Fluorescence modeling of droplets intersecting a focused laser beam

Laser-induced fluorescence (LIF) has been previously used in various forms to characterize droplets in a spray, according to either size or temperature. A rigorous examination is presented of the LIF signal obtained when a water droplet seeded by Rhodamine 6G passes through one or two highly focused laser beams, i.e., with a beam waist of the order of the droplet diameter or smaller. The calculations are performed with a fluorescence model based on the generalized Lorenz-Mie theory and on ray tracing methods, assuming that the droplet is spherical and nonabsorbing. A parametric study reveals a size dependent signal as the droplet passes through the focused beam(s). The observed signal features suggest several avenues for the use of LIF scattering for size and velocity measurements of individual droplets.