An Ultracompact Underwater Digital Holographic Camera for Study of Marine Microorganisms

A submersible pulsed digital holographic camera (named weeHoloCam) has been developed for monitoring microparticles in the ocean, mainly microorganisms. It is the most compact and lightweight of its kind. We have built two versions, both of the same dimensions: 9 cm diameter x 60 cm long. The first version with aluminium housing (500 m depth rating) weighs 3.5 kg while the second with titanium (2000 m depth rating) housing weighs 4.7 kg. They can be connected to a 50 m subsea cable for real-time analysis onboard a vessel or to a battery unit for autonomous deployment. weeHoloCam can be easily adapted for mounting on autonomous underwater vehicles (AUVs) as well. The two main optical components of the holographic system are a 532 nm, 3 ns pulsed laser and a CMOS sensor with 2464 × 2056 pixels, each of 3.45 µm size. The pulsed laser allows for dynamic particles with a relative speed of up to 100 m/s to be recorded without image degradation. weeHoloCam can record water samples up to 240 ml/s with the ability to image particles from 25 µm to 5 mm. Its 1 TB internal memory allows a maximum of 200,000 holograms to be captured in each deployment. This paper presents the design and development of weeHoloCam and results from a number of its ocean deployments. Image resolution of the system is evaluated in the laboratory and compared against those derived from the plankton images.