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

Quantitative Phase Microscopy (QPM) by interferometric
techniques can require a multiwavelength configuration to remove
2 pi ambiguity and improve accuracy. However, severe chromatic
aberration can affect the resulting phase-contrast map. By means of classical
interference microscope configuration it is quite unpractical to correct such
aberration. We propose and demonstrate that by Digital Holography (DH) in
a microscope configuration it is possible to clear out the QPM map from the
chromatic aberration in a simpler and more effective way with respect to
other approaches. The proposed method takes benefit of the unique feature
of DH to record in a plane out-of-focus and subsequently reconstruct
numerically at the right focal image plane. In fact, the main effect of the
chromatic aberration is to shift differently the correct focal image plane at
each wavelength and this can be readily compensated by adjusting the
corresponding reconstruction distance for each wavelength. A procedure is
described in order to determine easily the relative focal shift among
different imaging wavelengths by performing a scanning of the numerical
reconstruction along the optical axis, to find out the focus and to remove at the same time the chromatic aberration.