Takashi Muramatsu

Bio: Takashi Muramatsu is an academic researcher from Tohoku University. The author has contributed to research in topics: Grating & Moiré pattern. The author has an hindex of 3, co-authored 7 publications receiving 118 citations.

Accuracy of the Sampling Moiré Method and its Application to Deflection Measurements of Large-Scale Structures

Abstract: Measuring accurate displacement distributions for large-scale structures is an important issue and a very challenging task. Recently, a simple and accurate phase measurement technique called sampling moire method [Exp Mech 50–4:501–508, (2010)] has been developed for small-displacement distribution measurements. In this method, the phase distribution of moire fringes can be analyzed from a single grating image by simultaneously performing down-sampling image processing and intensity-interpolation to generate multiple phase-shifted moire fringe images. In addition, the phase of the original grating can also be obtained from the phase of the moire fringe by adding the phase of the sampling grating. In this study, the measurement accuracy of the sampling moire method was analyzed through computer simulations and a displacement measurement experiment. Four factors of the sampling moire method were investigated, including the sampling pitch, the order of the intensity-interpolation, random noise, and the form of grating. The results show that determining the optimal sampling pitch is an important factor for obtaining better results but it is not critical. In addition, a practical application of the sampling moire method is presented that involves a deflection measurement on a 10-meter-long crane. The experimental results demonstrate that submillimeter deflections of the crane can be successfully detected.

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

Abstract: Recently, a rapid and accurate single-shot phase measurement technique called the sampling moire method has been developed for small-displacement distribution measurements. In this study, the theoretical phase error of the sampling moire 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 moire fringe. Moreover, an effective phase compensation methodology is developed to reduce the periodic phase error. Single-shot phase analysis can perform accurately even when the sampling pitch is not matched to the original grating pitch exactly. The primary simulation results demonstrate the effectiveness of the proposed phase compensation methodology.

A simple technique for measuring thickness distribution of transparent plates from a single image by using the sampling moiré method

Abstract: A simple measurement technique for the thickness distribution of transparent plates is proposed by using the sampling moire method. The thickness distribution is automatically analyzed by measuring the phase difference of the moire fringe obtained from a single image, consistent with light refraction. The relationship between the actual thickness and phase difference of the moire fringe is calibrated by using glass plates of known thicknesses. The thickness measured by the present method was found to be in agreement with measurements using a digital micrometer. In our experiment, the average error in glass plate thickness was 1.9% over a 3.5 mm measurement range. Experimental results indicate that the present method can be useful for nondestructive measurements of the thickness distribution of various transparent plates.

Fast and accurate shape measurement system utilizing the fringe projection method with a ferroelectric liquid-crystal-on-silicon microdisplay

Abstract: Fringe projection methods using the phase-shifting technique have the advantages of fast 3-D shape measurement and high accuracy. The performance of the fringe projection system, including image quality, nonlinearity of the projected intensity, stability, and switching time for multiple phase-shifted patterns, is essential for fast and accurate shape measurement. A fast and accurate measurement system using a ferroelectric liquid-crystal-on-silicon microdisplay and a high-powered light emitting diode light source is developed. Our results indicate that the nonlinearity of the projected intensity and the stability of the fringe projection were dramatically improved compared with an ordinary commercial liquid crystal display projector. The rapid measurement of the warpage distribution of a flip chip ball grid array electronic package was performed by using the developed system. Nine phase-shifted fringe images with a resolution of 1280×960 pixels were recorded in 1.6 s. In addition, the measurement results obtained by our system agreed well with the results obtained from a micrometer and laser focus sensor. The average error was 2.6 µm, and the standard deviation was less than 10 µm with a 6-mm measurement range.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

Abstract: A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Digital wavefront measuring interferometer for testing optical surfaces and lenses

Abstract: A self-scanned 1024 element photodiode array and minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100. The photodiode array samples intensities over a 32 x 32 matrix in the interference pattern as the length of the reference arm is varied piezoelectrically. Using these data the minicomputer synchronously detects the phase at each of the 1024 points by a Fourier series method and displays the wavefront in contour and perspective plot on a storage oscilloscope in less than 1 min (Bruning et al. Paper WE16, OSA Annual Meeting, Oct. 1972). The array of intensities is sampled and averaged many times in a random fashion so that the effects of air turbulence, vibrations, and thermal drifts are minimized. Very significant is the fact that wavefront errors in the interferometer are easily determined and may be automatically subtracted from current or subsequent wavefrots. Various programs supporting the measurement system include software for determining the aperture boundary, sum and difference of wavefronts, removal or insertion of tilt and focus errors, and routines for spatial manipulation of wavefronts. FFT programs transform wavefront data into point spread function and modulus and phase of the optical transfer function of lenses. Display programs plot these functions in contour and perspective. The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.

The Grid Method for In‐plane Displacement and Strain Measurement: A Review and Analysis

Abstract: The grid method is a technique suitable for the measurement of in-plane displacement and strain components on specimens undergoing a small deformation. It relies on a regular marking of the surfaces under investigation. Various techniques are proposed in the literature to retrieve these sought quantities from images of regular markings, but recent advances show that techniques developed initially to process fringe patterns lead to the best results. The grid method features a good compromise between measurement resolution and spatial resolution, thus making it an efficient tool to characterise strain gradients. Another advantage of this technique is the ability to establish closed-form expressions between its main metrological characteristics, thus enabling to predict them within certain limits. In this context, the objective of this paper is to give the state of the art in the grid method, the information being currently spread out in the literature. We propose first to recall various techniques that were used in the past to process grid images, to focus progressively on the one that is the most used in recent examples: the windowed Fourier transform. From a practical point of view, surfaces under investigation must be marked with grids, so the techniques available to mark specimens with grids are presented. Then we gather the information available in the recent literature to synthesise the connection between three important characteristics of full-field measurement techniques: the spatial resolution, the measurement resolution and the measurement bias. Some practical information is then offered to help the readers who discover this technique to start using it. In particular, programmes used here to process the grid images are offered to the readers on a dedicated website. We finally present some recent examples available in the literature to highlight the effectiveness of the grid method for in-plane displacement and strain measurement in real situations.

Dynamic characteristics of suspension bridge hanger cables using digital image processing

Abstract: In this study, a non-contact measurement method is proposed to estimate the tension in hanger cables by using digital image processing. Digital images were acquired through a vision-based system using a portable digital video camera (camcorder), which was used to remotely measure the dynamic responses and was chosen because it is convenient and cost-efficient. Digital image correlation technique, as one of digital image processing algorithms, was applied to develop an image processing algorithm. An image transform function was used to correct the geometric distortion between the deformed and undeformed images and to calculate the subpixels. The motion of the vision-based system caused by external wind or vibration at camera location was corrected considering a fixed object in the image without any additional sensors. Using this algorithm, the dynamic response of the hanger cable and the resolution of the modal frequencies were improved. It was also confirmed that the dynamic characteristics of the hanger cables can be estimated with only the cable shape not attaching any target.