Optical sensor uses LED technology for fitness tracking

Optical sensor uses LED technology for fitness tracking

Technology News |
By eeNews Europe

The SFH 7050 sensor, which can be used in mobile devices, such as smart watches and fitness armbands, and simplifies personal heart rate or pulse rate measurements during jogging, contains three light emitting diodes with different wavelengths, based on efficient chip technology to save power and also offer high signal quality for reliable measurements. A built-in photodetector receives the reflected optical signals and is separated from the emitters by an opaque barrier.

The three emitters built into the sensor have wavelengths of 530 nm (green), 660 nm (red) and 940 nm (infrared). These enable the pulse at the wrist or fingertip and the oxygen content of the blood at the fingertip to be measured. The emitters can be controlled individually so the sensor can be used for different applications. Green light has become established as the best option for measuring the pulse at the wrist. Red or infrared emitters are generally used for measuring the pulse at the finger as they can then also measure the oxygen content of the blood (pulse oximetry).

Each of the three emitters is based on highly efficient thin-film chip technology with narrow spectral bandwidths of around 30 nm. The red LED has a specified wavelength tolerance of only ±3 nm.

“As far as measuring the oxygen content of blood is concerned, the absorption of light by blood depends largely on the wavelength of the light. The small tolerances of the red LED mean that precise measurements can be achieved with the new sensor”, explained Dr. Jörg Heerlein, Senior Manager Product Marketing at Osram Opto Semiconductors.

The photodiode integrated in the sensor (4.7 mm x 2.5 mm x 0.9 mm) has an active surface of 1.3 mm x 1.3 mm. On the one hand the diode is therefore very sensitive to light, and on the other it enables an extremely compact component to be produced. Its high linearity and excellent signal-to-noise ratio are particularly noteworthy.

The infrared LED can also be used in combination with the photodiode as a proximity sensor to start or stop the measurement automatically as soon as the sensor touches or is removed from the skin. The integrated optical barrier prevents crosstalk from the three LEDs to the photodiode and therefore corruption of the optical signal or the entire measurement.

The SFH 7050 will be presented at electronica 2014 (hall A3, booth 110) which takes place in Munich from November 11 to 14, 2014.

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