## 2 Methods and Equipment  
### Digital Holographic Camera  
DHC is designed to measure dispersed particles using the principles of digital holography according to the in-line scheme [22; 23] (Gabor scheme); Figure 1. Note that this scheme does not impose high requirements for the coherence of radiation during hologram recording, since the path difference between the object wave (radiation scattered on particles) and the reference wave (radiation passed without scattering) is quite small. Therefore, we use a laser diode as a source of radiation. The DHC registers a digital hologram of the entire volume of the medium with studied particles per exposure (laser diode pulse). The particle images are reconstructed numerically from a preprocessed digital hologram by calculating the diffraction integral [1; 2; 3], and further processing of holographic images [23; 24], including determining the best image plane of the particle, allows detecting the spatial distribution of particles (three-dimensional coordinates of each particle), size, shape, speed, and direction of movement of each particle, and ensuring their recognition [24; 25] (Figure 1d). In other words, the DHC technology, which is digital hologramrecording, reconstruction of particle images from it, and processing of holographic images, makes it possible to create a virtual 3D image of the volume with test particles.  
Model particles are used for physical modeling in laboratory experiments and for calibration in field conditions. They represent opaque figures of regular shape (Figure 1b,c) placed by a photolithographic method on a 2.65 mm thick glass plate fixed in a metal frame (together we will call them the caliber, since in real experiments they are used to calibrate the DHC on magnification). The DHC design assumes that the calibers are placed in four positions [23], but for the purposes of this study, only two end positions were chosen--near the portholes (Figure 1a).  
Figure 2 shows one of the modifications of the DHC with a folded in-line scheme using prisms to reduce the size of the DHC while maintaining the length of the studied volume (depth of scene). The length of the studied volume is the total distance between the surfaces of optical parts bordering water ((3) and (6)). The length of the test volume and thus the volume itself are changed by replacing the calibrated control rods (8). The design provides for the installation of four calibers (7) on the path of a laser beam in the working volume to perform calibration at magnification. The main parts of the DHC are the illuminating (9) and recording (10) modules connected by a synchronization line, located in two strong deep-sea cases with portholes (3) and connectors. The lighting module is  
Figure 1: Digital holographic camera (DHC) optical scheme (**a**) 1—semiconductor laser diode with fiber output; 2—beam expander; 3—portholes; 4—complementary metal-oxide semiconductor (CMOS) camera; 5—calibers with model particles; 6—housing of the recording module; 7—housing of the lighting module; \(z_{\text{H}}\)—working volume length. Red arrows point the direction of laser light propagation. (**b,c**) Photographs of model particles. (**d**) Block diagram of the algorithm for digital hologram processing.  
designed to position a laser (1) and an optical system to form the optical radiation beam (2). The recording module includes an optical system to receive optical radiation (5), as well as a CMOS camera (4) and a control system for operating modes (synchronization). The modules are optically connected via the mirror-prism system (6) that provides registration information about up to 0.75 l of the studied volume (marked red) per exposure.