Red light, green light: Why Fitbit's sensor shake-up is a huge deal

Switching up tech could lead to much more than improved HR performance
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We learned a couple of things from those Fitbit smartwatch pictures that leaked this week – and a bigger picture of what the company's new device is emerging.

For starters, it's going to look like a Fitbit Blaze, which isn't great news. But design complaints aside, there's something much more interesting. I'm talking about those red lights on the back of Fitbit's watch.

Look at any other Fitbit press renders in the past and you'll spot that the lights on the back are always green. Is it a really big deal? We think it is and here's why.

Fitbit's sensor story so far...

Red light, green light: Why Fitbit's sensor shake-up is a huge deal

Since Fitbit introduced the Charge HR – its first fitness tracker to include a wrist-based heart rate monitor – the company has taken a lot of heat over the accuracy of its heart rate data.

In fairness, it's not just Fitbit that has had this kind criticism levelled at it. But when you are the poster child for wearable tech, you can expect to bear the brunt of the flak because you're the most popular one out there.

Read this: The best smartwatches to buy right now

So Fitbit has taken action. It seems highly unlikely that the company would move its tech away from the wrist and into the ears, on the head or nearer to the chest, which could lead to improved readings and reduce the artefacts or 'noise' that can impact on readings like the fit or movement. Fitbit is all about wrist-based devices right now.

Where it could be making moves is on the optical sensor that's already packed into its devices potentially switching from a green light based sensor technology to a red one. Can a change in colours really make a difference? Here's what we know about the whole green light/red light debate and what the startups, companies and experts have to say about it.

Green light versus red light

Red light, green light: Why Fitbit's sensor shake-up is a huge deal

From the many health and fitness companies we've spoken to over the past year or so, one thing is clear and that is optical sensors are here to stay. It's been predicted that in 2017, 90% of wearable shipments will include optical sensors. When we spoke to startup BSX Athletics, who raised just under $1 million to build its hydration monitoring wearable, this is what founder and CEO Dustin Freckleton told us:

"Optical sensors are so powerful because they have that ability to non invasively measure things in the body. These biosignals are far more important than the mechanical signals that we are currently measuring like the simple steps. It actually reflects what the body is trying to tell us, things it's saying but we can't hear."

Optical sensors are here to stay but the exact make up of those optical sensors whether they're from Fitbit, Apple or Garmin are up for debate. Most of the wearables you can get your hands on use green flashing LED lights and take measurements via method called PPG or photoplethysmography to give its full name. The tech has been around for years and has quickly become the standard for wearable heart rate sensors that are not chest straps. This is how the Apple Watch or the Garmin Vivoactive HR provides heart rate readings, shining light on the surface of the skin, detecting the volume of blood flow that helps produce the data and displays it on your wearable.

What has been found is that these green lights can experience problems when trying to get a signal – namely reaching the depths the body to pull out that data. Skin tone or even tattoos have been known to also cause issues for green light sensor tech as well.

Red light, green light: Why Fitbit's sensor shake-up is a huge deal

So let's talk about red light sensor technology.

It's a method that can be found in most hospitals and has been used in the medical industry for tests like pulse oximetry, which measures oxygen levels in the body. The body is a poor absorber of red light allowing the light to pass much deeper into the body and a larger volume of tissues to help provide more insightful data and could lead to improved accuracy with biometric data like heart rate.

Biostrap is another wearable that embraces red LEDs and infrared PPG instead of the green tech and CEO Sameer Sontakey recently explained to us why it opted for red over green-based sensor tech:

"Everyone is using a green LED, with a 540 nanometer wavelength," says Sontakey. "The body absorbs green really well, it's great for reducing signal distortion, but it doesn't penetrate deep. A lot of it is absorbed by your body so you don't get anything deeper than heart rate."

We will get onto that whole "deeper than heart rate" thing in a moment, but it's similar story to what Freckleton told us – calling the accuracy of green light monitoring "incredibly poor" believing that it's unfortunate that it's become such a staple in wearable devices. David Carr, associate strategy director at Frog who has worked with Freckleton on his hydration monitoring wearable also waded in on the red versus green debate:

"The red light technology enables optical sensing, particularly for heart rate that's vastly more accurate than anything else on the market today. That's one of the biggest complaints about the green light technology. That's really just scratching the surface of what the red technology can do. Using the longer wavelengths, we can move the signals much deeper inside of the body and we can see more interesting signals."

Dr. Steven LeBoeuf, co-founder and president of Valencell, a company that develops biometric sensors for the likes of Samsung and Suunto, sees things differently when we start talking about the benefits of using different coloured light sensors.

"We've found that the full spectra of purple-to-IR light can be used to assess heart rate in a wearable device near the skin. However, some wavelengths work better than others, depending on the use case," he explained.

"Each wavelength of light has a benefit depending on what biometric you're trying to measure, how you're trying to measure it, the use case in which it is measured, and the measurement accuracy requirements.

"Using a combination of red and IR wavelengths can be used to assess SpO2 (blood oxygenation) when worn at the finger, forehead, or ear, and to assess muscle oxygenation when worn at the wrist, arm, or leg. Moreover, using IR light can help with reducing sunlight noise (the sun generates a much higher percentage of green light than IR light), helping folks sleep at night (no green light glow in bed), and with helping to remove motion noise from signals."

It's about so much more than HR

Red light, green light: Why Fitbit's sensor shake-up is a huge deal

Now we can talk about how the red light sensor tech could go beyond heart rate, which could really open up the possibility that Fitbit is exploring delivering other biometric data.

BSX Athletics is using red lights because it's able to recognise the water signals at various depths of the body to measure hydration and it's understood that the red light method will enable sensors to look not only deeper into the body, but at multiple tissues beds at the same time to capture richer data. Biostrap is using it to monitor heart rate variability, oxygen saturation and respiratory rate with its clinical grade PPG sensors. What is clear is that it has the potential to really make our fitness trackers and smartwatches a whole lot smarter.

Ilkka Korhonen, VP of technology at heartbeat data analytics firm Firstbeat believes that the possibility Fitbit could change from green to red could actually be tied to the idea of serving up new biometric data. "Based on the physiology and physics, red seems to bring little benefit over green in wellness and sports monitoring, especially during motion," he told us. "However, multi-wavelength solutions may give new insights e.g. for health applications." This could well tie into Fitbit's greater push into the health space with the company's lead research scientist and founder James Park having already confirmed that it is working on devices to track and diagnose sleep apnea for instance.

Is the future red or green?

So why haven't the likes of Apple, Garmin and Fitbit done this before? This is a key question. According to this Medium piece by BSX, it comes down to the fact that the science behind the green light tech is more straightforward and likely tied to getting devices quickly out to market, which clearly means the tech may not always be up to scratch for all users, just most of them.

Freckleton did tell us that Apple has put red light into sensors inside the Apple Watch, it's just not using them yet, but it's starting to put the hardware in there collecting the data. Garmin is apparently doing the exact same thing with their watches.

Valencell's LeBoeuf believes that there's every chance Fitbit might not be ditching the green lights altogether but combining it with the red light/IR sensor technology. "We should point out from the 'leaked photo' that it is not clear that green wavelengths are not being used at all," he told Wareable. They could be. Remember, the Apple Watch uses both Green and IR wavelengths."

So things are changing and if a change from green to red means we can actually have more wearables spitting out not only more reliable data but richer information, that can only be a good thing.

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Michael Sawh


Michael Sawh has been covering the wearable tech industry since the very first Fitbit landed back in 2011. Previously the resident wearable tech expert at Trusted Reviews, he also marshaled the features section of

He also regularly contributed to T3 magazine when they needed someone to talk about fitness trackers, running watches, headphones, tablets, and phones.

Michael writes for GQ, Wired, Coach Mag, Metro, MSN, BBC Focus, Stuff, TechRadar and has made several appearances on the BBC Travel Show to talk all things tech. 

Michael is a lover of all things sports and fitness-tech related, clocking up over 15 marathons and has put in serious hours in the pool all in the name of testing every fitness wearable going. Expect to see him with a minimum of two wearables at any given time.

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