How do smartwatches measure heart rate

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I wanted to share with you guys a text message I just got from my buddy and he asks “hey man is just bought a new fitness watch and I saw it had a bright green light on the back of it”. Do you know anything about it?

So in this article, I am going to describe the answer to this question.

How do smartwatches measure heart rate?

So the green light in the back of the watch, it’s based on a technology called photoplethysmography or PPGis a non-invasive way to measure health data and to give us biometrical information.

It works by an optical sensor in your watch that shoots a light onto the surface of your skin so when the light reaches your blood vessel some of the light is reflected back to a photodetector on the watch. The intensity of light captures a waveform of the amount of blood in your blood vessel, so depending on the color of light used you can analyze and capture different biometrics and waveform.

Since the depth of penetration of light is dependent on wavelength, red light with a longer wavelength has an increased depth of penetration than the shorter green light.

Greenlight PPG technology is what the Apple Watch and Fitbit used to measure heart rate but some trackers like bio strap use red light PPG for more in-depth measurement, So which light is better at capturing health data?

Well, it depends on what kind of data you looking for Greenlight.

The advantage of Greenlight:

  • Resilient to motion so it is great at measuring accurate heart rate during exercise.

The disadvantage of Greenlight:

  • It has a limited depth of penetration, so blood vessels are thin at the wrists. Any measurement you take at the wrist might be an inaccurate number
  • The limited depth of penetration also means a limitation on what you can measure. you are mostly limited to accurately measuring an active heart rate with a green light PPG number 
  • Greenlight is more absorbed by darker skin, tattoos, and freckles these individuals have more inaccuracies in their measurement

So how does red light compare to green light?

The advantage of Redlight:

  • The red light PPG penetrates the skin 10x times deeper than green light, so you can capture more clinically relevant biometric data and can measure it at the wrist
  • Differences in skin color, tattoos, and freckles don’t affect your measurement

The disadvantage of Redlight:

  • Redlight PPG is extremely sensitive to motion or movement so this means is doing exercise or any  activity what’s going to happen is you are going to get inaccuracies in your readings
  • Redlight is sensitive to environmental noise data that can only be captured every few minutes, so second by second reading such as active heart rate can be difficult to record.

So until the technology improves where one light source can give us comprehensive clinically accurate data. My recommendation is to utilize both light sources. At EC wellness we are using a green light PPG active heart rate monitor and a red light PPG ECA bio strap to give us clinically accurate biometric data.

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