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Now that the Apple Watch is being shipped to the general public, some users are encountering an unanticipated problem – darkly-colored tattoos may interfere with the watch’s heart rate sensor and wrist detection.
The problem was first brought to our attention on a Reddit thread. According to his post, the watch would “lock up every time the screen went dark,” prompting the wearer for a password and the wearer was unable to receive notifications. The issue cleared when the watch was placed against his un-tattooed skin. Users on Twitter and other social media sites reported similar issues, as well as the heart rate sensor wildly miscalculating the user’s heart rate.
The problem, it seems, has to do with photoplethysmography – the method the Apple Watch sensors use to measure your heart rate. The Apple Watch support page contains an explanation as to how this technology works:
“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.”
Apparently, darkly-colored ink (especially red and black ink) on the skin absorbs the light from the watch, hindering its heart-rate and wrist detection capabilities. It should be noted that this is issue is limited to tattoo ink, and is unaffected by skin pigmentation.
One temporary solution has been offered so far – turning off wrist detection allows for push notifications to reach the watch. However, with wrist detection turned off, Apple Pay will not work via the watch. Hopefully a firmware update can fix the issue. Until then, if you have tattoos on your wrist, be sure to demo the watch in store before buying.