Sensor integrates LEDs for fitness tracking wearables

September 11, 2015 // By Paul Buckley
Osram Opto Semiconductors has optimized the company's portfolio for the optical measurement of pulse rates and blood oxygen levels.

The SFH 7060 sensor from Osram Opto Semiconductors is designed to measure pulse rates and the oxygen saturation level of blood. The offers excellent signal quality and low energy consumption. The main applications for this component include mobile devices such as smartwatches and fitness armbands – also known as wearables – for continuously monitoring fitness levels. The integrated optical sensor contains five light emitting diodes with three different wavelengths and one high-sensitivity photodiode.

The SFH 7060 performs the same functions as its predecessor but has improved power consumption and signal quality. 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. The device works by shining light into the skin. Different amounts of this light are absorbed by blood and the surrounding tissue. The light 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. SFH 7060 is equipped with three green LEDs with a wavelength of 530 nm based on the latest high-efficiency UX:3 chip technology from Osram Opto Semiconductors. At their optimum operating point at a current of 20 milliamperes, they are particularly efficient and typically deliver an optical output of 3.4 milliwatts at a voltage of 3.2 V per chip. The higher light output compared with SFH 7050 results in better signal quality and more stable pulse measurements. The low power consumption also means long battery life in the device.

Oxygen saturation in your blood is calculated from the different absorption rates of red (660 nm) and infrared (940 nm) 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 1.3 mm x 1.3 mm 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 of its predecessor, the wavelength of the red transmitter is specified with a very narrow tolerance of ± 3 nm to ensure accurate measurements. The spectral bandwidth of the infrared and green LEDs is 30 nm in each case.