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The Underwater Vision Profiler (UVP) was developed to quantify the vertical distribution of macroscopic particles and zooplankton 100 mu m in size. The smaller size limit is fixed by optical resolution, whereas the larger size limit is determined by the volume of water illuminated per image. The new fifth generation instrument (UVP5) is compact (30 kg in air) and operates either as a stand-alone instrument with an independent power supply for use on a mooring or free-drifting array, or as a component of a Conductivity, Temperature, and Depth (CTD)-rosette package. Images are recorded at a frequency up to 6 Hz. If the UVP5 is interfaced with a CTD, these images are acquired and analyzed in real time. Images are recorded every 20 cm at the 1 m s(-1) lowering speed. The current maximum deployment depth is 3000 m. The recorded volume per image is 1.02 L, and the conversion equation from pixel area to size in mm(2) is S(m) = 0.003S(p)(1.3348) where S(p) is the surface of the particle in pixels and S(m) the surface in mm(2). Comparisons between the earlier UVP versions and UVP5 indicate that images ranging in size from 105 mu m to 2.66 mm are identical so historical and contemporary data sets can be compared.