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Principle of blood oximeter
血氧仪.rar
(2.27 MB, downloads: 13)
血氧仪.rar
(2.27 MB, downloads: 13)
-----Heart rate is obtained by injecting green light into the skin, PPG is obtained, and heart rate is obtained through algorithms. Blood oxygen is two light sources, IR and red light. By detecting the ratio of IR and red light reflected back from the skin, the relevant blood oxygen value is calculated through algorithms.
----- Principle of blood oxygen saturation measurement: The light absorption coefficient of hemoglobin changes according to the type and wavelength of hemoglobin. Hemoglobin in the blood is the main transporter of oxygen to body tissues. In addition to oxygenated hemoglobin (HbO2) and reduced hemoglobin (RHb), hemoglobin also combines to produce additional components, such as carboxyhemoglobin (COHb) and methemoglobin (MetHb), but the concentrations of these two are usually very low. Therefore, in non-invasive blood oxygen measurement technology, hemoglobin is assumed to be composed of only two substances, RHb and HbO2, and blood oxygen saturation (SpO2) is defined as the percentage of HbO concentration in the blood to the sum of RHb and HbO2 concentrations.
-----Heart rate detection uses the principle of photoelectric sensing elements absorbing light energy, recording the signal detected by the changes in light in blood vessels due to blood flow. Blood flow rate changes with the beating of the heart, and the photoelectric sensing element will cause the sensor to produce changes in brightness or light reflection as the blood vessels contract and expand. Currently, there are two types of sensors in the industry that can be used for presentation: one is the photodiode sensor (as shown in Figure 1), and the other is the complementary metal oxide semiconductor sensor (CMOS sensor as shown in Figure 2), both of which can be used to measure subcutaneous tissue blood changes:
------Photoelectric measurement method can be divided into two cases
There are two situations, one emits green light and the other emits red light. No matter which one, these lights are emitted by LED lights. Red light uses the change in the absorbance of hemoglobin in the blood vessels to measure the pulse, relying on the infrared emission beam loop and the receiving reflection loop, but the signal is extremely weak and very susceptible to external interference, resulting in inaccurate measurement data, and generally requires a quiet state to measure the green light, which is composed of two green wavelength LEDs and a photosensitive sensor. The principle is that the blood in the arm blood vessels will change in density when pulsating, causing changes in light transmittance. It can continuously measure heart rate (even during exercise), calculate average heart rate, record maximum heart rate, etc.
The reason why green light is chosen as the light source is that the following characteristics are taken into consideration:
1. Melanin in the skin absorbs a lot of shorter wavelengths
2. The moisture on the skin also absorbs a lot of UV and IR light
3. Most of the green light (500nm) and yellow light (600nm) that enter the skin tissue will be absorbed by red blood cells
4. Red light and light close to IR pass through skin tissue more easily than other wavelengths of light
5. Blood absorbs more light than other tissues
6. Green (green-yellow) light is absorbed by both oxyhemoglobin and deoxyhemoglobin compared to red light
In general, green light and red light can be used as measurement light sources. In the early days, red light was mostly used as the light source. With further research and comparison, green light as the light source has a better signal and a better signal-to-noise ratio than other light sources, so most wearable devices now use green light as the light source. However, considering the different skin conditions (skin color, sweat), high-end products will automatically use multiple light sources such as green light, red light and IR according to the situation. Although we know the above characteristics, it is not enough to figure out why heart rate, blood oxygen and other parameters can be measured by light?
When light passes through skin tissue and then reflects to the photosensor, there is a certain attenuation of the light. The absorption of light by muscles, bones, veins and other connecting tissues is basically unchanged (provided that the measuring part does not move significantly), but blood is different. Since blood flows in the arteries, the absorption of light naturally changes. When we convert light into electrical signals, the resulting signals can be divided into direct current (DC) signals and alternating current (AC) signals, precisely because the absorption of light by arteries changes while the absorption of light by other tissues remains basically unchanged. Extracting the AC signal can reflect the characteristics of blood flow.
Because it mainly uses green light for detection, when the heart contracts, blood flow at the wrist increases, blood absorbs more green light, and reflects less green light; while when the heart relaxes, the opposite is true, the absorption of green light decreases, and the reflection of green light increases; the bracelet sensor calculates the heart rate through the frequency of the reflected green light. This is to monitor the heart rate through photoplethysmography (PPG). The principle is very simple: blood is red, reflects red light, and absorbs green light. The heart rate sensor combines green LED light with a photosensitive photodiode to detect the amount of blood circulating in the wrist at a specific time, thereby obtaining heart rate information.
1. Technical Principle of Heart Rate Sensor
1. PPG Photoplethysmography Principle
Simply put, it is just reflection. The pulsating changes in the light transmittance in the blood are converted into electrical signals, which are converted through software algorithms and correspond to the heart rate. When a light beam of a certain wavelength is irradiated on the surface of the fingertip skin, the light beam will be transmitted to the photoelectric receiver by transmission or reflection. During this process, the light intensity detected by the detector will be weakened due to the absorption and attenuation of the fingertip skin muscles and blood. The absorption of light by the skin, muscle tissue, etc. remains constant throughout the blood circulation, while the volume of blood in the skin changes pulsatingly 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 in diastole, on the contrary, the detected light intensity is the largest, causing the light intensity received by the light receiver to change pulsatingly.
Calculation formula: SpO2 = HbO2/(HbO2+Hb)×100%
The content of either hemoglobin or oxyhemoglobin can indicate the content of blood oxygen. In this case, why can't the content of blood oxygen be determined by measuring either red light or infrared light? ? ? ?
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