The result may vary depends on the device (and can not have any sense)
Algorithm and image analysis from:
https://github.com/phishman3579/android-heart-rate-monitor
minSdkVersion 15 (Ice Cream Sandwich 4.0.3)
targetSdkVersion 25 (Nougat 7.1)
Uses camera api (deprecated but necessary to reach a decent number of devices)