Phase-Doppler interferometry in which a probe volume that is much smaller than the droplets being measured has been shown to work well when coupled with a phase-ratio and intensity-validation scheme that is capable of eliminating trajectory-dependent scattering errors. With ray-tracing and geometric-optics models, the type and magnitude of trajectory errors were demonstrated quantitatively through stochastic trajectory calculations. Measurements with monodispersed water droplet streams and glass beads were performed to validate the model calculations and to characterize the probe volume. Scattered-light intensity has also been shown to provide a robust means of determining the probe cross-sectional area, which is critical for making accurate mass flux measurements.