Light scattering device for sizing and velocimetry of large droplets utilizing a ring-shaped laser beam

A newly developed light scattering device for sizing and velocimetry of large droplets is described. A novel beam shaping technique is employed in conjunction with a laser operating in the circular TEM(01) (doughnut) mode as a light source to achieve a ring-shaped source beam. This particular beam geometry allows definition of a sheetlike scattering volume with an approximately constant power density across it resulting in improved sizing and sampling accuracy. The size resolution of the instrument is better than 7%, and the overall sizing accuracy for an input size range of 30 dB (droplets of ~45-1500 microm in radius) is ~10%. While the instrument reported in this paper was designed to analyze water droplets entrained in air, it should also be useful in other sizing applications if appropriate modifications are made.