The section "The science behind Apple's sensors" in the article contains some details about that. Quick google search returned this: https://en.wikipedia.org/w

The section "The science behind Apple's sensors" in the article contains some details about that. Quick google search returned this: https://en.wikipedia.org/wiki/Photoplethysmogram https://en.wikipedia.org/wiki/Photoplethysmogram According to this: http://bsn2014.org/poster/BSN2014-poster09s-maeda.pdf http://bsn2014.org/poster/BSN2014-poster09s-maeda.pdf Green light seems to be used to lessen the impact of motion on the measurements.