Experimental assessment of the optical transfer function for quantitative oblique back illumination microscopy (qOBM)

Quantitative oblique back illumination microscopy (qOBM) is a recently developed imaging technique that enables 3D quantitative phase imaging (QPI) and refractive index (RI) tomography of thick scattering samples. To quantify the phase and RI information with qOBM, the optical transfer function (OTF) of the system must be known or estimated, which requires knowledge of the angular distribution of light at an imaging plane inside a highly scattering medium. To date, this information has been estimated using a Monte Carlo photon transport method which relies on documented tissue scattering properties. While this numerical approach has shown high-fidelity quantitative results, it is limited by its dependence on published scattering parameters and simulated conditions. Here we propose a novel approach that allows experimental measurement of the angular distribution of the multiple-scattered light at the imaging plane inside a highly scattering medium. Experimental results using samples with known and unknown scattering properties are presented, including excised brain tissue, in-vivo skin, and formalin-fixed and paraffin-embedded (FFPE) tissues. Results further support qOBM's quantitative fidelity across different tissue types, and show how directly measuring the angular distribution of light can widen qOBM's utility to more complex samples with unknown or highly variable scattering properties.