Red Light versus Green Light
The Future of Optical Sensing in Wearable Devices
The wearable industry is on the verge of a disruptive shift in the way devices measure human physiology enabling new devices to monitor previously invisible biosignals at an unprecedented level of accuracy.
The State of the Wearables Industry
The wearable device market is continuing to grow at a rapid pace and does not show any signs of slowing. According to IDC,
The worldwide wearable device market will reach a total of 111.1 million units shipped in 2016, up a strong 44.4% from the 80 million units shipped in 2015. By 2019, the final year of the forecast, total shipments will reach 214.6 million units, resulting in a five-year compound annual growth rate (CAGR) of 28%.
Perhaps more interesting than these figures is the fact that optical sensing is here to stay–a technology which only a few years ago was unheard of in wrist-based wearables. This revolution began in 2012 when two small companies Mio Global and Basis shipped watches with optical heart rate monitors (OHRMs).
Since 2012 we’ve not only seen a proliferation of wearable devices–including offerings from the industry’s biggest players like Apple, Fitbit, and Garmin–but we’ve also seen an increased reliance on sensor technology with compounding growth in the number of sensors per wearable device.
In fact, according to IDTechEx, the average wearable shipped in 2019 will have 4.1 different sensors–up from 1.4 in 2013. It’s also expected that OHRMs will be included in over 90% of smartwatches sold by the end 2016.
With this proliferation of both devices and sensors, scrutiny is increasing as consumers race to find the most accurate devices. This demand for accuracy is a recent evolution since customers didn’t use to have as many options in the early days that OHRMs were on the market. The technology was new, the number of devices were limited, and the excitement to be able to measure heart rate from something other than a chest strap was overwhelming.
But as the market has grown, so too has consumer scrutiny. A recent survey by the MEMS and the Sensor Industry Group polled 706 US consumers and showed that 63% of respondents believed that accuracy was a highly important feature and 74% would consider buying a new wearable if the accuracy could be shown to help them better manage their health.
With a multitude of options, which devices are actually the most accurate? And can they meet the expectations of today’s modern consumer? Or are there other technologies on the near horizon that will exceed those expectations?
The optical sensors in most consumer wearable devices use green visible light to measure heart rate through a process called photoplethysmography (PPG). PPG involves shining light onto the surface of the skin, recording the variation in light intensity that is transmitted through the tissue as blood perfuses the superficial layers, and then using signal processing to convert the transmission variations into a heart rate.
While convenient, these green OHRM products do not perform as well as the gold standard chest strap, which measures electrical signals produced by the heart. So why do manufacturers keep using green light, and why is this technology failing?
Manufacturers continue to use green light OHRMs largely because the science is straight-forward and there is a wealth of knowledge from existing products to build on. This is especially important when you’re a product-oriented company focused on speed to market and creating shareholder value. PPG is almost 150 years old, but it’s experiencing a revival for the 21st century in new consumer wearable applications because the human body is a very good absorber of green light.
Since the body naturally reduces the amount of green light penetrating the skin from external light sources, the noise floor produced by ambient light is significantly reduced–thus making it easier to measure a meaningful signal. But this convenience is a double-edged sword. By efficiently absorbing the green light from external sources, the body also absorbs the green light intentionally emitted by the light emitting diodes (LEDs) in these wearable products, which severely limits the depth that light can pass through the body. This creates significant problems near the wrist where blood perfusion is limited, weakening the overall strength of the signal.
Another complicating factor for green light sensors is skin tone. Our skin color is produced by a natural pigment found in most organisms called melanin. This pigment is produced by a specialized group of cells in the skin known as melanocytes. Melanin is a very good absorber of green light, meaning that darker skin absorbs more green light, producing an obvious problem when you can’t get your signal through. This also presents a problem when measuring heart rate through tattooed skin (remember tattoogate? Apple does.)
Moreover, green light is strongly absorbed by hemoglobin, making it unable to penetrate deeper into tissue, rendering it useless in the determination of deeper tissue parameters such as hydration muscle saturation and total hemoglobin.
All this amounts to a performance ceiling on green OHRMs that is now creating headaches for consumers and manufacturers alike.
As an extreme example, Fitbit is now embroiled in a class action lawsuit due to the persistent inaccuracy of their optical heart rate measurements. The plaintiffs assert an average variance of 24 bpm with more accurate readings from chest straps. This variance increases to as much as 75 bpm in extreme cases. Though we have not personally seen variances this large (and we have extensively tested all OHRMS on the market), we have witnessed a significant and concerning deviation from true heart rate, typically between 7–18 bpm.
Since green light technology has hit this ceiling in the accuracy and breadth of its measurement, what new technologies are being developed to fuel the wearables of the future?
To answer this problem, we turned to the world of medical devices where accuracy and precision are tightly monitored and regulatory standards exist to protect patient safety. The last time you were in the hospital, you’ll probably recall patients with red lights on their finger reminiscent of the 1980s cult movie E.T. These sensors employ a technology called pulse oximetry which uses light in the red and near infrared space (typically referred to as near infrared spectroscopy or NIRS). Pulse oximetry is a noninvasive method of monitoring multiple aspects of patient wellbeing, including heart rate and arterial oxygen saturation. This red light has been used in hospitals for decades to safely and continuously monitor patients in many different settings including the Emergency Department, Operating Room, Nurseries and Intensive Care Units.
The advantages of the red light technologies are numerous.
First, the body is a very poor absorber of red light. This allows the light signal to pass much deeper into the body (by a factor of 10x), down to the tissue levels in the body where there are larger vascular and/or tissue beds of interest. These environments provide a rich source of physiological signals that the sensor can recognize.
Second, with greater optionality of depth, red light sensors can see multiple tissue beds at the same time. This is referred to as a multi-spectroscopic approach and it allows the ability to scan multiple layered tissue beds to appreciate the differences in signals at varying levels including the epidermis, hypodermis, subcutaneous and muscle layers. Multi-spectroscopy is important for both signal purification and noise reduction–both of which can significantly improve sensor accuracy.
Third, the range of wavelengths in the near infrared spectrum spans from around 650–1100nm. This broad window presents a rich window of opportunity by which to simultaneously record several physiological parameters, all from the same sensor. This is not dissimilar to the difference in absorbance and reflectance patterns in visible light that makes a shirt appear red, a sky blue, or the grass green. This unique pattern of passage through the body creates a unique ‘fingerprint’ that can be captured thousands of times a minute to form highly detailed ‘pictures’ of what’s happening inside the body. These pictures can contain many simultaneously physiologic signals, all from a single optical source.
Recall previously, we presented NIRS medical devices that measure heart rate and oxygen saturation. When you take the time to catalog all of the physiologic parameters that NIRS can simultaneously measure, the list is actually quite staggering, 10x that of green light: hydration, heart rate, respiratory rate, pulse oximetry, muscle oxygenation, total hemoglobin (blood flow), lactate threshold, and more.
Fourth, red light is transparent to melanin and is hence not appreciably affected by differences in skin color, tattoos, freckle patterns or other normal physiologic variations that traditionally wreak havoc for green light.
THE FUTURE IS RED
You may wonder why it has taken so long for red light technology to come to market. While there are many reasonable answers here, we must remember that it was only a few years ago that optical heart rate monitoring made its debut. At that time there were no real alternatives. In this vacuum, wearable companies discovered a window of opportunity.
These companies — predominantly product companies — began searching for meaningful solutions. The technology was still in its nascent form and product owners with very real profitability expectations and shareholder concerns leveraged what would provide them with the fastest speed to market. This followed the classic lean methodology of releasing quickly and iterating soon afterwards.
While these bigger companies were focusing on profit, however, others saw an opportunity to focus on the science. We at BSX Technologies are one such company. BSX is a small sensor startup based in Austin, Texas.
Our premiere product, the BSXinsight, leveraged this red light technology becoming the world’s first ever wearable lactate threshold sensor. It is a revolutionary tool for endurance athletes and has been a huge success. It’s been independently validated by third party research centers and is now used by the US Olympic teams and countless other athletes at all levels.
We have spent the past 4.5 years focusing on extensive research and development that is positioning us to solve the two most pressing problems with wearables today: accuracy and breadth of sensing capability, including — for the first time ever — hydration monitoring.
After four years studying sweat in our labs, LVL will be the next product to utilize our technology, measuring hydration like no other device can. It represents another radical shift in how people will approach their health and training, delivering unique insights by measuring what matters.
We took the long view on wearables and developed what we knew would be a superior (albeit, more difficult) technology. Now, 4.5 years later, that bet is paying off. With the pressure that consumers are rightly applying on wearable manufacturers, the market is now scrambling to find a solution. Apple and Garmin have already begun integrating red light LEDs into their wearables, as they search for implementation strategies to solve the hard development challenges that NIRS poses.
Product leaps often require the development of new technologies. Classic examples are micro hard drives enabling the portable music player and capacitive touch screen technologies enabling the smart phone. In the case of hydration monitoring, low-cost, high brightness near-infrared LEDs with narrow linewidths are the technology advancement underpinning new product advancements, as these LEDs enable wearable monitors to look deeper into tissue while maintaining high accuracy.
For all these reasons we believe the future of wearables is bright. Bright red.