Will the Apple Watch’s coolest feature work for people of color?
Will the Apple Watch’s coolest feature work for people of
color?
by Alexis C. Madrigal | March 9, 2015 2:57 PM
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.
applexecs
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?
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