The Phase-Doppler Method Applied to Very Small Particles

Using a computer model based on Lorenz‐Mie and generalized Lorenz‐Mie theory, various optical confiugurations of a phase‐Doppler system were analysed with regard to their suitability for diameter measurements in the sub‐micron range. The major concern in this size range is multi‐valuedness of the phase‐diameter characteristic, the relatively small signal‐to‐noise ratio obtained with the very low scattered intensity and the small value of the phase difference to be measured. It is shown numerically and by experiment that for particles in a free stream the multi‐valuedness and the shot noise need not prohibit measurements in the sub‐micron size range. The major source of phase error results from light scattered form objects or material other than the particle inside the measuring volume. Using an optical set‐up with nearly counter‐propagating incident beams and a large angle between the detectors, measurements were obtained for particle diameters down to 200 nm, and it is estimated that with some improvements in receiver optics measurement down to 100 nm will be possible.