Transmission fluctuation spectrometry with temporal or spatial correlation is being developed as a new method for particle size analysis, in which the particle size distribution and particle concentration can be measured simultaneously. Based on the assumption of low particle concentrations, an analytical expression of the transmission fluctuation correlation spectrum can be obtained with a layer model. It is found that the measurement matches the theory very well as long as the particle concentration is low enough. However, as the particle concentration increases, the measured transmission fluctuation spectrum deviates gradually from the theory, which is a result of the high concentration effects. High concentration effects include particle overlapping and monolayer structure. In this work, theory and simulations on particle overlapping are studied. As a result, an empirical expression is found.