Medical engineering on your wrist

Integrated optical sensor for measuring pulse rate and blood oxygen

Checking PulseThe new SFH 7050 sensor from OSRAM Opto Semiconductors makes it easier to monitor your own fitness levels. Pulse rates can be conveniently measured at the wrist or on a finger. There is no longer any need to wear a chest belt. The sensor can be operated so that pulse measurements on a finger can also provide information on the oxygen saturation of blood. Up to now, blood samples usually had to be taken to determine oxygen levels on the move, in other words with the aid of mobile units. The SFH 7050 needs very little power or space and is intended for use in fitness armbands, smartwatches or smartphones.

The integrated sensor consists on one green, one red and one infrared LED and one photodiode and is designed for optical measurements of the absorption of light by blood. It sits directly on the skin, sending light into the tissue. The detector records the amount of light reflected. The changes in the signal level correspond to the changes in the volume of blood flowing through the illuminated artery. The periodicity of the signal therefore indicates the pulse rate. The three LEDs can be individually controlled and are used to take measurements at different parts of the body – green at the wrist, and red or infrared on a finger. If red and infrared light is used in alternation for measurements on a finger the two reflected light signals can then be used to calculate the oxygen saturation of the blood. This function is useful, for example, for people with heart or lung problems, athletes and people engaged in activities at high altitudes. It is already possible to buy fitness armbands and smartphones that enable you to measure the oxygen saturation in your blood simply by placing your finger on a screen.

Thin-film chips are the key

Optical measurement methods for determining blood oxygen levels and pulse rate have been established for many years in the medical sector. What has so far been lacking is a light source with power consumption low enough to be installed in a wearable device. The LEDs have to have very narrow emission spectrums so that the sensor achieves the necessary signal quality. It was not until thin-film technology appeared on the scene that highly efficient LED chips could be produced with the required narrow emission bandwidths. In the SFH 7050 the green and infrared LEDs have bandwidths of +-30 nm, and the wavelength tolerance of the red LED is as low as +-3 nm. A bridge between the LED and the photodiode also helps to ensure that the sensor offers excellent signal quality. This bridge prevents light from the emitters directly reaching the receivers as this would reduce the background signal considerably. With the aid of the infrared LED and the photodiode the SFH 7050 can also function as a proximity sensor. This means it can automatically switch on or off as soon as the sensor touches or moves away from the skin. The component measures 4.7 x 2.5 x 0.9 mm so it will easily fit in fitness armbands and similar wearable devices. Chipsets for controlling the sensor are available on the market.

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