Abstract: It is shown in this paper that in geometrical space two polarization planes of the linearly polarized light waves scattered by particles in measuring ellipsoid constitute an angle of π/2, while in sequence of lime the signals sent out by two symmetric photodetectors in PLDA are separated by a phase angle π. This property of PLDA enables the improvement of SNR.The similarity between power spectrum of pholoeleclrical current of PLDA and probability density junction P_d (u_c) of investigated flow velocity has been proved theoretically and checked by agreement of obtained results with classical theory and generally accepted experiments.