  The evaluation and monitoring of cells health in the human retina is crucial
and follow time course of retinal diseases, detect lesions before irreversible
visual loss and to evaluate treatment effects. Towards this goal, a major
challenge is to image and quantify retinal cells in human eyes in a
non-invasive manner. Despite the phenomenal advances in Optical Coherence
Tomography (OCT) and Adaptive Optics systems, in vivo imaging of many of these
cells is limited by the fact that cell contrast in reflection is extremely low.
Here, we report on a major advance by proposing and demonstrating a radically
different method compared with OCT to visualize retinal cells with high
contrast, resolution and an acquisition time suitable for clinical use. The
method uses a transscleral illumination which provides a high numerical
aperture in a dark field configuration. The light backscattered by the Retinal
Pigment Epithelium (RPE) and the choroid 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 quasi-quantitative phase image with twice the numerical aperture
given by the eye's pupil. The retrieved quasi-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 apply the technique in vivo on human eye, without
pupil dilation, we demonstrated the feasibility to image retinal cells up to
the retinal pigment epithelium within a few seconds.
