With the continuous advancement of technology, health monitoring devices are becoming increasingly common in our daily lives. Devices using Photoplethysmography (PPG) technology have become an important method for detecting heart rate and blood oxygen saturation. This article provides a detailed introduction to the principles of PPG technology and its applications in measuring heart rate and blood oxygen.
Principle of PPG Technology
PPG technology is a non‑invasive technique that detects changes in blood volume within vessels by analyzing variations in light absorption in biological tissues. The basic principle is that when light is directed onto the skin surface, part of the light is absorbed by the skin, blood vessels, and blood, while another part is scattered and reflected. As the heart beats, the blood volume in the vessels changes periodically, causing corresponding changes in the light absorption characteristics of the tissues under the skin. By detecting these changes in light absorption, we obtain the PPG signal.
In practical implementation, a PPG device typically consists of a light source (such as an LED) and a photodetector (such as a photoresistor or photodiode). The light emitted by the source passes through the skin and is received by the photodetector. The detector converts the received optical signal into an electrical signal, which is then processed and analyzed to extract information related to blood volume changes.
Heart Rate Measurement
Heart rate is one of the vital indicators of human life activities, and PPG technology enables real‑time, non‑invasive measurement of heart rate. In the PPG signal, the waveform corresponding to each cardiac cycle includes a rising edge and a falling edge; the time interval between these two edges corresponds to the cardiac cycle. By calculating the time intervals between consecutive cardiac cycles, the heart rate can be derived.
To accurately detect heart rate, the PPG signal needs to undergo preprocessing, including denoising and filtering. Then, peak detection, wavelet‑based methods, or other advanced algorithms can be used to identify the cardiac cycles within the PPG signal.
Blood Oxygen Measurement
Blood oxygen saturation (SpO₂) is the ratio of oxygen‑bound hemoglobin to total hemoglobin in the blood, and it is an important indicator for assessing respiratory function and circulatory system status. Similar to heart rate measurement, PPG technology can also be used for real‑time, non‑invasive measurement of blood oxygen saturation.
For blood oxygen measurement, the PPG device uses two light sources of different wavelengths (typically red and infrared), because oxygenated hemoglobin and deoxygenated hemoglobin have significantly different absorption coefficients for these two wavelengths. By measuring the absorption ratios of these two wavelengths, the device can calculate the blood oxygen saturation.
The specific calculation method is: first calculate the absorption ratio (R value) of red and infrared light, and then convert the R value into an SpO₂ value using an empirical formula or a calibration curve. It should be noted that blood oxygen measurement is affected by various factors, such as light source stability, skin pigmentation, and tissue thickness. Therefore, effective preprocessing and calibration of the PPG signal are necessary to obtain accurate SpO₂ values.
Applications of PPG Technology
PPG technology is widely used in medical, health, and sports fields. Many modern medical devices, such as pulse oximeters and electrocardiographs, employ PPG technology for real‑time monitoring of patients' heart rate and blood oxygen saturation. In addition, many wearable health monitoring devices, such as heart rate/blood oxygen bands and smart watches, also integrate PPG sensors, allowing users to conveniently keep track of their physiological status at any time.
As a real‑time, non‑invasive physiological monitoring method, Photoplethysmography (PPG) technology has broad application prospects in heart rate and blood oxygen measurement. Through continuous optimization of device design, signal processing, and algorithmic techniques, PPG technology will provide more accurate and convenient services for our health management.