Red vs. Green: Does the Light Sensor in Your Wearable Matter?

Wearables are on the wrists of millions of people today, gathering information on everything from the simple metrics of steps taken to the advanced biosignals of heart rate variability and blood oxygenation.

Despite wearing these new technologies every day, many of us aren’t aware of the hardware and processes that go into tracking these metrics––or whether the information we’re getting is as accurate as it could be.

For example, wearable devices use either red or green lights to measure heart rate through a 150-year old process called photoplethysmography (PPG). PPG works in the following way: a LED light source shines onto the skin and light bounces back into the photodetector, recording how the light’s intensity changes as blood perfuses through the tissue. Data picked up by the photodetector use signal processing algorithms to convert the variations into a heart rate.

Why is Green So Prevalent?

Green light PPG sensors are used in the majority of optical heart rate monitor (OHRM) products for a few reasons: there is a vast amount of existing knowledge of the technology because of its common use, there is a great signal-to-noise ratio, and more resistance to motion artifacts.

Unfortunately, this convenience comes at a high price. Our skin absorbs green light from the light emitting diodes (LEDs) very well—this is problematic as it severely limits the amount of light that passes through the tissue and weakens the overall signal.

Skin tone, specifically the amount of melanin, affects the skin’s ability to absorb green light and further increases the variation in reporting accuracy. Finally, hemoglobin strongly absorbs green light, making it unable to reach the deeper tissue to extract any deeper physiological insights.

Betting on Red

Red light PPG sensors (also called pulse oximeters) utilize light in near-infrared spectroscopy (NIRS) and are widely used by doctors’ offices and hospitals, where accuracy is closely monitored and absolutely essential for medical use.

Our bodies do not absorb red light well which is actually a good thing; it allows the transmission to penetrate 10x deeper into multiple tissue layers in order to obtain a number of biometric signals (such as hydration, muscle saturation, total hemoglobin, and more) that a green light sensor can never see. Additionally, tattoos, freckles, and melanin in the skin do not affect readings by red light sensors.

Despite the massive benefits, red light PPG sensors have their development challenges, including a higher signal-to-noise ratio and susceptibility to motion artifacts. This creates a need for advanced and robust signal processing with the ability to filter out the noise (motion) in order to produce a high fidelity signal—not an easy task!

At Biostrap, we believe the future is bright red. We utilize red light PPG sensors to their full potential in order to achieve our mission of providing users with the tools to unlock total health.

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