Health Monitoring with LEDs and Photodiodes for SpO2 and Heart Rate

This article explores health monitoring systems that use LEDs and photodiodes. It highlights specific LEDs and photodiodes used for heart rate and SpO2 measurement.

Introduction

Wearable devices have become increasingly popular for monitoring vital signs and tracking various health parameters. These devices, including fitness trackers, smart bands, smartwatches, and wearable ECG monitors, commonly measure:

• Heart rate (bpm)
• Pulse rate (bpm)
• Systolic and diastolic blood pressure
• Body temperature
• SpO2 (Oxygen Saturation in %)
• Step detection

Optoelectronic components, specifically LEDs and photodetectors, play a crucial role in these measurements, as well as in various light therapy applications.

Health Monitoring System Architecture

The human heart pumps blood through our circulatory system, leading to volume changes in our arteries with each heartbeat. To measure these changes, LEDs illuminate the skin, and photodiodes detect the transmitted or reflected light.

Figure 1: Health Monitoring System Architecture

As illustrated in Figure 1, a typical health monitoring system consists of:

• LEDs
• Photodiodes
• Microcontroller with biometric algorithms
• Motion sensors (e.g., 3-axis or 6-axis accelerometer or gyroscope)
• Display
• Wireless IC (Bluetooth, Wi-Fi, or GSM)

An optical barrier is placed between the LED and photodiode to prevent direct crosstalk. The analog front-end IC includes modules such as:

• Analog-to-digital conversion (ADC)
• Photodiode signal amplification
• LED drivers
• DACs
• GPIOs
• Interrupt handling
• Built-in FIFO
• Digital SPI interface

LEDs and Photodiodes for Heart Rate and SpO2 Measurement

Light absorption in our blood is directly related to the amount of hemoglobin present. This absorption varies depending on the light’s wavelength and differs between normal hemoglobin (Hb) and oxygenated hemoglobin (HbO2). The spectral responsivity of the photodiode also factors into heart rate and SpO2 measurements.

Specific wavelengths used are:

• Green light: 530 nm
• Red light: 655 nm
• IR (Infrared) light: 940 nm

Shorter wavelengths are generally absorbed more strongly than longer wavelengths. Green light is often preferred for heart rate measurement, while red and IR lights are used for pulse oximetry.

Figure: LEDs and Photodiodes for Heart Rate and SpO2 measurement

To differentiate between Hb and HbO2, the skin is illuminated using green, red, and IR LEDs. The photodiode measures the light absorption, and the SpO2 level can be calculated using the following formula:

SpO2 = (HbO2) / (Hb + HbO2)

Where:

• Hb = Non-oxygenated hemoglobin
• HbO2 = Oxygenated hemoglobin

Figure: PPG sensor

PPG (photoplethysmogram) is a non-invasive technology that uses a light source and photodetector attached to the skin. It measures volumetric variations in blood circulation during heart pumping. The LED and photodetector sense blood flow to derive the heart rate.

As shown in the figure, a PPG sensor consists of an LED on one side of the tissue and a photodetector on the other. It measures changes in blood volume based on the obstruction and absorption of incident light.

• Examples of PPG sensors with integrated LEDs and photodiodes include the SFH 7050/7060 from Osram Opto Semiconductors, used for heart rate and pulse oximetry.
• 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.