The imaging properties of scattering particles in laser beams

The scattering properties of spherical particles crossing a laser beam deviate from the Lorenz-Mie solution for particles both smaller and larger than the beam dimensions. For smaller particles the local beam properties at the position of the particle can be used in the Lorenz-Mie calculations, whereas for larger particles the varying beam properties over the surface of the particle must be accounted for. The Fourier Lorenz-Mie theory and the extended geometrical optics theory offer two methods for analysing such situations. These two methods are used to investigate the scattering fields of various scattering particles and to demonstrate the agreement of the results. The results are useful for understanding the signal origins in laser-Doppler and phase-Doppler techniques, especially the `trajectory effect' or `Gaussian beam effect'.