Holographic particle image velocimetry: from film to digital recording

Holographic particle image velocimetry (HPIV) offers potentially the best solution to volumetric measurements of the three-dimensional velocity fields in complex flows. However, the traditional film-based HPIV measurement is rather cumbersome, limiting its use to only a handful of groups worldwide. The newly emerged digital HPIV revolutionizes flow measurement science by providing a practical 3D velocimetry tool. It commands simple hardware that is similar to regular two-dimensional particle image velocimetry (PIV), yet it provides continuous (time-series) three-dimensional, three-component flow field data. Not only is the need for chemical processing eliminated, but also the cumbersome optical reconstruction is completely replaced by numerical reconstruction algorithms. Several breakthroughs have led to the development of the first practical and integrated digital HPIV systems. To explain the transition from film to digital recording, fundamental issues in HPIV are reviewed in this paper. Axial accuracy in HPIV measurement is ultimately limited by an inherent depth-of-focus problem, while information capacity is limited by inherent speckle noise. Information capacity is an important concept in HPIV, comprising the maximum acceptable seeding density multiplied by the sample volume depth along the optic axis. Both the axial accuracy and the information capacity are limited by the effective hologram aperture. The pursuit of a large hologram aperture in the past has resulted in further complexity in film-based HPIV systems. Digital HPIV, on the other hand, enjoys great simplicity of implementation and operation. A digital HPIV is also far more compact and rugged compared to existing film-based HPIV systems, making it suitable for duplication and commercialization. However, since digital sensors suffer from inferior pixel resolutions compared to films, the effective hologram aperture is much smaller in digital HPIV than that achievable in film-based HPIV. Alleviating this problem, digital HPIV also presents new possibilities in data processing such as the use of the complex amplitude of the reconstructed light wave to improve depth sensitivity and signal-to-noise ratio. Two examples of digital HPIV systems and measurement results are given. We believe digital HPIV can revitalize holographic particle imaging and bring it into the mainstream in much the same way that digital PIV brought PIV into widespread use a decade ago.