Digital Holography Based Particle Tracking Velocimetry

Digital holography appears to be the next-generation technology for holographic diagnostics of particle fields and has been applied to holographic particle tracking flow measurements. In the digital holography, the Fresnel or Fourier holograms are recorded directly by the CCD or CMOS cameras and stored digitally. No film material involving wet-chemical or other processing is necessary as in optical holography. The reconstruction of the wave-field, which is done optically by illumination of a hologram, is performed by numerical methods. Digital holography also enables the use of complex amplitude information inaccessible by optical reconstruction, thereby allowing flexible reconstruction algorithms to achieve optimization of specific information. However, due to the poor pixel resolution of CCD or CMOS cameras as compared to that of holographic films, the digital holography gives poor depth resolution for images. This problem severely impairs the usefulness of digital holography especially in densely populated particle fields. In this paper the authors present a simple digital in-line holography recording setup and numerical reconstruction process based on the Fourier transform and convolution schemes. The use of the complex amplitude, available from these reconstructed waves, in particle extraction significantly improves particle axial-location accuracy compared with other numerical reconstruction schemes that are merely traditional optical reconstruction. This method also alleviates the speckle noise problem intrinsic to in-line particle holography since speckles and particles can be clearly differentiated in complex wave-field.