Development of two underwater imaging apparatuses for suspended particulate matter

Here we present two apparatuses to measure the size and shape of suspended particulate matter (SPM) in situ. Target size of the SPM ranges from several micrometers to several millimeters. For some groups of SPM within the size range, we can identify “what they are” when we know their shapes. Imaging of SMP is useful. The first one of the two apparatuses is a cost-effective microscope equipped with a commercially available camcorder. Advantage of this apparatus is that we can measure for a relatively long time (for example eight hours). Another one is a digital holographic imaging system, whose depth in focus is more than a hundred times wider than that of conventional microscopes, enabling us to know the distances between individual particles. In other words, by use of digital holographic imaging system, we can measure the microscopic distribution of SPM. With these imaging apparatuses, continuous imaging of SPM has become feasible, allowing us to know the SPM's spatial distribution and its temporal change. Note that since these apparatuses have been developed to the spatial distributions of zooplankton, for the lighting was adapted infrared radiation to avoid an artificial increase of abundance due to photo taxis.