Advanced subsea imaging technique of digital holography: in situ measurement of marine microscale plankton and particles

Studying the amount, variety, and distribution of microscale particles and plankton at the global scale at different times and seasons is very important to understand ocean environments, and by reference, the global environment. It is important to image them in their natural habitats. Many optical techniques have been developed to meet this requirement. Most techniques suffer from one or another drawbacks, such as limited depth-of-field or low volume. Digital holography is an advanced optical imaging technique to image micro-objects, and it provides high-resolution recording, large depth-of-field and recording volume, and 3D viewing and tracking. This paper describes basic principles of in-line digital holography and provides common image processing methods. Four submersible digital holographic cameras, eHoloCam, RamaCam, weeHoloCam, and LISST-Holo, are introduced, as well as their image processing software. At the end, some limitations and challenges in the underwater holographic imaging systems are mentioned, and some possible solutions are discussed.