Wearables sensor measures pulse & oxygen saturation

12th August 2015
Nat Bowers

Providing excellent signal quality and low energy consumption, the SFH 7060 sensor from Osram Opto Semiconductors is designed to measure pulse rates and the oxygen saturation level of blood. It is designed for wearables such as smart watches and fitness armbands for continuously monitoring fitness levels. This integrated optical sensor contains five light emitting diodes with three different wavelengths and one high-sensitivity photodiode.

The SFH 7060 is a further development of optical sensor SFH 7050, which Osram introduced in the fall of 2014. It performs the same functions as its predecessor but with improved power consumption and signal quality. The SFH 7060 consists of three green LEDs, one red LED, one infrared LED and one large-format photodiode, which is optically separated from the emitters by an opaque barrier. It works by shining light into the skin. Different amounts of this light are absorbed by blood and the surrounding tissue. The light that is not absorbed is reflected to the detector. Absorption measurements with different wavelengths are used to determine the pulse rate and the saturation level of oxygen in the blood.

Green light is best for measuring the pulse at the wrist. The SFH 7060 is equipped with three green LEDs with a wavelength of 530nm based on the latest high-efficiency UX:3 chip technology from Osram. At their optimum operating point at a current of 20mA, they are particularly efficient and typically deliver an optical output of 3.4mW at a voltage of 3.2V per chip. The higher light output, compared with the SFH 7050, results in better signal quality and more stable pulse measurements. The lower power consumption also means longer battery life in the device.

Oxygen saturation in the blood is calculated from the different absorption rates of red (660nm) and infrared (940nm) light. The quality of the measurements depends to a large extent on the achievable signal-to-noise ratio and on the linearity of the photodetector. The integrated photodiode, with its active surface of 1.3x1.3mm, meets these requirements perfectly. In addition, the distance between the two transmitters and the photodiode in the SFH 7060 is greater than in the SFH 7050. This means the light penetrates deeper into the skin before it is reflected to the detector, which leads to more stable signals and a better signal-to-noise ratio. As in the case with its predecessor, the wavelength of the red transmitter is specified with a very narrow tolerance of ±3nm to ensure accurate measurements.

“The market for fitness tracking is growing at a rapid pace, and some of the requirements that the components have to meet are also changing rapidly,” explained Christoph Goeltner, Product Marketing Manager, Osram Opto Semiconductors. “We are working very closely with our customers so that we are always in a position to offer suitable products continuously and quickly.”

Users can now choose between a highly efficient solution designed for pulse rate measurements, an extremely compact sensor for combined pulse rate and blood oxygen measurements, and a slightly larger version with the same features but with significantly improved functionality.

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