Wrist detection on the Apple Watch may not work properly for some people with lower-arm tattoos, as they appear to fool the device's sensors, some early adopters of the device have said.
One owner noted in a Reddit post on Monday that when he wore his Watch on a wrist tattooed with black ink, he couldn't receive any notifications, and the device would lock every time it went dark. Before trying to contact Apple, though, he tested the Watch against his unmarked hand, which suddenly allowed it to work as intended.
Turning off wrist detection entirely was said to solve the problem, but at the expense of features like Apple Pay.
Wrist detection relies on light sensors on the back of the device which also serve as a heart rate tracker through a method known as photoplethysmography. By flashing infrared and/or green LED lights, the Watch can detect blood flow. Dark skin color could theoretically affect accuracy, although Apple states that the Watch will increase LED brightness and sampling rate to deal with tough reads.
The issue could be down to the materials that tattoo ink is made from. Many pigment bases are derived from heavy metals, like mercury, copper, or nickel, which may interfere with the sensors.