Optical Heart Rate Sensors: Tech Explained
Just a couple years ago, fitness bracelets were basically glorified step counters worn on your wrist. Now, they’re doing everything from measuring your heart... Optical Heart Rate Sensors: Tech Explained

Just a couple years ago, fitness bracelets were basically glorified step counters worn on your wrist. Now, they’re doing everything from measuring your heart rate on a run to warning you to get out of the sun. And they’re everywhere. Every company from Fitbit to Jawbone and Amazfit makes a pretty advanced piece of wrist-worn technology now — all with the promise of better health.

But what’s in these things, anyway?

Even though fitness bands are fairly simple compared to full-blown smartwatches, a growing number of sensors crammed inside has turned them into complex labs on your wrist. For example, Amazfit Arc’s Band advertises 2 different sensors in the tiny package, an optical heart rate sensor and an accelerometer. Today we’ll look at the optical heart rate sensors and how do they work?

Unlike the EKG a doctor might use to measure your heart rate, an optical heart-rate monitor measures your heart rate using light. An LED shines through the skin, and an optical sensor examines the light that bounces back. Since blood absorbs more light, fluctuations in light level can be translated into heart rate – a process called photoplethysmography.

That’s an oversimplification, but PPG (photoplethysmography) is based on the fact that light entering the body will scatter in a predictable manner as the blood flow dynamics change, such as with changes in blood pulse rates (heart rate) or with changes in blood volume (cardiac output).

How does the technology work?

PPG uses four primary technical components to measure heart rate:

  1. Optical emitter – generally made up of at least 2 LED’s that send light waves into the skin. Because of the wide differences in skin tone, thickness, and morphology associated with a diversity of consumers, most state-of-the-art OHRM’s use multiple light wavelengths that interact differently with different levels of skin and tissue.
  2. Digital Signal Processor (DSP) – the DSP captures the light refracted from the user of the device and translates those signals into one’s and zero’s that can be calculated into meaningful heart rate data.
  3. Accelerometer – the accelerometer measures motion and is used in combination with the DSP signal as inputs into motion-tolerant PPG algorithms.
  4. Algorithms – the algorithms process the signals from the DSP and the accelerometer into motion-tolerant heart rate data, but can also calculate additional biometrics such as  VO2, calories burned, R-R interval, heart rate variability, blood metabolite concentrations, blood oxygen levels, and even blood pressure.

Unfortunately, these devices are still not as accurate as chest straps and nor are they as rapid in picking up changes. If you want the most accurate reading possible then, you should still probably opt for a chest strap, something that measures skin conductivity.

One of the issues with a wrist-worn optical heart rate monitor is that other things can also affect the oxygenation of your blood. Among these are muscle contractions – so if you are working out, this can actually upset the accuracy of the readings you’re getting.

Sellami Abdelkader Freelance Writer

Computer engineering student at the institute of Electrical and Electronics Engineering in Algeria. Passionate about Web design, Technology and Electronic Gadget.