Moire effect-based interference microscopy for biospecimen characterization

摘要

Interference microscopy for biospecimen characterization based on the moire phenomenon is described. Two scenarios of incoherent multiplicative superimposition are employed: (1) the object information carrying interferogram is superimposed with the reference one and (2) two object interferograms are superimposed (with equal or opposite phase signs). Second strategy provides additional doubling of the underlying phase function of interest. The superimposition can be performed using two experimental interferograms or single real interferogram and numerically designed reference structure in so-called digital moire regime. Multiplicative superimposition of two periodic intensity distributions yields moire pattern containing difference beat low spatial frequency term (moire fringes - the macrostructure) and sum beat high spatial frequency term (microstructure). The macrostructure was studied and exploited in great majority of previously reported moire techniques. In this contribution we turn the attention onto the sum frequency beat component and report efficient means for its recovery using numerically advanced digital filtering (variational/empirical decomposition approaches). Its single-frame (single-pattern) phase demodulation by 2D Hilbert spiral transform with enhanced accuracy follows - this feature comes from the fact that sum beat moire term has high spatial frequency which is generally beneficial in single-frame fringe analysis and its phase function is to be doubled. In particular the spatial phase change is better sampled by fringes for denser interferogram and more importantly numerical filtering of the fringe term of interest from background intensity is easier. We propose and preliminarily evaluate experimental proof-of-concept strategy where two interferograms with slight difference in spatial frequency are simultaneously recorded in two halves of CCD camera and superimposed multiplicatively. Proposed moire technique opens up new possibilities in interference microscopy based bio-phase imaging mainly due to its data-driven enhanced phase sensitivity (fringe doubling effect) and real-time operation. Numerical simulations and validation using experimental recordings of phase bio-samples, i.e., prostate cancer cells are enclosed.