A Method for Detecting Small Vibrations of Particles by LDV for Particle Size and Electrostatic Charge Measurements [Translated]†

A mathematical model based on an ideal LDV (Laser Doppler Velocimeter) signal indicates that the phase and amplitude of a light scattering particle vibrating sinusoidally within the interference fringes can be obtained from the Fourier integrals of the signal. Although actual LDV signals are distorted, it is found empirically that the use of a Hanning window is very effective for the removal of the undesirable effect of distortion. Hence the size and electrostatic charge of a particle being excited in an AC electric field can be calculated using the Fourier analysis of the LDV signal. A computer simulation shows that the phase can be determined very accurately, say, within 0.1º error if so desired. The lower limit of the amplitude measurement is ca. 0.2 μm within a few percent relative error. Therefore the present method is expected to provide a convenient device for measuring the size and the charge of particles of around 1 μm in diameter. † This report was originally printed in FUNSAI No.35, 11-20 (1991) in Japanese, before being translated into English with the permission of Hosokawa Micron Corporation, Japan.