A system is described to facilitate the characterization of particles within a fluid contained in a vessel using an illumination system that directs source light through each vessel. One or more optical elements may be implemented to refract the source light and to illuminate the entire volume of the vessel. As the refracted source light passes through the vessel and interacts with particles suspended in the fluid, scattered light is produced and directed to an imager, while the refracted source light is diverted away from the imager to prevent the source light from drowning out the scattered light. The system can therefore advantageously utilize an imager with a large depth of field to accurately image the entire volume of fluid at the same time, facilitating the determination of the number and size of particles suspended in the fluid.