When a certain wavelength of light beam irradiated to the surface of the skin at the end of the finger, the light beam will be transmitted to the photoelectric receiver by transmission or reflection, in the process due to the finger, the end of the skin muscle and blood absorption attenuation effect, the detector detected light intensity will be weakened. Among them, the absorption of light by skin, muscle tissue, etc. is constant throughout the blood circulation, while the blood within the skin, the volume of the heart under the action of pulsatile changes. When the heart contraction peripheral blood volume is the most light absorption is also the largest, the detected light intensity is the smallest. And in cardiac diastole, just the opposite, the detected light intensity is the largest, so that the light intensity received by the light receiver is then pulsating changes.
Why do we often see green LEDs?
Because in the presence of a red liquid such as blood, the absorption of green light is the greatest, and it is more accurate for data judgment.
When the user's heart beats, more blood flows through the user's wrist, and the more green light is absorbed. In between heartbeats, blood flow decreases, resulting in less absorption of green light as well.
As an example, suppose the luminous value of the bracelet is 100, the skin muscle tissue absorbs a constant 10, and the total blood absorption is 15, then the reflection is 100-10*2-15=65, then the arterial blood comes over, and the red blood cells increase in oxygen, and the total absorption of the blood becomes twice, then the reflection is 50, and then the reflection is 65-50-65-50-65-50-65-50-65 all the time. -50-65-50...... your heart rate by counting how many pulse changes per second.
I don't know if PPG reads this way?
It's principle then determines its drawbacks, what if there is a lot of sweat directly between the bracelet and your skin? Then the values would be inaccurate.
In summary:
This method, measuring the resting pulse and normal regular movement (running, etc.) is still relatively accurate, but for irregular sports, such as soccer and badminton and other irregular sports, for example, said that the assumption that the value will pop up, it will be a slight decrease in accuracy. But it is also a difference of a few values, for non-professionals, I feel it should be enough.