We first provide an overview of 3-D shape measurement us- ing various optical methods. Then we focus on structured light tech- niques where various optical configurations, image acquisition tech- niques, data postprocessing and analysis methods and advantages and limitations are presented. Several industrial application examples are presented. Important areas requiring further R&D are discussed. Finally, a comprehensive bibliography on 3-D shape measurement is included, although it is not intended to be exhaustive. © 2000 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(00)00101-X)

Overview of three-dimensional shape measurement using optical methods

PURPOSE: To adapt the so-called nonlocal means filter to deal with magnetic resonance (MR) images with spatially varying noise levels (for both Gaussian and Rician distributed noise). MATERIALS AND METHODS: Most filtering techniques assume an equal noise distribution across the image. When this assumption is not met, the resulting filtering becomes suboptimal. This is the case of MR images with spatially varying noise levels, such as those obtained by parallel imaging (sensitivity-encoded), intensity inhomogeneity-corrected images, or surface coil-based acquisitions. We propose a new method where information regarding the local image noise level is used to adjust the amount of denoising strength of the filter. Such information is automatically obtained from the images using a new local noise estimation method. RESULTS: The proposed method was validated and compared with the standard nonlocal means filter on simulated and real MRI data showing an improved performance in all cases. CONCLUSION: The new noise-adaptive method was demonstrated to outperform the standard filter when spatially varying noise is present in the images.

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Adaptive non‐local means denoising of MR images with spatially varying noise levels

A combination of phase-shift with gray-code light projection into a three-dimensional vision system based on the projection of structured light is presented. The gray-code method is exploited to detect without ambiguity even marked surface discontinuities, whereas the phase-shift technique allows the measurement of fine surface details. The system shows excellent linearity. An overall mean value of the measurement error equal to 40 microm, with a variability of approximately +/-35 microm, corresponding to 0.06% of full scale, has been estimated. The implementation of the technique is discussed, the analysis of the systematic errors is presented in detail, and the calibration procedure designed to determine the optimal setting of the measurement parameters is illustrated.

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Three-dimensional vision based on a combination of gray-code and phase-shift light projection: analysis and compensation of the systematic errors

Optical coherence tomography (OCT), a method for depth-resolved imaging within turbid media based on the concept of low-coherence interferometry, rapidly evolved in the recent years with the development of a multitude of new functionalities and modalities. Biomedical research and diagnostics have been up to now the main driving forces for the reported applications and progress in OCT. The characteristics of OCT, precisely the ability to provide high-resolution images in a contact-free way, render this technique also attractive for a broad spectrum of research topics and applications outside the biomedical field. Consequently, a variety of novel applications for OCT and developments for the method itself have started to emerge. In this review we will give a detailed overview of the so far presented OCT-based methods and applications, ranging from dimensional metrology, material research and non-destructive testing, over art diagnostics, botany, microfluidics to data storage and security applications, and include new data from a study on penetration depths in various polymer materials as well as on birefringence imaging of different crystalline polymer structures. Finally, advanced and related OCT techniques are presented with high potential for future applications outside the biomedical field.

Beyond biomedicine: a review of alternative applications and developments for optical coherence tomography

Digital holography (DH) has emerged as one of the most effective coherent imaging technologies. The technological developments of digital sensors and optical elements have made DH the primary approach in several research fields, from quantitative phase imaging to optical metrology and 3D display technologies, to name a few. Like many other digital imaging techniques, DH must cope with the issue of speckle artifacts, due to the coherent nature of the required light sources. Despite the complexity of the recently proposed de-speckling methods, many have not yet attained the required level of effectiveness. That is, a universal denoising strategy for completely suppressing holographic noise has not yet been established. Thus the removal of speckle noise from holographic images represents a bottleneck for the entire optics and photonics scientific community. This review article provides a broad discussion about the noise issue in DH, with the aim of covering the best-performing noise reduction approaches that have been proposed so far. Quantitative comparisons among these approaches will be presented.

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Strategies for reducing speckle noise in digital holography

Wavelength scanning profilometry suitable for real-time surface shape measurement is proposed. A phase slope of the interference signal generated by a wavelength scan is measured at an individual image pixel on-line. The parallel outputs of these on-line measurements show a map of surface height in real time. Experiments where a tunable dye laser was used were conducted to simulate the real-time measurements of step objects with specular and diffuse surfaces. The results have shown that a height map is available at any moment during the wavelength scan, and the measurement accuracy of height increases as the scanning proceeds. For a scanning width of 25 nm, the accuracy was as high as 1 mum. Analyses of the measurement accuracy are given.

Wavelength scanning profilometry for real-time surface shape measurement

The accuracy and the measurement range of surface profilometry by wavelength scanning interferometry applied to diffusely reflecting surfaces are investigated. The influences of surface roughness and the imaging system in the interferometer are theoretically analyzed by derivation of the autocorrelation function of interferograms arising from wavelength scanning. By using a dye laser with a tuning range of 4.2 nm to a yield resolution of 39.1 μm, we have observed interferograms and their Fourier transforms and autocorrelations to study effects of defocusing and the size ratio of speckle to the CCD pixel for a plane diffuse object positioned normal to the incident beam.

Speckle decorrelation in surface profilometry by wavelength scanning interferometry.

In the previous paper [AJ, 105, 665 (1993)], we described speckle spectroscopic instrumentation and presented some observational results with data acquired by our speckle spectrometer. The shift-and-add method was used for the data reduction in the previous paper. We have revised the speckle spectrometer to augment the performance. The cross-correlation method is newly introduced for data reduction. The spectra of single stars γ Cas and α Per are reconstructed in nearly diffraction-limited angular resolution with the resolved Hα emission and absorption lines, respectively. It is shown that the cross-correlation method is superior to the shift-and-add method in the narrow bandwidth region. The results of α Per are compared with the photometric measurement of Procyon's spectrum which has the similar spectral type as α Per and they are shown to be fairly consistent

Stellar spectra reconstruction from speckle spectroscopic data

An adaptive optics (AO) system for solar observations at the Hida Observatory is described, which is composed of commercially available low-cost devices and fully controlled with software in standard personal computers. This AO system operates at a frame rate of more than 250 Hz by virtue of the image processing procedures used. From laboratory experiments, it was confirmed that this AO system was useful for the temporal wavefront-variation of less than 64 Hz. During solar observations at the Hida Observatory the AO system worked well, except for a few cases.

Solar Adaptive Optics System Based on Software Control

A solar adaptive optics system has been improved by using a high-speed electromagnetic deformable mirror and adopting a modified sum-of-absolute-differences algorithm in wavefront sensing. Results of laboratory experiments clearly show that the use of the mirror raises the temporal performance of the system. In solar observations, wavefront compensation using solar granules as a target is realized.

Susumu Kuwamura

Papers

Wavelength scanning profilometry for real-time surface shape measurement

Speckle decorrelation in surface profilometry by wavelength scanning interferometry.

Stellar spectra reconstruction from speckle spectroscopic data

Solar Adaptive Optics System Based on Software Control

Solar adaptive optics system using an electromagnetic deformable mirror