Luc Joannes

Improved three-dimensional imaging with a digital holography microscope with a source of partial spatial coherence

A digital holographic technique is implemented in a microscope for three-dimensional imaging reconstruction. The setup is a Mach-Zehnder interferometer that uses an incoherent light source to remove the coherent noise that is inherent in the laser sources. A phase-stepping technique determines the optical phase in the image plane of the microscope. Out-of-focus planes are refocused by digital holographic computations, thus considerably enlarging the depth of investigation without the need to change the optical focus mechanically. The technique can be implemented in transmission for various magnification ratios and can cover a wide range of applications. Performances and limitations of the microscope are theoretically evaluated. Experimental results for a test target are given, and examples of two applications in particle localization and investigation of biological sample are provided.

Phase-shifting schlieren: high-resolution quantitative schlieren that uses the phase-shifting technique principle

A quantitative autocalibrated high-resolution schlieren technique for quantitative measurement of reflective surface shape is proposed. It combines the schlieren principle with the phase-shifting technique that is generally used in interferometry. With an appropriate schlieren filter and appropriately tailored setup, some schlieren fringes are generated. After application of the phase-shift technique, the schlieren phase is calculated and converted into beam deviation values. Theoretical and experimental demonstrations are given. The technique is validated on a reference target, and then its application in a fluid physics experiment is demonstrated. These two examples show the potential of the phase-shifting schlieren technique that in some situations can become competitive with interferometry but with a much better dynamic range and with variable sensitivity. The technique can also be used to measure refractive-index gradients in transparent media.

An integrated optical set-up for fluid-physics experiments under microgravity conditions

An integrated optical set-up for fluid-physics diagnostics is proposed. The combined diagnostic tools are direct visualization, the schlieren technique, electronic speckle-pattern interferometry, differential interferometry, digital holography and holographic interferometry with a photorefractive crystal. The sharing of most of the optical lenses allows a compactness of hardware compatible with space-application requirements (the Fluid Science Laboratory for the International Space Station). Performances and complementarity of techniques are discussed. Interferometric methods for refractive-index measurement are compared and preliminary experimental results are given.

Interfacial turbulence in evaporating liquids: Theory and preliminary results of the ITEL-master 9 sounding rocket experiment

Abstract Evaporation of a pure liquid into a inert gas is studied theoretically and experimentally. In contrast with the case where the gas phase is made of pure vapor, the thermocapillary (Marangoni) effect strongly destabilizes the system, and results in intensive and often chaotic forms of interfacial convection. Theoretically, a generalized one-sided model is proposed, which allows the solution of the thermo-hydrodynamic equations in the liquid phase only, still taking into account relevant effects in the gas phase. The equivalent heat transfer coefficient (Biot number) to be incorporated in this one-sided model appears to be high, which results in an acceleration of transitions to polygonal chaotic patterns. Chaotic interfacial patterns driven by the Marangoni effect have indeed been observed during the ITEL-Maser 9 sounding rocket experiment flown in March 2002, in preparation of the CIMEX (Convection and Interfacial Mass Exchange) experiment foreseen for the International Space Station.

The reproducibility of a new power mapping instrument based on the phase shifting schlieren method for the measurement of spherical and toric contact lenses

PURPOSE To assess a new method of power measurement of soft and rigid contact lenses. The method is the phase shifting schlieren method, as embodied in the Nimo TR1504 instrument. MATERIALS AND METHODS Three Nimo TR1504 instruments were used to measure the power related dimensions of: (a) a range of custom toric rigid lenses; (b) a range of commercially available spherical hydrogel lenses; and (c) a commercially available range of toric silicone hydrogel lenses. The measurements were carried out using a standard ISO ring test protocol where independent tests were carried out under conditions of reproducibility. The analysis of the measurements was carried out using ISO methods which enabled the reproducibility standard deviation, SR, of the method to be calculated. RESULTS The results show that this new method has SR of 0.048D for spherical soft (hydrogel) lenses. This means the back vertex power of spherical soft lenses having a power in the range +/-10.0D can be determined to current ISO product tolerances with a single measurement. The method has SR of 0.059D for sphere power and 0.093D for cylinder power for toric soft lenses having powers in the range +/-10.0D and cylinder powers in the range +/-2.0D. A single measurement will determine sphere power to current ISO tolerance limits with 95% confidence while two measurements are required to determine the cylinder power to the same confidence level.

Optical deflection tomography with the phase-shifting Schlieren

We present a new optical deflection tomography method that takes advantage of the phase-shifting schlieren. The reconstruction algorithm is based on filtered backprojection. The instrument is well adapted for three-dimensional imaging of spatially sparse objects exhibiting large refractive index variations. It achieves a 35 μm resolution with a 3 mm depth of field. Its performance is illustrated with a bundle of fibers immersed in a matching index solution.

Compressive Imaging and Characterization of Sparse Light Deflection Maps

Light rays incident on a transparent object of uniform refractive index undergo deflections, which uniquely characterize the surface geometry of the object. Associated with each point on the surface is a deflection map (or spectrum) which describes the pattern of deflections in various directions. This article presents a novel method to efficiently acquire and reconstruct sparse deflection spectra induced by smooth object surfaces. To this end, we leverage the framework of compressed sensing (CS) in a particular implementation of a schlieren deflectometer, i.e., an optical system providing linear measurements of deflection spectra with programmable spatial light modulation patterns. In particular, we design those modulation patterns on the principle of spread spectrum CS for reducing the number of observations. Interestingly, the ability of our device to simultaneously observe the deflection spectra on a dense discretization of the object surface is related to a particular multiple measurement vector mode...

Compressive schlieren deflectometry

Schlieren deflectometry aims at characterizing the deflections undergone by refracted incident light rays at any surface point of a transparent object. For smooth surfaces, each surface location is actually associated with a sparse deflection map (or spectrum). This paper presents a novel method to compressively acquire and reconstruct such spectra. This is achieved by altering the way deflection information is captured in a common Schlieren Deflectometer, i.e., the deflection spectra are indirectly observed by the principle of spread spectrum compressed sensing. These observations are realized optically using a 2-D Spatial Light Modulator (SLM) adjusted to the corresponding sensing basis and whose modulations encode the light deviation subsequently recorded by a CCD camera. The efficiency of this approach is demonstrated experimentally on the observation of few test objects. Further, using a simple parameterization of the deflection spectra we show that relevant key parameters can be directly computed using the measurements, avoiding full reconstruction.

Optical tomography based on phase-shifting schlieren deflectometry

We present a new optical tomography technique based on phase-shifting schlieren deflectometry. The principle is that of computerized tomography. The three-dimensional profile is reconstructed from the deflection angles of rays passing through the tested object. We have investigated optical phantoms chosen in view of the characterization of dendritic growth in a solidification process. Promising results have been obtained with a homogeneous sphere and a bundle of 200μm fibers. The deviation angles exceed two degrees with a variation of the refractive index ▵n=0.025.

High-resolution shape measurements with phase-shifting Schlieren (PSS)

A new technique to measure shapes and deformation with a high resolution is proposed. It combines the conventional Schlieren technique principle with the phase-shifting approach generally used in interferometry. By an adequate Schlieren filter and an adapted set-up, some Schlieren Fringes are generated. After the application of the phase shift technique, the Schlieren phase is calculated and converted into beam deviation values, which are integrated to deduce the objects shape. Both theoretical and experimental demonstrations are given. The technique is first validated on a reference target. With a setup working in reflection, we have measured the curvature radius of a lens surface with accuracy better than 1%. Then an application in a fluid physics experiment is given. The shape of a liquid-gas interface in a conventional Marangoni-Benard experiment has been measured with a resolution of 30nm and amplitudes up to 50μm. The shape of MEMS has also been measured in a PSS microscope with a nanometric resolution. Finally, we propose an adaptation of the setup to make it possible the measurement of fast phenomena at video frame rate.