Klaus D. Hinsch

Holographic particle image velocimetry

Holography is truly the key to three dimensions in particle image velocimetry, i.e. the measurement of all spatial components of the velocity vector—and this over a deep measuring field. Sophisticated instruments have been designed that successfully tackle practical problems such as the low scattering efficiency of particles, the inferior depth resolution or the aberrations and distortions in the reconstruction. Furthermore, efficient strategies are introduced to interrogate the holographic storage and process the huge amount of data towards a final flow field representation. Recently, phase-sensitive metrology, familiar in many fields of experimental mechanics, has been examined for use in particle velocimetry. Suitable methods are holographic and speckle interferometry or the optical processing of data for three-dimensional correlation. While in these techniques the power of optics is unrivalled, the practical advantage of video and digital techniques over photographic recording is obvious. The electronic version of speckle interferometry (ESPI/DSPI) is a well-established method used in laser metrology and has received further exploitation for applications in flow analysis recently. Finally, the state-of-the-art of digital particle holography is reviewed to allow estimates of its future in experimental flow analysis.

Electronic speckle pattern interferometry system for in situ deformation monitoring on buildings

An electronic speckle pattern interferometry system has been developed for in situ measurements of microdeformations on buildings or monuments. Design of the optomechanical setup received special attention to allow firm and steady contact between object and optical head. Features are the miniaturization of the light source (laser diode) and pickup (CCD camera). Image processing was carried out with a personal computer. Deformation monitoring was demonstrated successfully over periods of several weeks in the study of decay mechanisms of walls in an historic church.

Spatial phase shifting for pure in-plane displacement and displacement-derivative measurements in electronic speckle pattern interferometry (ESPI)

Hitherto no method, to our knowledge, was known to incorporate spatial phase shifting for the measurement of pure in-plane displacements. We demonstrate that the modified Duffy two-aperture configuration [Opt. Lett. 22, 1958 (1996)], which is sensitive to only the in-plane displacement component and offers increased sensitivity, lends itself to measurement with spatial phase shifting. The configuration can also be used for obtaining displacement derivatives by the introduction of shear with the tilt of a mirror.

Light-in-flight holographic particle image velocimetry for wind-tunnel applications

Holographic particle imaging techniques for air-flow investigations are mainly limited to small-scale laboratory experiments. The two main reasons are the limited light power available in conjunction with small tracer-particle sizes, which must be in the order of 1 µ mt o properly probe air flows, and the increased background noise from out-of-focus particles in deep volumes preventing investigations with higher particle densities. To ensure a good accuracy of the velocity measurements by faithful reconstruction geometry, th ee valuation of particle images is often conducted in the original recording set-up. The time-consuming scanning process, however, blocks the flow facility during evaluation—a disadvantage for measurements in costly industrial wind tunnels. For an alternative, we have introduced off-site reconstruction and evaluation. In recent papers (Hinrichs et al 1997 Opt. Lett. 22 828–30, Herrmann and Hinsch 2001 DLR-Mitteilung 2001–03: 4th

Speckle metrology with novelty filtering using photorefractive two-beam coupling in BaTiO3 crystal

Abstract Two-beam coupling in BaTiO 3 crystal is suggested for the tracking novelty filtering in speckle metrology. Two schemes are discussed: one for point measurement of displacement and another for full-field display. The processing is in real time and in line. The optimal conditions for a high contrast of fringes are introduced. Experimental results are given, too.

Deformation monitoring on ancient terracotta warriors by microscopic TV-holography

Abstract In order to investigate deterioration processes on the famous 2000-yr-old Chinese Terracotta warriors, deformation measurements were performed with TV-holography. The aim of the investigations is to estimate the suitability of several conservation agents and procedures, which are developed to preserve the extremely endangered paint layers of the figures. Due to the occurrence of small-scale deformation areas the field of view had to be reduced. A microscopic system was constructed and typical object deformations due to shrinkage and swelling are presented. To investigate the individual layers of the multilayered objects a short-coherent superluminescence diode instead of a laser diode is used in a further modification. By changing the path length of one of the interfering beams it is possible to select a region limited in depth where deformations are measured even if it is located below the surface. Results on an artificial test object and on original terracotta fragments demonstrate the capabilities of this new short-coherence TV-holography system.

Two-wavelength electronic speckle-pattern interferometry for the analysis of discontinuous deformation fields.

Ambiguity in the conversion of phase measurements to deformation values restricts the applicability of electronic speckle-pattern interferometry. The use of two wavelengths greatly relaxes this restriction.

Light-in-flight holography for visualization and velocimetry in three-dimensional flows

Flow visualization and particle image velocimetry of deep volumes are achieved by holographic recordings. Light-in-flight holography is applied to avoid noise from out-of-focus regions during interrogation of the reconstructed image by use of a ruby laser source of small coherence. The scheme permits reconstruction of thin layers in depth without disturbance by the rest of the field. The location of the layers is selected by the position of a reconstructing slit aperture on the hologram. Applications to tracer particles in water and visualization by smoke of an air flow are given.

Signal quality improvements by short-coherence holographic particle image velocimetry

Particle holography has proven to be a useful metrological tool for three-dimensional flow velocimetry. To cope with the problem of noise from out-of-focus particles the technique of light-in-flight holography (LiFH) has been introduced that utilizes properties of a laser source of short coherence. While the feasibility of the method has been shown earlier, a more profound quantitative analysis of its performance was still required. The present paper briefly summarizes some essential knowledge on noise in particle holograms, reviews recent approaches for handling noise in deep-field particle holography and presents first experimental checks of these concepts on short-exposure holographic recordings of particle fields in a wind-tunnel flow. The performance of ordinary and short-coherence particle holography are compared directly by operating the same laser in either long-coherence or short-coherence mode. It is established experimentally that performance is also related to laser beam quality.

Mural inspection by vibration measurements with TV-holography

A commonly encountered problem in the conservation of historical murals is the identification of sections in the plaster that have detached from the wall and thus threaten to fall off. Commonly, walls are inspected by the acoustic response to a gentle finger-tapping (percussion method). Since this is a costly and cumbersome technique, means for a more automatic inspection are searched for. A TV-holography system of increased sensitivity in combination with acoustic excitation of the object is shown to be a new and powerful tool for monitoring of loose areas. It has the advantage of non-contact and remote operation which, for example, is extremely useful in large buildings. Principles of the method, experimental results obtained at an artificial wall in the laboratory, and a thorough comparison of results from historical sites gained by the traditional percussion method and the new technique are presented. The latter shows very good agreement in the assessment of wall quality and thus is evidence of the suitability of the optical equipment for tasks in conservation.