The articles in this special feature of Measurement Science and Technology are devoted to an exciting area of fluid metrology pursuing the registration of flow velocities in three dimensions by particle holography—commonly termed holographic particle image velocimetry (HPIV) (Hinsch 2002). Already in 1993 this technique was considered to 'revolutionize the acquisition of velocity data in much the same way as did the inventions of hot wire anemometry and laser Doppler velocimetry' as E P Rood states in his foreword to the proceedings of the first workshop dedicated to the topic at the Washington ASME Fluid Engineering Conference (Rood 1993). The big step forward is to eliminate most of the depth-of-focus restrictions of classical PIV by a holographic recording of tracer particles. Thus, even non-stationary flows can be registered in a single record.A central concern of the early days was to explore optical set-ups suitable for improving particle-position resolution by using large recording apertures and for suppressing coherent noise. Furthermore, the evaluation of the holographic images required efficient hardware and software to scan and process the coordinates of particle images in a reasonable time. A sophisticated system relying on the state-of-the art experience and the utmost in processing hardware was producing first fields of thousands of three-dimensional velocity vectors (Barnhart et al 1994). Much profound research work on the main issues has been carried out in the meantime. Advances toward practical systems, however, needed fuelling by the recent technological developments of high-energy pulsed lasers and electronic image acquisition as well as the increasing performance of digital image processing.This recent progress led to a session on HPIV during the international PIV'01 conference at Göttingen, Germany (Kompenhans 2001), the creation of a worldwide working group (photon.physik.uni-oldenburg.de/hpiv) and in May 2003 an international workshop on holographic metrology in fluid mechanics at Loughborough University, UK (Coupland 2003). These workshop presentations have been elaborated and supplemented in the present special feature.The holographic velocimetry work presented here can be grouped into two sections according to the type of hologram recording—using either a physical carrier material or an electronic image sensor. Most researchers still use the somewhat anachronistic silver-halide emulsion of photographic film, especially when high resolving power is needed as in several application-specific topics. It offers still an unequalled resolution of up to 5000 line-pairs/mm at reasonable sensitivities to record even the low-power light scattered by tiny tracer particles, yet it requires laborious wet chemical processing.A good impression of the huge amount of data that can be stored on photographic film and the immense effort needed to analyse the reconstructed holographic images is given in the paper by E Malkiel et al. A straightforward in-line recording layou...

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 µm to 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, the evaluation of particle images is often conducted in the original recording setup. 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 (

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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 summarises some essential knowledge on noise in particle holograms, reviews recent approaches to handle 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. Some interpretations are checked by continuous-wave recordings in a model environment.

The purpose of the work presented here is to analyze the transition of an initially laminar air-into-air free jet to turbulence. An holographic PIV system based on light-in-flight holography (LiFH) combined with a switched reference beam is used to measure velocity vector fields of a whole volume. The analysis of the velocity fields reveals that the transition to turbulence is marked by a separation of vorticity and shear.