In PIV, tracer particles are added to the flow, which are illuminated by a double pulse. The scattered light of a light section is then recorded photographically. This produces sharply defined particle pairs from which the direction and magnitude of the particle displacement and thus two-dimensional velocity vectors can be determined if the pulse spacing is known [1,2]. It is immediately obvious why this technique is initially limited to only two dimensions. If a particle field is illuminated in depth, scattered light is obtained from the entire volume, and the depth assignment of the respective scattered light causes difficulties. To avoid this problem, the beam of a pulsed laser is formed into a light section in two-dimensional PIV. This mainly produces scattered light from a clearly defined plane in the flow volume, which is easy to localise. If the scattered light of the double particles is recorded with a CCD camera, the displacement vectors in this two-dimensional section can be determined in the computer using correlation techniques. Click here for film

If the principle of PIV is also to be applied in three dimensions, the scattered light from a deep volume must be imaged sharply and at the same time it must be possible to assign the depth of the tracers.

This can be realised with holographic recording techniques. The scattered light of the tracers is superimposed with a reference beam as object light at the location of the hologram disc. The entire particle field is thus recorded holographically and can be reconstructed and analysed with the corresponding reference beam after the hologram has been developed. If the hologram is recorded with a double pulse, the double particles can be reconstructed accordingly with the aid of the reference beam.

The images of the double particles in the deep volume are to be analysed layer by layer, analogous to two-dimensional PIV. Using a camera, planes lying one behind the other are imaged separately and analysed using correlation techniques [3].

If the hologram is recorded with a long coherence length, the scattered light of the tracer particles is reconstructed from the entire illuminated volume when the hologram is illuminated with the reference beam. When focussing on a plane within the flow volume, scattered light from areas outside the focal plane is therefore always inevitably obtained. The sharply focussed particle images are superimposed with a noise background, which reduces the reliability of the method. Holographic recording is based on the interference of object and reference beams at the location of the hologram disc under the condition that the path length difference between the two beams is smaller than the coherence length of the light source used. If a short coherence length and an obliquely incident reference beam are used for holographic recording, only the object light originating from a limited layer in the object space that fulfils the condition of maximum path length difference is stored at each point of the hologram plate. Object light from other volume areas is suppressed during the holographic reconstruction of the recorded image. This means that the reconstruction can be analysed layer by layer without the information within this layer being overlaid by a noise background from another position in the depth. A similar technique is used in shape measurement or as optical coherence tomography (OCT) under the heading of coherence radar. It has its origins in light-in-flight holography, which enables the visualisation of the propagation of optical wavefronts in space and thus serves as a high-speed camera [4].

1. Hinsch, K. D.: Particle Image Velocimetry. In: Sirohi, R. S. (ed.): Speckle Metrology, Marcel Dekker, New York, 235-324, 1993.
2. Kompenhans, J., Raffel, M., Willert, C. E.: Particle Image Velocimetry - A practical Guide, Springer, 1997.
3. Hinsch, K. D.: Three dimensional particle velocimetry. In: Dracos, T. (ed.): Three-dimensional velocity and vorticity measuring and image analysis techniques, Kluwer Academic Publishers, Dordrecht, 129-152, 1996.
4. Abramson, N.: Light-in-Flight or the Holodiagram: The Columbi egg of Optics, SPIE Optical Engineering Press, Bellingham, Wash. 1996.

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