Experimental assessment of the optical transfer function for quantitative oblique back-illumination microscopy

Quantitative oblique back illumination microscopy (qOBM) is a recently developed phase imaging modality that enables 3D quantitative phase imaging and refractive index (RI) tomography of thick scattering samples. The approach uses four oblique illumination images (acquired in epi-mode) at a given focal plane to obtain cross sectional quantitative information. In order to quantify the information, qOBM uses a deconvolution algorithm which requires an estimate of the angular distribution of light at the focal plane to obtain the system's optical transfer function (OTF). This information is obtained using Monte Carlo numerical simulations which uses published scattering parameters of tissues. While this approach has shown robust results with high quantitative fidelity, the reliance on available published scattering parameters is not optimal. Here we present an experimental approach to measure the angular distribution of the back-scattered light at the focal plane. The approach simultaneously obtains information from the imaging plane and the Fourier plane to provide insight into the overall angular distribution of light at the focal plane. Together with the pupil function, given by the known numerical aperture of the system, this approach directly yields the OTF. A theoretical analysis and experimental results will be presented. This approach has the potential to widen the utility of qOBM to also include tissues and samples whose scattering properties are not well documented in the literature.