The Apple Watch has a bit of trouble monitoring users’ heart rates when they have tattoos on their wrists, according to a TechCrunch article by Sarah Perez. Apple changed its support documentation when it discovered the problem via user feedback in April.
The problem has to do with the sensor’s use of LED lights, which flash hundreds of times per second in order to read (together with light-sensitive photodiodes) blood flow in a user’s wrist. Tattoos in that area – especially very dark ones – can obscure these light readings, and thereby throw off the cardiac monitor.
Interestingly, this technology suggests that the Apple Watch’s biometric sensor is actually more sophisticated than advertised. Perez quotes an iFixit article which noted that the cardiac sensor is “actually a plethysmograph—it looks and acts like a pulse oximeter, but Apple isn’t claiming it can measure your blood oxygen level,” and added that the company may not have wanted to promote such technological capabilities to avoid running into problems with respect to FDA regulations.
It’s a validation of previous speculation that the Apple Watch would feature advanced biometric capabilities, and it may also be a hint of future software advancements that could take advantage of those capabilities, perhaps in the form of an Apple HealthKit upgrade or a similar app.