Quantitative Phase Microscopy of microstructures with extended measurement range and correction of chromatic aberrations by multiwavelength digital holography

Quantitative Phase Microscopy (QPM) by interferometrictechniques can require a multiwavelength configuration to remove2 pi ambiguity and improve accuracy. However, severe chromaticaberration can affect the resulting phase-contrast map. By means of classicalinterference microscope configuration it is quite unpractical to correct suchaberration. We propose and demonstrate that by Digital Holography (DH) ina microscope configuration it is possible to clear out the QPM map from thechromatic aberration in a simpler and more effective way with respect toother approaches. The proposed method takes benefit of the unique featureof DH to record in a plane out-of-focus and subsequently reconstructnumerically at the right focal image plane. In fact, the main effect of thechromatic aberration is to shift differently the correct focal image plane ateach wavelength and this can be readily compensated by adjusting thecorresponding reconstruction distance for each wavelength. A procedure isdescribed in order to determine easily the relative focal shift amongdifferent imaging wavelengths by performing a scanning of the numericalreconstruction along the optical axis, to find out the focus and to remove at the same time the chromatic aberration.