  We present a non-invasive, label-free method of imaging blood cells flowing
through human capillaries in vivo using oblique back-illumination
capillaroscopy (OBC). Green light illumination allows simultaneous phase and
absorption contrast, enhancing the ability to distinguish red and white blood
cells. Single-sided illumination through the objective lens enables 200 Hz
imaging with close illumination-detection separation and a simplified setup.
Phase contrast is optimized when the illumination axis is offset from the
detection axis by approximately 225 um when imaging 80 um deep in phantoms and
human ventral tongue. We demonstrate high-speed imaging of individual red blood
cells, white blood cells with sub-cellular detail, and platelets flowing
through capillaries and vessels in human tongue. A custom pneumatic cap placed
over the objective lens stabilizes the field of view, enabling longitudinal
imaging of a single capillary for up to seven minutes. We present high-quality
images of blood cells in individuals with Fitzpatrick skin phototypes II, IV,
and VI, showing that the technique is robust to high peripheral melanin
concentration. The signal quality, speed, simplicity, and robustness of this
approach underscores its potential for non-invasive blood cell counting.
