Measurement of suspended particle and plankton with a digital in-line holographic system

Study of the concentration and the distribution of underwater suspended particles is vital for understanding the mechanism of material transport in oceanographic. However, existing in situ monitoring instruments and methods (based on optical and acoustic method) have certain limitations. The holography technique provides a high-resolution, three-dimensional and non-intrusive measurement method to monitor the dynamics of marine particulate matter in situ. Underwater holography systems have been used to monitor plankton and other millimeter-sized marine organisms. In this paper, a digital in-line holographic particle tracking system which consists of a pulsed laser illumination, in-line optical structure, a quartz glass tank and a high resolution CMOS camera were developed. With a high-resolution CMOS sensor (4096×3072 pixels, 6 μm per pixel), the instrument generates sharply focused images of particles from approximately 100μm to 1000μm in diameter at up to 8 frames per second. the Fresnel transform to reconstruct the hologram and implemented automatic focusing is adopted. By calculating the coordinate values of the target particle, we can conclude the movement speed and further track its trajectory. Some plankton and standard polystyrene particles diameter of 205μm were used to assess the system performance and it was showed that the digital holographic system has the advantages over conventional image systems in analyzing location, shape, size distribution and trajectory of the particles. It is demonstrated that the holographic system is an inexpensive, rapid and unobtrusive approach for size variability of suspended sediment monitoring over time and space scale. This provides effective analysis method for analyzing the suspended sediment, plankton and other fine particles in ocean.