The transmission fluctuation spectrometry (TFS) is a recently-developed method for real-time, online/inline particle analysis in two-phase flows, whereby the particle size distribution (PSD) and particle concentration can be measured simultaneously. This study presents a new technique of data processing to the fluctuating transmission signal. Instead of low-pass filters, band-pass filters are employed to improve the resolution of the measurement on particle size distribution. Based on the layer model, an analytical expression of the spectrum of the fluctuating transmission through a monolayer is derived and hence the spectrum of the fluctuating transmission through a 3-dimensional suspension is formulated. The comparison between simulation and theory at low concentrations shows a satisfactory match. Measurements on a mono-modal suspension are presented. It is found that the measurements using band-pass filters are of better resolution in the PSD than those with low-pass filters. (c) 2007 Elsevier B.V. All rights reserved.