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The great challenge is encountered in photographic measurement, with regard to distinctly distinguishing the particle profiles from the background (continuous phase) in poorly illuminated multiphase reactors. In this work, a novel approach utilizing particle scattering is proposed to enhance the brightness contrast. The theoretical basis of this approach is described in detail. A single particle scattering model based on noncoherent scattering hypothesis is built up to evaluate the scattering property. The quantitative relationships between scattering flux and scattering coefficient/absorption coefficient are established. Furthermore, the partial light fluxes projected onto the focal plane by single particles of different sizes are also calculated. Combined with the particle scattering and dispersion, an optimal particle size of ∼0.5–1.5 μm is designated. On this basis, four representative oxide powders are tested as scattering particles, i.e., TiO2, Al2O3, CuO, and SiO2; in addition, the TiO2 particles are found to perform the best, based on the theoretical simulation.

A transmission signal measured on a flowing suspension of particles with a high spatial and temporal resolution shows significant fluctuations, which contain the complete information on particle size distribution and particle concentration. In Parts 1 and 2, the basic properties of signal fluctuations were studied for temporal averaging by a gliding time window and for spatial averaging by a circular beam of uniform intensity. However, the experimental implementation of such conditions is difficult. Now, the theory is extended for Gaussian beams of variable diameter and averaging by signal filtering in the frequency domain. This provides the basis for an experimental implementation by transmission of a laser beam and analog signal processing by an array of low pass filters.

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