Remote photoplethysmography (rPPG): the real optical effect behind the scan, and exactly where its limits are.
Every heartbeat pushes a small pulse of blood through the capillaries near the skin's surface. That changes, very slightly, how much light the skin absorbs versus reflects -- a real optical effect, not a guess.
The color shift per heartbeat is far too small to notice by eye. A phone camera recording at normal frame rates captures it anyway, frame by frame, as a faint periodic signal in the video.
Sentinel extracts a heart rate from that signal three separate ways -- an FFT (frequency analysis), an autocorrelation pass, and a peak-detection pass -- then reconciles the three into one consensus reading rather than trusting whichever ran first.
Phone cameras constantly auto-adjust exposure and white balance, which would otherwise swamp the tiny pulse signal. For a finger scan, Sentinel locks exposure outright; for a face scan it takes a lighter touch (locking white balance instead) because a hard exposure lock caused black frames on some mid-range Android hardware.
Standard (~20 seconds, one surface, a normal daily reading). Robust (~25 seconds, a steadier reading for lower light or more movement). Dual-surface (~27 seconds, finger and face in sequence, cross-checked against each other).
Quantum Khor is a wellness companion, not a medical device. It does not diagnose, treat or prevent any condition.