Wearable Device Technology for Accurately Monitoring Heart Rate from the Wrist

Posted on November 2014

Measuring Heart Rate from the Wrist

In 2011 Epson began providing a service to corporate health insurance associations in support of healthy lifestyles. Association members diagnosed with, or at risk of, obesity wear an Epson heart rate monitor that tracks their heart rate and allows them to regulate the intensity of exercise to keep them in the fat-burning zone. They are also provided with exercise and dietary advice and guidance.

The heart rate monitor has a sensor that accurately tracks the wearer's heart rate from the wrist pulse even as they walk.

Our Pulsense activity trackers and Runsense sports monitors employ improved heart rate measurement technology that is based on original heart rate measurement technology we have developed over two decades.

Epson's heart rate monitors measure heart rate at an accuracy of ±2%* even when the user is walking or running. That's as good as a chest strap solution. A key feature of our heart rate monitors is convenience. Just strap one on, and you can count on it to count on you.

* Measurements were taken under testing conditions specified by Epson. Compared to an Epson chest strap heart rate monitor when sampling at 10 second intervals. Excludes cases where heart rate changes rapidly.

Engineer Interview

"I want to create products that benefit people and change lives."

Heart rate measurement technology developer, S Planning Design Department, Sensing Systems Operations Division

*The division name is as of November 2014.

Watch the interview

Optical Heart Rate Monitoring

Monitoring Slight Variations in Light

The light-absorbing property of hemoglobin is used in the measurement of heart rate. Light from a green LED on the underside of the monitor is shone on blood vessels just under the skin. The light that is not absorbed but reflected back is captured by a photodetector.

The photodetector produces an electrical signal when light strikes it. This analog signal is converted into a digital signal, and slight changes of this signal are used to measure heart rate.

Most wrist-worn heart rate monitors operate on this same principle. However, Epson's sensing system, which includes a unique photodetector manufactured by Epson in Japan, provides accurate heart rate readings at low power.

Semiconductor and Optical Technologies Combined to Produce a Unique Optical Filter

Filtering Unwanted Light

To increase measurement accuracy, Epson provides two types of optical filters on top of the photodetector that collects reflected LED light: an angle limiting filter and a multi-layer thin film filter. Light from the LED is not the only light that strikes the photodetector. The photodetector may be hit with unwanted sunlight when outdoors, artificial light when indoors, and LED light reflected from the surface of the skin or from the monitor's case. The filters remove this unwanted ambient light.

The angle limiting filter and multi-layer thin film filter enable efficient, accurate heart rate measurement.

Advanced semiconductor technology is used to form the angle limiting filter as a tiny MEMS structure on the photodetector. This filter cuts unwanted LED light that is reflected from the skin or the monitor case. The multi-layer thin film filter is comprised of dozens of layers deposited on top of the angle limiting filter. This thin film filter contributes to accuracy by blocking noise generated by sunlight and other sources of intense light.

These two filters are essential for accurate measurement and were made possible only by combining the semiconductor and optical technologies that we developed over decades.

One way to effectively prevent sunlight and other ambient light from reaching the photodetector is to make the monitor larger and increase the area that is in contact with the skin, but that solution sacrifices comfort.

Our filter technology allowed us to produce small monitors that are nonetheless highly accurate.

The accuracy of our measurement technology also permitted us to reduce the amount of light output by the green LED, thus saving power and extending battery life.

A Dual-Sensor Construction

On the Runsense GPS sports monitors, we used a dual-sensor construction to track heart rate even more accurately during vigorous exercise, such as running. In this construction there are two photodetectors: a main sensor and a secondary sensor.

The dual sensor used in Runsense sports monitors.
Light emitted by the LED shines on blood vessels under the skin, and the reflected light is collected by the two photodetectors (the rectangular, pinkish-purple parts in the center of the photograph).

Motion Noise Removed with an Algorithm

Our heart rate monitors are also equipped with accelerometers to further increase heart rate measurement accuracy. When a monitor is jostled about due to arm motion when the user exercises, the reflected LED light scatters and becomes motion noise, which interferes with heart rate measurement. To compensate for this, we use the accelerometer to track the user's movements and a built-in algorithm to remove this motion noise.

Accurate measurement, comfortable fit

We at Epson engineer our wearable devices for both accuracy and comfort. For example, we constantly tweak the designs of mechanical structures and wristbands to ensure a snug, stable fit that will provide superior accuracy as well as maximum comfort.

Our heart rate measurement technology is a key technology for vital sign sensing, and we will strive to further develop this technology to bring new value to customers in the sports, healthcare, and medical fields.