With the increasing popularity of “quantitative life movementsâ€, there is growing interest in medical measurement methods that can be integrated into so-called wearable gadgets such as watches, smartphones or fitness bracelets. This interest begins with a fitness tracker that uses an acceleration sensor to measure the stride frequency. Now, optical sensors can directly measure heart rate and blood oxygen saturation, further expanding the possibility of self-observation. In addition, pulse measurement on the wrist or finger is more convenient than wearing a chest strap. Last but not least, optical methods are also economically advantageous, as new technologies for efficient LEDs enable higher energy efficiency and more compact sensors.
OSRAM Opto Semiconductors offers a wide range of innovative components for modern fitness tracking and health monitoring, including green and red LEDs, photodiodes and infrared LEDs in different package, size and performance levels. All of our sensor products are based on high-efficiency chip technology, ensuring low power consumption and high signal quality for extremely reliable measurements. Choose the components you need based on your particular application. Or maybe our latest innovation is exactly what you are looking for: the new SFH7050 is the first integrated optical sensor from Osram Opto Semiconductors for automatic fitness tracking that combines three different wavelengths of light-emitting diodes and a built-in photodetection Device.
An optical sensor suitable for measuring heart rate and blood oxygen saturation utilizes the absorption of light by blood, more specifically, the absorption of light by hemoglobin contained in blood.
Heart rate monitoring
Light illuminates the body tissue and then undergoes processes such as transmission, absorption, and reflection (Figure 1). The greater the amount of blood received, the lower the amount of light reflected. The amount of blood in the arteries varies with the cardiac cycle, so the heart rate can be derived from the detector signal cycle (Figure 2). This method of measuring light in blood vessels is called photoplethysmography (PPG). The sensor consists of a parallel source and detector. In practice, the sensor is placed directly on the skin (usually the wrist or finger). The wavelengths used for the measurements vary depending on the location: green light has proven to be the best choice for wrists, while red and infrared light are ideal for finger areas.
Pulse oximetry
When using both infrared and red light, blood oxygen saturation can be measured (Figure 3). The so-called pulse oximetry is based on the fact that the absorption behavior of hemoglobin (Hb) when combined with oxygen (oxygenated hemoglobin HbO2) changes. The concentration of these two hemoglobin variants can be determined by measuring the absorption at two different wavelengths. This will give you a hemorrhagic oxygen saturation. In this application, red light (660 nm) and infrared light (940 nm) are ideal because at these two wavelengths, the absorption behavior of the two hemoglobin molecules is the largest. In contrast to pulse measurements that only consider changes in light absorption, in this case the absolute value of the light absorption of the arterial blood must be measured. In practice, blood oxygen saturation can be expressed by a ratio function (Imin/Imax) of the minimum and maximum detector signals at the respective wavelengths.
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