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Apple ink? Why the Apple Watch may not work great with tattoos.

As a security measure, the Apple Watch unlocks only when it detects a pulse via its optical sensor, which can be confused by tattoo ink and movement.

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Reuters journalist Matt Siegel inputs his passcode onto his Apple Watch, as his tattoos prevent the device's sensors from correctly detecting his skin, in Sydney, Australia, Thursday.

Jason Reed/Reuters

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If your wrist sports a tattoo, then it might not be the best place for an Apple Watch. 

Apple wearable computer, which first went on sale last week, has a sensor on the back that reads the wearer's heart rate. When it can't detect a pulse, it locks and asks for a passcode.

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Apparently this feature poses a problem for those with ink on their wrists. Beginning on Tuesday, users on Reddit with tattooed arms and wrists began reporting that their devices were locking up on them. The sensors had trouble reading their pulses through the ink. 

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That's because the sensors work by shining light on the skin and measuring the light that bounces back, explains Steven LeBoeuf, president of Valencell, a company that develops biometric sensor technology, though not for the Apple Watch.

“What they’re doing is shining green light at the skin and measuring the optical scatter that comes back,” Dr. LeBoeuf says in an interview.

LeBoeuf writes in an email that, “obtaining accurate biometric data in the wrist lies in the wearable’s ability to measure pulsatile blood flow – especially pulsatile blood flow that carries blood pressure information with it. The best way to do this is with photoplethysmography (PPG) using infrared light and with advanced algorithms to remove motion noise and sunlight noise.

“However, infrared light faces serious challenges at the wrist because it travels deep within the body and interacts with not just flowing blood but also (and unfortunately) skin, muscle tissue, tendons, cartilage, bone, etc. Since these tissues all move at nearly the same rate as blood flow, it is challenging for a wearable sensor to sense the difference between heart rate and wrist motion,” he writes.

To overcome this limitation, Apple and others with smart watches and wrist wearables have attempted to measure blood flow using PPG with green light.

“Green light doesn't travel as deeply within the body as does infrared light, and it modulates well with blood flow. However, green light, unlike infrared light, is absorbed strongly by tattoos,” he says. “Thus, very little green light reaches the flowing blood, and heart rate readings can be almost impossible in persons having very dark skin or tattoos. So the problem gets worse when you put in ink or have darker skin.”

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The solution is to use different wavelengths and pair yellow light, which penetrates deeper, with green light, he says.  

LeBoeuf’s company's makes a biosensor, found on the Scosche Rhythm+ fitness tracker, that uses measure at the arm and wrist. The yellow light helps penetrate through tattoos so that accurate readings can be generated “even for folks who are all tatted up,” he says.

Apple is aware of this issue. A representative of the company offered this explanation, available on the company’s website, which says: “Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance. The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.”

The company also notes that the watch's functionality can be limited by  “exercising in the cold” and “motion.” The watch works best with rhythmic movements, such as running, says Apple.

The sensor's limitations might extend beyond just tattoos. Typists, for instance, could find their watches giving them bogus data, says LeBoeuf.

“The problem is that the light that returns to the detector scatters with everything – when your skin moves, when your muscles move, when you’re typing, etc,” LeBoeuf says. “As a matter of fact if you take an Apple watch and you start to type with it, the heart rate it will report is your typing rate, not your heart rate because it’s just measuring the skin and the skin has a an optical scatter signal that’s so much stronger than that of the tiny little blood flow signal.”

Even worse, LeBoeuf also speculates that the watch's functionality could also be hampered by dark skin.

“If someone has really high melanin content, such as many African-Americans, the signal to noise ratio of the blood flow vs. the noise of everything else – muscle movement, skin movement, etc – is much lower. What that means is, if you have really dark skin or a heart rate sensor that’s using only green light you may be able to pick-up the person when they’re not moving, when they’re at rest, and get the heart rate accurately, but when they’re moving, because that signal to noise ratio is so much lower to begin with now it’s below the noise-floor.  So for someone with high melanin or ink in their skin, you may not be able to pick them up at all during exercise.”

It should be emphasized that this is just a conjecture, as there are no known instances of the Apple Watch failing to take the pulses of black people.