PhotoBad news for tattooed iFans who want an Apple Watch: A growing body of complaints seems to indicate that tattoo ink, especially in dark colors, interferes with the Watch's sensors, thereby disabling some of the device's functions.

The Daily Dot first called attention to a complaint posted on reddit's /r/apple forum yesterday — a tattooed redditor going by the handle “guinne55fan” started a thread to report problems with his new Watch:

So I thought my shiny new 42mm SS watch had a bad wrist detector sensor. The watch would lock up every time the screen went dark and prompted me for my password. I wouldn't receive notifications. I couldn't figure out why especially since the watch was definitely not losing contact with my skin. ... I was about to give up and call Apple tomorrow when I decided to try holding it against my hand (my left arm is sleeved and where I wear my watch is tattooed as well) and it worked. My hand isn't tattooed and the Watch stayed unlocked. Once I put it back on the area that is tattooed with black ink the watch would automatically lock again. Just wanted to give anyone a heads up about this issue because I don't see it mentioned anywhere in Apple's support documents.

In addition, guinne55fan also included  photos  of his tattooed left wrist. Sure enough, the part that would be in contact with the underside of an Apple Watch is almost completely colored with black tattoo ink.

Similar complaints can also be found on Twitter; just yesterday, @stroughtonsmith Tweeted that “Turns out people with wrist tattoos will be unable to use Apple Watch for Apple Pay because it can't sense you're alive. Fun!”

No Apple comment

So there's at least two people with tattooed wrists reporting identical complaints with the Apple Watch's functionality, both on the same day. Is this mere coincidence — or is there an actual connection?

Although Apple has not, as of press time, publicly commented on whether or how tattoos interfere its watches, Apple's own support page suggests that tattoo interference genuinely could be a problem, in its explanation of “How Apple Watch measures your heart rate”:

The heart rate sensor in Apple Watch uses what is known as photoplethysmography. This technology, while difficult to pronounce, is based on a very simple fact: Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate.

The heart rate sensor can also use infrared light. This mode is what Apple Watch uses when it measures your heart rate every 10 minutes. However, if the infrared system isn't providing an adequate reading, Apple Watch switches to the green LEDs. In addition, the heart rate sensor is designed to compensate for low signal levels by increasing both LED brightness and sampling rate.

What does this have to do with tattoos? Simple: tattoo ink, especially the darker colors, can serve as a “barrier” between those LED or infrared lights and your bloodstream, absorbing some of the light before it hits your bloodstream, and some more of it as it bounces back.

Dark tattoos

For the record: the problems reported with Apple Watch only affect people whose wrists have dark tattoos; it does not affect people with naturally dark skin due to high melanin levels. Dark tattoo inks are made of a variety of ingredients, including ground-up minerals, which can obviously interfere with light reflection or absorption in ways which melanin does not.

The iMore blog tested various Twitterers' and redditors' complaints and concluded that, yes: tattoo inks do interfere with the Apple sensor readings, especially the darker inks – and in some cases, thick (yet non-tattooed) scar tissue can interfere with the sensors as well:

We tested the Watch's sensors against tattooed and non-tattooed sections on both the wrist and elsewhere on the body. On non-tattooed non-wrist sections, the sensors gave identical readings as when also tested on the wrist; on tattooed sections, sensor readings varied wildly depending on colors and shading.

Dark, solid colors seem to give the sensor the most trouble — our tests on solid black and red initially produced heart rate misreadings of up to 196 BPM before failing to read skin contact entirely. Tests on lighter tattoo colors including purple, yellow, and orange produced slightly elevated heart misreads of 80 BPM (compared to 69 BPM on the wearer's non-tattooed wrist), but otherwise did not appear to interfere with skin contact registration. … It's also worth noting that prominent scars and other potential skin aberrations can trip the Watch's sensors.

Granted, the Watch's sensors can be turned off, but doing so will disable certain of the Watch's functions, including Apple Pay (plus heart-monitoring and similar personal-fitness apps, of course). If nothing else, Apple has a two-week return policy; Watch owners with tattooed wrists might need to take advantage of that.

It might not be possible for Apple to fix this particular problem, short of abandoning light-based sensors altogether; there's nothing Apple or anyone else can do, to change such facts as “Crushed minerals packed together densely enough to look black in full daylight will block or absorb far more light than regular human skin and blood can.”

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