Time-shift technique for simultaneous measurement of size, velocity, and relative refractive index of transparent droplets or particles in a flow

The time-shift technique, also known as the pulsed-displacement technique, is revisited as a method to measure size, velocity, and relative refractive index of spherical, transparent particles. Building on the basic measurement principle, we introduce several new innovations, making the technique significantly more attractive for use outside of the laboratory. These innovations include two possibilities for velocity measurement, validation criteria for one- and two-detector arrangements, and approaches to achieve higher bandwidths, and, in particular, lower measurable sizes. We discuss numerous optical configuration examples to illustrate the flexibility of this technique to meet various application requirements.