# Study of Marine Particles Using Submersible Digital Holographic Camera during the Arctic Expedition
## 2 Methods and Equipment  
### DHC Engineering  
Holography is used as a noninvasive method that registers a hologram and reconstructs an image of an ensemble of particles. Thus, information on each particle (individual plankton, settling non-living particle, oil droplet, gas bubble, etc.) contained in the registered volume of the aqueous medium can be stored, transmitted, and reproduced at a given time. Figure 1 shows an in-line hologram recording scheme used in the DHC (in-line scheme or Gabor scheme) [35; 36; 37; 38]. The interference pattern of the reference wave (radiation that passed by the particles) and the object wave (radiation scattered on the particles) is formed as a result of illuminating the studied volume with laser radiation. The camera registers this interference pattern and transmits it to the computer's memory. The distinctive feature of the DHC is the use of a lens for hologram recording to increase the field of view (the diameter of a beam illuminating the volume of the medium with particles). The lens represents a serial movie projection lens, 35 KP-1.8/70 (USSR) with the following characteristics: focal length (f = 70 mm), relative aperture (1/1.8), and a diameter of the circle of confusion in the field at a wavelength of 0.55-10 \(\upmu\)m.  
Figure 2 shows the 3D model of the DHC horizontal version. The horizontal design implies the vertical scanning of the water column for free flow of plankton in a vertical direction through the working volume of the camera (3). The design provides for four test objects (4) to be placed on the path of the laser beam in the working volume for further magnification calibration. The laser lighting module (2) and the recording module (1) with the CCD/CMOS camera are the main parts of the DHC, which are connected by a synchronization line. These parts are located in two strong, deep-water cases with  
Figure 1: In-line scheme of digital hologram recording: 1—laser diode, 2—beam expander, 3—lens, 4—CCD/CMOS camera, 5—computer. proproduces and connectors. All other DHC equipment is also placed in these two strong, deep-water sealed cases, which are structurally linked by a frame (6) to the base surfaces for interchangeable assembly. The cases and other devices of the complex are electrically connected by sealed connectors and offshore cables. The lighting module (2) (Figure 2) has a laser-diode fiber module (9) and an optical system to create an optical radiation beam (7, 8). The recording module (1) includes an optical system that receives the optical radiation (7, 8), a CCD/CMOS camera (10), and a control system for operating modes (synchronization). The units are optically connected by a mirror-prism system (5, 11) that provides the recording of data in 0.75 L of the studied volume per one exposure. The design of lighting and recording modules ensures the interchangeability of components (Figure 3). This principle creates the conditions for maintainability and redundancy in unfavorable field conditions as well as upgradability, thus updating or expanding the DHC capabilities or adapting them to various research tasks. Specifications of the DHC with a horizontal submersible device:  
Figure 3: DHC lighting (**a**) and recording (**b**) modules outside sealed cases. Figure 2: Horizontal version of the DHC: 1—recording module, 2—lighting module, 3—studied medium volume (working volume), 4—calibers (test objects for magnification calibration), 5—mirror-prism system to create a measuring channel in the medium (working volume), 6—submersible device frame, 7—35 KP-1.8/70 lens, 8—proproholes, 9—laser-diode fiber module, 10—CMOS camera, 11—measuring channel prisms.