The extinction of radiation in suspensions is traditionally described by the Bouger-Lambert-Beer law (BLBL). Based on a quasicontinuum approach, the BLBL does not account for the discrete nature of particles or their spatial extension and arrangement. If an extinction measurement is made with a high spatial and temporal resolution, the transmitted intensity signal shows signi®cant uctuations. The strength of¯uctuation is related to the physical properties of the suspension and the process of spatial and temporal averaging. Exploiting this connection, it is possible to calculate the particle size distribution and the particle concentration from transmission measurements. This part of the series of papers provides a method for the temporal decomposition of the transmission's power spectrum, which permits the information on the particle size and concentration to be extracted from the seemingly irregular¯uctuation of the transmission signal.

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The extinction of radiation in suspensions is traditionally described by the Bouguer‐Lambert‐Beer law (BLBL). Based on a quasi‐continuum approach, the BLBL does not account for the discrete nature of particles or their spatial extension and arrangement. If an extinction measurement is made with a high spatial and temporal resolution, the transmitted intensity signal shows significant fluctuations. The strength of fluctuation is related to the physical properties of the suspension and the process of spatial and temporal averaging. Exploiting this connection, it is possible to calculate the particle size distribution and the particle concentration from transmission measurements. This second part provides an analytical solution for the description of transmission fluctuations in terms of the expectancy of the transmission square, including both variations of the beam diameter to particle diameter ratio and variation of the temporal resolution of the signal capture. The result also provides the basis for describing the transmission fluctuations for beams of variable diameter and allows to define a measurement volume for the case of focussed beams.

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