Finger Beat

It's called photoplethysmography (PPG), the same trick as the clip on a hospital fingertip. Every beat pushes a little blood through your fingertip, changing how much light it absorbs. With the lens lit and your finger over it, the camera catches that tiny rhythmic flicker, and we read your rate from the green channel in the frequency domain, so a desk or a wobble can't fake a pulse.

Rest a fingertip gently but fully over the back camera lens so it's completely covered, and hold your hand still. We light it for you: the flash on the back, or a bright screen on the front. Press softly, because mashing it squeezes the blood out and the signal fades.

Cover the lens closest to the flash, and make sure the flash itself is shining into your fingertip. That lit lens gives the strongest reading. We automatically detect which lens you're covering, so just keep your finger pressed there while it finds it.

Hold the phone and your hand steady, keep the lens fully covered, and stay out of bright, flickering light. The bar only fills while it sees a real pulse, and it never rewinds. If it pauses, just settle your finger and it picks up where it left off.