Well, that inks.

The newly released Apple Watch may be problematic for people with dark tattoos on their wrist, the company acknowledged after complaints from customers.

“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,” according to the official Apple Watch support page, updated on Wednesday.

Apple did not respond to requests for comment.

The issue comes down to light.

The Apple Watch’s green light sensor that tracks the wearer’s heartbeat needs to be bounced off blood flow for measurement, said Steven LeBoeuf, president of Valencell, which develops biometric sensor technology – though not specifically for Apple.

“The key to obtaining accurate biometric data in the wrist lies in the sensor’s ability to measure pulsatile blood flow in blood vessels less than 1 to 2 millimeters from the surface of the skin, while also rejecting noise,” he said.

Green light is considered safer because it does not penetrate deep into the skin. “But that’s its blessing and its curse,” LeBoeuf said. The green light, while less harsh, is easily absorbed by dark ink.

“The darker the ink and the deeper the tattoo, the more problems you will have,” he said. Scars may also be a problem for some users because they pose a thicker layer to penetrate, he added.

A combination of green and yellow light wave sensors, and amplified blood flow detection, probably would fix the tattoo hurdle, LeBoeuf said.

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