Apple Watch and FitBit light sensors not working properly on dark-skinned and obese users

Light sensors in wearable devices don’t work on obese and darker-skinned people, a new study shows.

Light sensors do not work on obese and dark-skinned users

The finding is a bad sign for wearable devices like Fitbit and Apple Watch that use light sensors for new applications, like monitoring the user’s blood pressure, according to The Verge.

Study author Jessica Ramella-Roman, an associate professor at Florida International University, said the architecture of the device needed to change.

The study focused on signal photoplethysmography (PPG), which is a technique that uses changes in the way light is reflected to measure the user’s blood flow, on three wearable devices: Fitbit Versa 2 , Apple Watch Series 5 and Polar M600, as published on NCBI.

A model was used to simulate how light travels through tissue and demonstrate how wearable light sensors behave with different skin tones.

Since obese people have thicker skin, it is more difficult for light from the sensor to pass through body tissues.

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While previous wearable device accuracy and bias study focused on user skin tone, Ramella-Roman said many studies had not included many obese users despite physiological differences.

The PPG signal from wearable devices drops by 60% if used by an obese person, while it drops by 10% if used by a darker-skinned person, according to Nature.

Signal loss appears to be due to changes in skin thickness in obese users. There were changes in the peak of the PPG signal, which is used to calculate heart rate but whose signal strength should not change depending on the heart rate value.

The shape of the signal has also changed, which different groups use to track blood pressure.

Decreasing signal

The study noted that as they increased the BMI level and increased the user’s skin tone, the signal decreased and other features also began to disappear.

The study only modeled how the wearable devices would detect signals in the lab. The team has yet to use the devices on real people to confirm their findings. They are currently doing the study and they have recruited 100 participants.

However, the issues uncovered by studies like this complicate projects that seek to use wearable devices to detect the cardiovascular health of underserved groups.

The new study shows that researchers should be careful in using PPG, especially for projects aimed at monitoring the health of people with cardiovascular problems, most of which are associated with obesity.

Wearable devices also use light sensors to detect flu in users, which can be compromised.

The results also suggest that other devices that use light and PPG sensors, such as blood oxygen measurements in clinics or hospitals, may not work well for obese people.

Related article: Fitbit files patent for smart ring that tracks blood oxygen and pressure levels

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Written by Sophie Webster

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Maria D. Ervin