Determination of size and complex index of refraction of single particles with elastic light scattering

We show that for spherical particles greater than ca. 5 µm, the differential scattering cross section is only weakly dependent on the real and imaginary parts of the refractive index (m=n+iκ) when integrated over angle ranges near 37±5^∘ and 115±5^∘, respectively. With this knowledge, we set up an arrangement that collects scattered light in the ranges 37±5^∘, 115±5^∘, and 80±5^∘. The weak functionality on refractive index for the first two angle ranges simplifies the inversion of scattering to the particle properties of diameter and the real and imaginary refractive indices. Our setup also uses a diamond-shaped incident beam profile that allows us to determine when a particle went through the exact center of the beam. Application of our setup to droplets of an absorbing liquid successfully determined the diameter and complex refractive index to accuracies ranging from a few to ten percent. Comparisons to simulated data derived from the Mie equations yielded similar results.