You’re asking for an explanation of how optical sensors work in wearable devices. Optical sensors in wearables are key components that use light to detect various biosignals and environmental information. The main principle used is analyzing changes in light reflected or transmitted through the skin. These changes can be used to measure heart rate, blood oxygen saturation, blood pressure, and more. The operation can be broadly described as follows:

1. Light Emission:

• LED (Light Emitting Diode): Most wearable devices use LEDs as a light source. LEDs emit light of a specific wavelength, and various wavelengths of LEDs, such as green, red, and infrared, are used depending on the measurement purpose.
  • Green LED: Has a wavelength that is well absorbed by hemoglobin in the blood and is mainly used for heart rate measurement.
  • Red and Infrared LEDs: Used for measuring blood oxygen saturation. They utilize the principle that the absorption rate of red and infrared light varies depending on oxygen saturation.

2. Light Reflection and Transmission:

• The light emitted from the LED penetrates the skin or is reflected by blood vessels. Blood has the property of absorbing light, and hemoglobin, in particular, selectively absorbs light of certain wavelengths.
• With each heartbeat, the volume of blood vessels changes, and accordingly, the amount of light absorbed also changes. By analyzing these changes in light, heart rate can be measured.
• Blood oxygen saturation is measured using the principle that the absorption rate of red and infrared light varies depending on the oxygen saturation in the blood.

3. Light Detection:

• Photodiode: The light reflected or transmitted from the skin is detected by an optical sensor called a photodiode. The photodiode generates an electrical signal proportional to the intensity of the light.

4. Signal Processing:

• The electrical signal generated by the photodiode is very weak and needs to be amplified. It is then converted into a digital signal through an analog-to-digital converter (ADC).
• The converted digital signal is analyzed by the wearable device’s processor. In the case of heart rate measurement, the heart rate is calculated by analyzing the pattern of light changes. In the case of blood oxygen saturation measurement, the oxygen saturation is calculated by analyzing the ratio of red and infrared light.
• Signal processing processes such as noise removal and filtering are performed to improve measurement accuracy.

Key Applications:

• Heart Rate Monitoring: Uses green LEDs and photodiodes to detect changes in blood flow and measure heart rate.
• SpO2 Monitoring (Blood Oxygen Saturation Measurement): Uses red and infrared LEDs and photodiodes to measure blood oxygen saturation.
• Blood Pressure Monitoring: Technology is being developed to measure blood pressure using both optical and pressure sensors.
• Skin Condition Measurement: Technology is being researched to analyze skin moisture, oil content, and pigmentation using light of specific wavelengths.

Advantages and Limitations of Optical Sensors:

• Advantages: Convenient to use as they can measure biosignals non-invasively.
• Limitations: Measurement accuracy can be affected by movement, skin color, and ambient lighting. In particular, heart rate measurement can become inaccurate during vigorous exercise.

It is expected that optical sensors in wearable devices will provide more accurate and diverse information through continuous technological development. In the future, it is expected that technologies capable of non-invasively measuring more diverse biosignals such as blood pressure and blood glucose will be commercialized.