  The health of cells found in the inner retinal layers of the human eye is
crucial to understand the onset of diseases of the retina such as macular
degeneration and retinopathy. A challenge is to periodically image these cells
in human eyes to detect abnormalities well before physiological and
pathological changes occur. However, in vivo imaging of many of these cells is
still elusive despite the phenomenal advances in Optical Coherence Tomography
(OCT) and Adaptive Optics systems. It stems from the fact that cell contrast in
reflection is extremely low. Here, we report a major advance towards this goal
by proposing and demonstrating a method to visualize these cells with high
contrast and resolution. The method uses a transcleral illumination which
provides a high numerical aperture in a dark field configuration. The light
backscattered by the Retinal Pigment Epithelium (RPE) and Choroid layer
provides a forward illumination for the upper layer of the retina, thus
providing a transmission illumination condition. By collecting the scattered
light through the pupil, the partially coherent illumination produces dark
field images, which are combined to reconstruct a quantitative phase image with
twice the numerical aperture given by the eye's pupil. The retrieved
quantitative phase images (QPI) of cells in ex vivo human and pig's eyes are
validated with those taken with a standard QPI system. We then report, to our
knowledge, the very first human in vivo phase images of inner retinal cells
with high contrast.
