RF Wireless World

Health Monitoring System | LEDs and Photodiodes for SpO2 and Heart rate measurement

This article covers health monitoring system using LEDs and photodiodes. It mentions specific LEDs and Photodiodes used for Heart rate measurement and SpO2 measurement.

Introduction: Wearable devices have become popular to monitor vital signs for tracking various health parameters of human beings. Wearable devices include fitness trackers, smart bands, smart health watches, wearable ECG monitors etc.

The common parameters measured by health monitoring system are heart rate, pulse rate (in bpm), Systolic or diastolic blood pressure, body temperature, SpO2 (Oxygen Saturation in %), step detection etc. Opto electronic components (LEDs and photo detectors) are used for these measurements and various other light therapy applications.

Health Monitoring System

Human heart pumps blood through circulatory system of our body. This results into increase or decrease of volume of our arteries with every heartbeats. In order to measure this change in volume our skin is illuminated by LEDs. The transmitted or reflected light is detected using photodiode.

The figure-1 depicts health monitoring system architecture. As shown it consists of LEDs, photodiodes, microcontroller with biometric algorithms, motion sensors (such as 3-axis or 6-axis accelerometer or gyroscope), display and wireless IC (Bluetooth, wifi or GSM). Optical barrier is provided between LED and photodiode to prevent direct cross talk.

Analog front end IC consists of various modules such as Analog to digital conversion (ADC), photodiode signal amplification, LED drivers to drive LEDs, DACs, GPIOs, interrupt, built-in FIFO, digital SPI interface etc.

LEDs and Photodiodes for Heart rate and SpO2 measurement

As we know light absorption in our blood depends on amount of hemoglobin. The amount of absorption varies based on light wavelength and it is different for normal hemoglobin (Hb) and oxygenated hemoglobin (HbO2). The spectral responsivity of Photodiode is also considered for measurement of heart rate and SpO2. Specific wavelengths for green, red and IR lights are 530 nm, 655 nm and 940 nm respectively.

Light with shorter wavelengths are absorbed very strongly than light with longer wavelengths. Green light is preferred for heart rate measurement where as red/IR (Infrared) lights are used for pulse oximetry.

In order to detect the difference between these, skin is illuminated using green, red and IR LEDs. Photodiode is used to measure the absorption.

SpO2 level can be calculated as per following formula.
➨SpO2 = ((HbO2)/(Hb + HbO2))
Where,
Hb is non-oxygenated hemoglobin
HbO2 is oxygenated hemoglobin

PPG (photoplethysmogram) is non-invasive technology. PPG sensor uses light source and photodetector attached on skin. Using these devices, it measures volumetric variation in blood circulation during heart pumping. LED and photodetector are used to sense blood flow rate to derive heart rate. As shown, PPG sensor consists of LED attached on one side of tissue and photodetector on the other side. PPG sensor measures changes in blood volume based on obstruction and absorption of incident light.

Refer difference between ECG sensor and PPG sensor >> and advantages and disadvantages of PPG sensor >> for more information.

• Examples of PPG sensor consisting of integrated LEDs and photodiode are SFH 7050/7060 from Osram Opto Semiconductors. They are used for heart rate and pulse oximetry applications.
• Multiple light emitters from OSRAM include SFH 7015 (Red and IR) and SFH 7016 (Green, Red and IR).
• Photo detectors from OSRAM include SFH 2200/2201, SFH 2240, SFH 2440, SFH 2704 etc.

➨Refer advantages or benefits of light therapy >> using Green, Red, Blue and IR LEDs.

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