Five major problems facing photoelectric heart rate measurement technology

Since the first generation of Apple Watch, there has been a function to monitor the user's heart rate. In this function, Apple has adopted photoelectric measurement technology.

Not only Apple, the vast majority of smart watches and bracelets use this technology, but usually non- medical grade products, can only be used as a reference data. In heart rate measurement, there is another way - electrode type, most electrocardiographs and other products that can reach the medical device level will adopt this technology. The photoelectric type is mainly used to measure the heart rate by light reflection: it is based on the principle that the body's whole body blood vessels will have slight fluctuations when the heart beats, and a beam of light is emitted onto the wrist of the person, and the reflected shadow is observed by the camera. If there is a slight change, the heart is considered to be beating once.

Compared with electrode type measurement, photoelectric measurement can be operated with one hand. It does not need two contacts, that is, two hands to detect data, as in the electrode type mode. This can realize active reading of data and remote reading of data, which is more suitable for Cooperate with cloud big data services. This is why it is widely used in wearable devices.

The seemingly sought after technology is actually facing many problems.

main problem

The use of optoelectronic measurement of heart rate in an activity must overcome five fundamental questions that affect accuracy:

1, light interference

2, skin color

3, cross problem

4, the position of the sensor on the human body

5, low perfusion

Let's take a closer look at the specific issues in these five areas.

Light interference

In fact, the biggest technical obstacle of photoelectric heart rate measurement equipment is how to separate biometric signals from interference, especially motion interference. Unfortunately, when light is incident on a person's skin, only a small fraction of the light quantum is returned to the sensor, and only one or one-thousandth of the collected light quantum is regulated by the blood flow of the heart contraction. The rest are scattered on non-pulsating physiological substances such as skin, muscles, tendons and so on. Therefore, when these non-pulsating physiological substances move around, such as in exercise or daily life activities, the resulting light pulsing with time changes is difficult to distinguish from the dispersion of light with real blood flow. Ambient light interference also exacerbates the severity of the problem. For example, as time passes, the interference of sunlight can completely penetrate into the photodetector and even create a pulse signal with similar physiological properties.

color

Humans have a very wide variety of beautiful skin tones, so many that the Fischer scale provides seven types of standards for skin color classification and response to UV light. Different skin tones absorb light differently, so each skin color is characterized by a different absorption spectrum. Then, this means that the intensity and wavelength of the light captured by the photoelectric heart rate measuring device sensor is dependent on the skin color of the person wearing the sensor. For example, darker skin absorbs more green light, which also indicates why most devices use green LEDs as light emitters, limiting the ability to accurately measure heart rate through dark skin. This also exposes the problem of measuring heart rate through the skin of the tattoo. This is also the "tattoo door" that Apple has been criticized. Apple watch users with tattoos on their wrists find that the data on the display is very weak or not.

Cross problem

Photoelectric heart rate monitors have problems with cross-interference due to motion during periodic activity. The biggest challenge for this problem is that this activity brings about the same repeated action. This is most common when recording the pace of the pace during jogging and running, as these data are usually in the same basic interval as the heartbeat frequency (140-180 down/steps per minute). This problem faced by many photoelectric heart rate monitoring devices makes it easy for algorithms to interpret the rate of pace of data entry through photoelectric monitoring as a heart rate. This is known as the “crossover problem” because when viewing these data on a chart, when heart rate and pace rate overlap, many photoelectric heart rate monitoring devices tend to lock the pace rate and display it as heart rate, although heart rate may A huge change occurred after the overlap. This cross-interference problem is evident on the Apple Watch.

Compared with the photoelectric heart rate measurement device technology of other wrists, it is obvious that the heart rate monitoring of the apple watch when it is "crossed" fails. The pace of the label is similar to the heart rate of 1 to 4 people, and the digital signal of Apple cannot distinguish them. Come. The second intersection has more than two minutes to read the heart rate into a pace rate.

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