Some formulations of light scattering in laser flowmeters employing differential Doppler heterodyning

Abstract

The theory for the Doppler shifted light is developed by diffraction theory combined with galilean transformation. The theory developed is applied to a differential Doppler heterodyning technique in which two beams are brought simultaneously, but at different angles, into the same area, and the cross region made by the two beams becomes a probing volume. When the concentration of scattering particles is so low that only one particle is found in the probing volume, the theory can be classified into two cases of a single particle and many particles depending on the analysing time for the Fourier transform, i.e. whether the time, during which one particle passes through the probing volume, is longer than the observing time or not. When the concentration of particles in the probing volume becomes dense, the light scattered by the different particles interferes and the theory shows great complexity. The cases of two particles and many particles showing a very high density are studied. Comparisons are made for the difference between differential Doppler heterodyning and normal heterodyning techniques.