Interferometric out-of-focus imaging of ice particles with overlapping images

Liquid lens for adaptive interferometric particle imaging

A liquid lens is used to realize an adaptive interferometric particle imaging system. The defocus parameter of the system can be changed instantaneously by adjusting the voltage applied to the lens. This enables adjusting the imaging conditions in real time. The setup is first tested on programmable rough particles generated by a digital micromirror device (DMD), and then with real glass particles under nanosecond pulse illumination.

Interferometric images of irregular rough particles in the presence of time-dependent index inhomogeneities in front of the imaging system

A digital micromirrors device is used to reproduce the speckle-like interferometric images that would produce rough particles. Time-dependent index inhomogeneities induced by a flame are added between the particle and the imaging system. The size measurements deduced from 2D-Fourier analysis of the interferometric patterns show a less than 10% error when the programmed object is fixed, and a less than 20% error when a scintillation of the object is programmed.

Corrected equations for the determination of a particle position using the interferometric particle imaging technique

A new interferometric particle imaging (IPI) simulator is developed based on Fourier optics. With this model, the optical field distribution of any kind of IPI setup at any location, such as on the focused and the defocused image planes, can be easily generated. In addition, the geometric center of a particle image and the image of the particle center on focused and defocused image planes can then be assessed in detail with the IPI simulator by the optical ray tracing method. The size effect of the defocused image is investigated, and the new expression of the calculated defocused image size is given, which can produce higher accuracy in particle locating.

3D-reconstructions for the estimation of ice particle's volume using a two-views interferometric out-of-focus imaging set-up

The characterization of ice crystals has important applications in airborne research and civil aviation. Interferometric out-of-focus imaging is a promising technique. We investigate in this study the impact of the 3D shape reconstruction of the particles from a pair of interferometric images on the estimation of the ice particle's volume. An interferometric image gives indeed the 2D autocorrelation of the particle's shape. As different shapes can exhibit a similar 2D autocorrelation, particles of different shapes can have similar interferometric images. In this study, the volume of ice particles is estimated from a pair of interferometric images (with two perpendicular angles of view). The relative error in the particle's volume estimation is shown to be around ΔV V≈30% depending on the choice of the initial 3D particle's shape. It appears that the choice of the shape of the particle for both angles of view has a lower impact on the estimation of the particle's volume than the other sources of errors due to image acquisitions themselves.

Interferometric particle imaging of ice particles using a multi-view optical system

Interferometric particle imaging enables particle size estimation in a wide range [from 10 μm to a few millimeters] depending on the optical system design. With a multi-view optical system, it is possible to extract more information about the 3D morphology of the particle. In this study, multi-view interferometric out-of-focus imaging of ice particles is performed in a backward-scattering configuration. It is used to estimate ice particle volume and thus to reduce uncertainty concerning particle size estimation. It is further used to better analyze the presence of nearby particles whose images overlap.

Two-dimensional shape retrieval from the interferometric out-of-focus image of a nonspherical particle-Part I: theory

Shape is one of the important characteristics of nonspherical particles. Herein, the two-dimensional shapes of several micrometer/millimeter-sized particles with a variety of geometrical forms are retrieved from simulated interferometric out-of-focus images using the Hybrid Input-Output algorithm. The particle concerned can be either a single particle or a complex particle with separate parts. The impact of the axial size of the three-dimensional particle on the retrieval of the two-dimensional shape (i.e., the projection of the particle on the image sensor) is analyzed, showing that an increase of the axial size increases the deviation of low frequencies in the interferometric out-of-focus image and eventually degrades the quality of the reconstructed shape. This study demonstrates the capability of the interferometric out-of-focus imaging technique on shape information retrieval for micrometer/millimeter-sized nonspherical particles.

Instrumentation for ice crystal characterization in laboratory using interferometric out-of-focus imaging

Airborne characterization of ice crystals has important applications. The extreme difficulty of realizing in situ tests requires the development of a complete instrumentation in the laboratory. Such an installation should enable design, development, test, and calibration of instruments in conditions as close as possible to real ones. We present a set of numerical and experimental tools that have been developed to realize ice crystal sensors based on interferometric particle imaging. The set of tools covers the development of complementary simulators for crystal growth and interferometric particle imaging predictions, experimental generation of "programmable" ice crystals, and instrumentation of a freezing column where different techniques as in-focus imaging, out-of-focus imaging, and digital in-line holography can be combined simultaneously for test and calibration.

Digital micromirror device as programmable rough particle in interferometric particle imaging

The 2D autocorrelation of the projection of an irregular rough particle can be estimated using the analysis of its interferometric out-of-focus image. We report the development of an experimental setup that creates speckle-like patterns generated by "programmable" rough particles of desired-shape. It should become an important tool for the development of new setups, configurations, and algorithms in interferometric particle imaging.

3D-shape recognition and size measurement of irregular rough particles using multi-views interferometric out-of-focus imaging

We realize simplified-tomography experiments on irregular rough particles using interferometric out-of-focus imaging. Using two angles of view, we determine the global 3D-shape, the dimensions, and the 3D-orientation of irregular rough particles whose morphologies belong to families such as sticks, plates, and crosses.