The issue isn't limited to the Apple Watch, however. A device using similar hardware can also come unstuck if it meets a dark tattoo on a wearer's wrist. There have been reports that Fitbit's Charge HR suffers from the same issues. It comes down to how the Watch's green LED lights and photodiode sensors penetrate the skin and detect the changes in capillaries and blood vessels. The ink in darker tattoos can impact the absorption of both light and infrared, causing the issues Apple's describing above. It's certainly something you'll need to consider if you're looking to buy an Apple Watch and have tattoos on your favored wrist, but Apple notes you can connect it to external heart rate monitors over Bluetooth (not that it really helps). It might also be worth heading to your local Apple Store to check for yourself before putting down your heard-earned for a new Apple smartwatch.