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 determ ine the optimized optical observation angles for different relative refract ive indices of media was derived. The sensitivity of the spatial frequency varying with the relative refractive index of media and the polarization an gle of the incident beams was simulated by using generalized Lorenz-Mie the ory (GLMT) [Gouesbet et al., Localized interpretation to compute all the co efficients g(n)(m) in the generalized Lorenz-Mie theory, J. Opt. Sec. Am. A , 7 (6) (1990) 998-1007 [1]; Gouesbet, Generalized Lorenz-Mie theory and ap plications, Part. Part. Syst. Charact., 11 (1994) 22-34]. 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 h as 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 parti cle diameter at a certain scattering angle. This study also demonstrated th at the beating effect between the refractive and the reflective rays can be ignored which was a critical issue for the conventional laser phase-Dopple r anemometry. By using fast Fourier transform, the spatial frequency can be measured accurately. (C) 2000 Published by Elsevier Science B.V. All right s reserved.