Determination of the orientation of transparent spheroids using interference technology

The distribution of light scattered by a spheroidal particle is sensitive to the orientation of the particle. Based on interference technology, we present a method for obtaining the orientation of the spheroidal particle from in-focus and out-of-focus images. We simulate the in-focus images using the LightTools. From the optical transfer matrix theory, we obtain both the in-focus and corresponding out-of-focus images using Matlab. We find that the glare-point distribution of the in-focus image exhibits axial symmetry for a spheroidal particle and that it is perpendicular to the speckle orientation of the out-of-focus image. We establish an interferometric particle imaging system to experimentally acquire the out-of-focus and in-focus images of the transparent spheroidal particles. The experimental results agree with the simulations. We are thus able to propose a method for obtaining the orientation of a spheroidal particle using either the in-focus image or out-of-focus image. The method has potential applications in particle measurements.