The transmission signal of a narrow laser beam attenuated by particles in multiphase flow shows strong fluctuations, due to the presence and absence of particles in the beam path. The fluctuating transmission signal can be used for the measurement of both the particle size distribution (PSD) and particle concentration. In this paper, the theory of transmission fluctuation spectrometry with temporal correlation (TFSTC) is developed on the basis of the layer model and with the assumptions of geometric ray propagation and completely absorbent spherical particles. Experiments are realized in a focused Gaussian beam and the acquisition of transmission signals is achieved by using a high-resolution digital oscilloscope. Measurements on spherical and non-spherical particles over very wide ranges of particle concentrations are presented. The high concentration effects are discussed in detail and are used in the inversion algorithm to obtain reasonable results. As a result, the method of TFSTC is hopeful to be used for real-time, online applications.