Abstract: An apparatus and method that can allow a standard PIV systems to obtain particle size as well as the third velocity component with minimal hardware modifications. The invention is based on using two radiation sheets of different wavelength ranges, overlapped with a known offset. By obtaining simultaneous images filtered for each wavelength, the scattering particle's location within the radiation sheet can be established. Once its location is known, its size can be determined through intensity measurements, and the third velocity component determined from position change between exposures.