Compared with electrode-based measurements, photoelectric measurements can be operated with one hand and do not require two contacts, i.e., both hands, to detect data as in the electrode-based method, which allows for active and remote reading of data, and is more suitable for use with cloud-based big data services. This is why it is widely used in wearable devices.
The seemingly sought-after technology faces many problems.
Major issues
There are five foundational issues that must be overcome to have an impact on accuracy when utilizing optoelectronic heart rate measurements during an event:
1. Light interference
2. Skin color
3. Crossover issues
4. Positioning of sensors on the body
5. Low perfusion
Below Let's look specifically at these five specific areas.
Light interference
In fact, the biggest technical hurdle for optoelectronic heart rate measurement devices is how to separate biometric signals from interference, especially motion interference. Unfortunately, when light is directed into a person's skin, only a small fraction of the light quanta are returned to the sensor, and of all the light quanta collected, only one percent or one thousandth are regulated by the blood flow of the contracting heart, with the rest being dispersed over nonpulsatile physiological substances such as skin, muscles, tendons, and so on. Thus, when these nonpulsatile physiological substances move around, such as during exercise or activities of daily living, the resulting agitated dispersion of light over time is difficult to distinguish from the dispersion of light with real blood flow. The problem is also exacerbated by interference from ambient light, such as that from sunlight over time, which can fully penetrate the photodetector and even create pulsatile signals of near-physiological nature.
Skin color
Humans have a very wide variety of beautiful skin tones, so many that the Phi Scale provides seven types of criteria for numerical classification of skin color and response to ultraviolet light. Different skin tones absorb light differently, so each skin tone is characterized by a different light absorption profile. This means, then, that the intensity and wavelength of light captured by the sensor of an optoelectronic heart rate measurement device is dependent on the skin color of the person wearing the sensor. For example, darker skin absorbs more green light, which points to the issue of why most devices use green LEDs as light emitters, limiting the ability to accurately measure heart rate through darker skin. This also exposes the problem of measuring heart rate through tattooed skin, which is what led to Apple's much-criticized "tattoo-gate," in which users of Apple Watches with tattooed wrists found that the data on the display was very faint, if not non-existent.
Crossover issues
Optoelectronic heart rate monitors have issues with crossover interference due to movement during cyclical activity, and the biggest challenge with this issue is that this activity creates a constant repetition of the same movements. This is most commonly seen when recording pace frequency during jogging and running, as these data are usually in the same basic range as heart rate (140-180 beats/steps per minute). This problem with many optoelectronic heart rate monitoring devices makes it easy for algorithms to misinterpret the pace rate recorded through the optoelectronic monitoring data as heart rate. This is known as the "crossover problem" because when viewing this data on a chart, when the heart rate and pace rate overlap, many optoelectronic heart rate monitors tend to lock onto the pace rate and display it as the heart rate, even though the heart rate may change dramatically after the overlap. This cross-talk issue is evident in the Apple Watch.