Drone-based displacement measurement of infrastructures utilizing phase information.

Drone-based inspections provide an efficient and flexible approach to assessing aging infrastructures while prioritizing safety. Here, we present a pioneering framework that employs drone cameras for high-precision displacement measurement and achieves sub-millimeter accuracy, meeting the requirements for on-site inspections. Inspired by the principles of human auditory equilibrium, we have developed an effective scheme using a group of strategical reference markers on the bridge girders to measure structural displacements in the bridge.

Stereo sampling moiré method for three-dimensional deformation mapping with a stereomicroscope.

Three-dimensional (3D) deformation distribution measurement is of great interest in applications of materials evaluation. In this study, we propose a stereo sampling moiré method for full-field 3D deformation measurement based on a stereomicroscope. From the phase analysis on the left and right image planes as well as the relationship between displacements in the world and image coordinate systems, the 3D displacements and the in-plane strains of the specimen can be acquired.

Three-dimensional dynamic measurement of unstable temperature fields by multi-view single-shot phase-shifting digital holography.

A multi-view phase measurement system based on single-shot phase-shifting digital holography is proposed to dynamically obtain three-dimensional (3-D) information of an unstable temperature field. The proposed system consists of a laser, three polarization imaging cameras, and the corresponding optical components. The laser beam emitted from the laser is separated by the fibers into three pairs that contain three object beams and three reference beams.

Dynamic deformation measurement of dual-wavelength arbitrary phase-shifting digital holography with automatic phase-shift detection.

Dual-wavelength arbitrary phase-shifting digital holography with automatic phase-shift detection is first proposed in this study. Holograms with two wavelengths and the interference fringes used to detect the phase-shifting amount for each wavelength were simultaneously recorded in one image using the space-division multiplexing technique. Compared with conventional methods, the proposed approach can achieve simultaneous phase shifting of the reference beams of two wavelengths, which substantially reduces recording time and does not require excessive phase-shifting device precision.

Multiplication sampling moire method for full-field deformation measurement of composite materials.

A multiplication sampling moire (MSM) method was proposed for robust deformation distribution measurement by performing phase analysis of the second harmonic (second-order frequency) of a single grating pattern. The MSM method has a very strong noise immunity because the second harmonic spectrum is far from the low-frequency region of the background noise in the frequency domain. Phase analysis of an experimental grid image on a carbon fiber-reinforced plastic (CFRP) specimen indicated that the MSM method effectively solved the problem of non-negligible phase measurement errors of conventional methods that extract the fundamental frequency of the grating, in the case of severe local noise.

Point defect detection and strain mapping in Si single crystal by two-dimensional multiplication moiré method.

Although defect detection is critical for evaluating the manufacturing processes of semiconductor materials and metals, the detection of crystal defects, especially point defects, over a large field of view still faces considerable challenges. Herein, we report on the development of a two-dimensional (2D) multiplication moiré method using digital image processing to simultaneously detect point and line defects in a wide field of view. Defect locations were automatically detected by employing the concept of a hybrid strain, that is, the absolute value of the product of the strain distributions in different principal directions.

Wide-view and accurate deformation measurement at microscales by phase extraction of scanning moiré pattern with a spatial phase-shifting technique.

A scanning-based second-order moiré method is proposed for high-accuracy deformation measurement in a large field of view (FOV) by analyzing the phase distribution of a single-shot scanning moiré fringe image using a spatial phase-shifting technique. In this method, the grating pitch can be as small as around one pixel in the scanning moiré image to ensure a wide FOV, while high-precision phase measurement is achievable. The strain measurement accuracy has been verified from simulations at different grating pitches, applied strains, and noise levels.

Random phase-shifting digital holography based on a self-calibrated system.

Random phase-shifting digital holography based on a self-calibrated system is proposed. In the proposed method, the hologram and the calibration interference fringes can be recorded simultaneously in a single image based on the space-division-multiplexing technique. Three randomly phase-shifted holograms and corresponding interference fringes are recorded, and the phase-shifting amount between each two adjacent holograms is calculated by the sampling Moiré method from the calibration interference fringes.

Second-order moiré method for accurate deformation measurement with a large field of view.

In this study, we propose a second-order moiré method by performing digital sampling at two stages to realize high-accuracy deformation measurement in a wide field of view, where a grid image is recorded at a low magnification. Simulations have verified that this method has high strain measurement accuracy when the grid pitch is close to or even smaller than two pixels for both parallel and oblique grids with random noise. As an application, the two-dimensional microscale strain distributions of a carbon fiber reinforced plastic specimen when the grid pitch was about 2.

Data of dynamic microscale strain distributions of Ti-6Al-4V alloys in dwell fatigue tests.

Dynamic microscale strain distributions with temporal resolution of 1 s in a smooth and a cracked Ti-6Al-4V alloys during one-cycle dwell fatigue tests are illustrated in videos (URL: https://drive.google.com/drive/folders/1pit_VV2apGOpETVfaJAAtL5Xl2CNOiJ3?usp=sharing).

Comparative study of sampling moiré and windowed Fourier transform techniques for demodulation of a single-fringe pattern.

Phase measurement techniques using a single-shot carrier fringe pattern play an important role in optical science and technology and have been widely used for various applications. In this paper, we focus on the comparative study of two major fringe analysis techniques, the sampling moiré (SM) and the windowed Fourier transform (WFT). While SM converts a single-fringe pattern to multiple phase-shifted moiré fringe patterns to extract the phase information in the spatial domain, WFT obtains the phase information in the windowed Fourier domain; thus, the two methods look entirely different.

Interlaminar Shear Behavior of Laminated Carbon Fiber Reinforced Plastic from Microscale Strain Distributions Measured by Sampling Moiré Technique.

In this article, the interlaminar shear behavior of a [±45°] laminated carbon fiber reinforced plastic (CFRP) specimen is investigated, by utilizing microscale strain mapping in a wide field of view. A three-point bending device is developed under a laser scanning microscope, and the full-field strain distributions, including normal, shear and principal strains on the cross section of CFRP, in a three-point bending test, are measured using a developed sampling Moiré technique. The microscale shear strain concentrations at interfaces between each two adjacent layers were successfully detected and found to be positive-negative alternately distributed before damage occurrence.

Nanometer-order thermal deformation measurement by a calibrated phase-shifting digital holography system.

A thermal deformation measurement system based on calibrated phase-shifting digital holography is proposed. Two synchronized ordinary CMOS cameras are used in the calibrated phase-shifting digital holography system. One is to record the holograms including the object information, and the other is to record the interference fringes to evaluate phase-shifting errors.

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

A calibrated phase-shifting digital holography system based on the sampling Moiré 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.

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

Aimed at the low accuracy problem of shear strain measurement in Moiré methods, a two-dimensional (2D) Moiré 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 Moiré technique to get the Moiré 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.

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

This work describes the measurement procedure and principles of a sampling moiré technique for full-field micro/nano-scale deformation measurements. The developed technique can be performed in two ways: using the reconstructed multiplication moiré method or the spatial phase-shifting sampling moiré method. When the specimen grid pitch is around 2 pixels, 2-pixel sampling moiré fringes are generated to reconstruct a multiplication moiré pattern for a deformation measurement.

Digital sampling Moiré as a substitute for microscope scanning Moiré for high-sensitivity and full-field deformation measurement at micron/nano scales.

This study proposed to generate digital sampling Moiré fringes by two-pixel down-sampling as a substitute for microscope scanning Moiré fringes, and further reconstruct multiplication Moiré fringes for micron/nano-scale deformation measurement. The displacement and strain sensitivities of the proposed reconstructed multiplication Moiré method are 2 times higher in a wide field of view. Besides, two-dimensional deformation is easily measurable without rotating the sample stage or the scanning lines, no matter whether the scanning resolution is adjustable or not.

Accurate full-field optical displacement measurement technique using a digital camera and repeated patterns.

In this study, a novel, fast, and accurate in-plane displacement distribution measurement method is proposed that uses a digital camera and arbitrary repeated patterns based on the moiré methodology. The key aspect of this method is the use of phase information of both the fundamental frequency and the high-order frequency components of the moiré fringe before and after deformations. Compared with conventional displacement methods and sensors, the main advantages of the method developed herein are its high resolution, accuracy, speed, low cost, and easy implementation.

Theoretical error analysis of the sampling moiré method and phase compensation methodology for single-shot phase analysis.

Recently, a rapid and accurate single-shot phase measurement technique called the sampling moiré method has been developed for small-displacement distribution measurements. In this study, the theoretical phase error of the sampling moiré method caused by linear intensity interpolation in the case of a mismatch between the sampling pitch and the original grating pitch is analyzed. The periodic phase error is proportional to the square of the spatial angular frequency of the moiré fringe.

Intensity range extension method for three-dimensional shape measurement in phase-measuring profilometry using a digital micromirror device camera.

Phase-measuring profilometry is an accurate and effective technique for performing three-dimensional (3D) shape and deformation measurements of diffuse objects by fringe projection. However, phase analysis cannot be performed in underexposed or overexposed areas of the detector when an object with wide reflectance is measured. A novel intensity range extension method using a digital micromirror device (DMD) camera is proposed.

Accurate pixel-to-pixel correspondence adjustment in a digital micromirror device camera by using the phase-shifting moiré method.

A camera based on the digital micromirror device (DMD) technology has been previously developed. In this optical system, the correspondence of each mirror of the DMD to each pixel of the CCD cannot readily be done since the pixel sizes of the DMD and the CCD are very small. An accurate pixel-to-pixel correspondence adjustment in the DMD camera by means of the phase-shifting moiré method is proposed.