puls reads your heart rate from the colour of your face. every beat pushes blood into the skin and darkens it by well under one percent — invisible to you, just large enough to measure by averaging a few hundred pixel patches of the picture.

nothing leaves this device. no frame is uploaded, recorded, or kept. there is no server to send anything to; after loading, the page works with the network unplugged.

it needs light on your face and it needs you to sit still. it takes about twenty seconds to settle.

it works less well on darker skin. melanin absorbs the green light the method depends on; that is the physics.