The Apple Watch is designed to automatically detect when it’s on your wrist so that you can receive notifications, access the watch’s functions, and track your heart rate. But Apple fans who already have an Apple Watch in hand — or on their wrist — report that the watch doesn’t always work properly on tattooed skin.
In a thread begun on Reddit’s Apple subreddit on April 28, early adopters of the Apple Watch complained that they are having issues receiving notifications on their Apple Watches. The problem seemed to be that these users have tattoos on the skin where the watch sits against their wrists. As Quartz’s Mike Murphy points out, the Apple Watch locks and requires a passcode when it doesn’t detect skin contact, and a tattoo may fool the watch into thinking that there isn’t skin contact when there really is. Some in the Reddit thread suggested that the issue is caused by the sensor technology that Apple uses to check for skin contact and measure the wearer’s heart rate.
Apple’s website explains that the heart rate sensor on the Apple Watch uses a technology called photoplethysmography. “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 therefore the green light absorption in the area, is greater. The Apple Watch flashes its green LEDs hundreds of times per second, and based on how the light scatters, the sensor can determine how oxygenated your blood is and calculate the number of times your heart beats each minute. The Apple Watch’s sensor can also use infrared light, which it does when it reads your heart rate every 10 minutes.
Apple warns that “Even under ideal conditions, Apple Watch may not be able to get a reliable heart rate reading every time for everybody. And for a small percentage of users, various factors may make it impossible to get any heart rate reading at all.” Apple says that factors like skin perfusion, or how much blood flows through your skin, or motion can impact the ability of the sensors to take an accurate reading. The page, modified on April 29, also notes that “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.”
Steven LeBoeuf, a mechanical engineer scientist and the co-founder of Valencell, which supplies biometric sensors to companies like Jabra and Scosche, tells Murphy that Apple is just one of many manufacturers of wearable technology to rely on sensors that send green light toward the skin. (And Reddit users noted that other wearables, like the Basis Peak and Fitbit Surge, don’t function properly on tattooed skin.) Green light is absorbed by the ink used in most tattoos, and LaBoeuf says that “Green light is a problem for anything dark, especially for tattoos.”
The issues could affect not only Apple Watch wearers with tattooed wrists, but also those who comprise a much larger potential audience: those with darker skin. LaBoeuf says that the green light used by the Apple Watch is more likely to be absorbed by skin with higher melanin content. And even if the sensors work when a person is sitting down, the darker the wearer’s skin is, the less likely the sensors are to capture data when the person is moving. “The signal to noise ratio will be much lower for people having higher melanin content.”
Writing for Fusion, Alexis C. Madrigal explains that in the medical world, photoplethysmography often uses a pulse oximeter in the form of a finger cuff to measure the components of the blood; the light goes on one side of the finger and the sensor on the other. Scientific reports have offered conflicting assessments of whether skin color impacts the accuracy of those readings. And getting the technology right with a gadget worn on the wrist, not wrapped around the finger, is even more complex.
Unlike a finger-worn sensor, a wrist-mounted sensor relies on light that’s reflected back from the blood, not shining through it. The skin on the back of the wrist tends to be darker, particularly for people with higher melanin. The higher a person is on the Fitzpatrick scale, which measures skin tone, the more difficult it is for the light to bounce back. Apple likely tries to compensate for skin tone by shining brighter light when the wearer’s complexion is darker, but there are also likely limitations on the Apple Watch’s ability to work with different skin tones. And using optical technology to measure heart rate could mean that users with darker skin will see a greater drain on their watch battery thanks to the more intense light needed to power the sensor.
For users already dealing with an Apple Watch that doesn’t work as planned on their wrists, you can switch off wrist detection to prevent the watch from asking for a passcode each time the screen is unlocked. But that will also switch off Apple Pay. And though you can use a Bluetooth-enabled chest strap to get an accurate reading during workouts, that won’t solve the issue of the Apple Watch being unable to tell that it’s on your wrist. A major challenge in creating wearable devices is the diversity of the bodies on which they’ll be worn. The problems with the Apple Watch indicate that tech companies may not yet be able to deal with that diversity as seamlessly as they’d planned.