N. Takai

Bio: N. Takai is an academic researcher from Hokkaido University. The author has contributed to research in topics: Speckle pattern & Diffraction. The author has an hindex of 17, co-authored 38 publications receiving 659 citations.

Laser speckles produced by a diffuse object under illumination from a multimode optical fiber: an experimental study

Abstract: The size and displacement of speckles produced by a diffuse object under illumination from a multimode optical fiber are investigated experimentally to confirm the theory presented in a previous paper [ J. Opt. Soc. Am. A2, 1282 ( 1985)]. The experimental results show good agreement with the theory. It is verified that the mean size of speckles at the exit face of the fiber can be determined from measurements of the mean size of speckles at the observation plane. Furthermore, it is particularly emphasized that the speckle displacement is given by a relation, which is simpler than the conventional method where a Gaussian laser beam is used for the illumination.

Simultaneous Magnitude and Direction Measurements of a Diffuse Object's Velocity Using the Rotating Directional Detecting Aperture in a Laser Speckle Zero-crossing Method

Abstract: The zero-crossing number of speckle intensity fluctuations produced in the diffraction field by a diffuse object moving in a plane and integrated spatially by a finite-size aperture having the directional forms of, for example, the ellipse and the rectangle is studied theoretically and experimentally. The theoretical and experimental results indicate that the magnitude and direction of the moving object's velocity can be simultaneously measured by counting the zero-crossings of the spatially integrated speckle intensity fluctuations.

Experimental Study of the Laser Speckle Phase in the Image Field

Abstract: Statistical properties of the laser speckle phase in the image field were experimentally studied under the condition that the speckle field obeys Gaussian statistics. The speckle phase was measured in relation to the phase variations of light passing through a diffuse object and the defocusing of the imaging system. It is found that the phase variations of speckles at the image plane are smaller than those of light passing through the object. The anomaly of the speckle phase in the neighbourhood of the Gauss image plane is also found and interpreted in relation to the dip phenomenon of the speckle contrast.

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.

Laser Doppler, speckle and related techniques for blood perfusion mapping and imaging

Abstract: Laser Doppler velocimetry uses the frequency shift produced by the Doppler effect to measure velocity. It can be used to monitor blood flow or other tissue movement in the body. Laser speckle is a random interference effect that gives a grainy appearance to objects illuminated by laser light. If the object consists of individual moving scatterers (such as blood cells), the speckle pattern fluctuates. These fluctuations provide information about the velocity distribution of the scatterers. It can be shown that the speckle and Doppler approaches are different ways of looking at the same phenomenon. Both these techniques measure at a single point. If a map of the velocity distribution is required, some form of scanning must be introduced. This has been done for both time-varying speckle and laser Doppler. However, with the speckle technique it is also possible to devise a full-field technique that gives an instantaneous map of velocities in real time. This review article presents the theory and practice of these techniques using a tutorial approach and compares the relative merits of the scanning and full-field approaches to velocity map imaging. The article concludes with a review of reported applications of these techniques to blood perfusion mapping and imaging.

Current time-domain methods for assessing tissue motion by analysis from reflected ultrasound echoes-a review

Abstract: The Doppler technique has traditionally been the method used to extract motion information from ultrasonic echoes reflected by moving tissues. The Doppler technique has been around for a long time, and has been extensively reviewed and analyzed in the literature. Recently, time-domain methodologies for estimating tissue motion have gained in popularity. Time-domain methods have advantages over Doppler methods in many applications, and as of yet have not been comprehensively reviewed. An overview of time-domain techniques that have appeared in the literature over the past few years is presented. Their potential advantages over Doppler are examined, and the individual techniques are compared. >

Digital speckle-displacement measurement using a complex spectrum method

Abstract: An alternative approach to fully automatic speckle-displacement measurement is described. Two speckle patterns of a specimen, one before and one after deformation, are captured by a CCD camera and registered by a frame grabber. Two series of small subimages are obtained by segmenting the two speckle patterns. The corresponding subimage pairs extracted from both series are analyzed pointwise. The interrogation of each subimage pair involves a two-step fast-Fourier transform. While the first-step fast-Fourier transform achieves a complex spectrum characterized by the local displacement information, the second-step one generates a signal peak in the second spectral domain that resolves the local displacement vector. A rough estimate of the displacement vector is achieved by detecting the maximum pixel of the discrete spectrum. A more accurate determination is attained by a subpixel-maximum determination through a biparabolic fitting near the signal peak. The u- and v-displacement fields are deduced by analyzing all subimage pairs. A large rigid-body displacement can be overcome by introducing an artificial rigid shift of the two speckle patterns toward each other before the numerical process. The technique retains all the advantages of optical speckle photography and provides an extended range of measurement. Dynamic incremental deformations can be inspected by registering more speckle patterns at many consecutive deformation stages by using a high-speed CCD camera. The system was applied successfully to the study of crack-tip deformation fields.