Droplet sizing with a wavelength modulated laser.

The light scattered by spherical droplets much larger than the wavelength may be modeled with rays associated with scattering orders that follow the laws of geometrical optics. These rays travel different paths as they are reflected and refracted by the droplet. Therefore, there is an optical pathlength difference (OPD) between these rays as they arrive on the detector. By illuminating the droplet with a frequency or wavelength modulated laser, the OPD of any pair of rays can be measured from the beat frequency that results when the rays interfere on the detector. Sweep wavelength lasers are well suited for this application given their rapid sweep rate and broad sweep wavelength band.