Calibration for the magnification of a submersible digital holographic camera during the study of particles in natural conditions

One of the key tasks in the study of plankton by underwater digital holography is to determine the coordinates and sizes of particles based on their digital images reconstructed from digital holograms recorded in the medium. The use of a lens in the object beam of a submersible digital holographic camera complicates the process of obtaining accurate data on the size and spatial characteristics of particles. To solve this problem, we proposed earlier the concept of an equivalent imaging optical system for obtaining an image in a digital holographic camera, based on a geometric-optical model of the image formation. The concept includes a mathematical model that establishes a one-to-one correspondence between the dimensional and spatial parameters of the digital holographic image and the displayed particles. This paper presents the results of using this model in a submersible digital holographic camera (DHC) during laboratory and field experiments. The proposed mathematical model requires a single calibration, since the error in determining the position and size of particles for different salinities changes slightly (within 5%). The results of the laboratory calibration can be used for field experiments without the calibration directly in the water area, taking into account the range of change in natural salinity.