Suezou Nakadate

Fringe scanning speckle-pattern interferometry

Digital speckle-pattern interferometry systems for automatic measurement of deformations of a diffuse object are presented, which are based on a fringe scanning method with phase-shifted speckle interferograms. A digital speckle pattern before deformation of an object is recorded in the mass storage device of a computer facility. After deformation, four digital speckle patterns are recorded as changing the phase of reference light such as 0, π/2, π, and 3π/2, respectively. Four speckle interferograms, whose phases are shifted by 0, π/2, π, and 3π/2, are generated by calculating the square of the differences between speckle patterns before and after deformation. These interferograms are low-pass filtered to reduce speckle noise. The calculation of the arctangent with four phase-shifted speckle interferograms gives the optical path difference which is proportional to the deformation. A correction of the discontinuity of the calculated phase gives the numerical data of the deformation in the whole object area. Some experimental results for the measurement of out-of-plane, in-plane, and 3-D deformations are presented.

Electronic speckle pattern interferometry using digital image processing techniques.

The use of digital image processing techniques for electronic speckle pattern interferometry is discussed. A digital TV-image processing system with a large frame memory allows us to perform precise and flexible operations such as subtraction, summation, and level slicing. Digital image processing techniques make it easy compared with analog techniques to generate high contrast fringes. Some experimental verifications are presented in the cases of surface displacement and vibration amplitude measurements.

Digital speckle-pattern shearing interferometry.

An application of digital image processing techniques to speckle-shearing interferometry is described. A present system consists of an image-shearing camera using a Fresnel biprism of small angles and a digital TV-image processing facility. This interferometer makes it easy to measure in quasi-real-time spatial derivatives of surface displacement and modal vibration amplitude of objects. A statistical theory is applied to analyze the formations of these fringes due to 3-D displacement. Interpretation of the result leads to conditions of maximum fringe contrast and the limitation of this technique. Experimental results in the three cases of slope of normal displacement, surface strain, and slope of vibration amplitude measurements are presented.

Automatic Fringe Analysis Using Digital Image Processing Techniques

An automatic computer analysis system for interferometric patterns is developed. A digital interferogram is stored on disk memory, and fringe skeletons of the interferogram are extracted, based on the two-dimensional gray level fringe peak detection method. Extracted fringe skeletons are thinned, and then their fringe orders are determined, during which process the fringe pattern is visually inspected. The least-square interpolation method is applied to determine the fractional fringe orders of pixels between fringe skeletons. All the procedures of analyzing fringe data, except for the fringe order determination, are automatically performed. A man-machine interactive method with a light pen is used to inform the determined fringe orders to the computer. A representative example of the fringe pattern analysis is presented.

Vibration measurement using phase-shifting speckle-pattern interferometry

Digital speckle-pattern interferometries for the measurement of a vibration amplitude are presented, based on phase shifting of a speckle interferogram. The calculation of the square root and division with phase-shifted speckle interferograms give a high contrast fringe pattern, which contours a vibration amplitude regardless of a bias deformation of the vibration. The arctangent calculation with phase-shifted fringe patterns gives the phase distribution proportional to the bias deformation of the vibration, which is obtained by the correction of phase discontinuities of ±2π and ±π in the phase image. The detection of the phase discontinuity of ±π in the phase image gives the center line of a dark fringe in the calculated fringe pattern. The fringe-order numbers are determined by referring to the new calculated fringe pattern. The distribution of the vibration amplitude is obtained by the interpolation of the fringe-order numbers.

Simultaneous formation of four fringes by using a polarization quadrature phase-shifting interferometer with wave plates and a diffraction grating

We present a new type of quadrature phase-shifting interferometer, which utilizes wave plates, a diffraction grating, and two lasers with different wavelengths, in order to acquire two sets of two quadrature fringe patterns in each wavelength formed on a single image sensor. This method for calculating with four phase-shifted fringe patterns gives us the phase sum and difference distributions between the phases in two wavelengths. This is also substantiated by results of our experiments.

Vibration measurement using phase-shifting time-average holographic interferometry

Computer image processing techniques for the measurement of vibration amplitude are presented, which utilize phase-shifted time-average holographic interferograms. The calculation of the square root and division with phase-shifted interferograms gives a high contrast fringe pattern which contours the vibration amplitude. The arctangent calculation with phase-shifted fringe patterns also gives the phase distribution proportional to the optical path difference of the measured object before and after vibration. The detection of the phase discontinuity of ±π in the phase image gives exactly the center line of the dark fringe. The distribution of the vibration amplitude can be obtained by the fringe-order determination and its interpolation. The bias deformation of the vibration is obtained by the correction of phase discontinuities of ±2π and ±π in the phase images before and after vibration.

Phase-shifting interferometer based on changing the direction of linear polarization orthogonally.

We present a new type of phase-shifting interferometer, which utilizes a polarizing prism to form quadrature phase-shifted fringe patterns onto a single imaging sensor. By changing the direction of linear polarization of the incident light orthogonally, four phase-shifted fringe patterns in quadrature are obtained by taking images twice; thus it is possible to reduce phase errors caused by mechanical vibrations and air turbulence that occur in temporal phase-shifting interferometers. We present the principle of this interferometer with its theoretical analysis, using the Jones matrix, along with experimental results.

Three-dimensional displacement measurement for diffuse object using phase-shifting digital holography with polarization imaging camera

The amount of displacement of a diffused object can be measured using phase-shifting digital holography with a polarization imaging camera. Four digital holograms in quadrature are extracted from the polarization imaging camera and used to calculate the phase hologram. Two Fourier transforms of the phase holograms are calculated before and after the displacement of the object. A phase slope is subsequently obtained from the phase distribution of division between the two Fourier transforms. The slope of the phase distribution is proportional to the lateral displacement of the object. The sensitivity is less than one pixel size in the lateral direction of the movement. The longitudinal component of the displacement can be also measured separately from the intercept on the phase axis along the phase distribution of the division between two Fourier transforms of the phase holograms.

Computer-Aided Speckle Pattern Interferometry

An on-line computer system for measuring a deformation of a diffuse object with a speckle interferometer is presented. Methods for evaluating a speckle interferogram using digital image processing techniques are also discussed. The system consists of an interferometric optical setup and a computer TV-image processing facility. A speckle interferogram is generated by arithmetic operation between two digitized speckle patterns before and after deformation of the object. The informations about the deformation are extracted by two modes of procedures in analyzing the interferogram. One of them is a method for automatic analysis using digital image processing techniques such as gray scale modification, linear spatial filtering, thresholding, and skeletoning. The alternative is a man-machine interactive method for a simple and high-speed processing of the interferogram by using a light-pen facility. The determined fringe order numbers are interpolated and differentiated spatially to give strain, slope, and bending moment of the deformed object. Some examples of processed patterns are presented.