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pted icle e thr n mu disto tain ver more small apertures produce the same number of formation over a 3D volume with good optical effi-1220 OPTICS LETTERS / Vol. 31, No. 9 / May 1, 2006images of each particle over an extended depth of field.2 The small apertures used make this optically inefficient and sensitive to all sources of wavefront slope error; hence this method is typically used with large tracers in liquid flows. Scanning techniques are optically efficient but impose limits on the temporal scales within the field.3 Holographic methods yield high resolution 3D data, and 3C optical correlators provide rapid data processing.4 However, the record-ing and analysis systems are complex and vibration sensitive. Further, this approach cannot be applied to self-luminous, e.g., fluorescent, particles typically re-quired in microfluidic applications.5 In this Letter we present new techniques for the measurement of a 3D particle position based on wavefront sensing (WFS). WFS was developed to measure turbulence-induced aberrations in ground-based astronomical imaging.6 We use a phase-diversity7 (PD) WFS technique for which the raw data is the image intensity distribution mea-sured in two planes within the imaging system. Par-ticle depths are determined from the wavefront cur-ciency and using a single detector or single view. The techniques can be used for 3C, 3D flow velocimetry by suitable processing of a temporal image sequence. Figure 1 illustrates the principles of particle rang-ing using a quadratically distorted diffraction grating (off-axis Fresnel zone plate) with a spherical lens to create an imaging system where the focal length is aThree-dimensional wavefr