Interferometric signals involving speckle waves invariably exhibit phase indeterminations. These indeterminations
arise at the zero-intensities of the speckle fields, or singularities, and show themselves as a net loss of modulation depth
of the interferometric signals. To bypass the difficulty associated with the processing of low modulated parts of speckle
interferometry signals, we propose a novel approach based on the Delaunay triangulation (DT). The method applies in
both situations of static and dynamic regimes, and is designated respectively by "sine-cosine DT filter" and "3D
piecewise processing" or 3DPP - 3D denoting the temporal and the two spatial coordinates of the recording. The task
consists in discarding purely and simply the under-modulated parts of the signal according to a user-defined binary
criterion, and filling the missing parts by interpolation. This first step provides a grid with nodes randomly occupied by
reliable phase values or empty. At the empty nodes, the computed phase values result from a DT ensuring that the
interpolation relies on the three closest well-behaved neighbors, followed by spline-fitting a smooth surface over them.
In a dynamic regime - where the benefits of the temporal approach are unanimously acknowledged - the empirical mode
decomposition is used to select the valid intervals and the Hilbert transform to compute phase data therein. We give a
detailed description of the DT filtering techniques, show their ability to offer the optimal compromise between spatial
and measurement resolutions depending on the user-chosen binary criterion and highlight some definite advantages over
classical filtering methods in terms of phase error reduction and algorithmic complexity.
The dynamics of liquid-liquid mixing is a difficult problem, encountered in many scientific and engineering branches.
Experiments in this field are mandatory to help building sound mathematical models, finding out the best fit parameters,
evaluating the degree of confidence of these models, or detecting traces of unwanted dangerous substances. The
investigations reported here are driven by water pollution concerns. For analyzing the water-pollutant blending behavior,
dynamic speckle interferometry has been preferred to more standard optical full field methods, like deflectometry, or
classical and holographic interferometry. The choice of this technique is vindicated. The opto-fluidic system is described.
A first series of results is presented, demonstrating the effectiveness of the technique and showing qualitatively how two
liquids blend in controlled conditions. In the last part of the paper, recently appeared processing schemes, including
empirical mode decomposition, Hilbert transform and piecewise treatment, give access to the numerical values of the
phase maps computed for each frame of the recorded sequence. These phase maps represent the refractive index
distributions integrated along the line of sight. They provide a better visualization of the dynamics of the blending
behavior and therefore an improved understanding of the phenomena. These encouraging preliminary results should
open the door to a full characterization of the method and to further flow investigations and diagnostics.
Many types of optical tweezers arrays have been proposed and developed for use in conjunction with microfluidics for
bio-chemical essays. Trap arrays rely on different methods allowing various degrees of flexibility and relative trapping
efficiencies. Among the different techniques currently employed, it is not simple to distinguish which ones produce
adequate performances for a given task in bio-chemistry. Experimental results for trapping efficiently diverse biological
specimens allow distinguishing between the properties of optical trap arrays based on techniques as different as
interferometry, holography, Fresnel or Fraunhoffer diffraction of diffractive structures, generalized phase contrast,
microlens assemblies, micro-mirrors matrices, and also clusters of individual tweezers. The bulkiness of those systems is
another important factor in the design of labs-on-a-chip; in particular the use of cumbersome microscope objectives can
be detrimental to chip optimization. Arrangements of tweezers produced with different concepts should be compared in
terms of efficiency, ease of use, and number of traps simultaneously exploitable
The basic convolution integral, Uf = Uo crossed circle h where Uo is a random object complex amplitude and h the impulse response of the system under consideration, serves to model the observed speckle field Uf. Depending on the choice of h, the simulated field is an objective or a subjective speckle pattern. The computation makes use of two consecutive Fast Fourier Transforms. In the reported examples, the object function represents a pure phase diffuser ruled by a uniform distribution. The probability density functions (PDF) of the simulated intensity and phase patterns fit very well with their analytical counterparts obtained under the classical Gaussian hypotheses. Phase maps exhibit the awaited singularities. Moreover, elements of second order statistics, as the autocorrelation functions, are in very good agreement too. Furthermore, subtle effects, as the dip of contrast in the focused image plane of partially developed speckle patterns, are also suitably disclosed. The linear model thus appears, all together, as conceptually easy, very flexible, computationally simple, very accurate for a wide range of experiments, and endowed with excellent predictive and speculative potentials.
In biological investigations, many protocols using optical trapping call for the possibility to trap a large number
of particles simultaneously. Interference fringes provide a solution for massively parallel micro-manipulation of
mesoscopic objects. Concurrently, the strength of traps can be improved by raising the slope of fringe profiles,
such as to create intensity gradients much higher than the ones formed by sinusoidal fringes (Young's fringes). We
use a multiple-beam interference system, derived from the classical Fizeau configuration, with semitransparent
interfaces to generate walls of light with a very high intensity gradient (Tolansky fringes). These fringes are
formed into a trapping set-up to produce new types of trapping templates. The possibility to build multiple trap
arrays of various geometries is examined; a high number of particles can be trapped in those potential wells. The
period of the fringes can easily be changed in order to fit traps sizes to the dimensions of the confined objects.
This is achieved by modifying several parameters of the interferometer, such as the angle and/or the distance
between the beam-splitter and the mirror. It is well known that optical trapping presents a great potential when
used in conjunction with microfluidics for lab-on-a-chip applications. We present an original solution for multiple
trapping integrated in a microfluidic device. This solution does not require high numerical aperture objectives.
In a two-beam interference experiment involving at least one speckle wave, intensity and phase are rapidly fluctuating distributions. There is no way to make a prediction of the evolution of the interference pattern aver distances greater than the correlation volume - as small as 3×3×100 μm3 for visible wavelengths and usual apertures. Most of the difficulties associated with a correct understanding and a good practice of speckle interferometry (SI) arise from this observation. It also explains why a technique simply ruled by the elementary two-beam interference or triangle formula raises nonetheless many problems. This contribution reviews some of the fundamentals of SI, mainly those concerned with the consequences of the random nature of the speckle phenomenon. It discusses what is thought to be the most interesting optical arrangements, modi operandi and phase extraction schemes, and finally presents selected applications. Constantly kept in mind is the idea to try to cope with the apparent disorder of the analyzed speckle distributions.
Speckle interferometry (SI) is used for the measurement of the shape change of x-ray mirrors. Initially flat under thermal equilibrium, the mirror, or "thermal bender", is deliberately and adaptively bent by means of well-controlled temperature gradients. As the deflection of an optically polished surface can be obtained by a number of methods, the choice of SI and its subsequent advantages are discussed. Quantitative results are reported, referring to four kinds of tests: conformity, stability, sensitivity and repeatability tests. SI is recognized to meet the expectations: it provides a simple, complete, sensitive and accurate control of the shape of the bent x-ray mirrors.
KEYWORDS: Doppler effect, Blood circulation, Skin, Imaging systems, 3D metrology, 3D image processing, In vivo imaging, Tissue optics, Wound healing, Digital micromirror devices
We present a combination of topography measurements based on digital fringe projection and blood flow imaging based on Laser Doppler Imaging (LDI). Both techniques are optical, non-contact and high-speed whole-field methods well suited for in-vivo measurements on the skin. Laser Doppler perfusion imaging is an interferometric technique used for visualization of two-dimensional (2D) maps of blood flow. Typically the measured sample has a surface with a specific 3D relief. In many cases the sample relief can be of importance for correct interpretation of the obtained perfusion data. We combined the topography and the blood flow data obtained from the same object. The structural information provided by the topography is completed by the functional images provided by LDI.
Shaping optical fields is the key issue in the control of optical forces that pilot the manipulation of mesoscopic polarizable dielectric particles. The latter can be positioned according to endless configurations. The scope of this paper is to review and discuss several unusual designs which produce what we think are among some of the most interesting arrangements. The simplest schemes result from interference between two or several coherent light beams, leading to periodic as well as pseudo-periodic arrays of optical traps. Complex assemblages of traps can be created with holographic-type set-ups; this case is widely used by the trapping community. Clusters of traps can also be configured through interferometric-type set-ups or by generating external standing waves by diffractive elements. The particularly remarkable possibilities of the Talbot effect to generate three-dimensional optical lattices and several schemes of self-organization represent further very interesting means for trapping. They will also be described and discussed. in this paper. The mechanisms involved in those trapping schemes do not require the use of high numerical aperture optics; by avoiding the need for bulky microscope objectives, they allow for more physical space around the trapping area to perform experiments. Moreover, very large regular arrays of traps can be manufactured, opening numerous possibilities for new applications.
Very high frequency oscillations of intense light fields interact with micron-size dielectric objects to exert dc optical forces that allow polarizable particles to levitate, to be trapped and to be bound. Such optical forces are also suitable to arrange cold atoms in optical lattices. Various assemblages of optical traps, including periodic arrays, can be constructed either with independent lasers, or with a single laser beam split into different parts later recombined by interference, as well as through the use of diffractive elements. These optical-well arrays serve as templates for writing and erasing dynamic two-dimensional and three-dimensional "optical crystals", composed of mono-dispersed polystyrene spheres in water. Subsequently, the crystals become diffractive structures themselves.
The association of micro-fluidics and optical trapping allows for the formation of optical traps into micro-channels. This leads to perform microchemistry experiments, such as fluorescence detection, on individual bodies attached to trapped particles. Self-trapping due to the optical binding force relates to the interaction between different dielectric objects located in an electromagnetic field; each one reacts not only to the field of the incident beam, but also to the induced fields radiated coherently by all other particles. Optical binding strongly influences the equilibrium state and the behavior of optical crystals. It must have the potential for creating collective effects.
Graphite-epoxy laminates are subjected to the "incremental hole-drilling" technique in order to investigate the residual stresses acting within each layer of the composite samples. In-plane speckle interferometry is used to measure the displacement field created by each drilling increment around the hole. Our approach features two particularities (1) we rely on the precise repositioning of the samples in the optical set-up after each new boring step, performed by means of a high precision, numerically controlled milling machine in the workshop; (2) for each increment, we acquire three displacement fields, along the length, the width of the samples, and at 45°, using a single symmetrical double beam illumination and a rotary stage holding the specimens. The experimental protocol is described in detail and the experimental results are presented, including a comparison with strain gages. Speckle interferometry appears as a suitable method to respond to the increasing demand for residual stress determination in composite samples.
The measurement of shape, displacement and deformations is often performed using interferometric methods, featuring nm to mm sensitivities and very high spatial and temporal resolutions. We first give a brief overview of interferometric techniques. Emphasis is laid on the wide purposes of these techniques. Then, we present a novel method using wavelet analysis to process live interference patterns. Further developments of the method are then presented. Finally, through two practical examples, we intend to highlight the interest of fringe processing by wavelet transform.
Speckle interferometry is an interesting tool for the measurement of micro-deformations and has found application in many different fields ranging from material testing to structural assessment. This kind of applications, however, has often been confined inside optical laboratories where operational conditions are optimal. This paper is devoted to the extension of speckle interferometry to various measurements--performed not inside well protected rooms but in testing halls dedicated to experimentation in civil engineering--where the environmental conditions are severe for an interferometric method.
The mechanical behavior of satellite telescope structures, subjected to non-uniform heating, as this occurs in orbit, is examined. In the laboratory, temperature gradients are created either by resistors or by IR lamps. Telescope structures are made of a material of very low thermal expansion coefficient--INVAR or carbon-carbon composite--in order to cope with stringent long term stability requirements in relation to temperature variation. Real time holographic interferometry is used to disclose the micro- deformation of the telescope. A careful implementation of the method is necessary, since, within the fixed temperature range for this study, very small deformations are expected. Monitoring of the heating conditions is achieved by an IR camera and thermocouples. Maps of temperature and of the corresponding interferometric out-of-plane deformation are obtained in parallel. The dynamic phase shifting technique allows to produce time-sequences of such deformation maps. These movies, and the temperature/deformation maps, highlight the structure deformation in a quantitative and dynamic way and suitably determine whether the structures meet their specifications.
A new method is presented that allows continuous deformation measurements to be performed. It is based on the use of the deformation-induced phase change produced at each pixel to compute an instantaneous phase with a dedicated phase- shifting algorithm. Examples in holographic, speckle and decorrelated speckle interferometry illustrate the interest of the method which provides both relative displacement phase maps and absolute temporal phase evolution curves.
An interferometer based on a single multi-functional holographic optical element (HOE) is presented. The interferometer is meant for flatness testing of quite large objects, not necessarily optically polished. Other features include two beam common-path arrangement, desensitization as compared to the classical (lambda) /2) figure, white-light illumination. Emphasis is then laid on automatic fringe pattern interpretation which makes use of an ad hoc phase-shifting procedure. Results obtained with computer disks are shown.
Two holographic methods suited for the measurement of flatness deviation are presented. The first one takes advantage of the basic holographic interferometry arrangement usually meant for deformation analysis of rough bodies. Simple modifications allow flatness measurement of polished surfaces with sub-wavelength sensitivity. Secondly, a new desensitized interferometer is described allowing the measurement of rougher objects, more frequently encountered in engineering practice. The key component of this interferometer is a diffractive optical element produced by recording two wave interference patterns. Desensitization factors ranging from 1 to 100 with respect to a Fizeau interferometer can be achieved. Flatness checks of computer disks demonstrate the possibilities of both interferometers. Deformation measurements performed with the desensitized interferometer are presented.
Moire-holographic techniques are utilized to analyse the mechanical behaviour of structural elements made of thermoplastic materials in the critical temperature domain. Several extensions of the basic set-up are described, which consist mainly in the computation of in-plane displacement components by a phase-shifting technique and in the computation of the derivatives of these components, either by pure numerical means or by a hybrid procedure involving an analog optical recording followed by an automatic phase calculation. The results of tests carried out with ring shaped perspex specimens subjected to compression, under various boundary conditions and different temperature domains, are also provided.
A holographic method suited for the measurement of flatness deviation is presented. This method takes advantage of the basic Fizeau arrangement usually meant for contouring analysis of relatively flat bodies. A desensitized interferometer is described allowing the measurement of rough objects, as frequently encountered in engineering practice. The key component of this interferometer is a diffractive optical element produced by recording two-wave interference patterns. Desensitization factors ranging from 1 to 100 with respect to a Fizeau interferometer can be achieved. Flatness checks of computer disks demonstrate the possibilities of the interferometer; deformation measurements performed with the desensitized interferometer on classical experimental mechanics specimens are presented: static testing on notched C-rings and on welds, and fatigue testing on notched three-point bend tests.
Two holographic methods suited for the measurement of flatness deviation are presented. The first one takes advantage of the
basic holographic interferometry arrangement usually meant for deformation analysis of rough bodies. Simple modifications
allow flatness measurements of polished surfaces with sub-wavelength sensitivity. Secondly, a new desensitized
interferometer is described allowing the measurement of rougher objects, more frequently encountered in engineering practice.
The key component of this interferometer is a diffractive optical element produced by recording two wave interference
patterns. Desensitization factors ranging from 1 to 100 with respect to a Fizeau interferometer can be achieved. Harness
checks of computer disks demonstrate the possibilities of both interferometers. Deformation measurements performed with
the desensitized interferometer are presented.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.