How does the Smartwatch measure heart rate?
Smartwatches have a photoelectric sensor and use a technique called Photoplethysmography (PPG) to measure heart rate. Sensors use green LED lights paired with light sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment.
When your heart beats, the blood flows in your wrist, and the blood vessels expand and contract. Since the Blood is red color, it reflects red light and absorbs green light. The expanded blood vessels absorb more green light compared to contracted blood vessels. Meaning the absorption of green light is greater during the heartbeat, compared to between the beats.
The sensors measure the reflected light, and a software algorithm converts the changes in light intensity into your pulse rate.
Why use a green light?
Since the Blood is red color, it reflects red light and absorbs green light and additionally, the green light is less susceptible to ambient light. This means this green light will not get affected by the ambient light compared to others and in this way, this light can give more accurate results while performing the pulse sense activity.
Is green or red light for Heart rate measurement harmful to the body?
The green and red LEDs for heart rate belong to visible spectrum, like the LEDs used in home or office lighting and LCD. And the power of green light and red light is very low, and the brightness is generally kept at the level ensuring it is safe for the skin. The light emitted does not contain ultraviolet or any other potentially harmful frequencies. Light energy in contact with the skin is comparable to normal room lighting conditions and far less than the energy that a person would receive from sunlight.
