Effect of refractive index in optical particle sizing by using spatial frequency method

Investigation of optical particle sizing by using spatial frequency method was conducted by analytical and experimental methods. An equation to determine the optimized optical observation angles for different relative refractive indices of media was derived. The sensitivity of the spatial frequency varying with the relative refractive index of media and the polarization angle of the incident beams was simulated by using generalized Lorenz-Mie theory (GLMT). It was found that a large observation angle has very little effect on the spatial frequency. The relationship between the spatial frequency and the particle diameter has been successfully described. Experimental study was conducted to measure the light scattered from a spherical particle and it demonstrated that the spatial frequency of the fringe pattern is uniquely dependent on the particle diameter at a certain scattering angle. This study also demonstrated that the beating effect between the refractive and the reflective rays can be ignored which was a critical issue for the conventional laser phase-Doppler anemometry. By using fast Fourier transform, the spatial frequency can be measured accurately.