  A method is presented to infer simultaneously the wavelength-dependent real
refractive index (RI) of the material of microspheres and their size
distribution from extinction measurements of particle suspensions. To derive
the averaged spectral optical extinction cross section of the microspheres from
such ensemble measurements, we determined the particle concentration by flow
cytometry to an accuracy of typically 2% and adjusted the particle
concentration to ensure that perturbations due to multiple scattering are
negligible. For analysis of the extinction spectra we employ Mie theory, a
series-expansion representation of the refractive index and nonlinear numerical
optimization. In contrast to other approaches, our method offers the advantage
to simultaneously determine size, size distribution and spectral refractive
index of ensembles of microparticles including uncertainty estimation.
