Moiré effect-based interference microscopy for biospecimen characterization

Abstract

Interference microscopy for biospecimen characterization based on the moire phenomena is described. Two scenarios of incoherent multiplicative superimposition are employed: (1) object information carrying interferogram is superimposed with reference one or (2) two object interferograms are superimposed (with equal or opposite phase signs). Second strategy provides additional doubling of underlying phase function of interest. 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 distribution yields moire pattern containing low spatial frequency difference beat term (moire fringes – macrostructure) and high spatial frequency sum beat term (microstructure). The macrostructure was studied in great majority of previously reported moire techniques. In this contribution we point attention onto the sum beat spatial frequency 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 be doubled. In particular spatial phase change is better sampled by fringes for denser interferogram and more importantly numerical filtering of 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. Evaluation employing numerical simulations and validation using experimental recordings of phase bio-samples, i.e., prostate cancer cells are enclosed.