Underwater holography - past and future

摘要

Holography is a well-known optical technique which can provide valuable information on the location and distribution of small particles in three-dimensional space. For several years now, we have utilised holography for high-precision subsea inspection and measurement. One specific application which spurred much of our work was the need for high-precision inspection and analysis of plankton sizes, distribution and species identification. To this end we have developed a subsea holographic camera (HoloMar) for recording of plankton and other marine organisms in situ in their natural environment. This camera is unique in that it is able to record simultaneous in-line and off-axis holograms to cover a range of size of marine organisms from a few microns to tens of millimetres and at concentrations from a few particles per cubic centimetre to dense aggregates. Holograms of aquatic systems of up to 50000 cm~3 volume (off-axis) and 9500 cm~3 (in-line), have been recorded in situ, using a pulsed laser (Q-switched, frequency-doubled Nd-YAG, 532 nm). The use of a pulsed laser effectively "freezes" the scene at a given instant. Although the recording of the holograms takes place in water, replay of the image is carried out in the laboratory in air, using the projected (real) image mode of reconstruction. By precision translation of a computer-controlled video-camera through the replayed image volume and performing "optical sectioning" on the image, individual organisms can be isolated and their size, shape and relative location precisely determined. Image processing algorithms, will allow optimisation of the holographic image together with automated identification of individual species and enumeration of concentrations. The local interactions between different organisms and particles can be observed, recorded and quantitatively determined. Following initial laboratory and observation tank testing, the holo-camera was deployed in a sea loch in the West of Scotland to a depth of 100 m and over 300 holograms recorded. However, the HoloMar camera is physically large and heavy and difficult to deploy. It is also based on the use of photographic emulsions to record the holograms. To overcome some of these difficulties we are now developing a new holographic camera (eHoloCam) based on digital holography. In digital or "eHolography", a hologram is directly electronically recorded onto a CCD or CMOS sensor and then numerically reconstructed by simulation of the optical hologram reconstruction. The immediate advantages of this new camera are compactness, ease-of-use and speed of response, but at the expense of restricted off-axis recording angles and reduced recording volume. In this paper we describe both approaches, the use of holography for analysis of marine organisms and the results obtained in the field. We also describe recent work, using both photo and digital holography, to study the behaviour of sediments in river estuaries and outline future applications of underwater holography.