The principles and practice of holographic recording of plankton

We review the application of optical holography to the recording of plankton in both captive (tank) and natural (open-water) environments. Holography offers a unique ability to record the true three-dimensional spatial coordinates of both zooplankton and phytoplankton with high optical resolution in large volumes of water. Recent developments in the technology now enable volumes approaching one hundred litres to be recorded using the off-axis technique at a resolution of a hundred micrometres and smaller volumes of a few litres to be recorded using the in-line technique at a resolution of 10 μm or better. We discuss the principles of holographic recording and replay, and the special factors that must be considered when the objects are recorded in water but their images are replayed in air. Finally, we present an overview of the development of the technique from the first work of Katz, performed in 1966, to the latest deployment of the new HOLOCAM camera in October 2001, and speculate on future trends in the deployment of holographic cameras.