Novel illumination system for off-axis holography of small particles

This illumination system is one component of a holographic camera, which is part of a multi-disciplinary, multi-national project (funded by the European Commission MAST III initiative) to use holography for the in situ recording and subsequent analysis of holograms of marine organisms and particles. Laboratory experiments have shown that the best results for off-axis recording are obtained using side illumination. Our design goal has been to provide even side illumination throughout the recording volume. The illumination system, the 'lightrod', is based on a cylinder made from a transparent material, containing a series of partial reflectors inclined and spaced along its length. An unexpanded laser beam is fed into the cylinder along its axis. Each reflector diverts a fraction of the beam into the recording volume. Two versions of the lightrod have been designed and built. In one version, the cylinder uses solid Perspex spacers with a small air gap in between; in the other, the spacers are hollow with thin glass windows in between. Both lightrod designs are discussed together with an analysis of the illumination profile, and their practical implementation is outlined. Both lightrod prototypes have been tested in an observation tank. Results from these tests are presented.