Quantitative differential phase microscopy based on structured illumination

Abstract

Structured illumination microscopy (SIM) is a well-known super-resolution imaging technique, which exploits moire patterns created when a sample is illuminated with periodic stripes. Conventional SIM often applies to fluorescent samples, or the samples which have absorption on illumination light. Here we report quantitative phase imaging of transparent samples with a SIM apparatus in transmittance-mode. For this purpose, two sets of fringe patterns, which have two orthogonal orientations and five phase-shifts for each orientation, were generated by a digital micro-mirror device (DMD) and projected on a sample. Under different fringe illuminations slightly-defocused images of the sample were recorded sequentially by a CCD camera, where the object waves along the ±1st orders of the illumination interfere with each other with a lateral shear in-between. The phase derivatives of the sample along the shear direction can be reconstructed from the phase-shifted intensity patterns. Eventually, the quantitative phase distribution of the sample was obtained by integrating the two phase derivatives. Furthermore, an iterative algorithm was used to enhance the resolution of the phase image, considering the structured illumination synthesizes a larger spectrum in the Fourier domain, similar to oblique illuminations in digital holography. This apparatus can also work in the conventional SIM mode, which images fluorescent samples in an in-focus manner. We believe such simple and versatile apparatus will be widely applied to biological imaging or industrial inspection.