A method of obtaining a measurement signal representative of the particle size distributions in nanocrystal suspensions that includes a step of providing a first light beam along a first axis to a first micro-retarder array to generate polarization wavefront shaped light. The shaped light is applied to an objective configured to focus two orthogonally polarized components of the polarization wavefront shaped light to produce first and second axially offset foci along the first axis. A sample having particles in suspension is disposed in one foci to produce a measurement optical signal having phase and intensity values corresponding to at least some of the particles in suspension. The method also includes determining intensity and quantitative phase information as a function of time based on the optical signals.