Heart Rate Measurement Technology

Easy Heart Rate Measurement

Epson's GPS sports watches employ heart rate measurement technology that enables wearers to measure heart rate easily.

Epson provides a program to corporate health insurance associations in support of healthy lifestyles. Association members diagnosed with, or at risk of, obesity wear a device using heart rate measurement technology that enables them to track their heart rate and allows them to regulate the intensity of exercise to keep them in the fat-burning zone. Association members are also provided with exercise and dietary advice and guidance.

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.

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.