An invention is described which allows measurement of the concentration of fluorescent particles in a flowing (laminar) fluid by imaging the flow with a video camera. A beam of illumination is used to illuminate the target particles. Imaging optics are arranged to view the focal plane to form an image of the multiple fluorescent sample particles in the flow stream; a camera records the image formed by the imaging optics, and a counting algorithm enumerates the particles. Operational parameters of the system are adjusted according to an initial estimate of particle density, for example flow rate, exposure time, and sampling interval. In addition, the counting algorithm is selected according to the estimated particle density.