remote photoplethysmography can also be performed by digital holography, which is sensitive to the phase of light waves, and hence can reveal sub-micron out-of-plane motion. in particular, wide-field imaging of pulsatile motion induced by blood flow can be measured on the thumb by digital holography. the results are comparable to blood pulse monitored by plethysmoraphy during an occlusion-reperfusion experiment . a major advantage of this system is that no physical contact with the studied tissue surface area is required.