Wearables don’t work the same on dark skin. It’s time to change thatWearables don’t work the same on dark skin.
It’s time to change that
By Andy Boxall
February 28, 2021
Ever since wearables equipped with heart rate tech began entering the mainstream, there have been questions about the accuracy of optical heart rate sensors when taking readings on different skin tones. From Apple’s infamous so-called “tattoogate” shining a green LED light on the problem in 2015 to various academic studies during the years that followed, there has been a lot of discussion, but not a lot of conclusions.
What happened? Have sensors changed to cope with different skin tones, or has the industry failed to change? It turns out the early research has prompted change, and may also inspire new standards for wearable heart rate sensors.
The heart rate sensor on your wrist
The optical heart rate sensor on the back of your smartwatch or fitness tracker works using an established technology called photoplethysmography (PPG), where a very bright light shines through your skin and tissue to measure blood flow. The information is then reflected back and interpreted to provide your heart rate.
PPG optical sensors on wearables use an infrared green light for cost effectiveness and efficiency, as it’s cheap to produce an established tech, our bodies and blood are good at absorbing it, and it’s less affected by movement. All this makes it a good choice for inclusion on a nonmedical-grade wearable.
However, the use of green IR light causes a problem. Melanin, a natural skin pigment, is also very good at absorbing green light, and the increased presence of melanin in darker skin tones reduces the accuracy of green PPG sensors. Green light can also have problems with dark tattoos, sweat, and even arm hair.
An underreported problem
It seems the darker your skin, the less accurate a green PPG heart rate sensor could be, because it can be blocked by melanin in the skin’s base cell layer. The subject has received attention for several years, but mainly through academic papers and studies.
Nature, as part of an investigation into the accuracy of the heart rate sensors, lists seven papers discussing the subject. And Valencell, a company that produces biometric sensors, including PPG sensors, and lists Suunto and Scosche as partners, lists skin tone as one of its top five challenges related to optical heart rate sensors. An Oxford Academic Journal paper on PPG sensors and sleep writes the following: “We note with deep concern that there is increasing evidence that [PPG sensors] are not as accurate, and may not work at all, in people with darker skin tones.”