Recently, we reported that you might have a problem if your wrist is heavily tattooed and you own an Apple Watch. We discovered this problem via a thread on Reddit, where users with wrist tattoos reported difficulties using some of the wearable device’s key functions. Since then, Apple has quietly updated one of its support pages for the smartwatch, confirming the problem.
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 heart rate sensor in Apple Watch uses a technology called photoplethysmography, which is based on the simple fact that blood is red because it reflects red light and absorbs green light. The green LED lights on the bottom of Apple Watch are paired with light-sensitive photodiodes to detect the amount of blood flowing through your wrist.
If you have tattoos on your wrist with deep red or black ink, this can prevent the photodiodes from functioning properly. If that is the case for you, Apple recommends pairing your smartwatch with an external heart rate monitor like a Bluetooth chest strap.
If your tattoos are causing problems with authorizing and unlocking your Apple Watch, you can disable wrist detection to solve the problem. According to a report from 9to5Mac, Apple Watch will continue to switch on when you raise your wrist, even with detection switched off.