Looks like another junk paper. There's also so much verbiage that is common that it is a clear ripoff, despite no reference being made by the later one to the earlier one.it's a weird case you have https://acadpubl.eu/hub/2018-119-15/1/55.pdf
both those doc.-'s reference list is somewhat "weird" - no much advancement since 2018 ? -- further more one would expect some calibration for given (likely precisely chosen pair of iR emitter/detector ← there are variations inside the patch . . . for both . . . )
all this really much does not sound correct ??
by Jeff Child
by Jake Hertz
by Jake Hertz
by Duane Benson