44 research outputs found

    Tomographic sensing of displacement fields

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    A wavelength scanning interferometry system is used to measure all the orthogonal components of the displacement field inside semitransparent scattering materials. A near infrared tunable laser illuminates a sample from multiple directions. The image of the sample is recombined with a reference beam on a photodetector array. As the laser frequency is linearly tuned during a scan, a sequence of speckle interferograms is recorded. In order to reconstruct the sample structure, Fourier transformation is performed on a pixel by pixel basis along the temporal axis of the 3-D data cube obtained. Multiple displacement sensitivities are achieved by introducing different optical delays between the reference and the illumination beams, which separate the reconstruction signals in the frequency domain. Phase changes due to mechanical loading of the sample can finally be measured and combined to obtain all orthogonal components of the displacement field in a convenient coordinate system. Controlled rigid body rotations of an epoxy phantom have been used to validate the methodology

    Depth-resolved phase imaging

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    Traditional full-field interferometric techniques (speckle, moiré, holography etc) encode the surface deformation state of the object under test in the form of 2-D phase images. Over the past 10 years, a family of related techniques (Wavelength Scanning Interferometry, Phase Contrast Spectral Optical Coherence Tomography (OCT), Tilt Scanning Interferometry and Hyperspectral Interferometry) has emerged that allows one to measure the volume deformation state within weakly-scattering objects. The techniques can be thought of as combining the phase-sensing capabilities of Phase Shifting Interferometry and the depth-sensing capabilities of OCT. This paper provides an overview of the techniques, and describes a theoretical framework based on the Ewald sphere construction that allows key parameters such as depth resolution and displacement sensitivity to be calculated straightforwardly for any given optical geometry and wavelength scan range. Finally, the related issue of robust phase unwrapping of noisy 3-D wrapped phase volumes is also described

    Single-shot areal profilometry using hyperspectral interferometry with a microlens array

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    We describe a single-shot technique to measure areal profiles on optically smooth and stepped surfaces for applications where rapid data acquisition in non-cooperative environments is essential. It is based on hyperspectral interferometry (HSI), a technique in which the output of a white-light interferometer provides the input to a hyperspectral imaging system. Previous HSI implementations suffered from inefficient utilisation of the available pixels which limited the number of measured coordinates and/or unambiguous depth range. In the current paper a >20-fold increase in pixel utilisation is achieved through the use of a 2-D microlens array, that leads to a 35Ă—35 channel system with an unambiguous depth range of 0.88 mm

    High-throughput single-shot hyperspectral interferometer for areal profilometry based on microlens array integral field unit

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    A single-shot technique to measure areal profiles on optically smooth and rough surfaces and for applications in non-cooperative environments is presented. It is based on hyperspectral interferometry (HSI), a technique in which the output of a white-light interferometer provides the input to a hyperspectral imaging system. Previous HSI implementations suffered from inefficient utilisation of the available pixels which limited the number of measured coordinates and/or unambiguous depth range. In this paper a >20-fold increase in pixel utilization is achieved through the use of a 2-D microlens array as proposed for integral field units in astronomy applications. This leads to a 35Ă—35 channel system with an unambiguous depth range of 0.88 mm

    Tomographic imaging of all orthogonal components of the displacement field in weakly scattering materials using wavelength scanning interferometry

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    This paper presents a wavelength scanning interferometry system which provides displacement fields inside the volume of semi-transparent scattering materials with high spatial resolution and threedimensional displacement sensitivity. This technique can be viewed as frequency multiplexed sweptsource OCT in which different channels carry information for specific displacement sensitivities

    Correction of refraction induced distortion in optical coherence tomography corneal reconstructions for volume deformation measurements

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    In this project, the depth-resolved full-field deformation of the porcine cornea under changing intraocular pressure was investigated by performing digital volume correlation (DVC) on the reconstructed volume images generated through swept source optical coherence tomography (SS-OCT). Posterior inflation test of porcine cornea sample for two load steps were performed and the distribution patterns of displacement and strain fields were produced. The error sources for the measurements were analyzed. The refraction induced OCT image distortion is a main error source for the measurement results. Then, a methodology was developed to correct the OCT distortion based on the Fermat’s principle

    Single-shot areal profilometry: Towards real-time surface quality control in additive manufacturing

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    © 2018 The authors and IOS Press. All rights reserved. Hyperspectral Interferometry (HSI) is a recently-proposed technique for measuring 3-D point clouds from an opaque object in a single shot. We propose a new application of HSI enabling single-shot 3D surface measurements of optically rough surfaces commonly found on additively manufactured and machined components. Using an additively manufactured sample, single-shot surface profiles were taken at a fixed distance to capture and reconstruct the surface profile. This enables the single-shot measurements of rough surfaces over many independent channels in a short time

    Depth-resolved imaging and displacement measurement techniques viewed as linear filtering operations

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    The last 5 years have seen the emergence of a family of optical interferometric techniques that provide deformation measurements throughout three-dimensional (3-D) weakly scattering materials. They include wavelength scanning interferometry (WSI), tilt scanning interferometry (TSI), phase contrast spectral optical coherence tomography (PC SOCT) and hyperspectral interferometry (HSI) and can be thought of as a marriage between the phase sensing capabilities of Phase Shifting Interferometry and the depth-sensing capabilities of Optical Coherence Tomography. It was recently shown that some closely related 3-D optical imaging techniques can be treated as shift-invariant linear filtering operations. In this paper, we extend that work to include WSI, TSI, PC SOCT and HSI as spatial filtering operations and also relate the properties of their transfer functions in the spatial frequency domain to their spatial resolution and phase sensitivity, for depth-resolved displacement measurements

    Single-shot profilometry of rough surfaces using hyperspectral interferometry

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    The combination of white light interferometry with hyperspectral imaging (“hyperspectral interferometry”) is a recently proposed technique for single-shot measurement of 3D surface profiles. We consider for the first time its application to speckled wavefronts from optically rough surfaces. The intensity versus wavenumber signal at each pixel provides unambiguous range information despite the speckle-induced random phase shifts. Experimental results with samples undergoing controlled rigid body translation demonstrate a measurement repeatability of 460 nm for a bandwidth of approximately 30 nm. Potential applications include roughness measurement and coordinate measurement machine probes where rapid data acquisition in noncooperative environments is essential

    Double-shot 3-D displacement field measurement using hyperspectral interferometry

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    A combination of a Michelson interferometer, a micro-optic element and a hyperspectral imager is used with broadband illumination to measure depth-resolved out-of-plane displacements without any scanning. Reference and deformed states of a transparent sample are recorded in single shots and used to evaluate the displacement field at different interfaces
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