Optical sensor simplifies wearable measurements

9th October 2014
Siobhan O'Gorman

Designed for automatic fitness tracking, the SFH 7050 optical sensor has been introduced by Osram Opto Semiconductors. The sensor, which is suitable for smart watches and fitness armbands, simplifies personal heart rate or pulse rate measurements. To reduce power consumption and increase signal quality, the sensor contains green, red and infrared light emitting diodes.

The emitters are based on thin-film chip technology which provides spectral bandwidths of approximately 30nm. To ensure an accurate measurement of the pulse at the wrist or fingertip and the oxygen content of the blood at the fingertip, the emitters provide wavelengths of 530nm (green), 660nm (red) and 940nm (infrared). The emitters can be controlled individually, allowing the sensor to be used for a variety of applications. The green light emitter is recommended for measuring the pulse at the wrist and the red and infrared for the pulse at the finger and the oxygen content of the blood at the fingertip.

The reflected optical signals are received by an integrated photodetector, which is separated from the emitters by an opaque barrier. This barrier prevents crosstalk from the three LEDs to the photodiode, and corruption of the optical signal or measurement. The photodiode, which measures 4.7x2.5x0.9mm, offers high linearity and signal-to-noise ratio. Featuring an active surface of 1.3x1.3mm, the photodiode enables a compact sensor.

A specified wavelength tolerance of only ±3nm is offered by the red LED. To start or stop the measurement automatically, the infrared LED can be used in combination with the photodiode as a proximity sensor. 

“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,” explained Sevugan Nagappan, Product Marketing Manager, Osram Opto Semiconductors. “The small tolerances of the red LED mean that precise measurements can be achieved with the sensor.”

Osram Opto Semiconductors will be presenting the SFH 7050 at electronica in hall A3, booth 110.

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