformation, it is necessary to rotate a sample stage or a main scan line of a microscope by 90°, and there is a problem that during that time the sample is affected by deformation, and hence, a secondary deformation strain distribution of a structural material cannot be measured promptly and correctly.SOLUTION: The number of scan lines is reduced, and a moire image is acquired at a higher speed without lowering a measurement sensitivity of displacement and strain. Scan point intervals in two directions of x and y are appropriately arranged by using a crossed grating for a sample grating and separated into primary scan moire fringes from one moire fringe image acquired without rotating the sample, and a secondary displacement strain distribution of the structural material is promptly and correctly combined with a high speed moire method.SELECTED DRAWING: Figure 1

Digital Moire fringe phase extraction method based on nonlinear optimization

Abstract: The invention relates to a digital Moire fringe phase extraction method based on nonlinear optimization, belonging to the technical field of optical measurement and image processing. According to the method, a Moire synthesis light intensity distribution graph mathematical model is established, a fringe phase is used as an optimization variable, with actual Moire synthesis graph light intensity distribution as an optimization target, phase continuity is taken as a boundary condition, through nonlinear optimization calculation, the light intensity distribution of the mathematical model is equal to the light intensity distribution of an actual Moire synthesis graph, and the fringe phase at that time is the Moire fringe phase of the actual Moire synthesis graph. According to the method, the Moire fringe phase can be obtained without low pass filtering, a filtering error in a spectrum aliasing condition can be avoided, and a phase measurement precision is improved.

Digital grating and physical grating combined tiny object surface three-dimensional measurement system

Abstract: The invention provides a digital grating and physical grating combined tiny object surface three-dimensional measurement system which is composed of a computer, collimated laser beams, a digital micromirror device (DMD), digital gratings, physical gratings, three lens groups, an object to be measured, an object table and a CCD image detector The computer generates light and dark graphs to be transmitted to the DMD The collimated laser beams irradiate the DMD to be reflected and then generate the digital gratings through the first lens group, and then the digital gratings reach the surface of the object to be measured on the object table through the second lens group The digital gratings modulated by the object to be measured are imaged on the surface of the physical gratings through the second and third lens groups The digital grating and the physical gratings are superposed at a certain gap, and multiple times of diffraction occurs so that the amplified Moire interference fringes are formed and then imaged on the CCD image detector, and image acquisition and processing are performed through the computer so that three-dimensional information of the object to be measured can be acquired The system is high in precision, easy to operate and high in anti-interference capacity and has high application value

Representation of image distortion by Moiré fringes at phase singularity state.

Abstract: When a grating is imaged by an optical imaging system, due to the aberrations of the system, the parameters of the image grating suffer minute gradual changes across the image. Superimposing an ideal grating image over the real grating image at the phase singularity state of the two gratings leads to phase contours, special Moire fringes, which directly represent the distortions over the image. In this report, after a brief review of the required theoretical bases, we show when the parameters of a grating change linearly the corresponding Moire fringes at the singularity state are represented by quadratic functions, and for nonlinear changes higher order functions are involved. Thus, by imposing desired changes on the parameters of a grating one can produce Moire fringes satisfying functions of required orders. In the experimental part of the report we apply the technique to evaluate the image distortions imposed by a conventional camera and cameras installed in a mobile and in a tablet.

Talbot Interferometry for Focal Length Measurement Using Linear and Circular Gratings

Abstract: Several methods for measurement of focal length of a positive lens involve the measurement of distances of the object and image planes from principal planes, which are mathematical planes; the positions of which are difficult to figure out accurately. The Talbot method for focal length measurement is a method, which does not require the distances from the principal planes. Further Talbot interferometry can be used to measure focal length variation over the surface of a progressive power lens. A technique for the measurement of focal length using Talbot interferometry and moire effect has been described. The Talbot image of a Ronchi grating placed just after a test lens is superimposed on the second grating to give us the moire fringes. The angle of tilt of the moire fringes can be used to find out the focal length. The effect of unequal pitches of the gratings on moire formation is also investigated. The investigation is also carried out using binary gratings having equidistant circular rings (circular gratings). The theory and experimental results obtained with both types of gratings have been discussed.

Synthetic wavelength phase extracting method based on circular carrier frequency Moire fringe theory

Abstract: The invention discloses a synthetic wavelength phase extracting method based on a circular carrier frequency Moire fringe theory. On the condition that an aspheric surface is tested by means of a double-wavelength interference testing device based on the Moire fringe theory, a circular carrier frequency Moire fringe graph after overlapping of two wavelength interference fringes can be obtained, and a method for directly extracting the synthetic wavelength phase from a circular carrier frequency Moire fringe graph is presented. Through performing phase shifting through using pi/2 of the synthetic wavelength as a phase shifting stepping amount, DC component eliminating is performed on a circular carrier frequency Moire fringe phase shifting interferogram, and then squaring is performed. Furthermore secondary polar coordinate transformation is performed for obtaining a linear carrier frequency Moire fringe graph. According to a carrier frequency overlapping reconstruction theory, extraction of a low-frequency synthetic wavelength component in a frequency spectrum domain is realized, and finally a synthetic wavelength phase is extracted. The synthetic wavelength phase extracting method settles a problem of incapability of recovering the phase on the condition of overhigh fringe density in single-wavelength detection.

Small-scale noise-like moiré pattern caused by detector sensitivity inhomogeneity in computed tomography.

Abstract: We report a new type of moire pattern caused by inhomogeneous detector sensitivity in computed tomography. Defects in one or a few detector bins or miscalibrated detectors induce well-known ring artifacts. When detector sensitivity is not homogenous over all detector bins, these ring artifacts occur everywhere as distributed rings in reconstructed images and may cause a moire pattern when combined with insufficient view sampling, which induces a noise-like pattern or a subtle texture in the reconstructed images. Complete correction of the inhomogeneity in detectors can remove the pattern and improve image quality. This paper describes several properties of moire patterns caused by detector sensitivity inhomogeneity.

Vibration error-compensation technique in phase shifting moiré interferometry

Abstract: Undesirable vibrations can greatly degrade the accuracy of phase shifting moire interferometry. Although moire interferometry is always equipped with a somehow elaborate system to dampen or to eliminate vibration, such as granite slab and air table, it is common to see the phase fluctuation of moire diagram attributable to the various vibration during the measurement process. The sensitivity of moire interferometry to vibration is rooted in the fundamental principles of the instrument. Typically, an imaging system collects interference patterns for a sequence of reference phases, and then a computer unwraps the original wave-front phase by analyzing the variations of intensity. Compared with classical interferometry, an experiment by grating rosette moire interferometry with different principle of image formation is carried out and a corresponding efficient phase calculation method is developed to eliminate or compensate the effects of mechanical vibrations. In the decomposition and reconstitution of phase-shifting fringe patterns using discrete Fourier series, the phase-shifting fundamental coefficients (a 0, a 1 and b 1) are extracted from 10 samples in one phase shift cycle, with which three images with 120 degree intervals are obtained. Besides, a mathematical model for the phase error is derived by vibration frequency content or spectrum. Combined with traditional phase shifting algorithm, the proposed data post-processing technique can effectively reduce the phase ripple caused by vibrations. The result is supported by numerical simulations in MATLAB software and experiments with grating rosette moire interferometry.

Projection lens testing with Moiré effect

Abstract: The application of Moire effect for testing of a lithographic projection lens is reported. The arrangement presented allows measuring magnification, distortion, field curvature and telecentricity of the lens and can be used for its fine tuning. The method is based on two matched two-dimensional gratings, positioned in mutually conjugated planes; one of them can be translated. Visual interpretation of Moire fringe pattern allows quick diagnostics of position errors exceeding critical dimension, whereas lateral scanning is applied for measuring of smaller magnitude errors. Field curvature and telecentricity are measured by 3D scanning. Presented results are in a good agreement with those obtained elsewhere.

Method for decreasing Moire fringes in glasses-free 3D directional backlight system

Abstract: The invention provides a method for decreasing Moire fringes in a glasses-free 3D directional backlight system. The method comprises: calculating Moire fringes generated on an LCD (liquid crystal display) screen when a Fresnel lens set and the LCD screen are under different angles, distances and cycle ratios, and drawing a graph; finding Fresnel lens set and LCD screen distance and angle optimal values from a visual contrast distribution graph for Moire fringes generated by Fresnel lens set and the LCD screen periodical information in the glasses-free 3D directional backlight system; constructing the 3D directional backlight system according to the found Fresnel lens set and LCD screen distance and angle optimal values. The method can decease Moire fringes without affecting the visual enjoinment from the glasses-free 3D system, and the quality of visual sensory views can be improved greatly.

Radiographic imaging system

Abstract: PROBLEM TO BE SOLVED: To generate an absorption image by a simple imaging without removing a grating from a field of view, in a radiographic imaging device using a Talbot interferometer or a Talbot-Lau interferometerSOLUTION: The radiographic imaging device includes a control part of inserting a scatterer body into an irradiation field in a low-definition imaging mode to lower definition of a moire fringe relative to that imaged by a fringe scanning method and acquire a moire fringe image A controller generates an absorption image based on the imaged moire fringe imageSELECTED DRAWING: Figure 10

Experimental approach for optical registration of circular time-averaged moiré images

Abstract: This paper presents the development of an experimental stand for the registration of time-averaged geometric circular moire images. Optical interpretation of patterns of time-averaged moire fringes allows to identify the parameters of circularly vibrating structures. The developed scheme has a strong potential for designing experimental schemes capable to measure angular vibrations of rotating shafts in different mechanisms, and do not require any mountable sensors.

Measurement of amplitude of the moiré patterns in digital autostereoscopic 3D display.

Abstract: The article presents the experimental measurements of the amplitude of the moire patterns in a digital autostereoscopic barrier-type 3D display across a wide angular range with a small increment. The period and orientation of the moire patterns were also measured as functions of the angle. Simultaneous branches are observed and analyzed. The theoretical interpretation is also given. The results can help preventing or minimizing the moire effect in displays.