Optical limits of particle concentration for multi-dimensional particle sizing techniques in fluid mechanics

The interferometric particle imaging and the global phase Doppler techniques for multi-dimensional particle sizing are based on out-of-focus images of the particles illuminated in laser light sheets of finite width. For a given optical arrangement, an increase in particle number concentration will eventually lead to overlapping particle images on the recording medium, and this in turn will degrade or even prohibit the correct processing of individual particle images for size. The probability of overlapping images should therefore be minimized, which places constraints on the design of the optical system. These constraints are rigorously derived, using Poisson statistics to describe the particle spatial distribution in the flow. The main result consists of an overlap probability coefficient as a function of number density and the optical parameters of the system. However, the resolution of the third position coordinate normal to the laser sheet and the measurement size range also result from this analysis. These results are a prerequisite for designing such systems.