Optical Methods for Data Processing in Heat and Fluid Flow

8

A Particle Imaging and Analysis System for Underwater Holograms

J J Nebrensky, G Craig, G L Foresti, S Gentili, P R Hobson, H Nareid, G G Pieroni, and J Watson

Abstract

Pulsed holography is an important technique for the study of particle fields: it allows instantaneous, non-invasive high-resolution recording, and the later replay of real images from which one can obtain the size, shape, position, and – if multiple exposures are made – velocity of every object in the sample volume. This Chapter will discuss various issues encountered during the design of a hologram replay machine, with particular emphasis on the physical design of the instrument, and on the volume scanning and image processing techniques needed to pick out the useful information from the terabyte amounts of raw data produced on each hologram.

8.1 Introduction

Pulsed laser holography is an extremely powerful technique for the study of particle fields as it provides instantaneous, non-invasive high-resolution records, avoiding the distortion of the flow field associated with inserted probes, while from the replayed real images one can obtain the size, shape, position and – if multiple exposures are made – velocity of every object in the sample volume. The value of such experiments depends crucially on the quality of the reconstructed image: not only will poor resolution degrade size and shape measurements, but aberrations such as coma and astigmatism can change the perceived centroid of a particle, affecting position ...