Seeing Through the Skin

Working for a manufacturer of electronic components, I’m always wondering where all these “parts” end up being used. And in doing so, I learn about really exciting applications and feel happy to have somehow contributed to the development of a particular end-product. For example, Würth Elektronik manufactures LEDs that emit infrared light and matching detectors. Now many of you may wave this aside and say: “Remote controls, light barriers – that is nothing new.” However, allow me to highlight my own personal favorite among infrared applications to you: pulse oximetry.

Pulse oximetry is a non-invasive method for determining hemoglobin (Hb) saturation through oxygen in the blood and heart frequency. Simply put: Infrared light pointed at the fingertip. The absorbed light allows drawing conclusions about blood circulation.

This is how it works: For the determination of the fractional oxygen saturation, the different wavelengths for red (660 nm) and infrared (940 nm) light come into play.

Light emitted from a light source is absorbed differently by veins, capillaries and tissue. Photodetectors measure the non-absorbed light. A micro-controller processes the signals to calculate the percentage of oxygen saturation (SpO2), red hemoglobin with oxygen (oxyhemoglobin HbO2) and hemoglobin without oxygen (deoxyhemoglobin Hb).

HbO2 absorbs more infrared light and less red light than Hb and due to the higher reflection of red light it appears bright red. By contrast, Hb absorbs more red light and thus appears darker. The processes in the pulse oximetry devices are based on this difference between HbO2 and Hb. There are two different measurement methods: the transmissive oximetry, the screening of a fingertip or earlobe, and the reflective oximetry, which is used for example in a fitness tracker on the wrist.

I Can Hear Your Heartbeat

It is also very interesting how these devices take pulse measurements. It’s all down to the fact that veins, bones and tissue show relative constant absorption characteristics. The light absorption in arterial blood, however, varies due to the pulsating volumetric change within one heartbeat. These fluctuations in relation to the percentages of the absorption wavelengths also provide an indication of oxygen saturation in the blood.

Want to find out more? This technical article explains everything in more detail.

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