Mr. Wolfgang,

I see you posted this quite some time ago, but I'll put in my two cents. I'm not sure what you're doing, so I'll go ahead and post a few solutions depending on application.

1. You're doing laser-light diffraction and you need "accurate" real and imaginary refractive index values. The RI of the material can be obtained using the Becke line method. If birefringent, then the average of two RIs is typically used. The imaginary part is then assumed to be in one of three categories: perfectly transparent, semi-transparent or opaque. Simply take a guess as to what category your sample belongs and go with it. For most non-spherical, anisotropic particles, calculated imaginary refractive indeces won't make a dent in the total error emanating from the irregular shape and birefringence of real world samples.

2. You're working with laser-light diffraction and perfectly spherical, isotropic and transparent particles. In this case, it may make sense to find truer RI values. Keep in mind that true RI values do not arise from Mie theory per se, but from wave propagation theory. What you're really after is not to calculate the scattering function (the instrument does that), but to find to what extent your sample absorbs the incident light while it is refracting. To do this you'll have to find an accurate RI using the Becke line or other methods. Then you must calculate the imaginary component using the diameter of your particles, the electrical conductivity of your material and the wavelength of your incident light. I highly recommend the book "Light scattering by Small Particles" by van de Hulst.

3. You're building an instrument and need to construct a rugged model. Can't help you there.

I hope this helps you. Good luck. ■