Your pulse, by camera light.
Rest a fingertip over the rear camera and flash. Your blood dims the glow a tiny bit with every beat — I count those dips for about thirty seconds.
on-device only · frames averaged & discarded · a wellness toy, not a doctor
starting the camera…
the last few beats you saved
what your fingertip tells the lens
Every heartbeat pushes a small wave of blood into your fingertip. Blood is very good at absorbing light, so for a fraction of a second your finger becomes a slightly better curtain. Press it over the camera with the flash on, and the camera sees the scene get faintly dimmer, then brighter, then dimmer — a rhythm that is your pulse. The trick has a lovely name, photoplethysmography, and it's the same one the little clip at the hospital uses.
Under the hood, pulsebycam averages the red channel of each frame, locks the camera's exposure so it stops hunting, band-passes the signal to the range a heart can beat in, and then finds the dominant rhythm by autocorrelation — matching the wave against a shifted copy of itself. The upshot: it shows a number only when the signal is genuinely periodic, and stays quiet (rather than guessing) when it isn't.
getting a reading that behaves
- Warm hands win. Cold fingertips hold less blood at the surface, and the signal fades to a whisper. Rub your hands together first if you've been outside.
- Cover camera and flash together. The pad of your finger should sit over both, like a tiny blanket.
- iPhone? Find the right lens. Phones with two or three rear cameras only hand this page one of them. Cover each lens in turn and watch the little preview circle — the one that goes dark red is the live camera. (And Safari won't let a web page switch the flash on, so a bright lamp behind your fingertip helps; if it still won't settle, the tap counter is the sure thing on iPhone.)
- Barely press. Squashing your fingertip squeezes the blood out of it — the gentlest touch that blocks outside light is perfect.
- Be a statue for thirty seconds. Every wobble looks like a heartbeat to a camera. Rest your elbow on a table if you can.
little questions, honest answers
Is this a medical device?
No — a wellness toy with real signal maths inside. Curiosity: yes. Diagnosis: never. Worried about your heart? See a human with a stethoscope.
Does the camera record me?
Never. Each frame becomes one averaged redness number and is discarded on the spot. No video is stored, and there's no server here to send it to anyway.
Why the flash?
Your finger has to glow for the trick to work. The flash lights it from behind the lens; your blood dims that glow once per heartbeat, and that dimming is what gets counted.
What about iPhones?
iOS Safari won't let a webpage turn the flash on. Bright ambient light over the lens often works anyway — and the tap counter is the trusty plan B.
Where do my readings go?
Into your browser's local storage on your device, and nowhere else. Export them as CSV or wipe them anytime from your log.