By combining aspects of optical fiber probes used to determine local particle concentrations and local particle velocities and by rapid signal processing, it is shown that a single probe can determine local instantaneous particle fluxes. Providing a transparent cover to prevent a "blind zone" is shown to be critical in improving the probe performance and linearity. A simple mechanistic model is successful in predicting the performance of the optical system. The results of the probe are validated using particles glued onto both a rotating disk and a flat vibrating surface.