The brightness (the depth of red) changes periodically like a wave. This regularly changing interval is quickly monitored by the detector, and then amplified by the scanning applied in the mobile phone, and then converted, so as to judge the systolic frequency of the heart.
Some mobile phones that can detect heart rate are equipped with some sensors. Such as blood oxygen heart rate sensor, ultraviolet sensor, etc. Mobile phones equipped with these sensor chips can also provide the detection of heart rate, blood oxygen and ultraviolet intensity. Of course, this function is only available in the application that comes with the mobile phone.
Principle of mobile phone heart rate test;
1, PPG photoplethysmography (photoplethysmography)
This is the most widely used working principle in smart phones or bracelets. Simply put, it is the reflection of light, which is converted into an electrical signal according to the fluctuation of light transmittance in blood, and then converted into a heart rate through a certain algorithm.
It is based on the principle that when a light beam with a fixed wavelength irradiates the skin surface, the light beam will transmit the light to the photoelectric receiver through transmission or reflection, and the light intensity detected by the detector will be attenuated due to the absorption of skin muscles and blood in this process.
However, the absorption of light by skin and muscle tissue remains unchanged during the whole blood circulation process, but the blood volume in the skin will fluctuate under the action of the heart.
When the heart contracts, the peripheral blood volume is the largest, the light absorption is also the largest, and the detected light intensity is the smallest. When the heart is diastolic, on the contrary, the maximum light intensity is detected, which makes the light intensity received by the receiver fluctuate accordingly.
The reason why the LED lights of most smart bracelets on the market are green is because red blood has the largest absorption rate of green light, and the data is the most accurate by green light detection.
When a person's exercise is in a relatively stable state, his heart rate is also relatively stable. As long as the pulse intensity of the heart is detected at different times, the corresponding heart rate can be calculated through the algorithm.
But this is limited to sitting, sleeping, walking, running and other movements are relatively stable. If the heart rate fluctuates during acceleration, deceleration and playing basketball, it is difficult to calculate the accurate heart rate by this method.
In addition, during strenuous exercise, due to a large amount of sweat on the skin, the light absorption rate of sweat is not fixed, which will also affect the measurement accuracy. However, considering that most users of smart bracelets do not need to detect the heart rate during strenuous exercise as a parameter of health management, this defect is also dispensable.
2, ECG signal measurement, similar to electrocardiogram (ECG)
This method uses the potential difference of the human body to detect the heart rate, similar to the electrocardiogram in the hospital. The tissues and body fluids around the heart can conduct electricity, so the human body can be regarded as a three-dimensional volume conductor: length, width and thickness.
The heart is like an energy source. Every heartbeat, the function of myocardial cells will cause tiny electrode changes on the surface of human body, and these electrode changes can be captured, and then the heartbeat frequency can be restored by algorithm.
Baidu encyclopedia-heartbeat detection
Reference to the above content: Baidu Encyclopedia -PPG
Refer to the above content: Baidu Encyclopedia-Electrocardiogram (ECG)