# Study of Marine Particles Using Submersible Digital Holographic Camera during the Arctic Expedition
## 2 Methods and Equipment
### DHC Software  
The principle of building a complex (probe) as an information measuring system assumes the intelligent functionality of the sensor. In the DHC, these functions are fulfilled in the information unit using an on-board single-board computer with the basic DHC software supplemented by software focused on a measurement problem. This separation makes it possible to easily adapt the DHC to a particular task within the framework of one technical solution. The problem-oriented software can have three options:* To solve operational tasks according to the assessment of the integral characteristics of plankton with a high degree of averaging by volume;
* To solve monitoring tasks related to the classification of plankton individuals and other particles according to preliminary defined criteria, a set of features, and databases of plankton in the studied water area;
* To solve monitoring tasks related to the analysis of plankton behavior by identifying preliminary signs of behavioral responses. The result of problem-oriented software is a stream of digital data on marine particles. In this regard, the DHC serves as the same data \"provider\" as other sensors of the information measuring system. The information unit has an SSD drive to store data on board the probe. Problem-oriented software (DHC technology) makes it possible to automatically register a digital hologram (or a sequence of digital holograms) according to specified parameters and then, by mathematical processing, restore the image of each particle according to special computational algorithms. The mathematical processing of a digital hologram makes it possible to obtain images containing information on the distribution of both amplitude and phase of light waves [36; 37]. The phase-contrast image of a particle gives information on the volume of the particle, but in order to obtain it, we need to use one of the unwrapping methods [36; 37; 40; 41]. The three-dimensional coordinates of each particle (spatial distribution of particles in the studied volume), dimensions, space orientation, speed, and direction of movement of each particle can be determined within the in situ tasks according  
Figure 4: A hydrobiological DHC probe in combination with hydrophysical sensors of temperature, conductivity, and pressure on the deck of the Academician [PERSON] (1). Operator’s workstation for data acquisition and processing (2). to the amplitude images of particles. Thus, the DHC technology allows for the creation of a virtual 3D image of the volume with the studied particles. A lens makes the magnification ratio during the reconstruction of holograms dependent on the position of a particle and the refractive index of water. This dependence is described using four coefficients determined during calibration. The magnification ratio taken for different particle positions is one of the functions of the DHC software [42]. The DHC software includes a particle recognition and classification unit that is used to obtain information on individual particles of the medium and their belonging to various taxa. The values of features (decision tree) for automatic classification of particles are presented in Table 1. A rectangle is circumscribed around the reconstructed holographic image of a particle to ensure its automatic classification. Here it is critical to consider the length of this rectangle \(H\) and the morphological parameter \(M\), which is the width-to-length ratio of the rectangle, as well as the presence of outgrowths in the zooplankton individual. The values of the relevant criteria for the taxonomy of certain particles are taken on the basis of an experiment, which is one of the tasks of the expedition measurements. Taxa 1-7 in Table 1, characterizing mesoplankton, are chosen based on their highest prevalence in the aquatic medium. The _Suspension_ taxon is used to determine the turbidity of the medium.