Apple Watch owners with tattooed wrists have been reporting that their smartwatches are not functioning properly as the LED-based heart monitor built into the devices experiences difficulty detecting that the watch is being worn — an issue which impacts the use of Apple Pay among other features such as receiving notifications, placing phone calls and using some applications.
“I have black ink tattoos just under the watch’s sensors and have very spotty results with notifications sometimes,” reports one Reddit user. “But Apple Pay is unusable since I have to re-enter my passcode each and every time the watch screen wakes up.”
Some have discovered that turning off the device’s wrist detection feature enables notifications to be received on the watch, but that Apple Pay remains unusable as it requires that the wrist detection setting be turned on to function.
“If the watch can’t detect that you’re wearing it, the screen locks with your passcode,” Digital Trends reports. “Many users are also unable to use Apple Pay due to interference from their tattoos, because, for an extra level of security, Apple Pay will only work when the watch is being worn.”
“The heart rate sensor in Apple Watch uses what is known as photoplethysmography,” explains Apple. “This technology, while difficult to pronounce, is based on a very simple fact: 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.”
“The ink, pattern and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings,” says the company.