This is the first in a three-part series on optical heart rate sensors for biometric wearables. This first installment focuses on how these sensor systems work and what you can measure with them.
Most wearables use photoplethysmography (PPG) to measure heart rate and other biometrics. PPG is the methodology of shining light into the skin and measuring the amount that scatters based on blood flow. That’s an oversimplification, but optical heart-rate sensors work based on the fact that light entering the body will scatter predictably as the blood-flow dynamics change, such as with changes in blood pulse rates (heart rate) or changes in blood volume (cardiac output). Figure 1 below depicts the primary components and basic working methodology of an optical heart rate sensor.
Figure 1: Optical heart rate sensor’s basic structure and operation
Optical heart-rate sensors use four primary technical components to measure heart rate:
What can an optical heart-rate sensor measure?
Optical heart-rate sensors produce a PPG waveform that can measure heart rate as a foundational metric, but there’s much more that can be measured from a PPG waveform. Although it is very difficult to achieve and maintain accurate PPG measurements (more on that in the next section), when you do get it right, it can be very powerful. A high-quality PPG signal is foundational to a wealth of biometrics that the marketplace is demanding today. Figure 2 is a simplified PPG signal marking the measurement of several biometrics within that signal.
Figure 2: Typical PPG waveform
Here’s further detail on some of the measurements possible with optical heart rate sensors:
Optical Heart Rate Sensor Challenges
Designing an optical heart-rate sensor can be very challenging on a wearable device, because the methodology is sensitive to motion. To compensate, you need to have strong optomechanics and signal-extraction algorithms. Figure 3 shows some of the primary challenges you might face when designing with optical heart rate sensors.
Figure 3: Primary challenges when integrating optical heart rate sensors
Here’s further detail on the optomechanical considerations for PPG sensor integration:
Here’s further detail on the signal-extraction considerations for PPG sensor integration:
I hope this post provided some insight on how PPG sensor systems operate and what they can measure. In the next post in this series, I’ll explore best practices in integrating this technology into devices of all kinds – watches, fitness bands, earbuds and more.
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