I want to do laser particle size analysis (Mie scattering) on a material . The literature lists two values for index of refraction, n_epsilon (1.79) and n_omega (1.88). What do these mean, and which one should I use for particle size analysis?
First one important thing. It depends on the size of the particles if you are allowed to apply Mie theory in this situation. If they are smaller than one tenth of the wavelengths, you take the 2/3 of the index of refraction of the ordinary ray + 1/3 of the index of the extraordinary (or 1/3 of each if the symmetry is orthorhombic or lower). If they are too large (larger than 1/10 of the wavelength), then Mie theory is not applicable. It seems to me your particles are uniaxial, so you have to find out which one is the ordinary and which one the extraordinary index of refraction (probably epsilon = extraordinary and omega = ordinary).
Thomas, thank you for the information. My particles are definitely greater than 1/10 the wavelength, but less than 2 microns.
Ok, what happens is that you get double refraction inside the particles when they are larger than about one tenth of the wavelength (or larger than the resolution limit). Accordingly I would expect that in particular depolarization values should be far off from the values predicted by Mie theory. How strong this affects particle sizing I cannot tell. Possibly you can still get reasonable values if you take an averaged index of refraction and assume Mie behavior, but I am not sure about this.have to admit that I don't know if this is correct.
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