Interferometric Mie Imaging (IMI) is a sizing technique for the evaluation of the diameter of spherical particles, droplets and bubbles similar to PDI. The working principle is based on the out-of-focus imaging of particles illuminated by a laser light sheet. The optical setup of a standard IMI system consists of a laser-light sheet and a digital camera with a high quality lens. Moving the camera chip to an out-of-focus position an interference fringe pattern becomes visible.
The visible fringe pattern corresponds exactly to the far field scattering which can be calculated by the Mie theory. The number of fringes within the aperture image depends on the diameter of the droplet and the aperture angle. With increasing particle size, the number of fringes increases. The exact size of the particles is determined by analysis of the fringe patterns. The size of the aperture image is a measure for the z-position of the particle along the line-of-sight.