Satoru Toyooka

Vector-subspace model for color representation

In multispectral imaging it is advantageous to compress spectral information with a minimum loss of information in a way that permits accurate recovery of the spectrum. By use of the simple vector-subspace model that we propose, spectral information can be stored and recovered by the use of a few inner products, which are easy to measure optically. Two large data sets, the first consisting of 1257 Munsell colors and the other of 218 naturally occurring spectral reflectances, were analyzed to form two bases for the model. The Munsell basis can be used to represent the natural colors, and the basis derived from the natural data can be used to represent the Munsell data. The proposed vector-subspace model includes a simple relation between the inner products and conventional color coordinates. It also provides a way to estimate the spectrum of an object that has known chromaticity coordinates.

Automatic profilometry of 3-D diffuse objects by spatial phase detection.

A method automatically processing a projected grating to profile a 3-D diffuse object is proposed. A deformed grating pattern projected on the object is regarded as a phase modulated pattern with a constant spatial carrier frequency. To retrieve phase modulation, acquired data in a microcomputer are sinusoidally fitted using a phase detection algorithm similar to that used in communication techniques. High sensitivity measurements of height distribution can be done using simple optical geometry. Random and systematic errors inherent in the method are discussed in detail.

Spectral vision system for measuring color images

We present a spectral vision system that can be used to measure a color spectrum and two-dimensional spectral images. First, a low-dimensional color filter set was designed by an unsupervised neural network. Then a compact optical setup for the spectral synthesizer was constructed to synthesize the light that corresponds to the spectral characteristics of the color filter. In the optical setup a liquid-crystal spatial light modulator was used to implement color filters. A sample was illuminated by the synthesized lights, and the intensity images that correspond to the inner products between the color filter and the sample were detected by a CCD camera. From the detected inner products the sample’s color spectra were reconstructed by use of a pseudoinverse matrix. Experimental results of measuring a single color spectrum and spectral images are presented.

Phase shifting common path interferometer using a liquid-crystal phase modulator

Abstract A common path phase shifting interferometer is proposed using a liquid-crystal cell as a phase modulator. In the proposed common path interferometer, the specular component of the object light, separated in the Fourier transform plane, plays the role of a reference wave for the diffracted component of the object. A homogeneous-type nematic liquid-crystal cell is used as a phase only modulator, to introduce an arbitrary phase modulation between the specular and the diffracted components to achieve phase shifting interferometry. This makes quantitative measurements possible as well as qualitative observations for phase objects like in a Zernikes phase contrast microscope. Finally, experiments were carried out to show the validity of the proposed method. Experimental results show that the present method is quite stable against mechanical shocks and air fluctuation owing to its common path arrangement. The accuracy of the present method was experimentally verified to be better than 1 30 of wavelength by comparing the results with those obtained by the heterodyne interferometer.

Dynamic electronic speckle pattern interferometry (DESPI) phase analyses with temporal Hilbert transform

In this study, we propose the Hilbert transform (HT) method for phase analysis of a Dynamic ESPI signal. The data processing is performed in the temporal domain, using the temporal history of the interference signal at every single pixel. The final results give a temporal development of the two-dimensional deformation field. To reduce the influence of the fluctuations of bias intensity on the calculated phase, it was removed prior to performing the HT. This method was demonstrated for defects distinction and the determination of the sign change in the deformation field in two different experiments. The range of measurement lies between submicrons and tens of microns and the spatial resolution is better when compared to the fringe analysis method and the spatial carrier method.

Spatial fringe scanning for optical phase measurement

Abstract A new method of analyzing interference fringes using a microcomputer is proposed. Interference fringes with a tilted wavefront are spatially scanned perpendicularly to the spatial carrier by a TV camera. Irradiance distribution along a raster scan is sinusoidally interpolated in each carrier interval and optical phases are calculated over the whole area under test.

Speckle-shearing interferometry using a liquid-crystal cell as a phase modulator

Fundamental properties of the phase-modulation ability of a nematic liquid-crystal cell are studied. Based on these phase-modulation properties of the liquid-crystal cell, a new type of speckle-shearing interferometer is proposed and studied experimentally. A liquid-crystal cell is employed as a phase shifter to implement the phase-shifting method for the conventional speckle-shearing interferometer. From the experiments used to measure the deformation of an object, the usefulness of the method is confirmed. Finally, a compensation method for phase-shift error is proposed on the basis of the statistical properties of the fully developed speckle field. In this method the speckle phase is regarded, in a statistical sense, as a standard phase object used to calibrate the measuring system. Experiments to confirm the error-compensation method are performed, and it is shown that the phase-shift error can be determined with an accuracy of as much as λ/100.

Statistical interferometry based on a fully developed speckle field: an experimental demonstration with noise analysis

A novel interferometric method named statistical interferometry is proposed and studied. In the method, in contrast to the conventional deterministic interferometry, the complete randomness of the two interfering light fields, i.e., the random interference of the fully developed speckle fields, plays an essential role and is used as a standard of phase in a statistical sense. Preliminary experiments were conducted to verify the validity of the method, followed by a computer simulation. As an experimental result, the accuracy of the measurements of an out-of-plane displacement was confirmed up to lambda/800 by comparison with the heterodyne interferometer. The method has the advantage of simplicity of the optical system required, while at the same time providing high accuracy.

Dynamic observation of localized strain pulsation generated in the plastic deformation process by electronic speckle pattern interferometry

An electronic speckle pattern interferometry (ESPI) system, which makes it possible to observe dynamic phenomena, was applied to investigate the plastic deformation process in tensile experiments of aluminum alloy samples. The dynamic behavior of a strain-localized band which propagated through a specimen was thoroughly investigated. A pulsating fringe variation of the order of 1 Hz was discovered to be the fine structure of the propagating band. Fringe pulsation is caused by localized strain pulsation which suggests that the stress relaxation process propagates periodically with a time constant which governs the dissipative characteristics of a heterogeneous material.

Dynamic ESPI with subtraction–addition method for obtaining the phase

Abstract Dynamic electronic speckle pattern interferometry (DESPI) was developed for in situ observations. The quantitative evaluation of the deformation field was performed through 2-D subtraction–addition method (SAM) for phase analysis. This method does not require additional phase modulation, which makes it applicable to studying dynamic events. The method utilizes the ratio of the subtraction and addition correlation fringe patterns. The bias intensity that has to be removed from the addition images was determined by temporal averaging of a sequence of speckle patterns. To reduce the error, Gaussian filter was applied to the correlation fringe patterns. The deformation field was evaluated with accuracy of λ /10.