This is how Optical HR work as explained by Dr Steve from Valencell for in-ear usage.
Valencell make the technology for JABRA, SCOSCHE and others.
https://youtu.be/-1WVpxZQCbU
Last Updated on 18 June 2015 by the5krunner
tfk is the founder and author of the5krunner, an independent endurance sports technology publication. With 20 years of hands-on testing of GPS watches and wearables, and competing in triathlons at an international age-group level, tfk provides in-depth expert analysis of fitness technology for serious athletes and endurance sport competitors.
