Cardiio is an app that measures your heart rate when you look straight into the front camera of your iPhone.
How does this technology work? From the website: “Every time your heart beats, more blood is pumped into your face. This slight increase in blood volume causes more light to be absorbed, and hence less light is reflected from your face. Using sophisticated software, your iPhone’s front camera can track these tiny changes in reflected light that are not visible to the human eye and calculate your heart beat!”
You have to look directly at the front camera (or else results will be abnormal), but in general results seem pretty expected!