Compact Osram sensor improves fitness monitoring

Osram Opto Semiconductors presents its first integrated optical sensor

SFH 7050 is the first integrated optical sensor from Osram Opto Semiconductors for automatic fitness tracking. The sensor is used in mobile devices, such as smart watches and fitness armbands, and simplifies personal heart rate or pulse rate measurements during jogging for example. The sensor contains three light emitting diodes with different wavelengths, based on highly efficient chip technology. This helps save power and also offers high signal quality for particularly 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).

Minimum tolerances for consistent measurements

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.

Compact photodiode for small components

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.

SFH 7050 is one of the innovations that Osram Opto Semiconductors will be presenting at electronica 2014 (hall A3, booth 107). The fair will take place in Munich from November 11 to 14, 2014.