Monitoring Slight Variations in Light
The light-absorbing property of hemoglobin is used in the measurement of heart rate. Light from a green LED on the underside of the monitor is shone on blood vessels just under the skin. The light that is not absorbed but reflected back is captured by a photodetector.
The photodetector produces an electrical signal when light strikes it. This analog signal is converted into a digital signal, and slight changes of this signal are used to measure heart rate.
Most wrist-worn heart rate monitors operate on this same principle. However, Epson's sensing system, which includes a unique photodetector manufactured by Epson in Japan, provides accurate heart rate readings at low power.