What is PPG? How a camera reads your pulse

Each heartbeat pushes a pulse of blood through the small vessels in your fingertip, and blood absorbs more light when there's more of it. Photoplethysmography measures that change.

What the app does with the signal

• The flash lights your finger and the camera sees what comes through.
• Averaging the red channel across each frame gives a waveform that rises and falls once per beat.
• The signal is filtered to the range of a human pulse, and slow drift from finger pressure and warming is removed.
• Exposure, focus and white balance are locked for the recording — a camera adjusting itself would create beats that were never there.
• Peaks are found, and the gaps between them become your pulse and variability.