  We present a single-image numerical phase retrieval method for Zernike
phase-contrast microscopy (ZPM) that addresses halo and shade-off artifacts, as
well as the weak phase condition, without requiring hardware modifications. By
employing a rigorous physical model of ZPM and a gradient descent algorithm for
its inversion, we achieve quantitative ZPM imaging. Our approach is
experimentally validated using biological cells and its quantitative nature is
confirmed through comparisons with digital holography observations.
