@TanMath ...glow, coming from the infrared spectrum. The most typical test is holding an IR remote in front of the camera and watching the IR LED flash prominently.
I am not saying that the authors have not done something properly. I am saying that maybe it has not been described accurately.
The easiest way to control for this is with an infrared cut-off filter (https://en.wikipedia.org/wiki/Infrared_cut-off_filter). Once you have your system up and running, try to check BPM with and without the filter. Mind you, the filter would diminish your signal (worsen the SNR) so, you might have to…