This image was removed due to legal reasons.

On the back of the new Apple Watch, there are two sets of lights and sensors. One sends out and receives visible light, while the other works in the infrared part of the spectrum. Together, they form the technical core of the watch's coolest feature—a heart rate monitor.


This kind of design—using optical sensors to measure pulse—is not unprecedented, but Apple, of course, claims to have improved the system over competitors like the Basis Peak, Samsung Gear Fit, or Mio's line of activity monitors. Which would be a good thing, because these systems have struggled with their accuracy.

"Everyone has been joking about the inaccuracy of the prior technologies and that's what we've got to get past to get people really comfortable with this," UCSF's Michael Blum told me last year at a Samsung event.


And because of the technology that most smart watch makers, including Apple, have chosen, the problems may be exacerbated if you have darker skin.

Why?

The technology is based on a laboratory technique called photoplethysmography, which uses a "pulse oximeter" to measure the components of the blood. A light pulse is sent through the skin and picked up by a sensor. Based on how the light scatters, the sensor can measure how oxygenated the blood is, which allows one to calculate heart-rate based on how often fresh, arterial blood is pumping through.

In the medical context, this oxygen level tends to be measured by a little finger cuff. So the light is on one side of the finger and the sensor is on the other. Even in this more controlled setting, there have been conflicting scientific reports about whether skin color impacts the accuracy of readings.

But no one wants to wear a finger cuff around while they work out, so fitness band companies have had to make do with the wrist. Even a flashlight can shine easily through one's fingers, which makes the sensing task easier. A wrist-mounted sensor has to rely on the light that's reflected back from the blood, not shining through it. That's harder.


The skin on the back of the wrist tends to be darker, too, especially for people with more melanin. "The light has to penetrate through several layers…and so the higher the person is on the Fitzpatrick scale (a measure of skin tone), the more difficult it is for light to bounce back," explained Basis COO Bharat Vasan to CNET.

Basis, and presumably Apple, try to compensate for skin tone by shining brighter light when someone's complexion is darker.


Without knowing the precise nature of Apple's lights and sensors, it's hard to know whether the device will have hard limits on its ability to work with different skin tones. Apple probably had to choose a center point for calibrating the device, too, making the trade-off between lighting power and battery drain.

Like many technology companies, Apple's executives form a key early testing team. The company's top people become the company's default customers. And if people with dark skin do have problems with the watch's heart-rate sensor, none of Apple's top executives would have experienced it.


This image was removed due to legal reasons.

Using optical technology to measure heart rate might also mean that people with darker skin will experience a greater drain on their watch battery, because of the more intense light required to power the sensor. The light required to do accurate measurements was enough to push Apple-competitor Jawbone away from an optical system to an electrical one: "Because bioimpedance requires significantly less power compared to optical sensors for same level of accuracy, we can deliver a smaller form factor and longer battery life," they claim.


We won't know for sure until lots of these watches are strapped onto lots of different wrists, but consider the broader idea: wearables want to be intimate devices that interact deeply with our bodies. Are companies like Apple really ready to reckon with the diversity of bodies that exist in the world?