Poor axial resolution in holographic particle imaging applications makes particle positioning in 3D space more complex since the positions are not directly obtained. In this paper we estimate the axial position of micrometer particles by finding the location where the wavefront curvature from the scattered light becomes zero. By recording scattered light at 90 degrees using off-axis holography, the complex amplitude of the light is obtained. By reconstruction of the imaged scene, a complex valued volume is produced. From this volume, phase gradients are calculated for each particle and used to estimate the wavefront curvature. From simulations it is found that the wavefront curvature became zero at the true axial position of the particle. We applied this metric to track an axial translation experimentally using a telecentric off-axis holographic imaging system with a lateral magnification of M = 1.33. A silicon cube with molded particles inside was used as sample. Holographic recordings are performed both before and after a 100 mu m axial translation. From the estimated positions, it was found that the mean displacement of particles between recordings was 105.0 mu m with a standard deviation of 25.3 mu m. (C) 2016 Optical Society of America
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