H. Nareid

HoloCam: a subsea holographic camera for recording marine organisms and particles

The HoloCam system is a major component of a multi-national multi- discipline project known as HoloMar (funded by the European Commission under the MAST III initiative). The project is concerned with the development of pulsed laser holography to analyse and monitor the populations of living organisms and inanimate particles within the worlds oceans. We describe here the development, construction and evaluation of a prototype underwater camera, the purpose of which is to record marine organisms and particles, in-situ. Recording using holography provides several advantages over conventional sampling methods in that it allows non-intrusive, non-destructive, high- resolution imaging of large volumes (up to 105 cm3) in three dimensions. The camera incorporates both in-line and off-axis holographic techniques, which allows particles from a few micrometres to tens of centimetres to be captured. In tandem with development of the HoloCam, a dedicated holographic replay system and an automated data extraction and image processing facility are being developed. These will allow, optimisation of the images recorded by the camera, identification of species and particle concentration plotting.

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. This 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.

Data extraction system for underwater particle holography

Pulsed laser holography in an extremely powerful technique for the study of particle fields as it allows instantaneous, non-invasive high- resolution recording of substantial volumes. By relaying the real image one can obtain the size, shape, position and - if multiple exposures are made - velocity of every object in the recorded field. Manual analysis of large volumes containing thousands of particles is, however, an enormous and time-consuming task, with operator fatigue an unpredictable source of errors. Clearly the value of holographic measurements also depends crucially on the quality of the reconstructed image: not only will poor resolution degrade the size and shape measurements, but aberrations such as coma and astigmatism can change the perceived centroid of a particle, affecting position and velocity measurements. For large-scale applications of particle field holography, specifically the in situ recording of marine plankton with Holocam, we have developed an automated data extraction system that can be readily switched between the in-line and off-axis geometries and provides optimised reconstruction from holograms recorded underwater. As a videocamera is automatically stepped through the 200 by 200 by 1000mm sample volume, image processing and object tracking routines locate and extract particle images for further classification by a separate software module.

Optimizing replay intensity and resolution in aberration-compensated off-axis holograms by ambient humidity control

Abstract In hologrammetry it is desirable to reconstruct the real image rather than the virtual image as the latter must be viewed at a distance through the window of the holographic plate itself. When a scene is located in water but the image is replayed into air, it is necessary to correct for the refractive index difference by reconstructing the image with shorter wavelength illumination and changing the beam angle to satisfy the grating equation. However, this means that the Bragg condition may no longer be satisfied during replay, reducing the diffraction efficiency and decreasing the signal-to-noise ratio of the reconstructed images. Changing the replay beam angle to satisfy better the Bragg condition makes the images brighter but also renders them unusable by increasing the optical aberrations. Our solution is to alter the Bragg properties of the hologram by altering the humidity of the surrounding atmosphere. This approach has been experimentally demonstrated for Agfa 8E56HD emulsions by measuring the brightness and resolution of a reconstructed real image from an off-axis hologram over a humidity range from 6 to 93 per cent. The emulsion swelling and its effect on the Bragg properties of the hologram were modelled using the Flory-Huggins theory of polymer swelling.

Development and sea trials of a subsea holographic camera for large volume in-situ recording of marine organisms

We describe the development, construction and sea testing of An underwater holographic camera (HoloCam) for in situ recording of marine organisms and particles in large volumes of sea water. HoloCam comprises a laser, power supply, holographic recording optics, and plate holders, a water- tight housing and a support frame. Added to this are control electronics such that the entire camera is remotely operable and controllable from ship or dock-side. Uniquely the camera can simultaneously record both in-line and off-axis holograms using a pulsed frequency double Nd:YAG laser. In- line holography is capable of producing images of organisms with a resolution of better than 10 micrometers . Off-axis holograms of aquatic systems of up to 50,000 cm3 volume, have been recorded. Following initial laboratory testing, the holo-camera was evaluated in an observation tank and ultimately was tested in Loch Etive, Scotland. In-line and off-axis holograms were recorded to a depth of 100 m. We will present result on the ste dives and evaluation of the camera performance.

A camera to record underwater particles using simultaneous in-line and off-axis hologrammetry and its associated replay facility

Summary form only given. The HOLOMAR collaboration have designed and built an underwater holographic camera specifically designed to record marine plankton to a maximum depth of 100 m below sea level. The camera records holograms using both the in-line and the off-axis techniques simultaneously. The laser is a specially designed frequency doubled Nd-YAG laser, using a passive Q-switch and LBO doubler. It produces a single pulse with an energy of 700 mJ and duration of less than 10 ns. The oscillator-amplifier design results in a single longitudinal mode with a measured coherence length of over 2 m. Considerable care was taken in the optical design to minimise the effects of distortion arising from the change in refractive index. The computer controlled, fully automated replay machine and image processing and analysis systems allow large amounts of data to be extracted from the holograms without manual intervention.