Did you know that the Cardiio app can accurately measure your heart rate, by using the camera of your iPhone?
Here’s how it works:

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!

This video below shows the process: