Didier Beghuin

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.

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.

Hyperspectral imager for components identification in the atmosphere

Several applications require the identification of chemical elements during re-entry of material in the atmosphere. The materials can be from human origin or meteorites. The Automated Transfer Vehicle (ATV) re-entry has been filmed with conventional camera from airborne manual operation. In order to permit the identification of the separate elements from their glow, spectral analysis needs to be added to the video data. In a LET-SME contract with ESA, Lambda-X has built a Fourier Transform Imaging Spectrometer to permit, in a future work, to bring the technology to the readiness level required for the application. In this paper, the principles of the Fourier Transform Imaging spectroscopy are recalled, the different interferometers suitable for supporting the technique are reviewed and the selection process is explained. The final selection of the interferometer corresponds to a birefringent prism based common path shear interferometer. The design of the breadboard and its performances are presented in terms of spatial resolution, aperture, and spectral resolution. A discussion is open regarding perspective of the technique for other remote sensing applications compared to more usual push broom configurations.

Interferometric precision with Fourier-based deflectometry

Optical components are routinely tested with inteferometric based techniques. It is show in this paper that Fourier based deflectometry method can be used for optical component inspection through very sensitive and precise wavefront reconstruction. The wavefront is expressed from the raw measurements of the wavefront derivatives as a Zernike polynomial expansion. The form of the polynomials permits absolute instrumental error characterization by repeated measurement of the element under test oriented at several azimuthal angles. It is shown that nanometric precision of Zernike based reconstructions can be performed and that the air turbulences are the experimental limiting factor to the instrumental precision.

Interferometric Sensitivity with Fourier Based Deflectometry

A precise characterization of optical components generally occurs via interferometric measurements. It is show in this paper that Fourier based deflectometry method can be used for very sensitive and precise wavefront reconstruction. The wavefront is expressed from the raw measurements of the wavefront derivatives as a Zernike polynomial expansion. The form of the polynomials permits absolute instrumental error characterization by repeated measurement of the element under test oriented at several azimuthal angles. It is shown in this paper that nanometric precision of Zernike based reconstructions can be performed and that the air turbulences are the experimental limiting factor to the instrumental precision.

Real-time phase shift schlieren

The phase shift schlieren (PSS), an improved method of the conventional schlieren imaging technique, enables direct quantitative measurements of optical path length gradients. Like all phase-shifting imaging methods, a successive acquisition of multiple images is required, therefore limiting the PSS use to quasi-steady-state phenomena. We show that PSS can be easily modified to access real-time data. A possible implementation is disclosed, as well as experimental results that validate the method. An application on gas jet is illustrated. We present the features and limitations of the real-time phase shift schlieren (RTPSS).